Zinc titanate nanoarrays with superior optoelectrochemical properties

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Applications of Polymer, Composite, and Coating Materials

Zinc titanate nanoarrays with superior optoelectrochemical properties for chemical sensing Syed Sulthan Alaudeen Abdul Haroon Rashid, Ylias M. Sabri, Ahmad Esmaielzadeh Kandjani, Christopher James Harrison, Ram Kumar Canjeevaram Balasubramanyam, Enrico Della Gaspera, Matthew R. Field, Suresh K. Bhargava, Antonio Tricoli, Wojtek Wlodarski, and Samuel J Ippolito ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08704 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 27, 2019

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ACS Applied Materials & Interfaces

Zinc Titanate Nanoarrays with Superior Optoelectrochemical Properties for Chemical Sensing Syed Sulthan Alaudeen Abdul Haroon Rashid 1, Ylias M. Sabri

1,*,

Ahmad E. Kandjani

1,*,

Christopher J. Harrison 2, Ram Kumar Canjeevaram Balasubramanyam1,2,3, Enrico Della Gaspera 4, Matthew R. Field 5, Suresh K. Bhargava 1, Antonio Tricoli 6, Wojtek Wlodarski 2, and Samuel J. Ippolito 1,2,* 1Centre

for Advanced Materials and Industrial Chemistry (CAMIC), School of Science,

RMIT University, GPO Box 2476, Melbourne, Victoria 3001, Australia. 2School 3CNRS,

of Engineering, RMIT University, Melbourne, Victoria 3001, Australia. Institut de Chimie de la Matière Condensée de Bordeaux (ICMCB), University of

Bordeaux, UMR 5026, 87, Avenue du Docteur Schweitzer, Pessac Cedex F-33608, France. 4School

of Science, RMIT University, GPO Box 2476, Melbourne, Victoria 3001, Australia.

5RMIT

Microscopy and Microanalysis Facility (RMMF), RMIT University, Melbourne,

Victoria 3001, Australia. 6Nanotechnology

Research Laboratory, Research School of Engineering, Australian National

University, Canberra, Australian Capital Territory 2601, Australia. * Corresponding authors: [email protected], [email protected] and [email protected]

Keywords: ZnTiO3 nanoarrays, acetone sensing, amperometry, light-assisted, bias

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Abstract In this report, the gas sensing performance of zinc titanate (ZnTiO3) nanoarrays (NAs) synthesized by coating hydrothermally formed zinc oxide (ZnO) NAs with TiO2 using lowtemperature chemical vapour deposition (CVD) is presented. By controlling the annealing temperature, diffusion of ZnO into TiO2 forms a mixed oxide of ZnTiO3 NAs. The uniformity and the electrical properties of ZnTiO3 NAs made them ideal for light-activated acetone gas sensing applications for which, such materials are not well studied. The acetone sensing performance of the ZnTiO3 NAs is tested by biasing the sensor with voltages from 0.1-9 V DC in an amperometric mode. An increase in the applied bias was found to increase the sensitivity of the device toward acetone under photoinduced and non-photo induced (dark) conditions. When illuminated with 365 nm UV light, the sensitivity was observed to increase by 3.4 times towards 12.5 ppm acetone at 350 °C with an applied bias of 9 V, as compared to dark conditions. The sensor was also observed to have significantly reduce the adsorption time, desorption time and limit of detection (LoD) when excited by the light source. For example, LoD of the sensor in the dark and under UV light at 350 °C with a 9 V bias is found to be 80 and 10 ppb, respectively. The described approach also enabled acetone sensing at operating temperature down to 45 °C with repeatability of > 99% and LoD of 90 ppb when operated under light, thus indicating that the ZnTiO3 NAs are a promising material for low concentration acetone gas sensing applications.

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1. Introduction Developing a highly sensitive and selective gas sensor for detecting a low concentration of volatile organic compounds (VOCs) for various applications has become extremely urgent in various fields.1,2 There have been numerous efforts to develop acetone sensors for medical and industrial applications.2 Inhaling acetone concentrations higher than ~170 ppm can cause harm to the central nervous system resulting in serious health issues.3 Conversely acetone is also a by-product of the human metabolic reaction, and its concentration in the human breath will depend on whether the individual suffers from a condition such as diabetic.4 Therefore, the development of highly sensitive, selective and miniaturized acetone sensors at an affordable price is important not just for the non-invasive diagnosis of diseases such as diabetes, but also for various industrial sensing applications.1-4 Solid-state devices, especially solid-state amperometric gas sensors (AGS)5-6 based on semiconducting materials is a promising field.7-9 AGS could potentially offer reduced instrument size, low fabrication cost, improved sensitivity, better selectivity, low LoD with short response times to enable real-time gas monitoring. This work explores the potential of an interdigitated AGS (which measures current as a sensor response), wherein the acetone interacts with the gas sensing layer.5-6 Probing the AGS with different voltages will alter the electrical properties of the of the gas sensing layer which results in the change of the gas adsorption properties at the surface.5, 10 Applying different bias to the sensor can also act as an additional input parameter that can influence the sensor performance. To attain effective gas sensing performance, sensitive layers with high surface area, uniform structures/morphology and high crystallinity are generally preferred. Solid-state sensors using various wide bandgap semiconductors like zinc oxide (ZnO)1,

11-12,

titanium dioxide (TiO2)3,

13-14,

tungsten trioxide (WO3)15 and tin oxide

(SnO2)16-17 with different surface morphologies have been considered for acetone sensing applications in the past. Among various semiconductors, ZnO (with bandgap 3.1-3.37 eV) 3



and TiO2 (with bandgap 3.0-3.4 eV) have attracted huge interest. Both, ZnO and TiO2 have been individually explored for acetone sensing due to their excellent optoelectronic properties, good chemical stability, low cost and non-toxicity. ZnO-TiO2 composites have also been reported for gas sensing. However, the use of ZnO-TiO2 mixed oxides for acetone sensing is not yet explored.13 At operating temperatures above 600 °C, ZnO and TiO2 are known to diffuse in to each other to form wide bandgap ZnTiO3 materials18-20, which has been widely been investigated for applications such as sorbents for desulfurization of hot coal gases21, microwave dielectric ceramics22, paint pigments and LPG gas sensing.20 It is expected that, this diffusion process could lead to ZnO-TiO2 composite sensitive layers with enhanced acetone sensing properties13. ZnO based NA structures are also highly controllable regarding size and also grow uniformly over different substrates.23-24 Thus, ZnTiO3 in an NA structure is expected to have excellent electrical properties and a high surface area to volume ratio, making it a promising sensitive layer for acetone sensing, however, their synthesis, thus far, has been a major challenge.25 The major drawback of the semiconductor based gas sensors is their requirement for an external heat source for the devices to operate between 200-400 °C for enhancing surface reaction. As an alternative, a photoactivated gas sensing approach through AGS has been proposed to attain sufficient surface reaction using a light source of different wavelengths.1, 3, 26 Inspired by the excellent material properties of ZnTiO3 NAs and considering the advantages light-assisted AGS can offer, this work has explored the acetone gas sensing by applying different biases (0.1-9 V) with a specific goal of lowering the required temperature for acetone sensing applications.

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Experimental Chemicals and Substrates The chemicals used in this work were purchased from Sigma-Aldrich and utilised as received. Milli-Q water (18.2 MΩ·cm-1) was used for all synthesis processes. Interdigitated transducer (IDT) substrates fabricated on borofloat glass substrates were purchased from Micrux Technologies. Each substrate had 120 pairs of electrode fingers within a circular sensing area of 3.5 mm in diameter. The electrode configuration was fabricated using 150 nm thick platinum with each electrode being 10 µm wide and spaced 5 µm apart. Sensor substrates were washed with acetone, ethanol and water then dried under nitrogen to remove any residual organic traces. The sensor substrates were subjected to 10 minutes of plasma cleaning under pure oxygen before the material deposition process. TiO2 Adhesion Layer The IDT patterned substrate was coated with a TiO2 adhesion layer (~30 nm) before the deposition of the ZnO intermediate layer. A previously reported low-temperature CVD method was used to deposit the overlaying TiO2 film.3,

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Briefly, 500 μL of titanium

(IV) isopropoxide (TTIP) was added to a 100 mL round bottom flask, and was kept at 65 °C under a nitrogen atmosphere. The sensor substrate was placed over the TTIP solution at a distance of 1.5 cm with the electrodes side facing down toward the TTIP solution. In order to commence the formation of the TiO2 coating, water vapour was introduced into the vessel containing the TTIP solution, once a stable vessel temperature of 65 °C was reached. The deposition was carried out for 10 minutes to achieve a ~30 nm thick TiO2 layer. The reaction was ceased by stopping the water vapour flow and introducing dry N2 into the reaction flask. The flask containing the TTIP was then cooled down to room temperature and, later the 5



substrate was removed. Further, the substrate was annealed at 650 °C under air to attain a crystalline TiO2 film.

ZnO Seed layer The intermediate ZnO layer was prepared using a sol-gel protocol which is described in the literature.23-24, 28 Briefly, this involves 15 mL of ethanol containing a mixture of 0.1 M zinc acetate with an equivalent weight proportion of monoethanolamine (MEA) being prepared. The mixture was kept at a temperature of 55 °C for 24 hours. The prepared solution was then deposited over the TiO2 adhesion layer coated sensor substrate using three cycles of spin coating at 3500 rpm for a period of 30 seconds each. The thickness of the ZnO seed layer was ~100 nm (as determined by a surface profile of a control substrate, see Supporting Information, Figure S-1a). The prepared ZnO coated samples were dried at 95 °C for 10 minutes and then annealed at 450 °C for 1 hour under air. Hydrothermal Growth of ZnO NAs To grow ZnO NAs, a solution containing 25 mM of zinc nitrate hexahydrate and an equimolar quantity of hexamine was prepared (approximately 40 mL of the solution) and transferred into a sealed glass autoclave. The substrate was kept facing down at the top of the surface of the solution (i.e. suspended at the air/water interface).23-24 The autoclave temperature was maintained at 95 °C for 6 hours. The substrate was then removed from the autoclave, washed with milli-Q and dried utilising dry nitrogen gas at room temperature. Formation of ZnTiO3 NAs The ZnO NAs were then coated with a ~50 nm TiO2 layer (as determined by a surface profile of a control substrate, see Supporting Information, Figure S-1b) to form a ZnO-TiO2 coreshell NA structure. This step was followed by annealing the ZnO/TiO2 core-shell structure at

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650 °C under air for an hour to form a mixed oxide, namely ZnTiO3 NAs with the mild traces of ZnO and TiO2.14, 18

Control Sensor Synthesis The control sensors are the replica of the above synthesis steps except that the hydrothermal growth of the ZnO NAs was omitted from the procedure. Therefore the control sensor surface comprised of the TiO2 adhesion layer (~30 nm) followed by the ZnO intermediate layer (~100 nm) and finally the TiO2 layer (~50 nm), hence the final thin film did not have the NA morphology. Characterization Morphological analyses were characterised using scanning electron microscopy (SEM) on an FEI Verios 460L SEM instrument which was operated at an accelerating voltage of 18 kV and a beam current of 50 pA. Energy-dispersive X-ray spectroscopies (EDX) were performed using (Oxford XMax-80T) detector, which is in build within FEI Verios 460L SEM at an accelerating voltage of 25 kV. The transmission electron microscopy (TEM) samples were prepared by lamella procedure with carbon as the protection layer using a FEI Scios Dual Beam FIB-SEM. High-resolution transmission electron microscopy (HR-TEM) analyses were performed using JEOL-2100F with an acceleration voltage of 200 kV. HR-TEM images were obtained by operating in the TEM mode using a Gatan Orius SC1000 CCD camera. The Energy-dispersive X-ray spectroscopy (EDX) elemental line profile is acquired along the NAs in TEM mode were done using an Oxford instruments X-MaxN 80 detector. The crystallinity of the materials was analysed using a Bruker D8 Discover micro diffraction system, which was equipped with Cu Kα radiation source (40 kV, 40 mA) and a general area detector diffraction system (GADDS). X-ray photoelectron spectroscopy (XPS) analyses 7



were performed to study the chemical bonding using a Thermo K-Alpha XPS instrument equipped with an Al Kα monochromated X-ray radiation source. All spectra were background corrected using the Shirley algorithm, and all binding energies (BE) were aligned considering adventitious C 1s has a BE of 285 eV. Diffuse reflectance measurements for samples deposited on Si substrates were acquired with a Cary 7000 UV-Vis-NIR spectrometer equipped with an integrating sphere. The optical band gap was estimated using Tauc analysis assuming direct allowed transition after converting the reflectance spectra using the Kubelka-Munk function. The thicknesses of the deposited TiO2 layers were measured using a surface profiler (Tencor P-16+). The thickness of the depositions were measured using a piece of the silicon wafer covered in half with photoresist.27 The photoresist was removed after the deposition by liftoff procedure using acetone under ultrasonic to produce a step profile required for thickness estimation. This process is depicted using a schematic diagram in Supporting Information, Figure S-2. I−V Characteristics and Photoresponse Studies Current-voltage (I-V) characteristics were analysed using an Ivium stat electrochemical workstation. A homemade pulse generator was used to obtain time-resolved photoresponse signals of each device to support the acetone sensing results. The experiments were conducted using the same chamber used for gas sensing experiments which isolated the device from external or environmental light sources. The UV LED source had a wavelength of 365 nm with a maximum intensity of 2024 µW·cm-2. The light source intensity was calibrated with a PM16-140 power meter from Thor Labs. The dynamic responses of the sensors were measured with the UV illumination turned ON for 5 s and OFF for 10 s at room temperature. The repeatability of the photocurrent is calculated using; repeatability (100%) = 100-CoV (Coefficient of variance).3 The rise (t90-r) and decay (t90-d) times were calculated 8


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based on the t90 parameter, which refers to the time required for the current to increase or decrease to 90% of the steady-state photocurrent value, respectively.

Acetone Sensing A Teflon based gas chamber was designed in-house and built with a quartz window lid, allowing a UV light source to illuminate the sensitive layer during gas sensing experiments. The acetone vapour sensing event encompasses an acetone exposure and recovery process (flushing the chamber with dry air), each set at 5 minutes while maintaining a total flow rate of 200 sccm and constant temperature. A certified G-size gas cylinder of 25 ppm acetone balanced in the air along with the second cylinder of pure air were purchased from BOC and used for gas sensing experiments with flow rate control provided through a computer controlled mass flow controller. The sensors were tested against acetone under operating temperatures ranging from 45 to 350 °C. The operating temperature of the sensor was controlled by applying a potential ranging from 1 to 24 V (achieving 45 to 350 °C, respectively) across a ceramic heater (2 cm x 2 cm) that was monitored by a thermocouple. The sensor performance was tested by applying an external bias of 0.1, 3, 6 and 9 V DC to the sensor under both in dark and UV illuminated situations at different operating temperatures. The external bias that was applied to the sensor was observed to increase the operating temperature of the sensor by a few degrees higher than the thermal excitation of the ceramic heater, however, in all cases, the actual operating temperature of the sensor is reported (as measured by the K-type thermocouple placed at the sensor surface). Before acetone exposure, the stability of the sensor was monitored by maintaining the sensor at the desired operating temperature under a dry air atmosphere while tracking the sourced current over a period of two hours in dark and one hour under 9



illumination. The UV source was placed in direct contact with the outside surface of the quartz window on the chamber which in turn was ~1.5 cm above the sensor mounted inside the chamber. The UV source of 365 nm with the intensity of 2024 µW·cm-2 was used for light-assisted gas sensing (same used for the photoresponse studies). The repeatability of the acetone response was calculated using Repeatability (100%) = 100 - CoV. The term t90 is usually defined as the time required to reach 90% of the response signal of the equilibrium value of sorption (t90-ads)/desorption (t90-des) phase of a sensing event. The LoD of the developed sensor for acetone vapour was calculated by determining three standard deviations (±3σ) of their noise profiles and applying the method described elsewhere.29 Any memory effect posed by the sensors was tested by exposure to abrupt acetone concentration changes (A=1.2 ppm, B=5 ppm and C=12.5 ppm) in a designed sequence of 9 pulses defined as a memory test.29 Sensor responses are normalised based on the highest response for clarity wherever necessary. Results and Discussion Figure 1 shows the schematic diagram of ZnTiO3 NA synthesis procedure. Briefly, the electrodes are coated with a TiO2 adhesion layer and ZnO intermediate layer (Figure 1a). The ZnO NAs are then grown (Figure 1b) and coated with a TiO2 layer to form ZnO-TiO2 core-shell NAs (Figure 1c). This step was followed by annealing at controlled temperatures to form a mixed oxide, namely ZnTiO3 NAs (Figure 1d).

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Figure 1: Schematic diagram of the synthesis procedure of ZnTiO3 NAs: (a) TiO2 adhesion/ZnO seed layer, (b) hydrothermal growth of ZnO NAs, (c) TiO2 deposition via CVD and (d) annealing process at 650 °C to form ZnTiO3 NAs. The amount of TiO2 added to the ZnO NAs and the annealing temperature plays a crucial role in determining the crystal phase of the material.14, 19 Annealing ZnO and TiO2 above 600 °C could potentially form other zinc titanate phases like ZnTiO3, Zn2TiO4 and Zn2Ti3O8 with traces of ZnO and TiO2 (depending on the composition of ZnO and TiO2 added).19 The system in this study has a much higher ZnO content in comparison to TiO2. Due to the concentration difference in Ti and Zn in both layers, the increase in temperature to 650 °C results in the formation of a diffusion gradient of Zn from ZnO core into TiO2 shell and Ti from the TiO2 shells into ZnO core. The titania amorphous layers diffuse into the ZnO core and form ZnTiO3. There is the possibility of forming other components, however as the time and calcination temperature are the main factors in diffusion of Zn and Ti and also crystal arrangement of mixed oxide structures, these parameters can be used to control the mixed oxide that is formed. The formation of other titanate phases like (Zn2TiO4 and Zn2Ti3O8) may require more activation energy (more time and higher temperatures) for crystal formation. This could be the reason for restricting the growth of the secondary phase (Zn2TiO4 and Zn2Ti3O8) resulting in pure ZnTiO3 phase on the surface which is supported by the XRD in Figure 4a. Hence ZnTiO3 is the prominent structure and is in agreement with the crystal structure expected from the phase diagram ( see Supporting Information, Figure S-3).19 On the other hand, when insufficient time or temperature is provided to reach full equilibrium, the Ti can diffuse into ZnO core, and Ti4+can substitute with Zn2+ in the structure thus making it electron rich ZnO core.

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Materials Characterisation The SEM images of the ZnO NAs, ZnO/TiO2 core-shell NAs and ZnTiO3 NAs surface are presented in Figure 2.

Figure 2: SEM images ZnO NAs (a) top view; (b) side view; ZnO/TiO2 core-shell NAs (c) top view; (d) side view; ZnTiO3 NAs (e) top view; (f) side view.

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The ZnO NAs were found to be uniform in size with the high packing density. The average length of the NAs were found to be 1.5 µm with an average diameter of ~30 nm (see Figure 2a (top view) and b (side image)). These ZnO NAs were then coated with ~50 nm TiO2 to form a ZnO/TiO2 core-shell NAs. It was noted that the thickness of the ZnO NAs increased significantly after the (~50 nm) TiO2 coating (see Figure 2c (top image) and d (side image)). Further, the ZnO/TiO2 core-shell NAs were annealed at 650 °C for 1 hour under air to attain ZnTiO3 NAs which are shown in Figure 2e (top image) and f (side image). Figure S-4a (Supporting Information) shows the control surface (without NAs) being relatively flat with patches of islands (98%. This was the case for all biases (2-9 V) with the non-linear increase in photocurrent being attributed to the I-V characteristics of the semiconductor36 (see Supporting Information, Figure S-6 for repeatability profile for biases ranging from 2-8 V). ZnTiO3 NAs had a rapid rise in photocurrent in 1 second37-38 (See Figure 5e). It could be noted the current increases rapidly once the UV is turned ON and it continues to increase gradually until the end of UV illumination. This indicates two different mechanisms occurs (rapid raise and gradual raise) once the UV light is turned ON. The rapid raise is attributed to instant generation of electron-hole pair leading to an increase in carrier density on the ZnTiO3 NAs structure. The gradual raise is due to surface effects.36 The decay time of the ZnTiO3 NAs is 5 seconds (See Figure 5f). Once the UV light is turned OFF, the current decay also has two different mechanisms like (rapid decay and gradual decay). The rapid decay is related to the instant recombination of electron-hole pair while the gradual decay is mainly due to surface effects like gas adsorption and desorption.36,

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The longer

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decay time is also possibly due to a lack of trap free surface which could potentially delay the electron-hole recombination.40

ZnTiO3 NA Based Acetone Sensor The higher surface area relative to the control substrate as well as the high photoactivity and repeatability in photocurrent generation makes the ZnTiO3 NAs a suitable sensitive layer for UV-assisted AGS applications, in this case, acetone sensing was explored. When ZnTiO3 NA based sensors are exposed to different concentrations of acetone at 350 °C with a 0.1 V bias in dark conditions, the response magnitude increased with the increase in acetone concentration as shown in Figure 6a.To describe the gas sensing mechanism briefly, when the ZnTiO3 NAs are exposed to air, leading to oxygen molecules are being adsorbed on the surface. The oxygen molecules can capture the electrons present in the ZnTiO3 NAs conduction band and form O2-, O-, and O2- ions .38 The mobility of the carriers are restricted leading to an increase in resistance, hence a high resistance depletion layer is formed near the surface. As a consequence, an upward band bending is formed near the surface.41 When acetone gas molecules react with the chemisorbed oxygen ions, electrons are injected back into the conduction band by releasing water vapour and carbon dioxide, which is observed as an increase in the current output. When the concentration of acetone increases, it interacts with more number of oxygen ions on the ZnTiO3 NAs. Hence more number of electrons is released back in to the conduction band with the increase in acetone concentration, thus, an overall increase in the response magnitude38 (see Figure 6a). The response magnitude can be further be increased by illuminating the ZnTiO3 NAs surface with photon energy higher than the band gap energy of the semiconductor. This increases the number of active sites on the surface and hence enhances the chemical activity of the surface.26 When exposed to 12.5 ppm acetone at 350 °C with 0.1 V bias under UV illumination the ZnTiO3 NAs response 19



magnitude increased 7.5 times higher relative to dark conditions as is shown in Figure 6b. At 350 °C with 0.1 V bias under UV illumination, the ZnTiO3 NAs based sensor had a sensitivity of 0.3 µA/ppm toward acetone. The sensitivity was observed to reduce to 0.04 µA/ppm under dark conditions. The illumination also increased the dynamic range of the sensor compared to the dark conditions, as shown in Figure 6b.

Figure 6: ZnTiO3 NAs sensor response towards different acetone concentrations at an operating temperature of 350 °C (a) dynamic sensor response profile in the dark and under UV with an intensity of 2024 µW·cm-2 with an applied bias of 0.1 V; (b) calibration curve representing the sensor response magnitudes versus acetone concentrations in (a); (c) normalised sensor response profiles towards an acetone concentration of 12.5 ppm in dark and under light with different applied biases (0.1 – 9 V); (d) sensor response towards acetone 20


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concentrations of 1.2, 5 and 12.5 ppm in dark and under light with different applied biases (0.1 – 9 V). Upon UV illumination, electron-hole pairs are photogenerated instantly [hν = e- + h+] (Reaction A). Holes migrate towards the ZnTiO3 NAs surface and interact with the oxygen ion resulting in desorption of oxygen molecules from the surface38 [h+ + O2- (ad) = O2 (g)] (Reaction B). Meanwhile, photo-induced electrons react with the oxygen molecules and ionosorb it38, 42 [O2 (g) + e- = O2- (ad)] (Reaction C). Thus,UV illumination increases a larger number oxygen ions formed on the surface relative to dark conditions.26 When acetone interacts on the ZnTiO3 NAs surface under UV illumination, it releases the electrons from the oxygen ions back into the conduction band through the release of carbon dioxide and water as the reaction products

43.

The decomposition of acetone into water and CO2 furthur enhance

the sensor response in comparison to the dark conditions. An increase in the electron density on the surface leads to an increase in response magnitude38 [CH3COCH3 + 4O- = 3CO2 + 3H2O + 4e-] (Reaction D). Thus the photoactivation can assist the sensors with the much increased response facilitating the decomposition reaction of the acetone. The ZnTiO3 NA surface is recovered to the initial state when exposed to air environment following the sensing event. Furthermore, the sensitivity towards acetone was observed to increase rapidly with an increase in the applied bias ranging from 0.1 to 9 V. The normalized response data when operated at 350 °C in the presence of 12.5 ppm acetone is presented in Figure 6c, (response without light illumination is presented separately in Supporting Information, Figure S-7). It is evident that the increase in bias significantly increased the response magnitude. The increase in response magnitude due to the increased applied bias may be the result of operating the amperometric gas sensor in the diffusion controlled mode rather than the kinetically controlled mode of the electrode.5 That is, the rate of reaction at the 21



surface is the limiting step and the current output is governed by the reaction rate. The three factors that can increase the rate of reaction (hence convert the loDS ing step to the gas diffusion rate at the surface) are the operating temperature, illumination and/or application of a positive potential to the metal oxide semiconductor surface. All of these factors promote a significant surface accumulation of electrical charge brought by oxygen ions.44 Under the influence of different biases, the materials exhibit different adsorption balances on the surface and hence different sensing response to different gases.44 The Schottky diode junction is formed when the n-type ZnTiO3 NAs are grown on the platinum sensor electrodes. Once they get in contact, an equilibrium state is established and their Fermi levels become equal. Electrons from the semiconductor lower their energy level by flowing into the metal creating an energy barrier known as the Schottky barrier. The generated Schottky barrier prevents the flow of electrons from ZnTiO3 NAs into the metal without assistance from an external energy source to elevate their energy above that of the Schottky barrier height. When a positive bias of 0.1 V is supplied to the metal, the semiconductor band bends to flow electrons in to the metal which enhance the response magnitude of the sensor.45 Further, when the bias is increased from 0.1 to 9 V, the resistance of the materials is decreased nonlinearly by overcoming the defects of the material.46 Hence an easy migration of electron to the surface to participate on the surface reaction (to form oxygen ions) and from the surface to electrodes is achieved leading to the increase in sensor response in dark and under illumination.47-48 The change in response magnitude with an increase in bias was tested against other acetone concentrations (i.e. 1.2 and 5 ppm) and is presented in Figure 6d. The results demonstrate that the increase in bias plays a significant role in the increase in response magnitude and further, the illumination enhances the response magnitude and results in a better dynamic range. 22


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Figure 7a presents the sensor data of acetone of different concentrations with 9 V bias in dark and under illumination at 350 °C. ZnTiO3 NAs exhibited a sensitivity of 1.05 mA/ppm under illumination and 0.2 mA/ppm in dark conditions. The dynamic range towards different concentrations of acetone under illumination was found to be higher than in dark conditions (See Figure 7b). The response magnitude of the control sensors is compared with the ZnTiO3 NAs under an applied bias of 9 V at 350 °C in both dark and under light conditions as is shown in Figure 7b. The ZnTiO3 NAs had 25 times and 21.5 times higher response magnitude than the control sensor when operated in the dark and under light conditions, respectively. The sensitivity of the ZnTiO3 control sensors was found to be 0.05 mA/ppm under illumination and 0.01 mA/ppm in the dark conditions. The significant increase in response magnitude could be due to high surface area of the NA morphology than the flat film35, the uniform structure of the NAs which can facilitate a fast charge transfer of the charge carriers leads to an increase in current flow, when compared to a rough flat film.40

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Figure 7: ZnTiO3 NA sensor response towards acetone gas at an operating temperature of 350 °C with an applied bias of 9 V (a) dynamic sensor response towards acetone in dark and under with UV incident intensity of 2024 µW·cm-2; (b) response magnitude of ZnTiO3 NAs in comparison to control sensors towards different acetone concentrations in dark and light; repeatability of ZnTiO3 NA 12.5 ppm acetone concentrations in (c) dark and (d) light; (e) dynamic response of the sensor by exposing abrupt concentrations (memory test) (A = 1.2 ppm, B = 5 ppm and C = 12.5 ppm);(f) bar chart representation of (e).

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One of the most important criteria for a gas sensor is to have high repeatability.3 The sensor repeatability was studied by exposing 12.5 ppm acetone vapour over 7 sequential pulses at different operating conditions. At 350 °C with 9 V bias the sensor had a repeatability of >98% in dark and under illumination as presented in Figure 7c and d. As can be seen in Figure 7e and a bar chart representation in Figure 7f, when the sensor is operated at 350 °C with 9 V bias under the illumination of light the sensors exhibited similar response magnitudes for each repeated concentration of acetone vapor tested even when the lowest concentration (A) followed the highest concentration tested (C) indicating that the sensor had little memory effect associated with it. The CoV of the response magnitude toward each concentration of acetone during the test are calculated to be lower than 1 thus indicating that the developed ZnTiO3 NAs had almost no memory effect. The sensing performance of the pristine ZnO NAs is compared with the ZnO-TiO2 core-shell NAs (before annealing) and ZnTiO3 NAs at 350 °C with 9 V bias in dark and under illumination is presented in Figure S8. It can be noted the ZnO-TiO2 core-shell NAs has a much higher response in comparison to pure ZnO NAs in dark and under illumination. The ZnTiO3 NAs had almost 100 times high response than the ZnO-TiO2 core-shell NAs (before annealing) under illumination. Before annealing, the TiO2 on the surface is amorphous on the ZnO-TiO2 core-shell NAs and acts as a poor conducting material and it does not show response to the light or temperature activation unlike ZnTiO3 NAs. While annealing the ZnO NAs coated TiO2, the TiO2 on the surface not only reacts with ZnO along grain boundaries to form ZnTiO3 crystal phase, it also diffuses into the ZnO bulk, which it substitutes (dopes) for Zn2+ ions with Ti4+ ions (ionic radius of Ti4+ ions and Zn2+ ion are 0.068 and 0.074 nm, respectively) thereby making the ZnO in the core electron rich. It was noted that the resistance of ZnO NAs decreased remarkably by adding TiO2, which is attributed to the increase in electrons concentration achieved by doping the Zn2+ ions with Ti4+. The introduction TiO2 to the ZnO primarily could 25



increase the affinity to adsorb more oxygen on the surface because of Ti4+ ions being introduced in to the ZnO lattice.14, 49 Hence a higher response magnitude could be obtained with ZnTiO3 NAs when compared to pure ZnO and the ZnO-TiO2 core-shell NAs (before annealing). The effect of operating temperature (45 to 350 °C) on the ZnTiO3 NAs based sensor response performance when operated under a 9 V applied bias and exposed to 12.5 ppm acetone is shown in Figure 8a. The temperature profile demonstrates that the sensitivity of the developed sensor toward acetone gas is reduced when the operating temperature is reduced from 350 to 45 °C. The response towards different concentrations of acetone are tested at 45 °C with a 9 V bias under illumination, the data of which is shown in Figure 8b. The sensor showed an increase in response magnitude with the increase in acetone concentration though the baseline had some drift associated with it. At the same temperature, the sensor showed a poor response (i.e, similar response to different concentration of acetone) without illumination (is presented in Supporting Information, Figure S-9). This clearly shows that the implementation of UV based illumination and bias can effectively enhance the sensing performance when detecting low concentrations of acetone at temperatures as low as 45 °C. When a bias of 9 V was applied to the sensor, the operating temperature increased from room temperature to 45 °C due to the resistive heat. Any bias over 9 V was not considered because it tended to increase the operating temperature of the sensors above 45 °C. The response magnitude of the ZnTiO3 NAs comparison with the control sensor at 45 °C with 9 V bias in dark and under illumination is presented Figure 8c. The sensitivity of the ZnTiO3 NAs sensors was found to be 0.15 mA/ppm under illumination and 0.009 mA/ppm in the dark conditions. The response magnitude of ZnTiO3 NAs was observed to be 30 times higher than the control sensor in the dark and 20 times higher under light when tested toward 12.5 ppm acetone. This indicates that the NAs structure has improved the response magnitude at both dark and light 26


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conditions. At 45 °C the ZnTiO3 NAs sensor exhibited an excellent repeatability of >98% with 9 V bias under illumination is presented in Supporting Information, Figure S-10. It can be observed that the developed sensor exhibited almost identical response for all 7 pulses, with a CoV of 2.01 (i.e. repeatability= 100-CoV%= 98%). This indicates that the developed sensor has the capability to measure low concentrations of acetone (12.5 ppm) in a repeatable manner, which is imperative for a sensor. Memory tests were conducted at 45 °C with 9 V bias with and without illumination. Under the illumination, the memory test response of ZnTiO3 NAs sensors is presented in Figure 8d (bar graph representation). The CoV of the response magnitude toward each concentration of acetone during the test are calculated to be lower than 1. These values indicate the developed ZnTiO3 NAs almost had almost no memory effect even at temperature as low as 45 °C. In the dark conditions, previous sensing event immensely influenced the successive sensing events could be due to the poor recovery of the sensor as shown in Figure 8e (bar graph representation). Figure 8f shows the sensor profile comparison toward 12.5 ppm acetone at 350 °C and 45 °C with 9 V bias in the dark and under the light. It can be observed that the sensitivity obtained at 350 °C (dark conditions) can be achieved when undergoing light-assisted acetone detection at 45 °C. Thus, the light-assisted acetone gas sensing approach can considerably reduce the operating temperature of the sensor device (by over 300 °C), which is very promising for attaining high sensitivity at lower temperatures. The reduction of operating temperature for low concentration gas sensing is also an important for many applications. For example, the importance of low-temperature gas sensing in the petroleum industry is often discussed due to possible explosions from thermal heat sources when devices operate at elevated temperatures (>400 °C). Furthermore, acetone is a flammable gas and can cause an explosion at concentrations > 2.5% v/v.3 In these above cases, it is highly preferred not to use hightemperature heaters which could become an ignition source. Experiments were conducted in 27



dark conditions toward 12.5 ppm acetone at 350 and 45 °C to compare and understand the behaviour of ZnTiO3 NAs (see Figure 8g). It is evident that a 20% higher response magnitude can be obtained in the dark at 45 °C with 9 V bias relative to 350 °C with 3 V bias which has effectively reduced the input energy (heater voltage from 24 to 1 V) to the system. Therefore, the approach of undergoing light-assisted sensing and/or application of bias (amperometric sensing) can reduce the operating temperature yet allow for low concentration gas sensing to be conducted. This phenomenon is least explored in the field of gas sensing and has been possible due to the physio-chemical properties of the ZnTiO3 NAs that has been developed in this work.

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Figure 8: (a) ZnTiO3 NA sensor response towards an acetone concentration of 12.5 ppm at operating temperatures ranging from (45-350 °C) with a 9 V bias in dark and under illumination; (b) sensor response at 45 °C with a 9 V bias under light; (c) response magnitude comparison between the ZnTiO3 NAs and the control sensor towards different concentrations of acetone at 45 °C with a 9 V bias in dark and under light; response of the 29



sensor towards abrupt concentrations of acetone (memory test) (A = 1.2 ppm, B = 5 ppm and C = 12.5 ppm) at 45 °C with 9 V bias under (d) light and (e) dark conditions; (f) comparison of ZnTiO3 NA sensor response towards an acetone concentration of 12.5 ppm at operating temperatures of 350 °C and 45 °C with a 9 V bias in dark and under light; (g) comparison of ZnTiO3 NA sensor response in the dark at 350 °C with a 3 V bias and 45 °C with a 9 V bias. Long term stability of the ZnTiO3 NAs sensor is tested by conducting dynamic acetone sensing in which a repeated and prolonged exposure to acetone sensing events are monitored for a significant amount of time. The dynamic acetone profile on Day 1 and Day 30 at 350 °C with 9 V bias under light illumination and the normalized sensor response is presented in Figure S-11 (Supporting information). It can be noted the sensors are almost completely repeatable for 1.2ppm and only slightly reduced for other concentrations even after 30 days, proving the robustness and long term stability of the AGS. Low level detection of target gases below 1 ppm is a prerequisite for effective gas sensing, especially when developing ultra-sensitive healthcare monitoring systems.1 The LoD under different bias and in dark/light conditions while operating at 350 °C and 45°C with 9 V bias is presented in Table 1. It is worth observing from Table 1 that the LoD significantly increases with increasing bias both in the presence and absence of illumination. At 350 °C, it is understandable to achieve a higher LoD value as the noise is considerably less with the stable baseline. When the bias is increased from 0.1 to 9 V, the baseline tends to be less noisy (with the decrease in noise magnitude for 0.1 V = 2.2 µA, 3 V = 1.3 µA, 6 V = 0.9 mA, 9 V = 0.1 mA) along with the increase in response magnitude of sensor leading to increase in LoD. Further, under illumination, a stable lesser noise baseline was achieved with the much higher sensor response, when compared to dark condition leads to achieving a much higher LoD under illumination. It is to be acknowledged at 350 °C, the increase in LoD is attributed to the 30


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combined effect of having a baseline of lower noise magnitude and a higher response magnitude. The LoD of ZnTiO3 NAs with 9 V bias at 45 °C under light is 90 ppb which is 8 times lower when compared to 350 °C indicating that the temperature has a lower effect on the noise magnitude and sensitivity of the sensor effecting the LoD at lower temperature. The LoD performance of ZnTiO3 NAs is compared with the literature (is presented in Table 2) and it was found that the ZnTiO3 NAs can be sensitive to low concentration of acetone at a relatively low operating temperature. The sorption (t90-ads) and desorption (t90-des) times for 12.5 ppm acetone with different biases in the dark and under light at an operating temperature of 350 °C and 45 °C is presented in Table 1. Overall, dark and light-assisted sensing showed that the sorption (t90-ads) and desorption (t90-des) times decreased with the increase in bias. This is attributed to the sweep in charge carriers leading to a faster raise in decay in sensor response.47-48 Under illumination, with the increase in bias from 0.1-9 V, the adsorption and desorption time tend to decrease in comparison to dark condition. This could be due to the generation of electron-hole pairs leading to faster adsorption and desorption time. The sorption (t90-ads) and desorption (t90-des) times at 45 °C were found to be 117 and 99 seconds in the dark and 141 and 131 seconds under illumination, respectively. Overall ZnTiO3 NAs sensing performance under the illumination shows an absolute promise in detecting low concentrations of acetone down to sub-ppm levels at near room temperature.

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Table 1: LoD, Adsorption and desorption times of ZnTiO3 NAs in the dark and under light at 350 °C with different biases and at 45 °C with a 9 V bias.

T (°C)

Bias (V)

LoD (Dark) (ppb)

LoD (Light) (ppb)

(t90-ads) (s) Dark

(t90-des) (s) Dark (s)

(t90-ads) (s) Light (s)

(t90-des) (s) Light (s)

350

0.1

150

40

96

96

165

78

350

3

120

30

84

75

165

75

350

6

100

20

80

61

132

73

350

9

80

10

75

50

81

69

45

9

900

90

117

99

141

131

Table 2: Comparison of acetone sensing in the literature with ZnO, TiO2 and ZnO/TiO2 composites based sensitive layers to ZnTiO3 NAs sensor. Material & Synthesis route

Sensor Temperature

Acetone LoD Concentration

References

ZnO Fibers

Room Temperature

10-60 ppm

10 ppm

38

ZnO Hollow Nanofibers (Single Capillary Electrospinning)

220 °C

1-200 ppm

1 ppm

50

ZnO Nanostructure (Hydrothermal Method)

300 °C

50-300 ppm

50 ppm

51

TiO2 Nanoparticles

270 °C

500 ppb-

500 ppb

52

1 ppm

53

(Electro-spinning)

( Electro-spinning) TiO2 Nanorods

500 ppm 500 °C

50-500 ppm 32


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(Electro-spinning) 400 °C

20-200 ppm

20 ppm

54

350 °C

25-100 ppm

1.5 ppm

13

ZnTiO3 NAs

350 °C

1.2-12.5 ppm

10 ppb

This Work

ZnTiO3 NAs

45 °C

1.2-12.5 ppm

90 ppb

This Work

TiO2 NPs (Matrix Assisted Deposition)

Pulsed

ZnO-TiO2 Nanocomposites

Laser

(High Temperature CVD)

The selectivity performance of the ZnTiO3 NAs sensor was tested by exposing the sensor to humidity and CO2 with/without the presence of 12.5 ppm acetone gas under different operating conditions. The choice of cross-interfering gases was limited due to experimental setup and the relevance of humidity and CO2 for breath diagnostic applications. Furthermore, given that both ZnO and TiO2 are widely known to be sensitive towards CO2 and humidity, precedence was given to them over other cross-interfering gas species Mixing TiO2 with other semiconductor is said to increase the defect density on the surface and promotes the dissociate H2O molecules.55 The formation of ZnO and TiO2 composites has been shown to be an active sensing material as they induce coupling transport in the interface of the two different materials.56 From a materials perspective the high surface to volume ratio of one dimensional ZnTiO3 NAs and the easier electron transport makes them highly sensitive to humidity.56 Few possible mechanisms to elucidate the humidity sensing are available in the literature.55, 57-58 The Kulwicki mechanism

59

states that the sensor response depends on the

reaction of water molecules on the surface. The basic concept is being the reaction of humidity with the adsorbed oxygen on the surface thus releasing the electron back in to the conduction band and resulting in an increase of the response magnitude.60 In another humidity sensing mechanism, the water molecules are said to rapidly occupy the available 33



surface active sites. Once the surface is covered with enough water molecules it is not further affected by additional exposure to humidity as they are physically adsorbed. The water molecules can be desorbed by exposing dry air or by thermal desorption. Charge transport happens when H3O+ releases a proton to a nearby water molecule which accepts it before releasing the proton to another molecule, and so forth. This is known as the Grotthuss chain reaction.59 The behaviour of humidity towards materials mainly depends on the operating temperature of the sensor.61 In our case, the response magnitudes increased when the humidity concentration increased. At lower temperature, the water molecules get adsorbed to the material surface either in the form of a hydrogen bond or get physisorbed. This normally results in poor desorption and further leads to a poor sensor response.60 At 45 °C with 9 V applied to bias the ZnTiO3 NAs are highly sensitive to H2O molecules in the dark and under illumination conditions. The acetone mixture with humidity (A + H) increased the response magnitude in dark and interestingly it was an opposite phenomenon under illumination (see Figure 9a). The sensing profile of a typical selectivity test with humidity as the cross-interfering gas, with and without UV-illumination is shown in Supporting Information, Figure S-12. This shows the selectivity towards acetone is not favored by the water molecules at lower temperatures 45 °C, possibly because of mechanism like self-ionization or through an impede reaction.62-63 At 350 °C with 3 and 9 V bias, the obtained response magnitudes for acetone and the acetone humidity mixture (A + H) in dark and under illumination are presented in Figure 9a. In dark conditions, the developed ZnTiO3 NAs are not selective towards acetone under the influence of humidity. Under illumination, with 3 V bias was closer to get selective to acetone and was almost selective to acetone with 9V bias. Under illumination the electron-hole pair is generated instantly, it plays a significant role in participating in the surface reaction. Under UV illumination the H2O produce ionic products of water molecules like H+, H3O+, and OH− ions.58, 64-65 The produced ions (positive 34


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and negative) of water molecules engage in capturing the photo-generated electrons and holes on the surface leading to decrease the response magnitude towards humidity the ZnTiO3 NAs in comparison to dark conditions. This could restrict the influence of humidity towards acetone leading to achieve selectivity towards acetone. This mechanism needs further investigation however the data suggest that exploring different conditions could potentially improve the selectivity of the sensor toward humidity. The adsorption of CO2 on the ZnTiO3 NAs sensitive layer depends on the operating temperature as it dictates the electrical properties of the semiconductor. The sensing mechanism of CO2 by an n-type material (ZnTiO3 NAs) is exactly similar as the acetone and humidity sensing mechanisms.66 At 45 °C, CO2 has much higher influence when exposed as separately (CO2) and as a mixture with acetone (A + C) makes the ZnTiO3 NAs not selective towards acetone under its influence (See Figure 9b). The sensing profile of a typical selectivity test with CO2 as the cross-interfering gas, with and without UV-illumination is shown in Supporting Information, Figure S-13. At 350 °C with 3 and 9 V bias, it can be noted that the response magnitude of CO2 is decreased under the light in comparison to dark. Under illumination, the photoinduced electrons react with CO2 to form CO2-.67 Also under UV illumination, CO2 can break down in to carbon monoxide and an oxygen atom.66 The produced oxygen atom combines with the molecular oxygen and form ozone O3. Both carbon monoxide and ozone are reducing gases. Hence under illumination CO2 is transformed in to other gas forms in certain conditions. At 350 °C with 9 V bias in dark conditions (when the CO2 is thought to have not been influenced by UV) the ZnTiO3 materials seem to be more selective to acetone in the presence of CO2. Therefore, as in the case of humidity, the exploration of various operating condition could deem the sensor more selective toward acetone, not just in the presence of CO2 and humidity but also a combination of different gas species. 35



Figure 9: Summary of the selectivity performance of ZnTiO3 NAs towards acetone with (a) H2O and (b) CO2 as an interfering gas under different conditions.

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Conclusions To summarise, we have developed ZnTiO3 NAs and demonstrated its high photoactivity and repeatability (> 99%) in detecting low UV intensities (µW range). Considering the advantage of the high photoactivity, uniformity and high surface area of the ZnTiO3 NAs relative to the control substrate, the developed material was used as a sensitive layer to investigate lightassisted amperometric gas sensing for effectively detecting low concentrations of acetone such as 1.2 ppm. Light-assisted amperometric sensing was shown to improve the sensing performance of the ZnTiO3 NAs towards acetone, enabling higher sensitivity, lower LoD and lower sorption/desorption times. Furthermore, light-assisted amperometric sensing was shown to allow for operation at low temperatures (down to 45 °C). The developed ZnTiO3 NAs based sensor was calculated to have a LoD of 90 ppb under light at 45 °C with 9 V bias, which makes it a promising material for the detection of trace quantities of acetone at low temperatures. Furthermore, we have shown that light assisted AGS could effectively enhance sensitivity while reducing the LoD and operating temperature. Supporting Information Thickness profile of ZnO seed layer and TiO2 coating on the ZnO NAs, schematic diagram to illustrate the step edge formation, phase diagram of bulk Zi-Ti-O, SEM and XRD of ZnTiO3 control sensors, photoresponse repeatability of ZnTiO3 NAs with different biases (2-8 V), normalised sensor response profiles towards an acetone concentration of 12.5 ppm under dark conditions with different applied biases at 350 °C, gas sensing comparison between ZnO NAs and ZnTiO3 NAs, ZnTiO3 NAs sensor responses at 45 °C with 9 V bias in dark, repeatability under illumination, typical selectivity test pattern involving the exposure of cross-interfering gas species humidity and CO2 with/with UV illumination at different conditions are provided as supplementary information.

37



Conflicts of Interest The authors declare that they have no conflict of interest. Acknowledgements The support from the Australian Research Council through Discovery Project DP150101939 is gratefully acknowledged. All authors acknowledge the RMIT Microscopy and Microanalysis Facility (RMMF) for their scientific and technical assistance and for providing access to their comprehensive infrastructure and research facilities. RKCB acknowledges the financial assistance of the RMIT University HDR publication grant. AEK acknowledges the RMIT Vice-Chancellor fellowship scheme. We also acknowledge Dr Blake Plowman for proofreading and significantly enhancing the quality of our manuscript.

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Zinc titanate nanoarrays with superior optoelectrochemical properties

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