Size-controlled TiO2 nanoparticles on porous hosts for enhanced photocatalytic hydrogen production

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Size-controlled TiO2 nanoparticles on porous hosts for enhanced photocatalytic hydrogen production Chaoran Jiang a,1 , Ki Yip Lee a,1 , Christopher M.A. Parlett b , Mustafa K. Bayazit a , Chi Ching Lau a , Qiushi Ruan a , Savio J.A. Moniz a , Adam F. Lee b , Junwang Tang a,∗ a b

Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE,UK European Bioenergy Research Institute, Aston University, Birmingham B4 7ET, UK

a r t i c l e

i n f o

Article history: Received 31 August 2015 Received in revised form 17 November 2015 Accepted 2 December 2015 Available online xxx Keywords: TiO2 ZSM-5 SBA-15 Porous supports Photocatalysis Hydrogen evolution

a b s t r a c t Nanocystalline TiO2 particles were successfully synthesized on porous hosts (SBA-15 and ZSM− 15) via a sol–gel impregnation method. Resulting nanocomposites were characterized by XRD, TEM, BET surface analysis, Raman and UV–vis diffuse reflectance spectroscopy, and their photocatalytic activity for H2 production evaluated. XRD evidences the formation of anatase nanoparticles over both ZSM-5 and SBA-15 porous supports, with TEM highlighting a strong particle size dependence on titania precursor concentration. Photocatalytic activities of TiO2 /ZSM-5 and TiO2 /SBA-15 composites were significantly enhanced compared to pure TiO2 , owing to the smaller TiO2 particle size and higher surface area of the former. TiO2 loadings over the porous supports and concomitant photocatalytic hydrogen production were optimized with respect to light absorption, available surface reaction sites and particle size. 10%TiO2 /ZSM-5 and 20%TiO2 /SBA-15 proved the most active photocatalysts, exhibiting extraordinary hydrogen evolution rates of 10,000 and 8800 ␮mol gTiO2 −1 h−1 under full arc, associated with high external quantum efficiencies of 12.6% and 5.4% respectively under 365 nm irradiation. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Several decades after the discovery of titanium dioxide (TiO2 ) as a photochemical electrode for splitting water [1], it remains one of the benchmark materials for photocatalytic hydrogen production from water due to its advantageous properties including low cost, low toxicity and high chemical stability [2–5]. However, the limited efficiency of TiO2 due to its inherently poor utilization of the solar spectrum, fast bulk recombination of photo-excited charge carriers, and low surface area via conventional syntheses, constrains the practical application of tiania for large scale hydrogen production [6,7]. Fabrication of composite materials, for example through the incorporation of TiO2 nanostructures into high area, nanoporous materials such as zeolites (e.g., ZSM-5) and mesoporous silica (e.g., SBA-15, HMS,and MCM-41) proved to offer an effecient route to TiO2 /porous host nanocomposites with enhanced photocatalytic performance towards the decomposition of organic pollutants in water by TiO2 /ZSM-5 [8], TiO2 /SBA-15 [9], TiO2 /HMS

∗ Corresponding author. E-mail address: [email protected] (J. Tang). 1 These authors contributed equally to this work.

[10], and TiO2 /MCM-41 composites [11,12]. Hybrids of porous substrates and TiO2 semiconductor photocatalysts may provide several benefits for photocatalytic reactions including: (i) enhanced light absorption due to trapping (scattering and stop band effects) within the porous structure; [13] (ii) the formation of ultrafine TiO2 particles with high aspect ratios for reduced bulk charge carrier, i.e., electron (e− )/hole (h+ ) recombination; [14,15] and (iii) superior surface areas and an attendant higher surface density of active sites for photocatalytic reactions with adsorbates [16]. Among porous substrates, SBA-15 and ZSM-5 are interesting support materials for TiO2 functionalization. SBA-15 is a mesoporous silica with two-dimensional ordering of hexagonal close-packed pore channels with diameters typically spanning 5–14 nm [17], larger than HMS and MCM-41 and with a superior hydrothermal stability attributed to its thicker walls [18], providing a high area framework of 500–1000 m2 g−1 over which to disperse titania either as nanocrystals or conformal films [17,19]. In contrast, ZSM-5, a microporous silica aluminate, is widely used in heterogeneously catalyzed thermal catalysis due its strong acidity, high area, chemical stability and ion-exchange capacity [20]. However, ZSM-5 possesses far smaller pore dimensions of only 0.5 nm, rendering it unsuitable for the genesis of TiO2 nanocrystallites within the pore architecture [21]. It

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was therefore envisaged that these two distinct host materials would enable an exploration of the impact of porosity on the photocatalytic performance of entrained TiO2 photocatalysts. The performance of titania photocatalysts is known to be strongly dependent on their size/morphology, proximity to (and nature of) host supports, and their specific surface area. Particle size effects reflect a combination of both charge carrier dynamics (rates of undesired bulk and surface e− /h+ recombination versus transport to the titania surface and transfer to adsorbates), surface active site density (and thus specific surface area) [22,23], and variations in the ratio of exposed facets [24], which influence the photocatalytic activity. Size-controlled TiO2 nanoparticles on porous supports can be readily synthesized by varying the concentration of titanium species during wet-chemical routes [25], or number of deposition cycles through vapor routes such as ALD [26], and also through changing the dimensions of the host pore network(s). To date, there has been little focus on the synthesis of size-controlled TiO2 nanoparticles on porous substrates for photocatalytic hydrogen production via water splitting, which has significant potential to delivering solar fuels, compare with many reports related to environmental remediation/dye degradation. Herein, we report the post-synthetic functionalization of commercial ZSM-5 and an in-house SBA-15 with anatase TiO2 nanoparticles via a facile sol–gel impregnation route, which confer significantly enhanced photocatalytic hydrogen production compared to bulk anatase TiO2 . Various TiO2 /ZSM-5 and TiO2 /SBA-15 composite photocatalysts with varying titania contents ranged from 5 wt% to 40 wt% were prepared and their activities investigated. The effect of precursor concentration on particle size, light absorption, surface area were characterized by TEM, UV–vis absorption spectra and BET surface area analysis. The interdependent interactions of titania particle size and loading, and support porosity and surface area upon light absorption and resulting photocatalytic activity are highlighted.

2. Experimental details 2.1. Sample preparation SBA-15 was prepared adapting the procedure reported by Zhao et al. [18]. Pluronic P123 (10 g) was dissolved in water (74.5 ml) and hydrochloric acid (2 M, 291.5 ml) with stirring at 35 ◦ C. Tetraethoxysilane (23.4 ml) was added and left for 20 h with agitation. The resulting gel was aged under sealed conditions for 24 h at 80 ◦ C without agitation. The solid was filtered, washed with water (1000 ml) and dried at room temperature before calcination at 500 ◦ C for 6 h in air (ramp rate 1 ◦ C min−1 ). A commercially available zeolite ZSM-5 (Alfa Aesar, ammonium salt, 425 m2 g−1 , SiO2 :Al2 O3 molar ratio of 80) was used as purchased. A modified literature protocol was employed to incorporate nanocrystalline TiO2 into the porous supports via sol–gel impregnation [25,27]. Briefly, precursor solution with the requisite amount of titanium (IV) tetraisopropoxide (TTIP, Sigma–Aldrich, 97%) in 10 ml absolute ethanol (Merk, 99.5%) was prepared, which was hydrolyzed by HCl (37%) addition to form Ti4+ ions as indicated by the solution turning clear. A fixed amount of each support (ZSM-5 or SBA-15) was added slowly to the preceding sol and stirred at room temperature until evaporation of the ethanol was complete, yielding titanium impregnated composite materials. The resultant white solids were subsequently calcined in a muffle oven at 550 ◦ C for 4 h at a ramp rate of 5 ◦ C min−1 . A reference, pure TiO2 was also prepared via the sol–gel procedure described above. These materials are denoted throughout as wt% TiO2 /support, e.g., 10%TiO2 /ZSM-5 or 20%TiO2 /SBA-15.

2.2. Sample characterization Wide angle powder X-ray diffraction (XRD) patterns were obtained using a Bruker D4 powder diffractometer with a current of 30 mA, voltage of 40 kV, and Cu K˛ ( = 1.540562 and 1.544398 nm) radiation. Small angle X-ray scattering (SAXS) was performed using a Siemens D500 diffractometer using Ni-filtered Cu K˛ radiation at 40 kV and 30 mA. Raman spectra were recorded on a Renishaw InVia Raman spectrometer in a back scattered confocal configuration using 514 nm laser excitation. All spectra were recorded on solid samples in the range of 100–2000 cm−1, and referenced to the silicon line at 520 cm−1 . Transmission electron microscopy (TEM) was performed using a Jeol JEM− 1010 microscope coupled with an Oxford Instruments EDS detector. TEM samples were prepared from stable dispersions of particles dispersed in ethanol and dropped on carbon coated copper grids. Particle size distributions were estimated through measurement of more than 100 particles using ImageJ software. Statistical analysis was undertaken in Origin9.1 OriginLab software. Diffuse reflectance UV–vis spectra (DRUVS) were obtained on a Shimadzu UV–vis 2550 spectrophotometer with an integrating sphere. Reflectance spectra were recorded on powdered samples using barium sulphate as a reference, and subsequently converted to absorption spectra through application of the Kubelka-Mulk formalism. Nitrogen adsorption–desorption isotherms were recorded at 77 K using a MicromeriticsTristar 3000 model on samples degassed at 130 ◦ C for overnight. Specific surface areas were determined according to BET model, with pore diameters, volumes and distributions determined through the BJH method for mesoporous TiO2 /SBA-15 composites. 2.3. Photocatalytic activity test Hydrogen production was performed in a bespoke Pyrex batch reactor with quartz window (diameter 3.6 cm, total volume 700 ml), employing a 300 W Xe lamp (Newport). Evolved H2 was measured through hourly sampling of gas in the reactor headspace via a gas-tight syringe and gas chromatography (Varian GC 430, TCD detector, Ar carrier gas 99.999%). For a typical reaction, 0.1 g of photocatalyst was suspended in a 200 ml aqueous solution with 10% methanol as a hole quenching agent (sacrificial electron donor). The solution was subsequently sonicated in an ultrasonic bath (UW U500H) for 30 min to ensure a homogeneous catalyst dispersion. 2 wt% platinum (fixed Pt to TiO2 mass ratio), which was in situ photodeposited on the titania nanocomposites as a H2 evolution co-catalyst from an aqueous solution of chloroplatinic acid hexahydrate (H2 PtCl6 ·6H2 O, Sigma–Aldrich, 37.5% Pt basis) following literature methods [28]. The reactor was then sealed and purged with Ar for 30 min to remove air and degas the solution completely prior to irradiation. While photocatalytic performance is dependent on the number of incident photons, and quantum efficiency of their utilization in chemical conversion, the overwhelming majority of photocatalytic studies report activity in units of mmolproduct gcatalyst −1 time−1 , i.e., without reference to the number of incident photons. In large part this reflects the wide variance in light source (notably power and spectral wavelength) and photoreactor design (notably size and aspect ratio) employed, which hamper quantification of the light flux impinging on catalysts within reactors. Hence a similar unit of activity is used in the present work to facilitate comparison with literature, although we also report quantum efficiencies for the two optimal catalyst formulations (see below). 2.4. Quantum efficiency (QE) measurement Quantum efficiencies were determined through the suspension of either a 10%TiO2 /ZSM-5 or 20%TiO2 /SBA-15 sample in aque-

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Fig. 1. Wide angle XRD patterns of: (a) TiO2 /ZSM-5 composites; and (b) TiO2 /SBA-15 composites. Inset shows low angle XRD patterns of pure SBA-15 and 20%TiO2 /SBA-15.

ous solution with 10% methanol and subsequent irradiation by a 300 W Xe lamp with 365 nm band-pass filter for 1 h. Hydrogen evolution measured by gas chromatography revealed 3.7 ␮mol and 1.6 ␮mol H2 respectively for the 10%TiO2 /ZSM-5 and 20%TiO2 /SBA15 photocatalysts. The average light intensity measured using a silicon photodiode and Newport Optical Meter (Model 1918-R) was 165 × 10−6 W cm−2 across an irradiated area of 32.17 cm2 . The number of incident photons (N) was calculated as 3.5 × 1019 from Eq. (1): N =

E 165 × 10 − 6W cm−2 × 32.17cm2 × 3600S × 365 × 10 − 9m = 3.5 = hc 6.626 × 10−34 J S × 3 × 108 m s−1 × 1019

(1)

The resulting quantum efficiency was calculated from Eq. (2): QE of10%TiO2 /ZSM − 5 =

2 × evolved H2 molecules × 100% N

Fig. 2. Raman spectra of exemplar TiO2 /ZSM-5 and TiO2 /SBA-15 composites.

(2)

3. Results and discussion Wide angle XRD patterns of TiO2 /ZSM-5 composites are shown in Fig. 1a as a function of titania loadings (up to 40 wt%), which exhibit characteristic diffraction peaks associated with the zeolitic structure [29]. Only the 40%TiO2 /ZSM-5 sample displayed strong diffraction peaks indicative of anatase TiO2 at 2 = 25.5◦ , 37.7◦ , 48.1◦ , 53.9◦ , 55.0◦ and 62.9◦ , assigned to (1 0 1), (0 0 4), (2 0 0), (1 0 5), (2 1 1) and (2 0 4) reflections respectively [30], in good agreement with JCPDS 21-1272. The TiO2 /SBA-15 nanocomposite photocatalysts were characterized by both low and wide angle XRD. The low angle patterns of the parent SBA-15 and TiO2 /SBA15 composites exhibit three diffraction peaks indexed as (1 0 0), (1 1 0) and (2 0 0) reflections (Fig. 1b, inset) in accordance with the expected 2D hexagonal mesoporous structure [14]. It is noteworthy that the (1 0 0) reflection is suppressed and broadened after TiO2 incorporation, indicating a slight lowering of the degree of mesopore ordering [14,31]. Fig. 1b shows the corresponding wide angle XRD patterns for TiO2 /SBA-15, which show a weak, broad feature around 2 = 20–25◦ for the parent support [32], and these features are consistent with anatase titania for higher TiO2 loadings, akin to the TiO2 /ZSM-5 composite samples. These observations are in accordance with reports that sol–gel routes to titania generally produce phase pure anatase [33]. The absence of titania features for low loadings (5%) on SBA-15 likely indicates the presence of highly dispersed titania as crystallites with dimensions below the instrumental sensitivity limit. Raman spectroscopy is a powerful technique for the analysis of dilute titanias (0.05 wt%) [34,35], and hence was employed to confirm the presence and identify the phase of titania within the low loading ZSM-5 and SBA-15 materials. Consistent with previous literature, characteristic Raman bands were observed

for both TiO2 /ZSM-5 and TiO2 /SBA-15 samples at 147, 397, 515 and 639 cm−1 , attributed to the Eg(1) , Blg(1) , Alg + Blg(2) , and Eg(2) vibrational modes of anatase TiO2 (Fig. 2) [36], confirming the successful synthesis of anatase nanoparticles on the porous supports in agreement with XRD and other reported TiO2 /ZSM-5 [37] and TiO2 /SBA-15 materials [38]. Fig. 3a and b shows the DRUVS spectra of TiO2 /ZSM-5 and TiO2 /SBA-15 photocatalysts as a function of TiO2 content, alongside the corresponding reference supports. No optical absorption was observed for either pure ZSM-5 [37] or SBA-15 [39]. Hence the absorption band edges observed between 360 and 400 nm in the titania functionalized materials must be associated with anatase. The band gap (Eg ) of samples was determined via a Tauc plot of the square of the Kubelka-Munk function versus photon energy [11]. The band-gap decreased from 3.5 to 3.1 eV with decreasing titania loading from 5% to 40% over TiO2 /ZSM-5, and 3.6–3.3 eV for the TiO2 /SBA-15 composite; these compare with 3.2 eV for bulk anatase (see Tables 1 and 2). Furthermore, the absorption bands of both photocatalyst families shift to longer wavelength with increasing titania, commensurate with an increasing in TiO2 particle size [40–42], as previously reported for supported TiO2 [43] and ascribed to quantum effect that emerge for TiO2 semiconductor with dimensions <10 nm. Hence, DRUVS confirms the presence of dispersed titanias over the ZSM-5 and SBA-15 mesoporous frameworks, whose crystallite size increases with TiO2 loading. N2 adsorption–desorption isotherms of all TiO2 /ZSM-5 and TiO2 /SBA-15 composites were indicative of the parent microporous and mesoporous materials (Fig. S1) [44]. Surface areas and mean mesopore diameters derived from porosimetry (Fig. S1b and S2) are summarized in Tables 1 and 2. The BET surface area of TiO2 /SBA15 composites decreased with increasing TiO2 loading, attributed to the growth of TiO2 particles within the framework (in addition to external surface) and associated partial pore blockage [9]. Com-

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Fig. 3. DRUV spectra of (a) TiO2 /ZSM-5 and (b) TiO2 /SBA-15 composites as a function of titanium loading. Table 1 Physicochemical properties of TiO2 /SBA-15 composite photocatalysts and corresponding photocatalytic hydrogen production. Support

TiO2 loading/wt%a

Band gap/eVb

Particle size/nmc

Surface aread /m2 g−1

Mesopore diameter/nme

H2 evolution rate/␮mol gTiO2 −1 h−1

SBA15

0 5 10 20 40 100

– 3.6 3.6 3.5 3.3 3.2

– – – 5 – 21

755.4 – – 527.2 390.3 30.8

5 – – 4 5 7

0

a b c d e

7640 8790 4470 1440

ICP. DRUVS. TEM. BET. BJH.

Table 2 Physicochemical properties of TiO2 /ZSM-5 composite photocatalysts and corresponding photocatalytic photocatalytic hydrogen production. Support

TiO2 loading/wt%a

Band gap/eVb

Particle size/nmc

Surface aread /m2 g−1

H2 evolution rate/␮mol gTiO2 −1 h−1

ZSM5

0 5 10 20 40 100

– 3.5 3.4 3.3 3.1 3.2

– 5 7 – 13 21

425 420 401 – 248 31

0 6829 10060 8862 2836 1435

a b c d

ICP. DRUVS. TEM. BET.

pared with the SBA-15 parent, the mesopore diameter narrowed even for low TiO2 loadings likely due to the growth of TiO2 nanoparticles within the mesopore channels. The increased mesopore diameter observed for the 40%TiO2 /SBA-15 sample is attributed to the formation and aggregation of discrete TiO2 nanoparticles and associated interparticle voids [40]. Similar trends were observed for TiO2 /ZSM-5 composites, in agreement with previous work [45], with mesoporosity again arising from interparticle voids due to aggregation of separate TiO2 /ZSM-5 crystallites. TEM analysis confirmed the formation of TiO2 nanoparticles over the support surface. Fig. 4a and b shows typical TEM micrographs of the parent ZSM-5 and TiO2 /ZSM-5, highlighting TiO2 particles uniformly dispersed across the zeolite surface without disrupting the ZSM-5 framework. The TiO2 particle size in TiO2 /ZSM-5 composites increased with titanium loading consistent with DRUVS. Although only a few isolated TiO2 particles were observed by TEM for low TiO2 loadings, the number of such unsupported particles increased at higher loadings (ESI Fig. S3). A concomitant rise in titania particle size was observed from 4.6 to 12.9 nm as the loading rose from 10% to 40%.

Fig. 5 shows the corresponding TEM images for pure SBA-15 and 20%TiO2 /SBA-15 samples. A well-ordered hexagonal mesopore array can be observed for both samples, characteristic of the SBA15 p6mm structure [46]. Mesopores were approximately 5.2 nm, in agreement with the values from porosimetry (∼5.39 nm). TiO2 nanoparticles with mean sizes of 4.8 nm were deposited randomly both on the external silica surface and within the two–dimensional pore channels, which induced some disordering within mesopores near the surface. As the TiO2 loading increased from 20% to 40% a small number of isolated TiO2 particles were observed, probably due to extensive particle agglomeration (Fig. S4). Compared with unsupported TiO2 crystals prepared using the same method which were around 21 nm (Fig. S5), the oxide supported titania nanoparticles were far smaller over both ZSM-5 and SBA-15, demonstrating that the porous supports suppressed particle aggregation, particularly at low titania loadings. Fig. 6a and b display reaction profiles for the time-dependent evolution of H2 over platinized TiO2 /ZSM-5 and TiO2 /SBA-15 composite photocatalysts as a function of titania loading. All samples were tested in the presence of methanol as a hole scavenger, with

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Fig. 4. TEM micrographs of (a) pure ZSM-5 and (b) TiO2 /ZSM-5 composite.

Fig. 5. TEM micrographs of (a) pure SBA-15 and (b) 20%TiO2 /SBA-15 composites.

Fig. 6. Hydrogen evolution of various TiO2 based samples under a 300W Xe lamp illumination in the presence of 2 wt% Pt, and methanol as an hole scavenger of (a) Pure TiO2 and TiO2 /ZSM-5, (b) Pure TiO2 and TiO2 /SBA-15. Control experiment on the photoactivity of pure ZSM-5(left) and SBA-15(right) are also shown.

2 wt% Pt as a co-catalyst under broad spectrum UV–vis light irradiation. Control experiments evidenced no hydrogen evolution over either pure ZSM-5 or pure SBA-15 under illumination, indicating that TiO2 was the only photoactive component of both composites. For all samples, H2 evolution increased nearly linearly with reaction time, indicating that platinized TiO2 /ZSM-5 and TiO2 /SBA-15 composites are stable photocatalysts under our reaction conditions. Fig. 6a shows that H2 productivity over platinized TiO2 /ZSM-5 decreased in the order: 10% > 20% > 5% > 2.5% > 40%> unsupported TiO2 , with a similar trend observed for TiO2 /SBA-15 composites in Fig. 6b (20% > 10% > 40% > 5%>unsupported TiO2 ). TiO2 nanoparticle size generally increased with loading over both porous supports

(Fig. S3), and is anticipated to be a crucial factor influencing photocatalytic hydrogen production. In general, smaller TiO2 particles exhibit higher activity due to suppressed charge carrier bulk recombination [47]. Smaller particles also afford higher surface areas and hence active site densities for heterogeneously catalyzed reactions [48]. However, photocatalytic activity does not monotonically increase with particle size. This is attributed to the tendency of extremely small TiO2 particles to suffer enhanced surface holeelectron recombination due to surface defects [47]. A volcano dependence of photocatalytic performance on nanocrystalline TiO2 particle size is thus anticipated. In our synthetic strategy, increasing the TiO2 loading from 5 to 20% had little impact on nanoparticle size,

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which remained small over both supports (Tables 1 and 2), resulting in an approximately first order hydrogen productivity dependence on titania loading: Fig. 6a shows that the H2 evolution rate of 10%TiO2 /ZSM-5 is almost twice that of 5% TiO2 /ZSM-5, with both samples possessing very similar particle sizes and surface areas (see Fig. 3 and Table 2). This observation confirms that our catalytic measurements are free from undesired mass-transport limitations. XRD, DRUVS, and TEM demonstrate that subsequent increases in TiO2 loading to 40 wt% lowered the H2 evolution rate due to an increase in particle size. The performance of any specific composite is thus observed to be proportional to titania loading, and inversely proportional to nanoparticle size, resulting in a volcano dependence for both composite families in accordance with the literature. High TiO2 loadings afford a large number of active TiO2 sites, and improved light absorption, at the expense of nanoparticle agglomeration and enhanced (undesired) charge carrier recombination. Zhang et al. reported 10 nm crystallites as optimal for the photocatalytic decomposition of chloroform [49], while Anderson et al. synthesised TiO2 /SBA-15 composites for the photocatalytic destruction of R-6G and observed similar trends to the current work [50], consistent with previous literature reports [51–53]. In this work, the 10%TiO2 /ZSM-5 sample (6.7 nm anatase crystallites) exhibited the highest photocatalytic activity with respect to H2 evolution of 10,000 ␮mol gTiO2 −1 h−1 , comparable to the benchmark efficiency of visible light excited black TiO2 nanocrystals [54]. Hydrogen productivity of 8800 ␮mol gTiO2 −1 h−1 was achieved for 20%TiO2 /SBA-15 (4.8 nm anatase). The 10 wt% TiO2 /ZSM-5 and 20 wt% TiO2 /SBA-15 catalysts are almost an order of magnitude more active than unsupported TiO2 , attributed to their enhanced surface areas, lower rates of photogenerated e− /h+ recombination and increased thermal stability [45,55]. It is worthwhile noting that the optimal 20%TiO2 /SBA-15 photocatalyst is slightly less active than 10%TiO2 /ZSM-5 counterpart, which may arise from the higher density of homogenously dispersed TiO2 particles across ZSM-5, compared with the randomly dispersed particles observed on the external and internal surfaces (channels) of SBA-15 (Figs. 4 and 5); the latter in-pore titania particles may be less effective for H2 production due to poor light transmission and slow water diffusion throughout the mesopores. Fig. 5b shows that even in the optimal 20%TiO2 /SBA-15 the majority of TiO2 nanoparticles are located near the SBA-15 selvedge, which may introduce structural disordering [56]. Accordingly, the 20%TiO2 /SBA-15 photocatalyst exhibits an external quantum yield of 5.4% at 365 nm, half of the 10%TiO2 /ZSM5 sample at 12.6% at the same wavelength. In summary, sol–gel impregnation of microporous zeolite and mesoporous SBA-15 silica supports afford a simple means to create and optimize the density of photoactive anatase nanoparticles, and hence their optical and photocatalytic performance in H2 production.

4. Conclusions We have demonstrated a facile and efficient approach to significantly improve the photocatalytic activity of TiO2 for hydrogen production via incorporation into microporous ZSM-5 and mesoporous SBA-15 via a sol–gel impregnation route. The surface area of anatase TiO2 nanoparticles dramatically increase through interaction with these porous supports, with the titania loading emerging as the key parameter influencing photocatalytic actvity: 10%TiO2 /ZSM-5 and 20%TiO2 /SBA-15 photocatalysis with respective particle sizes of 6.7 and 4.8 nm, are the most active for hydrogen evolution, with rates of 10,000 and 8800 ␮mol gTiO2 −1 h−1 . These performances are comparable to that of visible light driven black TiO2 , offering very high quantum yields reaching 12.6% at 365 nm. The enhanced photocatalytic performance is attributed to the high support surface areas, which enable the stabilization of well-

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