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Optimization of a Hydrothermal Growth Process for Low Resistance 1D Fluorine-Doped Zinc Oxide Nanostructures
Design of Experiment (DOE) has been used for the optimization of a hydrothermal growth process of one-dimensional fluorine-doped zinc oxide (1D-FZO). Box-Behnken design was used in the DOE which includes three design points on each of the synthesis condition parameters. The condition parameters were the gold sputtering time (10 s, 15 s, and 20 s), hydrothermal reaction time (3 hours, 6.5 hours, and 10 hours), and hydrothermal temperature (50°C, 75°C, and 100°C). This statistical method of DOE was used to study the effects of these hydrothermal conditions on the quality of 1D-FZO produced. Au nanoparticles were used as the catalyst to enable the growth of the 1D-FZO. The XRD and EDX analysis confirmed the formation of polycrystalline FZO with the presence of fluorine, zinc, and oxygen elements. SEM observations indicated that the sputtering time of the Au nanoparticles has significant effect on the morphology and growth process of 1D-FZO. The lowest resistance value of 22.57 Ω was achieved for 1D-FZO grown with the longest Au sputtering time at growth temperature below 100°C.
Nanostructured zinc oxide (ZnO) has been attracting a lot of attention due to its capability of working in a variety of applications such as dye-sensitised solar cells , biosensors , gas sensors , and organic light-emitting diodes . This n-type semiconductor has demonstrated experimentally that it has fast electron transfer kinetic, high isoelectric point, and wide band gap of 3.37 eV at ambient condition [5, 6]. Furthermore, its unique morphology especially one-dimensional structures (nanorod) made it as one of the promising nanomaterials. In fact, 1D-ZnO can provide a stable and direct electron transfer . In addition, the presence of the dopant element in the ZnO system can affect its electrical properties. Previously, influence of nitrogen, copper, aluminium, and fluorine dopant on ZnO was extensively studied [8–11]. Among them, the fluorine element is the most effective dopant for the ZnO matrix as it could avoid the lattice distortion and produce an efficient charge carrier without an electron trap . Since 1948, the demand of low-cost and environment-friendly fabrication processes for nanostructures leads to the development of a hydrothermal process . This method plays an important role in producing crystalline 1D-ZnO on various substrates including glass, silicon, and polymer [14, 15]. Furthermore, the nanostructures’ properties can be tuned by varying different parameters of a hydrothermal process such as the reaction time, temperature, concentration of a precursor, and the pH value of the solutions. Boubenia et al.  focused on the best concentration of ammonia (as an additive) for producing a high aspect ratio of ZnO nanowires. Meanwhile, the experiment conducted by Jiao et al.  found that the growth duration of ZnO nanorods influenced their structural size and electrical properties. Wahid et al. used gold nanoparticles (Au-np) as a nucleation site for the hydrothermal growth of 1D-ZnO and at the same time improved the adhesive surface layer of the substrate . From their results, Au nanoparticles could promote the growth of 1D-ZnO in shorter time . However, they disregarded to optimize the amount of Au-np on the substrate. The high concentration of Au-np may lead to excess of impurities and can become the dominant layer which is able to influence the electrical performance of the device. It was found that the previous work only focused on one parameter at one time. Although, this optimization approach improved performance of the nanostructures, it limits to a fixed (certain) condition.
The Response of Surface Methodology (RSM) has been used to indicate the unknown relationship between the independent variables (input factor) and the process response. This method actually a combination of statistical and mathematical techniques to analyze, model, and optimize processes . Initially, the observations are made to fit either a linear function also known as a first-order model or a second-order model which is suitable to be used based on the results produced . Choosing the best experimental design significantly influences the efficiency of the response analysis. Therefore, Box-Behnken and Central Composite Design (CCD) are the most favourable class of design to fit response surfaces. To optimize the quality of ZnO film deposition, Sultan et al.  applied the Box-Behnken design in their study to determine the impact of remote plasma parameters on the film deposited.
Similarly, Design of Experiment (surface response methodology) is used in this study to identify the impact of hydrothermal conditions (reaction time and temperature) and Au sputtering time to produce a good quality 1D-FZO. However, CCD design is not compatible since there is a concern regarding the process of constraints to test each factor at extreme levels. As a result, the Box-Behnken technique was selected to design the experiment, since its factorial combinations do not include any points at the vertices of the cubic region and the resulting design is still rotatable as shown in Figure 1. The electrical, structural, elemental, and morphological properties of 1D-FZO produced were examined using a semiconductor parameter analyzer, X-ray diffractometer (XRD), energy dispersive X-ray spectroscope (EDX), and a variable-pressure scanning electron microscope (VPSEM), respectively.
2. Experimental Method
1D-FZO nanostructures were fabricated on a glass substrate via hydrothermal process. For optimization, the Box-Behnken design was used which includes three design points to correspond to each synthesis condition parameter. There were three design factors or condition parameters considered in this work which were the gold sputtering time (10 s, 15 s, and 20 s), hydrothermal reaction time (3 hours, 6.5 hours, and 10 hours), and hydrothermal temperature (50°C, 75°C, and 100°C). Unblocked Box-Behnken design for three factors was chosen with 15 experiments to run. The precursors used for FZO were zinc nitrate (alfa aesar, 99%), hexamethylenetetramine (alfa aesar, 99+%), and ammonium fluoride (R&M) with equimolar of 0.01 M in DI water. The precursor solutions were stirred for 1 hour at ambient temperature. At the same time, glass substrates were immersed in 2-propanol and ethanol solution inside an ultrasonic cleaner for 15 minutes. Next, the substrates were coated with gold nanoparticles by sputter coating at constant current (60 mA) with three different sputtering times to enhance its adhesiveness on the surface layer. Then, the substrates were rinsed with 2-propanol solution before drying in an oven under 100°C for 15 minutes to remove any moist on its surface. The cleaned substrates were placed inside the autoclave by facing downwards as illustrated in Figure 2(a). Finally, the precursor solution was poured gently into the autoclave for the subsequent growth process in the oven. After growing, an aluminium contact was deposited on FZO as a metal contact. The distance between the two metal contacts was fixed at 1 cm as shown in Figure 2(b).
The electrical characteristics were measured using a B1500 Semiconductor Parameter Analyzer. The current response was recorded between -1 V and 1 V. The surface morphology of 1D-FZO synthesized with different parameters was studied in a scanning electron microscope (VPSEM) (JEOL JSM-IT300LV). The element composition was examined by using an energy dispersive X-ray spectroscope (EDX) which is attached with the SEM system. For structural analysis, measurements were conducted by X-ray diffraction (XRD) (Rigaku SmartLab).
3. Results and Discussion
3.1. Optimization of Hydrothermal Growth
To determine the optimal growth parameters, a series of preliminary hydrothermal growth experiments were conducted. These initial experiments were conducted to determine the effect of different process parameters during the growth towards the electrical properties of FZO. In this experiment, the concentration of Zn(NO3)2·6H2O, HMT, and NH4F were fixed at 0.01 M. The Au sputtering time, hydrothermal reaction time, and hydrothermal temperature were varied systematically based on Box-Behnken design. After each synthesis, the FZO sample was measured for its resistance using the parameter analyzer. Table 1 shows the DOE parameters generated with its corresponding resistance values measured for each sample. The lowest resistance values measured for the sample grown with Au sputtering time of 10 s, 15 s, and 20 s were 571.43 Ω, 94.42 Ω, and 22.57 Ω, respectively. As Au nanoparticles have contributed to the electrical properties, three contour plots were produced separately based on three sputtering times, 10 s, 15 s, and 20 s as shown in Figures 3(a) and 3(b), respectively. The -axis and y-axis represent the hydrothermal reaction time and the hydrothermal temperature, respectively. Samples with 10 s of Au sputtering time showed that low-resistance-nanostructured material can be produced at a . Samples with 15 s of Au sputtering time showed that low reaction times are required to produce low-resistance-nanostructured material at a wider temperature range.
Interestingly, from Figure 3(b), it only needs 3 hours of reaction time in order to obtain the lowest resistance value. In contrast, for samples with 20 s of Au sputtering time, the lowest resistance was achieved at 7 hours~10 hours of reaction time as shown in Figure 3(c).
Figure 4(a) shows the three main effects plot towards the resistance of the samples produced. The Au sputtering time shows a significant effect on the electrical resistance values of the FZO nanostructures. This indicates that Au sputtering time plays an important role to tune the electrical properties of the samples. Initially, the resistance decreases with increasing sputtering times. However, at 20 s of sputtering time, the resistance increases slightly forming an arch. The initial measurements on substrate with Au nanoparticles before the FZO growth process exhibit no electrical response. Figure 4(b) shows the effect of resistance against the sputtering time with three different temperatures. When the hydrothermal temperature increases, the resistance of the FZO nanostructures also reduces.
Based on the main effect plot in Figure 4(a), the range of the optimized conditions to obtain the lowest resistance nanostructures is as follows: sputtering time of 15~20 s, hydrothermal reaction time of 3 hours, and the hydrothermal temperature of 75~100°C.
To study in detail the effect of Au sputtering time and hydrothermal temperature on the produced FZO nanostructures, further characterizations were executed such as their morphological, structural, and elemental properties.
3.2. Structural and Elemental Analysis
The XRD analysis was carried out in order to investigate the crystal structure of fabricated 1D-FZO. An X-ray source operated at 40 kV and 30 mA was used. The diffraction patterns were collected over 0°-100° with a step width 0.1° and scan speed of 10.0619°/min. Figure 5 depicts XRD spectrum for FZO fabricated on 15 s of Au sputtering time, 100°C of hydrothermal temperature for 10 hours of reaction time. As can be seen, the major XRD peaks were detected with diffraction of (100), (002), and (101) planes and all the peaks are indexed to hexagonal wurtzite ZnO according to the 01-082-3143 ICDD file. From the observation of the XRD spectrum, the peaks only showed the presence of the ZnO phase without the fluorine phase. A similar result was found in FTO film structure because the fluorine phase overlapped with the SnO2 peaks . This is due to the level of fluorine doping which is relatively small with respect to the host lattice. However, the existence of the fluorine element can be detected using EDX analysis.
The EDX spectrum of fabricated FZO with three different Au sputtering times is presented in Figures 6(a)–6(c). All samples show the presence of fluorine (F), zinc (Zn), and oxygen (O) elements as main composition in FZO. Furthermore, weight percentage of the Au material was decreased when Au nanoparticles were sputtered at longer time. This confirmed that samples prepared consist of polycrystalline FZO as supported by XRD result.
3.3. Surface Morphology
The Au nanoparticles improve the adhesiveness of the surface layer thus promoting nanostructure growth. Since sputtering time significantly impacted to the initial growth process as reported in , the surface morphology presented in this section focused on three different Au sputtering times: 10 s, 15 s, and 20 s (Figures 7(a)–7(c)). The samples shown in Figures 7(a)–7(c) exhibited the lowest resistance based on the contour plot shown earlier for different sputtering times of the Au nanoparticles. Therefore, the growth process conditions for the three samples were not the same. Figure 7(a) shows the formation of FZO nanoparticles for shortest Au sputtering time, 10 s. Meanwhile, 15 s of sputtering time indicated that FZO nanostructures starting to grow into 1D formation. However, the lengths of the nanostructures formed were not uniform (Figure 7(b)). The size of the FZO increased when the substrate was coated with Au nanoparticles at 20 s. In this sample, it only needs low temperature and low reaction time to grow 1D-FZO compared to the other samples produced with shorter sputtering time.
The growing process for FZO nanostructures using the hydrothermal method is based on reactant species and the crystallographic orientation of hexagonal wurtzite FZO. HMT was used to maintain the neutral behaviour of solution at pH 7 and provided a hydroxyl group for ZnO formation . First, formaldehyde (6CH2O) was formed when HMT solution was heated. Then, ammonia (NH3) continues to hydrolyze and produce ammonium (N) and hydroxyl ions (OH-) . The chemical reaction can be described as equations (1) and (2) .
HMT decomposition reaction
As the concentration of these Zn2+ and OH- ions exceeds a critical value, the precipitation of ZnO nuclei starts. The Zn(OH)2 was transformed into ZnO crystals via these simple chemical reactions.
Formation of complex compound of Zn(OH)2
Formation of halo acid
The chemical reaction between the growing ZnO and the halo acid leads to equation (6).
3.4. Growth Mechanism
A possible growth formation of FZO on three different Au sputtering times is proposed. Au nanoparticles have a function similar to that of seed layers which is to promote the further growth of 1D structures . It also acted as nucleus site for initial growth after the nucleation process . As stated by Demes et al. , the nucleation process can occur at two possible conditions which are on seed layer surface or at their grain boundaries . Au nanoparticles were sputter coated on the glass substrate at three different times to produce three different samples. It was believed that, after a shorter time, the Au nanoparticles deposited would not be distributed uniformly on the substrate surface.
The growth mechanisms of FZO are illustrated in Figures 8(a)–8(c). During the early stage of a hydrothermal process, there were two types of processes occurred which were physisorption (adsorb) and surface diffusion. Physisorbed species could move at finite distances on the substrate . Since the glass substrate has high activation energy, the surface diffusion process was difficult to achieve. As a result, physisorbed species has high tendency to desorb. Here, Au nanoparticles functioned as large nuclei attracting physisorbed species by forming chemical bonds. Through coalesces with nearby nuclei, a thin first layer is formed . The increase of FZO thickness was achieved by increasing the hydrothermal temperature which promote a nucleation process as shown in Figure 8(a). At this stage, the nucleation rate was higher and grains were easier to coalesce . A similar mechanism for substrate with 15 s of Au sputtering time. However, the coating process for this sample took longer than the previous sample. Therefore, there is a high chance for more nucleus sites deposited on the substrate. As a result, the growth species assembled each other into 1D formation. Some parts on the substrate surface only produced nanoparticles illustrated in Figure 8(b). This phenomenon is clearly observed in the SEM image in Figure 7(b). The 1D nanostructures were grown with different lengths and sizes. Interestingly, after 20 s of Au sputtering time, the 1D-FZO with lowest resistance was achieved in 3 hours at 75°C as shown in the SEM image (Figure 7(c)). This suggested that, after the longer times (coating process), Au nanoparticles were well distributed along the substrate surface (Figure 8(c)). As a result, 1D-FZO was easier to grow even with shorter reaction time. Then, as the temperature increased, their size were also increased and this finding is similar to the results of a previous study by Heinonen et al. .
3.5. Effect of 1D-FZO Morphology towards Electrical Properties
One of the factors that affect the resistance values of the FZO is the grain boundary. It presents a discontinuity in the crystalline of the FZO. Between the grain boundaries, a space charge region formed resulting in a potential barrier scattering of the electrons and reducing their mobility . This condition can occur for samples coated with Au nanoparticles at 10 s and 15 s of sputtering time. As the temperature increased, the grain size with improvement in crystallinity contributed to the increase of carrier mobility as well. The total interfacial grain boundary was reduced when the grain size increases . These findings agree with the results of the previous studies conducted by Kim et al.  and Napi et al. . For substrate coated with Au nanoparticles at longest sputtering time, the nanostructures grown are more uniform but nanoneedle structure attached on nanorods as clearly seen in Figure 7(c). In fact, the 1D-FZO can provide a stable and direct electron transfer than the ZnO nanoparticles . However, the nanoneedles which were attached on nanorods affected their resistance. Furthermore, the increase in temperature increased the number of nanoneedles. Consequently, the resistance of the FZO also increased. This is verified from the contour plot (Figure 3(c)) which indicated that at low temperature, the measured resistance values were low. Subsequently, the resistance values recorded increased as the temperature increased.
From the Design of Experiment analysis, the Au sputtering time showed a significant effect on the resistance of 1D-FZO nanostructures compared to the other two parameters which are hydrothermal reaction time and temperatures. From the main effect plots and the contour plots, the optimized grow low resistance 1D-FZO are with sputtering time of 15~20 s, hydrothermal reaction time of 3 hours, and at temperature range of 75~100°C. It was evident from the SEM observations that by increasing the sputtering time, the 1D-FZO nanostructures are easier to grow with less reaction time (3 hours) and at lower temperature (75°C). This can be due to the increase density of the Au nanoparticles when the sputtering time increases. Samples coated with Au nanoparticles with less sputtering time form many grain boundaries which form a potential barrier for the electrons, hence reducing the mobility. However, the increase in temperature increased the grain size with improvement in crystallinity which contributed to the decrease of the nanostructure resistance. From the growth mechanism proposed, Au nanoparticles form nucleus sites to promote growth species to assemble each other into 1D formation. Longer Au sputter time caused the Au nanoparticles to be well distributed along substrate surface. As a result, 1D-FZO was easier to grow even with shorter reaction time. This discovery could facilitate the fabrication of various 1D-FZO nanomaterial-based devices such as transparent conducting oxide, biosensor, and optical detector.
All data obtained from characterization technique used to support this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
This work was supported by the Ministry of Education Malaysia through Fundamental Research Grant Scheme (FRGS) vot 5F053, Research University grant of Universiti Teknologi Malaysia GUP Tier 1, vot no. 19H33, and UoSM 2018/18 seed fund. The authors would like to acknowledge the fabrication and experimental support from the Micro-Nano Systems Engineering, School of Electrical Engineering, Universiti Teknologi Malaysia, and University-Industry Research Lab of UTM. The authors also acknowledge the support from the UTM Research Management Centre.
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