BiOI heterojunction

Journal Pre-proof Highly sensitive detection of chromium (VI) by photoelectrochemical sensor under visible light based on Bi SPR-promoted BiPO4 /BiOI heterojunction Mengyin Li, Guangxue Zhang, Chuanqi Feng, Huimin Wu, He Mei

PII:

S0925-4005(19)31648-X

DOI:

https://doi.org/10.1016/j.snb.2019.127449

Reference:

SNB 127449

To appear in:

Sensors and Actuators: B. Chemical

Received Date:

31 July 2019

Revised Date:

15 November 2019

Accepted Date:

20 November 2019

Please cite this article as: Li M, Zhang G, Feng C, Wu H, Mei H, Highly sensitive detection of chromium (VI) by photoelectrochemical sensor under visible light based on Bi SPR-promoted BiPO4 /BiOI heterojunction, Sensors and Actuators: B. Chemical (2019), doi: https://doi.org/10.1016/j.snb.2019.127449

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Highly sensitive detection of chromium (VI) by photoelectrochemical sensor under visible light based on Bi SPR-promoted BiPO4/BiOI heterojunction

Mengyin Lia, Guangxue Zhangc,*, Chuanqi Fenga, Huimin Wua,*, He Mei b,*

Collaborative Innovation Center for Advanced Organic Chemical Materials &

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a Hubei

Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Hubei Key Laboratory of Polymer Materials, National & Local Joint Engineering Research Center of High-throughput Drug Screening Technology, College of Chemistry &

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Chemical Engineering, Hubei University, Wuhan 430062, PR China

Department of Environmental Sciences, Zhejiang Provincial Key Laboratory of Watershed

c School

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Science and Health, Wenzhou Medical University, Wenzhou, 325035, P.R. China of Nuclear Technology and Chemistry & Biology, Hubei University of Science and

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Highlights

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Technology, Xianning, 437100, PR China

 Bi surface plasmon resonance (SPR)-promoted BiPO4/BiOI is synthesized by

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hydrothermal method.

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 The Bi-BiPO4/BiOI composites have excellent PEC performance.  The PEC sensor shows outstanding performances for hexavalent chromium detection.

Abstract In this work, Bi surface plasmon resonance (SPR)-promoted BiPO4/BiOI

heterostructures (Bi-BPI) are synthesized through a one-step hydrothermal method. The structure and morphology have been measured by a series of physical characterizations. Furthermore, the photoelectrochemical (PEC) experiments show that the PEC properties of BiPO4/BiOI are higher than those of pure BiPO4 and BiOI, while BiPO4/BiOI-6% has the best PEC performance. At the same time, it is found that Bi metal could improve the PEC properties of BiPO4/BiOI-6%, and that 0.05-Bi-BiPO4/BiOI-6% has the highest performance among all materials.

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Ultraviolet-visible diffuse reflection spectra (DRS) and photoluminescence (PL) also prove that 0.05-Bi-BiPO4/BiOI-6% features excellent visible light absorption and a

lower electron recombination rate than the other composites. Therefore, PEC sensors

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based on 0.05-Bi-BiPO4/BiOI-6% have been fabricated for the detection of

hexavalent chromium (Cr (VI)). They exhibit a wide linear range from 0.5 to 180 μM

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application in Cr (VI) detection.

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with high stability for sensing Cr (VI). Thus, the PEC sensor has a great potential

Keywords: Surface plasmon resonance, Heterostructure, Photoelectrochemical,

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Detection, Hexavalent chromium

1. Introduction

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Heavy metal-ion pollution has become increasingly serious in the aquatic

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environment [1]. Amongst the different heavy metal ions, hexavalent chromium (VI) has attracted the attention of many researchers because of its high toxicity, and its carcinogenic and mutagenic properties, with their impact on human health [2]. The permissible limit for total chromium in drinking water has been set to 0.96 μM by the World Health Organization (WHO) [3]. The maximum allowable total chromium in ground water in the USA is 2 μM [4]. Therefore, it is crucial to use a rapid and

reliable technique for detecting Cr (VI) concentrations in environmental processes. A wide variety of methods have been employed for detecting Cr (VI), including ion-chromatography (IC) [5], energy dispersive X-ray fluorescence spectrometry (EDXRF) [6], fluorescence [7], atomic adsorption spectroscopy (AAS) [8], and photoelectrochemical (PEC) analysis [9]. Among them, PEC analysis, which possesses the advantages of an ideal signal ratio and high sensitivity from its optical and electrical chemistry, has attracted considerable interest due to its low cost, fast

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response, and good stability [10-12]. Measurements by the PEC method are based on the optical and electrical properties of semiconductor materials [9]. So, developing semiconductor materials with an easily synthesized and wide optical response is

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critical to the realization of ultra-sensitive PEC methods.

To date, various semiconductors, such as oxide-, sulfide-, and Bi-based

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photocatalysts, has been applied in PEC analysis [13-15]. Among of them,

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bismuth-system oxides, with 6s Bi orbitals and 2p oxygen orbitals, display excellent optical and electrical properties [16]. At the same time, they have some wider advantages, such as abundance, low toxicity, and low cost [17]. Based on these merits,

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various bismuth-system oxides have attracted wide attention in the PEC field, including BiOX (X=Cl, I, Br) [18-20], Bi2MoO6 [21], Bi2WO6 [22], BiPO4 [23], etc.

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Much attention has been given to BiOI, a p-type semiconductor with great visible

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light absorption capacity because of its small band gap (1.8 eV) [24]. Moreover, the unique layered structure of BiOI is conducive to excellent photocatalytic activity [25]. Recently, BiOI has been widely applied in PEC systems, such as for photoelectrocatalytic hydrogen production [26], PEC sensors [27], and PEC CO2 reduction [28]. Unfortunately, it suffers from low energy conversion efficiency due to its rapid charge recombination rate [29]. Combinations of other semiconductor

materials and BiOI can accelerate the electron-hole pair separation, however, and enhance the electron transfer efficiency, which will improve the energy conversion efficiency [20-33]. Alternatively, as an oxyacid salt photocatalyst, BiPO4 with its special structure of nonmetallic oxygen has exhibited favorable photocatalytic performance [34]. Furthermore, recent research has indicated that BiPO4 displays superior photocatalytic activity compared to P25 (TiO2) due to the rich negative charges in PO43-, which

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could prevent the recombination of photogenerated electron-hole pairs [35]. BiPO4 limits its absorption of visible light due to its wide bandgap (4.1 eV) [36], but as a

representative n-type semiconductor, it has been used to couple with several narrow

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bandgap materials to improve photocatalytic performance, such as WO3 [37], CdS [38] and Bi2SiO5 [39], because forming a heterojunction between an n-type and a p-type

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semiconductor can improve the absorption of visible light and promote charge carrier

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separation [40].

Bismuth (Bi) metal has some advantages due to its low cost and widely distributed resources, and it is also a semimetal, which not only has a narrow band gap and a

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small carrier effective mass, but also has a highly anisotropic Fermi surface and low carrier density [41]. Furthermore, Bi shows the same characteristics of surface

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plasmon resonance (SPR) as the precious metals, which can promote the separation of

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electron-hole pairs, because it can be used as electron contributor and conductor [41]. New charge transfer pathways have been demonstrated along the path of BiOI → Bi metal → BiPO4, which shows that the SPR effect of Bi improves the PEC detection. In this manuscript, we report a PEC sensor for Cr (VI) based on Bi SPR-promoted BiPO4/BiOI heterostructures. The PEC experiments show that 0.05-Bi-BPI-6% has excellent PEC performance, and it exhibits a wide linear range, low detection limit

(0.3 μM, signal to noise ratio (S/N) = 3) and excellent selectivity towards detecting Cr (VI). The results show that the sensor based on Bi-BPI possesses excellent application prospects for the practical detection of Cr (VI).

2. Experimental section 2.1. Reagents All reagents were of analytical grade and used without further purification. Bismuth

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nitrate pentahydrate (Bi(NO3)3·5H2O), potassium hydrogen phosphate anhydrou (K2HPO4·3H2O), potassium iodide (KI), ethanol, and glucose were purchased from

2.2. Preparation of the Bi-BPI heterojunctions

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Sinopharm Chemical Reagent Co., Ltd. (www.sinoreagent.com).

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The Bi-BPI heterojunctions were prepared by one-step hydrothermal methods: An

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appropriate amount of KI was dissolved in a mixed solution (H2O: CH3COOH = 1:1 v/v). Then, appropriate amounts of K2HPO4·3H2O aqueous solution, Bi(NO3)3.5H2O, and glucose were added to the KI solution and continuously stirred for half an hour.

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The mixed solution was heated at 160 °C for 24 h in a 100 ml Teflon-lined autoclave. Finally, the product was washed and then dried at 60 °C. The prepared materials

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corresponding to the amounts of constituent materials are displayed in Table S1 in the

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Supporting Information.

2.3. Characterization X-ray diffraction (XRD) was performed on a Bruker D8 Advance diffractometer with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was conducted on an ESCALAB 250Xi. Diffuse reflection spectra (DRS) of the materials were collected on

a UV-visible (UV–vis) spectrophotometer with BaSO4 as reference. The morphology of the samples was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The conductivity of the measurement solutions was tested on a conductivity meter (AZ 8362).

2.4. Electrochemical experiments All electrochemical experiments were carried out on a CHI 660E electrochemical

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workstation with a three-electrode system. Pt wire and saturated calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. Indium tin oxide (ITO) glass was used as the working electrode. A xenon lamp (PLS-SXE 300,

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100 mW.cm-2, λ ≥ 420 nm) was used as the light source. Electrochemical impedance spectroscopy (EIS) was conducted over a frequency range from 1 Hz to 1 MHz in

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nitric acid (HNO3, 0.1 mM, 91 μs·cm-1, 25.1 °C) purged with N2. The ITO electrodes

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(10×15 mm) were washed separately with water, acetone, and ethanol for 5 min. Then 3 mg catalyst powders were dispersed in a mixed solution of chitosan and ethanol (0.5 mL) to form a homogeneous suspension. Then, 20 µl of the suspensions were coated

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on ITO electrodes (0.5 cm2).

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3. Results and discussion

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3.1. Choice of Materials

The phases of all the synthetic materials were investigated by XRD. In Fig. S1, for

the BPI-X composites (X = m(BiPO4)/[m(BiPO4)+m(BiOI)] = 4%, 6%, 8%)), all diffraction peaks of BiOI (JCPDS 10-0445) are clearly observed. The three unobtrusive peaks at around 20.06o, 25.47o, and 31.31o correspond to the (101), (110), and (102) planes of BiPO4 (JCPDS 10-0766). In Fig. 1, the four characteristic peaks

of Y-Bi-BPI-6% (where Y represents the amount of glucose and Y = 0.01, 0.05, 0.1) at around 29.64o, 31.66o, 45.67o and 55.15o correspond to the (102), (110), (104), and (212) planes of BiOI (JCPDS 10-0445). The two characteristic peaks at 20.06o and 31.31o correspond to the (101) and (102) planes of BiPO4 (JCPDS 15-0766). After adding Bi metal, the characteristic peaks of BiPO4 and BiOI show no obvious change. Furthermore. the characteristic peaks at around 33.56o correspond to the (110) planes of Bi (JCPDS 26-0214). All results indicate that the synthesized materials are

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perfectly indexed as Bi-BPI heterostructures. The PEC properties of all the synthesized materials were further evaluated by

photocurrent response experiments (Fig. 2a, Fig. S2a-b), which were repeated 5 times

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every 20 s in 0.1 mM HNO3 under visible light irradiation. The photocurrents of all the materials changed under visible light irradiation, and the photocurrent value of

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BPI-6% was 1.4 μM.cm-2. This is significantly higher than those of BiPO4, BiOI, and

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BPI-X (Fig. S2a). Furthermore, Fig. S2b shows that 0.05-Bi-BPI-6% has a higher photocurrent in Y-Bi-BPI-6%. Therefore, the photocurrent of 0.05-Bi-BPI-6% is higher than for the other materials.

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Electrochemical impedance spectroscopy (EIS) was employed to further evaluate the electron transfer kinetics of all the composites (Fig. 2b, Fig. S2c-d). The EIS

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spectra are composed of a squeezed semicircle portion and a linear portion. According

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to our current understanding, the equivalent circuit should include the solution resistance (Rs), double layer capacitance (Cdl), charge transfer resistance (Rct), and a diffusion element known as the Warburg impedance (Zw). The charge transfer resistance (Rct) can be quantified using the semicircle diameter [13]. The values for the samples can be ranked as: BiPO4 (4451 Ω) > BiOI (3687 Ω) > BPI-6% (705 Ω) > 0.05-Bi-BPI-6% (425 Ω), with 0.05-Bi-BPI-6% showing the lowest impedance

among the composites. All the results indicate that suitable amounts of BiPO4 and Bi metal coupled with BiOI can efficiently promote the separation of electron-hole pairs and improve the absorption of visible light. Excessive BiPO4 and Bi metal, however, will reduce the PEC properties of the materials as they hinder exposure of the active sites [42]. Based on the above results, 0.05-Bi-BPI-6% was selected for further investigation.

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3.2 Physical characterization XPS was performed to confirm the chemical composition and valence states of

0.05-Bi-BPI-6%. The high resolution of Bi 4f, O 1s, I 3d, and P 2p spectra are shown

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in Fig. 3a-d. As shown in Fig. 3a, the four main peaks of the Bi 4f XPS spectrum are located at 158.8 eV, 156.3 eV, 164.2 eV, and 161.7 eV. The peaks at 158.8 eV and

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164.2 eV are assigned to Bi 4f7/2 and Bi 4f5/2, respectively, for the standard Bi3+ ions

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[43]. The peaks at 156.3 eV and 161.7 eV are ascribed to the zero valence state of metallic Bi [44]. The O 1s spectrum in Fig. 3b shows a wide characteristic peak at 530.8 eV, which is assigned to the O element of the heterojunctions [42]. The two

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peaks for 3d3/2 (630.2 eV) and 3d5/2 (618.5 eV) of BiOI correspond to I-1 (Fig. 3c) [33]. The P 2p core-level spectrum (Fig. 3d) reveals an obvious peak at 132.7 eV related to

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the P element in the P5+ ion of BiPO4 [35]. All of the results indicate that the

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composite has coexisting metallic Bi, BiPO4, and BiOI. The morphology of the BiPO4, BiOI, and Bi-BPI composites was characterized by

SEM. The pure BiPO4 is rendered as regular blocks in Fig. 4a. In Fig. 4b, it can be clearly observed that BiOI consists of a uniform sheet composed of microsphere structures. The microstructures of BPI-6% and 0.05-Bi-BPI-6% are shown in Fig. 4c-d. It can be clearly observed that the BiPO4 blocks are attached to the BiOI

microspheres. Moreover, the sheets of 0.05-Bi-BPI-6% are more densely packed than those of pure BiOI, which may be beneficial for improving the photoelectrochemical performance. Fig. 4e-g shows TEM images of 0.05-Bi-BPI-6%. The high resolution TEM (HRTEM) image in Fig. 4h displays three lattice spacings of 0.267 nm, 0.285 nm, and 0.301 nm, which are consistent with the Bi (110), BiPO4 (102), and BiOI (102) planes in Fig. 4h. All these results further demonstrate that the Bi-BPI heterojunctions were successfully prepared.

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The absorption properties of BiPO4, BiOI, and 0.05-Bi-BPI-6% were analyzed by UV-vis DRS, as shown in Fig. 5a. The absorption edge of pure BiPO4 is at about 270

nm, which illustrates that BiPO4 only has strong ultraviolet absorption. In addition, it

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can be clearly seen that BiOI and 0.05-Bi-BPI-6% have stronger absorption than

BiPO4 in the visible light range from 400 to 600 nm. The visible light absorption

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capacity of 0.05-Bi-BPI-6% is significantly higher than that of BiOI. On the basis of

applying Equation (1). αhν = A(νh – Eg)n/2

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the basic electronegativity concept [45], the band-gap energy (Eg) is computed by

(1)

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where α: absorption coefficient, h: Planck’s constant, ν: incident light frequency, A: constant, and n: the type of optical transition, BiPO4 is an indirect semiconductor, and

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the value of n is 4 [46]. As shown in Fig. S3a, the Eg gap energy of BiPO4 is 4.3 eV.

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BiOI is a direct semiconductor, and therefore, n is 1 [31]. The corresponding band gap of BiOI is about 1.7 eV in Fig. S3b. Subsequently, photoluminescence (PL) spectra were further used to study the ability of the synthesized materials to separate electrons and holes. Fig. 5b shows that 0.05-Bi-BPI-6% has lower intensity compared with BiOI and BiPO4, suggesting that 0.05-Bi-BPI-6% has a low electron recombination rate [46]. Moreover, Fig. S4 presents the PL delay fitting curves of the samples

through the time-resolved PL spectra. The lifetimes of BiPO4, BiOI, and 0.05-Bi-BPI-6% are 0.91, 1.01, and 1.92 ns, respectively, which demonstrate that 0.05-Bi-BPI-6% has superior charge carrier transfer properties [35]. According to the above discussion, 0.05-Bi-BPI-6% presents excellent PEC performance.

3.3 Photoelectrochemical sensor Fig. 6a shows the PEC performances of BiOI, BiPO4, BPI-6%, and

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0.05-Bi-BPI-6% at -0.2 V vs. SCE with the addition of 20 μM Cr (VI) under visible light excitation in 0.1 mM HNO3 [9]. The photocurrents of all the synthesized

materials are increased correspondingly after adding Cr (VI). Meanwhile, the change

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in the value of BPI-6% is about 2 times larger than that of BiOI, while that of

0.05-Bi-BPI-6% is about 3 times larger than that of BPI-6%, which is attributed to the

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and the SPR of Bi metal [9, 40].

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significant enhancement of the PEC efficiency for Cr (VI) by the p-n heterostructures

To assess the PEC performance of the 0.05-Bi-BPI-6% towards Cr (VI), the photocurrent was measured at various concentrations of Cr (VI). Upon each addition

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of Cr (VI), the photocurrent response (I) to the Cr (VI) concentration (c) rises steeply in Fig. 6b. A linear relationship between the photocurrent and the Cr (VI)

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concentration is obtained (Fig. 6c). Fig. 6c exhibits two linear relationships of the

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sensor for Cr (VI) based on 0.05-Bi-BPI-6%. From 0.5-10 μM, the linear regression equation is △I = 1.146 + 0.315c (coefficient of determination, R2 = 0.9911), and in the range of 10-180 μM, the linear relationship is △I = 3.556 + 0.111c (R2 = 0.9973). The detection limit of the sensor is 0.15 μM (S/N = 3). The linear concentration range down to 0.5 μM makes it easy to achieve the World Health Organization (WHO) specified permissible limit of 0.96 μM. In addition, a comparison of 0.05-Bi-BPI-6%

with other Cr (VI) sensors reported in the literature is shown in Table S3. It is obvious that our sensor is superior to other Cr (VI) sensors in certain aspects. The stability of 0.05-Bi-BPI-6% was also examined by monitoring the photocurrent during repeated photoexcitation over 620 s (Fig. 6d). The response photocurrent of 0.05-Bi-BPI-6% retained 95.7% of its initial value towards 20 μM Cr (VI) over 14 days (Fig. S5a). The reproducibility of 0.05-Bi-BPI-6% was tested through detecting 20 μM Cr (VI) by five parallel electrodes (Fig. S5b), and the photocurrents do not

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show any obvious change, indicating its good reproducibility. The photocurrent responses of 0.05-Bi-BPI-6% towards 20 μM Cr (VI) and interfering substances are

exhibited in Fig. S6, where the photocurrent responses to interference are negligible,

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suggesting that 0.05-Bi-BPI-6% has effective selectivity towards the detection of Cr (VI).

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To verify the practical reliability of the prepared Cr (VI) sensor, tap and lake water

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containing Cr (VI) were used as real samples. The results are shown in Table 1, where the relative standard deviation (RSD) values are less than 5.7%, and the recoveries are found to be 99.97-100.6% and 99.2-100.77%. These results suggest that the sensor

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based on 0.05-Bi-BPI-6% is reliable for Cr (VI) detection in real samples.

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3.4 Proposed mechanism of PEC sensing Cr (VI)

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In order to explore the mechanism for the PEC sensor to detect Cr (VI) based on Bi-BPI, the conduction band energy (ECB) and valence band energy (EVB) of BiPO4 and BiOI can be investigated using the follows equation [47]: EVB = X-Ee + 0.5Eg

(2)

ECB = EVB-Eg

(3)

Where the value of Ee is about 4.5 eV, which refers to the energy of free electrons on

the hydrogen scale. X is the electronegativity of the semiconductor, and the values for BiPO4 and BiOI were 6.49 eV and 5.94 eV. The CB edge potential of BiPO4 and BiOI were counted as -0.26 eV and 0.59 eV, and moreover, the VB edge potentials of BiPO4 and BiOI were calculated as +4.14 eV and +2.29 eV. On the basis of previously reported heterojunction formation processes [46], the energy band of BiOI rises, and the energy band of BiPO4 decreases, while the Fermi levels (EF) of BiOI and BiPO4 will be in equilibrium after contact under visible light excitation, which could be

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advantageous for promoting charge carrier separation. Accordingly, the migration of photogenerated electrons will proceed from the conduction band of BiOI to plasmonic Bi metal. Due to the excellent conductivity and SPR effect of Bi metal, Bi can act as

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an electron contributor and conductor to improve PEC performance. Therefore, more photogenerated electrons from BiOI and Bi metal could be easily transferred to the

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4. Conclusion

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inference is explained in Fig. 7.

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valence band of BiPO4, where they are available to reduce Cr (VI) to Cr (III). This

In summary, in this work, we designed a BiPO4/BiOI heterostructure that is

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enhanced by Bi surface plasmon resonance (SPR) by using a one-step hydrothermal

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method. Due to the benefits of its p-n heterojunction structure and Bi metal SPR effect, its energy conversion efficiency has been effectively improved for detecting Cr (VI). The sensor based on 0.05-Bi-BPI-6% exhibits a wide linear range (0.5 to 180 μM), low detection limit (0.3 μM, S/N = 3) and excellent stability towards detecting Cr (VI). Moreover, the linear concentration range 0.5 μM achieve the World Health Organization (WHO) specified permissible limit of 0.96 μM. The study indicates that

the sensor based on 0.05-Bi-BPI-6% has great application prospects for Cr (VI) PEC detection.

All authors declared no conflict of interest.

Acknowledgements We acknowledge financial support from the National Natural Science Foundation of China (Grant

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No. 21205030), from the Key Project of the Hubei Provincial Education Department (D20171001), and from the Hubei Key Laboratory of Ferro & Piezoelectric Materials and Devices (201710) and

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the 111 Project (B12015).

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[21] J.L. Lv, J.F. Zhang, J. Liu, Z. Li, K. Dai, C.H. Liang, Bi SPR-promoted Z-scheme Bi2MoO6/CdS-diethylenetriamine composite with effectively enhanced visible light photocatalytic hydrogen evolution activity and stability, ACS Sustainable Chem. Eng. 6 (2018) 696-706. [22] B.S. Li, C. Lai, G.M. Zeng, L. Qin, H. Yi, D.L. Huang, C.Y. Zhou, X.G. Liu, M. Cheng, P. Xu, C. Zhang, F.L. Huang, S.Y. Liu, Facile Hydrothermal Synthesis of

Z-scheme Bi2Fe4O9/Bi2WO6 Heterojunction Photocatalyst with Enhanced Visible-Light Photocatalytic Activity, ACS Appl. Mater. Interfaces 10 (2018) 18824-18836. [23] R.Y. Wu, H.B. Song, N. Luo, G.J. Ji, Hydrothermal preparation of 3D flower-like BiPO4/Bi2WO6 microsphere with enhanced visible-light photocatalytic activity, J. Colloid Interface Sci. 524 (2018) 350–359. [24] X.M. Sun, J. Lu, J. Wu, D.Y. Guan, Q.Z. Liu, N.Q. Yan, Enhancing

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synthesized by one-step reaction and its capacity to degrade bisphenol A under simulated solar light irradiation, Chem. Eng. J. 233 (2013) 305–314.

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fabrication of Bi2S3/BiVO4 hybrid nanostructure through photoelectrochemical transformation process for solar hydrogen production, ACS Sustainable Chem. Eng. [27] H. Xu, D.D. Huang, Y. Wu, J.W. Di, Photoelectrochemical determination of Cu2+

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One dimensional SnO2 NRs/Fe2O3 NTs with dual synergistic effects for photoelectrocatalytic reduction of CO2 into methanol , J. Colloid Interface Sci. 486 (2017) 232-240. [29] Z.Y. Jiang, X.Z. Liang, Y.Y. Liu, T. Jing, Z.Y. Wang, X.Y. Zhang, X.Y. Qin, Y. Dai, B.B. Huang, Enhancing visible light photocatalytic degradation performance and bactericidal activity of BiOI via ultrathin-layer structure, Appl. Catal. B 211 (2017)

252–257. [30] B.O. Orimolade, B.A. Koiki, G.M. Peleyeju, O.A. Arotiba, Visible light driven photoelectrocatalysis on a FTO/BiVO4/BiOI anode for water treatment involving emerging pharmaceutical pollutants, Electrochim. Acta 307 (2019) 285-292. [31] B. Li, X.W. Chen, T.Y. Zhang, S. Jiang, G.H. Zhang, W.B. Wu, X.Y. Ma, Photocatalytic selective hydroxylation of phenol to dihydroxybenzene by BiOI/TiO2 p-n heterojunction photocatalysts for enhanced photocatalytic activity, Appl. Surf. Sci.

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sources, J. Hazard. Mater. 364 (2019) 429–440. [37] H. Lv, X.X. Wu, Y.M. Liu, P. Zhang, Y.F. Cao, H. Ren, Photoreactivity and mechanism of BiPO4/WO3 heterojunction photocatalysts under simulant sunlight irradiation, Ceram. Int. 44 (2018) 6786-6790. [38] J. Zhao, K. Ge, L.F. Zhao, S. Zhang, Y.W. Zeng, Enhanced photocatalytic properties of Cd -decorated BiPO4 heterogeneous semiconductor catalyst under UV-light irradiation, J. Alloy. Compd. 729 (2017) 189-197.

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[43]X. Zhang, X.B. Wang, L.M. Wang, W.K. Wang, L.L. Long, W.W. Li, H.Q. Yu, Synthesis of a highly efficient BiOCl single-crystal nanodisk photocatalyst with exposing {001} Facets. ACS Appl. Mater. Interfaces 6 (2014) 7766-7772. [44] J.X. Xia, J. Di, H.T. Li, H. Xu H, H.M. Li, S.J. Guo, Ionic liquid-induced strategy for carbon quantum dots/BiOX (X = Br, Cl) hybrid nanosheets with superior

visible light-driven photocatalysis. Appl. Catal. B 18 (2019) 260–269. [45] W.S. Kuo, P.H. Ho, Solar photocatalytic decolorization of dyes in solution with TiO2 film, Dyes Pigments 71 (2006) 212–217. [46] J. Cao, B.Y. Xu, H.L. Lin, S.F. Chen, Highly improved visible light photocatalytic activity of BiPO4 through fabricating a novel p–n heterojunction BiOI/BiPO4 nanocomposite, Chem. Eng. J. 228 (2013) 482–488. [47] Y. Xu, M.A.A. Schoonen, The absolute energy positions of conduction and

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Biographies

Mengyin Li She is currently working toward the M.S. degree in College of Chemistry & Chemical Engineering, Hubei University. Her research interests include synthesis of nanomaterials for nanobiosensors and fuel cell.

Guangxue Zhang He is an Associate Professor from Hubei University of Science and

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Technology. His research interests include synthesis of nanomaterials and applications.

Chuanqi Feng He is an Professor of Hubei University. His active areas of research

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include synthesis of inorganic materials and application.

Huimin Wu is an Associate Professor of Physical Chemistry at Hubei University. She

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holds a Ph.D. degree in Physical Chemistry from University of Wollongong, Australia. She received her M.S. degree in Physical Chemistry from Wuhan University. Her

and nanomaterials.

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active areas of research include sensors, fuel cell, hydrogen production and storage,

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He Mei He is a teacher of Wenzhou Medical University. His research interests include

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synthesis of nanomaterials for non-enzymatic glucose sensors.

Figure Captions: Fig. 1 XRD spectra of the Y-Bi-BPI-6% composites. Fig. 2 Photocurrent responses (a), and EIS spectra, with the equivalent circuit in the left inset (b) of the BiPO4, BiOI, BPI-6%, and 0.05-Bi-BPI-6% in 0.1 mM HNO3. Fig. 3 XPS spectra of 0.05-Bi-BPI-6% composites: (a) Bi 4f (magenta Bi3+ 4f7/2, blue Bi3+ 4f5/2, red Bi0 and green Bi0 plots), (b) O 1s, (c) I 3d, and (d) P 2p. Fig. 4 SEM images of BiPO4 (a), BiOI (b), BPI-6% (c), and 0.05-Bi-BPI-6% (d); (e-g)

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TEM images of 0.05-Bi-BPI-6%; (h) HRTEM image of the 0.05-Bi-BPI-6%. Fig. 5 UV-vis diffuse reflectance spectra (a), and PL spectra (b) of the BiPO4, BiOI and 0.05-Bi-BPI-6% composites.

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Fig. 6 (a) Photocurrent responses of the BiPO4, BiOI, BPI-6%, and 0.05-Bi-BPI-6% in the absence and presence of 20 μM Cr (VI); (b) Photocurrent responses of

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0.05-Bi-BPI-6% towards Cr (VI) at increasing concentrations: (a) 0, (b) 0.5, (c) 1, (d)

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2, (e) 4, (f) 6, (g) 8, (h) 10, (i) 20, (j) 30, (k) 40, (l) 60, (m) 80, (n) 100, (p) 140, and (q) 180 μM Cr (VI); (c) The corresponding calibration plot (enlargement in inset) of the Cr (VI) concentration; (d) The stable photocurrent response curve of the

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0.05-Bi-BPI-6% in the presence of 20 μM Cr (VI) in 0.1 mM HNO3 at -0.2 V vs. SCE with visible light excitation.

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Fig. 7 PEC mechanism of Bi-BPI for the detection of Cr (VI).

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Table. 1 PEC detection of Cr (VI) in tap and lake water samples.

0.01-Bi-BPI-6% 0.05-Bi-BPI-6% 0.1-Bi-BPI-6% BiOI: JCPDS-No.10-0445 BiPO4: JCPDS-No.15-0766 Bi: JCPDS-No.26-0214

Intensity(a.u) 30

31

32

33

34

35

36

37

38

39

40

2 Theta/degree

15

30

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Intensity(a.u)

0.01-Bi-BPI-6% 0.05-Bi-BPI-6% 0.1-Bi-BPI-6% BiOI: JCPDS-No.10-0445 Bi: JCPDS-No.26-0214

45

60

2 Theta/degree

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Fig. 1 XRD patterns of the Y-Bi-BPI-6% composites with enlargement in inset.

BiPO4 BiOI

BiPI-6% 0.05-Bi-BPI-6%

500

(b)

400 300

3000 -6

200 100

2000

-4

200

0 100

150

200

600

800

BPI-6% 0.05-Bi-BPI-6%

0 50

400

Z'/ohm

BiOI BiPO4

1000

-2

0

BPI-6% 0.05-Bi-BPI-6%

-Z''/ohm

(a) -8

4000

-Z''/ohm

Photocurrent(A.cm-2)

-10

0

3000

6000

9000

Z'/ohm Fig. 2 Photocurrent responses (a), and EIS spectra, with the equivalent circuit in the t(s)

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left inset (b) of the BiPO4, BiOI, BPI-6%, and 0.05-Bi-BPI-6% in 0.1 mM HNO3.

161.7eV

160

162

166

630

530

Intensity (a.u.) 625

620

532

534

536

538

540

Binding Energy (eV)

615

Binding Energy (eV)

610

125

130

135

140

Binding Energy (eV)

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635

528

(d) P 2p

I-3d3/2

Intensity (a.u.)

526

168

I-3d5/2

(c) I 3d

640

164

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158

Binding Energy (eV)

-p

156.3eV

156

164.2eV

Intensity (a.u.)

Intensity (a.u.) 154

(b) O 1s

158.8eV

(a) Bi 4f

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Fig. 3 XPS spectra of 0.05-Bi-BPI-6% composites: (a) Bi 4f (magenta Bi3+ 4f7/2, blue

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Bi3+ 4f5/2, red Bi0, and green Bi0 plots), (b) O 1s, (c) I 3d, and (d) P 2p.

(b)

5μm

2μm

2μm

2μm

(e)

(g)

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(d)

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(c)

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(a)

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(f)

500nm

2μm

20nm

(h)

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Bi (110) d=0.267

BiOI (102) d=0.301

BiPO4 (102) d=0.285

5nm Fig. 4 SEM images of BiPO4 (a), BiOI (b), BPI-6% (c), and 0.05-Bi-BPI-6% (d); (e-g)

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TEM images of 0.05-Bi-BPI-6%; (h) HRTEM image of the 0.05-Bi-BPI-6%.

0.05-Bi-BPI-6% BiOI BiPO4

0.05-Bi-BPI-6% BiOI BiPO4

(b)

Intensity(a.u.)

Absorbance(a.u.)

(a)

400

500

600

Wavelength(nm)

700

800

400

500

600

Wavelength(nm)

700

800

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300

Fig. 5 UV-vis diffuse reflectance spectra (a), and PL spectra (b) of the BiPO4, BiOI,

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and 0.05-Bi-BPI-6% composites.

Photocurrent(A.cm-2)

BiPO4+20μM Cr(Ⅵ) BPI-6%

-8

BPI-6%+20μM Cr(Ⅵ) 0.05-Bi-BPI-6%

-6

0.05-Bi-BPI-6%+20μM Cr(Ⅵ)

-4 -2 0

45

60

120

180

t(s)

4.5

40 △I(A.cm-2)

30 25 20

-15

2.5 2.0 1.5

0

2

4

[Cr(Ⅵ)](M)

6

8

10

R2=0.9973

10 5 40

60

80 100 120 140 160 180

[Cr(Ⅵ)](M)

0

0

200

400 600 Time(s)

800

1000

0.05-Bi-BPI-6% + 20M Cr(Ⅵ)

-14 -12 -10 -8 -6 -4 -2 0

0

100

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20

-5

(d)

3.0

15

a

-10

-16

1.0

0

-20

240

R2=0.9911

3.5

35

-25

(c)

4.0

q

(b)

ro of

-10

-30

Photocurrent(A.cm-2)

Photocurrent(A.cm-2)

-12

0

△I(A.cm-2)

(a)

BiOI+20μM Cr(Ⅵ) BiPO4

-p

BiOI

-14

200

300

400

500

600

t/s

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Fig. 6 (a) Photocurrent responses of the BiPO4, BiOI, BPI-6%, and 0.05-Bi-BPI-6% in the absence and presence of 20 μM Cr (VI); (b) Photocurrent responses of

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0.05-Bi-BPI-6% towards Cr (VI) at increasing concentrations: (a) 0, (b) 0.5, (c) 1, (d) 2, (e) 4, (f) 6, (g) 8, (h) 10, (i) 20, (j) 30, (k) 40, (l) 60, (m) 80, (n) 100, (p) 140, and (q)

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180 μM Cr (VI); (c) The corresponding calibration plot (enlargement in inset) of the Cr (VI) concentration; (d) The stable photocurrent response curve of the

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0.05-Bi-BPI-6% in the presence of 20 μM Cr (VI) in 0.1 mM HNO3 at -0.2 V vs. SCE with visible light excitation.

Before contact

0

e- e- e-

CB

Bi

CB

After contact

EF

1.7 eV

2 VB

e- eBi e- e- eSPR

EF

h+ h+ h+

4.3 eV

Cr (VI) Cr (III)

EF

BiOI

4

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VB BiOI BiPO4

-p

BiPO4

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Fig. 7 PEC mechanism of Bi-BPI for the detection of Cr (VI).

Table. 1 PEC detection of Cr (VI) in tap and lake water samples.

Cr (VI) concentration (μM) Sample

Detected

Added

Found

Recovery

RSD (%)

(%) 10.06

100.60

2.1

2

60.00

59.95

99.91

3.5

3

140.00

139.96

99.97

4.9

99.2

2.6

100.77

4.8

1

Not found

10.00

9.92

2

Not found

60.00

60.46

3

Not found

140.00

140.12

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10.00

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Lake

1

-p

Tap

100.08

5.7