Chapter 6 Fabrication of CO 2 sensors based on graphene oxide based 2D materials and
6.2. Experimental details
6.2.1. Synthesis of RGO and GQDs
The graphite oxide was synthesized by using the simplified Hummer’s method and further, exfoliated into GO sheets using a mild heating technique. The exfoliated GO sheets are drop-casted on the SiO2/Si substrate and dried on the hotplate at 85 ˚C. Subsequently, GO sheets are converted into RGO using thermal annealing at 360 ˚C. GQDs are synthesized from the GO solution using a hydrothermal method assisted by a tip-sonication process. More details of the synthesis process of graphite oxide, exfoliation of GO, and GQDs were presented in Chapter 2.
Table 6.1: Summary of the samples studied and sample codes.
Sample codes Descriptions
SO SnO2 Nanoparticles
SO-GO SnO2 on GO nano hybrid SO-RGO SnO2 on RGO nano hybrid GQDs-SO GQDs on SnO2 nano hybrid
6.2.2. Fabrication of SnO2 NPs, nano hybrids of RGO, and GQDs devices
The interdigitated electrode (IDE) is fabricated using UV lithography. A 10 µm channel IDE is made together of multiple numbers of metal fringes; the minimum distance between the two electrodes is 10 µm. After the patterned IDE on the SiO2/Si substrate by UV lithography, Au/Cr are deposited by a thermal/e-beam evaporator. The detailed process steps for the fabrication of IDE are explained in section 5.2.2 of chapter 5.
The SO sensor is made by depositing the SnO2 NPs on the IDE pattern. An average of 100 nm size SnO2 NPs is purchased from Sigma Aldrich. 5mg of SnO2 NPs is added to a 50 ml beaker containing 30 ml of de-ionized water. A bath-sonicator disperses the solution for 30 minutes. Then, 10 µl of the solution is drop-casted on the IDE pattern and dried at 85 ˚C for 60 minutes. Thus, the SnO2 sensor (SO) is made and characterized for further analysis.
SO-RGO sensor is fabricated by deposition of SnO2 NPs on the RGO, which was placed on the IDE pattern. The exfoliated GO sheets are drop-casted on the IDE pattern and dried on the device on the hot plate at 85 ˚C for 60 minutes. Afterward, the device is placed inside a quartz tube using a quartz boat for vacuum annealing. The device is heated at 360 ˚C for 60 minutes to remove oxygen functional groups attached to the GO sheets. The process of vacuum annealing followed the same steps to synthesize RGO from GO using the thermal annealing method explained in chapter 2.
GQDs-SO sensor is fabricated using 10 µl of GQDs dispersed in the water solution and drop-casted on the SO sensor. The synthesis of the GQDs is explained in chapter 2. The device is dried at 85 ˚C on the hot plate for 60 minutes. This is the simplest method to fabricate the SnO2 NPs, RGO, and GQDs hybrid devices.
6.2.3. Characterization techniques
The morphology of the RGO, GQDs, SnO2 NPs and their nano hybrids on the SiO2/Si were observed using a field emission scanning electron microscope (FESEM) (JEOL, JSM- 7610F). High resolution image and selected area electron diffraction (SAED) pattern of the SnO2 NPs was examined using a field emission transmission electron microscopy (FETEM) (JEOL, JEM-2100F), operated at an accelerating voltage of 200 kV. The structural analysis of the RGO, GQDs, SnO2 NPs and their nano hybrids were studied using XRD (Rigaku, RINT 2500 TTRAX-III), with Cu Kα radiation as a source of x-ray and a scanning speed of 3 °/min.
The structural and functional groups attached to the RGO, GQDs, SnO2 NPs and their nano hybrids were studied using Raman spectroscopy (Horiba, LabRam HR) with the 532 nm laser
using a 100X optical lens. The functional groups attached to the RGO, GQDs, SnO2 NPs and their nano hybrids were studied using an X-ray photoelectron spectroscopy (XPS) (PHI X-tool, ULVAC-PHI INC) with Al Kα as an X-ray energy source at 20 kV and 54 W. The functional groups of the nano hybrids of SnO2 and RGO was studied using the transmittance mode of the Fourier transform infrared (FTIR) spectroscopy (Perkin Elmer, spectrum BX). The photoluminescence (PL) of SnO2 NPs was measured using the fluorimeter (Horiba, Fluromax- 4) using 360 nm excitation. I-V measurements for CO2 sensors were performed using a home- made set-up with a source measure unit (Keithley 2400, Germany) and the data were retrieved using KickStart Instrument control software (version 1.9.8).
6.2.4. Gas sensing set-up
A CO2 gas sensing system is a set-up by assembling various parts such as a probe station with a temperature controller, a source-measure unit (Keithley 2401), a gas flow reader and controllers (MKS), high purity gas cylinders (5000 ppm of CO2, and 99.999% of N2), a vacuum system, and a personal computer with KickStart tool. The schematic illustration of the customized system is shown in Fig. 6.1. The vacuum supported probe station with substrate holder is heated at 120 ˚C for 6 hours by using the metal hot plate placed inside the vacuum chamber. This process removes the attached water vapors and humidity inside the chamber with the support of a rotary vacuum pump. The pressure inside the chamber is reduced to 1.6
× 10-2 mbar.
Fig. 6.1: Schematic diagram of the CO2 gas sensing system.
Then N2 is pursued inside the chamber for over 30 minutes to remove the impurities/ unwanted gas presence inside the gas line tube. Similarly, CO2 (5000 ppm) is pursed inside the chamber for 30 minutes to remove the other gases trapped inside the tubes. The pursing of the gas process is done in vacuum environments. After that, the main gas valve is closed, and the sample is loaded inside the chamber. The fabricated device is placed on the probe station and probed the tips to measure the electrical characteristics.
Before measuring the gas sensing parameters, removing water vapors and flushing the impurity gas are done. Thus, the device is placed on the micro-probe station, and two gold- coated micro-tips are engaged on the metal pads of the device. The metal probes are connected to a source measure unit (Keithley, 2401) which is further connected to the personal computer through a GPIB cable. The ON/OFF response of the CO2 sensor is controlled by manually stopping the flow of CO2 gas supplied into the chamber. All the response activities are controlled and acquired using KickStart software.