Preparation of platinum-loaded cubic tungsten oxide: A highly efficient visible light-driven photocatalyst

Materials Letters 65 (2011) 1252–1256

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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Preparation of platinum-loaded cubic tungsten oxide: A highly efficient visible light-driven photocatalyst Zhanglian Xu a,⁎, Isao Tabata a, Kazumasa Hirogaki a, Kenji Hisada a, Tao Wang b, Sheng Wang b, Teruo Hori a a b

Fiber Amenity Engineering Course, Graduate School of Engineering, University of Fukui, 3-9-1, Bunkyo, Fukui-shi, Fukui-prefecture, 910-8507, Japan Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education of P. R. China, Zhejiang Sci-Tech University, Hangzhou 310018, P. R. China

a r t i c l e

i n f o

Article history: Received 5 October 2010 Accepted 7 December 2010 Available online 15 December 2010 Keywords: Tungsten oxide Cubic Platinum Visible light-driven

a b s t r a c t Multistructural tungsten oxide samples were prepared using the hydrothermal method in the presence of different sulfates. In this paper, we present WO3 nanorods, WO3 toothpicks and cubic WO3 samples prepared in the presence of Na2SO4, Li2SO4 and FeSO4, respectively. These catalysts were characterized by XRD, SEM, TEM, EDS and UV–vis DR. It is found that Fe2O3 was impregnated in the cubic WO3 which is different from other two samples. After Pt loading, Pt-loaded WO3 with different morphology acting as novel visible lightdriven photocatalysts showed remarkably high photocatalytic activity under visible light radiation. Significantly, the maximum efficiency of photodegradation was observed at 1 wt.% Pt loading amount in the cubic WO3 sample. The highest photocatalytic activity of the cubic Pt/Fe2O3/WO3 photocatalyst is attributed to the synergistic action of Pt nanoparticles and Fe2O3. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Water decontamination is an active area of research to date. So far, numerous pathways have been exploited to facilitate degradation of xenobiotic compounds in water body. Semiconductor photocatalysis, especially visible light-driven photocatalysis, is one of them and has grown as a “technology provider” for many current environmental and energy issues in recent years [1–4]. In general, the conduction band (CB) level of semiconductors should be more negative than the potential for the single-electron reduction of oxygen (O2 + e− = O− 2 (aq), −0.284 V; O2 + H+ + e− = HO2 (aq), −0.046 V) in order to efficiently degrade the organic compounds. In the case of titanium dioxide (TiO2), recent studies have focused on various modifications of TiO2 achieving both broad visible light absorption and a sufficiently negative CB level for O2 reduction, for example, nitrogen-doped and sulfur-doped TiO2 [1,2]. However, nitrogen doping causes localized states in the bandgap of TiO2, and the localized holes generated at the impurity level have a slower mobility than those in the valence band. Thus, the quantum efficiency of N-TiO2 is still low under visible light [5]. Therefore an imperative and challenging issue is to develop new and efficient visible light-driven photocatalysts. WO3 is a good candidate due to its many advantages for visible light-driven photocatalysis, including a deeper valence band, strong absorption within the solar spectrum and resilience to photocorrosion effect [6]. Unfortunately, pure WO3 has lower light energy conversion efficiency than TiO2 as the CB levers of WO3 (+0.5 V) are more positive than the

⁎ Corresponding author. Tel./fax: +81 776278641. E-mail address: [email protected] (Z. Xu). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.12.011

reduction potentials of O2. Most current efforts focus on the morphology of WO3 such as nanowire, nanotube and microsphere [7–9]. The oxidation of organic pollutants using WO3 is rarely reported [10,11]. The noble metal loading is an attractive solution to enhance the photocatalytic property of WO3 because the noble metal can act as the electron pool leading to enough negative potentials of O2 reduction. Moreover, photocatalytic properties of photocatalysts greatly depend on their morphology, chemical composition and surface modification. In this paper, WO3 samples with different morphologies have been selectively prepared by adding different sulfates under hydrothermal conditions, and on the base of as-prepared products, platinum-loaded WO3 is demonstrated to exhibit high photocatalytic activity for the decomposition of organic pollutants under visible light irradiation. 2. Experimental details 2.1. Materials All the chemicals were of analytical grade and used without further purification. Sodium sulfate (Na2SO4), Lithium sulfate (Li2SO4), Ferrous sulfate (FeSO4), oxalic acid (H2C2O4), and sodium tungstate (Na2WO4) were purchased from Wako Pure Chemical Industries, Ltd. Chloroplatinum acid hexahydrate (H2PtCl6, ACS reagent) was obtained from Sigma-Aldrich. Inc. Other chemicals including acetic acid (CH3COOH) and hydrochloric acid (HCl, 36%– 38%) were purchased from HangZhou Chemical Regent CO. and used as received. TiO2 (P25) and N-TiO2 used as reference samples were purchased from Degussa CO. Ltd. and Sumitomo Chemical CO., respectively.

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2.3. Preparation of Pt-loaded tungsten oxide Platinum-loaded WO3 samples were prepared with a photodeposition method. WO3 (100 mg) and hexachloroplatinic acid (H2PtCl6·6H2O, 100 ml/g) were added to an aqueous methanol (50 vol.%) and Pt loading amount of 0.5, 1.0, 1.5 and 2 wt.% tested in next photocatalytic reaction can be controllable through adjusting the H2PtCl6·6H2O concentration. The mixed solution was sonicated for 5 min, flushed by N2 for 10 min and then exposed to UV light (λN 300 nm) provided by a 500 W Hg lamp. After irradiation for 1 h, the aqueous methanol was centrifuged, and the samples were dried at 60 °C for 12 h. 2.4. Characterization XRD (D/max-2550pc) analysis of samples was performed. The morphologies and sizes of the samples were observed on SEM (S4800). The crystal lattice of sample was obtained by TEM measurements (JEM-2010 (HR) model). The compositions of samples were measured by EDS (Inca Energy-200). UV–Vis spectra were acquired with a Lambda 900 spectrophotometer. 2.5. Procedures of decomposition of acetic acid The visible light-driven photocatalysis was carried out with the mixture of 40 mg of the photocatalysts and acetic acid solution (4 ml, 0.9 mol/L) under visible light (λ N 400 nm) irradiation using an Hg arc lamp (500 W, Nanjing xujiang inc.) attached with UV cut filter. The amounts of CO2 generation from AcOH solution were measured by GC (Agilent 6890). The commercial N-TiO2 and TiO2 (P25) were used as the reference samples in the decomposition of acetic acid. 3. Results and discussion XRD patterns of the as-obtained products are shown in Fig. 1. All the diffraction peaks can be indexed to the pure hexagonal phase of WO3 with lattice constants of a = 7.340 Å and c = 7.668 Å, which agrees well with the data from the JCPDS card (85-2460). No peaks of impurities were detected from Fig. 1a and b. The peaks in the patterns are strong and narrow, indicating perfect crystallinity of the asprepared samples. The main peaks of pure hexagonal phase of WO3 also appear in the cubic WO3 samples (Fig. 1c) which, however, show worse crystallinity than WO3 nanorods and toothpicks and several additional peaks. These peaks were attributed to the formation of the hematite (γ-Fe2O3) [12] nanocrystal phase in the hydrothermal process, because γ-Fe2O3 has characteristic reflections of 110,111, 210, 211, and 220 according to JCPDS card (391346), as shown in

a

220

210 211

111

b 110

In a typical experiment, WO3 sol was prepared in advance as follows: sodium tungstate powder (4.075 g) was dissolved in distilled water (100 ml). Then Na2WO4 solution was acidified to a pH range of 1.0–1.2 by HCl (3 mol/L) solution. A flaxen precipitate was generated. H2C2O4 (3.15 g) was added to the mixture and diluted to 250 ml. After that, a translucent, homogeneous, and stable WO3 sol was formed. A 15 ml volume of WO3 sol was transferred to a 20-ml autoclave, and then FeSO4 (1 g) was added to the solution, sealed, and maintained at 180 °C for 24 h. Then the precipitates were filtered, washed sequentially with water and ethanol to remove ions possibly remnant in the final products, and dried at 60 °C. The whole process can be easily adjusted to prepare WO3 with other structures by simply adding Na2SO4 (1 g) and Li2SO4 (1 g) instead of FeSO4 while keeping other conditions unchanged. The addition of Na2SO4, Li2SO4 and FeSO4, respectively, to the WO3 sol leads to the formation of the following morphologies, WO3 nanorods, WO3 toothpicks and cubic WO3 samples.

Intensity

2.2. Preparation of tungsten oxide with different morphology

c d e

10

20

30

40

2 theta /

50

60

70

80

o

Fig. 1. XRD patterns of the as-prepared WO3 products. (a) WO3 nanorod, (b) WO3 toothpick, (c) cubic WO3, (d) Pt-WO3 nanorod and (e) Pt-loaded cubic WO3 samples (2 wt.% Pt loading amount).

Fig. 1c. After Pt loading, Pt traces in the WO3 nanorods and cubic WO3 samples around 46 o and 55 o can be detected in the XRD patterns, as shown in Fig. 1d and e by the black arrows. The morphology and nanostructures of the samples were investigated using SEM and TEM. Fig. 2a and b are the lowmagnification scanning electron microcopy (SEM) images, which clearly show that numerous WO3 nanorods and toothpicks with diameters ranging from 30 nm to 70 nm and lengths of ca. 500 nm were formed in the presence of Na2SO4 and Li2SO4. WO3 nanorods have a uniform size in the two ends, while the nanostructure with thick midst and thin ends exists in WO3 toothpicks. The cubic assemblies of WO3 can be selectively prepared in the presence of FeSO4, as shown in Fig. 2c. The width, thickness and length of cubic products are all in the range of 100–500 nm. The spacing of the lattice fringes are calculated to be about 0.384 nm (Fig. 2d) and this plane can be well indexed as (001) plane of WO3 crystal, confirming that the cubic samples are single crystals grown along the c-axis. On the basis of the above SEM analysis, it is obvious that different sulfates play a crucial role in controlling the morphology of final products. Our results show that the addition of Na2SO4 and Li2SO4 favors the formation of nanorods and toothpicks. This is likely to be attributable to the action of SO2− 4 as structure-directing agent to preferentially adsorb on the face parallel to the c axis of the WO3 nanocrystal, leading to the formation of c-axis oriented nanorod and toothpick, as suggested by Jiannian Yao et al. [7]. To understand the formation mechanism of the cubic products, we conducted experiments for different reaction time. Close observation shown in Fig. 2e revealed that the intermediates collected after 12 h were composed of assembles of smaller cubes. These smaller cubes were actually formed by assembling of numerous and highly aligned nanorods. The edge wrinkle at the interface between the two primary building nanorods provided direct evident for the growth mechanism, as indicated with an arrow in Fig. 2e. After reaction for 72 h, large and uniform cubic samples were obtained and the edge fringes demonstrated that these smaller cubes have assembled into larger crystals (Fig. 2f). In addition, the formation of different morphology under different sulfates leads us believe that the different radius of cations such as Fe2+ and Na+ also induce the different interactions between these cations and WO3 nanocrystals, contributing to various morphologies of the samples. To confirm the action of sulfates other inorganic salts, including NaCl, KCl, FeCl3 and KNO3, were tested in this work, while no or irregular WO3 particles were obtained in these cases under the same preparation conditions.

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Energy dispersive spectra (EDS) were carried out followed by SEM. The analysis showed that only W and O were contained in the nanorods, and same results appeared in WO3 toothpicks. No sodium and lithium elements existed in EDS spectra of final samples, indicating that Na and Li ions had been washed completely during the preparation. However, iron element was found besides W and O in cubic samples with the same treatment as nanorods and toothpicks samples. It is strongly suggested that hematite (γ-Fe2O3) had generated in the hydrothermal process. Fig. 3 presents the UV/Vis spectra of the cubic WO3 and WO3 nanorod. The cubic WO3 samples are bottle-green in color, and the final WO3 nanorod and toothpick appear light-green. It is known that the optical absorption coefficient near the band edge follows the equation (αhν)2 = A(hν − Eg) for a direct-bandgap material in which α, h, ν, Eg, and A are the absorption coefficient, Planck constant, light

frequency, band gap, and a constant, respectively [13]. This relationship gives the band gap (Eg) by extrapolating the straight portion of (αhν)2 against hν plot to the point α = 0, which are 2.62 eV and 2.81 eV for the cubic WO3 samples and WO3 nanorods. The band gap energy of WO3 toothpicks is not shown here because the datum is almost the same as that of WO3 nanorods. On the basis of the above analysis, it is believed that the impregnation of Fe2O3 acting as an impurity energy band contracted the valence band state and actually increased the effective band gap in the case of cubic samples, obtaining a broader respond in visible light region. It can be also observed that Pt loading to the cubic WO3 did not change the absorption range but strengthened the absorption intensity. The photocatalytic reaction was carried out under visible light irradiation (400 b λ b 500 nm) in order to clarify the photocatalytic activity of products. All the products were ensured to have the

Fig. 2. SEM images of multistructural WO3 using different sulfates at 180 °C for 24 h. (a) Na2SO4, (b) Li2SO4, (c) FeSO4, (d) TEM observation of the cubic samples, (e, f) SEM images of cubic samples collected after 12 h and 72 h.

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a 100

CO2 generation / umol

N-TiO2 Cubic WO3

80

WO3 toothpick WO3 nanorod

60

40

20

0 0

15

30

45

60

75

90

60

75

90

Time / min Fig. 3. Diffuse reflectance UV/Vis spectra of the cubic WO3, WO3 nanorod and Pt-loaded cubic WO3 (2 wt.%).

b 800 Pt-WO3(cube, full arc)

equimolar amounts of WO3. Acetic acid (AcOH) was selected as a model pollutant, and the overall photocatalytic activity was evaluated as the amount of CO2 that generated from photocatalytic decomposition of AcOH. To enable a quantitative comparison, the decomposition rates for the first 60 min were proposed to represent the photocatalytic activities because this region is most likely to be dominated by pure light-intensity-limited conditions. Fig. 4 shows these values and clearly indicates that 1.0 wt.% Pt-loading produces the most effective photocatalytic reactivity for all the WO3 samples. An excessive Pt loading can decrease the photocatalytic activity due to photon absorption by the photocatalytically inactive Pt nanoparticles. It can also be observed that the photocatalytic activity of cubic WO3 is much better than that of WO3 toothpick and nanorod at each Ptloading concentration. Therefore, we choose 1 wt.% Pt loading samples as the research object in the following parts. To investigate the photocatalytic activity of the as-prepared samples, TiO2 (P25) and N-TiO2 were chosen as the reference photocatalysts. Fig. 5 shows the time courses of CO2 generation over bare WO3 and Pt-WO3 with different nanostructures, TiO2 (P25) and N-TiO2 photocatalysts suspended in aqueous AcOH. Before Pt loading, the rates of CO2 generation over bare WO3 are inferior to that over nitrogen-doped TiO2 particles

300

CO2 generation / umol

250

cubic WO3

200 WO3 toothpick 150

100

50 0.0

WO3 nanorod

0.5

1.0

1.5

2.0

Pt loading / wt% Fig. 4. The relationship between Pt loading amount and decomposition rates at initial 60 min.

CO2 generation /umol

700

TiO2(full arc) Pt-WO3(cube, vis)

600

Pt-WO3(toothpick, vis) Pt-WO3(nanorod, vis)

500

Pt-N-TiO2(vis)

400 300 200 100 0 0

15

30

45

Time / min Fig. 5. Time course of CO2 evolution over bare WO3, N-TiO2, Pt-WO3 (1 wt.%) and TiO2 photocatalysts suspended in aqueous acetic acid solution (AcOH) under full-arc (300 b λ b 500 nm) and visible light irradiation (400 b λ b 500 nm) from an Hg lamp.

under visible light irradiation, as shown in Fig. 5(a). The fact shows that all the multistructured WO3 samples are unsuitable for achieving the efficient oxidative decomposition of AcOH. However, it is worthwhile to note that the photocatalytic activity of the cubic WO3 samples got relatively high improvement comparing to that of WO3 toothpicks and nanorods. Such a large photoacitivity enhancement probably arises from the impregnation of Fe2O3 in cubic WO3 samples, which acts as the impurity energy band leading to the easier charge separation with lower energy. In the meanwhile, the crystalline of photocatalysts greatly impacts the photocatalytic property [14]. It has been pointed out that the crystalline of WO3 toothpick and nanorod is much better than that of the cubic WO3 samples. Perfect crystalline may also become the recombination center of the photogenerated electrons and holes leading to low photocatalytic activity. The decomposition of AcOH on the three catalysts after Pt loading is compared in Fig. 5(b). A remarkable photocatalytic activity enhancement can be observed for the three Pt-loaded catalysts with respect to the bare ones under full-arc and visible light irradiation. Under full-arc irradiation, the rate of CO2 evolution over cubic Pt-WO3 photocatalyst (1 wt.% Pt) is higher than that over TiO2. Significantly, cubic Pt-WO3 exhibits considerably high activity under visible light irradiation, achieving a rate comparable to that over TiO2 under fullarc irradiation. The rate of CO2 generation of 1 wt.% Pt-loaded cubic WO3 samples upon 90 min irradiation with visible light is about 10, 1.8 and 2.2 times greater than that of the cubic WO3 sample, 1 wt.% Pt-

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loaded WO3 toothpicks and nanorods, respectively. Notably, only 1.3fold increase was observed for N-TiO2 even by optimal Pt loading. According to the above results, the questions that need to be addressed including why the bare WO3 samples exhibit so low photocatalytic activity? How do Pt nanoparticles enhance the activity of WO3 samples? And is the photocatalytic mechanism of Pt-WO3 same as that of N-TiO2? Herein, we try to make a reasonable explanation. In the case of bare WO3 samples under visible light irradiation, pentavalent tungsten (W5+) will be accumulated by capture of photoexcited electrons at the trapping sites on WO3 surface. This process is beneficial to visible light driven photocatalysis of WO3 because W5+ has a broad absorption range in the visible region [15]. However, W5+ is inevitably and quickly exhausted through electron transfer from W5+ back to the original W6+ by O2. As mentioned in the introduction section, the CB levels of WO3 are more positive than potential of single-electron reduction of oxygen (+0.5 V for WO3 vs −0.284 V for NHE). This single-electron has no enough energy to reduce O2. This is the reason why bare WO3 samples can't exhibit high efficiency for the decomposition of organic compounds under visible light. When WO3 is loaded with Pt, our results strongly elucidate that the photocatalytic activity of Pt-WO3 is reactive toward O2. This phenomenon contradicts the above understanding that the CB level of WO3 is insufficient to reduce O2. This can be interpreted by assuming that, with the loading of Pt nanoparticles on the surface of WO3, the photogenerated electrons react efficiently with O2 before being captured to form W5+. In general, the potential of the reduction of O2 through multi electrons is more positive than through singleelectron pathway, as following Eqs. (1) and (2): þ

−

þ

−

O2 + 2 H + 2e O2 + 4 H + 4e

= H2 O2 ðaqÞ; + 0:682 V = 2H2 O; + 1:23 V

ð1Þ; ð2Þ

It thus seems reasonable to consider that the coaction of multi electrons more favors to catalyzing O2 reduction than the bare WO3 samples. The high photocatalytic activity of Pt-WO3 is therefore likely to be attributable to the Pt-induced multi electrons reduction of O2 rather than single electron reduction. It is well known that the singleelectron reduction of O2 generally regarded as the main way for electron consumption over TiO2 and N-TiO2 [16]. This is reasonably supported by the Pt-induced improvement of the photoactivity of NTiO2 was remarkably less than that for WO3 (1.3-fold vs 10-fold). In addition, it is important to note that the existence of Fe2O3 in the cubic Pt-WO3 samples played a key role in higher photocatalytic activity compared to that of Pt-WO3 toothpicks and nanorods under full-arc and visible light. The impregnated Fe2O3 used as an impurity energy band shortened the valence band state of WO3 and actually enhanced the transition of photoinduced electrons involved in the reduction of O2. In the meanwhile, more generated holes were released to oxidate

organic compounds. The following (Eqs. (3) and (4)) are the possible reactions that Fe2O3 took part in.   − þ Fe2 O3 →Fe2 O3 ecb ; hvb −

−

H2 O2 + ecb →OH

+ OH⋅

ð3Þ ð4Þ

It can thus be concluded here that the highest photocatalytic activity of the cubic Pt-WO3 samples is likely to the synergy effect of loaded Pt and Fe2O3, leading to multi-electron reduction of O2 which rapidly accelerate the decomposition of aqueous AcOH. 4. Conclusions In summary, we have demonstrated that WO3 products with different morphologies were prepared in the hydrothermal process in the presence of different sulfates which act as structure-directing agent. Significantly, the cubic Pt-WO3 showed remarkably high photocatalytic activity under visible light which was almost comparable to that of commercial P25 under UV light irradiation but much higher than that of N-TiO2. We attribute this to the synergy effect of implantation of Fe2O3 and Pt loading, leading to broadening visible light absorption range and multi-electron reduction of O2 which rapidly accelerate the decomposition of aqueous AcOH. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10.1016/j.matlet.2010.12.011. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

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