Ultrathin BiOCl Single-Crystalline Nanosheets with Large Reactive

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Energy, Environmental, and Catalysis Applications

Ultrathin BiOCl Single-Crystalline Nanosheets with Large Reactive Facets Area and High Electron Mobility Efficiency: a Superior Candidate for HighPerformance Dye Self-Photosensitization Photocatalytic Fuel Cell (DSPFC) Lei Zhang, Cheng-Gang Niu, Xiu-Fei Zhao, Chao Liang, Hai Guo, and Guangming Zeng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14227 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on November 1, 2018

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ACS Applied Materials & Interfaces

Ultrathin BiOCl Single-Crystalline Nanosheets with Large Reactive Facets Area and High Electron Mobility Efficiency: a Superior

Candidate

for

High-Performance

Dye

Self-Photosensitization Photocatalytic Fuel Cell (DSPFC) Lei Zhang, Cheng-Gang Niu *, Xiu-Fei Zhao, Chao Liang, Hai Guo, Guang-Ming Zeng College of Environmental Science Engineering, Key Laboratory of Environmental Biology Pollution Control, Ministry of Education, Hunan University, Changsha 410082, China *To whom correspondence should be addressed. Tel: +86 731 88823820; E-mail: [email protected]

Keywords:

ultrathin BiOCl, {010} facet, fuel cell, electricity generation,

photosensitization degradation.

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Abstract: Strong dye adsorption and fast electrons transfer are of crucial importance to achieve high conversion efficiency of dye self-photosensitization photocatalytic fuel cell (DSPFC). In this study, we have experimentally achieved the enhanced cell performance in ultrathin BiOCl{010} (BOC(010)-U) nanosheets and provide an idea to investigate the relationship between the physical structure and the chemical performance of semiconductor materials. Experimental phenomenon showed that the exposed areas of highly active {010} facets were remarkable enhanced with the decrease of BiOCl thickness. The large area of {010} facets with abundant active sites and open channel characteristic

were

exposed

to

facilitate

photosensitization

process

and

the

atomically-thin structure was designed to speed up electrons transfer. By employing 40 mL of 5 mg/L RhB as fuel, it was found that the BOC(010)-U photoanode exhibited superior photovoltaic performance and photocatalytic degradation activity than other materials in DSPFC system, whose Jsc and Voc were measured to be 0.00865 mA/cm2 and 0.731 V. Besides, about 72% color removal efficiency and 10.77 % Coulombic efficiency were obtained under visible light irradiation for 240 min. The experimental results and multiple characterizations demonstrated that the strong dye adsorption ability and efficient charge migration were responsible for the sustaining generation of photocurrent and enhancement of pollutants degradation activity.


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1. Introduction Photocatalytic fuel cell (PFC) shows the direct conversion of chemical fuels into electric energy. In addition to possessing a free and unlimited supply of solar energy, the fascination also reflects by its working mechanism that the process of recovering energy is accompanied by the degradation of organic pollutants, which hold huge potential for linking and dealing with the environmental and energy problems

1-5.

To date, the PFCs

based on TiO2 and ZnO photoanodes have obtained impressive conversion efficiencies under UV light

6-8.

However, the low visible light absorption and fast recombination of

photogenerated charge carriers are not favorable for their wide application. In this context, dye self-photosensitization photocatalytic fuel cell (DSPFC) is developed to offset the drawback of PFC

9-11.

Different from the direct photoexcitation process of PFCs, the

generated photoelectrons in DSPFCs is indirectly derived from the excited dye molecules. Because the dye molecules possess strong absorption ability for visible light

12,

the

photoelectrons can be generated easily in DSPFCs. Furthermore, the unique charge carriers transfer process in DSPFCs will significantly reduce the loss of photoinduced electrons, namely, the valence band of the semiconductor not participates in the photochemical reaction 13. It is well known that high-performance DSPFCs need photoanodes with large surface areas for enough dye molecules adsorption and fast electrochemical reaction. In recent years, 2D structural bismuth oxychloride (BiOCl) has been attracted considerable



interest and considered as a candidate for a proper electrode material in DSPFC application because of its unique layered structure and face-dependent photocatalytic performance. On the one hand, an internal static electric field can be generated between the [Bi2O2] slabs and halogen anionic slabs in BiOCl, which are favorable for the migration of photoinduced charge carriers

13-14.

On the other hand, the {010} facet

possessed large surface areas and abundant active sites endows BiOCl with excellent dye self-photosensitization performance

9.

Furthermore, the negative conduction band

position of BiOCl (-1.1 eV) makes the injected photoelectrons with strong reduction capacity, which facilitates the generation of the O2·- active species (E (O2/O2·-) (-0.046 eV)) for pollutants degradation.

15-17.

Therefore, the design and synthesis of BiOCl with

high percentage of {010} may be the effective strategy to achieve high conversion efficiency of DSPFC. However, the {010} facets commonly reduce rapidly in the process of crystal growth due to its high surface energies

18-19.

In this regard, tremendous efforts

have been dedicated to control the growth of {010} facets in the synthesis process of BiOCl single crystal 9. Although the performance of BiOCl photoandes is obviously boosted when more {010} facets are exposed, the great loss of photogenerated electrons during the transfer on the electrode interface still limits its development and wide application. Therefore, further breakthroughs in the design and construction of BiOCl photoanode with a high chemical reaction activity and effective charge transfer hold the key to the development of DSPFC. Elevating charge carriers’ migration rate and shortening diffusion distance may be


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the effective strategy for decreasing the loss of photogenerated electrons and prolonging electrons lifetime. Up to now, many feasible methods have been proposed to improve the transfer efficiency of charge carriers, such as morphology engineering modification

22-23,

metal ions doping

24-26

and heterojunction constructing

20-21,

surface

27-31.

Among

them, the controllable synthesis of BiOCl nanosheets with atomically ultrathin structure can be a reliable strategy. According on the diffusion equation of t = d2/k2D (d is the particle size, k is a constant, D is the diffusion coefficient of charge carriers)

32,

the

ultrathin 2D structure can help BiOCl to achieve rapid photoelectrons transport in photoelectrodes owing to the shortened diffusion distance. Meanwhile, the ultrathin 2D configuration with large interlayer space, ultrahigh surface area and tunable electronic properties allows for intimate contact between the photoanodes and electrolyte, which is beneficial to the fast interfacial electron transfer and electrochemical reaction as well as low corrosion rates 33. Besides, when the thickness of BiOCl is decreased to atomic scale, the buried internal oxygen atoms of [Bi2O2] can easily escape to form oxygen vacancies. The negatively charged oxygen vacancy provides more active centers for dye molecules adsorption and photogenerated electrons migration

34-36.

Inspired by the principle

mentioned above, it is highly desirable to synthesize ultrathin structural BiOCl with high percentage of {010} crystal plane in efforts to obtain high-performance DSPFCs. Herein, the ultrathin BiOCl nanosheets enclosed with fully dominant {010} facets were successfully fabricated via a facile hydrothermal process in the mannitol solution containing PVP under the condition of pH 6.0. In order to prove the influence of physical



structure and surface property on the DSPFC performance, two other BiOCl(010) samples (BOC(010)) with different thickness were prepared for comparison with the ultrathin BiOCl(010) nanosheets. The detailed structural information about BOC(010) samples were clearly observed from HRTEM and AFM images. And, the photosensitive properties of the resulting BOC(010) samples were assessed by the the RhB removal under visible light irradiation. X-ray photoelectron spectroscopy (XPS) and electron spin trapping resonance (ESR) were conducted to measure the surface defects of the BOC(010) samples. Photoluminescence spectrum (PL), time-resolved fluorescence spectroscopy (TRPL), electrochemical impedance spectroscopy (EIS) were performed to investigate the photogenerated electrons transfer behavior on the BOC(010) surface. Commonly used dye, rhodamine B (RhB) was used as fuel to investigate the DSPFCs properties on electricity generation and dye degradation. Additionally, the Coulombic efficiency (Ec) was proposed to assess the energy conversion between the available fuel and electrical current.

2. Experimental section 2.1. Materials Bismuth nitrate pentahydrate (Bi(NO3)3∙5H2O), Sodium chloride(NaCl), Mannitol (C6H14O6), PVP-30K, Sodium hydroxide (NaOH), Rhodamine B (RhB), Sodium sulfate (Na2SO4). All chemicals used were purchased from Sinopharm Chemical Reagent Co., Ltd and were in analytical reagent grade without further purification.


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2.2 Preparation of Photocatalysts (1) The BiOCl (010) nanosheets were obtained via a facile hydrothermal reaction. 0.4851 g of Bi(NO3)3∙5H2O solids was dispersed in 25 mL deionized water with continuous ultrasonic. Then, 5 mL saturated sodium chloride solution was slowly added into above solution. The pH value mixture solution was adjusted to 6.0 by addition of 2 mM NaOH. The mixture was continuously stirred for 10 min at room temperature in air, and then poured into a Teflon-lined autoclave with a reaction at 160 °C for 3 h. When the reaction solution air cooled to room temperature, the resulting white products were collected and washed with deionized water and ethanol by filtering to remove the residual ions. The final product (BOC(010)-S) was dried at 80 °C overnight for further use. (2) The BiOCl(010) nanoplates were synthesized by a facile hydrothermal process according to the previous report 37. 0.4851 g of Bi(NO3)3∙5H2O solids were added slowly into 25 mL of 0.1 M mannitol solution containing 5 mL saturated sodium chloride solution. The pH value of mixture solution was adjusted to 6.0 by addition of 2 mM NaOH. The mixture was continuously stirred for 10 min at room temperature in air and poured into a Teflon-lined autoclave with a reaction at 160 °C for 3 h, and then air cooled to room temperature. The resulting white products were collected and washed with deionized water and ethanol to remove the residual ions. Finally, the BOC(010)-P samples were obtained after drying at 80 °C overnight. (3) The ultrathin BiOCl(010) nanosheets were prepared via PVP-assisted hydrothermal process. 0.4851 g of Bi(NO3)3∙5H2O solids were added slowly into 25 mL



of 0.1 M mannitol solution containing 0.4 g PVP. With the addition of 5 mL saturated sodium chloride solution, the white precipitates were formed immediately. Then, the pH value of mixture solution was adjusted to 6.0 by adding of NaOH (2 mM). After reacting at 160 °C for 3 h, the resulting faint yellow products were collected and washed with ethanol and water by centrifugation. Finally, the BOC(010)-U samples were obtained after drying at 80 °C overnight. For comparison, the ultrathin BOC(001) nanosheets (BOC(001)-U) were fabricated under the same condition without the addition of NaOH. 2.3. Characterization X-ray diffraction patterns (XRD) were recorded on a Rigaku D/max 2500 X-ray diffractometer with Cu Kα radiation (λ= 0.15406 nm). The morphologies of the samples were obtained by using a Hitachi S-4800 field-emission scanning electron microscope. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were performed on Tecnai G2 F20. Tapping-mode atomic force microscopy (AFM) images were recorded with a DI Innova Multimode SPM platform. UV-vis diffuse reflection spectrum (DRS) was collected with a UV-vis-NIR spectrophotometer (Hitachi U-4100) using an integrating-sphere accessory from 200 to 800 nm. The pure BaSO4 was used as the reflectance standard material. The specific surface areas (BET) were conducted on an ASAP-2020 accelerated surface area and porosimetry analyzer through the nitrogen adsorption-desorption isotherms at 77 K. X-ray photoelectron spectroscopy (XPS) spectra was recorded with the ESCALAB 250 photoelectron spectrometer by using Al Kα radiation source. The photoluminescence


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spectrum (PL) was conducted on a FluoroMax-4 Spetrofluorometer (HORIBA) under λex = 290 nm. Electron spin trapping resonance (ESR) spectra were performed on a Brucker A300 ESR spectrometer at room temperature. Time-resolved photoluminescence (TRPL) decay spectra were obtained on a FLS 980 fluorescence spectrometer (Edinburgh). 2.4. Photocatalytic Measurements The photosensitive performance of the resulting samples were evaluated by photocatalytic degradation of RhB under visible light irradiation by using a 300 W Xe lamp (CEL-HXF300, Beijing) with a cutoff filter (λ> 420 nm). The light intensity was measured to be 100 mW/cm2. Briefly, a 0.02 g of catalyst was added into 100 mL 40 mg L−1 RhB solution. The mixture solution was stirred in darkness for 1 h to ensure desorption-adsorption equilibrium before light irradiation. Each 10 min later, two milliliters of reaction solution was taken out and centrifuged. Subsequently, the concentration of RhB was recorded with an UV-Vis spectrophotometer (Shimadzu 2550, Japan). 2.5. Photoelectrochemical Measurements The photoelectric property characterization of BiOCl electrodes was carried out by three electrodes system using a CHI660D workstation. The Ag/AgCl electrode was used as reference electrode. platinum foil plate was employed as counter electrode. BiOCl/FTO was used as the working electrode. The preparation process of photoanode was as following

38.

100 mg as-prepared powder was suspended in 2 mL ethanol by

magnetic stirring, subsequently, 0.86 mL terpineol and 1.0 mL ethyl celluloses in ethanol



Page 10 of 47

(10 wt%) were added into the mixture followed by stirring and sonication. 200 μL of resulted paste was applied onto a FTO substrates with the available surface area of 1.0 cm2 by spin coating method. Finally, the electrodes were annealed at 450 °C in air for 30 min. A 300 W Xe arc lamp (CEL-HXF300, Beijing) with a 420 nm cutoff filter was utilized as visible light source. The light intensity was measured to be 100 mW/cm2. The supporting electrolyte was consisted of Na2SO4 (0.05 mol/L). The photocurrent response (I-T), electrochemical impedance spectroscopy (EIS) and current density-voltage (J-V) were measured to evaluate the DSPFC property assembled with BiOCl. 40 mL of 5 mg/L RhB dye solution was used as fuel during investigation. The initial pH of solution was 2.0 in the whole experiment if not mentioned. Before light irradiation, the photoanode was vertically immersed in dye solution with stirring continuously for 30 min.

3. Results and Discussion 3.1. Structure and morphology characteristics In this study, the BiOCl crystals with dominant {010} facets are successfully synthesized via controlling the pH of initial solution to 6.0

13.

The XRD patterns of the

resulting samples were conducted to investigate their crystal structure and the related results were clearly shown in Fig. 1(a). It can be seen that all of the diffraction peaks are in line with the BiOCl tetragonal crystals (JCPDS NO: 06-0249)

39.

And no any other

phases are observed on all the BiOCl samples, indicating a high crystallinity of the as-prepared products. The (110) peaks, as the characteristic diffraction peak of {010}


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facets, are sharp and strong in all samples, suggested that the obtained BiOCl samples mainly enclose with dominant {010} facets. Among all samples, BOC(010)-U possesses the largest percentage areas of {010} facet exposed, which can be reflected by the specific values of the I(110)/I(001) (Fig. 1(b)). Besides, the main diffraction peaks of {001} facets in BOC(010)-U samples nearly disappear, such as (001), (002), (003) and (112) faces, further demonstrating the large areas of {010} facets exposed. Meanwhile, it is found that the 2θ of (110) diffraction peak in BOC(010)-U sample is shifted to a low-angle of 31.85o, compared with the standard angle (32.50o). This phenomenon may be attributed to the enhanced surface tension as the thickness of BiOCl reduces to few-layer

40-41.

For comparison, the ultrathin BiOCl with dominant {001} facets was

obtained under the condition of pH=1.0. As depicted in Fig S1(a), the large percentage areas of {001} facets exposed is achieved and the specific value of the I(110)/I(001) in BOC(001)-U samples is much smaller than BOC(010) samples. The XRD result demonstrates that BiOCl crystals with dominant {010} facets are synthesized successfully. And it is notable that the areas of {010} facets increase with the decrease of thickness, which due to the reduction of surface energy. The {010} facets provide large specific surface areas for dye molecules anchoring and open channel for photogenerated electrons transfer, which are beneficial to enhance the photosensitization performance of BiOCl.



Fig. 1 XRD patterns of the prepared BOC(010) samples (a), and the specific values of (110) and (001) peak (b).

The morphology and detailed interior structure information about BiOCl samples were investigated by the high-resolution TEM images. As shown in Fig. 2, all prepared samples exhibit 2D nanosheet shape due to their intrinsic layered structure consisting of [Bi2O2]2+ layers sandwiched between two Cl atoms

14.

When water is used as reaction

solvent, the large-sized BiOCl nanosheets are obtained, whose thickness is estimated to be 200 nm (Fig 2(a)). Because the hydrolytic process of Bi(NO3)3 will accelerate the growth of BiOCl crystals, the bulk BiOCl nanosheets are generated in water. Fig 2(b) shows that the well-defined square-like BiOCl nanoplates are obtained with the addition of mannitol. Compared to BiOCl prepared in water, the BiOCl obtained in mannitol aqueous solution is thinner, whose thickness is estimated to be 27.9 nm. The formation of square-like BiCl nanoplates with thin structure is primarily attributed to two aspects: Firstly, Bi3+ ions prefer to absorb on oxygen terminated mannitol molecule to form a


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relatively stable complex, which lead to the decrease of BiOCl growth speed. secondly, mannitol worked as a directing agent will restrict the intrinsic anisotropic growth of BiOCl nuclei owing to the long chain and polyhydroxyl of mannitol molecules 37. When PVP is added into mannitol aqueous solution, the ultrathin BiOCl nanosheets with small size are obtained (Fig. 2(c)). More accurate information about the thickness of BOC(010)-U was obtained from the AFM image and the corresponding height histogram of nanosheets was presented in Fig 3(a). The thickness of these nanosheets is estimated to be 3.48 nm. Because the c parameter of BiOCl is 0.737 nm, therefore, each BiOCl nanosheet with a thickness is stacked together by nearly 5 [Cl-Bi-O-Bi-Cl] units

42.

All

specification of samples is displayed in Table 1. Fig. 2(d~f) shows the HRTEM images of the prepared samples. It can be clearly observed that all samples exhibit clear lattice fringes, indicating their good crystalline natures. The continuous lattice spacing of 0.37 nm matches well with the (002) lattice planes of the tetragonal BiOCl. The result indicates that these BiOCl nanosheets have similar crystal structure and enclose with dominant {010} facets. Compared with BOC(001)-U, the BOC(010)-U nanosheets are smaller (Fig. S1(b,c)). The decrease of size is mainly due to the growth of highly active {010} facets

[9].

With the decrease of BiOCl thickness, the diffusion distance of

photogenerated charge carriers will be shortened, which is beneficial for prolonging the electrons lifetime and enhancing the photocatalytic activity 13. Moreover, the VBi‴VO••VBi‴ vacancy will be produced as the thickness of BiOCl reduces to atomic scale, which can provide more active sites for dye molecules adsorption and photosensitive degradation 34.



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Fig. 2 HRTEM images of the BOC(010)-S (a,d), BOC(010)-P (b,e), and BOC(010)-U (c,f).

Table 1. Physical parameter and photosensitive degradation performance of the BiOCl samples

Catalyst

Ha (nm)

Lb (nm)

SBET (m2 g-1)

Kc (10-2 min-1)

Kd (10-2 g min-1 m-2)

BOC(010)-S

200

795

9.826

1.997

0.203

BOC(010)-P

27.9

138

13.154

2.712

0.206

BOC(010)-U

3.48

35

30.008

6.339

0.211

BOC(001)-U

5.16

49

17.302

2.033

0.118

aApproximate

thickness (nm). bDiameter of BOC(010) samples from AFM and TEM


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images (nm). cThe photosensitive degradation rate constant. dNormalized degradation rate constant of RhB.

3.2. Optical properties The optical absorption performance of the obtained BiOCl samples was reflected by the diffuse reflectance UV-vis absorbance spectra (DRS). As seen from Fig. 3(b), BOC(010)-S displays no absorption in visible region owing to its band gap of 3.34 eV. But, with the decrease of thickness, the BOC(010)-P and BOC(010)-U all have a small amount of absorption for visible light. And their absorption bands show a red-shift at the bottom portion, implying the narrowed band-gap

43.

It is well known that the optical

properties of semiconductor nanomaterials are greatly influenced by its structure, morphology and size

44-46.

When the size of semiconductor photocatalysts are reduced, a

significant shift in position of the band edge of the materials will occur, which may cause the strong absorption of visible light

47-49.

Moreover, once the thickness of BiOCl is

reduced to atomic scale, the oxygen atoms will easily escape from the lattice oxygen in [Bi2O2] units to form oxygen vacancy. It is clear that a weak absorption tail is appeared in the visible absorption spectrum of BOC(010)-U nanosheet, which is ascribed to the absorption of surface oxygen vacancies 50. The oxygen vacancy can not only function as light receptors to enhance light absorption ability but also introduce a new impurity levels in the valence band of BiOCl, resulted in the red-shift of the band gap. Therefore, BOC(010)-U exhibits the highest absorption intensity for visible light among all samples,



which is primarily attributed to its small-sized nanostructure and the existence of surface oxygen vacancies. As a crystalline semiconductor, the optical absorption near the band edge bases on the conversion formula: αhv = A(hv - Eg)1/2, where α, A, v, and Eg are the absorption coefficient, a constant, light frequency and band gap energy, respectively [20].

The band gap of BOC(010)-U is estimated to be 3.01 eV, which is smaller than that

of BOC(010)-S and BOC(010)-P (Fig. 3(c)). The DRS result indicates that the absorption ability for visible light of BiOCl(010) samples is significantly increased with the thickness decreased owing to the phonon confinement effects and the generation of surface oxygen vacancies. In order to demonstrate the existence of oxygen vacancy more clearly, the ESR spectra of prepared BiOCl samples were conducted. As seen from Fig. 3(d), no obvious ESR signal can be observed from BOC(010)-S. But, on the contrary, a remarkable ESR signals at g=2.001 appear in BOC(010)-P and BOC(010)-U, indicating the existence of oxygen vacancies

51.

A much more stronger ESR signal is detected in

BOC(010)-U than that of BOC(010)-P samples, implying that ultrathin BiOCl surface possesses higher concentration of oxygen vacancy. The oxygen vacancy can not only enhance visible light harvesting but also function as electron trapping centers to accelerate the migration of photoinduced charge carriers, which facilitate the photosensitization degradation process.


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Fig. 3 Atomic force microscopic image of BOC(010)-U (a), UV/vis diffuse reflection absorbance spectra (b), plots of (αhν)1/2 versus photon energy (hν) (c), and ESR spectra of the BOC(010) samples (d).

3.3. Surface chemical composition analysis X-ray photoelectron spectroscopic (XPS) characterizations were performed to reveal the chemical composition of the obtained samples. The survey spectra of BOC(010)-S, BOC(010)-P, and BOC(010)-U are depicted in Fig. S2. As seen, all samples contain Bi, O, and Cl elements, indicating that they are pure BiOCl crystal. Fig. 4(a) shows the high



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resolution Bi 4f XPS spectra of the BiOCl(010) samples. Two asymmetric Bi4f peaks with the gap of 5.3 eV are observed in all samples, which belong to Bi (4f7/2) and Bi (4f5/2) of Bi3+ photoelectron emissions, respectively 52. However, the peaks of Bi (4f7/2) and Bi (4f5/2) shift to lower binding energy position with the decrease of BiOCl thickness, implying that the chemical environment of Bi core has changed 42. This phenomenon can be ascribed to the presence of oxygen vacancies. When the thickness reduces to atomic scale, the predominant defects of BiOCl surface will change from VBi‴ to VBi‴VO••VBi‴. The negatively charged VBi‴VO••VBi‴ results in the shift of binding energy position of Bi core. The result also can be proved by O 1s spectra. As shown in Fig. 4(b), the binding energies of O (1s) located at 530.12 eV and 529.97 eV are detected in BOC(010)-S and BOC(010)-P, respectively. But, for BiOCl(010)-U sample, the O (1s) can be deconvolved into two peaks. The peak located at 529.32 eV is belonged to the oxygen atoms of BiOCl. And, the peak with high binding energy at 531.02 eV is assigned to the hydroxyl groups, chemisorbed oxygen and organic oxygen on the BiOCl surface. It was reported that this peak is developed with the increase of oxygen vacancy defects

53-54.

Therefore, the XPS

result further demonstrates that the surface of BOC(010)-U is exposed with plentiful oxygen vacancies, which can facilitate the photosensitization process.


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Fig. 4 XPS of BOC(010) samples. Bi 4f (a), and O 1s (b).

3.4. Photosensitization performance for RhB degradation The photosensitization performance of the as-obtained BiOCl(010) samples was evaluated by the photocatalytic degradation of RhB under visible light irradiation. It is well known that the photosensitization degradation process generally involves initial adsorption of dye molecules, photoexcitation of the dye molecules, transfer of photogenerated electrons on the conduction band of the semiconductor, and the generation of active species for pollutant degradation

13, 55.

Fig. S3 shows the adsorption

curves of RhB over the resulting samples. Obviously, the RhB adsorption efficiency of BOC(010)-U is the highest in all samples under the condition of continuous darkness for 60 min, and the adsorption capacity decreases with the increase of BiOCl thickness. This phenomenon can be ascribed to the following two aspects. On the one hand, the BOC(010)-U nanosheets with maximum areas of {010} facets have large surface areas, which can provide more active sites for the RhB molecules adsorption. Nitrogen



adsorption-desorption isotherm curves were conducted to demonstrate the large specific surface area of the prepared BiOCl samples. As clearly shown in Fig. 5(a), the BET of BOC(010)-U reaches up to 30.008 m2/g, which is much larger than that of BOC(010)-S and BOC(010)-P. On the other hand, the generation of negatively charged oxygen vacancies on the BOC(010)-U surface can facilitate the adsorption of cationic dye molecules. The above results indicate that the strong adsorption capacity of RhB by BOC(010)-U can be ascribed not only to the its large surface area but also to the existence of oxygen vacancy, which will generate more active species for pollutant degradation. The photosensitive degradation curves of RhB over BOC(010) samples were showed in Fig. 5(b). It can be observed that the degradation efficiencies of RhB enhance with the decrease of BiOCl thickness. Under visible light irradiation, RhB molecules adsorbed on BiOCl surface are excited to generate photoelectrons. Then, the photoelectrons will be transferred to the conduction band of BiOCl and captured by dissolved oxygen molecules

56.

Since the potential of BiOCl conduction band is much

negative than E (O2/O2·-) (-0.046 eV), the O2·- active species can be generated for RhB degradation. Fig. S4 shows valence-band XPS spectra of the prepared BOC(010) samples. It can be seen that the VB position of BOC(010)-S, BOC(010)-P and BOC(010)-U is estimated to be 2.36, 2.08 and 1.67 eV, respectively. Based on the optical absorption spectrum, their CB minimum energy will occur at about -0.98, -1.11, and -1.31 eV, respectively. Obviously, the CB minimum of BiOCl is up-shift with the decrease of


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Page 21 of 47 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


thickness. The elevation of the conduction band position can make the photoinduced electrons with strong reduction potential, which is beneficial for reacting with O2 molecules to produce active species for pollutant removal. Beside, more photoelectrons can be provided on the surface of BOC(010)-U nanosheets owing to its strong RhB adsorption capacity. As a result, the BOC(010)-U nanosheets show the highest degradation efficiency among all samples, which can degrade almost all RhB in 30 min. The inset of Fig. 5(b) is the RhB degradation curves over BOC(010)-U and BOC(001)-U. Although

BOC(001)-U

possesses

higher

oxygen

vacancy

concentration

than

BOC(010)-U (Fig. S5), the RhB degradation rate of BOC(010)-U is faster. The result indicates that the photosensitive degradation process of BiOCl is strongly related with the {010} facet which presents large surface areas and open channel characteristic. In order to describe the photosensitization degradation performance more accurately, the initial reaction rates of RhB degradation were conducted. As shown in Fig. 5(c), the reaction rate constant (k) on RhB degradation increases with the decrease of BiOCl thickness. But, after normalization by the surface area, the k value (k′) of BOC(010)-U obviously decreases, indicating the great effect of surface area on photosensitization process. The detailed dates are presented in Table 1. Fig. 5(d) shows the influences of pH value on RhB photosensitization degradation. In the pH range from 2.0 to 6.0, the degradation efficiencies of RhB over BOC(010)-U all reach up to 100% within 30 min. However, as pH increases to 10.0, the RhB degradation efficiency significantly decreases, while only 20% RhB can be removed at the same time. For the reason that RhB molecules prefer to



form a macromolecule structure by aggregating with each other at the high pH value 57, and these macromolecular compounds are not easily adsorbed on the BOC(010)-U surface (Fig. S6). Besides, the surface charge of nanomaterials is of crucial importance to investigate the interaction between catalysts and pollutants 58-59. As shown in Fig. S7, the point of zero charge of BOC(010)-U is determined to be pH 6.51. So, when the pH value of RhB solution is above 6.51, the adsorption process of RhB molecules will be inhibited due to the positively charged BOC(010)-U surface. As a result, the RhB photosensitization degradation process is significantly hindered at pH 10.0.

Fig. 5 Nitrogen adsorption-desorption isotherm curves of BOC(010) samples (a), the RhB photosensitive degradation (b), initial reaction rate constant and normalized reaction rate


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constant of RhB (c), and the effects of pH on RhB photosensitive degradation over BOC(010)-U nanosheets (d).

3.5. Charge kinetics Photoluminescence (PL) measurement was conducted to investigate the transfer efficiency of photoinduced electrons which is a key influence factor on photosensitization degradation process. The PL spectra of BOC(010) samples are obtained under the excitation wavelength of 290 nm. As seen from Fig. 6(a), the PL intensity decreases with the decrease of the BiOCl nanosheets thickness, indicating the longer lifetime of the photogenerated electrons with the higher separation rate

23.

This result is mainly due to

the shortened diffusion distance of charge carriers. Moreover, the oxygen vacancies as electron trapping centers can enhance the transfer efficiency of photoinduced electrons. In order to reveal the effective transfer of photoelectrons intuitively, time-resolved photoluminescence (TRPL) decay spectra were performed. As seen from Fig. 6(b), it is clearly seen that all of the TRPL spectra display a multi-exponential decay process. And their decay dynamics can be obtained through fitting to a biexponential function 60: I(t) = A1exp ( - t/τ1) + A2exp ( - t/τ2)

(1)

where I(t) is the PL intensity, A1 and A2 represent the amplitude (or weighting factors). τ1and τ2 are the corresponding lifetime. The average lifetime (τm) is often used to assess the charge carrier separation efficiency and can be calculated according to the following equation:



τm =

Page 24 of 47

∑Aiτ2i

(2)

∑A1τi

The short lifetime component (τ1) originates from the nonradiative recombination of the photogenerated electrons with the surface defects. And, the long lifetime component (τ2) is derived from the interband recombination of the free-excitons

61.

The experimental

decay profiles of BOC(010) samples are listed in Table. S1. For BOC(010)-U, the two exponential constants (τ1 and τ2) are 2.04 ns and 0.67 ns, which are longer than that of BOC(010)-P (τ1 = 1.67 ns and τ2 = 0.59 ns) and BOC(010)-S (τ1 = 1.44 ns and τ2 = 0.46 ns). Therefore, BOC(010)-U nanosheets possess a longer average lifetime (τm = 0.93 ns ) than that of BOC(010)-P (τm = 0.68 ns) and BOC(010)-S (τm = 0.51 ns). The reduced PL intensity and increased average lifetime demonstrate the effective separation and transfer of photogenerated electrons in the thin BiOCl nanosheets, especially in the ultrathin BiOCl material. The transient photocurrent signals were collected to analyze the generation and migration of photogenerated charges on the surface of BOC(010) samples during the photosensitization process. As shown in Fig. 6(c), no photocurrent signal is detected on the BOC(010)-U electrode, oppositely, a high photocurrent signals appear on the BOC(010)/RhB electrode under visible light irradiation, indicating the excellent photosensitization performance of BOC(010) samples. Moreover, the photocurrent intensity enhances with the decrease of BOC(010) nanosheets thickness. The increased photocurrent signal suggests vast photoelectrons generation and effective transfer. Owing to the large surface areas and more active sites, BOC(010)-U nanosheets can adsorb a


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mass of RhB molecules. Since photoelectrons are derived from the excited RhB molecules, a higher concentration of photogenerated electrons is thus obtained on BOC(010)-U/RhB electrode than that of other electrodes. Beside, abundant surface oxygen vacancies and short diffusion distance speed up the transfer of photoelectron and prolong photoelectrons lifetime. As a result, the highest photocurrent signal is achieved on BOC(010)-U/RhB electrode among all BOC(010) samples, suggesting that high electricity density will be generated and more active species will be formed for pollutants degradation. Electrochemical impedance spectra (EIS) was employed to investigate the transfer efficiency of photoelectrons on the surface of BOC(010) photoanodes. Commonly, the small arc radius indicates the highly efficient electrons transfer

44, 62.

From the Fig. 6(d), it can be seen that the arc radius of BOC(010) photoanodes decrease with the reduction of BiOCl thickness. Owing to the unique layered structure of BiOCl, an internal electric field can be formed between the [Bi2O2] slabs and the double slabs of halogen atoms, which is parallel to the {010} facets of BiOCl nanosheets. Therefore, the fastest electrons transfer rate is achieved on the surface of BOC(010)-U. The above results provide strong evidence that the outstanding photosensitization degradation activity of BOC(010)-U is ascribed to the effective transfer of photoinduced charges.



Page 26 of 47

Fig. 6 PL spectra (a), TRPL decay spectra (b), transient photocurrent responses (c), and EIS Nyquist plots of the prepared BOC(010) samples (d).

3.6. DSPFC application The dye self-photosensitization photocatalytic fuel cell systems (DSPFCs), including a BOC(010)/FTO photoanode, a Pt cathode, a Ag/AgCl reference electrode and RhB solutions, were constructed to further investigate the photosensitization performance of BOC(010) samples. In the DSPFC system, the electrons can be generated on BiOCl photoanode by using the dye molecules as fuel and transferred to a cathode through an external circuit, resulted in the generation of electricity

10.

Fig. 7 shows the

current-voltage curves (J-V) and current-power curves (J-JV) of DSPFCs based on


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different photoanodes. The open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factors (ff) of the cell are listed in Table 2. The fill factor can be calculated by the following formula 63: Pmax

FF = Jsc × Voc =

Jmax × Vmax

(3)

Jsc × Voc

Where Jmax × Vmax is the maximum power of BiOCl−Pt cell obtained from the J-JV curve. Jsc × Voc is the maximum power density of cell. A fuel cell performance is directly reflected by its fill factor

[9].

As shown in Fig. 7(a), Jsc of cell is 0.0011 mA/cm2 in the

absence of RhB. After the addition of 5 mg/L RhB, it increases from 0.00261, 0.00665 to 0.00865 mA/cm2 with the decrease of BiOCl thickness. Because the charge-transfer process at the electrode/electrolyte interface is connected with the Jsc of cell, the high Jsc value obtained on BOC(010)-U indicates its high concentration of photoelectrons generation and effective transfer. Besides, a comparison of the DSPFCs based on BOC(010), BOC(001)-U, P25 and SnO2 photoanodes is conducted. P25 and SnO2 are widely used as electrode material to assemble dye-sensitized solar cell (DDSC) owing to its excellent photosensitization performance

64-65.

Under the same experimental

conditions, it is clearly seen that a higher photovoltaic performance is achieved on BOC(010)-U photoanode than others, suggesting its huge potential on the application of DDSC. The performance of DSSC devices assembled with the BOC(010) photoanodes will be investigated in our further work. High-performance DSPFCs require photoanodes with high surface area for sufficient dye adsorption and open channel for photogenerated electrons transfer. The BOC(010)-U nanosheets possessed high percentage areas of



Page 28 of 47

{010} facets can provide large surface area and more active sites for dye anchoring. Furthermore, the unique layered ultrathin structure of BOC(010)-U facilitates the electrons efficient transfer. The output power of cell was evaluated by the current-power plots (J-JV). As displayed in Fig. 7(b), the highest actual power of cell is obtained on BOC(010)-U photoanode among all samples, further demonstrated its excellent photovoltaic performance. Table 2. Current-Voltage Characteristics of the BiOCl-Pt Cell with Different Photoanodes under Visible Light Irradiationa

aC

Catalyst

Jsc (mA/cm2)

Voc (V)

FF

No fuel

0.0011

0.114

0.24

P25

0.00151

0.42

0.42

SnO2

8.88E-4

0.512

0.32

BOC(010)-S

0.00261

0.468

0.34

BOC(010)-P

0.00665

0.661

0.17

BOC(010)-U

0.00865

0.731

0.17

BOC(001)-U

0.00391

0.453

0.30

RhB

= 10 mg/L, [Na2SO4] = 0.05 mol/L, pH 2.0.


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Fig. 7 Current-voltage (J-V) (a) and power-voltage (J-JV) (b) curves for DSPFCs with different BiOCl photoanodes under visible light irradiation: CRhB = 10 mg/L, 0.05 M Na2SO4 solution, pH = 2.0.

The RhB removal efficiency is another important indicator to estimate the performance of cell. In DSPFC system, the generated photoelectrons will be transferred to cathode and react with dissolved molecular oxygen to form active superoxide radicals for dye degradation, which simultaneously achieves the energy recovery and pollutant decomposition. Fig. 8(a) shows the RhB removal efficiencies (evaluated by color) by using different BOC(010) photoanodes under visible light irradiation. It is clearly seen that approximately 72% of RhB (40 mL, 5 mg/L) is removed within 240 min by photodegradation using BOC(010)-U photoanode and the removal efficiencies decrease with the increase of BiOCl thickness. The high adsorption ability for dye molecules and effective photoelectrons transfer are responsible for substantially enhanced RhB removal efficiency of BOC(010)-U photoanode. Owing to the large surface area and the presence of oxygen vacancy, more RhB molecules can be adsorbed on the BOC(010)-U surface,



Page 30 of 47

which can generate a mass of photoelectrons under light irradiation. Besides, rapid electrons transfer on the BOC(010)-U surface is achieved, thus the loss of electrons is reduced and the electrons lifetime is prolonged significantly. As a result, more active species can be generated for pollutants degradation. The comparisons of photocatalytic activity on RhB removal over different photoanodes were conducted. As seen from Fig. S8, the RhB degradation efficiencies on BOC (010) photoanodes are much higher than that of TiO2 and SnO2 photoanodes under the same conditions, indicating its excellent cell performance. Moreover, compared with the BOC(001)-U photoanode, the RhB removal efficiency of BOC(010)-U is higher, which further demonstrates the leading role of {010} facets on DSPFCs performance. In order to accurately evaluate the performance of a fuel cell, the Coulombic efficiency, Ec, is employed as an energy conversion indicator in this study. Ec is the ratio of total Coulombs to maximum possible Coulombs if all substrate removal produced current

66.

For dye wastewater, the chemical oxygen

demand (COD) is a more accurate indicator to describe how much of the dye is converted to electricity than using color removal. Mathematically, the Ec of the proposed DSPFC can be calculated as following formula 10: t

Ec =

M∫0 I dt

(4)

FbV∆COD

where M presents the molecular weight of oxygen. F is Faraday constant. b equals 4 (the number of electrons released per mole of oxygen). V is the volume of reaction solution. And ∆COD is the change of COD value after reaction time t. In this work, 40 mL of 5 mg/L RhB was employd as fuel and removed in the BOC(010)-Pt DSPFC with the


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reaction time of 4 h. According to the result of current-time curve shown in Fig. 8(b), the total Coulombs actually transferred in BOC(010)-S, BOC(010)-P, and BOC(010)-U DSPFC are calculated to be 0.0679, 0.20 and 0.421 C, respectively. The COD values of RhB solution before and after visible light irradiation are displayed in the inset of Fig. 8(b). According to the above formula, the resulting Ec values are 8.3%, 7.82% and 10.77%, respectively. Obviously, the BOC(010)-U photoanode possesses the highest Ec value among all DSPFCs, indicated the efficient energy conversion. Because of the strong RhB adsorption ability and efficient electrons transfer of BOC(010)-U photoanode, more photoinduced electrons can be generated and transferred to achieve high electricity density and RhB removal. The four repetitive degradation reaction processes were conducted to investigate the recyclability and stability performance of BOC(010)-U photoanode in DSPFC. As shown in Fig. S9(a), after four repeated experiments, the BOC(010)-U still exhibits relatively high RhB removal efficiency. And, there is no significant difference on crystalline structure of BOC(010)-U samples before and after light irradiation (Fig. S9(b)). The result indicates the BOC(010)-U photoanodes has good photocatalytic stability in the application of DSPFCs.



Fig. 8 RhB color removal efficiency (a) and current-time plot of DSPFC assembled with different BOC(010) photoanodes under visible light irradiation for 4 h. Inset is the COD removal of RhB (b).

4. Conclusion In summary, the ultrathin BiOCl nanosheets with fully exposed {010} facets were synthesized via a simple hydrothermal process in mannitol solution with the assist of PVP. The resulting BOC(010)-U possessed large surface area, atomic layer thickness and abundant surface oxygen vacancy, which is crucial for obtaining high-performance DSPFC. By employing 40 mL of 5 mg/L RhB as fuel, the Jsc and Voc of BOC(010)-U photoanode can reach up to 0.00865 mA/cm2 and 0.731 V. And, about 72% color removal efficiency of RhB and 10.77 % Coulombic efficiency were obtained under visible light irradiation for 240 min. The strong dye adsorption ability and fast electrons transfer are responsible for the enhanced DSPFC performance. The {010} facets with large surface area and open channel characteristic possess huge advantages on RhB adsorption, thus more photoelectrons can be generated. And, as the thickness of BiOCl


Page 32 of 47

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nanosheets reduces to atomic scale, the diffusion distance of photoelectrons is significantly shortened, resulted in the fast migration efficiency and long lifetime of electrons. Moreover, the presence of surface oxygen vacancies acted as active sites facilitates photosensitization process by enhancing dye adsorption and speeding up electrons transfer. Our works may provide a new idea for deep understanding the relationship between the surface structure and the chemical performance of semiconductor photocatalytic materials.

Associated Information Supporting Information XRD pattern and HRTEM images of the BOC(001)-U sample; XPS survey spectra; adsorption curves of RhB; Valence-band XPS spectra of the BOC(010) samples; ESR spectra of the BOC(010)-U and BOC(001)-U; Adsorption curves of RhB under different pH value over the BOC(010)-U samples; Zeta potential of BOC(010)-U under different pH conditions; RhB photocatalytic degradation curves over different photoanodes; Cycling curve of RhB degradation for DSPFCs and XRD patterns of BOC(010)-U before and after visible light irradiation; The fitted parameters of the TRPL decay profiles.

Notes The authors declare no competing financial interest.



Page 34 of 47

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (51541801, 51521006, 21507034), the Key R & D project in Hunan (2018SK2048).

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Graphic for TOC only


Ultrathin BiOCl Single-Crystalline Nanosheets with Large Reactive

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