Catal Lett DOI 10.1007/s10562-014-1194-8
Facile Synthesis of WO3 Nanorod Thin Films on W Substrate with Enhanced Photocatalytic Performance Hideyuki Katsumata • Kiichiro Inoue Tohru Suzuki • Satoshi Kaneco
•
Received: 22 November 2013 / Accepted: 7 January 2014 Ó Springer Science+Business Media New York 2014
Abstract Thin films containing aligned WO3 nanorods were grown directly onto a W metal plate via a hydrothermal reaction in the presence of Cs2SO4. The films exhibited a much higher photocatalytic activity than WO3 nanoplates and TiO2 thin films under visible light irradiation. Further, the films showed good stability during the photocatalytic reaction. Keywords WO3 nanorod Photocatalyst Thin film Visible light
1 Introduction Tungsten oxide (WO3) is an n-type semiconductor with a band gap of 2.4–2.8 eV, and it is used as a visible light responsive photocatalyst for water splitting and environmental remediation [1–3]. In particular, various WO3 nanostructures (nanoparticles, nanoplates, nanotubes, nanorods, and nanowires) attract extensive attention as a photocatalyst and for several other applications because of their high surface area and unique properties [4]. Recently, nanostructured WO3 thin films have been synthesized using several methods such as sol–gel [5, 6], template [7], anodic Electronic supplementary material The online version of this article (doi:10.1007/s10562-014-1194-8) contains supplementary material, which is available to authorized users. H. Katsumata (&) K. Inoue S. Kaneco Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie 514-8507, Japan e-mail:
[email protected] T. Suzuki S. Kaneco Environmental Preservation Center, Mie University, Tsu, Mie 514-8507, Japan
oxidation [8, 9], chemical vapor deposition [10, 11], and thermal evaporation [12]. A facile method, the hydrothermal method, has been applied to the synthesis of WO3 nanostructured particles. For example, Gu et al. [13] synthesized WO3 nanostructured particles by adding alkali sulfates to a hydrothermal solution. The alkali metal of the sulfate was found to influence the shape of the nanostructure, i.e., Rb2SO4 led to nanorod and nanoplate structures and K2SO4 led to ribbon-like structures. Peng et al. [14] prepared WO3 nanorod particles using a hydrothermal process with Na2SO4 as the directing agent. However, there are very few reports on aligned WO3 thin films with nanostructures grown directly on the substrates using a one pot hydrothermal synthesis. WO3 nanotree thin films on a W metal substrate have been synthesized by Shibuya and Miyauchi using a hydrothermal method with Rb2SO4 as the directing agent, and the film’s photoinduced super-hydrophilic property was investigated [15]. When (NH4)2SO4 was used as a directing agent in the hydrothermal solution, the morphology of the WO3 nanotree thin films changed from that observed in the earlier report [15]; that is, each nanotree was composed of several nanosheet-shaped branches [16]. Therefore, there is the possibility that thin films of various WO3 nanostructures can be directly grown on a W plate via a hydrothermal method, using different sulfates as directing agents. Further, no attention has been paid to the photocatalytic properties of the WO3 nanotree thin films. Herein, the hydrothermal synthesis of aligned WO3 nanorod thin films, directly grown on a metal W substrate, is presented. Briefly, a metal W plate was hydrothermally treated in a solution of oxalic acid, Cs2SO4, and HNO3. The obtained catalyst thin film was characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray
123
H. Katsumata et al.
photoelectron spectroscopy (XPS), UV–Vis diffuse reflectance spectroscopy (DRS), and photoluminescence (PL) spectroscopy. The aligned WO3 nanorod thin films showed superior photocatalytic activity in a visible light driven decolorization of Rhodamine B (Rh B).
2 Experimental Section 2.1 Preparation of WO3 Nanorod Thin Films WO3 nanorod thin films were synthesized according to a modified reported procedure [15]. In brief, oxalic acid (1.56 g) and Cs2SO4 (0.2 g) were added to 0.2 mol/L HNO3 (25 mL), and the mixture was placed in a Teflonlined autoclave. To enhance the adherence of WO3 film on the substrate, W foil (20 9 20 9 0.2 mm, 99.95 % purity) was pre-oxidized in air at 500 °C for 30 min. Then the oxidized W foil was vertically placed in the hydrothermal reaction autoclave, and was treated at 150 °C for 30 h. After cooling to room temperature, the substrate was rinsed with distilled water and dried at room temperature. After that, the sample was annealed at 500 °C for 30 min. Pt species loaded WO3 nanorod thin films were prepared by the photodeposition method. The thin film of WO3 nanorods was immersed into the mixture solution of H2PtCl66H2O (0.08 mmol/L, 27 mL) as a Pt source and methanol (3 mL) as a hole scavenger, and then the WO3 nanorod surfaces were irradiated with a 990 W Xe lamp for 8 h. Also, WO3 nanoplate thin films were synthesized by the hydrothermal method in the absence of Cs2SO4. For the synthesis of other nanostructured WO3 thin films, 0.2 g of Li2SO4, Na2SO4, K2SO4 and Rb2SO4 were used instead of Cs2SO4. TiO2 thin film was synthesized according to a reported procedure [17]. Ti foil used was 99.5 % purity (20 9 20 9 0.2 mm). Briefly, the Ti foil was soaked in 50 mL of 30 % H2O2 solution and was hydrothermally treated at 80 °C for 72 h. After cooling, the treated Ti foil was washed with water and dried in air. Thermal treatment of the Ti foil was conducted in an oven maintained at 450 °C for 1 h. Pt species loading onto the WO3 nanoplates and TiO2 thin films was same method as described for the WO3 nanorod thin films. 2.2 Characterization The UV–Visible DRS of the thin films were recorded using a Shimadzu UV-2450 spectrophotometer. PL spectra were obtained at an excitation wavelength of 270 nm using a Shimadzu RF-5300PC spectrofluorophotometer. XPS measurements were carried out with a PHI Quantera SXM photoelectron spectrometer using Al Ka radiation. XRD (Rigaku RINT Ultima-IV diffractometer) measurements
123
were carried out using Cu–Ka radiation. SEM and TEM observations were performed using a Hitachi S-4800 SEM and a Hitachi H-9000 TEM, respectively. 2.3 Photocatalytic Activity The obtained thin film was placed inside a 50 mL Pyrex glass cell filled with 30 mL of 5 mg/L Rh B aqueous solution. Before irradiation, the sample was allowed to equilibrate for 30 min in the dark. The original pH of the dye solution was ca. 5.6. The thin film surfaces were irradiated with a Xe lamp (990 W, Ushio Electronics) in conjunction with a UV cutoff filter (L-42, HOYA) and an IR cutoff filter (HA-50, HOYA). The light intensity was measured using a radiometer (UVR-300, Topcon) with a 360–490 nm wavelength sensor (UD-400, Topcon) and was adjusted to 5.5 mW/cm2. The photocatalytic reactions were carried out without stirring. The decolorization of Rh B was determined by measuring the absorbance of the solution at 554 nm using a UV–Visible spectrophotometer (Shimadzu UV-1650PC). All experiments were conducted in triplicates and the results showed at the mean values.
3 Results and Discussion 3.1 Characterization of WO3 Nanorod Thin Films The aligned WO3 nanorod thin films on W substrate can be selectively prepared in the presence of Cs2SO4. The XRD pattern of the obtained products is shown in Fig. 1a. From the diffraction pattern, the sample was assigned as the hexagonal phase of WO3 with lattice constants of ˚ and c = 7.669 A ˚ . These values correlate a = 7.350 A ˚, nicely with the reported values (a = 7.324 A ˚ c = 7.662 A) from the JCPDS card (85-2460). No impurity peaks were detected in this pattern. The XRD peaks of the Pt-species (Pts) loaded WO3 (Pts/WO3) nanorod thin film are in agreement with those of the WO3 nanorod (Fig. 1b), showing no diffraction peaks from low content and good dispersion of Pts on the WO3 surface. In addition, there were no diffraction pattern characteristics of the Pts loaded sample. This proved that the crystal structure of the WO3 nanorod thin film was not transformed by loading with Pts. The morphology of the as-prepared samples was observed using SEM and TEM. The overall morphology of the sample is shown in Fig. 2a, b, which indicate a large quantity of WO3 nanorods uniformly grown on a W substrate. Further, the deposition of Pts nanoparticles on the WO3 nanorod thin films was revealed in Fig. 2c. More details of the morphological and structural features of WO3 nanorods were studied using TEM. The nanorods had
Facile Synthesis of WO3 Nanorod Thin Films Fig. 1 XRD pattern of a WO3 nanorod thin films and b Pts/ WO3 nanorod thin film
W
a)
W
Intensity (a.u.)
W b)
10
20
30
40
50
60
70
80
2θ (degree)
(b)
(a)
(c)
2 µm (d)
(e)
500 nm WO3 (002)
(f)
PtO2 (101)
0.382 nm
200 nm
2 nm
200 nm
0.223 nm
4 nm
1 nm
Fig. 2 SEM images of a WO3 nanorods, b high magnification, c Pts/WO3 nanorods, d TEM image of WO3 nanorods, HRTEM images of e WO3 nanorods and f Pts/WO3 nanorods
uniform diameters and lengths of about 50 nm and 1 lm, respectively (Fig. 2d). As shown in Fig. 2e, the spacing of the lattice fringes was approximately 0.382 nm, corresponding to the (0 0 2) plane of a hexagonal WO3 crystal [13]. This result shows that the nanorod thin films are a single crystal grown along the c-axis. HRTEM observation was conducted to further investigate the interfacial structures of Pts/WO3 nanorod thin films. The HRTEM image in Fig. 2f shows two kinds of lattice fringes. The lattice fringes of WO3 and the Pts nanoparticles clearly showed the surface-junction of the composite thin film. The Pts nanoparticles, with diameters of about 8–15 nm, were deposited on the surface of the WO3 nanorods. In addition to the (0 0 2) lattice fringe of hexagonal WO3, spacing of 0.223 nm was observed for the lattice fringes. This plane can be indexed as the (1 0 1) plane of PtO2 [18]. Therefore, it can be concluded that much of the Pts was deposited in the PtO2 form on the surface of the WO3 nanorod thin films. The formation of the surface-junction could potentially promote nanoparticle and nanorod charge migration,
facilitating photogenerated electron transfer in the conduction band of WO3 nanorods to PtO2 or other Pts [19]. This improves charge separation efficiency and enhances photocatalytic activity. Furthermore, WO3 thin films with various nanostructures can be synthesized by the hydrothermal reaction in the presence of alkali sulfates (Fig. S1). As clearly seen in Fig. S1, the nanostructure of WO3 thin film with Rb2SO4 as a directing agent was different from that with Cs2SO4 under the same hydrothermal reaction conditions. The oxidation state of Pt, as well as W, O, and Cs, and the atomic ratio in the as-prepared sample were confirmed by XPS analysis, shown in Fig. 3. The W 4f XPS spectra displayed two peaks at 35.6 and 37.7 eV that could be attributed to W 4f7/2 and 4f5/2, respectively. These values were ascribed to the W6? state in oxides and are consistent with reported values [20]. The major peak around 530 eV was observed in the O 1 s XPS spectra of a Pt loaded WO3 nanorod thin film. After a reasonable fitting, two kinds of surface O species appeared in the O 1 s spectra. The peak
123
H. Katsumata et al. Fig. 3 XPS spectra of Pts/WO3 thin film
W 4f
(a)
W 4f7/2
(b)
O 1s
Intensity (a.u.)
W 4f5/2
45
40
35
Pt 4f
30
(c)
Pt 4f7/2
545
540
535
530
525
520
(d)
Cs 3d
Intensity (a.u.)
Pt 4f5/2 Cs 3d3/2
Pt4+
Cs 3d5/2
Pt2+ Pt0
85
80
75
70
Binding energy (eV)
at 532.3 eV was assigned to adsorbed water on the thin film surface [21]. The peak centered at 530.7 eV was assigned to the oxygen adjacent to W and Pt [22]. From the quantification analysis, the atomic ratio of W to O in the sample was estimated to be about 1:3. The Pt 4f spectra showed Pt in different oxidation states. The first peak at 71.2 eV (Pt 4f7/2) was due to metallic Pt [23]. The second peak for Pt 4f7/2 (73.6 eV) could be assigned to Pt(II) because of PtO or Pt(OH)2 [23]. The third peak of Pt with a very high intensity was observed at 75.0 eV (Pt 4f7/2) and assigned to Pt(IV) from PtO2 [23]. Therefore, the major Pts on the thin film surface was PtO2. The results were in agreement with HRTEM observation. In addition, the peaks for Pt 4f5/2, corresponding to each Pts were observed, except for metallic Pt which would overlap with the peak for Pt 4f7/2 from Pt(IV). The Pt atom percentage in the sample was 8.0 %. As shown in Fig. 3d, Cs was also detected in the thin film. It is well-known that the hexagonal phase of WO3 has a large tunnel structure due to octahedral sharing (WO6) in the crystal [24]. This characteristic structure means that ions can be quickly intercalated and extracted in the empty tunnels of hexagonal WO3. Therefore, Cs? ions would be intercalated into hexagonal WO3 during hydrothermal synthesis. The XPS results indicated that Cs? ions were intercalated into these tunnels at 6.3 atom%. The UV–Vis DRS of the WO3 nanorod thin films (Fig. S2a) showed that the absorption edge of the thin film was located around 460 nm. The band gap of the WO3 nanorod thin film was estimated from the spectra using a Tauc plot.
123
25
65
755
745
735
725
715
Binding energy (eV)
The band gap was 2.72 eV for the thin film. The UV–Vis DRS of Pts/WO3 nanorod thin film indicates that the absorption edge did not change from the bare-WO3 nanorod thin films (data not shown). To evaluate the charge separation of photoexcited WO3 nanorods and Pts loaded thin films, PL spectra of the samples excited at 270 nm were recorded (Fig. S2b). Although the PL spectra of both samples displayed a similar shape with two intense peaks at 363 and 468 nm, the samples did exhibit some differences in PL emission intensity. The higher energy peak (363 nm) was assigned to the electron/hole radiative recombination, while the lower energy peak (468 nm) was assigned to localized states in the band gap because of oxygen vacancies or defects [25]. The PL intensity at 363 nm for Pts/WO3 nanorod thin films was lower than that of bareWO3 nanorod thin films. In the Pts/WO3 composite, electron transfer occurred from the conduction band of lightactivated WO3 to Pts, which acted as an electron trapping center that could promote charge separation. This efficient charge separation can be expected to greatly increase the photocatalytic activity of the WO3 nanorod thin films. 3.2 Photocatalytic Activity of WO3 Nanorod Thin Films The photocatalytic reaction was conducted under visible light irradiation (k [ 400 nm) to clarify the photocatalytic activity of the WO3 nanorod thin films. Rh B was selected as a model compound, and the photocatalytic activity was
Facile Synthesis of WO3 Nanorod Thin Films Fig. 4 Photocatalytic decolorization of Rh B with Pts/ WO3 nanorod thin films under visible light irradiation
(a)
(b)
1
0.8
photolysis
0.8
without visible light
TiO2
0.6
C/C0
C/C0
0.6
1
0.4
WO3 nanoplates 0.4
under visible light 0.2
WO3 nanorods
0.2
0
0 0
1
2
3
4
5
Time (h)
evaluated by the decolorization of Rh B. The effect of Pts content on the photocatalytic activity of the thin film was examined in the range of 0–3.2 lmol. These results are shown in Fig. S3. The decolorization of the Rh B solution was very low in the presence of bare-WO3 nanorod thin films (0.056 h-1) under visible light irradiation, with only a 24 % decrease in Rh B concentration after a 5 h irradiation. In contrast, the rate constant of the photocatalytic reaction rapidly increased as the loading content of Pts increased up to 2.2 lmol (0.43 h-1) and then became almost constant. These results indicate that Pts loading improved the photocatalytic activity of WO3 nanorod thin films because the surface Pts accelerate the multielectron reduction of dissolved oxygen [19, 26]. The optimum loading content of Pts was determined to be 2.2 lmol. Figure 4 illustrates the photocatalytic activity of Pts/ WO3 nanorod thin films. The decolorization of Rh B was not observed in a direct photolysis under visible light irradiation without any photocatalysts (Fig. 4a). The photocatalytic decolorization of Rh B drastically increased with increasing irradiation time, with 100 % decolorization of Rh B with Pts/WO3 nanorods after 5 h, while the adsorption of Rh B onto the thin film was only 12 % after 5 h without visible light irradiation. Incidentally, the photocatalytic decolorization of Rh B was performed with the thin film at a different wavelength of visible light irradiation (k [ 520 nm) to eliminate the possibility that Rh B might be decolored by sensitization. With k [ 400 nm irradiation, both the sensitization of the dye and the band gap excitation of Pts/WO3 are allowed, whereas, only the sensitization of the dye should be allowed under k [ 520 nm [26]. Although Rh B was decolored by the sensitization (k [ 520 nm), the band gap excitation of Pts/ WO3 nanorod thin films under k [ 400 nm markedly enhanced the decolorization of the dye (Fig. S4). Additionally, the photocatalytic activities of Pts/WO3 nanorod thin films and other photocatalyst thin films were evaluated by the decolorization of Rh B under visible light irradiation
0
1
2
3
4
5
Time (h)
for comparison (Fig. 4b). SEM images and XRD patterns of the other thin films are shown in Fig. S5. Comparative studies indicated that the photocatalytic activity of Pts/ WO3 nanorod thin films was superior to those of Pts/WO3 nanoplates and Pts/TiO2 thin films under same conditions. The rate constant for Pts/WO3 nanorod thin films was approximately 1.5 and 7.7 times higher than those of Pts/ WO3 nanoplates (0.29 h-1) and Pts/TiO2 (0.056 h-1) thin films, respectively. The decolorization of Rh B with Pts/ TiO2 thin film would occur by sensitization because TiO2 can generally only absorb in the UV region. Fig S4 also supports this, therefore, the photocatalytic decolorization of Rh B was carried out under full-arc irradiation (Fig. S6). Although the photocatalytic decolorization of Rh B with Pts/TiO2 thin film could be observed, the rate constant of Pts/WO3 nanorod thin films (0.51 h-1) was approximately 2.7 times higher than that of Pts/TiO2 thin film (0.19 h-1). This showed that WO3 nanorod thin films are useful photocatalysts under UV and visible light irradiation. To investigate the photocatalytic activity stability of the WO3 nanorod thin films under visible light irradiation, the same sample was used five times after washing with water prior to each run. The photocatalytic activity of Pts/WO3 nanorod thin films was retained after five recycling runs (Fig. S7), suggesting that the thin film has high stability under visible light irradiation.
4 Conclusions In summary, highly ordered WO3 nanorod thin films on a W substrate were successfully synthesized via a hydrothermal reaction in the presence of Cs2SO4. The photocatalytic activity of the thin films was enhanced by loading Pts, which was mainly deposited as PtO2 on the WO3 nanorod surface. The WO3 nanorod thin films exhibited a much higher catalytic activity than the nanoplate and TiO2 thin films and had high stability under visible light irradiation.
123
H. Katsumata et al. Acknowledgments This work was partly supported by Scientific Research (C) No. 24510095 from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
References 1. Sadakane M, Sasaki K, Kunioku H, Ohtani B, Ueda W, Abe R (2008) Chem Commun 48:6552 2. Li L, Krissanasaeranee M, Pattinson SW, Stefik M, Wiesner U, Steiner U, Eder D (2010) Chem Commun 46:7620 3. Ma SSK, Maeda K, Abe R, Domen K (2012) Energy Environ Sci 5:8390 4. Ha JH, Muralidharan P, Kim DK (2009) J Alloys Compd 475:446 5. Yang B, Barnes PRF, Bertram W, Luca V (2007) J Mater Chem 17:2722 6. Balaji S, Djaoued Y, Albert AS, Ferguson RZ, Bruning R (2009) Chem Mater 21:1381 7. Yan AH, Xie CS, Zeng DW, Cai SZ, Li HY (2010) J Alloys Compd 495:88 8. Nah YC, Ghicov A, Kim D, Schmuki P (2008) Electrochem Commun 10:1777 9. Zhang J, Wang XL, Xia XH, Gu CD, Zhao ZJ, Tu JP (2010) Electrochim Acta 55:6953 10. Meda L, Breitkopf RC, Haas TE, Kirss RU (2002) Thin Solid Films 402:126 11. Deshpande R, Lee SH, Mahan AH, Parilla PA, Jones KM, Norman AG, To B, Blackburn JL, Mitra S, Dillon AC (2007) Solid State Ion 178:895
123
12. Cao BB, Chen JJ, Tang XJ, Zhou WL (2009) J Mater Chem 19:2323 13. Gu Z, Zhai T, Gao B, Sheng X, Wang Y, Fu H, Ma Y, Yao J (2006) J Phys Chem B 110:23829 14. Peng T, Ke D, Xiao J, Wang L, Hu J, Zan L (2012) J Solid State Chem 194:250 15. Shibuya M, Miyauchi M (2009) Adv Mater 21:1373 16. Zhang J, Wang XL, Xia XH, Gu CD, Tu JP (2011) Sol Energy Mater Sol Cells 95:2107 17. Wu JM, Zhang TW, Zeng YW, Hayakawa S, Tsuru K, Osaka A (2005) Langmuir 21:6995 18. Gao MR, Lin ZY, Jiang J, Cui CH, Zheng YR, Yu SH (2012) Chem Eur J 18:8423 19. Abe R, Takami H, Murakami N, Ohtani B (2008) J Am Chem Soc 130:7780 20. Qi H, Wang CY, Liu J (2005) Adv Mater 15:411 21. Li G, Liu ZQ, Lu J, Wang L, Zhang Z (2009) Appl Surf Sci 255:7323 22. Leftheriotis G, Papaeffthimiou S, Yianoulis P, Siokou A, Kefalas D (2003) Appl Surf Sci 218:276 23. Mane RB, Potdar AS, Nadgeri JM, Biradar NS, Rode CV (2012) Ind Eng Chem Res 51:15564 24. Gerand B, Nowogrocki G, Guenot J, Figlarz M (1979) J Solid State Chem 29:429 25. Lee K, Seo WS, Park JT (2003) J Am Chem Soc 125:3408 26. Kim J, Lee CW, Choi W (2010) Environ Sci Technol 44:6849
