Journal of Science: Advanced Materials and Devices (2018) 151e160 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Multi-functional Zn2TiO4:Sm3ỵ nanopowders: Excellent performance as an electrochemical sensor and an UV photocatalyst K.M Girish a, b, S.C Prashantha b, c, *, H Nagabhushana d, C.R Ravikumar c, H.P Nagaswarupa c, Ramachandra Naik e, H.B Premakumar f, B Umesh g a Department of Physics, Dayanand Sagar Academy of Technology and Management, Bengaluru 560082, India Research and Development Center, Bharathiar University, Coimbatore 641046, India Research Center, Department of Science, East West Institute of Technology, VTU, Bengaluru 560091, India d Prof CNR Rao Center for Advanced Materials, Tumkur University, Tumkur 572103, India e Department of Physics, New Horizon College of Engineering, Bengaluru 560103, India f Department of Physics, Ramaiah University of Applied Sciences, Peenya Campus, Bengaluru 560058, India g Department of Humanities, PVP Polytechnic, Dr AIT Campus, Bengaluru 560056, India b c a r t i c l e i n f o a b s t r a c t Article history: Received 25 December 2017 Received in revised form 11 February 2018 Accepted 12 February 2018 Available online 23 February 2018 Zn2TiO4:Sm3ỵ (1e9 mol %) nano powders (NPs) were prepared by a facile solution combustion route using oxalyl dihydrazide (ODH) as a fuel The obtained product was characterized by Powder X-ray diffraction (PXRD), Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and UV eVisible studies Cyclic voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) measurements were performed using a carbon paste electrode CV results indicated the reversibility of the electrode reaction, whereas EIS measurements revealed a reduction in the charge transfer resistance with increase in the double layer capacitance of the electrode The prepared electrodes also exhibited high sensitivity for detection of paracetamol The photocatalytic degradation of Zn2TiO4:Sm3ỵ on Titan yellow (TY) dye was evaluated under UV light irradiation The catalyst showed an excellent photocatalytic activity (PCA) for the degradation of TY dye due to reduction of photo generated electronehole pair recombination, thereby enhancing absorption The high electrode reversibility and excellent catalytic activity of Zn2TiO4:Sm3ỵ make it promising for multifunctional applications â 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Zn2TiO4:Sm3ỵ XPS Cyclic voltammetry EIS Photocatalysis Introduction Nowadays, a growing energy requirement for the production of all kinds of devices is an energy conservation topic all over Researchers focused on earth abundant energy alternatives and developing efficient energy storage devices with high power and energy densities to achieve this requirement [1,2] and hence developed the non conventional electrochemical energy devices such as batteries, capacitors and super capacitors for all electronic gadgets [3] However, the batteries suffer from low-power density though they have high-energy density whereas it is vice versa in capacitors To investigate these problems, researchers concentrate * Corresponding author Research Center, Department of Science, East West Institute of Technology, VTU, Bengaluru 560091, India E-mail address: scphysics@gmail.com (S.C Prashantha) Peer review under responsibility of Vietnam National University, Hanoi on supercapacitors with high power density and more energy storage capacity, which fulfil the gap between battery and capacitor [4] Supercapacitors are of two types; an electrical double layer capacitors (EDLC) stores the energy by non-Faradaic manner, and the pseudo-capacitors (PC) stores the energy electrostatically [5e7] Schematic representations of the electrochemical supercapacitors are shown in Fig With the development of the chemical industry, much industrial waste is being emitted into the environment, some of which has the probability of causing adverse effect on the human immune function, reproductive disorders, resulting in neural and behavioural changes in mankind Therefore, the development of new sensors for the detection of trace concentrations of chemicals has become a very important subject of research Electrochemical detection techniques have advantages over other conventional techniques, when nanomaterials were used to modify their surface [8] Electrochemical sensors represent the most rapidly growing class of chemical sensors Amperometric sensors exploit the use of https://doi.org/10.1016/j.jsamd.2018.02.001 2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) 152 K.M Girish et al / Journal of Science: Advanced Materials and Devices (2018) 151e160 Fig Schematic representations of the electrochemical supercapacitors potential applied between a reference and a working electrode, to cause the oxidation or reduction of an electroactive species [9] Semiconductor photocatalysts are attractive to researchers owing to its low energy consumption, strong oxidation ability and mild reaction conditions in the field of environmental protection in the present scenario Catalytic activities are highly influenced by the surface area, crystallinity, number of active catalytic sites, phase composition and size distribution of the catalyst [10,11] A cubic Zn2TiO4 semiconductor photocatalyst, gained abundant interest in the field of materials science, as a promising catalyst for deformation, desulphurization, dehydrogenation, isomerization of organic compounds, mineralization of pollutants etc, since ZnOeTiO2 based systems possess an inverse spinel structure and a high thermal stability [12] Particularly, the photocatalytic water splitting and photocatalytic oxidation reaction of Zn2TiO4 under UV light have been an interesting topic on account of its high reduction, and low oxidation potential [13], but it suffers poor quantum yield and is still a challenge to meet the requirement practical applications Presently, various methods are employed to enhance the photo reactivity of Zn2TiO4 by doping of transition metal, non-metal and rare earth (RE) ions for the degradation of organic pollutants [14,15] Various methods were reported to fabricate Zno-TiO2 based nano powders such as hydrothermal, solidestate reaction, chemical vapour deposition, solegel, direct precipitation, etc [16e20] However, these techniques require rigorous experimental conditions, sophisticated equipments, and procedure is tedious when compared to the solution combustion technique and the same is used to synthesize Zn2TiO4:Sm3ỵ (1e9 mol %) NPs In this paper, we report upon the electrochemical sensor, supercapacitor and photocatalytic properties of Zn2TiO4:Sm3ỵ (1e9 mol %) nanopowders in detail Experimental 2.1 Preparation of Zn2TiO4:Sm3ỵ NPs Stoichiometric amounts of analytic grade zinc nitrate (Zn(NO3)3.6H2O, Sigma Aldrich Ltd), samarium oxide (Sm2O3, Sigma Aldrich Ltd), tetra butyl titanate (Ti(OC4H9)4, Sigma Aldrich Ltd) and laboratory made ODH (C2H6N4O2) were used to synthesize a series of Zn2TiO4:Sm3ỵ (1e9 mol %) NPs, and the detailed synthesis procedure has been discussed elsewhere [21] 2.2 Preparation of carbon paste electrode High quality electrically conducting carbon paste electrode on homemade teflon cavity with high pressure was prepared by using the mixture of graphite powder, prepared powder and silicone oil (added to enhance the mechanical strength of the electrode) at 75%, 15% and 10% respectively [22] The schematic illustration of the electrochemical process with carbon paste electrode is as shown in Fig 2.3 Photo catalytic experimental procedure Photocatalytic experiments were performed for degradation of TY dye, a 20 ppm of 250 ml aqueous solution of TY and 60 mg of photocatalysts in 176.6 cm2 surface area of circular glass reactor under 125 W mercury vapour lamp as a source of UV light along with continuous stirring using magnetic stirrer The irradiation was carried out directly focusing UV light into the reaction mixture from top at a distance of 21 cm in the open air condition [23] Also, it was monitored by UVeVis absorption spectroscopy at RT in the range 200e800 nm using Shimadzu UV-Vis spectrophotometer model 2600 Results and discussion PXRD analysis was performed to study the phase purity and crystal structure of the Zn2TiO4:Sm3ỵ (1e9 mol %) NPs using Shimadzu diffractometer with CuKa radiation (1.541 Å) operating at 50 kV and 20 mA Fig depicts typical PXRD patterns of Zn2TiO4:Sm3ỵ (1e11 mol %) NPs with a scan rate of 2 minÀ1 It was observed that all PXRD peaks were well indexed and in good agreement with JCPDS card No.77-14 with space group Fd-3m (No 227), which indicates that Sm3ỵ ions have been incorporated successfully into the host lattice without disturbing the structure of the host lattice, since there is no shift in peak position with increase in Sm3ỵ concentration Also, slight shift in peak position towards K.M Girish et al / Journal of Science: Advanced Materials and Devices (2018) 151e160 153 Working Electrode Reference Electrode Counter Electrode Electrochemical cell 00 Fig The proposed electrochemical cell mechanism of electrodes Fig PXRD patterns of Zn2TiO4:Sm3ỵ (1e9 mol %) nanopowders lower angle side was observed with increase of dopant dosage, which may be attributed to expansion of unit cell volume and tensile stress when Sm3ỵcations capped into the host lattices [24] Morphological surfaces of the Sm3ỵ doped Zn2TiO4 NPs were ascertained by SEM using Hitachi table top (SEM) (Model TM 3000) (accelerating voltage up to 20 kV using Tungsten filament) The obtained results revealed the porous and agglomerated nature of the product [Fig (aed)] due to rapid evolution of the gasses during combustion process Further, it was noticed that Sm3ỵ concentration does not inuence the morphology of the phosphor [25] The surface composition and elementary oxidation states of the Sm3ỵ doped Zn2TiO4 NPs were investigated using XPS analysis using AXIS ULTRA from AXIS 165, integrated with Kratos patented magnetic immersion lens, charge neutralization system and spherical mirror analyzer The spherical mirror analyzer gives real time chemical compound state and basic imaging using the full range of pass energies and multi-point analysis from either real time or scanned pictures without the requirement for sample test interpretation translation, and the results were shown in Fig 5(aef) All the binding energies were calibrated to the C 1s peak at 284.8 eV of the surface adventitious carbon and the overall survey spectra are shown in Fig 6f It can be observed that all the peaks on the curve are attributed to Zn, Ti, O and C elements and no other element peaks are observed Fig 5(a) shows the binding energies for the Zn2p3/2 and Zn2p1/2 peaks, which are observed at ~1022 and 1045 eV respectively and can be associated with the Zn element in ZnO It conrms the oxidation state of ỵ2 for Zn in the synthesized samples [26,27] The peaks at ~464.0 and 458.4 eV were ascribed to Ti2p1/2 and Ti2p3/2 respectively [28] In this way, it could be deduced that titanium is in the form of Ti4ỵ, and Sm3ỵ doping does not change the state of titanium (Fig 5b), which indicates that Sm3ỵ ions occupy Zn2ỵ sites in the host matrix Further, we identified the presence of oxygen vacancies by the XPS spectra of O1s (536 eV) as shown in Fig 5(c) is asymmetric, showing that the multi-component oxygen species are present in the surface The curve was deconvoluted into two separate peaks situated at 530.2 (OeTi) and 532.1 eV (OeZn) The peak situated at ~530.2 eV, which was normally assigned to the low binding energy component (LBEC), and the high binding energy component (HBEC) was ascribed to two different kinds of O species in the sample [28] Fig 5(d) displays the Sm3d XPS spectrum; the peaks at ~1113.6 and 1080.0 eV can be ascribed to Sm3d3/2 and Sm3d5/2, respectively (Sm3d5/2 is 1083.2 eV in Sm2O3) [29] This difference may be ascribed to the chemical environment surrounding Sm3ỵ ions in Zn2TiO4, and it clearly indicates that samarium is in its ỵ3 state in the present case 154 K.M Girish et al / Journal of Science: Advanced Materials and Devices (2018) 151e160 Fig SEM images of Sm3ỵ doped Zn2TiO4 (a) mol % (b) mol % (c) mol %, and (d) mol % nanopowders Electrochemical studies were carried out to examine the conduction efficiency and charge transfer resistance of Sm3ỵ doped Zn2TiO4 nanopowders (Electrochemical analyser CH I604-E potentiostat with conventional tri-electrode system and M KOH solution as an electrolyte) The separation efficiency between the generated electrons and holes could be explained based on the charge transfer resistance Cyclic Voltammetry (CV) studies were carried out to investigate the reversibility of the electrode reaction and charge efficiency at room temperature by using Ag/AgCl electrode as a reference and platinum electrode as a counter The obtained CV studies of Zn2TiO4:Sm3ỵ (1e9 mol %) electrodes with different scan rate are depicted in the Fig CVs will provide valuable information on the reduction-oxidation (chargeedischarge) behavior of the electrodes Here, the capacitance was primarily based on the redox reaction, since the shapes of the CVs were derived from the shape of the electric double-layer capacitance [30,31] In CV scans, clearly one reduction and one oxidation peaks were observed, resulting in a pair of strong redox peaks as a consequence of Faradaic reactions of the Zn2TiO4:Sm3ỵ active electrode material The peak currents of both the oxidation and reduction peaks are found to increase with a consequent reduction in potential between the peaks Anodic peaks shift towards more positive side while the cathodic peak shifts towards the negative side, suggesting that the increase in Sm3ỵ concentration caused a marked development in the oxidation and reduction states reversibility [30] Further, the graph of peak current (ip) versus scan rate was plotted to confirm the electrode process, and as shown in Fig 7a, the obtained graph shows straight lines with good linearity which indicates that the electrode reaction was a diffusion-controlled process [32,33] A common scan rate of 10 mV/s was used to compare the CV plots of Sm3ỵ (1e9 mol %) doped Zn2TiO4 sample (Fig 7b), and the higher current was observed for the Sm3ỵ (3 mol %) sample, probably due to higher reversibility of electrode reaction In order to evaluate the applicability of the proposed method as a sensor, the modified CPE was used to sense the paracetamol content in commercially available Dolo-650 tablets (paracetamol- 650 mg, Micro labs, India limited), which was dissolved in a 10 mL of double distilled water without any treatment and the known quantity of the paracetamol standard was added into the electrolyte solution The proportional change in the peak current after the addition of the standard paracetamol (Fig 7c) clearly shows that the observed peak was surely due to paracetamol, and no extra voltammetric signal was observed in the potential window indicates that there was no significant interference by various inorganic cations, anions and some organic substances This shows that the paracetamol in pharmaceutical formulations (tablets) can be sensed by using the proposed voltammetric method without interference from other substances in the samples The reversibility of the electrode reaction can be measured by using difference between the oxidation potential (EO) and reduction potential (ER), all the measured values were given in Table From the table it can be observed that the difference between EO and ER for Zn2TiO4:Sm3ỵ (3 mol %) is small as compared to other dopants It shows that this electrode is more reversible [34] EIS were performed to study the charge transfer nature of the prepared electrode on capacitance, the impedance spectra were measured in the frequency range of Hze1 MHz using an AC voltage of mV The obtained Nyquist plots with simulation graphs using the fitted circuit were depicted in the Fig 8a and in an equivalent circuit (Fig 8b) Q1 indicates a constant phase element which can be expressed as [35,36] ZCPE ẳ Yjuịn (1) where u is an angular frequency in rad sÀ1, Y and n; the adjustable parameters of the constant phase element (Q1) and the value of n ¼ 1, and 0.5 correspond to the double layer capacitance, resistance and Warburg diffusion From the Nyquist plot, the semicircles on the real axis will provide the resistance at the electrodeeelectrolyte interface It is noticed that the impedance spectrum of Zn2TiO4:Sm3ỵ (1, 5, 7, mol %) consists of an elevated arc with a larger diameter in the K.M Girish et al / Journal of Science: Advanced Materials and Devices (2018) 151e160 (b) 2p states of Titanium in Zn2TiO4:Sm3+ (3 mol %) nanopowder Intensity (a.u.) 1022 Ti 2p1/2 Zn 2p3/2 Zn 2p1/2 1044 Intensity (a.u.) (a) 2p states of Zinc in Zn2TiO4:Sm3+ (3 mol %) nanopowder 155 Ti 2p1/2 464 eV 458.4 eV 1050 1040 1030 1020 470 465 460 455 Binding Energy (eV) Binding Energy (eV) (c)1S state of Oxygen in Zn2TiO4:Sm3+ (3 mol %) nanopowder (d) Samarium 3D states of Zn2TiO4:Sm (3 mol %) nanopowder 1113 3+ Sm 3d3/2 Intensity (a.u.) Intensity (a.u.) O 1s O-Zn Sm 3d5/2 1083 O-Ti 525 530 535 540 545 550 1120 1110 Binding Energy (eV) 1100 1090 1080 Binding Energy (eV) 3+ (e) 1S state of Carbon in Zn2TiO4:Sm (3 mol %) nanopowder O KLL Sm 3d3/2 Sm 3d5/2 Intensity (a.u.) Intensity (a.u.) C1s Zn 2p Zn 2p3/21/2 (f) Wide Scan spectra of Zn2TiO4:Sm3+ (3 mol %) nanopowder Ti 2s O1s Ti 2p C1s 300 298 296 294 292 290 288 286 284 282 Binding Energy (eV) 1200 1000 800 600 400 Zn2s Zn3p 200 Binding Energy (eV) Fig XPS spectra of (a) zinc 2p states, (b) titanium 2p states, (c) oxygen 1S state, (d) samarium 3d states, (e) carbon 1S state, and (f) wide scan XPS spectrum of Zn2TiO4:Sm3ỵ (3 mol %) nanopowders higher frequency region, showing the high charge transfer resistance (RCt) with low capacitance of the electrode Whereas, Zn2TiO4:Sm3ỵ (3 mol %) shows the depressed arc with a smaller diameter in the high frequency region, indicating that the low charge transfer resistance (RCt) with high capacitance of the electrode [22,37e37] The Warburg element ‘W’ was shown by a straight line in the low-frequency region in the Nyquist spectrum The diffusion of ions from the electrolyte as well as the electrons from the working electrode into the pores on the surface of the electrode was represented by ‘W’ During the transition from the high-frequency semicircle to the mid-frequency point, the electrolytic diffusion of ions took place Q1 represents the constant phase element which was parallel to the charge-transfer resistance (Rct) and the low frequency capacitance (Q2), and was also parallel to the leakage resistance (Rl) The charge transfer resistance (Rct) and the double layer capacitance (C) quantify the semicircle at high frequencies as evidenced from the Nyquist plots The capacity of this electrode was attributed to the synergistic effect of the electric double-layer capacitance on the high surface area of AC and pseudocapacitance via the intercalation/ extraction of ions in lattices [22,36] Fig shows the variation of the resistive part of impedance with Sm3ỵ (1e9 mol %) 156 K.M Girish et al / Journal of Science: Advanced Materials and Devices (2018) 151e160 (a) 3+ Zn2TiO4: Sm (1 mol %) 3+ (b) Zn2TiO4: Sm (3 mol %) 15 15 10 Current (mA) Current (mA) 10 10 mV/s 20 mV/s 30 mV/s 40 mV/s 50 mV/s -5 0.00 -0.18 -0.36 -0.54 -0.72 10 mV/s 20 mV/s 30 mV/s 40 mV/s 50 mV/s -5 -0.90 0.0 Potential (V) 20 -0.2 -0.4 -0.6 -0.8 Potential (V) 3+ (c) Zn2TiO4: Sm (5 mol %) 1.5 (d) Zn2TiO4: Sm3+ (9 mol %) 15 Current (mA) Current (mA) 1.0 10 10 mV/s 20 mV/s 30 mV/s 40 mV/s 50 mV/s -5 -10 0.2 0.0 -0.2 -0.4 -0.6 0.5 0.0 10 mV/s 20 mV/s 30 mV/s 40 mV/s 50 mV/s -0.5 -1.0 -0.8 0.00 -0.23 Potential (V) -0.46 -0.69 Potential (V) Fig CV plots of Zn2TiO4:Sm3ỵ (1e9 mol %) nanopowders with different scanning rates 10.8 (a) Zn2TiO4:Sm3+ (1-9 mol %) 12 mol % (b) Zn2TiO4:Sm3+ (1-9 mol %) Scan rate 10 mV mol % 10 Current (mA) mol % ip 5.4 mol % mol % 0.0 -5.4 0.0 -0.2 -1 -0.4 -0.6 Potential (V) Square root of scan rate ( mVs ) (c) Zn2TiO4:Sm3+ ( mol %) with paracetamol Current (mA) 10 mV/s 20 mV/s 30 mV/s 40 mV/s 50 mV/s -5 -10 -15 0.2 0.0 -0.2 -0.4 -0.6 -0.8 Potential (V) Fig a) Peak current (ip) versus square root of scan rate; b) CV plots of Sm3ỵ (1e9 mol %) doped Zn2TiO4 electrodes with a scan rate of 10 mV/s; c) CV plots of Sm3ỵ (1e9 mol %) doped Zn2TiO4 electrodes with a paracetamol standard K.M Girish et al / Journal of Science: Advanced Materials and Devices (2018) 151e160 Table Oxidation potential (EO), reduction potential (ER), the difference between EO and ER and diffusion co-efcient of Zn2TiO4:Sm3ỵ (1e9 mol %) electrodes Zn2TiO4:Sm3ỵ (mol %) Electrode E0 (V) ER (V) E0eER (V) D (cm2sÀ1) Â 10À5 0.1962 0.2323 0.1960 0.1920 0.1975 0.4575 0.4440 0.4863 0.5034 0.4624 0.2613 0.2117 0.2901 0.3114 0.2649 5.49 7.42 4.28 4.39 3.79 -250 mol mol mol mol mol (a) -30 Z" (ohm) -200 -20 -10 -150 10 -100 20 30 40 50 60 -50 20 40 60 80 100 Z' (ohm) 1.11 x10 -11 2.74 x10 -7 (b) Q Q R= 4.948 (Ohm) 0.00467 C 2.437 x10-8 C R R 35.29 (Ohm) 0.001015 3611 (Ohm) W Fig Nyquist plots of Zn2TiO4:Sm3ỵ (1e9 mol %) electrodes 60 3+ Zn2TiO4:Sm (1- mol %) mol % mol % mol % mol % mol % Z' (Ohm) 50 40 30 20 10 0 Frequency (10 Hz) Fig Variation of the resistive part of impedance with frequency for Zn2TiO4:Sm3ỵ (1e9 mol %) electrodes 157 composition as a function of frequency It is noticed that the magnitude of Z decreases with increase in frequency and in good agreement with the conductivity results The high value of Z at lower frequency was due to the total polarization caused by space charge, dipoles, ions and electrons [38] Photocatalytic decolourization of the TY dye was performed under UV light illumination at room temperature for the duration of 60 to investigate the photocatalytic activity of Zn2TiO4:Sm3ỵ (1e9 mol %) NPs The photocatalytic experiments were carried out in a circular glass reactor using a 125 W medium pressure mercury vapour lamp as the UV light source at room temperature The results show that the prepared nanopowder decomposed nearly 80% after 60 of irradiation in the presence of the TY dye Fig 10(a) shows the absorbance spectra of TY decomposition with a maximum wavelength at 405 nm The prepared material ensures the adequate detachment of the photogenerated carriers, which has an excellent charge-transport ability This property, along with less crystalline size, reduced the band gap and increased oxygen vacancies, making the sample an exceptional material for photocatalytic applications The detailed degradation percentage of TY catalysed by the Zn2TiO4:Sm3ỵ photocatalyst under UV light is given in Table Fig 10(b) illustrates the plot of % degradation as a function of time for the degradation of TY dye using Zn2TiO4:Sm3ỵ (1e9 mol %) photocatalysts under UV light It is noticed that the photocatalytic activity of Zn2TiO4:Sm3ỵ (7 mol %) showed an enhanced degradation of TY dye under UV light, and the photocatalytic activity of catalyst may be ranked in the following order of mol % Zn2TiO4:Sm3ỵ > mol% Zn2TiO4:Sm3ỵ > mol % Zn2TiO4: Sm3ỵ > mol % Zn2TiO4:Sm3ỵ > mol % Zn2TiO4:Sm3ỵ The degradation of TY follows the LangmuireHinshelwood first order kinetics model and its kinetics can be expressed as ln (C/Co) ¼ Àk t [39] (Fig 10c) Doping of Sm3ỵ ions into the catalyst resulted in a slight shift of the band edge positions and consequently changed the band gap energy, which was usually accompanied by the formation of defects that could play as trap centres to photoelectrons, and disproportionate amounts of doping in the host may lead to formation of defects This may lead to an upward bending of the conduction and valence bands at the interface, acts as the recombination centre which reduces the photocatalytic efficiency It was observed that incorporating of a precise (critical) quantity of doping element is essential for separation of charge carriers in the host lattice, which resulted in a higher surface barrier, and a narrower space charge region leads to enhanced PCA [40,41] At the lower dopant concentration, the space charge layer exceeded by increasing the penetration depth and at higher concentrations, dopant atoms covered the host surface, leading to the increase in the number of electronehole re-combinations, and hence lowering the PCA [42] Electrons are excited to the conduction band from the valence band under irradiation of UV light and simultaneously holes are created in the valence band [43,44] Excited electrons can be effectively trapped in the 4f level, which helps inhibit the recombination of holes and electrons Sm3ỵ ions can behave as an electron trap as well as a hole trap in the Zn2TiO4 catalyst Sm3ỵ reduces to Sm2ỵ when an electron traps Sm3ỵ and the trapped electron will be transferred to oxygen molecule, promoting the formation of hydroxyl radical Sm3ỵ gets oxidized to the Sm4ỵ state, when hole traps of Sm3ỵ are transferred to the hydroxyl anion, leading to the formation of hydroxyl radicals and finally the product gets mineralized by these oxidants [45] The proposed degradation mechanism of TY dye under UV light irradiation was illustrated in Fig 11 and the corresponding equations are as follow: 158 K.M Girish et al / Journal of Science: Advanced Materials and Devices (2018) 151e160 3+ (a)Zn2TiO4:Sm (7 mol %) 0 15 30 45 60 Absorbance (a.u.) 1.0 0.8 (b) Zn2TiO4:Sm3+ (1-9 mol %) under UV light mol % mol % mol % mol % mol % 20 Degradation (%) 1.2 0.6 322 nm 0.4 405 nm 40 undoped 60 0.2 80 0.0 300 400 500 10 20 (c) Zn2TiO4:Sm3+ (1-9 mol %) under UV light 1.0 80 Degradation (%) mol % mol % mol % mol % mol % 0.8 C/C0 30 40 50 60 Time (min) Wavelength (nm) 0.6 0.4 (d) Zn2TiO4:Sm3+ (7 mol %) 60 40 20 0.2 10 20 30 40 50 60 Time (min) Recycle runs Fig 10 a) Absorbance spectra of Zn2TiO4.:Sm3ỵ (7 mol %) photocatalyst, b) Plot of % degradation of TY dye under UV light, c) Plot of C/Co for the degradation of TY dye under UV light, d) Reusability of Zn2TiO4.:Sm3ỵ (7 mol %) photocatalyst for ve consecutive recycle runs Table Kinetics studies on Zn2TiO4:Sm3ỵ (1e9 mol %) under UV light illumination Photocatalysts Zn2TiO4:Sm3ỵ (x mol %) %D K Â 10 65 69 74 80 75 16 17 20 26 20 mol mol mol mol mol % % % % % O2.-ỵ2Hỵ/ 2OH Sm3ỵỵ hỵ/ Sm4ỵ Sm4ỵỵ OH/ Sm3ỵ ỵOH In order to investigate the photo stability of Sm3ỵ (7 mol %) doped Zn2TiO4 nanopowders, reusability test was performed and a negligible decrease in the degradation efficiency was observed even after the five successive cycles (Fig 10d) The above results indicate that the present nanopowder can be used as an efficient photocatalyst for the degradation of TY with high reusable potential, and in general it is highly useful for practical applications in secondary pollution removal Zn2TiO4 ỵ photon / Zn2TiO4 (hỵ ỵ e) Sm3ỵỵ e/ Sm2ỵ Sm2ỵỵ o2/ Sm3ỵỵO2.- e- e- e- e- e- e- e- e- Sm3+ electron driven hydroxide radicals O2 e.trap UV light h+ h+ h+ h+ h+ h+ h+ Sm3+ h+ trap Sm2+ O2 OH Decolourized product TY Sm4+ detrap HOdetrap OH Hole driven hydroxide radicals Fig 11 Proposed mechanism for the photocatalytic degradation of TY dye K.M Girish et al / Journal of Science: Advanced Materials and Devices (2018) 151e160 Conclusion A series of Sm3ỵ (1e9 mol %) doped Zn2TiO4 NPs were synthesized by the simple and economical facile combustion route using ODH as a fuel The obtained products were characterized by different techniques such as PXRD, SEM, XPS and characterized as the electrode material for all solid state flexible supercapacitors Electrochemical studies showed the high reversible electrode reaction, high charge transfer resistance, and exhibited the high sensitivity for detection of paracetamol The photocatalytic activity of Zn2TiO4:Sm3ỵ (1e9 mol %) nanopowders showed the enhanced activity in the degradation of TY dye under UV light irradiation, which was attributed to the effective separation of charge carriers without undergoing any significant loss, even after the five recycles Therefore, the prepared material is highly useful as a sensor for applications in supercapacitors, battery and energy storage devices It is also a prominent material for degradation of organic dye pollutants Acknowledgements The author SCP thanks to VGST, Govt of Karnataka, India (VGST/ 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