Shijing
Liang
,
Xiaowei
Wang
,
Yan
Chen
,
Jia
Zhu
,
Yongfan
Zhang
,
Xuxu
Wang
,
Zhaohui
Li
* and
Ling
Wu
*
State Key Laboratory Breeding Base of Photocatalysis, Research Institute of Photocatalysis, Fuzhou University, Fuzhou, 350002, P. R. China. E-mail: wuling@fzu.edu.cn; zhaohuili@fzu.edu.cn; Fax: +86-591-8377-9105; Tel: +86-591-8377-9362
First published on 6th September 2010
A photocatalyst Sr0.4H1.2Nb2O6·H2O (HSN) nanopolyhedra with high surface area has been successfully prepared by a simple hydrothermal method. The as-prepared samples were characterized by XRD, BET, SEM, TEM and XPS. The electronic structure of HSN determined by DFT calculations and electrochemical measurement revealed that HSN is an indirect-bandgap and n-type semiconductor, respectively. HSN samples showed high photocatalytic activities for both pure water splitting and the decomposition of benzene. The rate of H2 evolution over HSN was 15 times higher than that of P25 and the conversion ratio of benzene exceeded twice that of P25. The photocatalytic activities for water splitting can be greatly improved by loading various co-catalysts on HSN, such as Au, Pt, and Pd. The photocatalytic mechanisms were proposed based on the band structure and characterization results of the photocatalyst.
Nanostructured materials have received great interest primarily owing to their unique properties and morphologies. Their performances in a variety of applications can be significantly improved by controlling their surface area, size, shape, and composition.7 For example, nanostructured photocatalysts with large surface area, small particle size, and various morphologies exhibited much higher photocatalytic activity compared with their bulk counterparts, because of more active sites and lower recombination rate of photogenerated electron-hole pairs.8 Unfortunately, due to the limitation of the suitable precursors, the reported niobate photocatalysts have mainly been synthesized by a traditional high-temperature solid state reaction. Most recently, our group has developed a simple and versatile method for preparing various niobates, employing Nb2O5·nH2O as the precursor.8a Not only the stable phase niobates, but also the metastable phase niobates, like Sr0.4H1.2Nb2O6·H2O, can be prepared under mild reaction conditions using this strategy. It should be pointed out that the studies on the structure and the performances of the metastable phase photocatalyst can not only enrich our understanding of photocatalysis, but can also give us guidance in searching for a highly efficient photocatalyst. Our subsequent study has also demonstrated that Sr0.4H1.2Nb2O6·H2O nanopolyhedra showed photocatalytic activity on the decomposition of organic dyes like methyl orange (MO). However, to the best of our knowledge, the other properties of this newly developed photocatalyst Sr0.4H1.2Nb2O6·H2O, such as the electronic band structure and the photocatalytic activity for water splitting, etc., have not been investigated.
Herein, we present a hydrothermal method for the fabrication of Sr0.4H1.2Nb2O6·H2O nanopolydedra with high surface area. Its photocatalytic activities for both pure water splitting and the decomposition of benzene in the gas phase have been investigated. The effect of various co-catalysts on the photocatalytic water splitting over Sr0.4H1.2Nb2O6·H2O has also been studied. The mechanisms for both the water splitting and the degradation of benzene over HSN have been proposed in terms of its electronic band structure as determined by the DFT calculations.
The photocatalytic oxidation of benzene into carbon dioxide was measured with a fixed bed tubular quartz reactor operated in a single-pass mode. The catalyst (0.3 g, 50–70 mesh) was loaded in the reactor surrounded by four 4W UV-lamps with a wavelength centered at 254 nm (Philips, TUV 4W/G4 T5). Benzene diluted in a pure oxygen stream was used as the test reactant stream. The flow rate of the reactant stream was kept at 20 mL min−1. The concentrations of benzene and carbon dioxide were simultaneously determined by the flame ionization detector (FID) and thermal conductivity detector (TCD), respectively, with an online gas chromatograph (Agilent 6890N).
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Fig. 1 XRD patterns of the samples prepared at different temperatures for 48 h. |
T/°C | Crystallite size/nm | BET surface area/m2 g−1 | Pore structure |
---|---|---|---|
160 | 41.2 | 51.4 | Accumulated porous structure |
180 | 49.8 | 22.9 | |
200 | 56.8 | 18.4 |
XPS measurements were carried out to further investigate the surface compositions and chemical states of the as-prepared HSN and the results are showed in Fig. 2. No obvious peaks for impurities have been observed for any of the samples. As shown in Fig. 2b–c, the core lines were fixed at 134.9 eV (Sr 3d5/2) and 208.4 eV (Nb 3d5/2). For the Sr 3d and Nb 3d, the spin orbit separations (Δ) were 1.8 and 2.8 eV, respectively, and the ratios of two peak areas were 3:
2. These results demonstrated that the chemical states of the sample were Sr2+ and Nb5+. Fig. 2d showed the peak corresponding to O may be fitted to two kinds of chemical environments: crystal lattice oxygen and adsorbed oxygen. The peak at 531.6 eV is assigned to the crystal lattice oxygen, while the peak at 533.2 eV is related to the adsorbed oxygen. The ratio of two peak areas was about 5
:
2. The results indicated that the catalyst has an abundance of surface-adsorbed oxygen (surface OH groups). As we know, the surface OH groups play an important role in the photocatalytic oxidation process since they can react with photogenerated holes to form hydroxyl radicals when excited by UV light.12 No signal for F 1s was observed in Fig. 2e, indicating that the F− anion can be thoroughly removed during the preparation process.
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Fig. 2 XPS spectra of sample HSN: (a) survey XPS spectrum and (b–e) high-resolution spectra of Sr 3d, Nb 3d, O 1s, and F 1s. |
Fig. 3 shows the UV–vis DRS for HSN samples prepared at different temperatures. It could be seen that the samples had a steep absorption edge in the UV region, indicating that they were due to a band gap transition from valence band to conduction band.13 The wavelength at the absorption edge, λ, is determined as the intercept on the wavelength axis for a tangent line drawn on the absorption spectra. The absorption edges of the samples prepared at different temperatures were all located at about 300 nm, corresponding to a band gap of ∼4.1 eV. The wide gap of HSN might be a drawback; however, it is not a serious problem for treatment devices using bactericidal lamps (emission wavelength 254 nm). In addition, our initial experiments have shown that HSN can be easily modified by doping nitrogen into the crystal lattice under an NH3 gas flow at high temperature, making the materials responsive to the visible spectrum.
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Fig. 3 UV–vis DRS of Sr0.4H1.2Nb2O6·H2O synthesized at different temperatures. |
To analyse the electronic structure, we have carried out density functional theory (DFT) calculations for the cubic phase Sr0.4H1.2Nb2O6·H2O. The band structure and density of states (DOS) are displayed in Fig. 4. It can be seen clearly that the HSN sample is a wide band gap semiconductor. An indirect transition from the valence band at L point to the conduction band at Γ point as the minimum band gap, for which a value of 3.5 eV is determined. Due to the inherent deficiency of the DFT method,14 the calculated band gap is smaller than the experimental value although a portion of Hartree–Fock exchange (20%) is introduced in the hybrid B3LYP framework. According to the DOS shown in Fig. 4b, the top of the valence band of HSN is mainly derived from the O 2p orbitals, while obvious components of the Nb atom can also be found for the bands in the energy region between −6 and −1 eV, indicating the covalent interaction between Nb and O atoms. On the other hand, the bottom of the conduction band is dominated by the Nb 4d orbitals. The relatively delocalized Nb 4d states might give a high mobility to the photoinduced electrons.15
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Fig. 4 Band structure (a) and density of state (b) of the HSN sample (the top of the valence band was set to zero in the figures). |
In order to better understand the intrinsic electronic properties of HSN, a Mott–Schottky measurement was also performed in darkness using impedance techniques. Reversed sigmoidal plots were observed with an overall shape that is consistent with that of typical n-type semiconductors. The flatband potential of HSN estimated by typical Mott–Schottky plots at various frequencies (Fig. 5a) was −0.28 eV vs. SHE and close to that of TiO2 (−0.29 eV),16 whereas the edge energy of the valence band (EVB) of the HSN sample was estimated to be 3.82 eV, much lower than that of TiO2 (2.91 eV) (Fig. 5b). This indicates that the HSN thermodynamically enables photocatalytic water splitting into H2 (0 eV) and O2 (1.23 eV). Moreover, the higher energy levels of valence band edges allow the photogenerated holes to be kinetically more favorable to produce active OH˙ radicals. In addition, the HSN might show higher photocatalytic activities for environmental photocatalysis as compared with TiO2.
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Fig. 5 Typical Mott–Schottky plots (a) and band structure (b) of the HSN sample prepared at 160 °C. |
The morphology of HSN synthesized at 160 °C was characterized by SEM and TEM. SEM examination (Fig. 6a) showed that the sample consisted of nanopolyhedra ranging from 30 to 70 nm and was well-dispersed. The morphology was also confirmed by the TEM image (Fig. 6b). As shown in Fig. 6c, clear lattice fringes can be observed. The interplanar spacing was consistent with the d-spacing of the corresponding lattice plane. The fringes of d = 0.61 nm matched that of the (111) crystallographic plane of cubic Sr0.4H1.2Nb2O6·H2O. A typical selected area electron diffraction (SAED) pattern (Fig. 6d) revealed that the sample had single-crystalline character.
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Fig. 6 SEM (a), TEM (b), HRTEM images (c), and typical SAED pattern (d) of the HSN sample prepared at 160 °C. |
The above-mentioned discussions indicate that the HSN sample might be used as a good candidate for photocatalytic reduced reactions. Therefore, the photocatalytic activities of the samples without co-catalyst were evaluated by the pure water splitting into H2 under high-pressure Hg lamp irradiations, the results have been compared with that obtained over TiO2 (Degussa P25) (Fig. 7). At the beginning, the exhibited activity is low, probably due to a catalyst activation process. After the induction period, HSN showed higher activity and no noticeable loss of activity was observed during the whole reaction process. The crystal structure and the chemical surface state of HSN nanopolyhedra after photocatalytic splitting of water were also checked by XRD and XPS, which showed no observable change after the reaction (Fig. S4†).
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Fig. 7 Photocatalytic activities of the HSN sample and TiO2 without any co-catalyst (50 mg catalyst, 125 W high-pressure Hg lamp, 170 mL H2O). |
The photocatalytic efficiency of HSN samples showed strong dependency on the synthetic temperature and the surface area. A reduction in the efficiency was observed with the increase of the hydrothermal preparation temperature and the decrease of the surface area. The highest activity was obtained on the sample prepared at 160 °C, which shows a H2 evolution rate of 91.9 μmol h−1 gcatalyst−1. This value was 15 times higher than that of TiO2 (5.9 μmol h−1 gcatalyst−1). Since HSN and TiO2 have similar positions for the conduction band bottom, and the number of absorbed photons for HSN should be smaller than that of TiO2 under the same experimental conditions, the higher photocatalytic activity for water splitting observed over HSN should be due to its electronic structure, i.e. the composition of the conduction band.
In addition, to further obtain higher activity, a series of co-catalysts have been introduced into HSN as electron acceptors. The effect of different co-catalysts on the rate of hydrogen production from pure water is shown in Fig. 8. Compared with pure HSN, HSN loaded with 1.0 wt.% of various noble metals, such as Au, Pt, and Pd, exhibited a great improvement of the H2 evolution rate. This enhancement may be well explained by the work functions of different noble metals. Since the work functions of Au (5.1 eV), Pt (5.65), and Pd (5.55)10 are much higher than that of HSN, a Schottky barrier can be formed at the metal-HSN interface. Benefiting from the Schottky barrier, the photogenerated electrons can transfer from HSN to the noble metal, and the recombination of photogenerated charge carriers can be suppressed. The H2 evolution rate follows the order: Au/HSN (434.1 μmol h−1 gcatalyst−1) > Pt/HSN (385.6) > Pd/HSN (249.3) > HSN (91.9). Moreover, the amount of Au loaded was also an important factor for improving the activity (Fig. 8b). When the amount of Au increased from 0.2 to 1.0 wt.%, the rate was enhanced from 142.1 to 434.1 μmol h−1 gcatalyst−1. However, when the amount of loaded Au exceeded 1.0 wt.%, the rate was reduced. The decreased activity may be caused by the reduction in the light adsorption by excessive Au loading.
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Fig. 8 Photocatalytic activities of sample HSN prepared at 160 °C with various co-catalysts: (a) different co-catalysts; (b) different amounts of Au (50 mg catalyst, 125 W high-pressure Hg lamp, 170 mL H2O). |
The turnover number, usually introduced to determine if the reaction was one photocatalytic reaction, is defined by the number of reacted molecules to that of an active site.1c However, it is difficult to determine the number of active sites for heterogeneous photocatalysts. Here, the total amount of the catalyst was adopted as the number of active sites to ensure the reliability of the evaluation.8b,17 For H2 evolution with 1% Au as co-catalyst, the turnover number was estimated to be 3 after 10 h of reaction time. The result proved that the reaction was photocatalysis instead of a photocorrosion.
Interestingly, common co-catalysts like NiO and RuO2 that is usually used to enhance H2 evolution were not effective for HSN photocatalyst. Instead they showed a negative effect. This phenomenon can be well explained by their relative band positions (Fig. 9). The conduction band level of HSN was roughly estimated to be −0.28 eV while that of NiO is −0.96 eV.18 Therefore, the photogenerated electrons in the conduction band of HSN were not able to transfer to the conduction band of NiO co-catalyst which may cover the active site of hydrogen evolution and decrease the activity.
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Fig. 9 Mechanism of water splitting over the HSN samples. |
Our previous work8a demonstrated that HSN can decompose methyl orange effectively in aqueous solution since the valence band (VB) of HSN (3.8 eV vs. SHE, pH 0) is more positive than the ˙OH/H2O potential (2.8 eV). This implies that HSN can also be used to decompose other organic contaminants not only in the aqueous but also in the gas phase. To further confirm the photocatalytic activity of HSN nanopolyhedron, the photocatalytic degradation of benzene in the gas phase under UV light irradiation was chosen and the result is shown in Fig. 10. Under UV illuminations, about 17.8% benzene was converted and 65 ppm CO2 was produced over HSN. Considering the negligible conversion of excited-state benzene on dielectric oxides,19 these results suggested that the degradation of benzene proceeded photocatalytically on HSN. Although both samples were capable of oxidizing benzene with molecular oxygen, HSN showed much higher efficiency compared with TiO2. The CO2 production rate and benzene reaction rate of sample HSN was 2 times higher than that of TiO2. Since these two samples have similar BET surface areas, their different activities may be explained by the different oxidizing capability of the hole (h+) (Fig. 5b). It is well known that ˙OH, ˙O2− and h+ were commonly suggested as the primary oxidizing species in the gas-phase photocatalytic oxidation processes. The generation of ˙OH and ˙O2− radicals have been examined by ESR. Four characteristic peaks of DMPO–˙OH were obviously observed under the UV light irradiation as shown in Fig. 11a. However, the characteristic peaks of DMPO–˙O2− were very weak (Fig. 11b) and suggested that the photocatalytic degradation of benzene may proceed mainly via ˙OH or/and h+ instead of ˙O2− oxidation.
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Fig. 10 Photocatalytic decomposition of benzene on sample HSN prepared at 160 °C (0.3 g catalyst, 20 mL min−1 O2 flow, initial benzene concentration). |
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Fig. 11 ESR spectra observed for sample HSN prepared at 160 °C. The active species are (a) ˙OH irradiated for 80 s and (b) ˙O2− irradiated for 360 s. |
Footnote |
† Electronic supplementary information (ESI) available: Schematic structure of HSN; Nitrogen adsorption-desorption isotherm of HSN samples; Comparison of XRD and XPS spectra of HSN sample before and after reaction of water splitting. See DOI: 10.1039/c0nr00327a |
This journal is © The Royal Society of Chemistry 2010 |