Self-Doped Carbon Dots Decorated TiO 2 Nanorods: A Novel Synthesis Route for Enhanced Photoelectrochemical Water Splitting

: Herein, we have successfully prepared self-doped carbon dots with nitrogen elements (NCD) in a simple one-pot hydrothermal carbonization method, using L-histidine as a new precursor. The effect of as-prepared carbon dots was studied for photoelectrochemical (PEC) water splitting by decorating NCDs upon TiO 2 nanorods systematically by changing the loading time from 2 h to 8 h (TiO 2 @NCD2h, TiO 2 @NCD4h, TiO 2 @NCD6h, and TiO 2 @NCD8h). The successful decorating of NCDs on TiO 2 was conﬁrmed by FE-TEM and Raman spectroscopy. The TiO 2 @NCD4h has shown a photocurrent density of 2.51 mA.cm − 2 , 3.4 times higher than the pristine TiO 2 . Moreover, TiO 2 @NCD4h exhibited 12% higher applied bias photon-to-current efﬁciency (ABPE) than the pristine TiO 2 . The detailed IPCE, Mott–Schottky, and impedance (EIS) analyses have revealed the enhanced light harvesting property, free carrier concentration, charge separation, and transportation upon introduction of the NCDs on TiO 2 . The obtained results clearly portray the key role of NCDs in improving the PEC performance, providing a new insight into the development of highly competent TiO 2 and NCDs based photoanodes for PEC water splitting.


Introduction
Rapidly spiking global energy demands and pollution caused by the depletion of fossil fuels necessitated the development of natural and renewable sources of energy [1]. Hydrogen is an excellent contender capable of replacing fossil fuels owing to its both eco-friendly and reusable nature. Photoelectrochemical (PEC) water splitting is the most reliable and popular method employed for converting solar light energy into clean and sustainable chemical fuels, such as hydrogen [2,3]. The initial study on photocatalytic water splitting using TiO 2 was published way back in 1972 [4]. Since then, different types of semiconductor materials including ZnO, [5] [10], and Ta 3 N [11] were reported as photoelectrodes for PEC. The TiO 2 material is considered as the most competent semiconductor for investigating PEC devices due to its characteristics such as advantageous band-edge positions, ease of fabrication, abundance, excellent photocorrosion resistance, eco-friendliness, and cost effective nature [12]. However, application of TiO 2 in PEC has been constrained by comparatively greater band gaps for its rutile (3.0 eV) and anatase (3.2 eV) phases [12], severe bulk charge recombination, and slow OER kinetics [13]. As a result, numerous attempts were made to surpass the limitations, such as use of dopants [14], formation of heterojunctions [15], surface modification [16], introduction of defects [17], and quantum dot sensitization [15]. 1 and 2. The FE-SEM analysis of pristine TiO2 film (Figure 1a) has shown dense nanorod morphology of TiO2 which have perpendicularly grown on FTO glass showing an average length of ~2.8 μm and width of ~150 nm. Moreover, no obvious changes in the size and morphology of TiO2 were observed in Figure 1b, even after dipping for 8 h in NCDs solution. Further, HR-TEM (Figure 1c) analysis confirmed the nanorod morphology of TiO2. Moreover, the observed lattice fringes' distance in Figure 1d was 0.35 nm, which corresponds to the d-spacings of the rutile TiO2 (101) planes, which has well-matched with XRD results [12]. Further, HRTEM image of TiO2@NCD4h ( Figure 2) showed that NCDs are uniformly loaded on the TiO2 nanorods and appeared in a sphere and ellipsoidal morphology with particle size ranging from 4 to 10 nm. In addition, 0.21 nm lattice spacing was observed for NCDs particle, associated with the (100) facet of NCDs ( Figure S3) [15,18,34]. Moreover, to further investigate the distribution of Ti, O, C, and N elements, the elemental mapping analysis was executed, and the respective results are displayed in Figure 2b. The obtained results have shown even distribution of C and N elements on TiO2 nanorods' surface, suggesting the successful decoration of NCDs on the TiO2.   The crystalline structure of the as-synthesized TiO2 and the effect of NCDs loading time on TiO2 crystallinity (TiO2@NCD2h, TiO2@NCD4h, TiO2@NCD6h, and TiO2@NCD8h) and the orientation growth were examined using XRD analysis. The obtained XRD peaks are displayed in Figure 3. The diffraction peaks of pristine TiO2 films appeared at 36.10°, 41.27°, 54.39°, 62.86°, and 69.80° and correspond to the (101), (111), (211), (002), and (112) crystal planes of tetragonal rutile structure [15,18,35,36]. Moreover, regardless of the loading time of NCDs, the peak positions are the same, but the (101) plane intensity has increased with NCDs loading time. The results suggest that the TiO2 nanorod crystal structure does not get affected by loading NCDs but size of the crystal and preferred orientation directions sparsely get affected. Furthermore, no noticeable diffraction peak of NCDs was observed for TiO2-NCDs, which could be attributed to the modest load of NCDs, lower than the minimum limitation of XRD detection [15,35]. The crystalline structure of the as-synthesized TiO 2 and the effect of NCDs loading time on TiO 2 crystallinity (TiO 2 @NCD2h, TiO 2 @NCD4h, TiO 2 @NCD6h, and TiO 2 @NCD8h) and the orientation growth were examined using XRD analysis. The obtained XRD peaks are displayed in Figure 3. The diffraction peaks of pristine TiO 2 films appeared at 36 [15,18,35,36]. Moreover, regardless of the loading time of NCDs, the peak positions are the same, but the (101) plane intensity has increased with NCDs loading time. The results suggest that the TiO 2 nanorod crystal structure does not get affected by loading NCDs but size of the crystal and preferred orientation directions sparsely get affected. Furthermore, no noticeable diffraction peak of NCDs was observed for TiO 2 -NCDs, which could be attributed to the modest load of NCDs, lower than the minimum limitation of XRD detection [15,35]. As seen in Figure 4, the Raman peaks of TiO2 located at 615.2, 450.5, and 240 cm −1 correspond to (A1g), (Eg), and multi-photon scattering process, respectively, and represent the TiO2 rutile phase. The peaks which appeared at 1580 and 1333 cm −1 can be attributed to the D (disordered sp 2 ) band and G band of NCDs, respectively [37,38]  As seen in Figure 4, the Raman peaks of TiO 2 located at 615.2, 450.5, and 240 cm −1 correspond to (A 1g ), (E g ), and multi-photon scattering process, respectively, and represent the TiO 2 rutile phase. The peaks which appeared at 1580 and 1333 cm −1 can be attributed to the D (disordered sp 2 ) band and G band of NCDs, respectively [37,38]. Thus, the Raman spectrum of TiO 2 @NCD4h has shown five peaks, indicating successful fabrication of NCDs in TiO 2 nanorods. Furthermore, the enhancement of Raman intensity might be contributed by the increased crystallinity, and it is consistent with the XRD results. As seen in Figure 4, the Raman peaks of TiO2 located at 615.2, 450.5, and 240 cm −1 correspond to (A1g), (Eg), and multi-photon scattering process, respectively, and represent the TiO2 rutile phase. The peaks which appeared at 1580 and 1333 cm −1 can be attributed to the D (disordered sp 2 ) band and G band of NCDs, respectively [37,38]. Thus, the Raman spectrum of TiO2@NCD4h has shown five peaks, indicating successful fabrication of NCDs in TiO2 nanorods. Furthermore, the enhancement of Raman intensity might be contributed by the increased crystallinity, and it is consistent with the XRD results. The elemental composition and chemical binding of NCDs decorated on TiO2 catalyst were determined by the XPS analysis ( Figure 5). The survey scans illustrated the elements in two structures, e.g., C, N, Ti, and O elements in TiO2@NCDs4h, whereas C, Ti, and O elements were in the pristine TiO2 [12,39] (Figure 5a). An increase in the carbon content and the presence of N1s peak compared with the bare TiO2 evidently confirm that the NCDs were successfully decorated on TiO2. Ti2p spectra (Figure 5b) showed two representative peaks at 464.0 and 458.4 eV (difference: 5.6 eV), which correspond to the spin-orbit coupling for Ti2p1/2and Ti2p3/2, respectively, and was identical to those for TiO2 [15]. Furthermore,  The elemental composition and chemical binding of NCDs decorated on TiO 2 catalyst were determined by the XPS analysis ( Figure 5). The survey scans illustrated the elements in two structures, e.g., C, N, Ti, and O elements in TiO 2 @NCDs4h, whereas C, Ti, and O elements were in the pristine TiO 2 [12,39] (Figure 5a). An increase in the carbon content and the presence of N1s peak compared with the bare TiO 2 evidently confirm that the NCDs were successfully decorated on TiO 2 . Ti2p spectra (Figure 5b) showed two representative peaks at 464.0 and 458.4 eV (difference: 5.6 eV), which correspond to the spin-orbit coupling for Ti2p1/2and Ti2p3/2, respectively, and was identical to those for TiO 2 [15]. Furthermore, Ti2p peaks of the TiO 2 @NCDs4h structure have considerably shifted (by 0.2 eV) compared to bare TiO 2 ( Figure 5b). This is due to the electronegativity of C/Ns, which increased the binding ability of extra-nuclear electrons, hence raising the binding energy. The C 1s spectra (Figure 5c) is fitted with three peaks corresponding to C-C, C-O & C-N, and C=O & C=N bonds at 284.8, 286.0, and 288.3 eV, respectively [40]. The N1s spectra ( Figure 5d) shows three peaks which appeared at 396.7 eV, 401.4 eV, and 403.7 eV, ascribed to the pyridine-N, pyrrole-N, and Graphitic-N, respectively, indicating the carbon dot doped with the N element [41,42]. In Figure S4, the peak in the O1 s region of TiO 2 @NCDs@4h was deconvoluted into three peaks at 532.3, 530.1, and 529.6 eV, which are assigned to O-H, C-O, or O-N, and Ti-O bonds in TiO 2 @NCDs4h, respectively. In agreement with the above microstructure analysis, XPS results suggest that NCDs were successfully deposited on the TiO 2 [40,42,43].
bonds at 284.8, 286.0, and 288.3 eV, respectively [40]. The N1s spectra ( Figure 5d) shows three peaks which appeared at 396.7 eV, 401.4 eV, and 403.7 eV, ascribed to the pyridine-N, pyrrole-N, and Graphitic-N, respectively, indicating the carbon dot doped with the N element [41,42]. In Figure S4, the peak in the O1 s region of TiO2@NCDs@4h was deconvoluted into three peaks at 532.3, 530.1, and 529.6 eV, which are assigned to O-H, C-O, or O-N, and Ti-O bonds in TiO2@NCDs4h, respectively. In agreement with the above microstructure analysis, XPS results suggest that NCDs were successfully deposited on the TiO2 [40,42,43]. The absorption properties of the materials are important parameters to estimate the light harvesting nature and energy levels to be used in solar energy conversions. The optical properties of the prepared NCDs in solution and on TiO2 photoanode with changing time were analyzed systematically. The UV-vis spectra of NCDs in solution and TiO2@NCDs thin films are shown in Figure 6a,b and c by changing the loading time. The absorption band of pristine TiO2 ~400 nm represents the rutile TiO2 band edge [44]. After introduction of the NCDs on TiO2, the light harvesting property was enhanced with the The absorption properties of the materials are important parameters to estimate the light harvesting nature and energy levels to be used in solar energy conversions. The optical properties of the prepared NCDs in solution and on TiO 2 photoanode with changing time were analyzed systematically. The UV-vis spectra of NCDs in solution and TiO 2 @NCDs thin films are shown in Figure 6a,b and c by changing the loading time. The absorption band of pristine TiO 2~4 00 nm represents the rutile TiO 2 band edge [44]. After introduction of the NCDs on TiO 2 , the light harvesting property was enhanced with the increased amount of NCDs loading. The results suggest the successful decoration of NCDs and their contribution in improving the light harvesting property. Moreover, the Tauc plot Equation (S1) was employed to calculate the bandgap (Eg) of pristine TiO 2 and TiO 2 @NCD photoanodes [36,45,46]. The calculated Eg values of TiO 2 , TiO 2 @NCD2h, TiO 2 @NCD4h, TiO 2 @NCD6h, and TiO 2 @NCD8h were 3.12, 3.07, 3.03, 3.04, and 3.05 eV, respectively.

PEC Performance of the Photoanodes
The effect of newly prepared NCDs on photoelectrochemical water oxidation was studied systematically by changing the loading time of NCDs on TiO2 film and comparing it with the pristine TiO2. Figure 7a shows linear sweep voltammetry performance of TiO2, TiO2@2hNCD, TiO2@4hNCDs, TiO2@6h NCDs, and TiO2@8hNCDs, and their photocurrent data at 1.23 V are illustrated in Table S2. The pristine TiO2 film displayed a 0.73 mA.cm −2 photocurrent at 1.89 V vs. RHE. After loading NCDs upon the TiO2 film, an enhanced photocurrent was observed compared the pristine TiO2, which instigated to perform optimization studies to improve the NCDs loading and thereby achieve optimum PEC performance using TiO2 with NCDs. In order to optimize the TiO2@NCDs photoanodes, the decorated NCDs on TiO2 was controlled by monitoring the hydrothermal reaction. In Figure 7a, the photocurrents of four NCDs decorated TiO2 (TiO2@NCDs)based photoanodes showed higher photocurrent than pristine TiO2, indicating the contribution of NCDs in enhancing PEC performance of TiO2. Significantly, the photoanode corresponding to TiO2@NCD2h has displayed an improved photocurrent density of 2.33

PEC Performance of the Photoanodes
The effect of newly prepared NCDs on photoelectrochemical water oxidation was studied systematically by changing the loading time of NCDs on TiO 2 film and comparing it with the pristine TiO 2 . Figure 7a shows linear sweep voltammetry performance of TiO 2 , TiO 2 @2hNCD, TiO 2 @4hNCDs, TiO 2 @6h NCDs, and TiO 2 @8hNCDs, and their photocurrent data at 1.23 V are illustrated in Table S2. The pristine TiO 2 film displayed a 0.73 mA.cm −2 photocurrent at 1.89 V vs. RHE. After loading NCDs upon the TiO 2 film, an enhanced photocurrent was observed compared the pristine TiO 2 , which instigated to perform optimization studies to improve the NCDs loading and thereby achieve optimum PEC performance using TiO 2 with NCDs. In order to optimize the TiO 2 @NCDs photoanodes, the decorated NCDs on TiO 2 was controlled by monitoring the hydrothermal reaction. In Figure 7a, the photocurrents of four NCDs decorated TiO 2 (TiO 2 @NCDs)-based photoanodes showed higher photocurrent than pristine TiO 2 , indicating the contribution of NCDs in enhancing PEC performance of TiO 2 . Significantly, the photoanode corresponding to TiO 2 @NCD2h has displayed an improved photocurrent density of 2.33 mA.cm −2 at 1.89 V vs. RHE, while pristine TiO 2 photoanode has shown 0.73 mA.cm −2 at 1.89 V vs. RHE. Further, by increasing the loading time from 2 h to 4 h (TiO 2 @NCD4h), the photocurrent density has also increased to 2.51 mA.cm −2 at 1.89 V vs. RHE, which was 3.4 times greater than the pristine TiO 2 . Moreover, TiO 2 @NCD4h photoanode possesses both enhanced photocurrent density and smaller onset potential than pristine TiO 2 . The higher photoresponse of NCD decorated TiO 2 might be due to the addition of NCD which could effectively promote the separation of photogenerated electron-hole, and promote the capture of water molecules and intermediates in the process of water decomposition by electrons and holes at the interfaces [15,47,48]. However, further increasing the NCDs loading by increasing loading time to 6 h (TiO 2 @NCD6h) and 8 h (TiO 2 @NCD8h) showed declined photocurrent density of 1.99 and 1.85 mA.cm −2 at 1.89 V vs. RHE, respectively. In addition, onsite potentials also increased compared to the TiO 2 @NCD4h-based films, possibly due to the variation in conductivity by decorated NCDs. This phenomenon will be further discussed in electrochemical impedance spectroscopy (EIS) section [29].
1.89 V vs. RHE. Further, by increasing the loading time from 2 h to 4 h (TiO2@NCD4h), the photocurrent density has also increased to 2.51 mA.cm −2 at 1.89 V vs. RHE, which was 3.4 times greater than the pristine TiO2. Moreover, TiO2@NCD4h photoanode possesses both enhanced photocurrent density and smaller onset potential than pristine TiO2. The higher photoresponse of NCD decorated TiO2 might be due to the addition of NCD which could effectively promote the separation of photogenerated electron-hole, and promote the capture of water molecules and intermediates in the process of water decomposition by electrons and holes at the interfaces [15,47,48]. However, further increasing the NCDs loading by increasing loading time to 6 h (TiO2@NCD6h) and 8 h (TiO2@NCD8h) showed declined photocurrent density of 1.99 and 1.85 mA.cm −2 at 1.89 V vs. RHE, respectively. In addition, onsite potentials also increased compared to the TiO2@NCD4h-based films, possibly due to the variation in conductivity by decorated NCDs. This phenomenon will be further discussed in electrochemical impedance spectroscopy (EIS) section [29]. Chronoamperometric analysis was performed under chopped illumination for all the prepared electrodes at 1.89 V vs. RHE for 30 s to better understand photo response with time and stability. As depicted in Figure 7b, the photocurrent has rapidly increased immediately after illumination and sharply fell to zero upon stopping the illumination. It confirms that the prepared photoanodes have a well-reproducible photocurrent. Meanwhile, after decorating NCDs on TiO2, the response speed was boosted compared with the pristine TiO2, indicating that the presence of NCDs can significantly reduce the charge Chronoamperometric analysis was performed under chopped illumination for all the prepared electrodes at 1.89 V vs. RHE for 30 s to better understand photo response with time and stability. As depicted in Figure 7b, the photocurrent has rapidly increased immediately after illumination and sharply fell to zero upon stopping the illumination. It confirms that the prepared photoanodes have a well-reproducible photocurrent. Meanwhile, after decorating NCDs on TiO 2 , the response speed was boosted compared with the pristine TiO 2 , indicating that the presence of NCDs can significantly reduce the charge recombination in the intersection of electrolyte and photoanode surface. The highest photocurrent density was detected for the TiO 2 @NCD4h, which is consistent with the observed LSV results. In order to understand the durability of the prepared electrode, 1 h of continuous illuminated chronoamperometric analysis was performed with a high performing photoanode (TiO 2 @NCD4h) by comparing with the pristine TiO 2 based photo anode at same experimental condition. As displayed in Figure 7c, after continuous illumination for 1 h, TiO 2 @NCD4h has retained 99% of its initial activity, which supports the excellent stability of NCDs decorated TiO 2 photo anode. The current densities have matched well with the Catalysts 2022, 12, 1281 9 of 14 LSV data. Both TiO 2 and TiO 2 @NCD4h showed excellent stability after 1 h of continuous irradiation without any photocurrent decay.
The incident photon-to-current conversion efficiency (IPCE) was evaluated to analyze the contribution of various photons in obtaining solar photocurrent. The IPCE has been deduced by using the formula (1): where, P(λ), λ, and J(λ) are the intensity of a specific wavelength, wavelength of incident light, and photocurrent density at specific wavelength, respectively. Figure 8a shows enhanced IPCE after decorating NCDs on TiO 2 with highest IPCE of 29.76% at~390 nm for TiO 2 @NCD4h, which was~3 times greater than the pristine TiO 2 (9.76%). The IPCE trend was consistent with the obtained photocurrent density and the enhanced IPCE region is in good agreement with the optical absorption properties. The improved IPCE after the introduction of NCDs to the TiO 2 reveals the contribution of NCDs in obtaining enhanced photocurrent density.
recombination in the intersection of electrolyte and photoanode surface. The highest photocurrent density was detected for the TiO2@NCD4h, which is consistent with the observed LSV results. In order to understand the durability of the prepared electrode, 1 h of continuous illuminated chronoamperometric analysis was performed with a high performing photoanode (TiO2@NCD4h) by comparing with the pristine TiO2 based photo anode at same experimental condition. As displayed in Figure 7c, after continuous illumination for 1 h, TiO2@NCD4h has retained 99% of its initial activity, which supports the excellent stability of NCDs decorated TiO2 photo anode. The current densities have matched well with the LSV data. Both TiO2 and TiO2@NCD4h showed excellent stability after 1 h of continuous irradiation without any photocurrent decay. The incident photon-to-current conversion efficiency (IPCE) was evaluated to analyze the contribution of various photons in obtaining solar photocurrent. The IPCE has been deduced by using the formula (1): where, P(λ), λ, and J(λ) are the intensity of a specific wavelength, wavelength of incident light, and photocurrent density at specific wavelength, respectively. Figure 8a shows enhanced IPCE after decorating NCDs on TiO2 with highest IPCE of 29.76% at ~390 nm for TiO2@NCD4h, which was ~3 times greater than the pristine TiO2 (9.76%). The IPCE trend was consistent with the obtained photocurrent density and the enhanced IPCE region is in good agreement with the optical absorption properties. The improved IPCE after the introduction of NCDs to the TiO2 reveals the contribution of NCDs in obtaining enhanced photocurrent density. Besides, the applied bias photon-to-current efficiency (ABPE) has been calculated by using the Equation (2): where P, J, and V are the power density of incident light (100 mW cm −2 ), photocurrent density (mA cm −2 ), and the applied bias (V vs. RHE), respectively. As seen in Figure 8b, the pristine TiO2 reached maximum 0.11% ABPE at 0.88 V vs. RHE, while TiO2@NCD4h reached 1.37% photo conversion efficiency at 0.36 V vs. RHE, 12 times greater than the pristine TiO2 ABPE, suggesting an effective electron-hole pairs separation after the introduction of NCDs [12].
Besides, the applied bias photon-to-current efficiency (ABPE) has been calculated by using the Equation (2): where P, J, and V are the power density of incident light (100 mW cm −2 ), photocurrent density (mA cm −2 ), and the applied bias (V vs. RHE), respectively. As seen in Figure 8b, the pristine TiO 2 reached maximum 0.11% ABPE at 0.88 V vs. RHE, while TiO 2 @NCD4h reached 1.37% photo conversion efficiency at 0.36 V vs. RHE, 12 times greater than the pristine TiO 2 ABPE , suggesting an effective electron-hole pairs separation after the introduction of NCDs [12].
To further comprehend the interfacial charge transfer kinetics at the intersection of photoanode and electrolyte, EIS was employed under illumination and the respective Nyquist plots are displayed in Figure 9a. The decreased radius order of semi-circle was TiO 2 > TiO 2 @NCD8h > TiO 2 @NCD6h > TiO 2 @NCD2h > TiO 2 @NCD4h. The smallest arc of the TiO 2 @NCD4h compared with its counter parts demonstrates the improved interfacial charge transfer kinetics due to the introduction of NCDs [18]. Furthermore, using Zview software program, the EIS curves have been fitted with an analogous circuit model given in Figure 9a inset, where CPE, Rct, and Rs indicate the constant phase element, charge transfer resistance, and series resistance at the electrolyte/electrode interface, respectively. The observed Rct values of TiO 2 and NCDs decorated TiO 2 films (2 h-8 h) were 376.0 Ω, 273.6 Ω, 248.3 Ω, 277.2 Ω, and 297.4 Ω, respectively. The lowest R ct value of TiO 2 @NCD4h further demonstrates the advantage of NCDs decorated TiO 2 nanorods in enhancing the charge separation and transfer kinetics.
Nyquist plots are displayed in Figure 9a. The decreased radius order of semi-circle was TiO2 > TiO2@NCD8h > TiO2@NCD6h > TiO2@NCD2h > TiO2@NCD4h. The smallest arc of the TiO2@NCD4h compared with its counter parts demonstrates the improved interfacial charge transfer kinetics due to the introduction of NCDs [18]. Furthermore, using Zview software program, the EIS curves have been fitted with an analogous circuit model given in Figure 9a inset, where CPE, Rct, and Rs indicate the constant phase element, charge transfer resistance, and series resistance at the electrolyte/electrode interface, respectively. The observed Rct values of TiO2 and NCDs decorated TiO2 films (2 h-8 h) were 376.0 Ω, 273.6 Ω, 248.3 Ω, 277.2 Ω, and 297.4 Ω, respectively. The lowest Rct value of TiO2@NCD4h further demonstrates the advantage of NCDs decorated TiO2 nanorods in enhancing the charge separation and transfer kinetics. Mott-Schottky analyses have been executed to estimate the energy band position of pristine TiO2 and TiO2@NCDs, and the corresponding curves are displayed in Figure  9b and the data are depicted in Table S1. The positive slope of both curves indicates ntype semiconductor of TiO2 [18]. The flat band (VFB) potential can be calculated following the eq. (S1). The obtained VFB values of TiO2 and TiO2@NCDs (2 h, 4 h, 6 h, and 8 h) were 0.17, 0.27, 0.34, 0.22, and 0.19 V vs. NHE, respectively, which could be accomplished by the X-axis intercept. Moreover, NCDs decorated TiO2 films have shown decreased VFB than pristine TiO2, suggesting an increased band bending of the photoanode, favorable to enhance the charge transfer between the photoanode interfaces and electrolyte. Eventually, enhanced PEC was observed for NCDs decorated TiO2 films. As per the available Mott-Schottky analyses have been executed to estimate the energy band position of pristine TiO 2 and TiO 2 @NCDs, and the corresponding curves are displayed in Figure 9b and the data are depicted in Table S1. The positive slope of both curves indicates n-type semiconductor of TiO 2 [18]. The flat band (V FB ) potential can be calculated following the Equation (S1). The obtained V FB values of TiO 2 and TiO 2 @NCDs (2 h, 4 h, 6 h, and 8 h) were 0.17, 0.27, 0.34, 0.22, and 0.19 V vs. NHE, respectively, which could be accomplished by the X-axis intercept. Moreover, NCDs decorated TiO 2 films have shown decreased V FB than pristine TiO 2 , suggesting an increased band bending of the photoanode, favorable to enhance the charge transfer between the photoanode interfaces and electrolyte. Eventually, enhanced PEC was observed for NCDs decorated TiO 2 films. As per the available literature [49,50], the bottom of the conduction band (CB) was −0.1 V lower than the V FB of an n-type semiconductor [50]. Therefore, the CB of TiO 2 and TiO 2 @NCDs were estimated to be positioned at less than 0.1 V of their V FB [33]. Based on the V FB , the CB edge of TiO 2 and TiO 2 @NCDs (2 h, 4 h, 6 h, and 8 h) were determined to be at 0.07, 0.17, 0.24, 0.12, and 0.09 V, respectively. Particularly, doping of NCDs promotes a downward shift in energy levels towards higher potentials and enhances the carrier density in TiO 2 [51].

Preparation of Rutile TiO 2 Film (TiO 2 ):
The FTO coated glasses (1.5 mm × 2.5 mm, 8 Ω/cm 2 ) were cleaned ultrasonically using detergent, milli-Q water, ethanol, and acetone for 1 h, respectively. TiO 2 film was synthesized by following a reported hydrothermal method with certain modifications [18]. Under continuous stirring, 0.33 mL titanium (IV) butoxide was added dropwise to 20 mL equal volumes of HCl (35%) and milli-Q water mixed solution until it turned translucent. The solution was then moved to a 50 mL autoclave lined with Teflon, and the FTO glass was placed against the walls of Teflon vessel, conducted side down, for 12 h and heated to 150 • C. The TiO 2 layer was completely cleaned with milli-Q water and ethanol after cooling to RT, before being sintered in air at 450 • C for 1 h.

Preparation of TiO 2 @NCDs:
NCDs have been prepared using a hydrothermal approach (Scheme 1). First, 0.2 g of L-histidine was included in 20 mL mixture of milli-Q water and HCl in the ratio of 19:1. The solution was shifted to 50 mL autoclave, and two TiO 2 films on FTO glasses were inserted in the Teflon vessel with the TiO 2 side facing down. Then, the hydrothermal reaction was performed at 180 • C for 2, 4, 6, 8, and 10 h. The samples were denoted as TiO 2 @NCD2h, TiO 2 @NCD4h, TiO 2 @NCD6h, TiO 2 @NCD8h, respectively ( Figures S1 and S2). The TiO 2 @NCD films were extensively washed with milli-Q H 2 O and ethanol upon cooling to RT. Then, copper wires and as prepared photoanodes were adhered using silver paint. The samples were air dried for 3 h. Finally, the samples were encased by nonconductive epoxy with the illuminated area of 1 cm 2 and left to rest in air for at least 3 h.
of an n-type semiconductor [50]. Therefore, the CB of TiO2 and TiO2@NCDs were estimated to be positioned at less than 0.1 V of their VFB [33]. Based on the VFB, the CB edge of TiO2 and TiO2@NCDs (2 h, 4 h, 6 h, and 8 h) were determined to be at 0.07, 0.17, 0.24, 0.12, and 0.09 V, respectively. Particularly, doping of NCDs promotes a downward shift in energy levels towards higher potentials and enhances the carrier density in TiO2 [51].

Preparation of Rutile TiO2 Film (TiO2):
The FTO coated glasses (1.5 mm 2.5 mm, 8 Ω/cm 2 ) were cleaned ultrasonically using detergent, milli-Q water, ethanol, and acetone for 1 h, respectively. TiO2 film was synthesized by following a reported hydrothermal method with certain modifications [18]. Under continuous stirring, 0.33 mL titanium (IV) butoxide was added dropwise to 20 mL equal volumes of HCl (35%) and milli-Q water mixed solution until it turned translucent. The solution was then moved to a 50 mL autoclave lined with Teflon, and the FTO glass was placed against the walls of Teflon vessel, conducted side down, for 12 h and heated to 150 °C. The TiO2 layer was completely cleaned with milli-Q water and ethanol after cooling to RT, before being sintered in air at 450 °C for 1 h.

Preparation of TiO2@NCDs:
NCDs have been prepared using a hydrothermal approach (Scheme 1). First, 0.2 g of L-histidine was included in 20 mL mixture of milli-Q water and HCl in the ratio of 19:1. The solution was shifted to 50 mL autoclave, and two TiO2 films on FTO glasses were inserted in the Teflon vessel with the TiO2 side facing down. Then, the hydrothermal reaction was performed at 180 °C for 2, 4, 6, 8, and 10 h. The samples were denoted as TiO2@NCD2h, TiO2@NCD4h, TiO2@NCD6h, TiO2@NCD8h, respectively (Figures S1 and S2). The TiO2@NCD films were extensively washed with milli-Q H2O and ethanol upon cooling to RT. Then, copper wires and as prepared photoanodes were adhered using silver paint. The samples were air dried for 3 h. Finally, the samples were encased by nonconductive epoxy with the illuminated area of 1 cm 2 and left to rest in air for at least 3 h.

Conclusions
In conclusion, the new NCDs were successfully prepared in a simple one-pot hydrothermal synthesis method using L-histidine as an initial precursor and the as-prepared NCDs were decorated on the TiO 2 nanorod-based photoanode. The as-prepared NCDs and NCD decorated TiO 2 nanorods were well characterized using FE-SEM, HR-TEM, EDS elemental mapping, which revealed the nanorod morphology of TiO 2 and uniform distribution of CDs on TiO 2 surface while, XPS and Raman analyses have confirmed the successful self nitrogen element doping, preparation and decorating of NCDs on TiO 2 nanorod. The effect of NCDs decorated TiO 2 was tested for photoelectrochemical water splitting analysis systematically by changing the loading time of NCDs from 2 h to 8 h. The highest efficiency was observed for the TiO 2 @NCD4h-based photoanode (2.51 mA.cm −2 ), which was a 3.4 times higher photocurrent density than the pristine TiO 2 -based photoanode. It might be attributed to the increased light harvesting property with charge separation and transportation. The observed IPCE of TiO 2 @NCD4h has shown 3 times higher quantum yield (29.76%) than pristine TiO 2 (9.76). In addition, the calculated ABPE was 12% higher for TiO 2 @NCD4h than the pristine TiO 2 , which revealed the enhanced light harvesting property of photoanodes upon loading the NCDs. Moreover, the reduced charge transfer resistance and higher charge carrier density, as observed from EIS and Mott-Schottky analyses, respectively, further support the advantage of newly prepared NCDs in enhancing the PEC performance by promoting effective charge separation and transportation. This study may open up new insights into the rational design and synthesis of highly efficient photoanodes for PEC water splitting.

Data Availability Statement:
The data presented in this study are available upon request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.