In-situ synthesis of novel p-n heterojunction of Ag2CrO4-Bi2Sn2O7 hybrids for visible-light-driven photocatalysis

Accepted Manuscript In-situ synthesis of novel p-n junction of Ag2CrO4-Bi2Sn2O7 hybrids for visible-lightdriven photocatalysis Xiang-feng Wu, Yang Sun, Hui Li, Yi-jin Wang, Chen-xu Zhang, Jia-rui Zhang, Junzhang Su, Yi-wei Wang, Ying Zhang, Chao Wang, Mi Zhang PII:

S0925-8388(18)30101-4

DOI:

10.1016/j.jallcom.2018.01.100

Reference:

JALCOM 44564

To appear in:

Journal of Alloys and Compounds

Received Date: 16 November 2017 Revised Date:

30 December 2017

Accepted Date: 8 January 2018

Please cite this article as: X.-f. Wu, Y. Sun, H. Li, Y.-j. Wang, C.-x. Zhang, J.-r. Zhang, J.-z. Su, Y.-w. Wang, Y. Zhang, C. Wang, M. Zhang, In-situ synthesis of novel p-n junction of Ag2CrO4-Bi2Sn2O7 hybrids for visible-light-driven photocatalysis, Journal of Alloys and Compounds (2018), doi: 10.1016/ j.jallcom.2018.01.100. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.

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Graphical Abstract

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In-situ synthesis of novel p-n junction of Ag2CrO4-Bi2Sn2O7 hybrids for visible-light-driven

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photocatalysis Xiang-feng Wu§∗, Yang Sun§, Hui Li, Yi-jin Wang, Chen-xu Zhang,

Zhang

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Jia-rui Zhang, Jun-zhang Su, Yi-wei Wang, Ying Zhang, Chao Wang, Mi

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School of Materials Science and Engineering, Hebei Provincial Key Laboratory of Traffic Engineering Materials, Shijiazhuang Tiedao University, Shijiazhuang 050043, China Abstract

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Novel p-n heterojunction of Bi2Sn2O7-Ag2CrO4 photocatalysts with various contents of Bi2Sn2O7 were in-situ synthesized at 25 °C. X-ray

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diffraction, transmission electron microscopy, UV-vis diffuse reflectance

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spectra, photoluminescence emission and electrochemical impedance spectra were employed to characterize the as-prepared samples. Experimental results showed that, under the visible light irradiation, with ∗Corresponding

author: Dr. Xiang-feng Wu. Tel/Fax: +86 311 87936574.

E-mail: [email protected] (X.F. Wu) §

Xiang-feng Wu and Yang Sun contributed equally to this work and share

the first authorship.

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increasing the amount of Bi2Sn2O7 the degradation efficiency of the as-prepared hybrids was first increased and then decreased. It possessed the highest degradation efficiency of 97.5 % for rhodamine B in 120 min,

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which was obviously higher than that of 76.7 % of Ag2CrO4 and 11.8 % of Bi2Sn2O7, respectively. Moreover, it possessed the degradation efficiency of 90.4 % for methyl orange and 99.8 % for methylene blue. In

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addition, after it was circulated for 5 times, the as-prepared hybrids still

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possessed the degradation efficiency of 90.2 % for rhodamine B, which increased by 240.4 and 1950.0 % in comparison with Ag2CrO4 and Bi2Sn2O7, respectively. The enhanced photocatalytic activity could be attributed to the formation of the p-n junction at the interface of

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p-Bi2Sn2O7 and n-Ag2CrO4.

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Key words: Heterojunction; Photocatalysis; Bi2Sn2O7; Ag2CrO4

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1. Introduction Nowadays, semiconductor-based photocatalysis is taken as a promising avenue to solve the water pollution problems. To date, various

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semiconductors, such as metal oxides [1], suldes [2], phosphides [3] and their mixtures [4], have been synthesized as photocatalysts. However, their application under the visible light irradiation has been impeded due

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to their large band gap along with the rapid recombination of the photo

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generated charge carriers. Bi2Sn2O7 with a band gap of 2.86 eV is renowned as one of admirable semiconductors due to its high photosensitivity, non-toxic nature and low cost [5]. However, owing to its fast recombination rate of the photo generated electron-hole pairs, its

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photocatalytic activity is still hindered [6]. Some strategies, such as heterojunction formation with other semiconductors, have been

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developed to overcome the mentioned limitations [7-9]. This special structure will lead to generation of charge carriers and prolong their life

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time. Hence, this technology is a much more effective and non-expensive method for enhancing the photocatalytic activities of the traditional semiconductors. Inspired by this strategy, various Bi2Sn2O7-based photocatalysts, such as Bi2Sn2O7-ZnO [10], Bi2Sn2O7-TiO2 [11], Bi2Sn2O7-Bi2S3 [12], Bi2Sn2O7-BiOI [13] and Bi2Sn2O7-C3N4 [14], have been reported. Recently, silver-based semiconductors, such as AgI [15], AgBr [16],

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Ag3PO4 [17], Ag2CO3 [18], Ag2CrO4 [19] and Ag2O [20], have exhibited excellent photocatalytic performances in degradation of organic pollutants under the visible-light irradiation. Among them, Ag2CrO4 has

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also received extensive concerns [21]. However, similar to other silver-based semiconductors, its poor photo stability still needs to be improved. In order to display the synergy advantages and reduce the

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successfully

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disadvantages of Bi2Sn2O7 and Ag2CrO4, a facile method has been

Bi2Sn2O7-Ag2CrO4 hybrids. Rhodamine B (RhB), methyl orange (MO) and methylene blue (MB) solution were used to evaluate the photocatalytic activity of the as-prepared hybrids under the visible light

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irradiation. Experimental results showed that the as-prepared hybrids could obviously improve the photocatalytic performances and reusability

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compared with the pure samples. Moreover, the possible photocatalytic mechanism of the as-prepared hybrids under the visible light illumination

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was also presented.

2. Experimental section 2.1. Materials Sodium

stannate

tetrahydrate

(Na2SnO3⋅4H2O),

bismuth

nitrate

pentahydrate (Bi(NO3)3⋅5H2O), cetyltrimethyl ammonium bromide (CTAB), sodium hydroxide (NaOH), silver Nitrate (AgNO3), potassium

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chromate (K2CrO4), tert-Butanol (t-BuOH), ethylenediamine tetraacetatic acid (EDTA), p-benzoquinone (BQ) were purchased from Sinopharm Chemical Reagent Co., Ltd., China. RhB, MO, MB and titanium dioxide

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(TiO2, P25) were purchased from Macklin Biochemical Co., Ltd., China. 2.2. Preparation of Bi2Sn2O7 nanoparticles

Firstly, 3 mmoL Na2SnO3⋅4H2O, 3 mmoL Bi(NO3)3⋅5H2O and 0.3

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mmoL CTAB were fully dissolved together in 80 mL deionized water.

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Secondly, 3 moL⋅L-1 NaOH was used to adjust the pH of the above solution to 12. Thirdly, the obtained mixture was transferred into a 150 mL Teflon-lined autoclave and kept at 180 °C for 12 h. Finally, the precipitates were fully washed by using deionized water and dried in a

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vacuum oven at 80 °C for 6 h.

2.3. Preparation of Bi2Sn2O7-Ag2CrO4 hybrids

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A certain amount (0.25, 0.35, 0.45 or 0.55 g) of the as-prepared Bi2Sn2O7 particles was well dispersed in 50 mL deionized water at 25 °C. 0.154 g

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AgNO3 was slowly added into it with constant stirring for 60 min. Subsequently, 0.088 g K2CrO4 was dissolved in 30 mL deionized water and added dropwise into the above mixture. Finally, the precipitate was washed with distilled water and absolute ethanol, and dried in a vacuum oven at 60 °C for 24 h. The as-prepared products were defined as S-x, where x stands for the amount of Bi2Sn2O7 in the as-prepared hybrids.

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That was to say, the as-prepared samples were accordingly marked as S-0.25, S-0.35, S-0.45 or S-0.55. For comparison, pure Ag2CrO4 particles were prepared via the similar procedure with out using any Bi2Sn2O7.

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2.4. Characterization X-ray diffraction (XRD, Model: D8ADVANCE, Bruker Co., Germany) was used to characterize the structure of the pure Ag2CrO4, Bi2Sn2O7 and

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as-prepared Bi2Sn2O7-Ag2CrO4 hybrids. Field emission scanning electron

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microscopy (FE-SEM, Model: Hitachi S-4800, Japan) and transmission electron microscopy (TEM, Model: JEM-2100, JEOL Co., Japan) were employed to observe the morphologies of the samples. The UV-visible absorbance spectra of the products were obtained by using a UV-visible

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spectrophotometer (Model: UV-722N, Shanghai Precision Instrument Co., Ltd., China). The UV-vis diffuse reflectance spectra (DRS, Model:

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U-4100, Shimadzu Co., Ltd., Japan) of the samples was measured by using a UV/VIS/NIR spectrometer equipped with a diffuse reflectance

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accessory. The photoluminescence spectra (PL) of the samples were measured by using a spectrometer (Model: LS55, Perkin Elmer Co., Ltd., USA) with the excitation wavelength of 320 nm. 2.5. Photocatalytic properties tests The photocatalytic performance of the as-prepared samples was evaluated by degradation of RhB, MO, MB under the visible light irradiation. An aqueous solution of 10 mg⋅L-1 of dye was used. In each experiment, 50

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mg photocatalyst was dispersed in 150 mL dye aqueous solution. Before the illumination, the suspension solution was stirred in the dark for 1 h to reach the equilibrium of absorption-desorption between the sample and

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the dye solution. During the photocatalytic process, basing on previous reports, visible light was generated by a 300 W xenon lamp (Model: CE-HXF 300, Beijing Zhong Jiao Jin Yuan science and Technology Co.,

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Ltd., China) with an UV cutoff filter (λ > 420nm) [22, 23]. Moreover, 3

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mL of the suspension was with drawn in a 30 min interval. The degradation efficiency was calculated by using C / C0 . The photocatalytic decomposition kinetics was investigated and the Langmuir-Hinshelwood model was arranged as the following equation [24-26]:

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In(C / C0 ) = kt

where C0 and C are the adsorption equilibrium and dye concentration

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at reaction time t , respectively. k is the apparent reaction rate constant. 3. Results and discussion

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3.1. XRD analysis

The crystallinity and phase integrity of pure Bi2Sn2O7, Ag2CrO4 and the as-prepared Bi2Sn2O7-Ag2CrO4 hybrids with various contents of Bi2Sn2O7 were examined by XRD. It can be seen in Fig. 1 that the distinct diffraction peaks at 2θ = 31.14, 31.43, 32.30, 39.26, 44.26, 45.40, 52.07, 55.84, 57.07, 61.93 and 62.60°, which could be perfectly indexed to the

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orthorhombic Ag2CrO4 according to JCPDS card No. 26-0952 [27]. The strong and sharp diffraction peaks could clearly indicate that pure Ag2CrO4 sample was well-crystalline. Moreover, for Bi2Sn2O7, it showed

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that there were five distinct diffraction peaks at 2θ = 21.50, 30.58, 35.47, 51.04 and 60.68°, which could be attributed to the cubic Bi2Sn2O7 according to JCPDS card No. 870824 [28]. In addition, the as-prepared

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Bi2Sn2O7-Ag2CrO4 hybrids (S-0.25, S-0.35, S-0.45 and S-0.55) exhibited

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a coexistence of both Bi2Sn2O7 and Ag2CrO4 phases. With increasing the contents of Bi2Sn2O7, the intensities of diffraction peaks were strengthened continuously. No other peaks were found. This means that the as-prepared hybrids possessed high purity.

3.2. TEM analysis

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Fig. 1.

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It can be observed in Fig. 2 (a) and (b) that the morphology of pure Bi2Sn2O7 and Ag2CrO4 was homogeneous with an average size of about

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20 and 500 nm, respectively. Moreover, in Fig. 2 (c), it can be clearly found that the Bi2Sn2O7 nanoparticles were loaded on the surfaces of bulky Ag2CrO4. In addition, in Fig. 2 (d), it can be observed that the as-prepared hybrids displayed two types of lattice fringes as white arrows shown. One was 0.3095 nm, which was corresponding to (222) lattice plane of p-Bi2Sn2O7; another one was 0.28 nm, which was corresponding to (211) lattice plane of n-Ag2CrO4. These results confirmed that the

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heterojunctions were formed at the interface of Bi2Sn2O7 and Ag2CrO4. Fig. 2. 3.3. UV-vis DRS analysis

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The UV-vis absorption spectra of the samples are shown in Fig. 3 (a). It can be seen that Bi2Sn2O7 had weak absorption in the visible light region, while Ag2CrO4 had intense absorption in the visible to near infrared light

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region. Moreover, the as-prepared Bi2Sn2O7-Ag2CrO4 hybrids exhibited a

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mixed absorption property of Ag2CrO4 and Bi2Sn2O7. With increasing the amount of Bi2Sn2O7, the absorption edge of the as-prepared composites was approached to that of pure Bi2Sn2O7. Fig. 3.

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As is well known, the band gap of the samples can be estimated via the following equation [29]:

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αhν = A(hν − E g )

n 2

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where α , ν , E g and A are the absorption coefficient, light frequency, band gap energy and the proportionality constant, respectively. The value of n describes of the type if transition, which n = 1 for direct transition and n = 4 for indirect transition. For Ag2CrO4 and Bi2Sn2O7, both values of n are 4 [30, 31]. Therefore, the E g of Ag2CrO4 and Bi2Sn2O7 could be determined by a plot of (αhν )1 2 versus energy hν (as shown in Fig. 3 (b)). It can be seen that the band gap of all the as-prepared

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Bi2Sn2O7-Ag2CrO4 hybrids was distributed in the range of 2.79 to 1.67 eV. 3.4. Photocatalytic activity analysis

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Fig. 4. It can be observed in Fig. 4 that all the as-prepared Bi2Sn2O7-Ag2CrO4 hybrids exhibited obviously higher degradation efficiency than that of

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pure samples. With increasing the amount of Bi2Sn2O7, the degradation

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efficiency was firstly increased and then decreased. When the amount of Bi2Sn2O7 was 0.45 g (the as-prepared S-0.45 sample), it showed the highest degradation efficiency of 94.3 % in 60 min, which was much higher than that of 31.5 % for Ag2CrO4, 7.5 % for Bi2Sn2O7 and 11.1 %

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for P25, respectively. Moreover, in 120 min, it also possessed the highest degradation efficiency of 97.5 %, which was much higher than that of

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76.7 % for Ag2CrO4 and 11.8 % for Bi2Sn2O7, respectively. The possible reason was ascribed to the formation of heterojunction between

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p-Bi2Sn2O7 and n-Ag2CrO4. With increasing the contents of Bi2Sn2O7, much more heterojunction interface would be formed and suppress the recombination of photo induced electron-hole pairs. However, excessive Bi2Sn2O7 could be as impurities and cause to the decrease of photocatalytic activity. In Fig. 4 (b), it shows the temporal evolution of the absorption spectrum of RhB solution in the presence of the as-prepared S-0.45 sample. It can be seen that the absorption peak at 554

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nm gradually decreased as in 120 min. This phenomenon indicated that the conjugated structure of RhB was destroyed and might be decomposed into many small molecules, e.g., H2O and CO2 [32].

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Fig. 5 In Fig. 5 (a), it clearly shows that after it was circulated for 5 times, the as-prepared S-0.45 sample still possessed the degradation efficiency of

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90.2 % for RhB, which increased by 240.4 and 1950.0 % in comparison

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with Ag2CrO4 (26.5 %) and Bi2Sn2O7 (4.4 %), respectively. This indicated that the as-prepared S-0.45 sample possessed much better reusability than that of pure samples. In Fig. 5 (b), it can be seen that the degradation efficiency of the as-prepared S-0.45 sample for MO and MB

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reached to 74.8 % and 77.6 % in 60 min, and 90.4 % and 99.8 % in 120 min, respectively. The above results indicated that the as-prepared S-0.45

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sample not only possessed excellent photocatalytic performances for RhB, but also for MO and MB.

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3.5. PL analysis

Fig. 6

It has been generally believed that a weaker PL intensity means higher separation probability of the photogenerated charge carriers [33]. In Fig. 6 (a), it can be observed that the intensity of the PL signal for all the as-prepared hybrids, especially for the as-prepared S-0.45 sample, was lower than that of pure Bi2Sn2O7. This result indicated that the

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recombination of electron-hole pairs was effectively inhibited and caused to excellent photocatalytic properties. 3.6. EIS analysis

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It can be seen in Fig. 7 that the as-prepared S-0.45 sample possessed obviously lower charge transfer resistance than that of pure Ag2CrO4 and Bi2Sn2O7. This phenomenon indicated that more effective photo

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generated electron-hole pair separation and faster interfacial charge

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transfer at the as-prepared S-0.45 sample surface compared with the pure samples. It was also in keeping with an analogous trend of the photocatalytic activity [34].

3.7. The possible mechanism of photocatalytic activity

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Fig. 8

In order to gain insight into the reaction mechanism and ascertain the

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contribution of the active species, a series of control experiments of quenching active species were conducted. t-BuOH, EDTA and BQ were

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chosen as ·OH, h+ and ·O2- scavenger, respectively [35-37]. As indicated in Fig. 8, when the t-BuOH and EDTA were added into reaction solution, the photo degradation rate of RhB was almost invariable compared with that without using scavengers. This implied that ·OH and h+ was not the main active species. On the contrary, when the BQ was used, the photo degradation rate of RhB was seriously inhibited, meaning that ·O2- played an important role. Thus, it could be concluded that the photocatalytic

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degradation of RhB over the as-prepared Bi2Sn2O7-Ag2CrO4 hybrids was mainly governed by ·O2- rather than ·OH and h+ under the visible light irradiation.

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Fig. 9 The enhancement could be explained by the band regulation principle. The band position of the semiconductors can be calculated by the

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following formulas [38-41]:

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E CB = χ − E e − 0.5 E g EVB = E g + E CB

Among them, χ represents the Mulliken electronegativity value, which is the geometric mean of the absolute electronegativity of the constituent

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atoms. E e is the energy of free electrons on the hydrogen scale. ECB is the conduction band edge potential. EVB is the valence band edge

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potential. E g is the band gap energy of the semiconductor.

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Basing on the band gap, the conduction band (CB) and valence band (VB) edge potentials of n-Ag2CrO4 were calculated at 0.48 and 2.24 eV. Moreover, that of p-Bi2Sn2O7 were calculated at -0.59 and 2.20 eV. A possible photocatalytic mechanism of the as-prepared Bi2Sn2O7-Ag2CrO4 composites could be proposed. When the as-prepared Bi2Sn2O7-Ag2CrO4 hybrids were irradiated by visible light, electrons (e-) and holes (h+) were excited on the CB and VB of p-Bi2Sn2O7 and n-Ag2CrO4, respectively.

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Photo excited carriers could transfer smoothly due to the matching potentials of the composites, as shown as in Fig. 9. p-Bi2Sn2O7 possessed more negative potential of the CB ( ECB = −0.59eV ) than that of n-Ag2CrO4

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( ECB = 0.48eV ). Therefore, the excited electrons on p-Bi2Sn2O7 could be injected into the CB of n-Ag2CrO4. The enriched electrons on the CB of n-Ag2CrO4 reacted with oxygen to generate ·O2-. Moreover, ·O2- radicals

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could combine with H2O to further transform into ·OH. Meanwhile, h+

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could transfer from the VB of n-Ag2CrO4 (2.24 eV) to p-Bi2Sn2O7 (2.20 eV), which promoted the efficient separation of photo induced electrons and holes. In addition, their EVB were lower than the standard redox potential of ·OH / H2O (2.68 eV), indicating that the photo generated h+

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could not oxidize H2O into ·OH. Therefore, h+ would directly react with RhB. These reactive species of h+, ·O2- and ·OH were responsible for the

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degradation of organic pollutant. 4. Conclusion

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In summary, the p-n heterojunction of Bi2Sn2O7-Ag2CrO4 hybrids was prepared via in-suit synthetic method at 25 °C. Experimental results showed that the as-prepared Bi2Sn2O7-Ag2CrO4 hybrids exhibited obviously higher degradation efficiency and reusability than that of pure Bi2Sn2O7 or Ag2CrO4. The enhanced photocatalytic activity could be attributed to the formation of p-n heterojunction at the interface of

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p-Bi2Sn2O7 and n-Ag2CrO4. Moreover, ·O2- played an important role during the degradation of RhB. This technology might provide reference values for visible-light-driven photocatalysis.

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Acknowledgements This work was funded by Natural Science Foundation of Hebei Province, China (No. E2013210011).

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Conflicts of interest: none

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Wang, Y. Liu, X.Y. Yang, One-step hydrothermal synthesis of In2.77S4 nanosheets with efficient photocatalytic activity under visible light.

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Appl. Phys. A Mater. 123 (2017) 426.

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Figure Captions Fig. 1. XRD patterns of the samples Fig. 2. TEM images of the samples, (a) Bi2Sn2O7, (b) Ag2CrO4 and (c) the

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as-prepared S-0.45 sample; (d) HRTEM image of the as-prepared S-0.45 sample

Fig. 3. (a) UV-vis DRS and (b) band gap energy of the samples

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Fig. 4. (a) RhB concentration versus visible light irradiation time of the

the as-prepared S-0.45 sample

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samples; (b) representative degradation profile of RhB in the presence of

Fig. 5. (a) Photocatalytic cycle testing of Ag2CrO4, Bi2Sn2O7 and the as-prepared S-0.45 sample for RhB; (b) photocatalytic degradation of the

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as-prepared S-0.45 sample for MO and MB Fig. 6. PL spectra of the samples

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Fig. 7. EIS spectra of the samples

Fig. 8. Photodegradation of the as-prepared S-0.5 sample for RhB in the

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presence of various radical scavengers Fig. 9. Possible photocatalytic mechanism of the as-prepared hybrids under the visible light illumination

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Figure

30

40

50

60

2 Theta(degree)

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Fig. 1.

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70

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20

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Intensity(a.u.)

Ag2CrO4 S-0.55 S-0.45 S-0.35 S-0.25 Bi2Sn2O7

80

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Fig. 2.

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2.0

(a)

0.0 200

400 600 Wavelength(nm)

4

(b)

1

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1/2

(αhν) (eV)

1/2

3 2

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0.5

Ag2CrO4 S-0.25 S-0.35 S-0.45 S-0.55 Bi2Sn2O7

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1.0

2.79eV

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1.67eV

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0

2

800

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Absorbance

1.5

3 hν(eV)

Fig. 3.

Ag2CrO4 S-0.25 S-0.35 S-0.45 S-0.55 Bi2Sn2O7

4

5

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1.2 Bi2Sn2O7 P25 Ag2CrO4 S-0.25

C/C0

0.9 0.6

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1.0

(b)

554nm

0.8

0 min

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Abs.

0.4

11.1%

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0

0.6

11.8%

31.5% 76.7% S-0.35 S-0.55 97.5% S-0.45 94.3% 30 60 90 120 Time(min)

0.3 0.0

7.5%

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(a)

120 min

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0.2

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0.0 300

400

500

600

Wavenumber(nm)

Fig. 4.

700

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120 100

Bi2Sn2O7

90.2%

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80 60

26.5%

40

4.4%

20 0

1

2

3

4

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Degradation Efficency

(a)

S-0.45 Ag2CrO4

140

5

6

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Recycle Times

1.2

(b)

MB MO

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C/C0

0.9

0.6

77.6%

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0.3

0

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0.0

20

40

74.8%

60

99.8%

80

Time (min)

Fig. 5.

100

90.4%

120

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420

440

460

480

500

520

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Wavenumber(nm)

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S-0.25 S-0.35 S-0.55 S-0.45

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Bi2Sn2O7

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EP

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Fig. 6.

540

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50000

Bi2Sn2O7 Ag2CrO4

30000

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-Z''(ohm)

40000

20000

S-0.45

0

0

10000

20000

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10000

30000

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Z'(ohm)

40000

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EP

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Fig. 7.

50000

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1.2

BQ t-BuOH EDTA Without scavengers

0.6

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C/C0

0.9

0.0

0

20

40

60

80

100

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Time (min)

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0.3

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Fig. 8.

120

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Fig. 9.

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Highlights

The Bi2Sn2O7-Ag2CrO4 hybrids were in-situ synthesized at 25°C. It was an

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efficient photocatalyst for degradation of RhB, MO and MB. The enhanced degradation efficiency was ascribed to the formation of p-n

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heterojunction.