Ag-Modified BiOCl Single-Crystal Nanosheets: Dependence of

Article

Ag-Modified BiOCl Single-Crystal Nanosheets: The Dependence of Photocatalytic Performance on the Region-Selective Deposition of Ag Nanoparticles Huogen Yu, Cong Cao, Xuefei Wang, and Jiaguo Yu J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 01 Jun 2017 Downloaded from http://pubs.acs.org on June 2, 2017

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The Journal of Physical Chemistry

Ag-Modified BiOCl Single-Crystal Nanosheets: The Dependence of Photocatalytic Performance on the Region-Selective Deposition of Ag Nanoparticles

Huogen Yu,†,‡* Cong Cao,‡ Xuefei Wang,‡ Jiaguo Yu§

†

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of

Technology, Wuhan 430070, People’s Republic of China ‡

School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of

Technology, Wuhan 430070, PR China §

State Key Laboratory of Advanced Technology for Material Synthesis and

Processing, Wuhan University of Technology, Wuhan 430070, PR China

1

ACS Paragon Plus Environment


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ABSTRACT: For single-crystal photocatalysts, selective deposition of noble metals as electron cocatalysts has been demonstrated to be an effective method to improve their photocatalytic performance. However, the effect of selective-deposition location of noble metals on the photocatalytic performance of single-crystal photocatalysts has not been really revealed, because the noble metals can only be selectively deposited on the electron-enrichment facets and cannot be modified on the hole-enrichment facets due to the absence of effective deposition methods. In this study, a photosensitization-deposition strategy, as a new selective deposition method, has been developed to selectively deposit Ag nanoparticles on the hole-enrichment regions (the lateral surfaces) of BiOCl single-crystal nanosheets (referred to as Ag/BiOCl(Dye)), and the effect of selective-deposition locations of Ag nanoparticles on the photocatalytic performance of BiOCl single-crystal photocatalysts has been investigated. For comparison, Ag nanoparticles were also selectively deposited on the electron-enrichment regions (the outer-ring regions of dominant exposed {001} facets)

of

BiOCl

nanosheets

(referred

to

as

Ag/BiOCl(UV))

by

usual

photocatalytic-deposition method, and were randomly deposited on the whole surface of BiOCl nanosheets (referred to as Ag/BiOCl(NaBH4)) by a direct reduction method. It was found that the Ag/BiOCl(UV) photocatalysts with selective deposition of Ag nanoparticles on the electron-enrichment regions exhibited the highest photocatalytic performance, while the Ag/BiOCl(Dye) with selective deposition of Ag nanoparticles on the hole-enrichment regions displayed the lowest efficiency even lower than the unmodified BiOCl nanosheets. On the basis of our present results, a possible mechanism was proposed to account for the completely different photocatalytic 2


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performance of BiOCl nanosheets with site-selective deposition of Ag nanoparticles. This work may provide new insights into the flexible design and preparation of various high-performance photocatalytic materials.

3



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1. INTRODUCTION Photocatalysis technology has been considered to have great potential in the fields of solving environmental pollution and producing new energy.1-4 However, it is quite difficult for a photocatalytic material to achieve a high efficiency for practical applications due to its rapid recombination of photogenerated charges in its bulk and on its surface.5-7 To improve their photocatalytic activity, various strategies such as doping,8-10 coupling of semiconductor photocatalyst,11-14 surface modification,15-20 and morphology engineering21-22 (hollow nanostructures, core-shell nanostructures, etc.) have been widely designed and developed. Unfortunately, the above resultant photocatalysts usually exhibit a limited improvement of their performance owing to the random diffusion and transportation of photogenerated charges from bulk inside to their surface. In other words, the above usual strategies are still unable to significantly reduce the serious bulk recombination rate of photogenerated charges that widely exist in the traditional powdered and thin-film photocatalysts.6 Recently, it was reported that the bulk recombination rate of photogenerated charges could be clearly reduced by developing single-crystalline photocatalytic materials, as the self-built driving forces in single-crystal photocatalysts facilitate the rapid orientation transfer of photogenerated charges onto different facet surfaces.23-24 However, for a single-crystalline semiconductor, it is still quite possible that the photogenerated electrons and holes can recombine with each other on the photocatalyst surface owning to the existence of various surface defects, which limits the further enhancement of photocatalytic performance.6 Therefore, it is highly required to 4


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develop effective strategies to modify the surface of single-crystalline photocatalyst with the aim of reducing their surface recombination and promoting interfacial catalytic reactions. For a single-crystalline photocatalyst, recently, selective deposition of electron cocatalysts (such as Au, Pt, and Ag) on specific surface regions has been demonstrated to be one of the most effective and desirable routes to reduce the surface recombination of photogenerated charges and to promote their interfacial reduction/oxidation reactions.25-26 In this case, the synergistic effect of orientation transfer of photogenerated charges in the single-crystalline photocatalysts and their rapid interfacial catalytic reactions on the cocatalyst active sites usually contributes to the significantly enhanced photocatalytic performance by the well-coupling strategy of crystal-facet engineering and selective cocatalyst modification.6 As a result, many researchers have developed various high-efficiency single-crystalline photocatalysts by the selective deposition of electron cocatalysts.25-28 For example, Sun et al.26 reported that Ag nanoparticles could be successfully and selectively deposited on the {101} facets of TiO2 nanocrystals by a photodeposition process. Li et al.27 found that various reduction cocatalysts (such as Au, Ag and Pt) could be selectively deposited on the {010} crystal facets of BiVO4 through a photodeposition progress. Similarly, Zhang et al.28 also selectively deposited Au nanoparticles on the {001} crystal facets of BiOCl nanosheets. Apparently, all the above cocatalyst-modified photocatalysts clearly exhibited improved photocatalytic efficiencies not only for the decomposition of various organic substances but also for H2-evolution reaction. However, the above 5



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reported selective-deposition method for the electron cocatalysts is mainly restricted to the UV/Vis-light-induced photocatalytic progress, where the electron cocatalysts can only be loaded on the electron-enrichment facets of single-crystalline photocatalysts, while it is impossible for them to be selectively deposited on other surface regions.28-29 In fact, it should be noted that different surface regions for a single-crystalline photocatalyst possess different geometric and electronic structures, thus endowing them with distinctive properties.30-31 Therefore, it is quite believed that the photocatalytic performance of single-crystalline photocatalysts can be greatly influenced by the different selective-deposition location of electron cocatalysts. Unfortunately, up to now, seldom investigations have been performed to understand the effect of selective-deposition site of electron cocatalyst on the photocatalytic performance of single-crystalline photocatalysts. The main reason is that in addition to the above photodeposition method (or photocatalytic-induced method), no other effective methods have been developed to selectively deposit electron cocatalysts on different crystal facets of single-crystalline photocatalysts. Thus, it is quite necessary and important to develop new methods to realize the selective deposition of electron cocatalysts on different surface regions for a single-crystalline photocatalyst, which may help us to really realize the relationship between photocatalytic performance and site-selective deposition of electron cocatalysts. Recently, BiOCl nanosheets with a high producing rate can be easily prepared by a solution route, and has been demonstrated as an efficient photocatalyst due to its appealing electronic band structure, high physicochemical stability, and unique 6


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layered features.32-43 Herein, BiOCl single-crystalline nanosheets were employed as an excellent substrate to investigate the controllable site-selective deposition of Ag-electron

cocatalyst

by

different

loading

methods

and

the

effect

of

selective-deposition site of Ag nanoparticles on the photocatalytic performance of single-crystalline photocatalytic materials. In this study, BiOCl single-crystalline nanosheets were synthesized via a facile hydrothermal route, and then the Ag nanoparticles (as an electron cocatalyst) were selectively deposited on the different surface regions of BiOCl nanosheets via the following three methods: (1) selective deposition by a photocatalytic-induced progress, (2) selective deposition by a photosensitization-induced route, and (3) random deposition by a direct reduction method. Based on the above strategies, Ag nanoparticles were selectively deposited on the dominant exposed surface ({001} facet), lateral surfaces ({010} and {110} facets), and all exposed facets of BiOCl single-crystalline nanosheets, which can be referred to as Ag/BiOCl(UV), Ag/BiOCl(Dye), and Ag/BiOCl(NaBH4), respectively. Photocatalytic results demonstrated that the resultant Ag/BiOCl(UV) exhibited the highest photocatalytic performance, while the Ag/BiOCl(Dye) displayed the lowest efficiency even lower than the unmodified BiOCl nanosheets. A possible mechanism was proposed to account for the controllable photocatalytic performance of BiOCl single-crystalline nanosheets with site-selective deposition of Ag nanoparticles. To the best of our knowledge, this is the first report about the effect of site-selective deposition of Ag nanoparticles on the photocatalytic performance of single-crystalline BiOCl nanosheets. This work may provide new insights into the preparation and 7



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various potential applications of single-crystalline functional materials with controllable site-selective deposition of noble metals.

2. EXPERIMETNAL SECTION All the chemicals were of analytical grade supplied by Shanghai Chemical Reagent Ltd. (P.R. China) and used as received without further purification. Distilled water was used in all experiment. 2.1. Preparation of BiOCl Single-Crystal Nanosheets BiOCl single-crystal nanosheets (Figure 1a) were synthesized using a hydrothermal method.33 In a typical procedure, 1 mmol of Bi(NO3)3·5H2O and 1 mmol of KCl were firstly dissolved in 10 mL HNO3 solution (1 M) and 5 mL distilled water, respectively. A precipitation was obtained when KCl solution was added into the above Bi(NO3)3 solution under continuous stirring at room temperature for 30 min. The resultant suspension was then transferred into a Teflon-lined stainless autoclave and maintained at 160°C for 24 h. After cooling down to room temperature naturally, the resultant precipitates were collected, washed with ethanol and deionized water, and then dried at 60°C to obtain the BiOCl single-crystal nanosheets. 2.2. Preparation of Ag/BiOCl Photocatalysts Ag nanoparticles were selectively deposited on the different surface regions of BiOCl single-crystal nanosheets to prepare various Ag/BiOCl photocatalysts via the following three methods (as shown in Figure 1): (1) selective deposition by a photocatalytic-induced progress for the preparation of Ag/BiOCl(UV) with selective

8


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Ag-deposition on the dominant exposed {001} facets, (2) selective deposition by a photosensitization-induced route for the preparation of Ag/BiOCl(Dye) with selective Ag-deposition on the lateral surfaces, and (3) random deposition by a direct reduction method for the preparation of Ag/BiOCl(NaBH4) with random deposition of Ag nanoparticles. To simply their sample name, in this study, the above resultant samples are referred to as Ag/BiOCl(UV), Ag/BiOCl(Dye), and Ag/BiOCl(NaBH4), respectively. The Ag/BiOCl(UV) photocatalyst was prepared by a photocatalytic-induced selective deposition of Ag nanoparticles under UV-light irradiation (Figure 1b). Briefly, 100 mg of the BiOCl single-crystal nanosheets and a certain amount of AgNO3 solution (0.1 M) were added into 20 mL of 25 vol% ethanol solution in a cylindrical reaction vessel (25 mL in capacity). The above suspension was stirred for 30 min in the dark, and then exposed to UV light at room temperature for another 30 min. A 4-W 365-nm UV lamp (Shenzhen LAMPLIC Science Co. Ltd) was used as a light source and the average light intensity striking on the surface of reaction solution was about 80 mW cm-2. After light irradiation, the samples were collected, washed with ethanol and distilled water for several times, and finally dried at 40oC for 12 h to obtain the Ag/BiOCl(UV) photocatalysts with selective Ag-deposition on the dominant exposed {001} facets. To investigate the effect of metallic Ag amount on the photocatalytic performance of Ag/BiOCl(UV), the weight ratio of Ag to BiOCl was controlled to be 0, 0.5, 1, 3, 5, 10 wt%, respectively, and the resultant sample was referred to be Ag/BiOCl(UV-Xwt%), where the X represents the weight ratio of Ag to 9



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BiOCl. Photocatalytic experiments indicated that the Ag/BiOCl(UV-3wt%) sample showed the highest photocatalytic performance (Figure S1-A). Therefore, to simply the sample name, the Ag/BiOCl(UV-3wt%) was referred to be Ag/BiOCl(UV) in this work. The

Ag/BiOCl(Dye)

photocatalyst

was

synthesized

by

a

dye-photosensitization-induced selective deposition of Ag nanoparticles under visible-light irradiation (Figure 1c). Experimental details were shown as follows: 100 mg of the BiOCl single-crystal nanosheets were dispersed into 20 mL of RhB solution (10 mg/L) and then stirred in the dark at room temperature for 30 min to achieve an adsorption-desorption equilibrium. After that, a certain amount of 0.1 M AgNO3 solution was added into the above suspension solution and then was stirred for another 30 min. Finally, the resulting suspension was irradiated for 1 h by two 550-nm LEDs (24 mW cm-2, Shenzhen Lamplic Science Co. Ltd.). After visible-light irradiation, the samples were collected, washed with ethanol and distilled water for several times, and finally dried at 40oC for 12 h to obtain the Ag/BiOCl(Dye) photocatalysts with selective Ag-deposition on the lateral surfaces. To investigate the effect of metallic Ag amount on the photocatalytic performance of Ag/BiOCl(Dye), the weight ratio of Ag to BiOCl was controlled to be 0, 0.1, 0.5, 1, 3 wt%, respectively, and the resultant sample was referred to be Ag/BiOCl(Dye-Xwt%), where the X represents the weight ratio of Ag to BiOCl. Photocatalytic experiments indicated that all the Ag/BiOCl(Dye-Xwt%) photocatalysts showed a comparable photocatalytic performance (Figure S1-B), which is clearly lower than the pure BiOCl sample. In 10


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this study, to simply the sample name, the Ag/BiOCl(Dye-3wt%) was referred to be Ag/BiOCl(Dye). The Ag/BiOCl(NaBH4) photocatalyst was prepared by a random deposition of Ag nanoparticles via a direct reduction of AgNO3 in a NaBH4 solution (Figure 1d). Typically, 100 mg of the BiOCl single-crystal nanosheets and 279 µL of AgNO3 solutions (0.1 M) were dispersed into 10 mL of ethanol solution (here the weight ratio of Ag to BiOCl was controlled to be 3 wt%), and then 10 mL of NaBH4 ethanol solution (10 mM) were added. The resultant suspension was stirred in ice-water bath for 30 min to inhibit the self-hydrolysis of NaBH4. After that, the samples were collected, washed with ethanol and distilled water for several times, and finally dried at 40oC for 12 h to obtain the Ag/BiOCl(NaBH4) photocatalyst with random deposition of Ag nanoparticles. 2.3. Computational Details The calculations reported herein were performed on the basis of density functional theory (DFT) within the Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation (GGA), as implemented in the Vienna ab initio Simulation Package (VASP).44,45 The projector augmented wave (PAW) method with a plane wave basis set was used to describe the interaction between ion cores and valence electrons.31 Besides, the electron wave function was expanded in plane waves up to a cutoff energy of 380 eV and a gamma-centered (6×6×3) Monkhorst–Pack mesh of k points was used for the Brillouin zone sampling through careful volume optimization and atomic position relaxation with a primitive unit cell.46 11



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2.4. Characterizations The morphology of these samples was observed by a JEM-7500F field emission scanning electronic microscopy (FESEM, JEOL, Japan) equipped with an X-Max 50 energy-dispersive X-ray spectrometer (EDS, Oxford Instruments, Britain). Further morphological and structural characterizations were based on transmission electron microscopy (TEM) and high-resolution transmission microscopy (HRTEM) observation using a JEM-2100F transmitting electron microscope. X-ray diffraction (XRD) was used to identify the crystal structures and phase compositions of the samples on a D/MAXRB RU-200B X-ray diffractometer (Rigaku, Japan) using Cu Kα radiation. The chemical information of different elements on the sample surface was analyzed using X-ray photoelectron spectroscopy (XPS) on a ESCALAB 250Xi (Thermo Fisher Scientific, America) XPS system with Mg Kα source. All the binding energies were referenced to the C1s peaks at 284.8 eV for the surface adventitious carbon. UV–vis absorption spectra were obtained using a UV–vis spectrophotometer (UV-2450, Shimadzu, Japan). BaSO4 was used as a reflectance standard in a UV–vis diffuse reflectance experiment. 2.5. Photocatalytic Activity The photocatalytic activities of the as-prepared samples were evaluated by the degradation of methyl orange (MO) and phenol under UV-light irradiation at ambient temperature. In each experiment, 50 mg of the photocatalyst was dispersed into 10 mL of MO solution (20 mg L-1) or phenol solution (10 mg L-1) in a cylindrical reaction vessel (15 mL in capacity). Prior to irradiation, the solution was continuously stirred 12


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in the dark for 1 h to ensure the establishment of an adsorption-desorption equilibrium between the photocatalysts and degrading pollutants. For the evaluation of UV-light photocatalytic activity, two 365-nm LED lamps with an average light intensity of 80 mW cm-2 (Shenzhen LAMPLIC Science Co. Ltd.) were used as a light source. During the photocatalytic reactions, the MO or phenol solutions with photocatalysts were continuously stirred with magnetic stirrers, and at certain time intervals, 4 mL reaction solution was sampled and centrifuged to remove the photocatalysts. The concentrations of MO and phenol were determined by monitoring the change of absorbance at 460 and 270 nm, respectively, with a UV-visible spectrophotometer (UV-1240, SHIMADZU, Japan). Owing to a low concentration of MO aqueous solution or phenol solution, the photocatalytic decomposition is a pseudo-first order reaction and its kinetics may be expressed as ln(c0/c) = kt, where k is the apparent rate constant, and c0 and c are the MO or phenol concentrations at the initial state and after irradiation for t min, respectively.47-49 2.6. Photoelectrochemical Measurements Photoelectrochemical measurements and electrochemical impedance spectra (EIS) were performed on an electrochemical workstation (CHI660E) in a standard three-electrode configuration with a platinum wire as the counter electrode, saturated Hg/Hg2Cl2 (in saturated KCl) as a reference electrode, and Na2SO4 (0.5 M) aqueous solution as the electrolyte. The light source was provided with one 4-W LED (365 nm light source with an 80 mW cm−2 power). The working electrodes were prepared on fluorine-doped tin oxide (FTO) conductor glass. Typically, the sample (10 mg) was 13



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ultrasonicated in 2.5 mL of anhydrous ethanol, then transferred 0.5 mL of this solution to a Teflon tube containing 0.5 mL of Nafion D-520 dispersion (1%, w/w, in anhydrous ethanol, Alfa Aesar), and dispersed evenly to obtain suspension solution. The suspension was spread on the FTO glass with the side protected by Scotch tape and dried at 60oC for 12 h to obtain the final working electrodes. The transient photocurrent responses with time (i–t curve) of the working electrodes were measured at a 0.5 V bias potential during repeated ON/OFF illumination cycles, and EIS was determined over the frequency range of 0.001–106 Hz with an ac amplitude of 10 mV at the open circuit voltage.

3. RESULTS AND DISCUSSION

3.1. Strategies for the Controllable Synthesis of Ag/BiOCl Photocatalysts

Figure 1. Schematic illustration for the controllable synthesis of Ag-modified BiOCl samples: (a) BiOCl, (b) Ag/BiOCl(UV) with selective Ag-deposition on the dominant exposed {001} facets; (c) Ag/BiOCl(Dye) with selective Ag-deposition on the lateral surfaces, and (d) Ag/BiOCl(NaBH4) with random deposition of Ag nanoparticles. 14


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The schematic illustration for the controllable synthesis of various Ag/BiOCl photocatalysts is shown in Figure 1. First, the BiOCl single-crystal nanosheets (Figure 1a) were synthesized via a facile hydrothermal method using Bi(NO3)3·5H2O and KCl as the precursors. According to the FESEM images (Figure 2A and B), the resultant sample was composed of large-scale sheet-like nanostructures with a particle-size range of 2-5 µm and a thickness of ca. 400 nm. The corresponding XRD result (Figure 2C) clearly demonstrates that the resulting sample can be attributed to the tetragonal structure of BiOCl (JCPDS no. 06–0249) with an excellent orientation along the [001] direction, as the (001), (002) and (003) diffraction peaks are clearly sharper and stronger than that of the standard BiOCl result. To further demonstrate the crystal orientation of BiOCl nanosheets, their TEM images and corresponding selected-area electron diffraction are also provided, as shown in Figure 2D-F. The above results strongly suggest the formation of BiOCl single-crystal nanosheets with a high crystallization. In addition, it can be deduced that the dominant exposed surface of BiOCl single-crystal nanosheets is {001} facet, while their lateral surfaces can be ascribed to be {010} and {110} facets, as shown in Figure 1a. Subsequently, the resultant BiOCl single-crystal nanosheets were used as the precursor to selectively load metallic Ag nanoparticles on their different surface regions by the following methods.

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Figure 2. (A-B) FESEM images, (C) a typical XRD pattern, (D,E) TEM images and (F) its corresponding SAED pattern for the BiOCl nanosheets.

The Ag/BiOCl(UV) with selective Ag-deposition on the dominant exposed {001}

facets was first prepared via a photocatalytic-induced selective deposition progress by using AgNO3 as the precursor and ethanol as the sacrificial agent (Figure 1b). When the BiOCl sample was dispersed into AgNO3 solution and was then irradiated by UV light, the photogenerated electrons and holes can be produced by the band-gap excitation of the BiOCl photocatalysts. In this case, the photogenerated electrons can 16


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be spontaneously and preferentially transferred to the electron-enrichment regions of BiOCl single-crystal nanosheets via its self-built driving forces, causing the rapid and selective deposition of Ag nanoparticles on the specific electron-enrichment regions via the direct reduction of Ag+.50 Similar photocatalytic deposition mechanism has been widely reported and applied to selectively deposit various noble metals on specific facets of semiconductors materials, such as Ag/TiO26, Pt/BiVO427, and Au/TiO251. However, compared with the usual whole-facet deposition of noble metals on the photocatalysts, in this study, it is interesting to find that the Ag nanoparticles can only be selectively deposited on the outer-ring regions of {001} facet, but not the whole {001} facet (See below). The Ag/BiOCl(Dye) photocatalyst with selective Ag-deposition on the lateral surfaces was synthesized via a dye-photosensitization-induced selective-deposition method by using RhB as the sensitizer and AgNO3 as the precursor (Figure 1c), which includes the initial selective adsorption of RhB cationic dyes on the lateral surfaces ({010} and {110} facets) of BiOCl nanosheets and then in situ selective deposition of Ag nanoparticles via dye-photosensitization mechanism. To investigate the possible selective-adsorption facets of BiOCl single-crystal nanosheets for RhB cationic dyes, it is quite necessary to understand their electronegativity of {001} and {110} facets. A typical structure model of BiOCl was shown in Figure 3A. It is clear that BiOCl layered structure is composed of [Cl-Bi-O-Bi-Cl] sheet-like unit stacked together by the nonbonding Cl---Cl interaction (van der Waals forces) along c-axis, and the bonding distance between Bi-O and Bi-Cl are 2.3180 A and 3.0506 A, respectively.52 17



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According to the total density of states (DOS) and partial density of states (PDOS) for Bi, O and Cl atoms in Figure 3B, it is found that the overlap degree between Bi and O is significantly greater than that of Bi and Cl. Considering a shorter distance and greater overlap degree of Bi-O, it is very clear that there is a higher hybridization between Bi and O atoms compared with Bi and Cl atoms. In another words, the covalent bond of Bi-O is much stronger than that of Bi-Cl in the layered BiOCl structure. Therefore, the outermost of {001} facet with a positive charge is mainly exposed by Bi atoms (Figure 3C), while the {110} lateral surface with a negative charge is primarily composed of exposed O atoms (Figure 3D). In this case, when the BiOCl nanosheets were dispersed into RhB cationic-dye solution, RhB cationic molecules can be selectively adsorbed on the negatively charged {110} lateral facets of the BiOCl single-crystal nanosheets. Under visible-light irradiation by a 550-nm light source, only the selectively adsorbed RhB molecules on the {110} lateral facets of BiOCl nanosheets are excited to produce photogenerated electrons, where the electrons can be rapidly in situ captured by Ag+ to form Ag nanoparticles, as shown in Figure 1c. Therefore, it is clear that the preferential adsorption of RhB cationic dyes causes the effectively selective deposition of Ag nanoparticles on the lateral facets of BiOCl nanosheets. For comparison, the Ag/BiOCl(NaBH4) photocatalyst with random deposition of Ag nanoparticles was prepared by a direct reduction of AgNO3 in a NaBH4 solution. In this case, owing to the presence of large amount of NaBH4, the Ag nanoparticles can be gradually produced in the reaction solution and then are randomly deposited on 18


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the whole surfaces of BiOCl single-crystal nanosheets, as shown in Figure 1d. B Density of states (eV-1)

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Bi 6s Bi 6p

2 0

O 2s O 2p

2 0

Cl 3s Cl 3p

4 0

BiOCl

6 0 -20

-15

-10

-5

0

5

10

15

Energy (eV)

Figure 3. (A) The structure model of BiOCl nanosheets. (B) Total density of states (DOS) for BiOCl and partial density of states (PDOS) for Bi, O and Cl atoms, where the dashed lines represent the Fermi level at 0 eV. (C-D) Side view of atomic structure of (C) the {001} and (D) {110} facets.

3.2. Morphology and Microstructures The successful synthesis of above selective-deposited Ag/BiOCl photocatalysts can be clearly demonstrated by the following FESEM, XPS and UV-vis results. Figure 4 shows the typical FESEM images of the Ag/BiOCl(UV), Ag/BiOCl(Dye), and Ag/BiOCl(NaBH4) samples. For the Ag/BiOCl(UV) (Figure 4A), it is interesting to find that the Ag nanoparticles (ca. 10-50 nm) are only selectively deposited on the outer-ring regions of {001} facet, but not the whole {001} facet as previously reported results. For the Ag/BiOCl(Dye) (Figure 4B), the Ag nanoparticles (ca. 10-50 19



nm) are only selectively deposited on the lateral facets of single-crystal nanosheets. While for the Ag/BiOCl(NaBH4) (Figure 4C), the Ag nanoparticles (ca. 100 nm) are randomly distributed over the whole surface of BiOCl nanosheets. According to their corresponding EDS results (Figure S2), the amounts of Ag cocatalyst in the Ag/BiOCl(UV), Ag/BiOCl(Dye), and Ag/BiOCl(NaBH4) were about 1.24, 0.39 and 1.04 at%, respectively. A typical XRD pattern of the Ag/BiOCl nanosheets was shown in Figure 4D. It is clear that no obvious change about the diffraction-peak intensity and full width at half-maximum of the Ag/BiOCl photocatalysts can be found compared with the pure BiOCl sample, suggesting that the crystal structure of BiOCl nanosheets cannot be effected by the surface modification of Ag nanoparticles. Therefore, the above results clearly demonstrated that the Ag nanoparticles have been selectively deposited on the different regions of BiOCl single-crystal nanosheets by the present synthetic strategies.

10

220

104 114 005

212

113 211

112

200

a

110

101

002

102 003

001

D Relative intensity (a.u.)

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

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JCPDS no.06-0249

20

30

40

50

2θ (degree)

60

70

Figure 4. (A-C) SEM images of (A) Ag/BiOCl(UV), (B) Ag/BiOCl(Dye), and (C) 20


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Ag/BiOCl(NaBH4); (D) a typical XRD pattern of Ag/BiOCl(UV).

To further demonstrate the successful modification of Ag nanoparticles and their chemical states, the resultant samples are further characterized by XPS. Figure 5A shows the typical XPS survey spectra of the different samples. It is clear that all the prepared samples possess the main XPS peaks of Bi, O, Cl, and C elements according to their corresponding positions of binding energies. Herein, the C elements should be attributed to the adventitious hydrocarbon from the XPS instrument itself. Compared with the pure BiOCl, an additional XPS peak of Ag element at ca. 370 eV appears in the Ag/BiOCl(UV), Ag/BiOCl(Dye), and Ag/BiOCl(NaBH4) samples. To further reveal the chemical state of metallic Ag, its high-resolution XPS spectra are conducted and shown in Figure 5B. Compared with the unmodified BiOCl samples, new XPS peaks with binding energies of 368.4 eV for Ag 3d5/2 and 374.3 eV for Ag 3d3/2 are found in the three Ag/BiOCl samples, which can be well attributed to the metallic Ag phase.53,54 In addition, according to the element component analysis based on the XPS results (Table 1), the amounts of Ag cocatalyst in the Ag/BiOCl(UV), Ag/BiOCl(Dye), and Ag/BiOCl(NaBH4) were about 6.01, 0.96 and 6.09 at%, respectively. The high-resolution XPS results for the Bi 4f, O 1s and Cl 2p of various samples are shown in Figure 5C-E. It is found that the peak intensity and position of Bi, O, and Cl elements among different samples show no obvious shift, indicating that the deposition of Ag cocatalyst cannot significantly influence the surface structure and

21



chemical states of BiOCl photocatalyst, which is in good agreement with the XRD results.

Bi 3p

Bi 4d

O 1s

Ag 3d

C 1s

Bi 4f Cl 2p

Bi 6p

Relative intensity (a.u.)

A d c b a 0

200

400

600

800

Binding energy (eV)

C

363

Ag 3d5/2



B Ag 3d3/2 d c

5X

b a 366

369

372

375

378

156

Binding energy (eV)

Bi 4f7/2

Bi 4f5/2 d c b a

158

160

162

164

166

168

Binding energy (eV)

D

E


O 1S


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

Page 22 of 43

d c b a 528

530

532

194

534

Binding energy (eV)

Cl 2p3/2

Cl 2p1/2 d c b a

196

198

200

202

204

Binding energy (eV)

Figure 5. (A) XPS survey spectra and (B-E) high-resolution XPS spectra of (B) Ag 3d, (C) Bi 4f, (D) O 1s and (E) Cl 2p for various samples: (a) BiOCl, (b) Ag/BiOCl(UV), (c) Ag/BiOCl(Dye), and (d) Ag/BiOCl(NaBH4). 22


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Table 1. The composition (atom%) of the various samples according to XPS analysis. Samples

Bi

Cl

O

Ag

BiOCl

27.08

33.88

39.04

0

Ag/BiOCl(UV)

25.06

24.60

44.33

6.01

Ag/BiOCl(Dye)

27.69

30.76

40.59

0.96

Ag/BiOCl(NaBH4)

25.85

25.57

42.49

6.09

UV-vis spectra can provide further information about the selective modification of Ag nanoparticles on the BiOCl single-crystal nanosheets, as shown in Figure 6. It is found that the BiOCl nanosheets show band-gap absorption at ca. 370 nm, corresponding to a band gap of ca. 3.35 eV. After surface loading of Ag nanoparticles, the resultant Ag/BiOCl samples exhibit improved light absorption in the range of 370-800 nm. In addition, an additional shoulder peak in the range of 450-600 nm can be clearly observed in all the Ag-modified Ag/BiOCl(UV), Ag/BiOCl(Dye), and Ag/BiOCl(NaBH4) samples, which can be attributed to the localized surface plasmon resonance (LSPR) of metallic Ag nanoparticles.53,54 Moreover, the above various Ag/BiOCl samples show different LSPR of Ag nanoparticles due to their different size and distribution.55 The above results can be well supported by their digital photographs (inset), where the white color of BiOCl clearly changes into brown after the addition of Ag nanoparticles. Therefore, on the basis of the above FESEM, XPS and UV-vis results, it is quite reasonable that the Ag cocatalyst has been successfully 23



and selectively deposited on the different regions of BiOCl single-crystal nanosheets.

0.8

Absorbance (a.u.)

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

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a

0.6

c

b

d b

0.4

d c

0.2

a

0.0 200

300

400

500

600

700

800

Wavelength (nm) Figure 6. UV-Vis spectra of various samples: (a) BiOCl, (b) Ag/BiOCl(UV), (c) Ag/BiOCl(Dye), and (d) Ag/BiOCl(NaBH4).

3.3. Photocatalytic Performance and Mechanism The photocatalytic performances of BiOCl, Ag/BiOCl(UV), Ag/BiOCl(Dye), and Ag/BiOCl(NaBH4) samples were first evaluated by photocatalytic decolorization of MO aqueous solution. As shown in Figure 7A, the individual BiOCl photocatalyst (Figure 7A-a) displays a comparable photocatalytic activity, and the corresponding photocatalytic rate constant k value is about 0.017 min-1. The Ag/BiOCl(UV) with selective Ag-deposition on the outer-ring regions of dominant exposed {001} facet (Figure 7A-b) exhibits the highest photocatalytic performance (k= 0.044 min-1), which is higher than that of pure BiOCl by a factor of ca. 2.6. However, the Ag/BiOCl(Dye) photocatalysts with selective Ag-deposition on the lateral surfaces (Figure 7A-c) 24


Page 25 of 43

shows the lowest photocatalytic performance and its k value significantly decreases to 0.007 min-1, while Ag/BiOCl(NaBH4) with a random deposition of Ag nanoparticles (Figure 7A-d) exhibits a slightly enhanced activity. To eliminate the influence of Ag amount on the photocatalytic performance, the Ag/BiOCl(UV) and Ag/BiOCl(Dye) photocatalysts with different Ag amount have also been prepared and their photocatalytic performance was tested under an identical conditions, as shown in Figure S1. It is found that compared with the unmodified BiOCl, all the resultant Ag/BiOCl(UV)

photocatalysts

exhibit

a

remarkably

higher

photocatalytic

performance, while all the resultant Ag/BiOCl(Dye) samples show an obviously decreased photocatalytic performance. The above results can further be confirmed by the photocatalytic decomposition of colorless phenol solution, as shown in Figure 7B. It is very clear that compared with the unmodified BiOCl, all the Ag/BiOCl samples exhibit a similar tendency of their photocatalytic performance with the MO decomposition, and the Ag/BiOCl(UV) photocatalyst still preserves the highest photocatalytic activity and excellent stability.

A

B 0.006

0.04

a b c d

0.005

k (min-1)

0.004

k (min-1)

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


0.03

0.003

0.02

0.002

0.01

0.001

0.00

a

b

c

0.000

d

1st

25


2nd

3rd

4th


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

Figure 7. (A) The rate constant (k) of MO decomposition and (B) the repeated photocatalytic performance for phenol decomposition by various photocatalysts: (a) BiOCl, (b) Ag/BiOCl(UV), (c) Ag/BiOCl(Dye), and (d) Ag/BiOCl(NaBH4).

The above results clearly indicate that the selective deposition location of Ag nanoparticles on the BiOCl nanosheet surface has a vital influence on their photocatalytic performance. To investigate the potential photocatalytic mechanism of various Ag/BiOCl nanosheets with different selective deposition of Ag nanoparticles, it is highly required to consider the electron-enrichment and hole-enrichment regions on the pure BiOCl nanosheet surface after UV-light absorption. According to the photocatalytic mechanism, it has been widely reported that photocatalytic deposition method of cocatalysts via the electron/hole-induced reduction/oxidation reactions was an effective strategy to determine the electron- or hole-enrichment facets on the single-crystal photocatalyst surface.28 On the basis of the microstructure analysis of Ag/BiOCl(UV) (Figure 4A), in this study, the electron-enrichment regions of BiOCl nanosheets are only located on the outer-ring regions of dominant exposed {001} facet. To further determine the hole-enrichment regions of pure BiOCl nanosheets, the MnOx nanoparticles were selectively deposited on the BiOCl nanosheet surface by a similar photocatalytic progress via a hole-oxidation mechanism (the details are shown in the Supporting Information),27 and the corresponding results were shown in Figure 8. It is interesting to find that the MnOX not only can be selectively deposited on the lateral surfaces of BiOCl nanosheets, but also can be preferably modified on the 26


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center regions of {001} facets, clearly suggesting that the hole-enrichment regions of pure BiOCl nanosheets are simultaneously located on the lateral surfaces and the center regions of dominant exposed {001} facets (Figure 9a). In this case, under UV-light irradiation, the photogenerated electrons and holes are first produced inside the BiOCl nanosheets and then rapidly transfer to the electron-enrichment and hole-enrichment regions by the self-built driving force, respectively. For the unmodified BiOCl nanosheets, the sample shows a comparable photocatalytic activity owing to the lack of effective active sites for interfacial catalytic reactions (Figure 9a). As for the Ag/BiOCl(UV) photocatalysts (Figure 9b), the selective-deposited Ag nanoparticles on the electron-enrichment regions first acts as an electron sinks to rapidly capture photogenerated electrons from the BiOCl surface and then efficiently promote the interfacial catalytic reaction for oxygen reduction. As a consequence, the Ag/BiOCl(UV) photocatalysts exhibit the highest photocatalytic performance owing to the excellent synergistic effect of orientation transfer of photogenerated charges in the BiOCl single-crystalline structure and their rapid interfacial catalytic reactions on the Ag-cocatalyst active sites, which is in good agreement with our proposed well-coupling strategy of crystal-facet engineering and selective cocatalyst modification.6 On the contrary, for the Ag/BiOCl(Dye) sample (Figure 9c), the Ag nanoparticles on the hole-enrichment regions (the lateral surfaces of BiOCl nanosheets) not only capture photogenerated holes, but also can capture the photogenerated electrons from the electron-enrichment regions owing to the metal-semiconductor Schottky barrier,6 causing an obviously decreased photocatalytic 27



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performance. Therefore, the above results indicate that site-selective deposition of noble metal nanoparticles on different region of single-crystalline photocatalysts is an effective strategy to achieve the controllable photocatalytic performance.

Figure 8. FESEM images of MnOx-modified BiOCl photocatalysts by a UV-light photocatalytic mechanism.

Figure 9. Schematic drawing illustrating the possible photocatalytic mechanism: (a) the electron- and hole-enrichment regions of the BiOCl nanosheet; (b) the excellent synergistic effect of orientation transfer of photogenerated charges in the BiOCl nanosheets and the rapid interfacial catalytic reactions on the Ag-cocatalyst active sites for Ag/BiOCl(UV), and (c) the recombination of photogenerated electrons and holes on the Ag nanoparticles for Ag/BiOCl(Dye). 28


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The

above

photocatalytic

mechanism can further be

demonstrated

by

photoelectrochemical (PEC) analysis. The transient photocurrent responses of BiOCl, Ag/BiOCl(UV), Ag/BiOCl(Dye), and Ag/BiOCl(NaBH4) samples were shown in Figure 10A. It was found that the order of photocurrent density for the samples is in good agreement with the photocatalytic degradation performance, strongly suggesting that the selective Ag-deposition on the electron-enrichment regions can effectively boost photogenerated charge transfer and enhance the photocatalytic activity, while the selective Ag-deposition on the hole-enrichment regions possess negative effect due to the serious recombination of photogenerated electrons and holes. The typical electrochemical impedance spectroscopy (EIS) of these samples is shown in Figure 10B. The semicircle in the Nyquist plot can be simulated well by an electrical equivalent-circuit model (inset in Figure 10B), and the corresponding results are shown in Table S1. It is found that the semicircle radius sequence of as-prepared samples is Ag/BiOCl(UV) < Ag/BiOCl(NaBH4) < BiOCl < Ag/BiOCl(Dye), clearly suggesting the more efficient interfacial-charge transfer for the Ag/BiOCl(UV) and the serious recombination of photogenerated charges for the Ag/BiOCl(Dye).

A 1.2

B a

800K

b

on

0.9

c

0.6

off

b d

0.3

-Z`` (ohm)

Current density (µA cm-2)

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


a c

d

600K 400K Rt

200K Rs CPE

0

0.0 0

100

200

300

400

0

500

400K

Irradiation time (sec)

800K

Z` (ohm) 29


1200K


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

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Figure 10. (A) Photocurrent responses and (B) EIS Nyquist plots (the right inset represents a simulated circuit diagram) of various photocatalysts: (a) BiOCl, (b) Ag/BiOCl(UV), (c) Ag/BiOCl(Dye), and (d) Ag/BiOCl(NaBH4).

The above results highlight some interesting and important insights for the single-crystal photocatalytic materials. On one hand, for the BiOCl single-crystal nanosheets, it is interesting to find that the electron- and hole-enrichment regions can be coexisted on the dominant exposed {001} facets, where the electron-enrichment region is located on the outer-ring regions of dominant exposed {001} facet while the hole-enrichment region is located on the center region of {001} facets. The above results are

obviously

different

from

the

widely

reported

single-crystal

photocatalysts with various exposed facets, such as TiO2, WO3, and BiVO4.56-59 The real orientation-transfer mechanism of photogenerated electrons and holes onto the different specific regions of identical {001} facet is still unclear. On the other hand, it was found that compared with other deposition locations, the selective deposition of Ag-electron cocatalyst on the electron-enrichment regions of single-crystal photocatalysts showed the highest photocatalytic performance. In fact, according to the mechanism of photocatalytic-deposition method, the noble-metal electron cocatalysts

can

be

spontaneously

and

preferentially

deposited

on

the

electron-enrichment regions even for the usual powdered and film photocatalysts. Therefore, it is quite definite that the photocatalytic-deposition strategy should be one of the most effective methods for the surface modification of noble-metal electron 30


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cocatalysts to improve the photocatalytic efficiency of various photocatalytic materials.

4. CONCLUSION For the BiOCl single-crystal photocatalyst, it is quite interesting to find that the electron- and hole-enrichment regions can be coexisted on the dominant exposed {001} facets, which is obviously different from the widely reported results that the electron- and hole-enrichment regions can only be located on the different exposed facets. Ag nanoparticles were then selectively deposited on the different surface regions of BiOCl single-crystal nanosheets by different methods, including the Ag/BiOCl(UV) with selective Ag-deposition on the electron-enrichment regions (the outer-ring of dominant exposed {001} facet) by usual photocatalytic-deposition method, the Ag/BiOCl(Dye) with selective Ag-deposition on the hole-enrichment regions (lateral surfaces of nanosheets) by present photosensitization-deposition method, and Ag/BiOCl(NaBH4) with random deposition of Ag nanoparticles by a direct reduction method. It was found that the Ag/BiOCl(UV) photocatalysts exhibited the highest photocatalytic performance owing to the excellent synergistic effect of orientation transfer of photogenerated charges in the BiOCl single-crystalline structure and their rapid interfacial catalytic reactions on the Ag-cocatalyst active sites, while the Ag/BiOCl(Dye) displayed the lowest efficiency even lower than the unmodified BiOCl nanosheets, due to the fact that the Ag nanoparticles on the 31



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

hole-enrichment regions can become the recombination centers of photogenerated electrons and holes.

32


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ASSOCIATED CONTENT Supporting Information Additional experimental procedures; supplementary figures S1-S2; supplementary table S1. AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]; Tel: 0086-27-87756662, Fax: 0086-27-87879468

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (51472192, 21477094, and 51672203). This work was also financially supported by the Fundamental Research Funds for the Central Universities (WUT 2017IB002).

33



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Ag-Modified BiOCl Single-Crystal Nanosheets: Dependence of

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