Hydrothermal Growth of Zinc Oxide (ZnO) Nanorods (NRs) on Screen Printed IDEs for pH Measurement Application

There is considerable interest in nanostructured materials with interdigitated electrodes (IDEs) platforms to detect and monitor the level of various ions in numerous applications. Herein, we report the design and fabrication of IDEs based pH sensor by using hydrothermal growth of ZnO nanorods (NRs). A four-step deposition of ZnO seed layer followed by a hydrothermal treatment lead to the heavily ordered ZnO NRs patterns on the screen printed IDEs. The structural, chemical compositional and electrical properties of the NRs were investigated and examined by using field emission scanning electron microscopy (FeSEM), atomic force microscopy (AFM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) technique and Keithley 4200 semiconductor characterization system respectively. The sensor capacitance and pH were found to be inversely proportional at a working frequency of 1 kHz. The sensor displayed sensitivity of 1.06 nF/pH in the range of pH 4−10.

pH is a crucial parameter in regulating the reactivity of many chemical physical and biological species, thus its measurement is fundamental need in many fields ranging from agriculture 1,2 and environmental science to chemical engineering 3 and food science to biology and health applications. 4,5,6,7,8 Numerous attempts have been made to detect the pH concentration using different methods like glass probe based pH electrodes, 9 ion selective electrodes (ISEs) 10,11 , ion selective field effect transistor (ISFET) pH sensors 12,4 need pre conditionally prepared samples in laboratories, hence there is a time delay in processing and it will cause to the decreases in sensor detection performance.
Recently interdigitated electrodes (IDEs) based electrochemical pH sensors have attracted the interest of researchers due to their advantageous features i.e. small size, easy to operate, high sensitivity, fast response and economic with the lower cost of fabrication. 13,14,15,16 The emerging field of nanosensors, metal oxides based active nanolayers with IDEs structures on the variety of degradable substrates can easily detects the specific ion reactions. 17,18,15,19,20 In addition, nanostructured materials are offering the large surface to volume ratio to the specific ionic reactions and thereby improve the sensor performance. Among metal oxides, Zinc oxide (ZnO) is more versatile and widely studied metal oxide material due to easy synthesis method at room temperature and which exhibits the excellent physical, chemical, electrical and optoelectronic properties. One-dimensional (1D) Zinc oxide (ZnO) which has a direct wide bandgap (3.37eV) semiconductor with a large excitation binding energy (60 meV) at room temperature. The hydrothermal growing method is a simple and easy synthesis method to develop various nanostructures of ZnO for various applications. 21,22,23,24,25 We present a fabrication of ZnO NR based IDEs sensor for continuous electrochemical detection of pH. The Copper (Cu) based IDEs structure built using CIRCAD design tool and screen printing technology. The IDE surface was modified with the active layer of ZnO seed particle, further enhance the seed particle into the ZnO NRs. The structural, compositional and electrical properties were investigated with field emission scanning electron microscopy (FeSEM), atomic force microscopy (AFM) energy dispersive spectroscopy (EDS), Xray diffraction (XRD) technique and Keithley 4200 semiconductor * Electrochemical Society Member. z E-mail: akshayeliyana777@gmail.com characterization system respectively. The sensor displayed a sensitivity of 1.06 nF/pH in the range of pH 4 to pH 10.

Experimental and Methods
Chemicals.-Zinc acetate dehydrate (C 4 H 10 O 6 Zn), Zinc nitrate hexahydrate (H 12 N 2 O 12 Zn), Monoethanolamine (MEA) and Hexamethylenetetramine (HMT), the precursor material for the ZnO seed layer, were purchased from Sigma Aldrich. The IDEs structures are fabricated from Copper (Cu) metal and non-conducting fiber epoxy used as a substrate.
Device characterization.-The surface morphology of ZnO seed layer with NRs and IDE structure were characterized using field emission scanning electron microscopy (FeSEM) with energy dispersive X-ray spectroscopy (EDS). The crystallinity of ZnO NRs was examined using X-ray diffractometer (XRD) with peak intensities were in the range of 20°to 70°at 2θ degree angle in scanning rate. The ambient contact mode AFM imaging was performed on screen printed IDE using multimode nanoscope with IIIa controller (Bruker, Germany). The sensing characteristics of ZnO NRs grown IDE sensor was measured using a probe station (semiconductor parameter analyzer) connected to the computer at room temperature in ambient conditions. All the sensing data measurement were repeated and analyzed using four similar fabricated ZnO NRs grown IDEs sensors.

Fabrication of IDE based pH sensor.-
The complete fabrication process of ZnO NRs based IDEs sensor illustrated in Fig. 1. As a first step, the non-conducting fiber epoxy based printed circuit board (PCB) with the dimension of 16 × 25 mm 2 were ultrasonically cleaned. The IDEs structure has been screen printed on the fiber epoxy based printed circuit board with the finger length of 11mm, electrode width is 0.5 mm and spacing between two fingers is 0.5mm. The IDEs design has modeled and tested using CIRCAD software tool and prototype was shown in Fig. 2a. The copper (Cu) paste can be fired at a low temperature, was used for this purpose. After the screen printing process, the IDEs layers were dried at 100°C for 30 minutes. The fabricated IDEs consist of total 18 individual electrodes and it will provide a total sensing area of about 18 × 16 mm 2 for the ZnO seed layer as shown in Fig. 2b. The uniform and stable ZnO seed particle solution was prepared and deposited on the 18 × 16 mm 2 active area of the IDEs structure. A temperature controlled spin coating unit was used to deposit the ZnO seed solution on the IDEs at 4,000 rpm for 30 seconds with four step programming process. The electrodes were dried in a muffle furnace at 160°C for 15 minutes. The same deposition process repeated four times to obtain a uniform pattern of ZnO seed layer on the IDEs surfaces.
The low temperature hydrothermal growth process of ZnO NRs is illustrated in flow chart 1. The prepared growth solution consists of a mixed solution of the Zinc nitrate hexahydrate and hexamethylenetetramine (HMT).
In brief, 4.4 gm of zinc nitrate hexahydrate added with the deionized water and 3.25 gm of the hexamethylenetetramine (HMT). The ZnO NRs were grown by immersing the IDEs in the prepared aqueous solution using homemade Aluminium based sample holder. During this process, the solution was heated at 93°C for duration of 5 hours in a muffle furnace. 26 After the growth process, the samples were neatly cleaned in deionized water and dried in room temperature.  ≈60 ± 10 nm. Most of these NRs shows branched structure, parent NRs further grew into smaller and narrow NRs. The size of the ZnO NRs is strongly dependent on the seed size of the prepared ZnO seed layer. As the grain size of the particle layer decreases, smaller sizes of ZnO NRs in diameter are grown and vice versa. The screen-printed IDEs pattern modeled and designed for 500 μm finger gap of the electrodes. The finger gap of the IDEs patterns is approximately 460 μm from the FeSEM image analysis (Fig. 4). The 40 μm variations happened due to the non-aligned edges of the electrodes pattern.

Characterization of ZnO seed particles and ZnO
The EDS analysis shows the presence of Zn and O elements in the ZnO seed particles (Fig. 5a). Additionally, very small peaks for carbon element have been found in the spectrum and it is from the substrate material but no other impurity was detected in the chemical compositional studies. The weight percentage of Zn and O in ZnO seed particles are 314.19 and 203.91 respectively. The further, grown ZnO NRs also composed of Zn and O elements only. Except for Zn, O and C, no other element has been found in the spectrum. The weight percentage of Zn and O in ZnO NRs were 422.89 and 214.54 respectively (Fig. 5b). The ZnO NRs contain 10-15% more ZnO elemental composition compared to the ZnO seed particle.
Further, crystal phase and orientation of ZnO NRs were characterized using X-ray diffraction (XRD) technique. The peak intensities were measured in the range of 20°to 70°at 2θ degree angle in scanning rate. All the diffraction peaks observed in the ZnO NRs perfectly indexed and well matched with the standard Joint Committee on Powder Diffraction (JCPDS) card No.35-1451, which equivalent to the hexagonal morphology of ZnO.
The XRD result shows the ZnO NRs consist strong diffraction peak at the (002) plane and its location is in 35.44°degree angle as shown in Fig. 6. From that, ZnO NRs are in hexagonal wurtzite structures with high c-axis and the narrowest peak of full width at half maximum (FWHM). The obtained sharp peaks indicates that the ZnO NRs have an excellent crystal quality and its diffraction peaks were matching with the bulk ZnO material.
The average grain size of the ZnO NRs was calculated using Scherer's formula.
Here FWHM is the full width at half maximum of the respective peak in radian, and θ represents the diffraction peak angle. K is the Scherer constant, which is dependent on the crystallite shape and can be considered as 0.9; 27 λ is the X-ray wavelength of the incident Cu Kα radiation, which is 0.154056 nm. 28 The ZnO NRs that were grown on the surface of the IDEs produced the crystallite size of 54.18 nm and this result also was matching with the FeSEM studies.  The surface morphological features of IDE electrode structure were performed and presented in the AFM analysis.
The surface roughness is a predominant parameter in adsorption and desorption of ions in the sensitive layer for electrochemical sensor applications. The root mean square surface roughness of the layer was estimated to be 204 nm from the surface and three dimensional (3D) topography images of IDEs as shown in Fig. 7.

Evaluation of sensing performance of as fabricated IDE
sensor.-In order to measure the characteristics of the pH device, the two contact electrodes of the device was connected to the probe station using a Keithley 4200 semiconductor characterization system. The standard pH buffer solutions of pH 2 to pH 12 have been used for determine the pH sensors characteristics. The Fig. 8 shows the device characteristics measurement setup by using Keithley 4200 semiconductor   characterization system. To evaluate the sensing performance of the device, Capacitance-Voltage (C-V) and Capacitance-Frequency(C-F) characteristics of the sensor were performed with increasing pH concentrations from pH 2 to pH 12. Finally, the sensitivity of the device was estimated by plotting the graph of pH level (pH) versus capacitance (F) at applied frequency level of 1 KHz.
Here we have used two-contact electrode arrangement with IDEs platform for the pH sensor. When 0V to 3V range voltage is applied across the contact electrodes of the IDEs, the local electrical field generated on each digit induces in the micro gap electrodes, changes in the electrical properties of the active layer. While varying the pH concentration of the solution, the positive or negative charged surface groups of the electrical double layer (EDL) formed at the interface of ZnO NRs, electrolyte gets disturbed and which further leads to change in the electrical properties of the active layer. 14 Fig. 9 shows the capacitance measured between the interdigitated microelectrodes are plotted against the voltage frequency with voltage ranging from the −6V to 5V to confirm the capacitive nature of the fabricated IDEs. The value of the capacitance is almost stable in the range of 0V to 5V (1.9 × 10 −11 F). Fig. 10 reveals that the pH buffer solution of capacitance measurements obtained and plotted against the voltage frequency ranging from 1 KHz to 100 KHz. According to the Coulomb's law, C = q/v, the chance of electrons passing across the electrode will increases when there is a low frequency of a voltage applied. 29 The resistivity is inversely proportional to the conductivity which means, the conductivity will be increased, and hence interactions of more hydrogen ions will cause low resistivity according to the theory of conductivity. 30  From the obtained data the capacitance decreases with increasing frequency (1 KHz to 3 KHz) and further from 4 KHz onwards the capacitance value is almost constant. In addition, it can be observed from Fig. 11, the fabricated sensor exhibits the decreases capacitance value (9.65 nF to 13.07 pF) with increases in the pH value (pH 2 to pH 12) of the solution selected frequencies (1 KHz, 2 KHz, 3 KHz, 4 KHz, 5KHz).   Further to calculate the linear fit and sensitivity of the fabricated ZnO NRs based IDEs pH sensor, a corresponding curve (average capacitance Vs pH concentration) of C-pH characteristics as shown in Fig. 12. As seen in the plot, the fabricated IDE sensor detected pH in the linear pH range from 4−10 with a good correlation coefficient (R 2 ) of 0.8203. From the obtained slope, sensor showed a sensitivity of 1.06 nF/pH in the range of pH 4 to pH 10.
To estimate the response time of the fabricated pH device, the sensor was immersed in standard pH buffer solutions whose pH values were 4, 6, 8 and 10 respectively. The Keithley 4200 semiconductor characterization system used to record the response time of the sensor. The experiment was repeated four times and the response time of the fabricated pH device was found about 1 S to 10 S for acidic and alkaline solutions. Their response speed much faster in acid solutions than alkaline solutions.
The Fig. 13a reveals the relative experiments of fabricated device in order to observe the stability of the device in various acid and alkaline solutions. The every experiment was continued for the time of 2100 S, and the result was recorded every 300 S in the semiconductor characterization system. The analysis results confirm the fabricated device becomes highly stable in pH 6 to pH 8 and in higher acidic and alkali solutions it produce small variations in the results. Also the stability of the fabricated device tested for the long periods. In a week, the pH values of buffered solutions were measured in interval of a day and the obtained results once again confirms the good stability of the fabricated device.
To test the device reproducibility, we have fabricated three more IDE NRs based pH sensors namely IDE NRs 2 , IDE NRs 3 and IDE NRs 4 with IDE NRs 1 is the first fabricated device. The change in capacitance with respect to pH solution of the three devices with constant frequency of 1 KHz as shown in Fig. 13b. The results confirms the difference between fabricated IDE NRs 1 with the second third and fourth IDE NRs devices are 0.8%, 1.3% and 0.7%, which is an acceptable error margin of less than <2%. The obtained results confirms the fabricated pH sensor possess good stability for repetitive detection of pH and are remarkably reliable.

Conclusions
Herein, we report the design and fabrication of ZnO NRs based IDEs senor using hydrothermal growth of ZnO nanorods (NRs) were successfully implemented. Spin coating deposition of ZnO seed layer followed by the hydrothermal growth of ZnO nanostructures having NRs morphology lead to the fabrication and characterization of the sensor. The structural, compositional and electrical properties of the NRs based IDEs structures were investigated and examined by using field emission scanning electron microscopy (FeSEM), atomic  force microscopy (AFM), energy dispersive spectroscopy (EDS), Xray diffraction (XRD) technique and Keithley 4200 semiconductor characterization system respectively. The sensing performance of the IDEs, the sensor capacitance decreases with increasing frequency (1 KHz to 3 KHz) and further frequency from 4 KHz onwards the capacitance value is almost constant and negligible. It also exhibits the decreases capacitance value (9.94 nF to 13.07 pF) with increases in the pH value (pH 2 to pH 12) of the solution at selected frequencies (1 KHz, 2 KHz, 3 KHz, 4 KHz, 5 KHz). The sensor showed a sensitivity of 1.06 nF/pH in the range of pH 4 to pH 10. The ZnO NRs based IDEs pH sensor present it as with simple fabrication process, low-cost, and convenient device for measurement of pH in water.