A Hertz

M Drobek

J-C Ruiz

F Charton

S Sarrade

C Guizard

A Julbe

A detailed insight into the preparation of nanocrystalline TiO2 powders in supercritical carbon dioxide

Keywords: Supercritical CO2, sol-gel route, hydrolysis/condensation reactions, TiO2 nanophased particles, photocatalysis, water and wastewater treatment

This work reports detailed investigations for the preparation of nanostructured titania powders by a solvent-free sol-gel derived process, operated in supercritical CO2 (SC-CO2) at high pressures (10 -30 MPa) and large range of temperatures (373 K -823 K).

Depending on the processing temperature, the reaction between Ti(OiPr)4 and water performed in a single supercritical phase led to the formation of either amorphous (Ti(OH)4 -Titanium hydroxide) or crystalline (TiO2 -Titania or Titanium dioxide) nanostructured particles. Crystalline (anatase) mesoporous powders with high specific surface area were obtained directly in CO2 solvent under supercritical conditions at temperatures as low as 523 K. The effect of hydrodynamic key process parameters such as stirring and water injection rate on both powder morphology and aggregation degree was also investigated in details. The optimized TiO2 anatase powders exhibited attractive photocatalytic activity, with high potential for the degradation of water pollutants.

Introduction

Titania (TiO2) has been historically used in a number of industrial areas including paints, cosmetics and health care products [START_REF] Zhanga | Mesoporous titania: from synthesis to application[END_REF]; it is also a key material in photocatalytic applications [START_REF] Herrmann | Photocatalysis fundamentals revisited to avoid several misconceptions[END_REF][START_REF] Zhang | TiO2 photocatalysis and related surface phenomena[END_REF]. Indeed, TiO2 has been found to be an efficient material in environmental clean-up processes to decompose a wide variety of contaminants in both water and air [START_REF] Pelaez | A review on the visible light active titanium dioxide photocatalysts for environmental applications[END_REF][START_REF] Bernardini | Photocatalytic degradation of organic molecules in water: photoactivity and reaction paths in relation to TiO2 particles features[END_REF]. In addition TiO2 also deserves special attention for photoelectrochemical cell applications, with the recent discovery and developments of solar cells based on dye-sensitized TiO2 photo-electrodes [START_REF] Hu | Incorporating carbon nanotube in a low-temperature fabrication process for dye-sensitized TiO2 solar cells[END_REF]. Most of these applications require TiO2 nanopowders with specific characteristics, namely high specific surface area, large accessible (meso)porosity and the anatase crystalline form. TiO2 nanopowders can be prepared by various methods, including chemical vapor deposition (CVD) [START_REF] Ding | Novel Silica Gel Supported TiO2 Photocatalyst Synthesized by CVD Method[END_REF], thermal hydrolysis of titanium tetrachloride [START_REF] Lee | Synthesis of brookite TiO2 nanoparticles by ambient condition sol process[END_REF][START_REF] Fang | Preparation of titania particles by thermal hydrolysis of TiCl4 in n-propanol solution[END_REF], thermal decomposition/pyrolysis of metal-organic precursors [START_REF] Terada | Synthesis and characterization of TiO2 powders by electrospray pyrolysis method[END_REF][START_REF] Payakgul | Effects of reaction medium on the synthesis of TiO2 nanocrystals by thermal decomposition of titanium (IV) n-butoxide[END_REF][START_REF] Qourzal | Synthesis of TiO2 via hydrolysis of titanium tetraisopropoxide and its photocatalytic activity on a suspended mixture with activated carbon in the degradation of 2-naphthol[END_REF][13], and more frequently by sol-gel techniques [START_REF] Ding | Synthesis and microstructure control of nanocrystalline titania powders via a sol-gel process[END_REF][START_REF] Liu | Synthesis and characterization of titania prepared by using a photoassisted sol-gel method[END_REF][START_REF] Tanaka | Synthesis of highlymonodisperse spherical titania particles with diameters in the submicron range[END_REF][START_REF] Marugán | Synthesis, characterization and activity of photocatalytic sol-gel TiO2 powders[END_REF][START_REF] Kaper | Toward a Low-Temperature Sol-Gel Synthesis of TiO2 Using Mixtures of Surfactants and Ionic Liquids[END_REF][START_REF] Cheng | Synthesis of titania microspheres with hierarchical structures and high photocatalytic activity by using nonanoic acid as the structure-directing agent[END_REF][START_REF] Dittert | Phase content controlled TiO2 nanoparticles using the MicroJetReactor technology[END_REF][START_REF] Farbod | Synthesis of TiO2 nanoparticles by a combined sol-gel ball milling method and investigation of nanoparticle size effect on their photocatalytic activities[END_REF][START_REF] Loryuenyong | Sol-gel derived mesoporous titania nanoparticles: effects of calcination temperature and alcoholic solvent on the photocatalytic behavior[END_REF][START_REF] Duvarci | Preparation and characterization of nanocrystalline titania powders by sonochemical synthesis[END_REF]. The latter approach, which can be operated under more or less sophisticated conditions (e.g. ultrasounds, microwaves, pressure…), is a powerful method for the preparation of both titania powders and films. However, classical sol-gel methods often require the utilization of costly and polluting organic solvents and/or chemical additives which have to be eliminated by drying and firing treatments of the final material, thus increasing both the environmental impact and costs of the synthesis process. In addition, the formation of powders with high specific surface area is not straightforward as far as it is often tricky to avoid strong agglomeration of individual particles. Trying to overcome such bottlenecks, the thermal decomposition of titanium alkoxides in supercritical alcohols was reported at the beginning of the 90's as an efficient method to prevent agglomeration: spherical TiO2 submicron particles were produced with a hierarchical structure made of individual nanosized crystallites [START_REF] Chhor | Syntheses of submicron TiO2 powders in vapor, liquid and supercritical phases, a comparative study[END_REF][START_REF] Courtecuisse | Kinetics of the Titanium Isopropoxide Decomposition in Supercritical Isopropyl Alcohol[END_REF].

Unfortunately, this route suffers from many safety problems since supercritical alcohols are highly flammable and stringent security systems are therefore required in the reactor area.

For the sake of replacing organic solvents, other protocols were developed applying sub-and super-critical CO2 as a reaction media for operating the hydrolysis and condensation of titanium tetra-isopropoxide (Ti(O i Pr)4). Indeed, supercritical CO2 (SC-CO2) is an attractive, non-toxic, environmentally benign and low cost green solvent with readily accessible critical point (Tc = 304.3 K, Pc = 7.38 MPa). It has been used as a solvent or anti-solvent in a large variety of materials processing and synthesis protocols [START_REF] Zhang | Applications of supercritical carbon dioxide in materials processing and synthesis[END_REF] and it is particularly well adapted to the preparation of nanostructured materials [START_REF] Sanli | Synthesis of nanostructured materials using supercritical CO2. Part I[END_REF][START_REF] Bozbag | Synthesis of nanostructured materials using supercritical CO2. Part II. Chemical transformations[END_REF][START_REF] Cansell | Design of functional nanostructured materials using supercritical fluids[END_REF][START_REF] Sui | Synthesis of metal oxide nanostructures by direct sol-gel chemistry in supercritical Fluids[END_REF]. Titanium alkoxides are reasonably soluble in SC-CO2, and Ti(O i Pr)4 exhibits the highest solubility among the commercially available compounds. This alkoxide thus enables easy formation of uniform Ti(OH)4 particles when Ti(O i Pr)4 is reacted with water in the SC-CO2 reactor [START_REF] Tadros | Synthesis of titanium dioxide particles in supercritical CO2[END_REF]. Thereafter several other authors published on the influence of the reaction pressure, temperature and molar ratio water/Ti(O i Pr)4 on the characteristics of titania powders obtained under SC-CO2 conditions [START_REF] Reverchon | Synthesis of titanium hydroxide nanoparticles in supercritical carbon dioxide on the pilot scale[END_REF][START_REF] Stallings | Synthesis of nanostructured titania powders via hydrolysis of titanium isopropoxide in supercritical carbon dioxide[END_REF][START_REF] Hong | Synthesis of titanium dioxides in water-in-carbon dioxide microemulsion and their photocatalytic activity[END_REF][START_REF] Alonso | Synthesis of titanium oxide particles in supercritical CO2: effect of operational variables in the characteristics of the final product[END_REF][START_REF] Wakayama | Supercritical CO2 as a solvent for synthesis of nanoporous materials[END_REF]. A critical review summarizing the impact of these key-parameters and the basic principles of supercritical-based techniques applied to the production of nanostructured materials was published elsewhere [START_REF] Reverchon | Nanomaterials and supercritical fluids[END_REF]. In spite of large improvement in the synthesis protocols, several important process parameters are often neglected in the literature, and this might explain difficult up-scaling of the synthesis processes.

To shed further light into the key-parameters which have to be controlled for preparing TiO2 anatase nanocrystalline powders, with the desire structure and microstructure in SC-CO2, several strategies have been developed and compared by the authors. It must be noted that such synthesis approaches are nowadays of an utmost actuality as application of the supercritical fluids is attracting a constantly increasing interest of the scientific community. It is indeed recognized that this green and sustainable technology could become an alternative to many conventional synthesis techniques in organic solvents.

Hence, the present work is a detailed description of the results obtained in the framework of internal CEA research projects exploring the potential of supercritical fluids for preparing ceramic nanomaterials at large scale. Special emphasis was first devoted to controlling the effect of conventional processing parameters on powder characteristics, notably the role of the reaction temperature on the produced powders crystallinity degree, crystal phase, crystallite size, particle microstructure and specific surface area. In addition, other key process parameters such as SC-CO2 pressure, stirring and water injection rate in the reactor were also investigated. Finally, the photocatalytic activity of the powders has been assessed, as a typical functional property of TiO2 materials [START_REF] Zhou | Ordered mesoporous black TiO2 as highly efficient hydrogen evolution photocatalyst[END_REF][START_REF] Zhou | Well-ordered large-pore mesoporous anatase TiO2 with remarkably high thermal stability and improved crystallinity: Preparation, characterization, and photocatalytic performance[END_REF][START_REF] Herrmann | Heterogeneous photocatalysis: state of the art and present applications[END_REF][START_REF] Guillard | Photocatalytic degradation mechanism for heterocyclic derivatives of triazolidine and triazole[END_REF][START_REF] Carp | Photoinduced reactivity of titanium dioxide[END_REF][START_REF] Bickley | A structural investigation of titanium dioxide photocatalysts[END_REF][START_REF] Okamoto | Heterogeneous photocatalytic decomposition of phenol over TiO2 powder[END_REF][START_REF] Munuera | Photo-adsorption and photo-desorption of oxygen on highly hydroxylated TiO2 surfaces. Part 1.-Role of hydroxyl groups in photoadsorption[END_REF]. The performance of the nanocrystalline titania powders prepared by the "green" SC-CO2 process were compared with those of more conventional commercial TiO2 powders.

Experimental section

Powders were prepared by reacting titanium tetraisopropoxide (Ti(O i Pr)4) and water in supercritical CO2, according to the conventional hydrolysis and condensation reactions pathway of sol-gel route (Eq. 1, 2 and 2'):

(Eq. 1) Ti(O i Pr)4 + n H2O → Ti(O i Pr)4-n(OH)n + n i PrOH (Eq. 2) (O i Pr)4-n(OH)n-1Ti -OH + HO -Ti(O i Pr)4-n(OH)n-1 → (O i Pr)4-n(OH)n-1Ti -O -Ti (O i Pr)4-n(OH)n-1 + H2O (Eq. 2') (O i Pr)4-n-1(OH)n Ti O i Pr + HO -Ti (O i Pr)4-n(OH)n-1 → (O i Pr)4-n-1(OH)n Ti -O -Ti (O i Pr)4-n(OH)n-1 + i PrOH
In the presence of water, the formation of hydroxyl groups is enhanced by very fast hydrolysis step in such a way that the condensation reaction between hydroxides (Eq. 2)

is favoured, leading to hydroxide precipitation (Eq. 3):

(Eq. 3) TiNO2N-3-x/2(OH)6+x + (2N-3-x/2) H2O
Crystalline TiO2 can be obtained by increasing the temperature (Eq. 4), either by thermal post treatment of the hydroxide or directly in situ, by crystallization in supercritical CO2.

(Eq. 4)

TiNO2N-3-x/2(OH)6+x ∆𝑇 → N TiO2 + (3+x/2) H2O

Chemicals

Chemicals and solvents were of analytical grade and used without any further purification. Titanium tetraisopropoxide (Ti(O i Pr)4) (Fluka, 99 %) was selected as titanium precursor. Standard grade CO2 ([H2O] < 10 ppm) was supplied by Air Liquide.

Pure water, with a resistivity of 18.2 M, was produced by a Milli-Q system (Millipore USA). Commercial titania powder (Aeroxide ® P25) was supplied by Evonik-Degussa.

Methylene blue (MB) and Congo red (CR) dyes were purchased from Sigma Aldrich.

Experimental set-up and titania preparation protocol

Powders were synthesized in a lab scale device schematized in figure 1. The reaction was carried out either in a 500 ml or in a 1 litre high-pressure stirred vessel (Autoclave France). After introducing a specific amount of titanium alkoxide in the autoclave, liquid CO2 was pumped into the vessel up to the operating pressure ). An efficient mixing of the reactants was achieved with a magnetic turbine stirrer (stirring rate from 100 to 2000 rpm).

The reaction temperature was regulated from 323 K to 823 K using a thermocouple and an external electric heater. Water required for hydrolysis reaction was fed using a liquid chromatography pump with a flow ranging from 0.01 to 10 ml.min -1 . After the experiment, the pressure was released by evacuating CO2 to the vent and a dry powder was recovered directly from the autoclave. The thermal behaviour of the produced powders has been investigated by both TGA and TDA up to 1473 K (TA instrument system, 10 K/min in air flow). In addition, the sintering behaviour of TiO2 powders (compacted at 8 tons.cm -2 in a disc shape) has been investigated with a Setaram dilatometer, up to 1773 K in air.

The photocatalytic activity of the produced titania powders was evaluated by studying the degradation of Methylene Blue and Congo Red dyes (model pollutants) in water.

In order to eliminate possible impurities (e.g. unreacted precursors, carbonaceous residues) which might influence the photocatalytic efficiency of TiO2 powders, the samples were first washed in isopropanol under sonication for 10 min, rinsed with water and dried 24 h at 80°C.

For all experiments, titania powder suspensions (0.2 g.L -1 ) were prepared in deionized water and sonicated during 10 min. The surface charge (zeta potential) and size of the aggregates obtained in aqueous suspensions (Dynamic Light Scattering -DLS) were measured with the help of a Zetasizer (Malvern).

For each photocatalytic test, 100 ml of the aqueous suspension were mixed with 100 ml of dye solution (10 mg.L -1 ). The as-prepared suspensions were stirred for 5 minutes in a dark chamber and then submitted to UV irradiations (λ = 365 nm) up to 30 h. Samples (5ml) were regularly withdrawn from the mother solution and filtered before characterization by UV/vis spectroscopy (Lambda 40 Perkin Elmer).

Results and discussion

Two strategies have been studied in this work to prepare TiO2 nanopowders under supercritical conditions. The first strategy bears on the preparation of an amorphous Ti(OH)4 powder which was subsequently crystallized by post-thermal treatment, while the second one is based on the direct formation of crystalline TiO2 (anatase) in SC-CO2.

The key characteristics of both synthesis protocols have been examined by varying the processing parameters and studying their influence on the final powder characteristics in relation with their morphology, microstructure, crystallinity, thermal behaviour and photocatalytic activity for the degradation of model pollutants in water media.

Amorphous Ti(OH)4 powders synthesized in SC-CO2

Reactant solubility and operating parameters

In a first step, hydrolysis and condensation of titanium tetraisopropoxide (Ti(O i Pr)4) was studied at low temperature under supercritical conditions (30 MPa -323 K). In order to determine the appropriate amounts of reactants to be introduced into the reactor, the solubility data for both Ti(O i Pr)4 and water in SC-CO2 were evaluated.

The extraction yield of the alkoxide by SC-CO2 was measured in the aforementioned standard operating conditions and was determined to be ~4.5 wt% (a value of 4.25 wt% was reported in the literature in slightly different operating conditions [START_REF] Tadros | Synthesis of titanium dioxide particles in supercritical CO2[END_REF]). The alkoxide concentration in the reactor was consequently fixed at ~4 wt% in order to ensure a single phase reaction.

Because of the non-polar character of CO2, water solubility in SC-CO2 is known to be very low (typically 0.2 wt%). A low water injection flow rate ~10 ml/min was thus applied in the first series of experiments and pressure was released immediately after water injection to maintain a constant pressure (30 MPa). The above selected flow rate was five times lower than the maximum value allowed by water solubility (0.2 wt%) under the selected experimental conditions [START_REF] Wiebe | Vapor phase composition of carbon dioxide-water mixtures at various temperatures and at pressures to 700 atmospheres[END_REF].

The involved chemical reaction is given below (Eq 5):

(Eq. 5) Ti(OiPr)4 + 4 H2O Both the unreacted alkoxide and formed isopropyl alcohol (both soluble in SC-CO2)

were easily separated from the prepared hydroxide, during pressure release. The standard conditions (30 MPa -323 K) led to the production of an amorphous powder corresponding to submicron-sized spherical particles (Figure 2a).

No significant variation of the average particle sizes was observed when changing the alkoxide concentration while keeping the hydrolysis ratio constant. In other respect, when the alkoxide concentration was set at 4 wt% and the hydrolysis ratio was less than 4 (stoichiometry for complete hydrolysis of the Ti(O i Pr)4 alkoxide), the average size of particles was found to slightly decrease from 0.37 to 0.30 µm, but this variation was not considered as significant.

Characteristics of amorphous SC-Ti(OH)4 powders (323 K, 30 MPa, H2O: 10 ml/min)

As a consequence of high reactivity of the titanium alkoxide, large surface area and fluffy hydroxide powders were directly obtained in the reactor. No carbonate species were detected in the powders by FTIR spectroscopy, despite of using water in CO2 medium. In order to calculate the synthesis yield, the amorphous powder has been submitted to a 12 h thermal treatment at 1273 K in air. A molar yield around 70% has been calculated by considering the amount of TiO2 expected from the fed quantity of TTIP precursor. As expected at such low reaction temperature (323 K), TTIP was partially unreacted and residual precursor was vented with CO2 at the end of the experiment.

SEM and TEM observations (Figure 2a & 2b, respectively) revealed a hierarchical structure of the synthesized powders. The spherical particles, relatively uniform in size (diameters in the range 0.15 -0.5 µm), were partially agglomerated (Figure 2a). Each of them consisted of a compact stacking of titanium hydroxide nanoparticles (Figure 2b).

The apparent density measured for the as-prepared powder was 0.24 whereas the intrinsic density measured by helium pycnometry (Quantachrom system) was 2.80, thus suggesting large pore volumes between and within the spherical particles. The N2 adsorption-desorption isotherms measured for the powders dried at 373 K for 12 h, was mainly of type I, i.e. typical for a microporous material (results not shown). The total pore volume was 0.22 cm 3 .g -1-with 80 % microporosity and the BET specific surface area was 450 m 2 .g -1 . It is interesting to note that a higher value of specific surface area (750 m 2 .g -1 ) was measured for the same sample when applying a non-intrusive method such as Small Angle X-ray Scattering. The SAXS technique is sensitive to surface rugosity and is able to detect pores which are not accessible to N2 molecules (closed pores and ultramicropores). This technique also confirmed that the spheres were composed of nanometer-sized aggregated primary particles.

Influence of reaction time and CO2 density on powder characteristics

One of the advantages arising from the application of supercritical fluids bears on the possibility of adjusting the solvent pressure and thus its density (impacting on solutes diffusion rate) while maintaining a single phase medium. In the aforementioned reaction conditions, the characteristics of the produced powders could be affected by two key parameters: i) reaction time after water injection and ii) pressure of the CO2 phase. The influence of both parameters on particle sizes and specific surface areas of the titanium hydroxide powders are reported on table 1. The evolution of mean particle sizes and specific surface areas versus reaction time and SC-CO2 density is plotted in figures 2c and 2d, respectively. The observed increase in particle sizes with the reaction time suggests a growing mechanism comparable to the Ostwald ripening in suspensions where the smallest particles coalesce to feed the larger ones [START_REF] Voorhees | The theory of Ostwald ripening[END_REF]. Concurrently, both the specific surface area and microporous volume fraction decrease when the reaction time increases. On the other hand the total pore volume and mean pore sizes increase. For a reaction time of 20 h, a mesoporous powder was obtained (type IV -N2 adsorption/desorption isotherms) with a total pore volume of 0.36 cm 3 .g -1 .

When decreasing the system pressure from 30 MPa down to 10 MPa, the average particle sizes increased by a factor x2. This was attributed to the increasing diffusion coefficient of solutes when the SC-CO2 density decreases. Consequently, lower working pressures statistically increase the number of interactions between titanium hydroxide nanoparticles, favoring their aggregation and therefore increasing the average size of the final particles.

In conclusion, powders with very high specific surface area (~450 m²/g) and small cluster sizes (~ 1 nm) were obtained for amorphous titanium hydroxides synthesized in SC-CO2 at low temperatures (< 473K). Small nuclei were obtained as a result of SC-CO2 specific properties such as low viscosity and high density, enabling fast reaction kinetics. Hence, clusters tend to condense at low temperatures in very short reaction time. In addition, cluster growth is also limited due to pressurized media, low CO2

surface tension and CO2 sorption on hydroxide surface.

Conversion of amorphous Ti(OH)4 powders to crystalline TiO2 powders by thermal post-treatment

For the sake of obtaining crystalline TiO2 particles, a thermal post-treatment was conducted with the as-prepared amorphous titanium hydroxide powders. XRD analysis under vacuum revealed that the amorphous powder transformed to anatase crystalline phase between 613 and 633 K. The rutile phase transition was found to occur between 1023 and 1123 K. These results (not shown) fit the data reported in [START_REF] Edelson | Role of particle substructure in the sintering of monosized titania[END_REF] for microsized TiO2 particles. During the thermal treatment, the powder surface area decreased from 450 m 2 .g -1 at 373 K, down to 5 m 2 .g -1 at 813 K (Table 2). The anatase powder obtained at 813 K was found to be mainly macroporous/non porous (Type III-N2 adsorption/desorption isotherms). On the other hand, the thermal treatment changed very slightly the powder morphology: spherical particles were obtained with a slightly smaller diameter (typically around 460 m at 813 K). CO2 pellet: it starts at ~1473 K and corresponds to a sintering between individual particles (inter-particles sintering). For sol-gel derived powders, a more conventional sintering process is observed: pellet densification starts at temperature around 1323 K and it is almost completed at 1700 K. Densification of the SC-CO2 pellets could be facilitated by using a pressure-assisted sintering process, able to compact the individual particles during/after the intra-particle sintering step, as already demonstrated by our research group for zirconia powder prepared in SC-CO2 [START_REF] Hertz | Robust synthesis of yttria stabilized tetragonal zirconia powders (3Y-TZP) using a semi-continuous process in supercritical CO2[END_REF][START_REF] Klotz | Sintering and conductivity of nano-sized yttria-doped ZrO2 synthesized by a supercritical CO2-assisted sol-gel process[END_REF].

Direct synthesis of nanocrystalline TiO2 anatase powder in SC-CO2

In previous works, the authors synthesized crystalline zirconia from metal-organic precursors in SC-CO2 by applying high synthesis temperatures directly in the reactor [START_REF] Klotz | Sintering and conductivity of nano-sized yttria-doped ZrO2 synthesized by a supercritical CO2-assisted sol-gel process[END_REF][START_REF] Hertz | Synthesis and encapsulation of yttria stabilized zirconia particles in supercritical carbon dioxide[END_REF][START_REF] Hertz | Yttria Stabilized Zirconia Synthesis in Supercritical CO2: Understanding of Particle Formation Mechanisms in CO2 / Co-Solvent Systems[END_REF][START_REF] Klotz | High specific surface area YSZ powders from a supercritical CO2 process as catalytic supports for NOx storage-reduction reaction[END_REF]. Zirconia crystallisation was attained under supercritical conditions at lower temperatures compared to conventional sol-gel and thermal treatment protocols. In the present work, direct synthesis of crystalline TiO2 ultra-fine particles has been performed in SC-CO2 at 30 MPa and in the temperature range 323 -823 K. In order to modify the reaction kinetics and better control particle formation, two hydrodynamic keyparameters: water injection and stirring rates, have been investigated in addition to the reaction temperature. The influence of these parameters on the derived TiO2 powder characteristics is discussed hereafter.

Influence of reaction temperature on TiO2 powders structure and microstructure

Direct synthesis of TiO2 anatase powders in the reactor under supercritical conditions and mild synthesis temperatures revealed much more effective than hydroxide thermal post-treatment for obtaining uniform crystalline nanoparticles with high specific surface area. The anatase structure was detected by XRD already for synthesis temperature of 523 K whereas, in conventional thermal treatment at atmospheric pressure, titanium hydroxide was found to crystallize to anatase only at 613 K. Results reported on table 3 clearly demonstrate that anatase TiO2 can be obtained under supercritical conditions between 523 and 823 K. It is also worth noting that the amorphous Ti(OH)4 powder obtained at 423 K can be easily converted to anatase by a post-treatment at 573 K while maintaining a large specific surface area of 190 m 2 .g -1 (Table 3). This finding suggests that the crystallization process starts already below 523 K although it cannot be detected with XRD (too small crystallites). The lower crystallization temperature observed in SC-CO2 is attributed to the positive effect of solvothermal conditions on the oxide formation, due to the energy conveyed by the pressure in the supercritical phase. Interestingly, the hierarchical structure of amorphous Ti(OH)4 particles is maintained after crystallization. The anatase powders directly formed in SC-CO2 are mesoporous and their specific surface varies in the range 20 -100 m 2 .g -1 depending on the synthesis temperature (Figure 3b). For comparison, the powders obtained by thermal post-treatment of Ti(OH)4 in air at 613 K, are macroporous/non porous with a low specific surface area of about 5 m 2 .g -1 (Figure 3b). The size of primary crystallites estimated from both XRD line width and SBET values are reported in Table 4 for TiO2 powders obtained between 523 and 823 K.

As expected, when the synthesis temperature was increased, crystallite sizes increased and both specific surface areas and pore volumes decreased. Remarkably, brown and dark powders were obtained above 573 K (Figure 5c). This colour was attributed to carbon residues resulting from the thermal decomposition of the titanium alkoxide organic chain, released during the hydrolysis and condensation reaction. Indeed, white powders could be recovered after a thermal post-treatment in air above 573 K. Accordingly, TGA & DSC analysis (Figure 5a & 5b, respectively) clearly demonstrated the existence of carbonaceous residues and their decomposition upon heating.

Comparison of TGA mass losses for powders prepared at different temperatures in SC-CO2 (Table 4) revealed the formation of a higher quantity of carbon for the highest synthesis temperatures. In fact, significant quantities of carbon were detected for synthesis temperatures ≥ 773 K. At 823 K, the quantity of carbon was abundant (24 % mass loss) and resulted in a very low specific surface area (3 m 2 .g -1 ) caused by pore clogging, as shown in figure 5d.

The carbonaceous film covering the titania spherical particles can be easily eliminated by a simple thermal post-treatment in air at 823 K. In fact, the pores and internal surface of the derived white powder become accessible to N2 and SBET values increased from 3 to 20 m².g -1 after thermal oxidation of carbon species. Indeed, similar pores blocking phenomenon by carbonaceous residues, resulting in powders with lower specific surface areas, has been also evidenced for samples prepared at 573 and 773 K (Table 3).

Another indication/consequence of carbon species formation is the increasing discrepancy between crystallite sizes calculated from SBET values and XRD lines analysis (Table 4) when synthesis temperature was increased. In addition, the synthesis yield (calculated from expected TiO2 amount and taking into account the TGA weight loss measured at 823 K) was almost 94 % at 573 K but drastically decreased when the synthesis temperature was increased. In order to study the thermal stability of a TiO2 powder prepared in SC-CO2 at 573 K, samples were treated for 3h in air at 873 and 1073 K, and analysed by N2 physisorption, XRD and SEM. The evolution of particle microstructure is shown in figure 6b. In comparison with the bare powder, the post-treatment at 873 K resulted in individual crystallite growth and simultaneous decrease of specific surface area (30 m 2 /g instead of 65 m 2 .g -1 initially). However, it must be noted that particle microstructure was still preserved at this temperature. When the starting powder was treated at 1023 K, intraparticle sintering occurred, yielding denser spherical particles and a dramatic decrease of their specific surface area (1 m 2 .g -1 instead of 65 m 2 .g -1 initially). In fact, the internal densification of individual particles also corresponds to the progressive transformation of anatase to rutile phase, as revealed by XRD (Figure 6a). In the supercritical process used in this work for the synthesis of crystalline TiO2 powders, both water injection rate and steering rate are key-parameters with a direct impact on powder morphology and specific surface area. Various combinations of these parameters were tested, as reported in table 5. The quantity of injected water was fixed to 4 moles H2O for 1 mole of titanium alkoxide. The temperature and pressure of the SC-CO2 phase were set at 523 K and 30 MPa, respectively. As mentioned above, both hydrolysis and condensation reactions of Ti(O i Pr)4 alkoxide are involved in Ti(OH)4 clusters formation and thus impact on the formation of TiO2 primary particles. The as-formed primary particles diffuse and collide in the supercritical phase, yielding secondary spherical particles. Considering the fact that hydrolysis of alkoxy groups is a very fast reaction in presence of a stoichiometric quantity of water, the formation of spherical secondary particles can be described as a diffusion-limited process. This is the case when the water injection rate is 10 ml/min.

On the contrary, for lower water injection rates (1 ml.min -1 ) it can be assumed that alkoxide hydrolysis is not fully completed before the formation of spherical particles. It can be thus ascribed to a reaction limited process. The effect of water injection rate on particle morphology can be clearly observed in figure 7. For a water injection rate ~ 1 ml.min -1 , small particles were obtained with narrow size distribution but irregular geometry. This finding results from the coincidence of clusters and particles formation in a reaction limited process. For higher water injection rate (~ 10 ml.min -1 ) and comparable stirring rate (700 rpm), powder specific surface areas were similar (90 -100 m 2 .g -1 ) but particles had a better defined spherical geometry. This, results from the fact that complete hydrolysis and cluster formation take place before the formation of spherical particles.

The second hydrodynamic parameter investigated in this study was the stirring rate. The specific surface areas of powders prepared at 523 K with gradually increasing the stirring rate (from 300 to 2000 rpm) are compared in table 5. The SBET values were found to increase insignificantly with the stirring rate (e.g. from 90 to 110 m 2 .g -1 with 10 ml.min -1 water injection rate). On the other hand, the stirring rate was found to strongly influence powders morphology as shown in figure 7. Large aggregates with undefined geometries were generally observed at high stirring rates. As shown in table 5, the highest value of the specific surface area (130 m 2 .g -1 ) was obtained when combining a medium stirring rate (~ 1000 rpm) and a low water injection (~ 1 ml/min).

The results obtained in this paragraph demonstrate the strong impact of hydrodynamic parameters on the quality and homogeneity of the formed powders. have been obtained, exhibiting controlled morphology (spherical particles from 0.1 to 0.8 µm in diameter), structure (amorphous or crystalline powders composed of < 15 nm primary particles) and microstructure (pore volume around 0.30 cm 3 .g -1 ; SBET of about 540 m².g -1 for Ti(OH)4 and 112 m².g -1 for anatase TiO2).

Titania powder behavior in aqueous suspensions

Since photo-catalytic treatment of liquid effluents takes place in aqueous solutions and photo-catalysis is a surface-dependent mechanism, it was important to investigate/compare first the behavior of TiO2 powders in suspensions. Actually, particle aggregation as well as surface charges and surface chemistry might vary depending principally on the effluent pH. Each powder suspension has been then mixed with the same volume of a methylene blue solution (concentration MB 10 mg.L -1 , pH = 6.3), chosen as a model compounds for the photodegradation tests. The suspensions characteristics (zeta potential and sizes of aggregates) are reported on table 6. In terms of aggregate sizes and surface accessible to UV, both crystalline samples (#TiO2 -573 K and #TiO2 -P25) were quasi-equivalent. In comparison, the amorphous sample contains significantly larger aggregates and thus 10 times lower accessible surface for UV irradiation. The intrinsic sorption capacity of the nanostructured powders has been first evaluated without any UV lightning (dark conditions). If was found out that ~10 mol% of methylene blue was adsorbed on all the prepared TiO2 powders after 24h treatment.

Sorption kinetics were faster for the powders synthesized in SC-CO2 in comparison with the P25 reference one, due to the high specific surface areas of the former.

Indeed, crystalline TiO2 anatase is known to be the most efficient titania form in view of UV-light photocatalytic applications. Results obtained with the crystalline powder #TiO2 -573 K emphasize the role of powder/suspension conditioning on their photocatalytic efficiency. A suspension of #TiO2 -573 K powder merely dispersed in water by gentle stirring was less active than #TiO2 -P25 suspension.

However, sonication-assisted dispersion (15 min in a lab-sonication bath) of #TiO2 -573 K powder was benefic to improve the photocatalytic perfomance and reach same efficiency and degradation kinetics as #TiO2 -P25 suspension (figure 9a1). The powder surface accessible for UV irradiation is directly related to the size of primary particles, the size of aggregates and their dispersion degree. Hence, both the powder isoelectric point and the pH of the treated solution have a strong influence on the process efficiency, in addition to the structure, microstructure and design of the photocalyst.

Additional photocatalytic tests for the degradation of Congo Red dye (CR model molecule) proved the efficiency of #TiO2 -573 K (Figure 9b). A CR solution at 15 mg/L was completely cleaned in less than 75 min with 0.5 g.L -1 #TiO2 -673 K, with degradation kinetics slightly better than those measured for #TiO2 -P25.

Conclusion

A simple eco-friendly route for the preparation of titania nanopowders with well controlled and reproducible characteristics has been thoroughly described in this work.

This green process, based on sol-gel processing of titanium isopropoxide (Ti(O i Pr)4)

and water as hydrolyzing agent in supercritical CO2 (single supercritical phase), yields either submicronic titanium oxide (TiO2 anatase) or titanium hydroxide spherical

Fig. 1

 1 Fig. 1 Experimental device used for titania powders synthesis in supercritical carbon dioxide

Fig. 2

 2 Fig. 2 Microstructural characteristics of amorphous Ti(OH)4 powders obtained by reacting Ti(O i Pr)4 with H2O in SC-CO2 at 323 K and 30 MPa: a) SEM observation, b) TEM observations, c) Evolution of mean particle sizes and SBET as a function of reaction time/maturation in SC-CO2 and d) as a function of SC-CO2 density. (Stirring rate: 1000 rpm, water injection rate: 10 ml.min -1 )

Fig. 3

 3 Fig. 3 Thermal evolution of titania powders: a) Comparison of sintering behavior of a powder prepared in SC-CO2 at 323 K, 30 MPa and a sol-gel derived powder (discs with green density ~ 50 %), b) Specific surface area vs. synthesis temperature in SC-CO2 (in situ crystallization) and specific surface area vs. post-treatment temperature of an amorphous powder prepared at 323 K, 30 MPa in SC-CO2

The morphologies of particles obtained at different synthesis temperatures are compared in figure 4 .Fig. 4

 44 Fig. 4 Evolution of TiO2 particles morphology and pore size distribution (PSD) as a function of the synthesis temperature at 30 MPa in SC-CO2: SEM images of powders

Fig. 5

 5 Fig. 5 Evidence of residual carbonaceous species in TiO2 powders prepared in SC-CO2 at 30 MPa: a) TGA, b) DSC analysis; c) powders prepared at different temperatures in the range 523K-823K; d) SEM image of TiO2 powder prepared at 823 K, before and after an additional post-treatment of 3 h in air at 823 K

Fig. 6

 6 Fig. 6 Thermal evolution (in air) of a TiO2 powder prepared in SC-CO2 at 573 K and 30 MPa, and post-treated for 3 h in air at 873 K and 1073 K: a) XRD patterns of powders with calculated crystallite sizes and b) SEM images of particles microstructure, with associated SBET values. (Stirring rate: 1000 rpm, water injection rate: 10 ml.min -1 )

Fig. 7

 7 Fig. 7 Influence of hydrodynamic parameters: water injection rate and stirring rates, on the morphology of TiO2 particles prepared in SC-CO2 at 523 K and 30 MPa, within 5 minutes reaction time

3. 4 .

 4 photo-catalyst. It was thus used in the present work to compare the performance of two TiO2 powders prepared in SC-CO2 at 30 MPa: an amorphous powder prepared at 423 K (#Ti(OH)4 -423 K) and a crystalline anatase powder prepared at 573 K (#TiO2 -573 K).

Fig. 8

 8 Fig. 8 Influence of pH on the characteristics of suspensions prepared by dispersing the powders in water Ti(OH)4 -423 K and TiO2 -573 K prepared in SC-CO2 and a commercial TiO2 -P25 powder: a) Zeta potential and b) size of aggregates (by DLS) versus pH

Table 6 : 3 )

 63 Photocatalytic activity evaluationResults of MB photocatalytic degradation tests are compared in figure 9a1. As expected, the amorphous powder #Ti(OH)4 -423 K clearly appears as the less efficient catalyst towards MB degradation.

Fig. 9

 9 Fig. 9 Photocatalytic performance of titania powders prepared at 423K and 573K in SC-CO2, compared with commercial TiO2 -P25, for the degradation of: a) methylene blue

Table 1 :

 1 Characteristics of Ti(OH)4 powders prepared in SC-CO2 at 323 K at different

	synthesis pressures and reaction times (stirring rate: 1000 rpm, water injection rate: 10
	ml.min -1 )
	Pressure (MPa)	CO2 density (kg.m -3 )	Reaction Time (min)	Particle size (µm)	SBET (m².g -1 )
	30	870	5	0.37	450
	30	870	15	0.38	360
	30	870	30	0.39	355
	30	870	60	0.42	330
	30	870	600	0.50	270
	30	870	1200	0.62	210
	27	850	5	0.43	370
	20	790	5	0.59	355
	13	630	5	0.69	330
	10	390	5	0.75	305

Table 2 :

 2 Influence of thermal post-treatment temperature (1 h) on the characteristics of

	Ti(OH)4 powders prepared in SC-CO2 at 323 K and 30 MPa
	Post-treatment
	temperature
	(K)

Particle size (µm) SBET (m².g -1 ) Cristalline phase Porous structure

  In order to further investigate the characteristics of the TiO2 powders obtained by thermal post-treatment of SC-CO2 Ti(OH)4 particles, a series of pellets have been prepared (compacted at 8 tons/cm 2 ) and studied by dilatometry up to 1773 K.

	No	0.37	450	Amorphous Microporous
	453	0.42	445	Amorphous Microporous
	523	0.43	105	Amorphous Mesoporous
	633	0.40	5	Anatase	Macroporous / non porous
	813	0.46	5	Anatase	Macroporous / non porous
	993	0.43	3	Anatase	Macroporous / non porous

Table 3 :

 3 Characteristics of TiO2 powders prepared in SC-CO2 within 5 min reaction time at 30 MPa in the temperature range 323 -823 K (stirring rate: 1000 rpm, water injection rate: 10 ml.min -1 ).

	Sample	Synthesis conditions		Powder characteristics
	reference (#)	Synthesis temperature (K)	CO2 density (kg.m -3 )	SBET (m².g -1 )	Crystalline phase	Color	Porous structure
	1	323	870	450
	2	373	660	475	Amorphous		Microporous
	3	423	490	480
	3.1	Sample #3+ thermal post treatment at 573 K -3 h	190	Anatase	White	Mesoporous
	4	473	390	455	Amorphous		Microporous
	5	523	330	100
	6	573	280	65		Beige
	6.1 6.2	Sample #6 + thermal post-treatment at 573 K -1 h Sample #6 + thermal post-treatment at 873 K -3 h	75 30	Anatase	White	Mesoporous
	6.3	Sample #6 + thermal post-treatment at 1073 K -3 h	1	80% Rutile 20% Anatase	White Non porous
	7	623	250	45		Beige
	8					Light
		673	230	40		Brow
						n
	9					Dark	Mesoporous
		773	200	35	Anatase	Brow n
	9.1	Sample #9 + thermal post-treatment at 773 K -1 h	45		White
	10	823	180	3		Black	Non porous
	10.1	Sample #10 + thermal post-treatment at 823 K -3 h	20		White Mesoporous

Table 4 :

 4 Comparison of pore volume, crystallite sizes, TGA weight loss and synthesis yields for TiO2 powders prepared in SC-CO2 at 30 MPa within 5 min reaction time in the temperature range 523 -823 K

	Sample reference (#)	Synthesis temperature (K)	Synthesis yield (%)	Pore volume (cm 3 .g -1 )	Pore size (nm)	crystallites from DRX (nm)	crystallites from SBET (nm)	Weight loss at 823K (%)
	5	523	94	0.20	4.8	16	17	8
	7	623	77	0.15	11.7	24	35	3
	9	773	65	0.10	10.2	24	43	9
	10	823	71	-	-	27	513	24
	10.1	Sample#10 + TT823K-3h	-	0.07	9.2	40	73	1

Table 5 :

 5 Influence of hydrodynamic parameters (water injection and stirring rates) on

	the specific surface area of TiO2 powders prepared in SC-CO2 at 30 MPa within 5 min
	reaction time at 523 K
	Water injection rate (ml.min -1 )	Stirring rate (rpm)	SBET (m².g -1 )
	1	700	100
	1	1000	130
	5	700	105
	10	300	95
	10	700	90
	10	1000	100
	10	2000	110

Acknowledgements

The authors would like to thank Dr. Sébastien PAPET for his contribution to amorphous powder preparation and characterizations, M. Mickael GARCES and M. Damien AVRIL for their work on crystalline TiO2 particles and Mlle Géraldine DIDERON for photo-catalytic tests.

The authors wish to confirm that there are no known conflicts of interest associated with this publication and there has been no significant financial support for this work that could have influenced its outcome.

particles, depending on the processing temperature. In fact, the low viscosity and high diffusivity of supercritical fluids counterbalance the low solvent power of supercritical CO2 for polar compounds such as metal alkoxides. As a result, an inferred effect of the rapid diffusion and aggregation of nanoparticles in SC-CO2 bears on the formation of spherical particles with a hierarchical microstructure. At low processing temperature (323 K), amorphous particles with sizes in the range 0.1 -0.5 µm and high specific surface area (450 m 2 .g -1 ) were obtained. These particles transform to TiO2 anatase by thermal treatment in air between 613 and 633 K. The rutile phase was formed between 1023 and 1123 K. Advantageously, it was possible to produce nanocrystalline spherical anatase particles directly in SC-CO2, using a processing temperature ~ 523 K, i.e. lower than those typically required for the thermal crystallization of amorphous particles in air. The immediate benefits are a higher specific surface area of the powders and lower particle aggregation due to supercritical drying. It has demonstrated that the homogeneity of both particle sizes and microstructure has to be controlled by an appropriate choice of hydrodynamic parameters, namely reactant injection rate and stirring rate in the reactor. The as-prepared TiO2 anatase powders synthesized in SC-CO2 have a photocatalytic activity comparable to that of commercial P25 powder from Degussa. Moreover, the characteristics of these TiO2 powders prepared under supercritical conditions are potentially attractive for a wide range of applications, e.g. catalyst support, chromatography static phase, pigment or ceramic membranes. Further investigations are currently in progress for studying their performance for the sorption of radionuclides or the photocatalytic degradation of biorefractory pollutants in liquid wastes.


Syreina Sayegh

Jae-Hyoung Lee

Dong-Hoon Yang

Matthieu Weber

Igor Iatsunskyi

Emerson Coy

Antonio Razzouk

Sang Sub Kim

Mikhael Bechelany
email: mikhael.bechelany@umontpellier.fr

Humidity-Resistant Gas Sensors Based on SnO2 Nanowires Coated with a Porous Alumina Nanomembrane by Molecular Layer Deposition

Keywords: metal oxide nanowire, semiconductor metal oxide, gas sensing mechanism, humidity resistance, porous shell formation

We report a novel route for enhancing the humidity resistance of SnO2 nanowires (NWs; widely used metal oxide semiconductor gas sensors) based on their coating with a humidity-resistant nanomembrane. A conformal thin film was deposited by Molecular Layer Deposition (MLD) on the SnO2 NWs using trimethylaluminum and ethylene glycol as precursors (100 MLD cycles), followed by oxidation in air at 400 °C. In conditions of increasing relative humidity, the SnO2 NW gas sensor coated with a microporous, 10-nm thick nanomembrane layer of converted alucone obtained by MLD and calcination showed a remarkable gas response compared with the pristine SnO2 NW gas sensor. This sensor was stable and displayed good recovery times in humid conditions (30 and 90% of relative humidity). This work shows the benefits and the practical application of MLD-prepared nanomaterials. This original strategy applied to NW-based gas sensors paves the way for other complex 3D designs and different applications that require gas or ion selectivity, such as photocatalysts and biosensors, in humid atmosphere.

Introduction

In recent years, the demand for gas detection and measurement devices has strongly increased, mostly for safety and environmental monitoring [1]. Semiconducting metal oxides (SMOs) are frequently used for the production of gas sensors because of their simple fabrication, good stability, stoichiometirc tunability, and excellent sensitivity towards many different gas molecules [1][2][3][4]. The gas sensor surface characteristics, e.g. composition, crystallinity, and microstructure (porosity, roughness), are directly related to its gas sensing performance, and therefore strongly influence the sensor features, particularly its selectivity, response and recovery times. Among the different metal oxide-based sensors, SnO2 nanowires (NWs) are fast, stable, sensitive and reliable gas sensors that can be easily combined into a multi-component array [5]. However, SnO2 and other metal oxide-based gas sensors present relatively low response and selectivity in realistic conditions of humid atmosphere, limiting their practical use. Hence, research efforts are still needed to tackle this key challenge. Among the different approaches, innovative nanomembrane materials could be used to improve the gas sensor performances in humid atmosphere conditions.

Recent studies have shown that aluminium alkoxide-based materials could be a potential candidate to improve the response of gas sensing in humid conditions [6][7][8][9][10][11]. In fact, Aluminium alkoxide, better known as alucone, is the most commonly synthesized material by Molecular Layer Deposition (MLD), using a well-developed process with trimethylaluminum (TMA) as precursor and ethylene glycol as co-reactant [12]. MLD is an Atomic Layer Deposition (ALD)-derived method based on sequential and self-limiting surface reactions [13].

ALD is a vapour phase deposition technique for the preparation of inorganic nanomaterials, such as oxides [14], nitrides [15,16] and metals [17,18], with sub-nanometric control of the thickness [19] and competitive mechanical response [20,21]. One of the main advantages of these deposition methods is the excellent substrate uniformity and conformality over the surface even for the most complex 3D nanostructures [19,22,23].

The deposition of nanoscale films has become essential for many applications, especially in microelectronics [24,25] and catalysis, [26,27] but also membranes [28][29][30] and sensors [31].

MLD allows the preparation of conformal thin films (either hybrid organic-inorganic or purely organic films) using an organic compound instead of the classical ALD precursors [13].

Moreover, the addition of an etching or calcination post-treatment allows removing the organic component from the film, resulting in a porous inorganic matrix. MLD conformality control and the transformation of the prepared organic-inorganic films into porous (inorganic) ultrathin layers allow the synthesis of porous oxide nanomembranes. This microporous alumina layer could be valuable for enhancing the selectivity and stability of sensors and membrane devices [32,33]. The layer thickness can be modulated by varying the number of MLD cycles, and the pore dimensions can be tuned by controlling other experimental parameters, such as the organic reactant chemistry [32,34]. Jiang et al. prepared nanomembranes for gas separation by MLD (200 cycles) following by calcination of the organic template. These membranes display an He permeance of 5.3 cm 3 /bar•cm 2 •min, and He/N2 and He/SF6 selectivities higher than 10 3 and 10 4 , respectively [35]. By coating zeolite membranes (initial support) with microporous nanomembranes using MLD, Yu et al. prepared defect-free, microporous Al2O3/SAPO-34 zeolite composite membranes with very high H2/N2 selectivity compared with uncoated SAPO-34 membranes (1040 versus 8) [36].

Here, we describe a novel approach for improving the gas sensing performance of widely used SMO-based sensors in humid atmosphere. Specifically, the SnO2 NW surface was coated with a microporous nanomembrane layer obtained by MLD of conformal alucone films using TMA and ethylene glycol as precursors, followed by oxidation in air at 400 °C. This approach for improving humidity resistance that relies on the protective effect of converted alucone nanomembranes has never been described before. The optimized alucone-coated SnO2 NW gas sensors exhibited good recovery time in humid atmosphere.

Experimental

Fabrication of the sensing devices:

To facilitate their potential integration into devices, sensors were produced using typical microelectronic fabrication routes. First, patterned interdigital electrodes (PIEs) were prepared on SiO2-Si (100) substrates using a photolithographic technique. PIEs consisting of Au (3 nm)/Pt (200 nm)/Ti (50 nm) were sequentially sputter deposited onto the substrates. The Ti layer was used to improve the adhesion between the substrate and the Pt electrode layer. Using the vapour-liquid-solid growth technique, networked SnO2 NWs were successively obtained on the previously deposited PIEs. For SnO2 NW growth, Sn powder (99.9%) was put in a crucible that was introduced with the Si substrates supporting the PIEs into a quartz tube furnace at 900 °C for 5 min in the presence of nitrogen (300 standard cubic centimetre per minute, sccm) and oxygen (10 sccm).

Molecular layer deposition and calcination:

MLD was performed using a home-built ALD reactor and TMA (98% purity, CAS: 75-24-1; Strem Chemicals, Inc.) and ethylene glycol (99% purity, CAS: 107-21-1). The container with ethylene glycol was heated to 70 °C and the TMA bubbler was maintained at room temperature.

The deposition chamber was set at 130 °C. Each MLD cycle consisted of 0.1s pulse of TMA, 10s exposure, and 30s purge, followed by a 2s pulse of ethylene glycol, 10s exposure and 60s purge with Argon. After MLD, samples were directly placed in a furnace and calcined at 400 °C (ramp rate = 1 °C/min) for 1h.

Physicochemical characterization

Field emission Scanning Electron Microscopy (SEM, Hitachi S-4800) and Transmission Electron Microscopy (TEM, JEOL ARM 200F (200 kV)) coupled to Energy-dispersive X-ray spectroscopy (EDX) were used for NW morphology and composition analysis. Pristine SnO2 NW and alucone-coated SnO2 NW samples were immersed in ethanol and then laser scratched in a low-power sonication bath. Ethanol drops were collected, casted drop-by-drop on commercial Cu-grids and dried in a membrane pump (<10 bar) overnight. The sample chemical composition was further analysed by SEM (Zeiss EVO HD microscope) coupled to EDX (Oxford, X-Max N) and X-ray Photoelectron Spectroscopy (XPS, ESCALAB 250 Thermal Electron) with Al-Kα (1486.6 eV). Binding energies were calibrated with carbon (C 1s = 284.4 eV), and a surface of 500 µm 2 was analysed by XPS.

Gas sensing measurements:

The gas sensing performances of pristine and alucone-coated SnO2 NW samples were compared by exposing them to different gases at various temperatures, in a horizontal gassensing system. The target gases were introduced in the gas-sensing system at specific concentrations obtained by mixing the dry air-balanced gas with dry synthetic air through accurate mass flow controllers. The total flow rate was set at 500 sccm. The sensor resistance was monitored using an electrical measurement system (Keithley 2400) interfaced with a computer. The sensing test was extensively described in our previous articles [9,23]. After recording the resistance in air (Ra) and the resistance in the presence of the target gas (Rg), the sensor response (R) was calculated as R = Ra/Rg (for reducing gases, such as CO, H2, NH3, C6H6, C7H8, ethanol) and R = Rg/Ra (for oxidizing gases, such as NO2). To study the effects of humidity, gas sensing measurements were performed in dry and in humid air (relative humidity (RH) of 0-90%, measured at 20 °C) at the sensor operating temperature of 300 °C. The highly networked SnO2 NWs extended from one side to the other of the electrodes (Figure 1b). SnO2 NW growth on the surface of the PIE pads was obtained using the vapourliquid-solid method, a catalyst route. Metallic Sn powder (99.9%) was heated to 900 °C, and the flow profiles of the source vapours were optimized, because they can affect the overall formation of NWs. Indeed, the careful tuning of vapour flows is crucial for solid sources of SnO2 vapours, because highly variable NW density and length can be obtained [37].

Results and discussion

Next, the growth of poly (aluminium ethylene glycol) polymer (better known as aluminium alkoxide, or simply alucone) films, was performed by MLD. Various films were prepared, by sequential exposures of TMA and ethylene glycol. Different numbers of MLD cycles (10, 50, and 100) were used to coat with alucone the SnO2 NW surface at 130 °C, resulting in conformal hybrid alucone layers of different thickness.

MLD was directly followed by calcination to convert the hybrid nanolayers to the desired alumina nanomembranes because the air and water present in the atmosphere quickly and permanently degrade the MLD-deposited alucone material [12,33]. Calcination was carried out at 400 °C to ensure the successful conversion of CH2-CH2 bonding to CO2 and H2O. It has been demonstrated that after oxidation in air at 400 °C of alucone films made of TMA and ethylene glycol, micropores with a diameter of around 0.6 nm are created [33,34,38]. This is due to the removal of the monolayer of (-CH2CH2-) groups, presenting this dimension [12,13]. However, it is important to note that although the length of the organic chains of the organic co-reactant has a major influence on the pore dimensions, [39] other parameters involved in etching/calcination also can have an effect. For example, the film porosity has been linked to the applied temperature ramp rate [34]. Therefore, many different porous oxide films with controlled thickness can be synthesized by thermal treatment of MLD-prepared thin films.

These oxide nanostructures can have high surface areas (more than 1000 m 2 /g) [32]. After sample preparation, EDX mapping was performed to verify the sensor elemental composition and the presence of the alumina nanomembrane. The NWs presented a diameter between 50 and 300 nm (mean value: ~175 nm), and their length typically reached several microns (Figure 3 and Figure S1). The forest of nanowires visible in the SEM image allowed their interconnection, which is beneficial for the good conduction of electrical signals. Moreover, the EDX mapping results (Figure 3) clearly highlighted the presence of C, O, Al, Si and Sn elements.

In addition, the composition and the oxidation level of alumina membrane coated SnO2

NWs were analysed by XPS. All the expected elements (Sn, Al, O) were found as well as carbon, which can be explained by the surface contamination between NWs. Figure 4 shows the XPS scans of Sn 3d and Al 2p in alumina membrane coated SnO2 NWs. Finally, the morphology of SnO2 NWs coated with alumina nanomembranes (10, 50 and 100 cycles of alucone) was studied by high resolution TEM (Figure 5a-c). This revealed the presence of amorphous alumina nanomembranes conformably covering the SnO2 NWs. The nanomembrane thicknesses was approximately 2 nm, 5 nm and 10 nm for the samples prepared using 10, 50 cycles and 100 cycles of alucone, respectively. Importantly, although the diameter distribution of the SnO2 NWs was large, the film conformal growth remained homogeneous over the whole sample. The alucone theoretical growth rate (∼0.50 nm per cycle) [40] should result in much thicker layers; however, the layers obtained in this work were much thinner. This is not surprising, because the oxidation of MLD-prepared layers leads to a structural collapse through the formation of the alumina nanomembrane. Moreover, the water present in the atmosphere also contributes to degrade MLD layers and to quickly decrease their thicknesses. In addition, our results are in good agreement with what previously reported. For example, Weimer's group deposited alucone by MLD on particles and showed that if a porous film persisted on the particle surface after oxidation, the film thickness was greatly reduced from 25 nm to 8 nm [33].

Gas sensing results and possible mechanism(s) of the enhanced humidity resistance

The gas sensing performance of the prepared devices was then tested. The alumina nanomembrane coated SnO2 NWs sensor is schematically described in Figure 6a. concentrations in dry air. This behaviour of the optimal sensitivity at a certain sensing temperature is often observed in chemiresistive-type gas sensors. This is because the interaction between adsorption and desorption of gas molecules onto the sensor material is optimized at that temperature. The SnO2 NWs gas sensor coated with alumina nanomembrane exhibits its highest response at 300 °C. In case the temperature is less than 300 °C, the NO2 gas molecules do not have sufficient energies for adsorption onto the sensing material. In sharp contrast, if the temperature is greater than 300 °C, the desorption of NO2 gas molecules becomes more dominant. The adsorption rate and desorption rate are likely to be balanced at 300 °C, resulting in the highest sensor response. Therefore, all the sensing responses were measured at this temperature. It should be mentioned that the operating temperatures vary between oxidizing and reducing gases. Typically, the highest gas-sensing response is observed at higher temperatures for reducing gases. Therefore, for a more detailed and precise analysis of the gas sensing performance, the operating temperature should be optimized for oxidizing and reducing gases before testing, which could be a new research topic. Next, the gas-sensing behaviour of the alumina coated SnO2 NW sensors was investigated in function of the alumina shell layer thickness (2 nm, 5 nm and 10 nm) and for different gases (NO2, CO, H2, NH3, C6H6, C7H8 and ethanol) (Figure 7). The resistance curves clearly varied in function of the alumina thickness. Similarly, the sensing response (Figure 8) decreased as the alumina layer thickness increased. It is likely that in sensors coated with thicker alumina layers, the direct interaction of gas molecules with the SnO2 NW surface of the active sensing material becomes more hampered in comparison with the case of thinner alumina layers, thus suppressing the sensor response as the thickness of the alumina layer increases. Overall, the response to NO2 was higher than all other tested gases with all sensors (Figure 8). NWs with 5nm-thick, and (d) SnO2 NWs with 10nm-thick alumina layer.

These data indicated that with the RH% increase, the gas sensor response decreased, as previously reported [41]. This effect was observed when using pristine SnO2 NWs and also, to a lower extent, with SnO2 NWs coated with 2nm-thick and 5nm-thick alumina layer (Figure 9a-c), but was negligible with SnO2 NWs coated with a 10nm-thick alumina layer (Figure 9d). For networked NW sensors, resistance changes can be explained by ( 1) modulation in contact resistance between NWs, and (2) radial modulation of the conduction layer in NWs [42,43]. In the case of smaller diameter than the Debye length, the whole region of NWs becomes depleted in carrier concentration, resulting in a high initial resistance, and the resistance change is likely to originate largely from the modulation in contact resistance.

Conversely, if the NW diameter is larger than the Debye length, the NW region will be only partially depleted, and the modulation of the conduction layer in NWs is the dominant source.

By taking into account the Debye lengths of SnO2 NWs (~18 nm) [44,45] in similar sensing measurement conditions, it is reasonable to conclude that the SnO2 NWs used in this study were partially depleted. This indicates that the main source of resistance change in the alumina membrane coated SnO2 NWs is the radial modulation of the conduction layer in NWs. The underlying sensing mechanism was extensively discussed in our earlier reports [46,47].

Therefore, it is reasonable to think that the resistance change is caused by the modulation in width of the depletion layer along the length direction of single-crystalline SnO2 nanowires.

However, the modulation in contact resistance between NWs also should be considered as a significant source of resistance change observed in alumina coated NWs. The contacts between NWs can play a similar role as grain boundaries in bulk or film-type sensing materials. Oxygen and target gas molecules can be adsorbed into contact areas between NWs, eventually resulting in a considerable resistance modulation during the adsorption and desorption of gas molecules.

Importantly, as NWs form a simple mechanical contact with each other and mutual displacements of NWs may occur during alucone deposition and annealing, the alucone layer can be deposited between SnO2 NW contacts. Accordingly, NW contacts can be a more important source of resistance modulation.

In addition, we investigate here the deposition of alucone by MLD on SnO2 NWs. TMA first reacts with hydroxyl groups on SnOH to deposit Al-CH3 surface species. Ethylene glycol then reacts with Al-CH3 species to deposit -CH2CH2OH surface species. Nevertheless, many parameters could affect the growth rate and the conformal layer deposition, such as the density of available reactive sites on the surface and the steric effects of the optimum saturation occupancy of those sites. Steric hindrance of the ligands and the number of reactive surface sites are among the major factors that could affect the growth per cycle [48]. In our experimental conditions, part of the surface of SnOH ligands might have been not accessible to the reactant because of steric hindrance, leaving available sites for gas detection. Moreover, the alucone layer deposited by MLD was calcined. The obtained porous alumina membrane had a pore size (~0.6 nm) that was large enough for all the tested gas molecules, except NH3, to penetrate toward the n-SnO2 surface. Therefore, it could be hypothesized that the SnO2 NWs are only partially covered rather than completely covered by the alumina layer. Thus, the improvement in gas sensing performance is due to the depletion region created on the interface of the heterojunction of the two semiconductors when the electron and hole carriers inter-diffuse [49].

The alumina membrane provide additional accessible surface area and grain boundaries that contribute to the enhanced sensing response via the effective and quick diffusion of gas vapour into the sensing membranes [50].

After heat treatment, alucone deposited on n-type SnO2 NWs is transformed into alumina membrane that behaves like a typical dielectric material with a very low concentration of charge carriers. Therefore, the band diagram of the insulating Al2O3-n-type SnO2 heterostructure must be constructed. Accordingly, NWs consisting of the insulation shell and semiconducting core should show higher resistance compared with pristine n-type SnO2 NWs without the insulation shell. For this reason, the deposition of alumina membrane resulted in the large increase in resistance of the sensor observed in this study. Of note, the large increase in resistance is disadvantageous for the actual sensor applications.

As the pores of the alumina membrane layer were large enough for the tested gas molecules to penetrate toward the surface of n-SnO2 (Figure 10b), the radial modulation of the conduction layer in core SnO2 NWs can be considered the dominant sensing mechanism, which is basically the same as for pristine n-SnO2 NWs, although the conduction layer is contracted. A similar sensing behaviour was observed in metal-organic framework-based membrane encapsulated ZnO NWs [51,52].

The kinetic diameters of the tested gas molecules (NO2, CO, H2, NH3, C6H6, C7H8, and ethanol) were 0.34, 0.376, 0.289, 0.260, 0.585, 0.585, and 0.45 nm, respectively [53,[START_REF] Wilcox | Carbon Capture[END_REF]. On the other hand, the kinetic diameter of water molecules is 0.28 nm, [START_REF] Wilcox | Carbon Capture[END_REF] which is smaller than what obtained with all tested gas molecules except NH3. As the pore diameter of the alumina membrane layer was estimated to be ~0.6 nm, it is reasonable to conclude that both the tested gases and water molecules can penetrate through the porous alucone layer (Figure 10b). It was shown that the alucone coating is hydrophilic, but its conversion into a microporous structure allows its conversion to a hydrophobic surface [56]. Interestingly, Mavric et al. have recently found that the structure of alumina could change with the synthesis conditions. In fact, they reported that hydrophobicity of alumina is directly affected by the crystallinity of the layer.

They found that polycrystalline structure were hydrophilic while amorphous form was hydrophobic. This funding is in good agreement with our research, whereas alumina membrane is amorphous [57]. As hydrophobic membranes "hate" or repel water, they are typically used in gas filtration processes [58]. For instance, Hu et al. found that the sensing response toward HCl remains unchanged when varying the relative humidity from 10 to 90%. They showed that their sensor surface is hydrophobic and this explains the sensor stability in different conditions of humidity [59]. Moreover, Liu et al. confirmed that higher hydrophobic materials have better waterproof effect. This means that due to the membrane hydrophobicity, water vapours cannot penetrate the membrane. However, the tested gases need to permeate the membrane to get in touch with SnO2. As all the tested gases had a diameter smaller than that of alumina micropores (except NH3), they could penetrate the membrane and get in touch with SnO2, whereas hydrophobic nanoporous alumina will inhibit water vapour penetration by repelling it on the surface to limit the influence of water vapour [60] (Figure 10b).

Comparison of the response and recovery times of the sensors to 100 ppm NO2 at 300 ℃ and RH 90% (Figure S7) showed that the 10 nm alumina coated SnO2 NWs sensor had the shortest response and recovery time in humid conditions. In general, the presence of an additional layer that prevents the penetration of gas molecules to the sensitive layer should be accompanied by an increase in the response and recovery times. However, this was not observed in the present experiments, suggesting that there is another sensing mechanism that controls the gas-sensing effect. In humid environments, water molecules are competitively adsorbed on the sensor material surface and this hampers the reaction of target gas molecules with the sensor material. Conversely, once a porous Al2O3 layer is formed on the sensor material, water molecules are preferentially blocked by the layer compared with other target gases. This facilitates the reaction of gas molecules with the sensor material. This explain the shorter response and recovery times of 10 nm thick alumina-coated NWs. Based on our results, it is possible to design other nanomembrane-coated SMO NW gas sensors that could be employed to improve their gas sensing activity in humid conditions for practical applications.

Humidity-resistant gas sensors based on SnO2 materials have been extensively studied. For example, mesoporous SnO2 hierarchical architectures show H2S detection at low concentrations (ppb) in highly humid atmosphere (RH = 85%) [61]. Thin-walled SnO2 nanotubes show good detection of H2S in very humid atmosphere by the formation of clustered pores [62]. A negligible humidity dependence of the sensor signal was observed using CuO-loaded SnO2 hollow spheres at RH = 80% [63]. In our study, a molecular nanomembrane was used to increase humidity resistance of SnO2 NW gas sensing properties, a novel and completely different approach compared with the previous reports [61][62][63].

Conclusions

In this work, we presented a promising strategy for improving the gas sensing properties of metal oxide sensors in humid atmospheres. This novel approach is based on the encapsulation of SnO2 NWs with a humidity-resistive nanomembrane obtained by calcinating MLD-prepared alucone films. The control of channel size in alumina membrane is useful for preventing the crossing by water molecules, which leads to highly efficient and industrially viable gas sensing devices. Our results showed that the response to NO2 gas of 10 nm alumina-coated SnO2 NWs is comparable in dry air conditions and in various conditions of relative humidity (RH 30 to 90%). The thickness of the alumina membrane layer should be minimized to secure the best sensing performance under humidity conditions. The present results open the way for the fabrication of humidity-resistive gas sensors that could be used in the production, transportation or storage of this upcoming energy source.

1 .

 1 Sensor fabrication and characterization SMO gas sensors are widely used and are typically integrated in miniaturized packages with other silicon-based elements. The main sensing device architecture, based on SnO2 NWs grown on PIEs, is shown in the SEM images of Figure 1.

Figure 1 .

 1 Figure 1. SEM images of the prepared SnO2 NW sensor. (a) Top view, and (b) SEM image of

  Figure 2 is a schematic illustration of the MLD process used in this work.

Figure 2 .

 2 Figure 2. Schematic illustration of the MLD process for the preparation of the alucone-coated

Figure 3 .

 3 Figure 3. SEM image of the sensor (left panel) and EDX mapping indicating the distribution

Figure 4 .

 4 Figure 4. High-resolution XPS spectra of the (a) Sn 3d and (b) Al 2p levels of the alumina

Figure 5 .

 5 Figure 5. TEM images of NWs coated with alumina nanomembranes prepared using (a) 10

  It shows the electrical conduction channel of the networked SnO2 NW structure grown on neighbouring PIE pads. The gas-sensing properties of the SnO2 NWs gas sensor coated with alumina nanomembrane (100 cycles of alucone, 10 nm thickness) was tested in the presence of three concentrations (10, 50 and 100 ppm) of NO2 gas in dry air at temperatures ranging from 250 to 400 °C. The transient normalized resistance curves of the sensor in the presence of various concentrations of NO2 gas clearly demonstrated the n-type sensing behaviour of the sensor (Figure6bfor 100 ppm). This means that the introduction of an oxidizing gas, like NO2, results in an increase of the sensor resistance that returns to baseline values upon interruption of the gas delivery, in good agreement with the well-known modulation behaviour in the conduction channel of NWs during the interaction of gas molecules and pre-adsorbed oxygen species on the NW surface. Analysis of the sensor response to NO2 in function of the operating temperature (Figure6c) indicated that the maximum sensing responses was obtained at 300 °C for all NO2

Figure 6 .

 6 Figure 6. (a) Schematic illustration of the alumina coated SnO2 NW gas sensor; (b) Normalized

Figure 7 .Figure 8 .

 78 Figure 7. Transient resistance curves of the SnO2 NW sensors coated with an alumina layer

  ).

Figure 9 .

 9 Figure 9. Transient resistance curves of SnO2 NW sensors for NO2 (100 ppm) at 300 °C and

  The transient resistance curves and the responses of pristine and alumina-coated SnO2 NW sensors for various gases (100 ppm) in function of the relative humidity are shown in Figures S2-S5 and Figure S6, respectively.

  Therefore, another reason to explain the improvement of humidity resistance by the formation of the alumina layer should be considered. It is known that Al2O3 or Al2O3/alucone layers deposited by ALD strongly decrease water vapour transmission[6,8,33,[START_REF] Xiao | [END_REF]. Al2O3 layers can drastically improve the water vapour diffusion barrier property possibly by increasing the water vapour diffusion path in the film and by decreasing the diffusion speed by trapping water vapour chemically. The hydrophobic nature of alumina/SnO2 prepared sensor plays vital role to avoid the water vapor adsorption on oxide surface during gas exposure, which leads to fast response/recovery and baseline stability of the sensor.

Figure 10 .

 10 Figure 10. (a) The responses of gas sensors to NO2 (100 ppm) in function of the relative

Acknowledgments:

We would like to thanks the French national research agency (ANR, program MeNiNA-ANR-17-CE09-0049) and the Lebanese University for funding this project. Support by the COST Action MP1402 "HERALD", a European cooperation program, is acknowledged. I.I. acknowledges the partial financial support from the NCN of Poland by the SONATA 11 project UMO-2016/21/D/ST3/00962. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2021R1A2C1009790).