detectivity (D) (Chapter 5). SnO2 NPs and their nanocomposite with RGO/GQDs are analyzed for their surface morphology, crystalline structure, and structural defects. SnO2 NPs and their nanocomposite with RGO/GQDs -based device are fabricated on the IDE pattern. The characteristics of the SnO2 and SnO2-RGO are measured in CO2 and air environments. Through the ON/OFF analysis of the SnO2-RGO sensor, we have studied its sensitivity and response time for CO2 sensing in gas different concentrations and it shows superior performance as compared to the individual counterparts (Chapter 6).
The important contributions of the present thesis are summarized as follows:
A. Synthesis and Characterization of Large Lateral-size Graphene Oxide and its Derivative Materials
Firstly, graphite oxide is synthesized using the simplified Hummer’s method; we have omitted continuous starring of solution during the oxidation process, which controls the lateral size of the graphite oxide flakes. We have demonstrated a cost-effective and facile approach using mild heating to exfoliate GO sheets with ultra-large lateral sizes up to 104 µm using a simplified technique, superior to the commonly used ultrasonic method. Further, GO is confirmed from the XRD and Raman analysis. Subsequently, the reduction was carried out to obtain RGO with fewer defects as indicated by ID/IG ratio from the Raman studies. The highly uniform and small size of the GQDs after 60 minutes of tip-sonication is validated from the FETEM imaging and its size distribution analysis. The structure of GQDs is analyzed using XRD and Raman spectroscopy. Finally, we successfully synthesized high-quality, super large lateral size GO and its derivative materials such as RGO and GQDs.
B. A Detailed Study of Layer Dependent Properties of Large Lateral-size Graphene Oxide and Reduced Graphene Oxide
Large lateral size GO as obtained from the mild heating technique is confirmed from the FESEM image and Raman analysis. Correlation between the number of GO layers and the ID/IG ratio was obtained from Raman spectra in which the ID/IG ratio decreases linearly with an increase in the number of layers in GO (monolayer, bilayer, <5, <10, and > 10 layers). Results from FTIR, XPS, UV-vis absorption, PL, and TGA/DTG confirmed the partial removal of oxygen-containing functional groups in RGO, suggesting the minimization of defect-induced disorder in graphitic carbon framework, which results in restoration of sp2 hybridized honeycomb skeleton. In RGO, the current flow was significantly improved from micro-ampere to milli-ampere, which is 3 orders of magnitude change, arising from the good ohmic contact
formation on RGO as confirmed by the linear I-V curve, indicating the high quality of the graphene-based sheets. The electrochemical impedance results corroborate with the I-V characteristics, exhibiting much lower charge transfer resistance for RGO than that of GO, which depicts the higher conductivity across the liquid/solid interface for the former as compared to the latter electrode materials. The large area GO and RGO sheets have been utilized for SERS application for detecting Rhodamine B (RhB) down to a concentration of 10 nM, and a large SERS enhancement factor of 104 is reported, which is significantly high considering its semiconducting nature. Thus, the present simplified and economical approach of large-area graphene oxide could potentially open up a new strategy for industrial-scale production for cutting-edge sensing applications. This work has been published in “RSC Advance, 2021, 11,9488”.
C.
Plasma Treated Graphene oxide surface for Trace Dye Detection using Surface- enhanced Raman Spectroscopy
The structure of GO and RGO is modified using gas plasma treatment on the GO and RGO, which was evidenced from the Raman analysis with shifting of peak positions, FWHM of peaks, D`-G, and ID/IG, and substantiated by FTIR analysis. We have shown that the SERS effect of the RhB on the GO/RGO is selectively more enhanced than the MO, MB, and RB dyes due to the high charge transfer between the RhB molecules and GO/RGO samples. Ar plasma-treated GO substrate exhibits the highest EF among all the substrates for RhB detection, owing to the strong coupling dye molecules on the defect sites of GO. The SERS effect is also improved on the composite AuGO than the individual Au NPs or GO/RGO owing to the contribution of EF from both electromagnetic enhancement (EE) and chemical enhancement (CE) for RhB dye detection. This study unveils the application of large-area graphene-based sheets for SERS application and the role of low-power plasma treatment on the metal-free substrates for its exploitation in low-cost SERS sensing applications. This work has been published in “ACS Appl. Nano Mater.2022, 5,5, 6352-6364”.
D. Fabrication and Analysis of Photodetector Devices based on Surface Modified Graphene Oxide and SnO2- Graphene Oxide Hybrid
We demonstrated the fabrication of an interdigitated electrode (IDE) pattern using the UV lithography technique on various substrates including GO. The channel width or minimum distance between the two electrodes is made of 10 µm, confirmed by the AFM and FESEM images. The thickness of the metal electrode is 30 nm, measured from the height profile analysis of the AFM image. Through the FESEM imaging, we observed the fabrication of the
GO-based IDE pattern. The changes in characteristics and properties of the GO after different temperatures treatment (such as 50 ˚C, 85 C, 120 ˚C, and 230 ˚C) are studied. The restoration of the graphitic structure after annealing at 230 ˚C is authenticated from the XRD and Raman spectra analysis. It shows the shifting of the 2θ peak towards the 25.6˚ of the XRD pattern and the reduction of the value of the ID/IG ratio in the Raman spectra. The presence of oxygen functional groups is studied using XPS and FTIR analysis; flattening of C-O peak after 230 ˚C annealing of GO and removal of oxygen functional groups related peaks in the FTIR spectra.
PL and UV absorption also fortified the rich presence of oxygen functional groups in the GO RT. I-V measurement of the GO and different annealed GO are studied and it was concluded that higher oxygen functional groups attached to GO disturbed the flow of current in the GO, which leads to its insulator-like behavior. The attached oxygen functional groups can regulate the electrical properties of the GO. Three photodetectors are made by using GO, Ar-GO, and SO-GO. The performance of the devices was evaluated and compared in terms of photocurrent (Iph), response time (τrise and τdec), responsivity (R), and detectivity (D). Ar-GO based photodetector performs better than the other two, showing that the surface modification by the Ar plasma treatment is beneficial for the improved photodetector performance.
E. Fabrication of CO2 Sensors Based on Graphene Oxide Based 2D Materials and its Nano Hybrid with SnO2
SnO2 NPs and their nanocomposite with RGO/GQDs are analyzed for their surface morphology by FESEM and FETEM images; The crystalline structure of the SnO2 is studied using SAED pattern, HRTEM image, and XRD spectra. The structural defects and signature of the material are confirmed by the Raman spectra. The structural bonds such as Sn-O-Sn, Sn- OH, Sn-O, C=O, and C=C are analyzed from the FTIR spectra. The elemental composition of the materials is examined using EDS and XPS spectra. Defects, daggling bonds, and oxygen vacancies are evaluated from the PL and UV absorbance data. Further, SnO2 NPs (SO) and their nanocomposite with RGO/GQDs -based device are fabricated on the IDE pattern. The CO2 sensing is measured and calibrated in a customized gas sensing system. IV characteristics of the SO and SnO2-RGO (SO-RGO) are measured in CO2 and air environments. The ON/OFF analysis of the SO-RGO and the sensitivity were studied for CO2 gas at different concentrations, such as 250 ppm, 500 ppm, 1000 ppm, and 5000 ppm. The response and recovery time of the SO, SO-RGO, and GQDs-SnO2 (GQDs-SO) are compared. The sensitivity of the devices is calculated using the commonly used formula; The sensitivity of the GQDs-