Enhanced visible light photocatalysis over Pt-loaded Bi₂O₃: an insight into its photogenerated charge separation, transfer and capture

Abstract

In this study, we have fabricated crystalline metallic Pt nanoparticles-loaded α-Bi₂O₃ microrods (Pt/Bi₂O₃) using a precipitation method, followed by an impregnation–reduction deposition route. The Pt/Bi₂O₃ catalysts have much higher photocatalytic activities than that of the pure Bi₂O₃ for the degradation of RhB and 2,4-DCP under visible light irradiation. The photogenerated charge separation, transfer, and capture in the photocatalysis of Pt/Bi₂O₃ were investigated in detail according to the various characterizations and analyses of the photogenerated active species and H₂O₂. It was revealed that the loaded Pt plays an important mediating role in the efficient separation of photogenerated electron–hole pairs by rapidly transferring electrons from the excited Bi₂O₃ to the surface oxygen. As a result, the reduction of O₂ to form H₂O₂ by the photogenerated electrons was promoted, leaving more holes on the deep valence band to drive the degradation of organic compounds and the production of ˙OH radicals, which were responsible for the enhanced photocatalysis of Pt/Bi₂O₃.

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Introduction

For the full utilization of solar energy, great efforts have been made to develop visible light photocatalytic materials for environmental pollution remediation in the past two decades.^1–5 It has been well documented that doping with anions or cations into large-band gap semiconductors, such as TiO₂ and SrTiO₃, can create localized levels above the valence band (VB) or below the conduction band (CB) in the forbidden band, leading to an extension of the photoactive region to visible light. A considerable number of visible light photocatalytic materials have been developed on the basis of this strategy.^1,3,6–8 In addition, some small-band gap semiconductors as potential visible light photocatalytic materials have also recently received significant attention due to their intense absorption in the visible region.^9–13 Among the various small-band gap semiconductors, α-Bi₂O₃ is an attractive material with some valuable merits including thermal stability, non-toxicity similar to TiO₂, chemical inertness in neutral water, dielectric permittivity, and wide applications in gas sensors, optical coatings, and photocatalysis.^14–16 Moreover, compared to TiO₂, α-Bi₂O₃ possesses a deeper valence band (VB) maximum (∼+3.13 V vs. NHE), which infers an adequately high oxidation power of valence holes. However, α-Bi₂O₃ alone exhibits a poor photocatalytic performance due to the inability of its conduction band (CB) electrons (∼+0.33 V vs. NHE) to scavenge the surface oxygen molecules through the one-electron reduction reaction process (E⁰(O₂/O₂^−•) = −0.33 V vs. NHE), which results in the fast recombination of photogenerated electron–hole pairs. In recent years, several approaches have been successfully developed for improving the photocatalytic performance of α-Bi₂O₃, which include microstructure control,¹⁶ fluorination,¹⁷ surface modification with Au or Fe(III) clusters,^18,19 and combination with other semiconductors such as ZnO and NiO.^20,21

Earlier investigations on the semiconductor–metal composites have revealed that semiconductor photocatalysts (TiO₂ and ZnO) after Pt loading exhibit highly efficient photocatalytic redox processes since the loaded Pt can act as an electron acceptor to effectively promote the separation of electron–hole pairs with an operation mechanism different from that for loaded Au.^22–24 Recently, it has also been reported that the deposition of Pt on several small bang gap semiconductors, such as Bi₂WO₆, g-C₃N₄, WO₃, and Bi₂O₃, can enhance their visible light photocatalytic performances for the degradation of organic compounds.^25–28 However, the photoinduced charge separation, transfer, and capture involved in the photocatalysis of these Pt-loaded small-band gap semiconductors have not been fully understood. Clarification of these mechanistic problems, we believe, is of great importance for the development of small-band gap semiconductors, especially α-Bi₂O₃ with a deeper valence band, as efficient visible light photocatalytic materials. In this work, we prepared crystalline metallic Pt nanoparticles-loaded α-Bi₂O₃ with different Pt loadings (Pt/Bi₂O₃) by the hydrolysis of bismuth nitrate under basic conditions followed by an impregnation–reduction deposition route. Pt/Bi₂O₃ has much higher photocatalytic activities for the degradation of Rhodamine B and 2,4-dichlorophenol in aqueous solutions than that of pure Bi₂O₃ upon visible light irradiation. Moreover, the role of loaded Pt in the separation of photogenerated electron–hole pairs, the transport pathway and capture of the photoinduced charge involved in the photocatalysis of Pt/Bi₂O₃ are well revealed according to various characterizations and analyses results.

Experimental

Sample preparation

To prepare the pure Bi₂O₃ sample, 10.78 g of Bi(NO₃)₃·5H₂O was dissolved in a 30 mL aqueous solution of 1.5 M HNO₃. NaOH aqueous solution (50% w/v) was slowly added with stirring until yellow precipitate formed at about pH = 13. Subsequently, the mixture was continuously stirred at 80 °C for 2 h, and then the resulting precipitate was collected by centrifugation and washed with deionized water and ethanol several times before drying at 120 °C for 12 h. Finally, the obtained precipitate was further calcined at 450 °C for 5 h to form crystalline Bi₂O₃ powder.¹² The deposition of Pt on Bi₂O₃ was carried out via an impregnation–reduction deposition route.^29,30 Typically, 0.5 g of the as-prepared Bi₂O₃ powder was impregnated with 10 mL aqueous solution of H₂PtCl₆ with stirring for 0.5 h. Subsequently, the Bi₂O₃ powder with adsorbed H₂PtCl₆ was treated with 2 mL aqueous solution containing NaBH₄ (0.1 M) and NaOH (0.5 M) for 0.5 h. During this process, Pt⁴⁺ in H₂PtCl₆ was reduced by NaBH₄ to form Pt⁰ on the Bi₂O₃ surface. Then, the sample was collected by centrifugation and thoroughly washed with deionized water before drying in an oven at 80 °C for 12 h. The obtained Pt-loaded Bi₂O₃ samples were denoted as 0.05, 0.075, and 0.1 wt% Pt/Bi₂O₃, based on the amount of H₂PtCl₆ used in the abovementioned impregnation–reduction deposition route. For comparison, the pure Bi₂O₃ was also treated in the same manner in the absence of H₂PtCl₆.

Characterization

The polycrystalline X-ray diffraction (XRD) patterns of the as-prepared pure Bi₂O₃ and Pt/Bi₂O₃ with different Pt loadings were obtained at room temperature by an X-ray diffractometer (Shimadzu, XRD-7000) using Cu Kα X-ray radiation at 40 kV and 30 mA. The UV-vis diffuse reflectance spectra of the samples were obtained by a UV-vis spectrophotometer (Hitachi U-3900) with BaSO₄ as the reference standard. The chemical states and composition of the surface elements in the samples were studied via X-ray photoelectron spectroscopy (XPS) (ESCSLAB, 250Xi) using 300 W Al Kα radiation. All binding energies were referenced to the C 1s peak (284.6 eV) of the surface adventitious carbon. Morphologies and microstructures were observed using field-emission electron microscopy (FESEM) (JEOL, JSM-7401) and transmission electron microscopy (TEM) (JEOL-2010), respectively. Photoluminescence (PL) measurements were carried out using a fluorescence spectrophotometer (Hitachi, F-4600).

Photocatalytic activity evaluation

The photocatalytic activities of the as-prepared pure Bi₂O₃ and Pt/Bi₂O₃ were evaluated with respect to the degradation of Rhodamine B (RhB) and 2,4-dichlorophenol (2,4-DCP) in aqueous solution, respectively. In a typical procedure, 100 mg of catalyst sample was suspended in 100 mL of RhB (∼10⁻⁵ M) or 2,4-DCP (∼10⁻⁴ M) aqueous solution. A 300 W Xe arc lamp (CHF-XM 150, Beijing Trusttech. Co. Ltd) with a λ > 420 nm cutoff filter was used as a visible light source. Prior to irradiation, the suspension was magnetically stirred in the dark for 1 h to ensure the establishment of an adsorption/desorption equilibrium. At the given time interval during the photoreaction, 3 mL of the reaction suspension was sampled and centrifuged to remove the catalyst particles. The RhB concentration was measured by monitoring the variation in the absorption spectrum for the absorbance at 554 nm with a spectrophotometer (Shimadzu, UV-3600). Quantitative analysis of 2,4-DCP was carried out using high-performance liquid chromatography (HPLC) (Agilent, 1100) with a C18 column. The eluent consisted of 65% acetonitrile, 35% water, and 0.1% phosphoric acid.

To study the active species involved in the photocatalysis, triethanolamine (TEOA, 10 mM) as an effective hole scavenger and tert-butyl alcohol (TBA, 10 mM) as an ˙OH radical scavenger were chosen, respectively, to participate in the photocatalytic degradation of RhB over these catalysts.

Analysis of the photogenerated H₂O₂

The in situ photogenerated H₂O₂ in the visible light irradiated (λ ≥ 420 nm) suspension of the catalysts (2.0 g L⁻¹) was analyzed in the presence of methanol (1.0 M) as an electron donor using the colorimetric DPD-POD method.³¹

Results and discussion

Phase structure and morphology

XRD was used to identify the phase structure and composition of the as-prepared samples. Fig. 1 shows the XRD patterns of the pure Bi₂O₃ and Pt/Bi₂O₃ samples with different Pt loadings. All the samples exhibited the single characteristic monoclinic phase of well-crystallized α-Bi₂O₃ (JCPDS No. 41-1449). No apparent characteristic XRD peaks associated with Pt were observed in all the detected samples. These results suggest that loading Pt does not alter the phase structure of the substrate Bi₂O₃ and that the loaded Pt particles are highly dispersed on the substrate. The presence and form of the loaded Pt are further confirmed by the TEM observation and XPS analysis mentioned below.

Fig. 1 Polycrystalline X-ray diffraction patterns of pure Bi₂O₃ and Pt/Bi₂O₃ with different Pt loadings.

The FESEM image (Fig. 2a) reveals that the pure Bi₂O₃ has microrod-shaped structures with diameters ranging from 500 to 800 nm and lengths of about several micrometers, which is similar to that of the previously reported α-Bi₂O₃.^17,18 No obvious differences in both morphology and size of the substrate were observed after the Pt loading (Fig. 2b). The TEM images (Fig. 2c) of the pure Bi₂O₃ and 0.075 wt% Pt/Bi₂O₃ samples show that numerous nanoparticles with the size of several nanometers are well dispersed on the surface of the Bi₂O₃ microrod substrate after 0.075 wt% Pt loading. As observed in the typical high-resolution TEM (HRTEM) image (Fig. 2d) corresponding to a region of 0.075 wt% Pt/Bi₂O₃ in Fig. 2c, both the nanoparticles and substrate are well crystallized. The measured lattice fringes of 0.23 and 0.26 nm well match the crystallographic planes of the metallic Pt(111) and α-Bi₂O₃(002), respectively.^32–34 These observations indicate that the crystalline metallic Pt nanoparticles were uniformly dispersed on the surface of the substrate Bi₂O₃.

Fig. 2 FESEM images of pure Bi₂O₃ (a) and 0.075 wt% Pt/Bi₂O₃ (b); TEM images of pure Bi₂O₃ and 0.075 wt% Pt/Bi₂O₃ (c); and typical HRTEM image of 0.075 wt% Pt/Bi₂O₃ (d).

XPS analysis and optical property

X-ray photoelectron spectroscopy (XPS) was employed to investigate the compositions and chemical states of the surface elements in the samples. The full-scale XPS spectra of pure Bi₂O₃ and 0.075 wt% Pt/Bi₂O₃, as shown in Fig. S1 (ESI†), clearly reveal the presence of Bi, O, and C in the pure Bi₂O₃, and Bi, O, Pt, and C in the 0.075 wt% Pt/Bi₂O₃. The atomic ratios of Bi to O in both the samples were approximately 2 : 3. These results further confirm the purity of Bi₂O₃ in both samples and the presence of Pt in the 0.075 wt% Pt/Bi₂O₃. The high-resolution XPS spectra of the Bi 4f core-level for pure Bi₂O₃ and 0.075 wt% Pt/Bi₂O₃ are displayed in Fig. 3a. It can be clearly seen that the Bi 4f spectrum of pure Bi₂O₃ consists of two well-defined photoelectron peaks located at 158.8 and 164.1 eV, which can be assigned to Bi 4f_7/2 and Bi 4f_5/2, respectively. The two peak positions and their separation (Δ = 5.3 eV) are well consistent with the characteristics of the Bi³⁺ species in α-Bi₂O₃.¹⁹ Note that the binding energies of Bi 4f_7/2 and Bi 4f_5/2 obviously shift to 159.0 and 164.3 eV, respectively, after Pt loading in the case of 0.075 wt% Pt/α-Bi₂O₃, whereas their separation value remains the same as that of the corresponding pure Bi₂O₃. As reported earlier,²² when a semiconductor and metal nanoparticles are in contact, electron migration between them occurs until Fermi level equilibration of the composite is established. In the present case, the substrate α-Bi₂O₃ is an n-type semiconductor with the Fermi level close to its conduction band (∼+0.33 V vs. NHE), whereas metal Pt has a Fermi level of about + 1.0 V vs. NHE.^35,36 Therefore, electron transfer from the substrate α-Bi₂O₃ to the loaded Pt occurs for Fermi level equilibration, which results in the observed shift to higher binding energies. The observed shift in the binding energy also reflects a well-contacted interface between the Pt nanoparticles and the substrate in the composite. Fig. 3b presents the high-resolution XPS spectrum of the Pt 4f core-level for the 0.075 wt% Pt/Bi₂O₃ sample. As revealed in Fig. 3b, the Pt 4f signal can be fitted into four symmetric peaks, which suggests that the Pt species are present in two forms. The Pt 4f_7/2 peak at 71.1 and Pt 4f_5/2 peak at 74.5 eV originate from metallic Pt⁰, whereas the other pair of Pt XPS peaks with binding energies at 72.5 and 75.9 eV correspond to Pt 4f_7/2 and Pt 4f_5/2 of Pt²⁺, respectively.³⁷ According to the relative XPS areas, the atomic percentages of Pt⁰ and Pt²⁺ in the total Pt species of the sample are estimated to be 91.2% and 8.8%, respectively, confirming that the loaded Pt is mainly present in the metallic state in Pt/Bi₂O₃. The presence of a small amount of Pt²⁺ species might be attributed to the formation of a Pt–O bond caused by the oxygen chemisorption on the surface of the Pt nanoparticles.^38,39

Fig. 3 (a) Bi 4f XPS spectra of pure Bi₂O₃ and 0.075 wt% Pt/Bi₂O₃. (b) Pt 4f XPS spectrum of 0.075 wt% Pt/Bi₂O₃.

The optical absorption properties of pure Bi₂O₃ and Pt/Bi₂O₃ with different Pt loadings were determined using UV-vis diffuse reflectance spectroscopy. As shown in Fig. 4, pure Bi₂O₃ exhibits an intense absorption with a threshold at the wavelength near 455 nm. This absorption originates from the excitation of electrons from the valence band to the conduction band in Bi₂O₃, which is well consistent with the intrinsic bandgap of α-Bi₂O₃ (E_g = 2.8 eV). Compared to the spectrum of pure Bi₂O₃, a broad absorption in the range of 450–800 nm is observed for all the Pt/Bi₂O₃ samples, which is caused by the change in the sample color from yellow to dark gray after Pt loading. Moreover, the weak surface plasmon resonance of the loaded Pt also contributes to the observed absorption based on the related report.⁴⁰

Fig. 4 UV-vis diffuse reflectance spectra of pure Bi₂O₃ and Pt/Bi₂O₃ with different Pt loadings.

Photocatalytic property

To explore the effects of the Pt loading on the photocatalytic properties, the photocatalytic activities of pure Bi₂O₃ and Pt/Bi₂O₃ with different Pt loadings were examined by the degradation of RhB and 2,4-DCP in aqueous solution under visible light irradiation (λ > 420 nm). Fig. 5a displays the photocatalytic degradation of RhB over these samples under visible light irradiation. The control experiment shows that RhB in an aqueous solution is stable against direct photolysis. With the excitation of Bi₂O₃ under visible light irradiation, all the Pt/Bi₂O₃ samples exhibit photocatalytic activities much higher than that of the pure Bi₂O₃ for the degradation of RhB, which indicates that Pt loading is an efficient method for improving the photocatalysis of Bi₂O₃. Moreover, it can be found that the enhanced activity is also dependent on the amount of loaded Pt. The 0.075 wt% Pt loading displays optimum activity. Approximately 85% of RhB was degraded over 0.075 wt% Pt/Bi₂O₃ under irradiation for 3 h. After this, the activity starts to drop when the Pt loading is 0.1 wt%. The observed decrease in the activity over 0.1 wt% Pt/Bi₂O₃ can be ascribed to the excess Pt nanoparticles that shield the incident light reaching Bi₂O₃ and diminish the active sites on Bi₂O₃. Similar enhancement in the photocatalytic activity after Pt loading was observed from the photocatalytic degradation of 2,4-DCP, which is an organic compound that does not absorb visible light, over 0.075 wt% Pt/Bi₂O₃ (Fig. 5b).

Fig. 5 (a) Photocatalytic degradation of RhB over pure Bi₂O₃ and Pt/Bi₂O₃ with different Pt loadings under visible light irradiation. (b) Photocatalytic degradation of 2,4-dichlorophenol (2,4-DCP) over pure Bi₂O₃ and 0.075 wt% Pt/Bi₂O₃ under visible light irradiation.

The photocatalytic stability and reusability of the Pt/Bi₂O₃ catalyst was determined by conducting cyclic degradation experiments with fresh RhB solution and the used catalyst from the previous runs under the same conditions. As shown in Fig. S2 (ESI†), no obvious decrease in the photocatalytic activity for RhB degradation over 0.075 wt% Pt/Bi₂O₃ is observed after recycle use for four times, which demonstrates the good stability of the catalyst.

Analysis of the photogenerated active species and H₂O₂

The active species in the photocatalysis of pure Bi₂O₃ and Pt/Bi₂O₃ was investigated by the addition of TEOA as an effective hole scavenger or TBA as an ˙OH radical scavenger (with a rate constant of k = 6 × 10⁸) in the photocatalytic reaction of RhB degradation over both catalysts.⁴¹ As depicted in Fig. 6a and b, over both catalysts, the degradation efficiencies of RhB obviously decreased with the addition of the ˙OH radical scavenger TBA, whereas no degradation of RhB occurred in the presence of TEOA, as an effective hole scavenger. These results reveal that both ˙OH radicals and photogenerated holes are responsible for the degradation of RhB over pure Bi₂O₃ and Pt/Bi₂O₃. From the viewpoint of thermodynamics, the photogenerated electrons on the conduction band of Bi₂O₃ (∼+0.33 V vs. NHE) can energetically reduce the surface oxygen molecules to form H₂O₂ through a two-electron reduction process (reaction (1), E⁰(O₂/H₂O₂) = +0.695 V vs. NHE). Thus, with the excitation of two catalysts under visible light irradiation, there are two possible production paths for ˙OH radicals over the two catalysts. One is the reductive path of the conduction band electron through reactions (1) and (2), the other is the oxidative path of the valence band hole through reaction (3).

2e_CB⁻ + O₂ + 2H⁺ → H₂O₂ (1)

H₂O₂ + e_CB⁻→ OH⁻ + ˙OH (2)

h_VB⁺ + H₂O (or –OH) → ˙OH (3)

No degradation of RhB over the two catalysts in the presence of the hole scavenger TEOA, as evidenced in Fig. 6, clearly reveals that reaction (3) is the sole source of ˙OH radical generation, whereas the production path through reaction (2) is negligible in both systems. This indicates that the photogenerated holes play a determining role in the photocatalytic activity by directly degrading RhB and oxidizing water or hydroxyl groups to form ˙OH radicals. The effective production yield of the photogenerated holes entirely depends on the separation extent of the photogenerated electron–hole pairs. According to the results of the photocatalytic experiments, as shown in Fig. 5a and b, it can be concluded that efficient separation over Bi₂O₃ is successfully achieved after Pt loading. This conclusion is also well supported by the photoluminescence (PL) spectra of pure Bi₂O₃ and 0.075 wt% Pt/Bi₂O₃ (Fig. 7). It is well known that PL emission is the result of the combination of photogenerated electrons and holes. From Fig. 7, the strong peak at around 455 nm for pure Bi₂O₃ is ascribed to the emission of the band gap transition. The intensity of the PL emission peak is dramatically reduced after Pt loading, indicating that Pt nanoparticles diminish the recombination of the photogenerated electron–hole pairs and that an efficient separation is achieved over Pt/Bi₂O₃.

Fig. 6 Photocatalytic degradation of RhB over pure Bi₂O₃ (a) and 0.075 wt% Pt/Bi₂O₃ (b) in the presence of different scavengers under visible light irradiation for 3 h.

Fig. 7 PL spectra of pure Bi₂O₃ and 0.075 wt% Pt/Bi₂O₃.

Previous studies on the semiconductor–noble metals (Au, Ag and Pt) indicate that loading noble metals on the semiconductors can enhance the separation of electron–hole pairs photogenerated on the semiconductor.^22,23 These noble metal nanoparticles play a mediating role to store and shuttle or directly transfer the electrons from the photoexcited semiconductor to an acceptor in a photocatalytic process. Therefore, in the present study, we need to understand that where the electrons go after they are transferred to Pt nanoparticles from the excited Bi₂O₃. The Fermi level of Pt (∼+1.0 V vs. NHE) is more positive than that of the reaction (1) (E⁰(O₂/H₂O₂) = +0.695 V vs. NHE); however, it shifts to a more negative position due to the establishment of Fermi level equilibration between Bi₂O₃ and Pt, as evidenced by the abovementioned XPS measurements. Thus, after the photoinduced electron transfer from the excited Bi₂O₃ to Pt nanoparticles, it becomes possible for the Pt nanoparticles to act as a reduction site to reduce the surface oxygen molecules to form H₂O₂ through reaction (1). The analysis of the in situ photogenerated H₂O₂ in the visible light irradiated suspensions of pure Bi₂O₃ and 0.075 wt% Pt/Bi₂O₃ (Fig. 8) confirms the production of H₂O₂ in both systems. The production of H₂O₂ is much more over Pt/Bi₂O₃ than that over pure Bi₂O₃, indicating that more photogenerated electron–hole pairs are efficiently separated in Pt/Bi₂O₃. In contrast, in the case of pure Bi₂O₃, although the conduction band level (∼+0.33 V vs. NHE) is much more negative than that of reaction (1), fast recombination of the photogenerated electron–hole pairs occurs in the absence of Pt. This result is in accordance with the photocatalytic performances of pure Bi₂O₃ and Pt/Bi₂O₃, as shown in Fig. 5a and b. Note that the Pt/Bi₂O₃ system shows a much higher initial production rate of H₂O₂ than that of pure Bi₂O₃, which is different from that of Au/Bi₂O₃ in our previous report.¹⁸ This observation is consistent with the role of Pt nanoparticles in the separation of photogenerated electron–hole pairs.²³ In the presence of Pt nanoparticles, rapid transfer of electrons from Pt to surface oxygen molecule occurs without electron accumulation on Pt. However, in the presence of Au nanoparticles, the electrons from the excited Bi₂O₃ accumulate on Au nanoparticles and do not transfer to the surface oxygen until their Fermi level merges with the conduction band of Bi₂O₃. The results of the analysis of photogenerated H₂O₂ clarify very well the transfer and capture of the electrons photogenerated over pure Bi₂O₃ and Pt/Bi₂O₃.

Fig. 8 In situ photogenerated H₂O₂ in the visible light irradiated suspensions of pure Bi₂O₃ and 0.075 wt% Pt/Bi₂O₃ in the presence of methanol as an electron donor.

According to the abovementioned analysis of the photogenerated active species and H₂O₂, the photocatalytic process over Pt/Bi₂O₃ is illustrated in Scheme 1. Due to the decomposition of H₂O₂ into H₂O and O₂ in the photoreaction,⁴² especially under the promotion of Pt catalysis,⁴³ the concentration of H₂O₂ is found to gradually decrease after reaching an optimum and the decay rate is faster over Pt/Bi₂O₃ than that over pure Bi₂O₃.

Scheme 1 Schematic for the photocatalysis over Pt/Bi₂O₃.

Conclusions

In summary, crystalline metallic Pt nanoparticles are successfully deposited on the surface of Bi₂O₃ microrods via an impregnation–reduction route. Upon visible light irradiation, the photocatalytic activities of Bi₂O₃ for the degradation of RhB and 2,4-DCP are significantly enhanced after Pt nanoparticle loading. The Pt nanoparticles loaded on Bi₂O₃ function as an electron mediator to effectively promote the separation of photogenerated electron–hole pairs. As a result, the reduction of O₂ to H₂O₂ by the photogenerated electrons is greatly enhanced, leaving more holes on the conduction band of Bi₂O₃ for the degradation of organic compounds and production of ˙OH radicals. This work, we believe, provides some useful information for the design and development of the small-band gap semiconductor Bi₂O₃ with a deep valence band as an efficient visible light photocatalyst.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21273281) and National Basic Research Program of China (973 Program, No. 2013CB632405).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Full-scale XPS spectra of pure Bi₂O₃ and 0.075 wt% Pt/Bi₂O₃, cyclic use of 0.075 wt% Pt/Bi₂O₃ for photocatalytic degradation of RhB. See DOI: 10.1039/c6cp06755g

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