Highly efficient photoanodes based on cascade structural semiconductors of Cu2Se/CdSe/TiO2: a multifaceted approach to achieving microstructural and compositional control.

Hydrogen produced by splitting water is receiving significant attention due to the rising global energy demand and growing climate concern. The photocatalytic decomposition of water converts solar energy into clean hydrogen, and may help mitigate the crisis of fossil fuel depletion. However, the photocatalytic hydrogen production remains challenging to obtain high and stable photoconversion efficiency. Here, we report a highly efficient photoanode based on coaxial heterogeneous cascade structure of Cu 2 Se/CdSe/TiO 2 synthesized via a simple room-temperature and low-cost electrochemical deposition method. The microstructure and composition of the Cu 2 Se top layer are regulated and controlled by doping Cu with various amounts in different zones of the CdSe/TiO 2 coaxial heterojunction and then using a simple integral annealing process. Surprisingly, a little effort made to achieve the Cu 2 Se top layer utilizing such doped CdSe/TiO 2 exhibits a significant enhancement in photocatalytic activity. The maximum stable photocurrent density of the sample with the optimal copper zone and doping concentration has reached up to 28 mA/cm 2 , which can be attributed to the success in the uniform dispersion of the three-layer heterogeneous nanojunctions among the anatase nanotube wall from top to bottom. This results in a stepwise structure of band-edge levels in the Cu 2 Se/CdSe/TiO 2 photoelectrode that is conducive to enhancing effectively the separation of the photogenerated electron-hole pair.


Introduction 1
The photocatalytic decomposition of water, which uses solar energy to split water and produce cheap hydrogen as a clean energy heterojunction research of CdSe/TiO 2 NTAs focuses mainly on the formation of single-junction nano-materials, which generally use integral annealing process. The microstructural and compositional characteristics of the materials can be controlled and manipulated

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Synthesis of highly ordered TiO 2 NTAs followed the typical two-step anodic oxidation method. 44 Titanium foils (99.8% purity) were mechanically ground using emery papers with different types and polished with the metallographic abrasive paper successively, then 1 and distilled H 2 O (2.0 vol %) in ethylene glycol at room temperature using platinum foil as the counter electrode. The two-step 2 anodic oxidation was conducted as follow: step-1, the titanium foil was firstly anodized at 60 V for 20 min. in the electrolyte, 3 followed by rinsing with ethanol and drying in ambient air, and then by annealing at 700 ℃ in a muffle furnace for 1 h with heating 4 rate of 7 ℃/min; step-2, the sample was re-soaked into the electrolyte and suffered the second anodization for 11 h, then was 5 annealed again at 450℃ in the muffle furnace for 2 h at a heating rate of 2℃/min after rinsing with ethanol and drying in an oven at 6 100℃ for 1 h.

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For electrochemical deposition of CdSe on TiO 2 NTAs, CdSO 4 and SeO 2 of the analytical reagent grade were used as the sources of 9 Cd and Se, respectively. EDTA-2Na (C 10 H 14 O 8 N 2 Na 2 •2H 2 O) and NH 4 OH were used to complex the ions and adjust the pH value for 10 obtaining a proper electrodeposition potential. The solutions were freshly prepared just before the beginning of each series of 11 measurements. The electrochemical deposition was carried out using a computer controlled electrochemical workstation that was 12 connected to a three-electrode system comprised of Ti foil as work electrode (WE), Pt foil as counter electrode (CE) and Hg 2 Cl 2 /KCl 13 (SCE) as reference electrode (RE). For copper doping, CuSO 4 (2 mM) solution was used as Cu source.

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The morphologies of the samples were studied by field emission scanning electron microscopy (FESEM) (Nova NanoSEM 450) and

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The optical response performance of the samples was investigated in a photoelectrochemical cell with a platinum foil counter electrode and the SCE reference electrode. 0.5 M Na 2 S was used as the electrolyte in photoelectrochemical measurements. The

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CorrtestTM CS350 electrochemical workstation was also used to control the potential and record the photocurrent generated. Xe 24 lamp (CHF-XM35-500W) coupled to an AM 1.5G filter was used as the standard light source throughout the tests. The illumination 25 intensity of 100mW/cm 2 was calibrated with a readout meter for solar simulator irradiance before the measurement. The sample size 26 was 1.0 cm×2.0 cm.

Results and discussion 28
This research focuses on synthesis of multi-junction nano-materials coated with highly ordered structure through a modified 29 electrochemical atomic layer deposition (ALD) route, and on studying and manipulating their microstructural and compositional 30 properties. The electrochemical ALD method as reported in the literature is based on underpotential deposition (UPD). 47-51 UPD is a 31 surface-limited phenomenon in which the deposition of one element occurs at a potential that precedes the Nerstian equilibrium 32 value, so that the resulting deposit is generally limited to one atomic layer. Electrochemical ALD utilizes alternating UPD of the 33 elements that form the compound semiconductor in a cycle. Each deposition cycle can form only a monolayer of heterogenous 34 elements, and the thickness of the deposit is controlled by the number of deposition cycles. To date, this method has been extensively 35 used to grow highly crystalline nanofilms of transition-metal chalcogenides at ambient temperature and pressure and is convenient 36 for industrial production. Before electrochemical ALD, it is pivotal to find the suitable UPD potential of each compositional element 37 of the compound. This can be determined by cyclic voltammetry (CV). The electrochemistry behaviour of Se(IV) was investigated in 38 an ammonia buffer medium. 49,52 In this regime, two competitive processes were observed: the first led to the formation of Se(0), and 39 the second resulted in further reduction of Se(0) to HSe -. Thus, it is important to understand which process dominates in the 40 competition, and which exerts a direct impact on the Se UPD behaviour. Apparently, the competition can be affected by a number of 41 factors, such as the type of electrolyte and buffers, pH, complexing agent, temperature, and so on. Therefore, in this study, the 42 addition of EDTA as a complexing agent, the content variation of the ammonia buffer, and a resultant suitable pH value are used to 43 adjust the UPD potential of Se and Cd for the deposition of the CdSe layer. Figure 1   agent. It can be seen that both CVs exhibit a reduction peak around -0.6 V, which is indication of the UPD potentials of both Se and procedures. 53,54 In the modified electrochemical ALD, a constant potential within the common UPD region was chosen and held

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The CdSe(7h)/TiO 2 electrode was then selected as the optimal seed layers on account of its best photocurrent density in this 16 work. Before doping Cu into the CdSe nanofilm, a suitable potential for Cu deposition on the CdSe underlayer should be determined.       Figure S1c,d). The tube inner diameter was shrunken and the wall was thickened relative to those in 9 Figure S1a,b. After CdSe electrodeposition, the average inner diameter of the tubes was ∼80 nm, suggesting that the CdSe coating 10 layer was ∼10 nm thick (the pure TiO 2 NT substrate had an average inner diameter of 100 nm). Figure S1(e, f) present the FE-SEM   Figure S2(b,c) when semiconductor was deposited on the TiO 2 TNAs using the electrochemical ALD 17 method, both the interior and exterior surfaces of TiO 2 NTAs were homogeneously coated with sensitizer without any obvious 18 particle agglomerations, suggesting the electrochemical ALD method in our work is rather efficient to grow well dispersed nanofilms 19 among the whole nanotube wall.

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The prepared samples were further investigated using transmission electron microscopy (TEM). Figure 6a   plane. This result demonstrates that the double-layer Cu 2 Se and CdSe co-sensitized TiO 2 NT electrodes with a coaxial heterogeneous Such stepwise energy band structure is advantageous to the electron injection and hole recovery in the system. When the copper 11 doping site is chosen near the outer layer, it gives rise to the formation of Cu 2 Se at the top level during the integral annealing process.

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As a result, a suitable architecture of Cu 2 Se/CdSe/TiO 2 can be obtained, which is favourable for reducing the charge-carrier

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Practical photoelectrolysis system consists of two electrolyte immersed electrodes with the bias voltage applied between the 13 working and counter electrodes, the overall chemical reaction in such a system is made of two independent half reactions. 56 To 14 understand the chemical changes at the photoelectrode, in laboratorial water photoelectrolysis experiments a three-electrode 15 geometry is used to measure photocurrent. This geometry involves a working electrode (photocathode or photoanode), a counter 16 electrode that generally is platinum and a reference electrode that is SCE electrode in our work. To investigate the difference of 17 photoconversion efficiency between the two-electrode and three-electrode configuration, the photochemical measurement was 18 carried out. Figure S3 shows the measurement results of CdSe/TiO 2 and Cu 2 Se/CdSe/TiO 2 in different configuration. The  To quantify the photoresponse of prepared samples, incident-photon-to-current-conversion efficiency (IPCE) measurements were 10 made to examine their photoresponses as a function of incident light wavelength. As revealed in Figure 9a  annealing, the absorption was further red-shifted and has an absorption tail to 800 nm, gaining a band-gap value of 1.5 eV. In 21 addition, the absorbance of the cascade structural Cu 2 Se/CdSe/TiO 2 NTAs film is apparently stronger than that of the CdSe/TiO 2 film in the visible region from 400 to 800 nm. The enhanced absorption is believed to result from the formation of the Cu 2 Se top

Conclusions
In this paper, we have proposed and demonstrated a new method for the fabrication of a novel semiconductor photoanode with a unique Cu 2 Se/CdSe/TiO 2 nanostructure. The fabrication is developed on the TiO 2 NTAs substrate in three steps. Firstly, the CdSe inner layer was successfully deposited on pure TiO 2 nanotubes substrate to form a nanotube-array coaxial heterogeneous structure by electrochemical ALD. It was found that CdSe deposition for 7 h had led to a significant improvement in the photocurrent density, reaching 7.5 mA/cm 2 . Secondly, the CdSe(7h)/TiO 2 nanotubes were selected as the optimal seed layer for Cu doping. Finally, integral annealing was applied, and the sample with the structural formula of CdSe(1h)/Cu(0.6C)CdSe(6h)/TiO 2 showed the best photoelectrochemical performance.
The product possessed a cascade of multiple heterogeneous junctions formed in the coaxial manner on the TiO 2 NTAs substrate. An accurate control and manipulation in microstructure and composition of the materials could be achieved by simply changing the b a