# Sulfur-Doped Carbon Nanohorn Bifunctional Electrocatalyst for Water Splitting
## 3 Results and Discussion  
The preparation of S-doped CNHs was accomplished by treatment of oxidized CNHs (ox-CNHs) with Lawesson's reagent, which was used as sulfur source, in the autoclave reactor, as shown in Figure 1. The doping of S atoms occurs mostly at the conical tips of ox-CNHs, where carbonyl groups are abundantly present. The doping mechanism can be briefly described as follows: initially, thioketone groups are formed, and afterwards, S atoms replace C atoms in the graphitic network [20; 21]. ATR-IR and Raman spectra give proof of the successful realization of S-doped CNHs. Specifically, while the IR spectrum of ox-CNHs is governed by the characteristic carbonyl-related band at 1705 cm\({}^{-1}\) due to stretching vibrations of the carboxylic acid moieties, the corresponding IR spectrum due to S-CNHs shows the decrease of the carbonyl band and the evolution of a new broad band at 1080 cm\({}^{-1}\) related to C-S vibrations (Figure 2a). On the other hand, the Raman spectrum of pristine CNHs exhibits the characteristic D-band at 1341 cm\({}^{-1}\), associated with defects, and the G-band at 1591 cm\({}^{-1}\), related to the sp\({}^{2}\) graphitic network. In the Raman spectrum of ox-CNHs, an increase of the intensity ratio I\({}_{\text{D/G}}\) compared to pristine CNHs from 1.3 to 1.7, due to the change of hybridization from sp\({}^{2}\) to sp\({}^{3}\) upon oxidation, is evident. Concerning S-CNHs, there is a decrease of the I\({}_{\text{D/G}}\) to 1.0 (Figure 2b) due to S-doping and partial restoration of the graphitic network [31]. Additionally, there is a shift of the G-band to lower frequencies by 8 cm\({}^{-1}\), which is indicative of n-doping in S-CNHs [20]. Figure 1: Illustrative preparation of S-CNHs. Figure 2c shows the thermographs of pristine CNHs, ox-CNHs and S-CNHs that were recorded under nitrogen atmosphere. Pristine CNHs present an almost stable thermal profile up to 800 \({}^{\circ}\)C, while ox-CNHs show a 9% mass loss up to 500 \({}^{\circ}\)C due to the thermal decomposition of the carboxylic groups. Conversely, this is not shown in the thermograph of S-CNHs, where the thermal mass loss starts after 330 \({}^{\circ}\)C. The mass loss at higher temperatures is attributed to the thermal decomposition of the graphitic framework at sp\({}^{3}\) defects close to where sulfur doping takes place. HR-TEM imaging and XPS were employed to determine the morphology and the chemical composition and elemental content of S-CNHs. In more detail, HR-TEM micrographs (Figure 3a,b) show the characteristic morphology of those nanostructures and their aggregates' organization. In the XPS survey spectrum of S-CNHs, characteristic peaks due to S\({}_{\text{2p}}\), C\({}_{\text{1s}}\), O\({}_{\text{1s}}\) located at binding energies of ca. 164, 285 and 533, respectively, are evident (Figure 3c). Moreover, the S content was determined to be 3%, higher than that reported for S-doped graphene [31] and previously prepared S-doped CNHs [26]. To probe the chemical state of sulfur in S-CNHs high-resolution S\({}_{\text{2p}}\) peaks were analyzed (Figure 3d). The S\({}_{\text{2p}}\) peaks can be resolved into four peaks at binding energies of 162.2 (8.4%), 164.1 (70.6%), 165.3 (11.3%) and 168.4 (9.8%) eV, respectively. While the second and third deconvoluted peaks are related to sulfur bound as heteroatoms to aromatics, such as thiophene [31; 32], the peaks at 162.2 and 168.4 eV arise from thiol and sulfone components [31; 32]. The high percentage of the peaks corresponding to C-S related bonds confirms that doping of sulfur atoms happens mostly at the conical tips of CNHs where five-membered rings are located [16]. Next, evaluating the electrocatalytic properties of S-CNHs against OER, linear sweep voltammetry (LSV) measurements were performed in O\({}_{\text{2}}\) saturated 1.0 M aqueous KOH. A very important factor for evaluating the performance of OER electrocatalysts is the onset potential.