Internal Electric Field Assisted Photocatalytic Generation of Hydrogen

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Internal Electric Field Assisted Photocatalytic Generation of Hydrogen Peroxide over BiOCl with HCOOH Yang Su, Ling Zhang, Wenzhong Wang, and Dengkui Shao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01023 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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ACS Sustainable Chemistry & Engineering

Internal Electric Field Assisted Photocatalytic Generation of Hydrogen Peroxide over BiOCl with HCOOH Yang Su, Ling Zhang*, Wenzhong Wang*, Dengkui Shao State Key Laboratory of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences 1295 Dingxi Road, Shanghai 200050, P. R. China University of Chinese Academy of Sciences, Beijing 100049, P.R. China Abstract: Hydrogen peroxide (H2O2) is a superb clean and versatile reagent. However, large-scale production of H2O2 is manufactured through non-green methods that motivating people to develop more efficient and green technologies as alternatives. As a novel and green technology used for H2O2 generation, the efficiency of photocatalysis is still far from satisfactory. Here, we demonstrate a novel and efficient path of the generation of H2O2 in BiOCl photocatalysis but not the direct electron reduction of O2 or holes oxidation of OH- to the H2O2. Super high production (685 µmol/h) of H2O2 by the addition of HCOOH as holes shuttle was realized over the BiOCl nanoplates. In this photocatalytic system, the BiOCl supplied abundant photo-induced holes to initiate HCOO· radical. The HCOO· further reacts with OHto ·OH which is approved as the source of H2O2. Apart from HCOOH, O2 also played important roles. The O2 not only promoted the reaction through the cycle between Bi3+ and Bi, which decreased the combination of carriers, but also avoided the carbonation of surfaces thus achieved the high production of H2O2 (1020 µmol/h). In *

Corresponding author, L. Zhang, [email protected]; W. Wang, [email protected], Tel: +86-21-5241-5295 1

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ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

this work, we shed light on the deep understanding photocatalytic evolution of H2O2 in a novel perspective and achieve high production.

Key words: BiOCl, H2O2 evolution, HCOOH, photocatalysis, electric field Introduction Hydrogen peroxide (H2O2) is a superb clean and versatile reagent which has been widely used not only in chemical synthesis, environmental cleaning and biological process but also in one-compartment cell for electricity generation1-3. Currently, large-scale production of H2O2 is manufactured through anthraquinone autoxidation which has some non-green features for the consumption of high energy due to the multistep hydrogenation and oxidation reactions4. It is high desirable to discover an energy-saving and environmental friendly method to meet the huge demand of H2O2. Semiconductors have been applied in numerous fields because it has many advantages such as clean, one-step generated and environmental friendly thus has attracted much attention. Efforts have been put into the investigation of photocatalytic H2O2 evolution. C.Y. Wang and his co-workers have demonstrated the photocatalytic H2O2 generation of g-C3N4 can be improved as much as 14 times through a carbon vacancy strategy thus achieved 90umol/h production5. As a typical photocatalyst, TiO2 has also been excavated for H2O2 generation via loaded with Au −Ag bimetallic alloy6 or bimodal Au nanoparticles7. Despite many semiconductor systems have been and still are explored for H2O2 generation, the production is still unsatisfied. The reaction mechanism of H2O2 evolution which hindered the improvement of production is still 2


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obscure. In the past few years, Bi-based materials, such as BiOIO38-9, Bi2O2[BO2(OH)]10 and Bi3O4Cl11, have been favored since it possesses efficient internal electric field as a separation ‘accelerator’ and [Bi2O2]2+ layer structure as a separation channel. As a typical Bi-based material, BiOCl have been dug deeply in photocatalytic field, for instance, water-splitting12-13, N2 fixation14-15 and selective alcohol oxidation16, etc. Nevertheless, there were rare reports about the achievement of high H2O2 generation. Typically, the generation of H2O2 conclude three paths: 1) direct one−electron reduction of O2 (O2 + e- = O2·-, -0.33 V vs. NHE)5, 17 to the H2O2; 2) multi−electron reduction of O2 to the H2O2 (O2 + 2H+ + 2e- = H2O2, +0.68 V vs. NHE)18-19; 3) hole-oxidized-·OH(OH- + h+ = ·OH, +2.8 V vs. NHE)20 to generated H2O2. However, BiOCl could not generate H2O2 from direct one−electron reduction of O2 to the H2O2 for the deficient reduction ability of electrons13. Meanwhile, the multi−electron reduction of O2 also cannot realize in BiOCl system for the incapability of electron accumulation. Apart from that, there are rare reports about hole-oxidized-·OH of BiOCl to generated H2O2 because its zeta potential is quite negative (~50mv) which would lead to low adsorption capacity of negative OH-21. Recently, an amount of photo-generated holes in the valence band of BiOCl could be utilized through a holes shuttle to initiate the radical22. Herein, enlightened by the above analysis, we demonstrate a new path of the generation of H2O2 in BiOCl photocatalysis by adding HCOOH as a hole shuttle to direct transfer the holes from valence band of BiOCl to HCOO- yield HCOO·. HCOO· further reacts with OH3



to ·OH which is considered as the source of H2O2. We utilize the BiOCl nanoplates to yield super high H2O2 production (685 µmol/h). Under constant irradiation, the BiOCl produced photo-generated carriers and rapidly separated by the internal electric field exist in the bulk. Apart from HCOOH, O2 also played important roles in the generation of H2O2. When blowing O2 into the system, the generation of H2O2 gives a significant enhancement. The O2 reacted with reduced Bi thus realized the cycle between Bi3+ and Bi0 that decreased the combination of carriers. Blowing O2 also avoided the carbonation of surfaces thus achieved the high production of H2O2 (1020 µmol/h). Through a series of characterization experiments, we elucidate the mechanism of H2O2 generation and optimize the photocatalytic reaction. This work opens new avenues towards achieving high H2O2 production and new perspectives of understanding the mechanism of H2O2 generation. Experimental Preparation of BiOCl photocatalysts The BiOCl photocatalyst was synthesized hydrothermal procedure according to a report23. Briefly, under constantly stirring, 2 mmol(0.97g) of Bi(NO3)3·5H2O and 2 mmol(0.149g) of KCl were dissolved in 30 mL distilled water at room temperature. After 1h vigorous stirring, the as-prepared slurry was sealed into a Teflon-lined autoclave, and maintained at 160 ° for 24h and the cooled down to room temperature. Then, the white products were collected, washed with deionized water, and dried at 330 K for 6 h in air. Characterizations

4


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The phase and composition of as-prepared sample were measured by X-ray diffraction (XRD) studies using an X-ray diffractometer with Cu Kα (λ=0.15418nm) radiation (Rigaku, Japan). The diffuse reflectance spectra (DRS) of the samples were obtained on a Hitachi U-3010UV-vis spectrophotometer using BaSO4 as the reference. The microstructures and morphologies of the samples were analyzed by Transmission Electron Microscopy and selected area electron diffraction (SAED) (TEM, FEI Tecnai G2 F30). The piezoelectric response of BiOCl was characterized by

an AFM

(Atomic Force Microscope, SPA-300HV) equipped with a ferroelectric test system. For the PFM (Piezoresponse Force Microscopy) test, a drop of the suspension of BiOCl dispersed on the Si substrate coated Pt by pulsed laser deposition (PLD). Electron paramagnetic resonance (EPR) analysis was performed at room temperature using a JEOL-FA200 instrument. The typical instrument conditions were as follows: central field, 324 mT; microwave power 0.99800 mW; 9094.532 MHz. EPR signals of the radicals were trapped by 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). After light irradiated for 10 min, the test solution has been used to take the EPR analysis. The degradation of nitroblue tetrazolium (NBT) measurement has been used to detect the O2·- using the UV-vis spectrophotometer (Hitachi U-3010). The degradation experiments of NBT were carried out for 1 h in the presence of BiOCl. Raman spectra were recorded on a microscopic Raman spectrometer (Labram HR Evolution) with an excitation of 532 nm laser light. Electrochemical measurement Electrochemical measurements were performed on a CHI660D electrochemical 5



workstation (Shanghai Chenhua, China) using a standard three-electrode cell containing a working electrode, the counter electrode platinum wire and a reference electrode saturated calomel electrode (SCE). The working electrodes were prepared by dip-coating: 10mg photocatalyst was suspended in ethanol solution containing 5% nafion and ultrasonicated for 15 min to form slurry. 0.1 mL of as-prepared slurry was dip-coated onto a 2.5 cm×1.5 cm fluorine-tin oxide (FTO) glass electrode which was deposited on 2 cm×1.5 cm square. The film was dried under 60 ℃ for 24 h. Here, the electrolyte solution was 0.5M KCl24. After the electrolyte solution was purged by nitrogen, the measurements of impedance potential method was performed under 100Hz to get the Mott-Schottky plots. The light resource for the photocurrent density-time test was from a 300W Xe-lamp. H2O2 production catalytic test With a water cooling jacket outside the reactor, the photocatalytic reaction was performed in an open gas circulation under 25±0.5 °C and the light resource was from a 500 W Xe-lamp. Then 0.05 g of photocatalyst was added in 100 mL aqueous containing HCOOH solution in a Pyrex reaction cell. The reaction was continued 1h or other times for H2O2 evolution. The amount of H2O2 in solution was determined by the titanium sulfate spectrophotometric method via a UV-vis spectrophotometer (Hitachi3010). The production of H2 and CO2 were measured by online gas chromatographs (Tianmei, GC-7890 and GC-7900, TCD, N2 carrier). Results and Discussion Characterization of BiOCl Photocatalysts 6


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Figure 1a shows the XRD patterns of the as-prepared samples. All of the diffraction peaks can be indexed to the standard pattern of tetragonal BiOCl (JCPDS 06-0249 a=b=3.890 Å, c=7.370 Å), which is consistent with the reported data of BiOCl, revealing the high purity of the product. The strongest diffraction peak positions of the obtained products appear at 11.98°, which correspond to the (001) crystal faces of BiOCl, respectively25. The strongest peak implied that (001) plane is the prior growth facet. Figure 1b, 1c and 1d show the morphology and microstructure of the as-prepared BiOCl. The panoramic TEM image in Figure 1b and 1c revealed the nanoplate structure of the sample. In Figure 1d, the corresponding selected-area electron diffraction (SAED) pattern shows the nanoplates with the same growth orientation and most exposed crystal facets of (001), indicating the single-crystalline characteristic of the as-prepared BiOCl. The angle labeled in the SAED pattern is 45° ,which is in agreement with the theoretical value of the angle between the (110) and (200) planes23. The set of diffraction spots can be indexed as the [001] zone axis of tetragonal BiOCl. Figure 2a and 2b show the AFM (Atomic Force Microscope) and PFM (Piezoresponse Force Microscopy) images of the BiOCl. The piezoelectric response and domain structures of BiOCl were characterized by PFM. The difference—dark(Ⅰ) and bright regions(Ⅱ)—indicated the negatively and positively polarized domains, respectively26-27. The PFM image of BiOCl revealed the piezoelectric response was indeed created from BiOCl nanoplates28. This internal electric field existed in piezoelectric BiOCl helps the separation of carriers thus promotes the photocatalysis 7



reaction. The UV-vis diffuse reflectance spectrum (DRS) of the sample is shown in Figure. S1. It was found that the optical adsorption of the BiOCl nanoplates from UV light to visible light and the fundamental absorbance edge of BiOCl is about 370 nm due to the intrinsic band-gap transition. According to the Tauc formula, a direct band gap of 3.6 eV (inset of Figure S1) is estimated for the as-prepared sample. Photocatalytic generation of H2O2 The catalytic activity of the as-prepared BiOCl sample was examined by measure the H2O2 production under constant irradiation in the HCOOH solution (in air atmosphere). Control experiments exhibited that H2O2 cannot be detected in the absence of BiOCl, indicating HCOOH could not self-decompose under irradiation but BiOCl is essential for the generation of H2O2. Fig.3a shows the time-dependent changes in the H2O2 concentration during reactions over the as-prepared BiOCl nanoplates. This result has given an excellent high performance (685 µmol/h). The electric filed inside the BiOCl that could help “pull” electrons and holes apart. The photo-generated electrons and holes would transported to the (001) facets and the (110) facets, respectively12. Furthermore, the concentration of HCOOH can be a factor which influences the production of H2O2. For comparison, different concentrations of HCOOH solution (none, 1.0%, 3.0% and 5.0%) have been used in this catalysis system (shown in Figure 3b). The results revealed that the addition of HCOOH is key for producing of H2O2 but increasing the amount of formic acid only enhanced the production of H2O2 production slightly. For the most possible reason is 8


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that the HCOOH is already sufficient for the generated-holes to oxidize even in 1.0% HCOOH solution. Mechanism for the generation of H2O2 on the BiOCl photocatalyst Based on the above experiment results, we further investigate mechanism of the generation of H2O2. The electrochemical experiments, which can measure the electronic characteristics and band position of the BiOCl, have been carried out. We have taken the electrochemical flat potential measurement and the data was plotted in Mott−Schottky curve (Figure 4a). Generally, flat−band potential values are determined by using the Mott−Schottky equation29. We found that the slope of linear 1/C2 potential curve was positive, which exhibited a typical n-type semiconductor characteristic. The Mott-Schottky measurement has given the conductive position of the as-prepared sample is about -0.30 V (vs. NHE) which means the electron of conduction band may fail to realize the reduction of O2 (O2 + e- = O2·-, -0.33 V vs. NHE)17. The direct one−electron reduction of O2 to the H2O2 would not be the main path to the generation of H2O2. A new path for the generation of H2O2 needs to be explored. In this reaction system, the crucial effect of HCOOH should not negelect based on the results in Figure 3. In order to evaluate the HCOOH effect on H2O2 evolution deeply, we employed BiOCl as photoanodes in a photoelectrochemical cell. A Pt counter electrode and 0.5M KCl as electrolyte were employed, and the BiOCl electrodes were illuminated with UV-visible light. When HCOOH was added into the photoelectrochemical cell, the photocurrent response to illumination is novel which is 9



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shown in Fig.4b. The photocurrent is higher than that in 0.5M KCl, indicating that the capture of holes by HCOOH is so fast that electrons cannot be extracted in a timely manner, resulting in the accumulation in CB of BiOCl30. The HCOOH immediately oxidized by holes to generated active HCOO· (eq.1)31. HCOO-+h+→HCOO·

(1)

HCOO·→CO2+H++eCB

(2)

Apart from that, the decomposition of HCOO· injected electrons into conduction band also enhanced the photocurrent response (eq.2)32. After near-surface-HCOOH exhausted for several seconds, the photocurrent is approaching to the no-HCOOH added process. This phenomenon indicated that HCOOH played an important role of promoting the separation of carriers and shuttling holes to initiate a radical reaction for the H2O2 evolution. In order to track how the HCOOH and HCOO· influence the reaction, EPR (Electron Paramagnetic Resonance) measurement has been carried out (shown in Figure 5a). The scavenging of ·OH formed upon irradiation of a BiOCl suspension in the presence of HCOOH yields the characteristic of HO-DMPO· complex (aN =aH =14.9mT)

22, 33-34

. Fig.5a reveals four characteristic signals of HO-DMPO· complex

generated over BiOCl nanoplates in the HCOOH solution after the irradiation of light, confirming the formation of ·OH radicals. HCOO·+OH-→·OH+HCOO-

(3)

2·OH→H2O2

(4)

The generated-HCOO· further reacts with OH- to ·OH (eq. 3). ·OH radical has been 10


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verified the existence in this system and demonstrated that the ·OH radical is the main source of H2O2 (eq. 4) (Scheme 1)35. None HO-DMPO· complex can be detected in the absence of HCOOH demonstrated the vital role of HCOOH (shown in Figure 5a). Considering the charge conservation, the behavior of photogenerated electrons has been investigated. Typically, the BiOCl could be self-reduced by the photogenerated electrons. Specifically, the Bi3+ can be reduced to Bi according to previous report12. In this photocatalytic system, the reduced-Bi also has been observed due to the color of the BiOCl surface changing from white to black (shown in Figure S2) under Ar/N2 atmosphere. Figure 5b is Raman spectra of BiOCl and BiOCl-HCOOH-light which has reacted in HCOOH solution under light irradiation. After 3h reaction, there appears a broad and weak band at 80-90 cm−1 on BiOCl, which corresponds to the acoustical part of the Bi spectrum36. When bubbled with O2, the catalyst keeps the white color which indicates the oxidation of Bi. The reduced-Bi is oxidized back to Bi3+. O2 consumes the electrons through the cycle between Bi3+ and Bi. This process decreases the combination of carrier thus promotes the holes participate the reaction. Meanwhile, the generation of H2O2 gives a 49% increase (1020 µmol/h) compared to that of not introduced O2 (685 μmol/h) (shown in Figure 6). It is worth to note, it is well known that oxygen vacancies, which can activate O2 to produce O2·-, generate over BiOCl under constant light irradiation37,38. The corresponding detection measurements of O2·- also have been carried out (shown in Table S1). All experiments were carried out in the presence of BiOCl with the reacting time of 1 h. And, the degradation rate of NBT is proportional to the yield of generated O2·-. Under constant 11



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light irradiation, O2 were activated and O2·- formed by the photo-induced oxygen vacancies over the surface of the BiOCl37. Without HCOOH, 80% of NBT was degraded meaning that large amount of O2·- were generated, but this system resulted in low H2O2 yield of ~34 µmol/h (without HCOOH, in air atmosphere). However, when HCOOH was added in the reaction solution, less NBT (~23%) was degraded (with HCOOH, in air atmosphere). It suggested the generation of O2·- was depressed in the presence of HCOOH. Apart from that, oxygen-rich atmosphere also depressed the generation of O2·- due to the restrictions on oxygen vacancies which were strongly inclined to be filled by activated oxygen, no matter HCOOH was introduced or not. To sum up, above results indicates that the reduction of O2 to O2·- process may not the main route for the generated H2O2. All results demonstrate that O2 participates in the reaction through the cycle between Bi3+ and Bi as following: e-

O2

Bi3+ (white) ሱۛሮ Bi ሺblackሻ ሱۛሮ Bi3+ (white)

(5)

Photogenerated electrons reduce Bi3+ to Bi (eq. 5). The blowing O2 oxidized Bi back to Bi3+ (Scheme 1). Furthermore, when exhausted the air by N2, H2 has been detected (~2 µmol/h) for the electrons of conduction band possessing the ability of hydrogen evolution reaction that can be regarded as one path of electron consumption. Apart from that, large amount of CO2 also generated that confirmed the eq 2. However, we found that the H2O2 evolution rate of this system was very high at the initial first hour but it would drop down along with the irradiation time. One reason could be responsible for the lower H2O2 evolution rate of subsequent hours. We assumed the HCOOH was decomposed into CO2 (eq. 2) which adsorbed onto the 12


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surfaces of BiOCl thus suppressed the charge transfer and generated photocatalytic inertial surface. To further clarify the surface states change, FT-IR characterizations were carried out. The FT-IR spectra shown in Figure 7a demonstrate that the surface of BiOCl is significantly changed after the photocatalytic reaction in HCOOH. In the FT-IR spectra, a broad band centered at 3440 cm−1 is observed in each sample, which is assigned to the O-H stretching of water for the water adsorption from air39. In the spectrum of pure BiOCl, the absorption peak at 529 cm-1 can be assigned to Bi–O bond stretching vibration40. The peak at 2400 cm-1 can be assigned as one feature signal of (BiO)2CO3 (the green circle)41-42. The feature peaks both existed in the (BiO)2CO3

and

used-BiOCl

which

revealed

the

surface-carbonated.

The

generated-CO2 adsorbed onto the surface of BiOCl thus covered the active site then reacted with BiOCl to generate (BiO)2CO3. The generation of (BiO)2CO3 should be responsible for the loss of H2O2 evolution rate. The FT-IR surface analysis clarified the phenomenon. In order to recover the evolution rate, diluted HNO3 has been used to sculpture the surface-carbonated layer (This sample is denoted as BiOCl-R). As is shown in Figure 7b, the H2O2 evolution rate has been recovered after the acid treatment. All results were in line with the proposed mechanism analysis. In order to further confirm the adsorption of CO2 on the surface, contrast experiments were carried out under constantly inletting Ar or CO2 for comparison, respectively. As is shown in Figure 7c, the photocatalytic H2O2 evolution is strongly dependent on the atmosphere. In the CO2 inletting-system, the H2O2 evolution rate 13



only maintains 464 µmol/h which decreases 32% compared to no-inletting O2 (685 µmol/h). Under the Ar atmosphere, the H2O2 evolution rate is about 531 µmol/h which is obvious higher than in inletting CO2 system. The decrease compared with the inletting-O2 system is that Ar cannot realize the cycle between Bi3+ and Bi. The O2 optimized-system achieved the highest H2O2 production (1020 µmol/h, shown in Figure 6) to the best of our knowledge (shown in Table S2). The blowing pure O2 timely carried off the generated-CO2 eliminating the carbonation of BiOCl and promoted the holes participate the reaction. Conclusion We utilize the BiOCl nanoplates to yield super high H2O2 prodution (1020 µmol/h) by the addition of HCOOH and O2. Under constant irradiation, the BiOCl produced photo-generated carriers and rapidly separated by the internal electric field exist in the bulk. HCOOH has played a shuttle to transfer the photo-generated holes oxidized HCOOH into HCOO· radicals further to react with OH to ·OH which is considered as the main source of H2O2. The HCOOH not only transfers the holes thus promote oxidation but also play a key role in providing abundant active species in this photocatalysis system. And the filling abundant O2 significantly promote holes participate the reaction, through the cycle between Bi3+ and Bi, which decreased the combination of carriers. Apart from that, O2 also prevented the carbonation of surfaces thus achieved even high production (1020µmol/h). Through the adjustment of atmosphere and recover process, we optimized the H2O2 evolution reaction of the BiOCl photocatalysis in the HCOOH solution. In this work, we elucidated a new 14


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understanding of H2O2 generation and achieved even high production as well. Supporting Information UV-vis diffuse reflectance spectrum of BiOCl; The image of the color change of BiOCl after reacted in HCOOH solution; The degradation experiments of NBT for the detection of O2·- yield; Summary of representative for generating H2O2.

Notes The authors declare no competing financial interest.

Acknowledgements We acknowledge the financial support from the National Natural Science Foundation of China (51472260, 51772312).

15



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Figure 1. (a) XRD patterns of BiOCl. TEM images of the (b) overall view of BiOCl, (c) single particle of BiOCl, (d) selected area electron diffraction (SAED) pattern of BiOCl.

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Figure 2. (a) AFM (Atomic Force Microscope) and (b) PFM (Piezoresponse Force Microscopy) images of the BiOCl.

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Figure 3. (a) The H2O2 evolution of BiOCl as time extended in the presence of HCOOH in air atmosphere, (b) The H2O2 evolution of BiOCl in different concentration of HCOOH solutions for 1h in air atmosphere.

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Figure 4. (a) Mott-Schottky (MS) plots of BiOCl electrodes; (b) Photocurrent density-time (i-t) curves of the BiOCl (before and after HCOOH was added into the system).

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Figure 5. (a) EPR (Electron Paramagnetic Resonance) spectra of HO-DMPO· spin adducts obtained by irradiation of BiOCl suspension. (b) Raman spectra of BiOCl and BiOCl-HCOOH-light which has reacted under light in HCOOH solution.

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Figure 6. The comparison of H2O2 evolution with BiOCl in air atmosphere and bubbled with O2 in air atmosphere, respectively.

Scheme 1. The mechanism of the generation of H2O2 in the presence of HCOOH over the BiOCl

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Figure 7. (a) FT-IR spectra of BiOCl, (BiO)2CO3 and BiOCl which has been irradiated for 3h in the HCOOH, the inset green circle peak at about 2400cm-1 should be assigned to the feature of (BiO)2CO3. (b) H2O2 evolution of BiOCl and recoverd-BiOCl (BiOCl-R) by diluted HNO3. (c) H2O2 evolution of BiOCl under different atmosphere.

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TOC

We utilize BiOCl nanoplates to yield excellent photocatalytic H2O2 production (1020 µmol/h) in HCOOH solution under O2 atmosphere.

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Internal Electric Field Assisted Photocatalytic Generation of Hydrogen

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