Photoelectrochemical study of carbon-modified p-type Cu2O nanoneedles and n-type TiO2−x nanorods for Z-scheme solar water splitting in a tandem cell configuration

Nanostructured photoelectrodes with a high surface area and tunable optical and electrical properties can potentially benefit a photoelectrochemical (PEC) water splitting system. The PEC performance of a nanostructured photoelectrode is usually quantified in a standard three-electrode configuration under potential-assisted conditions because of the additional overpotentials for the two half-reactions of water splitting. However, it is a necessity to fully recognize their potential to split water under unassisted conditions by designing a tandem cell that can provide sufficient voltage to split water. Herein, we present a tandem cell consisting of carbon-modified cuprous oxide (C/Cu2O) nanoneedles and oxygen-deficient titanium dioxide (TiO2−x) nanorods for unassisted solar water splitting. The synthesized photoelectrodes were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and electrochemical impedance spectroscopy (EIS) techniques. The tandem cell performance was analyzed by measuring the current–voltage responses in various photoelectrode configurations to validate the collective contributions of both photoelectrodes to unassisted solar water splitting. The PEC properties of C/Cu2O nanoneedles coupled with TiO2−x nanorods in a tandem configuration exhibited a photocurrent density of 64.7 μA cm−2 in the absence of any redox mediator and external bias. This photocurrent density can be further enhanced with an application of external bias. Moreover, the heterojunction formed by the above-mentioned nanostructured photoelectrodes in intimate contact and in the absence of water exhibited 2 μA cm−2 UV photoresponsivity at 1.5 V with promising rectifying characteristics of a diode.


Introduction
With the ever-increasing global energy demands and the negative environmental impacts of fossil fuels, renewable solar energy has been regarded as an alternative energy source because of its abundance and wide availability. 1,2Among many techniques [3][4][5] used to harvest and store solar energy, photoelectrochemical (PEC) water splitting offers an easy, low-cost and effective route to simultaneously produce hydrogen (H 2 ), a clean fuel energy source, and oxygen (O 2 ).First introduced in 1968, 6 then popularized in 1972 by Fujishima and Honda, 7 semiconductor-based solar water splitting has undergone considerable research and development. 8A typical PEC water splitting reaction, an uphill reaction, is shown as: very low because of the limited light absorption, charge separation, charge transport, sluggish kinetics of water splitting, and poor stability. 8,9,124][15][16] Inspired by nature's photosynthesis, a Zscheme solar water splitting system can be established that involves two different photoactive semiconductors, a p-and ntype material, via a two-step excitation mechanism to enhance charge carrier-separation and hence improve the overall photoconversion efficiency. 13,15,17Fig. 1A shows the schematic of a self-biased Z-scheme solar water splitting system involving pand n-type photoelectrodes.In this approach, electron-hole pairs are generated in both the p-and n-type semiconductors, which absorb in different regions of the solar spectrum.Due to the band-bending at the semiconductor/electrolyte interface, the photogenerated holes react with water molecules generating oxygen (O 2 ) molecules, while the photogenerated electrons in the photocathode move to the surface and reduce H + to generate H 2 . 18or a successful self-biased Z-scheme PEC water splitting system, it is prerequisite that the Fermi energy (E f ) level of the photoanode is at the higher energy level (more positive) than that of the photocathode. 18Our group recently reported unbiased Z-scheme solar water splitting system by combining an optimized Co-doped BiVO 4 /WO 3 photoanode and CuO/CuBi 2 O 4 photocathode in a two-electrode conguration; wherein both the photoelectrodes were illuminated with visible light. 19Such a two-electrode conguration holds a promise for providing enough voltage to split water in the absence of external bias.Titanium oxide (TiO 2 ) and cuprous oxide (Cu 2 O) are another such photoanode, 20 and photocathode 21 materials that can potentially be used for Z-scheme solar water splitting as their energy band positions are sufficient enough to form a type II band alignment. 22Both the materials are earth-abundant, environmentally benign and low-cost.Cu 2 O (E g ¼ 2-2.2 eV) is an attractive p-type semiconductor, whose CB is appreciably negative (more energetic) than that of commonly employed visible-light semiconductors and the hydrogen evolution potential.Although it suffers from photo-corrosion, 24 its surface can be protected by some means to utilize its full potential.The single or multi-layered protective materials such as TiO 2 , 23,24 MoS 2 , 25,26 carbon, 27,28 graphene, [29][30][31] molecular catalyst 32 have been employed to protect Cu 2 O from photo-corrosion for solar water splitting and CO 2 reduction reactions.TiO 2 (E g ¼ 3.0-3.2eV) is one of the best chemically stable materials as of yet with extensive applications.However, it has poor solar light absorptivity in the visible and NIR region. 33Although efforts were made to reduce its band gap to make it available in the visible region, TiO 2 performs best as a UV-absorbing material and delivers relatively good photocurrents even at a small portion of UV light.Additionally, being a wide bandgap material, it can be used as a window layer to direct visible light on to the visible-light-absorbing material such as Cu 2 O in tandem conguration.
Herein, we report the PEC performance of carbon-modied p-type Cu 2 O nanoneedles (C/Cu 2 O NNs) and n-type oxygen-decient TiO 2Àx nanorods (TiO 2Àx NRs) for unassisted Zscheme solar water splitting system in a tandem conguration to efficiently utilize the sunlight absorption and improve the charge separation efficiency.A modication of Cu 2 O NNs with a thin protective carbon layer enhanced its photostability.A solution-based approach has been adapted to modify the Cu 2 O NNs to obtain corrosion-resistant carbon layer with enhanced conductivity for hydrogen evolution reaction. 27,28On the other hand, the introduction of oxygen vacancies in semiconductors such as TiO 2 has proven to increase the electrical conductivity as well as charge transfer properties. 34,35In addition to this, the one-dimensional (1D) nanoscale morphology of both photocatalysts is employed to increase the surface area for multiplying scattering events to maximize the light absorption, promote the separation and migration of photogenerated charges, thereby accelerating the reaction rates to increase the photocurrent density. 14,23,36,37he efficiency of this tandem system is evaluated by placing the two photoelectrodes side by side (parallel) or one in the front of the other (tandem).On the other hand, since p-n heterojunctions have also received great interests due to their potential use in optoelectronic devices such as biosensors and photodiodes; [38][39][40] in the present work, we also analyzed the electrical characteristics of a p-C/Cu 2 O NNskn-TiO 2Àx NRs heterojunction photodiode behavior to further understand the charge transfer mechanism in the absence of water.0.0254 mm thick) were purchased from Alfa Aesar Co. Cu foils (1.5 cm Â 1.5 cm) were rst ultrasonically degreased in acetone, detergent water, and deionized water, respectively.The cleaned Cu foils were dried with nitrogen (N 2 ) then electropolished in a solution mixture containing 55% H 3 PO 4 and ethylene glycol (1.11 g mL À1 ). 41Then, the electropolished Cu foils were anodized in 2.0 M potassium hydroxide (KOH) aqueous solution at a current density of 10 mA cm À2 and a potential of 2.0 V at room temperature for 1-9 min. 42A light blue lm, indicating the formation of Cu(OH) 2 NNs, was formed on the surface of electropolished Cu foil; the anodized Cu foil was then rinsed with DI water and dried under N 2 stream.The Cu(OH) 2 NNs were then soaked in a dextrose solution (1-50 mg mL À1 ) for overnight, dried under N 2 stream, then annealed at 550 C for 4 h in N 2 atmosphere to obtain yellowish-orange colored carbon (C)-modied Cu 2 O NNs. 28 Fig. 1B illustrates the schematic design of the formation of C/Cu 2 O NNs. Amount of carbon modication was varied by the concentrations of dextrose solution at 1, 3, 5, 7, 10, 15, 20 and 50 mg mL À1 to optimize the photocurrent and electrode stability.C 10 /Cu 2 O represents C/Cu 2 O NNs synthesized with 10 mg mL À1 dextrose with optimum activity in our following discussions.

Synthesis of n-type TiO 2Àx nanorods array
TiO 2Àx NRs array photoanodes were fabricated using a hydrothermal synthesis route. 43In a typical process (Fig. 1C), a mixture of 12 mL DI water and 12 mL concentrated hydrochloric acid (36.0-38.0wt%) was poured into a Teon-lined stainless steel autoclave of 50 mL capacity.The mixture was stirred for 5 min under ambient conditions, then 0.4 mL of titanium(IV) butoxide (Ti(OBu) 4 , 97.0%) was added to it and stirred for another 5 min.A cleaned piece (1.5 cm Â 1.5 cm) of uorine-doped tin oxide (FTO) substrate was then placed at an angle against the wall of the Teon liner with conducting side facing down.The hydrothermal reaction was conducted for different durations (6-20 h) in an electric oven maintained at 150 C. Aer the hydrothermal reaction, the autoclave was cooled to room temperature in the air; then the samples were rinsed with DI water and dried under N 2 stream.As-synthesized TiO 2Àx NRs were then annealed in air at 450 C for 2 h at the ramp rate of 2 C min À1 . 44

Sample characterization
The structure and morphology of the synthesized photoelectrodes were characterized by X-ray diffraction (XRD, Bruker D8 Discover with GADDS), scanning electron microscope (SEM, JEOL 7000 FE), transmission electron microscope (TEM, FEI Tecnai F-20), and Raman spectroscopy (Horiba Jobin Yvon LabRam HR800) using a 532 nm laser.The catalytic performances were tested electrochemically using cyclic voltammetry (CV) and linear sweep voltammetry (LSV) on a CHI760C electrochemical work station in a three or two-electrode cell conguration at room temperature.In a three-electrode conguration, C/Cu 2 O NNs or TiO 2Àx NRs were used as the working electrode, a graphite rod as the counter electrode, and an Ag/AgCl (saturated KCl) as the reference electrode.An aqueous solution made of 0.5 M sodium sulfate (Na 2 SO 4 ) and 0.1 M potassium phosphate monobasic (KH 2 PO 4 ) was used as the electrolyte solution (pH 5.0) and was deaerated by purging N 2 gas for at least 30 min before all the electrochemical and photoelectrochemical measurements.Electrochemical impedance spectroscopy (EIS) study of the samples was carried out using a three-electrode system on a CHI760C electrochemical work station in the frequency range of 100 kHz to 0.1 Hz with an AC potential amplitude of 10 mV and a DC potential of À0.3 V vs. Ag/AgCl for C/Cu 2 O samples and 0.6 V vs. Ag/AgCl for TiO 2Àx samples.The Mott-Schottky (MS) analyses of C/Cu 2 O and TiO 2Àx electrodes were performed at the 1 kHz frequency in their respective potential regions.The EIS and MS measurements were performed in buffered near-neutral 0.5 M Na 2 SO 4 electrolyte.The optical properties of C/Cu 2 O NNs and TiO 2Àx NRs samples were measured using UV-vis spectrophotometer (Varian Cary 50) and spectrouorometer (Jobin Yvon Horiba uoromax-3).The current-voltage (J-V) characteristics were measured using a Keithley 2400 source meter with the assistant of a LabVIEW program.A simulated solar light of the intensity of 100 mW cm À2 intensity generated from a solar simulator (Newport 66902, Xenon Arc lamp-modied with an Oriel 1.5 air mass, AM spectral lter).A slight shi towards lower 2q angle could be due to residual stress due to the presence of carbon.The XRD results of TiO 2Àx  2D) showed a rutile phase with prominent peaks at 441 cm À1 (E g ), 608 cm À1 (A 1 g), a weak peak at 140 cm À1 (B 1 g) and 236 cm À1 from a second-order effect (SOE). 46,47Thus, from Fig. 2D, it indicates that TiO 2Àx rst forms as amorphous NRs and then crystalline NRs grow on top of them.Although anatase TiO 2 is usually more photoactive than rutile TiO 2 , previous studies have shown that the rutile phase shows a lower charge recombination rate. 48Recent studies have also revealed an increase in photoactivity in TiO 2 samples that exhibit a mixture of anatase and rutile TiO 2 phases due to electron transfer from the rutile states to the anatase states which are lower in energy or vice versa. 49ig. 3A 3)).During this process, dextrose is believed to dehydrate, cross-link, aromatize, and then carbonize to form a thin protective layer (Fig. 3B) that covers and protects the NNs morphology. 28 + 2OH À / Cu(OH) 2 + 2e À

Results and discussion
(2) In the absence of the dextrose solution, the resulting Cu 2 O nanoneedles showed curly-like morphology but with similar dimensions (Fig. S4, ESI †).Synthesized at different durations (6-20 h), TiO 2Àx NRs rst form a TiO 2 thick lm having NR arrays that were approximately 5-7 mm in length and 200-400 nm in diameter.The NR length increased with the hydrothermal duration (Fig. S4 †).The photoelectrodes were further analysed by uorescence spectroscopy to investigate the recombination processes of photogenerated carriers.TiO 2Àx NRs, under excitation at 365 nm, showed a bluegreen emission centered around 436 nm and a shoulder peak around 534 nm (Fig. 4C).4][55][56] The C/Cu 2 O NNs (Fig. 4D), excited at 470 nm, show a major exciton peak around 621 nm and a shoulder peak centered around 539 nm both usually attributed to phonon-assisted transitions in Cu 2 O samples conrming that no carbon elements were doped into the Cu 2 O lattice. 57ig. 5A shows the individual photocurrent responses of ptype C/Cu 2 O NNs and n-type TiO 2Àx NRs measured with threeelectrode set-up.The photocurrent response of TiO 2 NRs is inverted for convenience in order to identify the operating the current of Z-scheme solar water splitting system with a common intersection point, an approach analogous to typical load-line analyses of photovoltaic cells and resistive loads. 58The photocurrent turn-on potential of C/Cu 2 O NNs is more positive than that of n-type TiO 2Àx NRs.The C/Cu 2 O NNs photocathode shows a net photocurrent density of $À300 mA cm À2 at 0 V vs. RHE (À0.5 V vs. Ag/AgCl).TiO 2 shows a stable photocurrent density of 60 mA cm À2 at 1.23 V vs. RHE (0.8 V vs. Ag/AgCl).The cathodic photocurrents ranging from À240 mA cm À2 to À2.45 mA cm À2 at 0 V vs. RHE for anodized Cu 2 O NWs 23,24,27,31,[59][60][61][62] and anodic photocurrents ranging from 20 mA cm À2 to 0.5 mA cm À2 at 1.23 V vs. RHE for hydrothermally grown TiO 2 NRs, [63][64][65][66][67][68][69][70] have been reported in the literature.Nevertheless, our main goal in this paper is to demonstrate and establish unassisted Z-scheme solar water splitting in tandem conguration.Moreover, we envisage that still there remains much opportunity to optimize the individual performances of C/Cu 2 O and TiO 2Àx photoelectrodes to improve the unassisted water splitting efficiency further.The intersection point in Fig. 5A    to the scenario when both electrodes were illuminated at simultaneously in a parallel conguration (Fig. 5C).It is clear that performances of both the photoelectrodes under individual illumination conditions while masking one another is minimal, even if they are added up.When both the electrodes are used and are illuminated simultaneously, there is a dramatic enhancement in the PEC performance.Hence, both photoelectrodes are needed to effectively produce and separate photogenerated holes and electrons that take part in the overall Zscheme solar water splitting.EIS measurements for C/Cu 2 O NNs and TiO 2Àx NRs photoelectrodes (Fig. 5D and E) were carried out in the dark and under simulated sunlight in a threeelectrode conguration to obtain insights into the chargetransfer properties and the recombination processes of the photogenerated electron-hole pairs.The experimentally measured Nyquist plots were tted using circuit elements consisting of one resistor and one RC circuit according to the standard Randles equivalent circuit (insets of Fig. 5D and E).Table S1 and S2 † list the EIS parameters obtained from the ttings of Nyquist plots.The series resistance (R s ) corresponding to the resistance of the electrolyte solution from working electrode to reference electrode is almost similar for all the samples ($430 U for Cu 2 O samples and $495 U for TiO 2Àx samples).The charge-transfer processes that dictate the photocurrent response of photoelectrodes is governed by the charge transfer resistance (R ct ), which is lower for C/Cu 2 O NNs prepared with 10 mg mL À1 dextrose and TiO 2Àx NRs (20 h) under both dark and light illumination conditions in comparison to unmodied Cu 2 O NNs and TiO 2Àx NRs (12 h), respectively.Additionally, the corresponding double layer capacitance (C dl ) is higher for both C/Cu 2 O NNs and TiO 2Àx NRs (20 h).A constant phase shi element (CPE) is used to t the equivalent circuit (insets of Fig. 5D and E) for capacitance C dl , meant for imperfect capacitance arising due to non-planar nature of the electrodes.Thus, carbon modication greatly improves the water splitting performance of Cu 2 O NNs due to efficient charge separation and the resistance of the TiO 2Àx NRs decreases as the length increases.
Fig. 6A shows the schematic setups of Z-scheme water splitting systems and the actual photographs of participating photoelectrodes in parallel and tandem congurations that were used to record the J-V curves of the two photoelectrodes in a non-sacricial environment under a standard simulated solar light.Both congurations showed photocurrent responses indicating that the photoelectrons were efficiently shied to the photocathode to produce the photocurrent. 51Additional J-V tests using TiO 2Àx NRs of different thicknesses in both tandem and parallel congurations are shown in Fig. S8.† The cell photocurrent in the parallel conguration is relatively higher for 12 h TiO 2 and drops for 20 h TiO 2 , but when compared at lower applied potentials, no discernible difference is seen for tandem conguration.The difference in tandem conguration is apparent only at higher applied potentials with relatively higher photocurrent for 20 h TiO 2 contrary to the parallel conguration.Such anomaly can be explained based on charge collection and light penetration depths owing to different lm thicknesses (TiO 2 NR lengths).Longer the NR length, slower the charge collection rate.In parallel conguration and with the necessity to traverse smaller optical distance (for 12 h TiO 2 ), the charge carriers are readily separated and contributes to the photocurrent without much losses than that for 20 h TiO 2 .On the contrary, in tandem conguration, the signicant contribution in tandem conguration comes from thicker 20 h TiO 2 than that of 12 h TiO O NNs, on the other hand, migrate to the photoelectrode-electrolyte interface to participate in the overall Z-scheme water splitting. 13,52,71,72The photocurrent response of Z-scheme system involving C/Cu 2 O NNs and TiO 2Àx NRs recorded with two-electrode set-up, shows relatively higher photoactivity for tandem conguration, especially at higher applied potentials (Fig. 6B).The photoelectrodes in tandem conguration showed better photoactivity compared to the parallel conguration, possibly due to the increased surface to volume ratio of the device.Fig. 6C shows the current response of unassisted water splitting cell involving p-type C/Cu 2 O NNskntype TiO 2Àx NRs in tandem and parallel congurations (using 20 h TiO 2Àx NRs) performed in N 2 -purged 0.5 M Na 2 SO 4 and 0.1 M KH 2 PO 4 electrolyte (pH 5.0) under simulated 1 sun illumination with no external bias and any sacricial reagents for 10 min.The photocurrent dropped to one-third of its initial photocurrent before reaching an average plateau of 20 mA cm À2 for the tandem conguration whereas the photocurrent for parallel conguration kept on decreasing over time indicating lower stability.Only a slight difference is seen for parallel and tandem congurations for unassisted tandem cell water splitting cell comprising 12 h TiO 2Àx NR electrode (Fig. S9 †), although the response is relatively stable in tandem conguration.In order to see the durability of unassisted Z-scheme solar water splitting system, a long-term photostability test was performed in both tandem and parallel congurations (Fig. 7).It was found that the electrodes are quite stable with no signs of any degradation over time.Although, tandem conguration response is better, the photoresponse of parallel conguration improved over time.The possible reason for this could be the release of loose C-layer on Cu 2 O NNs, which gets optimized over time.Chopped light response aer an hour-long stability test, still shows the sustained photoactivity.
To further conrm the charge transfer and performance of the proposed tandem PEC cell, a ip-chip method (Fig. 8A) 73 was used in the absence of any sacricial redox species such as water.A light-activated heterojunction diode between C/Cu 2 O NNs and TiO 2Àx NRs can be established with a low turn-on voltage around 0.15 V when these nanostructured electrodes are in intimate contact.Under forward bias, the photocurrent density increases nonlinearly up to $2 mA cm À2 and reaches almost six times more than the photoresponse from a planer Cu 2 O thin lm because of its larger interfacial area in the nanostructure heterojunction (Fig. 8B).A leakage current close to 62 nA cm À2 was observed in the reversed bias at À1.5 V. J-V characteristics of the fabricated p-n junction diode showed a photo-dependent rectifying behavior with high sensitivity towards the UV light and a small photoresponse in the visible region from 430 to 600 nm (Fig. 8C).Such behavior is because of the higher population of carriers generated from TiO 2 NRs than the bottom Cu 2 O NNs in the present light illumination conguration as shown in Fig. 8A.
In order to understand the charge transport and separation processes between C/Cu 2 O NNs and TiO 2Àx NRs and possible charge transfer mechanism of the photogenerated charge carriers, the respective CB or VB edge positions and carrier concentrations of C/Cu 2 O and TiO 2Àx are estimated from Mott-Schottky analysis using following relations. 74,75 ][84] The Fermi energy levels, CB or VB band positions are estimated based on MS analysis and optical absorption studies and   86 Although the p-n heterojunction photodiode does not suffer from sluggish redox reactions and the corrosion effects in water encountered in the proposed PEC tandem cell; further optimization to maximize contact between the two photoelectrodes could increase the charge separation, stability and ultimately the photocurrent response from the p-n heterojunction photodiode.To improve the tandem cell efficiency further, the absorption of TiO 2 NRs can be extended in the visible region by decreasing the band gap via hydrogen doping, 87 or through surface plasmon resonance (SPR) effect of Au nanostructures. 88Alternatively, a Z-scheme photocathode can be formed by depositing Cu 2 O on Auincorporated TiO 2 NRs array to improve the overall charge separation, the carrier density, and the kinetics of electron injection into electrolyte for enhancing the photoactivity toward solar-to-fuel energy conversion. 16Another approach would be to combine TiO 2 NRs with chalcogenide to improve the aligned hole transport and charge-transfer kinetics. 89

Conclusions
In this work, a self-biased, UV-vis light-responsive tandem cell for Z-scheme solar water splitting was established using two photoelectrodes, viz. a p-type carbon-modied Cu 2 O NNs and ntype TiO 2Àx NRs.The carbon layer was proven to protect the Cu 2 O NN morphology as well as improve the charge separation by imparting additional conductivity, while the presence of oxygen vacancies facilitated the charge transfer in TiO 2Àx NRs.Such tandem cell exhibited unassisted solar driven Z-scheme water splitting with a photocurrent activity of 64.7 mA cm À2 that gradually decreased over time.Although the tandem cell performance did not use any sacricial reagents or redox mediators, the overall tandem performance is still limited by the C/Cu 2 O NNs performance and poor visible-light response from TiO 2 .Further improvements to the system are still necessary to increase the overall performance.Simultaneously, the nanostructured heterojunction diode made of C/Cu 2 O NNs and TiO 2Àx NRs demonstrated a current density of 2 mA cm À2 at 1.5 V with a 62 nA cm À2 leakage current at À1.5 V.These ndings could open new pathways to develop low-cost and efficient unassisted solar water splitting systems.Photodiode characteristics of the nanostructured p-n junction electrode is not only promising towards PEC studies for solar water splitting but also would potentially benet other electronic devices such as ultrasensitive molecular sensing and optoelectronics.

Cu 2 O
NNs and TiO 2Àx NRs were characterized by XRD and Raman spectroscopy to conrm the structure and nanocrystalline nature.Although XRD pattern of the synthesized C/Cu 2 O NNs with 10 mg mL À1 dextrose showed a small shi in 2q angle compared to that of Cu 2 O NNs; they both showed elemental Cu peaks attributed to the Cu foil substrate and a single cubic Cu 2 O phase (ICDD#00-05-0667) with a preferential orientation along the (111) orientation (Fig. 2A).
and B show the respective SEM and TEM images of C/ Cu 2 O NNs synthesized using 10 mg mL À1 dextrose.C/Cu 2 O NNs are $5 mm in length and 400 nm in diameter.Fig. 3C and D reveal uniform hydrothermal growth of TiO 2Àx NRs grown on FTO for 20 h with average NR thickness of $2 mm.C/Cu 2 O NNs were synthesized by anodization of Cu foil from KOH solution at a constant current of 10 mA to obtain uniformly distributed Cu(OH) 2 NNs according to eqn (2).The SEM images of Cu(OH) 2 NNs prepared at different anodization durations, and their corresponding photocurrent responses are shown in Fig. S2 and S3A, † respectively.The formation of carbon modied-Cu 2 O NNs was achieved by the dehydration and oxygen removal of Cu(OH) 2 NNs upon annealing in the presence of dextrose and nitrogen (eqn ( 28 A series of carbon-modied Cu 2 O NNs were prepared by soaking Cu(OH) 2 NNs in different dextrose concentrations (1-50 mg mL À1 ) overnight.Although all C/Cu 2 O NNs showed better photocatalytic activity compared to unmodied Cu 2 O NNs, C/Cu 2 O NNs synthesized with 10 mg mL À1 dextrose showed optimum activity (Fig.S3B †).Cu 2 O samples were further characterized using cyclic voltammetry.As shown in Fig.S5, † a reduction peak of Cu 2+ to Cu + is observed around À0.20 V vs. Ag/AgCl for both Cu foil (Fig.S5A †) and Cu(OH) 2 NNs (Fig.S5B †) samples.The reduction peak is shied towards a more negative potential (À0.50 V vs. Ag/AgCl) for the unmodied Cu 2 O NNs (Fig.S5C †), and the shi can be attributed to the increase in charge diffusion polarization compared to Cu(OH) 2 NNs and Cu foil samples.50The absence of reduction peak for the carbon-modied Cu 2 O NNs (Fig.S4D †) conrms the role of the carbon layer in protecting the nanoneedle morphology.The photocurrent responses of different TiO 2Àx NRs (6-20 h) were recorded in a three-electrode system under the back-side (FTO side) and the front-side (NRs side) illuminations (Fig.S6 †).Although the average back-side illumination response is almost half the front-side response, TiO 2Àx NRs grown for 12 and 20 h were further investigated for effect of the NR length on the overall efficiency of the tandem PEC cell.The reason for varied photocurrent responses for different TiO 2Àx NRs under different illumination conditions is explained on the basis of few factors that includes light penetration depth (optical thickness), type of illumination (front or back), and hole diffusion length.According to Beer-Lambert's law, the lm thickness must be optimized for maximum electrode performance.The absorbed light ux depends on the absorption length of the light impinging onto the photoelectrode.In other words, the optical thickness decides the ability of the photons to penetrate inside the lm.Under frontside illumination, while traversing through the NRs, most photons are absorbed near the surface and are possibly used up in the water oxidation reaction.On the other hand, during backside illumination through FTO substrate, the photogenerated holes have to travel longer distance upon separation to reach the surface for water oxidation reaction, depending on the NR lengths.Under backside illumination, the sluggish water oxidation response from TiO 2 -12 h electrode at lower potentials compared to TiO 2 -15 h electrode is due to poor charge collection possibly due to the presence of defects trapping the holes and a poor interface between FTO and the base of TiO 2 NRs.However, both TiO 2 -12 h and TiO 2 -15 h electrodes perform similarly at higher applied potentials.The reason for higher performance from TiO 2 -12 h electrode under front-side illumination could be due to efficient charge separation from optimal length of TiO 2 NRs.

Fig. 3 (
Fig. 3 (A) SEM image of C/Cu 2 O NNs; (B) TEM image of a C/Cu 2 O NN; (C) top and (D) cross-sectional SEM images of 20 h TiO 2Àx NRs on FTO.

Fig
Fig.4A and Bshow the Tauc plots of C/Cu 2 O NNs and TiO 2Àx NRs, respectively.The DRS measurements were transformed using the Kubelka-Munk function (Fig.S7 †), and a Tauc plot was used to estimate the band gap (E g ) values of 2.18 eV for C/ Cu 2 O NNs and 3.26 eV for TiO 2Àx NRs.28,51,52The photoelectrodes were further analysed by uorescence spectroscopy to investigate the recombination processes of photogenerated carriers.TiO 2Àx NRs, under excitation at 365 nm, showed a bluegreen emission centered around 436 nm and a shoulder peak around 534 nm (Fig.4C).The emission spectra were deconvoluted into four peaks: (1) 404 nm, attributed to the rutile structure of bulk TiO 2 crystals; (2) 411 nm, attributed to the trapped excitons; (3) 461 nm and (4) 534 nm, which are due to oxygen vacancies at the surface of the NRs.[53][54][55][56]The C/Cu 2 O NNs (Fig.4D), excited at 470 nm, show a major exciton peak around 621 nm and a shoulder peak centered around 539 nm both usually attributed to phonon-assisted transitions in Cu 2 O samples conrming that no carbon elements were doped into the Cu 2 O lattice.57Fig.5Ashows the individual photocurrent responses of ptype C/Cu 2 O NNs and n-type TiO 2Àx NRs measured with threeelectrode set-up.The photocurrent response of TiO 2 NRs is inverted for convenience in order to identify the operating the current of Z-scheme solar water splitting system with a common intersection point, an approach analogous to typical load-line analyses of photovoltaic cells and resistive loads.58The photocurrent turn-on potential of C/Cu 2 O NNs is more positive than that of n-type TiO 2Àx NRs.The C/Cu 2 O NNs photocathode shows a net photocurrent density of $À300 mA cm À2 at 0 V vs. RHE (À0.5 V vs. Ag/AgCl).TiO 2 shows a stable photocurrent density of 60 mA cm À2 at 1.23 V vs. RHE (0.8 V vs. Ag/AgCl).The cathodic photocurrents ranging from À240 mA cm À2 to À2.45 shows the maximum operating current of 60 mA cm À2 for TiO 2Àx NRs photoanode and C/Cu 2 O NNs photocathode.The action spectra of p-C/Cu 2 O NNs and n-TiO 2Àx NRs (Fig. 5B) strongly suggests that the proposed tandem PEC cell can have broader sunlight absorption by extending the UV absorption response for the TiO 2Àx NRs to visible absorption for the C/Cu 2 O NNs.The individual electrode contribution towards overall solar water splitting in a PEC cell was evaluated by measuring the J-V curves by exclusively illuminating one photoelectrode at a time and compared

Fig. 4
Fig. 4 Tauc plots revealing band gap energies of (A) C/Cu 2 O NNs and (B) TiO 2Àx NRs.Normalized emission spectra of (C) TiO 2Àx NRs and (D) C/Cu 2 O NNs.The excitation wavelength of 365 nm and 470 nm for TiO 2Àx NRs and C/Cu 2 O NNs, respectively.For emission measurements, the nanostructured films were scratched off, dissolved in ethanol, then drop-casted onto an indium tin oxide (ITO) glass substrate.

Fig. 5 (
Fig. 5 (A) Individual photocurrent responses of p-type C/Cu 2 O and ntype TiO 2Àx (inverted) to determine the bias-free operating condition (a green dot depicts intersection point), (B) normalized spectral photocurrent responses of C/Cu 2 O NNs and TiO 2Àx NRs, and (C) influence of individual illumination on the J-V curves of p-type C/ Cu 2 O NNskn-type TiO 2Àx NRs (20 h) tandem cells in parallel configuration.Nyquist plots of (D) Cu 2 O NNs with and without a protective carbon layer and (E) TiO 2Àx NRs grown at 12 and 20 h in the dark and under the light.Electrolyte: N 2 -purged 0.5 M Na 2 SO 4 in 0.1 M KH 2 PO 4 (pH 5.0); light illumination: 1 sun (A.M 1.5, 100 mW cm À2 ).
2 and the photocurrent generation from C/ Cu 2 O is limited by the number of charge carriers available for C/ Cu 2 O upon their separation by TiO 2 .Under simulated solar light illumination, both C/Cu 2 O NNs and TiO 2Àx NRs of the tandem PEC cell absorb photons to produce electrons and holes.The photogenerated electrons in the CB of the TiO 2Àx NRs are transferred to the VB of the C/Cu 2 O NNs to recombine with the holes in C/Cu 2 O NNs.The holes in TiO 2Àx NRs and the electrons in C/Cu 2
170 for TiO 2 (ref.76) and 3 ¼ 7.6 for Cu 2 O (ref.26)), 3 0 is the permittivity of the free space, N is the donor or acceptor density (N D or N A ), E is the applied potential, E  is the at-band potential that equals the Fermi energy when semiconductor band bending vanishes at at-band conditions, 77,78 A* is the area of the electrode in contact, k is the Boltzmann constant, and T is the absolute temperature.When the linear region of the Mott-Schottky plot is extrapolated to X-axis at 1/C sc 2 ¼ 0, the intercept at the X-axis leads to the quantity (E  + kT/e), from which the at-band potential values are determined (Fig. S10 †).The E  values estimated from MS plot for TiO 2Àx NR and C/ Cu 2 O NN electrodes are 0.13 and 0.54 V vs. RHE, respectively, which are in agreement with the literature values for TiO 2 (ref.75, 76 and 79) and Cu 2 O. 80,81 The positive and negative slopes in the MS plots or +ve and Àve sign in eqn (2) conrm the n-type and p-type conductivity of TiO 2Àx and C/Cu 2 O electrodes, respectively.The corresponding N D and N A carrier concentrations estimated for TiO 2Àx and C/Cu 2 O are 2.103 Â 10 17 cm À3 and 2.262 Â 10 20 cm À3 , respectively, which are also in line with the reported values for TiO 2 (ref.75 and 76) and Cu 2 O. 82-84

Fig. 7
Fig. 7 A long-term durability test of unassisted Z-scheme solar water splitting system involving n-type TiO 2Àx NRs and p-type C/Cu 2 O NNs in tandem and parallel configurations, performed in N 2 -purged 0.5 M Na 2 SO 4 phosphate-buffered electrolyte (pH 6.85) under 1 sun illumination.Light was chopped at the end to check the photoactivity response and photostability (right panel shows the magnified view of the chopped light response).

Fig. 8 (
Fig. 8 (A) Schematic illustration and (B) J-V curves of p-type C/Cu 2 O NNskn-type TiO 2Àx NRs (20 h) heterojunction photodiode under 1 sun illumination (AM 1.5, 100 mW cm À2 ).(C) The spectral photocurrent response of p-type C/Cu 2 O NNskn-type TiO 2Àx NRs (20 h) heterojunction measured at different applied potentials in a two-electrode system with TiO 2Àx NRs biased positively for pronounced photocurrent response.

Fig. 9
Fig. 9 Energy band diagrams of p-type C/Cu 2 O NNs and n-type TiO 2Àx NRs (20 h) (A) before and (B) after intimate contact to form a pn heterojunction.