Zinc Oxide Nanoparticles from Waste Zn-C Battery via Thermal Route: Characterization and Properties

Disposable batteries are becoming the primary sources of powering day-to-day gadgets and consequently contributing to e-waste generation. The emerging e-waste worldwide is creating concern regarding environmental and health issues. Therefore, a sustainable recycling approach of spent batteries has become a critical focus. This study reports the detail characterization and properties of ZnO nanoparticles recovered from spent Zn-C batteries via a facile thermal synthesis route. ZnO nanoparticles are used in many applications including energy storage, gas sensors, optoelectronics, etc. due to the exceptional physical and optical properties. A thermal treatment at 900 °C under an inert atmosphere of argon was applied to synthesize ZnO nanoparticles from a spent Zn-C battery using a horizontal quartz tube furnace. X-ray diffraction (XRD), selected area electron diffraction (SAED) and X-ray photoelectron spectroscopy (XPS) results confirmed the formation of crystalline ZnO nanoparticles. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) analysis confirmed that the size of synthesised ZnO particles were less than 50 nm and mainly composed of sphere shaped nanoparticles. Synthesized ZnO exhibited BET surface area of 9.2629 m2/g and showed absorption of light in the UV region. Excitation of ZnO by UV light showed photoluminescence in the visible range. This study will create an opportunity for potential applications of ZnO nanoparticles from spent batteries and will benefit the environment by reducing the volume of e-waste in landfills.


Introduction
Zinc oxide (ZnO) is a material of special interest due to its wide band gap of~3.3 eV, n-type semiconducting properties, unique optical behaviour and excellent chemical and thermal stability [1]. Due to its exceptional properties, ZnO is used in many applications such as electronics, optoelectronics, sensors, lasers, solar cells, photo catalysts, pharmaceutical, energy harvesting applications [2][3][4]. It is a promising material for many optoelectronic applications like ultraviolet lasers, light emitting diode, thin film transistors etc. The properties of ZnO nanomaterials can be improved by doping, changing their size, shape, chemical composition and surface area [5,6]. Several methods including chemical vapour deposition, arc discharge, sol-gel, hydrothermal, microbial route, oxidation process, etc. have been employed to synthesise ZnO nanostructures, using conventional materials like zinc acetates or nitrates precursor [7][8][9][10]. Most of these techniques require complicated facilities and expensive precursors or chemicals which ultimately hinder low-cost and large-scale fabrication of ZnO nanostructures [3,11]. Thus, synthesis of ZnO nanoparticles from waste sources could provide an attractive and sustainable solution for the future. The closure of many mines and deficit in raw material has pushed the price of zinc (Zn) by 60% in 2016 and expected to be $2900/ton in 2019 [12].
crucible and placed on the graphite rod. The graphite rod was then pushed into the furnace where the temperature was 900 °C and kept there for 1 h. The greyish material which was agglomerated to the quartz tube surface at the low temperature zone (temperature 280 ± 20 °C) near the gas outlet was separated and collected. The obtained powder material was then kept at 500 °C in an air atmosphere to remove hydroxide impurities followed by subsequent analysis. An overall flow diagram to synthesise ZnO nanoparticle from waste Zn-C battery is shown in Figure 1.

Characterization Methods
Phase analysis of the raw material and synthesised nanoparticles were analysed by X-ray diffraction, XRD (Philips, PANalytical X'Pert Pro multipurpose, Australia) using CuKα radiation of 45 kV and 40 mA as the radiation source. Samples were scanned in the 2θ range from 10° to 100° diffraction angles, under the step size of 0.026° with 1° slit and 10 mm mask. Phase identification was done by using Xpert High Score Plus software (version 4.7). X-Ray Photoelectron Spectroscopy, XPS (ESCALAB250i, Thermo Scientific, UK) was conducted with standard conditions, mono-chromated AlKα (energy 1486.68 eV), 150 W (12 mA and 13 kV). Spot size was 500 micrometers, 90° photoelectron take off angle and 100 eV, 20 eV for survey scan and region scans respectively. Field emission scanning electron microscope, FESEM (FEI Nova NanoSEM 450) and transmission electron microscope, TEM (JEOL-1400) along with selected area diffraction (SAD) techniques are applied to analyse morphology of the ZnO nanoparticles. ZnO particles were coated with Platinum (Pt) for FESEM to make the sample conductive. ZnO particles were suspended in ethanol, dispersed ultrasonically to separate individual particles, and two drops of the suspension deposited onto holeycarbon coated copper grids for TEM analysis. BET surface area was conducted by N2 physisorption on a Micromeritics Tristar II Plus absorption analyser from relative pressure (p/p0) 0 to 1. The samples (~0.5 g) were dried at 110 °C in the oven and then degassed for at least 3 h at 150 °C under vacuum prior to analysis. The absorption spectrum was obtained by a computer interfaced UV-Visible spectrometer (PerkinElmer Lambda 35). Synthesized ZnO was dispersed in ethanol and ultrasonicated and used for optical absorption spectra using 1 mm pathlength cuvette from 600 to 200 nm wavelength. Photoluminescence (PL) analysis was conducted using Renishaw inVia Raman spectrometer using near UV lasers of excitation wavelength 325 nm coupled with an optical microscope having 15× objective lens.

Characterization Methods
Phase analysis of the raw material and synthesised nanoparticles were analysed by X-ray diffraction, XRD (Philips, PANalytical X'Pert Pro multipurpose, Australia) using CuKα radiation of 45 kV and 40 mA as the radiation source. Samples were scanned in the 2θ range from 10 • to 100 • diffraction angles, under the step size of 0.026 • with 1 • slit and 10 mm mask. Phase identification was done by using Xpert High Score Plus software (version 4.7). X-Ray Photoelectron Spectroscopy, XPS (ESCALAB250i, Thermo Scientific, UK) was conducted with standard conditions, mono-chromated AlKα (energy 1486.68 eV), 150 W (12 mA and 13 kV). Spot size was 500 micrometers, 90 • photoelectron take off angle and 100 eV, 20 eV for survey scan and region scans respectively. Field emission scanning electron microscope, FESEM (FEI Nova NanoSEM 450) and transmission electron microscope, TEM (JEOL-1400) along with selected area diffraction (SAD) techniques are applied to analyse morphology of the ZnO nanoparticles. ZnO particles were coated with Platinum (Pt) for FESEM to make the sample conductive. ZnO particles were suspended in ethanol, dispersed ultrasonically to separate individual particles, and two drops of the suspension deposited onto holey-carbon coated copper grids for TEM analysis. BET surface area was conducted by N 2 physisorption on a Micromeritics Tristar II Plus absorption analyser from relative pressure (p/p 0 ) 0 to 1. The samples (~0.5 g) were dried at 110 • C in the oven and then degassed for at least 3 h at 150 • C under vacuum prior to analysis. The absorption spectrum was obtained by a computer interfaced UV-Visible spectrometer (PerkinElmer Lambda 35). Synthesized ZnO was dispersed in ethanol and ultrasonicated and used for optical absorption spectra using 1 mm pathlength cuvette from 600 to 200 nm wavelength. Photoluminescence (PL) analysis was conducted using Renishaw inVia Raman spectrometer using near UV lasers of excitation wavelength 325 nm coupled with an optical microscope having 15× objective lens.

Results
X-ray fluorescence spectroscopy, XRF of spent battery powder is shown in Figure 2a. Zn (18.39 wt. %), Mn (34.96 wt. %) in oxide form, Cl (16.19 wt. %) were the major elements in the analysis and other minor oxides include Fe, Co, Ca, Si, Cr, Ni, K etc. XRD analysis of the waste battery powder is shown in Figure 2b, confirm the presence of mainly ZnMn 2 O 4 (hetaerolite) and Zn 5 (OH) 8 Cl 2 H 2 O (simonkolleite) phases. Formation of hetaerolite and simonkolleite is in agreement that the major elements in the battery are Zn, Mn, Cl, Oxygen in XRF analysis data and available literature of Zn-C battery. XPS analysis of spent battery powder showed Zn, Mn, Cl and O which is also in good agreement with materials analysis by XRF and XRD. Presence of C in XPS can be attributed to the presence of carbon in battery and/or carbon from carbon rod during battery dismantling.

Results
X-ray fluorescence spectroscopy, XRF of spent battery powder is shown in Figure 2a. Zn (18.39 wt. %), Mn (34.96 wt. %) in oxide form, Cl (16.19 wt. %) were the major elements in the analysis and other minor oxides include Fe, Co, Ca, Si, Cr, Ni, K etc. XRD analysis of the waste battery powder is shown in Figure 2b, confirm the presence of mainly ZnMn2O4 (hetaerolite) and Zn5(OH)8Cl2H2O (simonkolleite) phases. Formation of hetaerolite and simonkolleite is in agreement that the major elements in the battery are Zn, Mn, Cl, Oxygen in XRF analysis data and available literature of Zn-C battery. XPS analysis of spent battery powder showed Zn, Mn, Cl and O which is also in good agreement with materials analysis by XRF and XRD. Presence of C in XPS can be attributed to the presence of carbon in battery and/or carbon from carbon rod during battery dismantling. Simonkolleite starts to decompose upon heating at 900 °C under an inert atmosphere leaving a mole of water, and further prolonged heating decomposes it to ZnO and Zn(OH)Cl (zinc chloride hydroxide) [23]. ZnO reduced to Zn by carbothermal reduction (C present in battery mixture) and produced Zn vapor. Zn vapor again formed ZnO through in-situ oxidation [24]. In addition, at 900 °C ZnMn2O4 also decomposes to ZnO which also reduced to Zn vapor and oxidised into ZnO in the gas phase and finally condensed as ZnO nanoparticle [25,26]. ZnO formation may occur through decomposition of ZnO into Zn vapor and oxygen and recombination depending on different parameters [25,26]. Major elements such as Mn, along with other impurity elements (like Fe, Si) which were present in the battery powder, remain in the residue which was discussed in detail in a previous study [22]. Further oxidation of collected powder under air atmosphere at 500 °C removed hydroxide impurity leaving only ZnO as the residue. The thermal nanosizing mechanism [22] to synthesise ZnO nanoparticles from waste Zn-C battery is shown in Figure 3 and is also displayed in Figure 1. Simonkolleite starts to decompose upon heating at 900 • C under an inert atmosphere leaving a mole of water, and further prolonged heating decomposes it to ZnO and Zn(OH)Cl (zinc chloride hydroxide) [23]. ZnO reduced to Zn by carbothermal reduction (C present in battery mixture) and produced Zn vapor. Zn vapor again formed ZnO through in-situ oxidation [24]. In addition, at 900 • C ZnMn 2 O 4 also decomposes to ZnO which also reduced to Zn vapor and oxidised into ZnO in the gas phase and finally condensed as ZnO nanoparticle [25,26]. ZnO formation may occur through decomposition of ZnO into Zn vapor and oxygen and recombination depending on different parameters [25,26]. Major elements such as Mn, along with other impurity elements (like Fe, Si) which were present in the battery powder, remain in the residue which was discussed in detail in a previous study [22]. Further oxidation of collected powder under air atmosphere at 500 • C removed hydroxide impurity leaving only ZnO as the residue. The thermal nanosizing mechanism [22] to synthesise ZnO nanoparticles from waste Zn-C battery is shown in Figure 3 and is also displayed in Figure 1.  As-synthesised ZnO nanoparticles were analysed using FESEM, TEM, XRD and XPS analysis. The synthesised powder was white gray in colour, which was similar to the colour of ZnO obtained using conventional synthesis from zinc acetates or nitrates. Representative low and high magnification FESEM images (65,000× to 200,000×) of synthesised ZnO nanoparticles are shown in Figure 4. The microstructure observed at low and high magnification FESEM confirmed that the recovered ZnO particles are in the nano range and composed of sphere-shaped nanoparticles. The morphology of the nanoparticles was almost similar in shape and the size was within 50 nm. Particles were mainly homogenously distributed though aggregation of particles was observed in some areas of the SEM images. The TEM image in Figure 5 also confirmed the formation of nanoparticles in spherical shape and the represented size of the nanoparticles was in the range of 10-40 nm. As-synthesised ZnO nanoparticles were analysed using FESEM, TEM, XRD and XPS analysis. The synthesised powder was white gray in colour, which was similar to the colour of ZnO obtained using conventional synthesis from zinc acetates or nitrates. Representative low and high magnification FESEM images (65,000× to 200,000×) of synthesised ZnO nanoparticles are shown in Figure 4. The microstructure observed at low and high magnification FESEM confirmed that the recovered ZnO particles are in the nano range and composed of sphere-shaped nanoparticles. The morphology of the nanoparticles was almost similar in shape and the size was within 50 nm. Particles were mainly homogenously distributed though aggregation of particles was observed in some areas of the SEM images. The TEM image in Figure 5 also confirmed the formation of nanoparticles in spherical shape and the represented size of the nanoparticles was in the range of 10-40 nm. As-synthesised ZnO nanoparticles were analysed using FESEM, TEM, XRD and XPS analysis. The synthesised powder was white gray in colour, which was similar to the colour of ZnO obtained using conventional synthesis from zinc acetates or nitrates. Representative low and high magnification FESEM images (65,000× to 200,000×) of synthesised ZnO nanoparticles are shown in Figure 4. The microstructure observed at low and high magnification FESEM confirmed that the recovered ZnO particles are in the nano range and composed of sphere-shaped nanoparticles. The morphology of the nanoparticles was almost similar in shape and the size was within 50 nm. Particles were mainly homogenously distributed though aggregation of particles was observed in some areas of the SEM images. The TEM image in Figure 5 also confirmed the formation of nanoparticles in spherical shape and the represented size of the nanoparticles was in the range of 10-40 nm.    [27]. XRD analysis confirmed the low/no impurity of the obtained ZnO as there were no other characteristic impurities peaks. The lattice parameters for hexagonal crystal structure (a = b= 0.328 nm and c = 0.522 nm) and d spacing values were calculated for major peaks using Bragg's equation which matches with observed reference pattern are shown in Table 1. Particle sizes using different Miller indices were estimated by following the Debye-Scherrer formula (Equation (1)), where D is the crystallite size, k is shape factor (k = 0.9), β is the full width at half maxima (measured by Gaussian fit using Origin), λ is the wavelength of X-ray (Cukα) and θ is the diffraction angle. The average particle size ~27 nm is in agreement with the observed particle size from HRTEM image ( Figure 5).
The Bragg reflection of as-synthesized ZnO nanoparticles were also measured by SAED pattern (Figure 6b). Polycrystalline nature of ZnO nanoparticles was confirmed by the bright spots, making up rings coming from the Bragg reflection from each crystallite. The crystallite distances were well matched with (002), (101), (102) and (103) which are in agreement with XRD result. The presence of lattice fringes of ZnO nanoparticles in Figure 6c, represents the crystalline nature and distance between the fringes was 0.25 nm which corresponds to the dominant (101) plane and matched with the calculated d value from XRD.  The peaks of as-synthesised ZnO particles indicated the nanocrystalline nature and matches the pure ZnO standard peaks [27]. XRD analysis confirmed the low/no impurity of the obtained ZnO as there were no other characteristic impurities peaks. The lattice parameters for hexagonal crystal structure (a = b= 0.328 nm and c = 0.522 nm) and d spacing values were calculated for major peaks using Bragg's equation which matches with observed reference pattern are shown in Table 1. Particle sizes using different Miller indices were estimated by following the Debye-Scherrer formula (Equation (1)), where D is the crystallite size, k is shape factor (k = 0.9), β is the full width at half maxima (measured by Gaussian fit using Origin), λ is the wavelength of X-ray (Cukα) and θ is the diffraction angle. The average particle size~27 nm is in agreement with the observed particle size from HRTEM image ( Figure 5). D = kλ/β cos θ The Bragg reflection of as-synthesized ZnO nanoparticles were also measured by SAED pattern (Figure 6b). Polycrystalline nature of ZnO nanoparticles was confirmed by the bright spots, making up rings coming from the Bragg reflection from each crystallite. The crystallite distances were well matched with (002), (101), (102) and (103) which are in agreement with XRD result. The presence of lattice fringes of ZnO nanoparticles in Figure 6c, represents the crystalline nature and distance between the fringes was 0.25 nm which corresponds to the dominant (101) plane and matched with the calculated d value from XRD.  XPS results are shown in Table 2. Atomic % of Zn2p3, 29.96% and O1s, 43.77% were the highest and represented the formation of ZnO. Low atomic % of impurities, such as Cl2p3, Si2p, Ca2p3A, K2p3 etc. were also observed which could attributed to impurity during ZnO collection and can be removed by dissolution by acid if high purity material is required. C1s at 284.8 eV was used as binding energy reference therefore could be attributed to adventitious hydrocarbon. The highest atomic % at binding energy peaks at 1021.87 eV corresponding to Zn2p3 confirmed the presence of ZnO. O1sA with atomic percentage 26.53% at 530.36 eV, could be assigned to oxidized metal ions specifically Zn-O present in the ZnO lattice. O1sB at 531.6 eV with 13.63 atomic % is attributed to loosely bound oxygen (O 2− ions) on the surface or oxygen deficient region within ZnO matrix [28]. O1sC with small atomic % of 3.61, at 532.7 eV should be assigned to OH species of absorbed H2O molecules onto the surface of the ZnO nanoparticles [29].  XPS results are shown in Table 2. Atomic % of Zn2p3, 29.96% and O1s, 43.77% were the highest and represented the formation of ZnO. Low atomic % of impurities, such as Cl2p3, Si2p, Ca2p3A, K2p3 etc. were also observed which could attributed to impurity during ZnO collection and can be removed by dissolution by acid if high purity material is required. C1s at 284.8 eV was used as binding energy reference therefore could be attributed to adventitious hydrocarbon. The highest atomic % at binding energy peaks at 1021.87 eV corresponding to Zn2p3 confirmed the presence of ZnO. O1sA with atomic percentage 26.53% at 530.36 eV, could be assigned to oxidized metal ions specifically Zn-O present in the ZnO lattice. O1sB at 531.6 eV with 13.63 atomic % is attributed to loosely bound oxygen (O 2− ions) on the surface or oxygen deficient region within ZnO matrix [28]. O1sC with small atomic % of 3.61, at 532.7 eV should be assigned to OH species of absorbed H 2 O molecules onto the surface of the ZnO nanoparticles [29].
Properties of as-synthesized ZnO nanoparticles from spent Zn-C battery were observed by BET analysis, UV-Vis and Photoluminescence spectroscopy. Specific BET surface area is an important microstructural parameter of ZnO particles, which depends on the geometrical shape and porosity of the particles. BET surface area and porosity parameters are given in Table 3. Synthesized ZnO showed BET surface area of 9.2629 m 2 /g which is comparable with the literature [30,31]. The average BJH pore diameter was~5 nm which demonstrates that the ZnO nanoparticles comprise of micro and mesopores as per IUPAC definition. A type III isotherm (Figure 7) was observed with no/minor hysteresis loop. The BJH pore size distribution of the ZnO nanoparticle (inset of Figure 7) shows that major pores were within 5 nm and larger pores also coexist [22]. Properties of as-synthesized ZnO nanoparticles from spent Zn-C battery were observed by BET analysis, UV-Vis and Photoluminescence spectroscopy. Specific BET surface area is an important microstructural parameter of ZnO particles, which depends on the geometrical shape and porosity of the particles. BET surface area and porosity parameters are given in Table 3. Synthesized ZnO showed BET surface area of 9.2629 m 2 /g which is comparable with the literature [30,31]. The average BJH pore diameter was ~5 nm which demonstrates that the ZnO nanoparticles comprise of micro and mesopores as per IUPAC definition. A type III isotherm (Figure 7) was observed with no/minor hysteresis loop. The BJH pore size distribution of the ZnO nanoparticle (inset of Figure 7) shows that major pores were within 5 nm and larger pores also coexist [22].  The room temperature UV-Vis optical absorption spectrum for the ZnO nanoparticles is shown in Figure 8. The excitonic absorption peak of the ZnO nanoparticles was observed at ultraviolet region ~388 nm (3.2 eV), which originates from band edge absorption of synthesized ZnO and is in agreement with literature data [32,33]. The absorbance value is dependent on the various factors such as size of particles, flaws or deformities in grain structure, and oxygen deficiency [34,35]. The lower The room temperature UV-Vis optical absorption spectrum for the ZnO nanoparticles is shown in Figure 8. The excitonic absorption peak of the ZnO nanoparticles was observed at ultraviolet regioñ 388 nm (3.2 eV), which originates from band edge absorption of synthesized ZnO and is in agreement with literature data [32,33]. The absorbance value is dependent on the various factors such as size of particles, flaws or deformities in grain structure, and oxygen deficiency [34,35]. The lower band gap value of as-synthesized ZnO compared to bulk ZnO 3.3 eV (370 nm), could be attributed to the presence of oxygen vacancy defects. The particle size of the ZnO as a function of peak absorbance wavelength was measured by effective mass model by the following mathematical formula (Equation (2)) [36]. Here r is the particle radius and λ p is the peak absorbance wavelength in nm. The particle size was around 12 nm which is in broad agreement with TEM particle size. The absorption of ZnO nanoparticles in the UV region demonstrates the potential applications where UV absorption is required. band gap value of as-synthesized ZnO compared to bulk ZnO 3.3 eV (370 nm), could be attributed to the presence of oxygen vacancy defects. The particle size of the ZnO as a function of peak absorbance wavelength was measured by effective mass model by the following mathematical formula (Equation (2)) [36]. Here r is the particle radius and λp is the peak absorbance wavelength in nm. The particle size was around 12 nm which is in broad agreement with TEM particle size. The absorption of ZnO nanoparticles in the UV region demonstrates the potential applications where UV absorption is required. The optical properties of synthesized ZnO was also studied by PL spectroscopy and the spectra is shown in Figure 9. The excitation energy 3.8 eV (325 nm) which is higher than the bulk ZnO (3.3 eV) and as-synthesized ZnO (3.2 eV) band gap energy was used so that an electron in the valence band could directly be excited to the conduction band and to the deep levels within the band gap was possible. Room temperature PL spectra of synthesized ZnO showed emission band of visible range at ~439 nm corresponding to blue emission and at ~538 nm corresponding to the green emission. These peaks are found in literature and could be associated with the deep level emission in ZnO due to zinc and oxygen vacancy and the energy gap between the interstitials [36][37][38]. Photoluminescence of the as-synthesized ZnO in the blue-green region validate its use as photonic application in bluegreen spectral range. The optical properties of synthesized ZnO was also studied by PL spectroscopy and the spectra is shown in Figure 9. The excitation energy 3.8 eV (325 nm) which is higher than the bulk ZnO (3.3 eV) and as-synthesized ZnO (3.2 eV) band gap energy was used so that an electron in the valence band could directly be excited to the conduction band and to the deep levels within the band gap was possible. Room temperature PL spectra of synthesized ZnO showed emission band of visible range at~439 nm corresponding to blue emission and at~538 nm corresponding to the green emission. These peaks are found in literature and could be associated with the deep level emission in ZnO due to zinc and oxygen vacancy and the energy gap between the interstitials [36][37][38]. Photoluminescence of the as-synthesized ZnO in the blue-green region validate its use as photonic application in blue-green spectral range. Nanomaterials 2018, 8, x FOR PEER REVIEW 10 of 12 Figure 9. Photoluminescence spectra of synthesised ZnO nanoparticles.

Conclusions
The present study details the characterization and properties of synthesised ZnO nanoparticles from spent Zn-C battery via thermal nanosizing technique. Synthesized ZnO nanoparticles were spherical in shape, within 50 nm size and confirmed by XRD, XPS, SAED analysis. BET surface area of as-synthesized ZnO nanoparticles were 9.2629 m 2 /g with average BJH pore diameter ~5 nm. UV-Vis spectra showed UV absorbance at around 388 nm wavelength and PL spectra showed visible luminescence. As-synthesized ZnO nanoparticles could be potentially useful for optical applications and will simultaneously provide an economical route to produce ZnO nanoparticles and an effective solution to reduce waste battery in landfills.