Ni-Doped SnO2 Nanoparticles for Sensing and Photocatalysis - ACS

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Ni-Doped SnO2 Nanoparticles for Sensing and Photocatalysis Manikandan Kandasamy, Amreetha Seetharaman, Dhanuskodi Sivasubramanian, Arjunan Nithya, Kandasamy Jothivenkatachalam, Nallappan Maheswari, Muralidharan Gopalan, Sastikumar Dillibabu, and Ali Eftekhari ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01473 • Publication Date (Web): 03 Oct 2018 Downloaded from http://pubs.acs.org on October 4, 2018

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ACS Applied Nano Materials

Ni-Doped SnO2 Nanoparticles for Sensing and Photocatalysis Manikandan Kandasamy, † Amreetha Seetharaman, † Dhanuskodi Sivasubramanian, *, † Arjunan Nithya, ¶ Kandasamy Jothivenkatachalam, *, ¶ Nallappan Maheswari, ‡ Muralidharan Gopalan, ‡ Sastikumar Dillibabu, § and Ali Eftekhari ⁑ †Nonlinear

Optical Materials Laboratory, School of Physics, Bharathidasan University,

Tiruchirappalli - 620 024, Tamil Nadu, India. ¶Department

of Chemistry, Bharathidasan Institute of Technology, Anna University,

Tiruchirappalli-620 024, Tamil Nadu, India. ‡Department

of Physics, Gandhigram Rural Institute-Deemed University Gandhigram, Dindigul-

624 302, Tamil Nadu, India. §Department

of Physics, National Institute of Technology, Tiruchirappalli-620 015, Tamil Nadu,

India. ⁑Belfast

Academy, 2 Queens Road, Belfast, United Kingdom.

ABSTRACT Ni-doped SnO2 nanoparticles (NPs) were synthesized by a coprecipitation route, and showed superior properties for a series of different applications. The 1 at. % Ni doped SnO2 NPs exhibited an excellent electrochemical performance as a supercapacitor to deliver a specific capacitance of 793 F g-1 at an applied current density of 2.5 A g-1 in KOH electrolyte while retaining its capacitance over 1,250 cycles. Energy-dependent (100, 150 and 200 µJ) nonlinear absorption behavior of the Ni-doped SnO2 NPs was investigated and it is due to the effective 2PA (2PA with FCA). A clad modified fiber optic gas sensor setup is used to sense the ammonia at the ambient temperature. Further, the photocatalytic degradation of Rhodamine B (RhB),

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Congo red (CR), and Direct red (DR) dyes by the Ni-doped SnO2 NPs were examined under the visible light irradiation. KEYWORDS: Ni-doped SnO2, Supercapacitor, Optical limiter, Fiber optic gas sensor, Photocatalyst, Dielectric 1. INTRODUCTION Synthesis of nanomaterials is of particular importance in the realm of nanotechnology. Since approaching the atomic scale, the surface to bulk ratio is highly increased. Hence, the material properties are strongly controlled by the shape and structure of nano-objects. In the bulk, the crystal structure forms a uniform architecture. Therefore, the lattice units are similar, but this is not the case for the surface. On the other hand, the activity of irregular surfaces weakens the stability of nanomaterials in general. To control the properties of a nanomaterial, it is imperative to devise a suitable synthesis route. While the common strategy is to synthesize a nanomaterial for a specific application, here, we attempt to utilize a powerful synthesis route for the preparation of Ni doped SnO2, which is one of the key nanomaterials with various applications. Then, we examine the properties of the material synthesized here for a series of application to investigate the powerfulness of the synthesis approach devised. 1.1. Supercapacitors Owing to the growing demand for energy storage, supercapacitors are now competing with batteries.1 In this direction, the electrode materials play the central role, and thus, numerous electroactive materials have been examined as potential candidates for supercapacitors. The key advantage of supercapacitors is high power and cyclability.2 The high rate capability for charging and discharging is the absence of sluggish electrochemical processes as pseudocapacitance 2


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mostly occurs at the surface and subsurface.3 Similarly, in the absence of strong solid-state diffusion, the material structure is not altered in the course of cycling, and this guarantees a better cyclability as compared with battery counterparts. The surface structure of nanomaterial is evidently of vital importance in designing supercapacitors. This is the reason that the electrochemical performance of the Ni doped SnO2 NPs synthesized here can well reflect the usefulness of the synthesis approach followed here. Although carbon nanomaterials are an excellent choice for the so-called double layer capacitors, pseudocapacitance can provide a higher capacity for energy storage. This is the reason that metal oxides have recently attracted considerable for this purpose. The prototype of this class of pseudocapacitive materials is RuO2, which can deliver an excellent pseudocapacitive behavior, but unfortunately, its high cost has limited its practical potential. Therefore, the quest for cheaper metal oxides is an active area of research.4 SnO2 is a potential candidate due to its low cost and stability. Rutile-structured SnO2 is used as a promising multifunctional semiconductor (n-type) due to its unique properties such as direct bandgap (3.6 eV), exciton binding energy (130 meV), electron mobility (250 cm2/V s) and theoretical capacity (782 mAh g-1).5,6 The electronic properties of SnO2 can be controlled by metal doping for which Mn, Ni, Cr, and Fe are among the best choices. Ni can play a critical role to control the growth of the SnO2 NPs, which is indeed the centerpiece of the present work.7 Meng et al. synthesized 3D Ni/SnO2 nanoflowers by a hydrothermal route and the resulting material delivered a specific capacitance of 410 mF cm-2 at a current density of 1 mA cm-2 in a 1M NaOH electrolyte. This was due to the large surface area provided by the NPs.8 Bonu et al. reported a specific capacitance of 10 F g-1 for SnO2 quantum dots (QDs) with a 3



diameter of 2.4 nm. The Coulombic efficiencies of the SnO2 QDs and SnO2 NPs (25 nm in diameter) were 95 and 91%, respectively. A shorter diffusion length was responsible for this excellent behavior.9 Cui et al. reported a specific capacitance of 204.4 F g-1 with a potential scan rate of 5 mV s-1 for SnO2 QDs in a KOH electrolyte.10 Karthikeyan et al. prepared Co-substituted SnO2 hydrothermally. The resulting supercapacitor could deliver a specific capacitance of 840 F g-1 at the potential scan rate of 10 mV s-1 in an H2SO4 electrolyte. The enhanced crystallinity along with a porous morphology and mobility of the charge carriers were responsible for the reported performance.11 1.2. Human Eye Protection The fabrication of intense and short pulse lasers has recently demanded to aim at various applications such as medical, military and industries. It has been realized that a direct exposure of laser beams causes severe damage to the sensitive optical sensor and the human eye. The protection from lasers is, therefore, considered as a safety issue. An ideal optical limiter (OL) is strongly opaque at high intense laser beam and transparent to the lower intensity. Ideally, an OL should possess a rapid laser response time, great linear optical transmission, excellent physiochemical sustainability, higher laser damage, and lower limiting thresholds. Udayabhaskar et al. showed that ZnO: Au nanostructures exhibited a good optical limiting performance at 532 nm (Nd: YAG laser).12 Amreetha et al. reported that C, N, S-doped TiO2 NPs could be a potential OL for both nanosecond (λ=532 nm; Nd: YAG) and femtosecond (λ=800 nm; Ti: sapphire) lasers.13 1.3. Sensors for Environmental Detection Sensors play a central role in our everyday life. Following the emerging pollution of the environment, it is of utmost importance to develop reliable sensors for detecting toxic substances 4


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including toxic gases in the air. Ammonia, for example, is the target of environmental investigations due to its key role in agriculture. It is highly toxic, colorless with a pungent odor gas, and very harmful to human health and the environment. In explosives, ammonium nitrate is slowly decomposed through releasing a trace amount of ammonia.14 The traditional available resistive-type gas sensor is working at elevated temperatures (200-400 ºC). For this purpose, it is of practical interest to fabricate low-cost ammonia sensors with high sensitivity and selectivity, which can work at the ambient temperature. The fiber optic gas sensor is suited to fulfill these requirements. Kavinkumar et al. reported a clad-modified optical fiber sensor composed of silver NPs-decorated graphene sheets with a sensitivity of 17 counts/100 ppm towards ammonia at ambient temperature.15 1.4. Photocatalytic Degradation Organic dyes present in wastewater released from textile, leather tanning, paper, and food industries are very hazardous to aquatic life and the human body. During the dying process in the textile industry, one-kilogram of fabrics consumes about 120-180 L of water. According to the World Bank estimation, 17-20% of water pollution is made by the textile factories. Several methods are available to treat the wastewater containing organic pollutants such as biodegradation, physical adsorption, and reverse osmosis. However, the drawbacks of these methods are an incomplete degradation and the production of secondary solid pollution in wastewater. Most of the works published in the literature suggest that TMOs (Transition metal oxides)-based photocatalysis could be useful for the removal of contaminants in wastewater due to significantly strong oxidation properties, chemical stability, extended surface area, recyclability, and low cost.16 Zhang et al. studied Ni2+, Ti3+-codoped porous black anatase TiO2 5



for a visible light photocatalytic dye degradation.17 Amreetha et al. reported C, N, S-tri doped TiO2 NPs for a dye degradation under visible light.18 In the present work, Ni-doped SnO2 NPs are synthesized by a coprecipitation method. The microstructural parameters are estimated with X-ray line broadening analysis and the crystallite sizes are measured from TEM micrographs. Linear optical properties are studied by Ultraviolet-visible (UV-vis) and Photoluminescence (PL) spectroscopy. Dielectric properties of the pellets have been analyzed via impedance spectroscopy. The electrochemical properties of the NPs are examined for the supercapacitor application. The Z scan technique with a pulsed laser is used to investigate the OL. The Ni-doped SnO2 NPs coated fiber optic sensor setup is adopted for ammonia sensing. Furthermore, the visible light photocatalytic degradation efficiency of the samples is investigated for RhB, CR and DR dyes. 2. EXPERIMENTAL PROCEDURE 2.1 Materials preparation. Ni-doped SnO2 NPs were prepared by a coprecipitation method as illustrated in Figure 1. The required amounts of SnCl2.2H2O and Ni(CH3COO)2.4H2O were added to double distilled water (100 ml) under constant stirring (1 h). For the preparation of 1, 3, 5 and 7 at. % Ni-doped SnO2, the reagents of Ni: Sn was used in the ratios of 0.13:13.1, 0.39:12.8, 0.66:12.5 and 0.92:12.2. Then, 8 M NH4OH was added dropwise (pH=9) while keeping the reactor at the ambient temperature to allow the precipitation process to proceed for 24 h. In the next step, the settled solid was collected and washed with double distilled water several times, and then by ethanol. Afterward, the obtained product was kept in an oven at 80 ºC for 4 h, and then, calcined at 600 ºC for 4 h. Finally, the prepared samples were labeled as SN1 (1 at. %), SN3 (3 at. %), SN5 (5 at. %) and SN7 (7 at. %). The bare SnO2 has been previously reported and it is named as SN0 for the sake of comparison.19 6


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2.2 Materials characterization. The crystal structures of the Ni-doped SnO2 NPs were examined by a PANanalytical X’ Pert Pro Powder X-Ray Diffractometer with Cu Kα irradiation. Raman spectra were recorded using a Witec CRM-200 confocal Raman spectrophotometer with an excitation wavelength of 514.5 nm (Ar ion laser). The molecular structures were confirmed by a Jasco-460 plus FTIR spectrophotometer utilizing a KBr pellet technique. XPS measurements were performed using a Shimadzu AXIS ULTRA-XPS spectrophotometer for elements confirmation. The morphology of the Ni-doped SnO2 NPs was inspected by TEM and HRTEM (JEOL-JEM-2100F). The absorption property was studied using an Agilent Cary 60 UV-vis spectrophotometer. The Perkin Elmer LS-55 PL spectrophotometer was used to investigate the emission property. 2.3. Measurements. The dielectric properties were studied by an LCR HiTESTER (HIOKI 3532-50) in the frequency range of 42 Hz-5 MHz. For this purpose, the powder samples were pressed into pellets (diameter: 6 mm and thickness: ~1 mm) and sintered at 500 ºC for 4 h. The pellets were coated with silver on both sides for a parallel plate geometry. The electrochemical measurements were carried out using a CHI660D electrochemical workstation consisting of three electrodes. The working electrode was a series of Ni-doped SnO2 NPs coated graphite sheets, and Pt and Ag/AgCl used as the counter and the reference electrodes, respectively. All electrodes were dipped into a 2 M KOH solution. For making a working electrode, the active material (Ni-doped SnO2, 85 wt. %), activated carbon (10 wt. %) and polytetrafluoroethylene (5 wt. %) were mixed and dispersed in ethanol, and then, coated on 1x1cm2 graphite sheets, which were dried at 80 ºC for 8 h. Open aperture (OA) Z scan experiment was used to determine the nonlinear absorption of the samples. A Q-switched Nd: YAG laser (532 nm, 9 ns, 10 Hz) (Spectra-Physics) was utilized 7



to excite the samples. The 1 mm cuvette was mounted on the translation platform which was moved through Z-axis direction. This cuvette was filled by a dispersed form of the sample. Here, ethylene glycol was taken as a dispersing medium. Consequently, the transmittance of the samples was about 71%. The laser beam was focused on the powder suspension (1mm cuvette) by 20 cm convex lens. The sample was moved from -Z to +Z direction and the corresponding transmittance was measured with the aid of a pyroelectric energy detector. The experiment was performed for all the samples for the input laser pulse energies of 100, 150 and 200 µJ. The fiber optic gas sensor experimental arrangement is shown in Figure S1. In this set up, 30 cm PMMA step index optical fiber (diameter 750 µm, numerical aperture 0.51) was connected between the light source (Model:SL1, Stellar Net Inc. USA, Emission range:100-2000 nm) and the detector (Model: EPP - 2000, Stellar Net Inc. USA, Detection limit: 100-1100 nm). The 3 cm mid-region of the fiber clad was removed and it was coated by the prepared Ni-doped SnO2 NPs serving as a sensing region, which was kept in the gas chamber (conical flask). The chamber inlet was connected to the gas reservoir and the outlet was contacted with the atmosphere. Different concentrations (0-500 ppm) of gases such as ammonia, ethanol, methanol, acetone, and isopropyl alcohol were taken in the flask. These vapors were injected into the gas chamber. The output light intensity spectrum variation was then recorded. The whole measurement was performed in a dark room at the ambient temperature with a 71% relative humidity and the normal atmospheric surroundings. The 8500-lumen model tungsten halogen lamp source (300 W power, visible light: 400750 nm) was utilized to test the photocatalytic activity of the samples. The schematic diagram of the whole setup has been reported elsewhere.18 RhB, CR, and DR dyes were taken as model pollutants in the present investigations. The concentration of the dye was fixed to be 5 ppm. The 8


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amount of the catalyst was varied for the optimization process (0.1, 0.2, 0.3 and 0.4 g/L). The catalyst was added into the dye solution and this mixture was kept in a dark place under magnetic stirring for 30 min to attain the equilibrium state through adsorption and desorption process. The solution was placed under a continuous visible light irradiation. At regular time intervals (every 30 min), about 3.5 ml of the aliquot was taken from the mixture and centrifuged. The absorbance maximum of the solution was recorded by a UV-Vis spectrophotometer (Shimadzu UV-2450). 3. CHARACTERIZATION 3.1. Crystal structure confirmation (XRD). X-Ray diffraction patterns of the Ni-doped SnO2 NPs show a single-phase rutile tetragonal crystal structure with the space group P42/mnm (JCPDS No. 41-1445) as depicted in Figure S2a. The XRD peaks are analyzed using XRDA software program and the values for the unit cell parameters are presented in Table S1. Due to the similar ionic radii of both ions (0.71 Å for Sn4+ and 0.69 Å for Ni2+), no significant variation is observed in the unit cell parameters as a function of the dopant concentration.7 The microstructural parameters were calculated (Table S2) by Debye Scherer, W-H (UDM), Modified W-H (USDM and UDEDM) analyses and size-strain plot (SSP) method as demonstrated in Figure S3. The above methods have been discussed in detail in our previous report.19 The calculated crystallite size is found to be 15.6, 15.1, 20.1 and 16.3 nm for 1, 3, 5 and 7 at. % Ni-doped SnO2 NPs, respectively. The crystallite size decreases for 1, 3 and 7 at. % Ni doping because of the replacement of Sn4+ by Ni2+ resulting in the formation of Ni-O-Sn bonds in the SnO2 lattice. This phenomenon inhibits the crystal growth controlling the grain size. Notwithstanding, the crystalline size is increased for the case of 5 at. % Ni dopant. This

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fluctuation is due to the fact that the strain and the number of dislocations decrease for high Ni contents in the NPs during the synthesis process.20 3.2. Molecular and elemental analyses (FTIR, Raman, and XPS). The appearance of a broadband at 3443 cm-1 is ascribed to the O-H stretching and a weak band at 1637 cm-1 is associated with the O-H bending vibrations of H2O (KBr pellet rapidly adsorbs water molecules from the atmosphere). The characteristic Sn-O bond vibrations at 651 and 546 cm-1 (SN1) can be considered as convincing evidence for the formation of SnO2. The vibration shifts (651-627 cm-1 and 546-537 cm-1) with increasing the dopant content (1-7 at. % Ni) imply the incorporation of Ni ions within the SnO2 lattice (Figure S2b).21 The crystallinity, structural defects, nanometric size, and dopant distribution effect of nanomaterials can be identified using Raman spectroscopy (Figure S4a). The normal vibrational modes of the rutile tetragonal SnO2 are represented by Г=A1g+ A2g + A2u + B1g + B2g+2B1u+ Eg+ 3Eu. Whereas, A1g, B1g, B2g (Non-Degenerate) and Eg (Doubly-Degenerate) modes are Raman active. A2u (Singlet: Longitudinal Optical) and Eu (Triply-Degenerate: Transverse Optical) modes are IR active. A2g and B1u are silent modes. The peak appeared at 468 cm-1 is attributed to the Eg mode arising from the oxygen vibration. A1g (630 cm-1) and B2g (769 cm-1) modes indicate the expansion-contraction of the Sn-O bond. These observed modes are the same as that of undoped rutile tetragonal SnO2 NPs indicating the absence of secondary phases in the Ni-doped SnO2 NPs. The number of hydroxyl groups on the Ni dopant is different during the nucleation process, and this leads to the formation of oxygen vacancies and local disorders in the NPs. These defects are accountable for the A1g peak shift (Figure. S4b) and the generation of additional vibrational bands in the spectra. The bands located at 248, 306, 341, 505, 543 and 693 cm-1 correspond to Eu (2) TO, Eu (3) TO, Eu (2) LO, A2u TO, B1u (3) and A2u LO modes.22,23 10


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The elemental compositions and the oxidation states of SN0 and SN7 were investigated by XPS (Figure 2a-f). Figure 2a displays a spectrum of SN0 and SN7 confirming the presence of Sn, O and Ni elements. Figure 2b confirms the Sn4+ oxidation state with the binding energies of 485.4 and 493.9 eV for Sn 3d5/2 and Sn 3d3/2. respectively. For SN7, however, these peaks are shifted to higher binding energies (486.9 and 495.4 eV) because of the Ni ions in the SnO2 lattice (Figure 2c). The high-resolution O 1s spectrum (Figure 2d) of SN0 is deconvoluted into three peaks: (i) the peak positioned at 529.4 eV indicates the surface adsorbed O2- ions, (ii) the peak centered at 530.7 eV demonstrates O2-/O-/OH- ions in the oxygen vacancies, and (iii) the peak at 531.9 eV is ascribed to the lattice oxygen. Nonetheless, for SN7, only two peaks at 530.8 and 531.6 eV are observed. This is due to the doping effect (Figure 2e). The Ni 2p core-level spectrum (SN7) has doublet components (Ni 2p3/2 and Ni 2p1/2) and is demonstrated in Figure 2f. The Ni 2p3/2 component is resolved into four peaks at 855, 856.4, 861.6 and 865.7 eV. The binding energies of 855 and 856.4 eV correspond to the Ni2+ and Ni3+ oxidization states, respectively. The presence of both Ni2+ and Ni3+ shows that some of the Ni2+ ions have been transferred to Ni3+ in the Ni-doped SnO2. The other two binding energies at 861.6 and 865.7 eV are ascribed to the satellite peaks. The peak of 874.7 eV in Ni 2p1/2 component denotes the Ni3+ oxidation state of Ni ions.24,25 3.3. Morphological analysis (TEM and HRTEM). Figure 3a-d display the TEM images (SN1, SN3, SN5, SN7) of the Ni-doped SnO2 NPs. The SAED patterns (SN1, SN7) are also depicted in Figure 3e, f. It is evident that the particles are spherical (Figure 3a-d). It has been observed that the agglomeration of NPs increases with increasing the Ni concentration (1-7 at. %). Generally, the nanoparticles tend to agglomerate at short interparticle distances and they attract each other by van der Waals forces as well as electrostatic or magnetic forces. Hence, the attractive 11



magnetic force between the Ni-doped SnO2 NPs is the cause of this agglomeration.26,27 Figure S5 shows the particle size distribution histogram and Table S2 summarizes the average particle size. In the SAED patterns (Figure 3e, f), the indicated (h k l) planes are in a good agreement with the SnO2 JCPDS No. 41-1445. HRTEM images of the Ni-doped SnO2 are shown in Figure S6. Clear lattice fringes indicate that the prepared NPs are highly crystalline with a single phase. The measured inter-planner spacings are 0.26 and 0.33 nm corresponding to the (1 0 1) and (1 1 0) planes of the rutile-tetragonal crystal structure of SnO2. In HRTEM, the lattice fringes have some distortions which are marked by yellow square boxes and can be attributed to the presence of a strain in the prepared NPs. These results corroborate the XRD line broadening analysis. 3.4. Optical studies (UV-vis and PL). The absorption spectra (Figure S7a) of the Ni-doped SnO2 NPs indicate a cutoff region of 287-306 nm, which is ascribed to an electron transition between the oxygen (O2p) and tin (Sn3d). The absorption edge redshifts with respect to the undoped (SN0) SnO2 (284 nm)19 suggests the alteration of local electronic structure on Ni dopant. The s, p-d hybridization and local disorder cause this modification. The bandgap energy estimated by the extrapolation to the zero-absorption coefficient from the Tauc’s relation ( h ) 2  A(h  Eg ) where  , A and Eg are absorption coefficient, constant, and bandgap,

respectively. Figure S7b (Tauc’s plot) shows that the bandgap decreases from 3.46 to 3.20 eV because of the induced bandgap renormalization and screening effects of free carriers.25 Figure S8 provides room temperature photoluminescence spectra of the Ni-doped SnO2 NPs for the excitation wavelength of 275 nm. The emission peaks are deconvoluted by the Gaussian peak fitting procedure. The UV emission is observed at the range of 380-394 nm. The visible region contains three shoulder peaks centered in the regions of 413-422, 442-446, and 486-491 nm. Figure 4 displays a possible emission mechanism for the Ni-doped SnO2 NPs. The 12


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calculated bandgaps energy from the Tauc’s plots are 3.46 (SN1), 3.39 (SN3), 3.27 (SN5) and 3.20 eV (SN7). The emitted UV region energies from PL are 3.26, 3.15, 3.18 and 3.15 eV for SN1, SN3, SN5, and SN7, respectively. Therefore, the emitted energy from the UV region is attributed to the near band edge emission (NBE). In general, for semiconducting oxide materials, the visible emission at 400-800 nm is due to the oxygen vacancies such as neutral ( VO0 ), singly ionized ( VO ) and doubly ionized ( VO2 ). In this case, VO0 and VO are the shallow donors and VO2 is a deep trap. After the photon excitation, the valence band (VB) electrons are excited to the conduction band (CB) and leaving a hole in the VB. These holes can be trapped by the VO0 and

VO directly forming VO and VO2 , respectively. The recombination of an electron in the VO0 to the VB is indeed the NBE (3.26 eV). The energy obtained at 3.00 eV is originated from the recombination of the CB electron to the VO2 . The electron transition from VO to the surface trapped hole emits the energy at 2.78 eV. The emission energy at 2.55 eV is related to the transition between the VO0 to VO2 . The emission intensity decreases with the Ni concentration due to a decrease in the electron-hole recombination rate (Figure S9).28,29 3.5. Electrical properties. Impedance spectroscopy is a powerful technique to understand the electrical characteristics of the samples synthesized here. The Cole-Cole plots of the Ni-doped SnO2 NPs at different temperatures are displayed in Figure S10. These plots have two successive semicircular arcs. Low-frequency side semicircle resembles the grain boundaries influence, whereas high-frequency side semicircle reflects the grain interiors effect. Then, the overall electrical transportation process in the sintered pellet is due to both grain boundary and grain interior effects.

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Z-view software is employed to fit the Cole-Cole plot data with the help of suitable electrical equivalent circuit model as depicted in Figure S11. The fitted parameter values are shown in Table S3. This equivalent circuit is modeled by making a series connection between the contact resistor (Rs) and two subcircuits where each subsequent is comprised of parallel connected constant phase element (CPE) and resister (R). Here, the contact resister Rs indicates the contact resistance of electrode-pellet response. The first subcircuit (R1/CPE1) implies the grain interior and the second subcircuit (R2/CPE2) indicates the grain boundary of the pellet. In this equivalent circuit, CPE element is used instead of capacitance (C) due to the depression of the semicircular arcs. By knowing the values of thickness (l), resistance (R: Rb or Rgb) and area (A) of the pellet, the dc conductivity (σdc) is calculated using the relation   l RA m-1 (Table S4). The frequency-temperature dependence for the real part of the impedance (Zʹ) spectra is shown in Figure S12. As the temperature increases, the low-frequency Zʹ decreases indicating an enhancement in the a.c conductivity but at the high-frequency region, Zʹ is constant denoting the release of space charge polarization of the samples. Figure S13 represents the temperature and frequency dependence of the Zʹʹ spectra. These plots contain grain boundaries and grain interior relaxation peaks at lower and higher frequency region, respectively. Zʹʹ maximum loss and its position shift towards higher frequencies by increasing the temperature. This is ascribed to the temperature dependence of the electrical relaxation arising in the samples. This is due to the oxygen deficiencies and immobile charges at higher and lower temperatures, respectively. The relaxation time (τ) of the grain interior is estimated from the expression   1/   1/ 2f , where f is the relaxation frequency obtained from the Zʹʹ maxima (grain interior).19,30 The calculated values are given in Table S4. 14


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The dielectric property of the SnO2 NPs is the result of a space charge polarization (SCR) and rotation dielectric polarization (RDP). The detailed explanation of these polarization processes has been reported earlier by our group.19 Figure S14 represents the dielectric constant (εʹ) versus frequency at different temperatures. The enhancement of εʹ is noticed in the lowfrequency region because of RDP and SCP, and the εʹ value decreases as the frequency increases. This behavior is because of the polarizability bound to lagging the applied electric field. When the temperature increases, εʹ decreases up to 343 K but starts to increase beyond this temperature. This is because of the palletization and followed by the sintering of the samples have reduced pore space volume and increased grain size. These bulk grains decrease the energy barrier between the grains and enhance the diffusion of atoms. The εʹ decreases with an increase in the dopant concentration in SnO2.31 The dielectric polarizability of the Ni2+ ions (1.23 Å3) is lower than that of the Sn4+ ions (2.83 Å3). Therefore, more substitution of the Ni ions within SnO2 decreases the dielectric polarization, which in turn, decreases the dielectric constant.32 Figure S15 displays the imaginary part of the dielectric constant as a function of frequency at different temperatures. It has been observed that εʹʹ decreases with increasing the Ni concentration and frequency. This is attributed to the molecules contain large relaxation time with delayed polarization. This behavior shows that the prepared materials can be used for highfrequency applications.33 Loss factor tan δ indicates the energy dissipation in the Ni-doped SnO2 pellets. Figure S16 shows that tan δ decreases with an increase in frequency which is due to the space charge polarization. All the samples show the relaxation peaks except for SN7. The appearance of relaxation peaks is due to the hopping frequency of the charge carriers matched with the frequency of the applied ac electric field. The disappearance of the relaxation peak in SN7 reveals the high resistivity nature of the sample.34 15



Figure S17 depicts the frequency dependence of the AC conductivity for Ni-doped SnO2 NPs at different temperatures. The AC electrical conductivity was obtained from the expression, ( ac   0 '' ) . The AC conductivity plots are divided into two parts such as frequency-

independent region (plateau domain) and frequency-dependent region (dispersion domain). In Figure S17, two frequency-dependent regions (a and c) and two frequency-independent regions (b and d) are observed which confirm the presence of grain interior and grain boundary activated conductivity. The  ac data have been fitted by Johnscher power law fit ( a.c ( )   d .c  A( ) n ) to explain the electrical conductivity mechanism. Here,  dc and  ac are extracted from the plateau and dispersion regions, respectively, and A is the temperature-dependent constant. The exponent n is frequency-dependent quantity, which can vary from 0 to 1 and denotes the degree of interaction among mobile ions and the lattices. When n does not exceed 1, the charge carriers have a rapid hopping with a translational motion. If n exceeds 1, this indicates that the charge carriers hopping motion is localized and the species are not leaving the neighborhood. The fitted curves suggest that all samples have n values lesser than 1. This clearly exhibits the charge carriers’ motion of hopping associated with the translation. Using  ac , the hopping frequency (ωb), carrier concentration (N), and mobility (µ) are evaluated. The obtained values are given in Table S4. It is seen that  ac decreases with increasing the Ni concentration in SnO2. Defects are formed in the SnO2 system by doping. While sintering the pellets, defects tend to separate at the grain boundaries through the diffusion process. Hence, the defects hindering the transportation of charge carriers and decreases the conductivity of the sample.34,35 Figure S18 and Figure S19 indicate the electric modulus spectra. An electric modulus is suitable to bring out the electrode polarization and conductivity relaxation times of the materials. 16


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It is expressed as, M *  M ' M '' . For all temperatures at the low-frequency region, both Mʹ and Mʹʹ reach zero indicating a long tail geometry. This is related to the large capacitance of the electrodes.36 The modulus peaks of Mʹ and Mʹʹ move towards the high-frequency side with respect to a temperature depict that the dielectric relaxation process exists in the samples and it is considered as a thermally activated process. 4. APPLICATIONS 4.1. Energy storage (Supercapacitors). The fabricated Ni-doped SnO2 NPs electrodes are examined by cyclic voltammetry (CV) and Galvanostatic charge-discharge (GCD). Figure S20 reports the CV curves of the Ni-doped SnO2 NPs with various potential scan rates (5-100 mV s1).

The CVs resemble an ideal rectangular shape with no distinguishable redox peaks and sustain

the geometry even at 100 mV s-1. The charge storage mechanism for the SnO2 based electrode is as follows SnO 2  K   e   SnOOK

(1)

From the CVs, the specific capacitance (Cs ) is calculated by the formula, Cs  I / mV

(2)

where, I, m and V are the average current of both anodic-cathodic scans, active material mass, and scan rate. The specific capacitance decreases by increasing the scan rate (Table S5) which is due to the interaction of the electrolyte ions with the outer face of the electrode material only. However, lower scan rates enhance the specific capacitance which is because of the electrolyte ions interact with both inner and outer faces of the electrode material.

17



GCD curves of the Ni-doped SnO2 NPs for various current densities are displayed in Figure 5. The curves show nearly symmetric profiles which means the electrode materials have an excellent charge-discharge characteristic. The specific capacitance is evaluated using, Cs  I t / mV

(3)

where, I and t correspond to the discharge current and the discharge time, respectively.37-39 As the current density increases, the specific capacitance decreases (Table S6). At 2.5 A g-1, the SN1, SN3, SN5, and SN7 electrodes exhibit specific capacitance values of 793, 412, 245 and 176 F g-1 respectively. 1 at. % Ni-doped SnO2 (SN1) shows the highest specific capacitance. However, the specific capacitance of 3-7 at. % Ni-doped SnO2 NPs decreases. According to the TEM observation, SN1 shows a well-distributed NPs, and this structure enhances the intercalation with the counterions resulting in a high specific capacitance. Agglomerated SN3, SN5, and SN7 reduce the intercalation with the counterions. Furthermore, an excess amount of Ni increases the resistance (Rs) resulting in a lower specific capacitance.38 Figure S21 demonstrates the intercalation and deintercalation process of SN1 and SN7. Table 1 shows the superior pseudocapacitive performance of the Ni-doped SiO2 NPs as compared with other metal oxides.40-43 A long-term cycling stability is an important parameter for the practical implementation of supercapacitors. The SN1 electrode was examined during 1,250 chargedischarge cycles at a current density of 15 A g-1. The specific capacitance was calculated on 50cycle intervals. The electrode was even capable of maintaining its specific capacitance after 1,250 cycles, but the first 1,250 cycles are sufficient to represent the cyclability for the electrode under consideration. The stability profile is shown in Figure S22 (red color). The Coulombic efficiency (η) is determined by the expression, (%)  t c t d *100 , where t c and t d represent the

18


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charging and discharging times, respectively.37 SN1 electrode shows 100% Coulombic efficiency over 1,250 cycles. 4.2. Human eye protection (Optical limiters). The experimental results are shown in Figure S23a-c. The Z scan traces of all samples exhibit a valley geometry at the focus which is the characteristics of reverse saturable absorption (RSA). RSA indicates that samples possess an optical limiting (OL) behavior. Commonly, the nonlinear absorption (NLA) and nonlinear scattering (NLS) are considered as the origin of this optical limiting phenomena.44 In this Z scan setup, the 10 Hz repetition rate laser pulses are employed to irradiate the samples and a duration of 0.1 sec is sufficient for the thermal relaxation process.45 The low repetition rate and good dispersion of the media (thermally homogeneous) prevent the formation of the scattering centers. Consequently, it is concluded that the obtained results are not attributed to the NLS contribution. Typically, a material can exhibit any of the following nonlinear mechanisms in NLA: twophoton absorption (2PA), three-photon absorption (TPA), saturable absorption (SA), excited state absorption (ESA), and free carrier absorption (FCA).46 In the present case, the estimated bandgap energies of undoped (SN0:3.51 eV) and Ni-doped samples (SN1:3.46 eV) are greater than the single photon energy of laser pulse (2.33 eV). Based on these, it is assumed that all samples exhibit nonlinear absorption process due to the 2PA. To further confirm the 2PA behavior as well as a quantitative evaluation of nonlinear absorption coefficient, the experimental data were fitted theoretically by the equation, 

T ( z , s  1)    q0 ( z )) m (m  1)3 2  for | q0 (0)  1 (4) m0

Where, q0 ( z )    eff I 0 Leff 1  ( z 2 / z02 ) 

(5)

Leff  1  e  L 

(6) 19



Where,  eff = nonlinear absorption coefficient, I 0 = axial intensity (focal point), Leff = sample’s effective length, z0 = Rayleigh range,  =linear absorption coefficient and l = sample thickness.44 The estimated  values are given in Table S7. In addition to that, the obtained β values decrease with respect to the input laser pulse energies (100, 150 and 200 µJ). In general, β is constant as a function of the input intensity since a weak multiphoton absorption cross-section cannot entirely reduce the ground state population. In the present case, the decreasing β indicates a significant reduction of the ground state population at higher incident intensities due to the FCA cross sections. Therefore, the samples raised nonlinearity are indeed effective 2PA (2PA with FCA).47-49 Figure. S24 expresses the proposed mechanism of the NLA process in the Nidoped SnO2 NPs. FCA can generally occur in semiconductors. In the 2PA process, carriers (electron/hole) can be excited to higher states (CB/VB) in which may absorb an additional photon from the laser pulse resulting in the excitation to the next higher energy level. In the Nidoped SnO2 NPs, the FCA is induced by the defect states (below the CB of Sn 3d). β value decreases by the Ni dopant which is due to an increase in the agglomeration, and this can reduce the nonlinear absorption cross-section. To investigate the OL capability of the Ni-doped SnO2 NPs, the OL graphs are drawn between the input fluence versus normalized transmittance. Therefore, input fluence was calculated from the given relation, Fin 

4 ln 2 Ein  3/2 ( z ) 2

(7)

Where, Ein = the energy of the input laser pulses,  ( z ) = radius of the laser beam and it is expressed as

20


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 z   ( z )   (0) 1     z0 

2

(8)

Where,  (0) indicates the beam radius at the focus and z0 depicts the Rayleigh range and is given by z0 

02 

(9)

Where,  is the wavelength of the laser. The energy-dependent optical limiting curves are portrayed in Figure 6a-o. The best optical limiting performance of the material is revealed by knowing the limiting thresholds. The materials maintaining a lower limiting threshold can be a better optical limiter. From the OL curves, the limiting thresholds were estimated by the input fluence at which the transmission drops to 50% of the linear transmission.47 The calculated OL thresholds for the samples synthesized here are presented in Table S7. The obtained nonlinear optical parameters are compared with those reported in the literature (Table 2).50,12,46,51,52 4.3. Smart gas sensing (Fiber optic gas sensors). The gas sensing mechanism has been explained through a simple schematic representation as displayed in Figure 7. Generally, the light propagation in the fiber is due to the total internal reflection which occurs when the refractive index of the core (ncore) is greater than the clad (nclad) region. In the present case, the clad region (3 cm) is modified by the NPs. Here, the refractive index of the core (ncore:1.49) is lesser than the modified clad (nmclad :2.61), thus, light enters the modified cladding. The coremodified cladding interface (Interface I) will act as a leaky mode due to the higher refractive of the modified clad. When the light enters the outer medium (air), again the total internal reflection rises in the modified cladding region due to the lower refractive of the air (nair=1) and the evanescent field forms in this modified clad-air interface (Interface II). Finally, the light re-enters 21



the core via core-modified cladding interface (Interface I). The total output light intensity of the fiber is due to the propagation of light through the fiber and the re-entered light. When the gas molecules/air interact with the evanescent field, the output intensity of the fiber optic sensor varies. In the presence of gas molecules, the evanescent wave absorption is neither low nor high when compared to the reference value (air).53 The clad modified fiber-optic gas sensors output characteristics of spectral variation with different concentration of ammonia gas (SN1: 0-1000 ppm; SN3, SN5, SN7: 0-500 ppm, step size: 100 ppm) at room temperature are shown in Figure S25. All sensors exhibit three characteristic peaks of the fiber, among them the prominent peak intensity variations are in the range of 650-700 nm and their magnified views are shown in Figure S26. The spectral intensity decreases with respect to the gas concentration indicating that the magnitude of evanescent wave absorption in the ammonia gas atmosphere is higher than in the air atmosphere. The sensitivity of the gas sensor is calculated from the ratio of the variation of the peak intensity counts to the gas concentration (Figure S27a). The estimated sensitivity values are 23, 17, 13 and 11 counts/100 ppm for SN1, SN3, SN5 and SN7, respectively. From this observation, SN1 exhibits a higher sensitivity than all other sensors and it is comparable to those reported for other nanomaterials in the literature (17, 9, 16, 4 and 17 counts/100 ppm) as summarized in Table S8.54-56,53,15 The ammonia gas sensing mechanism is demonstrated in Figure 7. The existence of oxygen vacancies in the sensing material (Ni: SnO2) offers the adsorption of oxygen molecules (O2) over its surface. These O2 molecules are ionized to form oxygen ions (O-, O2-) by capturing free electrons from the sensing material. Upon exposure of the sensor to ammonia, these oxygen ions react with ammonia ( 2NH 3  3O   N 2  3H 2 O  3e  ), and hence, this process is realized through the output intensity variations.57 However, the surface to volume ratio is reduced in SN3, SN5 and 22


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SN7 due to the particle agglomeration leading to a decrease in the number of active sites interacting with the gas molecules, and thus, a lower sensitivity. Singkammo et al. also reported that a higher amount of the Ni dopant reduced the sensing response extensively as a result of a high-level surface disorder on the nanoparticle surfaces.24 For the gas selectivity purpose, the SN1-based sensor was exposed to a series of gases such as ethanol, methanol, acetone, and isopropyl alcohol. The spectral responses are illustrated in Figure S28a-d. The prominent peak intensity decreases by increasing the concentration of gases (Figure S29a-d). The intensity versus gas concentration plots of these gases are shown in Figure S27b. The calculated sensitivity of SN1 is low for ethanol, methanol, acetone, and isopropyl alcohol, and the values are 7, 4, 2 and 6 counts/100 ppm, respectively compared to ammonia (23 counts/100 ppm). Possible reactions responsible for the aforementioned sensors can be expressed as.58-61 Ethanol, CH 3CH 2 OH  6O   2CO 2  3H 2 O  6e 

(10)

Methanol, CH 3OH  2O   CO 2  2H 2 O  2e 

(11)

Acetone, CH 3COCH 3  4O   CH 3COOH  CO 2  H 2 O  4e 

(12)

CH 3COOH  7O   3CO 2  3H 2  O 2  7e 

(13)

Isopropyl alcohol, 2C3 H8O  9O 2  6CO 2  8H 2 O  9e 

(14) 23



To determine the maximum detection limit of the SN1 sensor, the gas concentration was increased up to 1000 ppm. Exceeding 500 ppm, the sensor response becomes saturation which is indicated by the square boxes in Figure S25. The higher gas concentration leads to the accumulation of the gas molecules on the sensor surface. Hence, the adsorption sites are saturated leading to a saturation of the output light intensity. This result indicates that the higher detection limit of the sensor under consideration is 500 ppm. The calibration curve is drawn for I0-I (counts) versus gas concentration (ppm) which is shown in Figure S30a. Where I and I0 are the intensity of the spectra with and without the presence of the gas, respectively. The limit of detection (LOD) and the limit of quantification (LOQ) are determined from the corresponding relations to be 3SD/m and 10SD/m, respectively; where SD denotes the standard deviation of the intercept and m is the slope of the calibration curve.62 The calculated LOD is 19 ppm, LOQ is 62 ppm and the linearity of the sensor is 100-500 ppm (R2=0.99). Theoretically determined LOD (19 ppm) is lower than the US Occupational Safety and Health Administration (USOSHA) announced ammonia exposure limit toward working place (25 ppm for 8h. exposure).19 Figure S30b shows the time response curve of the sensor for ammonia at 300 ppm concentration. For the response time measurement, the gas is allowed inside the gas chamber while keeping the inlet valve opened and the outlet valve closed. This procedure is reversely followed for the recovery time measurement. The response and recovery times of the sensor are found to be 1.7 and 5.7 min, respectively. The experiment is performed twice with an interval of 30 days to examine the long-term stability of the SN1 sensor as shown in Figure S31a, b. The magnified view of the prominent peak is shown in Figure S31c, d. The sensitivities calculated after 30th and 60th days are 22 and 21 counts/100 ppm respectively (Figure S27c). This observation implies that the sensor has an acceptable stability. 24


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Real sample analysis: For the real sample analysis, the ammonia-containing water is collected from the maintained fish aquarium water tank by the Animal Science Department, Bharathidasan University, Tiruchirappalli. The collected water is used for the ammonia sensing test. The output light intensity of the sensor is unchanged (Figure S32 a) and the magnified view of the prominent peak is presented in Figure 32 b. This is due to the real sample has a lower concentration of ammonia. Further, the ammonia concentration of the sample is examined by the commercially available API ammonia test Kit. The color chart indicates the water sample has 0.5 ppm ammonia (Figure S32 c). The present fiber optic gas sensor setup is working at ambient temperature without carrier gas flow regulator and hence, low level detection is not possible. To overcome this limitation, the sensor setup will be modified in future. 4.4. Environmental remediation (Dyes degradation). Photocatalytic activities of undoped and Ni-doped SnO2 samples (0.3g/L) were investigated for RhB dye (5ppm) under a visible light irradiation. The degradation of the dye solution was identified by monitoring the changes in the UV-vis absorbance spectrum during the photodegradation process. The dye degradation percentage (%) is estimated by the relation, [(C0  C ) / C0 ]*100 , where, C0 is dye concentration at an initial time and C dye concentration at the final time.63 The calculated dye degradation percentage (%) is displayed as a bar graph (Figure S33) and the absorbance spectra of 5 ppm RhB (SN5) are shown in Figure S34a. The SN5 (50.3 %) and SN7 (35.1 %) samples show a higher efficiency as compared with the SN0 clearly revealing the beneficial impact of the dopant on the photocatalytic performance. Moreover, the photocatytic degradation was examined for the maximum concentrations of RhB dye (15 and 20 ppm) at 0.3g/L (SN5) catalyst amount. Figure 34b, c shows the absorbance spectra of RhB degradation. The estimated degradation percentage 25



(%) decreases from 19.3 (15 ppm) to 11.4 (20 ppm). An increasing the dye concentration, the dye molecules adsorbed on the catalyst surface gets saturation which reduces the number of active sites. Consequently, the photons from the light are blocked and cannot reach the catalyst surface. Thus, the generation of hydroxyl radicals is reduced which results in the decrease of degradation efficiency.64 In addition, SN5 and SN7 were also utilized to examine the dye degradation performance of CR and DR for a catalyst amount of 0.3g/L. The absorbance maxima observed for CR (SN5) and DR (SN5) are 496 and 492 nm, respectively. It is seen that the peak maxima gradually decrease by increasing the irradiation time (Figure S35a, b). Influence of catalyst amount: The influence of catalyst amount on the degradation efficiency is shown in Figure S35c. The degradation of the dyes obeys a pseudo-first-order kinetics and its linear transformation is In(C0 / C )  kt , where, k is the apparent pseudo-first order reaction rate constant and t the reaction time. The kinetic plots are drawn for In(C0 / Ct ) as a function t (Figure S36a-f). The slope simply gives the k value. A higher k value was obtained for 0.3 g/L catalyst for all dyes. The degradation rate increases up to 0.3 g/L and then decreases by 0.4 g/L. The adsorption of the dye molecules is saturated at the catalyst surface, and hence, the extra catalyst presence in the solution leads to the suspension opacity. This, in turn, blocks the penetration of the light and retard the degradation rate.16 Plausible photocatalytic mechanism. The photocatalytic degradation process can be explained by the Ni: SnO2 band structure schematic model illustrated in Figure 8. The VB and CB potentials are calculated by the following empirical equation which is derived from the Mulliken electronegativity theory, E VB    E e  0.5E g , where, EVB is the valance band edge potential, χ 26


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the electronegativity of the semiconductor, Ee the free electrons energy on the hydrogen scale (4.5 eV), and Eg the bandgap energy. The conduction band potential energy is related to E CB  E VB  E g . The estimated EVB and ECB potentials are 2.8 eV (vs. NHE) and -0.4 eV (vs.

NHE), respectively. Pathway I: In this direct mechanism, the electrons are excited from the valence band to the oxygen vacancy-induced defect energy levels (-0.15, 0.01 and 0.28 eV vs. NHE) by the visible light irradiation (Figure. 8). The defect level potential (-0.15 eV vs. NHE) is negative enough to produce O2   comparing with the standard redox potential of O2/O2   (-0.046 eV vs. NHE). The valence potential (2.8 eV vs. NHE) is more positive than the standard redox potential of the OH  /OH  (2.38 eV vs. NHE). Therefore, the photogenerated holes in the valence band can migrate to the catalyst surface and react with the surface hydroxyl (OH  ) molecules to form strong oxidizing OH  radicals.65,66 Pathway II: The RhB dye is excited by the visible light illumination as illustrated in Figure 8 (indirect mechanism). The excited dye (-1.42 eV vs. NHE) transfers the electrons to the conduction band (-0.4 eV vs. NHE), and the defect states of SnO2 lead to a semi-oxidized radial cation (Dye   ). Then, the oxygen molecules (O2) adsorbed on the photocatalyst surface are combined with these electrons to form anionic superoxide radicals (O2   ). This O2   reacts with a proton to produce a hydroperoxyl radical (O2   (ads) + H+ ƒ

HOO  ) resulting in the

formation of hydrogen peroxide (H2O2), which dissociates to a highly reactive hydroxyl radical (H2O2 + e   OH  ) by taking up an electron. Finally, these OH  radicals oxidize the dye molecules and converts them to carbon dioxide (CO2) and water (H2O).16,18 The photocatalytic degradation efficiency of SN5 and SN7 is superior as compared with other samples because of the high concentration of the Ni dopant, which facilitate the charge 27



separation. The potential of Ni2+/Ni is about -0.26 eV (vs. NHE) which is below the conduction band and suitable to effectively capture the photoinduced electrons while enhancing the electronhole separation. Kumar et al. have reported that the Ni-containing ZnO suppressed the electronhole recombination process and enhanced the degradation efficiency.67 Lui et al. observed that the presence of surface defects in ZnO also greatly improved the dye degradation activity.68 These two possibilities exist in our present dye degradation process as evidenced by the observations of defect-related emissions as well as a decrease in the emission intensity with increasing the dopant concentration in PL spectra. 5. CONCLUSION In summary, SN1 exhibited excellent electrochemical, nonlinear optical and gas sensing properties. The formulated electrode material showed a higher specific capacitance of 793 F g-1 at 2.5 A g-1 in a KOH electrolyte. This enrichment is associated with a low-level doping prevents the agglomeration of the NPs, and thus, increase the counterion interaction with the working electrode. From the nonlinear optical investigations, this material demonstrated a high βeff (2.07x10-10 m/W) and a low OLthreshold (3.59) values at 150 µJ energy. Fabricated fiber optic gas sensor illustrated a sensitivity of 23 counts/100 ppm for ammonia at the ambient temperature. In the visible light photocatalytic activity, SN5 and SN7 samples showed a higher percentage of degradation for RhB, CR and DR dyes. The suppression of electron/hole recombination process at higher Ni doping (5 and 7 at. %) and defects of NPs is responsible for this enhancement in the degradation efficiency. These inspections established that the prepared Ni doped SnO2 NPs can be employed for various applications.

28


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ASSOCIATED CONTENT Supporting Information Schematic-Fiber optic gas sensor experimental set up; Characterization: XRD, FTIR, RAMAN, TEM-histogram, HRTEM, UV-vis and PL; Dielectric spectra: Cole-Cole, equivalent circuit, impedance, dielectric constants, dielectric loss, a.c conductivity and electric modulus; Supercapacitor: CV, schematic-charge-discharge process, stability and Coulombic efficiency; Optical limiter: Open aperture Z scan traces and schematic-2PA with FCA; Fiber optic gas sensor: Ammonia gas sensor spectra (SN1, SN3, SN5, SN7), sensitivity plot, gas sensor spectra of SN1 (ethanol, methanol, acetone and isopropyl alcohol), calibration curve, time response curve, and ammonia gas sensor spectra of SN1(30th and 60th day) and real sample analysis sensor spectra; Photocatalyst: Degradation efficiency, absorbance spectra (RhB, CR and DR) and kinetic plots. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. Phone: +91-9790533134 *E-mail: [email protected]. Phone: +91-9443215423 ORCID Manikandan Kandasamy: 0000-0003-2391-7252 Amreetha Seetharaman: 0000-0003-1612-8925 Dhanuskodi Sivasubramanian: 0000-0001-6985-0095 Kandasamy Jothivenkatachalam: 0000-0001-5758-9109 Ali Eftekhari: 0000-0003-3568-4812

29



Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS K. M gratefully acknowledges the DST-FIST Government of India for providing Nd: YAG laser and FTIR experimental facilities and thankful to Bharathidasan University, Tiruchirappalli, for the award of University Research fellowship (Ref. No.5441/URF/K7/2013). K. M acknowledges Dr. Anuradha Ashok and Mr. T. Vijayaraghavan, PSG Institute of Advanced Studies, Coimbatore, for the HRTEM measurements. K. J thankful to SERB, New Delhi, Govt. of India (EMR-2016-003074) for the financial support. REFERENCES 1. Eftekhari, A. Metrics for Fast Supercapacitors as Energy Storage Devices. ACS Sustainable Chem. Eng., 2018, 6, DOI:10.1021/acssuschemeng.7b04532. 2. Eftekhari, A. The Mechanism of Ultrafast Supercapacitors. J. Mater. Chem. A., 2018, 6, 2866-2876. 3. Eftekhari, A.; Mohamedi, M. Tailoring Pseudocapactive Materials from a Mechanistic Perspective. Mater. Today. Energy, 2018, 6, 211-229. 4. Zhi, M.; Xiang, C.; Li, J.; Li, M.; Wu, N. Nanostructured Carbon-Metal Oxide Composite Electrodes for Supercapacitors: A Review. Nanoscale 2013, 5, 72-88. 5. Bhardwaj, N.; Pandey, A.; Satpati, B.; Tomar, M.; Gupta, V.; Mohapatra, S. Enhanced CO Gas Sensing Properties of Cu Doped SnO2 Nanostructures Prepared by a Facile Wet Chemical Method. Phys. Chem. Chem. Phys. 2016, 18, 18846-18854.

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6. Wang, Q.; Zong, Q.; Zhang, C.; Yang, H.; Zhang, Q. L. Network Structure of SnO2 Hollow Spheres/PANI Nanocomposites for Electrochemical Performance. Dalton Trans. 2018, 47, 2368-2375. 7. Lavanya, N.; Radhakrishnan, S.; Sekar, C. Fabrication of Hydrogen Peroxide Biosensor Based on Ni doped SnO2 Nanoparticles. Biosens. Bioelectron. 2012, 36, 41-47. 8. Meng, X.; Zhou, M.; Li, X.; Yao, J.; Liu, F.; He, H.; Xiao, P.; Zhang, Y. Synthesis of SnO2 Nanoflowers and Electrochemical Properties of Ni/SnO2 Nanoflowers in Supercapacitor. Electrochim. Acta 2013, 109, 20-26. 9. Bonu, V.; Gupta, B.; Chandra, S.; Das, A.; Dhara, S.; Tyagi, A. K. Electrochemical Supercapacitor Performance of SnO2 Quantum Dots. Electrochim. Acta 2016, 203, 230237. 10. Cui, H.; Liu, Y.; Ren, W.; Wang, M.; Zhao, Y. Large Scale Synthesis of Highly Crystallized SnO2 Quantum Dots at Room Temperature and Their High Electrochemical Performance. Nanotechnology 2013, 24, 345602. 11. Karthikeyan, K.; Amaresh, S.; Kalpana, D.; Selvan, R. K.; Lee, Y.S. Electrochemical Supercapacitor Studies of Hierarchical Structured Co2+ Substituted SnO2 Nanoparticles by a Hydrothermal Method. J. Phys. Chem. Solids 2012, 73, 363-367. 12. Udayabhaskar, R.; Karthikeyan, B.; Sreekanth, P.; Philip, R. Enhanced Multi-Phonon Raman

Scattering

and

Nonlinear

Optical

Power

Limiting

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Figure 1. Schematic diagram for Ni doped SnO2 NPs synthesized by coprecipitation method.

Figure 2. XPS survey spectra (a), Gaussian deconvolution Sn 3d spectrum of SN0 (b) and SN7 (c), O 1s spectrum of SN0 (d) and SN7 (e), and Ni 2p spectrum of SN7 (f).

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Figure 3. TEM micrographs of SN1 (a), SN3 (b), SN5 (c), SN7 (d) and SAED of SN1 (e), SN7 f).

Figure 4. Possible emission mechanism for Ni doped SnO2 NPs

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Figure 5. GCD curves of SN1, SN3, SN5, SN7 with different current densities

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Figure 6. Optical limiting curves of SN1, SN3, SN5, SN7at 100 µJ (a-e), 150 µJ (f-j), 200 µJ (ko) input energies. Circles indicate experimental data and dense lines display theoretical fitting.

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Figure 7. Fiber optic gas sensing mechanism of Ni doped SnO2 NPs for ammonia gas

Figure 8. Energy band diagram of Ni doped SnO2 NPs for explaining possible mechanism for photocatalytic degradation of Rh B dye molecules under visible light irradiation.

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Table 1. The specific capacitance of our present work is compared with previous reported work. Electrode Material Ni: WO3 MnCo2O4 Ni: NiCo2O4 Mn: ZnCo2O4 Ni: SnO2 (SN1)

Electrolyte

Current density (A g-1) 0.25 1.00 1.00 0.50 2.50

2 M KOH 2 M KOH 2 M KOH 2 M KOH 2 M KOH

Specific capacitance (F g-1) 171.2 539.0 597.0 707.4 793.0

Ref. 40 41 42 43 Present work

Table 2. The comparison of nonlinear optical parameters of Ni doped SnO2 NPs with recent reported works Sample

NiO Au: ZnO 1(1-x) CaFe2O4xBaTiO3 ZnFe2O4 CdO Ni: SnO2 (SN1) Ni: SnO2 (SN1) Ni: SnO2 (SN1)

Excitation wavelength (nm) 532 532 532

Pulse width (ns) 5 5 5

Energy (μJ) 80 150 55.8

β x 10-10 m/W 0.35 0.51 1.15

Ref.

532 532 532 532 532

5 5 9 9 9

100 100 100 150 200

1.45 2.00 2.16 2.07 1.84

51 52 Present work Present work Present work

Table of contents:

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50 12 46

Ni-Doped SnO2 Nanoparticles for Sensing and Photocatalysis - ACS

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