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Research Article pubs.acs.org/journal/ascecg

Comparative Study of TiO2/CuS Core/Shell and Composite Nanostructures for Efficient Visible Light Photocatalysis Sunita Khanchandani,† Sandeep Kumar,†,‡ and Ashok K. Ganguli*,†,§ †

Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India Department of Chemistry, University of Delhi, Delhi 110007, India § Institute of Nano Science & Technology, Habitat Centre, Phase X, Sector 64, Mohali, Punjab 160062, India ‡

ACS Sustainable Chem. Eng. 2016.4:1487-1499. Downloaded from pubs.acs.org by OPEN UNIV OF HONG KONG on 01/25/19. For personal use only.

S Supporting Information *

ABSTRACT: An enduring impediment in the photocatalysis domain is the rapid recombination of photoinduced charge carriers. One viable strategy to realize efficient separation of photoinduced charge carriers is to design core/shell nanostructures. In this context, our work explains the substantial separation of photocarriers and enhanced light harvesting in TiO2 nanostructures following the realization of core/shell geometry with CuS. We demonstrate the design of the TiO2/CuS core/shell nanostructures, utilizing a surface-functionalizing agent, 3-mercaptopropionic acid, and offering commendable visible light driven photocatalytic performance for degradation of virulent organic pollutants of dye wastewater, like methylene blue (MB). To validate the merits of the TiO2/CuS core/shell nanostructures, we have also designed TiO2/CuS composite nanostructures under similar conditions (without utilizing the surface-functionalizing agent, 3-mercaptopropionic acid). Successful realization of TiO2/CuS nanostructures (core/shell and composite) was concluded from the PXRD, FESEM, TEM, HRTEM, EDS elemental mapping, and DRS studies. The resulting core/shell nanostructures have propitious photocatalytic performance (∼90%) over composite nanostructures (∼58%), which could be scrutinized in terms of core/shell geometry, maximizing the interfacial contact between TiO2 and CuS and enabling retardation in the recombination rate of the photoinduced charge carriers by confining electrons mainly in one component (core) and holes in the other component (shell). To have the best photocatalytic performance from the TiO2/CuS core/shell nanostructures, we also determined the optimum amount of photocatalyst (0.3 g/L) and organic pollutant dye concentration (0.003 g/L) required for visible light driven degradation of MB. A credible mechanism of the charge transfer process and mechanism of photocatalysis supported from trapping experiments in the TiO2/CuS nanostructures for the degradation of an aqueous solution of MB is also explicated. Degradation intermediates analysis performed using mass spectroscopy (MS) studies showed that MB dye degradation is initiated by a demethylation pathway. Our work also highlights the stability and recyclability of a core/shell nanostructures photocatalyst and supports its potential for environmental applications. We thus anticipate that our results bear broad potential in the photocatalysis domain for the design of a visible light functional and reusable core/shell nanostructures photocatalyst. KEYWORDS: Core/shell, TiO2 nanostructures, Type-II band structure, Visible light photocatalysis, CuS

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INTRODUCTION

band gap (Eg = 3.2 eV) that sternly precludes the utilization of visible light and results in restricted viability of TiO2 as a visible light photocatalyst.5−7 Another formidable confront still persisting with TiO2 is the rapid recombination of photo-

1

Owing to the milestone research by Fujishima and Honda, semiconductor photocatalysis has garnered global acclaim as a potential solution to degrade virulent organic pollutants for environmental sustainability.2 Among the plethora of semiconductor photocatalysts, TiO2 with its unrivaled efficiency and stability has materialized as a workhorse photocatalyst.3,4 Nevertheless, the efficacy of TiO2 is impeded by its wide © 2016 American Chemical Society

Received: November 8, 2015 Revised: December 25, 2015 Published: January 15, 2016 1487

DOI: 10.1021/acssuschemeng.5b01460 ACS Sustainable Chem. Eng. 2016, 4, 1487−1499

ACS Sustainable Chemistry & Engineering

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induced charge carriers, which substantially curtails its quantum efficiency.8,9 To alleviate these two enduring issues, attempts have been directed to reinforce the photocatalytic efficiency of TiO2 under visible light.10−12 Toward this goal, designing onedimensional nanostructures (e.g., nanorods, nanotubes, nanobelts, and nanowires, etc.) has been evidenced to manifest inherent merits, including fast collection of photoinduced charge carriers and the probability of enhanced light harvesting, thus tailoring the light absorption of TiO2 to the visible region.13,14 Further, integrating TiO2 with other semiconductor components into a single nanoscale heterostructure has proven to be a thriving strategy in enhancing the photocatalytic efficiency and separation of photoinduced charge carriers to a great extent.15−18 Of particular interest, is the integration with metal chalcogenides (e.g., PbSe, PbS, CdS, CdTe, CdSe), which are taking new strides as sensitizers and offer high extinction coefficients, broad spectral ranges, and size tunable band gaps for exploiting low energy photons.19,20 Although significant performances have been realized following the employment of cadmium and lead chalcogendies sensitizers, which post arduous environmental concerns, the quest for benign sensitizers has become imperative for clean energy demands.21−23 In this view, CuS, a benign sensitizer, with a band gap (Eg = 2.2 eV) comparable to CdS (Eg = 2.4 eV) offers a potential platform to tailor the light absorption in TiO2 nanostructures.24,25 The additional prospect of enhancing photocatalytic efficiency could be realized by constructing a core/shell geometry that confines electrons mainly in one component and holes in the other component, thus leading to retardation in the recombination rate of photoinduced charge carriers.26,27 The formation of a CuS shell onto the TiO2 nanostructures leads to a type-II band gap configuration of core/shell heterostructures, which renders support for the separation of photoinduced charge carriers through the core/ shell interface. Thus, it is envisaged that integrating a CuS shell onto the TiO2 nanostructures core unveils a type-II system, realizing commendable photocatalytic efficiency and substantial retardation in the recombination rate of photoinduced charge carriers, which are of prime relevance in the photocatalysis domain. Understanding from the above analysis, for the first time, we proposed to design type-II TiO2/CuS core/shell nanostructures following the surface functionalization route. These core/ shell nanostructures manifest commendable visible light driven photocatalytic performance for the degradation of virulent organic pollutants of dye wastewater, methylene blue (MB). To highlight the merits of TiO2/CuS core/shell nanostructures, we also designed TiO2/CuS composite nanostructures under similar conditions (without utilizing the surface-functionalizing agent, 3-mercaptopropionic acid). The propitious photocatalytic performance of TiO2/CuS core/shell nanostructures over composite nanostructures is scrutinized in terms of core/ shell geometry, which enables closer contact between TiO2 and CuS and facilitates the efficient separation of photoinduced charge carriers. Our work also unveils the admirable stability and recyclability of a core/shell nanostructures photocatalyst and thus supports the conjecture that the system may be designated as a potential, viable, and stable photocatalyst for environmental sustainability. We thus anticipate that our results bear broad potential in the photocatalysis domain for the design of a visible light functional and reusable core/shell nanostructures photocatalyst.

Research Article

EXPERIMENTAL SECTION

Synthesis of TiO2 Nanorods. Synthesis of TiO2 nanorods was accomplished through a hydrothermal method.28 In a typical procedure, 5.71 mmol pure anatase TiO2 powder was added to 80 mL of 10 N NaOH solution under continuous stirring for 2 h at room temperature (RT). The resulting milky white solution was subsequently put into a 100 mL Teflon-lined autoclave, and the reaction was performed at 180 °C for a period of 72 h. Afterward, the autoclave was cooled to RT. The resulting product (white precipitate) was separated through centrifugation and washed with double distilled water and absolute alcohol several times. Lastly, the product was washed with HCl solution at pH ∼ 7 and calcined at 700 °C for 4 h. Synthesis of TiO2/CuS Core/Shell and Composite Nanostructures. The surface-functionalization route was adopted to synthesize the TiO2/CuS core/shell nanostructures.29 3-Mercaptopropionic acid (MPA) was utilized as the functionalizing ligand for the growth of the shell onto the TiO2 nanorods. MPA (20 μL) was introduced to the 1.4 mmol of TiO2 nanorods and suspended in 100 mL double distilled water under continuous stirring for 2 h at RT. Subsequently, 0.7 mmol of Cu(NO3)2·3H2O aqueous solution was added dropwise to the reaction mixture under continuous stirring at RT for 2 h. Finally, 0.7 mmol of sulfur precursor (Na2S·9H2O) was introduced to the reaction system and stirred continuously for 2 h. The resulting green product was collected through centrifugation and washed several times with double distilled water and absolute alcohol. Synthesis of TiO2/CuS composite nanostructures was performed under similar conditions (using the same amount of precursors); however, the experiments were executed without employing MPA. Synthesis of CuS Nanoparticles. Synthesis of CuS nanoparticles was conducted following the solvothermal reaction.30 In a typical procedure, 2 mmol of Cu(NO3)2·3H2O was dissolved in 80 mL of ethylene glycol, resulting in the formation of a green colored solution. Subsequently, 4 mmol of thiourea was introduced to the above reaction mixture and stirred vigorously for 30 min. Afterward, this solution was transferred to a 100 mL Teflon-lined autoclave and maintained at 150 °C for 24 h. The autoclave was cooled to RT naturally after the completion of reaction. The resulting green product was extracted through centrifugation and washed several times with double distilled water and ethanol. Characterization. Powder X-ray Diffraction (PXRD). To identify the purity, phase composition, and structure of the resultant samples, we have utilized powder X-ray diffraction studies using an Advance diffractometer (Bruker D8) working in Bragg configuration with Cu Kα radiation (Ni-filtered). The resultant samples were gently ground to a fine powder employing a mortar and pestle. Scans were collected for a 2θ range from 10−70° with a scan rate of 0.02°/s for analysis. Field-Emission Scanning Electron Microscopy (FESEM). Morphology and surface features of the resultant samples were explored by employing the assistance of a FEI QUANTA 3D field-emission scanning electron microscope (FESEM) performing at 5 kV (accelerating voltage). Samples for FESEM studies were ultrasonicated in ethanol for 2 min and finally drop-casted onto the carbon tape supported onto the aluminum stub. Samples were subsequently sputter coated with a thin film of gold to avert the charging effects, and the FESEM images were recorded at original magnifications of ×20,000. Transmission Electron Microscopy (TEM) and High Resolution TEM (HRTEM). To achieve a detailed understanding of the morphology and corroborate the existence of core/shell and composite geometries in TiO2/CuS nanostructures, TEM and HRTEM studies were utilized. Samples were prepared by ultrasonicating the resultant samples in isopropanol and subsequently casting a drop of the sample onto an entirely carbon-coated 400-mesh copper grid. Images were recorded on a Technai G2 20 (FEI) performing at 200 kV (accelerating voltage). Energy Dispersive X-ray Spectroscopy (EDS) Elemental Mapping. Additional support for the existence of core/shell and composite geometries in TiO2/CuS nanostructures was finally furnished by employing energy dispersive X-ray spectroscopy (EDS) elemental mapping studies. The TEM instrument was accompanied by an EDS 1488


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core/shell nanostructures). The trapping experiments were performed analogous to the above photodegradation experiments of MB. Degradation Intermediates Analysis Using Mass Spectroscopy (MS). To support the photocatalysis mechanism and suggest a probable degradation pathway of MB over the TiO2/CuS core/shell nanostructures, we have analyzed the intermediates generated during photocatalysis using mass spectroscopy (MS) studies. The intermediates in the photocatalytic degradation of MB were analyzed at a regular interval by a Bruker micrOTOF-QII mass spectrometry instrument. The scanned range utilized was m/z 50−750, and only positive ions were monitored. Photoluminescence (PL) Spectroscopy. To corroborate the retardation in the recombination rate of photoinduced charge carriers of the resultant samples, photoluminescence (PL) measurements were executed. The PL studies for solid resultant samples were investigated on a fluorescence spectrophotometer (Fluoromax-4, Horiba Jobin Yvon Japan) at room temperature using an excitation wavelength (λex) of 340 nm, and the excitation and emission slit widths employed were 2 and 5 nm, respectively. The emission spectrum was scanned over a wavelength range of 250−800 nm. Electrochemical Impedance Spectroscopy (EIS). For a better understanding of the origin of commendable photocatalytic performance of TiO2/CuS core/shell nanostructures and to support PL results, we utilized electrochemical impedance spectroscopy (EIS) studies. EIS results were obtained on a Metrohm Autolab 302/ PGSTAT electrochemical work station. A three-electrode configuration comprising a working electrode (resultant samples), reference electrode (saturated Ag/AgCl), and counter electrode (platinum) is utilized. A Na2SO4 solution was utilized as the electrolyte, and the measurements were acquired in the light at an open circuit voltage over a frequency range from 100 kHz to 0.1 Hz with an AC voltage of 50 mV. Stability and Recyclability of TiO2/CuS Core/Shell Nanostructures Photocatalyst. To evaluate the stability and recyclability of the photocatalyst, five successive runs of the photodegradation process was performed. Following the completion of each run, the photocatalyst was recycled by washing with water and absolute alcohol several times and then dried at 70 °C for the next cycle of photocatalysis. The stability of the photocatalyst was scrutinized through powder X-ray diffraction studies.

detector with the capability to acquire elemental mapping images of the resultant samples. Diffuse Reflectance Spectroscopy (DRS). The light harvesting ability of the resultant samples as a dry-pressed disk was assessed by diffuse reflectance spectroscopy (DRS). DRS studies were performed on UV-2401 Shimadzu spectrophotometer accompanied by an integrating sphere accessory and utilizing BaSO4 as a diffuse reflectance standard in the wavelength spanning from 300 to 700 nm. The diffuse reflectance data is transformed to F(R∞) values, employing the Kubelka−Munk relation and expressed as F(R∞) = (1 − R∞)2/2 R∞. A transformed Kubelka−Munk relation, (F(R∞) hυ)1/2, is plotted against hυ, and extrapolation of (F(R∞) hυ)1/2 to zero F(R∞) calculates the band gap energy, unraveling the light harvesting ability of the resultant samples. Test for Photocatalytic Ability of TiO2/CuS Nanostructures for Degradation of Colored Dye, Methylene Blue (MB). Evaluation of the photocatalytic performance of TiO2/CuS nanostructures (core/shell and composite nanostructures) and their bare counterparts (TiO2 and CuS) was explored by scrutinizing the degradation of a heteropolyaromatic dye, methylene blue (MB), a virulent organic pollutant of dye wastewater under visible light illumination, activated using a 500 W xenon lamp accompanied by a UV cutoff filter to eliminate the radiations below the wavelength of 420 nm. The photocatalytic ability tests were accomplished in a 100 mL glass beaker. In visible light activated photocatalytic ability tests, 30 mg of the resultant samples was dispersed in 100 mL of 10−5 M aqueous solution of MB and then stirred magnetically for 12 h in the dark to establish complete equilibrium of adsorption/desorption between the photocatalyst (resultant samples) and the MB dye prior to visible light illumination. Appropriate aliquots of the dispersion were extracted and centrifuged to separate the residual photocatalyst particulates. Blank experiments were executed either in the absence of resultant samples (photocatalyst) or in the dark to determine that degradation of MB is truly operated by a photocatalytic mechanism. The photocatalytic ability was examined by monitoring the change in the absorbance of the characteristic wavelength at 664 nm utilizing a UV-2401 Shimadzu spectrophotometer. The photocatalytic ability or degradation efficiency (DE) of the resultant samples was assessed from the following relation31

DE (%) = (C0 − C)/C0 × 100

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(1)

RESULTS AND DISCUSSION The optimum thickness of a CuS shell on TiO2 nanorods was realized by utilizing the varying concentration of precursors (copper and sulfur) as well as modulating the amounts of the surface-functionalizing ligand, 3-mercaptopropionic acid (MPA). Analysis of the results discussed below determine that the TiO2/CuS core/shell nanostructures have pronounced absorption in the visible region and possess commendable visible light driven photocatalytic efficiency toward the degradation of virulent organic pollutant of dye wastewater, methylene blue (MB). Results of TiO2/CuS core/shell nanostructures followed by comparison with TiO2/CuS composite nanostructures and bare TiO2 are scrutinized here. PXRD Studies. Identification of the phase composition and structure of the resultant samples was performed by powder Xray diffraction (PXRD) studies, and the results are shown in Figure 1. Analysis of the PXRD studies uncovers that the reflections corresponding to TiO2 align with the welldocumented anatase phase of TiO2 (JCPDS no. 211272), along with the additional phase in the PXRD patterns of the TiO2/CuS composite and core/shell nanostructures, characterized by the reflections at 27.68°, 29.28°, 31.78°, 47.94°, 52.71°, and 59.34°, and are unquestionably assigned to the hexagonal phase of CuS (JCPDS no. 060464). A close analysis of the PXRD studies of the TiO2/CuS core/ shell nanostructures unveils a shift in the reflections

where C0 refers to the absorbance of MB after the adsorption equilibrium is achieved prior to the visible light illumination, and C refers to the absorbance of the MB at time interval t under visible light illumination. Effect of Photocatalyst (TiO2/CuS Core/Shell Nanostructures) Dosage and MB Concentration on Degradation of Aqueous Solution of MB under Visible Light Illumination. Analogous photocatalytic ability tests were conducted under visible light illumination by varying the amount of photocatalyst (0.1 to 0.5 g/L) and MB dye concentration (0.001 to 0.005 g/L) to decipher the optimum amount of photocatalyst and organic pollutant dye concentration required to avail the best photocatalytic performance from a TiO2/CuS core/shell photocatalyst. Test for Photocatalytic Ability of TiO2/CuS Core/Shell Nanostructures for Degradation of Colorless Dye, Salicylic Acid (SA). To rule out photosensitization of the organic dye pollutant, MB, under visible light illumination, the photocatalytic ability of the TiO2/CuS core/shell nanostructures was explored by performing analogous photocatalytic ability tests with the colorless organic dye, salicylic acid (SA), under similar conditions. Mechanistic Insights into Photocatalytic Degradation of MB. To achieve a full understanding of the photocatalysis mechanism of MB over the TiO2/CuS core/shell nanostructures, trapping experiments utilizing active species trappers were conducted. AgNO3, a trapper for electrons (e‑CB), ammonium oxalate, a trapper for holes (h+VB), benzoquinone, a trapper for superoxide radical anion (O2•−), and tBuOH, a trapper for hydroxyl radicals (OH•), were introduced in the reaction system prior to the inclusion of the photocatalyst (TiO2/CuS 1489


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uniform, with an average length of 700 ± 15 nm and diameter of 100 ± 6 nm. Following the introduction of the surfacefunctionalizing ligand, 3-mercaptopropionic acid (MPA), and the shell (Cu and S) precursors, it is noticed that uniform secondary CuS nanoparticles are covered on the TiO 2 nanostructures (Figure 2b), and thus, TiO2/CuS nanostructures (core/shell) exhibit larger diameters compared to the bare TiO2 nanostructures, pointing at the existence of core/shell geometry between TiO2 and CuS. To understand the importance of introducing the surface-functionalizing ligand, MPA, in the growth of uniform CuS nanoparticles onto the TiO2 nanostructures, experiments were executed without employing MPA, and the results are presented in Figure 2c, which reveal the formation of TiO2/CuS composite nanostructures. The FESEM image reveals that the TiO2/CuS composite nanostructure is a blend of agglomerated nanorods and clusters of nanoparticles. This supports the pivotal role played by MPA in surface functionalization and assists in segregation of the agglomerated nanorods, probably by electrostatic interactions, thus offering uniform growth of core/shell geometry.29 From the FESEM studies, we conclude that the inclusion of MPA results in uniform coverage of CuS nanoparticles onto the TiO2 nanostructures, thus resulting in core/shell geometry; however, performing experiments without employing MPA results in the formation of TiO2/CuS composite nanostructures. TEM and HRTEM Studies. The existence of core/shell and composite geometries in TiO2/CuS nanostructures was ultimately corroborated by TEM and HRTEM studies. Examination of TEM results shown in Figure 3a (TiO2/CuS

Figure 1. Powder X-ray diffraction patterns of bare TiO2, TiO2/CuS composite, and core/shell nanostructures.

corresponding to TiO2 toward larger angles. Analyzing from Bragg’s equation, the noticed shift of the X-ray reflections reveals the lattice compression of TiO2 with growth of the CuS shell. This discloses that the growth of the shell (CuS) contracts the lattice planes of the core (TiO2) and thus lowers the lattice constant of TiO2 in the TiO2/CuS core/shell nanostructures as compared to uncoated TiO2 nanostructures.32 Nevertheless, no shift in the X-ray reflections of TiO2 is discernible in the TiO2/CuS composite nanostructures, which hints at the absence of core/shell geometry. The calculated value of the lattice constants of TiO2 (in core/shell and composite) is summarized in Table 1. Thus, from the PXRD Table 1. Parameters Obtained from XRD Investigation of TiO2, TiO2/CuS Composite, and Core/Shell Nanostructures samples

shell thickness (nm)

cell parameter (Å)

TiO2 TiO2/CuS composite TiO2/CuS core/shell

− − 20

a = 3.786(3) c = 9.512(2) a = 3.787(3) c = 9.512(3) a = 3.780(4) c = 9.505(5)

studies, we infer the anatase and hexagonal phases for TiO2 and CuS, respectively, in TiO2/CuS core/shell and composite nanostructures and also propose the possibility of growth of the CuS shell onto the TiO2 nanostructures (in TiO2/CuS core/ shell nanostructures) from the lattice constant values. However, the convincing evidence of the core/shell and composite geometries in the TiO2/CuS nanostructures is furnished by additional studies as explored below. FESEM Studies. To gain insights into the morphology and surface features of the resultant samples, we utilized fieldemission scanning electron microscopy (FESEM) studies. As shown in Figure 2a, the morphology of TiO2 is described as rod-type with smooth surfaces. The nanorods of TiO2 are

Figure 3. TEM images of (a) TiO2/CuS core/shell and (b) TiO2/CuS composite nanostructures.

core/shell nanostructures) unveils the uniform distribution of CuS nanoparticles with an average thickness of ∼20 nm onto the surface of TiO2 nanorods (diameter 100 nm). The contrasting difference between the TiO2 core nanostructures (dark) and shells of the CuS nanoparticles (light) endorses the existence of core/shell geometry. However, the TEM results

Figure 2. FESEM images of (a) TiO2, (b) TiO2/CuS core/shell, and (c) TiO2/CuS composite nanostructures. 1490


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nanostructures is homogeneous; however, the mapping distribution of shell elements, Cu and S, displays a larger region compared to the core elements, Ti and O, supporting the results obtained from TEM and HRTEM studies that the CuS nanoparticles are uniformly covered onto the TiO2 core nanostructures. Elemental mapping studies of the TiO2/CuS composite nanostructures (Figure 5e−h) also illustrate the coexistence of Ti, O, Cu, and S elements. Noticeably, the distribution of CuS is not uniform and thus results in the scattering of CuS nanoparticles onto the core TiO 2 nanostructures. In summary, analysis from the mapping studies of the TiO2/CuS nanostructures conveys that the homogeneous distribution and larger mapping area of CuS onto the TiO2 nanostructures results in core/shell geometry; however, the uneven presence of CuS nanoparticles onto the TiO2 nanostructures results in composite geometry in the TiO2/ CuS nanostructures. DRS Studies. To calculate the band gap energy, revealing the light harvesting ability of the resultant samples, diffuse reflectance spectroscopy (DRS) studies were executed, and the results are presented in Figure 6. Bare TiO2 nanorods unveil a

portrayed in Figure 3b (TiO2/CuS composite nanostructures) illustrate the CuS nanoparticles of an average size of 20 nm scattered onto the surface of a TiO2 nanorod, pointing toward the evidence of composite geometry in the TiO2/CuS nanostructures. Further, decisive evidence for the presence of core/shell and composite geometries in the TiO2/CuS nanostructures was discerned from the HRTEM analysis. A HRTEM analysis of TiO2/CuS core/shell nanostructures (Figure 4a) acquired through a core/shell interface distinctly

Figure 4. HRTEM images of (a) TiO2/CuS core/shell and (b) TiO2/ CuS composite nanostructures.

reveals the presence of two sets of lattice fringes with spacings of 0.350 and 0.282 nm in the core and shell region, respectively. The lattice spacing of 0.350 nm in the core area correlates to the (101) plane of anatase TiO2, and the shell area identified with spacing of 0.282 nm accords well with (103) plane of hexagonal CuS. However, HRTEM studies of TiO2/CuS composite nanostructures (Figure 4b) reveal the presence of randomly distributed lattice fringes with spacing of 0.282 nm (correlating to the (103) plane of hexagonal CuS) along with the lattice fringes with spacing of 0.350 nm (corresponding to (101) plane of anatase TiO2) in the whole area, which clearly endorses the composite geometry in the TiO2/CuS composite nanostructures. Thus, TEM and HRTEM studies unambiguously support the fact that two types of TiO 2 /CuS nanostructures are synthesized: one with core/shell geometry and the other with composite geometry. EDS Elemental Mapping Studies. Information regarding the elemental composition and extent of distribution of elements in the TiO 2 /CuS core/shell and composite nanostructures was determined by energy dispersive X-ray spectroscopy (EDS) mapping studies. Results of mapping studies (Figure 5a−d) of TiO2/CuS core/shell nanostructures portray the coexistence of Ti, O, Cu, and S elements. The distribution of all the elements in the TiO2/CuS core/shell

Figure 6. UV−vis DRS of TiO2, TiO2/CuS composite, and core/shell nanostructures. Inset shows the Kubelka−Munk plot for band gap calculation of TiO2, TiO2/CuS composite, and core/shell nanostructures.

distinct absorption edge in the UV region corresponding to the band gap of 3.18 eV stemming through the transfer of valence band electrons to the conduction band. The DRS profile of the TiO2/CuS composite nanostructures present a combination of the spectral features of TiO2 and CuS alone, with TiO2 absorbing mainly in the UV region (3.13 eV) while CuS absorption is in the visible range (1.98 eV), which could originate through the weak contact between the TiO2 nanorods and CuS nanoparticles. However, this feature is not present in the TiO2/CuS core/shell nanostructures. DRS studies of the TiO2/CuS core/shell nanostructures conveys a broadened absorption onset at 2.62 eV, which could be interpreted in terms of type-II core/shell structures,33 which allows access to longer wavelengths that could not be possible with one of the two materials (either core or shell) alone. A shift in the absorption onset of TiO2 at 3.18 eV to longer wavelengths (2.62 eV) and broadening of the absorption edge with the formation of a CuS shell is the signature of type-II core/shell geometry between TiO2 and CuS, which maximizes the interfacial contact34 between the core (TiO2) and shell (CuS) and favors the confinement of one of the photoinduced charge carriers in the core and the other in the shell, leading to retardation in the recombination rate of photoinduced charge

Figure 5. EDS elemental mapping of (a−d) TiO2/CuS core/shell and (e−h) TiO2/CuS composite nanostructures. 1491


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Figure 7. (a) Photocatalytic performances and (b) degradation efficiency of TiO2, CuS, TiO2/CuS composite, and core/shell nanostructures for the degradation of aqueous solution of MB under visible light illumination.

ance of MB vs time were almost linear, implying that the reaction kinetics of photodegradation of MB concur with the first-order reaction model. The degradation constants acquired from the slope of the plot of ln(C0/C) vs t presented in Figure 8 were utilized to have a more precise comparison of the

carriers. Thus, the TiO2/CuS core/shell nanostructures offer considerable light harvesting in the visible region of the solar spectrum as discernible from the DRS results, and thus, the core/shell nanostructures are anticipated to hold photocatalytic abilities under visible light illumination. However, in the case of composite nanostructures, owing to the weak contact between TiO2 and CuS,34 an unsubstantial absorption is obtained in the visible region, and therefore, the prospect of visible light photocatalysis is unpropitious for TiO2/CuS composite nanostructures. Thus, directed by the PXRD, FESEM, TEM, HRTEM, EDS elemental mapping, and DRS studies, we univocally conclude the successful formation of two types of TiO 2 /CuS nanostructures: one with the core/shell geometry and other with composite geometry. Visible Light Activated Photocatalytic Ability for Degradation of Methylene Blue (MB). Dye wastewater poses a pervasive threat to the environment.35 In this context, to explore the worth of the TiO2/CuS nanostructures, we have performed photocatalytic tests using an aqueous solution of methylene blue dye, a virulent organic pollutant of dye wastewater under visible light illumination. The photocatalytic results shown in Figure 7 reveal that for bare TiO2 nanorods and CuS nanoparticles the photocatalytic performance was unpropitious, affording only ∼13% and 45% degradation of MB under visible light illumination for 60 min, respectively. However, introduction of the TiO2/CuS nanostructures boosts the photodegradation of aqueous solution MB, validating the fact that integrating TiO2 nanostructures with other semiconductor components is an appealing approach to boost photocatalytic efficiency. The TiO2/CuS core/shell nanostructures unveil commendable photocatalytic efficiency, affording ∼90% photodegradation of the MB solution after 60 min; however, the TiO2/CuS composite nanostructures reveal ∼58% photodegradation of an aqueous solution of MB under similar conditions. To illustrate the photocatalytic performance of the resultant samples more precisely, the kinetics of the photodegradation of an aqueous solution of MB over the resultant samples were fit to a pseudo-first-order reaction model utilizing the following integral form of the first-order equation36

ln(C0/C) = kt

Figure 8. Plot of ln(C0/C) as a function of visible light irradiation time for the degradation of aqueous solution of MB containing TiO2, CuS, TiO2/CuS composite, and core/shell nanostructures under visible light illumination.

photocatalytic performances of the resultant samples. From the analysis of degradation constants (Table 2) of the resultant samples, it is understood that the TiO2/CuS core/shell nanostructures offer the best degradation constant (3.6 × 10−2 min−1) for the degradation of an aqueous solution of MB under visible light illumination. The commendable photocatalytic performance of the TiO2/CuS core/shell nanostructures (compared to the TiO2/CuS composite nanostructures) could be credited to the core/shell geometry with a type-II band structure and enhanced light harvesting ability, which are described below: Core/Shell Geometry with Type-II Band Structure. The major factor accounting for the origin of photocatalysis in the TiO2/CuS core/shell nanostructures is the core/shell geometry, which not only maximizes the interfacial contact between TiO2 and CuS and promotes the suppression of the electron− hole recombination through the core/shell interface but also offers an effectual passivation of the surface of the TiO2 core. Utilizing core/shell geometry with a type-II band structure further provides an additional degree of freedom to enhance the spectral response. The type-II core/shell nanostructures result in a spatially indirect transition, which occurs at longer wavelengths that cannot be accessible with one of the two

(2)

where C0 is the absorbance of MB after the adsorption equilibrium is achieved prior to visible light illumination, C is the absorbance of MB at time interval t under visible light illumination, and k is the degradation rate constant of the pseudo-first-order reaction. The logarithmic plots of absorb1492


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Table 2. Optical and Photocatalytic Properties and Calculated CB and VB Positions of TiO2, TiO2/CuS Composite, and Core/ Shell Nanostructures sample

shell thickness (nm)

band gap (eV)

photocatalytic efficiency (%)

rate constant (min−1)

CB (eV)

VB (eV)

TiO2 TiO2/CuS composite TiO2/CuS core/shell

− − 20

3.18 3.13, 1.98 2.62

13 58 90

1.9 × 10−3 (±2.8 × 10−4) 1.4 × 10−2 (±1.1 × 10−3) 3.6 × 10−2 (±1.2 × 10−3)

−0.28 −0.26, −0.38 −0.28, −0.39

2.90 2.92, 1.60 2.90, 1.61

Figure 9. Schematic of charge transfer mechanism in (a) TiO2/CuS core/shell and (b) TiO2/CuS composite nanostructures under visible light illumination.

materials (either core or shell) alone.26 Thus, the retardation in charge carriers recombination and access to longer wavelengths stemming through the core/shell geometry followed by the type-II band structure imparts commendable photocatalytic activity to the TiO2/CuS core/shell nanostructures. Noticeably, a type-II band structure is also present in the TiO2/CuS composite nanostructures; however, following a weak contact between TiO2 and CuS, the collection and transportation of the carriers is inefficient, resulting in unpropitious photocatalytic performance. To validate the fact that a type-II band structure is formed between TiO2 and CuS (in TiO2/CuS core/shell and composite nanostructures), we have calculated the band gap positions of the conduction and valence band of TiO2 and CuS using the following equations37 ECB = X + E0 − 0.5Eg

shell nanostructures. The band gap of the bare CuS nanoparticles of a size of 20 nm was calculated from the DRS studies, depicted in Figure S1). From the calculated values, it is apparent that the CB and VB energies of CuS are higher energies than that of TiO2 correlating with the type-II band structure, in which both the conduction and valence bands of the shell are either lower or higher than those in the core. The band diagram of the TiO 2/CuS core/shell nanostructures presented in Figure 9a reveals that the transfer of photoinduced charge carriers takes place through the core/ shell interface. Electrons move from CB of CuS to CB of TiO2, provided by the lowered energy, and holes move from VB of TiO2 to VB of CuS under visible light illumination. As a consequence, the photoinduced charge carriers were separated at the core/shell interface of the TiO2/CuS nanostructures. However, owing to the composite geometry, which results in weak contact between TiO2 and CuS, collection and transportation of photoinduced charge carriers is not efficient in the TiO2/CuS composite nanostructures as reflected in Figure 9b. Thus, the charge transfer process will be identical (electrons move from CB of CuS to the CB of TiO2, provided by the lowered energy, and holes move from the VB of TiO2 to the VB of CuS under visible light illumination) in both the TiO2/CuS nanostructures owing to the type-II band structure, however, the presence of core/shell geometry and type-II band structure together results in superior photocatalytic activitiy in the TiO2/ CuS core/shell nanostructures through the retardation in the recombination rate of photoinduced charge carriers compared to the composite nanostructures, in which collection and transportation of photoinduced charge carriers is not efficient. Enhanced Light Harvesting Ability. As shown from the DRS studies (Figure 6), substantial visible light absorption by the TiO2/CuS core/shell nanostructures is another factor allowing commendable photocatalysis performance. However, visible light utilization by the TiO2/CuS composite nanostruc-

(3)

and E VB = ECB + Eg

(4)

where Eg is the band gap of the material, ECB is the conduction band energy, EVB is the valence band energy, E0 is the scaling factor relating the reference electrode redox level to the absolute vacuum scale (E0 = −4.5 eV for normal hydrogen electrode), and X is the electronegativity of the material, which can be expressed as the geometric mean of the absolute electronegativity of the constituent atoms. Utilizing the above equations, the calculated values of CB and VB energies of TiO2 and CuS are provided in Table 2. (In TiO2/CuS composite nanostructures, CB and VB values are calculated using the band gap of TiO2 and CuS, obtained from DRS data of the TiO2/ CuS composite nanostructures; however, in the TiO2/CuS core/shell nanostructures, CB and VB values are calculated using the band gap of bare TiO2 used during the synthesis of core/shell nanostructures and bare CuS nanoparticles of a size of 20 nm, identical to the thickness of the CuS shell in core/ 1493


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Figure 10. (a) Effect of photocatalyst (TiO2/CuS core/shell nanostructures) dosage and (b) MB concentration on degradation of aqueous solution of MB under visible light illumination.

the optimal concentration of MB solution to impact the degradation of MB solution over TiO2/CuS core/shell nanostructures under visible light illumination, experiments were executed by varying the concentration of MB solution (0.001 to 0.005 g/L) and taking a constant amount of photocatalyst (0.3 g/L). Figure 10b shows that with increasing the concentration of MB solution the rate of degradation reaches a maximum up to a certain limit; however, further increase in MB concentration caused a decline in the degradation rate. This could be assigned to the fact that increasing the MB concentration increases the probability of active species (generated during the photocatalysis of MB over the TiO2/CuS core/shell nanostructures) reacting with MB, resulting in enhanced photocatalytic performance. A decline in the photocatalytic performance with an increase in MB concentration is credited to the fact that absorption of light by the MB dye is more than that of the photocatalyst (TiO2/ CuS core/shell nanostructures). Consequently, the light absorbed by the MB dye is ineffective to trigger the degradation reaction. Additionally, with an increase in MB concentration, the active sites on the surface of the photocatalyst were enveloped with MB molecules, diminishing the active sites for the generation of active species and consequently scaling down the photocatalytic efficiency.39 Visible Light-Activated Photocatalytic Ability for Degradation of Colorless Dye, Salicylic Acid (SA). To exclude the photosensitization of the MB dye under visible light illumination, we also assessed the photocatalytic performance of the TiO2/CuS core/shell nanostructures toward photodegradation of the colorless organic dye, salicylic acid (SA), and the results are presented in Figure S2. As presumed, the photocatalyst, TiO2/CuS core/shell nanostructures, showed an impressive photocatalytic ability to degrade SA (∼86%) under visible light illumination. A convincing interpretation of the observation is that the photocatalyst (TiO2/CuS core/shell nanostructures) has true photocatalytic activity under visible light illumination, as proposed by the photocatalysis of an aqueous solution of MB. However, the mechanism of degradation of MB may not be similar to that of SA due to the structural difference between SA and MB.40 Mechanistic Insights into Photocatalytic Degradation of MB over TiO2/CuS Core/Shell Nanostructures under Visible Light Illumination. The underlying mechanism of semiconductor photocatalysis is well comprehended. In summary, when photon energy is equal to or exceeds the band gap of the semiconductor photocatalyst, electrons are promoted from the valence band (VB) to the conduction band (CB) leaving an electron vacancy or hole in the VB.41 If the

tures is discouraging for carrying out photocatalytic reactions under visible light illumination. Thus, the enhanced light harvesting ability of the TiO2/CuS core/shell nanostructures also imparts commendable photocatalytic activity to the TiO2/ CuS core/shell nanostructures. Thus, the above factors comprising core/shell geometry with a type-II band structure and enhanced light harvesting ability account for the remarkable photocatalytic performance of the TiO2/CuS core/shell nanostructures over TiO2/CuS composite nanostructures. Optimum Amount of Photocatalyst and Organic Pollutant Dye Concentration. Subsequent to understanding the factors credited for the remarkable photocatalytic performance of the TiO2/CuS core/shell nanostructures, we also determined the effect of photocatalyst dosage and MB concentration on the photodegradation of an aqueous solution of MB employing the TiO2/CuS core/shell nanostructures under visible light illumination to decipher the optimum amount of photocatalyst and organic pollutant dye concentration required to have the best photocatalytic performance from the TiO2/CuS core/shell nanostructures. Effect of Photocatalyst (TiO2/CuS Core/Shell Nanostructures) Dosage on Degradation of MB Solution under Visible Light Illumination. To gain understanding of the optimal amount of photocatalyst to influence the degradation of organic pollutants, we performed experiments by varying the amount of the photocatalyst (0.1 to 0.5 g/L) at pH 7 at 27 °C. It is evident from Figure 10a that an increase in catalyst loading enhances photocatalytic performance, which could be assigned to the fact that an increase in the number of TiO2/CuS core/shell nanostructures will increase the number of photons absorbed, accessible active sites, and consequently, number of MB dye molecules absorbed. However, a further increase in photocatalyst amount (to 0.5 g/L) shows a decline in photocatalysis performance. This is ascribed to the fact that sedimentation and agglomeration of the photocatalyst will result under large photocatalyst loadings and the accessible surface of the photocatalyst for photon absorption will diminish.38 In fact, the opacity and screening effects of a substantial number of TiO2/CuS core/shell nanostructures serve as shields and consequently impede the penetration of light, resulting in a loss of the available surface area for harvesting of light and a decline in the photocatalytic performance. Thus, the optimal dosage of the TiO2/CuS core/shell nanostructures for degradation of MB solution under visible light illumination was discerned as 0.3 g/L. Effect of MB Concentration on Degradation of MB Solution under Visible Light Illumination. To determine 1494


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ACS Sustainable Chemistry & Engineering separation of the charge carriers (electrons and holes) is retained, the electrons and holes migrate to the surface of the photocatalyst and are engaged in redox reactions leading to the generation of active species such as superoxide radical anions (O2•−) and hydroxyl radicals (OH•), which participate in the oxidation of organic dye pollutants. Thus, for a full understanding of the mechanism of photocatalysis, it is imperative to identify the leading active species involved in the degradation of MB over the TiO2/CuS core/shell nanostructures. A series of scavengers or trappers were utilized to identify the active species involved in photodegradation of MB over the TiO2/ CuS core/shell nanostructures. We introduced AgNO3 to trap conduction band electrons (e‑CB), ammonium oxalate (AQ) to trap valence band holes (h+VB), benzoquinone (BQ) to trap superoxide radical anions (O2•−), and t-BuOH to trap hydroxyl radicals (OH•) prior to the inclusion of the TiO2/CuS core/ shell nanostructures photocatalyst.42 As a consequence of introducing trappers, the reduction in photocatalytic performance implies the dominance of the corresponding active species. From the results shown in Figure 11, it is apparent that the

Offering visible light illumination to the TiO2/CuS core/ shell nanostructures, CuS acts as a visible light photosensitizer, exciting electrons from VB of CuS to CB, thus creating photoexcited species, electrons in CB, and holes in VB. Compelled by the decreased potential energy, the conduction band electrons of CuS are transferred to CB of TiO2, and in contrast, valence band holes of CuS are moved to VB of TiO2, mitigating the recombination of the photoinduced charge carriers. Afterward, the conduction band electrons are captured by molecular oxygen (O2) in the reaction system to generate the superoxide radical anions (O2•−), which consecutively results in the formation of hydroxyl radicals participating in the photocatalysis of MB. The valence band holes are also engaged in the photocatalysis process to a slight extent as supported from the trapping experiments. Thus, the above findings allowed us to conclude that the hydroxyl radicals (OH•) are the ruling active species (resulted through superoxide radical anion, which in turn formed by conduction band electrons) with minor assistance from the valence band holes in degradation of MB over the TiO2/CuS core/shell nanostructures under visible light illumination. Analysis of Degradation Intermediates Formed during Photocatalysis of MB. MS is an effective tool to analyze the degradation intermediates of MB under visible light illumination over the TiO2/CuS core/shell nanostructures. Preliminary analysis of MB using MS showed a major peak at m/z = 284.12, which corresponds to the MB dye (without catalyst). The photodegradation intermediates of MB were identified to be azure B (m/z = 270.10), azure A (m/z = 256.09), azure C (m/z = 242.07), and thionine (m/z = 228.13), which eventually fragmented into smaller molecules that are difficult to detect by mass spectroscopy (Figure 12). The formation of these degradation intermediates through demethylation of MB is consistent with the literature.43,44 Directed by the evidence provided from MS studies, we proposed a probable degradation pathway of MB involving demethylation of the MB molecule as shown in Scheme 1. Thus, analysis of intermediates through mass spectroscopy studies supports the demethylation pathway for degradation of methylene blue dye. PL Studies. We have credited the commendable photocatalytic performance of the TiO2/CuS core/shell nanostructures to retardation in the recombination rate of the photoinduced charge carriers resulting through the core/shell geometry and type-II band structure. Thus, to validate the retardation in the recombination rate of the photoinduced charge carriers of the resultant samples, photoluminescence (PL) measurements were performed with an excitation wavelength of 340 nm. Because the PL signal is contributed through recombination of the free charge carriers, we have utilized photoluminescence studies to understand the fate of the photoinduced charge carriers. Analysis of Figure 13 shows that the PL peaks of the TiO2 nanorods and CuS nanoparticles at 394 and 635 nm are credited to the band gap transition corresponding to the band gap energies of TiO2 and CuS, respectively. However, following the realization that there is a CuS shell on the TiO2 nanorods, an instant suppression of the emission of TiO2 is noticed, and a new PL peak at 478 nm emerges. This considerable shift to longer wavelengths in the absorption (DRS studies, Figure 6) and emission (PL) spectra also corroborate the existence of a shell. Moreover, the PL intensity of the TiO2/CuS core/shell nanostructures illustrates a significant decline compared to TiO2 and CuS and is ascribed to retardation in the recombination rate of the photoinduced

Figure 11. Trapping experiments using different active species trappers for photodegradation of MB over TiO2/CuS core/shell nanostructures under visible light illumination.

introduction of AgNO3 (electrons trappers), benzoquinone (superoxide radical anions trappers), and t-BuOH (hydroxyl radicals trappers) causes a considerable decline in the photocatalytic performance of the TiO2/CuS core/shell nanostructures for degradation of MB under visible light illumination. Nonetheless, only a trivial decline is discerned on introducing hole trappers (ammonium oxalate, AQ). On the bais of the information provided from the trapping experiments, we propose the following operative mechanism for the photocatalysis of MB over the TiO2/CuS core/shell nanostructures under visible light illumination based on charge separation, which is well supported by photoluminescence (PL) studies explained later. CuS + hν → CuS (e−CB.....h+ VB) CuS (e−CB) + TiO2 → CuS + TiO2 (e−CB)

TiO2 (e−CB) + O2 → TiO2 + O2·−

O2−̇ + H 2O → HO2 ̇ + OH−

HO2 ̇ + H 2O → OH· + H 2O2 H 2O2 → 2OH·

OH· + MB → H 2O + CO2 CuS (h+ VB) + MB → Degraded products 1495


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Figure 12. Mass spectra of degradation intermediates generated during photocatalysis of MB over TiO2/CuS core/shell nanostructures.

charge carriers driven by the type-II band structure and core/ shell geometry between TiO2 and CuS. The PL spectra of the TiO2/CuS composite nanostructures, however, illustrate the emission originating from the association of TiO2 (at 400 nm) and CuS (at 638 nm) alone, owing to the weak contact between the materials. The decreased emission intensity of the composite nanostructures (enabling retardation in the recombination rate of the photoinduced charge carriers) compared to the bare counterparts is accounted for by the

type-II band structure formed by the TiO2 and CuS materials. Thus, the analysis drawn from the PL studies substantiates the fact that although both the nanostructures (core/shell and composite) have a type-II band structure the presence of core/ shell geometry could efficiently slow recombination of the photoinduced charge carriers over composite nanostructures and thus accountable for the commendable visible light driven photocatalytic performance of the TiO2/CuS core/shell nanostructures. 1496


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EIS Studies. To provide a reasonable interpretation accounting for the commendable photocatalytic performance of the TiO2/CuS core/shell over composite nanostructures and to support PL analysis, electrochemical impedance spectroscopy (EIS) studies were conducted. Analysis of the results obtained from impedance spectroscopy (Figure 14) show that a

Scheme 1. Demethylation of Methylene Blue (MB) Dye over TiO2/CuS Core/Shell Nanostructures under Visible Light Illumination

Figure 14. Electrochemical impedance spectra (EIS) of TiO2/CuS core/shell and composite nanostructures under visible light illumination.

larger impedance arc radius is observed for a TiO2/CuS core/ shell over composite nanostructures, which corresponds to a larger recombination resistance as the radius of the impedance arc implies recombination resistance.45 It is well comprehended that the recombination resistance is inversely proportional to the recombination rate of the photoinduced charge carriers.46 In this regard, the TiO2/CuS core/shell nanostructures have a lower recombination rate of the photoinduced charge carriers compared to composite nanostructures. Thus, the formation of core/shell geometry between TiO2 and CuS significantly retards the recombination rate of the photoinduced charge carriers leading to the commendable visible light driven photocatalytic performance of the TiO2/CuS core/shell nanostructures over composite nanostructures as supported by our EIS results, which also are in line with the PL analysis. Stability and Recyclability of TiO2/CuS Core/Shell Nanostructures Photocatalyst. It is of supreme importance to explore the photostability and recyclability of the photocatalyst as it could appreciably reduce the cost of the photocatalytic process and thus promising photocatalysis, a fascinating strategy for the destruction of virulent organic pollutants. Toward this end, we carried out five successive runs of photodegradation of MB. Analysis of the recyclability results shows (Figure 15a) an imperceptible reduction (∼8%) in the photocatalytic performance of the TiO2/CuS core/shell nanostructures for degradation of MB under visible light illumination, which could stem through unpreventable loss in the recycling process. As illustrated in the PXRD data (Figure 15b), there is no discernible change in the pattern following the five successive cycles of photocatalysis. Thus, recycling results mirror the stability of the TiO2/CuS core/shell nanostructures photocatalyst and supports its potential for environmental applications. Thus, our work explains the development of visible light functional core/shell nanostructures affording substantial suppression in the recombination rate of photoinduced charge carriers and offers commendable photocatalytic performance for the degradation of a virulent organic pollutant of dye wastewater. The importance of core/shell geometry is also shown by synthesizing composite nanostructures under similar

Figure 13. Photoluminescence (PL) spectra of TiO2 nanorods, CuS nanoparticles, TiO2/CuS core/shell, and composite nanostructures.

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Figure 15. (a) Degradation efficiency of TiO2/CuS core/shell nanostructures with increasing number of catalytic cycles. (b) PXRD patterns of TiO2/CuS core/shell nanostructures before and after photocatalytic runs.

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conditions, which offers unpropitious photocatalytic performance owing to the composite morphology. Further, the results obtained from the stability and recyclability experiments of the core/shell nanostructures support their viable potential for environmental applications. We anticipate that our work bears vast potential in the photocatalysis domain for the development of efficacious and recyclable core/shell nanostructures.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01460. Figure S1: UV−vis DRS of CuS. Figure S2: Degradation efficiency of TiO2/CuS core/shell nanostructures for the degradation of aqueous solution of MB and SA under visible light illumination. (PDF)

CONCLUSIONS

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Our work provides a potential platform for tailoring light absorption and retardation in the recombination rate of photoinduced charge carriers in TiO2, a benchmark photocatalyst, by realizing core/shell geometry with CuS. The importance of core/shell nanostructures was determined by synthesizing the composite nanostructures of TiO2/CuS under similar conditions. Collectively, all the results acquired from the PXRD, FESEM, TEM, HRTEM, EDS elemental mapping and DRS studies conclude the successful realization of core/shell and composite nanostructures. We have probed the prospect of photocatalysis in core/shell and composite nanostructures. The TiO2/CuS core/shell nanostructures offer commendable photocatalytic performance over the corresponding composite nanostructures for degradation of MB under visible light illumination. The propitious photocatalytic performance of the core/shell nanostructures could be credited to core/shell geometry, which maximizes the interfacial contact between core and shell, thus ensuring the fast collection, transportation, and lower recombination rate of photoinduced charge carriers, as evidenced from photoluminescence and impedance spectroscopy. A credible photocatalysis mechanism for degradation of MB over core/shell nanostructures under visible light illumination is also proposed from the findings discerned through trapping experiments. Results of the trapping experiments using active species trappers conclude that hydroxyl radicals (OH•) are the ruling active species with minor assistance offered by the valence band holes in degradation of MB over the TiO2/CuS core/shell nanostructures under visible light illumination. The results offered here illustrate new insights into the photocatalysis domain for exploring novel core/shell nanostructures photocatalyst for environmental sustainability.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 91-11-26591511. Fax: 91-11-26854715. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.K.G. thanks DeitY, Department of Science & Technology (DST) and Council of Scientific and Industrial Research (CSIR), Govt. of India for financial support. S. Khanchandani and S. Kumar thank CSIR and DST, Govt. of India for a fellowship, respectively.

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REFERENCES

(1) Fujishima, A.; Honda, K. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature 1972, 238, 37−38. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Environmental Applications of Semiconductor Photocatalysis. Chem. Rev. 1995, 95, 69−96. (3) Schneider, J.; Matsuoka, M.; Takeuchi, M.; Zhang, J.; Horiuchi, Y.; Anpo, M.; Bahnemann, D. W. Understanding TiO2 Photocatalysis: Mechanisms and Materials. Chem. Rev. 2014, 114, 9919−9986. (4) Banerjee, S.; Pillai, S. C.; Falaras, P.; O’Shea, K. E.; Byrne, J. A.; Dionysiou, D. D. New Insights into the Mechanism of Visible Light Photocatalysis. J. Phys. Chem. Lett. 2014, 5, 2543−2554. (5) Yan, H.; Wang, X.; Yao, M.; Yao, X. Band structure design of semiconductors for enhanced photocatalytic activity: The case of TiO2. Prog. Nat. Sci. 2013, 23, 402−407. (6) Kurian, S.; Seo, H.; Jeon, H. Significant Enhancement in Visible Light Absorption of TiO2 Nanotube Arrays by Surface Band Gap Tuning. J. Phys. Chem. C 2013, 117, 16811−16819. (7) George, S.; Pokhrel, S.; Ji, Z.; Henderson, B. L.; Xia, T.; Li, L.; Zink, J. I.; Nel, A. E.; Mädler, L. Role of Fe Doping in Tuning the Band Gap of TiO2 for the Photo-Oxidation-Induced Cytotoxicity Paradigm. J. Am. Chem. Soc. 2011, 133, 11270−11278. 1498


Research Article

ACS Sustainable Chemistry & Engineering (8) Long, R.; English, N. J.; Prezhdo, O. V. Minimizing Electron− Hole Recombination on TiO2 Sensitized with PbSe Quantum Dots: Time-Domain Ab Initio Analysis. J. Phys. Chem. Lett. 2014, 5, 2941− 2946. (9) Pesci, F. M.; Wang, G.; Klug, D. R.; Li, Y.; Cowan, A. J. Efficient Suppression of Electron−Hole Recombination in Oxygen-Deficient Hydrogen-Treated TiO2 Nanowires for Photoelectrochemical Water Splitting. J. Phys. Chem. C 2013, 117, 25837−25844. (10) Dimitrijevic, N. M.; Tepavcevic, S.; Liu, Y.; Rajh, T.; Silver, S. C.; Tiede, D. M. Nanostructured TiO2/Polypyrrole for Visible Light Photocatalysis. J. Phys. Chem. C 2013, 117, 15540−15544. (11) Dong, J.; Han, J.; Liu, Y.; Nakajima, A.; Matsushita, S.; Wei, S.; Gao, W. Defective Black TiO2 Synthesized via Anodization for VisibleLight Photocatalysis. ACS Appl. Mater. Interfaces 2014, 6, 1385−1388. (12) Lee, W. J.; Lee, J. M.; Kochuveedu, S. T.; Han, T. H.; Jeong, H. Y.; Park, M.; Yun, J. M.; Kwon, J.; No, K.; Kim, D. H.; Kim, S. O. Biomineralized N-Doped CNT/TiO2 Core/Shell Nanowires for Visible Light Photocatalysis. ACS Nano 2012, 6, 935−943. (13) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. One-Dimensional Nanostructures: Synthesis, Characterization, and Applications. Adv. Mater. 2003, 15, 353−389. (14) Cheng, J. Y.; Zhang, F.; Chuang, V. P.; Mayes, A. M.; Ross, C. A. Self-Assembled One-Dimensional Nanostructure Arrays. Nano Lett. 2006, 6, 2099−2103. (15) Xiao, F. Construction of Highly Ordered ZnO−TiO2 Nanotube Arrays (ZnO/TNTs) Heterostructure for Photocatalytic Application. ACS Appl. Mater. Interfaces 2012, 4, 7055−7063. (16) Zhang, J.; Bang, J. H.; Tang, C.; Kamat, P. V. Tailored TiO2− SrTiO3 Heterostructure Nanotube Arrays for Improved Photoelectrochemical Performance. ACS Nano 2010, 4, 387−395. (17) Zhou, W.; Liu, H.; Wang, J.; Liu, D.; Du, G.; Cui, J. Ag2O/TiO2 Nanobelts Heterostructure with Enhanced Ultraviolet and Visible Photocatalytic Activity. ACS Appl. Mater. Interfaces 2010, 2, 2385− 2392. (18) Mu, J.; Chen, B.; Zhang, M.; Guo, Z.; Zhang, P.; Zhang, Z.; Sun, Y.; Shao, C.; Liu, Y. Enhancement of the Visible-Light Photocatalytic Activity of In2O3−TiO2 Nanofiber Heteroarchitectures. ACS Appl. Mater. Interfaces 2012, 4, 424−430. (19) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. Quantum Dot Solar Cells. Harvesting Light Energy with CdSe Nanocrystals Molecularly Linked to Mesoscopic TiO2 Films. J. Am. Chem. Soc. 2006, 128, 2385−2393. (20) Pan, Z.; Mora-Seró, I.; Shen, Q.; Zhang, H.; Li, Y.; Zhao, K.; Wang, J.; Zhong, X.; Bisquert, J. High-Efficiency “Green” Quantum Dot Solar Cells. J. Am. Chem. Soc. 2014, 136, 9203−9210. (21) Kolny-Olesiak, J.; Weller, H. Synthesis and Application of Colloidal CuInS2 Semiconductor Nanocrystals. ACS Appl. Mater. Interfaces 2013, 5, 12221−12237. (22) Li, W.; Pan, Z.; Zhong, X. CuInSe2 and CuInSe2−ZnS based high efficiency “green” quantum dot sensitized solar cells. J. Mater. Chem. A 2015, 3, 1649−1655. (23) Khanchandani, S.; Srivastava, P. K.; Kumar, S.; Ghosh, S.; Ganguli, A. K. Band Gap Engineering of ZnO using Core/Shell Morphology with Environmentally Benign Ag2S Sensitizer for Efficient Light Harvesting and Enhanced Visible-Light Photocatalysis. Inorg. Chem. 2014, 53, 8902−8912. (24) Zhang, J.; Yu, J.; Zhang, Y.; Li, Q.; Gong, J. R. Visible Light Photocatalytic H2-Production Activity of CuS/ZnS Porous Nanosheets Based on Photoinduced Interfacial Charge Transfer. Nano Lett. 2011, 11, 4774−4779. (25) Lee, M.; Yong, K. Highly efficient visible light photocatalysis of novel CuS/ZnO heterostructure nanowire arrays. Nanotechnology 2012, 23, 194014−194019. (26) Lo, S. S.; Mirkovic, T.; Chuang, C. H.; Burda, C.; Scholes, G. D. Emergent Properties Resulting from Type-II Band Alignment in Semiconductor Nanoheterostructures. Adv. Mater. 2011, 23, 180−197. (27) Reiss, P.; Protiére, M.; Li, L. Core/Shell Semiconductor Nanocrystals. Small 2009, 5, 154−168.

(28) Das, K.; De, S. K. Optical Properties of the Type-II Core−Shell TiO2@CdS Nanorods for Photovoltaic Applications. J. Phys. Chem. C 2009, 113, 3494−3501. (29) Datta, A.; Panda, S. K.; Chaudhuri, S. Synthesis and Optical and Electrical Properties of CdS/ZnS Core/Shell Nanorods. J. Phys. Chem. C 2007, 111, 17260−17264. (30) Li, F.; Wu, J.; Qin, Q.; Li, Z.; Huang, X. Controllable synthesis, optical and photocatalytic properties of CuS nanomaterials with hierarchical structures. Powder Technol. 2010, 198, 267−274. (31) Wu, T.; Zhou, X.; Zhang, H.; Zhong, X. Bi2S3 nanostructures: A new photocatalyst. Nano Res. 2010, 3, 379−386. (32) Peng, X.; Schlamp, M. C.; Kadavanich, A. V.; Alivisatos, A. P. Epitaxial Growth of Highly Luminescent CdSe/CdS Core/Shell Nanocrystals with Photostability and Electronic Accessibility. J. Am. Chem. Soc. 1997, 119, 7019−7029. (33) Kim, S.; Fisher, B.; Eisler, H.-J.; Bawendi, M. Type-II Quantum Dots: CdTe/CdSe(Core/Shell) and CdSe/ZnTe(Core/Shell) Heterostructures. J. Am. Chem. Soc. 2003, 125, 11466−11467. (34) Kim, H.; Moon, G.; Monllor-Satoca, D.; Park, Y.; Choi, W. Solar Photoconversion Using Graphene/TiO2 Composites: Nanographene Shell on TiO2 Core versus TiO2 Nanoparticles on Graphene Sheet. J. Phys. Chem. C 2012, 116, 1535−1543. (35) Vinodgopal, K.; Bedja, I.; Kamat, P. V. Nanostructured Semiconductor Films for Photocatalysis. Photoelectrochemical Behavior of SnO2/TiO2 Composite Systems and Its Role in Photocatalytic Degradation of a Textile Azo Dye. Chem. Mater. 1996, 8, 2180−2187. (36) Weng, S.; Pei, Z.; Zheng, Z.; Hu, J.; Liu, P. Exciton-Free, Nonsensitized Degradation of 2-Naphthol by Facet-Dependent BiOCl under Visible Light: Novel Evidence of Surface-State Photocatalysis. ACS Appl. Mater. Interfaces 2013, 5, 12380−12386. (37) Su, Y.; Zhu, B.; Guan, K.; Gao, S.; Lv, L.; Du, C.; Peng, L.; Hou, L.; Wang, X. Particle Size and Structural Control of ZnWO4 Nanocrystals via Sn2+ Doping for Tunable Optical and Visible Photocatalytic Properties. J. Phys. Chem. C 2012, 116, 18508−18517. (38) Madhu, G. M.; Lourdu, M. A.; Raj, A.; Pai, K. V. Titanium oxide (TiO2) assisted photocatalytic degradation of methylene blue. J. Environ. Biol. 2009, 30, 259−264. (39) Borji, S.; Nasseri, S.; Mahvi, A. H.; Nabizadeh, R.; Javadi, A. H. Investigation of photocatalytic degradation of phenol by Fe(III)-doped TiO2 and TiO2 nanoparticles. J. Environ. Health Sci. Eng. 2014, 12, 101−110. (40) Zhang, S.; Li, J.; Niu, H.; Xu, W.; Xu, J.; Hu, W.; Wang, X. Visible-Light Photocatalytic Degradation of Methylene Blue Using SnO2/α-Fe2O3 Hierarchical Nanoheterostructures. ChemPlusChem 2013, 78, 192−199. (41) Teoh, W. Y.; Scott, J. A.; Amal, R. Progress in Heterogeneous Photocatalysis: From Classical Radical Chemistry to Engineering Nanomaterials and Solar Reactors. J. Phys. Chem. Lett. 2012, 3, 629− 639. (42) Wang, T.; Li, C.; Ji, J.; Wei, Y.; Zhang, P.; Wang, S.; Fan, X.; Gong, J. Reduced Graphene Oxide (rGO)/BiVO4 Composites with Maximized Interfacial Coupling for Visible Lght Photocatalysis. ACS Sustainable Chem. Eng. 2014, 2, 2253−2258. (43) Huang, H.; He, Y.; Du, X.; Chu, P. K.; Zhang, Y. A General and Facile Approach to Heterostructured Core/Shell BiVO4/BiOI p-n Junction: Room-Temperature in Situ Assembly and Highly Boosted Visible-Light Photocatalysis. ACS Sustainable Chem. Eng. 2015, 3, 3262−3273. (44) Rauf, M. A.; Meetani, M. A.; Khaleel, A.; Ahmed, A. Photocatalytic degradation of Methylene Blue using a mixed catalyst and product analysis by LC/MS. Chem. Eng. J. 2010, 157, 373−378. (45) Kim, H.; Jeong, H.; An, T. K.; Park, C. E.; Yong, K. HybridType Quantum-Dot Cosensitized ZnO Nanowire Solar Cell with Enhanced Visible-Light Harvesting. ACS Appl. Mater. Interfaces 2013, 5, 268−275. (46) Kim, H. S.; Park, N.-G. Parameters Affecting I−V Hysteresis of CH3NH3PbI3 Perovskite Solar Cells: Effects of Perovskite Crystal Size and Mesoporous TiO2 Layer. J. Phys. Chem. Lett. 2014, 5, 2927−2934.

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