Shell and Composite

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Comparative Study of TiO2/CuS Core/Shell and Composite Nanostructures for Efficient Visible-Light Photocatalysis Sunita Khanchandani, Sandeep Kumar, and Ashok Kumar Ganguli ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01460 • Publication Date (Web): 15 Jan 2016 Downloaded from http://pubs.acs.org on January 21, 2016

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ACS Sustainable Chemistry & Engineering

Comparative Study of TiO2/CuS Core/Shell and Composite Nanostructures for Efficient Visible-Light Photocatalysis Sunita Khanchandania, Sandeep Kumara,b and Ashok K. Gangulia,c* a

Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India b

c

Department of Chemistry, University of Delhi, Delhi 110007, India

Institute of Nano Science & Technology, Habitat Centre, Phase X, Sector 64, Mohali, Punjab 160062, India

Corresponding Authors *A.K.G.: e-mail: [email protected] Tel: 91-11-26591511 Fax: 91-11-26854715

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ABSTRACT An enduring impediment in the photocatalysis domain is the rapid recombination of photoinduced charge carriers. One viable strategy to realize the efficient separation of photoinduced charge carriers is to design core/shell nanostructures. In this context, our work unravels 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 TiO2/CuS core/shell nanostructures, utilizing surface-functionalizing agent, 3mercaptopropionic acid and offering commendable visible light driven photocatalytic performance for the degradation of virulent organic pollutants of dye waste water, like methylene blue (MB). To validate the merits of TiO2/CuS core/shell nanostructures, we have also designed TiO2/CuS composite nanostructures under similar conditions (without utilizing surfacefunctionalizing 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 manifest 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 enables the retardation in the recombination rate of photoinduced charge carriers by confining electrons mainly in one component (core) and holes in the other component (shell). To avail the best photocatalytic performance from TiO2/CuS core/shell nanostructures, we also deciphered the optimum amount of photocatalyst (0.3 g/L) and the organic pollutant dye concentration (0.003 g/L) required for the visible light driven degradation of MB. A credible mechanism of the charge transfer process and mechanism of photocatalysis, supported from the trapping experiments in TiO2/CuS nanostructures, for the degradation of aq. solution of MB is also explicated. Degradation intermediates analysis was 2 ACS Paragon Plus Environment

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performed using mass spectroscopy (MS) studies showed that the MB dye degradation is initiated by demethylation pathway. Our work also highlights the stability and recyclability of these 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 visible light functional and reusable core/shell nanostructures photocatalyst. Keywords:

Core/shell,

TiO2

Nanostructures,

Type-II

band

structure,

Visible-light

Photocatalysis, CuS INTRODUCTION Owing to the milestone research by Fujishima and Honda,1 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 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 persists with TiO2 is the rapid recombination of photoinduced 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 Towards this goal, designing one dimensional 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 visible



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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 offers high extinction coefficient, broad spectral range and size tunable band gaps for exploiting low energy photons.19-20 Though, 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 to seek for clean energy demand.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 Additional prospect of enhancing the photocatalytic efficiency could be realized by constructing core/shell geometry which confines electrons mainly in one component and holes in the other component, thus leading to the retardation in the recombination rate of photoinduced charge carriers.26-27 The formation of CuS shell onto the TiO2 nanostructures leads to a type-II band gap configuration of the core/shell heterostructures which renders a support for the separation of photoinduced charge carriers through the core/shell interface. Thus, it is envisaged that integrating CuS shell on to TiO2 nanostructures core unveils 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 4 ACS Paragon Plus Environment

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for the degradation of virulent organic pollutants of dye waste water, 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 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 the 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 these core/shell nanostructures photocatalyst and thus supports the conjecture that the system may be designated as 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 visible light functional and reusable core/shell nanostructures photocatalyst. EXPERIMENTAL SECTION Synthesis of TiO2 nanorods: Synthesis of TiO2 nanorods was accomplished through 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 to a 100 mL Teflon-lined autoclave and reaction was performed at 180 oC for a period of 72 h. Afterwards, the autoclave was cooled to RT. The resulting product (white precipitate) was separated through centrifugation, 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.



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Synthesis of TiO2/CuS core/shell and composite nanostructures: 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 shell onto the TiO2 nanorods. MPA (20 µL) was introduced to the 1.4 mmol of TiO2 nanorods, 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 sulphur precursor (Na2S.9H2O) was introduced to the reaction system and stirred continuously for 2 h. The resulting green product was collected through centrifugation, washed several times with double distilled water and absolute alcohol. Synthesis of TiO2/CuS composite nanostructures was performed under similar conditions (using 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 ethylene glycol, resulting in the formation of green colored solution. Subsequently, 4 mmol of thiourea was introduced to the above reaction mixture and stirred vigorously for 30 min. Afterwards, this solution was transferred to 100 mL Teflon-lined autoclave and maintained at 150 oC for 24 h. The autoclave was cooled to RT naturally, after the completion of reaction. The resulting green product was extracted through centrifugation, 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


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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 2θ range from 10°−70° with the 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 FEI QUANTA 3D fieldemission 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 x 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 geometry 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 a holey 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 geometry in TiO2/CuS nanostructures was finally furnished by employing energy dispersive X-ray spectroscopy (EDS) elemental mapping studies. The TEM instrument was accompanied with an EDS 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 dry-pressed disk were assessed by diffuse reflectance spectroscopy (DRS). DRS studies were 7 ACS Paragon Plus Environment


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performed on UV-2401 Shimadzu spectrophotometer, accompanied with an integrating sphere accessory and utilizing BaSO4 as a diffuse reflectance standard in the wavelength spanning from 300-700 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 the Photocatalytic Ability of TiO2/CuS Nanostructures for the Degradation of Colored Dye, Methylene Blue (MB): Evaluation of photocatalytic performance of TiO2/CuS nanostructures (core/shell and composite nanostructures), and its bare counterparts (TiO2 and CuS) was explored by scrutinizing the degradation of heteropolyaromatic dye, methylene blue (MB), a virulent organic pollutant of dye waste water under visible light illumination, activated using a 500 W xenon lamp, accompanied with a UV cutoff filter, to eliminate the radiations below wavelength of 420 nm. The photocatalytic ability tests were accomplished in a 100 mL glass beaker. In a visible-light activated photocatalytic ability tests, 30 mg of the resultant samples was dispersed in 100 mL of 10-5 M aq. solution of MB and allowed to stir magnetically for 12 h in dark to establish complete equilibrium of adsorption/desorption between the photocatalyst (resultant samples) and the MB dye prior to the 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 dark to evince 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


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spectrophotometer. The photocatalytic ability or degradation efficiency (DE) of the resultant samples was assessed from the following relation31 DE (%) = (C0 – C) / C0 x 100

(1)

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 aq. 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 the organic pollutant dye concentration required to avail the best photocatalytic performance from TiO2/CuS core/shell photocatalyst. Test for the Photocatalytic Ability of TiO2/CuS Core/Shell Nanostructures for the Degradation of Colorless Dye, Salicylic Acid (SA): To rule out the photosensitization of the organic dye pollutant, MB under visible light illumination, photocatalytic ability of TiO2/CuS core/shell nanostructures was explored by performing the analogous photocatalytic ability tests with the colorless organic dye, salicylic acid (SA) under similar conditions. Mechanistic Insights into the Photocatalytic Degradation of MB: To achieve a full understanding of the photocatalysis mechanism of MB over 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 t-BuOH, a trapper for hydroxyl radicals (OH•), were introduced in the reaction system prior to the inclusion of photocatalyst (TiO2/CuS core/shell



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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 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 250800 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 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 light at open circuit voltage over a frequency range from 100 kHz to 0.1 Hz with an AC voltage of 50 mV. 10 ACS Paragon Plus Environment

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Stability and recyclability of TiO2/CuS core/shell nanostructures photocatalyst: To evaluate the stability and the 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 dried at 70 oC for the next cycle of photocatalysis. The stability of the photocatalyst was scrutinized through powder X-ray diffraction studies. RESULTS AND DISCUSSION The optimum thickness of CuS shell onto the TiO2 nanorods was realized by utilizing the varying concentration of precursors (copper and sulfur) as well as modulating the amounts of surface-functionalizing ligand, 3-mercaptopropionic acid (MPA). Analysis of the results discussed below manifest that the TiO2/CuS core/shell nanostructures have pronounced absorption in the visible region and possesses commendable visible light driven photocatalytic efficiency towards the degradation of virulent organic pollutant of dye waste water, 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 X-ray diffraction (PXRD) studies and the results are manifested in Figure 1. Analysis of the PXRD studies uncovers that the reflections corresponding to TiO2 accords with the well documented anatase phase of TiO2 (JCPDS no. 211272), along with the additional phase in the PXRD patterns of TiO2/CuS composite and core/shell nanostructures, characterized by the reflections at 27.68o, 29.28o, 31.78o, 47.94o, 52.71o and 59.34o, is unquestionably assigned to hexagonal phase of CuS (JCPDS no. 060464).



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A close analysis of the PXRD studies of TiO2/CuS core/shell nanostructures unveils the shift in the reflections corresponding to TiO2 towards larger angles. Analyzing from Bragg’s equation, the noticed shift of the X-ray reflections reveals the lattice compression of TiO2 with the growth of CuS shell. This discloses that the growth of shell (CuS) contracts the lattice planes of core (TiO2) and thus lowers the lattice constant of TiO2 in 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 TiO2/CuS composite nanostructures which hint at the absence of core/shell geometry. The calculated value of lattice constants of TiO2 (in core/shell and composite) has been summarized in Table 1. Thus, from the PXRD studies, we infer the anatase and hexagonal phase for TiO2 and CuS, respectively in TiO2/CuS core/shell and composite nanostructures and also propose at the possibility of growth of CuS shell onto the TiO2 nanostructures (in TiO2/CuS core/shell nanostructures) from the lattice constants values. However, the convincing evidence of core/shell and composite geometry in 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 field-emission scanning electron microscopy (FESEM) studies. As noticeable from Figure 2a, the morphology of TiO2 is described as rod-type with smooth surfaces. The nanorods of TiO2 are uniform evincing an average length of 700 ± 15 nm and diameter of 100 ± 6 nm. Following the introduction of surface-functionalizing ligand, 3mercaptopropionic acid (MPA), and the shell (Cu and S) precursors, it is noticed that uniform secondary CuS nanoparticles are covered onto the TiO2 nanostructures (Figure 2b) and thus TiO2/CuS nanostructures (core/shell) exhibit larger diameter compared to the bare TiO2 nanostructures, pointing at the existence of core/shell geometry between TiO2 and CuS. To


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understand the importance of introducing 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 reveals 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 assisting in segregation of the agglomerated nanorods, probably by electrostatic interactions, thus, offering the uniform growth of core/shell geometry.29 In nutshell, 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 the core/shell geometry, however, on performing experiments without employing MPA results in the formation of TiO2/CuS composite nanostructures. TEM and HRTEM Studies: Existence of core/shell and composite geometry in TiO2/CuS nanostructures was ultimately corroborated by TEM and HRTEM studies. Examination of TEM results revealed in Figure 3a (TiO2/CuS core/shell nanostructures) unveils the uniform distribution of CuS nanoparticles of average thickness of ~ 20 nm onto the surface of TiO2 nanorod (diameter 100 nm). The contrast difference between TiO2 core nanostructures (dark) and shell of CuS nanoparticles (light) endorses the existence of core/shell geometry. However, the TEM results portrayed in Figure 3b (TiO2/CuS composite nanostructures) illustrate the CuS nanoparticles of average size of 20 nm scattered onto the surface of TiO2 nanorod, pointing towards the evidence of composite geometry in TiO2/CuS nanostructures. Further, decisive evidence for the presence of core/shell and composite geometry in TiO2/CuS nanostructures was discerned from the HRTEM analysis. A HRTEM analysis of TiO2/CuS core/shell nanostructures (Figure 4a) acquired through core/shell interface distinctly reveals the presence of two sets of



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lattice fringes with spacing of 0.350 and 0.282 nm in the core and the shell region respectively. The lattice spacing of 0.350 nm in the core area correlates to the (101) plane of anatase TiO2 and 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), reveals 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 clearly endorses the composite geometry in TiO2/CuS composite nanostructures. Thus, TEM and HRTEM studies unambiguously support the fact that two types of TiO2/CuS nanostructures are synthesized; one with the core/shell geometry and the other with the composite. EDS Elemental Mapping Studies: Information regarding the elemental composition and the extent of distribution of elements in TiO2/CuS core/shell and composite nanostructures were unfolded 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 TiO2/CuS core/shell 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 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 resulting in the scattering of CuS nanoparticles onto the core TiO2 nanostructures. To sum up, analysis from the mapping studies of TiO2/CuS nanostructures conveys that homogeneous distribution and the larger mapping area of CuS onto the TiO2


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nanostructures results in the core/shell geometry, however, uneven presence of CuS nanoparticles onto the TiO2 nanostructures results in composite geometry in 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 distinct absorption edge in the UV region corresponding to the band gap of 3.18 eV, stemming through the transfer of valence band electron to the conduction band. The DRS profile of 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 offering in the visible range (1.98 eV), which could be originated through the weak contact between TiO2 nanorods and CuS nanoparticles. However, this feature is not present in the TiO2/CuS core/shell nanostructures. DRS studies of 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 the 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 the broadening of the absorption edge with the formation of CuS shell is the signature of existence of type-II core/shell geometry between TiO2 and CuS, which maximizes the interfacial contact34 between 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 the retardation in the recombination rate of photoinduced charge carriers. Thus, 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 core/shell nanostructures are anticipated to hold photocatalytic abilities



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under visible light illumination. However, in case of composite nanostructures, owing to the weak contact between TiO2 and CuS,34 an unsubstantial absorption is obtained in 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 TiO2/CuS nanostructures; one with the core/shell and other with the composite geometry. Visible-Light Activated Photocatalytic Ability for the Degradation of Methylene Blue (MB): Dye waste water poses a pervasive threat to the environment.35 In this context, to explore the worth of the TiO2/CuS nanostructures, we have performed the photocatalytic tests using the aqueous solution of methylene blue dye, a virulent organic pollutant of dye waste water under visible light illumination. The photocatalytic results reflected in Figure 7, reveals 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, the introduction of TiO2/CuS nanostructures boost the photodegradation of aqueous solution MB, validating the fact that integrating TiO2 nanostructures with other semiconductor components is an appealing approach to boost the photocatalytic efficiency. The TiO2/CuS core/shell nanostructures unveils commendable photocatalytic efficiency, affording ~ 90 % photodegradation of MB solution after 60 minutes, however, TiO2/CuS composite nanostructures reveals ~ 58 % of photodegradation of aqueous solution of MB under similar conditions. To illustrate the photocatalytic performance of the resultant samples more precisely, the kinetics of the photodegradation of aq. solution of MB over the resultant samples were fit to a pseudo first –order reaction model utilizing the following integral form of first–order equation36


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ln (C0/C) = kt

(2)

where C0 is the absorbance of MB after the adsorption equilibrium is achieved prior to the visible light illumination, C is the absorbance of the 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 absorbance 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 photocatalytic performances of the resultant samples. From the analysis of degradation constants (Table 2) of the resultant samples, it is comprehended that the TiO2/CuS core/shell nanostructures offer the best degradation constant (3.6E-2 min-1) for the degradation of aqueous solution of MB under visible light illumination. The commendable photocatalytic performance of TiO2/CuS core/shell nanostructures (compared to the TiO2/CuS composite nanostructures) could be credited to the core/shell geometry with type-II band structure and the enhanced light harvesting ability, which are described below: Core/Shell Geometry with Type-II Band Structure: Major factor accounting for the origin of photocatalysis in 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 offer an effectual passivation of the surface of TiO2 core. Utilizing core/shell geometry with 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 materials (either core or shell) alone.26 Thus, the retardation in charge carriers recombination and access to longer wavelengths



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stemming through the core/shell geometry followed by the type-II band structure imparts commendable photocatalytic activity to TiO2/CuS core/shell nanostructures. Noticeably, type-II band structure is also present in TiO2/CuS composite nanostructures; however, following a weak contact between TiO2 and CuS, the collection and transportation of the carriers is inefficient, resulting in the unpropitious photocatalytic performance. To validate the fact that 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 conduction and valence band of TiO2 and CuS using the following equations37 ECB = X + E0 – 0.5Eg and

EVB = ECB + Eg

(3) (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 TiO2/CuS composite nanostructures, however in 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 size 20 nm, identical to the thickness of CuS shell in core/shell nanostructures. Band gap of bare CuS nanoparticles of size 20 nm was calculated from the DRS studies, depicted in Figure S1) From the calculated values, it is apparent that the CB and VB energy of CuS is at higher energy than that of TiO2 correlating with the type-II band 18 ACS Paragon Plus Environment

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structure, in which both the conduction and valence bands of shell are either lower or higher than those in the core. The band diagram of TiO2/CuS core/shell nanostructures presented in Figure 9a reveals that the transfer of photoinduced charge carriers takes place through the core/shell interface: electron moves from CB of CuS to the CB of TiO2, provided by the lowered energy, and holes moves from the VB of TiO2 to the VB of CuS under visible light illumination. As a consequence, the photoinduced charge carriers were separated at the core/shell interface of TiO2/CuS nanostructures. However, owing to the composite geometry, which results in weak contact between TiO2 and CuS, hence collection and transportation of photoinduced charge carriers is not efficient in TiO2/CuS composite nanostructures as reflected in Figure 9b. Thus, the charge transfer process will be identical (electron moves from CB of CuS to the CB of TiO2, provided by the lowered energy, and holes moves 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 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 reflected from the DRS studies (Figure 6), substantial visible light absorption by TiO2/CuS core/shell nanostructures is another factor endorsing the commendable photocatalysis performance. However, the visible light utilization by TiO2/CuS composite nanostructures is dissuading for carrying out photocatalytic reactions under visible light illumination. Thus, the enhanced light harvesting ability of TiO2/CuS core/shell



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nanostructures also imparts commendable photocatalytic activity to TiO2/CuS core/shell nanostructures. Thus, the above factors comprising core/shell geometry with type-II band structure and the enhanced light harvesting ability accounts for the remarkable photocatalytic performance of 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 to the remarkable photocatalytic performance of TiO2/CuS core/shell nanostructures, we also unfold the effect of photocatalyst dosage and MB concentration on the photodegradation of aqueous solution of MB employing TiO2/CuS core/shell nanostructures under visible light illumination, to decipher the optimum amount of photocatalyst and the organic pollutant dye concentration required to avail the best photocatalytic performance from 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 the 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 oC. Evident from the Figure 10a, that increase in catalyst loading enhances the photocatalytic performance, which could be assigned to the fact that increase in number of TiO2/CuS core/shell nanostructures will increase the number of photons absorbed, the accessible active sites and consequently, the number of MB dye molecules absorbed. However, further increase in photocatalyst amount (to 0.5 g/L) unveils a decline in photocatalysis performance. This is ascribed to the fact that, sedimentation and agglomeration of the photocatalyst will result under large photocatalyst


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loadings and the accessible surface of photocatalyst for photon absorption would diminish.38 In fact, the opacity and screening effect of substantial number of TiO2/CuS core/shell nanostructures serve as a shield, and consequently impede the penetration of light, resulting in the loss of the available surface area for the harvesting of light and decline in the photocatalytic performance. Thus, the optimal dosage of TiO2/CuS core/shell nanostructures for the 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 unravel 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 modulating the concentration of MB solution (0.001 to 0.005 g/L) and taking constant amount of photocatalyst (0.3 g/L). Figure 10b portrays that with increasing the concentration of MB solution, the rate of degradation reaches to maximum up to a certain limit, however, further increase in MB concentration induced the decline in 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 TiO2/CuS core/shell nanostructures) reacting with MB, resulting in the enhanced photocatalytic performance. A decline in the photocatalytic performance with 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 increase in MB concentration, the active sites on the surface of the photocatalyst were enveloped with the MB molecules, diminishing the active sites for the generation of active species, and consequently scaling down the photocatalytic efficiency.39 21 ACS Paragon Plus Environment


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Visible-Light Activated Photocatalytic Ability for the 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 TiO2/CuS core/shell nanostructures towards the 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 unveiled 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 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 the Photocatalytic Degradation of MB over TiO2/CuS core/shell nanostructures under visible light illumination: The underlying mechanism of semiconductor photocatalysis is well comprehended. In a nut shell, offering photon energy equal to or exceeding the band gap of the semiconductor photocatalyst, electrons are promoted from the valence band (VB) to the conduction band (CB) leaving electron vacancy or hole in the VB.41 If the separation of the charge carriers (electrons and holes) is retained, the electrons and holes migrate to the surface of the photocatalyst and engaged in the redox reactions and led to the generation of active species such as superoxide radical anion (O2•−) and hydroxyl radicals (OH•), which participate in the oxidation of organic dye pollutants. Thus, to develop a full understanding of the mechanism of photocatalysis, it is imperative to identify the leading active species involved in the degradation of MB over TiO2/CuS core/shell nanostructures. A series of scavengers or trappers were utilized to identify the active species involved in the photodegradation of MB over TiO2/CuS core/shell nanostructures. We introduced AgNO3 to trap conduction band electron (e-


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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 manifested in Figure 11, it is apparent that the introduction of AgNO3 (electrons trappers), benzoquinone (superoxide radical anions trappers) and t-BuOH (hydroxyl radicals trappers) unveil a considerable decline in the photocatalytic performance of TiO2/CuS core/shell nanostructures for the degradation of MB under visible light illumination. Nonetheless, only a trivial decline is discerned on introducing holes trappers (ammonium oxalate, AQ). Based on the information provided from the trapping experiments, we propose the following operative mechanism for the photocatalysis of MB over 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˙¯ + H2O → HO2˙ + OH¯ HO2˙ + H2O → OH˙ + H2O2 H2O2 → 2OH˙ OH˙ + MB → H2O + CO2



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CuS (h+VB) + MB → Degraded products Offering visible light illumination to TiO2/CuS core/shell nanostructures, CuS act as a visiblelight photosensitizer, exciting electrons from the VB of CuS to the CB, thus creating photoexcited species, electrons in the CB and holes in VB. Compelled by the decreased potential energy, the conduction band electrons of CuS are transferred to the CB of TiO2 and in contrast, valence band holes of CuS are moved to the VB of TiO2, mitigating the recombination of the photoinduced charge carriers. Afterwards, the conduction band electrons are captured by the 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 made 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 the minor assistance offered by the valence band holes in the degradation of MB over TiO2/CuS core/shell nanostructures under visible light illumination. Analysis of the degradation intermediates formed during photocatalysis of MB : MS is an effective tool to analyze the degradation intermediates of MB under visible light illumination over TiO2/CuS core/shell nanostructures. Preliminary analysis of MB using MS showed a major peak at m/z = 284.12 which corresponds to 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 and eventually fragmented to smaller molecules which are difficult to be detected by mass spectroscopy (Figure 12). The formation of these degradation intermediates through demethylation of MB is well consistent with 24 ACS Paragon Plus Environment

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literature.43,44 Directed by the evidence provided from MS studies, we proposed a probable degradation pathway of MB involving demethylation of MB molecule as shown in Scheme 1. Thus, analysis of intermediates through mass spectroscopy studies supports the demethylation pathway for the degradation of methylene blue dye. PL Studies: We have credited the commendable photocatalytic performance of TiO2/CuS core/shell nanostructures to the 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. Understanding from the fact that PL signal is contributed through the recombination of free charge carriers, we have utilized photoluminescence studies to understand the fate of photoinduced charge carriers. Analysis of Figure 13 unravels that the PL peak of TiO2 nanorods and CuS nanoparticles at 394 and 635 nm is credited to the band gap transition corresponding to the band gap energy of TiO2 and CuS, respectively. However, following the realization of CuS shell onto TiO2 nanorods, an instant suppression of the emission of TiO2 is noticed and 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 shell. Moreover, the PL intensity of TiO2/CuS core/shell nanostructures illustrate a significant decline compared to the TiO2 and CuS, is ascribed to retardation in the recombination rate of photoinduced charge carriers driven by the type-II band structure and core/shell geometry between TiO2 and CuS. The PL spectra of TiO2/CuS composite nanostructures, however illustrates 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 composite nanostructures (enables



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the retardation in the recombination rate of photoinduced charge carriers) compared to the bare counterparts is accounted by the type-II band structure formed by the TiO2 and CuS materials. Thus, the analysis drawn from the PL studies substantiate the fact that although both the nanostructures (core/shell and composite) have type-II band structure, however the presence of core/shell geometry could efficiently slows down the recombination of photoinduced charge carriers over composite nanostructures and, thus accountable for the commendable visible light driven photocatalytic performance of TiO2/CuS core/shell nanostructures. EIS Studies: To provide a reasonable interpretation accounting for the commendable photocatalytic performance of TiO2/CuS core/shell over composite nanostructures and to support PL analysis, electrochemical impedance spectroscopy (EIS) studies were conducted. Analysis of the results discerned from impedance spectroscopy (Figure 14), unveils that the larger impedance arc radius is observed for TiO2/CuS core/shell over composite nanostructures, which corresponds to the larger recombination resistance as the radius of the impedance arc implies the recombination resistance.45 It is well comprehended that the recombination resistance is inversely proportional to the recombination rate of photoinduced charge carriers.46 In this regard, TiO2/CuS core/shell nanostructures have lower recombination rate of the photoinduced charge carriers compared to the 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 the 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 the recyclability of the photocatalyst as it 26 ACS Paragon Plus Environment

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could appreciably reduce the cost of the photocatalytic process and thus promising the photocatalysis, a fascinating strategy for the destruction of virulent organic pollutants. Towards this end, we carried out five successive runs of the photodegradation of MB. Analysis of the recyclability results depicts (Figure 15a) an imperceptible reduction (~ 8 %) in the photocatalytic performance of TiO2/CuS core/shell nanostructures for the 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 unravels the development of visible light functional core/shell nanostructures affording substantial suppression in the recombination rate of photoinduced charge carriers and thus offering commendable photocatalytic performance for the degradation of virulent organic pollutant of dye waste water. The importance of core/shell geometry is also unveiled by synthesizing composite nanostructures under similar conditions, which offer unpropitious photocatalytic performance owing to the composite morphology. Further, the results discerned from the stability and recyclability experiments of the core/shell nanostructures support its viable potential for environmental applications. We anticipate that our work bear vast potential in the photocatalysis domain for the development of efficacious and recyclable core/shell nanostructures. CONCLUSIONS Our work provides a potential platform to tailoring the light absorption and retardation in the recombination rate of photoinduced charge carriers in TiO2, a benchmark photocatalyst, by 27 ACS Paragon Plus Environment


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realizing core/shell geometry with CuS. The importance of core/shell nanostructures was unraveled 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 the degradation of MB under visible light illumination. The propitious photocatalytic performance of core/shell nanostructures could be credited to core/shell geometry, which maximizes the interfacial contact between core and shell, thus ensuing in the fast collection, transportation and lower recombination rate of photoinduced charge carriers, as evidenced from Photoluminescence and Impedance spectroscopy. A credible photocatalysis mechanism for the degradation of MB over core/shell nanostructures under visible light illumination is also proposed from the findings discerned through the trapping experiments. Results of trapping experiments using active species trappers evince that the hydroxyl radicals (OH•) are the ruling active species with the minor assistance offered by the valence band holes in the degradation of MB over TiO2/CuS core/shell nanostructures under visible light illumination. The results offered illustrated here offer new insights in the photocatalysis domain for exploring novel core/shell nanostructures photocatalyst for environmental sustainability. ACKNOWLEDGEMENT A.K.G. thanks DeitY, Department of Science & Technology (DST) and Council of Scientific and Industrial Research (CSIR), Govt. of India for financial support. Sunita Khanchandani and Sandeep Kumar thank CSIR and DST, Govt. of India for a fellowship, respectively. 28 ACS Paragon Plus Environment

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SUPPORTING INFORMATION Figure S1. UV-vis DRS of CuS. Figure S2. Degradation efficiency of TiO2/CuS core/shell nanostructures for the degradation of aq. solution of MB and SA under visible light illumination. This material is available free of charge via the Internet at http://pubs.acs.org



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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1 Parameters obtained from XRD investigation of TiO2, TiO2/CuS composite and core/shell nanostructures

Samples

Shell thickness (nm)

Cell parameter (Å)

TiO2

-

a=3.786(3) c=9.512(2)

TiO2/CuS composite

-

a=3.787(3) c=9.512(3)

TiO2/CuS core/shell

20

a=3.780(4) c=9.505(5)

Table 2 Optical, 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

-

3.18

13

1.9E-3 (±2.8E-4)

-0.28

2.90

TiO2/CuS composite

-

3.13, 1.98

58

1.4E-2 (±1.1E-3)

-0.26,-0.38

2.92, 1.60

TiO2/CuS core/shell

20

2.62

90

3.6E-2 (±1.2E-3)

-0.28,-0.39

2.90, 1.61


(101)

Page 38 of 54

* TiO2 # CuS

#

#

(204)

(116)

(200)

*

*

(108)

## #

(004)

(101) (102) (103)

#

* Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(110)


*

TiO2/CuS Core/Shell

TiO2/CuS Composite

TiO2

20

30

40 50 2θ (deg)

60

70

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


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100 nm

200 nm

(a)

100 nm

(b) (b)

200 nm

100 nm

(c) (c)

200 nm

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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(b) (b)

(a)

Av. Diameter 100 nm Av. Size of CuS Nanoparticles 20 nm

Av. Diameter 100 nm Av. Shell Thickness 20 nm

50 nm

50 nm

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


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b) (b)

(a) CuS

CuS

CuS

(Shell)

0.350 nm

0.350 nm 0.282 nm

(101)

(101)

TiO2

(103) 0.282 nm 1 nm

TiO2 (Core)

1 nm

CuS

(103)

CuS

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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(e) (e)

(b) (b)

(a)

Ti

Ti

O (d) (d)

(c) (c)

Cu

Page 42 of 54

(f) (f)

O (g) (g)

S

Cu

(h) (h)

S

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


1.98 eV 3.18 eV

F (R∞)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[F (R∞) hυ]2

Page 43 of 54

2 2.62 eV

3

hυ (eV)

4 3.13 eV

TiO2 TiO2/CuS Composite TiO2/CuS Core/Shell

300

400

500

600

700

Wavelength (nm)

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.


1.0

(a)

0.8

TiO2

0.6 CuS 0.4

Page 44 of 54

(b) (b) Degradation Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

C/C0


TiO2/CuS Composite

0.2

TiO2/CuS Core/Shell

90 TiO2/CuS Composite CuS

58

45 TiO2

13

TiO2/CuS Core/Shell

0.0 0

10

Figure 7.

20 30 40 50 Irradiation Time (min)

60

(a) Photocatalytic performances and (b) degradation efficiency of TiO2, CuS,

TiO2/CuS composite and core/shell nanostructures for the degradation of aq. solution of MB under visible light illumination.


Page 45 of 54

3.0

-1

TiO2 : k = 1.9E-3 (± 2.8E-4) min

-1

CuS : k = 1.1E-2 (± 9.1E-4) min -1 TiO2/CuS Composite : k = 1.4E-2 (±1.1E-3) min

2.5

-1

TiO2/CuS Core/Shell : k = 3.6E-2 (±1.2E-3) min

2.0 ln(C0/C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.5 1.0 0.5 0.0 0

10

20 30 40 50 Irradiation Time (min)

60

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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a)

Page 46 of 54

(b) (b)

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


0

90

88

85 0.005 g/L

81

83

0.004 g/L

Effect of MB Concentration

0.003 g/L

(b) (b)

0.002 g/L

80

Degradation Efficiency (%)

86

0.5 g/L

90

0.4 g/L

87

0.3 g/L

0.1 g/L

82

Effect of Photocatayst

0.2 g/L

(a) Degradation Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.001 g/L

Page 47 of 54

At 60 minutes

At 60 minutes

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



Effect of Introducing Active Species Trappers

AgNO3 (e−trapper)

t-BuOH (OH• trapper)

BQ (O2•− trapper)

86 Amm. Oxalate (h+ trapper)

No Trappers

90 Degradation Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 48 of 54

5

5

5

At 60 minutes

Figure 11.

Trapping experiments using different active species trappers for the

photodegradation of MB over TiO2/CuS core/shell nanostructures under visible light illumination.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Scheme 1. Demethylation of methylene blue (MB) dye over TiO2/CuS core/shell nanostructures under visible light illumination.


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Page 51 of 54

TiO2

λex = 340 nm

635 nm

CuS TiO2/CuS Composite

PL Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TiO2/CuS Core/Shell 394 nm 638 nm

400 nm

478 nm

300

400 500 600 Wavelength (nm)

700

800

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



10 TiO2/CuS Core/Shell

8 6

TiO2/CuS Composite

3

-Z'' x 10 (ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

4 2 0 0.0

2.0

4.0

6.0 8.0 10.0 12.0 14.0 3 Z' x 10 (ohm)

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


Page 52 of 54

84

Cycle 4

86

Cycle 3

87

Cycle 2

89

Cycle 1

TiO2/CuS Core/Shell

90

(a)

* TiO2

(b) (b) *

# CuS

82

#

Intensity (a.u.)

Stability of Photocatalyst Degradation Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cycle 5

Page 53 of 54

*

# # #

*

#

#

*

After 5 runs of Photocatalysis

Before Photocatalysis

20

At 60 minutes

30

40 50 2 θ (deg)

60

70

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



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents Comparative Study of TiO2/CuS Core/Shell and Composite Nanostructures for Efficient Visible-Light Photocatalysis Sunita Khanchandania, Sandeep Kumara,b and Ashok K. Gangulia,c* a

Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India b

c

Department of Chemistry, University of Delhi, Delhi 110007, India

Institute of Nano Science & Technology, Habitat Centre, Phase X, Sector 64, Mohali, Punjab 160062, India

A novel TiO2/CuS core/shell nanostructures, offering commendable photocatalytic performance for the degradation of methylene blue under visible light is unraveled.


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Shell and Composite

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