Platinum Nanoparticle Co-Catalyst-Induced Improved

Journal of the Korean Physical Society, Vol. 55, No. 6, December 2009, pp. 2470∼2475

Platinum Nanoparticle Co-Catalyst-Induced Improved Photoelectrical Properties in a Chromium-Doped SrTiO3 Photocatalyst J. S. Jang and J. S. Lee Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784

Pramod H. Borse Center for Nanomaterials, International Advanced Research Centre for Powder Metallurgy and New Materials (ARC International), Balapur PO, Hyderabad, AP, 500 005, India

K. T. Lim Department of Imaging System Engineering, Pukyong National University, Busan 609-735

O.-S. Jung Department of Chemistry (BK21), Pusan National University, Busan 627-706

E. D. Jeong, M. S. Won and H. G. Kim∗ Busan Center, Korea Basic Science Institute, Busan 609-735 (Received 31 July 2009, in final form 21 September 2009) SrTi1−x Crx O3 (0 ≤ x ≤ 0.5) photocatalysts were prepared by using conventional solid state reactions at 1300 ◦ C for 5 h and were characterized by using ultraviolet-visible (UV-vis) absorption spectroscopy. SrTi1−x Crx O3 samples not only absorb UV light photons, but also visible light photons due to substitutional doping of chromium at titanium sites in the SrTiO3 lattice. Apparently, the band transition from the Cr 3d to the Cr 3d + Ti-3d hybrid orbital induces visible light absorptivity in SrTi1−x Crx O3 samples. The photocatalytic activity of Cr-doped SrTiO3 samples for hydrogen production from water-methanol mixtures under UV light irradiation decreased with increased Cr doping concentration in SrTiO3 . Conclusively, there exists an optimum chromium and platinum concentration in SrTiO3 for achieving high photocatalytic activity under visible light (λ ≥ 420 nm) PACS numbers: 61.66.Fn, 61.72.Ww, 81.05.Zx, 85.40.Ry Keywords: Cr-doped SrTiO3 , Solid state reaction, Photocatalysis, Hydrogen production DOI: 10.3938/jkps.55.2470

I. INTRODUCTION Hydrogen production by use of semiconductor photocatalysts has recently received much attention because, SrTiO3 has been a promising material for photocatalytic water splitting as well as photocatalytic oxidation reactions under UV light by virtue of its higher reduction potential and lower oxidation potential [1–5], as shown in Fig. 1. It can be observed that compared to other materials viz. TiO2 , ZnO, SrTiO3 is a fascinating material having an ideal band gap energy and band positions for water splitting, but it has a wide band gap (3.2 eV); consequently it shows photocatalytic activity only under ∗ To

whom correspondence should be addressed. [email protected]; Tel: +82-51-974-6104; Fax: 82-51-517-2497

UV light. Since visible light accounts for the largest portion (ca. 46 %) of the solar spectrum, visible-light-driven photocatalysts that can produce hydrogen from water splitting or aqueous electrolyte solutions under solar light are needed [6–10]. The development of visible-light photocatalytic materials, therefore, has become an important topic in the photocatalysis research today. One of the promising approaches to develop new photocatalysts is tuning or modification of the optical properties of UV-light-active catalysts by substitutional doping, as demonstrated in Nix In1−x TaO4 [11], SrTix M1−x O3 (M = Ru, Rh, Ir, Pt, Pd) [12], TiO2−x Crx O2 [13], TaON [14], TiO2−x Cx [15,16], and TiO2−x Nx [17] for cationic and anionic doping. Here, we tuned the band gap of SrTiO3 by substitutional doping of Cr metal ion in the host lattice during

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Platinum Nanoparticle Co-Catalyst-Induced Improved Photoelectrical · · · – J. S. Jang et al.

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Diffraction Standards (JCPDS) data for phase identification. The optical property viz. band gap energy of the as-prepared material was studied by using an UVVisible diffuse reflectance spectrometer (Shimadzu, UV 2401). The morphology was observed by using scanning electron microscopy (SEM, Hitachi, S-2460N) and highresolution transmission electron microscopy (HR-TEM, Philips, CM 200).

3. Photocatalytic Activity

Fig. 1. Position of valence and conduction band edges for several oxide materials in contact with an aqueous electrolyte at pH = 0.

the conventional solid state reaction and further characterized it with UV-vis diffuse reflectance spectroscopy (UV-vis DRS) and X-ray diffraction (XRD). In addition, the most important issue here is related to the co-catalyst loading over the SrTiO3 photocatalyst. These paper not only describes the work on Cr-doped SrTiO3 but it also specifically demonstrates how the optimization of the platinium- co-catalyst concentration can improve the efficiency of hydrogen evolution under UV and visible light irradiation (λ ≥ 420 nm).

II. EXPERIMENTAL 1. Preparation of Nanocrystalline SrTi1−x CrO3

The SrTi1−x Crx O3 (0 ≤ x ≤ 0.5) powders were synthesized by using the conventional solid state reaction (SSR) method. For the preparation of SrTi1−x Crx O3 (0 ≤ x ≤ 0.5) sample of various Cr doping concentrations, the stoichiometric amounts of SrCO3 (99.99 %, Aldrich) and TiO2 (99.99 %, Aldrich) and Cr2 O3 (99.9 %, Aldrich) were mixed and ground in methanol. The pelletized powders were calcined at 1300 ◦ C for 5 h in a static furnace. On the other hand, for the purpose of comparison, TiO2−x Nx nanoparticles were also prepared by using the hydrolytic synthesis method (HSM) [18].

2. Characterization

SrTi1−x Crx O3 (0 ≤ x ≤ 0.5) samples thus obtained were characterized by an X-ray diffractometer (Mac Science Co., M18XHF). X-ray diffraction (XRD) results were compared with the Joint Committee Powder

Photocatalytic reactions were performed in an outerirradiation-type pyrex reactor equipped with a cutoff filter (λ ≥ 420 nm) and liquid filter to remove IR and using a high-pressure Hg lamp (Oriel, 500 W). The rate of H2 evolution was determined for water-methanol solution (distilled water 70 ml and methanol 30 ml) while stirring with 0.1 g of the catalyst loaded Pt. The amounts of H2 evolution were analyzed by using a gas chromatograph equipped with a thermal conductivity detector (molecular sieve 5-˚ A column and Ar carrier). Pt-metal-loaded catalysts were prepared by using in-situ photodeposition method. For this, the photocatalyst was added to an aqueous methanol solution containing the required amount (0.1 ∼ 1.0 wt%) of H2 PtCl6 and was filtered and then dried in a static oven.

III. RESULTS AND DISCUSSION Figure 2 shows the XRD patterns of SrTi1−x Crx O3 (0 ≤ x ≤ 0.5) samples prepared by sintering the ground mixture of SrCO3 , TiO2 , and Cr2 O3 at 1300 ◦ C for 5 h in air. The undoped SrTiO3 crystal formed at 1300 ◦ C exhibits a tetragonal structure without any impurity phase. With increasing concentration of chromium (x = 0.03), the X-ray diffraction patterns of SrTi0.97 Cr0.03 O3 are slightly shifted to higher angles. The gradual shift of the main (112) peak toward higher diffraction angles with increasing chromium content is correlated with increases in the lattice constants. The small difference in the ionic radii of the Ti4+ (0.61 ˚ A) and the Cr3+ (0.62 ˚ A) ions suggests that there is a low probability of deformation of the tetragonal unit cell, but a shift in the 2θ angle of diffraction is expected due to the lattice expansion. Indeed, no other impurity phase was detected in the XRD pattern except for the last sample with the highest chromium content. However, a further increase in chromium (x > 0.03) didn’t show a gradual shift in the peak position. The SrTi1−x Crx O3 samples prepared with compositions of 0.1 ≤ x ≤ 0.5 represent a mixture of the SrTiO3 cubic structure and unknown materials, as shown Fig. 2(e) ∼ (i). This indicates that there exists a maximized amount of Cr substituted into the lattice at the Ti site in the SrTiO3 crystal structure.

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Fig. 4. SEM images of SrTi1−x Crx O3 photocatalysts for x = (a) 0.05, (b) 0.07, (c) 0.1, and (d) 0.2. Fig. 2. X-ray diffraction patterns of SrTi1−x Crx O3 photocatalysts for x = (a) 0, (b) 0.03, (c) 0.05, (d) 0.07, (e) 0.1, (f) 0.2, (g) 0.3, (h) 0.4, and (i) 0.5.

The optical properties were probed by using UVVis diffuse reflectance (UV-DRS) spectroscopy for chromium-doped SrTiO3 samples with different compositions. Fig. 3 shows the UV-visible diffuse reflectance spectra of SrTi1−x Crx O3 samples (0 ≤ x ≤ 0.5). In the case of the undoped SrTiO3 sample, the absorption edge appeared near 388 nm, corresponding to 3.2 eV and consistent with the literature. However, the absorption spectra of the Cr-doped SrTiO3 samples depend on the content of chromium and exhibit a new absorption shoulder in the visible region. The absorption edge around 630 nm was ascribed to charge transfer from Cr3+ to Ti4+ while the broad absorption ranging from 630 to 800 nm was ascribed to a d-d transition of 4A2 → 4T2 in Cr3+ ions in the octahedral system [19]. To get the band gap energy of the undoped and the Cr-doped SrTiO3 samples we drew a line with a steep slope in the direction of the wavelength around a point that represented the maximum absorptivity in the UV-DRS. We calculated the band gap energy of the as-prepared samples with the obtained wavelength and the following simple conversion equation; Band gap energy (in eV) = 1240/wavelength (in nm).

Fig. 3. UV–vis diffuse reflectance spectra of SrTi1−x Crx O3 photocatalysts for x = (a) 0, (b) 0.03, (c) 0.05, (d) 0.07, (e) 0.1, (f) 0.2, (g) 0.3, (h) 0.4, and (i) 0.5.

The increase in the amount of Cr in the SrTiO3 structure formed an impurity phase and prevented the crystal growth of SrTiO3 .

Figure 4 shows a scanning electron microscopy (SEM) image of the SrTi1−x Crx O3 (x = 0.05, 0.07, 0.1, 0.2) samples with different contents of chromium calcined at 1300 ◦ C for 5 h. All samples have the morphology of a well-developed crystal, and the grain sizes of assynthesized samples are a similar, 5 – 10 µm. However, with increasing Cr doping level, there exists crystals with small sizes between the grain boundaries of the sintered ceramic samples. This result affects the specific surface area and, consequently, the photocatalytic activity of the as-prepared samples. We investigated the photocatalytic hydrogen produc-


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Table 1. Photocatalytic H2 production from a methanol-water solution over 1-wt% Pt/SrTi1−x Crx O3 (0.0 ≤ x ≤ 0.5) and 1-wt% Pt/TiO2−x Nx samples. H2 evolution (mmol/g·cat) Catalyst Pt/SrTiO3 Pt/SrTi0.97 Cr0.03 O3 Pt/SrTi0.95 Cr0.05 O3 Pt/SrTi0.93 Cr0.07 O3 Pt/SrTi0.9 Cr0.1 O3 Pt/SrTi0.8 Cr0.2 O3 Pt/SrTi0.7 Cr0.3 O3 Pt/SrTi0.6 Cr0.4 O3 Pt/SrTi0.5 Cr0.5 O3 Pt/TiO2−x Nx

Energy Bandgap Eg (eV)-1 Eg (eV)-2 3.25 3.25 2.00 3.25 2.00 3.25 2.00 3.25 2.00 3.25 2.00 3.25 2.00 3.25 2.00 3.25 2.00 3.2 2.73

UV light irradiation (λ ≥ 210 nm)

Visible light irradiation (λ ≥ 420 nm)

76 33 32 27 26 6 5 3 1 8

0 1.1 1.3 2.8 1.6 Trace Trace

tion from the methanol-water solution by using the asprepared samples under UV and visible light irradiation to examine the extent of the improvement in the efficiencies under for these different regions. Table 1 shows the results of H2 evolution and the respective band gaps of the samples, an calculated from the respective spectra. All samples showed photocatalytic activity for hydrogen production from methanol-water solution under UV light irradiation (λ ≥ 210 nm). Among the as-prepared samples, undoped SrTiO3 showed the highest photocatalytic activity, as compared to the other samples. The doped samples, SrTi1−x Crx O3 (0.03 ≤ x ≤ 0.1), with low doping level only showed H2 production as high as 1.1 ∼ 2.8 mmol/gcat·hr, TiO2−x Nx and undoped and doped SrTiO3 (0.2 ≤ x ≤ 0.5) samples with high doping levels showed only a trace amount of H2 production under visible light irradiation. The SrTi0.93 Cr0.07 O3 sample showed the highest photocatalytic activity for hydrogen production under visible light irradiation (λ ≥ 420 nm). This indicates that the SrTi0.93 Cr0.07 O3 seems to be an optimum concentration for producing a relatively highactivity material as compared to the other Cr concentrations. Possibly, a higher Cr doping concentrations than SrTi0.9 Cr0.1 O3 leads to an impurity phase, which could play the role of a recombination center. From the result of the above-mentioned photocatalytic activity, we thought that the photocatalytic activity over the 1 wt% Pt-loaded SrTiO3 sample was lower than the general photocatalytic activity under UV light irradiation. This low activity of the as-prepared samples comes from the amount of Pt loading on SrTiO3 , meaning that the amount of Pt loading could be optimized as a co-catalyst for photocatalytic activity [20,21]. We investigated the dependence of the photocatalytic activity on the amount of Pt loading on the SrTi0.93 Cr0.07 O3 sample, which showed the highest photocatalytic activity under visible light irradiation (λ ≥ 420 nm). As shown in Fig. 5, the photocatalytic activity showed a maximum activity

Fig. 5. Effects of the amount of platinum loaded on SrTi0.93 Cr0.07 O3 on the rate of hydrogen production under visible light irradiation (λ ≥ 420 nm).

at 0.1 Pt wt% and then decreased gradually with further increases in the amount of Pt loading. Thus, an optimum level of Pt loading on SrTi0.93 Cr0.07 O3 is necessary for an efficient hydrogen production. We also investigated the photocatalytic activity for H2 production using all SrTi1−x Crx O3 (0 ≤ x ≤ 0.5) samples with the optimized amount (0.1 wt%) of Pt loading on the SrTi0.93 Cr0.07 O3 sample in the presence of methanol as hole scavengers under UV and under visible light irradiation. Table 2 shows the results of H2 evolution and the respective band gaps of the samples calculated from respective spectra. The photocatalytic activity of 0.1-wt% Pt-loading SrTi1−x Crx O3 increased 2 ∼ 4 times and 10 ∼ 20 times than those of 1.0 wt% Pt loading SrTi1−x Crx O3 under UV and visible light irradiation, respectively.

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Table 2. Photocatalytic H2 production from methanol-water solution over 0.1-wt% Pt /SrTi1−x Crx O3 (0.0 ≤ x ≤ 0.5) and 0.1-wt% Pt/TiO2−x Nx samples. H2 evolution (mmol/g·cat) Catalyst Pt/SrTiO3 Pt/SrTi0.97 Cr0.03 O3 Pt/SrTi0.95 Cr0.05 O3 Pt/SrTi0.93 Cr0.07 O3 Pt/SrTi0.9 Cr0.1 O3 Pt/SrTi0.8 Cr0.2 O3 Pt/SrTi0.7 Cr0.3 O3 Pt/SrTi0.6 Cr0.4 O3 Pt/SrTi0.5 Cr0.5 O3 Pt/TiO2−x Nx

Energy Bandgap Eg (eV)-1 Eg (eV)-2 3.25 3.25 2.00 3.25 2.00 3.25 2.00 3.25 2.00 3.25 2.00 3.25 2.00 3.25 2.00 3.25 2.00 3.2 2.73

UV light irradiation (λ ≥ 210 nm)

Visible light irradiation (λ ≥ 420 nm)

122 83 77 71 64 9 3 1 1 13

0 17 22 41 33 Trace Trace

Fig. 7. Schematic band structure of SrTi1−x Crx O3 and its mechanism for photocatalytic hydrogen production from a methanol-water solution.

Fig. 6. HR-TEM images show platinum nanoparticles welldispersed over the surface of the SrTi0.93 Cr0.07 O3 sample with contents of (a) 0.1 and (b) 1.0 wt%.

In Figure 6, HR-TEM images show well-dispersed platinum nanoparticles over the surfaces of the SrTi0.93 Cr0.07 O3 samples with contents of (a) 0.1, (b) 1.0 wt%. This indicates that platinum nanoparticles are well deposited on the surfaces of the SrTi0.93 Cr0.07 O3 samples. The dispersion of the 0.1-wt% Pt-loaded

SrTi0.93 Cr0.07 O3 sample looks better than the dispersion on the 1-wt% Pt-loaded SrTi0.93 Cr0.07 O3 sample, as shown in the HTEM images of Fig, 6. We think that this result is important evidence to validate why the photocatalytic activity over the 0.1 wt% Pt-SrTi1−x Crx O3 (0 ≤ x ≤ 0.5) samples could be improved. Figure 7 shows the schematic band structure of SrTi1−x Crx O3 and its mechanism for photocatalytic hydrogen production from a methanol-water solution. Crdoped SrTiO3 produced H2 photocatalytically in the presence of an aqueous methanol-water solution under visible light. However, the role of Pt loading looks more important to enhance the photo-reduction efficiency. As in case of Pt/Cr-doped SrTiO3 , an electron excited to the conduction band acquires sufficient reduction potential to reduce H+ ion, and a hole in the valence band has a lower oxidation potential for CH3 OH degradation to CO2 . Therefore, as compared to Cr-doped SrTiO3 , a Pt-loaded photocatalyst tenders a higher probability to


transfer the electron for photoreduction. Additionally, Pt-loading further enhances the probability of charge separation, thereby hindering the electron-hole recombination. Thus, it is evident that an optimum Pt-loading is necessary to acquire an improved photocatalyst.

IV. CONCLUSIONS SrTi1−x Crx O3 photocatalysts were successfully synthesized by using the solid state reaction method. Consequently, lower absorption bands were obtained by Cr doping in SrTiO3 . The SrTi1−x Crx O3 (0.03 ≤ x ≤ 0.1) samples with low doping level were synthesized without an impurity phase and showed a photocatalytic activity under visible light. A further increase in the amount of Cr SrTiO3 led to the formation of as unknown impurity phase. The photocatalytic activity of the 0.1-wt% Pt/SrTi0.93 Cr0.07 O3 sample was much higher than that of 1-wt% Pt/SrTi0.93 Cr0.07 O3 , or even an earlier-known visible-light-responsive photocatayst (TiO2−x Nx ) for H2 production under UV and visible light. Therefore, it is evident that even though Cr doping can reduce the band gap of SrTiO3 , an optimum Pt-nanoparticle-loading concentration is very important for achieving an improved visible light photocatalyst for SrTiO3 .

ACKNOWLEDGMENTS This work is supported by KBSI grant T29320, MKERTI04-0201, KOSEF grant (NCRCP, R15-2006-02201002-0), and by the Hydrogen Energy R&D Center, Korea.

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