Solar Driven 15.7% Hydrogen Conversion by Harmony of Light Harvesting, Electron Transporting Bridge, and S‐Defection in a Self‐Assembled Microscale CuS/rGO/CP Photoanode

CuS is an encouraging photoelectrode candidate that meets the essential requirements for efficient solar‐to‐hydrogen production, but it has not been thoroughly studied. A CuS light absorber layer is grown by the self‐assembly of copper and sulfur precursors on a carbon paper (CP) electrode. Simultaneously, rGO is introduced as a buffer layer to control the optical and electrical properties of the absorber. The well‐ordered microstructural arrangement suppresses the recombination loss of electrons and holes owing to enhanced charge‐carrier generation, separation, and transport. The potential reaching 10 mA cm−2 in 1.0 m KOH solution is significantly lowered to 0.87 V, and the photocurrent density at 1.23 V is 94.7 mA cm−2. The computational result reveals that the potential‐determining step is sensitive to O* stability; the lower stability of O* in the thin layer of CuS/rGO decreases the free‐energy gap between the initial and final states of the potential‐determining step, resulting in a lowering of the onset potential. The faradaic efficiency for the photoelectrochemical oxygen evolution reaction in the optimized 2CuS/1rGO/CP photoanode is 98.60%, and the applied bias photon‐to‐current and the solar‐to‐hydrogen efficiencies are 11.2% and 15.7%, respectively, and its ultra‐high performance is maintained for 250 h. These record‐breaking achievement indices may be a trigger for establishing a green hydrogen economy.


Introduction
Importance of hydrogen energy in the global industrial community, which is preparing to meet the goals of carbon neutrality and 100% renewable energy (RE100) by the year 2050, is emphasized on a daily basis.Although the thermodynamic energy required to decompose water and produce hydrogen is 1.23 V, actual water-splitting requires a higher energy of approximately 1.9 V because of the overpotential for the oxygen evolution reaction (OER) in the electrochemical cell and the resistances introduced by cell assembly. [1]To overcome this electrochemical overpotential, researchers have attempted to induce an electrochemical water-splitting reaction under sunlight, [2] utilizing photoelectrochemical (PEC) systems that combine light and electric energy.PEC systems have the advantages of high theoretical efficiency, device simplicity, low cost, and no additional required separation of products. [3]ven though, the biggest reason that such systems have not yet been commercialized is that photoelectrode corrosion is accelerated by the high energy required for water splitting and the necessity of immersing the electrode into the electrolyte. [4]We all know that it is not easy to develop a stable photoelectrode with high resistance to photocorrosion while maintaining a band position suitable for the watersplitting oxidation-reduction reaction.Although a method of coating the electrode surface with a protective film is sometimes used to prevent photocorrosion, it is difficult to find a conductive electrode material that can exist safely in acidic or alkaline solutions for the desired period of time.
Until now, PEC hydrogen and oxygen production using p-type semiconductor electrodes has not been studied as extensively as that using n-type semiconductor electrodes.In particular, p-type copperbased oxide photoanodes exhibit relatively good conductivity but are unstable in aqueous solutions under sunlight. [5]Many studies were reported on photoelectrodes based on CuO, [6] metal-or nonmetaldoped CuO, [7] and CuO-containing heterojunction structures [8] in PEC water-splitting.However, in terms of the electronic structures, defects, bandgaps, and hierarchical structures of CuO-based photoanodes in PEC water splitting, many researchers have critically evaluated their use. [9,10]Nonoxide-based p-type CuS, another copper-based CuS is an encouraging photoelectrode candidate that meets the essential requirements for efficient solar-to-hydrogen production, but it has not been thoroughly studied.A CuS light absorber layer is grown by the self-assembly of copper and sulfur precursors on a carbon paper (CP) electrode.Simultaneously, rGO is introduced as a buffer layer to control the optical and electrical properties of the absorber.The well-ordered microstructural arrangement suppresses the recombination loss of electrons and holes owing to enhanced charge-carrier generation, separation, and transport.The potential reaching 10 mA cm −2 in 1.0 M KOH solution is significantly lowered to 0.87 V, and the photocurrent density at 1.23 V is 94.7 mA cm −2 .The computational result reveals that the potential-determining step is sensitive to O* stability; the lower stability of O* in the thin layer of CuS/rGO decreases the free-energy gap between the initial and final states of the potential-determining step, resulting in a lowering of the onset potential.The faradaic efficiency for the photoelectrochemical oxygen evolution reaction in the optimized 2CuS/1rGO/ CP photoanode is 98.60%, and the applied bias photon-to-current and the solar-to-hydrogen efficiencies are 11.2% and 15.7%, respectively, and its ultrahigh performance is maintained for 250 h.These record-breaking achievement indices may be a trigger for establishing a green hydrogen economy.
semiconductor, is reported to have a stronger visible-light absorption capacity than CuO, and thus has been applied in various fields. [11,12]owever, because of its low stability in aqueous solutions and photocorrosion, it has not yet been used in PEC systems.Moreover, the band position of CuS is located such that photogenerated electrons participate in photocorrosion by reducing themselves, rather than participating in the hydrogen generation reaction. [13]In conclusion, it is unreasonable to apply CuS to the PEC-HER system as a photocathode.
Thus, this study proposes the use of a p-type CuS-based photoanode as a strategy to overcome the limitations of the photoelectrochemical OER.To improve the light stability of p-CuS, it is combined with reduced graphene oxide (rGO).Additionally, to match the energy band position with the p-CuS/rGO junction particle, a carbon paper (CP) electrode is used as the base electrode, and junction particles are grown directly thereon.Here, we explore two possibilities.First, rGO with abundant electrons is junctioned to the VB of the CuS, where it donates electrons to fill the holes in the VB continuously, thus raising the VB position.As a result, much electrons are gathered in the CB of CuS and eventually transferred to an external circuit, which is expected to increase the generation of hydrogen at the counter electrode.The second is that the defective rGO is junctioned to the CB of CuS; in this case, the electrons excited from the VB of CuS naturally migrate to rGO via CuS-CB, and these electrons are transferred to the counter electrode through an external circuit.This is expected to increase the adsorption of OH − in the holes in CuS-VB, and eventually promote oxygen generation.
The synthesis sequences of the powdered CuS/rGO junction particles or electrodes are shown in detail in Scheme S1, Supporting Information, and the EC-OER and PEC-OER evaluation methods and computational details are presented in Scheme S2 and Figure S1, Supporting Information, respectively.

Physicochemical Properties of xCuS/yrGO Powders and Electrodes
The XRD patterns of the rGO, CuS, 1CuS/2rGO, 1CuS/1rGO, and 2CuS/1rGO powder samples are shown in Figure 1a.The (002) and (001) characteristic diffraction peaks of rGO were broadly observed at 2θ values of 25 and 43°. [14]Although the peaks corresponding to rGO in the junctioned samples were not clearly observed, the presence of rGO could be predicted because of the baseline exhilaration; greater baseline destabilization implied a higher amount of junctioned rGO.Peaks corresponding to the (100), (101), (102), (103), (106), (110), (108), and (116) diffraction planes were observed at 2θ = 27.2,27.7, 29.3, 32.9, 33.8, 47.9, 52.8, and 59.4°, respectively, which converge on the hexagonal covellite CuS crystal (ICDD no.01-078-0876). [15]In the XRD patterns of samples in which rGO and CuS were junctioned, no new peaks were generated or CuS peaks shifted despite the junction of rGO.This is evidence that the junction of rGO does not affect the crystal structure of CuS.However, the much rGO junctions, the wider the full width at half maximum (FWHM) of CuS peaks.According to the Debey-Scherrer equation, FWHM = Kλ/Lcosθ (L is the crystal size, θ is the peak value obtained from analysis, K is a constant value, and λ is the wavelength used in the XRD analysis), the FWHM and crystal size have an inversely proportional relationship. [16]Therefore, the fact that the FWHM of CuS peaks broadens as the junctions of rGO increases means that the grown crystal size is small.Perhaps this is because the CuS crystal seeds were dispersed far from each other as rGO was much junctioned, resulting in less agglomeration between CuS seeds.On the contrary, when the rGO is less junctioned, since the CuS crystal seeds are more closely located, the agglomeration between the CuS seeds becomes stronger and the size of the grown CuS crystal increases, resulting in a sharp FWHM in the XRD peak.The Raman spectra of rGO, CuS, and 2CuS/1rGO, are shown in Figure 1b-1.It is well known that the primitive CuS unit cell contains 12 atoms, six each of copper and sulfur, in a covellite CuS hexagonal crystal system (P63/mmc), [17] and a total of 36 vibrational modes are possible.Of these, only 14 are Ramanactive modes, Γ Raman = 2A 1g + 4E 2g + 2E 1g , which can be classified as ΓCu 1 = A 1g + E 2g + E 1g and ΓCu 2 = E 2g for copper and ΓS 1 = A 1g + E 2g + E 1g , and ΓS 2 = E 2g for sulfur. [18]For CuS and 2CuS/1rGO, the recorded spectra exhibited less prominent peaks at 265 cm −1 and primary sharp peaks at 474 cm −1 .The former is typical of crystalline CuS, but the latter peak is attributed to the S-S stretching mode of S 2À 2 , which is known to be a specific peak in the spectrum of Cu 2 S. [19] Therefore, Cu + and Cu 2+ are coexisted in CuS.In particular, as the S-S peak increased in intensity after the junction compared to the Cu-S peak, it was determined that Cu + was more abundant than Cu 2+ in CuS after the junction.In rGO, the D band at 1331 cm −1 represents the defect of sp 2 graphite, and the G band at 1592 cm −1 represents the characteristics of sp 2 graphite.By comparing the D band value and the G band value, if I D /I G is >1.0 it means a carbon structure with many defects.It suggests that the degree of defects is reduced, as the I D /I G ratio decreases as more CuS is junctioned on the rGO (Figure 1b-2). [20]Bands converging to CuS and rGO were also observed in the junctioned 2CuS/1rGO sample, and the intensity ratios (I D /I G ) of the D and G bands in pure rGO and 2CuS/1rGO were 1.16 and 1.07, respectively.These numbers are significantly higher than the reference value of 0.84 for typical graphene oxide (GO), [21] which may be related to the structural defects of GO generated during hydrothermal processes.However, compared to the value from pure rGO, the I D /I G value is smaller in junctioned 2CuS/ 1rGO, suggesting that the carbon defects in rGO are reduced by CuS junction.It can be predicted that defects in rGO were repaired by grafting CuS to the torn or damaged parts of the rGO sheet.Figure 1c shows the SEM images of rGO, CuS, and CuS/rGO junctioned compounds.rGO shows a sheet structure in the form of a thin cloth, and CuS has a shape similar to that of a dandelion flower formed by the self-assembly of thin panels to a structure of 10 μm in size.In the image of the CuS/ rGO junction particles, it can be seen that the flower-shaped CuS microparticles were uniformly distributed on the rGO sheet.The average diameter of the CuS flower-shaped particles loaded on rGO was approximately 3-5 μm, which was smaller than that for separate CuS particles.It is presumed that rGO served as a template when they were formed, such that the CuS seeds were distributed somewhat more closely on the rGO sheet template and reached smaller sizes as they grew competitively; the larger the CuS concentration, the smaller the size.
The activity of the catalyst has a strong correlation with the specific surface area, and in general, the larger the specific surface area, the higher the catalyst activity. [22]Compared to 8.45 m 2 g −1 of CuS, the specific surface area of 2CuS/1rGO junction particles increased to 14.0 m 2 g −1 , which provides many active sites enhancing catalytic performance (Figure S2, Supporting Information). [23]Around 13% of O atoms in EDS result were present in rGO, indicating defect sites in rGO.

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Sulfur defects in CuS in 1CuS/2rGO, 1CuS/1rGO, and 2CuS/1rGO junction particles were 22.32%, 29.89%, and 28.57%, respectively, and carbon defects in rGO were 11.13%, 8.59%, and 7.22%, respectively (Figure S3, Supporting Information).The XPS results show that two Cu oxidation states, +1 and +2, coexist in CuS (Figure S4, Supporting Information).In CuS/rGO junction particles, the Cu 2p peak shifted toward higher binding energy, and the peak area converging to the +2 oxidation state became larger.This means that electrons in Cu atoms moved toward rGO due to junctions, or S vacancies were created in the CuS lattice. [24]This result supports the results of Raman spectra that the existence of isolated S-S on the surface due to the partial exposure of S from the CuS crystal or that the exposed S is oxidized and exists as SO 2À 4 .Xu et al. claimed that the SO 2À 4 group implying an S defect could break the adsorption energy between the reaction intermediates OH* and OOH* and reduce the OER overpotential. [25]In pure rGO with mostly sp 2 C=C structures, the abundances of C=O and C-OH peaks were similar.However, after junction, the C=O peak was greatly reduced compared to the C-OH peak.This is because the CuS grains were grown on the C=O defects of rGO.This is in line with the result that defects in rGO are reduced by CuS junctions in Raman spectra.In UV-visible absorption spectra, CuS absorbed orange light around 650 nm, and rGO absorbed all wavelengths in the test range (Figure S5a, Supporting Information).The band gap of CuS calculated by the Kubelka-Munk plot was about 1.36 eV (Figure S5b, Supporting Information), and it was predicted that all particles were p-type semiconductors from the inversely proportional slope of the Mott-Schottky plot (Figure S5c, Supporting Information). [26]The potential of the VB maximum of CuS obtained from the XPS-VB curve was 1.99 eV, and the VBM of 2CuS/1rGO was 1.91 eV (Figure S5d, Supporting Information).
The work functions (W f ) measured by ambient-pressure photoemission spectroscopy (APS) for CP and 2CuS/1rGO/CP are displayed in Figure 2a.The W f of pure CP was 5.39 eV, and that of the junctioned 2CuS/1rGO/CP electrode has risen to 5.73 eV.Based on the Figure S5, Supporting Information results, energy diagrams can be drawn for the band positions before and after the junctioning of the two particles, as shown in Figure 2b.Before the junction, the bandgap of CuS contained only the oxidation potentials for water splitting.In the junctioned particle 2CuS/1rGO, the VB and CB positions shifted slightly upward, and the bandgap slightly increased from 1.36 to 1.44 eV.Because the position of the CB of the two particles, whether pure or junctioned, is higher than the Fermi level of the CP electrode, it is judged that the electrons excited from CuS may gather easily at the CP electrode.If an FTO electrode (W f = 4.4 eV) were used as the reference electrode, electrons would not collect at it, because its W f was higher than the CB position of the junction particle; therefore, no electrons would move to the platinum counter electrode.However, in this study, we used a CP electrode with a lower W f as the reference electrode to match the band energy of the p-CuS.Because the strong oxidizing power of CuS−VB may increase the corrosion of the CP electrode, it was necessary to raise the VB position of the catalyst electrode to weaken its oxidizing power.Compared to pure CuS, it was confirmed that the oxidative power was weakened by shifting the VB position slightly upward in the rGO-junctioned electrode.However, it still contains the potential at which water is oxidized to generate oxygen, so there is no difficulty in carrying out the OER reaction.Figure 2c shows the photoluminescence curves of the particles.Four emission peaks located at 543, 630, 685, and 765 nm were observed in pure rGO, which are known to arise from the recombination of electron-hole pairs between sp3 and sp 2 -C.For completely reduced rGO, the oxide functional groups decrease and the sp 2 -C cluster expands, and the blue emission peaks at wavelengths above 630 nm disappear. [27]In pure CuS, broad emission peaks appeared at 540, 632, 740, and 761 nm.The peak at 540 nm arises from electron-hole recombination in the surface state, and the emission bands at 632, 740, and 761 nm are due to surface defects and radiative recombination between the CBs and copper or sulfur vacancy-related acceptor levels around the VB edges. [28]Generally, as the PL intensity decreases, the degree of electron-hole recombination decreases, and the photoactivity of the particles increases. [29]The PL intensity decreased the most for the 2CuS/1rGO particles.In addition, the decay rates in the time-resolved PL curves in Figure 2d were 3.74 and 7.64 ns for the CuS and 2CuS/ 1rGO particles, respectively.The slow PL decay observed in the 2CuS/ 1rGO junction particles suggests a potentially higher photocatalytic performance compared to pure CuS particles, as the reduced electron-hole recombination rate can lead to improved efficiency of the photocatalytic reaction.Figure 2e shows the photocurrent density curve used to analyze the effective separation of photogenerated charge carriers.In the 5th cycle of pure CuS, the photocurrent density was 0.5 μA cm −2 , but the photocurrent density in the 2CuS/1rGO junction particles increased rapidly to 118.7 μA cm −2 .In the time-resolved photocurrent density decay curve in Figure 2f, all particles almost maintained their initial current densities.However, with the addition of rGO to the junction particles, the photocurrent density gradually increased rather than decreased with time, and as the amount of CuS increased, the photocurrent density gradually decreased with time, although this decrease was insignificant.These results show that rGO has a positive effect on the charge separation in the junction particles.Even after 330 s, a photocurrent of 109 μA cm −2 continued to flow in the 2CuS/1rGO junction particles, indicating that the photoinduced charge separation in the junction particles was very effective.
On the contrary, although the peak intensity was low in the XRD patterns of the photoanodes prepared by growing individual particles on the CP electrode, the same peaks as the powder sample were observed in all electrodes except for the rGO/CP electrode (Figure S6, Supporting Information).Therefore, it is determined that the active particles are stably loaded on the CP electrode.For junction particles grown on CP electrodes, the peaks of C 1s and O 1s in rGO or Cu 2p and S 2p in CuS shifted to higher binding energies.This indicates that the CuS and rGO present in the junction particles are more oxidized than those in the pure ones.Moreover, CuS was more oxidized while rGO was less oxidized.This is a good evidence showing that electrons moved from CuS to rGO and from rGO to CP in the 2CuS/1rGO junction particles grown on the CP electrode surface (Figure S7, Supporting Information).The SEM images of the CuS/rGO particles grown on the CP electrode show that rGO was preferentially grown on the CP electrode and then CuS loaded on the rGO.In the 2CuS/ 1rGO particles grown on the CP electrode, the CuS particle size was about 5-7 μm.Similar to powder particles, dandelion flower-shaped CuS particles grew in a uniform line with rGO as the central axis (Figure S8, Supporting Information).From the HRTEM images obtained from the 2CuS/1rGO particles carefully scraped from the 2CuS/1rGO/CP electrode, it was predicted that CuS and rGO were single crystals and uniformly formed polycrystals, respectively (Figure S9, Supporting Information).

Electrochemical/Photoelectrochemical Oxygen Generation Performance Evaluations
We explain whether the electrodes developed in this study could be used as electrochemical OER electrodes from now.The CV measurement results for all the electrodes are performed (Figure S10, Supporting Information).The representative electrode, 2CuS/1rGO/CP, is separately summarized in Figure 3a.Through repeated CV measurements, the electrode activity, effective electrochemically active surface area (ECSA), and long-term reliability were estimated. [30]All electrodes were measured 100 times at a scan rate of 50 mV s −1 in the range of −0.6 to 1.6 V.In all electrodes, the redox potential was significantly unstable until the 75th CV cycle, but it remained stable thereafter.Therefore, this phenomenon after the 75th cycle should be explained.In the voltammogram of the rGO/CP electrode, a peak corresponding to the reduction of oxygen in rGO is seen at ~0.6 V.In the CuS/CP electrode, oxidation peaks from Cu 0 →Cu 2+ were observed at ~0.85 V, and two reduction peaks corresponding to Cu 2+ →Cu + and Cu + →Cu 0 were observed at 0.3 V and −0.15 V, respectively.In the CP electrode coated with CuS/rGO particles junctioned with CuS and rGO, two strong oxidation peaks were observed at 0.2 V and 0.85-1.2V, which converged into one peak at 0.35-0.4V.This indicates that the Cu oxidation state in CuS become various owing to the junction with rGO.In addition, the current density is strongly related to the activity of the electrode catalyst; the higher the current density, the greater the catalytic activity. [31]In the CP electrodes coated with CuS/rGO junction particles, the CV current density decreased as the cycle number increased, but the current density was significantly increased compared to that of the pure rGO/CP or CuS/ CP electrodes.It is considered that the oxidation-reduction potential shifted because the oxidation state of copper ions was more divalent than monovalent, and the current was increased because more hydroxyl groups in the electrolyte were adsorbed to the divalent copper ions.In particular, the current density was increased the most in the 2CuS/1rGO/CP electrode after 100 cycles, and it can be predicted that this electrode has increased catalytic activity owing to its large storage capacity.To understand the mass transfer and charge-transfer resistance at the electrolyte and electrode surfaces, Figure 3b shows the Nyquist plot obtained via electrochemical impedance spectroscopy (EIS) from the electrodes in 0.1 M KOH.The y-axis represents the impedance related to the recombination rate between charges inside the electrode; the higher the recombination rate, the larger the charge loss inside the electrode.In contrast, the x-axis represents the impedance related to charge transfer on the electrode surface; such charge transfer is hampered by an increase in the resistance. [32]onsequently, electrodes with smaller semicircles in their Nyquist plots have lower recombination rates and lower resistances, indicating higher electrical conductivity.Here, R s and R ct represent the solution resistance of the double-layer capacitor and charge-transfer resistance at the electrode, respectively.R s was almost equal for each electrode, but R ct differed significantly for the different electrodes.R ct decreased significantly in the order of rGO/CP, CP, CuS/CP, 1CuS/ 2rGO/CP, 1CuS/1rGO/CP, and 2CuS/1rGO/ CP electrodes.In particular, the 2CuS/1rGO/ CP electrode exhibited the lowest resistance, and thus, the highest charge transfer efficiency.As shown in Figure 3c, the electrochemical performance of the electrode was evaluated in 0.1 M KOH as the electrolyte, and the LSV was compared in the potential range 0.6-1.5 V.For the CP and rGO/CP electrodes, the overpotentials when reaching 10 mA cm −2 were very high at 2.12 and 2.08 V, respectively, indicating that they have poor function as OER electrocatalysts.In contrast, the overpotentials at which the CuS/CP, 1CuS/2rGO/CP, 1CuS/1rGO/CP, and 2CuS/ 1rGO/CP electrodes reached a current density of 10 mA cm −2 were 2.03, 1.97, 1.93, and 1.83 V, respectively.These results suggest that the 2CuS/1rGO/CP electrode, with the lowest overpotential, exhibited the best OER functionality in 0.1 M KOH.However, this overpotential is too high for it to be used as an electrochemical OER electrode.The ECSA of the electrode in Figure 3e was determined using the C dl presented in Figure 3d (for all electrodes in Figure S11, Supporting Information).C dl was measured as the CV in the nonfaradaic region, where the catalyst did not undergo a redox reaction, at scan rates of 10, 20, 30, 40, and 50 mV s −1 .The anode and cathode currents were measured at the intermediate potential in the CV scans.The density was fitted linearly with the CV scan rate to obtain positive and negative slopes: [33] C dl = (slope positiveslope negative)/2.The ECSA was calculated by dividing C dl by the specific surface area capacitance (C s ) of the electrode surface: [34] ECSA = C dl /C s .The ECSA of an electrocatalyst is generally directly related to the electrode activity; the larger the ESCA, the better the solution reaction that occurs on the catalyst surface. [35]The rGO/CP electrode shows a wider CV curve than the CuS/CP electrode does.The CuS/rGO/CP junction electrode exhibited a wider CV curve as more rGO was added.In particular, the 2CuS/1rGO/CP junction electrode exhibited the widest CV curve.If the CV width is plotted and expressed as the C dl value, as shown in Figure 3e, the difference between the current density (ΔJ) and ECSA obtained at 0-100 mV s −1 shows a linear trend.The 2CuS/ 1rGO/CP electrode showed the highest ECSA value of 10.74 mF cm −2 , which proves that the 2CuS/1rGO/CP electrode exhibits excellent electrocatalytic activity owing to its high C dl .Chronopotentiometric data of time versus potential measurements of the 2CuS/1rGO/CP electrode at 10 mA cm −2 over 10 h are presented in Figure 3f to evaluate the long-term durability and provide an indicator of electrode stability over time.In the 0.1 M KOH electrolyte, the electrode stably maintained a constant overpotential of 1.88 V for 10 h.These results suggest the applicability of the 2CuS/1rGO/CP electrodes as anodes for OER in alkaline electrolytes.However, the overpotential of the 2CuS/1rGO/CP electrode was too high to allow its use as an OER electrode in electrochemical water-splitting.Here, we investigated the synergistic effect of the driving force of light and electricity in chemical reactions induced by light irradiation.Surprisingly, upon irradiation with light, the overpotential dropped significantly to 1.07 V, and this value remained stable for 10 h without any change.From these results, it was confirmed that the CuS/rGO/CP electrodes prepared in this study were more advantageous for the PEC-OER reaction than for the EC−OER reaction.Now, the PEC−OER performance will be investigated in earnest.First, the external quantum efficiencies (EQEs) and integrated photocurrent densities for the CuS/CP and 2CuS/1rGO/CP electrodes in the range of 300-900 nm were measured using the IPCE spectrum, and the results are shown in Figure 4a.In the 400-700 nm range, the CuS particle showed an EQE of approximately 4.78% in 0.1 M KOH electrolyte, whereas that of the 2CuS/1rGO junction particle was approximately 58.2%, which was 12 times higher.Moreover, in 1.0 M KOH electrolyte, the 2CuS/1rGO junction particle showed a maximum EQE of about 92% at 550 nm.These results demonstrate that the conjugation of rGO to p-semiconductors is a promising strategy for increasing EQE.In particular, it was also found out that EQE is greatly affected by electrolyte concentration.Figure 4b shows the LSV curves obtained while irradiating all electrodes with only 1-sun light, to confirm that the OER response is driven by light and by applied potential.When irradiating a 0.1 M KOH solution with one-sun light and sweeping the potential of the working electrode from the initial to final potential at a constant speed in the positive or negative direction, oxidation or reduction occurs at a specific potential.As a result, current flows, and the current value according to the applied voltage is shown.In general, the current gradually increases as the voltage is applied, and after reaching the maximum value, the current does not increase or decrease even if the applied voltage remains. [36]In the CP and rGO/CP electrodes, almost no current flowed at 0 mA cm −2 , even when the voltage was continuously applied.In other words, the oxidation of water did not occur even with a voltage applied to these two electrodes.On the contrary, under one-sun irradiation, the overpotential at 10 mA cm −2 in the CuS/CP electrode was ~1.34 V, and the overpotentials at 10 mA cm −2 in the CP electrodes coated with CuS/rGO junction particles were significantly lowered to 1.33, 1.25, and 1.07 V in the 1CuS/2rGO/CP, 1CuS/1rGO/CP, and 2CuS/1rGO/CP electrodes, respectively.In general, the overpotential at 10 mA cm −2 in the EC−OER is significantly higher than the theoretical 1.23 V, and is at least 1.4 V. [37] However, in this study, the overpotential corresponding to OER was significantly reduced under light irradiation.In particular, the overpotential of the 2CuS/1rGO/CP electrode was lower than the theoretical value of 1.23 V in EC−OER.Thus, it was confirmed that when light was added to the EC reaction, a synergistic effect occurred for water splitting, and the OER overpotential was significantly lowered.Figure 4c shows the current density-potential (J-V) curves obtained by measuring the open-circuit potential (V oc , −0.158 V) of the full cell under one-sun light and then applying a potential based on the measured value.At 1.23 V, where water-splitting occurs, the photocurrents at the CP and rGO/CP electrodes were 0.6 and 1.15 mA cm −2 , respectively, and 5.87 mA cm −2 at the CuS/CP electrode.In contrast, for the CuS/rGO/CP electrodes, the photocurrent increased dramatically as the applied potential increased.In particular, the 2CuS/1rGO/CP photoelectrode exhibited the highest photocurrent at 1.23 V of 28.4 mA cm −2 , nearly five times higher than that of the CuS/CP electrode.In the J-V curves (corrected to the V oc ) under on/off chopped light in Figure 4d, the flow and block of current on/off switching for the 2CuS/1rGO/CP electrode occurred perfectly.As the applied potential increased, the photocurrent increased; however, it remained almost unchanged at potentials of ≥1.4 V. Chronopotentiometric analysis can evaluate the long-term durability of electrodes and provides an indicator of the electrode stability over time.Figure 5a shows the time versus photoamperometric data for the 2CuS/1rGO/CP electrode at 1.23 V for 10 h.Data were collected in a 0.1 M KOH electrolyte without V oc correction, and a constant photocurrent of 18.0 mA cm −2 was stably maintained.These results suggest that the 2CuS/1rGO/CP electrodes are stable photoanodes in alkaline electrolytes.Meanwhile, realtime reaction images were recorded every hour to obtain time-to-photocurrent data while irradiating the 0.1 M KOH electrolyte with one-sun light at 1.23 V and mA cm −2 , respectively, for the 2CuS/1rGO/CP photoanode.Real-time reaction records against current density and potential for 10 h are exhibited in Video S1 and S2, Supporting Information, respectively.The faradaic efficiencies obtained in 0.1 M KOH based on actual and theoretical oxygen evolution performed for up to 300 min at 1.23 V using the 2CuS/1rGO/CP electrodes are presented in Figure 5b.The theoretical amount of oxygen generation and faradaic efficiency of the electrode were calculated using the following equation: [38] faradaic efficiency = experimental μmol of O 2 gas/theoretical μmol of O 2 gas × 100.Here, the theoretical amount of O 2 gas was calculated from Faraday's law: [39] n = I × t/z × F, where n is the number of moles, I is the current, t is the electrolysis reaction time of water, z is the transfer of electrons (for O 2 , z = 4), and F is the Faraday constant (96485.3s•A mol −1 ).The 2CuS/1rGO/CP electrode showed a faradaic efficiency of 98.60% in the 0.1 M KOH electrolyte.This means that 98.60% of the electrons used were involved in the OER without side reactions.Figure 5c is the J-V curve for the 2CuS/ 1rGO/CP photoanode in the three-electrode system in 1.0 M KOH electrolyte, the photocurrent is 94.7 mA cm −2 and V oc is 0.153 V, and the overpotential at 10 mA cm −2 was significantly lowered to 0.87 V.This figure is approximately 3.5 times greater than the current value of 28.39 mA cm −2 generated by the 0.1 M KOH electrolyte (in Figure 4c).On the contrary, photoelectrochemical conversion efficiencies are largely expressed in four ways; the solar into hydrogen energy (STH), the applied bias photon to current (ABPE, in the state where external power is applied), the incident photon to current (IPCE = EQE), and the absorbed photon to current (APCE = IQE).The OER in this study was performed in an electrochemical system composed of three electrodes by a reference electrode as a calomel electrode, a working electrode as a photoanode, and a counter electrode as a photocathode.Therefore, it is desirable to adopt the half-cell type ABPE as an efficiency in this study.The ABPE, applied to determine the photoelectrochemical efficiency in the threeelectrodes is presented in Figure 5d.The maximum efficiency of the 2CuS/1rGO/CP photoanode is 2.83% (at 1.07 V) at 0.1 M KOH and 11.2% (at 1.03 V) at 1.0 M KOH, a difference of about four times.Moreover, it is about seven times higher than that of pure CuS/CP (0.39% at 1.10 V) photoanode in the same electroyte concentration.From this result, it can be seen that rGO addition improves the OER efficiency and ABPE value.In addition, 2CuS/1rGO/CP has a higher efficiency of ABPE at lower voltages than CuS/CP, which means that the system can produce the same amount of oxygen with less energy input.The overpotential required for the reaction is related to the activation energy required to overcome the kinetic barrier and initiate the reaction.A lower overpotential means lower activation energy, which means faster reaction rate and higher efficiency.Also, in PEC−OER, a lower voltage at the peak efficiency point of ABPE can lead to better charge transfer efficiency and lower resistance in the electrochemical system, resulting in improved performance.Therefore, in PEC−OER, it is desirable to achieve the highest efficiency at the lowest possible voltage.This indicates that 2CuS/ 1rGO/CP can generate oxygen with minimal energy input and high efficiency.On the contrary, we tried to obtain the STH for the 2CuS/ 1rGO/CP electrode, and in Figure 5e, the J-V curve for the full cell in the two-electrode (photocathode//photoanode) is presented.The STH conversion efficiency was evaluated using the following equation: [40] STH (%) = J ph (mA cm −2 ) × (1.23 − │V app │)V × 100/P light (mW cm −2 ), where J ph is the measured photocurrent density, 1.23 V is the standard-state reversible potential of water, |V app | = (V meas − V oc ), V meas is the applied potential during the measurement of J ph , V oc is the opencircuit potential of the working electrode, and P light is the illuminating light power density.In 0.1 M KOH media, V app = 1.23 V − (−0.154V) = 1.388V, P light = 100 mW cm −2 , and J ph = 30.95mA cm −2 , thus the calculated STH with these values was 4.76%.Otherwise, in the 1.0 M KOH solution, the photocurrent is 98.07 mA cm −2 and V oc is 0.160 V; the calculated STH with these values reached 15.69%.Excepting of the tandem solar cell system and the PV−EC system having STH efficiencies reaching as high as 10-20%, [41] the STH efficiency obtained from pure PEC-water splitting system using a particle-based PEC device to date is very low, within 2%. [42]Theoretically, an STH efficiency of up to 30% can be achieved with a semiconductor band gap of 1.6 eV; however, despite the tremendous efforts of many researchers, the realization of an STH of 30% has been a dream until now.It is well known that when the STH energy conversion efficiency is stably maintained at 10%, the commercial application of PEC water splitting becomes possible. [43]Therefore, the 2CuS/rGO/CP photoanode (particle-based catalyst-coated PEC single device) developed in this study shows an advanced STH level in the photochemical water decomposition system, which is a very encouraging result to the extent that the system can be commercialized.However, in general, carbon electrodes are weak in strong alkaline solutions, and thus may deteriorate in activity when used in alkaline solutions for long time periods.The time versus photoamperometric data for the 2CuS/1rGO/CP electrode at 1.23 V was obtained in a 1.0 M KOH electrolyte without Voc correction, and the results are displayed in Figure 5f.A constant photocurrent of 92.4 mA cm −2 was stably maintained until 250 h.These results confirm that the 2CuS/1rGO/CP electrode is very stable and excellent as a photoanode in strong alkaline electrolytes.In the near future, we believe that it is worth exploring whether this electrode has commercial potential.For showing reliability of the photoelectrochemical activity result in this study, a digital photograph for reaction system is presented in Figure S12, Supporting Information.All photoelectrochemical data were evaluated using a 1 cm × 1 cm electrode (catalyst-coated part).In addition, consistent data was always obtained under a constant active area by using an optical aperture to define an accurate active area.As a result of comparing the performances of photoanodes containing rGOcatalyst grafted to metal oxide and metal sulfide previously reported (Table S1, Supporting Information), the CuS/rGO/CP photoanode in this study shows the best photoelectrochemical performance by far.

Details of the Theory and Computational Results
DFT calculations of the potential active sites (Cu cus , S cus and brd) of the various reaction intermediates of HO, OH, O, and OOH in the bulk system (thick-layer) predicted that O* and OH* were preferentially adsorbed at the S cus site (Figure S13, Supporting Information).Although CuS in CuS/rGO suggested slightly more activity than CuS alone, the effect of rGO is small enough to be negligible.To understand the origin of the experimentally observed enhancement in reactivity by rGO, we investigated the OER reactivity of a thin layer of CuS on rGO.The modeled thin-layer CuS/rGO consisted of two layers of Co and one layer of S, as shown in Figure 6a.This structure is the thinnest layer possible on the rGO structure because sulfur could not be present at the bottom or top layers; 1) repulsive interactions between bottomlayer sulfur and rGO destabilize the system, 2) the presence of toplayer sulfur triggers blocking of copper active sites.The same site analysis was performed on the thin-layer CuS/rGO, and the same favorable configurations were predicted as for CuS/rGO.With the predicted favored configurations, the potential-dependent free energies were evaluated, as shown in Figure 6b.The results show that the trends of the free energy changed slightly, but the potential determining steps remained still identical (O*+H 2 O(g)→OOH*+H + +e − ).However, the predicted onset potentials are different; thin-layer CuS/rGO requires the much lower onset potential of 2.15 V compared to the potential of 2.92 V (CuS/rGO), which agrees well with the experimental results.Based on the computational results of thick-and thin-layer CuS/rGO, it is proposed that the enhanced reactivity stems from the thin layer of CuS on rGO.We further investigated the origin of the enhancement factor of the thin-layer CuS/rGO by conducting an electronic structure analysis, as shown in Figure 6c.Interestingly, the thin and thick layers of CuS/rGO still provide similar DOSs, meaning that rGO does not considerably affect the electronic structure of CuS (Figure 6d).The orbital hybridization is strongly related to the covalent bonds between the reactant and surface-active sites (related to the DOS), and the contributions of ionic bonds to the interactions between reactants and the catalytic surface depend on electron transfer (related to the W f ). [44]rom our OER calculations, it was found that O* stability plays an important role in determining the onset potential because the potential determining step of (O* + H 2 O(g) → OOH* + H + + e − ) is sensitive to O* stability.The electronically unsaturated O* considerably abstracts electrons from the catalytic surface, resulting in significant ionic contribution to its adsorbed stability.To quantify the contributions of ionic bonds, we evaluated the W f difference (ΔW ¼ WÀW 0 ) between the systems (W) and the OER intermediate of O* (W 0 ).The W f of the system can be evaluated as W = E v − E f , where E v and E f are the vacuum energy and Fermi level, respectively.Additionally, the W f of the adsorbate can be regarded as the highest occupied molecular orbital (HOMO) of the adsorbate because E f −E v of the molecule is the same as |HOMO|.Shen et al. proposed that the ionic contributions are well quantified by ΔW 2 , which has also been confirmed for heterojunction catalytic systems. [45,46]More specifically, a higher (lower) ΔW 2 triggers stronger ionic contributions, thereby forming a higher (lower) stability of O* on the surface.Our simulation predicted that ΔW 2 of the thin-layer CuS/rGO was lower than that of the thick-layer CuS/ rGO (22.75 vs 28.09).In other words, a lower O* stability is expected compared with the thick-layer CuS/rGO because of the lower ΔW 2 for the thin-layer CuS/rGO.The lower stability of O* decreases the freeenergy gap between the initial and final states of the potentialdetermining step, thereby lowering the onset potential.Based on our fundamental results, we conclude that the high reactivity of CuS/rGO is driven by its thin CuS layer.On the other hand, the results of the DFT calculations need to be further explained in connection with the defects of the particles.as already mentioned, the energy barrier of the OER path at the photoanode was estimated through DFT calculations.The complex OER process is dynamically slow because four electrons are involved, which is described by the following Equations (1-4). [47] 3 or SO 2À 4 resulting from the oxidation of S À 2 , which means that S defects were formed in CuS during synthesis.In CuS/rGO junction particles, the S 2p peak of CuS shifted to a higher binding energy due to the junction with rGO, indicating that S was oxidized.Here, the Cu 2p peak was not reduced by receiving electrons from S, but rather moved to a higher binding energy and was oxidized.This means that the electrons from S moved to rGO instead of to Cu.On the contrary, the S 2 O 2À 3 or SO 2À 4 peaks representing defects are somewhat smaller due to junction.This means that the S defects are reduced and the oxidation state of CuS is stabilized by the rGO junction.After junction, of course some defects still remain.However, overall excessive S defects were reduced, which gave stability in the OER response.Consequently, the adsorption-energy scaling relationship between reactive intermediates in the OER reaction of CuS/rGO/CP could be broken more easily by some remaining nonexcessive SO 2À 4 with an appropriate level of defects.Additionally, by lowering from 3.1 (CuS) to 2.15 eV (CuS/rGO) in onset potential, the OER overvoltage largely reduced.In the end, our theoretical calculations support that the presence of suitable defect levels of SO 2À 4 groups enhances the OER kinetics.However, it should be noted here that the computational discussion is simply the adsorption free energy relationship between the OER intermediate reactants and the catalyst surface by CuS and rGO junctions.A more important key to this study is that, as described in the experimental results, the electron transfer performance was maximized by using a carbon electrode that matches the energy band location of the CuS/rGO catalyst as the base electrode.In addition, it should not be overlooked that various factors such as light harvesting, electron transporting bridge, and structural defects are acting synergistically between the elements constituting the electrode.

Charge-Transfer and Exciton Recombination Rates, and Charge-Transfer Reaction Mechanism
As shown in Figure 7a,b (Based on experimental results in Figure S14a,b, Supporting Information), the electron-hole recombination rate was higher for the 2CuS/1rGO junction particles than for CuS.As the light intensity increased, the recombination rate rapidly increased in CuS but it mildly increased in the 2CuS/1rGO junction particles.These results show that the electron-hole recombination rate of the pure CuS particles is strongly affected by the light source intensity (depending on the wavelength of the absorbed light), but that in the junction particles is not significantly changed.The electron transport rate in the CuS particles was very slow compared to that in the 2CuS/1rGO junction particles; the difference was a factor of 4.5 at low light intensity.Moreover, as the light intensity on the CuS particles increased, the electron transport rate also decreased very quickly, which means that CuS was very sensitive to the light intensity.The 2CuS/1rGO junction particles showed a fast electron transport rate, but the rate was almost constant even with an increase in light intensity.This result made it possible to predict that the electron transport rate would remain almost constant, even if the light intensity was changed.From this result, it is recognized that the junction of CuS and rGO can play a role in accelerating the electron transport rate very quickly, maintaining a constant electron transport rate sensitive to light intensity, and inhibiting recombination by separating the induced charges.
Unusually, signals corresponding to DMPO(5,5-Dimethyl-1pyrroline N-oxide)-ÁOH were observed in both the CuS and 2CuS/ 1rGO particles, even without light irradiation, and the signals gradually increased in strength with light irradiation (Figure S15a,b, Supporting Information).However, the signal increase differed, and a larger signal was observed for the 2CuS/1rGO junction particles than for the single CuS particles.This result indicates that ÁOH radicals in the CuS − VB of the 2CuS/1rGO junction particles were generated well, which clearly proves that rGO helps to produce ÁOH radicals.Meanwhile, the characteristic signal intensity corresponding to DMPO−ÁO À 2 was not observed in the dark, but it appeared clearly in both CuS and 2CuS/1rGO after 5 min of simulated solar irradiation.However, comparing the magnitudes of the signals in the two particles, the ÁO À 2 radical is better generated in 2CuS/1rGO than in the CuS particle.When rGO is grafted to CuS, it attracts electrons to facilitate radical generation.As CuS and rGO 2 signal slightly increased in the 2CuS/1rGO junction particle, indicating that the rGO junction affected the ÁO À 2 radical or electron accumulation.This ultimately had a positive effect on the OER response.That is, when light is irradiated for water oxidation in the 2CuS/1rGO junction particle, electrons are excited in the CuS−VB and holes are created.At this time, the electrons excited to the CuS−CB easily migrate to the rGO, thereby forming more ÁOH radicals in the CuS−VB.
A predictive mechanism for the PEC−OER in the CuS/rGO/CP electrodes is presented in Scheme 1.When the CuS/rGO/CP photoelectrode is irradiated with light, the electron-hole pairs generated from CuS are separated and migrated to the electrode catalyst surface to participate in the surface redox reaction.The photoelectrons excited by CuS−CB move to rGO, gather at the lower energy level of the CP electrode, and then transfer to the platinum counter electrode to generate hydrogen.In this case, the generation of the photocarrier generates a driving potential and assists the external bias required for oxygen generation to reduce the overpotential, which greatly reduces the overpotential compared to EC. [48] In the holes of CuS−VB, ÁOH radicals, which are reactive oxidizing agents for oxygen generation, are generated; therefore, the PEC−OER under irradiation can be further improved compared to the EC-OER.Although photogenerated electron-hole recombination can reduce the light-harvesting efficiency, rGO accelerates hole and electron diffusion, preventing recombination at the electrode/solution interface.In particular, because the energy levels of CuS−CB, rGO, and CP are low and decrease in that order, excited electrons easily migrate to rGO to overcome the short diffusion length of photogenerated holes and suppress electron-hole recombination.Moreover, the sulfur defection in CuS must have promoted the adsorption of OH − formed on the particle surface during the OER process, thereby further activating oxygen generation.In the end, rGO was junctioned with CuS and acted as a conductive layer, which was expected to help transport the charge carriers generated in CuS and inhibit recombination, which in turn improved the PEC function.

Conclusions
In conclusion, this study highlights an effective strategy for PEC−OER electrode design.In order to facilitate the energy band and electron transfer of the self- assembled p-CuS/rGO, a CP electrode rather than FTO was used as the base electrode.CP exerted its light harvesting ability to the fullest, and rGO inserted in the middle act as an electron transporting bridge so that electrons flow without loss.Electron-hole recombination was greatly suppressed, and electron transport rate was accelerated.As a result, the Faraday efficiency of PEC−OER was 98.60%, and the electrolyte concentration-dependent STH efficiency was 4.76% and 15.7% in 0.1 M and 1.0 M KOH solutions (in two-electrode system), respectively.This value is the highest PEC−STH ever reported for a PEC system.DFT calculations revealed that the intermediate O* stability plays an important role in determining the enhanced reactivity in thin layers of CuS in CuS/rGO, and the low stability of O* lowers the onset potential by reducing the free energy gap between the initial and final states in the potential-determining step.that lowers the initiation potential by reducing The results of this study will serve as a signal for the development of new electrodes that dramatically increase OER performance, which has been limited in PEC systems.

Experimental Section
Detailed information related to the synthesis of active electrodes, physicochemical characterization, and electrochemical evaluation of bifunctional electrodes towards UOR and supercapacitor application is provided in Supporting Information.

Figure 4 .
Figure 4. a) IPCE, b) LSV curves, c) J-V curves obtained by LSV in 0.1 M KOH under one-sun light, and d) J-V curves under on/off chopped light.

Figure 5 .
Figure 5. a) Chronopotentiometric curve for 2CuS/1rGO/CP electrode in 0.1 M KOH, b) Faradaic efficiency for 2CuS/1rGO/CP electrode, c) J-V curve obtained by LSV in 1.0 M KOH for 2CuS/1rGO/CP electrode under one-sun light, d) ABPE plots of three photoanodes, e) J-V curve obtained by LSV with two-electrode system under one-sun light, and f) Chronopotentiometric curve for 2CuS/1rGO/CP electrode in 1.0 M KOH.
CuS can break the adsorption-energy linear scaling relationship between the reactive intermediates, O*, OH* or OOH* (probably O* in this study), and reduce the OER overpotential.However, adequate defect levels are required as excessive defects can degrade catalytic performance.In the XPS results of this study, the additional peaks at 163.22 and 163.90 eV in CuS correspond to S 2 O 2À

Figure 6 .
Figure 6.a) Top and side views of thin-layer structure of CuS/rGO.DFT-evaluated Gibbs free energy of CuS (001) (blue) and CuS/rGO (001) thin-layer (red) reactions at b) RHE (0 V) and c) onset potential, respectively, and d) DOS (d orbitals for Cu, s and p orbitals for S) for surface active sites of CuS and thin-layer CuS/rGO.
the active site, control the adsorption of intermediates, and promote surface carrier migration.In particular, the Energy Environ.Mater.2024, 7, e12631