Photocatalytic Mechanism Regulation of Bismuth Oxyhalogen via

Aug 31, 2017 - So, BiOBr0.5I0.5 showed higher activity than 0.5BiOBr/0.5BiOI for dye degradation because the 1O2 was the major active species with ext...
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Photocatalytic Mechanism Regulation of Bismuth Oxyhalogen via Changing Atomic Assembly Method Yang Bai,& Xian Shi,& Ping-Quan Wang,*,& Haiquan Xie,# and Liqun Ye*,&,# &

State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Southwest Petroleum University, Chengdu 610500, China Henan Key Laboratory of Environmental and Energy Photocatalysis, Engineering Technology Research Center of Henan Province for Solar Catalysis, Collaborative Innovation Center of Water Security for Water Source Region of Mid-route Project of South-to-North Water Diversion of Henan Province, College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China

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S Supporting Information *

ABSTRACT: Exciton and carrier photocatalytic processes have been proved in bismuth oxyhalogen photocatalysts. But, there are no reports about how to regulate the different mechanisms to improve photocatalytic activity for different reaction. Here, we found that the photocatalytic mechanisms could be regulated by changing the assembly method of bismuth, oxygen, and halogen atoms. Reactive oxygen species (ROS) experimentals results concluded that solid solution BiOBr0.5I0.5 showed enhanced exciton photocatalytic process, and coupling 0.5BiOBr/0.5BiOI displayed improved carrier photocatalytic proces. This work promoted the understanding about solid solution and coupling for bismuth oxyhalogen. KEYWORDS: BiOX, photocatalysis, carrier, exciton, solid solution, coupling

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solid solution and coupling with same element ratio and different atomic assembly method. In this work, we intended to regulate the photocatalytic mechanisms of BiOX via atoms assembly method. And the preliminary judgment was checked by basic redox experiments. First, BiOBr0.5I0.5 and 0.5BiOBr/0.5BiOI, with the same elements rates but formed by entirely different assembly method of bismuth, oxygen and halogen atoms, were in situ synthesized and their different structures were described in Figure 1a. The XRD patterns (Figure 1b) and TEM images (Figure 1c, d) were used to understand the different structures of BiOBr0.5I0.5 and 0.5BiOBr/0.5BiOI, For BiOBr0.5I0.5, only one series XRD characterization peaks between that of BiOBr and BiOI were entirely different from 0.5BiOBr/0.5BiOI that contained two XRD characteristic peaks of BiOBr and BiOI. To differentiate the XRD signals more clearly, the enlarged XRD patterns of 0.5BiOBr/0.5BiOI and BiOI0.5Br0.5 are shown in Figure S1a. It can be seen that the (001) peak of 0.5BiOBr/ 0.5BiOI had two signals at 9.65 and 10.86°, which were the (001) peaks of pure BiOBr and BiOI, respectively. But, BiOI0.5Br0.5 had only one (001) signal at 10.06°. The same phenomenon about (102) peak was also found as shown in Figure S1b. It proved that BiOI0.5Br0.5 and BiOBr/0.5BiOI were single-crystal structure solid solution and coupling, respectively.

ismuth halide-based photocatalysts have been widely explored for their appreciable photocatalytic performances in environmental remediation and solar fuels generation.1−4 Along with the comprehensive study of the photocatalytic mechanisms of BiOX, beyond the traditional carrier photocatalytic process, the existence of exction photocatalysis in BiOX has been proved.5 The previous viewpoint confirmed that [Bi2O2]2− sandwiched structures induced internal electric fields between halogen atoms slabs of BiOX, which provided effective separation of photogenerated electron−hole pairs spontaneously.6 Meanwhile, the unique layered structure was also favorable for the formation of excitons, thus affecting the photocatalytic process.7 The above two photocatalytic mechanisms influence the photocatalytic properties of BiOX,but how to regulate them for better photocatalytic properties is still a challenge and has not been reported yet until now. Forming solid solution and coupling structures are efficient methods to optimize the photocatalytic properties for semiconductors. A lot of solid solution and coupling photocatalysts even composed of the same elements ratio have been reported for their enhanced performances.8−12 For example, coupled 0.5CdS/0.5ZnS and Cd0.5Zn0.5S solid solution show the same elements ratio, but different atomic assembly method, and it have been reported that they all behaved better than CdS and ZnS monomers.13,14 However, though the two structures both enhanced photocatalytic activities, there is no report to compare the enhanced photocatalytic activity mechanism of © XXXX American Chemical Society

Received: July 14, 2017 Accepted: August 31, 2017 Published: August 31, 2017 A

DOI: 10.1021/acsami.7b10233 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces

Figure 1. (a) Simulated diagram of 0.5BiOBr/0.5BiOI and BiOBr0.5I0.5; (b) XRD patterns of BiOBr, BiOI, 0.5BiOBr/0.5BiOI, and BiOBr0.5I0.5; (c) HRTEM image of BiOBr0.5I0.5; and (d) HRTEM image of 0.5BiOBr/0.5BiOI.

HRTEM images (Figure 1c and 1d) also support the above conclusion. 0.5BiOBr/0.5BiOI contained two groups of (001) interplanar spacing indexed with BiOBr (0.81 nm) and BiOI (0.91 nm), respectively, which was different from the only (001) interplanar spacing of BiOI0.5Br0.5 of 0.87 nm.15−19 Figures S2−S6 show the more detailed characterizations about morphology, surface valence, light absorption, and specific surface area. All of the basic characterizations revealed that BiOBr0.5I0.5 and 0.5BiOBr/0.5BiOI expressed different structures through the change of atomic assembly methods. The redox experimental results were consistent with our expectations. Figure 2 showed the photocatalytic activity comparison of BiOBr0.5I0.5 and 0.5BiOBr/0.5BiOI for dye degradation and CO2 conversion. Over 120 min light irradiation, it can be seen that 0.5BiOBr/0.5BiOI did not showed obviously enhanced activity for RhB photodegradation. However, BiOBr0.5I0.5 performed best activity (87.5% of RhB degradation) than that of BiOI (55.8%), BiOBr (61.0%) and 0.5BiOBr/0.5BiOI (64.5%). For CO2 conversion, 0.5BiOBr/ 0.5BiOI behaved best with about 14.9 μmol g−1 reduced products after 2 h irradiation, which was about 2 times that of BiOBr (6.3 μmol g−1). However, BiOBr0.5I0.5 did not show obviously improved activity for CO2 photoreduction into CO (7.1 μmol g−1). The above experimental results implied that BiOBr0.5I0.5 and 0.5BiOBr/0.5BiOI showed different photocatalytic mechanisms. Figure S7 show the photocatalytic stability experiments of BiOBr0.5I0.5 and 0.5BiOBr/0.5BiOI for RhB degradation and CO2 conversion. It can be seen that the activities did not decrease obviously. It confirmed the reproducibility and stability of the time course reaction. Singlet oxygen (1O2) and superoxide radical (O2•−) were the exciton photocatalytic process and carrier photocatalytic process products from O2, respectively. Therefore, primary active species produced by BiOBr0.5I0.5 and 0.5BiOBr/0.5BiOI may be different for their different photocatalytic mechanisms, respectively. It has been reported that 1O2 was generated by transformation of 3O2, whereas O2•− was produced by the

Figure 2. (a)RhB photodegradation of BiOI, BiOBr, BiOBr0.5I0.5, and 0.5BiOBr/0.5BiOI under visible-light irradiation, (b) photocatlytic CO2 conversion of BiOBr0.5I0.5, 0.5BiOBr/0.5BiOI, BiOBr, and BiOI under Xe lamp.

reaction of photogenerated electron and O2.20−25 So, for analyzing the origin of different photocatalytic mechanisms, trapping experiments were done next step. As shown in Figure S8, the main reactive oxygen species (ROS) of BiOBr0.5I0.5 and B

DOI: 10.1021/acsami.7b10233 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 3. (a) Total absorbance of TMB oxidation; (b) total absorbance of NBT reduction; singlet oxygen and superoxide radical ESR tests of (c) BiOBr0.5I0.5 and (d) 0.5BiOBr/0.5BiOI.

0.5BiOBr/0.5BiOI were singlet oxygen (1O2) and superoxide radicals (O2•−), respectively. So, BiOBr0.5I0.5 showed higher activity than 0.5BiOBr/0.5BiOI for dye degradation because the 1 O2 was the major active species with extremely high oxidizability. On the other hand, trapping experiments also revealed that photoinduced electron affected the activity of 0.5BiOBr/0.5BiOI more strongly than BiOBr0.5I0.5. So, 0.5BiOBr/0.5BiOI displayed higher activity than BiOBr0.5I0.5 for CO2 reduction. To confirm the conclusion, we finalized the quantitative experiments of 1O2 and O2•−. Through comparing the photogenerated amounts of 1O2 and O2•−, exciton and carrier photocatalysis could be determined. 3,3′,5,5′-tetramethylbenzidine (TMB), nitrotetrazolium (NBT), and electron spin resonance (ESR) were used as efficient exploration methods and the experimental results were shown in Figure 3. We combined all the experimental results to verify above conclutions: solid solution structure enhanced its photocatalytic properties by exciton process and coupling by strengthen carrier photocatalysis. Namely, we can regulate photocatalytic mechanism by changing assembly method of bismuth, oxygen, and halogen atomics. The total TMB oxidation rates (evaluated by absorbance peaks at 380 nm) after 15 min irradiation were shown in Figure 3a.26 It could be seen clearly that larger amount of 1O2 generated by BiOBr0.5I0.5 structure than 0.5BiOBr/0.5BiOI. These four samples all revealed an increasing TMB oxidation rate (Figure S9) and BiOBr0.5 I0.5 exhibited a definite superiority, indicating more 1O2 generation of solid solution structure. As for 0.5BiOBr/0.5BiOI, BiOBr, and BiOI, almost the same weaker growth trend of the adsorption peak meant less singlet oxygen generation (Figure S9). Meanwhile, ESR, as the most direct evidence of photogenerated singlet oxygen, was used to compare solid solution and coupling (Figure 3c). No discernible 1O2 signal was revealed in dark. When under the light irradiation, BiOBr0.5I0.5 had stronger 1O2 signal than 0.5BiOBr/0.5BiOI. It demonstrated that there was a promotion

of exciton process in BiOBr0.5I0.5, which was in agreement with TMB measurement results. NBT measurements (evaluated by absorbance peaks at 259 nm) had been finished to estimate the O2•‑.27 As shown in Figure 3b, 0.5BiOBr/0.5BiOI presented an obvious total NBT reduction rate than other samples after 50 min reaction. It demonstrated that more O2•‑ generation (details were shown in Figure S10). ESR was also used for direct tests of O2•‑. The stronger signal of 0.5BiOBr/0.5BiOI revealed more O2•‑ generated, which implied more dissociative electrons excited (Figure 3d). 0.5BiOBr/0.5BiOI provided a platform for the generation of O2•− because of its better separation efficiency of electrons and holes, which also explained it is enhanced reduction activity for photocatalytic CO2 conversion. Beyond that, transient photocurrent responses (TPR) (Figure S11) directly showed better e− and h+ separation of coupling than solid solution. Combining with above test results, the enhanced exciton photocatalysis and carrier photocatalysis process by solid solution structure and coupling were accredited, respectively. Photocatalytic schemes of 0.5BiOBr/0.5BiOI and BiOBr0.5I0.5 are shown in Scheme 1. For 0.5BiOBr/0.5BiOI, electrons of BiOI could be excited from the valence band (VB, 1.23 eV) to the conduction band (CB) of BiOBr at position of −0.99 eV Scheme 1. (a) Enhanced Carrier Photocatalytic Process of 0.5BiOBr/0.5BiOI, (b) Enhanced Exciton Photocatalytic Process of BiOBr0.5I0.5

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DOI: 10.1021/acsami.7b10233 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

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under 465 nm light irradiation. Similarly, the electrons of BiOBr could jump to the CB potential of BiOI at −0.71 eV. The hetrostructure made the electrons of the BiOI nanoflakes easily jump to the CB of BiOBr easily. And so, more electrons were used to reduce O2 into O2•−. But for solid solution, the separation efficiency of electron−hole pairs was lower than coupling, wheras the unique layered structure of BiOBr0.5I0.5 solid solution possessed intensive electron−hole interactions and promoted exciton photocatalysis process. Therefore, more 1 O2 was generated via enhanced exciton photocatalytic process. In summary, we provided a new thought of photocatalytic mechanisms for bismuth oxyhalogen. In previous reports, solidsolution BiOBr0.5I0.5 and coupling 0.5BiOBr/0.5BiOI were proved that they can improve the photocatalytic activity. In this letter, we proved that solid solution and coupling enhanced their photocatalytic properties via different mechanisms. Singlet oxygen generated by exciton process was promoted by solid solution structure, and solid solution pholocatalyst may be more suitable for environmental photocatalysis field. On the other hand, coupling structure generated more dissociative electrons on the surface for enhanced carrier photocatalysis process. And coupling pholocatalyst may be more suitable for energy photocatalysis field. Namely, the photocatalytic mechanism of bismuth oxyhalogen could be regulated by different atomic assembly methods.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b10233. Materials preparation and characterization, photocatalytic RhB degradation, CO2 conversion, TMB measurement, and NBT measurement experimental procedures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Liqun Ye: 0000-0001-6410-689X Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.Y. thanks the National Natural Science Foundation of China (51502146, U1404506, 21671113). Y.B. thanks National Natural Science Foundation of China (51702270), the scientific Research Starting Project of SWPU (2015QHZ001), Young Scholars Development Fund of SWPU (201499010100), and Open Fund (201601) of State Key Laboratory of physical chemistry of solid surfaces (Xiamen University).

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REFERENCES

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DOI: 10.1021/acsami.7b10233 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX