Synthesis of Surface Oxygen-deficient BiPO₄ Nanocubes with Enhanced Visible Light Induced Photocatalytic Activity

Abstract

The visible light driven BiPO₄ nanocubes with sufficient surface oxygen deficiency were fabricated by a hydrothermal process and subsequently ultrasonic assistant Fe reduction process. The products were characterized by XRD, DRS, XPS, SEM and TEM which showed that the BiPO₄ had cuboid-like shape with a smooth surface and clear edges and the oxygen vacancies were succesfully introduced on the surface of the BiPO₄ nanocubes. The as prepared oxygen-deficient BiPO₄ nanocubes showed greatly enhanced visible light induced photocatalytic activity in degradation of Rhodamine B. The enhanced photocatalytic performance and expanded visible light response of BiPO₄ may be due to the introduction of surface oxygen vacancies which can generate the oxygen vacancies mid-gap states lower to the conduction band of BiPO₄.

Keywords:
BiPO₄; Nanocubes; Photocatalysis; Oxygen vacancy

1. Introduction

Photocatalysis has attracted considerable attention ever since the photocatalytic splitting of water on TiO₂ electrodes was investigated by Fujishima and Honda¹, which provides a new promising way to meet the challenges of the environment, energy and sustainability with abundant solar light^2-4. Although TiO₂ is the most promising photocatalyst and is widely investigated due to its non-toxicity, chemical stability and low cost, low solar energy conversion efficiency and high recombination of photogenerated electron-hole pairs make it difficult for practical applications^5,6. Therefore, it is necessary to fabricate efficient new types of photocatalysts capable of responding to visible light in order to improve the utilization of solar energy.

As is known that visible light response of the photocatalyst needs the narrow band gap in the semiconductors. However, the highly photocatalytic efficiency always requires the higher valence band (VB) and lower conduction band (CB) of the semiconductor photocatalyst, which can provide enough oxidation and reduction power of the holes and electrons respectively^7,8. The discrepancy always makes the highly photocatalytic efficient photocatalyst less visible light response. For instance, BiPO₄, with a novel nonmetal oxy-acid salt structure, has the suitable VB and CB position for highly photocatalytic performance which showed twice higher activity than that of TiO₂ (P25, Degussa) for the degradation of organic dye under UV light⁹. However, the large band gap energy of BiPO₄ (3.85 eV) prevented it from utilizing the visible light to meet the requirement of practical application^10,11. Therefore, great efforts have been made to broaden the visible light absorption region and obtain high visible light induced photocatalytic performance of BiPO₄. As is known coupling of a narrow band gap semiconductor such as CdS¹², BiOI¹³ and Ag₃PO₄¹⁴ on the BiPO₄ surface to form the p-n heterojunctions can increase the visible light induced photocatalytic activities of the catalyst. However, the charge separation in this p-n heterojunctions always undergoes the photogenerated electrons transferring to the CB of BiPO₄ leaving holes on the VB of narrow band semiconductors^12-14, which can hardly use the high oxidation ability of holes in the higher VB of BiPO₄. Recently, the oxygen defects have been reported can greatly enhance the photocatalytic activity and expand the range of photoresponse in the semiconductor photocatalysts^15-18. As investigated, the oxygen vacancies can not only serve as photoinduced charge traps to prevent the electron−hole recombination, they are also effective adsorption sites to adsorb active species which may greatly increase the photocatalytic activity^17,18. Moreover, oxygen vacancies can also form oxygen vacancy states lying close to the conduction band of the photocatalyst, which can enhance its visible light response¹⁷. Except for TiO₂ and BiOCl, Zhu et al.^18,19 also reported that oxygen vacancies enriched BiPO₄ nanorod, prepared by vacuum deoxidation, showed enhanced photocatalytic activity and expanded photocatalytic response (more than 365 nm). However, the investigation of the oxygen-deficient BiPO₄ is still insufficiency and the interior mechanism and some details of the visible light induced photocatalysis are yet not very clear. Therefore, it is still necessary to develop oxygen-deficient BiPO₄ with different morphology using more facile and milder process and further investigate the interior mechanism of oxygen vacancies effect on the photocatalytic activity.

In this work, we reported a facile method to prepare the oxygen-deficient BiPO₄ nanocubes through an ultrasonic assistant Fe reduction process. The BiPO₄ nanocubes were firstly synthesized by a hydrothermal process and then surface oxygen vacancies were generated by ultrasonic assistant Fe reduction process. The oxygen-deficient BiPO₄ nanocubes showed greatly enhanced visible light induced photocatalytic activity. The organic pollutant was mainly photodegraded by •OH and holes in the VB of BiPO₄, which may be due to the visible light induced electron and holes separation originated from oxygen vacancy states.

2. Experimental

2.1 Preparation of the BiPO₄ nanocubes

In a typical procedure, 5mmol bismuth nitrate (Bi(NO₃)₃·5H₂O) and 5mmol ethylenediamine tetraaceticacid disodium salt (EDTA) was added to 100 ml distilled water under magnetic stirred at ambient temperature. Then the nirtic acid (HNO₃, 1.8ml) was slowly added dropwisely into the solution. Until the precursor solution became transparent, 5 mmol potassium monophate (K₂HPO₄) was then added and continuously stirred magnetically for 10 min. The resulting precursor suspension was transferred into a Teflon-lined stainless steel autoclaveand maintained at 180 °C for 12h. The product was then separated by centrifugation and washed by distilled water and ethanol for several times. Finallly, the BiPO₄ nanocubes were obtained after the products dried at 70 ºC for 12h which were denoted as WBiPO₄.

2.2 Preparation of the oxygen-deficient BiPO₄ nanocubes

As prepared BiPO₄ nanocubes (0.6g) and reduced iron powder (0.6g) were directly dispersed in 40ml distilled water at ambient temperature. The mixture was mechanically stirred and kept under ultrasonic for 5h (power of ultrasonication was 800W). After reduction, the yellow samples and excess Fe powder were separated by an external strong magnetic field, and then the sample was maintained in dilute hydrochloric acid (0.01M) for 30min in order to fully remove the Fe powder residue. The product was collected and washed with absolute alcohol and deionized water for several times and finally dried at 70 ºC for 12h. The oxygen-deficient BiPO₄ nanocubes were denoted as GBiPO₄.

2.3 Characterization

The morphologies and microstructures of the photocatalysts were characterized using scanning electron microscope (SEM, Hitachi S-4700) and transmission electron microscopy (TEM, JEOL 200CX). X-ray diffraction (XRD, Thermo ARL SCINTAG X’TRA) was used to characterize the phase structures of the samples. UV-vis diffuse reflectance spectra were obtained on UV-vis spectrophotometer (UV-Vis, Shimadzu UV-2550) with BaSO₄ as reference. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a PHI-5400 spectrometer using the C1s level at 284.6 eV as an internal standard to correct the shift of binding energy. The conventional three-electrode system was used to measure the photocurrent and electrochemical impedance spectroscopy (EIS) on an electrochemical workstation (Zennium, Zahner, Germany). FTO glass coated with the as-prepared samples (0.1 mg) served as the working electrode. The counter and the reference electrodes were a platinum plate and a saturated Ag-AgCl electrode, respectively. NaOH aqueous solution (0.1 M) was used as electrolyte. The working electrode (the active area 0.5 cm × 0.5 cm.) was prepared using an amended doctor blade method. Firstly 0.1 g of sample was ground with 1 mL terpineol. The slurry was then coated onto FTO glass by the doctor blade method which was cleaned in distilled water and ethanol by ultrasonication. These electrodes were preliminarily dried at 70°C and calcinated at 200°C for 1 h under Ar atmosphere.

2.4. Evaluation of the photocatalytic performance

The photocatalytic performance of oxygen-deficient BiPO₄ nanocubes was evaluated by the degradation of Rhodamin B (RhB) solution under visible light irradiation. The light source was obtained by a 300W Xe lamp with a 420 nm cut off filter used to get the visible light. In each experiment, 0.1 g of the as-prepared photocatalyst was added into 100 mL of RhB solution (5mg/L). The well dispersed suspension was magnetically stirred in the dark condition for 30min to reach the adsorption/desorption equilibrium. After irradiation, about 5 mL of suspension was sampled at every interval and centrifuged to remove the photocatalysts. The concentration of filtrates was analyzed by measuring the maximum absorbance at 554 nm for RhB using a UV-2550 UV-vis spectrophotometer.

3. Results and discussion

The morphologies of oxygen-deficient BiPO₄ nanocubes (GBiPO₄) and unreduced BiPO₄ (WBiOP₄) are firstly observed by SEM. Figure 1 shows SEM images of WBiPO₄ and GBiPO₄. It is clearly seen that the as synthesized WBiPO₄ had a cuboid-like shape with a smooth surface and clear edges (Figure 1a). The enlarged SEM image (Figure 1b) shows that the width of the nanocubes ranging from 400-800nm and length is about 600nm-1µm. Moreover, the cuboid-like crystal with a different facet was also observed on the enlarged SEM image. Figure 1c shows the SEM image of GBiPO₄ which displays the similar morphology of WBiPO₄ suggesting the reduction of BiPO₄ by Fe did not evidently change the morphologies of the photocatalyst. The chemical composition of WBiPO₄ was confirmed by energy dispersive X-ray analysis (Figure 1d) which shows that Bi, O, P are major elements (except for the elements Pt and C from the Pt coating and conducting wafer used in SEM analysis respectively). No other impurities were found in the samples suggesting the relatively pure BiPO₄ can been obtained by this process.

Figure 1
SEM images of (a), (b) WBiPO₄ and (c) GBiPO₄. (d) EDS analysis of GBiPO₄.

TEM and HRTEM images were further demonstrated to observe the fine structure of WBiPO₄ and GBiPO₄ and the results were shown in Figure 2. TEM images of WBiPO₄ (Figure 2a) and GBiPO₄ (Insert of C) show the similar cuboid-like shape which is in consistent with the results of SEM analysis. Moreover, the evidently morphologies change for the two photocatalysts was still undetectable on the TEM images. On the HRTEM images of both WBiPO₄ (Figure 2b) and GBiPO₄ (Figure 2c) the distinct lattice spacing of about 0.196 nm can be clearly investigated which is consistent with the (212) crystallographic plane of BiPO₄²⁰. Further investigation reveals that unreduced BiPO₄ displays perfect lattice features, however the edge of oxygen-deficient BiPO₄ becomes dim and disordered, which indicates that the surface structure of oxygen-deficient BiPO₄ is damaged and surface oxygen vacancies are formed. Similar phenomena has also been investigated in the previous research¹⁹.

Figure 2
(a) TEM and (b) HRTEM images of WBiPO₄. (c) HRTEM images of GBiPO₄. (d) XRD patterns of WBiPO₄ and GBiPO₄. (Insert in C is the TEM image of GBiPO₄)

The XRD pattern was used to investigate the phase structures of the samples, and the typical diffraction patterns are shown in Figure 2d. It can be observed that the same XRD patterns were obtained on both WBiPO₄ and GBiPO₄, which could be indexed to the pure monoclinic phase of well-crystallized BiPO4, well consistent with the reported data (JPCDS 80-0209). The peaks at 2θ values of 19.01°, 21.31°, 27.13°, 29.04°, and 31.16° are corresponding to the diffraction peaks of (011), (l 11), (200), (120), and (012) crystal planes of BiPO₄, respectively. The similar XRD patterns suggested that the Fe reduction of BiPO₄ did not change the crystal phase of the photocatalyst. Moreover, there is no any trace of impurity phase detected, indicating the high purity and high crystallinity of the samples.

The XPS analysis was further carried out to analyze the chemical composition and elucidate the chemical state of the element in the oxygen-deficient BiPO₄ (Figure 3). The XPS survey spectrum (Figure 3a) clearly indicates that the WBiPO₄ was mainly composed of elements Bi, P and O (the C 1s peak at 284.8 eV may originate from adventitious hydrocarbon in the XPS instrument). A broad signal peak at about 132.7 eV in the high resolution XPS spectra of P (Figure 3b) suggests that the P in the sample exists in the oxidation state of P⁵⁺. The peak in the high resolution XPS spectra of O (Figure 3c) locates at 531.2eV which can be attributed to O1s in BiPO₄. In the high resolution XPS spectrum of Bi (Figure 3d), the peak can be deconvoluted into two pair separate peaks. The peaks at 165.8 eV and 160.5 eV are indexed as Bi³⁺ in BiPO₄. The lower binding energies at 163.3 eV and 158.2 eV can be indexed to lower charged Bi ions which are due to the oxygen vacancies present^17,21. The same phenomenon has also occurred in other oxygen vacancies enriched BiOCl and TiO₂ systems, namely, the appearance of oxygen vacancies induced a lower binding energy peak which was due to Bi^3-x or Ti^3+22-24.For comparison the high resolution XPS spectrum of Bi in WBiPO₄ has also been showed in Figure 3d. It can be seen that the peak is more symmetrical than that of GBiPO₄ which may indicate that there is no lower valence state Bi in WBiPO₄.

Figure 3
XPS profiles of GBiPO₄. (a) survey scan, (b) P2p core levels, (c) O1s core levels, (d) Bi4f core levels.

The structure of WBiPO₄ and GBiPO₄ was also further characterized by Raman spectroscopy as shown in Figure 4. In all spectra, the observed intense band at 165 cm^-1 with a shoulder at 231 cm^-1 corresponds to the Bi O stretching vibration. The bands centered at 1033 and 964 cm^-1 are ascribed to the asymmetric (ν₃) and symmetric (ν₁) stretching vibrations of the PO₄ group, respectively. The ν₄ bending vibration modes of PO₄ groups occur at 554 and 596 cm^-1, and ν₂ bending vibration occur at 403 and 458 cm^-1. It can be seen that although the positions of these Raman peaks were not changed but the intensities were decreased on GBiPO₄ which may be attributed to the contribution of oxygen vacancies²⁵.To further confirm the existence of oxygen vacancies the electron paramagnetic resonance (EPR) of WBiPO₄ and GBiPO₄ was also performed. As investigated from Figure 4b the intensity of the EPR signal at g ~ 2.001 of GBiPO₄ is much higher than that of WBiPO₄ under the same conditions. As reported previously, g equal to 2.003 ± 0.001 can be attributed to oxygen vacancies on the surface^18,19,25 indicating that surface oxygen vacancies were produced on GBiPO₄.

Figure 4
Raman spectrum (a) and EPR spectra (b) of WBiPO₄ and GBiPO₄.

It is well known that the light absorption and the migration of the light-induced electrons and holes are the key factors in determining photocatalytic activity, which rely on the electronic structure characteristics of the material. Diffuse reflectance spectroscopy (DRS) is a useful tool for characterizing electronic states in optical materials. Figure 5 displays the diffuse reflectance spectroscopy spectra of WBiPO₄ and GBiPO₄. It can be seen that WBiPO₄ only can absorb UV light due to its large bandgap energy. Moreover, the DRS of GBiPO₄ presents distinctly red shift with the increased absorption in the visible light region, which is probably due to the oxygen vacancies in the oxygen-deficient BiPO₄^18,19. From the inset of Figure 5A, the color of the as-prepared photocatalysts changed from white WBiPO₄ to light-yellow oxygen-deficient GBiPO₄, which is caused by the formation of oxygen vacancies. As a crystalline semiconductor, the estimated value of the band gap (Eg) of the samples could be calculated by the following formula:²⁰

(1) $A{\left(\mathrm{hv}-\mathrm{Eg}\right)}^{n/2}=\mathrm{ahv}$

Figure 5
UV-vis diffuse reflectance spectra (a) and Band gap calculation by Tauc’s method (b) of WBiPO₄ and GBiPO₄.

where A, a, v, and Eg are the constant, absorption coefficient, light frequency, and band gap energy, respectively. In addition, n is determined by the type of optical transition of a semiconductor (n = 1 for a direct transition and n = 4 for an indirect transition). As BiPO₄ exhibits the characteristic of indirect band transition²⁶, the value of n is 4. The corresponding Eg values of WBiPO₄ and GBiPO₄ were determined from a plot of (ahv)² versus energy (hv) (Figure. 4b) and estimated to be 3.92 and 2.62 eV, respectively, which implied that GBiPO₄ nanocubes may have the best visible light induced photocatalytic activities.

The photocatalytic degradation of RhB by WBiPO₄ and GBiPO₄ was investigated, and the removal efficiencies as a function of reaction time are shown in Figure 6. As can be investigated in Figure 6a, RhB is degraded slightly under visible light illumination in the absence of photocatalysts, suggesting that the photolysis of RhB is negligible. Moreover, the photocatalytic activities of WBiPO₄ was also very low under visible light irradiation indicating no visible light photocatalytic activity of pure BiPO₄. In contrast, GBiPO₄ presented much higher photocatalytic activity for RhB degradation under the same conditions. The linear relationship between ln(C₀/C) and t shown in Figure 6b confirms that the photocatalytic degradation process of RhB followed the apparent pseudo-first-order model expressed in Eq. (2):

(2) $\ln \left(C_0/C\right)=\mathrm{kappt}$ ²⁷

Figure 6
Photocatalytic degradation of rhodamine B (a) and plots of ln(C₀/C) versus time (b) with WBiPO₄ and GBiPO₄ under visible light irradiation.

Where C₀ and Ct are the RhB initial equilibrium concentration and reaction concentration at time (t), and kappt is the reaction rate constant (min^-1). Thus, the kapps of WBiPO₄ and GBiPO₄ were calculated to be 0.000835, 0.00962 min^-1 respectively. The much higher rate constants indicated the oxygen-deficient BiPO₄ nanocubes had much stronger visible light induced photocatalytic activities than that of pure BiPO₄, which may be due to the oxygen vacancies induced enhancement of the visible light absorption and charge separation.

Photocurrent is an effective method to reflect the generation, separation and migration efficiency of photogenerated carriers and the photocurrent of WBiPO₄ and GBiPO₄ were recorded for several on-off cycles with a pulse of 40 s under visible light irradiation (λ> 420 nm). It can be seen from Figure 7a that the photocurrent appeared promptly when the samples were irradiated, indicating the generation of photogenerated electrons, and then decreased sharply as soon as the irradiation of light was turned off. In comparison with pure BiPO₄, the oxygen-deficient BiPO₄ nanocubes exhibits an obvious increased current density(The slight photocurrent response of pure BiPO₄ may arise from a small quantity of unfiltered UV light). In general, the value of the photocurrent indirectly reflects the ability in generation and transfer of the photoexcited charge carrier under irradiation. The higher photocurrent demonstrates the higher electrons and holes separation efficiency¹³. Therefore, the result of the photocurrent measurement may suggest that the oxygen-deficient BiPO₄ nanocubes have stronger ability in separation of electron-hole pairs than pure BiPO₄ which is consistent with the results of the visible light photocatalytic activities. Electrochemical impedance spectra (EIS) measurements were also conducted to investigate the charge transfer resistance and the separation efficiency of WBiPO₄ and GBiPO₄. As can be investigated in Figure 7b, the EIS Nyquist plots of WBiPO₄ and GBiPO₄ displayed similar impedance spectra, being composed of semicircle. However, compared with the WBiPO₄ and GBiPO₄, the arc radius of GBiPO₄ was smaller than that of WBiPO₄. As is known that a smaller impedance arc radius in Nyquist plots always indicates better electron-hole pair separation efficiency of the photocatalysts²⁸. Therefore, the EIS results may also suggest that a more effective separation of photogenerated electron-hole pairs and faster interfacial charge transfer occurred on the oxygen-deficient BiPO₄ nanocubes.

Figure 7
Comparison of (a) transient photocurrent response and (b) EIS Nyquist plots of WBiPO₄ and GBiPO₄.

It is well known that the reactive species •O^2-, h⁺ and •OH acted as a bridge, play the important roles in the photodegradation process of organic pollutants under the light irradiation. In order to investigate the possible mechanism that occurs on the oxygen-deficient BiPO₄ nanocubes the photodegradation experiments were carried out with p-benzoquinone, tert-butanol, and KI as scavengers for the reactive species of •O^2-, •OH, and h⁺, respectively. Figure 8 shows that the photodegradation of RhB was not suppressed in the presence of p-benzoquinone which may suggests that •O^2- was not the main active species in the degradation of the RhB. However, when the scavengers of •OH (tert-butanol) and h⁺ (KI) were added into the solution, the photodegradation of RhB were significantly suppressed, which may suggested that •OH and h⁺ played important roles in this photocatalytic reaction. Moreover, the •OH was investigated to have stronger inhibition effect than h⁺ in this system.

Figure 8
The influence of active species capturers on photocatalytic performance of GBiPO₄.

In order to understand the photocatalytic mechanism occurring on the oxygen-deficient BiPO₄ nanocubes, it is necessary to calculate the conduction band (CB) and valence band (VB) of BiPO₄ because they played an important role in the photocatalytic oxidation process of organic molecule. Thus, the band edge positions of BiPO₄ were estimated in this study according to the following empirical formulas:²⁹

(3) $E_{\mathrm{VB}}=X-\mathrm{Ec}+0.5\mathrm{Eg}$

(4) $E_{\mathrm{CB}}=E_{\mathrm{VB}}-\mathrm{Eg}$

Where E_VB is the valence band (VB) edge potential, E_CB is the conduction band (CB) edge potential. Eg is the band gap energy of semiconductor which can be obtained from DRS (3.92 eV for BiPO₄) and Ec is the energy of free electrons on the hydrogen scale (about 4.5 eV). X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms (6.49 eV for BiPO₄). Based on the band gap positions, the E_VB and E_CB of BiPO4 were calculated to be 3.95 and 0.03 eV, respectively. The above results suggest that BiPO₄ has the higher valence band which can provide enough oxidation power of the holes.

The energy band structure diagram and possible mechanism of electron-hole separation and transportation at the BiPO₄ photocatalyst interface are illustrated in Figure 9. As can be investigated the pure BiPO₄ can not be excited under the visible light illumination due to its higher band gap. However, on the oxygen-deficient BiPO₄ nanocube photocatalyst, the generation of oxygen vacancy could result in the formation of the mid-gap states lying lower to the conduction band. The electrons can be excited up to oxygen vacancy states from the VB of BiPO₄ under visible light irradiation. Furthermore, the photo-induced electrons on oxygen vacancy states are not easy to recombine with photo induced holes as the oxygen vacancies are active electron traps. By trapping the active species during the photocatalytic reaction, •OH and h⁺ were suggested as the main active species. Under visible light irradiation, electrons were excited to oxygen vacancy states and could produce •OH through a photogenerated electron-induced multistep reduction route. Photogenerated holes can oxidize RhB directly due to higher valence band of BiPO₄ and could also react with OH^- to form •OH. Therefore, the oxygen vacancies in the oxygen-deficient BiPO₄ nanocubes can not only result in the enhancement of the separation efficiency of photoinduced electron-hole pairs but also expand the photoresponse range by forming the mid-gap states.

Figure 9
Schematic illustration of the charge separation and transfer in the GBiPO₄ photocatalysts under visible-light irradiation.

4. Conclusions

In summary, surface oxygen-deficient BiPO₄ nanocubes were synthesized by a hydrothermal process and subsequently an ultrasonic assistant Fe reduction process. The HRTEM and XPS results suggested that the oxygen vacancies were introduced on the surface of the BiPO₄ nanocubes. The surface oxygen-deficient BiPO₄ nanocubes showed greatly enhanced visible light induced photocatalytic activities. The enhancement of the photocurrent and photocatalytic performance and expanding of the photoresponse wavelength range were analytically due to the formation of mid-gap states lying lower to the conduction band of BiPO₄. This surface oxygen-deficient BiPO₄ nanocubes photocatalyst may provide new insights into the fabrication of photocatalysts with highly visible light induced photocatalytic performance.

5. Acknowledgment

This work is financially supported by National Nature Science Foundation of Zhejiang Province (No. LY12B07001).

6. References

• ¹
  Fujishima A, Honda K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972;238:37-38.
• ²
  Muñoz-Batista MJ, Fernández-García M, Kubacka A. Promotion of CeO₂-TiO₂ photoactivity by g-C₃N₄: Ultraviolet and visible light elimination of toluene. Applied Catalysis B: Environmental 2015;164:261-270.
• ³
  Fontelles-Carceller O, Muñoz-Batista MJ, Fernández-García M, Kubacka A. Interface Effects in Sunlight-Driven Ag/g-C₃N₄ Composite Catalysts: Study of the Toluene Photodegradation Quantum Efficiency. ACS Applied Materials & Interfaces 2016;8(4):2617-2627.
• ⁴
  Mohaghegh N, Tasviri M, Rahimi E, Gholami MR. A novel p-n junction Ag₃PO₄/BiPO₄-based stabilized Pickering emulsion for highly efficient photocatalysis. RSC Advances 2015;5:12944-12955.
• ⁵
  Zhang W, Jia B, Wang Q, Dionysiou D. Visible-light sensitization of TiO₂ photocatalysts via wet chemical N-doping for the degradation of dissolved organic compounds in wastewater treatment: a review. Journal of Nanoparticle Research 2015;17(5):221.
• ⁶
  Iliev V, Tomova D, Eliyas A, Rakovsky S, Anachkov M, Petrov P. Enhancement of the activity of TiO₂-based photocatalysts: a review. Bulgarian Chemical Communications 2015;47(Special Issue C):5-11.
• ⁷
  Yu H, Irie H, Hashimoto K. Conduction Band Energy Level Control of Titanium Dioxide: Toward an Efficient Visible-Light-Sensitive Photocatalyst. Journal of the American Chemical Society 2010;132(20):6898-6899.
• ⁸
  Dong S, Feng J, Fan M, Pi Y, Hu L, Han X, et al. Recent developments in heterogeneous photocatalytic water treatment using visible light-responsive photocatalysts: a review. RSC Advances 2015;5:14610-14630.
• ⁹
  Pan C, Zhu Y. New Type of BiPO4 Oxy-Acid Salt Photocatalyst with High Photocatalytic Activity on Degradation of Dye. Environmental Science & Technology 2010;44(14):5570-5574.
• ¹⁰
  Pan C, Zhu Y. A review of BiPO4, a highly efficient oxyacid-type photocatalyst, used for environmental applications. Catalysis Science & Technology 2015;5:3071-3083.
• ¹¹
  She L, Tan G, Ren H, Xu C, Zhao C, Xia A. BiPO4@glucose-based C core-shell nanorod heterojunction photocatalyst with enhanced photocatalytic activity. Journal of Alloys and Compounds 2016;662:220-231.
• ¹²
  Chen D, Kuang Z, Zhu Q, Du Y, Zhu H. Synthesis and characterization of CdS/BiPO4 heterojunction photocatalyst. Materials Research Bulletin 2015;66:262-267.
• ¹³
  Liu L, Yao W, Liu D, Zong R, Zhang M, X. Ma X, et al. Enhancement of visible light mineralization ability and photocatalytic activity of BiPO₄/BiOI. Applied Catalysis B: Environmental 2015;163:547-553.
• ¹⁴
  Lin H, Ye H, Xu B, Cao J, Chen S. Ag₃PO₄ quantum dot sensitized BiPO₄: A novel p-n junction Ag₃PO₄/BiPO₄ with enhanced visible-light photocatalytic activity. Catalysis Communications 2013;37:55-59.
• ¹⁵
  Yin HY, Wang XL, Wang L, Lin Q, Zhao HT. Self-doped TiO₂ hierarchical hollow spheres with enhanced visible-light photocatalytic activity. Journal of Alloys and Compounds 2015;640:68-74.
• ¹⁶
  Henkel B, Neubert T, Zabel S, Lamprecht C, Selhuber-Unkel C, Rätzke K, et al. Photocatalytic properties of titania thin films prepared by sputtering versus evaporation and aging of induced oxygen vacancy defects. Applied Catalysis B: Environmental 2016;180:362-371.
• ¹⁷
  Ye L, Deng K, Xu F, Tian L, Peng T, Zan L. Increasing visible-light absorption for photocatalysis with black BiOCl. Physical Chemistry Chemical Physics 2012;14:82-85.
• ¹⁸
  Lv Y, Zhu Y, Zhu Y. Enhanced Photocatalytic Performance for the BiPO4-x Nanorod Induced by Surface Oxygen Vacancy. Journal of Physical Chemistry C 2013;117(36):18520-18528.
• ¹⁹
  Lv Y, Liu Y, Zhu Y, Zhu Y. Surface oxygen vacancy induced photocatalytic performance enhancement of a BiPO₄ nanorod. Journal of Materials Chemistry A 2014;2:1174-1182.
• ²⁰
  Lin H, Ye H, Chen S, Chen Y. One-pot hydrothermal synthesis of BiPO₄/BiVO₄ with enhanced visible-light photocatalytic activities for methylene blue degradation. RSC Advances 2014;4:10968-10974.
• ²¹
  Weng S, Hu J, Lu M, Ye X, Pei Z, Huang M, et al. In situ photogenerated defects on surface-complex BiOCl (0 1 0) with high visible-light photocatalytic activity: A probe to disclose the charge transfer in BiOCl (0 1 0)/surface-complex system. Applied Catalysis B: Environmental 2015;163:205-213.
• ²²
  Ye L, Jin X, Leng Y, Su Y, Xie H, Liu C. Synthesis of black ultrathin BiOCl nanosheets for efficient photocatalytic H₂ production under visible light irradiation. Journal of Power Sources 2015;293:409-415.
• ²³
  Jiang J, Zhang L, Li H, He W, Yin JJ. Self-doping and surface plasmon modification induced visible light photocatalysis of BiOCl. Nanoscale 2013;5:10573-10581.
• ²⁴
  Chen X, Liu L, Yu PY, Mao SS. Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals. Science 2011;331(6018):746-750.
• ²⁵
  Wei Z, Liu Y, Wang J, Zong R, Yao W, Wang J, et al. Controlled synthesis of a highly dispersed BiPO₄ photocatalyst with surface oxygen vacancies. Nanoscale 2015;7:13943-13950.
• ²⁶
  Li G, Ding Y, Zhang Y, Lu Z, Sun H, Chen R. Microwave synthesis of BiPO₄ nanostructures and their morphology-dependent photocatalytic performances. Journal of Colloid and Interface Science 2011;363(2):497-503.
• ²⁷
  Xu J, Li L, Guo C, Zhang Y, Meng W. Photocatalytic degradation of carbamazepine by tailored BiPO₄: efficiency, intermediates and pathway. Applied Catalysis B: Environmental 2013;130-131:285-292.
• ²⁸
  Liu H, Cheng S, Wu M, Wu H, Zhang J, Li W, et al. Photoelectrocatalytic Degradation of Sulfosalicylic Acid and Its Electrochemical Impedance Spectroscopy Investigation. The Journal of Physical Chemistry A 2000;104(30):7016-7020.
• ²⁹
  Ye H, Lin H, Cao J, Chen S, Chen Y. Enhanced visible light photocatalytic activity and mechanism of BiPO₄ nanorods modified with AgI nanoparticles. Journal of Molecular Catalysis A: Chemical 2015;397:85-92.

Publication Dates

• Publication in this collection
  13 Mar 2017
• Date of issue
  May-Jun 2017

History

• Received
  05 Aug 2016
• Reviewed
  19 Jan 2017
• Accepted
  18 Feb 2017