Defect Engineering of Bismuth Oxyiodide by IO3â Doping for

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Defect Engineering of Bismuth Oxyiodide by IO3- Doping for Increasing Charge Transport in Photocatalysis Yongchao Huang, Haibo Li, Wenjie Fan, Fengyi Zhao, Weitao Qiu, Hong-Bing Ji, and Yexiang Tong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10653 • Publication Date (Web): 03 Oct 2016 Downloaded from http://pubs.acs.org on October 4, 2016

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ACS Applied Materials & Interfaces

Defect Engineering of Bismuth Oxyiodide by IO3Doping

for

Increasing

Charge

Transport

in

Photocatalysis Yongchao Huang, Haibo Li, Wenjie Fan, Fengyi Zhao, Weitao Qiu, Hongbing Ji* and Yexiang Tong* MOE of the Key Laboratory of Bioinorganic and Synthetic Chemistry, KLGHEI of Environment and Energy Chemistry. The Key Lab of Low-carbon Chem & Energy Conservation of Guandong Province, School of Chemistry, Sun Yat-Sen University, 135 Xingang West Road, Guangzhou 510275, China. E-mail: [email protected] (H.B.Ji); [email protected] (Y.X.T)

KEYWORDS: Bismuth Oxyhalide, Oxygen Vacancy, Photocatalysis, IO3- Doping, Charge Transport

ABSTRACT: Defect engineering is regarded as part of the most active projects to monitor the chemical and physical properties of materials, which is expected to increase the photocatalytic activities of the materials. Herein, oxygen vacancies and IO3- doping are introduced into BiOI nanosheets via adding NaH2PO2, which can impact the charge carrier dynamics of BiOI photocatalysts, such as its excitation, separation, trap, and transfer. These oxygen-deficient BiOI

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nanosheets display attractive photocatalytic activities of gaseous formaldehyde degradation and methyl orange under visible light irradiation, which are 5 and 3.5 times higher than the BiOI samples, respectively. Moreover, the co-modified BiOI also displayed superior cycling stability and can be used for practical application. This work did not only develop an effective strategy for fabricating oxygen vacancies but also offer a deep insight into the impact of surface defects in enhancing photocatalysis.

1. Introduction Photocatalysis is a promising strategy to solve environmental deterioration and worldwide energy shortages because it is an environmentally friendly and economically considerable approach.1-9 In photocatalytic reaction, light irradiation causes the semiconductor materials to be excited and produce electron-hole pairs which are then separated and transferred to active sites for redox reactions.10-12 Hence, the photocatalytic ability of materials in this process mainly depends on three steps: electron-hole pair separation, molecular absorption and reactions.13-14 As we know, recombination of electron-hole needs several picoseconds, much faster than charge transportation, meaning that photogenerated electrons and holes prefer to recombine.

15-16

This

high bulk-charge recombination efficiency leads to the poor performance of photocatalysts. Thus, it is necessary to boost the electron-hole separation and transportation abilities for the applications of photocatalysts. In order to solve these problems, several strategies have been proposed, such as preparing materials with high specific surface area, inducing oxygen vacancies on the surface of photocatalysts 17-20 and visible light sensitized heterojunction. 21-25 Active sites play the vital role in catalytic process. Materials with high surface area tend to display more active ends for


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reaction. Additionally, moderate defects can promote the separation of photoinduced electrons and holes because photoinduced electrons can preferentially activated the oxygen in oxygen defects states below the conduction band rather than recombining with photoinduced holes.26-28 Interestingly, element doping is a general and efficient strategy to change the lattice of substrates, which can also boost the photocatalytic performance of materials. For example, Iself-doping can extend the absorption of BiOI, and result in promoting photocatalytic activity, which was demonstrated by theoretical calculation and experiment.29 CO32- self-doped can broaden the absorption range of Bi2O2CO3 to visible light region, consequently enhance photocatalytic degradation of NO.2 This process may modulate the crystal structure, enhance the optical properties and separate the photoinduced electrons and holes, and finally improve the photocatalytic performance of the semiconductors. Nevertheless, most of the researches have focused on the combined action of optical properties and charge separation, but which one plays the main act in the photocatalytic process? As a promising photocatalytic material, BiOI has received increasing interest as a result of to its high photocatalytic efficiency, low toxicity, and visible-light response.30-32 In the BiOI structure, anionic iodide slabs interleave into positive BixOyn+ slabs and the internal static electric fields are perpendicular to each layer.33-34 The advantages of inherent electric fields play important role in BiOI photocatalytic performance, which is believe to reduce the combination of photogenerated electrons and holes.

35-37

However, its photodegradation is limited by the poor

charge migration, and immediate recombination of the electrons and holes. Thus, many methods such as fabrication of heterojunctions, noble metal deposition and elemental doping stand to increase the photocatalytic capability of BiOI.38-40 In this study, we exhibit the simultaneous realization of IO3- doping and oxygen vacancies in BiOI nanosheets by a one-pot hydrothermal


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method with sodium hypophosphite. The NaH2PO2 plays an essential part in the formation of oxygen vacancies and IO3--doped BiOI. The presence of oxygen vacancies (Ov) and IO3- doping considerably enhance the charge transfer to the surface of BiOI materials, and thus substantially boost the photocatalytic response for the degradation of methyl orange (MO) and gaseous formaldehyde (HCHO) under visible light (λ ≥ 420 nm) irradiation. In addition, the insight into the photocatalytic mechanism of Ov/IO3- co-modified BiOI is also discussed. 2. Experimental 2.1 Preparation of Ov/IO3- comodified BiOI. The BiOI samples were prepared by a typical solvothermal method. 33 Under magnetic stirring, 1 mmol Bi(NO3)3·5H2O and 1 mmol NaI were added into 20 mL ethylene glycol and 20 mL methyl alcohol, respectively. The solution was mixed to form the precursor solution, with constantly stirring for 2 h. Carbon cloth (3×5 cm2) was inserted into a 40 mL Teflon-lined stainless autoclave containing the precursor solution and maintained at 160 °C for 8 h. When cooling down to room temperature, the carbon cloth was taken out and washed (deionized water and ethanol) 3 times respectively, and dried at 60 °C for 12 h. Ov/IO3- comodified BiOI was obtained when amounts of NaH2PO2 (0.05 mmol) were dissolved in the Bi(NO3)3 solution. 2.2 Materials characterizations. The morphologies of the photocatalysts were characterized by SEM (FE-SEM, JSM-6330F) and TEM (TEM, JEM2010-HR, 200 kV). The structure and composition of the photocatalysts were characterized by XRD (D8 ADVANCE) and X-ray photoelectron spectroscopy, XPS (ESCALab250, Thermo VG). UV–vis spectrophotometer (UV2450) with BaSO4 as the reflectance standard was used to record the optical properties of photocatalysts. The photoluminescence spectrometers testing photocatalysts was performed under the ultraviolet excitation of 400 nm (RF-5301PC). The Brunauer–Emmett–Teller (BET)


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method (ASAP 2020 V3.03 H) was used to calculate the specific surface of the photocatalysts. Electron spin resonance (ESR) tests were performed in the X-band (9.45 GHZ) at 77 K. EPR spectra of DMPO-•O2- (25 mM DMPO methanol solution) and DMPO-•OH (25 mM DMPO water solution) with CM-BiOI samples were detected under visible light irradiation (λ ≥ 420 nm).48,49 A standard three electrode configuration system wax employed for the photoelectrochemical measurements where the BiOI samples, platinum plate and an Ag/AgCl electrode (or calomel electrode) are the working, counter and reference electrodes, respectively using the CHI 660C electrochemical station. The electrolyte was 0.5 mol L-1 Na2SO4 aqueous solution. A 300 W Xe lamp was used as the visible light source with a 420 nm UV light cut filter. The light intensity is 100 mW cm-2.

Scheme 1 Photocatalytic experimental device. 2.3 Measurement of the Photocatalytic performance. HCHO removal were first examined using 0.01g of the photocatalysts. The photocatalytic experimental device was showed in scheme 1. A purified air flow (N2/O2 = 4,100 mL⋅min-1) was injected in an incubator that filled with a


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solution of 37% HCHO and kept at 0 oC to produce HCHO gas. The gas hourly space velocity was kept at 60,000 mL⋅h-1. The products obtained after reaction were evaluated online by a gas chromatograph (Agilent 7890A) that consist of a TCD detector and a Porapak-Q column. CO2 is the only product that was detected in the entire products of all the tested photocatalysts. The percentage of HCHO conversion was determined from the CO2 content as shown: HCHO conversion (%) = [CO2] out/[HCHO] in×100 Where [CO2] out represents the CO2 concentration in the final products and [HCHO] in represents the HCHO concentration in the reactor. About 0.01g of the photocatalyst was added to a 100 mL solution that contain 10 mg⋅L-1 MO. The mixture was magnetically stirred for 60 min in the dark to establish equilibrium for the absorption-desorption. A 300 W Xe lamp was employed as the visible light source with a 420 nm UV light cut filter. The light intensity of the entire tests is 100 mW cm-2. After light have been irradiated on the solution, 3.0 mL of the reactive solution was taken out by the syringe at different time. Then, UV-vis spectrophotometer was employed to test the concentration of the residual dyes. For the residual MO concentration, an intensity of 465 nm absorbance was used for evaluation. The photocatalytic efficiency was calculated by the C/C0 × 100%, in which the C stands for the final dyes concentrations and C0 is the initial concentration of dyes. 2.4 Calculating the donor density. The donor density of these n-type BiOI samples can be calculated from the slopes of Mott-Schottky plots using the following equation; 53 Nd = (2/e0εε0)[d(1/C2)/dV]-1 Where Nd is the donor density, e0 is the electron charge, ε is the dielectric constant of BiOI, ε0 is the permittivity of vacuum, and V is the applied bias at the electrode. The exact donor density of


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these BiOI samples can not be calculated due to the lacking of the ε of BiOI. Nevertheless, it is reasonable to compare the donor densities between pristine BiOI and CM-BiOI electrodes using the slopes of Mott-Schottky plots. 3. Results and discussion 3.1 Synthesis and characterization of all the catalysts. BiOI nanosheets are prepared by a solvothermal method as reported in our previous report (details can be seen in the Experimental Methods).33 Scanning electron microscopy (SEM) images show that BiOI consists nanosheets that self-combined to form microspheres with diameter of ≈1.5 µm. (Figure 1 and Supporting Information Figure S1). Interestingly, Ov/IO3- co-modified BiOI (CM-BiOI) is obtained by adding sodium hypophosphite in hydrothermal solution. The SEM results reveal there are no obvious changes in morphology for the CM-BiOI. Compared to the BiOI nanosheets, CM-BiOI colour changed from orange red to yellow, implying a successful change after adding NaH2PO2 (Supporting Information Figure S1). The morphologies of BiOI and CM-BiOI are also identified by transmission electron microscopy (TEM). It can be seen in Figure 1 that the microspherical structure of BiOI is confirmed and the selected-area electron diffraction analysis suggested the polycrystalline of the BiOI nanosheets. Moreover, the clear lattice fringes with a spacing of 0.301 nm corresponds to the (102) plane of the tetragonal BiOI (Figure 1c). Significantly, the surface and inside of CM-BiOI nanosheets have some disorders, suggesting the existence of oxygen vacancies (Figure 1g). Meanwhile, the EDS line-scanning spectra of CM-BiOI samples show that Bi, I and O signals emerged simultaneously and no other elements appear, confirming the high purity of the samples (Supporting Information Figure S2).


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Figure 1 (a) SEM images; the bottom left inset: the high magnification image, (b) TEM image, (c) HRTEM image and (d) SAED pattern of BiOI. (e) SEM images; the bottom left inset: the high magnification image, (f) TEM image, (g) HRTEM image and (h) SAED pattern of CM-BiOI. To classify the possible phase transformation, XRD patterns of the BiOI and CM-BiOI are displayed in Figure 2a. The diffraction peaks collected from BiOI and CM-BiOI can be indexed to the tetragonal BiOI (PCPDF no.: 10-0445).29 No other peaks are observed for CM-BiOI, further identifying the high degree of purity of the samples. Noticeably, compared to the (102) peak of BiOI, a slight shift to low degree could be found in the CM-BiOI sample, also suggesting the existence of oxygen vacancies.41 In order to identify the defects in the CM-BiOI sample, Raman, X-ray photoelectron spectroscopy (XPS) and Electron spin resonance (EPR) analyses were carried out. Bi vacancies on CM-BiOI sample are found in Raman spectra (Figure 2b). A new band at 98 cm-1 can be detected, which can be designated to vibration modes A1g of Bi, suggesting that the valence of bismuth changed on the surface. It is well-known that oxygen vacancies give rise to a change in cation valence states.42 Figure 2c displays the core-level Bi XPS spectra of BiOI and CM-BiOI samples. There are two strong peaks of BiOI at 159.1 and


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164.4 eV, which can be respectively corresponding to Bi 4f7/2 and Bi 4f5/2. For the CM-BiOI samples, the Bi 4f7/2 and Bi 4f5/2 show the lower binding energy peaks at 156 and 161 eV, which is indicative of Bi centers with a lower valency in CM-BiOI sample. This result clearly confirms the presence of oxygen vacancies in CM-BiOI samples.33 The high resolution O 1s XPS spectra of BiOI and CM-BiOI samples are also displayed in Figure 2d. There are two asymmetric features (the lattice oxygen and surface active oxygen). It is known that surface active oxygen component increases with the increase in the number of oxygen vacancies, which is important for photocatalysis.43 The peak area of surface active oxygen in the CM-BiOI sample is obviously larger as compared to the BiOI sample, clearly suggesting the existence of some number of oxygen vacancies in the CM-BiOI sample. Moreover, more evidence of defect state could be observed from the electron spin resonance spectra in the crystal structure. As shown in Figure 2e, in contrast to the case of BiOI sample, the CM-BiOI sample displays an obvious signal peak at g ≈ 2.001, which is considered to be the typical signal of surface oxygen vacancies.38, 44 The entire results mentioned discussed above definitely demonstrate that the oxygen vacancies are present in the CM-BiOI sample. Undoubtedly, the crystal structure distortion of CM-BiOI sample mainly originates from the IO3- doping, which can be observed from the core-level I 3d XPS spectra (Figure 2f). Two peaks locate at 618.9 and 630.4 eV are indexed to the I- 3d5/2 and I- 3d3/2 of BiOI sample, respectively. In comparison to the BiOI sample, the CM-BiOI sample also shows the peaks of I- 3d5/2 and I- 3d3/2. Additionally, the peaks at 622.8 and 634.4 eV in CM-BiOI sample are detected, which are attributed to the I5+ 3d5/2 and I5+ 3d3/2.45 In order to identify the presence of IO3-, we doped IO3- into the BiOI sample (BiOI-IO3). Surprisingly, the peaks at 622.8 and 634.4 eV in BiOI-IO3 sample are also detected, which indicates that IO3- has been generated in CM-BiOI sample (Supporting Information S3). Combining with the XRD results, it


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can be concluded that the as-generated IO3- may be incorporated into the crystal lattice of CMBiOI.

Figure 2 (a) XRD patterns, (b) Raman patterns, (c) Bi 4f XPS spectra, (d) O 1s XPS spectra, (e) EPR spectra, (f) I 3d XPS spectra collected for BiOI and CM-BiOI samples. 3.2 Photocatalytic ability of the tested catalysts. To determine the impact of oxygen vacancies and IO3- doping effect in photocatalysis, it is fundamental to achieve the photocatalytic properties of the CM-BiOI sample and compare with that of pristine BiOI. The photocatalytic degradation of HCHO gas (a priority hazardous volatile organic compound substance in the


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indoor atmosphere

46-47

) was first performed to evaluate the activity of BiOI and CM-BiOI

samples, which is displayed in Figure 3a. Without the photocatalysts, decomposition of HCHO is inconsiderable after 90 min test, revealing the photolysis of HCHO is negligible under visible light irradiation. After 90 min of visible light irradiation, the HCHO degradation over P25 is only 9.1%. Notably, CM-BiOI sample could eliminate more than 99% after 45 min, while about 20% of degraded molecules are decomposed with BiOI sample at the same testing condition, certainly confirming that oxygen vacancies and IO3- doping can markedly improve the photocatalytic performance of BiOI. During the reaction processing, the amount of CO2 in the product increased (Supporting Information Figure S4). More intriguingly, the photodegradation rate attained a constant without any decay after 10 continuous cycles, revealing it has marked stability under visible light irradiation. This is very vital for its practical application. These results clearly demonstrate that the CM-BiOI sample has excellent photocatalytic performance compared with BiOI sample as a result of the induced of oxygen vacancies and IO3- doping for effective photocatalytic activity enhancement. Moreover, the stability of CM-BiOI sample is further confirmed by its phase and morphology stability (Supporting Information Figure S5). The XRD pattern and XPS spectra of CM-BiOI sample after 10 consecutive cycles have no change, suggesting its phase and compositional stability. The morphology is also well-maintained after 800 min irradiation. Based on the HCHO removal activity in the laboratory, the field test for practical application could be performed by using the catalysts in a commercial mask for the decorating worker. Figure 3c presents a photograph of the CM-BiOI sample inserted in a commercial mask. The HCHO gas can not be breathed in due to the presence of our CM-BiOI catalysts. It could can convert the HCHO into CO2 and H2O and protect the decorating worker's health.


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It is worth noting that the MO adsorption capabilities of BiOI and CM-BiOI samples are quite different, which are showed in Figure 3d. The MO adsorption onto CM-BiOI briskly reaches 70.55 mg/gcat in the first 5 min and then reaches the equilibrium in 45 min, with an adsorption of 83.97 mg/gcat, while the BiOI sample only reaches the equilibrium with 20.21 mg/gcat after 60 min. As it is well-known that the specific surface area and the charge of photocatalysts are crucial for the adsorption of dye molecules, so the specific surface area of the BiOI and CM-BiOI samples are determined by the Brunauer–Emmett–Teller (BET) test (Supporting Information Figure S6). However, the result shows that they have the similar specific surface area (29.95 m2/g for CM-BiOI and 29.15 m2/g for BiOI), suggesting that the charge of BiOI and CM-BiOI samples may be have different. Indeed, the CM-BiOI sample is more positively charged than BiOI, which favors the negatively charged MO molecules adsorption (Supporting Information Figure S7). To further identify that more positively charged in CM-BiOI sample, another positively charged RhB molecules was introduced as a target dye to compare its positivity. Indeed, it is not easy for the positive dye to undergo adsorption on the CM-BiOI sample (Supporting Information Figure S8). Figure 3e displays the photodegradation of MO under the illumination of visible light. CM-BiOI sample could eliminate more than 99% after 90 min, and

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