Facile and Controllable Modification of 3D In2O3 Microflowers with

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Facile and Controllable Modification of 3D InO Microflowers with InS Nanoflakes for Efficient Photocatalytic Degradation of Gaseous Ortho-Dichlorobenzene 2

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Fei Zhang, Xinyong Li, Qidong Zhao, and Aicheng Chen J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b03618 • Publication Date (Web): 02 Aug 2016 Downloaded from http://pubs.acs.org on August 6, 2016

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The Journal of Physical Chemistry

Facile and Controllable Modification of 3D In2O3 Microflowers with In2S3 Nanoflakes for Efficient Photocatalytic Degradation of Gaseous Ortho-Dichlorobenzene Fei Zhang,† Xinyong Li,*,† Qidong Zhao,† and Aicheng Chen*,‡ †State Key Laboratory of Fine Chemicals, Key Laboratory of Industrial Ecology and Environmental Engineering (MOE), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China ‡Department of Chemistry, Lakehead University, 955 Oliver Road, Thunder Bay, Ontario P7B 5E1, Canada

ABSTRACT: Novel 3D In2S3/In2O3 heterostructures comprised of 3D In2O3 microflowers and In2S3 nanoflakes were synthesized via a facile hydrothermal process followed by an in situ anion exchange reaction. In the In2S3/In2O3 heterostructures, the In2S3 nanoflakes were in-situ generated and uniformly assembled on In2O3 microflowers. The microstructures, optical properties, oxygen vacancy concentration, and photoreactivity of the heterostructures could be tuned by adjusting the amount of sulfide source. The effect of In2S3-nanoflakes modification on the oxygen vacancy concentration, optical properties, charge carrier separation, and charge carrier lifetime of In2O3 were investigated systematically. The catalytic activity of the proposed heterostructures for degradation of gaseous ortho–dichlorobenzene (o–DCB, a representative chlorinated volatile organic compounds) was higher than that of either unmodified In2O3 or TiO2 (P25). Meanwhile, oxygen vacancies, systematically explored by Raman, X-ray photoelectron spectroscopy (XPS), and low-temperature electron spin resonance (ESR) spectroscopy, were demonstrated to have a two-side effect on the

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photocatalytic

performance.

Particularly,

the

main

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reaction

products

including

o–benzoquinone type species, phenolate species, formates, acetates, and maleates were verified with in situ FTIR spectroscopy. Additionally, ESR examination confirmed that •OH and •O2− were the predominant reactive oxygen species involved in the degradation of gaseous o–DCB. The current research provides new insight into utilizing In-based heterostructures as promising and efficient visible-spectrum-responsive catalysts for the removal of harmful chlorinated volatile organic compounds.

1. INTRODUCTION Chlorinated volatile organic compounds (Cl-VOCs), as well as conventional combustion products (CO, NOx, SOx, and particulate matter), are released to the atmosphere during the incineration of medical and municipal wastes.1 It is highly desirable to develop efficient strategies for the abatement of Cl-VOCs due to their high toxicity,

environmental

persistence,

bioaccumulation,

and

carcinogenicity.2–4

Conventional techniques, such as catalytic oxidation or combustion, might function well for the elimination of Cl-VOCs because of their superior selectivity towards the formation of desired decomposition products.5 However, there is a number of demerits involved with the degradation process, such as the production of various secondary pollutants, high energy consumption, and additional heat sources that are required for supplying high temperatures. Photocatalysis based on semiconductor materials, as an eco-friendly and effective technology, has been widely applied for the abatement of gaseous organic compounds under mild conditions.6 It is well known that visible light occupies ~44% of the ambient sunlight spectrum. Hence, a key to the widely practical 2


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application of this technology is to develop stable, low cost, and high-performance photocatalysts under visible-light illumination.7–11 Very recently, numerous visible-light-responsive

photocatalysts have

been

developed, including Fe, Bi, Ag, and In-based photocatalysts.12–16 In particular, indium oxide (In2O3), a III–VI group oxide with a bandgap of 2.8 eV, has garnered extensive attention due to its stable photochemical properties and low toxicity.17 Additionally, In2O3, as a novel visible-light-driven photocatalyst, has been reported for the removal of VOCs under visible-light irradiation.18,19 Despite these substantial virtues, the photocatalytic application of isolated In2O3 is impeded by the high recombination rate of photogenerated e–/h+ pairs and inadequate photosensitivity to the visible portion of the solar spectrum, leading to poor quantum efficiency and low photocatalytic activity under visible-light illumination. To address these limitations, the construction of heterojunctions at the interface of In2O3 and other narrow-band-gap semiconductors with well-matched bandgap potentials (e.g., α-Fe2O3/In2O3,19 In2O3/g-C3N4,20 In2O3/ZnO,21 In2O3/graphene,22 and CuO QDs/In2O3

23

) has been investigated as the

most effective strategy for circumventing the drawbacks of In2O3. Currently, semiconductor metal sulfides have been extensively studied and proven to be a group of promising photocatalysts for the removal of pollutants. Typically, indium sulfide (In2S3), an n-type III–VI metal sulfide, possesses three different polymorphic forms: α-In2S3 (defect cubic structure), β-In2S3 (defect spinel structure), and γ-In2S3 (layered hexagonal structure).24 β-In2S3 has received significant attention due to its narrow bandgap (2.0–2.3 eV), stable physical and chemical characteristics,

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high

photosensitivity

and

photoconductivity,

and

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luminescence

properties.25

Moreover, in comparison with the toxic CdS and PbS, the green nature/low toxicity of In2S3

makes

it

an

excellent

visible-light

sensitizer

of

the

wide-band-gap

semiconductors in photocatalytic decontamination and water splitting.26 Up to now, several In2S3/In2O3 composites have been constructed. Sirimanne and his co-workers synthesized

In2S3/In2O3

compound

electrodes

with

significantly

enhanced

photocurrent via the sulfurization of In2O3 in a H2S atmosphere.27 Li et al. reported on an In2O3@In2S3 core–shell nanocube photoanode with type-II band alignment for photoelectrochemical water splitting.28 In addition to the photoelectrode form of these composite materials, powder forms of these composites have been also fabricated. Sun et al. have adroitly designed a reverse type-I In2O3@In2S3 core–shell nanoparticles by reacting In2O3 with CS2 at 250 oC.29 More recently, Yang et al. prepared In2O3–In2S3 core–shell nanorods using a facile hydrothermal procedure under relatively mild conditions, which exhibited good H2 evolution performance in contrast to other In-based photocatalysts under UV–vis light illumination.30 It is envisaged that the construction of 3D In2S3/In2O3 heterostructures with type-II staggered band alignment may lead to attractive

photocatalytic

activity towards the

degradation

of

ortho–dichlorobenzene (o–DCB). To the best of our knowledge, this has not been explored in previous reports. Herein, novel 3D In2S3/In2O3 heterostructures have been successfully prepared through a two-step reaction procedure: a hydrothermal process for synthesizing 3D In2O3 microflowers, and a subsequent in situ anion exchange process, which leads to the generation of In2S3

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nanoflakes on In2O3 microflowers. Subsequently, the microstructures, oxygen vacancy concentration, and photophysical properties of the 3D In2S3/In2O3 heterostructures were systematically investigated. Meanwhile, the presence, formation mechanism, and relative concentration of oxygen vacancies were also elaborately investigated. Photocatalytic performance was further evaluated via the degradation of o–DCB, a model compound that is structurally similar to the more toxic 2,4,7,8–tetrachlorodibenzodioxin. The resulting heterostructures exhibited significantly enhanced catalytic performance in comparison to the pristine In2O3 and In2S3 under visible-light irradiation (λ > 400 nm). More importantly, oxygen vacancies were proven to have an important influence on the photoactivity of the proposed materials. Furthermore, the reactant intermediates and final products of gaseous o–DCB were identified with in situ Fourier transform infrared (FTIR) spectroscopy. A possible photocatalytic reaction mechanism of o–DCB over In2S3/In2O3 heterostructures was finally proposed, in view of the separation of photo-induced charge carriers and the influence of oxygen vacancies and crucial reactive oxygen species. 2. EXPERIMENTAL SECTION Synthesis of 3D In2S3/In2O3 Heterostructures. All chemical reagents used in this work were of analytical grade and used as purchased without further purification. The 3D In2O3 microflowers were initially synthesized via a facile hydrothermal method, according to a strategy reported elsewhere.31 The detailed synthetic process is presented in the Supporting Information (SI). For the synthesis of 3D In2S3/In2O3 heterostructures, the subsequent hydrothermal approach was adopted, and this in situ anion exchange process led to the formation and growth of In2S3 nanoflakes on the surface of In2O3 microflowers. The overall 5



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synthetic strategy is depicted in Figure S1 (SI). Typically, 0.10 g of In2O3 microflowers was dispersed into 32 mL of deionized water by ultrasonication for 30 min to form a homogeneous suspension. Subsequently, a calculated amount of thioacetamide (TAA, acting as sulfide source to provide S2– ions) was introduced into the above suspension, which was stirred for an additional 30 min, followed by its transfer into a 40 mL Teflon-lined stainless steel autoclave. The autoclave was then heated at 150 oC for 5 h and cooled down naturally to room temperature. The final products were obtained by centrifugation, rinsed thoroughly with deionized water, and dried at 60 oC for 12 h prior to use. On the basis of this procedure, a series of In2S3/In2O3 heterostructures were synthesized and denoted as ISO(X, X = I, II, III). Herein, the ISO(I), ISO(II), and ISO(III) composites refered to the dosages of TAA were 0.02, 0.05, and 0.08 g, respectively. For comparison, pure In2S3 was prepared via a facile hydrothermal approach.32 Characterization. All of the proposed catalysts were characterized by multiple techniques, e.g., field-emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) spectroscopy, transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), X-ray powder diffraction (XRPD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), N2 adsorption–desorption isothermals, UV–vis absorption spectroscopy, steady-state photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopy, surface photovoltage spectroscopy (SPV), and electron spin resonance (ESR) spectroscopy. The detailed detection methods are presented in the SI. Photocatalytic Activity Tests and Degradation Products Analysis. The catalytic performance of the proposed catalysts was evaluated by measuring the degradation of

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gaseous o–DCB.19 A 500 W xenon lamp, equipped with a filter to cut off light wavelengths that were shorter than 400 nm, was used as the illumination source for the induction of the catalytic reaction. The photocatalysts (0.03 g) were pressed into a self-supporting wafer of ~13 mm in diameter and affixed onto a quartz holder at the center of a custom-made quartz cell (volume, ca. 120 mL), followed by sealing the cell. Subsequently, 5 µL of liquid o–DCB was injected into the cell using a microsyringe. Once the o–DCB was fully volatilized in the dark, and the adsorption–desorption equilibrium was established between the gaseous o–DCB and the surface of the catalysts, the lamp was switched on. Simultaneously, in order to explore the reactant intermediates and the final degradation products of gaseous o–DCB, in situ FTIR spectra were continuously collected in the range of 4000–600 cm–1 using a BRUKER Vertex 70 FTIR spectrometer. It should be noted here that the spectrum of the clean photocatalyst surface was collected and used as the background. The concentration of o–DCB was measured at regular time intervals by sampling 1 µL of the gaseous mixture from the cell into a gas chromatograph (Agilent 7890A). 3. RESULTS AND DISCUSSION Morphology Analysis. Figure 1a,b show the morphology of pure In2O3 for the purpose of comparison. The pure In2O3 possesses a flower-like microstructure, which is composed of numerous, dense, and thin interconnected nanosheets. The average diameter of these microstructures is in the range of 4.0–6.5 µm. Figure 1c–h show typical SEM images of the ISO(I), ISO(II), and ISO(III) composites. It can be seen that numerous In2S3 nanoflakes are attached uniformly on the surface of In2O3 microflowers. When the dosages of TAA were increased from 0.02 to 0.08 g, the number densities of the In2S3 nanoflakes also gradually 7



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increased. Particularly, for the ISO(II) and ISO(III) composites (Figure 1e–h), the flower-like morphology of the pure In2O3 could not be observed, which might be due to the uniform distribution of In2S3 nanoflakes. Meanwhile, Figure S2a indicates that the pure In2S3 nanoparticles are ~40 nm in diameter, and a number of some large blocks with sizes ~100 nm are also obtained. The typical EDX spectrum (Figure S2b) illustrates that the ISO(II) composite consists of In, S, and O elements. Moreover, the corresponding elemental mapping images of In, S, and O for a typical ISO(II) microflower (Figure 1i) reveal that the S element is uniformly distributed across the surfaces of the In2O3 microflowers, and In2S3 nanoflakes are intimately contacted with the In2O3 microflower.

Figure 1. FESEM images of (a, b) In2O3 microflower, (c, d) ISO(I) composite, (e, f) ISO(II) composite, and (g, h) ISO(III) composite. (i) The elemental mapping image of In, O, and S in a typical heterostructured ISO(II) microsphere. Figure 2a,b present the low-magnification TEM images of In2O3 microflowers. The In2O3 8


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microflowers are derived from the copious presence of thin interconnected nanosheets, which have relatively smooth surfaces. Meanwhile, the TEM images of the ISO(II) composite (Figure 2c,d) further indicate that the In2S3 nanoflakes are uniformly dispersed across the surfaces of In2O3, which is in good agreement with the above SEM results. HRTEM image of the ISO(II) composite (Figure 2e) reveals that the (222) crystal plane of cubic In2O3 are closely contact to the (400) plane of In2S3, and the interfacial connections could serve as migration paths to facilitate the charge separation, and induce synergistic effects for the enhanced photocatalytic performance.

Figure 2. TEM images of (a, b) In2O3 microflower and (c, d) ISO(II) composite. (e) Local HRTEM images of ISO(II) composite. Composition and Structural Analysis. The XRPD patterns of the In2S3/In2O3 heterostructures, as well as those of pure In2O3 and In2S3, are shown in Figure 3. It can be observed that the diffraction patterns of pure In2O3 and In2S3 samples match well with those of the body-centered-cubic (bcc) In2O3 (JCPDS No. 06-0416) and ത m, a0 = b0 = c0 = cubic-structured β-In2S3 (JCPDS No. 65-0459, space group: Fd3 10.774 Å), respectively.33,34 As for the ISO(I, II, III) heterostructures, all of the 9



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diffraction peaks could be well indexed to bcc-In2O3 and β-In2S3, and no extra peaks could be observed, Moreover, the bcc-In2O3 diffraction peaks at 30.8, 35.5, 51.5, and 60.8o are obviously weakened, whereas the β-In2S3 diffraction peaks at 27.7, 33.5, 43.8, and 47.9o gradually become stronger with increasing the In2S3 content. Furthermore, in conjunction with Raman spectra (Figure S3), it can be concluded that the In2S3/In2O3 heterostructures are two-phase hybrids.

Figure 3. XRPD patterns of In2O3, In2S3, and ISO(I, II, III) heterostructures. The diffraction peaks marked with “” and “”are for In2O3 and In2S3, respectively. The Raman peak centred at 367 cm–1 could further reveal the generation of oxygen vacancies (Vo) in the as-prepared In2O3 structure.33 Herein, the intensity ratio between the 368 and 132 cm–1 (caused by the In–O vibration of InO6 structure units) bands, denoted as I368/I132, was used to evaluate the relative quantity of Vo in the catalysts.35 Obviously, the I368/I132 value of the ISO(II) composite is significantly higher than that of the pristine In2O3. Vo were considered to be generated during the calcination process of In(OH)3, which can be expressed as follows:

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calcination

2InሺOHሻ3 ሱۛۛۛۛۛሮ In2 O3-x+

1 2

O2 ሺgሻ+ xH2Oሺgሻ

(1)

The hydroxyl species in the In(OH)3 lattice would be removed in the form of H2O. While some of the lattice oxygens can also be released in the form of O2(g), thus leaving Vo. The relation described below illustrates the formation of Vo: Oxo →

1 2

O2 ሺgሻ+ Vo

(2)

Where Oxo is the oxide anion in the regular In2O3 lattice site. The surface chemical composition and valence states of the as-synthesized samples were further determined by XPS spectra. The survey XPS spectra of the In2S3/In2O3 heterostructures (Figure S4) verify the presence of In, S, and O atoms. The presence of O in pure In2S3 survey spectrum was due to the adsorption of carbon dioxide or oxygen on the surface of In2S3, because of their exposure to the atmosphere.36 High-resolution XPS spectra of In 3d for In2O3 and ISO(II) samples are shown in Figure 4a. As for In2O3, the XPS spectrum of In 3d reveals two symmetrical peaks at the binding energies of 451.6 eV for In 3d3/2 and 444.0 eV for In 3d5/2, respectively. While for the ISO(II) composite, the two peaks shift to higher binding energies, 452.1 eV for In 3d3/2 and 444.5 eV for In 3d5/2, respectively. Figure 4b shows the S 2p spectra of pure In2S3 and ISO(I, II, III) heterostructures. As for the pure In2S3, two strong peaks at 161.7 and 162.7 eV in the S 2p spectrum are attributed to the S 2p3/2 and S 2p1/2 binding energies, respectively. While for the In2S3/In2O3 heterostructures, the intensity of S 2p peaks increases with greater In2S3 content, and the peak positions shift to lower binding energies as compared to pure In2S3. All the shifts for binding energies in In 3d and S 2p spectra indicate possible chemical bonding formation

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between In2O3 and In2S3; this strong electronic interaction is essential to the transfer of photogenerated charge carriers and the

improvement of

photoreactivity.37,38

Furthermore, according to the quantitative analysis of XPS results, the content of the surface S element in the ISO(II) composite is 15.6 at. %. Obviously, the S content of the surface is higher than the average content (6.65 at. %), which is characterized by EDX. Therefore, it can be inferred that S accumulated primarily on the surface layer of the composites.39

Figure 4. (a) High-resolution XPS spectra of In 3d for In2O3 microflowers and ISO(II) composite, (b) high-resolution XPS spectra of S 2p and (c) O 1s for In2O3 microflowers and ISO(I, II, III) heterostructures. (d) ESR spectra of In2O3 and ISO(II) samples measured at T = 100 K. Instrument conditions: microwave power, 1.76 mW; center field, 3370 G; modulation frequency, 100 kHz.

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As shown in Figure 4c, the asymmetrical O 1s spectrum for each sample could be deconvoluted into two peaks. The binding energy at 531.1 eV is ascribed to the O-atoms in the vicinity of an O-vacancy in the In2O3 and In2S3/In2O3 composites (denoted as Ov), while the other at 529.6 eV originates from the oxygen band of In–O–In (denoted as Olatt).40–42 The quantity of Olatt gradually decreases with increasing the In2S3 content, which indicates that more lattice oxygens in In2O3 are replaced by S via the in situ anion exchange reaction, leading to the formation of more In2S3 nanoflakes. Additionally, the Ov/Olatt ratio (estimated through the integration of peak areas) could provide a sensitive indicator of the level or concentration of oxygen vacancy in the catalysts.43 The Ov/Olatt ratios of In2O3, ISO(I), ISO(II), and ISO(III) heterostructures are calculated to be 1.51, 1.92, 3.24, and 3.88, respectively. It has been reported that the formation of oxygen vacancies contributed the separation of photogenerated

charge

carriers

and

thus

improved

the

photocatalytic

performance.33,43,44 Furthermore, more evidence for the presence of oxygen vacancies could be observed from the low-temperature ESR spectra. As shown in Figure 4d, both of the In2O3 and ISO(II) composites exhibit a remarkable ESR signal at g = 2.0003, corresponding to a typical signal of oxygen vacancies.41 The signal intensity (H) increases from 88517 for In2O3 to 152878 for the ISO(II) composite, indicating that the concentration of oxygen vacancies in ISO(II) sample is higher than that in the pristine In2O3, which is consistent with the aforementioned Raman and XPS analysis. The large amount of oxygen vacancies in ISO(II) composite is related to the in situ anion exchange reaction process. Previous researches have been demonstrated that the

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oxygen vacancies can not only improve the photoabsorption ability of semiconductors, but also can act as traps for photogenerated charges to constrain the recombination of electron/hole pairs, thereby enhancing the photoreactivity.45,46

Optical and Luminescent Decay Properties. The optical absorption property of a semiconductor, which is relevant to its electronic structure, is recognized as a key factor in the assessment of its photoactivity. The UV–vis absorption spectra of the proposed samples are illustrated in Figure 5a. The In2O3 microflowers exhibit strong absorption in the range of 200–450 nm, but weak absorption in the visible range spanning 500–700 nm, which is consistent with our previous report.19 Pure In2S3 shows significant absorption from ultraviolet up to the visible region, and its absorption band edge is ~610 nm. Meanwhile, in contrast to pure In2O3, the In2S3/In2O3 heterostructures demonstrate obviously enhanced absorption in the visible region, from 450 to 630 nm, and the absorption intensity gradually increases with increasing the In2S3 content. These results verify that the in situ generated In2S3 nanoflakes with narrow bandgap and large absorption coefficient may effectively improve the light absorption properties of In2O3. Additionally, the bandgap energy (Eg) values could be calculated from the Kubelka–Munk expression, i.e., αhv = A(hv – Eg)n/2, where Eg, α, hv, and A represent the bandgap energy, absorption coefficient, absorption energy, and proportionality constant, respectively. For pure In2O3 and In2S3, the values of n are 1, as they are direct gap semiconductors.19,47 The Eg values are determined from the x-intercept in (αhv)2 versus hv plots in Figure 5b. As a consequence, the Eg of In2O3 and In2S3 are roughly estimated to be 2.80 and 2.12 eV,

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respectively, which are practically equivalent to that in the previous literatures.19,48 Meanwhile, the Eg of ISO(I), ISO(II), and ISO(III) samples are 2.50, 2.30, and 2.21 eV, respectively. These data demonstrate that the In2S3/In2O3 heterostructures have the capacity to absorb lower energy photons than In2O3, which also provide a more quantitative view of the improving trends of their photon absorption performance.

Figure 5. (a) UV–vis absorption spectra and (b) Tauc plots of In2O3, ISO(I, II, III) heterostructures, and In2S3. (c) Room-temperature PL spectra of In2O3 and ISO(I, II, III) heterostructures with the excitation wavelength of 350 nm. (d) SPV spectra of In2O3 and ISO(II) composite. Inset: schematic setup of the SPV measurement. Photoluminescence (PL) is the emission of light that originates from the recombination of photogenerated electrons and holes. It is well acknowledged that lower PL intensities may indicate higher separation efficiencies of electrons and holes, which results in higher photoreactivity.26 Figure 5c illustrates a comparison of the PL spectra of In2O3 microflowers and In2S3/In2O3 heterostructures. The traces of the four PL spectra are quite similar. The strongest luminescent peak, centered at 470 nm, might have resulted from the radiative recombination of the photogenerated holes with 15



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the electrons that occupied the oxygen vacancies.49 It is clear that the PL intensity of pure In2O3 is the strongest. However, considerable PL quenching is observed once the In2S3 nanoflakes are formed on the surface of In2O3, suggesting that the recombination rate of the photogenerated carriers was significantly constrained in the In2S3/In2O3 samples. Enhanced charge separation efficiencies contribute to prolong the lifetime of charge carriers and improve the efficacy of interfacial charge transfer to target pollutants,

thus

improving

the

photocatalytic

activity

of

the

In2S3/In2O3

heterostructures.17 The SPV amplitude as a function of the incident wavelength reveals the photoactive spectral range and the separation efficiency of the photogenerated charges in the semiconductor materials.50 Generally, a stronger SPV response indicates a higher separation efficiency of photoexcited charge.51 Figure 5d presents the SPV amplitude spectra of the In2O3 microflowers and ISO(II) composite. For pure In2O3, their is no SPV response, while the SPV response of the ISO(II) composite is extremely increased, indicating that the formation of In2S3 nanoflakes is beneficial for the separation of photogenerated charge carriers. In order to acquire an in-depth insight into the physical mechanism of SPV response, a schematic diagram illustrating the spacial distribution of built-in electric field in both of the In2O3 and In2S3/In2O3 composite is shown in Figure S5. As for the pure In2O3, the flower-like microstructure is composed of numerous, dense, and thin intertwined nanosheets. Meanwhile, the disordered built-in electric field of the component petals within the microflower scatter in all directions and counteract with each other in space. Thereby, the macroscopically

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generated SPV by the whole microflowers is negligible. For the ISO(II) composite, due to the staggered type-II band alignment formed between In2O3 and In2S3, the electrons and holes could be separated followed by directionally transfer of holes to the surface of the microspheres. Therefore, an distinctly increased SPV response is observed for the ISO(II) composite. To further understand the photophysical characteristics of photogenerated charge carriers, time-resolved PL decay spectra of the In2O3 microflowers and ISO(II) composite were measured. Figure S6 shows the PL decay traces and the corresponding fit curves via biexponential decay functions. The lifetimes and the corresponding relative intensities are listed in Table S1. The short lifetime (τ1) of the In2O3 microflowers and ISO(II) composite are 243 and 409 ps, respectively. Significantly, the long lifetime (τ2) increase to 1648 ps for the In2O3 microflowers, and 1940 ps for the ISO(II) composite, respectively. The obviously prolonged lifetime after in situ formation of In2S3 nanoflakes indicates that the recombination rate of the photogenerated electrons and holes is suppressed by In2S3.52 Such long-lived charge carriers might most likely contribute to the enhancement of the photoreactivity.53

Evaluation of Photocatalytic Performance. To compare the photocatalytic activities of In2O3 prior to and after modification with In2S3, a series of degradation experiments were performed under visible-light irradiation by using gaseous o–DCB as the model pollutant. As shown in Figure 6a, in the absence of photocatalysts, the direct self-photolysis of gaseous o–DCB is inappreciable within the test period. The conversion ratios over various samples are summarized in Table S2. Significantly, the

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conversion ratios of o–DCB in the presence of pure In2O3 and In2S3 only attain 50.4 and 53.0% after visible illumination for 8 h, respectively. As expected, all of the ISO(I, II, III) heterostructures exhibit higher photocatalytic activities than pure In2O3 and In2S3 under identical conditions. When the quantity of In2S3 is relatively low, ISO(I) composite provides relatively less available surface active sites for o–DCB adsorption and inefficient heterojunction interfaces with low photoactivity. As the In2S3 content is increased, the visible-light harvesting ability of the heterostructures enhances and additional electron–hole pairs could be generated. Furthermore, the formation of the heterojunctions contributes to the rapid separation of photogenerated electron–hole pairs. However, when the In2S3 content is above an optimal value, excess In2S3 nanoflakes within the composites may serve as recombination centers for photogenerated charges, and thus eventually restrain the photocatalytic performance. Therefore, it is vital to prudently adjust the content of In2S3 in In2S3/In2O3 composites. Meanwhile, the photocatalytic degradation kinetics towards the degradation of gaseous o–DCB over various samples was investigated by fitting the experimental data to the Langmuir–Hinshelwood model.54 Since the concentration is low, the pseudo-first-order kinetic model might be assumed as ln(Co/Ct) = kt, where k (h–1) is the pseudo-first-order rate constant, Co and Ct are the initial concentration and the instantaneous concentration after irradiation for t h, respectively. The results indicate that the reaction kinetics for all of the catalysts could be fitted to the pseudo-first-order kinetic model with satisfactory correlation coefficients (R > 0.998), as presented in Figure 6b and Figure S7. Obviously, the degradation rate is improved through the introduction of In2S3. The ISO(II) composite exhibits the highest

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degradation rate, which is ~2.1 and 16.4 times as high as that of pure In2O3 and TiO2 (P25), respectively. These results indicate that the ISO(II) composite possesses superior visible-light-induced activity for the removal of gaseous o–DCB.

Figure 6. (a) Photocatalytic degradation and (b) kinetic fit curves for the degradation of gaseous o–DCB over various samples under visible-light irradiation. Furthermore, the influence of oxygen vacancy concentration on the photocatalytic activities has also been specially studied. As depicted in Figure S8, the photocatalytic activities gradually improve with greater Ov/Olatt ratios. However, when the amount of oxygen vacancy is too high, the excess oxygen vacancies could act as recombination centres of the photogenerated charges, and lower the free carrier’s mobility,44 thus leading to a

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negative effect on the catalytic activity. In summary, the oxygen vacancies have a two-side effect on the photoreactivity. An optimum concentration of oxygen vacancies could enhance the separation of photogenerated charge carriers and improve the photoactivity. While excess oxygen vacancies would result in lower photocatalytic activity.

BET Specific Surface Area Analysis. To investigate the effects of In2S3 on the BET specific surface area (SBET) of In2O3, N2 adsorption–desorption isotherms were further carried out (Figure S9). Meanwhile, the quantitative details in regard to SBET, pore size, and pore volume of the resulting photocatalysts are listed in Table S3. Compared with the pure In2O3, the SBET of the In2S3/In2O3 heterostructures gradually decreased as the content of In2S3 was increased, which might be attributed to the blockage of In2O3 channels that were caused by the In2S3 nanoflakes during the ion exchange process. This result can also be reflected by the observation of SEM images. In general, catalysts with higher SBET could supply more molecule transport channels and surface active sites for the adsorption of contaminant molecules, which is beneficial for the enhancement of catalytic activity.55 Nevertheless, the photocatalytic performance of the unmodified In2O3 microflowers is not optimal. Accordingly, it can be concluded that the SBET of the samples may not be the predominant factor in the current work.

In Situ FTIR Investigation and Degradation Pathway Analysis. The in situ FTIR technique may provide real-time monitoring of the transient intermediates formed during the degradation of gaseous o–DCB. A series of FTIR spectra collected during the decomposition of o–DCB over ISO(II) composite are shown in Figure 7. As shown

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in Figure 7a,c, prior to visible-light illumination, three significant bands at 1577, 1462, and 1438 cm–1 can be assigned to the C=C degenerated stretching vibration of the aromatic ring.4 The bands at 748 and 1255 cm–1 (Figure 7b,d) are ascribed to the C–H bending vibration,56 and the bands at 1130 and 1039 cm–1 to the C–Cl stretching vibrarions.57 It is obvious that the characteristic bands discussed above gradually decreased during 8 h of visible illumination, suggesting that the o–DCB was oxidized gradually. According to a previous report,4 o–DCB was adsorbed at the active sites through the formation of π-complexes with aromatic rings, where the active oxygen attacked the adsorbed intermediates to generate various products.

Figure 7. In situ FTIR spectra collected as a function of irradiation time for the degradation of gaseous o–DCB over ISO(II) composite under visible-light irradiation. Spectra in the range of (a, c) 1700–1300 cm–1 and (b, d) 1300–700 cm–1, respectively.

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Meanwhile, two bands at 2355 and 2338 cm–1 (Figure S10) corresponding to CO2 obviously increased, indicating that the o–DCB molecule was mineralized to CO2.19 More interestingly, multiple new bands at 1700, 1685, 1654, 1639, 1618, 1562, 1544, 1528, 1510, 1419, 1406, 1400, 1365, and 1340 cm–1 appeared after 1 h of illumination. Among them, the bands centered at 1340, 1528, 1510, and 1700 cm–1 (formates, acetates, and maleates), 1365, 1400 and 1562 cm–1 (carbonate bidentate), and 1544 cm–1 (asymmetric stretching vibration of carboxylates with the acetate type), indicate that phenolates are the intermediate products for o–DCB degradation. This result was similar to that of o–DCB oxidation on Mn-modified Co3O4 materials.4 Previous investigations have proposed that the initial step in the degradation of o–DCB was the nucleophilic substitution process, during which the Cl atoms of the aromatic rings were abstracted and substituted by surface oxygen species, thus surface phenolates were formed. Additionally, the bands at 1639 and 1419 cm–1 could be attributed to the surface enolic species.58 Weak band at 1618 cm–1 is assigned to chlorinated surface acetate and/or acetyl halides, such as CH3COCl– or CH2ClCOO–. The bands at 1654 and 1406 cm–1 are attributed to surface o–benzoquinone type species, generated through the rearrangement of surface catechol as o–DCB was fully dechlorinated during photocatalytic degradation by nucleophilic substitution.57 Together with the previous report,4 a schematic degradation pathway is elaborately proposed (Figure 8). o–DCB is oxidized into surface carboxylates with the acetate type, o–benzoquinone, maleates, formates, and acetates step by step, followed by further oxidization to CO2 and H2O.

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Figure 8. Left: schematic degradation pathway of gaseous o–DCB. Right: schematic of the band gap positions illustrating the type-II band alignment and the electron–hole separation mechanism for the 3D In2S3/In2O3 heterostructures upon visible-light excitation.

Discussion of Catalytic Reaction Mechanism. In order to identify the photo-induced reactive oxygen species in In2S3/In2O3 heterostructures, DMPO spin-trapping ESR spectra were collected. As shown in Figure 9a, the weak characteristic peaks of DMPO–•OH could be detected following the irradiation of pristine In2O3 microflowers under visible light. However, subsequent to the formation of In2S3 nanoflakes on In2O3 microflowers, distinct characteristic peaks for DMPO–•OH (with an intensity ratio of 1:2:2:1) are observed under identical conditions.59 Meanwhile, as shown in Figure 9b, the weak peaks of DMPO–•O2− adduct could be detected for pure In2O3, whereas stronger characteristic peaks associated with such adduct are observed for the ISO(II) composite, which benefit from the efficient charge separation and long-lived charge carries. In contrast, no •OH and •O2− signals were detected in the dark, indicating that visible-light illumination is essential for the generation of •OH and •O2−.

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Figure 9. DMPO spin-trapping ESR spectra of In2O3 and ISO(II) composite for (a) •OH and (b) •O2− radicals under visible-light illumination (λ > 420 nm). To further verify the mechanism of the enhancement photoactivity, the relative band positions of the two components were investigated, as the band edge positions play a key role in determining the transfer of the photo-induced charge carriers. The valence band potential (EVB) may be calculated empirically according to the Mulliken electronegativity theory: EVB = X – Ee + 0.5Eg. Where Ee is 4.5 eV for normal hydrogen electrode (NHE), Eg is the bandgap energy value, and X is the geometric mean of the electronegativity of the constituent atoms.60 Consequently, the EVB of In2O3 and In2S3 are 2.18 and 1.26 eV vs NHE, respectively. Correspondingly, the 24


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conduction band bottom (ECB) of In2O3 and In2S3 are –0.62 and –0.86 eV vs NHE, respectively. XPS-VB spectra of pure In2O3 and In2S3 are depicted in Figure 10. It is inferred that the EVB of In2O3 and In2S3 are 2.20 and 1.23 eV below their corresponding Fermi levels, respectively. As a consequence, the values from XPS-VB are consistent with the aforementioned calculated data, indicating that the In2S3/In2O3 heterostructures possess a type-II staggered heterojunctions.

Figure 10. XPS-VB spectra of the pure In2O3 and In2S3. On the basis of the above analysis, the EVB of In2S3 is more negative than that of In2O3, whereas the ECB of In2O3 is more positive than that of In2S3. Thus, a well-defined type-II band alignment and efficient heterostructures could be formed for the rapid spatial separation of the photo-induced charge carriers when the two semiconductors were coupled. Figure 8 illustrates the energy level diagram of the In2S3/In2O3 heterostructures and the possible catalytic mechanism. Under visible-light irradiation, photons may be absorbed by both In2O3 and In2S3, to induce excess e– and h+. The photoexcited e– migrates from the surface of In2S3 to In2O3, whereas the h+ are transferred in the opposite direction from In2O3 to In2S3. As a consequence, the 25



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photoexcited e– and h+ could be effectively separated in space. The stored e– can react with the adsorbed O2 molecules to generate •O2−. Subsequently, the •O2− experiences protonation to generate •OOH, and the •OOH could be further transformed into •OH radicals, which possesses strong oxidation ability. According to the aforementioned ESR results, it can be further confirmed that •OH radicals could be generated though the •O2− → •OOH → •OH route.61 It should be noted here that the h+ in In2S3 may be not oxidative enough to react with H2O (or OH–) to form •OH radicals, due to the lower VB edge potential of 1.26 eV vs NHE (2.38 and 1.99 eV for H2O/•OH and OH–/•OH, respectively).62 Therefore, both •OH and •O2− radicals, in conjunction with most h+ on the VB of In2S3 participate in the degradation of gaseous o–DCB. 4. CONCLUSIONS In summary, novel 3D In2S3/In2O3 heterostructures were synthesized via a hydrothermal process combined with a subsequent in situ anion exchange reaction. In2S3 nanoflakes, which were uniformly assembled on the surface of In2O3 microflowers, not only enhanced the visible-light harvesting ability, but also facilitated the separation and transfer efficiency of the photogenerated e–/h+ pairs. The In2S3/In2O3 heterostructures exhibited higher visible-light photoreactivity than the unmodified In2O3 microflowers for the degradation of gaseous o–DCB. More importantly, a moderate amount of oxygen vacancies was demonstrated to be beneficial for the degradation of o–DCB. ESR detection and band alignment analyses collectively confirmed that •O2−, •OH, and photogenerated h+ were the dominant reactive species responding for the degradation of gaseous o–DCB over the 26


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In2S3/In2O3 composites. Our findings herein demonstrate the potential application of In2O3-based hybrid heterostructures as efficient visible-spectrum-responsive catalysts for the eradication of harmful chlorinated volatile organic compounds. ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (X. Y. Li). *E-mail: [email protected] (A. C. Chen). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the Major Program of the National Natural Science Foundation of China (No. 21590813), the National Natural Science Foundation of China (No. 21377015, No. 21577012) and the Key Laboratory of Industrial Ecology and Environmental Engineering, China Ministry of Education. Supporting Information Chemicals; synthetic procedure of In2O3 and In2S3/In2O3 heterostructures; characterizations; SEM image of In2S3 and EDX spectrum of ISO(II) composite; Raman spectra; survey XPS spectra; TRPL decay spectra; reaction rate constants; relationship between photoreactivity and Ov/Olatt ratio, N2 adsorption–desorption isotherms; in situ FTIR spectra in the range of 2450–2250 cm–1; photogenerated charge lifetime; conversion ratios of o–DCB over various catalysts; SBET, pore size, and pore volume data. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Poplawski, K.; Lichtenberger, J.; Keil, F. J.; Schnitzlein, K.; Amiridis M. D. Catalytic Oxidation of 1,2-Dichlorobenzene over ABO3-Type Perovskites. Catal. Today 2000, 62, 329–336. 27



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Table of Contents

3D In2O3 microflowers were controllably modified with In2S3 nanoflakes, and the resulting In2S3/In2O3 heterostructures were used as efficient photocatalysts for the abatement of gaseous ortho-dichlorobenzene.

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Facile and Controllable Modification of 3D In2O3 Microflowers with

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