Introduction
In the past decade, graphene has grabbed a great deal of attention in the 2D materials research community [
1,
2]. Among the several remarkable properties that this wonder material boasts of are its high mechanical strength, ultra-high mobility of its conducting electrons, efficient thermal conductivity and its surface conformability. While many of these properties pertain to graphene studied in its pristine form, it is worthwhile to note they arise from investigations of micrometer-scale sample sizes that are obtained by the mechanical exfoliation of graphite using the infamous scotch-tape technique [
2]. On the contrary, large-scale and pristine graphene have also been successfully realized via bottom-up synthesis techniques such as chemical vapor deposition (CVD) [
3‐
5], but the technique suffers from the associated high-cost and technological challenges of scalability [
5]. To circumvent these problems, top-down synthesis routes of graphene
alternatives such as reduced graphene oxide (rGO) have been gaining increasing attention in the recent past.
The starting step in the process of obtaining rGO is the synthesis of graphene oxide (GO). GO is obtained through the chemical [
6‐
8] or electrochemical exfoliation [
9,
10] of graphite, based on methodologies proposed by Hummers [
6], Brodie [
8] and others [
11‐
13]. More recently, the modified Hummer’s method [
12,
13] has been accepted as the new standard for obtaining GO solutions, owing to its lower toxicity levels, explosion safety and improved yields. A typical chemical exfoliation process starts by the introduction of strong oxidizers that intercalate the carbon layers in the graphite. This causes a disruption in the crystalline network of sp
2 carbon atoms due to the occupation of oxygenated functional groups such as the hydroxyl (–OH), epoxy (–O–), carbonyl (C=O) and carboxylic (–COOH) groups [
14‐
18]. The GO is therefore rendered defective and highly electrically insulating in comparison to its graphite parent.
Fortunately, these detrimental functional groups may be eliminated by a variety of reduction processes that finally yields rGO, a highly conductive material akin to graphene. A few commonly used routes to achieve this transformation are through reduction via chemical means [
19‐
23], thermal reduction under vacuum [
17,
24‐
26] or other atmospheres [
27‐
29], joule heating assisted reduction [
30‐
32], reduction using microwave radiation [
33‐
36] and by employing a few other novel methods [
37‐
40]. A recent review article [
41] can be referred to by the reader for a comparative study on the various scalable routes to synthesize rGO. rGO can be processed into various three-dimensional structures such as free-standing rGO papers [
42‐
49], foams [
50,
51], gels [
52,
53] and as films deposited on desired substrates [
54]. The various forms of rGO are then used in wide ranging applications such as solar cells [
55], sensors [
56,
57], supercapacitors [
58,
59], thin-film conductors [
60], membranes [
61], water purification [
62,
63] and biomedical applications [
64]. In particular, scalable and flexible free-standing rGO offers a significant advantage when it pertains to its use in energy storage devices, for instance, as a replacement material for electrodes in supercapacitors [
58,
59,
65,
66]. This is possible due to the presence of closely stacked graphene layers that provide a high surface area for electron or ion diffusion. Moreover, due to the absence of supporting substrates or other added blending materials, free-standing rGO papers possess superior electrical [
44,
67,
68] and thermal conductivities [
45]. The obtained rGO paper may therefore be viewed as a dialed-down version of intrinsic graphene but with the extremely important advantage of being a low-cost alternative that can also be mass-produced. For these reasons, it is imperative to comprehensively understand the structural and electrical properties of this material.
Of the various reduction techniques available, thermal reduction offers a very simple and environmentally friendly route to obtain rGO. While a complete elimination of the functionalities is difficult to achieve, as is evident from ab initio studies [
69,
70], the reduction process is nonetheless highly effective, and recovers to a large extent, the intrinsic properties of graphene. Additionally, the extent of reduction can be controlled through experiment [
71,
72], allowing for a direct means of tuning the different properties such as its thermal or electrical conductivity and solubility.
Despite the apparent advantages of thermal reduction and free-standing rGO, most studies in the literature have focused on the thermal reduction effect on GO that is obtained either in powder form [
27,
73,
74] or via means of drop-casting [
75] onto a substrate of choice. There have been only a handful of reports that focuses on the thermal reduction process of free-standing GO papers [
42,
45‐
47]. Systematic studies to determine critical temperatures or crossover behaviors, important for GO-based applications sensitive to high temperatures, have not been reported. Moreover, correlation studies of the temperature-dependent structural and morphological variations and the electrical properties such as the film resistance and current–voltage behavior is still lacking.
In this work, we aim to carry out a comprehensive analysis to understand the evolution of structural, morphological and electrical changes occurring in free-standing GO papers, as a function of reduction temperature. To obtain free-standing GO, we follow a facile vacuum filtration [
76,
77] approach that has the advantage of yielding scalable, uniform and large area sheets of the material in thin-film form. The vacuum-filtered GO papers are then subject to thermal reduction to convert them to free-standing rGO papers. The choice of thermal reduction ensures good experimental control and tunability and therefore allows for the reliable and systematic characterization of the samples at various stages of the reduction process.
Structural changes occurring in the GO papers in response to thermal reduction are captured using Fourier transform infrared spectroscopy (FTIR), Raman, X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) techniques. FTIR is used to identify the various oxygen functional groups that intercalate the GO layers and qualitatively analyze the change in their compositions as reduction temperature is varied. The extent of disorder is quantified by studying the ratio of the characteristic D and G peaks obtained from the Raman spectroscopy while the dominant crystalline phases and their transformations are identified using XRD. EDX analysis of the film surface reveals the reduction efficiency in terms of the carbon and oxygen content present in the GO papers post reduction. Surface morphological changes are analyzed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Film thickness and layer exfoliation are studied using cross-sectional SEM imaging and the electrical resistance and current–voltage (I-V) behavior are analyzed using the four-point probe method.
Strong correlations are found between the structural, morphological and electrical properties of the thermally reduced GO papers. Four-point resistance measurements reveal a dominant crossover from an insulating to highly electrically conducting behavior beyond a reduction temperature of 200 °C. The orders of magnitude drop in resistance corroborates well to the onset of major structural changes observed from the FTIR, Raman, XRD and EDX analyses. SEM images reveal notable surface morphology changes occurring beyond the crossover temperature. At the highest reduction temperature of 600 °C, the rGO surface is dominated by a grainier appearance which is suggestive of an uptick in vacancy defect formation and signals the occurrence of a bottleneck in the reduction efficiency. Overall, the temperature-dependent correlations identified in this work is expected to be useful for exploitation in practical device applications based on GO and rGO.
Conclusions
In this work, we have conducted systematic studies to understand the effect of thermal reduction temperature on GO papers obtained via a facile vacuum filtration process. To this end, we have obtained and analyzed the FTIR, Raman and XRD patterns of GO papers subject to various reduction temperatures ranging from 100 to 600 °C and a reduction time of 10 min. The various oxygen functional groups (hydroxy, carbonyl, carboxyl and epoxy) are identified via FTIR and the intensities of these peaks, with the exception of the epoxy group, is found to wash out completely at reduction temperatures of 400 °C. The epoxy peaks are persistent and shows signs of disappearing only above 400 °C, consistent with the known fact that the epoxy groups have a higher dissociation energy. The Raman spectra reveals that the ID/IG ratio fluctuates in the temperature range of 100–500 °C which is reflective of competing contributors to the average defects. One is the lowering of defects due to removal of oxygen functionalities and the other leads to an increase due to the creation of lattice defects as carbon atoms are expelled in the form of CO and CO2 gases. The contribution from the lattice defects is seen to dominate the average defect at the highest temperature of 600 °C and is reflected as a sizeable increase of the ID/IG ratio. The XRD patterns reveal clear shifting of the crystallographic phase from < 001 > to < 002 > as temperature is increased, confirming the reduction of GO to rGO. The < 002 > peek is sharpest at a temperature of 500 °C and broadens at 600 °C, in agreement with the Raman analysis.
SEM surface scans revealed that the original GO paper had wrinkles or curvy features which seem to diminish for the GO papers reduced at higher temperatures. This made the surface flatter, which we corroborated with the AFM scans showing a reduction of the rms value by 40%. At the same time, it was observed that the surface of the GO became more granular with sharp edges that were especially pronounced at the highest temperatures. This is indicative of release of functional groups out of the plane of the GO paper. The elemental analysis showed that the carbon content was enhanced by 25% at the highest reduction temperature of 600 °C. At this temperature, the remaining oxygen content was only 13.31 wt. % indicative of the effectiveness of higher temperatures in the removal of oxygen functionalities.
Finally, we have analyzed the 4-point probe resistance of four batches of vacuum-filtered GO papers. We identified crossover behavior, characterized by a drop in the resistance by several orders of magnitude, in the temperature range of 200–250 °C. The I-V curves of the thermally reduced GO papers exhibited ohmic behavior between 300 and 500 °C, as expected for good conducting materials while the GO papers treated at temperatures below the crossover temperature showed strong insulating behavior with nonlinear I-V. The reduction time was also shown to have a noticeable effect on the average resistance of the samples.
In conclusion, we have presented a comprehensive and corroborative analysis of the evolution of structural and electrical properties of free-standing GO papers as a function of reduction temperature. The crossover between insulating GO and conductive rGO is found to strongly depend on the reduction temperature and correlates well with the structural and morphological analysis.
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