Morten Hertzum
email: hertzum@hum.ku.dk

Looking for "the Big Picture" on a Small Screen: A note on Overview Support in Cooperative Work

Keywords: overview, awareness, cooperative work, small-screen devices

Large, shared displays -such as electronic whiteboards -have proven successful in supporting actors in forming and maintaining an overview of tightly coupled cooperative activities. However, in many developing countries the technology of choice is mobile phones, which have neither a large nor a shared screen. It therefore appears relevant to ask: How may mobile devices with small screens support, or fail to support, actors in forming and maintaining an overview of their cooperative activities? This note contrasts the strengths of large, shared displays with those of small, mobile devices and briefly describes the mKrishti app for providing fishers in the Maharashtra state of India with an overview of the locations where there is likely to be many fish.

Introduction

In tightly coupled collaboration, the actors coordinate their activities by monitoring what the others are doing and by displaying their own activities for others to monitor [START_REF] Schmidt | The problem with 'awareness[END_REF]. The ways in which this monitoring and displaying is accomplished vary across contexts, as evidenced by the considerable research on awareness [e.g., 2] and overview [e.g., 3]. Unless the actors are permanently co-located, awareness and overview must be mediated by technology. These technologies include large, shared displays, which are becoming increasingly common in settings where the actors are locally mobile but co-located part of the time. Hospitals are a prominent example of such work settings. In European and North American hospitals wall-mounted electronic whiteboards are replacing dry-erase whiteboards [START_REF] Rasmussen | Electronic whiteboards in emergency medicine: A systematic review[END_REF], and the clinicians who use these large, shared electronic displays experience an improved overview of their work [START_REF] Hertzum | Electronic emergency-department whiteboards: A study of clinicians' expectations and experiences[END_REF]. In contrast, the technology of choice in many developing countries is mobile phones with comparatively small screens [START_REF] Nottebohm | Online and upcoming: The internet's impact on aspiring countries[END_REF][START_REF] Etzo | The mobile phone 'revolution' in Africa: Rhetoric or reality?[END_REF]. Thus, in systems that target developing countries the need for supporting actors in maintaining an overview of their cooperative work will often have to be accomplished on a small screen. Before proceeding it should be noted that it obviously is a simplification to associate display size with country. The argument is neither that large, shared displays such as electronic whiteboards are non-existent in developing countries, nor that small interfaces are rare in developed countries. Rather, the argument is that the ways in which large, shared displays support actors in maintaining an overview are irrelevant in settings characterized by small interfaces. To develop for these settings we need to understand how small interfaces may support, or fail to support, actors in maintaining an overview of their cooperative work. Clearly, this need is accentuated if the application of large, shared displays is not feasible, economically or otherwise.

2

Overview and Large, Shared Displays

Hertzum and Simonsen [START_REF] Hertzum | Visual overview, oral detail: The use of an emergency-department whiteboard[END_REF] find that in a collaborative setting with an electronic whiteboard the users adopted a strategy that could be described as: visual overview, oral detail. That is, they glanced at the whiteboard to get "the big picture" and augmented this visually acquired overview with asking their colleagues for clarification and detail. This finding can be seen as a cooperative-work extension of Shneiderman's [START_REF] Shneiderman | The eyes have it: A task by data type taxonomy for information visualizations[END_REF] visual information-seeking mantra (overview first, zoom and filter, then details-on-demand). Specifically, the focus on cooperative work emphasizes that an overview is the users' awareness and understanding of the information relevant in the situation; it is not merely a property or component of a user interface [START_REF] Hornbaek | The notion of overview in information visualization[END_REF]. The overview is a collaborative accomplishment in that the individual actors consult each other for information that elaborates and supplements the information they glean from the whiteboard. Apart from the obvious difference in screen real estate between a 52inch whiteboard and a 4-inch smartphone the large, shared displays have at least three strengths that appear to be absent on small screens:

• Artefactual multiplicity. The whiteboard may hold different pieces of information that are relevant to different groups of users, and it may also interrelate these pieces of information, thereby facilitating the coordination among user groups [START_REF] Bjørn | Artefactual multiplicity: A study of emergencydepartment whiteboards[END_REF]. The interrelating of the pieces of information is accomplished through their simultaneous presence on the display. • Social translucence. Because the whiteboard is shared it makes the same information visible to all actors. The actors are, however, not simply made aware of information they are also held accountable: As an actor I know that everybody knows what information I can read on the whiteboard [START_REF] Erickson | Social translucence: An approach to designing systems that support social processes[END_REF]. Thus, actors can rely on each other to glance regularly at the whiteboard and react on its content. • Information hotspots. In addition to being an information display, the whiteboard also creates a physical place where actors meet [START_REF] Scupelli | Supporting coordination in surgical suites: Physical aspects of common information spaces[END_REF]. They may visit the area around the whiteboard to interact with the whiteboard or to consult a colleague, who is there to interact with the whiteboard, consult a colleague or make herself available for consultation [START_REF] Hertzum | Procedures and collaborative information seeking: A study of emergency departments[END_REF]. While the three strengths are described on the basis of studies of electronic whiteboards, it appears likely that the same strengths exist for wall-size displays, tabletop interfaces, and other large, shared displays. The affordances of small, mobile devices are different.

Overview and Small, Mobile Devices

On a mobile device the functionality of the applications is narrowly focused to fit the small screen. This narrow focus reduces the possibilities for artefactual multiplicity.

In addition, the personal nature of the device reduces social translucence because it is less apparent to others what information I have available and when I have the opportunity to access it. Finally, the mobility of the device prevents it from functioning as a physical location for actors to meet. While it is tempting to presume that actors who collaborate using small, mobile devices need other means of achieving these three ends, it is also possible that they transmute artefactual multiplicity, social translucence, and information hotspots into alternative ways of gaining and maintaining an overview. Either way, it is important to human work interaction design to understand how the actors gain and maintain the overview they need to conduct their activities cooperatively and competently. Studies of the use of mobile phones in developing countries are beginning to address these issues, but tend to investigate loosely coupled activities. The studied activities include societal as well as local collaborations that exploit the widespread adoption of mobile phones:

• Nearly everybody has a mobile phone, thus making it possible to reach most people with information and include many people in cooperative activities. For example, multiple African initiatives use mobile phones as tools to disseminate and collect health information via text messages, to improve the transparency and accountability of elections by sending local observations about polls to central monitoring groups, and to promote reforestation by transferring payments to rural farmers for planting trees [START_REF] Etzo | The mobile phone 'revolution' in Africa: Rhetoric or reality?[END_REF]. • The actors carry their mobile phones everywhere, attend to them repeatedly, and may, thereby, interact with each other when needed rather than when they happen to be in the same place at the same time. For example, geographically distributed herders of livestock in rural Kenyan communities use mobile phones to share information about the changing location of water resources for the livestock and of rangers likely to disrupt herding practices [START_REF] Butt | Herding by mobile phone: Technology, social networks and the "transformation" of pastoral herding in East Africa[END_REF]. • Mobile phones can broadcast information about the whereabouts and activities of actors, thereby providing information for others to monitor. However, this possibility may primarily have been exploited in developed countries. For example, studies of collaborative web search have found that such activity information supports remotely located actors in aligning their search activities and progressing on a shared task [START_REF] Morris | SearchTogether: An interface for collaborative web search[END_REF]. The ways in which mobile devices may support actors in forming and maintaining an overview of their cooperative activities appear an important research area. Similarly, it is important to research the ways in which cooperative work arrangements may transmute what overview is about or what role technology plays in supporting it. This research should, in particular, attend to the conditions in developing countries, in which mobile phones are widespread whereas large-display technologies are not. The app, mKrishti, is freely available to the fishers. Its main feature is a map indicating the locations at which the concentration of fish is predicted to be high at the moment. The predictions are derived from satellite data about the thermal fronts in the water and the water color. For example, the water color gives an indication of the amount of plankton, which is a crucial source of food for the fish. Thus, a large amount of plankton is likely to coincide with a high concentration of fish. A second important feature of the app is weather forecasts, specifically forecasts of the speed and direction of the wind. See Fig. 2 for screenshots of these two features of the mKrishti app. In addition, the app provides information about good fishing practices and a few other issues. With respect to gaining an overview the app has affected the fishers' ways of working in multiple ways:

• After the introduction of the app the fishers use less diesel. It has been possible to document this reduction at the community level because the distribution of diesel is the responsibility of the local fishery societies and they keep careful records. The reduced diesel consumption indicates that (a) the fishers engage in less searching to find the fish and (b) select routes with more tailwind when they are returning with their catch to one of the landing centers along the coastline. The former suggests a better overview of the location of the fish, the latter a better overview of the weather conditions. • Previously, fishers would often try to keep it a secret if they had a hunch about where to go to get a good catch. Now, up-to-date information about where fish can most likely be found is available to all the fishers. The chairperson of the Alibaug fishery society considered this equal access to information a benefit; individual fishers may agree or disagree. With the app the fishers have an increased need to coordinate, or negotiate, who goes where to avoid going to the same place. The app provides no support for this coordination and creates no physical meeting place for it. • In its current version the app appears to bypass the cooperative aspects of the fishers' work. The fishers have proposed to extend the app with information about the price at which different fish can currently be sold at nearby landing centers. Another possible extension would be to track the location of the fisher boats. Knowing their location could help in coordinating who goes where and in coming to rescue during emergencies, which are all too frequent. Instead of artefactual multiplicity, the app pursues a fairly singular purpose. Activities related to, but different from, this purpose are handled through other arrangements, such as the fishery society for coordination/negotiation and the control center for emergency response. The mKrishti app does not have the strengths of a large, shared display but it has enabled the fishers to gain a better overview of the location of fish and the possibilities for tailwind back to the coast. The app is but one component in a large sociotechnical network and has been designed to improve, not obliterate, this network. While many things are best handled outside the app, it appears worthwhile to consider including some support for the cooperative aspects of the fisher's work.

Conclusion

Forming and maintaining an overview is pertinent to the competent performance of cooperative work. While technical support for forming and maintaining an overview has received considerable research attention in developed countries with ample access to large, shared displays, less is known about how small, mobile devices may support -or fail to support -actors in developing countries in overviewing their cooperative activities. The mKrishti app illustrates that at least some aspects of an overview can be provided on a small, mobile device but also that it is important to consider how overview support is distributed between the app and the other systems and arrangements that form the sociotechnical network in which the app is embedded.

4 Example:

 4 Use of mKrishti App by Alibaug FishersIn coastal Indian villages fishing provides jobs and income for nearly a million fishers and even more people in the processing and marketing of the landed fish. To support the fishers in the Maharashtra state an app for mobile phones has been developed in a decade-long collaboration between Tata Consultancy Services (TCS), Innovation Lab Mumbai, Central Marine Fisheries Research Institute (CMFRI), Indian National Center for Ocean Information Services (INCOIS), Indian Council of Agricultural Research (ICAR), and local fishery societies. Maharashtra has a coastline from Mumbai in the north and 720 kilometers southward. One of the fishery villages along the coastline is Alibaug (Fig.1), which we visited in September 2017.

Fig. 1 .

 1 Fig. 1. Fisheries landing center in Alibaug village. Photos by the author.

Fig. 2 .

 2 Fig. 2. The mKrishti app showing the predicted location of fish (left) and the forecast for wind speed and direction (right). Screenshots courtesy of TCS, Innovation Lab Mumbai, ICAR, CMFRI Mumbai, and INCOIS.


Patrick R Kelleher

Gary L Haller

Sandra Casale

Christophe Méthivier

Cyril Thomas
email: cyril.thomas@upmc.fr

Assessing carbon or tungstates coverage of ZrO2 nanoparticles supported on MWCNT via NOx-TPD

Keywords: Multi-walled Carbon Nanotubes (MWCNT), Tungstated-zirconia, NO x -TPD, TEM, XPS

est destinée au dépôt

Introduction

Since the original discovery of carbon nanotubes (CNT) by Ijima in the early nineties [START_REF] Ijima | Helical Microtubules of Graphitic Carbon[END_REF] the properties of these materials have been extensively studied in various fields [START_REF] Ayranci | Enhanced Optical and Electrical Properties of PEDOT via Nanostructured Carbon Materials: A Comparative Investigation[END_REF][START_REF] Demir | Highly Efficient Pt Nanoparticles and f-MWCNT Nanocomposites Based Counter Electrodes for Dye-Sensitized Solar Cells[END_REF][START_REF] Abdolhi | Preparation, Characterization and Toxicity Evaluation of Co 3 O 4 and NiO-Filled Multi-Walled Carbon Nanotubes Loaded to Chitosan[END_REF][START_REF] Rajaura | Structural and Surface Modification of Carbon Nanotubes for Enhanced Hydrogen Storage Density[END_REF]. In the catalysis domain, CNT and multi-walled carbon nanotubes (MWCNT) have been suggested to be promising catalyst supports or promoters, in particular when modified with metals and/or oxides [START_REF] Ai | Noncovalently Functionalized Carbon Nanotubes Immobilized Fe-Bi Bimetallic Oxides as a Heterogeneous Nanocatalyst for Reduction of Nitroaromatics[END_REF][START_REF] Zhang | Multiwalled Carbon Nanotubes as Novel Support or Promoter of Catalysts[END_REF]. Regarding the modification of MWCNT by addition of ZrO 2 , very few studies have been reported in the literature to our knowledge [START_REF] Lupo | Microstructural Investigations on Zirconium Oxide-Carbon nanotube Composites Synthesized by Hydrothermal Crystallization[END_REF][START_REF] Luo | Addition of Carbon Nanotubes During the Preparation of Zirconia Nanoparticles: Influence on Structure and Phase Composition[END_REF][START_REF] Shan | Synthesis and Characterization of Phase Controllable ZrO 2 -Carbon nanotube nanocomposites[END_REF][START_REF] Song | Promotion of Carbon Nanotube-Supported Pt Catalyst for Methanol and Ethanol Electro-Oxidation by ZrO 2 in Acidic Media[END_REF][START_REF] Guo | Synthesis of Sulfated ZrO 2 /MWCNT Composites as New Supports of Pt Catalysts for Direct Fuel Cell Application[END_REF][START_REF] Liang | Direct Electrochemistry and Electrocatalysis of Myoglobin Immobilized on Zirconia/Multi-Walled Carbon Nanotube Nanocomposite[END_REF][START_REF] Lee | Characterization of Multi-Walled Carbon Nanotubes Catalyst Supports by Point of Zero Charge[END_REF][START_REF] Liu | Controlling the Particle Size of ZrO 2 Nanoparticles in Hydrothermally Stable ZrO 2 /MWCNT Composites[END_REF][START_REF] Liu | Synthesis and Characterization of Nanocomposites with Strong Interfacial Interactions: Sulfated ZrO 2 Nanoparticles Supported on Multiwalled Carbon Nanotubes[END_REF][START_REF] Liu | The Electronic Structure or Charge Delocalization of Sulfated Zirconia (Supported on Multi-Walled Carbon Nanotubes): Acid Sites Probed by X-Ray Absorption Spectroscopy[END_REF][START_REF] Malolepszy | Deactivation Resistant Pd-ZrO 2 Supported on Multiwall Carbon Nanotubes Catalyst for Direct Formic Acid Fuel Cells[END_REF][START_REF] Baytak | Sensitive Determination of Terbutaline in Pharmaceuticals and Urine Samples Using a Composite Electrode Based on Zirconium Oxide Nanoparticles[END_REF][START_REF] Amin | Electrocatalytic Activity of Pt-ZrO 2 Supported on Different Carbon Materials for Methanol Oxidation in H 2 SO 4 Solution[END_REF], although ZrO 2 has long been of interest in catalysis [START_REF] Yamaguchi | Application of ZrO 2 as a Catalyst and a Catalyst Support[END_REF], in particular for acid-catalyzed reactions when promoted by sulfates [START_REF] Hino | Synthesis of Solid Superacid Catalyst with Acid Strength of H 0 ≤ 16.04[END_REF] and tungstates (WO x ) [START_REF] Hino | Synthesis of Solid Superacid of Tungsten Oxide Supported on Zirconia and Its Catalytic Action for Reactions of Butane and Pentane[END_REF][START_REF] Kim | Molecular/Electronic Structure-Surface Acidity Relationships of Model-Supported Tungsten Oxide Catalysts[END_REF][START_REF] Barton | Structural and Catalytic Characterization of Solid Acids Based on Zirconia Modified by Tungsten Oxide[END_REF]. Characterization of the ZrO 2modified MWCNT materials has been carried out by electron microscopies [START_REF] Lupo | Microstructural Investigations on Zirconium Oxide-Carbon nanotube Composites Synthesized by Hydrothermal Crystallization[END_REF][START_REF] Luo | Addition of Carbon Nanotubes During the Preparation of Zirconia Nanoparticles: Influence on Structure and Phase Composition[END_REF][START_REF] Shan | Synthesis and Characterization of Phase Controllable ZrO 2 -Carbon nanotube nanocomposites[END_REF][START_REF] Song | Promotion of Carbon Nanotube-Supported Pt Catalyst for Methanol and Ethanol Electro-Oxidation by ZrO 2 in Acidic Media[END_REF][START_REF] Liang | Direct Electrochemistry and Electrocatalysis of Myoglobin Immobilized on Zirconia/Multi-Walled Carbon Nanotube Nanocomposite[END_REF][START_REF] Liu | Controlling the Particle Size of ZrO 2 Nanoparticles in Hydrothermally Stable ZrO 2 /MWCNT Composites[END_REF][START_REF] Liu | Synthesis and Characterization of Nanocomposites with Strong Interfacial Interactions: Sulfated ZrO 2 Nanoparticles Supported on Multiwalled Carbon Nanotubes[END_REF][START_REF] Malolepszy | Deactivation Resistant Pd-ZrO 2 Supported on Multiwall Carbon Nanotubes Catalyst for Direct Formic Acid Fuel Cells[END_REF][START_REF] Baytak | Sensitive Determination of Terbutaline in Pharmaceuticals and Urine Samples Using a Composite Electrode Based on Zirconium Oxide Nanoparticles[END_REF][START_REF] Amin | Electrocatalytic Activity of Pt-ZrO 2 Supported on Different Carbon Materials for Methanol Oxidation in H 2 SO 4 Solution[END_REF][START_REF] Almeida | Enhanced Mechanical properties in ZrO 2 multi-walled carbon nanotube nanocomposites produced by sol-gel and high-pressure[END_REF] and XRD [START_REF] Lupo | Microstructural Investigations on Zirconium Oxide-Carbon nanotube Composites Synthesized by Hydrothermal Crystallization[END_REF][START_REF] Luo | Addition of Carbon Nanotubes During the Preparation of Zirconia Nanoparticles: Influence on Structure and Phase Composition[END_REF][START_REF] Shan | Synthesis and Characterization of Phase Controllable ZrO 2 -Carbon nanotube nanocomposites[END_REF][START_REF] Song | Promotion of Carbon Nanotube-Supported Pt Catalyst for Methanol and Ethanol Electro-Oxidation by ZrO 2 in Acidic Media[END_REF][START_REF] Guo | Synthesis of Sulfated ZrO 2 /MWCNT Composites as New Supports of Pt Catalysts for Direct Fuel Cell Application[END_REF][START_REF] Liang | Direct Electrochemistry and Electrocatalysis of Myoglobin Immobilized on Zirconia/Multi-Walled Carbon Nanotube Nanocomposite[END_REF][START_REF] Liu | Controlling the Particle Size of ZrO 2 Nanoparticles in Hydrothermally Stable ZrO 2 /MWCNT Composites[END_REF][START_REF] Liu | Synthesis and Characterization of Nanocomposites with Strong Interfacial Interactions: Sulfated ZrO 2 Nanoparticles Supported on Multiwalled Carbon Nanotubes[END_REF][START_REF] Malolepszy | Deactivation Resistant Pd-ZrO 2 Supported on Multiwall Carbon Nanotubes Catalyst for Direct Formic Acid Fuel Cells[END_REF][START_REF] Amin | Electrocatalytic Activity of Pt-ZrO 2 Supported on Different Carbon Materials for Methanol Oxidation in H 2 SO 4 Solution[END_REF][START_REF] Almeida | Enhanced Mechanical properties in ZrO 2 multi-walled carbon nanotube nanocomposites produced by sol-gel and high-pressure[END_REF], as long as crystalline ZrO 2 particles are large enough for detection. From these studies it can be concluded that the size of the ZrO 2 particles were rather widespread [START_REF] Luo | Addition of Carbon Nanotubes During the Preparation of Zirconia Nanoparticles: Influence on Structure and Phase Composition[END_REF][START_REF] Shan | Synthesis and Characterization of Phase Controllable ZrO 2 -Carbon nanotube nanocomposites[END_REF][START_REF] Song | Promotion of Carbon Nanotube-Supported Pt Catalyst for Methanol and Ethanol Electro-Oxidation by ZrO 2 in Acidic Media[END_REF] and higher than or equal to 10 nm in the majority of the synthetized samples [START_REF] Lupo | Microstructural Investigations on Zirconium Oxide-Carbon nanotube Composites Synthesized by Hydrothermal Crystallization[END_REF][START_REF] Luo | Addition of Carbon Nanotubes During the Preparation of Zirconia Nanoparticles: Influence on Structure and Phase Composition[END_REF][START_REF] Song | Promotion of Carbon Nanotube-Supported Pt Catalyst for Methanol and Ethanol Electro-Oxidation by ZrO 2 in Acidic Media[END_REF][START_REF] Liang | Direct Electrochemistry and Electrocatalysis of Myoglobin Immobilized on Zirconia/Multi-Walled Carbon Nanotube Nanocomposite[END_REF][START_REF] Malolepszy | Deactivation Resistant Pd-ZrO 2 Supported on Multiwall Carbon Nanotubes Catalyst for Direct Formic Acid Fuel Cells[END_REF][START_REF] Baytak | Sensitive Determination of Terbutaline in Pharmaceuticals and Urine Samples Using a Composite Electrode Based on Zirconium Oxide Nanoparticles[END_REF]. Recently we proposed that narrowly distributed, highly dispersed, ZrO 2 nanoparticles (2-3 nm) could be obtained by grafting zirconium acetylacetonate (Zr(acac) 4 ) on MWCNT that were previously functionalized in nitric acid [START_REF] Lee | Characterization of Multi-Walled Carbon Nanotubes Catalyst Supports by Point of Zero Charge[END_REF][START_REF] Liu | Controlling the Particle Size of ZrO 2 Nanoparticles in Hydrothermally Stable ZrO 2 /MWCNT Composites[END_REF][START_REF] Liu | Synthesis and Characterization of Nanocomposites with Strong Interfacial Interactions: Sulfated ZrO 2 Nanoparticles Supported on Multiwalled Carbon Nanotubes[END_REF][START_REF] Liu | The Electronic Structure or Charge Delocalization of Sulfated Zirconia (Supported on Multi-Walled Carbon Nanotubes): Acid Sites Probed by X-Ray Absorption Spectroscopy[END_REF].

Although estimating the size of the ZrO 2 nanoparticles by XRD and TEM remains reliable, these physical methods are limited when interest lies in characterization of their surface, which is of fundamental importance when a catalytic application is considered.

Recently, we have shown that the accessible surface of ZrO 2 in complex materials can be accurately estimated with the use of a novel chemisorption technique, namely the adsorption of nitrogen oxides (NO x = NO + NO 2 ) at RT followed by their temperature-programmed desorption (NO x -TPD) [START_REF] Law | NO x -TPD as a Tool to Estimate the Accessible Zirconia Surface of ZrO 2 -Containing Materials[END_REF][START_REF] Thomas | Should W Surface Density of WO x -ZrO 2 Catalysts Be Calculated With Respect to the Specific Surface Area of the Sample or That of ZrO 2 Only?[END_REF]. Thus, we have found of particular interest to characterize the size and accessible surface of ZrO 2 nanoparticles supported on MWCNT (ZrO 2 /MWCNT) by this newly-developed technique.

The aforementioned ZrO 2 /MWCNT composite has been further functionalized by adding W to make tungsten-promoted ZrO 2 nanoparticles supported on MWCNT (WO 3 -ZrO 2 /MWCNT). Due to their intrinsic acid properties, tungstated zirconias have attracted the attention of the catalysis community for about three decades [START_REF] Zhou | Nature of Catalytically Active Sites in the Supported WO 3 /ZrO 2 Solid Acid System: A Current Perspective[END_REF]. Several studies have shown that the W surface density (W/nm 2 ), also reflecting the coverage of ZrO 2 by tungstates [START_REF] Scheithauer | Genesis and Structure of WO x /ZrO 2 Solid Acid Catalysts[END_REF][START_REF] Barton | Structure and Electronic Properties of Solid Acids Based on Tungsten Oxide Nanostructures[END_REF], is the critical parameter when determining performance of these catalysts in various catalytic reactions [START_REF] Barton | Structural and Catalytic Characterization of Solid Acids Based on Zirconia Modified by Tungsten Oxide[END_REF][START_REF] Zhou | Nature of Catalytically Active Sites in the Supported WO 3 /ZrO 2 Solid Acid System: A Current Perspective[END_REF]. Consequently, much work has been devoted to determining the W surface density for which monolayer coverage of WO x on a ZrO 2 support is achieved.

Physical probes such as Raman [START_REF] Barton | Structure and Electronic Properties of Solid Acids Based on Tungsten Oxide Nanostructures[END_REF][START_REF] Wachs | Raman and IR Studies of Surface Metal Oxide Species on Oxide Supports: Supported Metal Oxide Catalysts[END_REF][START_REF] Wachs | Catalysis Science of the Solid Acidity of Model Supported Tungsten Oxide Catalysts[END_REF][START_REF] Kim | Molecular/Electronic Structure-Surface Acidity Relationships of Model-Supported Tungsten Oxide Catalysts[END_REF], Ion Scattering [START_REF] Vaidyanathan | Surface Coverage of WO 3 /ZrO 2 Catalysts Measured by Ion Scattering Spectroscopy and Low Temperature CO Adsorption[END_REF] and X-ray Photoelectron [START_REF] Vaidyanathan | Surface Characterization of WO 3 /ZrO 2 Catalysts[END_REF][START_REF] Valigi | WO x /ZrO 2 Catalysts: Part 1. Preparation, Bulk and Surface Characterization[END_REF][START_REF] Gregorio | Activation and Isomerization of Hydrocarbons Over WO 3 /ZrO 2 Catalysts: I. Preparation, Characterization, and X-ray Photoelectron Spectroscopy Studies[END_REF][START_REF] Ross-Medgaarden | New Insights into the Nature of the Acidic Catalytic Active Sites Present in ZrO 2 -Supported Tungsten Oxide Catalysts[END_REF][START_REF] Feng | Selective Hydrogenolysis of Tetrahydrofurfuryl Alcohol on Pt/WO 3 /ZrO 2 Catalysts: Effect of WO 3 Loading Amount on Activity[END_REF] spectroscopies have proven to be useful when characterizing bulk tungstated zirconias, however, there is inherent difficulty when using Raman spectroscopy to characterize lowly-loaded samples, such as the WO 3 -ZrO 2 /MWCNT samples described in this study. Furthermore, Ion Scattering and X-ray Photoelectron spectroscopies (ISS and XPS) are not readily available to all scientists characterizing these types of materials and this offers potential for new methods that are capable of characterizing these complex materials. The adsorption of chemical probes such as CO and CO 2 followed by FT-IR (CO [START_REF] Ferraris | WO x /ZrO 2 Catalysts: Part 3. Surface Coverage as Investigated by Low Temperature CO Adsorption: FT-IR and Volumetric Studies[END_REF]) or volumetric analysis (CO [START_REF] Vaidyanathan | Surface Coverage of WO 3 /ZrO 2 Catalysts Measured by Ion Scattering Spectroscopy and Low Temperature CO Adsorption[END_REF][START_REF] Ferraris | WO x /ZrO 2 Catalysts: Part 3. Surface Coverage as Investigated by Low Temperature CO Adsorption: FT-IR and Volumetric Studies[END_REF], CO 2 [START_REF] Barton | Structure and Electronic Properties of Solid Acids Based on Tungsten Oxide Nanostructures[END_REF]) has also been used to characterize the coverage of ZrO 2 by tungstates. FT-IR becomes less favorable when a carbon support is introduced due to carbon absorbing the IR beam. In addition, the dependence of CO and CO 2 adsorption on the nature of the ZrO 2 polymorph being investigated [START_REF] Pokrovski | Investigation of CO and CO 2 Adsorption on Tetragonal and Monoclinic Zirconia[END_REF] increases the uncertainty in methods relying on CO and CO 2 adsorption. For these reasons, the WO 3 -ZrO 2 /MWCNT samples in the current study were characterized by NO x -TPD to determine the coverage of the ZrO 2 nanoparticles by tungstates. It is our understanding that NO x -TPD can be performed by scientists everywhere and on tungstated zirconia samples of any composition, making it a sensitive and robust technique to determine the W surface density on ZrO 2 supports. To our knowledge, determination of the coverage of the ZrO 2 surface by tungstates in WO 3 -ZrO 2 /MWCNT materials has not been reported in the literature to date, whereas their use as substitutes of sulfated ZrO 2 materials, which are known to exhibit leaching issues in biomass processing applications [START_REF] Ni | Esterification of Free Fatty Acids in Sunflower Oil Over Solid Acid Catalysts Using Batch and Fixed Bed-Reactors[END_REF][START_REF] Suwannakarn | Stability of Sulfated Zirconia and the Nature of the Catalytically Active Species in the Transesterification of Triglycerides[END_REF][START_REF] Park | Esterification of Used Vegetable Oils Using the Heterogeneous WO 3 /ZrO 2 Catalyst for Production of Biodiesel[END_REF][START_REF] Mongkolbovornkij | Esterification of Industrial-Grade Palm Fatty Acid Distillate over Modified ZrO 2 (with WO 3 -, SO 4and TiO 2 -): Effects of Co-Solvent Adding and Water Removal[END_REF][START_REF] Cavani | Unexpected Events in Sulfated Zirconia Catalyst During Glycerol-to-Acrolein Conversion[END_REF][START_REF] Dosuna-Rodríguez | Glycerol Acetylation on Sulphated Zirconia in Mild Conditions[END_REF][START_REF] Huang | Characterization of an Alkaline Earth Metal-Doped Solid Superacid and Its Activity for the Esterification of Oleic Acid With Methanol[END_REF][START_REF] Popova | Synthesis of Biomass Derived Levulinate Esters on Novel Sulfated Zr/KIL-2 Composite Catalysts[END_REF], is of great interest.

Experimental

Catalyst preparation

The MWCNT used in this study were purchased from Cheap Tubes Inc. (>95% purity). These MWCNT (or pristine MWCNT, as described later) have an outer diameter of 10-20 nm, an inner diameter of 3-5 nm, and a length of 10-30 μm, as described by the supplier. The pristine MWCNT were functionalized by refluxing in nitric acid at ~120 °C for 2 h using ∼69 wt % (15 M HNO 3 ) nitric acid (purchased from J.T. Baker). The mixture was then filtered and washed with 1000 mL of deionized water in five batches (5 × 200 mL) to remove excess acid and soluble oxidation products. The resulting material was then dried at 60 °C overnight in a thermostated oven and ground to a fine powder. These functionalized MWCNT were used as the starting material to prepare the ZrO 2 /MWCNT composites. Synthesis of ZrO 2 /MWCNT was completed by grafting Zr(acac) 4 (98%, purchased from Sigma-Aldrich) onto functionalized MWCNT in refluxing toluene (300 mL) at ~111 °C for 3 h in a 500 mL round-bottom flask being purged with N 2 (ultrahigh purity, purchased from Airgas). The sample was filtered, washed three times with toluene, and dried overnight at 60 °C. A thermal treatment (annealing) was then performed in flowing He (ultrahigh purity, purchased from Airgas) at 450 °C for 2 h in a quartz reactor to decompose the acetylacetonate precursor and form ZrO 2 particles. Two batches of ZrO 2 /MWCNT were prepared differing in the loading of ZrO 2 (8.1 and 6.8 wt %) and the starting functionalized MWCNT. From here on, these samples will be denoted as ZrO 2 (x)/MWCNT, where x represents the ZrO 2 loading (wt %).

The ZrO 2 (6.8)/MWCNT composite was then impregnated with aqueous solutions of ammonium metatungstate hydrate (99.99%, purchased from Sigma-Aldrich) prepared at various concentrations to achieve WO 3 loadings of 2.33, 4.57 and 8.74 wt %. The impregnated material was thermally treated again at 450 °C in flowing He for 2 h in order to form tungstated ZrO 2 /MWCNT (WO 3 -ZrO 2 /MWCNT). The tungstated samples will be denoted as WO 3 (y)-ZrO 2 (x)/MWCNT, where y and x represent the loadings (wt %) of WO 3 and ZrO 2 , respectively. Note that when ZrO 2 /MWCNT of x wt% is tungstated, the wt% ZrO 2 is adjusted to x' < x to account for the added WO 3 , e.g., ZrO 2 (6.8)/MWCNT becomes WO 3 (2.3)-ZrO 2 (6.6)/MWCNT.

Characterization Techniques

The samples were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS) and NO x -temperature-programmed desorption (NO x -TPD).

Thermogravimetric Analysis (TGA). The final weight loadings of ZrO 2 and WO 3 were measured using TGA which was conducted on a Setaram Setsys 1750 instrument in flowing air (Ultra Zero grade, purchased from Airgas). During the measurement, the temperature was held at 200 °C for 30 min to remove adsorbed water and ramped to 1000 °C at a rate of 10 °C/min and then held at 1000 °C for 30 min. A second ramp on the oxidized sample was repeated to establish the baseline.

X-ray Diffraction Analysis (XRD). XRD patterns were collected using a Rigaku

Miniflex 600 powder X-ray diffractometer with Cu Kα radiation at λ = 0.154 nm. XRD data was collected in the range 2Ɵ = 10 -80 ° with a continuous scan mode at 0.5 °/min and counts recorded every 0.02 °. The XRD patterns have been scaled to the MWCNT (002) diffraction peak to clearly depict differences in each sample.

Transmission Electron Microscopy (TEM). Bright field TEM was performed at the Plate-forme of Institut des Matériaux de Paris Centre using a JEOL 2010 microscope operating at 200 kV with a LaB 6 filament and equipped with an Orius CCD camera (Gatan).

The TEM samples were prepared by dropping ethanol suspensions of the materials on a copper grid. The ZrO 2 particle-size distributions were estimated by counting from 497 to 1133 particles.

X-ray Photoelectron Spectroscopy (XPS). XP spectra were collected on an Omicron

Argus X-ray photoelectron spectrometer, using a monochromated Al K (h = 1486.6 eV) radiation source having a 300 W electron beam. Photoelectrons emitted from the sample were analyzed at a takeoff angle of 45° under ultra-high vacuum conditions (1 x 10 -8 Pa). XP spectra were collected at a pass energy of 20 eV with 0.1 eV steps. After data collection, the binding energies were referenced to the C 1s line (284.5 eV). The peak areas were determined after subtraction of a Shirley background and the atomic ratio calculations were performed after normalization using Scofield factors. Spectrum processing was carried out using the Casa XPS software package and Origin 7.1 (Origin Lab Corporation).

NO x Temperature-Programmed Desorption (NO x -TPD). The NO x -TPD experiments were carried out in a U-shape quartz reactor (15 mm i.d.). The samples were held on a plug of quartz wool and the temperature was controlled by a Eurotherm 2408 temperature controller using a K-type thermocouple. Reactant gases, used as received, were fed from independent gas cylinders (purchased from Air Liquide) by means of mass flow controllers (Brooks 5850TR) with a total flow rate of 230 mL NTP /min. Due to the presence of the MWCNT support, and its known sensitivity to oxidation at high temperature in the presence of O 2 [START_REF] Luo | Addition of Carbon Nanotubes During the Preparation of Zirconia Nanoparticles: Influence on Structure and Phase Composition[END_REF], the pretreatment and desorption feeds were modified compared to those used in our earlier studies [START_REF] Law | NO x -TPD as a Tool to Estimate the Accessible Zirconia Surface of ZrO 2 -Containing Materials[END_REF][START_REF] Thomas | Should W Surface Density of WO x -ZrO 2 Catalysts Be Calculated With Respect to the Specific Surface Area of the Sample or That of ZrO 2 Only?[END_REF]. O 2 was removed from these feeds and replaced with He to maintain identical flow rates. It was verified on a bulk ZrO 2 sample tested earlier (ZrO 2 -600 fa [START_REF] Law | NO x -TPD as a Tool to Estimate the Accessible Zirconia Surface of ZrO 2 -Containing Materials[END_REF]), that replacing the O 2 -He pretreatment and desorption feeds with He did not alter the desorption profiles (not shown) or the amount of NO x desorbed (475.5 µmol/g compared to 445.5 µmol/g [START_REF] Law | NO x -TPD as a Tool to Estimate the Accessible Zirconia Surface of ZrO 2 -Containing Materials[END_REF]). Prior to the NO x -TPD experiments, the MWCNT samples (~0.15 g) were annealed in situ in He at 500 °C (3 °C/min) for 2 h with a flow rate of 100 mL NTP /min. Following the insitu annealing, the sample was exposed to the adsorption mixture which consisted of 400 ppm NO x (mainly NO with some NO 2 ) in He. The reactor outlet was continuously monitored by a chemiluminescence NO x analyzer (Thermo Environmental Instruments 42C-HL) that allowed the simultaneous detection of both NO and NO 2 . The samples were exposed to the adsorption mixture at RT until the outlet NO x readout was equivalent to the inlet NO x readout. This latter parameter was set to ensure that each sample was saturated under the present experimental conditions. The time after which no change was observed in the gas phase NO x concentration was strongly dependent on the W surface density of the samples. It has been clearly demonstrated that the nature of formed ad-NO x species on such materials is affected to a significant extent by the presence of O 2 in the NO-containing mixture [START_REF] Gorce | On the Role of Organic Nitrogen-Containing Species as Intermediates in the Hydrocarbon-Assisted SCR of NO x[END_REF]. As the presence of ppm levels of O 2 could not be excluded, NO x adsorption was thus carried out in the presence of a large excess of O 2 (8 %). Before the NO x -TPD experiments, the samples were flushed in He at RT to remove weakly chemisorbed species until the NO and NO 2 concentrations detected at the outlet of the reactor were found to be negligible. NO x -TPD experiments were carried out from RT to about 400 °C at a heating rate of 3 °C/min under a flow of 230 mL NTP /min of He. After each NO x -TPD experiment, the NO x analyzer was calibrated with a NO/He standard mixture containing 933 ppm of NO. As reported earlier [START_REF] Law | NO x -TPD as a Tool to Estimate the Accessible Zirconia Surface of ZrO 2 -Containing Materials[END_REF][START_REF] Blanchard | On the Detrimental Effect of Tungstates on the n-C 10 -SCR of NO x on Ag/-Al 2 O 3[END_REF][START_REF] Bonnotte | Insights into the WO x Coverage-Dependent Location and Oxidation State of Noble Metals Supported on Tungstated Oxides: The Case of Rh/WO x -Ce 0[END_REF], it should be noted that NO x chemisorption does not occur on WO x species. The reactor outflow was also analyzed periodically using a µ-GC (Agilent Technologies, CP4900) equipped with a 5A molecular sieve column (80 °C, 150 kPa He, 200 ms injection time, 30 s backflush time) and a poraplot Q column (60 °C, 150 kPa He, 200 ms injection time). Under the NO x -TPD conditions, the only detected product was CO 2 at ppm levels on the second column. The NO x and CO 2 amounts were deduced from the integration of the corresponding TPD profiles. For the ZrO 2 -containing materials, the amount of NO x released per g of ZrO 2 was obtained by correcting the amount of NO x released per g of sample for the amount of WO 3 and/or the background contribution of the MWCNT support.

Results and Discussion

MWCNT supports

TEM images show the MWCNT diameter's range from 9 to 24 nm and wall thicknesses of 5 ± 2 nm (Fig. 1a), consistent with the specifications indicated by the provider.

Prior to studying the WO 3 -ZrO 2 /MWCNT materials, the corresponding MWCNT supports were submitted to NO x -TPD experiments. As indicated in Fig. 2, which displays the NO x and CO 2 profiles recorded in the course of a NO x -TPD experiment carried out on the functionalized MWCNT support used for the preparation of ZrO 2 (8.1)/MWCNT, the carbon support releases measurable amounts of NO x from 50 to 400 °C with a maximum at 80 °C.

The NO x species are mainly released as NO except at low temperatures for which NO 2 is the dominant species. The production of CO 2 increased monotonously as the temperature increased. From a quantitative point of view, the amounts of NO x released and CO 2 produced were found to be 58 and 41 µmol/g MWCNT , respectively. under He, were also performed on the functionalized MWCNT support used for the preparation of ZrO 2 (6.8)/MWCNT. The NO x and CO 2 profiles (not shown) were found to be similar to those reported in Fig. 2. The amounts of NO x released and CO 2 produced in these successive experiments were found to be reproducible (70 ± 4 µmol NO x /g MWCNT and 54 ± 0 µmol CO 2 /g MWCNT ) and slightly higher than those obtained on the first batch of functionalized MWCNT investigated in this work (Table 1).

ZrO 2 (8.1)/MWCNT

In agreement with our earlier studies [START_REF] Liu | Controlling the Particle Size of ZrO 2 Nanoparticles in Hydrothermally Stable ZrO 2 /MWCNT Composites[END_REF][START_REF] Liu | Synthesis and Characterization of Nanocomposites with Strong Interfacial Interactions: Sulfated ZrO 2 Nanoparticles Supported on Multiwalled Carbon Nanotubes[END_REF], TEM images show that the outer surface of the MWCNT becomes uniformly decorated with ZrO 2 nanoparticles after grafting with Zr(acac) 4 and annealing in He (ZrO 2 (8.1)/MWCNT, Fig. 1b). The ZrO 2 particle size distribution of this sample was found to be rather narrow (Fig. 1c) and the average ZrO 2 particle size was estimated to be 2.1 nm. This average size is consistent with our earlier work in which it was found that sintering of the ZrO 2 particles only occurred for highly-loaded samples and when thermally treated at high annealing temperatures [START_REF] Liu | Controlling the Particle Size of ZrO 2 Nanoparticles in Hydrothermally Stable ZrO 2 /MWCNT Composites[END_REF]. The lower ZrO 2 particle density observed in Fig. 1b compared to those observed earlier [START_REF] Liu | Controlling the Particle Size of ZrO 2 Nanoparticles in Hydrothermally Stable ZrO 2 /MWCNT Composites[END_REF] is also consistent with the lower ZrO 2 loading used in the present study. Fig. 1d,e indicate that the particle size was hardly affected after four successive NO x -TPD experiments, as the average particle size was estimated to be 2.3 nm for this particular sample. In these NO x -TPD experiments the annealing temperature never exceeded 500 °C and this result is in agreement with the finding that limited sintering of the ZrO 2 particles supported on MWCNT was previously reported for annealing temperatures lower than or equal to 550 °C [START_REF] Liu | Controlling the Particle Size of ZrO 2 Nanoparticles in Hydrothermally Stable ZrO 2 /MWCNT Composites[END_REF].

Four NO x -TPD experiments were performed successively on ZrO 2 (8.1)/MWCNT (Fig. 3). Prior to each NO x -TPD experiment, the sample was annealed in He for 2 h at 500 °C following the original pretreatment procedure (100 mL/min He, 3 °C/min ramping rate). It must be highlighted however that this pretreatment was slightly modified after the third NO x -TPD experiment as we anticipated that it would be of interest to monitor the CO 2 production in the course of the annealing treatment. For this purpose, the He flow rate was changed from 100 to 230 mL/min at 150 °C and the annealing treatment was performed under these particular conditions from 150 to 500 °C before carrying out the fourth NO x -TPD experiment.

Fig. 3a shows that the NO x -TPD profiles obtained for ZrO 2 (8.1)/MWCNT are significantly different from that obtained on MWCNT (Fig. 2). The NO x contribution near 85 °C is of slightly higher intensity and an intense NO contribution is observed close to 290 °C on ZrO 2 (8.1)/MWCNT when compared to blank MWCNT (Fig. 2). The NO x -TPD profiles, corrected for the contribution of the MWCNT support (not shown), are found to be qualitatively consistent with that found on bulk ZrO 2 with low and high NO x temperature desorption peaks [START_REF] Law | NO x -TPD as a Tool to Estimate the Accessible Zirconia Surface of ZrO 2 -Containing Materials[END_REF]. Fig. 3a-d show an increase in the formation of NO 2 in the lowtemperature desorption peak and a change in the shape of the NO x peak as the number of successive NO x -TPD experiments increases, in particular for the third and the fourth NO x -TPD experiments (Fig. 3c,d). This change is clearly illustrated in Fig. 4a in which an increase in the intensity of the low temperature NO x desorption peak can be easily observed for temperatures higher than 85 °C. Let us note that the shape of the low-temperature NO x desorption peak recorded in the third and the fourth NO x -TPD experiment (Fig. 4a) is closer to that reported earlier on bulk ZrO 2 [START_REF] Law | NO x -TPD as a Tool to Estimate the Accessible Zirconia Surface of ZrO 2 -Containing Materials[END_REF] than those recorded in the two first NO x -TPD experiments. The main difference between bulk ZrO 2 and ZrO 2 (8.1)/MWCNT is related to the nature of the NO x species released. Whereas NO 2 was found to primarily be released from bulk ZrO 2 [START_REF] Law | NO x -TPD as a Tool to Estimate the Accessible Zirconia Surface of ZrO 2 -Containing Materials[END_REF], NO was almost exclusively released from ZrO 2 (8.1)/MWCNT (Fig. 3).

Before discussing this difference, it is interesting to note that an intense CO 2 contribution was observed in parallel with the NO contribution at 290 °C (Fig. 3) and that this CO 2 contribution was only recorded in the NO x -TPD experiments. Pretreatment of ZrO 2 (8.1)/MWCNT under He only showed the presence of a CO 2 background from 150 to 500 °C (Fig. 4b, black trace).

The nature of the NO x adsorbed species (NO + , NO 2 -, N 2 O 3 , N 2 O 4 , NO 3 -) at the origin of the NO x released at low temperatures on bulk ZrO 2 remains complex [START_REF] Hadjiivanov | Surface Species Formed after NO Adsorption and NO + O 2 Coadsorption on ZrO 2 and Sulfated ZrO 2 : An FTIR Spectroscopic Study[END_REF], whereas those responsible for the release of NO 2 at high temperatures have been attributed unequivocally to nitrates (NO 3 -) [START_REF] Hadjiivanov | Surface Species Formed after NO Adsorption and NO + O 2 Coadsorption on ZrO 2 and Sulfated ZrO 2 : An FTIR Spectroscopic Study[END_REF][START_REF] Tsyntsarsky | FT-IR Study of the Nature and Reactivity of surface NO x Compounds Formed after NO adsorption and NO + O 2 Coadsorption on Zirconia-and Sulfated Zirconia-Supported Cobalt[END_REF][START_REF] Weingand | Nature and Reactivity of the Surface Species Formed after NO Adsorption and NO + O 2 Coadsorption on a WO 3 -ZrO 2 Catalyst[END_REF][START_REF] Schott | Reduction of NO x by H 2 on Pt/WO 3 /ZrO 2 Catalysts in Oxygen-Rich Exhaust[END_REF]. The observed release of NO at high temperatures, instead of NO 2 as expected from the experiments carried out on bulk ZrO 2 [START_REF] Law | NO x -TPD as a Tool to Estimate the Accessible Zirconia Surface of ZrO 2 -Containing Materials[END_REF] can thus be attributed to the reduction of NO 2 to NO assisted by the concomitant oxidation of the MWCNT support (y NO 2 + *CO x = y NO + CO 2 , where x + y = 2 and *C designates a carbon atom from MWCNT), in agreement with the conclusions drawn in the field of automotive exhaust soot oxidation [START_REF] Van Setten | Science and Technology of Catalytic Diesel Particulate Filters[END_REF][START_REF] Müller | The Oxidation of Soot Particulate in the Presence of NO 2[END_REF]. Such an attribution is supported by the much greater CO 2 production observed for ZrO 2 (8.1)-MWCNT (Fig. 4b) compared to MWCNT (Fig. 2).

Of much more interest is the evolution of the amounts of NO x released and CO 2 produced in the successive NO x -TPD experiments reported in Fig. 3 and Fig. 4. Overall, an increase in the amounts of NO x released and CO 2 produced was observed from 220 to 286 µmol NO x /g sample and from 164 to 210 µmol CO 2 /g sample , respectively, as the number of NO x -TPD experiment increased (Table 1). It must be emphasized that the increase in the amounts of NO x released and CO 2 produced in the NO x -TPD experiments were found to be more pronounced for the third experiment and seemed to stabilize after 3 consecutive NO x -TPD experiments (Table 1). By correcting the amount of NO x released per g of sample for the contribution of the MWCNT support (58 µmol NO x /g MWCNT ), the amount of NO x released per g of ZrO 2 was deduced to increase from 2063 to about 2834 µmol NO x /g ZrO2 as the number of successive NO x -TPD cycles increased (Table 1). Given that the NO x surface density on ZrO 2 was determined to be 6 µmol NO x /m 2 ZrO2 [START_REF] Law | NO x -TPD as a Tool to Estimate the Accessible Zirconia Surface of ZrO 2 -Containing Materials[END_REF][START_REF] Thomas | Should W Surface Density of WO x -ZrO 2 Catalysts Be Calculated With Respect to the Specific Surface Area of the Sample or That of ZrO 2 Only?[END_REF], the accessible surface of the ZrO 2 nanoparticles of ZrO 2 (8.1)/MWCNT was determined to increase from 344 to about 470 m 2 /g ZrO2 as the number of successive NO x -TPD cycles increased (Table 1). This indicates that the accessible surface of the ZrO 2 nanoparticles increased after three successive NO x -TPD experiments and then stabilized. By considering spherical ZrO 2 nanoparticles and a ZrO 2 density of 5.8 x 10 6 g/m 3 , the size of the corresponding particles was estimated to decrease from 3.0 to 2.2 nm as the number of successive NO x -TPD increased (Table 1). TEM showed that the size of the ZrO 2 particles was hardly affected by successive NO x -TPD experiments and remained close to 2.2 nm (Table 1, Fig. 1b-e). The apparent decrease in the size of the ZrO 2 particles estimated from the NO x -TPD data can be assigned to the occupation of the NO x adsorption sites on the ZrO 2 particles by carbonaceous deposits that were progressively oxidized by NO 2 in the course of the successive NO x -TPD experiments. The surface coverage of the ZrO 2 particles by these carbonaceous deposits is estimated to be about 30 %. It should be noted that the source of the carbonaceous deposits is likely the acetylacetonate decomposition of the zirconium precursor, not the MWCNT. From the fact that after four cycles the catalyst continues to produce significant quantities of CO 2 (Fig. 3d), it can be concluded that most of the CO 2 produced in the successive NO x -TPD experiments (Fig. 3) is a result of oxidation of the MWCNT support by NO 2 released from the ZrO 2 particles, as discussed previously. This series of NO x -TPD experiments was duplicated on a second aliquot of ZrO 2 (8.1)/MWCNT and the results obtained were found to be fairly reproducible with the NO x -TPD data reported in Fig. 3 and Table 1. Finally, it is of particular interest to highlight that the size of the ZrO 2 particles, freed of their initial C deposits, in ZrO 2 (8.1)/MWCNT estimated by NO x -TPD was found to be in remarkable agreement with the size of the ZrO 2 particles estimated by TEM (3 rd and 4 th NO x -TPD, Table 1, 2.2 nm).

ZrO 2 (6.8)/MWCNT

TEM indicates the ZrO 2 (6.8)/MWCNT and ZrO 2 (8.1)/MWCNT samples contain ZrO 2 particles of similar size (Fig. 1f,g, ~2.2 nm). In addition, the NO x and CO 2 profiles recorded in the three successive NO x -TPD experiments (Fig. 5) were found to be similar to those reported for ZrO 2 (8.1)/MWCNT (Fig. 3). In contrast to what was observed on ZrO 2 (8.1)/MWCNT (section 3.2.), the successive NO x -TPD experiments performed on ZrO 2 (6.8)/MWCNT were found to be reproducible, as indicated by the amounts of NO x released and CO 2 formed in these experiments, 257 ± 6 µmol NO x /g and 192 ± 5 µmol CO 2 /g (Table 1), respectively. The size of the ZrO 2 particles of ZrO 2 (6.8)/MWCNT was estimated to be ca. 2.2 nm from the NO x -TPD data, thus in excellent agreement with the size of the ZrO 2 particles determined by TEM (Table 1). Considering all of the above, these results strongly suggest the absence of carbonaceous deposits on the ZrO 2 particles of ZrO 2 (6.8)/MWCNT. Two successive NO x -TPD experiments were performed on a second aliquot of ZrO 2 (6.8)/MWCNT and confirmed the absence of carbonaceous deposits on this particular sample. The reason for the presence of carbonaceous deposits on ZrO 2 (8.1)/MWCNT and their absence on ZrO 2 (6.8)/MWCNT remains unclear at present. One explanation could stand in the change of the annealing reactor for ZrO 2 (6.8)/MWCNT, which may have resulted in differences in the annealing procedure.

The measurements performed on the ZrO 2 /MWCNT samples in the present study therefore provide further evidence to the robustness and the sensitivity of the NO x -TPD method. The absence of carbonaceous deposits on ZrO 2 (6.8)/MWCNT led us to select this sample as the starting material for the preparation of the tungstated samples studied below.

WO 3 -ZrO 2 /MWCNT

Fig. 6 shows XRD patterns for nitric acid treated MWCNT, ZrO 2 (6. MWCNTs diffraction peak at 24° and a peak at 34° that is most likely due to diffraction from WO 3 domains (JCPDS 43-1035), in agreement with the corresponding contributions observed on a WO 3 (8.0)/MWCNT sample (not shown) that was synthesized and treated thermally under similar conditions as those described for WO 3 (y)-ZrO 2 (x)/MWCNT (section 2.1.).

Among the tungstated samples, only WO 3 (2.3)-ZrO 2 (6.6)/MWCNT was investigated by TEM (Fig. 1h,i). This sample exhibited WO 3 -ZrO 2 particles of about 2.4 nm, thus of a size comparable to that of the ZrO 2 particles present on the parent ZrO 2 (6.8)/MWCNT composite (Table 1). Characterization methods such as XRD and TEM did not prove to be extraordinarily useful when investigating these complex materials therefore XPS and NO x -TPD were explored.

XPS measurements found the binding energies of Zr 3d5/2 and W 4f7/2 to be 182.5±0.1 and 35.7±0.1 eV, respectively, which is in good agreement with the presence of Zr 4+ (zirconia) [START_REF] Feng | Selective Hydrogenolysis of Tetrahydrofurfuryl Alcohol on Pt/WO 3 /ZrO 2 Catalysts: Effect of WO 3 Loading Amount on Activity[END_REF] and W 6+ (tungstates) [START_REF] Valigi | WO x /ZrO 2 Catalysts: Part 1. Preparation, Bulk and Surface Characterization[END_REF][START_REF] Gregorio | Activation and Isomerization of Hydrocarbons Over WO 3 /ZrO 2 Catalysts: I. Preparation, Characterization, and X-ray Photoelectron Spectroscopy Studies[END_REF][START_REF] Feng | Selective Hydrogenolysis of Tetrahydrofurfuryl Alcohol on Pt/WO 3 /ZrO 2 Catalysts: Effect of WO 3 Loading Amount on Activity[END_REF] species in the samples. Fig. 7 shows the evolution of the W/Zr atomic ratio determined by XPS as a function of the bulk W/Zr atomic ratio estimated from the ZrO 2 and WO 3 weight loadings of the tungstated samples. It is observed that the W/Zr atomic ratio determined by XPS does not vary linearly (Fig. 7, dotted line) with the known W/Zr atomic ratio of the samples investigated in this study. One explanation to account for this result is that W does not deposit in a highly dispersed state in the series of samples investigated, as is the case for bulk tungstated zirconias for comparable W surface densities, i.e. for W surface densities greater than or equal to that required to achieve monolayer coverage of ZrO 2 by tungstates [START_REF] Vaidyanathan | Surface Characterization of WO 3 /ZrO 2 Catalysts[END_REF][START_REF] Valigi | WO x /ZrO 2 Catalysts: Part 1. Preparation, Bulk and Surface Characterization[END_REF][START_REF] Gregorio | Activation and Isomerization of Hydrocarbons Over WO 3 /ZrO 2 Catalysts: I. Preparation, Characterization, and X-ray Photoelectron Spectroscopy Studies[END_REF][START_REF] Ross-Medgaarden | New Insights into the Nature of the Acidic Catalytic Active Sites Present in ZrO 2 -Supported Tungsten Oxide Catalysts[END_REF][START_REF] Feng | Selective Hydrogenolysis of Tetrahydrofurfuryl Alcohol on Pt/WO 3 /ZrO 2 Catalysts: Effect of WO 3 Loading Amount on Activity[END_REF]. In the case of the WO 3 -ZrO 2 /MWCNT materials, tungstates may also deposit onto the MWCNT support as WO 3 nanoparticles with diameters larger than the escape depth of excited electrons from the W 4f transition (~2.63 nm [START_REF] Ross-Medgaarden | New Insights into the Nature of the Acidic Catalytic Active Sites Present in ZrO 2 -Supported Tungsten Oxide Catalysts[END_REF]). This would contribute to the deviation of W/Zr atomic ratio measured by XPS from linearity (Fig. 7) despite that monolayer coverage of the ZrO 2 nanoparticles by the tungstates was not achieved. The material complexity therefore makes the use of XPS as a means to probe the coverage of the ZrO 2 nanoparticles by tungstates difficult. In contrast to bulk tungstated zirconias, these XPS data cannot be taken as direct evidence of monolayer coverage of the ZrO 2 nanoparticles by tungstates. To our knowledge, the only method capable of demonstrating monolayer coverage of ZrO 2 by tungstates is NO x -TPD.

As observed for ZrO 2 (6.8)/MWCNT, the two successive NO x -TPD experiments performed for each tungstated zirconia composite were found to be reproducible (Table 1).

Fig. 8 only shows the NO x and CO 2 profiles recorded in the first NO x -TPD experiment for each tungstated zirconia composite as the second cycle is nearly identical. As expected from our earlier investigation on bulk tungstated zirconias [START_REF] Law | NO x -TPD as a Tool to Estimate the Accessible Zirconia Surface of ZrO 2 -Containing Materials[END_REF][START_REF] Thomas | Should W Surface Density of WO x -ZrO 2 Catalysts Be Calculated With Respect to the Specific Surface Area of the Sample or That of ZrO 2 Only?[END_REF] the amount of NO x released was found to decrease on the introduction of increasing quantities of tungstates (Fig. 9a and Table 2). In parallel, the amount of CO 2 produced was also found to be lower on tungstate incorporation (Fig. 9b and Table 1). The fact that the amounts of NO x released, decreased on the introduction of increasing quantities of tungstates indicates that the tungstates interact with the ZrO 2 nanoparticles. The tungstate coverage, defined as the fraction of ZrO 2 covered by tungstates, is estimated with the NO x -TPD data and found to be 40.2, 64.6 and 77.3 % (Table 2) for WO 3 (2.3)-ZrO 2 (6.6)/MWCNT, WO 3 (4.6)-ZrO 2 (6.5)/MWCNT and WO 3 (8.7)-ZrO 2 (6.2)/MWCNT, respectively.

The theoretical W surface density of the tungstated zirconia composites can be estimated on the basis of their ZrO 2 and WO 3 loadings and considering that the size of the spherical ZrO 2 particles of the ZrO 2 (6.8)/MWCNT parent material is 2.2 nm (Fig. 1f,g, Table 1). The method used to determine such a theoretical W density will be detailed for WO 3 (2.3)-ZrO 2 (6.6)/MWCNT. The specific surface area of spherical ZrO 2 particles with a diameter of 2.2 nm is about 470 m 2 /g (Table 1). Hence, 1 g of WO 3 (2.3)-ZrO 2 (6.6)/MWCNT contains 31.2 m 2 of ZrO 2 and 100.5 µmol of WO 3 . The W surface density, which is the ratio of the number of atoms of W to the specific surface of ZrO 2 [START_REF] Thomas | Should W Surface Density of WO x -ZrO 2 Catalysts Be Calculated With Respect to the Specific Surface Area of the Sample or That of ZrO 2 Only?[END_REF] is therefore deduced to be 1.9 W/nm 2 ZrO2 . Applying this method to the two other tungstated zirconia composites resulted in theoretical W surface densities of 3.9 and 7.8 W/nm 2 ZrO2 for WO 3 (4.6)-ZrO 2 (6.5)/MWCNT and WO 3 (8.7)-ZrO 2 (6.2)/MWCNT, respectively.

From the difference between the amount of NO x released from the ZrO 2 (6.8)/MWCNT parent material and those released on the tungstated zirconia composites, and the fact that it was found that the addition of 1 W atom prevented the adsorption of 0.77 NO x molecule in our earlier investigation on bulk tungstated zirconia materials [START_REF] Thomas | Should W Surface Density of WO x -ZrO 2 Catalysts Be Calculated With Respect to the Specific Surface Area of the Sample or That of ZrO 2 Only?[END_REF], the amount of W interacting with ZrO 2 can be estimated and the corresponding W surface density deduced.

This resulted in W surface densities of 1.9, 3.0 and 3.6 W/nm 2 ZrO2 (Table 2) for WO 3 (2.3)-ZrO 2 (6.6)/MWCNT, WO 3 (4.6)-ZrO 2 (6.5)/MWCNT and WO 3 (8.7)-ZrO 2 (6.2)/MWCNT, respectively. It is remarkable that the W surface density determined by means of the NO x -TPD method agrees perfectly with that predicted theoretically for WO 3 (2.3)-ZrO 2 (6.6)/MWCNT (1.9 W/nm 2 ZrO2 , Table 2). This indicates that W interacted preferentially with the ZrO 2 particles in this sample. In contrast, the W density was found to be lower than that corresponding to monolayer coverage (4.1 W/nm 2 ZrO2 [START_REF] Thomas | Should W Surface Density of WO x -ZrO 2 Catalysts Be Calculated With Respect to the Specific Surface Area of the Sample or That of ZrO 2 Only?[END_REF]) for the more highly-loaded tungstated samples (Table 2). The fact that the high temperature NO x desorption peak did not vanish even for the most highly-loaded sample (Fig. 9a) suggests that excess tungsten interacted with MWCNT rather than ZrO 2 and started to form tridimensional WO 3 crystallites in agreement with the XRD data (Fig. 6). Finally, it can be concluded that monolayer coverage of ZrO 2 by W on the studied tungstated zirconia composites is almost complete when introducing W loadings twice as high as that theoretically needed.

Conclusions

In this study, the coverage of ZrO 2 nanoparticles supported on MWCNT by carbonaceous deposits or tungstates was investigated. For this purpose, the materials were characterized by NO x -TPD and these results were combined with XRD, XPS and TEM analyses. It was found that carbon deposition may occur on the ZrO 2 nanoparticles in the course of the synthesis process. These carbonaceous deposits, which covered approximately

Fig. 1 .

 1 Fig. 1. TEM micrographs and particle size distributions of (a) MWCNT, (b) and (c) as-

Fig. 2 .Fig. 3 .Fig. 4 .Fig. 5 .Fig. 6 .Fig. 7 .Fig. 8 .Fig. 9 .Fig . 1

 234567891 Fig.2. NO x (magenta), NO 2 (red), NO (blue) and CO 2 (green) profiles obtained on 0.1399 g

  ZrO 2 , comparison of the blank MWCNT and ZrO 2 (6.8)/MWCNT diffraction patterns only shows subtle differences in the 2Ɵ = 15 -60° range. The most noticeable deviation between ZrO 2 (6.8)/MWCNT and the blank MWCNT is between 28 -38° and is likely due to the

	presence of small ZrO 2 particles. After addition of W to the ZrO 2 (6.8)/MWCNT material,
	more signal is detected between 28 -38° and a shoulder appears at 24° that grows with
	increasing W loading. WO 3 (2.3)-ZrO 2 (6.6)/MWCNT and WO 3 (4.6)-ZrO 2 (6.5)/MWCNT,
	which contain W loadings lower than that required to achieve a monolayer coverage of ZrO 2
	by the tungstates, have very similar diffraction patterns although twice as much W is present
	in the latter sample. The additional signal in both samples when compared to
	ZrO 2 (6.8)/MWCNT, is most likely a result of diffuse but coherent diffraction from the
	tungstates. WO 3 (8.7)-ZrO

8)/MWCNT and ZrO 2 (6.8)/MWCNT impregnated with three different W loadings. At such a low loading of 2 (6.2)/MWCNT contains above the required amount of W to achieve a monolayer of tungstates and the XRD pattern exhibits a strong shoulder on the

Table 1 .

 1 30 % of the ZrO 2 surface, can be removed by successive NO x -TPD experiments, most likely via oxidation by NO 2 released from ZrO 2 . The size of the ZrO 2 particles estimated by NO x -TPD on ZrO 2 /MWCNT free of carbonaceous deposits and on a carbon-free ZrO 2 /MWCNT material were found to be in excellent agreement with that determined by TEM. Incorporation of increasing quantities of W on carbon-free ZrO 2 nanoparticles supported on MWCNT revealed that tungstate coverage of ZrO 2 increased from 40 to 77 % and that W interaction with the ZrO 2 nanoparticles decreased as the amount of W added increased. Remarkably, it was found that W interacted preferentially with the ZrO 2 nanoparticles for a W surface density of 1.9 W/nm 2 ZrO2 (half monolayer coverage of the ZrO 2 nanoparticles by tungstates).It was also found that monolayer coverage of ZrO 2 by tungstates on the studied tungstated zirconia composites was nearly complete when introducing W loadings twice as high as that theoretically needed. To our knowledge, these insights into the surface coverage of ZrO 2 by carbon or tungstates could not be obtained by any means other than NO x -TPD, which makes it a unique method to provide advanced characterization of the surface of oxides supported on MWCNT, in particular, and carbon supports, to a broader general interest. Summary of the NO x -TPD and TEM data. values estimated by considering spherical particles and a ZrO 2 density of 5.8 x 10 6 g/m 3

		NO x -
	Sample	TPD run	CO 2	NO x released	Accessible ZrO 2 surface a	ZrO 2 crystallite size (nm)
			µmol/g µmol/g µmol/g ZrO2	m 2 /g	m 2 /g ZrO2 NO x -TPD b TEM
	MWCNT 1	41	58	-		-	-	-	-
	As-prepared ZrO 2 (8.1)/MWCNT		-	-	-		-	-	2.1
	ZrO 2 (8.1)/MWCNT 1	164	220	2063		27.8	344	3.0	-
	ZrO 2 (8.1)/MWCNT 2	186	236	2252		30.4	375	2.8	-
	ZrO 2 (8.1)/MWCNT 3	218	280	2795		37.7	466	2.2	-
	ZrO 2 (8.1)/MWCNT 4	210	286	2874		38.8	479	2.2	2.3
	MWCNT 1	54	74	-		-	-	-	-
	MWCNT 2	54	66	-		-	-	-	-
	ZrO 2 (6.8)/MWCNT 1	188	263	2901		32.9	483	2.1	-
	ZrO 2 (6.8)/MWCNT 2	192	251	2735		31.0	456	2.3	-
	ZrO 2 (6.8)/MWCNT 3	197	257	2826		32.0	471	2.2	2.2
	WO 3 (2.3)-ZrO 2 (6.6)/MWCNT 1	118	171	1622		17.9	270	-	-
	WO 3 (2.3)-ZrO 2 (6.6)/MWCNT 2	124	180	1751		19.4	292	-	2.4
	WO 3 (4.6)-ZrO 2 (6.5)/MWCNT 1	78	127	991		10.7	165	-	-
	WO 3 (4.6)-ZrO 2 (6.5)/MWCNT 2	79	127	1004		10.9	167	-	-
	WO 3 (8.7)-ZrO 2 (6.2)/MWCNT 1	59	99	640		6.6	107	-	-
	WO 3 (8.7)-ZrO 2 (6.2)/MWCNT 2	55	96	579		6.0	97	-	-
	a values estimated by considering a NO x surface density of 6 µmol/m 2	ZrO2 [27,28]

b

Table 2 .

 2 Tungstate coverage and W surface density of the WO 3 (y)-ZrO 2 (x)/MWCNT samples estimated from NO x -TPD data calculated on the basis of the loadings of ZrO 2 and WO 3 in the samples, and considering a mean ZrO 2 particle size of 2.2 nm (470 m 2 /g ZrO2 , Table1) b based on the amount of NO x released from the ZrO 2 (6.8)/MWCNT parent sample and those released on the tungstated zirconia composites, and considering that the addition of 1 W atom prevented the adsorption of 0.77 NO x molecules[START_REF] Thomas | Should W Surface Density of WO x -ZrO 2 Catalysts Be Calculated With Respect to the Specific Surface Area of the Sample or That of ZrO 2 Only?[END_REF]

	Sample	Tungstate	NO x released W density (W/nm 2	ZrO2 )
		coverage
		(%) µmol NO x /g ZrO2 Theoritical a Estimated b
	MWCNT	-	-	-	-
	ZrO 2 (6.8)/MWCNT	-	2821	-	-
	WO 3 (2.3)-ZrO 2 (6.6)/MWCNT	40.2	1687	1.9	1.9
	WO 3 (4.6)-ZrO 2 (6.5)/MWCNT	64.6	998	3.9	3.0
	WO 3 (8.7)-ZrO 2 (6.2)/MWCNT	77.3	640	7.8	3.6

a

Acknowledgments

GLH and PRK acknowledge the DOE, Office of Basic Energy Science, Grant No. DE-FG02-01ER15183, for financial support of this research at Yale University/PNNL. PRK also acknowledges EMSL, a DOE Office of Science User Facility sponsored by the office of Biological and Environmental Research, and Dr. Mark Bowden for assistance collecting the XRD data in this study.

CT acknowledges UPMC and CNRS for providing financial support to this study at the Laboratoire de Réactivité de Surface.


J Desomberg

F Fournel

H Moriceau

A Roule

E Barthel

F Rieutord

Influence of water diffusion in deposited silicon oxides on direct bonding of hydrophilic surfaces

Direct bonding of PECVD SiOx films is of great interest in the field of microtechnologies for applications such as stacked structures, thin film transfers, whose fabrication processes still deserve to be investigated. This work deals with the influence of water diffusion in the SiOx films deposited onto 200mm Si wafers and its impact on adherence of hydrophilic surfaces. The as-deposited film nature induces various stresses. Stresses evolutions and water penetration are characterized and a correlation is made between the kinetics of both stress variations and water diffusion through the oxide thin films. Impacts of room temperature (RT) storage prior to bonding and thermal treatments applied for strengthening the bonding is shown.

. As the mechanical properties of silicon oxide material are greatly influenced by internal water concentration, aging and water diffusion on asperity have a real impact in term of direct bonding energy. Moreover at bonding interfaces, it is observed by X-ray reflectivity (XRR) that aging of deposited SiOx prior to bonding enables shallower bonding gaps, indicating better bonding interface closure which is coherent with the bonding energy enhancement.

All these results confirm the link between water diffusion and hydrophilic surface adherence of deposited SiOx films.

Introduction

Direct wafer bonding is a technique which consists in bringing into contact two mirror-polished wafers which are subsequently spontaneously held together by van der Waals forces at room temperature without addition of any polymer coupling material between the surfaces. This technique is more and more used in microelectronic applications such as film transfer, 3D integration and MEMS heterostructure assembly. A large distribution of insulator materials can be used in microelectronic fields but silicon dioxide material still remains widely used nowadays. As-deposited silicon oxides could be processed at low temperature and the sealing of the bonding interface could also be done at low temperature to allow some new applications for direct bonding of insulator materials. Direct bonding of thermal silicon dioxides was widely investigated for the last decade, but deposited silicon oxides direct bonding stills deserve to be investigated.

In this way Plasma-Enhanced Chemical Vapor Depostion (PECVD) silicon oxides (SiOx, x lower or equal to 2) is investigated for direct bonding. Main challenges in this study consist in getting an efficient bonding of deposited silicon oxides in terms of both bonding energy and defects density within the temperature range [RT-500°C] and in developing a better understanding of phenomena involved in direct bonding of deposited silicon oxides. Especially, the main objective is to understand the influence of trapped species in deposited silicon oxide thin films. Our investigation is focused on the water influence in those thin films. The nature of these deposited films is also investigated. This nature which depends on deposition parameters and post-deposition preparation has a great impact on the water absorption kinetic throughout the oxide films. Then the internal water concentration could be tuned and could have an impact on the mechanical or physicochemical surface properties of those films. These property modifications are shown to have a strong impact on wafer direct bonding of deposited silicon oxides. Deposited silicon oxides bonding strength dependence on various parameters are reported here.

Experiments

This study is carried out on 200 mm, lightly p-doped Si(100) wafers. Two kind of PECVD silicon oxides are deposited at 400°C on these wafers with tetraethyl orthosilicate (TEOS) precursor depending on the injection system: one is based on liquid direct injection and the other one uses a bubbler injection. Different thicknesses from 150 nm to 1 µm are deposited. It will allow having a better understanding of the trapped species density influence in these films for bonding applications.

After the film deposition these wafers are submitted to different treatments prior bonding. For instance a thermal treatment (450°C, 2 hours under O2) is applied after deposition on a few samples in order to study a silicon oxide drying step prior bonding. Many films are stored at RT in cleanroom environment with 45% of relative humidity (RH) to see the aging influence. Chemical Mechanical Polishing (CMP) is then performed (50 nm removed) in order to reach the needed roughness for direct bonding at room temperature and to enhance hydrophilic behavior of silicon oxide films just before the bonding in cleanroom. The sample behaviors are investigated after various thermal treatments in the [RT-500°C] range for 2 hours allowing the partial sealing of the bonding interface.

Many characterizations are done on the thin films (prior bonding and CMP) such as stress measurements over time (with RT storage in cleanroom environment at 45%RH). These stress measurements are done by an optical interferometer which measure the bow before and after deposition and give us the residual stress. From the difference of wafer curvature radius before and after deposition, the stress is calculated thanks to the Stoney formula. In addition, the film chemical compositions are given by Fourier Transform Infrared (FTIR) spectroscopy in transmission mode. This allows an aging monitoring by moisture absorption characterization.

Once bonding are annealed at various temperature in the [RT-500°C] range, bonding energy, defect density and bonding interface closure are investigated. The bonding energy is measured by the double cantilever beam (DCB) technic under prescribed load on 2 cm wide strip samples, which consists in a razor blade insertion at the bonding interface and the induced debonded length measurement. The measurement are done in anhydrous atmosphere to avoid any external water stress corrosion. The bonding energy G is calculated with the debonded length using the following El-Zein equation , Where a is the debonded length, t the wafer thickness, δ the inserted blade thickness, and Sij the general term of compliance tensor in the opening direction [START_REF] El-Zein | Evaluation of G IC of a DCB specimen using an anisotropic solution[END_REF] in order to take into account the silicon anisotropy [START_REF] Fournel | Measurement of bonding energy in an anhydrous nitrogen atmosphere and its application to silicon direct bonding technology[END_REF]. The defect density is investigated by acoustic microscopy. Otherwise high energy interfacial xray reflectivity (XRR) which gives the electron density profile of the bonding interface is used to get a better understanding of bonding closure mechanisms.

Results and discussion

In this paper, a global investigation of deposited silicon oxides films evolution over the aging time is done and it impacts on direct bonding are reported here. In a first part, the influence of the material nature and the aging duration on the water amount inside those thin films and its impact on direct bonding is shown. This is highlighted by high energy XRR results to characterize the bonding interface sealing. In a second part, a link is demonstrated between residual stresses kinetic evolutions of deposited silicon oxides thin films and water absorption kinetic evolution. In a third part, the impact of deposited silicon oxide thickness on water absorption and its impact on direct bonding is investigated. Finally some additional investigations were done concerning stress variation in order to go more deeply into this absorption phenomenon. Contrary to thermal silicon oxides which are structurally well defined overall their thickness, PECVD silicon oxides may have various nature of material depending on the nature of the technical deposition, the RF power chosen, the O2/TEOS ratio flow chosen [START_REF] Sabbione | Low temperature direct bonding mechanisms of tetraethyl orthosilicate based silicon oxide films deposited by plasma enhanced chemical vapor deposition[END_REF] or other technical deposition parameters such as the deposition temperature. Moreover PECVD silicon oxides may have an evolution of their intrinsic structure in thickness. All these parameters which could be tuned for deposited silicon oxides may have a great impact on the moisture absorption. This moisture absorption capability of deposited silicon oxides have a significant impact on direct bonding energy as higher values could be reached (above 5 J/m² in the post-bonding annealing temperature [200°C-500°C] range) with them compared to the thermal silicon oxides (bonding energy from 0.8 J/m² to 5 J/m² in the post-bonding annealing temperature [200°C-500°C] range) [START_REF] Fournel | Measurement of bonding energy in an anhydrous nitrogen atmosphere and its application to silicon direct bonding technology[END_REF].

In order to investigate the material nature influence in the case of PECVD silicon oxides, the capability of two different films to absorb moisture over the time is studied. Different kinds of TEOS injection systems are chosen such as Liquid direct injection in which TEOS injection debit in the reactor is accurately controlled or Bubbler injection in which TEOS injection debit is depending on the saturated vapor pressure.

FTIR measurements are performed on those two materials (1 µm deposited here on Si bulk) at different time after silicon oxide deposition in order to get the time evolution of the -OH and H2O absorption band. The FTIR spectra i.e. the wavenumber dependence in the [3000 cm -1 -3800 cm -1 ] range of the absorption coefficient of both materials is plotted in Figure 1. FTIR spectra of all deposited films show the presence of isolated Si-OH groups vibrating at 3650 cm -1 . Regarding the as-deposited films, both PECVD silicon oxides thin films have a certain capability to absorb moisture just after deposition as demonstrated here by the detection of a broad absorption band around 3300-3400 cm -1 which can be assigned to the associated Si-OH bonds and free H2O. All aged thin films were stored 2 months in cleanroom environment at a monitored 40% room humidity (RH). Noteworthy, the bubbler injection silicon oxide thin film (red curve for the as-deposited material and pink curve for the aged material) has a higher propensity to absorb moisture over the time than liquid direct injection silicon oxide thin film (black curve for the as-deposited material and grey curve for the aged material). A huge behavior difference regarding the moisture absorption could then be seen just by changing the PECVD injection type.

Moreover, even just after the deposition the bubbler injection silicon oxide thin films has a higher absorption bands in the 3300-3400 cm -1 range than both 2 months aged and as-deposited liquid direct injection materials. It emphasizes again the difference of those two materials in their intrinsic nature and composition. This should have an impact in terms of physicochemical and mechanical parameters of these oxides surfaces and sub-surfaces and should impact the sealing of the bonding interface and then the direct bonding energy.

Moreover the 2 months aged bubbler injection thin films a higher amount of isolated Si-OH groups (3650 cm -1 ) than all other materials (aged and as deposited liquid direct injection materials and as deposited bubbler injection material). These isolated Si-OH groups are probably located at the surface of the silicon oxides and should then have a great impact on subsurfaces physicochemical properties and on direct bonding. This effect will not be studied in this work as a CMP step which remove roughly 50 nm of material is performed just before the bonding. This process allows a reduction of the root mean squared (RMS) roughness and have a chemical action on the surface and the subsurface which facilitates direct hydrophilic direct bonding. First of all, as shown on Figure 2, the direct bonding energies of symmetrical as deposited liquid direct injection bonding and as deposited bubbler injection bonding are measured regarding different post bonding annealing temperature.

Above 200°C, the direct bonding energy of the as-deposited bubbler injection silicon oxide bonding is higher than the corresponding bonding energy of the liquid direct injection silicon oxide bonding. This is due to a difference in terms of global material structure. Indeed as previously shown, FTIR spectra of as-deposited bubbler injection materials got a higher amount Si-OH and H2O than the as-deposited liquid direct injection materials. Moreover, an outstanding increase of direct bonding energy between 100°C and 200°C can be seen for bubbler injection material. This thin film has absorbed a certain amount of water and this water is used to improve the sealing of the bonding interface. As demonstrated later in Figure 11 and 12, 200°C is needed to allow this water to be used at the bonding interface to strengthen it.

Finally as-deposited bubbler injection bonding has a bonding energy at 500°C far above 5 J/m 2 which is known to be the energy silicon fracture. As the sample aren't broken during the blade insertion and the sample was opened at the bonding interface it proves that a little amount of energy is lost throughout the silicon oxide volume. The system is not purely elastic.

In order to understand more deeply some mechanisms involved in the sealing of the bonding interface for PECVD SiO2 thin films depending of ageing and deposited silicon oxide thickness some additional investigations were performed. Some bonding were characterized by high energy XRR. This experimentation allow to measure the gap profile of the bonding interface. The gap profile of these bonding depending on the post-bonding annealing temperature is plotted in Figure 3. It appears that as-deposited PECVD SiO2 bonding have a higher gap profile at the bonding interface rather than aged PECVD SiO2 thin films, even at room temperature. This underline the impact of aging prior bonding for those deposited films, even at room temperature which means there is a physicochemical impact on the silicon oxide surface of aging. Aging allow a better closure of the bonding interface which means a higher bonding area. For aged thin films a high decrease of gap profile operates between 200°C and 300°C which correspond to a motion of water from the silicon oxide volume to the bonding interface. This is demonstrated later in Figure 11 and 12. Moreover 100 nm as-deposited PECVD liquid direct injection bonding have a lower gap profile than 1 µm as-deposited PECVD liquid direct injection bonding. This will confirm following results (Figure 6) related to bonding energy of thinner thin films.

It is then demonstrated that deposited silicon oxides thin films are able to absorb moisture depending on their deposition technic (bubbler or liquid injection) and it seems that this absorbed water have an impact on the sealing of the bonding interface. Then it appears interesting to investigate more deeply the influence of aging in those thin films. For this purpose, the time dependence of the stress variation in those films is measured. As shown on Figure 4, there is an important difference between the two material deposition types.

Bubbler injection silicon oxide is tensile with a residual stress of 40 MPa just after deposition and become more and more compressive over the time. On the other side liquid direct injection silicon oxide is compressive just after deposition with a residual stress around -70 MPa and become a little more compressive over the time but its dynamic is much smaller than the bubbler injection one. Moreover, after 65 days, the bubbler stress is even lower than the liquid one. There is then a strong correlation between the kinetic of stress variation of PECVD SiO2 thin films and the kinetic of water diffusion shown by the FTIR spectra. Various stress variation kinetics induces various water absorption kinetics. Because of its tensile stress as deposited bubbler material have an ability to facilitate the water absorption and become more and more compressive as a result of water penetration inside.

In order to see the impact of the silicon oxide deposited thickness on water absorption and on direct bonding, a 150 nm bubbler material is compared to the 1 µm thin film.

As shown on Figure 5, there is a higher kinetic of stress variation concerning the thinner film compared to the thicker one. This means that thinner bubbler injection materials have a better capability to absorb moisture to the thicker one and this may have an impact on direct bonding energy. Indeed as shown on Figure 6, the symmetrical direct bonding energy of both material thickness exhibit quite different values even with as deposited films. Asdeposited 100 nm bubbler materials exhibits a very high bonding energy close to 7 J/m² with a 400°C post-bonding annealing compared to 1 µm as-deposited one which has a bonding energy close to only 5 J/m² with the same postbonding annealing temperature. This results concerning the better bonding energy of thinner films is confirmed by XRR measurements which have shown that thinner films bonding have a lower gap profile at the bonding interface.

The same thickness impact is also observed for the liquid injection material. The residual stress variation over the time of a 150 nm PECVD SiO2 thin film obtained by liquid direct injection compared to a the residual stress variation over the time of a 1 µm PECVD SiO2 thin film obtained by liquid direct injection is plotted in Figure 7.

As seen in Figure 7, both PECVD SiO2 thin films thick (1 µm) and thin (150 nm) have a residual stress just after deposition close to -80 MPa. This is normal as this value depends only on the nature of the material and on the deposition parameters. However both films haven't the same stress variation kinetic over the time. While 1 µm thin film have a slight decrease of residual stress over the time the 150 nm thin film have a huge decrease of residual stress over the time which means that this film is able to absorb moisture quite quickly. In order to confirm this assumption, these thin films undergo FTIR measurements. The evolution in time of -OH and H2O absorption bands of a 1 µm PECVD SiO2 thin film obtained by liquid direct injection is plotted in Figure 8. In the same way corresponding evolution in time of -OH and H2O absorptions bands of a 150 nm PECVD SiO2 thin film obtained by liquid direct injection is plotted in Figure 9. As seen before thin films with a high kinetic variation of residual stress have also a high capability to absorb moisture as observed here for this 150 nm PECVD thin film rather than 1 µm one.

It is then very interesting to note that a very thin thickness enhance the water absorption kinetics. This allows us to produce direct bonding structure with important bonding energy even with the liquid direct injection material. After a CMP step, wafer bonding is operated with a as-deposited and a 2 months aged 150 nm thin film The evolution of the direct bonding energy is plotted on Figure 10. Aged 150 nm liquid direct injection silicon oxide bonding shows a bonding energy of 7.5 J/m2 after a bonding consolidation at 400°C during two hours against only 5.5 J/m2 for it corresponding as deposited material. This results emphases the significant role of water in the proceeds of the bonding interface sealing. It seems there is an impact of water since room temperature and this effect is increased above 200°C. The reason of this increased bonding energy is either due to a better concentration of Si-OH group at bonding interface which increase the dangling bonds density at the surfaces or due to some mechanical properties of those thin films which has been impacted by a high concentration of water in the subsurface allowing a higher bonding area during bonding consolidation.

It is then demonstrated that water diffusion in PECVD oxide film is quite interesting for direct bonding energy. In order to go more deeply into this absorption phenomenon, some additional investigations are done concerning stress variation. An aged bubbler injection silicon oxide is investigated and undergoes two in-situ subsequent stress variation measurements versus temperature from room temperature to 500°C and backward. Two sequential thermal cycle are done during the same experiment under nitrogen atmosphere. These two in-situ subsequent stress variation measurements versus temperature are plotted in Figure 11.

As shown on Figure 11 the aged thin film has an initial residual stress of -150 MPa and this thin film become less and less compressive with the increase of temperature. From 200°C to 500°C water desorption operates. After this increase of temperature, the sample is cooling down until room temperature and get the standard residual as deposited stress (cf. Figure 4). An additional thermal cycle is done and the residual stress variation correspond to the silicon oxide coefficient of thermal expansion (CTE) difference with the silicon wafer one which means without any water impact.

Corresponding measurements of -OH and H2O absorption bands after deposition, 3 months after deposition and after first and second thermal cycle are plotted in Figure 12.

As shown on Figure 12, after a first thermal cycle there is no longer water inside the thin film which is coherent with a residual stress back to the initial stress measured just after deposition as shown on Figure 11. However, after those thermal treatments the silicon oxide films are again able to absorb moisture. The residual stress variation over the time of PECVD SiO2 thin films without thermal cycle and after two thermal cycles is plotted in Figure 13.

After water "drying", PECVD silicon oxides thin films has a residual stress variation over the time which follow almost the initial residual stress variation after deposition. As proved before a similar kinetic of stress variation means a similar kinetic of water absorption over the time.

Conclusion

A global investigation of PECVD silicon oxide thin films is made in order to see the impact of intrinsic properties of those thin films on direct bonding. It is shown that deposited silicon oxides have various deposition parameters which can be tuned. Depending on these deposition parameters and how these parameters are tuned various kinetic of residual of stress variations could be induced. It is demonstrated a special relationship between the stress variation kinetic over the time with the water absorption kinetic which have a significant impact on direct bonding.

Water management inside deposited silicon oxide is then a key point to improve direct bonding of these oxides. Water induces special mechanical or physicochemical properties on asperities in contact at the bonding interface which can increase the bonding energy. Water could reduce the stiffness of asperities in contact at the bonding interface and increase the bonding area.

Figure 1 :

 1 Figure Captions :Figure 1 : Time evolution of the -OH and H2O band observed by FTIR for different natures of thin films and aging duration after deposition ......................................................................................................................................... 8 Figure 2 : Direct bonding energy evolution regarding the post bonding annealing temperature of as-deposited PECVD thin films. .................................................................................................................................................. 8 Figure 3 : Gap Profile of bonding interface of various as-deposited and aged PECVD SiO2 bonding regarding postbonding consolidation temperature ......................................................................................................................... 9 Figure 4 : Residual stress variation over the time of different nature of PECVD thin films of SiO2 ...................... 9 Figure 5 : Residual stress variation over the time of a 150 nm and 1 µm PECVD SiO2 thin film obtained by bubbler injection. .................................................................................................................................................................. 9 Figure 6 : Direct bonding energy evolution regarding the post bonding annealing temperature of bonded structures with as-deposited bubbler PECVD SiO2 thin films of various thicknesses ........................................................... 10 Figure 7 : Residual stress variation over the time of a 150 nm PECVD SiO2 thin film obtained by liquid direct injection (red curve) and a 1 µm PECVD SiO2 thin film obtained by liquid direct injection (black curve) ......... 10 Figure 8 : Evolution in time of -OH and H2O absorption bands of a 1µm PECVD SiO2 thin film obtained by liquid direct injection....................................................................................................................................................... 10 Figure 9 : Evolution in time of -OH and H2O absorption bands of a 150 nm PECVD SiO2 thin film obtained by bubbler injection material ..................................................................................................................................... 11 Figure 10 : Direct bonding energy regarding the post bonding annealing temperature with as-deposited and 2 months aged 150 nm PECVD SiO2 thin films ...................................................................................................... 11 Figure 11 : Two in-situ subsequent stress variation measurement versus temperature from room temperature to 500°C and backward (thermal cycle) of a 3 months aged bubbler injection deposited SiO2 thin film. ................ 11 Figure 12 : Measurement of -OH and H2O absorption bands observed by FTIR of a bubbler injection material after deposition (black curve), 3 months after deposition (red) and after a first (blue) and second (purple) thermal cycle .............................................................................................................................................................................. 12 Figure 13 : Residual stress variation over the time of PECVD SiO2 thin films without thermal cycle and after two thermal cycles ....................................................................................................................................................... 12

Figure 3 :

 3 Figure 3 : Gap Profile of bonding interface of various as-deposited and aged PECVD SiO2 bonding regarding post-bonding consolidation temperature

Figure 6 :

 6 Figure 6 : Direct bonding energy evolution regarding the post bonding annealing temperature of bonded structures with asdeposited bubbler PECVD SiO2 thin films of various thicknesses

Figure 9 :

 9 Figure 9 : Evolution in time of -OH and H2O absorption bands of a 150 nm PECVD SiO2 thin film obtained by bubbler injection material

Figure 12 :

 12 Figure 12 : Measurement of -OH and H2O absorption bands observed by FTIR of a bubbler injection material after deposition (black curve), 3 months after deposition (red) and after a first (blue) and second (purple) thermal cycle

Acknowledgments

This work was funded by the Commissariat à l'Energie Atomique et aux Energies Alternatives (CEA) of Grenoble.