KBiO3 as an Effective Visible-Light-Driven Photocatalyst: Stability

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Kinetics, Catalysis, and Reaction Engineering

KBiO3 as an Effective Visible-Light-Driven Photocatalyst: Stability Improvement by In-situ Constructing KBiO3/BiOX (X = Cl, Br, I) Heterostructure Hao Zhang, Hanxiao Zheng, Ying Wang, Runhua Yan, Dingyuan Luo, and Wei Jiang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05658 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 14, 2019

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KBiO3 as an Effective Visible-Light-Driven Photocatalyst: Stability Improvement by Insitu Constructing KBiO3/BiOX (X = Cl, Br, I) Heterostructure Hao Zhang, Hanxiao Zheng, Ying Wang, Runhua Yan, Dingyuan Luo, Wei Jiang* Low-carbon Technology and Chemical Reaction Engineering Laboratory, School of Chemical Engineering, Sichuan University, Chengdu, 610065 (PR China) E-mail: [email protected]

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Abstract

Petal-bismuth salt semiconductors are promising as visible-light-driven photocatalysts, but their short service times due to strong oxidation limit their application. In this research, the in-situ construction of a p-n heterojunction, by growing BiOX on a KBiO3 surface, is used as a strategy to improve the performance and stability. Evaluation of the photocatalytic degradation of crystal violet and phenol shows that the KBiO3/BiOX (X = Cl, Br, I) heterostructures exhibit significant improvement in photocatalytic performance and stability compared to KBiO3. The best one, KBiO3/(5.85%)BiOI, shows 98% degradation of crystal violet within 20 min, and of phenol within 1.5 h. No significant performance decay can be detected after repeated use. The development of KBiO3/BiOX heterostructure improves the applicability of KBiO3 as an efficient visible-light-driven photocatalyst.

KEY WORDS: Degradation mechanism, Heterostructure, In-situ formation, Stability

1. Introduction With the rapid development of industry, organic pollutants are produced, posing a threat to human health and the ecological environment1,2, photocatalysis technology, as an advanced oxidation technology, has a good application prospect in the treatment of organic pollutants3. As a typical photocatalyst, TiO2 has strong oxidizing ability, but its wider forbidden band width and photogenerated electron-hole recombination frequency limit the actual application, and promotes the development of visible-light-driven 2



photocatalysts4-6. Therefore, various semiconductors such as perovskite7,8, carbon nitride (C3N4)9, silver-containing compounds10-12, vanadium-containing compounds1315,

and doped TiO216,17, have been developed. Therein, bismuth-based semiconductors

exhibits an attractive application prospect due to their excellent performance and stability resulted from the particular electronic structure and wide light absorption spectrum18,19. Presently, three types of bismuth-containing compounds have been developed: multicomponent (BixMOy), such as Bi2WO620, Bi2O321 BiVO422, and Bi2O2CO323; oxyhalides (BiOX), such as BiOX(X=Cl,Br,I)24-26; and pentavalent bismuthates (MBiO3), such as MBiO3(M=Na, Li, K)

27-29.

Compared with other compounds,

pentavalent ruthenium stands out from these Bi-containing compounds due to its high oxidizing activity and narrow band gap, which is due to the existence of empty 6s orbital in the valence band and conduction band30. As a typical pentavalent bismuthates, sodium bismuthate (NaBiO3) has been used to photodegrade different typical organic persistence pollutants31-33 indicating that it is an effective visible light driven photocatalysts. However, KBiO3, another pentavalent bismuthate, is rarely reported, even though its band gap (2.04 eV) is narrower than NaBiO3 (2.46 eV). The narrow band gap, resulting from the tunnel structure of KBiO3, suggests wider spectrum utilization and possibly higher photocatalytic activity than NaBiO318. The research results conducted by Takei et al confirmed that KBiO3 is a better visible light driven photocatalyst than NaBiO3 after comparing a series of pentavalent bismuthates30. Some anionic dyes also can be effectively photodecomposed with KBiO3 under ultraviolet 3


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light irradiation and solar radiation28. These two studies confirm the promising prospect of using KBiO3 as a photocatalyst. However, the stability of KBiO3 is unsatisfactory owing to the strong oxidation ability of the pentavalent bismuth ion34,35. The phenomenon of co-existence of photocatalysis, redox, and adsorption in the photodegradation process of different organics with KBiO3 as the photocatalyst has been confirmed in our previous work. The cause of KBiO3 performance attenuation due to the generation of Bi2O3 was detected36. Therefore, it is necessary to improve the stability of KBiO3 and further improve its photocatalytic activity to enable its future applications. In the field of photocatalysis, constructing heterojunction structures is an ingenious and effective strategy to significantly improve photocatalyst activity and stability. Bismuth oxyhalides (BiOX, X = Cl, Br, I), have been the most popular candidates in recent years owing to their narrow band gap and strong photosensitivity properties. Various heterojunctions, such as NaBiO3/BiOCl37, BiOI/BiPO438, (BiO)2CO3/BiOI39, (BiO)2CO3/BiOX40,

BiOCl/BiVO441,

BiOCl/g-C3N442,

BiOI/BiVO443

and

BiOBr/Bi2WO644, have been reported. The addition of BiOX can intensify the spectrum response and expand the response range effectively. The special [X-Bi-O-Bi-X] layered structure of BiOX improves the separation of photoproduced electrons/holes45. Besides, due to the existence of van der Waals forces and covalent interactions between the layers, BiOX compounds possess higher stability than other semiconductors, and prevent secondary pollution due to their trace solubility (e.g. solubility constant of BiOCl is less than 1.8 × 10−31)19. Since KBiO3 is a typical n-type semi-conductor, and 4



BiOX compounds are one of the few kinds of p-type semiconductors, it is possible to effectively improve the activity and stability of KBiO3 by building a p-n heterojunction after combination with BiOX. In this study, KBiO3 prepared by a solid-phase heating method was used as the Bi resource. Three KBiO3/BiOX (X = Cl, Br, I) heterostructures were constructed by obtaining BiOX on KBiO3 surface based on an in-situ strategy. Two typical organics, crystal violet (CV) and phenol, were selected as the model target pollutants for degradation to evaluate the activity and stability of the heterostructures under visible light irradiation, in comparison with those of KBiO3.

2. Experimental 2.1. Reagents and materials All materials and chemicals were analytical grade and used directly, without any pretreatment. Sodium bismuthate dehydrate was purchased from Aladdin Reagent Co. Ltd., while other analytical purity reagents were obtained from ChengDu KeLong Chemical Reagent Co. Ltd.

2.2 Preparation of photocatalyst The preparation of KBiO3 was referenced to our previous work36. BiOX (X = Cl, Br, I) was synthesized by an in-situ formation strategy46. About 1.0 g KBiO3 powder was dispersed in a homogeneous mixture of 10 mL of deionized water and 20 mL of absolute ethanol stirred at room temperature. HCl aqueous solution (7.2 wt%) was 5


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added dropwise to synthesize pure BiOCl and KBiO3/BiOCl composites with a BiOCl molar ratio of 8.68%, 10.4%, 12.2% and 13.9% by controlling the amount of HCl added. After 1 h constant stirring, the substrate was washed alternately with deionized water and absolute ethanol, and then dried at 80 ˚C for 12 h. Similarly, pure BiOBr and KBiO3/BiOBr composites were prepared with HBr solution (7.2 wt%), and pure BiOI and KBiO3/BiOI composites were prepared with HI solution (7.2 wt%).

2.3 Characterization The components and crystal structure of the acquired composite sample were analyzed by a X-ray diffraction (DX-2007 SSC) with a Cu Kα 40 kV/35 mA X-ray source (wavelength λ = 0.15406 nm). The morphology was determined by a scanning electron microscope (JSM-6610LV, JEOL). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were obtained with a Tecnai G2 F20 STwin microscope. The optical absorption spectrum of composites was obtained by UVvisible diffusion reflection spectrometry (UV-Vis DRS, TU-1901 UV−Vis spectrophotometer), and their photoluminescence (PL) spectra were obtained with a fluorescence spectrometer (F-7000, Hitachi, Japan). Surface areas were determined by the nitrogen adsorption method with a BET analyzer (SSA-3500, China). Photoelectrochemical measurements and electrochemical impedance spectroscopy (EIS) measurements of the samples were performed by using a Gamry electrochemical workstation. The test was carried out using a three electrode in which an Ag/AgCl electrode was used as a reference electrode and a Pt wire was used as a counter electrode, 6



and a catalyst uniformly coated on the surface of the (FTO) conductor glass was used as a working electrode. A quartz cell filled with 50 mL of Na2SO4(0.5M) electrolyte was used as a measurement system.

2.4. Photocatalytic experiments The photocatalytic activities of KBiO3/BiOX (X = Cl, Br, I) with different bismuth oxychloride molar ratios were evaluated in an OCRS-IV photoreactor system with a 500 W Xe lamp as the source of analog visible light (filter, λ> 420nm). Crystal violet (16 mg/L) and phenol (25 mg/L) were used as the target organics for photodegradation. In a typical case, 0.01 g sample was dispersed into 10 mL of dye solution with continuous magnetic stirring in a dark room to achieve the absorption-desorption equilibrium. Subsequently, the suspension was illuminated and sampled at intervals. After centrifugation, the concentration was measured using a TU-1901 UV-Vis spectrophotometer. The characteristic peaks of crystal violet and phenol are 585 nm and 270 nm, respectively.

2.5 Free radical trapping experiment In order to further determine the active components of the reaction, a free radical trapping experiment was carried out in the study. In this experiment, methanol (MeOH), isopropanol(IPA), and benzoquinone(BQ) at a concentration of 1.0 mmol/L were selected as the traps for holes(h+), hydroxyl radicals (•OH), and superoxide radicals (•O2−), respectively. This method is the same as the previous photocatalytic performance test. 7


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3. Results and discussion 3.1. Characteristics of the Prepared KBiO3/BiOX (X=Cl, Br, I) Samples The prepared photocatalysts of pure KBiO3 and KBiO3/BiOX (X=Cl, Br, I) samples were characterized by XRD(Fig. 1) to definitude the crystal structures of the three composite catalysts. Among them, the peaks at 12.5°, 17.7°, 28.2°, 33.5°, and 46.3° can match the (110), (200), (310), (321), and (510) crystal faces of KBiO3( PDF 47-0879), respectively. In the three composite catalysts, the diffraction peaks of KBiO3 appear, and there is also a strong diffraction peak of bismuth oxyhalide. For KBiO3/BiOCl, the characteristic peaks at 32.5°, 33.5°, and 46.7° are allocated to the (110), (102), and (200) facets of BiOCl (JCPDS No. 85-0861). For KBiO3/BiOBr, the typical peaks at 31.7°, 32.4°, 57.2° are in agreement with the (102), (110), and (212) facets of BiOBr (JCPDS No. 78-0348). For KBiO3/BiOI, the peaks at 29.6°, 31.7°, and 55.2° coincide with the (102), (110), (212) facets of BiOI (JCPDS No. 10-0445). Simultaneously, no diffraction peaks of other impurities were detected. The crystal structures of all three BiOX compounds can be regarded as tetragonal phases (space group P4/nmm(129)).

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Figure 1. The XRD pattern of as-prepared pure KBiO3, KBiO3/(12.2%)BiOCl, KBiO3/(6.14%)BiOBr and KBiO3/(5.85%)BiOI. To investigate the surface morphology of the three composite photocatalysts, SEM and TEM graphics of KBiO3/BiOX samples were shown in Fig. 2. Pure KBiO3 is made up of many irregular small particles with a size of 0.5-2μm (Fig. 2-a). The SEM pictures of KBiO3/BiOX shown in Fig. 2-b to Fig. 2-d. The accumulation of flakes with similar appearance but different sizes can be observed on the bulk surface, and can be considered as the BiOX layer structure grown in-situ on the KBiO3 surface. The thickness of the implanted flakes ranged between 100–300 nm with 0.5-2.0μm dimensions, causing a noticeable increase in grain size and the corresponding coarseness. However, the determined specific surface areas of the KBiO3, 9


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KBiO3/BiOCl, KBiO3/BiOBr and KBiO3/BiOI samples examined by BET measurement were 7.5246m2/g, 10.8645m2/g, 13.6004m2/g and 5.7269m2/g respectively. This result implied that the variation in size of the composite structures did not affect the available surface area significantly. Moreover, the TEM images of the three composites shown in Fig. 2-e to Fig. 2-g show a similar close relationship between KBiO3 and the supported BiOX flakes, indicating the formation of a heterojunction structure. This close relationship can be attributed to the in situ growth of BiOX with KBiO3 as the Bi resource, which facilitates the efficient transfer of the photogenerated carriers between the two semiconductors. HRTEM analysis was performed by selecting a specific interface between KBiO3 and BiOX, which was marked as a white dot square. In Fig. 2-h, the lattice fringe width of the KBiO3/BiOCl composite photocatalyst in the sheet structure and the bulk structure is d=0.275nm and d=0.316nm. It is consistent with the (110) crystal plane of BiOCl and the (310) crystal plane parameter of KBiO3, respectively. The (101) crystal plane of BiOCl and the (200) crystal plane of KBiO3 are observed in the selective area electron diffraction (SAED) diagram. Similarly, in Fig. 2-i, the (310) plane of KBiO3 and the (102, 101) planes of BiOBr were detected by HRTEM and SAED, respectively. In Fig. 2-j, the (310) plane of KBiO3 and the (102) plane of BiOI were detected. The HRTEM and SAED results are in agreement with the XRD results, further confirming the co-existence of KBiO3 and BiOX in the as-prepared composites.

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Figure 2. (a) SEM image of pure KBiO3; (b) SEM, (e) TEM and (h) HRTEM images of KBiO3/BiOCl; (c) SEM, (f) TEM and (I) HRTEM images of KBiO3/BiOBr; (d) SEM, (g) TEM and (j) HRTEM images of KBiO3/BiOI

To determine the forbidden band width of KBiO3/BiOX (X=Cl, Br, I) composite photocatalyst and compare it with the corresponding light absorption characteristics of KBiO3, BiOCl, BiOBr and BiOI photocatalysts, UV-vis DRS detection was performed. The result is shown in Figure 3. Pure BiOCl has almost no absorption in the visible range, pure BiOBr has less absorption in the visible range, and pure KBiO3 and pure BiOI have obvious absorption in the ultraviolet-visible region. The absorption bands of 11


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KBiO3, BiOCl, BiOBr, and BiOI are 635nm, 354nm, 460nm, and 650nm, respectively. The band gap calculated by the intercept method are 1.95eV, 3.50eV, 2.70eV, and 1.91eV, respectively, which consistent with other reported values28,47. The calculated ECB and EVB of KBiO3, BiOI, BiOBr and BiOCl are summarized in Table 1. It can be observed that the CB value of KBiO3 is higher than the CB values of the three composites, and its VB value is also higher than the other VB values. This situation makes it possible to build an n-p heterojunction between them. In addition, the prepared composite photocatalysts of KBiO3/BiOX (X=Cl, Br, I) also have obvious absorption in the ultraviolet-visible region. Among them, the absorption bands of KBiO3/BiOCl KBiO3/BiOBr and KBiO3/BiOI are 650nm, 670nm and 685nm, respectively, and the band gaps are 1.91eV, 1.85eV and 1.81eV, respectively. Compared with KBiO3 photocatalyst, the absorption edges of KBiO3/BiOX all showed blue shift. The response spectrum of KBiO3 is effectively widened after coupling with BiOX due to the narrower wide gap of obtained hybrids compared with pure KBiO3, and the absorption band edges of these composites increase with increasing atomic number X. It can thus be regarded that the in-situ loading of BiOX results in the formation of KBiO3/BiOX heterojunctions.

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Figure 3. (a) UV–vis absorption spectrum of samples, (b)the plot of (ahυ)1/2 vs. hυ of samples

Table 1. Electronegativity (χp), conduction band position (ECB), valence band position (EVB) and band gap energy (Eg) of different samples.

Samples

𝜒𝑃 (eV)

ECB (eV)

EVB (eV)

Eg (eV)

KBiO3

5.32

-0.16

1.79

1.95

BiOCl

6.36

0.11

3.61

3.5

BiOBr

6.17

0.32

3.02

2.70

BiOI

5.94

0.49

2.40

1.91

PL can response the recombination rate of photogenerated hole-electron pairs, which is an important indicator for evaluating photocatalysts. Figure 4 shows the PL spectrum of heterojunction catalysts. The three composite photocatalysts of KBiO3/BiOX (X=Cl, Br, I) produced continuous fluorescence in the ultraviolet-visible range from 300nm to 800nm, and the fluorescence signal was significantly weaker than pure KBiO3 and pure BiOX(X=Cl, Br, I), and the fluorescence signal of KBiO3/BiOX gradually decreases with the increase of atomic number, indicating that the quantum efficiency of the heterojunction photocatalyst formed by in-situ growth of ruthenium oxyhalide by 13


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KBiO3 is significantly improved. This result is in agreement with the UV-Vis DRS result.

Figure 4. PL spectrum of KBiO3/BiOX (X=Cl, Br, I)

Photocurrent response analysis also is usually conducted to exhibite the generation and migration capacity of photogenerated electron-hole pairs of a photocatalyst48. As shown in Fig. 5-a, the rapid photocurrent response of pure KBiO3 and three heterojunctions all were observed under visible light irradiation, which confirms their noticeable photocatalytic performance. However, the photocurrent intensity of three composite catalysts are significantly enhanced, and the sequence is according to KBiO3

KBiO3 as an Effective Visible-Light-Driven Photocatalyst: Stability

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