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Alkali-Induced in Situ Fabrication of BiO-Decorated BiOBr Nanosheets With Excellent Photocatalytic Performance Dan Wu, Liqun Ye, Singtao Yu, Bo Wang, Wei Wang, Ho Yin Yip, and Po Keung Wong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02365 • Publication Date (Web): 24 Mar 2016 Downloaded from http://pubs.acs.org on March 28, 2016

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The Journal of Physical Chemistry

Alkali-Induced In Situ Fabrication of Bi2O4Decorated

BiOBr

Nanosheets

with

Excellent

Photocatalytic Performance Dan Wu,† Liqun Ye,†,‡,* Songtao Yue,┴ Bo Wang,† Wei Wang,┴,* Ho Yin Yip,† Po Keung Wong†,* †

School of Life Sciences, The Chinese University of Hong Kong, Hong Kong SAR, China

‡

College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang

473061, China ┴

College of Materials Science and Engineering, Huazhong University of Science & Technology,

Wuhan 430074, China

ACS Paragon Plus Environment

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ABSTRACT

A simple alkali (NaOH) post-treatment approach assisted with light irradiation to in situ obtain Bi2O4 nanoparticles decorated BiOBr nanosheets, brown colored BiOBr-0.01, is presented for the first time. Bi2O4 nanoparticles are formed due to a combined action of NaOH induced dehalogenation and light triggered photogenerated hole (h+) oxidation processes on the surface of BiOBr nanosheets. Importantly, by varying the NaOH concertation in the post-treatment, the content of Bi2O4 phase in the hybrid structures can be easily tuned and predominantly exposed highly reactive {001}-facet of BiOBr nanosheet can be well preserved. Significantly, without any foreign elements, the light absorption of as-prepared BiOBr-0.01 is extended to near-infrared (NIR) region. In comparison with normal BiOBr, brown BiOBr-0.01 nanosheet shows superior photocatalytic activity for the dye degradation and microbial disinfection. Particularly, it exhibit excellent capability to photocatalytically reduce CO2 into CO and CH4, whereas the normal BiOBr is completely incapable for CO2 conversion under simulated sunlight irradiation. The exceptional enhancement is due to the Bi2O4 extended light absorption, efficient photogenerated e−/h+ pair separation, as well as the increased surface-adsorbed ability to reactants. This facile post-treatment method is promising for different bismuth-based systems and hence offers a path to a large variety of materials.


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Introduction In recent years, the intractable crises of environmental pollution and fossil resource shortage are becoming an overwhelming concern throughout the world. Photocatalysis has been gained considerable attention for offering a promising solution in environmental remediation and storable renewable fuels supplies.1-3 In order to sufficiently utilize abundant solar energy, the exploitation of semiconductor photocatalysts that can harvest a wide range of solar spectrum with high efficiencies remains challenging, yet highly desirable. Bismuth oxybromide (BiOBr) have recently drawn intensive attention in photocatalysis owing to its unique layered-structure mediated fantastic optical and electronic properties.4-8 However, BiOBr still suffers from relative low quantum efficiency, which severely hinders its practicability in environmental and energy technologies. Therefore, further endeavors have been attempted to achieve desired photocatalytic activity of BiOBr. Semiconductor photocatalysis involves two major processes of which are absorption of light to generate electron-hole (e−/h+) pairs and charge carriers separation and migration to the surface for subsequent redox reactions. Subject to these major steps, efficient separation and migration of photoexcited e−/h+ pairs as well as strong light adsorption with broad spectrum of solar energy are crucial criteria for BiOBr to obtain high photocatalytic efficiency. Nevertheless, BiOBr possesses bandgap energy of about 2.8 eV, allowing it to utilize the sunlight only far to wavelength of 443 nm. Since ultraviolet (UV, λ< 400 nm), visible (VL, 400 nm 700 nm) light respectively accounts for 5%, 52% and 43% of the solar spectrum,9-10 thus most of the solar energy is wasted in photocatalysis of BiOBr. Therefore, numerous efforts including defect introduction and impurity introduction have been focused on enhancing the limited optical absorption of BiOBr.11-13 Nevertheless, they remain insufficient


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and not compatible to harvest the wide spectrum of sunlight from UV to NIR wavelength to achieve efficient solar-to-chemical energy conversion. In particular, extending BiOBr active to NIR utilizing full-spectrum of the sunlight is still a blank area. Besides the light absorption, another key issue in promoting the overall photocatalytic efficiency of BiOBr is to gain efficient photoexcited e−/h+ pairs separation and charges should promptly migrate across the surface/interface to suppress their recombination. Due to the unique crystal structure with stacked [Bi2O2] layers and bilayer-inserted Br atoms, BiOBr is easily to form two-dimensional nanosheets with preferentially exposed facets. As well-documented, BiOBr nanosheets predominantly exposed with {001} facets exhibit highly photoreactivity because of a cooperative effect between the internal electric fields and surface atoms structure, which is favorable of e−/h+ pairs separation and migration to surface.4-5,11,14 However, the photogenerated e−/h+ pairs will recombine rapidly if there are no appropriate active sites available on the surface.3 Therefore, suitable modification of the BiOBr surface is frequently necessary to facilitate the photocatalytic reactions. In this regard, impurity incorporation, such as C3N4,15 graphene,16 Ag/AgBr,17 Fe,18 C,19 TiO2,20 ZnWO4,21 BiPO422 and halogen element crystal substitution23-24 have widely been demonstrated to improve the photocatalytic activity of BiOBr. However, due to the inadequate contact interface between the introduced components and BiOBr, various energy band alignments appear25 and the barriers for charge transport across the interface would be enlarged, which is disadvantageous for the effective separation of carriers. Furthermore, although some deep impurity states should promote the light absorption,26 foreign components can act as carrier recombination and thus adversely increase the recombination probability of photogenerated e−/h+ pairs, which is undesirable for photocatalytic improvement. In particular, the crystal structure is distorted or damaged when the proportion of substituted constituents


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exceeds its tolerance,26 so that the highly reactive {001} facets of BiOBr can be destroyed. Even if the {001} facets are retained, a lack of suitable control over the interfacial contact facets would diminish this intrinsic feature on subsequent photocatalytic process. Additionally, impurities doping strategies usually require complex procedures, rigorous conditions, and special apparatus.27 The aforementioned disadvantages result in the limited success for further photocatalytic improvement of BiOBr. Hence, the pursuit of a simple and controllable strategy of surface engineering to acquire suitable energy band levels and efficient interfacial charge separation and transfer is a particularly important mission. However, to the best of our knowledge, no attempts in photocatalysis of BiOBr are gained versatile achievements of remarkably increasing of solar absorption extending to NIR region and efficient charge separation as well as well-preserved {001} facets, especially without introducing additional elements. Herein, we reported a discovery of a simple alkali post-treatment on BiOBr nanosheets to obtain brown colored BiOBr nanosheets (BiOBr-0.01) with increasing solar absorption extended to NIR region. The crystal phase, morphology, optical property and electronic structure of the BiOBr-0.01 nanosheets were systematically investigated. The photocatalytic performance of the brown BiOBr was respectively evaluated by organic dye decomposition, bacterial inactivation and CO2 reduction. The formation mechanism and photocatalytic enhancement mechanism of the brown BiOBr were proposed accordingly.

Experimental Section Synthesis of BiOBr nanosheets


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The normal BiOBr nanosheets were synthesized through a hydrothermal procedure. In Brief, an amount of 4 mmol Bi(NO3)3·5H2O was added to 30 mL of distilled water and bathsonicated for 30 min to achieve a uniform suspension. Then an equivalent molar amount of KBr (30 mL) was added to the above suspension followed by stirring at room temperature (ca. 25 °C) for 30 min. Subsequently, the mixture was transferred into the Teflon sealed autoclave and maintained at 160 °C for 12 h. BiOBr nanosheets were obtained after centrifugation, washing with distilled water several times and finally drying at 60 °C for 24 h. Synthesis of brown BiOBr nanosheets The brown BiOBr nanosheets were obtained through a simple alkali post-treatment assisted with light irradiation. The above BiOBr nanosheets (0.5 g) was dispersed in NaOH aqueous solution (20 mL) under an I300C Xenon lamp (Perfect Light Technology Co. Ltd., Beijing, China) irradiation at room temperature for 2 h with continuous stirring. The suspension were collected and washed with distilled water several times and dried at 60 °C for 24 h. The concentration of NaOH aqueous solution was changed to 0, 0.001, 0.005, 0.01, 0.05, 0.5, 1 and 5 M, respectively, and the resulting samples were named as BiOBr-x, where x referred to the NaOH concentration. Materials characterizations The X-ray diffraction (XRD) patterns of samples were recorded on a SmartLab X-ray diffractometer (Rigaku, Japan) operating at 40 mA and 40 kV with Cu Kα. A Quanta 400F fieldemission scanning electron microscope (SEM, FEI, USA), Tecnai G2 Spirit transmission electron microscope (TEM, FEI, USA), and Tecnai F20 high resolution TEM (HRTEM, FEI, USA) were used for morphology observations. UV-vis diffuse reflectance spectra (DRS) of samples were recorded on a Lambda 35 UV-vis spectrophotometer (PerkinElmer, USA). The


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Brunauer-Emmett-Teller (BET) specific surface area of samples was analysed with an ASAP 2020 volumetric adsorption analyser (Micromeritics, USA). X-ray photoelectron spectroscopy (XPS) was measured by an AXIS-ULTRA DLD-600 W spectrometer (Shimadzu-Kratos, Japan). Steady-state and transient-state photoluminescence (PL) spectra were measured using a FP-6500 fluorescence spectrometer (Jasco, Japan) with λexc=315 nm, and LP920 fluorescence spectrometer (Edinburgh Instruments Ltd., UK) with λexc= 255 nm, respectively. The generation of hydroxyl radicals (·OH) and superoxide radicals (·O2-) was detected using terephthalic acid (TA) and nitroblue tetrazolium (NBT) as a probe, respectively. The sample was analyzed after filtration on an Infinite® M200 microplate reader with fluorescence peak at 425 nm (λexc=315 nm) for the ·OH measurement and on Bluestar A UV-vis spectrophotometer with absorption wavelength at 259 nm for the (·O2-) measurement. The transient photocurrent responses and electrochemical impedance spectroscopy (EIS) measurements were recorded on a CHI760C electrochemical working station (CH Instruments, China) in Na2SO4 (0.1 M) electrolyte solution. Raman spectra were obtained on LabRAM HR800 Raman spectrometer (Horiba JobinYvon). Thermogravimetric (TG) and differential thermal analysis (DTA) analysis were performed by a Diamond TG/DTA (PerkinElmer Instruments, China) with a heating rate of 10 °C/min in nitrogen (N2) atmosphere. Photocatalytic Measurements Photocatalytic activities of brown and normal BiOBr nanosheets were initially evaluated by photodecomposition of methyl orange (MO, 20 mg/L). Typically, MO (50 mL) and photocatalyst (50 mg) were mixed together followed by stirring in the dark for 1.5 h to establish adsorption equilibrium. An I300C Xenon lamp (PefectLight, China) was used to simulate the solar light and located ∼20 cm above the photocatalytic reactor. The focused intensity was measured to be 153,


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1.4, 0.8 and 0.1 mW/cm2 respectively for VL, UVA, UVB and UVC. At each irradiation time interval, the mixed solution (0.1 mL) was collected and centrifuged, and the supernatant was analyzed by Bluestar A UV-vis spectrophotometer (LabTech, China). For the bacterial inactivation experiment, the procedure was same except that the MO solution was replaced by bacteria suspension (50 mL). After sampling, the mixture (0.1 mL) was serially diluted with sterilized distilled water, spread on Nutrient Agar (NA, Lab M, Lancashire, UK) plates and finally incubated at 37 °C for 24 h to determine the number of survival cells. The CO2 photoreduction experiment was conducted in a PLS-SXE300 Labsolar-IIIAG closed gas system (Perfectlight, China) with a total reaction volume of 500 mL. The photocatalyst (50 mg) evenly dispersed on a watch-glass with an area of about 28 cm2 and NaHCO3 (1.712 g) was put into the reaction cell followed with thorough vacuum treatment. Prior to light irradiation, H2SO4 (4 M, 5mL) was injected into the cell to achieve CO2 gas (1 atm). The reaction temperature was kept at 20 °C with DC-0506 low-temperature thermostat bath (Sunny Hengping Scientific Instrument Co., Ltd., China). At given time intervals, resulting gas (1 mL) was collected and then qualitative analyzed by a GC9790II gas chromatography (Zhejiang Fuli Analytical Instrument Co., Ltd., China) equipped with a GDX-502 flame ionization detector and a TDX-01 thermal conductivity detector. The outlet gases were determined to be CO2, CH4 and CO, respectively. The production yield was calculated according to a calibration curve. All the above photocatalytic experiments were conducted triplicates.


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Figure 1. (a) Full and (b) partial enlarged XRD patterns of BiOBr and BiOBr-0.01 samples.

Results and discussion Structure and morphology of brown BiOBr-0.01 nanosheets The crystal structure and phase purity of normal and alkali post-treatment modified BiOBr nanosheets were confirmed by XRD. As shown in Figure 1a, all the diffraction peaks of normal BiOBr sample can well be indexed to the tetragonal phase of BiOBr (JCPDS 01-078-0348). After post-treatment modification, the peaks are still intense and sharp without observable angleshift, revealing representative BiOBr-0.01 sample retains the benefits of crystalline BiOBr structures. Significantly, an additional peak located between 27.5° and 28.2° appears in the enlarged XRD pattern of BiOBr-0.01 compared with normal BiOBr (Figure 1b), which can be the characteristic (111) diffraction peak of cubic Bi2O4 (JCPDS 96-101-0312). This is highly related to the brown-colored appearance of post-treated BiOBr sample, which is totally different from the commonly observed yellowish normal BiOBr (the inserted photos). Additionally, the intensity ratio I(001)/I(110) of brown BiOBr-0.01 is 0.67, which is much smaller than that of normal


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BiOBr (1.11), probably because the newly appeared Bi2O4 induces specific changes on the preferred orientations of BiOBr crystals.

Figure 2. SEM images of (a) top view and (b) side view, and TEM images of (a) top view and (b) side view for BiOBr-0.01 nanosheets.

The morphology of the samples was confirmed by SEM and TEM images. As shown in Figure S1, the normal BiOBr is with a large amount of square-like nanosheets, which display flat and smooth surfaces on the planar and lateral sides with clear edges. Moreover, as shown in Figure S2a, the perpendicular lattice fringes with inter-planar lattice spacing (d) of 0.278 nm accords with the theoretical value of (110) atomic plane of BiOBr. The angle labeled in the


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corresponding selected-area electron diffraction (SAED) pattern is estimated to be 45°, which is identical to the theoretical angle between the (110) and (200) planes (Figure S2b), indicating that normal BiOBr crystal grows along the [001] orientation. Accordingly, normal BiOBr nanosheets exposes with well-defined {001} facets on the top and bottom surfaces.

Figure 3. HRTEM images of the (a) overall view, (b) enlarged view at the edge, (c) enlarged view at the center, and (d) corresponding SAED pattern for BiOBr-0.01 nanosheet.

Comparatively, BiOBr-0.01 sample exhibits similar dimensions and overall sheet-shaped morphology (Figure S3). However, significant distinction on the surface of the individual


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nanosheet is observed. As shown in Figure 2, abundant of small nanoparticles are found to be uniformly dispersed on the entire planar sides as well as the lateral sides. Moreover, the fuzzy edges observed on the top view and the crimped surfaces along the planar surfaces indeed validate the presence of the additional nanoparticles on the surface of BiOBr-0.01 nanosheets. From the HRTEM image in Figure 3a, it can be clearly seen that the individual nanosheet is surrounded with compactly arranged nanoparticles that are quasi-sphere with diameter around 6 nm. As displayed in Figures 3b, 3c and 3d, the observed fringes of d=0.279 nm with 90° angle and corresponding SAED pattern identified (200) and (110) planes conjointly confirm the original crystal characteristics. That is to say, the exposed {001} facets on the top and bottom nanosheet surfaces are well-preserved for BiOBr-0.01 with respect to normal BiOBr nanosheets. Significantly, closer observations in the HRTEM images at different locations (labeled in red) show clear lattice fingers of d= 0.324 nm, which matches the theoretical value of Bi2O4 with (111) planes being 0.319 nm. This is consistent with the XRD result. The acquired SAED pattern (Figure 3d) indicates the monocrystalline nature of the Bi2O4 nanoparticles and overlapped lattice feature in the contact region between Bi2O4 and BiOBr host. In addition, EDX mappings reveal that the elements of Bi, O and Br are homogeneously distributed on the surface of BiOBr0.01 nanosheets (Figure S4). Additionally, the estimated atomic molar ratio of Br/Bi by EDX analysis is about 0.22:1 for BiOBr-0.01, which is much lower than that of pure BiOBr (about 0.97:1). Consequently, small-sized Bi2O4 nanoparticles are evidenced to yield on the surfaces of BiOBr nanosheets, especially without destruction of predominately exposed {001} facets.


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Figure 4. XPS spectra of (a) survey, (b) Bi 4f, (c) O 1s and (d) Br 3d for BiOBr and BiOBr-0.01.

XPS analysis was employed to compare the chemical compositions and element chemical bonding states of BiOBr and BiOBr-0.01 nanosheets. The calibration of binding energies was referred to C 1s at 284.95 eV (Figure S5). Figure 4a shows the survey XPS spectra of BiOBr and BiOBr-0.01, which mainly consist of Bi, O and Br elements, indicating that the post-treatment does not introduce any heteroatom-related impurities on BiOBr-0.01 nanosheets. As shown in Figure 4b, two deconvoluted peaks located at 164.80 and 159.48 eV can be assigned to Bi 4f5/2 and Bi 4f7/2, respectively, referring to the normal state of Bi3+ in BiOBr nanosheets. However, the spectral profile of Bi 4f for BiOBr-0.01 nanosheets is asymmetric with a shoulder at higher binding energies, which can be deconvoluted into two new bimodal peaks. The peaks centered at 159.78 and 165.12 eV can be attributed to Bi3+, while the peaks at higher binding energies of


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165.78 and 160.48 eV can be attributed to Bi5+. These mixed oxidation states can be referred to the Bi3+/Bi5+ in Bi2O428-29 on the surface of BiOBr-0.01 nanosheets. Moreover, the existence of Bi5+ is also confirmed via observable color changes by iodometric titration method. Unfortunately, it is difficult to quantify the Bi5+ owing to its extremely low content and appreciable brown-colored interference in BiOBr-0.01 suspension. Notably, the stoichiometric formula of Bi2O4 is Bi3+Bi5+O4,29 which infers that the Bi3+ and Bi5+ amount is approximately equal in Bi2O4. Anticipatedly, the Bi3+/Bi5+ atomic ratio related to Bi2O4 in BiOBr-0.01 sample calculated from XPS is about 0.96, which is highly agreed with the theoretical value. In addition, two characteristic peaks associated with Bi3+ in BiOBr host shifts to lower binding energies of 164.25 and 158.92 eV, respectively. This shift can be resulted from the strong interface interaction of Bi2O4 and BiOBr host in BiOBr-0.01 nanosheets. The peaks of O 1s for BiOBr nanosheets at 530.24 and 531.80 eV in Figure 4c are respectively ascribed to the lattice O and absorbed oxygen species on the surface. In contrast, the O 1s region for BiOBr-0.01 nanosheets exhibits a broadened range extending from 528 to 536 eV. The two peaks located at 530.96 and 532.02 eV correspond to the O−Bi3+ and O−Bi5+ bonds in Bi2O4, respectively. Notably, the peak for lattice O in BiOBr-0.01 also shifts to low binding energy (529.70 eV), which might be ascribed to the polarization changes induced surface charge effect,30 further signifying that the newly appeared Bi2O4 have strong interaction with the host BiOBr. In addition, the peak at 533.41 eV is related to the surface absorbed hydroxyl groups. As depicted in Figure 4d, the Br 3d XPS spectra of BiOBr can be divided into two typical peaks, of which 68.50 and 69.51 eV are assigned to the characteristic Br−. Similarly, Br− associated peaks for BiOBr-0.01 nanosheets can be observed at 68.16 and 69.19 eV, with a slight shift to low binding energies compared with normal counterpart. This is probably because the strongly attached Bi2O4 drastically leads to


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some distortions of Br atoms on the surface and affect the spatial van der Waals force of Br atoms between each slabs in the subsurface of BiOBr host crystals of BiOBr-0.01 nanosheets. Interestingly, two other peaks associated with Br− at higher binding energies of 70.13 and 71.23 eV are found, revealing that there is another chemical status of Br atom on the surface of BiOBr0.01, besides lattice Br in BiOBr host crystal structure. In fact, the estimated molar ration of Br/Bi from XPS spectra dramatically decreases from 0.99 for the normal BiOBr nanosheets to 0.2 for BiOBr-0.01 nanosheets, which is comparable to the result of EDX. Although it is hard to quantify the accurate ratio of Br/Bi, the considerable reduction of Br atoms implies that the BiOBr at the contact interface with Bi2O4 is nonstoichiometric for BiOBr-0.01 nanosheets. This also can well explain the newly appeared bimodal XPS peaks of Br 1s for BiOBr-0.01 nanosheets. Since XPS measurements reflect the detailed surface information of brown BiOBr0.01 nanosheets, it is reasonably deducted that the occurrence of Bi2O4 is highly in connection with the reduction of interfacial Br atoms on the surface of BiOBr crystals.

Figure 5. TG-DTA curves of (a) BiOBr and (b) BiOBr-0.01 nanosheets.


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The thermal properties of normal and brown BiOBr nanosheets were characterized by TGDTA analysis performed in N2 atmosphere. As shown in Figure 5a, an obvious weight loss (30.3%) ranging from 600 to 700 °C is observed accompanying with an endothermic peak at 690 °C in the DTA curve, which is due to the decomposition and release of Br from normal BiOBr nanosheets.31 Similarly, this Br release induced weight loss in 600-700 °C temperature region with an endothermic peak at 710 °C in the DTA curve is also found for brown BiOBr nanosheets (Figure 5b). However, the weight loss is significantly decreased to 2.6%, which is consistent with the result of considerable reduction of Br on BiOBr-0.01 nanosheet surface according to the XPS and EDX analysis. As reported in previous study,28-29 Bi5+ in Bi2O4 is gradually reduced to Bi3+ by release of oxygen in the range of 250-400 °C. Unfortunately, an unnoticeable weight loss (< 0.1%) is observed in this temperature stage, since the amount of surface Bi2O4 takes an extremely small proportion of the whole BiOBr-0.01 nanosheets. Nevertheless, an obvious exothermic peak at 292 °C is a receivable indication of the aforementioned Bi5+ reduction process. In order to examine the surface structure changes between normal BiOBr and BiOBr-0.01 nanosheets, Raman spectroscopy was applied in Figure S6. The prominent bands at 56.7, 89.7 and 112.6 cm-1 can be assigned to the A1g internal Bi−Br stretching modes, while the bands at 160.3 cm-1 can be assigned to the E1g internal Bi–Br stretching modes.4,32 Compared to BiOBr nanosheets, no detectable shifts of these peaks are found for BiOBr-0.01, suggesting that the formation of Bi2O4 does not destroy the remaining deep internal region of the Br inserted [Bi2O2] layered crystal structures. Furthermore, the bands at 384.3, 422.6 and 478.6 cm-1 for BiOBr nanosheets should be originated from the motion of oxygen atoms.4,33 Nevertheless, these stretching modes are nearly not observable and a small new sharp peak emerges at 436.9 cm-1 for


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the brown counterpart. These results strongly support that the Bi2O4 nanoparticles occur on the surface of BiOBr-0.01 nanosheet without damaging its basic {001} facets predominately exposed nanostructure.

Figure 6. SEM images of (a) BiOBr-0.001, (b) BiOBr-0.005, (c) BiOBr-0.05, (d) BiOBr-0.5 nanosheets, and (e) schematic illustration of the morphological evolution process.

Formation mechanism of brown BiOBr nanosheets


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Based on the above results, the Bi2O4 nanoparticles generated on the BiOBr nanosheets are confirmed convincingly. Brown BiOBr-0.01 nanosheets can be in situ obtained through posttreatment in NaOH solution upon light irradiation, without any additional foreign elements. Therefore, the formation mechanism of BiOBr-0.01 is explored accordingly. In spite of the dimensional differences, we mainly focus on the surface details of BiOBr nanosheets. Figure 6 illustrates the SEM images of BiOBr nanosheets obtained with different NaOH concentration under xenon lamp irradiation. When the NaOH concentration is low (BiOBr-0.001), some ricelike Bi2O4 nanoparticles are formed individually on the planar surfaces, meanwhile, some of them align as necklace along the middle raised region of lateral surfaces (Figure 6a). When increasing the NaOH concentration to 0.005 M and 0.01 M, the surface Bi2O4 nanoparticles on the planar surfaces become small-sized and sphere-shaped with high density (Figures 6b and 2). In parallel, the lateral surfaces are gradually fully covered with small spherical nanoparticles. Because the {001}-facet predominantly exposed BiOBr nanosheets in present system is stacked along [001] orientation, the formation of Bi2O4 nanoparticles on the lateral surfaces are observed in parallel to the {001} facets accordingly. Noting that the {001}-facets of BiOBr on the top and bottom surfaces of these samples are well retained. However, if the NaOH concentration increases to 0.05 M, the Bi2O4 nanoparticles are assembled into various pieces and stacked as a layer on the planar surfaces (Figure 6c). The host {001} facets are overlaid and some of the nanosheets turn into fragmentations for BiOBr-0.05 nanosheets. Meanwhile, the lateral surfaces change to concave grooves. Interestingly, as the NaOH concentration is up to 0.5 M, the sheetlike morphology is completely destroyed and no intact nanosheets are observed (Figure 6d). As expected, in the presence of Bi5+, the colors of corresponding samples deepen from the light brown to the dark brown (Figure S7). These appearances also meet the general understanding


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that trivalent bismuthates have a yellow color due to broad hybrid orbital of Bi 6s and O 2p,34 further verifying the formation of Bi5+ in brown samples. A more visually schematic illustration of the formation process is depicted in Figure 6e. Therefore, the size, shape and distribution of Bi2O4 nanoparticles on BiOBr nanosheets are highly dependent on the NaOH concentration. That is, the present alkali post-treatment is controllable to in situ achieve different final products.

Figure 7. TEM images of the samples prepared in (a) 0.1 M, (b) 1 M, (c) 5 M NaOH solution without Xenon lamp irradiation, and (d) corresponding XRD patterns.


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In the present system, the newly Bi2O4 emerges during the post-treatment process in the presence of NaOH solution and xenon light illumination. Firstly, samples were prepared following the same procedure as BiOBr-0.01 nanosheets in the absence of NaOH or xenon light, respectively. As shown in Figure S8, the surfaces of the obtained nanosheets are both smooth without any observable small nanoparticles, implying that the formation of Bi2O4 nanoparticles on BiOBr nanosheets is necessarily triggered by the combined action of NaOH solution and xenon light. Hence, the effect of NaOH and light are further separately investigated to in-depth understand the formation mechanism of Bi2O4. In terms of the effect of NaOH, samples were prepared under alkali condition in the absence of xenon light. As shown in Figures S8b and 7a, as-obtained samples keep the integral nanosheet morphology without obvious collapses until the applied NaOH concentration up to 0.1 M. However, when further increasing the NaOH concentration, the nanosheet-shaped morphologies of the as-prepared samples undergo drastic evolution and finally changes to rod-assembled nanostructures (Figures 7b and 7c). Meanwhile, the colors of these samples gradually deepen from the pale yellow to bright yellow owing to the nanostructure variation and phase transformation. This situation is completely different from the above-mentioned cases with xenon irradiation. Significantly, as can be seen from the XRD patterns in Figure 7d, the diffraction peaks of BiOBr gradually disappear and the peaks of the sample post-treated with high concentration of NaOH (5 M) is found to be well indexed to the monoclinic Bi8O12 phase (JCPDS 96-901-2547). In brief, the Br atoms in the original BiOBr gradually decayed due to the NaOH-dependent cleavage. Therefore, the major effect of NaOH in the present system is dehalogenation.


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Figure 8. Proposed formation mechanism of BiOBr-0.01 nanosheets.

In terms of light sources, samples were prepared as the same procedure as BiOBr-0.01 nanosheets except that xenon light is replaced by green or red light emitting diode (LED) lamp. No Bi2O4 nanoparticles are found generated on the surfaces of these resulting samples (data are not shown). The xenon light possesses light spectra with wavelength onset of 385 nm, whereas the green and red LED lamp respectively possesses light spectra with wavelength onset of 450 and 556 nm (Figure S9). The bandgap of about 2.8 eV for BiOBr determines that it can only be activated by the light of wavelength less than 442 nm. Thus, xenon light is able to excite BiOBr whereas green and red LED lamps cannot. More importantly, no other oxidizing agent is involved during the materials preparation procedure. Taking into account that more oxidative state of Bi5+ exists in the brown BiOBr-0.01 nanosheets, the appearance of Bi2O4 nanoparticles should be assigned to the oxidative process induced by BiOBr nanosheets themselves. That is to say, only when the BiOBr photocatalyst are excited by the light to produce highly oxidative species (e.g. h+ and ⋅O2−), Bi3+ can be oxidized to form specific Bi2O4 nanoparticles on the


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surface of BiOBr nanosheets. In addition, the sample is also prepared in the presence of NaOH (0.01 M) under xenon light irradiation with argon purging to eliminate ⋅O2− generation. Similar morphology of small-sized Bi2O4 decorated BiOBr nanosheet is observed (Figure S10). Since h+ excited from BiOBr is incapable to produce ⋅OH,4,11 the light-excited highly oxidative h+ is believed to predominantly account for the Bi3+ oxidation process. Based on the above discussions and results, both NaOH aqueous solution and xenon light source are prerequisites to yield Bi2O4 nanoparticles on the BiOBr nanosheet surfaces. Accordingly, the formation mechanism of brown nanosheets is proposed, as illustrated in Figure 8. The BiOBr possesses a unique crystal structure with [Bi2O2]2+ slabs inserted with Br slices driven by van der Waals force (Figure S11). The surface atoms should be more easily to escape from the lattice than the inner atoms.35 On the other hand, due to the long bond length and low bond energy,36 the Bi−O bond is easy to be destroyed. Therefore, when the OH− ion contacts with BiOBr in solution, anion ion exchange between Br− on the crystal surface and OH− is presumed to proceed. During this Br atoms release process, free Bi3+ are available closely adjacent to the surface of BiOBr nanosheets. Simultaneously, BiOBr is excited to generate oxidative h+ and rapidly migrate to the surface. Subsequently, the photogenerated h+ speedily oxidizes Bi3+ in the presence of OH− at the solid-liquid interface to form small Bi2O4 nanoparticles on the BiOBr nanosheet surfaces. It has to point out that in the presence of excess OH−, the rate to exchange Br− is so fast to exceed the tolerance of BiOBr crystals, leading to the subsequent prompt collapse of nanosheet nanostructures. On one hand, the BiOBr nanosheet would not keep its {001}-facet featured integrity and photogenerated h+ oxidation process on the nanosheet surface would be completely different. On the other hand, significant dehalogenation effect leads to a large amount of Bi3+ and OH−, so that it is possible to form various bismuth


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oxides, as confirmed by Figure 7. It is worth mentioning that these bismuth oxides also likely to have oxidation reaction to form Bi5+ involved oxide structures under suitable light irradiation.37 As shown in Figure S12, the morphology, crystal structure and appearance of BiOBr-1 and BiOBr-5 samples are different from their counterparts in Figure 7. The diffraction peaks in the XRD patterns cannot match the original BiOBr, while most of peaks can be indexed as Bi2O3 (JCPDS 00-027-0053). In conjunction with their brown-like appearance, BiOBr-1 BiOBr-5 samples are might be Bi3+/Bi5+ oxide composites. Notably, due to different atom arrangement, the surface atom diffusion, adsorption of reactant molecules, sensitivity to the reaction environment varies on the different crystal facets.38 In this respect, the intrinsic properties of different facets on planar and lateral surfaces of BiOBr nanosheet determine the ion exchange and oxidation process, further leading to the different morphology evolution between its planar and lateral surfaces. Investigations related to the facet-dependent behaviors are still under exploration. Photocatalytic activity The photocatalytic activities of different BiOBr nanosheets were initially evaluated by MO degradation under simulated sunlight irradiation. The MO is stable without photolysis (Figure S13a). As shown in Figure 9a, normal BiOBr nanosheets show quite slow efficiency to decompose MO, with only 55% degradation even after 30 min. It is obvious that with increasing NaOH concentration in the post-treatment, the MO decomposition ability of the resulting nanosheets successively increases. Surprisingly, the photocatalytic performance of BiOBr-0.01 nanosheets is dramatically improved, and the degree of decomposition reaches 100% only in 10 min. However, the MO degradation efficiency decreases for the BiOBr-0.05 sample, which is mainly due to the {001}-facet destruction. Furthermore, the reaction kinetics can be perfectly


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fitted to the pseudo-first order model (Figure S13b). As depicted in Figure 9b, BiOBr-0.01 nanosheets exhibit the highest reaction rate (0.45452 min-1), which is nearly 15 times than that of BiOBr (0.03108 min-1). Prior to the light irradiation, the mixture containing the photocatalyst and MO was stirred in the dark for 1 h to ensure that MO can be adequately adsorbed onto the surface of photocatalysts. Notably, the adsorption capacity of the post-treatment modified BiOBr photocatalysts towards MO increases with different degrees (Figure S13c). In particular, the elevation for BiOBr-0.01, the best one, is up to 42% in comparison with BiOBr nanosheets of only 4%. The exceptional incensement is because that abundant Bi2O4 nanoparticles offer more active adsorption sites and result in undulating rough surfaces, so that BiOBr-0.01 nanosheets can efficiently interact with reactants, leading to high photocatalytic performance. Expectedly, the BET specific surface area of BiOBr-0.01 (2.6 m2/g) is much larger than that of BiOBr (0.6 m2/g). As the morphological change mainly happens on the nanosheet surfaces, the increased specific surface area is absolutely Bi2O4-dependent. These results also agree well with the proposed morphology evolution process. As a result, the significant photocatalytic enhancement of post-treatment modified BiOBr samples highly relies on the generation of Bi2O4 nanoparticles and the integrity of predominant {001}-facet of BiOBr nanosheets. The representative BiOBr-0.01 nanosheets are also utilized to photocatalytically inactivate bacteria and converse CO2 under simulated sunlight irradiation. Expectedly, BiOBr-0.01 nanosheets show superior inactivation efficiency towards a typical Gram-negative bacteria of E. coli, with 7-log being completely inactivated within 15 min (Figure 9c). On the contrary, more than 4-log bacteria still remain in 20 min of inactivation time for normal BiOBr nanosheets. Thus, BiOBr-0.01 nanosheets also exhibit excellent photocatalytic activity of bacterial inactivation. Furthermore, the utilization renewable solar energy to trigger photocatalytic


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reduction of CO2 into valuable fuels provides a promising solution for both the global warming problem and the shortage of fossil fuels.39-40 Unfortunately, BiOBr nanosheets are incapable for CO2 photoreduction (Figure 9d). However, it is exciting to find that BiOBr-0.01 nanosheets are active to photocatalytic convert CO2 into CH4 and CO, with 3.7 and 5.2 µmol.g-1 within 2 h irradiation, respectively. The breakthrough for BiOBr nanosheets in photocatalytic CO2 reduction is undoubtedly attributed to the surface decorated Bi2O4 nanoparticles.

Figure 9. Photocatalytic (a) degradation efficiency of MO, (b) corresponding reaction rates (k) fitting with pseudo-first-order model, (c) bacterial inactivation of E. coli and (d) CO2 reduction activity of BiOBr and BiOBr-0.01 nanosheets under simulated solar light irradiation.


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Photocatalytic enhancement mechanism Hence, in comparison with normal BiOBr nanosheets, brown BiOBr-0.01 nanosheets show superior photocatalytic activity for the dye degradation and microbial disinfection. Particularly, it exhibits excellent capability to photocatalytically reduce CO2 into renewable fuels, whereas BiOBr is completely incapable for CO2 conversion. Based on the aforementioned discussions, this exceptional enhancement is definitely ascribed to the specific nanoarchitectures of Bi2O4 nanoparticles decorated BiOBr nanosheets. Considering that the BiOBr nanosheets before and after post-treatment shows distinct colors, their optical properties should differ from each other. The UV-vis DRS spectra in Figure 10a reveal that BiOBr nanosheets possess a bandgap of 2.83 eV with the sharp optical absorption onset at 438 nm. Surprisingly, BiOBr-0.01 nanosheets not only exhibit much higher UV absorption but also extend the light absorption to the full range of VL. Not limited to that, it even has strong light absorption far to NIR region. According to the optical absorption onset at 738 nm, the bandgap of BiOBr-0.01 nanosheets is substantially narrowed to 1.68 eV. This exciting optical property suggests that the generated Bi2O4 can promote the light absorption in both intensity and spectral range, thus enhance the photocatalytic ability of BiOBr-0.01 nanosheets. It is well-known that various disorders should greatly affect the band structure of semiconductors, leading to different photocatalytic activities. The valence band XPS analysis was carried out to locate the location of conduction band (CB). As shown in Figure 10b, BiOBr nanosheets demonstrate a VB with the maximum energy edge at 2.21 eV. Because its bandgap is 2.83 eV, the CB minimum is estimated to be −0.62 eV. For BiOBr-0.01 nanosheets, the VB maximum energy up-shifts to 1.49 eV and the CB minimum would occur at −0.19 eV. The significant up-shift of the VB edge is probably attributed to the surface Bi2O4 induced mid-gap states,41-42 which not only affects the optical transition to promote efficiently


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light harvest, but also prevent the fast recombination of photoexcited e−/h+ pairs.41 It is worth mentioning that the up-shift of the VB edge and down-shift of the CB edge produce a wide solar absorption range but also lower the redox potentials simultaneously. That is to say, the photocatalytic performance of BiOBr-0.01 is accommodated between the wide-ranged solar absorption and lowered redox potentials.43 Photoluminescence (PL) is an effective and reliable approach to assess the recombination efficiency of charge carriers.44-45 Clearly, the intensity of BiOBr-0.01 nanosheets in steady-state PL spectra is far lower than that of BiOBr (Figure 10c), implying a significantly decreased recombination rate of photogenerated e−/h+ pairs. To understand the dynamic photogenerated electron process, time-resolved PL measurement was carried out to estimate decay components, as shown in Figure 10d. By fitting the time-resolved PL decay curves with the exponential fitting equation,44 the decay time (τ1) for BiOBr and BiOBr-0.01 nanosheets are 0.6639 and 0.7817 ns, respectively. The decay time is indicative to the recombination rate of photogenerated carriers under light excitation. Therefore, the prolonged fluorescence lifetime suggests that the charge recombination rate for BiOBr-0.01 nanosheets is effectively restrained after introduction of Bi2O4 nanoparticles on the surface. To confirm the favored impact of Bi2O4 on transportation and separation of the photoexcited e−/h+ pairs for BiOBr-0.01 nanosheets, the electrochemical measurements were also performed. Figure S14a displays the transient photocurrent response curves under several switch-on and switch-off intermittent light cycles. Predictably enough, BiOBr-0.01 nanosheets show enhanced photocurrent density in comparison with BiOBr nanosheets, demonstrating higher transfer and separation efficiency of photogenerated e−/h+ pairs.9 Furthermore, the arc radius of BiOBr-0.01 is obviously smaller than that of BiOBr in the EIS Nyquist plots (Figure S14b), reflecting that the


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introduction of Bi2O4 can promote the interface charge separation and transfer process of BiOBr0.01 nanosheets, further boosting the corresponding photocatalytic activity. Based on the above results, the exciting photocatalytic enhancement of BiOBr-0.01 nanosheets is attributed to its unique Bi2O4 nanoparticles featured nanosheet structure. Firstly, the highly active predominately exposed {001} facets of the BiOBr nanosheets are entirely preserved. This intrinsic nature of charge transfer and separation play a crucial role to enhance its photocatalytic activity. Secondly, the photogenerated Bi2O4 not only provides extended VL and NIR light absorption, but also leaves large space to reserve the light-responsive capability of pristine BiOBr (λ ≤ 438 nm), so that the resulting BiOBr-0.01 nanosheets possess increasing solar absorption with wide range for photocatalytic reaction to proceed. Thirdly, the small-sized Bi2O4 nanoparticles are densely distributed, offering more active adsorption sites, which is beneficial for the interface contact between photocatalyst and reactants, further accelerating the photocatalytic reaction. Finally, but the most important one, the surface decorated Bi2O4 nanoparticles on the BiOBr-0.01 nanosheets can efficiently suppress the recombination and facilitate the separation of photogenerated e−/h+ pairs. In brief, the synergetic effects of above advantages after introduction of Bi2O4 lead to the exceptional enhancement in photocatalysis. Additionally, to further investigate the universality of this photoinduced alkali posttreatment approach for the photocatalytic activity enhancement of bismuth oxyhalogen semiconductor, BiOCl and BiOI samples before and after post-treatment are also prepared with the similar procedures to BiOBr. Their photocatalytic activities are compared accordingly. The BiOCl-0.01 sample also possess considerable increasing solar absorption (Figure S15) compared to normal BiOCl. Although the prepared procedure needs further optimization, the modified BiOCl-0.01 presents superior photocatalytic activity with k (0.02418 min-1) almost 5-fold than


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that of normal BiOBr (0.00506 min-1). Hence, not only limited to BiOBr, this strategy also can enhance the photocatalytic performance of BiOCl nanosheets. Unfortunately, the BiOI-0.01 sample fails to display obvious photocatalytic enhancement compared with normal BiOI. This is because the I− ions during the ion exchange process scavenge photoexcited h+ of BiOI to prohibit the oxidation of Bi3+ to Bi5+, leading to the failure of Bi5+ oxides generation.

Figure 10. (a) UV-vis diffuse reflectance spectra (DRS), (b) valence band XPS spectra, (c) steady PL spectra and (d) time-resolved fluorescence decay curves of BiOBr and BiOBr-0.01.

Conclusions


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In summary, brown BiOBr-0.01 samples are successfully achieved via a simple alkali posttreatment approach under light irradiation. Bi2O4 nanoparticles in situ generate on the surface of BiOBr nanosheets with well-preserved highly active {001} facets. The as-obtained BiOBr-0.01 sample displays remarkably increased solar absorption through the entire UV/VL light and partial NIR light region, compared to BiOBr, which only exhibits an absorption onset at 438 nm. It shows superior photocatalytic activity for the dye degradation and microbial disinfection. Significantly, exhibits excellent ability to photocatalytically reduce CO2 into CO and CH4, whereas BiOBr is completely incapable for CO2 conversion. The exceptional enhancement is due to the Bi2O4 extended light absorption, efficient photogenerated e−/h+ pair separation, as well as the increased surface-adsorbed ability to reactants. Hence, the in situ formation of Bi2O4 nanoparticles not only brings novel insights into in-depth understanding the light-induced photocatalytic process and also provide new pathways to enhance the photocatalytic performance in environmental remediation and energy conversion of BiOX (X=Br and Cl) and other Bi-based materials and their derivatives.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * L. Ye: Tel.: +86 15188472063, E-mail: [email protected]; * W. Wang: Tel: +86-87541540, E-mail: [email protected];


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* P.K. Wong: Tel: +852-39436383, E-mail: [email protected].

ACKNOWLEDGMENT The research was supported by research grant (GRF14100115) of the Research Grant Council and ITSP Tier 3 Scheme (ITS/216/14) of Innovation and Technology Commission, Hong Kong SAR Government to P.K. Wong and by research grants (No. 51502146, U1404506) of National Natural Science Foundation of China to L. Ye. P.K. Wong was also supported by the CAS/SAFEA International Partnership Program for Creative Research Teams of Chinese Academy of Sciences, China.

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