Photocatalytic H2 and O2 evolution over tungsten oxide dispersed on silica

Journal of Catalysis 293 (2012) 61–66

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Photocatalytic H2 and O2 evolution over tungsten oxide dispersed on silica Gang Liu, Xiuli Wang, Xiang Wang, Hongxian Han, Can Li ⇑ State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National Laboratory for Clean Energy, 457 Zhongshan Road, Dalian 116023, China

a r t i c l e

i n f o

Article history: Received 19 March 2012 Revised 12 May 2012 Accepted 5 June 2012 Available online 9 July 2012 Keywords: Tungsten oxide Photocatalysis Water splitting Semiconductor Sol–gel

a b s t r a c t Tungsten oxide dispersed on silica (WO3/SiO2) as photocatalysts was prepared by sol–gel method and characterized with various techniques, including X-ray diffraction (XRD), UV Raman spectroscopy (UV Raman), UV–vis spectroscopy (UV–vis), N2-adsorption, X-ray photoelectron spectra (XPS), and high-resolution transmission electron microscopy (HRTEM). The characterization results show that most of tungsten oxide species are present as nanoparticles, which are highly dispersed on the surface of silica. Compared to bulk WO3, WO3/SiO2 exhibits relatively high visible-light-driven O2 evolution activity in the presence of Fe3+ ions as sacrificial reagents. Interestingly, it is found that WO3/SiO2 also shows H2 evolution activity with methanol as sacrificial reagents under UV light. The high photocatalytic O2 evolution and unexpected H2 generation properties of WO3/SiO2 could be correlated with the presence of highly dispersed WO3 nanoparticles and their interface W–O–Si like species. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Sunlight-driven water splitting into hydrogen and oxygen on semiconductor based photocatalysts has received increasing attention as a means of storing solar energy in chemical bonds [1–4]. Developing highly efficient semiconductor photocatalysts is a key to realize this process [5,6]. It is widely accepted that an excellent photocatalyst should possess the ability to minimize the recombination of photoexcited electron–hole pairs and to optimize electron hole diffuse to the surface to catalyze chemical redox reactions [7,8]. Thus, high crystallinity, sufficient surface active sites as well as suitable band structures are required for constructing semiconductor based photocatalysts. Compared to the bulk materials, nanostructured semiconductors exhibit many exceptional qualities, including high surface-tobulk atomic ratio, altered surface energies, quantum confinement effects, etc. [9,10]. These features significantly influence the charge separation and optical properties of semiconductors [11,12]. Recently, many efforts have been devoted to the synthesis of nanostructured semiconductors with controllable particle size and morphology, aiming for enhancing the photocatalytic activity or understanding the fundamental relations between the morphologies of semiconductors and their photocatalytic behaviors [13– 15]. Besides, introducing nanocystals into a macroscopic matrix is an alternative strategy for fabricating efficient photocatalysts [16–18]. For example, Shangguan and Yoshida incorporated CdS particles in the interlayer region of layered metal oxides by direct ⇑ Corresponding author. E-mail address: [email protected] (C. Li). 0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcat.2012.06.003

cation exchange reaction and a sulfurization process [16]. White and Dutta dispersed TiO2 and CdS nanoparticles on zeolite Y to construct binary combination photocatalysts for H2 production in the presence of sacrificial agents [18]. The improvement of photocatalytic H2 evolution in these works should be mainly attributed to the suppressing effects of supports on the growth of semiconductor particles, which can reduce the recombination between the photoinduced charge carriers and facilitates the transfer of the photogenerated electrons to the surface of photocatalysts [16,18]. Tungsten oxide (WO3) is a chemically stable semiconductor, possessing a bandgap between 2.4 and 2.8 eV [19–21]. It has been demonstrated that WO3 could act as an efficient visible-light-driven photocatalyst for O2 evolution from water splitting. However, this compound is not suitable for H2 production due to its positive conduction band level (+0.5 V vs NHE) [22]. In this work, a series of tungsten oxide highly dispersed on silica (WO3/SiO2) materials are prepared by sol–gel method. Interestingly, we found that these materials not only exhibit relative high photocatalytic activity for O2 evolution, but also can produce H2 in corresponding half reactions. 2. Materials and methods 2.1. Photocatalyst preparation WO3/SiO2 with different WO3 contents were prepared with a sol–gel method in the presence of citric acid, which has been proved previously an effective method for the synthesis of thermally stable mesoporous aluminophosphate materials or mesoporous silica-based materials [23,24]. The detailed process is as

G. Liu et al. / Journal of Catalysis 293 (2012) 61–66

Powder X-ray diffraction (XRD) patterns were recorded on Rigaku D/Max-2500/PC (40 kV, 100 mA) using Ni-filtered Cu Ka radiation. UV Raman spectra were recorded on a home-assembled UV Raman spectrograph using a Jobin–Yvon T64000 triple-stage spectrograph with spectral resolution of 2 cm 1. The laser line at 325 nm of a He–Cd laser was used as an exciting source. N2 adsorption–desorption isotherms were measured at 77 K, using a Micromeritics ASAP 2000 analyzer. High-resolution transmission electron microscopy (HRTEM) images were taken with a Tecnai G2 F30 S-Twin (FEI company) with an acceleration voltage of 300 kV. The ultraviolet–visible (UV–vis) absorption spectra were obtained using a JASCO V-550 spectrophotometer. The electrochemical measurements were performed using a PARSTAT 2273 electrochemical system in a conventional three-electrode cell with a platinum counter electrode and a saturated calomel reference electrode (SCE). X-ray photoelectron spectra were recorded on a Thermo Escalab 250 Xi with a monochromatic Al Ka X-ray source.

30%WO3/SiO2 20%WO3/SiO2 10%WO3/SiO2 10

20

30

40

50

60

70

80

Fig. 1. XRD patterns of various WO3/SiO2 and reference bulk WO3.

contents. The relatively broad diffraction peaks with low intensities indicate that the crystallite sizes of WO3 are very small. The N2 adsorption–desorption isotherms show that all the WO3/ SiO2 samples possess mesoporous characteristics with pore size distributions in a range of 2–8 nm (Fig. 2, 20%WO3/SiO2 as a representative sample). The specific surface areas decrease gradually with the increase of WO3 contents (Table 1). As for 20%WO3/SiO2, STEM image shows that WO3 mainly present as nanoparticle state and highly dispersed on the surface of silica (Fig. 3a). Energy dispersive spectroscopy (EDS) spots #1 and #2 confirm co-presence of silica and tungsten species on the selected areas. High-resolution transmission electron microscopy (HRTEM) reveals that most WO3 nanoparticles have diameters of 2–4 nm and well-disperse throughout the silica supports (Fig. 3b). Only a few relatively large

500

4

Volume (cm3g-1)

3

2.3. Photocatalytic reactions The photocatalytic O2 evolution from an aqueous Fe2(SO4)3 solution (4.0 M) and photocatalytic H2 generation from aqueous methanol solution (20 vol.%) were separatively carried out in a closed gas circulation system. The catalyst powder (0.3 g) was dispersed by a magnetic stirrer in an aqueous solution (100 mL) in a reaction cell made of a Pyrex glass. The light source was an ozone-free 300 W Xe illuminator (Ushio-CERMAX LX300). Optical filter (Kenko, l-42; l > 420 nm) was used in O2 evolution reaction to cut off the ultraviolet radiation. The amount of evolved O2 and H2 was determined by an on-line gas chromatograph (Agilent 7890A, TCD) equipped with a 4 m 5A column. The O2 evolution activity was calculated according to the amount of O2 evolved at 1 h. The H2 generation activity is the average rate of gas evolution in 4 h.

50%WO3/SiO2 40%WO3/SiO2

-1

2.2. Characterization

WO3

dV/dlog (D) (cm g )

following. A certain amount of citric acid and ammonium metatungstate are dissolved in 7.5 mL water. An aqueous ammonia solution (25 wt.%) was used to adjust the pH value of the solution to 5.0. After that, 20 mL of tetraethyl orthosilicate (TEOS) was added under vigorous stirring at ambient temperature. The pH value was then adjusted to 2.0 by adding aqueous HNO3 solution (4.0 M). After standing for 2 h, the mixture was heated at 50 °C in open air to remove water and all other volatiles to obtain the assynthesized precursor. Then, the dried solid was calcined at 600 °C for 3 h to obtain the WO3/SiO2 (heating rate 10 K min 1). The resulting materials with different WO3 contents are denoted as x%WO3/SiO2, where x is the percentage by weight of WO3 (x = 0, 10–50). As a reference sample, bulk WO3 was prepared by the calcination of ammonium metatungstate at the same temperature.

Intensity (a.u.)

62

400

300

3 2 1 0 0 2 4 6 8 10 12 14 16 18 20

Dp/nm

200

100

0 0.0

0.2

0.4

0.6

0.8

1.0

P/P0 Fig. 2. N2 adsorption–desorption isotherms and BJH pore size distribution (inset) of 20%WO3/SiO2.

3. Results and discussion Fig. 1 shows the XRD patterns of WO3/SiO2 with different WO3 contents and reference bulk WO3. As for bulk WO3, all of the diffraction peaks could be indexed to monoclinic WO3 (JCPDS card 83-0951). The dominant peaks at 23.2°, 23.7°, and 24.5° are corresponding to (0 0 2), (0 2 0), and (2 0 0) diffraction peak of WO3, respectively [20]. Sample of SiO2 prepared under the same condition is amorphous, since only one broad band from 15° to 30° appeared in the detected 2h region. As for WO3/SiO2, the diffraction peaks indexed to monoclinic WO3 can be observed. The intensity of diffraction peaks increases slightly with the increase of WO3

Table 1 Texture properties of WO3/SiO2 prepared by sol–gel method. Sample

SBET (m2g

SiO2 10%WO3/SiO2 20%WO3/SiO2 30%WO3/SiO2 40%WO3/SiO2 50%WO3/SiO2

633 612 472 397 388 306

1

)

Pore vol (cm3g 0.62 0.79 0.69 0.52 0.52 0.39

1

)

Pore sizea (nm) 3.6 4.9 5.2 4.9 4.9 4.9

a Average pore diameters calculated from desorption branches by using BJH model.

G. Liu et al. / Journal of Catalysis 293 (2012) 61–66

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Fig. 3. STEM and HRTEM images of 20%WO3/SiO2 (a–c) and 50%WO3/SiO2 (d–f).

WO3 particles (>10 nm) can be observed in the whole detected region (Fig. 3c). Compared with 20%WO3/SiO2, more particles with relatively large size and well crystalline phase can be observed in the sample of 50%WO3/SiO2 (Fig. 3f).

Fig. 4 shows the UV Raman spectra of various WO3/SiO2, SiO2, and reference bulk WO3. It has been reported that Raman spectroscopy is an effective measurement in revealing information of supported tungsten oxide catalysts owing to its high sensitivity to the

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G. Liu et al. / Journal of Catalysis 293 (2012) 61–66

SiO2

983

Intensity (a.u.)

490 711 804

269

996

10% WO3/SiO2 20% WO3/SiO2 30% WO3/SiO2 40% WO3/SiO2 50% WO3/SiO2 WO3

200

400

600

800

1000

1200

1400

Raman shift (cm-1) Fig. 4. UV Raman spectra of various WO3/SiO2, SiO2 and reference bulk WO3.

state of tungsten atoms [25]. The very weak band at 490 and 983 cm 1 are assigned to the silica support, and the intensities of these bands decrease upon increasing the tungsten oxide contents. The bands centered at 269, 711, and 804 cm 1 can be assigned to the crystalline WO3 with monoclinic phase [26,27]. Among them, the 804 and 711 cm 1 vibrations arise from the bridging W–O–W stretching frequencies, and the 269 cm 1 vibration is the related bridging W–O–W bending mode [26,27]. Compared with bulk WO3, WO3/SiO2 materials exhibit relatively weak and broad Raman bands, which should be due to the small nanoparticle size and high dispersion of tungsten oxide species. An additional weak Raman bands at 996 cm 1 can be observed in the spectra of WO3/SiO2, which can be ascribed to the symmetric stretching mode of the terminal W@O band (ms(W@O)) [27]. It has been reported that Keggin-type anions, such as [SiW12O40]4 , possess significantly distorted WO6 units and exhibit Raman features between 950 and 1015 cm 1 [27]. The ms vibration will shift to higher wavenumber values as the W–O bond is shortened by distortion. Raman spectra reported for [SiW12O40]4 and metatungstate anions exhibit intense bands at 998 and 977 cm 1, respectively [28]. According to these literatures, the appearance of band at 996 cm 1 in our case may indicate that small amount of distorted WOx species similar to [SiW12O40]4 clusters present in the WO3/SiO2. This result shows that a relatively strong interaction should be present between tungsten oxide species and silica supports. Fig. 5 shows the UV–vis absorption spectra of various WO3/SiO2, SiO2, and reference bulk WO3. The sample of SiO2 exhibits only

week absorption in deep UV region and no obvious absorption in the range larger than 350 nm. It means that visible light and most of UV light could irradiate guest semiconductors incorporated inside of silica matrix. As for bulk WO3, a wide range light absorption with a sharp absorption edge onset at 480 nm can be observed, corresponding to the bandgap (Eg) of about 2.6 eV. Compared with bulk WO3, WO3/SiO2 materials show a blue shift of the absorption edge, which might be correlated with the smaller particle sizes of WO3 species in these materials. It may also indicate that the bandgap become widened due to the formation of WO3 nanoparticles. Besides, another absorption edge at about 370 nm can be observed in all WO3/SiO2 samples (with Eg of 3.3 eV), which is absent in bulk WO3 spectrum. This signal should reflect the influence of silica supports. It has been reported that the corresponding Eg values for Keggin heteropolytungstates compounds occur between 3.0 and 3.6 eV [27]. Combined with the result of Raman spectra, it further confirms the presence of certain amount of WOx species similar to [SiW12O40]4 clusters on the surface of silica supports. Water splitting is a tough uphill reaction, accompanying with a largely positive change in the Gibbs free energy (237 kJ/mol) [7]. It is composed of two half electrochemical reactions, water oxidation, and proton reduction. For designing efficient photocatalysts for water splitting, separate half reactions in the presence of sacrificial reagents are often employed to evaluate the photocatalytic H2 and/ or O2 evolution behaviors of the semiconductor materials, which could give some valuable fundamental information. Fig. 6 shows the visible light driven O2 evolution activity of WO3/SiO2 and reference bulk WO3 from an aqueous solution containing Fe3+ ions as sacrificial agent. It can be seen that all the WO3/SiO2 samples exhibit photocatalytic O2 evolution properties under tested conditions. The O2 evolution activity increases with the increase of WO3 contents in the WO3/SiO2 materials. When the WO3 contents increase to 50 wt.%, a relatively good photocatalytic activity can be observed, which is higher than that of bulk WO3. It has been detected that SiO2 itself exhibits no photocatalytic H2 and O2 evolution activity in both half reactions. The relatively high-specific surface area and highly dispersed tungsten oxide species may be essential to the high catalytic performance of WO3/SiO2. Since most of known semiconductors are expensive, the cost of semiconductor photocatalysts will become a key issue for their large-scale application in the future. This work shows that introducing semiconductor nanocystals into cheap macroscopic matrixes could reduce the amount of semiconductors required and may offer a promising strategy for developing low-cost photocatalysts.

O2 evolvtion activity (µmolh-1)

18

Absorbance (a.u.)

50% WO3/SiO2 40% WO3/SiO2 10% WO3/SiO2 30% WO3/SiO2 20% WO3/SiO2 WO3 SiO2

200

300

16 14 12 10 8 6 4 2 0 10%

400

500

600

700

Wavelength (nm) Fig. 5. UV–vis absorption spectra of various WO3/SiO2, SiO2 and reference bulk WO3.

20%

30%

40%

50%

WO3

WO3/SiO2 with different WO3 contents Fig. 6. O2 evolution activity of WO3/SiO2 with different WO3 contents and reference bulk WO3 under visible light irradiation. Reaction condition: catalyst, 0.3 g; reactant solution: 100 mL of Fe3+ aqueous solution; light source: 300 W Xe lamp with cutoff filters (k > 420 nm).

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H generation (µmol) 2

H2 generation activity (µmolh-1)

0.6

0.4

2.5 2.0 1.5

Besides, the electronic band structure of the dispersed tungsten oxide may also be influenced by the support of silica. Generally, WO3 crystals are formed by corner and edge sharing of WO6 octahedral. The valence band is dominated by O2p states while the conduction band is determined mainly by the W5d states. Inserting positive center (such as cation) often causes distortion of WO3 crystal phases, accompanying with the bandgap increasing (i.e., increase of the W5d level occupation) [20]. Silica is a high cation electronegativity support, which could reduce the electron density residing on the bridging W–O–support bond [32]. Therefore, in the WO3/SiO2 composite materials, silica support can be regarded as a positive center for the highly dispersed WO3 nanoparticles. In this work, Raman and UV–vis characterizations have confirmed that an interaction presents between tungsten oxide species and silica supports, which is quite similar to Keggin-type units found in dodecatungstosilicates. This result suggests that silica support could cause the distortion of adjacent tungsten oxide structure and change their electronic band structure, which may contribute to the photocatalytic H2 generation behavior of WO3/SiO2 materials. To understand the mechanisms of O2 or H2 evolution, electronic band structure of WO3/SiO2 are investigated. UV–vis absorption spectra have shown that two absorption edges can be observed in WO3/SiO2 materials at ca. 470 and 370 nm, respectively (Fig. 5). It might reflect that two types of tungsten oxide species with different band gaps present in this sample. One has the properties similar to that of WO3 (Eg = 2.6 eV) and the other possesses the relatively wide band gap (denoted as W–O–Si, Eg = 3.3 eV). To determine the relative positions of conduction band (CB) and valence band (VB) edges, the total densities of states of XPS valence band spectra of 20%WO3/SiO2 and reference bulk WO3 are measured (Fig. 8). It reveals that VB maxima of both 20%WO3/SiO2 and bulk WO3 are at 2.8 eV. Combined with the results of UV–vis

Intensity (a.u.)

Photocatalytic H2 generation reaction with methanol as sacrificial reagent is carried out to detect the photoreduction properties of various WO3/SiO2 materials (Fig. 7). All these WO3/SiO2 materials exhibit photocatalytic H2 generation activities in the presence of 0.5 wt.% Pt as cocatalyst (prepared via in situ photodeposition) under full-arc irradiation of Xe lamp. Among them, 20% WO3/ SiO2 exhibits the highest H2 production behavior. The photocatalytic activities of the WO3/SiO2 composites decrease gradually with further increase of WO3 contents. It should be pointed out here that bulk WO3 material does not show any photocatalytic H2 evolution activity under the same reaction condition (with Pt as cocatalyst). It is well known that photocatalytic proton reduction often requires the conduction band of semiconductor to meet the thermodynamic potentials for the reaction, in which the bottom level of the conduction band should be more negative than the potential of H+/H2 (0 V vs. NHE) [7]. As for bulk WO3, the conduction band locates at +0.5 V (vs NHE), which is more positive than the reduction potentials of H+. Therefore, bulk WO3 is thermodynamically not suitable for photocatalytic H2 production. In recent years, many researches show that semiconductor nanocrystals exhibit size-dependent optical and electronic properties [11,29,30]. This is quite different from bulk ones with a composition-dependent band gap energy. When the size of a semiconductor nanocrystal is smaller than the size of the exciton, the charge carriers become spatially confined, which will raise their energy. This quantum confinement can shift the band gap of most semiconductors by over 1 eV, giving an enormous range of continuous tunability through size and shape of the particles [11]. Very recently, Imai et al. found that tungsten oxide nanoparticle (with particle size of 1.4 nm) dispersed on mesoporous silica exhibited a relative high photocatalytic performance in the decomposition of benzene [31]. A widened bandgap can be observed in these materials. They believed that the quantum confinement effect plays an important role for the enhancement of photocatalytic activity. In our case, a great deal of highly dispersed WO3 nanoparticles (2–4 nm) can be observed in the WO3/SiO2 composite materials. UV–Vis spectra show that blue shift of absorption edges can be observed in the spectra of the WO3/SiO2 composite materials, which indicates the widened bandgap of these WO3 nanoparticles (see in Fig. 5). Under irradiation, electrons may be excited to energy greater than the normal band gap of WO3, which can drive the proton reduction and contribute to H2 generation.

20%WO3/SiO2

10% WO3/SiO2 20% WO3/SiO2 30% WO3/SiO2 40% WO3/SiO2

16

50% WO3/SiO2

1.0

14

12

10

8

6

4

2

0

Binding energy (eV)

0.5 0.0 1

2

3

4

Reaction time (h)

Fig. 8. Total densities of states of XPS valence band spectra of 20%WO3/SiO2 and reference bulk WO3.

0.2

0.0 10%

20%

30%

40%

50%

WO3/SiO2 with different WO3 contents Fig. 7. Photocatalytic H2 generation activity on various WO3/SiO2 materials under full-arc irradiation of Xe lamp. Reaction condition: catalyst, 0.3 g; 0.5 wt.% Pt as cocatalyst; reactant solution: 100 mL of 20% methanol aqueous solution; light source: 300 W Xe lamp. Insets show the photocatalytic H2 evolution activities depending on the reaction time.

Scheme 1. Electronic structure of WO3/SiO2 and its photoexcitation mechanisms for O2 or H2 evolution. CB: conduction band; VB: valence band; A: electron acceptor; D: electron donor.

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WO3 10 6 8

20%WO3/SiO2 6

4

of China (21090340/21090341, 21061140361), National Basic Research Program of the Ministry of Science and Technology (2009CB220010), and Solar Energy Action Plan of Chinese Academy of Sciences. Gang Liu thank China Postdoctoral Foundation (20100480073 and 201104614) and K.C. Wong Post-doctoral Fellowships for financial supporting. References

4 2 2

-0.35 V 0 -1.0 -0.8 -0.6 -0.4 -0.2

0.34 V

0.0

0.2

0.4

0.6

0.8

0 1.0

Potential (V vs. SCE) Fig. 9. Mott–Schottky plots of 20%WO3/SiO2 and reference bulk WO3 electrodes in 0.5 M Na2SO4 (pH = 7.0), frequency: 1 kHz.

spectra, the relative positions of CB minimum of two types of tungsten oxide species (WO3 and W–O–Si) in 20%WO3/SiO2 can be determined (Scheme 1). The CB minimum of W–O–Si species is 0.7 eV upshift in contrast to that of WO3 species, which should be negative than the potential of H+/H2. The trend of CB minimum upshift is also evidenced in the recorded Mott–Schottky plots of 20%WO3/SiO2 and bulk WO3 (see Fig. 9). It is well known that bulk WO3 is an efficient visible-light-driven photocatalyst for O2 evolution from water splitting due to its valence band is deep enough to oxidize water. The presence of WO3 species in WO3/SiO2 should be responsible for the O2 evolution under visible light irradiation with Fe3+ as sacrificial reagent. The upshift of CB minimum for W–O–Si species could meet the thermodynamic potentials for photocatalytic proton reduction, contributing to the photocatalytic H2 generation in the presence of methanol as sacrificial reagent. 4. Conclusion WO3/SiO2 with highly dispersed tungsten oxide species has been prepared by sol–gel method. WO3/SiO2 exhibits relatively high photocatalytic O2 evolution activity and unexpected H2 generation activity in the presence of sacrificial reagents. These properties could be correlated with the presence of highly dispersed WO3 nanoparticles and their interface W–O–Si like species. Acknowledgments The authors thank Dr. Mingrun Li for HRTEM measurements. This research is supported by National Natural Science Foundation

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