Oxygen vacancy induced superior visible-light-driven photo-catalytic performance in the BiOCl homojunction

Siying Niu, Ruoyu Zhang and Chongfeng Guo*
State Key Laboratory of Photoelectric Technology and Functional Materials, International Collaborative Center on Photoelectric Technology and Nano Functional Materials, Institute of Photonics & Photon-Technology and Department of Physics, Northwest University, Xi’an 710069, China. E-mail: guocf@nwu.edu.cn

Received 28th March 2020 , Accepted 27th April 2020

First published on 27th April 2020


The photocatalytic performance of semiconductors can be enhanced by expanding the spectral response range and accelerating the photo-induced charge separation. Introduction of oxygen vacancies and construction of homo-junctions in BiOCl were adopted to widen the absorption spectra and reduce the photogenerated electron–hole pair recombination, further enhancing its photocatalytic activity. Black BiOCl with oxygen vacancies and a homo-junction was fabricated using a simple one-pot hydrothermal method. Meanwhile, white BiOCl nanosheets and nanorods with crystal growth in different orientations were simultaneously synthesized to compare with the black BiOCl homo-junction, which exposed the same crystal facets as those of black BiOCl. High resolution transmission electron microscopy (HRTEM) was performed to confirm the formation of the crystal-facet homo-junction, while the existence of oxygen vacancies in black BiOCl was proved by X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) spectroscopy. The spectral absorption range and the photo-generated electron–hole pair separation ability of catalysts were characterized by UV-vis diffuse reflectance absorption spectroscopy (UV-vis DRS), transient photocurrent density and electrochemical impedance spectroscopy (EIS). The photo-catalytic activities of the black BiOCl homo-junctions were evaluated through degradation of RhB and disinfection performance towards E. coli and S. aureus, and they can degrade 99.9% of RhB solution and disinfect 97% of bacteria under simulated sunlight irradiation. Further, the possible photo-catalytic mechanism and the improvement of photo-catalytic performance could be ascribed to the introduction of oxygen vacancies and the construction of homo-junctions, which was also confirmed by theoretical computation.


Introduction

Water is the most vital and strategic resource for human survival and development. However, water pollution has become a high priority global concern in this age of rapid industrialization, which urgently requires an environmentally friendly purification technology.1,2 Photo-catalysis technology is a green non-polluting and promising technique for purifying industrial wastewater because the key pollutant dyes and bacteria in wastewater could be degraded by some sunlight-driven photo-catalysts. As the most promising photo-catalysts, narrow-band semiconductor-based photo-catalysts have received more attention because they can make full use of abundant solar energy to efficiently degrade harmful organic-pollutants into H2O and CO2.3,4 However, traditional and popular photocatalysts (such as TiO2 and ZnO) with broad band gaps have a limited ability to utilize the solar spectrum in the ultraviolet (UV) region (∼5%), which inhibits their practical applications and leads to low solar energy utilization efficiency.5,6 Thus, a sunlight-driven narrow band-gap photo-catalyst is an essential step forward for taking advantage of the inexhaustible clean solar energy.

Driven by the increasing interest in two-dimensional (2D) layered materials, bismuth oxychloride (BiOCl) has been extensively investigated as an effective semiconductor-based photo-catalyst to deal with water pollution in recent years.7,8 BiOCl is a member of the family of 2D compounds with a sandwiched layered structure and can effectively separate the photo-generated carriers to promote photo-catalytic reaction.9 The indirect band-gap of BiOCl forces the electrons to travel a certain k layer to the valence band (VB), which decreases the probability of recombination of the induced charge pairs.10 However, due to its wide band gap (3.1–3.4 eV) it exhibits poor visible-light-driven photo-catalytic performance11,12 which has led to the development of many strategies to tailor BiOCl with a broadened spectral response range and with enhanced sunlight-driven photo-catalytic activity. Most of the described methods are effective to some extent by adopting a post-synthesis treatment approach, such as precious metal or quantum dot deposition, hetero-ion doping and constructing hetero/homo junctions by coupling with other narrow-band semiconductors or relying on morphological variation.13–17 However, homo-junctions generally composed of the same compound with different crystal phases18 or morphologies19 contribute to retard the recombination of photo-generated electrons and holes but cannot efficiently broaden the visible light absorption because of their closed band gaps.20

The introduction of oxygen vacancies in semiconductor photo-catalysts could improve their photo-catalytic activity in the visible light region; meantime, the bandgap of the semiconductor is also reduced by the impurity energy levels formed by the oxygen vacancy state to promote visible light absorption.21,22 Simultaneously, an oxygen vacancy can also act as an electron trap center to enhance charge separation.23,24 Black BiOCl with oxygen vacancies has been prepared and it showed over 20 times higher visible-light-driven photo-catalytic activity.25 According to the above mentioned facts, it is possible to achieve a black BiOCl homo-junction with strong visible-light absorption and higher photo-induced charge carrier separation efficiency by preparing variform black BiOCl.

In this work, black BiOCl composites with nanorods and nanosheets were synthesized via a one pot hydrothermal process; here the oxygen vacancies in BiOCl broaden the absorption from UV light to visible light while the crystal facet junction could speed up the separation of charge carriers between {010} and {001} facets. The appearance of oxygen vacancies and the homo-junction was confirmed through XPS, ESR and electrochemical properties. The photo-catalytic performance of black BiOCl composites was also assessed through the degradation of RhB and lethality to E. coli and S. aureus under simulated sunlight irradiation.

Experimental

Preparation of photo-catalysts

All chemicals were used without any further purification, including analytical reagents (A. R) BiCl3, KMnO4 and ethylene glycol. The white BiOCl samples were fabricated using a hydrothermal method. Initially, 5 mmol KMnO4 and 5 mmol BiCl3 were dissolved in 40 mL of de-ionized water with constant stirring for 30 min at room temperature. The mixed solution was then transferred into a 50 mL Teflon-lined autoclave and heated at 160 °C for 12 hours and allowed to cool to room temperature. The obtained precipitate was separated by centrifugation at 6000 rpm and washed three times with ethanol and de-ionized water; BiOCl nano-rods (designated as BOC-R010) were obtained after drying at 60 °C. For comparison, white BiOCl nano-sheets were also prepared under the same conditions without KMnO4 (named as BOC-S001).

For the synthesis of black BiOCl, 5 mmol BiCl3 and 5 mmol KMnO4 were each dissolved in a mixture of 20 mL of ethylene glycol and 20 mL of de-ionized water. Following this, the prepared BiCl3 solution was added into the KMnO4 solution dropwise under continuous stirring; after two hours, the obtained mixture was transferred into a 50 mL Teflon-lined autoclave and heated at 160 °C for 6 hours. Afterwards, the product was obtained as a black powder (named as BOC-black) through centrifugation, washing and drying, as mentioned in the above procedure.

Characterization

The powder X-ray diffraction (XRD) data were collected by using a Rigaku-Dmax 3C powder diffractometer (Rigaku Crop, Tokyo, Japan) with Cu Kα radiation (λ = 1.5406 Å) to determine the structure of all samples. Field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and high resolution transmission electron microscopy (HR-TEM) were performed using JEOL JSM-6700F and FEI TF-20 to characterize the morphology and microstructure of the samples. Fourier transform infrared spectroscopy (FTIR) was also performed to confirm the structure of the samples recorded on a Bruker Optics EQUINOX55 infrared absorption spectrometer in the range of 500–4000 cm−1. The optical absorption spectra of the samples were collected on a Cary 5000 spectrophotometer with a resolution of 1.0 nm. The chemical composition and oxygen vacancies of the samples were characterized using X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD) with a C 1s (E = 284.5 eV) level internal standard and electron spin resonance (ESR) spectroscopy (Bruker A300 spectrometer, Germany), respectively. The Brunauer–Emmett–Teller (BET) surface area of the BiOCl powder was calculated from N2 adsorption isotherms obtained on a 3H-2000PM2 analyzer at 77 K (Beishide Instrument Technology, Beijing). Furthermore, photocurrent density analysis and electrochemical impedance spectroscopy (EIS) were performed to determine the separation of electrons–holes through a three-electrode system of a CHI 760E electrochemical workstation (Shanghai, China).

Theoretical computation of BiOCl with oxygen vacancies

The density functional theory (DFT) calculations were performed using the plane-wave pseudopotential method in the Vienna ab initio simulation package (VASP).26 Generalized gradient approximation (GGA)27 in the scheme of Perdew–Burke–Ernzerhof (PBE) parameterization28 was employed to describe the exchange and correlation function. The kinetic energy cutoff energy was 520 eV and 6 × 6 × 4 Monkhorst–Pack k-points were used in all the calculations. As an input structure for all calculations, a 2 × 2 × 3 supercell with 20 Å vacuum layer thickness along the non-periodic direction was used in order to avoid interaction between layers. In this case, a single oxygen vacancy situation is considered, because higher oxygen vacancy concentration on the surface would cause lattice distortion. It is proved that BiOCl nanorods and nanosheets exist in (001) and (010) orientation, so we cut the crystal along the corresponding direction to represent the experimental surface. It is well known that d-orbital systems cannot be accurately simulated by the standard GGA functional. A GGA+U method was adopted in the calculation of density states and the U number was set to 4.8 eV for the p-states of Bi and O and 7 eV for the p-states of Cl.

Evaluation of photocatalytic degradation and disinfection activity

The photocatalytic activities of BiOCl samples were assessed by the degradation of RhB (2 × 10−5 mol L−1) with UV light (375 nm UV-emitting LED, 5 W) and simulated sunlight (white-emitting LED, 5 W) illumination, respectively. 10 mg of catalysts was dispersed in 50 mL of RhB aqueous solution and stirred for 30 min in the dark to establish an adsorption–desorption equilibrium. After every 10 min, 5 mL of solution was collected and centrifuged, and the residual concentration of RhB was determined through its characteristic absorbance peak (554 nm) on a spectrophotometer (TU-1901). The trapping experiments were performed by adding isopropyl alcohol (IPA), benzoquinone (BQ), silver nitrate (AgNO3) and sodium oxalate (Na2C2O4) to quench the hydroxyl radicals (˙OH), superoxide radicals (˙O2), electrons (e) and holes (h+), respectively.

The photo-catalytic ability of the samples was also investigated by disinfection of Escherichia coli (E. coli, A10103B) and Staphylococcus aureus (S. aureus, A1006B) under the irradiation of simulated sunlight for 60 min. 20 mg of photocatalysts was added into 30 mL of bacterial suspension solution and then diluted by PBS buffer (1.0 mg mL−1); the mixture solution was quickly coated on nutrient agar plates and incubated at 37 °C for 1 day, and then the number of viable bacterial cells was calculated. The experiments were repeated three times in order to reduce the experimental error.

Results and discussion

Structure and morphology

The white BiOCl nanorods and nanosheets as well as black BiOCl were prepared via the one-step hydrothermal method, respectively, and their digital images and XRD patterns together with the standard profile of the tetragonal phase BiOCl (JCPDS 06-0249) are displayed in Fig. 1a. The XRD patterns show that all diffraction peaks match well with those of the standard data but the relative intensities of diffraction peaks (001) and (110) are different, which indicates that all samples are pure phase and adopt a tetragonal structure but with different crystalline orientations.29 Furthermore, the highest diffraction intensity ratio of (001)/(110) in BOC-S001 among the three samples implied that BOC-S001 preferred orientation along the {001} facet.30 For BOC-R010, the intensity ratio of (110)/(001) is larger than that of BOC-S001, which illustrates that the oriented growth of BOC-R010 is along the {010} facet.31 FTIR spectra were used to further confirm the formation of BiOCl (Fig. 1b); the characteristic peaks at 525 cm−1 can be assigned to the Bi–O bond32 while the peaks at 1450 cm−1 can be ascribed to the asymmetric stretching vibration of the Bi–Cl bond in BiOCl.33 The band at 3450 cm−1 comes from the stretching of O–H in the absorbed water molecule.34 The above results reveal that the structures of as-synthesized BiOCl are the same; their crystal structure and different exposed facets are presented in Fig. 1c. The layered structure of BiOCl consists of [Bi2O2] slabs through van der Waals interaction together with the chlorine atoms along the [001] direction. The outermost layer of the BiOCl {001} facet mainly consists of O atoms while the {010} facet of BiOCl is constituted by the Bi, O and Cl ternary composite. The different facets of BiOCl expose different active sites that provide the channel to adsorb dye molecules.24
image file: d0qm00187b-f1.tif
Fig. 1 (a) XRD patterns, (b) FT-IR spectra and (c) the crystal structure with different crystal facets of BiOCl samples.

SEM and TEM were used to determine the morphology and microstructure of as-synthesized BiOCl, respectively. It was found that the sample BOC-R010 consisted of many nanorods with a diameter of about 200–500 nm (in Fig. 2a1 and a2), and the corresponding selected area electron diffraction (SAED) pattern implied that BOC-R010 nanorods are uniform single crystals (Fig. 2a3). The angle between the (102) and (002) planes is 43.4°, which is in accordance with the theoretical value, and the bottom/top surfaces of the BOC-R010 nanorods were identified as the {010} facet. But the as-prepared BOC-S001 was composed of large smooth nano-sheets with 200–500 nm width and 50–200 nm thickness (Fig. 2b1 and b2). Moreover, its SAED pattern (Fig. 2b3) revealed that the nano-sheet is tetragonal phase with the [001] growth direction and the bottom/top surfaces of BOC-S001 are identified as the {001} facet. The angle between the (200) and (110) planes is 45°, which is identical to the theoretical value. The BOC-black sample is composed of rods and sheets (Fig. 2c1); the TEM and HRTEM images in Fig. 2c2 and c3 show that both nanorods and nano-sheets exhibit clear lattice fringes with interplanar distances of 0.267 nm in nanorods and 0.275 nm in nanosheets, which correspond to the (102) and (110) planes of BiOCl, respectively. Therefore, a homo-junction could be formed between nanorods and nanosheets.


image file: d0qm00187b-f2.tif
Fig. 2 SEM images of (a1) BOC-R010, (b1) BOC-S001, (c1) BOC-black; TEM images of (a2) BOC-R010, (b2) BOC-S001, (c2) BOC-black; SAED patterns of (a3) BOC-R010, (b3) BOC-S001 and HR-TEM images of (c3) BOC-black; (d) N2 adsorption–desorption isotherms and (e) the pore size distribution curves as well as BET surface area of BiOCl samples.

The specific surface area of a photo-catalyst strongly affects its photo-catalytic performance; thus the three samples were also investigated by N2 adsorption–desorption experiments and the results are displayed in Fig. 2d and e. According to the Brunauer–Deming–Deming–Teller (BDDT) classification,35 all N2 adsorption–desorption isotherms of BiOCl samples are type IV isotherms, which indicates that it belongs to micro-mesoporous materials. The BET specific surface areas of BOC-R010, BOC-S001 and BOC-black are 3.77, 8.15 and 9.95 m2 g−1, respectively. As shown in Fig. 2e, the three BiOCl samples are mesoporous and the pore size of BOC-R010, BOC-S001 and BOC-black is 14.2, 17.9 and 22.4 nm, respectively. Therefore, BOC-black has the biggest specific surface area and pore size, which is beneficial to improve the absorption ability and photo-catalytic ability of samples.36

UV-vis DRS spectra and oxygen vacancies

The UV-vis reflection spectra were determined to analyze the spectral response range of photo-catalysts and the corresponding results of samples BOC-R010, BOC-S001 and BOC-black are given in Fig. 3a. Distinctly, the samples BOC-R010 and BOC-S001 could not be efficiently excited by visible light due to their wide band gap (absorption edge at 365 and 400 nm), while the absorption range of BOC-black covers UV-vis and near-infrared light. Unlike the conventional white BiOCl which responds only in the UV light region, the absorption edge of BOC-black shifts to visible light and induces a broad absorption almost covering the full spectra of sunlight.37,38 According to previous publication, the color change of BOC-black results from the oxygen vacancies in BiOCl.39 Therefore, the ESR technique was applied to confirm the oxygen vacancies in black BiOCl, and a remarkable oxygen vacancy signal ranging from 3450 to 3470 G was detected in the ESR spectrum of BOC-black compared to white BOC-R010 and BOC-S001, as shown in Fig. 3b.40,41 Moreover, the elemental composition and chemical states of BiOCl with different colors were also proved by the XPS spectra and are displayed in Fig. 3c–f. The peaks at 156.5 and 161.8 eV in Bi 4f spectra are attributed to Bi 4f7/2 and Bi 4f5/2, respectively (Fig. 3d). However, the Bi 4f7/2 and Bi 4f5/2 peaks for BOC-black shift toward lower energy by approximately 0.5 eV due to the presence of oxygen vacancies Bi(+3−x).42 The high resolution Cl 2p spectrum of the samples exhibits the peaks at 199.3 and 197.7 eV belonging to Cl 2p3/2 and Cl 2p1/2 in BiOCl, respectively (Fig. 3e). In terms of high resolution O1s spectra (Fig. 3f), the binding energy peaks at 529.3 eV are attributed to the Bi–O bond in the BiOCl samples.43 As expected, the new peak of oxygen vacancies appeared at about 531 eV in BOC-black, which also implied the existence of oxygen vacancies.44 All the results demonstrate the existence of oxygen vacancies in black BiOCl.
image file: d0qm00187b-f3.tif
Fig. 3 (a) UV-vis diffuse reflectance spectra as well as the plots of (αhν)1/2 vs. photon energy for different samples; (b) ESR spectra of three samples. The XPS spectra of three samples: (c) survey, (d) Bi 4f, (e) Cl 2p, (f) O 1s.

In order to theoretically explain the reason for the wide spectral response range and the role of oxygen vacancies in the black samples, the calculated band structures of white and black BiOCl nanorods/nanosheets, together with their total and partial density of states (TDOS and PDOS), are shown in Fig. 4. In a comparable method, BOC-R010/BOC-S001 represented white BiOCl nanorods/nanosheets, whereas BOC-R010-OV/BOC-R010-OV represented black BiOCl nanorods/nanosheets with oxygen vacancies. The conduction band minimum (CBM) of BOC-S001 was located at the k-point of G and the valence band maximum (VBM) was located at the k-point line of S–X (in Fig. 4a), which means that BOC-S001 is an indirect semiconductor with 3.25 eV band gap. The BOC-S001 valence band (VB) edge mainly consists of the Cl-3p orbital but the conduction band (CB) edge is mainly composed of Cl-3p, O-2p and dominant Bi-6p orbitals. For BOC-R010 (in Fig. 4b), the CBM was located at the k-point of G while the VBM was located at the k-point line of G–Y, suggesting that BOC-R010 is also an indirect semiconductor with about 3.66 eV band gap. The band-gap of BOC-R010 is larger than that of BOC-S001, which is consistent with the result of the above spectral absorption. The corresponding density of states (DOS) also displayed that the top of the BOC-R010 VB contains Cl-3p and few contributions of O 2p, while the bottom of the CB was mostly dominated by Bi 6p. The band structure and density of states of oxygen induced black BOC-S001-OV and BOC-R010-OV are also presented in Fig. 4c and d. In comparison with those of BOC-S001 and BOC-R010, a new oxygen vacancy level appeared in BOC-S001-OV and BOC-R010-OV, which is mainly composed of the Bi-6p orbital. After introducing oxygen vacancies, the photo-generated electrons could be easily excited into the conduction band or oxygen vacancy level under light illumination with low energy, resulting in reduction of the band gap of BiOCl nano-rods/nano-sheets.


image file: d0qm00187b-f4.tif
Fig. 4 The calculated band structure and density of states plots of (a) BOC-S001, (b) BOC-R010, (c) BOC-S001-OV and (d) BOC-R010-OV.

Photocatalytic activity and disinfection performance

Rhodamine B (RhB) is widely selected as an organic pollutant model to evaluate the photo-catalytic performance of photo-catalysts, and the ratio value (C/C0) of the RhB concentration at irradiation time t (C) to the initial concentration of RhB after adsorption–desorption equilibrium (C0) is used to measure the residual rate of organic dyes. The photo-catalytic activity of BOC-R010, BOC-S001 and BOC-black was assessed via the degradation of RhB solution under the irradiation of UV light and simulated sunlight. Fig. 5a shows the time-dependent degradation curves of RhB under UV light irradiation. It was noticed that about 50%, 55% and 92% RhB is degraded after adding BOC-R010, BOC-S001 and BOC-black within 50 minutes, respectively. Obviously, BOC-black exhibited the best photo-degradation ability among the three samples though they all presented good photo-catalytic performance under UV light irradiation. Differently, the degradation efficiency of BOC-black could achieve 99.9% after simulated sunlight irradiation for 80 min (Fig. 5b), while the degradation efficiency of BOC-R010 and BOC-S001 reached only 38% and 40%, respectively. The poor photo-catalytic performance of BOC-R010 and BOC-S001 under simulated sunlight irradiation could be attributed to their inefficient responses to visible light. The blank test in Fig. 5a and b proves that RhB is degraded very slightly without the addition of any catalysts, suggesting that the photolysis of RhB can be neglected under the irradiation of UV light or simulated sunlight. The photo-catalytic activities of catalysts could also be distinguished by the rate constant k obtained from the pseudo-first-order model;40 the larger k value means better photo-catalytic activity. Under UV light irradiation, the k value for RhB degradation over BOC-R010, BOC-S001 and BOC-black is approximately 0.006 min−1, 0.008 min−1 and 0.01 min−1, respectively. However, under simulated sunlight irradiation, the removal rate constant k for RhB degradation over BOC-black is 0.013 min−1, which is about 3.6 and 3.25 times as much as that of BOC-R010 (0.0036 min−1) and BOC-S001 (0.004 min−1), as shown in Fig. 5c. The above results imply that BOC-black exhibited more excellent photo-catalytic performance than BOC-R010 and BOC-S001 under the irradiation of UV light or simulated sunlight.
image file: d0qm00187b-f5.tif
Fig. 5 Photodegradation curves of different samples under (a) UV light and (b) simulated sunlight irradiation; (c) evolution of the kinetic constant over different BiOCl samples; (d) trapping test and (e) recyclability experiments of BOC-black under simulated sunlight irradiation; (f) XRD patterns of BOC-black before and after recyclability experiments; (g) photographs and (h) relative viability of E. coli and S. aureus colonies under simulated sunlight irradiation. (Error bars represent the standard deviation.)

Furthermore, a series of trapping agents were used in the active species trapping experiments to investigate the active species during the photo-catalytic degradation procedure of the black BOC homo-junction, as shown in Fig. 5d. No evident difference is observed in the photo-catalytic behavior on exposure to simulated sunlight with the introduction of IPA and AgNO3 in the RhB solution, implying that ˙OH and e radicals may not be the main active species. The degradation efficiencies of RhB are significantly inhibited with the addition of BQ and Na2C2O4, respectively. These quenching tests implied that ˙O2− and h+ radicals are the main active species responsible for the degradation process. Besides the photo-catalytic efficiency, the photo-catalytic stability and reusability of the BOC-black catalyst were also evaluated by recycling experiments by monitoring the degradation of RhB solution under simulated sunlight irradiation, and the results are displayed in Fig. 5e. No significant decrease in the photo-catalytic efficiency of the BOC-black catalyst was observed after three cycles. Moreover, the XRD pattern of the BOC-black catalyst after three cycles as well as that of unused BOC-black is displayed in Fig. 5f; it was found that the structure of the BOC-black catalyst did not change. All the above results demonstrated that the BOC-black catalyst had superior stability and reusability in the photo-degradation process.

The disinfection performance of the different BiOCl samples was investigated using E. coli and S. aureus cells in synthetic saline solution under simulated sunlight irradiation for 60 minutes. As illustrated in Fig. 5g, a number of E. coli and S. aureus bacterial colonies grow on the agar plates in the presence of different catalysts. It is found that the bacterial colonies did not undergo significant changes, which hinted that both BOC-R010 and BOC-S001 exhibited very low disinfection efficiencies. However, the density of E. coli and S. aureus bacterial colonies decreased obviously under simulated sunlight irradiation in the presence of BOC-black. The corresponding E. coli and S. aureus viability in different catalysts was measured after simulated sunlight irradiation and the results are displayed in Fig. 5h. The viability of E. coli for BOC-R010, BOC-S001 and BOC-black is 95%, 97% and 3%, respectively. As for S. aureus, the viability for BOC-R010, BOC-S001 and BOC-black is 96%, 94% and 3.5%, respectively. Herein, the results suggested that BOC-black shows the best simulated sunlight driven photocatalytic disinfection performance among the three samples.

Photo-electrochemical measurements, electronic structure and photo-catalytic mechanism

Electrochemical experiments were performed to detect the charge transfer efficiencies of different samples; the photocurrent–time (It) curves of BOC-R010, BOC-S001, and BOC-black as electrodes are given in Fig. 6a. With the irradiation of white light, the photocurrents were quickly generated in three working electrodes, but the photocurrent density of BOC-black (0.78 μA cm−3) is approximately 3-fold and 2-fold that of BOC-R010 (0.25 μA cm−3) and BOC-S001 (0.4 μA cm−3), implying that the photo-induced electrons and holes have higher transfer efficiency in BOC-black. EIS Nyquist plots for BiOCl samples are also displayed in Fig. 6b, in which BOC-black displays the smallest arc radius and the arc radius increased in the order BOC-black < BOC-S001 < BOC-R010, demonstrating that charge carrier transfer is more effective in BOC-black. The above results corroborate that the recombination of photo-induced electron and hole pairs is efficiently prohibited in BOC-black, and thus the photo-catalytic activity of black BiOCl was greatly enhanced.
image file: d0qm00187b-f6.tif
Fig. 6 (a) Photocurrent response and (b) EIS spectra for all BiOCl samples under visible-light irradiation; (c) the calculated work function of BOC-S001-OV and (d) BOC-R010-OV; (e) the energy band diagram of BOC-S001-OV and BOC-R010-OV before and after contact. (f) Diagram of the working mechanism for the BOC-black homo-junction.

In order to comprehensively understand the experimental mechanism, the work functions of oxygen vacancy induced BOC-S001-OV and BOC-R010-OV were calculated based on density functional theory (DFT). According to the equation ϕ = EvacEf, where Evac and Ef are the vacuum energy and Fermi energy, respectively,45 the vacuum energy of BOC-S001-OV and BOC-R010-OV is 5.91 and 3.687 eV while their Fermi energy is 2.03 and −1.16 eV, respectively. Therefore, the work functions of BOC-S001-OV and BOC-R010-OV were calculated to be 3.88 and 4.847 eV, respectively (Fig. 6c and d). Due to the Fermi energy of BOC-S001-OV (2.03 eV) being higher than that of BOC-R010-OV (−1.16 eV), interfacial band bending will occur and the electrons will flow from BOC-S001-OV to BOC-R010-OV after contact until their Fermi levels steady (Fig. 6e). Under simulated sunlight irradiation, the photo-generated electrons from the CB of BOC-R010-OV transfer to that of BOC-S001-OV while the holes move from the VB of BOC-S001-OV to that of BOC-R010-OV, which builds the directed built-in electric field from BOC-S001-OV to BOC-R010-OV in the interface of the junction. Based on the above analysis, the degradation mechanism of RhB and the potential transfer direction of electron–hole pairs are illustrated in Fig. 6f. Under simulated sunlight irradiation, the nano-rods were excited and produced electrons in the CB which were then transferred to the CB of nano-sheets; the holes remaining in the VB of nano-sheets could simultaneously migrate to the VB of nano-rods. Afterwards, the electrons in the CB of nano-sheets can reduce O2 to ˙O2 and the holes in the VB of nano-rods can oxidize organic pollutants. Meanwhile, the produced ˙O2 and h+ can react with oxygen or water to produce hydroxyl radicals (˙OH) to oxidize the bacterial cells.46 The above process can be described as follows:

 
BOC-black + → e + h+ (1)
 
e + O2 → ˙O2 (2)
 
˙O2 + RhB → degradation products (3)
 
h+ + RhB → degradation products (4)
 
OH + h+ → ˙OH (5)
 
˙O2 + H2O → ˙OH (6)
 
˙OH → E. coli and S. aureus (7)

In general, there are three main reasons for the superior photo-catalytic performance of BOC-black. Firstly, BOC-black with oxygen vacancies displayed an excellent absorption ability from UV light to the full spectrum, so can use sunlight more effectively. Secondly, the enlarged surface area of BOC-black can provide more active sites to adsorb more RhB molecules. Lastly, the superior performance of BOC-black may be due to the formation of crystal facet junctions between the nanorods and nanosheets, which are in favor of separating the photo-induced charge carriers.

Conclusion

In conclusion, an oxygen vacancy induced black BiOCl homo-junction was prepared through a one-step hydrothermal method with ethylene glycol as a solvent, which was composed of BiOCl rods and sheets with exposed {010} and {001} crystal-facets, respectively. In comparison with individual white BiOCl nanosheets and nanorods, the black BiOCl homo-junction exhibited about 3.25 and 3.6 times higher photocatalytic degradation for RhB under simulated sunlight irradiation, respectively. Meanwhile, about 96% of bacteria could be disinfected by the black BiOCl homo-junction under simulated sunlight irradiation within 60 min. The improved photocatalytic degradation and disinfection ability of black BiOCl can be ascribed to the enhanced visible light absorption resulting from the oxygen vacancies and the effective separation of photo-generated electron–hole pairs caused by the crystal-facet homo-junction.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 11974278, 51672215) and Youth Innovation Team of Shaanxi Universities.

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