Figureure. 1(a) shows field emission-scanning electron microscopy (FE-SEM) image of rGO demonstrates the individual few layer sheets are having 400–500 nm size. In Ce/CeO2-rGO hybrid nanostructure as shown in Figure. 1(b) and (c), Ce/CeO2 NPs decorated entirely on GO framework like in this reference,45 which is able to attribute the better dispersion. The FE-SEM image of Ce/CeO2-rGO hybrid nanostructure evidently shows the nanoparticles are having size of 50–100 nm in this nanocomposite as shown in Figureure. 1(b). The formation of highly dispersed metallic Ce/CeO2 NPs between narrow size distributions of 50–100 nm on rGO support. As shown FE-SEM in Figure. 1(c) having some particles are agglomerated.48 The surface morphology of Ce/CeO2-rGO is shown in Figure. 1(b) and 1(c). The surface of rGO was covered by Ce/CeO2 having 50–100 nm size of NPs and as hierarchical interconnected nanoparticles of a porous network like morphology shown in Figure.1(b) and (c). These Ce/CeO2 nanoparticles are vertically grown on sheets of graphene. Moreover, the SEM for bare Ce/CeO2 is having homogeneous distribution of particles having range of 10–100 nm (Figure. S1).
The elemental composition of the as-synthesized hybrid was analyzed using energy dispersive analysis of X-ray (EDAX). Accordingly, Figure. 2, consisted of carbon (C), cerium (Ce), oxygen (O), sulphur (S) and nitrogen (N) with the wt% of 62.67, 14.47, 13.72, 7.54 and 1.59 respectively conforms the formation of Ce/CeO2-rGO hybrid nanostructure and is in good agreement with XRD analysis shown in Figure. 3. The presence of N, S, and O in presence of C and Ce confirms the existence of defective sites through surface functionalities for anchoring Ce/CeO2 NPs on rGO sheets. The presence of signals, their binding energy values and representative wt% is in good agreement with the similar systems from literature.48
Figure. 3 shows the superimposed X-ray diffraction (XRD) for the as-synthesized rGO and Ce/CeO2-rGO hybrid nanostructures. Accordingly, XRD pattern shown in Figure. 3(a) corresponding to a characteristic peak at 2θ = 26° which can be assigned to (002) plane of rGO. Moreover, as shown in Figure. 3(b) corresponding planes (111), (002), (200), (220), (311), (400) (331) and (420) plane for Ce/CeO2-rGO having crystalline lattice planes of CeO2 and Ce of Ce/CeO2 NPs on rGO. These peaks agreed well with the cubic structure of Ce/CeO2. In XRD pattern of Ce/CeO2-rGO hybrid, both characteristic peaks for rGO and Ce/CeO2 were observed, indicating that the as-prepared hybrid is composed of high purity rGO and Ce/CeO2 phases and is in good agreement with literature.48, 49
The as-synthesized hybrid Ce/CeO2-rGO nanostructures were further characterized to confirm acid functionalization followed by its concern towards anchoring of Ce/CeO2 NPs by using FTIR analysis shown in Figure.4. In accord with the literature, the acid treatment results into introduction of -OH, -COOH, -SO3H etc. functional groups on the GO surface and these functionalities are responsible for further decoration of Ce/CeO2 NPs on the surface. Consequently, the FTIR spectra confirms well defined bands at 1642–1829 cm− 1, 1516 cm− 1, 1182 cm− 1 which are characteristic of -C-H, -CO-OH, -C = C- and -C-OH stretching frequencies corresponding to partial availability of oxidative functionalities on rGO. The band at 3355 cm− 1 determines the -O-H containing groups shown in Figure. 4(a). Interestingly, after hybrid formation additional signal appeared at ∼665 − 400 cm− 1 corresponding to Ce-O along with disappearance of bands corresponding to -O-H, -COOH and C-OH and change in the frequencies of the representative bands supports the formation of Ce/CeO2-rGO shown in Figure. 4(b). These results are also in good agreement with XRD, FE-SEM and similar systems from literature.48 Additionally, FTIR spectra of Ce/CeO2 NPs obtained using KBr pellet methods are shown in SI (Figure. S2), with appropriate scientific discussions.
The N2-adsorption/desorption isotherms and pore size distribution of rGO and Ce/CeO2-rGO hybrid electrocatalytic systems are shown in Figure. 5. Accordingly, rGO (I) shown in purple curve presented type-II isotherm with hysteresis loop, the precise surface area, the average pore size and the total pore volume of the rGO were analyzed through N2-adsorption/desorption isotherms. The hysteresis loop ranging from 0.4 to 0.9 P/P0. It is well known that the type-II isotherm is the characteristic isotherm of mesoporous materials.50,51
The porosity of the material is favours to the gas sensitivity and results reflected on quick response towards gas analyte. In this line the porosity of synthesized composite material is measured by using Brunauer-Emmett-Teller (BET) technique and as shown in Fig. 5. The results indicated that the surface area observed was 41.701 m2 g− 1v and pore volume 0.065 cc/g with total pore radius of 2.8 nm observed for rGO confirms its porous nature. Typical N2-adsorption/desorption isotherm of red (II) curve has been shown for Ce/CeO2-rGO hybrid. Accordingly, the enhancement in surface features after hybrid formation of Ce/CeO2 with GO i.e. of Ce/CeO2-rGO also evaluated and is found to be ∼33.297 m² g− 1 at P/P0∼1, this could be due to the porous nature of Ce/CeO2-rGO which is probably developed due to the formation of defects on rGO followed by decoration of Ce/CeO2 at nanodimensions. The isotherm appears to be nearly of type IV in nature and which is characteristic of solids containing both micro and mesoporous. The specific BET surface area, total pore volume and average pore radius were calculated and to be 24.780 m²/g, 1.405 e− 01cc/g and 0.84 nm respectively, and is in good agreement with hybrids from literature.52
2.5 Electrochemical Hs Sensing On Ce/ceo-rgo:
Electrochemical studies on CH-Electrochemical workstation CHI660D (USA) instrument. The Linear sweep voltammetric (LSV) curves of Ce/CeO2-rGO shows potential between − 0.5 to 1.1 V vs. SCE in 0.5 M KOH solutions. Once the potential exceeds 0.2 V vs. SCE, a significant decrease in over potential is observed corresponding to oxidative sensing of H2S on Ce/CeO2-rGO with respect to GO, which could be due to over oxidation of H2S and the interaction of H2S molecules at electrified interface respectively.
Accordingly, Figure. 6(a), shows the typical linear sweep voltammetry (LSV) of Ce/CeO2-rGO beyond potential range of -0.5 V to 1.1 V vs. SCE, the increase in the current originates from the oxidation of the H2S. Herein, Fig. 6(a) curve (iii) small oxidation peak observed for the changes in oxidation state of Ce metal i.e. from Ce+ 2 to Ce+ 3. LSV on the prepared Ce/CeO2-rGO in the presence of H2S was performed to examine electrocatalytic sensitivity towards oxidation of H2S. Practically, in 0.5 M KOH, all H2S species converts to HS−, as it follows the first acid dissociation of H2S molecules and schematically shown in scheme 2 and also reported elsewhere for similar systems.53–56Hydrogen sulfide is a weak acid here firstly dissociates H2S gas (H2S ↔ H+ + HS− ↔ 2H+ + S2−). Also the electric potential between H2S and O2 is really high, making it extremely favorable for H2S sensing (oxidation reactions); it may be oxidation of (H2S + 2O2 → SO42− + 2H+) which is catalyzed by metals or metal oxides. Herein, Fig. 6(a) curve ii the current density of surface oxide formation signal decreases along with positive shift in potential could be due to adsorption/interaction of H2S molecules hence hinders further surface oxidation. Here in SI added table of Comparative study of electrochemical H2S sensing.
To further confirm the electrochemical sensing (oxidation), and H2S concentration dependent studies have been carried out shown in Figure. 6(b). Accordingly, it represents with H2S concentrations the oxidation current density increases along with positive shift in potential in the range of 1–5 ppm concentration confirms the H2S sensing at electrified interface of Ce/CeO2-rGO and is diffusion controlled process, when number of HS− ions increases concentration peak are broad appears. Additionally, linearity of concentration vs. current has been determined and it has been shown in SI Figure S3, and conforms the current has been increased linearly with addition of H2S concentration, thus this methodology highly applicable for electrochemical sensing of other toxic gases of H2S class. Also in SI added Figure.S4, Linearity data at Ce/CeO2-rGO for different H2S concentrations Error bar between 1 ppm to 5 ppm.
Furthermore, the influence of the scan rate on the electrocatalytic oxidation peak potential (Epa) and peak current for H2S at 2 ppm concentration on Ce/CeO2-rGO in 0.5 KOH was studied using LSV and as shown in Figure. 6(c). The current density variation along with positive shift in potential values were found tobe increases with an increase in the scan rate from 10 to 100 mV/s and also with concentration corresponding to diffusion controlled electron transfer, while here in current density increases peak was broad observed and is in good agreements with literature.53–57
The typical linear sweep voltammetry in the absence and presence of 2 ppm H2S gas in 0.5 M KOH solution on bare (i) and Ce/CeO2-rGO modified (ii) GC electrodes are shown in Figure. 7. Interestingly, there is no peaks corresponding to the oxidation and reduction of H2S observed on the bare GC electrode. The LSV performance of Ce/CeO2-rGO in 0.5 M KOH without H2S shows oxidation signal at∼-0.2 V vs. SCE for surface oxide formation. Further, Ce/CeO2-rGO in the presence of H2S gas in same solution, additional oxidative signal appeared at 0.2 V vs. SCE corresponding to oxidative sensing of H2S.
Furthermore, the applcablity of sensor the analytical parametrtrs like LOD and LOQ has been calculated and were found to be 0.92 and 9.22 µM respectively as shown in Table number S1. The details of the calculations has been given in supporting information.
2.6 Comparative Electrochemical Studies For Hs And Cogases On Ce/ceo-rgo:
The number of dissolved H2S molecules in electrolytic solutions results into increase in adsorption of the gas molecules on Ce/CeO2-rGO surface produces electro active HS– ions is the further important species represents the qualitative and quantative sensing of H2S. There is a superior linear correlation between the peak current and H2S content in the tested range of 1–5 ppm. In each 20 mL of 0.5 N KOH electrolytes taken and dissolved 1–5 ppm H2S gas separately and was run the sample on given potential for LSV. Moreover, this response also compared with other representative gas i.e. of CO2 and it shows no response in the potential range of H2S oxidation and is shown in Figure.7, further it confirms the selectivity of the sensor in the given range of potential. The results also reveal a new type of low cost Ce/CeO2-rGO hybrid sensor for detection of H2S gas with ultra-high sensitivity and selectivity.
3.1 Presumable Mechanistic Path for Electrochemical H2S Sensing
3.1 Presumable Mechanistic Path for Electrochemical H2S Sensing:
Scheme 3 shows the H2S is first adsorbed on the Ce/CeO2-GO electrode surface, then H2S is dissolved in electrolyte and dissociates further in aqueous electrolytic solutions.58
$${H}_{2}S\left(g\right)\to {H}_{2}S\left(ads\right)$$
$${H}_{2}S\left(ads\right)\to {H}_{2}S\left(ads-liq\right)$$
$${H}_{2}S\left(ads-liq\right)\to H{S}^{-}\left(ads\right)+{H}^{+}$$
The probable reaction mechanism is remarkably different when the Ce/CeO2-rGO is in KOH electrolyte with different pH. When the Ce/CeO2-rGO exposed to high pH i.e. pH > 7, huge number of active sites available, hence chemisorptions dominates and this is the beginning for H2S adsorption.56
$${H}_{2}S\left(g\right)+ KOH\left(q\right)\to KHS\left(q\right)+ {H}_{2}O$$
$$KHS\left(q\right)\leftrightarrow {K}^{+}\left(q\right)+ {HS}^{-}\left(q\right)$$
$${HS}^{-}\left(q\right)\leftrightarrow {HS}^{-}\left(ads\right)$$
However, Ce/CeO2 based nanomaterials have been commonly used in numerous fields including catalysis, adsorption, sensing, H2 production, photo and electrocatalysis, semiconductor devices, fuel cells as well as in biomedical devices. Cerium (Ce) Ce3+, 0.1143 nm; Ce4+, 0.097 nm, favours extensive solubility with the ceria lattice, and it increases the trivalent state of Ce, which may further enhances the activity of ceria. Herein, the electrochemical H2S sensing studies on Ce/CeO2 showing a strong oxidizing agent in association with oxygen atoms. Cerium oxide (CeO2) one of the important transition metal oxides which is acting as n-type semiconductors. It exists as both cerous Ce3+, trivalent state and ceric Ce4+, tetravalent state in the form of compounds. As an electrochemical H2S sensing performance of Ce/CeO2-rGO hybrids is better as compared with reports from literature. For example, Qing et al. reported mono-ethanolamine hydroxyl-functionalized ionic liquid enabled electrochemical sensor for detection of H2S and our report reflects better oxidation performance towards at H2S sensing at very lower detection (ppm) limit.59 The main exclusive characters of CeO2 involve a band gap of 3-3.6 eV and ability to turn their oxidation state in a gas environment and easy path for electron transfer through rGO.