Hydrogen production from water splitting under visible light irradiation using sensitized mesoporous-assembled TiO2–SiO2 mixed oxide photocatalysts

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Hydrogen production from water splitting under visible light irradiation using sensitized mesoporous-assembled TiO2eSiO2 mixed oxide photocatalysts Natee Rungjaroentawon a, Surakerk Onsuratoom a, Sumaeth Chavadej a,b,* a b

The Petroleum and Petrochemical College, Chulalongkorn University, Soi Chula 12, Phyathai Road, Pathumwan, Bangkok 10330, Thailand Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn University, Bangkok 10330, Thailand

article info

abstract

Article history:

This work focused on hydrogen production from the photocatalytic water splitting under

Received 15 February 2012

visible light irradiation using Eosin Y-sensitized mesoporous-assembled TiO2eSiO2 mixed

Received in revised form

oxide photocatalysts, of which the mesoporous-assembled TiO2eSiO2 mixed oxides with

19 April 2012

various TiO2-to-SiO2 molar ratios were synthesized by a solegel process with the aid of

Accepted 24 April 2012

a structure-directing surfactant. The effects of SiO2 content, calcination temperature, and

Available online 26 May 2012

phase composition of the mixed oxide photocatalysts were investigated. The experimental

Keywords:

ratio of 97:3 and calcined at 500
C provided the maximum photocatalytic hydrogen

TiO2eSiO2 mixed oxide

production activity. The characterization results supported that the 0.97TiO2e0.03SiO2

results showed that the TiO2eSiO2 mixed oxide photocatalyst with the TiO2-to-SiO2 molar

Mesoporosity

mixed oxide photocatalyst (with the suitable SiO2 content of 3 mol%) possessed superior

Photocatalysis

physicochemical properties for the photocatalytic reaction as compared to the pure TiO2,

Hydrogen production

particularly higher specific surface area, lower mean mesopore diameter, higher total pore

Visible light

volume, and lower crystallite size, which led to an enhanced photocatalytic activity. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

The combustion of conventionally available energy resources, especially coal and fossil fuels, produces a large amount of carbon dioxide (CO2), causing the global warming. In addition, their consumption is rapidly increasing while their reserves are decreasing [1,2]. A possible solution is to use renewable energy resources, such as wind energy, tide power, geothermal energy, and solar energy. Particularly, many attempts have focused on hydrogen as an alternative and renewable energy carrier to meet future demands as it has been recognized as a potential energy source due to its versatile applications and environmentally friendly properties

without generating harmful products (e.g. CO2, smog, and particulates) to the environment [3e5]. The photocatalytic water splitting is an ideal method for producing hydrogen by using two major renewable sources, i.e. water and solar energy [6e10]. The use of a semiconductor photocatalyst for this reaction is a promising technique with many advantages: it is in a solid-phase form, it is relatively inexpensive, it is safe for operation, and it is resistant to deactivation [11e14]. Titanium dioxide (TiO2) has been considered as the most powerful photocatalyst because it is highly photoactive, inexpensive, non-toxic, chemically stable, and environmentally friendly. However, it can be activated only under UV light irradiation (l < 400 nm, w4% of the

* Corresponding author. The Petroleum and Petrochemical College, Chulalongkorn University, Soi Chula 12, Phyathai Road, Pathumwan, Bangkok 10330, Thailand. Tel./fax: þ66 2 218 4139. E-mail address: [email protected] (S. Chavadej). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.04.120

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coming solar energy) because of its large energy band gap (i.e. 3.2 eV for anatase TiO2) [12,15e17]. For further improvement of effective utilization of the solar energy, considerable efforts have been made to shift its photocatalytic hydrogen production activity to the visible light region (l > 400 nm, w42% of the coming solar energy) [18e20]. The development of photocatalytic water splitting system capable of using the visible light region of the solar spectrum can be achieved by many methods. Dye sensitization is an efficient method, in which a dye sensitizer added to a photocatalytic system is excited by visible light while the excitedstate dye sensitizer molecules inject electrons into the adjacent conduction band of a semiconductor photocatalyst. Afterward, the electrons in the conduction band move along the photocatalyst crystalline structure to its surface and reduce water molecules to produce hydrogen [21e24]. In this regard, an efficient electron transport to reach such substrate molecules strongly depends on physicochemical properties of a TiO2-based photocatalyst, e.g. specific surface area, crystallinity, and porous structure. Doping/incorporating of a TiO2 with the other suitable metal oxides is an effective technique to improve its physicochemical properties and consequent photocatalytic activity. Particularly, doping TiO2 with SiO2 has been verified to improve the photocatalytic activity of the pristine TiO2 for various photocatalytic reactions due to the thermal stability-enhancing effect of the doped SiO2 [25e32]. The use of TiO2eSiO2 mixed oxide photocatalysts for the photocatalytic water splitting under UV light irradiation has been studied in only a few literature [33e35]; however, to our knowledge, the application of a TiO2eSiO2 mixed oxide photocatalyst for the dye-sensitized water splitting under visible light irradiation has never been reported. Therefore, the purpose of this work was, for the first time, to optimize the composition of TiO2eSiO2 mixed oxides for achieving a maximum hydrogen production activity from the Eosin Y-sensitized water splitting under visible light irradiation in the presence of diethanolamine electron donor. The mesoporous-assembled TiO2eSiO2 mixed oxide nanocrystal photocatalysts with various TiO2-to-SiO2 molar ratios were synthesized by a solegel process with the aid of a structuredirecting surfactant and calcined under different thermal conditions. The effects of mixed oxide composition and calcination temperature on the photocatalyst’s physicochemical properties and hydrogen production activity were mainly investigated.

2.

Experimental

2.1.

Materials

Tetraisopropyl orthotitanate (TIPT, Ti(OCH(CH3)2)4, Merck), tetraethyl orthosilicate (TEOS, Si(OC2H5)4, Merck), laurylamine hydrochloride (LAHC, CH3(CH2)11NH2,HCl, Merck), acetylacetone (ACA, CH3COCH2COCH3, Carlo Erba), diethanolamine (DEA, (HOCH2CH2)2NH, Ajax), and Eosin Y (E.Y., C20H6Br4Na2O5, Aldrich) were used for the photocatalyst synthesis and photocatalytic activity tests. They were analytical reagent grade and used without further purification. TIPT and TEOS were used as titanium and silicon

precursors for synthesizing TiO2eSiO2 mixed oxide photocatalysts. LAHC was used as a structure-directing surfactant, behaving as the mesopore-forming agent. ACA served as a modifying agent to moderate the hydrolysis processes of the Ti and Si precursors. DEA and E.Y. were employed as an electron donor and a dye sensitizer for the photocatalytic sensitized hydrogen production, respectively. The commercial P-25 TiO2 (J.J. Degussa Hu¨ls Co., Ltd.) was used for comparative study on photocatalytic activity.

2.2.

Photocatalyst synthesis

The mesoporous-assembled TiO2eSiO2 mixed oxide nanocrystal photocatalysts were synthesized via a solegel process with the aid of a structure-directing surfactant. Firstly, the TIPT was homogeneously mixed with the TEOS at different TIPT-to-TEOS molar ratios. A specified amount of the ACA was then introduced into the TIPT/TEOS mixture with a (TIPT þ TEOS)-to-ACA molar ratio of 1:1. Then, the mixed solution was homogenized by continuously stirring at room temperature. Afterward, a 0.1 M LAHC aqueous solution was added into the ACA-modified TIPT/TEOS solution, in which the (TIPT þ TEOS)-to-LAHC molar ratio was controlled at 4:1. The mixed solution was kept continuously stirring at 40
C to obtain transparent yellow sol due to the hydrolysis process. The gel was then formed by placing the sol-containing solution into an oven at 80
C for a week for complete gel formation. Afterward, the gel was dried at 80
C and finally calcined between 500 and 800
C for 4 h to remove the LAHC surfactant and subsequently obtain the desired photocatalysts.

2.3.

Photocatalyst characterizations

The simultaneous thermogravimetric and derivative thermogravimetric analyzer (TGeDTA, Perkin Elmer, Pyris Diamond) was used to study the thermal decomposition behavior of the as-synthesized dried gels and obtain a suitable calcination temperature for removing the LAHC surfactant. A dried gel was heated from 50 to 800
C with a heating rate of 10
C/ min in a static air atmosphere with a-Al2O3 as the reference. The surface area was measured by a BET surface area analyzer (Quantachrome, Autosorb I). A photocatalyst sample was outgassed to remove the humidity and volatile species adsorbed on its surface under vacuum at 150
C for 4 h prior to the analysis. Then, N2 was purged to adsorb on the surface, and the quantity of gas adsorbed onto or desorbed from the surface at some equilibrium vapor pressures was measured by static volumetric method. The volume-pressure data were used to determine the BET specific surface area, mean mesopore diameter, and total pore volume. The X-ray diffraction (XRD) was used to identify crystalline phases present in the samples by using a Bruker AXS system (D8 Advance) with ˚ ) at a copper tube for generating Cu Ka radiation (1.54056 A 40 kV and 30 mV and a nickel filter. A photocatalyst sample was pressed into a hollow of glass holder and held in place by glass window. Then, it was scanned in the 2q range of 20
e60
in the continuous mode with the rate of 1
/min and scan step of 0.02 (2q). The crystallite size was calculated from the XRD data using X-ray line broadening. The UVevisible spectrophotometer (Shimadzu, UV-2550) was used to identify

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absorption ability of the photocatalysts. The analysis was operated under scanning wavelengths of 300e600 nm using BaSO4 as the reference. The sample morphology was observed by a scanning electron microscope (SEM, Hitachi, S-4800) and a transmission electron microscope (TEM, JEOL, JEM 2100). For the SEM analysis, the sample was coated with Au before measurement for improving conductivity of sample. For the TEM analysis, the photocatalyst samples were ground into fine powder and ultrasonically dispersed in ethanol. A small droplet of the suspension was deposited on a copper grid with polyvinyl desiccate, and the solvent was evaporated prior to loading the sample into the microscope. The SEM and TEM analyses were carried out at accelerating voltages of 1.5 kV and 200 kV, respectively. An energy-dispersive X-ray (EDX) analyzer attached to the SEM was also employed to analyze for the existence of chlorine on the synthesized photocatalysts, and no chlorine was found on the samples, indicating that the LAHC surfactant molecules were completely removed during the calcination step.

2.4.

Photocatalytic hydrogen production system

Photocatalytic activity tests were performed in a closed-gas Pyrex glass reactor. In a typical run, a specified amount (0.2 g) of each of the photocatalysts was suspended in 150 ml of 15 vol.% DEA aqueous solution containing dissolved 0.1 mM E.Y. sensitizer by using a magnetic stirrer. Prior to the reaction, the mixture was purged with Ar gas for 30 min in dark environment to establish the adsorption equilibrium, as well as to remove the air from the system. The reaction was started by exposing the mixture to visible light irradiation from a 300 W Xe arc lamp (Type KXL-300, WACOM Electric, light intensity ¼ 2.6 mW/cm2), emitting light with wavelength longer than 400 nm using a UV cut-off filter (B-48S, ATG). The gaseous sample in the reactor headspace was periodically withdrawn by a gas-tight syringe and analyzed for hydrogen content by a gas chromatograph (GC, Perkin Elmer, ARNEL, Argon gas) equipped with a thermal conductivity detector (TCD).

3.

Results and discussion

3.1.

Photocatalyst characterization results

3.1.1.

TGeDTA results

Fig. 1 e TGeDTA curves of the dried synthesized photocatalysts: (a) pure TiO2 and (b) 0.97TiO2e0.03SiO2 mixed oxide.

the photocatalyst crystallization process. Therefore, the calcination temperature in the range of 500e800
C was used to investigate its effect on physicochemical properties and consequent photocatalytic hydrogen production activity of the synthesized photocatalysts.

3.1.2.

Fig. 1 exemplifies the TGeDTA curves of the dried pure TiO2 and 0.97TiO2e0.03SiO2 mixed oxide gels. The DTA curves show three main exothermic regions, as summarized in Table 1. The first region with its position lower than 150
C is attributed to the removal of physisorbed water molecules. The second region between 150 and 320
C is attributed to the burnout of the LAHC surfactant molecules [7]. The third region between 320 and 500
C corresponds to the crystallization process of the photocatalysts, as well as the removal of organic remnants and chemisorbed water molecules [36,37]. The TG curves reveal that the weight losses ended at a temperature of approximately 500
C for both dried photocatalysts. Therefore, the calcination temperature of 500
C was sufficient for both the complete surfactant removal and

N2 Adsorptionedesorption results

The N2 adsorptionedesorption isotherms of the pure TiO2 and 0.97TiO2e0.03SiO2 mixed oxide photocatalysts calcined at 500
C are exemplified in Fig. 2. Both of the samples exhibit typical IUPAC type IV pattern with H2-type hysteresis loop, which is the major characteristic of a mesoporous material (mesoporous size between 2 and 50 nm) according to the classification of IUPAC [38]. A sharp increase in the adsorption curves at a high relative pressure (P/P0) implies a capillary condensation of N2 molecules inside the mesopores, implying the well-uniform mesopores and narrow pore size distributions since the P/P0 position of the inflection point is directly related to the pore dimension. The insets of Fig. 2 show the pore size distributions calculated from the desorption branch of the isotherms. The pure TiO2 and 0.97TiO2e0.03SiO2 mixed oxide photocatalysts possess very narrow pore size distributions entirely locating in the mesoporous region. Table 2 shows the textural properties

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Table 1 e Thermal decomposition results of the dried synthesized pure TiO2 and 0.97TiO2e0.03SiO2 mixed oxide photocatalysts from TGeDTA analysis. Photocatalyst

TiO2 0.97TiO20.03SiO2

Position of exothermic peak (
C)

Corresponding weight loss (wt.%)

1st region

2nd region

3rd region

1st region

2nd region

3rd region

Total

30e150 30e150

150e320 150e320

320e500 320e500

1.80 1.72

19.51 24.03

17.00 14.99

38.31 40.74

from the N2 adsorptionedesorption analysis of the pure TiO2 and TiO2eSiO2 mixed oxide photocatalysts calcined at various temperatures. It is clearly observed that at the calcination temperature of 500
C, the incorporation of SiO2 into TiO2 resulted in an increase in the specific surface area, as can be seen from the results that for example, by incorporating 3 mol% SiO2 into TiO2 to obtain the 0.97TiO2e0.03SiO2 mixed oxide (which exhibited the highest photocatalytic activity among the mixed oxide series, as shown later), the specific surface area increased from 55.3 to 162 m2 g1. The presence of this second metal oxide with an appropriate amount could retard crystallization process and affect the growth of bulk material, which was confirmed by the XRD analysis in the next section, resulting in higher specific surface areas of the mixed oxide photocatalysts [25e32]. In case of increasing calcination temperature from 500 to 800
C, it can be seen from Table 2 that the 3 mol% SiO2-incorporated TiO2 (i.e. 0.97TiO2e0.03SiO2 mixed oxide)

Fig. 2 e N2 adsorptionedesorption isotherms and pore size distributions (inset) the synthesized mesoporousassembled photocatalysts calcined at 500
C: (a) pure TiO2 and (b) 0.97TiO2e0.03SiO2 mixed oxide.

could retain its specific surface area much more than the pure TiO2 at high calcination temperatures. The specific surface area of the pure TiO2 was smaller and decreased more quickly from 55.3 to 3.5 m2 g1 when increasing calcination temperature from 500 to 800
C as compared to the 0.97TiO2e0.03SiO2 mixed oxide, of which its specific surface area decreased from 162 to 73.5 m2 g1 when increasing calcination temperature from 500 to 800
C. As expected, the observed loss in the specific surface area with increasing calcination temperature beyond 500
C for both photocatalysts is possibly because of the pore collapse due to both the destruction of walls separating the mesopores upon the crystallization and the grain growth of the photocatalyst crystallites, which consequently led to an increase in the mean mesopore diameter with a simultaneous decrease in the total pore volume (Table 2).

3.1.3.

XRD results

The XRD patterns of the mesoporous-assembled TiO2eSiO2 mixed oxide photocatalysts with different TiO2-to-SiO2 molar ratios calcined at 500
C are shown in Fig. 3. The XRD pattern of the pure TiO2 shows crystalline structure of the pure anatase phase. The dominant peaks at 2q of about 25.2
, 37.9
, 48.3
, 53.8
, and 55.0
, which represent the indices of (101), (103), (200), (105), and (211) planes, respectively [39], correspond to the crystalline anatase TiO2 phase. The mixed oxide samples with TiO2-to-SiO2 molar ratios of 99:1, 97:3, 95:5 and 93:7 also show diffraction peaks attributed to the anatase TiO2. Although the SiO2 was incorporated up to 7 mol%, the crystalline structure of the mixed oxide was still the anatase TiO2. Hence, the incorporated SiO2 at such low contents did not significantly affect the crystalline structure of the synthesized TiO2eSiO2 mixed oxide photocatalysts. However, the peak intensities gradually decreased with increasing SiO2 content possibly because a further increase in the SiO2 content more greatly inhibits the crystallization process of the TiO2. Fig. 4 shows the XRD patterns of the synthesized mesoporous-assembled pure TiO2 and 0.97TiO2e0.03SiO2 mixed oxide photocatalysts calcined at various temperatures between 500 and 800
C. As shown in Fig. 4a, the XRD pattern of the pure TiO2 photocatalyst calcined at 500
C shows crystalline structure of the pure anatase phase, as mentioned above. The pure TiO2 photocatalyst underwent the anataseto-rutile phase transformation beginning at 600
C, resulting in the combination of the anatase and rutile phases, and the complete transformation from the anatase to rutile was found at 800
C. The occurrence of the dominant peaks at 2q of about 27.5
, 36.0
, 39.0
, 41.2
, 44.1
, 54.2
, and 56.7
, which correspond to the indices of (110), (101), (200), (111), (210), (211), and (220) planes, respectively [39], indicates the presence of the rutile phase. The rutile ratio (WR) in terms of its weight

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Table 2 e N2 adsorptionedesorption results of the synthesized mesoporous-assembled pure TiO2 and TiO2eSiO2 mixed oxide photocatalysts calcined at various temperatures. Photocatalyst Pure TiO2 0.99TiO20.01SiO2 0.97TiO20.03SiO2 0.95TiO20.05SiO2 0.93TiO20.07SiO2 Pure TiO2

0.97TiO20.03SiO2

Calcination temperature (
C)

Specific surface area (m2 g1)

Mean mesopore diameter (nm)

Total pore volume (cm3$g1)

500 500 500 500 500 500 600 700 800 500 600 700 800

55.3 122 162 181 186 55.3 12.0 4.4 3.5 162 123 70.7 73.5

5.60 4.92 4.30 4.31 3.83 5.60 3.81 ea ea 4.30 5.62 6.58 6.56

0.114 0.203 0.240 0.256 0.282 0.114 0.035 ea ea 0.240 0.208 0.150 0.161

a N2 adsorptionedesorption isotherms correspond to IUPAC type II pattern.

kl bcosðqÞ

fraction was estimated from the XRD intensity data by using the following equation [40]:

L¼

WR ¼ ½1 þ 0:8IA =IR 1

where L is the crystallite size, k is the Scherrer constant usually taken as 0.89, l is the wavelength of the X-ray radiation (0.15418 nm for Cu Ka), b is the full width at half maximum (FWHM) of the diffraction peak measured at 2q, and q is the diffraction angle. The crystallite sizes of the

where IA and IR represent the integrated intensities of anatase (101) and rutile (110) diffraction peaks, respectively. All calculated values of the rutile ratio (WR) are presented in Table 3. A steadily increased phase transformation was observed until the pure TiO2 photocatalyst contained the pure rutile phase after calcined at 800
C. However, this phase transformation behavior did not occur for the 0.97TiO2e0.03SiO2 mixed oxide photocatalyst. Fig. 4b reveals that the incorporation of 3 mol% SiO2 can delay the phase transformation of TiO2 from the meta-stable anatase phase to the thermallystable rutile phase [30,41], since the mixed oxide photocatalyst displays the dominant XRD peaks corresponding to only the pure anatase phase even when it was calcined at as high as 800
C. The crystallite size of the photocatalysts was calculated from the line broadening of the most preferentially oriented diffraction peak of each crystalline phase according to the Scherrer equation [42], as follows:

Fig. 3 e XRD patterns of the synthesized mesoporousassembled pure TiO2 and TiO2eSiO2 mixed oxide photocatalysts calcined at 500
C (A [ Anatase TiO2).

Fig. 4 e XRD patterns of the synthesized mesoporousassembled photocatalysts calcined at 500e800
C: (a) pure TiO2 and (b) 0.97TiO2e0.03SiO2 mixed oxide (A [ Anatase TiO2, R [ Rutile TiO2).

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Table 3 e XRD results of the synthesized mesoporous-assembled pure TiO2 and TiO2-SiO2 mixed oxide photocatalysts calcined at various temperatures. Photocatalyst

Pure TiO2 0.99TiO20.01SiO2 0.97TiO20.03SiO2 0.95TiO20.05SiO2 0.93TiO20.07SiO2 Pure TiO2

0.97TiO20.03SiO2

Calcination temperature (
C)

500 500 500 500 500 500 600 700 800 500 600 700 800

Phase from XRD pattern

Anatase Anatase Anatase Anatase Anatase Anatase Anatase þ Rutile Anatase þ Rutile Rutile Anatase Anatase Anatase Anatase

synthesized mesoporous-assembled pure TiO2 and TiO2eSiO2 mixed oxide photocatalysts are also given in Table 3. The results reveal that the incorporation of SiO2 led to the decrease in crystallite size due to the role of SiO2 in retarding the growth of the TiO2 crystallites. With increasing calcination temperature, a larger crystallite size was observed for both the pure TiO2 and 0.97TiO2e0.03SiO2 mixed oxide photocatalysts due to the grain growth induced by an increasing temperature.

3.1.4.

UVevisible spectroscopy results

UVevisible spectroscopy was used to examine the light absorption ability of the synthesized mesoporous-assembled TiO2eSiO2 mixed oxide photocatalysts, as well as that of Eosin Y (E.Y.) solution, which was used as a sensitizer for the photocatalytic hydrogen production in this work. Fig. 5 shows the UVevisible spectra of the mesoporous-assembled TiO2eSiO2 mixed oxide photocatalysts with different TiO2to-SiO2 molar ratios calcined at 500
C. It is clearly seen that the absorption band of the synthesized mesoporousassembled TiO2eSiO2 mixed oxide photocatalysts was mainly in the UV light region in the range of low wavelength up to 400 nm. The band gap energy (Eg, eV) was determined by

Fig. 5 e UVevisible spectra of the synthesized mesoporous-assembled photocatalysts calcined at 500
C: (a) pure TiO2 and (b)-(e) TiO2eSiO2 mixed oxide.

Rutile ratio (WR)

e e e e e e 0.44 0.74 1 e e e e

Crystallite size (nm) Anatase (101)

Rutile (110)

15.62 8.62 7.41 6.40 6.52 15.62 28.53 29.75 e 7.41 7.84 13.02 13.78

e e e e e e 36.31 36.63 36.63 e e e e

extrapolating the absorption onset of the rising part to x-axis (lg, nm) of the plots [7] and calculated by the following equation: Eg ¼ 1240 =lg where lg is the wavelength (nm) of the exciting light. The results of absorption onset wavelength and corresponding band gap energy of all the photocatalysts obtained from the UVevisible spectra are summarized in Table 4. With increasing SiO2 content in the mixed oxide photocatalysts, the band gap energy gradually increases from 3.22 eV (lg ¼ 385 nm) for the pure TiO2 to 3.32 eV (lg ¼ 373 nm) for the 0.97TiO2e0.03SiO2 mixed oxide. The shift of the absorption onset edge in the TiO2eSiO2 mixed oxides can be attributed to quantum-size effect for smaller crystallites [43], since it is known that TiO2 crystallization and its crystallite growth are inhibited in the presence of SiO2, as shown above in the XRD results. In case of increasing calcination temperature from 500 to 800
C for the 0.97TiO2e0.03SiO2 mixed oxide as shown in Fig. 6, the gradual shift of the absorption onset edge toward a longer wavelength (red shift) can be observed. As also included in Table 4, the band gap energy of the 0.97TiO2e0.03SiO2 mixed oxide slightly decreases from 3.26 eV (lg ¼ 380 nm) at the calcination temperature of 500
C to 3.20 eV (lg ¼ 388 nm) at the calcination temperature of 800
C. This shift is normally due to the narrowing of the band gap energy, which results in a lower energy required for electrons to be excited from the valence band to conduction band [8,9]. This can be possibly explained in that the increase in crystallite size of the 0.97TiO2e0.03SiO2 mixed oxide (Table 3) due to the increase in calcination temperature leads to the observed decrease in its band gap energy. Interestingly, the band gap energy of the 0.97TiO2e0.03SiO2 mixed oxide was maintained at the anatase-TiO2 band gap energy, even though it was calcined at as high as 800
C. It can also be clearly observed that most of the TiO2eSiO2 mixed oxide photocatalysts could absorb only UV light of wavelength shorter than 400 nm. Therefore, in order to confirm that Eosin Y (E.Y.) is the visible light-responding sensitizer, its UVevisible spectrum was also measured, as shown in our previous work [44]. It is clear that E.Y. could mainly absorb the visible light with

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Table 4 e Absorption onset wavelength and band gap energy results of the synthesized mesoporous-assembled TiO2eSiO2 mixed oxide photocatalysts calcined at various temperatures. Photocatalyst

Calcination temperature (
C)

Pure TiO2 0.99TiO2e0.01SiO2 0.97TiO2e0.03SiO2 0.95TiO2e0.05SiO2 0.93TiO2e0.07SiO2 0.97TiO2e0.03SiO2

Absorption onset wavelength, lg (nm)

Band gap energy, Eg (eV)

385 384 380 375 373 380 385 385 388

3.22 3.23 3.26 3.31 3.32 3.26 3.22 3.22 3.20

500

500 600 700 800

the maximum absorption centered at 516 nm. This absorption feature strongly suggests that the sensitizer can be activated by the visible light for the sensitized photocatalytic hydrogen production system.

be attributed to the increases in the thermal stability and the resistance to sintering caused by the incorporated SiO2, as mentioned above.

3.1.5.

3.2.

SEM and TEM results

Photocatalytic hydrogen production results

The morphology of the photocatalysts was observed by the SEM analysis. Fig. 7 exemplifies the SEM image of the mesoporous-assembled 0.97TiO2e0.03SiO2 mixed oxide photocatalyst calcined at 500
C. The image clearly reveals the presence of agglomerated clusters formed by an aggregation of several uniform-sized photocatalyst nanoparticles. Therefore, the nanoparticle aggregation plausibly leads to the formation of mesoporous-assembled structure in the synthesized photocatalysts. The TEM analysis was also performed in order to obtain insight information about the particle sizes of TiO2eSiO2 mixed oxide nanoparticles. Fig. 8 shows the exemplified TEM images of the synthesized mesoporous-assembled pure TiO2 and 0.97TiO2e0.03SiO2 mixed oxide photocatalysts calcined at 500
C. The TEM images also reveal the formation of aggregated photocatalyst nanoparticles. The average particle sizes of the pure TiO2 and 0.97TiO2e0.03SiO2 mixed oxide photocatalysts were in the range of 8e15 nm and 5e10 nm, respectively, where the observed particle sizes are in good accordance with the crystallite sizes estimated from the XRD analysis (Table 3). The smaller particle size of the 0.97TiO2e0.03SiO2 mixed oxide can

The results of specific hydrogen production rate of the mesoporous-assembled TiO2eSiO2 mixed oxide photocatalysts with different TiO2-to-SiO2 molar ratios in terms of different SiO2 contents and calcined at various temperatures are shown in Fig. 9. It can be clearly observed that the specific hydrogen production rate reached a maximum value at the SiO2 content of 3 mol% (i.e. the TiO2-to-SiO2 molar ratio of 97:3 or the 0.97TiO2e0.03SiO2 mixed oxide) and the calcination temperature of 500
C. According to the specific surface area analysis (Table 2), the addition of SiO2 with an appropriate amount increased the specific surface area of the photocatalyst, consequently resulting in more available active sites on the photocatalyst surface. The obvious decrease in the photocatalytic activity at higher SiO2 contents is possibly because the SiO2 itself has a very large band gap

Fig. 6 e UVevisible spectra of the synthesized mesoporous-assembled 0.975TiO2e0.03SiO2 mixed oxide photocatalysts calcined at various temperatures.

Fig. 7 e SEM image of the synthesized mesoporousassembled 0.97TiO2e0.03SiO2 mixed oxide photocatalyst calcined at 500
C.

3.2.1. Effect of TiO2-to-SiO2 molar ratio in mixed oxide photocatalysts

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Fig. 8 e TEM images of the synthesized mesoporous-assembled photocatalysts calcined at 500
C: (a) pure TiO2 and (b) 0.97TiO2e0.03SiO2 mixed oxide.

Fig. 9 e Effect of TiO2-to-SiO2 molar ratio in terms of SiO2 content on specific hydrogen production rate over the mesoporous-assembled TiO2eSiO2 mixed oxide photocatalysts calcined at various temperatures (Photocatalyst, 0.2 g; total reaction mixture volume, 150 ml; DEA concentration, 15 vol.%; E.Y. concentration, 0.1 mM; and irradiation time, 5 h).

Fig. 10 e Effect of calcination temperature on specific hydrogen production rate over the mesoporous-assembled pure TiO2 and 0.97TiO2e0.03SiO2 mixed oxide photocatalysts (Photocatalyst, 0.2 g; total reaction mixture volume, 150 ml; DEA concentration, 15 vol.%; E.Y. concentration, 0.1 mM; and irradiation time, 5 h).

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energy of approximately 8.0e8.9 eV [45]. Therefore, too much SiO2 incorporation led to an undesirably large increase in the band gap energy of the TiO2eSiO2 mixed oxide photocatalysts (Table 4), especially with SiO2 contents higher than 3 mol%. In addition, this too much SiO2 incorporation resulted in very small crystallite size of the mixed oxides (Table 3), which possibly facilitates carrier charge recombination at the surface traps. Hence, the SiO2 incorporation higher than 3 mol% was found to be unfavorable in achieving high photocatalytic hydrogen production activity. Besides, the photocatalytic hydrogen production activity of the mesoporousassembled 0.97TiO2-0.03SiO2 mixed oxide photocatalyst calcined at 500
C (specific hydrogen production rate of 0.27 cm3/h$gcat) is higher than that of the widely investigated commercial P-25 TiO2 photocatalyst (specific hydrogen production rate of 0.17 cm3/h$gcat). From the overall results, the mesoporous-assembled 0.97TiO2e0.03SiO2 mixed oxide photocatalyst was used for further experiments.

3.2.2.

Effect of calcination temperature

Calcination temperature has a significant effect on the physicochemical properties and crystalline structure of a photocatalyst that definitely lead to the change in photocatalytic hydrogen production activity. Fig. 10 shows the effect of calcination temperature on the specific hydrogen production rate over the mesoporous-assembled 0.97TiO2e0.03SiO2 mixed oxide photocatalyst as compared to the mesoporousassembled pure TiO2 photocatalyst. It can be observed that the mesoporous-assembled 0.97TiO2e0.03SiO2 mixed oxide photocatalyst exhibited a higher photocatalytic hydrogen production activity than the mesoporous-assembled pure TiO2 photocatalyst over the entire calcination temperature range of 500e800
C. In case of the mesoporous-assembled 0.97TiO2e0.03SiO2 mixed oxide photocatalyst, its photocatalytic hydrogen production activity decreased with increasing calcination temperature. The highest photocatalytic hydrogen production activity was observed at the optimum calcination temperature of 500
C with the specific hydrogen production rate of 0.27 cm3/h$gcat. The lower photocatalytic activity with the increase in calcination temperature is mainly because of a large decrease in the specific surface area (Table 2). In addition, a significant increase in the crystallite size was observed when increasing calcination temperature (Table 3), resulting in a higher probability of charge carrier recombination at the bulk traps. This suggests that a good control of crystallite size is required in order to prevent any charge carrier recombinations. In case of the mesoporous-assembled pure TiO2 photocatalyst, the highest photocatalytic hydrogen production activity was also observed at the same optimum calcination temperature of 500
C with the specific hydrogen production rate of 0.21 cm3/ h$gcat, which was lower than that of the 0.97TiO2e0.03SiO2 mixed oxide photocatalyst. Apart from the similar reasons of the negative effects of the dramatic decrease in specific surface area and the increase in crystallite size, the pure TiO2 photocatalyst also underwent the anatase-to-rutile phase transformation starting at the calcination temperature of 600
C (Fig. 4a). It can be implied that the presence of a greater extent of rutile phase also exerts a negative effect on the photocatalytic activity. Since the rutile phase has a lower flat

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band potential as compared to NHE potential (Hþ/H2 level) than the anatase phase [44], this leads to a smaller driving force of the rutile TiO2 for water reduction to produce hydrogen than the anatase TiO2. In overall, the synthesized mesoporous-assembled 0.97TiO2e0.03SiO2 mixed oxide nanocrystal calcined at 500
C is a promising photocatalyst for hydrogen production from the sensitized water splitting under visible light irradiation. Further investigation will be focused on the loading of some selected non-precious active metals (e.g. Ag, Ni, and Cu) on the surface of this promising photocatalytst in order to greatly enhance its hydrogen production activity.

4.

Conclusions

In this work, the mesoporous-assembled TiO2eSiO2 mixed oxide nanocrystal photocatalysts with various TiO2-to-SiO2 molar ratios were synthesized by a solegel process with the aid of a structure-directing surfactant. They were comparatively used for hydrogen production from photocatalytic sensitized water splitting under visible light irradiation from aqueous diethanolamine solution containing dissolved Eosin Y sensitizer. The incorporation of TiO2 by SiO2 with a suitable content had a positive effect on the physicochemical properties and photocatalytic activity of the TiO2eSiO2 mixed oxide as compared to the pure TiO2. The incorporation of this secondary SiO2 phase could effectively stabilize the mesoporous structure of the TiO2, increase the specific surface area of the TiO2, and reduce the TiO2 crystallite size, which consequently enhanced the photocatalytic hydrogen production efficiency. The experimental results revealed that the mesoporous-assembled TiO2eSiO2 mixed oxide photocatalyst with the TiO2-to-SiO2 molar ratio of 97:3 calcined at 500
C possessed the highest photocatalytic hydrogen production activity as compared to the other mixed oxides as well as the commercial P-25 TiO2.

Acknowledgments The authors would like to thank the Sustainable Petroleum and Petrochemicals Research Unit, Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn University, Thailand.

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