TY - JOUR
T1 - Modulating chemical composition and work function of suspended reduced graphene oxide membranes through electrochemical reduction
AU - Rodriguez, Jan Sebastian Dominic
AU - Ohigashi, Takuji
AU - Lee, Chi Cheng
AU - Tsai, Meng Hsuan
AU - Yang, Chueh Cheng
AU - Wang, Chia Hsin
AU - Chen, Chi
AU - Pong, Way Faung
AU - Chiu, Hsiang Chih
AU - Chuang, Cheng Hao
N1 - Funding Information:
In Fig. 3(h), the surface potential images of dropcast GO and rGO are plotted in a histogram of the VCPD distribution, obtained from the pixel values of the whole image of each KPFM image. The histogram distributions are fitted with the Gaussian function [39]. The acquired data over an area enclosing the dropcast GO exhibit the peak at −498 ± 0.13 mV with a full width at half maximum (FWHM) of 38 mV. In total, the fitting result from the rGO sample consists of four Gaussian peaks centered at −382 ± 2.35, −327 ± 1.68, −272 ± 0.74, and −225 ± 0.25 mV with FWHM of 52, 80, 42, and 56 mV, respectively. Next, we converted the obtained surface potential into WF using the following equation:>[Formula presented] where VCPD is the contact potential difference between the tip and the sample, and e is the electron charge [39,40]. The fitting WF values are exhibited in Table S2 of supporting information, and the obtained values are shown in the top axis of Fig. 3(h). In the literature [41,42], the WF of freestanding single-/few-layer G and graphite is 4.37/4.43 and 4.6 eV, respectively. A similar saturation tendency is observed for the thickness dependence in G, GO, and rGO, with an increase of 0.1–0.2 eV of WF after 4–7 layers of thickness [39,43]. The WF value (5.78 eV) in our dropcast GO was much higher than that reported (4.4–4.6 eV) [44,45], inclusive of the thick area of the membrane; this was mostly due to the random spatial distribution, high degree of oxidation, and diverse oxidation states present on the surface. The literature [43] indicated a decrease (140–220 meV) in the WF of GO from vacuum to a high RH condition because the hydrophilic surface causes water molecule attachment as a shielding effect. In addition, the metal contact with GO shifts the WF of GO by a difference of 100–200 meV WF between Si, HOPG, and Au, depending on the charge transfer effect and Fermi-level modulation, which is similar to p- and n-doping effects [43,46]. Thus, we attribute the large WF values of dropcast GO to five causes: (1) thick layers that superpose the surface potential; (2) high and diverse oxidation that induces surface dipoles; (3) the random-oriented GO sheet, which enhances the edge effect; (4) suspended membrane without the substrate influence; and (5) precise measurement allowed by frequency-modulated KPFM [40], even under the shielding effect of water at a RH of 45%. Because the rGO membrane was reduced by the electrochemical CV process, comparing the WF and STXM results was worthwhile to recognize the bonding state transition and sp2 structure reformation during deoxidation. The SEM image of rGO exhibits a pattern remarkably similar to that seen in the surface potential map. The area inside the pattern, with a high WF value (5.70 and 5.65 eV), had higher degree of oxidation than that of the outer area, which caused lower electron conductivity and secondary electrons emission in SEM. Two small peaks of low WFs (5.59 and 5.55 eV) were recognized for the less-oxidized area among rGO membrane, with respect to the higher emission intensity of SEM.
Publisher Copyright:
© 2021 Elsevier Ltd
PY - 2021/11/15
Y1 - 2021/11/15
N2 - Electrochemical reduction in aqueous graphene oxide (GO) dispersion has emerged as an alternative route to producing a reduced GO (rGO) membrane on Au mesh. Under scanning electron microscopy, an interesting pattern formed by distinct differences was discovered from the deoxidization evolution. Scanning transmission X-ray microscopy shows the chemical composition coordination mixing of C–OH, C–O–C, HO–C[dbnd]O, and C[dbnd]O bonds at nanoscale resolution. The electrochemical reduction of C–OH, new bonding of C–O–C, and structure recovery of C[dbnd]C were obtained from GO transformation into the rGO membrane. In Kelvin probe force microscopy, the same pattern of rGO was also observed for the diversity of work functions ranging from 5.55 to 5.70 eV compared with the uniform distribution of GO of 5.78 eV. Density functional theory calculations predicted that the work function variation originated from the dependence of O atom number and functional group species. A high (low) diversity in work function values was ascribed to the C–O–C (HO–C[dbnd]O) bond even with increasing oxygen numbers, accounting for the peak variation. Controlling the work function holds great significance for photovoltaic behavior and band alignment in photoelectric devices. Thus, growing large-area rGO membranes offers a new route to obtaining membranes for applications requiring transparent materials.
AB - Electrochemical reduction in aqueous graphene oxide (GO) dispersion has emerged as an alternative route to producing a reduced GO (rGO) membrane on Au mesh. Under scanning electron microscopy, an interesting pattern formed by distinct differences was discovered from the deoxidization evolution. Scanning transmission X-ray microscopy shows the chemical composition coordination mixing of C–OH, C–O–C, HO–C[dbnd]O, and C[dbnd]O bonds at nanoscale resolution. The electrochemical reduction of C–OH, new bonding of C–O–C, and structure recovery of C[dbnd]C were obtained from GO transformation into the rGO membrane. In Kelvin probe force microscopy, the same pattern of rGO was also observed for the diversity of work functions ranging from 5.55 to 5.70 eV compared with the uniform distribution of GO of 5.78 eV. Density functional theory calculations predicted that the work function variation originated from the dependence of O atom number and functional group species. A high (low) diversity in work function values was ascribed to the C–O–C (HO–C[dbnd]O) bond even with increasing oxygen numbers, accounting for the peak variation. Controlling the work function holds great significance for photovoltaic behavior and band alignment in photoelectric devices. Thus, growing large-area rGO membranes offers a new route to obtaining membranes for applications requiring transparent materials.
KW - Density function theory
KW - Kevin probe force microscopy
KW - Membrane
KW - Oxygen functional group
KW - Reduced graphene oxide
KW - Scanning transmission X-ray microscopy
KW - Work function
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U2 - 10.1016/j.carbon.2021.09.015
DO - 10.1016/j.carbon.2021.09.015
M3 - Letter
AN - SCOPUS:85115747337
SN - 0008-6223
VL - 185
SP - 410
EP - 418
JO - Carbon
JF - Carbon
ER -
