Efficient Charge Separation in Plasmonic ZnS@Sn:ZnO

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Efficient charge separation in plasmonic ZnS@Sn:ZnO nanoheterostructure: Nanoscale Kirkendall effect and enhanced photophysical properties. Sumana Paul, Sirshendu Ghosh, and Subodh Kumar De Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00442 • Publication Date (Web): 23 Mar 2018 Downloaded from http://pubs.acs.org on March 25, 2018

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Efficient charge separation in plasmonic ZnS@Sn:ZnO nanoheterostructure: Nanoscale Kirkendall effect and enhanced photophysical properties. Sumana Paul,† Sirshendu Ghosh,† and Subodh Kumar De * Department of Materials Science, Indian Association for the Cultivation of Science, Kolkata700032, India

ABSTRACT

Tetravalent Sn doped ZnO nanocrystals show excellent plasmonic absorbance in visible region. Plasmonic ZnS@Sn:ZnO core shell heterostructures have been synthesized by anion exchange process where the O2- is exchanged by S2- anion. An increase of sulphur concentration induces interior hollow structures arising from the different diffusion rates of O2- and S2- ions. Gradual transformation of wurtztie ZnO nanocrystals in anion exchange process stabilizes the wurtzite crystalline phase of ZnS. Carrier concentration and various types of intrinsic defect states in both ZnO and ZnS result in ultraviolet, blue and green emissions. Co-existence of exciton –plasmon coupling in same nanopaticle and efficient electron-hole separation in type II heterostructure increase the photocatalytic activity and photo current gain.

ACS Paragon Plus Environment

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KEYWORDS: Aliovalent doping, plasmon, exciton-plasmon coupling, nanoscale Kirkendall effect, photophysical property.

INTRODUCTION Nanoscale heterostructures (NHS) in which two semiconductors are integrated into one nanoscale hybrid structure, have recently received more attention because of their intriguing interfacial charge transfer process under photoexcitation. The proper energy band alignments across the interface can effectively improve the charge separation on both semiconductor units by retarding the recombination process of photogenerated electron–hole pairs.1,2 Depending upon the relative band positions of two different semiconductors, NHS can be classified as type-I, type-II or z-scheme heterostructure.3-5 Moreover, crystallographic epitaxial growth of one semiconductor over another one facilitates the charge separation process.6,7 Among the various types of interfaces, type-II heterostructures are most desirable as photogenerated electrons and holes moved to the opposite direction, giving a spatial separation of the electrons and holes in different semiconductors. Such type of combination of semiconductors is very useful for photocatalytic and photovoltaic applications to improve the efficiency. We explored the type-II semiconductor heterostructure consisting of nature abundant, chemically stable and nontoxic ZnO and ZnS. Several theoretical calculations and experimental results predicted that the ZnOZnS heterostructure exhibit better performance than the individual components.8-14 Bulk ZnO and ZnS crystallize either in zincblende or wurtzite structure. ZnO is a promising semiconductor for photocatalytic degradation of pollutant and photoresponse device as it exhibits a higher electron mobility and longer photoexcited electron lifetime (compare to TiO2).15-17 ZnS is also an important semiconductor due to its excellent luminescence and photocatalytic


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properties. However, the major drawback for ZnO and ZnS is the large band gap of 3.20 eV and 3.60 eV respectively. Both semiconductors absorb ultraviolet (UV) light only for solar spectrum. Considering the small portion of UV light in solar spectrum (~4%), it is essential to develop new material with efficient visible light absorbance capability. Maximisation of photoactivity of this heterostructure can be achieved if it is able to absorb visible light (~48% of solar light). The absorption of solar light can be enhanced by introducing plasmon in wide band gap semiconductor. The localised surface plasmon resonance (LSPR) due to the collective oscillation of free carriers in presence of electromagnetic radiation at the interface of a dielectric and nanoparticle gives rise to additional absorption. The energetic position of LSPR primarily depends on the selection of proper aliovalent dopant and its concentration. Plasmonic semiconductors which show LSPR absorbance in visible and NIR region are one of the most promising candidates to absorb a major percentage of solar light (Vis + NIR ~ 96%). There are several reports on the appearance of LSPR in wide band gap semiconductors. For examples, tetravalent Sn doped ZnO (Sn:ZnO),18 MoO3-x,19 WO3-x,20,21 Nb doped TiO2,22 Sn doped CdO

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are the most promising n-type semiconductors to absorb light in Vis-NIR region

due to LSPR phenomenon. Cu based binary and ternary chalcogenides show plasmonic NIR absorbance for high p-type carrier concentration originating from Cu vacancy.24-27 Cheng et. al.19 reported enhanced catalytic H2 evolution from NH3BH3 by Pd/MoO3-x nanocomposite and enhancement has been explained by plasmonic absorbance of MoO3-x. Our recent extensive study [18] established that Sn4+ doping in ZnO nanostructures can generate LSPR in visible region (~ 675 nm). We also demonstrated that LSPR can be tunable up to NIR region by precise control of Sn doping concentration and cadmium alloying. Sn doped plasmonic ZnO can serve as an attractive and promising system in which plasmonic and excitonic absorptions occur in the same


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nanoparticle. In general, metal-semiconductor heterostructure plasmon and exciton reside in different nanocrystals (NCs) and there coupling (or enhanced absorbance of semiconductor) depends on physical distance between two types of NCs.28 The strong electric field generated by LSPR increases the absorption cross-section of semiconductor and is capable of harvesting light at the nanoscale, and has found potential and novel applications in diverse areas. Therefore plasmon-exciton interaction may enhance photophysical properties in comparison to conventional metal-semiconductor heterostructure. The fabrication of engineered heterostructure can be done employing core-shell process by covering shell on the core with adjustable core size and shell thickness.29,30 Successive ion layer adsorption and reaction technique has been used for the design of complex semiconductor heterostructure.[31] Ion-exchange has proved to be an effective tool for chemical transformation of inorganic nanocrystals. The nanoscale Kirkendall effect, where the cation or anion sub lattice of a pre-synthesized nanocrystal is replaced with different ions, is a power full method to generate interior voids in heterostructure.32-36 In this report, we have prepared highly monodispersed Sn doped ZnO nanocrystals and ZnS@Sn:ZnO heterostructures through the exchange of S2- for O2- in presynthesized Sn doped ZnO nanocrystals. Here we investigate the effect of plasmon -exciton coupling and heterostucture formation on the photophysical properties of the materials.

Experimental Section Materials


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Zinc stearate [Zn(STA)2, 90%], 1-octadecanol [ODANOL, 97%], tin(IV) acetate [Sn(OAc)4], and 1-octadecene [ODE] were purchased from Alfa Aesar. Myristic acid [MTAH, 99%], stearic acid [STAH, 95%], 1-octadecanol [ODANOL, > 97%], oleic acid [OLAH > 99%] and oleylamine [OLAM, 70%, Tech] were purchased from Sigma-Aldrich. Rhodamine-B (RhB), nbutylamine, ethanol, acetonitrile (GR), tetrachloroethylene (TCE), carbon disulfide (CS 2), dichlorobenzene (DCB), chloroform, hexane and other solvents were purchased from Merck India. Synthesis Preparation of Tin Myristate [Sn(MTA)4] solution: 1 mmol Sn(OAc)4 and 4 mmol MTAH were mixed in 20 ml of ODE. Then the solution was evacuated for 30 min. After that the temperature was raised to 110˚C and kept at this temperature for 4 hr under continuous evacuation condition. This tin myristate solution was kept at N2 atmosphere for further use. Preparation of OLAM-S stock solution: 5 mmol of S powder was dissolved in 5 ml of oleylamine. The mixture was degassed at 110˚C for 30 min under constant evacuation condition. After that the temperature was rapidly raised to 200˚C and kept at this temperature until a clear solution was obtained. Then the solution was cooled down to room temperature and kept under inert atmosphere to use as OLAM-S stock solution. Preparation of Pure ZnO and Sn doped ZnO (Sn:ZnO) nanocrystals: In a typical synthesis procedure 0.5 mmol of Zn(STA)2 and 5 ml of ODE were loaded in a three neck round bottom flask fitted with a reflux condenser. This solution was degassed at 100˚C and kept for 20 min under complete dynamic evacuation condition. At the same time in another three neck flask 10 mmol ODANOL was dissolved in 10 ml of ODE and heated to 240˚C under dry N2 atmosphere.


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Then the metal precursor solution was purged with N2 gas and heated to 270˚C and when the temperature reached to 270˚C , the ODANOL-ODE solution heated at 240˚C was swiftly injected into the solution. The reaction was continued for 5 min at 270˚C temperature and then cooled down to 60˚C. The as synthesized nanocrystals (NCs) were collected by adding ethyl acetate as non solvent and centrifuged at 10,000 rpm for 4 min. This washing procedure was continued for 3-4 times. As collected NCs were dissolved in TCE for further characterization. For 4% Sn doped ZnO NCs, we added 0.4 ml of Sn[MTH]4 stock solution along with Zn(STA)2 keeping the total metal concentration to 0.5 mmol into 5 ml of ODE and followed the same steps as done for synthesizing pure ZnO. In case of doped sample a colour change of light yellow to greenish blue was observed at 270˚C which confirms the formation of Sn doped ZnO NCs. Preparation of ZnS@Sn:ZnO nanoheterostructure (NHS): ZnS@Sn:ZnO nanoheterostructures were prepared by anion exchange process. Before the anion exchange reaction, ODANOL lignad was replaced by oleylamine (OLAM) by heating the as-obtained NCs in OLAM at 180 °C. The OLAM capped NCs were collected by centrifugation with addition of mixed non solvent (ethyl acetate: ethanol=1:1). Mixture of OLAM capped oxide NCs (total product obtained after one synthesis procedure) along with 1 ml oleic acid in 7 ml ODE was heated to 220 °C under inert atmosphere and required amount of hot OLAM-S solution was injected to initiate the anion exchange reaction. After 1 min of reaction at 220 °C the reaction solution was cooled down to room temperature. The as prepared NHSs were collected by adding ethanol as non solvent and centrifuged for 4 min at 8000 rpm. By changing the Zn:S ratio (1:0.25, 1:1, 1:1.5) we designed ZnS@Sn:ZnO core shell with varying ZnS thickness (CS1; Zn:S=1:0.25, CS2; Zn:S=1:1)and finally hollow ZnS (Zn:S=1:1.5).


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Characterization The crystalline phases of the products were determined by X-ray powder diffraction (XRD) by using a Bruker AXS D8SWAX diffractometer, with Cu Kα radiation (λ = 1.54 Å), employing a scanning rate of 0.5˚ S-1 in the 2θ range from 30˚–80˚. For XRD measurement, the TCE solution of the NCs was drop cast over a glass slide sample holder till a thin layer visible to the naked eye was formed. Transmission electron microscopy (TEM) images and high angle annular dark field scanning TEM (HAADF-STEM) images were taken using an ultra-high resolution field emission gun transmission electron microscope (UHR-FEG TEM, JEM-2100F, Jeol, Japan) operating at 200 kV. For the TEM observations, the sample was dissolved in TCE and was drop cast on a carbon coated copper grid. Room temperature optical absorbance of the samples was measured by a Varian Cary 5000 UV-VIS-NIR spectrometer. Room temperature photoluminescence (PL) measurements were carried out with a fluorescence spectrometer (Hitachi, F-2500). To analyze the chemical states of the constituents, X-ray photoelectron spectroscopy (XPS) was recorded with an SSX-100 ESCA spectrometer using Al Kα, 1486.6 eV line, and a spot size of 800 mm. For XPS measurement the hexane solution of the NCs was also dropcast over Si substrate till a thin layer visible to the naked eye was formed. Photocatalytic dye degradation study The photocatalytic activities of the pure ZnO, Sn:ZnO NCs and ZnS@Sn:ZnO heterostructures were studied by photodegradation of RhB dye in aqueous solution at room temperature in the presence of a light source (KRATOS, Analytical instruments, universal arc lamp supply: 250 Watt, 150XE, model no. 1152, λ > 360 nm light irradiance of 0.65 W cm−2). Before the photocatalytic activity test, a ligand exchange process was performed to substitute long chain


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OLAM by formic acid. In the ligand exchange procedure: a hexane solution of nanocrystals or nanoheterostructures was added to the formic acid solution in acetonitrile and vigorously stirred for a few minutes. The formic acid capped samples were collected by centrifugation with excess acetonitrile. The ligand exchanged NCs or NHSs were further annealed at 150 °C under an Ar atmosphere. 20 mg of catalysts was taken in a 100 ml beaker containing 60 ml deionised water. After ultrasonication, RhB dye (1 mL, 0.005 gm L−1) was injected to the above solution and the mixture was continuously stirred in the dark for 10 min to ensure the establishment of an adsorption–desorption equilibrium. Photocatalytic tests were conducted and 3 ml samples at different reaction times were centrifuged for the absorbance measurement using a Varian Cary 5000 UV-VIS-NIR spectrophotometer. The distance of the samples was kept at 5 cm away from the light source. Wavelength dependent photocatalytic study was performed by red LEDs (wavelength range 600-700 nm, 0.57 mW/cm-2) and IR lamp (wavelength range 800-1000 nm,0.7 mW/cm-2). Photoresponse study To study photoresponse property of the materials, a device was fabricated by spin casting the chloroform solution of NCs and different heterostructures onto a patterned ITO coated glass substrate. At first 12 × 12 mm2 cleaned ITO coated glass substrate was etched at the middle using Zn dust/HCl to form a non-conducting area keeping the width of the ITO portion at two sides nearly ∼3 mm. These two ITO coated portions were adopted as two electrodes. Then the substrates were cleaned with water and alcohol respectively. Before being used for coating, the substrate was flashed with high pressure dry N2 gas. A total of 0.5 ml of the NCs or NHSs


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solution in the mixed solvent (n-hexane : n-octane= 1 : 1, solution concentration = 30 mg/ml) was spin cast onto the glass substrate at 1000 rpm for 60 s and dried under vacuum at 80°C for 1 h. Then the film was immersed in formic acid solution in acetonitrile for 5 min to exchange the long chain ligand with formic acid. The ligand exchanged film was annealed at 200°C for 2 h in a dry Ar atmosphere. Electrical contact was made by Ag paste with conducting portion of ITO. The photoresponse of the samples was measured using a Keithley Electrometer 6517A and a Keithley multimeter 2000 using a computerized program. The photoconductivity measurements were carried out under illumination with a light irradiance of 0.65 W cm−2 which was obtained by using a Kratos universal Arc Lamp, Model 1152/1144 (150XE/200HG-XE). Results and discussion Structural and Morphological Analysis: Structural and morphological analysis of the as synthesized NCs and NHSs were conducted using XRD analysis and transmission electron microscopy. Figure 1a shows TEM image of highly monodisperse Sn:ZnO NCs. Figure 1b depicts the HRTEM image of a single nanocrystal with the size of nearly 15 nm. The FFT (Fast fourier transform) pattern of the yellow region in Figure 1b as displayed in Figure 1d indicates three planes (002), (013), (011) identified for hexagonal wurtzite (WZ) ZnO. The d spacings corresponding to the (002) and (011) planes were determined from the reconstructed HRTEM image of a nanocrystal (Figure 1c) are 0.26 nm and 0.25 nm respectively.37 Figure 1e indicates the atomic arrangement of Zn2+ and O2- ions in wurtzite ZnO along the [2 0] viewing direction. Along the [002] and [011] directions, Zn2+ and O2- ions are stacking alternatively (Figure 1e). So that along these two directions there exists a permanent dipole moment. These two polar facets {002} and {011} are stabilized by the


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activating and capping agent ODANOL used in the synthesis procedure. The polar facets of ZnO are thermodynamically unstable facets but these facets become more exposed in this case due to the kinetic control over the growth process using the capping agent. Figure 1f shows the XRD pattern of the Sn-doped ZnO NCs where the 4% doped sample is found to be in wurtzite phase (JPCDS, No. 36-1451). Other impurity phases such as tin oxide or zinc stannate phases are not found in the XRD pattern. Figure S6 shows elemental mapping and EDS line scan over a few particles of Sn:ZnO system to show a homogeneous distribution of Sn into the ZnO matrix. Also XPS analysis suggests that Sn is in Sn4+ state (Figure S10c) as the two peaks at 486.5 eV and 495 eV are associated with Sn 3d5/2 and Sn 3d3/2 respectively. Anion exchange was performed on Sn:ZnO nanocrystals to obtain ZnS@Sn:ZnO heterostructure. We varied the Zn:S ratio to change the S2- ion concentration and finally hollow ZnS nanoparticles were grown. Gradual anion exchange process has been illustrated by schematic diagram shown in the Scheme I. Figure 2a shows TEM image for Zn:S = 1:0.25 (CS1) and Figure 2b demonstrates a closer view of the CS1 nanoheterostructures. The SAED (Selected Area Electron Diffraction) pattern (Figure 2c) shows the (010) plane of WZ ZnS indicating the formation of ZnS phase. Figure 2d shows the FFT pattern of the highlighted area in Figure 2b where we have identified the (002) planes of both WZ ZnO and WZ ZnS. Figure 2e depicts the reconstructed HRTEM image of a single NHS where the d spacing of (002) plane of WZ ZnO is 0.26 nm and WZ ZnS is 0.31 nm. Since in this case just sulfidation process occurs, the shell thickness is ~ 1 nm (Figure 2b). The XRD pattern shows characteristic diffraction peaks for both WZ ZnO and WZ ZnS (Figure 2f). TEM and XRD analysis clearly indicate that both ZnO and ZnS are in wurtzite phase in heterostructure. Also we have performed EDS line scan (Figure S7)


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over a few number of particles to show the heterostructure formation in CS1 where sulfur exists around ZnO core region and also Sn exists in the core region. When the Zn:S ratio was increased to 1:1 (CS2) the heterostructers formed into a core shell structure with voids in the core ZnO and the shell thickness is of the order of 2-3 nm. Figure 3a shows HAADF-STEM image of CS2. Figure 3b shows a rather closer view of the core-shell particles. SAED pattern shown in the Figure 3c depicts that major planes such as (010), ( 20) of WZ ZnO and (011), (013), (022) planes of WZ ZnS are present in the nanoscale heterostructure. FFT pattern shown in the Figure 3d of the nanoparticle in the green square (Figure 3b) also confirms the presence of lattice planes of both ZnO and ZnS. We get (002), (013), (011) planes for both ZnO and ZnS from FFT pattern. Interestingly we have identified the same planes for ZnO and ZnS and also along the same crystallographic direction [2 0].38 HRTEM images (Figure 3e and Figure 3f) indicate two sets of epitaxially aligned fringes at the interface of core and shell regions, corresponding to ZnO and ZnS lattice planes of (002) and (011), respectively. Such heteroepitaxy formation is also confirmed by in the atomic model of ZnO and ZnS along the [2 0] direction (Figure 3g and 3h). XRD pattern in the Figure 3i also confirms the formation of both WZ ZnO and WZ ZnS. The elemental mapping of CS2 (Figure 4) suggests the presence of Zn, Sn, O and S in the heterostructure. The presence of Sn is also confirmed from XPS analysis which is shown in the Figure S11d. Further increase of Zn:S ratio to 1:1.5, hollow ZnS nanoparticles are formed as shown in the TEM image (Figure 5a). Core ZnO nanoparticle is nearly disappeared in this case and hollow ZnS shell is formed. The shell of the hollow ZnS nanoparticles have a thickness of 3-5 nm. Figure 5b shows the closer view of the nanoparticle indicating a void at the centre of each nanoparticle. HRTEM images of a few hollow particles are shown in the Figure 5c and the inset


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shows a single hollow particle with a void at the core. Diffraction pattern (Figure 5d) consists of (011), (013), (022) planes of Wurtzite ZnS. The presence of (002), (013), and (011) planes are also confirmed from the FFT pattern (Figure 5e). The atomic model of ZnS along the [2 0] direction exhibits that the crystal structure and orientation of final hollow ZnS are the same as that of parent ZnO nanoparticles (Figure 5f). The XRD pattern shown in the Figure 5g is also consistent with the TEM analysis. The effect of anion exchange of ZnO with S2- yields hollow structures rather than solid nanoparticles driven by nanoscale Kirkendall effect. The nanoscale Kirkendall effect is a classical phenomenon involving a mutual diffusion of ions along two opposite directions through an interface. OLAM-S solution produces diffused S2- ions in the reaction mixture and they form a layer at the surface of the presynthesiszed ZnO nanocrystals. Since diffusion rate of cation Zn2+ is much faster than the anion S2-, the outer ion flow would be faster and ZnS layer will be formed at the surface. Further anion exchange will be continued through this layer. Since we have performed the anion exchange at about 220°C, we get cystalline phases of both ZnO and ZnS. At a lower temperature an amorphous layer of ZnS will be formed. Since the mobility of O2- ion is larger than that of S2- ion,39 the outward flow rate of O2- is much faster than that of inward flow rate of S2- which gives rise to void in the heterostructure. As we increased the Zn:S ratio the amount of diffused S2- increases and as a result the size of the void increases and finally we get the hollow ZnS nanocrystals. Interestingly, in the overall transformation from ZnO to ZnS mimics the initial nanoparticle’s shape. Moreover, the resulting ZnS nanoparticles are single crystalline and preserve the crystal symmetry and orientation of the pristine ZnO nanoparticles. Optical properties


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The optical band gaps of the as synthesized nanocrystals and nanoheterostructures were investigated using UV-VIS-NIR absorption spectra at room temperature. The band gaps of the samples were calculated using the classical Tauc relation of the direct band gap semiconductors15, (αhν)2 = C(hν-Eg)

(1)

Where α is the optical absorption coefficient, hν is the incident photon energy, C is a constant. The band gaps (Eg) were determined by plotting (αhν)2 vs. hν and extrapolating the linear absorption edge of the curve to intersect the energy axis. Figure 6a shows the absorption spectra of pure ZnO, Sn:ZnO NCs and ZnS@Sn:ZnO NHSs. Pure ZnO shows an absorption band edge near 369 nm which is blue shifted to 347 nm on 4% of Sn doping for Sn:ZnO NCs. For NHSs CS1 (Zn:S=1:0.25) and CS2 (Zn:S=1:1) exhibit two absorption edges due to the ZnO and ZnS counterparts. The absorption edge of ZnO was found to be 13 nm red shifted for CS1 and 15 nm for CS2. Hollow ZnS (Zn:S=1:1.5) NCs has an absorption edge near 330 nm. The calculated band gap of Sn:ZnO NCs (Figure 6b) is 3.35 eV which is higher than pure ZnO NC (Eg=3.26 eV). Incorporation of Sn4+ into ZnO matrix enhances the carrier density as well as creates a cationic disorder. This is the reason for blue shifting of absorption band edge on Sn doping which is called the Moss-Brustein effect.22-23 The high density electrons populate the states near the conduction band edge and thus the Sn:ZnO NCs exhibit a greater optical band gap. The excitonic absorption spectra of NHSs are found to be red shifted. The calculated band gaps for ZnO and ZnS counterparts are 3.33 eV and 3.51 eV respectively for CS1 which is shown in the Figure 6c. For CS2 the values are 3.32 eV and 3.50 eV respectively (Figure 6d). The ZnS shell modifies the electronic states of ZnO core by confining the electrons to the


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conduction band of ZnO core and as a result there will be a red shifting.40 Nanoscale core-shell heterostructures undergo a lattice mismatch and as result there will be asymmetric strains between the interfacing layers. The lattice mismatch between WZ ZnS and ZnO is ~36.7%. 41 Thus we can expect that ZnS layers execute a compressive strain on inner ZnO layer. This causes a shift in the absorption band edge towards higher wavelength. Figure 7 shows the LSPR absorption spectra of Sn:ZnO NCs and ZnS@Sn:ZnO NHSs. Heavily doped semiconductor materials show plasmonic absorption other than their band edge absorptions due to appreciable free carrier concentration like commercially used indium tin oxide. Aliovalent Sn4+ doping in ZnO introduces an absorption peak at about 675 nm which is the characteristic plasmonic peak of Sn:ZnO NCs.18 ZnS shell formation shifts the plasmonic peak to the higher wavelength region (675 nm to ~ 970 nm). The characteristic plasmon frequency of a material in a composite medium can be expressed as18,23:

(2)

where ωsp represents the LSPR frequency, εm is the dielectric constant of the dispersion medium, ωp is the bulk plasma frequency, and γ is the line width of the plasmon resonance band. Equation (2) suggests that a change in dielectric constant or refractive index of the medium might change the LSPR absorption position of the material. LSPR absorption is very much sensitive to the size , shape and dielectric medium in which they are dispersed. We have investigated the effect of different solvents with different refractive indices as shown in Table S1. The peak at 675 nm for Sn:ZnO is red shifted to 735 nm (Figure S1a) from TCE to CS2 and plasmonic sensitivity was calculated to be 493 nm per refractive index unit (nm/RIU) like plasmonic Cu2-xS nanocrystals.42 The plasmonic sensitivity of CS1 and CS2 are 281 nm/RIU and 250 nm/RIU respectively.


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(Figure S1b and S1c) This reasonable amount of decrease in plasmonic sensitivity value on ZnS shell formation can be attributed to the formation of ZnS dielectric layer around the plasmonic Sn:ZnO region and also for decrease in Sn4+ concentration in plasmonic core. The refractive index of ZnS (2.36) is higher than that of ZnO (1.99). When the ZnS was coated onto the surface of Sn:ZnO core, the effective refractive index of surrounding environment of Sn:ZnO is increased. For the presence of ZnS layer, core Sn doped ZnO is lesser exposed to solar photon which results in a damp and red shifting in plasmonic absorbance feature. Similar observation was also reported for Au nanorod where the silica shelling and increase in silica shell thickness results in red shift of longitudinal LSPR band of Au nanorod.43 Moreover, the decrease in carrier concentration for leaching out of Sn4+ during anion exchange process as concluded from EDAX study (Figure S2-S5) results in damping and red shift of LSPR band.44 Photoluminescence study: Room temperature photoluminescence spectra of pure ZnO, Sn:ZnO NCs and ZnS@Sn:ZnO NHSs in the wavelength range 350 nm to 650 nm are shown in the Figure 8a-e at an excitation wavelength of 340 nm. Photoluminescence is very powerful tool to identify band edge related emissions as well as defect chemistry of nanoscale materials. Deconvoluted PL spectra show a strong influence on Sn doping into the ZnO matrix and also ZnS layer formation changes the origin of defect states of ZnO. The valance band (VB) of ZnO is composed of oxygen 2p hybridized orbital with Zn 3d and conduction band (CB) is composed of Zn 2+ excited states. Normally the photoluminescence spectra of ZnO consist of emission bands in the ultraviolet (UV) region and visible region. The UV emission generally considered as the band edge emission or emission due to the excitonic recombination. The emissions in the visible region are associated with the intrinsic or extrinsic defects related emissions. The defect states in


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ZnO can be divided into two categories: (i) shallow trapped states which are closely situated to CB and are mainly composed of optically active zinc-related intrinsic defect states like zinc interstitial (Zni) and zinc vacancy (VZn), (ii) deep trapped defect states like oxygen vacancy (VO), oxygen interstitial (Oi), zinc vacancy, oxygen at zinc lattice site (OZn). The blue emission band mainly originates from the recombination of photogenerated electrons in extended Zni states to the valance band edge.45 Transition between electrons close to the conduction band and deeply trapped holes at oxygen vacancies and surface defects give rise to the green emission in ZnO. 46 In Figure 8a we can see the photoluminescence spectra of pure ZnO where band edge related emission is centered at 377 nm and the defect related luminescence is also observed at 421 nm and 525 nm related to the zinc interstitial and oxygen vacancy. In case of Sn:ZnO NCs the peak in the UV region is blue shifted to 375 nm and the same phenomenon was confirmed from the absorption spectra. This blue shifting is nothing but due to the Moss-Brustein effect. Figure 8b shows the blue-green emissions of the material centered at 440 nm and 487 nm which are also enhanced.47 4% Sn doping highly creates structural defects such as oxygen vacancies, zinc interstials when Zn ions are replaced by Sn ions. This is the reason for enhanced defects related emissions in Sn doped ZnO nanocrystals. This can be further proved by XPS analysis. As we vary Zn:S ratio in the NHSs, the near band edge emission of ZnO is red shifted from 375 nm to 407 nm as we increase the amount of S which can be shown in the Figure 8c to 8e. For CS1 (Zn:S=1:0.25) near band edge emission is at 380 nm and for CS2 (Zn:S=1:1) it is at 406 nm. When we compare the PL spectra of ZnS coated nanoheterostructures with pure ZnO, we can see that the near band edge related emission is highly suppressed and the defect related blue and green emissions are highly enhanced. The quenching of the near band edge related PL emission is due to the type II band alignment between ZnO core and ZnS shell where hole is


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transferred from core to shell region and we get the reduced excitonic band edge emission.48 The defect related emission properties are highly enhanced in the nanoheterosructure. Wurtzite ZnS and ZnO have a large lattice mismatch between them and as a result the ZnS layer creates an asymmetric strain on the interfacing layer. Figure 8c and 8d show that for both CS1 and CS2 the blue emission (at 431 nm) is enhanced. As the sulfidation process increases the zinc intersitial related defect states are increased and which is responsible for enhancement of the blue emission. On the same time oxygen vacancy related defect states are highly enhanced as there is a anion exchange process is ongoing during the growth of the nanocrystals and as a result population density of the oxygen vacancy is highly increased and PL peaks near 480 nm and 530 nm are enhanced. In addition, in the case of hollow ZnS NCs (Figure 8e) sulfur based blue emissions became intense and defect related green emission is also observed. This is related to the structural defects and some stacking faults present on the surface of ZnS layer (Zn:S is 1:1.5) with a void is created in the centre due to the nanoscale Kirkendall Effect.49 The above discussion indicates that the variation of the amount of sulfur in NHSs has a great influence on the relative intensities of defect related emissions as well as near band edge emissions of ZnO. We can conclude that ZnS@Sn:ZnO core–shell formation not only changes the band edge emissions but also tunes the defect chemistry of pure ZnO or ZnS. To further investigate the chemical bond formation and defect related states in the samples we have performed XPS spectra for Sn:ZnO and CS2 nanoheterostructure. We have found that Zn 2p peak for Sn:ZnO (Figure S10a) and CS2 (Figure S11a) are centered at 1021.6 eV and 1044.8 ± 0.1 eV. The spin orbit splitting between the two peaks are 23.2 ± 0.1 eV which confirms that Zn is in Zn2+ state. The O 1s peak is deconvoluted to confirm the existence of different oxygen related species such as lattice oxygen (OL), oxygen vacancy related species


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(OV) and chemically absorbed oxygen (OC). The peak centered at 530.3 ± 0.1eV for Sn:ZnO (Figure S10b) and CS2 (Figure S10b) is ascribed to OL related peak due to the Zn-O band formation in wurtzite ZnO. A decrease in the lattice oxygen related peak have been observed due to the ZnS shell formation in case of CS2 nanoheterostructure.10 The peak centered at 531.8 ± 0.1 eV is attributed to the oxygen vacancy related species (Figure S10b and S10b) and the peak is more pronounced for CS2 as the heterostructure formation which have been confirmed from PL analysis also. The peak at 533.1 ± 0.9 eV is for chemically absorbed oxygen species (Figure S10b and S11b). The prominent S 2p peak in Figure S11c for CS2 nanoheterostructure is deconvoluted and two peaks at 161.4 eV and 162.5 eV are associated with S 2p3/2 and S 2p1/2 respectively. The peaks are due to the S2- species and Zn-S bond formation. This confirms ZnS phase formation in nanoheterostructure. We also investigated the Zn LMM Auger spectra from XPS analysis to demostrate whether zinc interstitial (Zni) related defects are formed. Zn LMM peak is fitted by two peaks centered at 499.1 ± 0.9 eV and 497.1 ± 0.3 eV (Figure S10c and Figure S11d) for Sn:ZnO and CS2 samples which are ascribed to Zn-O bond i.e.; lattice zinc and Zni defect related peak respectively. The XPS analysis confirms the zinc interstitial related defect formation which is consistence with PL analysis.50 Photocatalytic activity: Heterostructured semiconductor materials emerge new physical properties or modify the properties of individual materials. Depending upon the band position of two materials the electron–hole separation can be possible which is very essential to enhance the physical properties. Here we have investigated photocatalytic dye degradation of RhB using pure ZnO, Sn:ZnO NCs and ZnS@Sn:ZnO NHSs (Figure S8). First of all we replaced long chain of oleylamine by formic acid. The changes in RhB concentration were monitored by change in the


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optical absorbance spectra at 553 nm of the suspension at 5 min time interval for all the photocatalysts. To compare the photodegradation rate of RhB in the presence of different catalysts, we plotted C/C0 vs. time as depicted in Figure 9a, where C0 is the concentration of dye at equilibrium established in dark condition and C is the concentration of nondegraded dye after different irradiation time intervals. The concentration of dye after dark condition remains nearly same.The photodegadation of pure ZnO shows poor degradation of ~10% for about 30 min of irradiation time. Nearly 80%, 90%, 98% degradations of RhB were found for Sn:ZnO NCs, CS1 (Zn:S = 1:0.25), CS2 (Zn:S = 1:1) NHSs respectively under the same 30 min irradiation time. But for the hollow ZnS NCs the degradation is found to be drastically decreased to nearly 22%. The photodegradation rate of RhB follows a pseudo-first order kinetics, expressed by ln(C0/C) = Kt, where K is the apparent rate constant. Figure 9b shows plots of ln(C0/C) vs. time for all the photocatalysts. Sn:ZnO NCs have a photocatatlytic rate constant of 0.0574 min-1 which is nearly ~17 times than that of pure ZnO NCs. We have found that ZnS@Sn:ZnO (CS2, Zn:=1:1) NHS gives rise to the highest photocatalytic rate constant of 0.1337 min-1. The increased photocatalytic activity may be attributed to the following reasons: (i) Sn doping lifts the conduction band edge of ZnO and increases the band gap of the material. (ii) Aliovalent Sn4+ doping increases free carrier density of ZnO matrix and gives rise to a prominent LSPR peak at about 675 nm which as discussed in optical absorption study. Therefore there is an exciton-plasmon coupling into a single material and it increases light harvesting property of the material. There are several recent reports where heavily doped or self doped plasmonic metal oxides are proved to be promising photocatalysts.51,52 Thus we can say that plasmonic property of Sn:ZnO is the reason for such higher photocatalytic activity of the material than their nonplasmonic counterpart. LSPR mediated local electro-magnetic field (LEMF)53 is the probable


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reasons for higher photocatalytic activity. The plasmonic absorbance region centred at 675 nm for Sn:ZnO and 690 nm for CS1 has been fitted according to the Drude model as demonstrated in Figure S9a and S9b.18 The extrapolation of excitonic and plasmonic absorption bands clearly illustrates a significant overlap between them. This indicates there is an exciton-plasmon coupling in a single nanoparticle. This significant overlap between exciton and plasmon increases the near field enhancement thus the light absorption efficiency of the material and also increases the rate of exciton pair formation.54(iii) Due to the favourable band positions of ZnO and ZnS, there must be electron -hole separation in the nanoheterostructure which is responsible for such high catalytic activity. Scheme II shows that conduction band position of ZnS is at -1.04 eV which is more negative than conduction band of ZnO at -0.31 eV. Therefore upon photoexcitation with solar light the photogenerated electrons are transferred to the conduction band of ZnO along their interface and photoexcited holes are transferred to the valance band of ZnS. Here ZnS@Sn:ZnO offers a type II band alignment which is very much preferable for electron-hole (e-h) pair generation. To prove electron and hole separation in the heterostructure we have calculated relative migration rates of the electrons and holes in each semiconductor. The charge separation in a semiconductor heterostructure depends upon relative migration rates of charge carriers. At first we have calculated the diffusivity (D) of charge carrier following the equation55:

(3)

where μ is the mobility of charge carriers, kB is Boltzmann coefficient, T is the absolute temperature, and qe is the charge of the electron. Now the calculated diffusivities of photogenerated electrons and holes inside ZnO crystal are 5.3×10-4 m2s-1 and 1.29 × 10-4 m2s-1


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respectively taking the mobility values of electrons and holes as 0.0205 m2/Vs and 0.005 m2/Vs respectively at 300 K.56 The time required for the migration of charge carriers from the core of ZnO to ZnS layer can be determined by55:

(4)

where r0 is the migration length, which is taken as 3 nm (ZnS shell is ~ 3 nm). So the calculated migration times of electron and hole are 1.7 fs and 7.07 fs respectively. In similar process we have calculated the migration time for photogenerated carriers of ZnS shell. The diffusivities in case of ZnS electrons and holes are 6×10-4 m2s-1 and 4.14 × 10-4 m2s-1 taking the mobility values of electrons and holes of ZnS as 0.023 m2/Vs and 1.6 × 10-4 m2/Vs respectively.57-58 The migration time for electron and hole from ZnS shell to ZnO core is found to be 1.5 fs and 220 fs respectively. So from the above calculations we have found that photoexcited electrons in ZnS shell can migrate to the surface of ZnO and the holes of ZnO will migrate to the ZnS shell as the migration time is 31 times lower than that of ZnS. So that a total electron-hole separation can be possible. Therefore heterostructure formation maximizes the photocatalytic activity of the heterostructures. To evaluate the effect of LSPR absorption on photocatalytic activity of the as synthesized materials, we have performed wavelength dependent catalytic activity. We choose red LED and IR lamp as our light sources for wavelength dependent study. As shown in the Figure 10a and 10c, CS1 and Sn:ZnO samples show better catalytic activity than that of CS2 (plasmon > 700 nm) under irradiation of red LED (wavelength range 600-700 nm). C/C0 vs. time plot (Figure


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S12a and S12b) shows that nearly 87% dye is degraded by CS1 whereas Sn:ZnO and CS2 degrade 70% and 55% of dye respectively. When the experiments were conducted under irradiation of IR lamp (wavelength range 800-1000 nm), CS2 shows best catalytic activity (Figure 10b and 10d). Sn:ZnO and CS1 also show good activity but less than that of CS2. Figure S8b in SI shows that nearly 97% dye is degraded by CS2 under IR lamp. Since IR lamp generates heat energy during photocatalytic activity , the experiments were carried out at room temperature by using cold water bath to minimize heat energy. The wavelength dependent study suggests that the photocatalytic activity caused by irradiation of red LED and IR lamp is induced by plasmon. But energy transfer from VIS-NIR plasmon resonance to much higher energy to generate exciton is energetically not favourable as exciton energy is much higher than the observed plasmon photon energy. To evaluate whether our developed plasmonic Sn:ZnO NCs and ZnS @Sn:ZnO NHSs are capable of showing nonlinear optical phenomena , we have performed photocatalytic degradation of RhB in presence of red LED with varying the power of the source. The single photon absorption is expected to give a linear relationship whereas multiphoton absorption give rise to a nonlinear relationship between rate constant and source power.59-61We have found that the rate constant varies with the square of the source power (Figure 11) which indicates a two photon absorption process where two plasmonic photon is converted to one higher energy photon which is sufficient to generate one exciton pair in the semiconductor. So from the above experiments we can conclude that the high photoactivity of these plasmonic photocatalysts are due to (i) the antena effect (exciton-plasmon coupling) in a single nanoparticle which increases the photoabsorption efficiency of the nanoheterostructured for the increased electric field enhancement and (ii) conversion of two


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plasmonic photon to a high energy photon to for exciton formation. Both these factors result the high photoactivity of these plasmonic systems. The longterm stability and recyclability of the photocatalysts are important issues for practical application in photocatalytic degradation. During recycling experiments the photocatalysts (CS2) were recollected and reused five times for photodegaration of RhB dye shown in the Figure 12. The XRD pattern of the Sn:ZnO and CS1 after photocatalytic activity is shown in the Figure S13 which shows that there is no detectable change in the XRD pattern. The rate constant for the first cycle was 0.1337 min-1 which was slightly decreased to 0.1278 min-1 after five cycles. Therefore the catalysts exhibit nearly constant activity after five cycles which indicates good photocatalytic stability and reusibility of the photocatalysts under solar light irradiation. Tabls S2 show the comparison of rate constants between ZnS@Sn:ZnO nanoheterostructures and some other photocatalysts towards the decomposition of RhB. The photocatalytic activity of the ZnS@Sn:ZnO nanoheterostructures interestingly shows superior photocatalytic activity than the other photocatalysts. Photocatalytic Mechanism: ZnS@Sn:ZnO nanoheterostructures show excellent photocatalytic activity under solar light irradiation. A type II band alignment supresses the electron-hole (e-h) pair recombination and the charge transfer between them has been shown in the Scheme II. When ZnS shell is illuminated with solar light , the photoexcited electrons are transferred to ZnO core and a total eh separation is possible. Now these photogenerated electrons in the ZnO core will react with the surface absorbed oxygen molecules to produce superoxide anion (˙O2−). Holes in the ZnS shell can react with H2O molecules to generate highly active hydroxyl radical (OH•). According to the


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literature survey these superoxide anion (˙O2−) and hydroxyl radical (OH•) are active species in the photodegradation of organic dye molecule. The detailed mechanism is given below: Charge separation ZnS +hv → ZnS(eCB−+ hVB+) Charge transfer ZnS(eCB−) + ZnO → ZnO (eCB−) + ZnS Superoxide radical (˙O2−).formation ZnO (eCB−) + O2 → ZnO + ˙O2− OH radical formation ˙O2− +H2O →

+ OH-

ZnS (hVB+) + OH− → OH˙ +ZnS Dye degradation OH˙ + RhB → CO2 + H2O The formation of active species OH• radical during photocatalytic process has been probed by terephthalic acid (TA). TA reacts with hydroxyl OH• radical to generate 2- hydroxyterephthalic acid (TAOH) which has a fluroscence peak at about 425 nm on excitation at 315 nm.62 Fluorescence intensity of TAOH is directly proportional to the amount of OH• radical produced during the photocatalytic process. Figure S14 shows the fluorescence spectra of different


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photocatalysts after 30 min of irradiation under solar light. The spectra demostrate that in presence of CS2 the fluorescence intensity of TAOH is highest i.e.; CS2 generates highest amount of OH• radical during photocatalytic experiment. Figure S15a-f show that photoluminescence intensity increases with increasing irradiation time.63 This clearly proves that OH• radical generation is directly proportional to the irradiation time. This experiment reveals that CS2 generates maximum amount of OH• radical and shows the highest photocatalytic activity.

Photoresponse Property: To investigate the enhanced photoefficiency of the as-synthesized NCs and NHSs, we fabricated a photodetector device using these materials. Figure 13 shows the photoresponse curves for all the samples at a bias voltage of 5 V in the presence of light irradiance of 0.65 W cm−2 power density with 15 s time lapse. The green shaded regions represent the current in presence of light (IPhoton) and the other areas indicate current in the absence of light (IDark). The photocurrent gain of the devices, which is the ratio of current in the presence of light and in the absence of light (IPhoton/IDark) is found to be 40 in case of pure ZnO and maximum current gain of 785 is found for the ZnS@Sn:ZnO (CS2; Zn:S=1:1) system. For the Sn:ZnO NCs it is 215 and for CS1(Zn:S=1:0.25), it is 720. But in case of hollow ZnS the current gain drastically decreases to 95. Doping with Sn enhances the photocurrent gain nearly 5 times than that of pure ZnO NCs. The spectral overlap between plasmon and excitonic band of ZnO can be exclusively concluded for the excitonic-plasmonic coupling. So the near field plasmonic modes of Sn:ZnO couple and


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enhance the absorbance of photons resulting in higher exciton pairs. This spectral region is also the maximum intense emission region of solar spectrum. Therefore current gain efficiency is found to be quite high for the plasmonic Sn:ZnO NCs than that of pure ZnO.64 ZnS@Sn:ZnO offers a type II band alignment which can be confirmed from the Scheme II. A type II band alignment is beneficial for electron hole charge separation which is very useful in photovoltaic and photocatalytic applications. To fabricate a fast and stable photodetector, the two major criteria are: (i) increase of the lifetime of an exciton pair, i.e. complete separation of electron and hole, and (ii) fast recombination of the electron and hole pair in the absence of light. The first factor increases the photocurrent gain value and the second one decreases the response time which makes the device fast. In the case of CS2 the two criteria have been achieved, so that we get a current gain of 20 times greater than that of pure ZnO. The electron hole pair recombination rate decreases which can be confirmed from PL study. For the hollow ZnS NCs the electron hole generation is not fruitful and that is why the photocurrent gain decreasesdrastically. This systematic observation demonstrates that plasmon in wide band semiconductor and band alignment across the interface play important roles to engineer photophysical properties. Conclusion: In summary, we have successfully fabricated Sn:ZnO NCs, ZnS@Sn:ZnO core-shell NHSs with variable shell thickness and hollow ZnS by controlled diffusion of sulfur to ZnO NCs driven by nanoscale Kirkendall effect. Dissimilar diffusion rates of O2- and S2- ions results in interior voids in NHSs. The emissions as a function of wavelength in ZnO NCs are significantly modified due to the introduction of more defects by Sn4+ dopant and ZnS shell formation. Efficient exciton-


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plasmon coupling in a single nanoparticle and NHS enhances the light absorbance efficiency of the material. The Sn doped ZnO NCs shows nonlinear optical phenomena which converts the plasmon photon to high energy photon for formation of exciton pair. Also the type-II band alignment in NHS leads to efficient charge separation. These combined effects result in high photocatalytic activity and photocurrent gain. ASSOCIATED CONTENT Supporting Information Supporting Information includes different solvents with refractive indices, plasmonic sensitivity data, EDAX and elemental mapping of the samples, photocatalytic degradation plots, fitted plasmonic absorption data, XPS of the samples, dye degradation under different light, XRD pattern after photocatalytic activity, TA experiments, comparative study of rate constants with different photocatalysts. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions † Authors contributed equally. ACKNOWLEDGMENT The author S. Paul acknowledges DST-INSPIRE fellowship, India for providing the fellowship during the tenure of work.


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Nanocrystals as a Material Platform for Near-Infrared Plasmonics and Cation Exchange

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Figure Caption

Figure 1: (a) Large area TEM image of monodisperse Sn:ZnO NCs. (b) A closer view of a single nanocrystal. (c) Reconstructed HRTEM image showing (002) and (011) plane of WZ ZnO. (d) FFT pattern obtained from yellow square region in (c) showing (002), (013), (011) crystalline planes of Sn:ZnO. (e) Atomic model of ZnO along the [2 0] zone axis showing the planes obtained from FFT. (f) XRD pattern of WZ ZnO. Figure 2: (a) TEM image of ZnS@Sn:ZnO (CS1) NHSs. (b) Closer view of two NHSs. (c) SAED pattern shows the (010) plane of ZnS. (d) FFT pattern obtained from the single NHS highlighted area in Figure 2b shows the presence of (002) plane for both WZ ZnO and ZnS.(e) HRTEM of shows the presence of (002) plane of WZ ZnO with yellow colour and


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(002) plane of ZnS outside with white colour. (f) XRD pattern showing both the phases of WZ ZnO and ZnS. Figure 3: (a) HAADF image of monodisperse CS2 NHSs. (b) A few NHSs showing coreshell structure. (c) SAED pattern showing the presence of both WZ ZnO and ZnS. (d) FFT pattern showing yellow spots for the planes of ZnO and white spots for the planes of ZnS. (e) Reconstructed HRTEM image shows the presence of (011) and (002) planes of ZnO by masking the yellow spots. (f) Similarly by masking the white spots we get reconstructed HRTEM image for ZnS and identified (002) and (011) plane of ZnS. (g-h) Atomic model of ZnO and ZnS along the [2 0] zone axis. (i) XRD pattern of CS2 showing the peaks related to ZnO and ZnS. Figure 4: Elemental mapping of CS2 shows the presence of Zn, Sn, O, S. Figure 5: (a) TEM image of hollow ZnS. (b) A closer view shows that NHSs are shell like structure with a hollow at its centre. (c) HRTEM image of a few number of NHSs and inset shows a single hollow NHS. (d) SAED pattern shows (011), (013) and (022) planes of ZnS. (e) FFT pattern shows (002), (013), (011) along the [2 0] zone axis. (g) XRD pattern of hollow ZnS. Figure 6: (a) Absorbance spectra of differnt samples. (b) Tauc plot of pure ZnO, Sn:ZnO, hollow ZnS showing the change of band gap. (c-d) Tauc plot of CS1 and CS2 showing the band gaps of ZnO and ZnS. Figure 7: Optical ansorption spectra in the range 400-1500 nm for Sn:ZnO and different ZnS@Sn:ZnO NHSs.


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Figure 8: (a-e) PL emission profiles of different nanocrystals and core-shell heterostructures. Figure 9: (a-b) Solar light driven photoctalytic degradation of RhB dye in presence of different samples. Figure 10: (a)-(b) Photodegradation of RhB dye with red LED and IR lamp irradiation. (c)(d) Histogram plot of rate constant under different red LED and IR lamp of different photocatalysts. Figure 11: Linear fit of the photocatalytic degradation rate constant vs square of the power of the incident light (red LED) using Sn:ZnO NCs as photocatalysts. Figure 12: Cyclic experiment of RhB using CS2 photocatalysts. Figure 13: Temporal photoresponses of different samples. Photocurrent gain values are given on the right pannel. Scheme-I: Schematic diagram of Hollow ZnS formation from Sn:ZnO NCs. Scheme-II: Band alignment of the core-shell heterostructures.


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(a)

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2 Figure 2 (a) TEM image of ZnS@Sn:ZnO (CS1) NHSs. (b) Closer view of two NHSs. (c) SAED pattern shows the (010) plane of ZnS. (d) FFT pattern obtained from the single NHS highlighted area in Figure 2b shows the presence of (002) plane for both WZ ZnO and ZnS.(e) HRTEM of shows the presence of (002) plane of WZ ZnO with yellow colour and (002) plane of ZnS outside with white colour. (f) XRD pattern showing both the phases of WZ ZnO and ZnS.


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Figure 3(a) HAADF image of monodisperse CS2 NHSs. (b) A few NHSs showing core-shell structure. (c) SAED pattern showing the presence of both WZ ZnO and ZnS. (d) FFT pattern showing yellow spots for the planes of ZnO and white spots for the planes of ZnS. (e) Reconstructed HRTEM image shows the presence of (011) and (002) planes of ZnO by masking the yellow spots. (f) Similarly by masking the white spots we get reconstructed HRTEM image for ZnS and identified (002) and (011) plane of ZnS. (g-h) Atomic model of ZnO and ZnS along the [2 0] zone axis. (i)

XRD pattern of CS2 showing the peaks related ZnO and ZnS. ACS ParagontoPlus Environment 41

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Figure 4 Elemental mapping of CS2 shows the presence of Zn, Sn, O, S.


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2 Figure 5 (a) TEM image of hollow ZnS. (b) A closer view shows that NHSs are shell like structure with a hollow at its centre. (c) HRTEM image of a few number of NHSs and inset shows a single hollow NHS. (d) SAED pattern shows (011), (013) and (022) planes of ZnS. (e) FFT pattern shows (002), (013), (011) along the [2 0] zone axis. (g) XRD pattern of hollow ZnS


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Pure ZnO Sn:ZnO ZnS@Sn:ZnO,CS1 ZnS@Sn:ZnO,CS2 Hollow ZnS

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Time (min) Figure 12: Cyclic experiment of RhB using CS2 photocatalysts. ACS Paragon Plus Environment

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Figure 13: Temporal photoresponses of different samples. Photocurrent gain values are given on the right pannel.


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Schematic diagram of Hollow ZnS formation from Sn:ZnO NCs.

Scheme II Band alignment of core-shell heterostructure.


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Nanoscale Kirkendall effect driven formation of hollow ZnS@Sn:ZnO heterostructure with enhanced photophysical properties for excitonplasmon coupling.


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Efficient Charge Separation in Plasmonic ZnS@Sn:ZnO

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