High-performance In2O3@PANI [email protected] architectures with ultralong charge carriers lifetime for photocatalytic degradation of gaseous 1,2-dichlorobenzene

Applied Catalysis B: Environmental 263 (2020) 118278

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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb

High-performance In2O3@PANI core@shell architectures with ultralong charge carriers lifetime for photocatalytic degradation of gaseous 1,2dichlorobenzene

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Fei Zhanga,b, Xinyong Lia,*, Qidong Zhaoa, Guohua Chenb,*, Qianzhe Zhanga,c a

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 b Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China c Institut de Ciència de Material de Barcelona ICMAB, Consejo Superior de Investigaciones Científicas CSIC, Campus UAB 08193 Bellaterra, Catalonia, Spain

A R T I C LE I N FO

A B S T R A C T

Keywords: In2O3-based photocatalyst Core@shell structure Charge carriers lifetime In situ FTIR 1,2-dichlorobenzene

In2O3-based nanocomposites with ultralong lifetime charge carriers were successfully prepared by deposition of a thin polyaniline (PANI) shell on the In2O3 monodispersed nanospheres and used as a visible-light responsive catalyst for the decomposition of gaseous 1,2-dichlorobenzene. The effect of PANI shell modification on the micro-structures, optical properties, as well as transfer dynamic behaviors and lifetime of charge carriers were investigated, respectively. Meanwhile, the catalytic oxidation process and the corresponding intermediates of 1,2-dichlorobenzene were investigated by in situ FTIR spectroscopy. More importantly, the optical absorption and the time-resolved photoluminescence investigation jointly corroborated that the enhancement of photocatalytic performance by modification of PANI shell was highly correlated with the substantially prolonged lifetime of charge carriers and simultaneously narrowing bandgap. Thus, it resulted in improved charge separation efficiency and visible-light absorption capability, respectively. This study provides an interesting insight and meaningful guideline for designing long-lifetime In2O3-based heterojunctions via conductive polymer coating.

1. Introduction Nowadays, given the increasing social concerns on air pollution and human health, removal of volatile organic compounds (VOCs) from atmosphere has attracted considerable attention because VOCs could lead to respiratory, neurological and central nervous system damage, even carcinogenic risk [1]. In particular, VOCs can react with nitrogen oxides or suspended particulate matter under the irradiation of sunlight (mainly UV light region) through photochemical reaction, resulting in photochemical smog which is very dangerous to human beings [2,3]. During the past few decades, many researchers have been extensively studying the abatement of VOCs by means of adsorption [4], photocatalytic oxidation [5], thermocatalytic oxidation [1,6,7], photothermocatalytic oxidation [8], non-thermal plasma treatment [3,9], and biological treatment [10]. Among them, photocatalytic oxidation, as an environmental friendly, cost-effective, and efficient method, can degrade a wide range of VOCs and potentially mineralize VOCs into harmless oxidation end-products (H2O and CO2) at room temperature

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[11]. Up to date, a large number of photocatalysts have been explored for VOCs oxidation, such as metal-free catalysts (g-C3N4) [12], spineltype structure catalysts [13–15], perovskites [16,17], metal sulfide [18], and metal oxide [19]. In contrast, transition metal oxide and/or composite oxides (e.g., TiO2 [5], ZnO [20], TiO2/WO3 [21], NiOx/ Co3O4 [22], and V2O5 [23]) have been considered as intriguing candidates and drawn much more attention due to their higher chemical stability, less secondary pollution and lower cost. These catalysts actually display noticeable oxidative reactivity for VOCs oxidation. Indium oxide (In2O3) is a typical n-type semiconductor (Eg = ∼2.8 eV) with excellent conductivity, intrinsic oxygen vacancies, and low toxicity, and it fulfills the requirements for photocatalytic applications, such as environmental remediation [24], photoelectrolysis of water [25], CO2 photoreduction [26], and nanodevices [27]. In2O3 also exhibits potential applications for the abatement of VOCs under visiblelight illumination [24,28]. However, the photocatalytic efficiency and mineralization rate of In2O3 alone is still low due to its inadequate visible-light responsive ability and low separation rate of photo-

Corresponding authors. E-mail addresses: [email protected] (X. Li), [email protected] (G. Chen).

https://doi.org/10.1016/j.apcatb.2019.118278 Received 26 July 2019; Received in revised form 7 October 2019; Accepted 9 October 2019 Available online 17 October 2019 0926-3373/ © 2019 Elsevier B.V. All rights reserved.

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generated charge carriers. To address these limitations, coupling In2O3 with other organic or inorganic materials to fabricate novel forms of heterostructures is a promising strategy. In the In2O3-based heterostructures, the photo-induced charges cross the junction due to the mismatch of electronic structure for the both components and the barriers heights alter in the migration process of charges [29]. In our previous reports, we have prepared a series of In2O3-based heterojunctions by modifying In2O3 with transition metal oxide, transition metal sulfide, and spine-type semiconductor. These catalysts demonstrate that the visible-light photocatalytic performance of In2O3 could be substantially improved [28,30,31]. In particular, the In2O3based composites possess a larger number of photo-induced charge carriers with longer lifetime in comparison with the pristine pure In2O3. However, the lifetime of charge carriers of these In2O3-based composites is still at nanosecond level. It is essential to prolong the lifetime to microsecond level in order for these catalysts to be practically feasible. Recently, conductive polymers with extensively delocalized π-conjugated electron system, such as polypyrrole (PPy), polyaniline (PANI), and polythiophene (PT), have shown great potential in electron transfer process because of their high mobility of charge carriers, superior stability, excellent hole-transporting property, and high absorption of visible light, and relatively slow charge recombination [32,33]. Previous studies have confirmed that the sensitization of semiconductor photocatalysts (e.g., Bi2O2CO3 [32], ZnO [33], α-Fe2O3 [34], TiO2 [35,36], Fe3O4 [37], CdS [38], Bi4O5Br2 [39], and Bi12O17Cl2 [40]) with PANI can promote their photocatalytic reactivity. Nevertheless, the modification of In2O3 nanospheres by PANI film and their photocatalytic application for VOCs elimination, to the extent of our knowledge, have not been reported. Besides, the role of PANI layer (as the photo-induced hole scavenger) on prolonging the lifetime of charge carriers to microsecond level is yet to be explored. Herein, we established an efficient photocatalytic system by crafting conductive PANI thin film onto In2O3 monodispersed nanospheres, which results in a well-defined In2O3@PANI core@shell architectures with intimately contacted interfaces. The modification of PANI outer shell effectively improved the visible-light responsive capability, inhibited the combination of electron/hole pairs, and more importantly, substantially prolonged the lifetime of charge carriers. As a consequence, the In2O3@PANI core@shell architectures possess superior photocatalytic activity under visible-light irradiation (λ > 400 nm), and correspondingly exhibit 3.1-folds larger reaction rate constant of 1,2-dichlorobenzene degradation than the pristine In2O3. Meanwhile, the photo-induced reactive oxygen species were confirmed by ESR technique. The enhancement mechanism of photocatalytic activity in In2O3@PANI core@shell architectures was also systematically elucidated in view of the optical properties, steady-state/time-resolved PL detection, bandgap alignments and ESR analysis. The present work makes a new design for the efficient In2O3-based photocatalytic system to achieve prolonged lifetime and promote the separation of charge carriers, as seen subsequently.

Fig. 1. Synthetic procedure of the typical In2O3@PANI core@shell architectures.

Finally, the resulting product was dried in a vacuum oven at 85 °C overnight for further characterizations. For comparison, the In2O3@ PANI core@shell composites loaded with different weight percentages of PANI (3, 5, and 8 wt.%) were also prepared and processed by following the similar procedure mentioned above. Hereinafter, the resultant samples were denoted as In2O3@3%PANI, In2O3@5%PANI, and In2O3@8%PANI, respectively. Detailed information about chemicals and materials are presented in Supporting Information. 2.2. Characterization methods All the resulting samples were systematically characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDS) detector, transmission electron microscope (TEM), X-ray photoelectron spectroscope (XPS), confocal Raman microscope, UV–vis spectrophotometer, steady-state photoluminescence (PL) emission spectrophotometer, surface photovoltage (SPV) spectroscope, time-resolved photoluminescence (TRPL) spectrophotometer, electron spin resonance (ESR) spectroscope. The detailed instrument information and detection conditions are given in Supporting Information. 2.3. Photocatalytic performance evaluation and degradation production analysis The visible-light-induced performance of the as-prepared photocatalysts were evaluated by measuring their degradation efficiency toward gaseous 1,2-dichlorobenzene in a customized quartz cell reactor (volume, 120 mL) [30]. The typical schematic graph of the in situ reaction device was shown Fig. S1. The detailed degradation procedure was as follows: 30 mg of powder photocatalysts was initially pressed into a round thin wafer (thickness, 0.5 mm; diameter, 12 mm) and inserted into an annular holder at the center of the quartz reactor; Then, 5 μL of liquid 1,2-dichlorobenzene was rapidly injected into the airtight reactor. Prior to photo-illumination, the liquid 1,2-dichlorobenzene was thoroughly volatilized in the dark for 2 h in order to establish an adsorption-desorption equilibrium at the interface of catalysts and 1,2dichlorobenzene molecules (Fig. S2). The xenon lamp was switched on afterwards. Here the irradiation condition was the same as that used for EPR detection. A series of in situ FTIR spectra of the reactant species over the catalysts were simultaneously recorded in the 4000–600 cm–1 region on a BRUKER Vertex 70 FTIR spectrometer in order to explore the intermediate species and end-product of gaseous 1,2-dichlorobenzene. The residual concentration (Ct) of 1,2-dichlorobenzene was monitored every five minutes by an offline Agilent 7890A-type gas

2. Experimental section 2.1. Synthesis of In2O3@PANI core@shell architectures In2O3 nanospheres were first synthesized by a hydrothermal reaction of InCl3·xH2O and Na3cit at 140 °C according to our previous report [30]. In2O3@PANI core@shell architectures were prepared by an impregnation method (Fig. 1). Typically, In2O3 suspension was prepared by dispersing the starting materials of In2O3 nanospheres (0.1 g) in 25 mL of N-methyl-2-pyrrolidone (NMP) in a sealed pot and ultrasonicated for 1 h. Then PANI were added into the above suspension (the weight percentage of PANI in the composite was 10 wt.%). After stirred for 24 h at room temperature (25 °C), the dark green In2O3@10%PANI precipitate was collected by centrifugation and purification with ethanol and water thoroughly until the supernatant became colorless. 2

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Fig. 2. SEM images of (a) In2O3 and (b) In2O3@10%PANI core@shell composite, (c) EDX spectrum and (d) the corresponding elemental mapping images of the In2O3@10%PANI core@shell composite.

3.2. Phase structure and composition analysis

chromatograph system during the degradation process. Therefore, the removal rate of gaseous 1,2-dichlorobenzene could be calculated according to η(%) = (1−Ct/C0)×100%, where C0 and Ct are the concentrations initially and after t minutes of illumination, respectively.

The XRD patterns of In2O3 nanospheres, In2O3@10%PANI composite, and PANI are presented in Fig. 4a. From the XRD pattern of PANI, it can be seen that a relatively sharp diffraction peak appears at 2θ = 25.2°, which could be attributed to the periodicity parallel to the polymer backbone chains of PANI [38,42,43]. As for the bare In2O3 nanospheres, all the diffraction peaks are well identified as body-centered-cubic In2O3 (JCPDS card No. 06–0416; space group, Ia3¯) [44–46]. By comparison, the XRD pattern of In2O3@10%PANI composite presents no discernible changes in peak positions and shapes, which demonstrates that the coating of PANI shell has no significant influences on the lattice structure of In2O3 and the uniform PANI layer is very thin. The similar result could also be found for the PANI/TiO2 system [47]. Furthermore, in the present case, the existence of PANI in the composite can be easily confirmed by Raman spectroscopy. Raman spectrum of the In2O3@10%PANI core@shell composite were collected in the region from 100 to 1800 cm–1. As depicted in Fig. 4b, for the pristine In2O3, six distinct Raman peaks around 130, 306, 363, 496, and 628 cm–1 can be observed, which are in agreement with the body-centered-cubic structured In2O3 reported previously [48]. The bands at 130 and 306 cm–1 are ascribed to the In–O vibrations and bending vibration of InO6 octahedra structural units, respectively. The weak band around 363 cm–1 is associated with the stretching vibration mode of In–O–In plane. While two bands 495 and 628 cm–1 are assigned to the stretching vibration modes of InO6 octahedrons [49]. After coating PANI shell, the intensity of the above peaks decreases, indicating that the coating of PANI could weaken the polarizability of In2O3 lattice field [50]. And some new Raman peaks belonging to PANI appear in the composite, which can be assigned as follows: The band at 415 cm–1 corresponds to C–N–C out of plane deformation mode of PANI. While the band around 577 cm–1 probably comes from a deformation mode of protonated amine groups [51]. The peak at 810 cm–1 is associated with C–C and C–H for benzenoid unit [52]. Besides, two peaks at 1595 and 1477 cm–1 are due to the C=N and C=C stretching modes for the quinonoid (Q) and benzenoid (B) units, respectively [53]. The bands at 1221 and 1343 cm–1 can be attributed to C–N+ stretching mode for benzenoid unit [37,50], while the band at 1166 and 1416 cm–1 are assigned to in-plane bending mode of C–H and vibration mode of quinonoid unit doped PANI, respectively [47]. Among these, the characteristic peaks of PANI around 1343, 1477, and 1595 cm–1 indicate the existence of leucomeraldine and peringraniline components of PANI [37,54]. After coupling with In2O3, it can be observed that these peaks shift to higher wavenumbers, suggesting strong

3. Results and discussion 3.1. Micro-morphological structure characterizations The micro-morphologies of the pristine In2O3 and In2O3@PANI composites were observed by SEM. As shown in Fig. 2a, the hydrothermally synthesized In2O3 exhibits a spherical morphology with diameter ranging from 200 to 300 nm. The monodispersed In2O3 nanospheres are constituted by numerous primary nanoparticles of about 25 nm in diameter. Upon the modification with PANI (Fig. 2b), the size of the nanospheres remains almost unchanged. However, the In2O3 nanospheres are seen wrapped by a thin PANI polymer film with the interfaces between the primary nanoparticles become a little bit blurred. Additionally, the EDX spectrum (Fig. 2c) and elemental mapping images (Fig. 2d) of the In2O3@10%PANI composite collectively confirm the presence and homogeneous distribution of In, O, C, and N elements. The uniformly distributed elements in the PANI modified In2O3 composite further indicate the coexistence of In2O3 and PANI components. It should be noted here that the Si signal in Fig. 2c originates from the silicon wafer used as the sample holder in the characterization. As aforementioned, the In2O3@PANI composite with core@shell architectures has been successfully prepared. The typical low-magnification TEM images of In2O3 and In2O3@ 10%PANI composite are shown in Fig. 3a and b, indicating clearly that the film-like PANI layer is conformally attached on the surface of the In2O3 nanosphere. Furthermore, the HRTEM image (Fig. 3d) was specially acquired for the In2O3@10%PANI core@shell composite as marked in Fig. 3c. It reveals obviously that the PANI layer with an average thickness of 5 nm attached on the surface of In2O3 core. PANI polymer is also found between neighboring nanospheres, which can act as binder to improve the adhesion of primary particles. The lattice spacing of 0.238 and 0.293 nm could be ascribed to the reflections from (440) and (222) planes of the body-centered-cubic In2O3, respectively [41]. It can be deduced that the In2O3@PANI composites possess a heterogeneous structure rather than a mechanical mixture of two individual phases of In2O3 and PANI.

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Fig. 3. TEM and HRTEM images of (a) In2O3 nanospheres and (b) In2O3@10%PANI core@shell architectures.

Fig. 4. XRD patterns (a) and Raman spectra (b) of In2O3 nanospheres, In2O3@10%PANI core@shell composite, and PANI.

chemical interactions took place between the In2O3 and the –NH group of PANI [52]. And this strong interaction is essential to facilitate the migration and separation of charge carriers and to induce synergetic effect on promoting the photocatalytic activity. More detailed information about the surface elemental composition and binding states of the elements in In2O3@10%PANI core@shell composite were acquired from XPS analysis. The XPS survey spectrum indicates that In, O, C, and N elements exist in the In2O3@10%PANI core@shell composite (Fig. S3). The high-resolution XPS spectra of these four elements in the composites were further analyzed. Fig. 5a presents the high-resolution XPS spectrum of In 3d. As for In2O3, two binding energies at 451.3 and 443.8 eV with a peak splitting of 7.5 eV are assigned to the spin-orbit split In 3d3/2 and In 3d5/2 levels of In2O3, indicating the existence of In3+ electronic state [41,55,56]. In contrast to the pristine In2O3, the corresponding In 3d peaks for the In2O3@10% PANI core@shell composite located at 452.0 and 444.4 eV. The slight positive shift of the peaks indicates that the chemical environment of indium has been changed. The shifts in binding energies could be

attributed to the charge the interfacial region arising from the mutual interaction (strong electronic interactions and chemical bonding) between PANI and In2O3, and accordingly there interactions are very essential for the effective surface-interface migration of photo-generated charge carriers and the enhancement of the catalytic activity [50,57]. Meanwhile, as shown in Fig. 5b, the O 1s spectrum of the In2O3@10%PANI composite can be deconvoluted into three major components, which corresponds to different oxygen containing species. The binding energies positioned at 532.7, 531.6, and 530.0 eV could be ascribed to the H–O–H, In–O–H, and In–O, respectively [34,58]. The C 1s spectrum is shown in Fig. 5c, binding energies at 287.5 and 285.6 eV correspond to C = N and C–N of PANI, respectively [35]. While another peak at 284.6 eV is assigned to the hydrocarbons from the XPS instrument or the residual carbon from the materials [59]. The N 1s core-level spectrum in Fig. 5d suggests that most of the nitrogen atoms are in the form of amine (–NH–) centered at 399.3 eV, i.e., either in the amide or benzenoid amine groups. While two additional weak peaks unveil that some nitrogen atoms exist as positively charged nitrogen (N+) form

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Fig. 5. (a) High resolution XPS spectra of In 3d for In2O3 nanospheres and In2O3@10%PANI core@shell composite. (b–d) High resolution XPS spectra of O 1s, C 1s, and N 1s for the In2O3@10%PANI core@shell composite.

significantly upon the modification of PANI. Therefore, it can be speculated that the modification of In2O3 by PANI film is beneficial to enhancing the photocatalytic performance. Photoluminescence (PL) is the emission of light, which generally originates from the recombination of photogenerated electrons and holes. Generally, weaker PL intensity suggests lower density of recombination centers and consequently longer lifetime of photogenerated carriers [64]. Fig. 6c presents the steady-state PL emission spectra of In2O3 and In2O3@PANI core@shell composites. The PL spectrum of pure In2O3 shows a main emission peak centered at 470 nm, resulting from the radiation recombination of photoinduced holes with the electrons occupying the oxygen vacancies (VO+) [65]. Compared with pristine In2O3 under identical irradiation conditions, all In2O3@PANI core@shell composites display much weaker PL emission intensity and decrease remarkably with the increasing PANI loading until 8 wt.%. Interestingly, further increase in the loading of PANI to 10 wt.%, the In2O3@10%PANI composite, shows an increase in PL intensity. This is possibly because excessive PANI or thick PANI film hinders the charges from transferring to the surface. Anyhow, the In2O3@PANI core@shell composites exhibit a relatively low recombination rate of charge carriers. This could also contribute to the enhanced photocatalytic activities of In2O3@PANI core@shell composites, which will be seen subsequently [36,42]. SPV measurement was further carried out to reveal the dynamic behaviors of the photogenerated charges in the In2O3@PANI composites (Fig. 6d). The pristine In2O3 exhibits a quite weak SPV response in the region of 300–430 nm, which is attributed to the electron transition from valence band (O2p) to conduction band (In3d) of In2O3. For In2O3@8%PANI core@shell composite, an apparent SPV response ranging from 300 to 470 nm is observed, which is slightly broader than that of pure In2O3. Additionally, the SPV response intensity is strengthened remarkably in the In2O3@8%PANI core@shell composite, indicating that the introduction of PANI layer is beneficial to the separation of photogenerated electron-hole pairs. The spatial distributions of the built-in electric field in the In2O3 nanospheres and In2O3@

centered at 400.7 eV and imine (=N–) form centered at 398.1 eV [60,61]. These XPS results further confirm that physicochemical interactions exist between In2O3 and PANI, which is in good agreement with Raman observations. 3.3. Optical absorption and photoluminescence decay properties The light-absorption ability plays a vital role in determining the photocatalytic activity of the photocatalysts. The UV–vis absorption spectra of the resulting photocatalysts are shown in Fig. 6a. In2O3 nanospheres have a strong absorption in the UV range of 250 − 400 nm, the onset of light absorption is around 480 nm. The In2O3@PANI core@ shell composites exhibit broader absorption in the visible-light region with the PANI shell serving as the visible-light sensitizer. Therefore, an enhancement of the photo-absorption intensity for the composites can be noticed in the range of 500–800 nm with the increasing PANI content. The optical absorption of PANI in visible spectrum region can be attributed to the π−π* transition in the benzenoid and quinonoid units. Incidentally, the color of the photocatalysts changed from pale yellow for In2O3 to dark green for In2O3@PANI core@shell composites, and the color became darker with the increasing PANI content (Fig. S4). Thus, superior photocatalytic performance of the In2O3@PANI core@shell composites is expected over that of pure In2O3 under visible-light irradiation. The bandgap energy of a semiconductor could be further estimated by Tauc formula [62], which can be expressed by (αhν)1/n = A(hν – Eg). Herein, hν, Eg, A, and α are the photonic energy, bandgap value, proportionality constant, and optical absorption coefficient, respectively. The electronic transition property determines the value of exponent, i.e., n = 1/2 for direct transition, and n = 2 for indirect transition [63]. In the present study, Eg was determined from the plots of (αhν)2 versus hν. As shown in Fig. 6b, the Eg of the In2O3 is 2.80 eV, while the Eg of In2O3@PANI core@shell composites ranges from 2.78 eV for In2O3@ 3%PANI to 2.39 eV for In2O3@10%PANI. The decrease of the Eg again demonstrates that the visible-light responsive range is broadened 5

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Fig. 6. (a) UV–vis absorption spectra of In2O3 nanospheres and In2O3@PANI core@shell composites with different contents of PANI. (b) Calculation of the bandgap energies by Tauc plots of (αhν)2 vs. hν. (c) Steady-state PL spectra of In2O3 nanospheres and In2O3@PANI core@shell composites, recorded at an excitation wavelength of 350 nm. (d) SPV spectra of In2O3 nanospheres and In2O3@8%PANI core@ shell composite.

Fig. 7. Schematic diagram illustrating the spatial distributions of the built-in electric field in the pristine In2O3 nanospheres (top) and In2O3@PANI core@shell heterostructures (bottom).

between In2O3 core and PANI shell. According to the previous reports, the suppression of PL spectra generally implies either a faster migration process with a shorter decay time or a slower recombination process with a longer decay lifetime for photogenerated charge carriers [66,67]. Based on this consideration, the lifetime of the photogenerated charge carriers for pure In2O3 and In2O3@8%PANI core@shell composite were further investigated by time-resolved transient PL decay spectroscopy. Typically, the short lifetime (τ1) is induced by quasi-free excitons, while the long lifetime (τ2) is attributed to the localized exciton recombination, which is caused by de-trapping of carriers [68]. As shown in Fig. 8a and b, the PL decay data of pure In2O3 and In2O3@8%PANI core@shell composite

PANI core@shell composites are shown in Fig. 7, from which an indepth physicochemical mechanistic insight into the photogenerated charge carriers could be acquired. As the monodispersed In2O3 nanospheres are composed of numerous primary nanoparticles, the disordered built-in electric field of each nanoparticle within the nanospheres scatter toward all directions and counteract with each other. Therefore, the macroscopically generated SPV signal by the whole nanospheres could be neglected. For the In2O3@PANI core@shell composites, the electrons and holes separated effectively due to the staggered band alignment formed between In2O3 and PANI. Thereby, a distinctly increased SPV response could be detected for the In2O3@ PANI composites. This result also confirms the necessity of tight contact 6

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Fig. 8. Time-resolved transient PL decay traces (a) and the corresponding fitted curves by multi-exponential decay functions (b) of In2O3 nanospheres and In2O3@8% PANI core@shell composite. The emission was collected in the range of 400–750 nm upon excitation at 380 nm.

were fitted by biexponential decay function I(t) = A1*exp(−t/τ1) + A2*exp(−t/τ2), where I(t) is the PL intensity at a certain delay time, A1 and A2 are the relative weights of the decay components at t = 0 [69]. Consequently, τ1 of the In2O3 and In2O3@8%PANI composite are 0.28 ns and 2.11 μs, respectively. While τ2 of the In2O3 and In2O3@8% PANI composite are 1.92 ns and 8.14 μs, respectively. Up to date, the longest lifetime of charge carriers reported previously is in the Agdoped Zn-In-S quantum dots. However, τ1 and τ2 of the Ag:Zn-In-S QDs were just prolonged to 12.58 and 316.91 ns, respectively [69]. More detailed comparison about the lifetime results could be found in Table S1. Obviously, in the In2O3@PANI core@shell composites, both the short and long lifetime of photogenerated charge carriers are dramatically prolonged to a microsecond level. The ultralong lifetime of photogenerated charge carriers manifests that the separation efficiencies of the photogenerated charge carriers in the In2O3@PANI composites can lead to a superior separation efficiency so as to increase the probability of charge carriers’ involvement in the photocatalytic degradation process before recombination occurs, and thus improve the photocatalytic performance.

performance compared to that of the pristine In2O3 under identical reaction conditions. After illuminated for 8 h, In2O3@8%PANI composite possesses the best photocatalytic activity with 82.7% of 1,2-dichlorobenzene degraded, which is higher than 9.1%, 40.2%, 50.2%, and 62.6% of degradation over commercial TiO2, pure In2O3, In2O3@ 3%PANI, and In2O3@5%PANI, respectively. Further increasing the loading mass of PANI to 10 wt.%, however, the degradation rate decreases to 66.7%. In addition, the linear relationship between ln(C0/Ct) and illumination time (t) was established, indicating that the kinetics of 1,2-dichlorobenzene degradation was fitted well with pseudo-first-order model: ln(C0/Ct) = kt. Herein, k (h−1) is the apparent reaction rate constant (Fig. 9b and Fig. S5). k value of In2O3@8%PANI composite is 3.1 times that of pristine In2O3, 2.4 times that of In2O3@3%PANI, and 1.5 times that of In2O3@10%PANI composite. The results confirm the expectation from the physicochemical analysis of the In2O3@PANI composites. First of all, PANI shell can enhance the photocatalytic performance of In2O3. Secondly, there exists an optimal amount of PANI applied. For the present case, 8 wt.% PANI gives the best performance. The superior activity of the In2O3@8%PANI composite is also associated with the specific core@shell structure, enhanced charge separation, improved visible-light absorption capability, and more importantly, ultralong lifetime of photogenerated charge carriers, as discussed above. The reusability of the In2O3@8%PANI core@shell composite was also investigated. The In2O3@PANI core@shell composites exhibits good reusability and catalytic stability (Fig. S6), stable crystal structure, as well as good structural stability (Fig. S7).

3.4. Photocatalytic reactivity of In2O3@PANI core@shell composites As is known, VOCs contain a large kind of organic compounds with high volatility at room temperature. Among them, chlorinated VOCs (Cl-VOCs) with aromatic rings are more toxic to the environment and human beings [6]. Typically, 1,2-dichlorobenzene, an important precursor of polychlorinated dibenzofurans and dibenzodioxins, has been utilized as the model molecule for the investigation of complex Cl-VOCs degradation [70]. Herein, photocatalytic degradation of 1,2-dichlorobenzene was performed under visible-light irradiation in order to demonstrate the potential capability of the In2O3@PANI composites in Cl-VOCs purification. As can be seen in Fig. 9a, all the In2O3@PANI core@shell composites exhibit obviously improved photocatalytic

3.5. In situ FTIR investigation In situ FTIR spectra can provide a real-time monitoring of the reaction process, i.e., transient intermediates and final products generated during the photo-oxidation of gaseous 1,2-dichlorobenzene, which

Fig. 9. Photocatalytic degradation profiles (a) and the corresponding kinetic fitting curves (b) for the decomposition of gaseous 1,2-dichlorobenzene over various catalysts under visible-light irradiation. 7

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Fig. 10. In situ FTIR spectra for the degradation of gaseous 1,2-dichlorobenzene over In2O3@8%PANI composite recorded at different irradiation time. Bands recorded in the range of (a) 1200 − 1300 cm−1, (b) 1700 − 1300 cm−1, (c) 3750 − 3600 cm−1, and (d) 2420 − 2280 cm−1.

Based on the above analysis, the photocatalytic degradation pathway for 1,2-dichlorobenzene could be finally proposed, as shown in Fig. 11e.

would have a great significance for investigating the catalytic mechanism. In the present work, a series of FTIR spectra were collected continuously over the In2O3@8%PANI composite. As shown in Fig. 10a, doublet bands located at 1040 and 1131 cm−1 are attributed to the C − Cl asymmetric and symmetric stretching vibrations, respectively [71]. In Fig. 10b, the bands centered at 1436, 1461, and 1579 cm−1 could be ascribed to the C]C stretching vibrations of the aromatic ring [21,70]. The intensities of the above bands decrease obviously during 8 h of irradiation (Fig. 11a and b), indicating that the 1,2-dichlorobenzene molecules are decomposed gradually. Interestingly, a series of new peaks appear as irradiation time prolonged (Fig. 10b). Herein, two new shoulder bands at 1673 and 1684 cm−1 confirm the generation of partially oxidized carboxylates group (e.g., formats and acetates). The weak band at 1637 cm−1 is attributable to the C–H inplane bending vibrations of the surface adsorbed enolic type species, while the bands at 1541 and 1559 cm−1 could be ascribed to the surface chlorinated acetates and acetyl halides (e.g., CH3COCl − and CH2ClCOO−) [71]. The band at 1507 cm−1 reflects the formation of surface carbonated species belonging to bidentate formate species [72]. And the bands at 1339 and 1384 cm−1 could be assigned to the surface adsorbed maleate species [19]. Meanwhile, multiple new bands corresponding to the surface hydroxyl groups also appear (Fig. 10c), and their intensities are strengthened as time prolonged (Fig. 11c), which are very important for the oxidation of 1,2-dichlorobenzene. The bands at 3734 and 3725 cm−1 could be ascribed to terminal adsorbed hydroxyls, which can trap photoinduced holes to form •OH species. The bands at 3715 and 3609 cm−1 are assigned to surface adsorbed water species, indicating the generation of water during the decomposition process. Additionally, the bands at 3698, 3685, and 3646 cm−1 are attributed to bridged −OH, providing adsorption sites for 1,2-dichlorobenzene molecules [18,73,74]. Furthermore, two new bands appearing at 2360 and 2341 cm−1 (Fig. 10d), with increasing intensity in the first five hours (Fig. 11d), manifest the generation of CO2 species.

3.6. Photocatalytic mechanism investigation and reactive oxygen species confirmation With the degradation mechanism know, the remaining question to answer is the photocatalytic mechanism, namely what oxidants are responsible for the pollutant degradation. In the composite photocatalysts, the separation and transfer approach could be elucidated from the view of band gap alignment. As an inorganic semiconductor, the valence band potential (EVB) and conduction band potential (ECB) of In2O3 relative to zero potential point could be calculated empirically according to Mulliken electronegativity theory: ECB = X – Ee – 0.5Eg. Where X is defined as the geometric mean of the electronegativity of the constituent atoms (5.28 eV for In2O3), Eg represents the bandgap energy of the semiconductor, and Ee is the energy of free electrons on the hydrogen scale (∼4.5 eV vs. NHE) [75]. Consequently, the ECB and EVB of In2O3 are –0.60 and 2.20 eV vs. NHE, respectively. Meanwhile, as shown in Fig. S8, the π-orbital edge (HOMO) and π*-orbital edge (LUMO) of PANI are calculated to be 0.65 and –2.07 eV, respectively. As illustrated in Fig. 12a, one can clearly observe that the VB (top) and the CB (bottom) of In2O3 are more positive than the π-orbital edge and π*-orbital edge of PANI, respectively. Therefore, based on the energy levels of In2O3 and PANI, a possible transfer and separation mechanism of the charge carriers in the In2O3@PANI core@shell composites could be proposed and schematically described as follows (see Fig. 12b): PANI + hν → π-orbital(h+) + π*-orbital(e−)

(1)

−

In2O3 + hν → In2O3VB(h ) + In2O3CB(e ) +

−

(2) −

π*-orbital(e ) + In2O3CB → π*-orbital + In2O3CB(e ) 8

(3)

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Fig. 11. Intensity changes of the in situ FTIR spectra for the degradation of gaseous 1,2-dichlorobenzene over In2O3@8%PANI composite. Bands recorded in the range of (a) 1200 − 1000 cm−1, (b) 1700 − 1300 cm−1, (c) 3750 − 3600 cm−1, and (d) 2420 − 2280 cm−1. (e) The proposed degradation pathway of gaseous 1,2dichlorobenzene over the In2O3@PANI composites.

In2O3VB(h+) + π-orbital → In2O3VB + π-orbital(h+)

carriers is remarkably restrained, and thus the lifetime of the charge carriers is prolonged substantially. To further ascertain the generation of reactive oxygen species (ROSs) responsible for the degradation of gaseous 1,2-dichlorobenzene, DMPO spin-trapping ESR technique was employed [76]. As a control experiment, the characteristic ESR signals were undetectable in dark for both DMPO–•OH and DMPO–•O2− adducts, corroborating that visiblelight irradiation is vital for the generation of ROSs in the photocatalytic system. As shown in Fig. 13a, upon irradiation for 5 min, weak

(4)

Under visible-light irradiation, both In2O3 and PANI could be activated into excited state and generate electrons and holes (Eqs. (1) and (2)). The excited state electrons generated on the π*-orbital of PANI thermodynamically migrate to the CB of In2O3 (Eq. (3)). While the holes in the VB of In2O3 are injected into the π-orbital of PANI (Eq. (4)). Therefore, the photoinduced electrons and holes are effectively separated in micro-space and accumulated on In2O3 core and PANI shell, respectively. Consequently, the recombination process of the charge 9

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Fig. 12. (a) Schematic diagram showing the band positions of In2O3 and PANI. (b) Representation of the separation of photogenerated electron-hole pairs and migration mechanism for In2O3@PANI core@shell architectures under visible-light irradiation.

DMPO–•O2− signal was detected for pure In2O3. By comparison, six stronger characteristic peaks of the DMPO–•O2− adducts can be observed for In2O3@8%PANI composite, which demonstrates that more photogenerated electrons accumulate on the CB of In2O3, and thus massive •O2− can be generated via the reduction of chemisorbed O2 by photogenerated electrons. Herein, plenty of electrons accumulated on the CB of In2O3 in the In2O3@PANI system could be attributed to two aspects: the efficient separation of photogenerated electrons and holes in In2O3 itself and the migration of electrons from PANI film to the CB of In2O3. Meanwhile, the signals of DMPO–•OH adducts were also detected. As can be seen from Fig. 13b, for pure In2O3, four distinct characteristic peaks (with an typical intensity ratio of 1 : 2 : 2 : 1) associated with DMPO–•OH could be clearly observed, and this is similar to the previous report [77]. Because the potential energies of OH–/•OH and H2O/•OH are 1.99 and 2.38 eV vs. NHE, respectively [78,79], only a few holes on the CB of In2O3 can oxidize OH– into %OH. On the other hand, the •OH might be generated via the O2 → %O2− → H2O2 → %OH reaction route. As for the In2O3@PANI composites, the signal intensity of DMPO–%OH adducts is much weaker as compared with that of In2O3. This is because the π-orbital edge of PANI is lower than that of E(OH–/%OH) and E(H2O/%OH), the holes accumulated in the π-orbital of PANI are not oxidative enough to react with OH– or H2O, so hardly any •OH radicals could be produced in the In2O3@PANI composites. This indicates that the holes might participate in the oxidation of 1,2-dichlorobenzene directly, while •OH plays a minor role in the degradation of 1,2-dichlorobenzene over the In2O3@PANI composites, based on the preceding results and discussion.

subsequent impregnation method. In this case, the PANI thin shell wrapping on the surface of In2O3 broadened the visible-light absorption region and intensity, as well as improved the separation and transfer efficiency of the photoinduced charge carriers at the interface between PANI and In2O3 resulted from their well-matched energy band. Moreover, the charge carriers lifetime of In2O3@PANI catalysts could dramatically prolong to a microsecond level, which is the decisive factor that allows much more photoinduced charges to participate in the photocatalytic reaction, and ultimately contributes to the supreme photocatalytic performance of In2O3@PANI composites. As a result, the degradation efficiency of gaseous 1,2-dichlorobenzene over the In2O3@ 8%PANI composite is 3.1 times enhancement over the bare In2O3 monodispersed spheres within 8 h of visible-light illumination. The generated ROSs, such as h+ and %O2− are responsible for the degradation of 1,2-dichlorobenzene. In addition, the In2O3@8%PANI composite performed good physicochemical stability after four rounds of recycling. Beyond the enhanced catalytic capacity and stability, the present study demonstrates the significance of prolonging the lifetime of charge carriers, which may inspire more ideas focusing on modulation of the dynamic behaviors of charge carriers for efficient photocatalytic reaction systems.

4. Conclusion

Acknowledgements

In conclusion, the In2O3@PANI composites with core@shell architectures that possessed ultralong charge carriers lifetime were successfully synthesized by combining a hydrothermal process with a

The authors gratefully acknowledge the financial support from the Major Program of the National Natural Science Foundation of China (21590813), the National Natural Science Foundation of China (Nos.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 13. DMPO spin-trapping ESR spectra recorded at ambient temperature with In2O3 and In2O3@8%PANI core@shell composite. (a) In methanol dispersion for DMPO–•O2− adducts, and (b) in phosphate buffer for DMPO–•OH adducts under visible-light irradiation for 5 min. 10

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21377015 and 21577012), the Key R&D Program of the National Ministry of Science and Technology (2016YFC0204204), the Program of Introducing Talents of Discipline to Universities (B13012), and the Key Laboratory of Industrial Ecology and Environmental Engineering, China Ministry of Education.

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