Vincent Génot
email: vincent.genot@irap.omp.eu

N André

B Cecconi

M Bouchemit

E Budnik

N Bourrel

M Gangloff

N Dufourg

S Hess

Ronan Modolo

Joining the yellow hub: Uses of the Simple Application Messaging Protocol in Space Physics analysis tools

Keywords: space physics, planetary sciences, virtual observatory, interoperability standards, analysis tools, scientific exploitation

  L'archive ouverte pluridisciplinaire

INTRODUCTION

With the growing amount of data from both observational and modeling origins, data centres are forced to develop and ease data access and scientific exploitation. This goes through user friendly data bases but also through the procurement of dedicated tools which are the key to optimal science return. In planetary sciences and space physics the study of a particular object (e.g. planets, moons, comets, solar wind, …) is seldom performed using a single dataset; on the contrary it is indeed through the combination of multiple dataset of heterogeneous origins that new insight can be gained. However designing a single analysis tool holding all functionalities to tackle this large variety of data would be unreachable; more reasonably and efficiently, the current trend is to couple or interoperate specifically designed tools. Data communication between these tools or software then remained the key to have such an organization working on the large scale. This is the frame of common protocols to which IVOA (International Virtual Observatory Alliance) has long been committed.

In this paper we show how one of these IVOA protocols, the Simple Application Messaging Protocol or SAMP, has been efficiently used outside its originating community (astronomy) to address science cases applied to space physics. The focus is therefore not on the specificity of the protocol, but rather its practical use, to show how it enables easy data communication between tools. We take advantage of such uses at the French Plasma Physics Data Centre (CDPP), but also companion tools, to present adequate science cases. Further technical information on SAMP may be obtained directly from IVOA http://www.ivoa.net/documents/SAMP/ . The paper starts by shortly presenting the CDPP tools involved, continues with a short note on SAMP, before proceeding to the science cases.

CDPP: A SHORT PRESENTATION

CDPP and interoperability

Since its creation by CNRS and CNES in 1998, the French Plasma Physics Data Centre (CDPP) has a continuous strategy to develop analysis tools for the wider space physics community. If data distribution and scientific tool development are at the core of CDPP activities, they will not reach their goal if they were not entangled in an international web of efforts. Several consortia indeed aim at unifying data description (or data model) and data communication (or protocols) in order to assemble interoperable bricks of services. The main actors are namely: IVOA (http://www.ivoa.net/ ) in the general astronomy field, IPDA (http://planetarydata.org/ ) where planetary sciences issues are discussed between space agency, SPASE (http://www.spase-group.org/ ) in the space physics field. At the French level ASOV (http://www.francevo.org/ ) coordinates the national effort.

In the recent year European FP7 projects have given an important frame for designing and implementing new standard and protocols.

In Europlanet-RI, the CDPP team participated with the VO-Paris team to the development of PSR-DM (Planetary Science Resource Data Model) and EPN-TAP (Europlanet-Table Access Protocol), both based upon IVOA standards. These data model and protocol are now used to share more and more datasets in the domain of planetary sciences (Erard et al., 2014a,b), using IVOA existing standards and infrastructure. At the same time, AMDA has turned SAMP compatible which enabled its connection to external tools such as Aladin, Topcat and query databases such as APIS (http://lesia.obspm.fr/apis/ ); this is demonstrated here in science cases section. Tutorials (as video or manual) of how this interplay can be performed are also available on the CDPP tutorial page.

In IMPEx (see a detailed project description at http://impex-fp7.oeaw.ac.at/ ) the CDPP participated to the development of a simulation data model together with the LATMOS (documents are available at http://impex-fp7.oeaw.ac.at/documentation.html ). This model, which is a SPASE extension, enables to describe simulation and analytical model results in a common way which is a prerequisite to build interoperable services on heterogeneous databases.

The following Figure presents how CDPP databases and tools are interlinked and connect to the outside world. Some of these value added services and databases are discussed in the next pages.

Tools at CDPP

Since 2006 and the development of the AMDA tool, CDPP followed its strategy to increase data exploitation. It led to the development of several tools both internally and by the means of close cooperation between CDPP researchers and SME.

AMDA: http://amda.cdpp.eu/

AMDA is developed at CDPP since 2006 [START_REF] Jacquey | AMDA, Automated Multi-Dataset Analysis: A web-based service provided by the CDPP, The Cluster Active Archive, Studying the Earth's Space Plasma Environment[END_REF], Génot et al., 2010). Its initial aim was to provide visualization capabilities to Earth magnetosphere data. Quickly it was enriched by a data mining tool and the functionality for the user to define its own physical parameters. The central philosophy is that the user manipulates parameters and does not directly manipulate files. The tool therefore acts both as a data base and a tool as its data holding has been progressively increased to include those from a wide variety of heliophysics missions. Recently in the frame of the FP7 IMPEx project access to simulation and model data has been also provided via AMDA.

AMDA can be connected to a SAMP hub in order to easily exchange files (in VOTable or CDF formats) with other analysis tools.

Data in AMDA

The AMDA database mostly consists in time series (scalar, vector, spectra) in homogeneous format to ease exploitation. Moreover data from other space physics centres are accessible via interoperable means:

-CDAWEB at GSFC, NASA (a large collection of data from space physics missions), -the European mirror data centre for MAPSKP/Cassini data at IRAP (key plasma parameters from the Cassini mission), -the European mirror data centre for NASA/Themis data at IRAP (a five-spacecraft mission to study the magnetosphere), -the Venus Express MAG database at IWF, Graz, Austria (the magnetometer instrument onboard the Venus orbiter).

Via these various centres and its own holding, CDPP renders accessible space plasma data from the ionospheric domain to the heliospheric domain also including planetary sciences; in this last domain it should be noted that the FP7 Europlanet-RI project enabled the ingestion of more than 300 new datasets in AMDA, mostly from NASA/PDS. Thanks to the FP7 IMPEx project the CDPP recently widened its data offer by providing modeling results: magnetic field models at planets (from SINP, Russia) and hybrid/MHD codes for solar wind/planet interactions (from LATMOS and FMI).

3Dview: http://3dview.cdpp.eu/ 3D orbit visualisation in maneuverable scenes is the main objective of 3DView. All celestial objects from the Solar System are included together with most of scientific spacecraft. Recently in the frame of the FP7 IMPEx project access to observation, simulation and model data has been also provided. This tool is developed by the SME "GFI" under CNRS contract and CNES license.

CONNECTING TOOLS: SAMP APPLICATIONS IN SPACE SCIENCE TOOLS

Besides CDPP tools a crowd of space physics tools are nowadays joining the interoperability and Virtual Observatory paradigm, following the pioneering astronomy-led software such as Aladin or Topcat.

If these tools commonly take advantages of web services for data communication, there is also a growing use of the communication protocol SAMP (http://www.ivoa.net/documents/SAMP/), developed by IVOA from its predecessor PLASTIC [START_REF] Taylor | PLASTIC -a protocol for desktop application interoperability[END_REF]. One of the goals behind SAMP is to ease data broadcast between tools without building a strong link between them (as web services do).

The purpose of the next section is to show how this protocol, initially designed for astronomy applications, is successfully employed for plasma and planetary sciences uses. In order to understand this attractiveness, let us first present the basis of this protocol (the dedicated IVOA page is a reference document; see also http://wiki.ivoa.net/twiki/bin/view/IVOA/SampPapers).

Basically, the protocol is based on the creation of a hub through which data files in common format can transit from one stand-alone or web-based application to another. When a hub is formed it is symbolized by the ¤ logo, usually visible on the desktop bar of your computer. Usually space physicists tend to launch an instance of the Topcat tool to create the tool, even if they do not use it for their application (see for instance the Science Case 1 below).

The semantics of messages exchanged between SAMP clients are defined by MTypes (Message Type).

Although VOTable is the standard way to convey tabular data with SAMP (from the IVOA recommendations), it should be noted that it is also possible to transmit FITS file, and more recently also CDF files, now a standard format in the space physics community. This stems from the fact that most of existing MTypes were initially defined in astrophysical context, and thus not usable in the case of space physics or planetary sciences; it follows that specific communities tend to define their own MTypes. For CDF a specific MType has been defined which is useful when both ends of the communication understand CDF, or when transformation to VOTable would not be convenient. The list of known Mtypes is maintained informally by the IVOA, and can be obtained from http://www.ivoa.net/samp/ .

SCIENCE CASES

Science case 1: magnetic field in GSE/LSE coordinate systems in the Moon environment

Description

The user wants to visualize magnetic field data (vectors) acquired around the Moon in a Moon-centered scene. He knows that such data exist on AMDA and wants to use 3DView for the 3D rendering.

Tools involved: AMDA, 3DView, TREPS

Procedure

The user goes to the database of the AMDA tool (Figure 4) and selects magnetic field data in the Moon environment obtained by the NASA/Themis mission; however in AMDA these data are only available in Earth-centered coordinates (GSE), as this mission was initially targeting Earth sub-storms physics; in a later phase of the mission, two of its probes (Themis B & C) have been later gravitationally captured by the Moon, thus reorienting the scientific objectives (a project called Artemis), but not how the database is built. The user first plots the data for the interval of interest and then thinks to use the coordinate transformation service, TREPS, to perform an operation between the original GSE system and the final LSE system in which he/she wants to display the data. The user hits the Get Data button. A download interface appears in which he/she can choose the time format, the structure of the file, …; actually the default selection is enough for the current purpose. Hitting the download button he/she can choose to send the data to a companion tool instead of getting a file. Here one assumes that a SAMP hub has already been opened (typically the Topcat software is used to launch a local hub on the user's computer) and that AMDA and TREPS are also connected to the hub. In the AMDA interface it is then possible to choose TREPS as a destination tool for the data. Now, in TREPS the GSE coordinate system is chosen as the source system while the target system is set to LSE. The transformation is then easily realized and, if the user correctly defines VOTable as the export format, he/she then has the possibility to send the transformation result via SAMP (Figure 5). The user goes to 3DView, sets up the scene (Moon-centered, in ECLIPJ2000) for the corresponding spacecraft (Themis B for the given interval, a scene). 3Dview must then also be connected to the SAMP hub. Back in TREPS, the user sends the transformed data to 3DView which finally displays the magnetic field vector as colored arrows along the Themis B trajectory (Figure 6). It should be noted that, in this process, the user did not manipulate data files at any time: SAMP first enabled the transfer of data between AMDA and TREPS, and second between TREPS and 3DView. At present this chain is operating only if the VOTable format is used throughout but it will be soon possible to widen this choice (to CDF and netCDF formats for instance in this community).

Science case 2: comparison of observed and simulated plasma velocity in the Mars environment

Description

The user wants to compare observations on the Mars environment with numerical simulations in order to get the broader context and/or to validate the numerical model.

Tools involved: LatHyS, AMDA, Topcat

Procedure

The user goes to the database of the AMDA tool (Figure 7) and selects several Mars Express datasets for plotting such as the proton energy spectrograms and moments together with orbital information on a time interval corresponding to one orbit. He/she chooses to compare these data with simulated data available on the LatHyS (Latmos Hybrid Simulation, htpp://impex.latmos.ipsl.fr/LatHyS.htm) database to reveal the plasma behavior in places adjacent to the specific observations by Mars Express. As LatHyS is a database, containing 1D, 2D, and 3D pre-computed data products, but with no plotting interface, and as AMDA only enables to display time series, the user decides to perform the comparison on a third tool, Topcat, which enables both kinds of plots. Here also it is assumed that Topcat has been opened from the start to initiate the hub; in reality it is not too late to do it now. The procedure to send data to Topcat from AMDA is similar to what has been explained in Science Case 1 (an displayed in Figure 4). On LatHyS (Figure 8) the user has the choice between several simulations of the Martian environment and finally selects a run depending on the solar wind input parameters. He/she connects LatHyS by hitting the appropriate button on the interface (note that similarly to AMDA LatHyS is a web-based application but can nonetheless be connected to the hub). He/she decides to compare proton velocity in the simulated XZ plane which he/she sends to Topcat. At this stage (Figure 9) Topcat has received two tables: one (IMPEx data) from LatHyS, and one (DDw7qb4q) from AMDA; these particular names are chosen by the sending applications depending on their internal mechanism. Here again all data are conveyed in VOTable. Manipulating Topcat allows to displayed both tables and finally to over-plot 1/ the proton velocity (here the xcomponent but this can be modified) measured by Mars Express along its trajectory (the velocity amplitude is here coded in color) on 2/ the XZ plane cut of the 3D simulation file of the same quantity. It should be observed that the two plots agree reasonably well (i.e. colors match in the same location) such as to differentiate solar wind from planetary environment plasmas.

Figure 8: the LatHyS database developed at LATMOS in the frame of European FP7 IMPEx project. Hybrid simulations of planetary environments in interaction with the Solar Wind are accessible as 3D data files, or sampled as plane cuts or along spacecraft trajectories. The SAMP functionality may be activated on the upper left part of the interface and data subsequently sent by hitting the Send button (in the form of a paper plane).

Figure 7: on the AMDA desktop the user has displayed the data tree (left) with Mars Express data directories, made a plot on the interval of interest (middle part of the image), clicked on the "Get Data" button to select data to be exported (right part of the image), clicked on the "Download" button and displayed the different client tools to which he/she can SAMP-send the data (here Topcat and LatHyS are proposed as possibilities).

Figure 9: the Topcat interface where data (here the x-component of the proton velocity) from LatHyS (simulated 2D plane cut) and AMDA (observed Mars Express parameter coded in colour along the actual trajectory) are compared.

In the "Clients" window (upper right) the SAMP hub logo ¤ is visible together with Topcat, LatHyS and AMDA logos.

Science case 3: imagery of auroral emission at Saturn in conjunction with interplanetary shock observed by Cassini

Description

The user wants to compare ultraviolet observations of the auroral emissions of Saturn by the Hubble Space Telescope (HST) with local observations of the solar wind parameters during the cruise phase of Cassini in order to estimate the influence of external conditions on the global dynamics of the magnetosphere of Saturn. The main functionality developed to that purpose in AMDA consists in answering to a request for HST observations between start time and stop time for a given planet name (in our case here Saturn). To this end:

1. HST metadata are first obtained from the MAST server or APIS database using the EPN-TAP protocol in the latter case, 2. Filters are then applied to extract the relevant datasets corresponding to the user request, and an interface enables the user to obtain a first glance (quick-looks) at the relevant HST observations, 3. Communication is finally established with Aladin using SAMP in order to display selected HST observations (FITS files).

Databases queried: AMDA, MAST (http://archive.stsci.edu/hst/ ), APIS (http://lesia.obspm.fr/apis/ ) Tools involved: AMDA, Aladin

Procedure

In AMDA the user selects several Cassini datasets when the spacecraft was, for example, in the solar wind during the month of January 2004 (Figure 10). The "Get HST data" button is then clicked (top right corner of the plot window): a new window appears where the user can, on the left part, query two different HST databases -MAST or APISand define his/her request by choosing 1/ the target (Jupiter, Saturn or Uranus) and 2/ the time period for the query, eventually extending or shifting the original in-situ data interval; this last option may be useful in order to time shift in-situ observations in the solar wind such as to take into account the distance of the spacecraft to the targeted planet.

The user can then visualize, on the right part of the window, the results of his/her query: HST images are sorted by date and organized in day by day folders. The user can preview HST images by passing the mouse on the dates and select the images he/she wants to download or visualize by ticking the box associated to each date. A detailed scientific description (and metadata) of the HST image can be obtained directly from the MAST or APIS website by clicking on the date. After choosing the relevant HST images, the user can either download them in a zip file or send them to Aladin or any other tools connected via the SAMP protocol (e.g. the astronomical imaging application DS9 http://ds9.si.edu/ ) by clicking the corresponding buttons at the bottom.

The connection to Aladin may be done in two ways (see corresponding buttons on Figure 10). Either an Aladin Web tab is opened with the selected images directly uploaded (as FITS) in the interface; or the selected images are sent via SAMP (also as FITS) to a local Java instance of Aladin. This last case is illustrated on Figure 11. Any other SAMP tools accepting FITS could also receive the images. The extension of the CDPP/AMDA service to astronomical data (HST observations) and IVOA tools (Aladin) therefore opens a new window in our understanding of comparative auroral and magnetospheric physics of giant planets and will enhance the scientific return of previous, current and future (e.g., NASA/Juno and ESA/JUICE) planetary missions to these environments. This extended service will ultimately serve and benefit the European planetology community (and beyond) as well as promote and enlarge the application domain of astronomical VO tools. Also, this consists of one of the first experiment to combine tools at the interface of the plasma (SPASE) and astronomy (IVOA) communities.

CONCLUSION

This paper aimed at presenting current space physics applications which take benefit of the Simple Application Messaging Protocol initially developed by IVOA in the context of astronomy and astrophysics. By "joining the ¤ hub" these applications gain mutual advantages of their respective specialized functionalities: each development team can stay focused on its own domain of expertise knowing that it can relate on some other specialized tools (for particular plots, rendering, mathematical or physical operation pipelines, …). The French Plasma Physics Data Centre (CDPP) has been among the pioneering teams in this domain and a particular emphasis is placed on its tool suite. Other CDPP tools will soon integrate interoperability functionalities: the Propagation and the SpaceWeather Tools, devoted to the analysis of solar perturbations propagation in the interplanetary medium (imagery and analytical models are used in conjunction to in-situ measurements). They will naturally be connected to AMDA, the Helio and Helcats infrastructures (both European FP7 project) and the solar MEDOC database. In this paper a number of science cases exposing different facets of CDPP and companions tools have been presented which all illustrate the attractiveness of SAMP. Together with its facility of use in stand-alone or web-based applications, it constitutes a central building brick of the current Virtual Observatories.

Figure 1 :

 1 Figure 1: CDPP is an ensemble of databases and services in interoperable connection with external centres and tools.

Figure 2 :

 2 Figure 2: the AMDA interface as of end 2013. It is based on the concept of "desktop in the browser" using ExtJS framework. The data tree is exposed on the left part, activities (plotting, data mining, time table management, …) are proposed on the bottom right and corresponding interfaces appear in the central part of the browser.

Figure 3 :

 3 Figure 3: 3Dview scene illustrating about 20 days of the trajectories of the spacecraft Themis B orbiting the Moon along its passing through the solar Wind, the bow shock (green surface), the magnetopause (blue surface) and the distant magnetotail. The measured magnetic field vectors are shown as arrows with the length and colour coding the magnitude of the field in GSE coordinate system. Current sheet flapping is evidenced in the tail and perturbations are observed at frontier crossings. Themis in-situ data are obtained via web service access to the AMDA database (right window). Cluster1 and Geotail orbits are also shown.

Figure 4 :

 4 Figure4: on the AMDA desktop the user has displayed the data tree (left) with THEMIS B data directories, made a plot on the interval of interest (middle part of the image), clicked on the Get Data button to select data to be exported (right part of the image), clicked on the Download button and displayed the different client tools to which he/she can SAMP-send the data (here TREPS and 3DView are seen as possibilities, together with Topcat which was used to create the hub). The desktop bar in AMDA (lower right corner) shows successively a green sign meaning that AMDA is connected to the hub, the SAMP hub logo ¤, and Topcat, TREPS and 3DView logos.

Figure 5 :

 5 Figure 5: the last step of the transformation performed on TREPS (step 4: export result); the user has now the choice to send the transformed data to 3DView (or eventually back to AMDA).

Figure 6 :

 6 Figure 6: the 3DView scene displaying the Moon, the orbit of THEMIS B during the interval of interest, and the magnetic field in LSE coordinate system shown as arrows (both the colour and the length code the amplitude of the field).

Figure 10 :

 10 Figure 10: on the AMDA desktop the user has displayed the 1 minute Cassini magnetic field data in RTN coordinates with radial distance to Saturn in planetary radii (left window), clicked on the Get HST Data button to open the second window on the right, select the APIS database to be queried and define the target and time period to request. The results of the request are shown on the right part of the second window. The two "Send …" buttons enable connexion to other tools.

Figure 11 :

 11 Figure 11: on the AMDA desktop the user has displayed a selected HST observations from the APIS database using a local instance of Aladin connected to AMDA via the SAMP protocol.

ACKNOWLEDGMENTS

The EuroPlaNet-RI project was funded by the European Commission under the 7 th Framework Program, grant#228319 "Capacities Specific Programme". We particularly thank S.Erard (LESIA), P. Le Sidaner and C. Chauvin (Observatoire de Paris) for fruitful discussions on EPN-TAP. The IMPEx project is funded by the European Commission under the 7 th Framework Program, grant #262863 "Collaborative project / SP1-Cooperation".We thank the APIS service at LESIA/VO-Paris Data Centre (Observatoire de Paris, CNRS) for providing processed UV observations obtained by the ESA/NASA Hubble Space Telescope (namely, F. Henry and L. Lamy). We thank M. Taylor (Bristol, UK) for developing and supporting the Topcat software, and for his continued action within IVOA, notably on SAMP and VOTable specifications. We thank CDS (Strasbourg, France) for developing and supporting the Aladin and Aladin-web tools. The CDPP is supported by CNES, CNRS, Université Paul Sabatier, and Observatoire de Paris; these organizations partly financed the AMDA, 3DView and TREPS tools used in this paper. The CDPP thanks R. Hitier for early implementation of SAMP in AMDA.


Jérémy Cure

Eric Mattson

Kévin Cocq

Hala Assi

Stephanie Jensen

Kui Tan

Massimo Catalano

Shuai Yuan

Hao Wang

Liang Feng

Peng Zhang

Sunah Kwon

Jean-François Veyan

Yasiel Cabrera

Guoyu Zhang

Jing Li

Moon Kim

Hong-Cai Zhou

Yves J Chabal

Timo Thonhauser

High Stability of Ultra-small and Isolated Gold Nanoparticles in Metal-Organic Framework Materials

Keywords: Metal-organic framework (MOF), ultra-small gold nanoparticles, Au NPs, localization, nanoparticle stability, HER

Gold nanoparticles (NPs) exhibit optical, catalytic, and physical properties that are scientifically fascinating and essential for many applications. However, the challenge is to synthesize and disperse ultra-small and highly stable NPs. We demonstrate here that isolated and ultra-small (~1 nm) Au NPs can be synthesized by photo-reduction of HAuCl 4 inside thiol-functionalized MOFs, including MIL-101 (Cr) and a novel class of MOFs, MOF-808-SH. A combination of physical, imaging, spectroscopic measurements, and ab initio calculations confirms that the Au NPs are dispersed inside the bulk of the MOFs and demonstrate that the thiol group is critical to stabilize the Au NPs deeply inside individual pores which overcomes the major issue of external nucleation on the outer surfaces. The thiolfunctionalized MOFs thus fulfill a dual purpose: they foster the nucleation of the Au NP and also provide confinement and a framework that keeps the NPs separate. Moreover, the stability of NPs is tested by a series of wet chemical processes (transmetalation) and photocatalytic water reduction. Spectroscopic and TEM studies of the system after these chemical tests show that the Au NPs are remarkably stable (<1.5% diameter change/hour), even under harsh aqueous and irradiation environments, necessary for catalytic applications.

Introduction

Gold nanoparticles (NPs), in contrast to standard metal NPs, have generated much interest due to their unique physico-chemical properties such as conductivity, 1 localized surface plasmon resonance (LSPR), 2 stability towards oxidation, 3 biocompatibility, 4 and catalytic activity. 5 Since the pioneering work of Haruta, 5 several studies have demonstrated the capacity of gold NPs (<5 nm) to catalyze a large range of chemical reactions like the aerobic oxidation of alcohols or the oxidation of CO into CO 2 . 5 There are several methods to synthesize Au NPs, all requiring stabilizing ligands. 6,7 The central issue is to prepare ultra-small Au NPs (i.e. ~1nm) that can remain stable and dispersed under harsh environments. Small ligand-free Au NPs (~3 nm diameter) have recently been grafted on metal oxides (TiO 2 , ZnO, CeO 2 ) through decomposition of the HAuCl 4 salt by urea, 8,9 allowing for catalytic applications. 10,11 However, without ligands, these materials have to be stored under dry conditions to avoid aggregation and degradation.

A more robust solution is to use metal-organic frameworks (MOFs) as hosts to stabilize metal NPs (e.g. Ru, Pt, Pd, Cu, Ni, Au). [12][13][14][15][16][17][18] MOFs are porous crystalline three-dimensional materials that can be synthesized with tailored pore sizes [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] and can even support internal functionalization, 25,[34][35][36][37][38][39] useful for applications such as pollutant sequestration, catalysis, photocatalysis, sensors, gas adsorption, fuel production, and biomedicine. [40][41][42][43][44][45][46] A few strategies have been proposed, such as impregnation of Au salt/alcohol mixtures with a gas-flow reduction, 36 direct incorporation of Au NPs during MOF synthesis, 47,48 or the growth of a MOF shell surrounding pre-existing Au NPs, [49][50][51] all of which suffer serious drawbacks such as contamination resulting from incomplete removal of metal precursor ligands or reductive agents. 36,49 For instance, there have been remarkably few studies of photocatalysis for the hydrogen evolution reaction (HER), 52 despite the fact that Au exhibits plasmonic properties in the visible range compared to Pt, because it is extremely challenging to stabilize Au NPs smaller than 2-3nm compared to Pt NPs. 53 Gold NPs are prone to block the channels by aggregation on the external surfaces of MOFs or in very close cavities, which drastically reduces the diffusion of the reactive species within MOFs. 12,[54][55][56] To overcome these drawbacks, functional groups have been placed inside the host pores to nucleate and stabilize ultra-small Au NPs [57][58][59][60][61][62] and a method was developed to precisely determine whether the nucleation is internal or on the surface of the guests MOF NPs. 50 A milestone was recently achieved by Yan et al. who report the first ultra-small Au Nanoparticles embedded in 2D mixed-ligand MOF nanosheets although in presence of external ligands. 57 We present here an alternative method to synthesize Au NPs inside water-stable zirconium-based MOFs, 63 based on the photo-reduction of HAuCl 4 in an alcoholic MOF suspension without the need for additional external reducing or stabilizing agents. 56,64,65 Zirconium-based MOFs are selected because the Zr-O bonds within their multinuclear secondary building units (SBUs, usually Zr 6 clusters) are particularly stable in aqueous environments. 63 Finally, we use photocatalytic HER to precisely measure the stability of Au NPs under harsh aqueous and irradiation environments, without focusing on the yield of such a reaction. We find that the Au NPs are remarkably stable (<1.5% diameter change/hour), opening the door for exploration of catalytic processes.

Results and Discussion

The photo-reduction method is based on the decomposition of HAuCl 4 under UV-visible light irradiation to nucleate and grow guest Au NPs in an ethanolic suspension of the host MOF micro-crystals. As illustrated in Figure 1, the solvent plays a role as the reduction of Au 3+ to Au 0 occurs through oxidation of ethanol. With MOF as a stabilizing template, UVvisible irradiation can induce reduction without additional or stabilizing ligands that typically degrade device performance. [67][68][69] As explained in Section C-1 of the SI, we focus on MOF-808 because we were able to develop a post-synthetic route to graft pmba functional groups on the SBUs of MOF-808 to obtain MOF-808-SH, i.e. thiol groups that are expected to efficiently stabilize Au NPs (SI Section C-2.2 and C-2.5). 70 In the following, we combine Brunauer-Emmett-Teller (BET) measurements, high-resolution transmission electronic microscopy (HRTEM), UV-vis spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Raman and IR spectroscopy to determine pore sizes, Au NP diameter and dispersion, and the nature of the interactions of the Au NPs with the MOF structure. The internal surface areas of pristine and Au-loaded MOF materials and the original pore sizes (mesopore distribution, Figure S1 and S2 in SI Section C-1) can be extracted from the adsorption-desorption isotherms following a BET analysis, see Table 1. After Au incorporation, the pore volumes decrease, which is expected after filling part of the pores.

The loss of surface area for MOF-808 and MOF-808-SH (~30%) suggests that the Au NPs are synthesized inside the MOF crystallites. By themselves, however, the BET results are not sufficient to prove that the majority of Au NPs are inside the MOF because Au agglomerated outside the crystallites would also decrease the overall active surface area. We therefore turn to HRTEM to image the NPs.

HRTEM images support the conclusion that NPs are inside the MOF and are most useful to estimate the size of these NPs (SI Section C-2). Quantitative image analysis yields 7.9 ± 3.6 nm and 1.0 ± 0.5 nm for the mean diameters in MOF-808 and MOF-808-SH, respectively (Figure 2b,c,f,g), with much more size dispersion in MOF-808. In the Au/MOF-808-SH material, the Au NPs mean size (1.0 ± 0.5 nm) is matching perfectly with the pore size of the host MOF-808-SH (1.07 ± 0.05 nm), which is expected for a preferred inclusion of the particles inside the cavities throughout the MOF volume.

With independent confirmation by XPS that Au is indeed completely reduced (i.e., Au 0 ; see Figures S10 andS12, Tables S3 andS5 A sensitive method to determine whether the Au NP diameter is greater than 1-2 nm is the detection of LSPR (SI Section C-5). 53,71,72 In MOF-808, there are both a ligand-to-metal charge transfer transition (LMCT) at < 300 nm and a LSPR band observed at ~ 537 nm (Figure 2d), consistent with diameters > 2nm; 73 in contrast, there is no detectable LSPR in Au/MOF-808-SH (Figure 2h), which indicates that the Au NP diameters are less than 2 nm in Au/MOF-808-SH only. 53,74 Importantly, this macroscopic optical measure confirms that the large majority of the Au NPs in Au/MOF-808-SH are smaller than 2 nm over the whole sample, not just in a small volume explored with HRTEM.

As reported by Esken et al., NPs are sometimes distributed throughout the volume of the MOF crystal but still exhibiting a broad size distribution with an average particle size exceeding the dimensions of the pores. 56 Such a situation might result in a substantial local damage of the MOF host, opening the door for an aggregation phenomenon of metal clusters inside the pore channels when the NPs sizes are significantly larger than the pore widths. 50,56 This is clearly not the case for the Au/MOF-808-SH material, where the pore widths and NPs mean size are matching.

In addition to our TEM and BET analyses, which already suggest that the Au NPs are located inside the MOF, we also use Low Energy Ion scattering (LEIS), 75 which is a more precise technique to validate this conclusion. 50 Since low energy ions can only penetrate the first few layers of a material (~1nm), LEIS is a sensitive probe of the near surface region of MOF crystals. It is used here to monitor the time evolution of MOFs after synthesis of Au NPs and the composition of the material after Ar ion sputtering (i.e. depth profiling). LEIS can therefore unambiguously discriminate between the formation of Au NPs at the MOFs surface and within its interior, as detailed in Section C-6 of the SI. The LEIS spectra of Au/MOF-808 and Au/MOF-808-SH (Figure 3) show a clear signature of Zr, but no surface peaks due to Au are detected in the as-introduced samples. After sputtering with a dose of 4×10 14 cm -2 of 8 keV Ar + ions, however, a clear Au peak is observed. These results prove that Au is localized below the surface of the as-prepared MOF crystals, as Au particles present at the surface would produce a clear surface peak in the spectra. This is further supported by the observation that, while there is no Au after synthesis, Au appears on the surface after one month of aging (Figure S24, SI Section C-6). This effect, attributed to the migration of Au NPs to the surface over time at ambient temperature, clearly demonstrates that Au is readily detected when it is at the surface and is therefore clearly absent upon synthesis. sputtering (4×10 14 cm -2 Ar + ), using 3 keV He + ions. Spectra corresponding to the sputtered sample are scaled for ease of comparison to the spectra of the as-prepared material.

A striking observation is that MOF-808 is unable to confine gold within its pore diameter without thiol functionalization, i.e. the mean Au NPs size exceeds the measured or calculated pore sizes given in Table 1 for all MOFs that are not thiol functionalized. On the other hand, the size of Au NPs (1.0 ± 0.5 nm) in Au/MOF-808-SH corresponds exactly to the size of the pores (1.07 ± 0.05 nm) and more importantly, the size dispersion is very narrow.

We therefore hypothesize that the thiol groups stabilize the Au NPs size by preventing Au migration during synthesis and upon ultrasonic treatment. This is reminiscent of Kimura's earlier observation that an increase in the S/Au ratio leads to a decrease of the mean diameters of Au NPs. 74 In the Au/MOF-808-SH material, each pore contains 24 -SH groups (Figure 2e)

and a 1 nm diameter Au NP would contain 31 Au atoms with > 95% of the Au atoms as surface atoms; 80% of the Au surface atoms are directly in interaction with -SH groups (Figure S5, SI Section C-2.2) and the rest with the walls. 76 Therefore, while Au-MOF interaction could provide additional stabilization besides the main Au-S interaction, the fact that the functional -SH groups are oriented toward the center of the windows (Figure S26, SI Section C-7) helps direct the initial Au nucleation at the center of the sulfur hexagons.

Furthermore, the presence of S in MOF-808-SH during the photo-reduction may also be a factor to stop the growth of the nanoparticles.

The above hypothesis was first explored by ab initio calculations, as detailed in Section C-7 of the SI. The results show that, during the nucleation, the first gold atom prefers to bind to the sulfur functional group (2.32 eV binding energy) rather than the carboxylate metal cluster node (1.93 eV binding energy) located in the periphery of the pore. The binding of the gold atoms to the sulfur groups likely helps the nucleation process as it initially localizes the first few (24) gold atoms to the ends of the functional groups. This finding is supported by Raman spectra (Figure 5b) of Au/MOF-808-SH where the Au-S stretch vibration (calculated at 254 cm -1 and measured at ~234 cm -1 ) is in close vicinity to the Au-O mode (calculated at 296 cm -1

and measured at ~259 cm -1 ) for the pmba/Au film system, 77 which is consistent with a larger width for the mode at ~ 259 cm -1 (Table S9). Interestingly, as gold agglomerates into a small cluster, the binding energy of such a cluster to carboxylate linkers and metal nodes increases (2.38 eV binding energy for 16 atoms). Although the binding energy of such a cluster is larger than on an isolated SH group (1.68 eV binding energy), there are 24 thiol groups in the pore vs 12 oxygen atoms in SBUs, so that the total S-Au interaction still dominates over that of the carboxylate groups. Nonetheless, for MOF-808-SH, stabilization of the Au NP to the pore size is enhanced by the Au interaction with the carboxylate groups. Therefore, as the cluster grows, the combined interaction with both thiols and SBUs leads to stabilization within the pore volume, i.e. the Au NP cannot expand beyond the MOF pore boundaries. This is not the case for un-functionalized MOF-808, in which the Au is not initially confined at the center of the pore and therefore can migrate beyond the MOF pore boundaries before a larger cluster is formed. Note that the binding energy of the cluster near the tip of the functional group is significantly less than that of an individual atom, making it very unlikely that existing clusters interfere with the nucleation of new ones. The calculations further suggest that the -OH and carboxylic chemical groups should be affected after Au NP nucleation in specific ways (SI Section C-7). We have therefore performed IR absorption and Raman scattering 78 experiments to characterize the interaction between Au NP surfaces and ligands btc and pmba after synthesis in both MOF-808 (Figure 4) and MOF-808-SH (Figure 5). As predicted, the narrow bands at 3675 cm -1 (Figure 4a) and 3671 cm -1 (Figure 5a) for MOF-808 and MOF-808-SH, respectively, assigned to the stretch of the four bridge O-H groups in the SBU nodes (Figure 2a, e), 79 disappear after synthesis, consistent with direct bonding of Au to O (SI Section C-8.1).

This finding is consistent with Raman spectra (Figure 4b, Figure 5b) that show the appearance of sharp modes at 262 and 259 cm -1 in the two MOFs respectively, assigned to the Au-O stretch; [80][81][82] this assignment is unambiguous since the precursor HAuCl 4 does not have vibrational modes in the 300-400 cm -1 region (Figure S29). 83 Our ab initio calculations find the Au-O mode at 296 cm -1 and show that both the carboxylate and bridging oxygen atoms are also involved (SI Section C-7). Experimentally, resolvable shifts observed for the carboxylate modes (see IR spectra in SI Section C-8.2) are also consistent with recent findings for Au stabilization of citrate, benzoic acid, or phthalic acid ligands. [84][85][86] Because optical spectroscopy probes the bulk system in contrast to HRTEM or surface spectroscopies (XPS and LEIS), these findings confirm that the Au NPs are located deep in the bulk of the MOF crystals.

At this point, it is important to determine the role of thiol groups and MOF pores in nucleating and confining the Au NPs. To that end, we explored a number of MOFs of various pore size and without thiol functionalization: MIL-101 (Cr) with larger pores, and UiO-66 and HKUST-1 with smaller pores than the MOF-808 series (Table 1, SI Section C-1). For all these MOFs, BET measurements show a decrease of the surface areas and pore volumes after the loading of gold (Table 1, SI Section C-1), and LEIS, IR and Raman spectra (SI Section C-6 and 8) confirm that gold is located inside the MOF crystals. However, in all these unfunctionalized MOFs, the average Au NPs size is much larger than the pore sizes (Figure S4), as confirmed by the appearance of LSPR bands in the UV-vis spectra (Figure S23). There is no mechanism for stabilization, resulting in continued growth of Au NPs beyond the confines of their pores. The inability of MOF-808 and other MOFs without thiol functional group to confine the Au NPs (SI Section C-2.3) indicates that, while the synthesis is clearly initiated within pores (acting as nano-reactors), the walls of the pore without -SH functionalization are not necessarily sufficient to prevent Au aggregation (Figure S6, SI Section C-2.4).

To determine whether other functional groups could also foster nucleation and confinement, we developed a method to attach -NH 2 groups in MOF-808 using paraaminobenzoic acid. The resulting average diameter of Au NPs in these MOF-808-NH 2 is 7.1 ± 3.6 nm (SI), much larger than the pore size (1.4 ± 0.1 nm) (Table 1, Figure S4). Therefore, in this structure which has the same SBU nodes [Zr 6 O 4 (OH) 4 ] and same number of functional groups as in MOF-808-SH, there is no confinement of Au NPs, most likely due to the fact that -NH 2 groups are less soft than -SH groups in the hard and soft acids and bases theory (HSAB). 70 This finding highlights the particular role of thiol groups in the MOF-808 series.

The generic role of thiol group is further explored by functionalizing another MOF with a larger pore size: MIL-101 (Cr), with thiol groups as described in the literature (SI Section C-9). 87 The pores size of MIL-101 (Cr) -SH is 1.4 ± 0.1 nm, i.e. larger than MOF-808-SH (1.07 ± 0.05 nm, Table 1). However, in MIL-101 (Cr) -SH, the diameter of Au NPs (1.0 ± 0.2 nm) is very similar to that in MOF-808-SH (1.0 ± 0.5 nm, Figure 2f, g and Figure S4), as confirmed by the absence of a LSPR band in the UV-vis spectrum (Figure S23), despite the difference in pore size. LEIS, IR and Raman measurements confirm that the NPs are inside the pores of MIL-101 (Cr) -SH (Figure S25, Figure S32-33, see SI Section C-9). Altogether, these findings confirm that the interaction with thiol groups is more important to stabilize Au NPs than the interaction with SBUs or walls of the MOF, although there is clearly a chemical interaction between Au and O (SI Section C-9) for MIL-101 (Cr) -SH as well.

Since the formation and stabilization of Au NPs appears to be very sensitive to the chemical environment (e.g. presence of an SH group), the question arises as to the stability of these NPs once formed. We address this question by testing the system under two harsh treatments, selected for their ultimate value in photocatalysis. The first is the incorporation of Ti into these Zr-based MOFs, since the recent discovery of an enhanced photocatalytic activity for Ti based MOFs was demonstrated. 52,88 The second is to subject the modified MOFs to an actual photocatalytic process in order to measure the stability of ultra-small Au NPs.

Inspired by a new post-synthetic route with a TiCl 4 precursor, 89 and with the knowledge that thorough characterization is critical to ensure that metal replacement takes place, 90 we developed a procedure to incorporate Ti atoms by transmetalation into both MOF-808 and MOF-808-SH (described in SI Section C-10). Importantly, we used IR spectroscopy and energy dispersive X-ray spectrometry (EDS) to prove that metal replacement takes place without residual titanium oxide nanoparticles or coatings onto MOFs, and to quantify that 18% of the SBU nodes Zr atoms are replaced by Ti atoms covalently bound with the btc carboxylate groups without degrading the MOF structure.

We then subjected 10 mg of MOF-808-SH, Au/MOF-808-SH, MOF-808-SH(Ti), and Au/MOF-808-SH(Ti) to a standard photocatalytic process (see Methods section) 52 and measured hydrogen production (Figure 6a). As previously described by Matsuoka et al., 52 under UV-visible irradiation electron-hole pairs are formed in the MOF photocatalyst; the photo-electrons reduce water molecules whereas the holes oxidize TEOA. 52 Since the LSPR properties of Au NPs co-catalysts can lead to an enhancement of the photocatalytic activity of the water reduction, 91 we monitored the hydrogen production as a function of time even though the initial size of the Au NPs is too small to support LSPR (Figure 6d) and consequently does not exhibit an efficient photo-reduction of water. Indeed, H 2 production is initially negligible for all samples. However, there is a notable increase in hydrogen production rate only for the Au/MOF-808-SH(Ti) material after a ~48 h incubation time (Figure 6e, see SI Section C-12). 85 We demonstrate that this enhancement results from the appearance of LSPR properties of our gold nanoparticles due to their increase in size during the observed incubation period of 48h (Figure 6e). Indeed, HRTEM observations show an increase of the Au NPs mean size and size dispersion from 1.0 ± 0.5 nm to 2.0 ± 0.9 nm after the completed photocatalysis experiment (i.e. after 66h UV-visible irradiation, Figure 6b,c).

A broad LSPR absorption band appears after 24h around 500 nm in the UV-vis spectra (Figure 6f), expected when the diameter reaches 1.94 nm; 74 this resonance is believed to be responsible for the photocatalytic enhancement due to the synergy between the LSPR properties of gold co-catalyst and the presence of titanium in the MOF nodes. 52,89,92,93 The actual rate of hydrogen production in Figure 6e is small compared to other systems 92,[94][95][96] (SI Section C-11) but the main point of our experiments is not to find a better system for hydrogen production but rather to show Au NP stability under harsh conditions.

It is clear that the hydrogen evolution reaction (HER) is initially very weak, until the size of the NPs becomes large enough to activate the hybrid photocatalyst during the observed incubation period. However, the focus of using HER in our study is not to yield high catalytic performance known e.g. for Pt NP, 52 but rather to use it as a probe: The sharp dependence of the reaction on the NP diameter makes it possible to use this method to finely tune the NP size and explore the photocatalytic process, which opens the door for studying other plasmonic NPs such as Pt, Ag, and Cu. From these measurements of hydrogen production (SI, Section C-11), the rate of NP diameter increase can be estimated to be 0.015 nm.h -1 . This is an upper limit as the growth is highly non-linear. From IR and EDS measurements after photocatalysis (Table S10 and SI Section C-12), we determine that the slow growth of the Au NPs is probably due to removal of part of the pmba functional groups since a ~25% loss of sulfur is measured by EDS (Table S10)-this removal opens up more space in the pores and is thus favorable to NP aggregation, leading to the observe increase of the Au NPs size. Moreover, the EDS spectra performed before and after the photocatalysis (Table S10) clearly evidence the same Au loading: 0.14% and 0.15%, respectively, which strongly support the conservation of larger Au NPs inside the pores after photocatalysis as more detailed in Section C-12 of the supplementary information.

Finally, a depth profile of the composition is performed by XPS for Au/MOF-808-SH (Ti) before and after photocatalysis (SI Section C-13). The XPS spectra show that gold is not only detected on the top surface of the MOF crystals but also deeply inside the pores of the MOF-808-SH (Ti) host since a larger quantity of gold is exhibited after sputtering whether before or after photocatalysis.

Overall, these data confirm that the Au NPs are stable under photocatalytic conditions, exhibiting only a small diameter increase which actually fosters photocatalytic activity.

Conclusion

In summary, the presented work not only demonstrates that ~1 nm diameter Au NPs can be synthesized within individual pores of MOF-808-SH and MIL-101 (Cr) -SH by a photoreduction without additional external stabilizing or reducing agents, but also provides a mechanistic picture of the growth mechanism and in particular the reason why confinement is not achieved in other MOFs, i.e. how the presence of thiol groups located in the pore leads to confinement. The difficult question of whether Au NPs are located inside or on the surface of the MOF, often not fully addressed in the literature, is unambiguously answered using spectroscopic tools (LEIS, IR, Raman and UV-vis) in addition to imaging (TEM) and physical (BET) measurements. Detailed information on the chemical interaction of Au with the functionalized MOF is further provided with input from ab initio calculations. This combination is particularly powerful as other techniques such as tomography, while very useful, can only be applied to a local (nm size) region of the material and not its bulk.

Furthermore, we demonstrate that the new Zr-based MOF-808-SH samples can be doped by titanium transmetalation before Au NP synthesis, allowing us to precisely measure the high stability of our Au NP by monitoring hydrogen production during the solar photocatalytic reduction of water and showcasing a novel way to accurately determine NP stability that may also be applicable to other hybrid materials. The slow growth of the Au NP diameter, i.e. 0.015 nm.h -1 , in an aqueous medium under solar irradiation indicates that these Au NPs are stable under particularly harsh environmental conditions.

Experimental and Theoretical Section:

Sample preparation: The synthesis of MOF-808, MOF-808-SH, MOF-808-NH 2 , MIL-101 (Cr), MIL-101 (Cr) -SH and UiO-66 samples and titanium doping on MOF-808-SH is described in Section A of Supporting Information, as they followed standard procedures reported in numerous publications. 66 The Au@MOFs were synthesized by directly mixing

Ab initio calculations:

The calculations of the binding energy and frequency were performed in VASP at the density functional theory (DFT) level, using the vdW-DF exchange-correlation potential to capture van der Waals interactions (SI Section C-7).

Photocatalysis: Each sample was immersed in an aqueous solution (10 mL) of triethanolamine (TEOA, 1.10 -2 mol.L -1 ) as sacrificial reagent in a quartz reactor and irradiated by a solar simulator (Xenon lamp, 300 Watts) for 66 h. The reactor was connected to a gas chromatography (GC) system to measure (every 6 h) the hydrogen production during the photocatalytic reduction of water (Figure 6a).

  After an extensive search (SI Section C), we focus on [Zr 6 O 4 (OH) 4 (H 2 O(OH)) 6 ](btc) 6 [MOF-808] with the SH-functionalized variation 66 [Zr 6 O 4 (OH) 4 ](btc) 6 (pmba) 6 [MOF-808-SH, btc=1,3,5-benzenetricarboxylate, pmba=paramercaptobenzoicacid]. Combining physical (BET), imaging (TEM) and spectroscopic (IR, Raman, UV-vis, XPS, LEIS) techniques with ab initio simulations, we demonstrate that dispersed and ultra-small Au NPs (~1nm diameter) can be synthesized and stabilized deep inside individual pores of MOF-808-SH. Thanks to these techniques, the fundamental chemical mechanisms leading to such a controlled confinement in MOFs functionalized with -SH groups is addressed. A thorough investigation is performed in order to identify the chemical groups (pores walls) that interact with the nanoparticles inside the MOF pores.

Figure 1 .

 1 Figure 1. MOFs as nano-reactors for the growth of Au NPs via photo-reduction.

  , SI Section C-3) and therefore that the features detected by HRTEM are indeed Au NPs (Figure2b, f), fast Fourier transform analysis of the pictures indicates that the Au NPs inside the MOF are crystalline. Importantly, there is no evidence for Au on the outer surfaces of the MOF crystallites, supporting the original conclusion based on our BET analysis. Note that the Raman and IR spectra of Au/MOFs are in very good agreement with the ones of the corresponding MOF hosts, indicating that the structural integrity of all studied MOFs is well preserved (SI Section C-8). This is further supported by the XRD data performed on MOFs and Au/MOF materials (SI Section C-4), which only show a partial modification of the crystallinity for HKUST-1 and MIL-101-Cr samples.

Figure 2 .

 2 Figure 2. Characterization of MOF-808 and MOF-808-SH: (a/e) Structures of the host MOF;

Figure 3 .

 3 Figure 3. LEIS spectra of Au/MOF-808 and Au/MOF-808-SH before (blue) and after (black)

Figure 4 .

 4 Figure 4. (a) Infrared spectra of the 4000-3000 cm -1 region; (b) Raman spectra; (c) schematic

Figure 5 .

 5 Figure 5. (a) Infrared spectra of the 4000-3000 cm -1 region; (b) Raman spectra; (c) schematic

Figure 6 .

 6 Figure 6. (a) Scheme of the reactor; (b) HRTEM observations and (c) Au NP size dispersion

  and stirring MOF (20 mg) and the gold salt precursor HAuCl 4 .3H 2 O (10 mg, 2.5.10 -5 mol) in ethanolic solution (2 mL) under UV-visible irradiation (UV Rayonet system:  = 253.7, 300, 350 and 575 nm, 224 Watts, 1 h 30 min). The resulting solution was centrifuged at 6000 rpm during 3 min and the colorless supernatant was removed. The resulting powder was dried under the vacuum of a Schlenk line (1 h).Characterization:The characterization methods include Brunauer-Emmett-Teller (BET), High-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), Ultraviolet-visible spectroscopy (UV-vis), Low energy ion scattering (LEIS) and infrared and Raman spectroscopies, as detailed in Section B of Supporting Information.

Table 1 .

 1 Experimental surface areas, pore widths and pore volumes for the MOF and Au/MOFs materials, structural windows and pore sizes, and mean Au NPs size for all studied MOFs.

Acknowledgement

All the synthesis and spectroscopic characterization and modeling were supported by the Department of Energy, Basic Energy Sciences, division of Materials Sciences and Engineering (DOE Grant No. DE-FG02-08ER46491). The photocatalytic experiments were supported by the Université de Toulouse (IDEX MUSE) and the CNRS.

Conflicts of interest

The authors declare no competting financial interest.

Author information

Corresponding author

Timo Thonhauser: thonhauser@wfu.edu Yves J. Chabal: chabal@utdallas.edu

Author Contributions