Enhanced photo-electrochemical activity of ZnO/Cu2S nanotube arrays photocathodes

ZnO/Cu₂S nanotube arrays are fabricated firstly by a facile and capping-agent-free method, and the photo-electrochemical performance has been studied systematically. The results show that ZnO/Cu₂S nanotube arrays achieve enhanced photo-electrochemical water splitting performance and the photocurrent densities of ZnO/Cu₂S are 7.9 times than that of ZnO at 0 V versus Ag/AgCl. The performance of the ZnO/Cu₂S nanotube arrays can be adjusted by changing the amount of Cu₂S microcrystals. The results confirm that the enhanced photo-electrochemical performance of ZnO/Cu₂S is due to the significantly improved visible light absorption, effective separation of photo-induced carriers due to the well band energy match and the formed p-n junction between ZnO and Cu₂S.     

Flame-assisted pyrolysis formation of Cu2O/Cu/TiO2 nanotube arrays to boost superior photo-electrochemical response

Herein, a flame-assisted pyrolysis technique is developed for one-step in-situ construction of Cu₂O/Cu/TiO₂ nanotube arrays. The structure and morphology of the obtained samples are systematically characterized. Interestingly, UV–vis absorption spectroscopy reveals that the introduction of Cu₂O and Cu considerably enhance light response of TiO₂ nanotube arrays in the visible region. Moreover, the composited electrode shows enhanced photo-electrochemical activity. Compared with blank TiO₂ nanotube arrays, not only the photocurrent and photo-voltage of composited electrodes are improved but also the stability is also enhanced. And, the maximum photo-conversion efficiency for composited electrode presents 4.71 folds larger than that of blank TiO₂ electrode. This enhancement is due to the faster charge separation/transportation ability of Cu and visible light response of Cu₂O. This research develops a novel and facile method for the in-situ fabrication of Cu₂O/Cu/TiO₂ nanotube arrays and demonstrates their improved utilization of solar energy.     

Cu2O/Cu/TiO2 nanotube Ohmic heterojunction arrays with enhanced photocatalytic hydrogen production activity

Cu₂O/Cu/TiO₂ nanotube heterojunction arrays were prepared by assembling Cu@Cu₂O core-shell nanoparticles on TiO₂ nanotube arrays (NTAs) using a facile impregnation-reduction method. SEM and TEM results show that Cu@Cu₂O plate-like nanoparticles with tens of nanometers in size are confined inside TiO₂ NTAs. Only the outmost several nanometers of the nanoparticles are Cu₂O and the predominant inner of the nanoparticles are Cu metals. Cu L₃VV Auger spectra of Cu₂O/Cu/TiO₂ NTAs suggest that Cu metals are enveloped by at least several nanometers Cu₂O on the surface, which further confirms the Cu@Cu₂O core shell structure of Cu nanoparticles. The ability of light absorption of Cu₂O/Cu/TiO₂ NTAs is enhanced. The range of absorption wavelengths changes from 400 to 700 nm due to the surface plasmon response of Cu metals core and Cu₂O nanoparticles shell. The photocatalytic hydrogen production rate of Cu₂O/Cu/TiO₂ heterojunction arrays is enhanced when compared with those of Cu₂O/TiO₂ NTAs and TiO₂ NTAs under UV light. Moreover, a stable H₂ generation property was obtained under visible light (λ gt; 400 nm). The Cu metal core is believed to play a key role in the enhancement of photocatalytic properties of Cu₂O/Cu/TiO₂ nanotube heterojunction arrays.     

Thin film deposition of Cu2O and application for solar cells

Deposition conditions of cuprous oxide (Cu₂O) thin films on glass substrates and nitrogen doping into Cu₂O were studied by using reactive radio-frequency magnetron sputtering method. The effects of defect passivation by crown-ether cyanide treatment, which simply involves immersion in KCN solutions containing 18-crown-6 followed by rinse, were also studied. By the crown-ether cyanide treatment, the luminescence intensity due to the near-band-edge emission of Cu₂O at around 680 nm was enhanced, and the hole density was increased from 10¹⁶ to 10¹⁷ cm⁻³. Finally, polycrystalline p-Cu₂O/n-ZnO heterojunctions were grown for use in solar cells. Two deposition sequences were studied, ZnO deposited on Cu₂O and Cu₂O deposited on ZnO. It was found that the crystallographic orientation and current–voltage characteristics of the heterojunction were significantly influenced by the deposition sequence, both being far superior for the heterojunction with structure Cu₂O on ZnO than for the inverse structure. We successfully obtained a photoresponse for the first time in the deposited thin film of Cu₂O/ZnO.     

Deposition of heterojunction of ZnO on hydrogenated TiO2 nanotube arrays by atomic layer deposition for enhanced photoelectrochemical water splitting

The heterojunction of ZnO was deposited on hydrogenated TiO₂ nanotube arrays (H–TiO₂) by atomic layer deposition (ALD) with various cycles. The ZnO was uniformly wrapped with the H–TiO₂ samples and the thickness could be accurately controlled by the cycle numbers of ALD. The higher growth rate ~2.7 Å/cycle was obtained due to the surface amorphous layer, compared with the air-treated samples (A-TiO₂), ~2.3 Å/cycle. When the cycle numbers increased to 200, nanowire arrays appeared. Interestingly, the absorption in the visible light region improved more significantly when ALD ZnO was employed for the H–TiO₂ rather than the A-TiO₂ samples. The H–TiO₂ samples with 42 nm of ALD ZnO exhibited enhanced photoelectrochemical water splitting performances, compared with the A-TiO₂ with 42 nm of ALD ZnO. This was related to the higher degree of the electronic band bending and improved photo-response in the UV and visible light region, resulting from the oxygen vacancies.     

The influence of the electrodeposition potential on the morphology of Cu2O/TiO2 nanotube arrays and their visible-light-driven photocatalytic activity for hydrogen evolution

The influence of the electrodeposition potential on the morphology of Cu₂O/TiO₂ nanotube arrays (Cu₂O/TNA) and their visible-light-driven photocatalytic activity for hydrogen evolution have been investigated for the first time in this work. The photocatalytic hydrogen evolution rate of the as-prepared Cu₂O/TNA at the deposition potential of −0.8 V was about 42.4 times that of the pure TNA under visible light irradiation. This work demonstrated a feasible and simple electrodeposition method to fabricate an effective and recyclable visible-light-driven photocatalyst for hydrogen evolution.     

Highly broadband plasmonic Cu film modified Cu2O/TiO2 nanotube arrays for efficient photocatalytic performance

To develop an efficient photocatalyst electrode for solar energy harvesting and photocatalysis application in the visible region, broadband plasmonic Cu film combined with Cu₂O/TiO₂ nanotube arrays heterojunction (Cu film/Cu₂O/TiNT) has been successfully fabricated by anodization combined with electrodeposition method. Interestingly, linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and UV–Vis diffuse reflectance spectroscopy reveal that the combined consequence of both Cu film and Cu₂O in the as-synthesized ternary composite considerably enhances light absorption in the visible spectral. This activity is attributed to the more efficient charge separation/transportation and the presence of Cu film with strong plasmon resonance (SPR) effect. Moreover, the combined effects of both Cu film and Cu₂O on TiNT approved highest catalytic current density and highest photocatalytic activity on methylene blue (MB). The efficiency and the rate of MB photodegradation over the Cu film/Cu₂O/TiNT were found to be triple compared to TiNT. Within only 30 min of reaction time, the photodegradation of MB reaches nearly 100%.     

Chemical bath deposition synthesis of TiO2/Cu2O core/shell nanowire arrays with enhanced photoelectrochemical water splitting for H2 evolution and photostability

In this study, we have developed a facile chemical bath deposition (CBD) method to grow p-type Cu₂O nanoparticles on n-type TiO₂ nanowire arrays (TiO₂ NWAs) to fabricate TiO₂/Cu₂O core/shell heterojunction nanowire arrays (TiO₂/Cu₂O core/shell NWAs). When used as photoelectrode, the fabricated TiO₂/Cu₂O core/shell NWAs show improved photoelectrochemical (PEC) water splitting activity to pure TiO₂ NWAs. The effects of the CBD cycle times on the PEC activities have been studied. The TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode prepared by cycling 5 times in the CBD process achieves the highest photocurrent of 2.5 mA cm^?2, which is 2.5 times higher than that of pure TiO₂ NWAs. In addition, the H₂ generation rate of this photoelectrode reaches to 32 μmol h^?1 cm^?2, 1.7 times higher than that of pure TiO₂ NWAs. Furthermore, the TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode shows excellent photostability and achieves a stable photocurrent of over 2.3 mA cm^?2 during long light illumination time of 5 h. The enhanced photocatalytic activity of TiO₂/Cu₂O core/shell heterojunction nanowire array photoelectrode is attributed to the synergistic actions of TiO₂ and Cu₂O for improving visible light harvesting, and efficient transfer and separation of photogenerated electrons and holes.     

Structural properties and enhanced photoelectrochemical performance of ZnO films decorated with Cu2O nanocubes

Combination of ZnO and Cu₂O semiconductors is remarkable for efficient photovoltaic cells and enhanced photoelectrochemical (PEC) performance due to the high electronic energy band alignment of these materials and their controllable electronic structure at the interface. This study reports on a systematic analysis of the effects of Cu₂O nanocube doping on the structural properties and PEC performance of ZnO films. ZnO samples doped with Cu₂O were prepared by a practical electrochemical method. Characterization of the materials was performed by XRD, Raman, FTIR spectroscopy and electrochemical techniques. The XRD, Raman, FTIR spectroscopy analyses indicated a single phase of ZnO for the lower Cu₂O deposition time, while a secondary phase of Cu₂O evolved for the 5 min deposition time. This study showed that ZnO doped with Cu₂O grown for 3 min had the best PEC performance. ZnO/Cu₂O photoelectrodes are recommended as an attractive, competitive and alternative candidate for advanced PEC sensing and this may be for the extended field of water splitting into oxygen and hydrogen under sunlight.     

Cu2O as an emerging photocathode for solar water splitting - A status review

Recently, cuprous oxide (Cu₂O) based photocathodes have gained research attention for hydrogen (H₂) production through photoelectrochemical (PEC) water splitting reactions due to marginally lower synthesis cost and low energy intensity fabrication processes. Unique properties of Cu₂O, such as tunable bandgap, appropriate band edge potentials with water redox levels and non-toxic nature makes it beneficial for PEC applications. Cuprite is mainly studied under visible light to facilitate enhanced H₂ gas production upon illumination. However, notoriously photocorrosion degrades the PEC performance and restricts the photoactivity of Cu₂O. Moreover, because of the redox potentials lies within the band gap of Cu₂O; self-photocorrosion or self-oxidation upon illumination is unavoidable. Improvement in the Cu₂O photocathodes was achieved by finding elegant solutions such as forming thin heterojunction layers by atomic layer deposition (ALD) as well other methods, co-catalyst deposition, tuning crystal facets and surface modifications with different synthetic methods. In this review, we discuss the improvements in Cu₂O photocathodes achieved over the years for enhanced H₂ production with recently studied photocathodes.     

Synergetic effect of carbon self-doping and TiO2 deposition on boosting the visible-light photocatalytic hydrogen production efficiency of carbon nitride

To improve the visible light utilization and photogenerated carriers separation, carbon self-doped carbon nitride (C-CN) supported TiO₂ photocatalysts were synthesized via a designed two-step strategy. After carbon self-doping, the colloid TiO₂ were in-situ deposited on C-CN surface and crystalized by calcination. Simultaneously, the bulk C-CN structure was thermally exfoliated to nanosheet morphology. This strategy ensured the saturated deposition of colloid TiO₂ nanoparticles on C-CN nanosheets to form well-constructed heterostructure with sufficient interfacial contact. The as-prepared TiO₂/C-CN (TCN) heterojunction photocatalysts showed enhanced visible light absorption capability, resulting in impressively high hydrogen production efficiency as 212.7 μmol h⁻¹, which was 10.8 times higher than that of CN. The remarkably enhanced photocatalytic performance may be mainly ascribed to synergetic effect of carbon self-doping and TiO₂ deposition on the improved visible light utilization and photogenerated carriers separation. The probable mechanism in such well-constructed heterojunction photocatalysts was proposed based on the structural analysis, electrical and photoelectrical properties, and photocatalytic process. The proposed strategy may be extended to the preparation of diverse heterojunction photocatalysts with excellent performance for solar energy conversion.     

Surface modification of aligned TiO2 nanotubes by Cu2O nanoparticles and their enhanced photo electrochemical properties and hydrogen generation application

In this work, we report the synthesis of cuprous oxide (Cu₂O) nanoparticles modified vertically oriented aligned titanium dioxide (TiO₂) nanotube arrays through wet chemical treatment of TiO₂ nanotubes and their multi-functional application as enhanced photo electrochemical and hydrogen generation. The synthesized samples were characterized by X-ray diffraction, SEM, TEM, and UV–Vis spectroscopy. The structural characterization revealed that the admixed Cu₂O nanoparticles on the TiO₂ surface did not alter its crystalline structure of vertically oriented aligned TiO₂ nanotube. The photocatalytic performance and hydrogen generation of as synthesized Cu₂O nanoparticles modified aligned TiO₂ nanotube was found to highly depend on the Cu₂O content. The optical characterizations reveal that the presence of Cu₂O nanoparticles extends its absorption into the visible region which improves the photocurrent density in comparison to pristine aligned TiO₂ nanotubes electrodes due to enhanced photoactivity and better charge separation. The optimum photocurrent density and hydrogen generation rate has been found to be 3.4 mA cm^?2 and 127.5 μmole cm^?2 h^?1 in 1 M Na₂SO₄ electrolyte solution under 1.5 AM solar irradiance of white light with illumination intensity of 100 mW cm^?2.     

Fabrication of highly ordered and vertically oriented TiO2 nanotube arrays for ordered heterojunction polymer/inorganic hybrid solar cell

Highly ordered and vertically oriented TiO₂ nanotube arrays with a length of 250 nm and a diameter of 70 nm were prepared by atomic layer deposition (ALD) coupled with anodic aluminum oxide (AAO) template. An ordered heterojunction (OHJ) polymer/inorganic hybrid solar cell was fabricated by successful infiltration of P3HT into the nanotube arrays. Structural features of the nanotube arrays enabling the interdigitated structure of the OHJ were discussed and the performance of the solar cell was characterized to be the power conversion efficiency of 0.50%.     

ZnO/Cu2O-decorated rGO: Heterojunction photoelectrode with improved solar water splitting performance

In present work, we report a facile fabrication process to improve the photoelectrochemical (PEC) performance of ZnO-based photoelectrodes. In order to achieve that, the Cu₂O nanocubes are cathodic-deposited on the as-prepared ZnO nanorods. Then rGO nanosheets are electrodeposited on the ZnO/Cu₂O heterostructures. The fabricated photoelectrodes are systematically studied in detail by different characterization techniques such as powder X-ray diffraction, micro-Raman, X-ray photoelectron spectroscopy, ultraviolet diffused reflectance spectroscopy and photoluminescence spectroscopy analysis. Morphologies of the fabricated photoelectrodes are investigated through electron microscopy in scanning and transmission mode. To evaluate the PEC performance of the fabricated photoelectrodes, the line scan voltammetry (LSV) measurement is performed using a three-electrode system in 0.5-M Na₂SO₄ electrolyte solution under stimulated light illumination at 100 mW/cm² from a 300-W Xenon Arc lamp coupled with an AM 1.5G filter using a three-electrode system. The photocurrent measurement demonstrates that the photoelectrodes based on ZnO/Cu₂O/rGO possess enhanced PEC performance compared to the pristine ZnO and ZnO/Cu₂O photoelectrodes. The photocurrent density of ZnO/Cu₂O/rGO-15 photoelectrode (10.11 mA/cm²) is ∼9 and ∼3 times higher than the photoelectrodes based on pristine ZnO (1.06 mA/cm²) and ZnO/Cu₂O (3.22 mA/cm²). The enhanced PEC performance of ZnO/Cu₂O/rGO photoelectrode is attributed to the excellent light absorption properties of Cu₂O and excellent catalytic and charge transport properties of rGO. Experimental results reveal that the proposed functional nanomaterials have a great potential in water splitting applications.     

Efficient light harvesting by NiS/CdS/ZnS NPs incorporated in C,N-co-doped-TiO2 nanotube arrays as visible-light sensitive multilayer photoanode for solar applications

Here, we report a significant enhancement in photo-electrochemical activity of co-doped/modified TiO₂ nanotube arrays (TNAs). First, TiO₂ nanostructures were sensitized with nitrogen and carbon via a single step/low cost anodization process and then modified with Nis/CdS/ZnS nano particles (NPs) by the successive ionic layer adsorption and reaction (SILAR) method at room temperature. Photo-electrochemical properties and physical/chemical characteristics of the pure and sensitized/modified TNAs were investigated using field emission scanning electron microscopy (FESEM), XRD, XPS and EDX, comprehensively. Electrochemical measurements and UV–Vis DRS spectroscopy of the photo-electrodes showed that co-doping with anions and modification with different NPs result in the broadening of the absorption region of visible light and the reduction of band gap energy. The mechanism responsible for the enhanced photo-electrochemical activity of the C, N-co-doped/NiS, CdS, ZnS NPs modified TNAs for the water reduction reaction using aqueous solutions of Na₂S/Na₂SO₃ as sacrificial electrolyte under the whole spectrum of simulated solar light irradiation has been presented. The highest photocurrent in presence of sacrificial agent (Na₂S/Na₂SO₃) was obtained as 18.79 mA/cm², for the optimized SILAR loading cycles and dopants concentration. Furthermore, a high incident photon to current efficiency (IPCE) of about 82% for the optimum photo-anode had been achieved. These results confirm that the C, N-co-doped/NiS, CdS, ZnS NPs modified TNAs nanocomposite may offer a promising strategy to attain maximum efficiency in a variety of solar energy conversion systems, along with reduced photo-corrosion in the semiconductor-semiconductor heterojunction.     

Bias-free visible light-driven photoelectrochemical water splitting of type II ZnO/CuS core/shell heterojunction nanotube arrays

Solar-driven water splitting of semiconductor photoelectrodes via photoelectrochemical (PEC) cell has been regarded as the most promising approach to mitigate the energy crisis and environmental issues in the future. In this work, CuS nanoparticles (NPs) are deposited on ZnO nanotube arrays (ZnO/CuS NTAs) via successive ion layer absorption and reaction method for PEC water splitting under visible light irradiation without applying bias. The excellent light harvesting capacity of CuS NPs from visible to near infrared region not only expands the light harvesting of ZnO NTAs into near infrared region, but also substantially boosts light absorption ranging from 300 to 800 nm. Moreover, CuS NPs coupled on ZnO NTAs can establish a type-II band alignment between ZnO and CuS. Consequently, the ZnO/CuS NTAs photoanode exhibits the significantly boosted PEC water splitting performance under visible light illumination (λ > 420 nm) without applying bias. The photocurrent density of the ZnO/CuS NTAs photoanode is 21.2 μA/cm², which is increased by 9 times compared to that of the pure ZnO NTAs photoanode. The enhancement in PEC water splitting performance for ZnO/CuS NTAs is attributed to (i) the cooperative actions of ZnO and CuS; (ii) significant enhancement in light absorption from the visible to near infrared region achieved by CuS NPs and (iii) efficient charge carrier separation achieved by type-II band alignment.     

A promising CuOx/WO3 p-n heterojunction thin-film photocathode fabricated by magnetron reactive sputtering

A CuO_x/WO₃ thin-film based on p-n heterojunction proposed as a highly performance and stable photocathode. The CuO_x/WO₃ thin-film was deposited by magnetron reactive sputtering layer by layer, followed with slow rate annealing in O₂ ambient. This is an excellent method for high-quality and uniform composite thin-film deposition with large areas at a high growth rate. The optimized CuO_x/WO₃ thin-film photocathode after slow rate annealing at 500 °C in O₂ provides an obviously enhanced photoinduced current density of −3.8 mA cm⁻² at a bias potential of −0.5 V (vs. Ag/AgCl), which value is 1.5 times higher than that of bared CuO_x thin-film. This highly enhanced photoelectrochemical performance is attributed to p-n heterojunction, which accelerates the photogenerated electrons and holes transfer to n-WO₃ and p-CuO_x, thereby accelerate the separation of photogenerated carries. In addition, WO₃ layer covered on the surface of CuO_x thin film can improve the stability of Cu₂O in electrolytes.     

Enhanced visible-light-photoconversion efficiency of TiO2 nanotubes decorated by pulsed laser deposited CoNi nanoparticles

The pulsed laser deposition (PLD) technique has been used to decorate TiO₂ nanotubes (NTs) with cobalt-nickel (CoNi) nanoparticles (NPs). The TiO₂ NTs were produced beforehand through the controlled anodic oxidation of titanium substrates. The effect of the nature of the PLD background gas (Vacuum, O₂ and He) on the microstructure, composition and chemical bondings of the CoNi-NPs deposited onto the TiO₂-NTs has been investigated. We found that the PLD CoNi-NPs have a core/shell (oxide/metal) structure when deposited under vacuum, while they are fully oxidized when deposited under O₂. On the other hand, by varying the CoNi-NPs loading of the TiO₂-NTs (through the increase of the number of laser ablation pulses (N_LP)), we have systematically studied their photocatalytic effect by means of cyclic-voltammetry (CV) measurements under both AM1.5 simulated solar light and filtered visible light. We show that depositing CoNi-NPs on the substrate under vacuum and He increases the photo-electrochemical conversion effectiveness (PCE) by 600% (at N_LP = 10,000) in the visible light domain, while their overall PCE degrades with N_LP under solar illumination. In contrast, the fully oxidized CoNi-NPs (deposited under O₂) are found to be the most effective catalyst under sunlight with an overall increase of more than 50% of the PCE at the optimum loading around N_LP ~1000. Such catalytic enhancement is believed to result from both an enhanced light absorption by CoO (of which bandgap is of ~2.4 eV) and the formation of a heterojunction between NiO/CoO nanoparticles and TiO₂ nanotubes.     

Nano-structured Cu2O solar cells fabricated on sparse ZnO nanorods

Nano-structured Cu₂O/ZnO nanorod (NR) heterojunction solar cells fabricated on indium tin oxide (ITO)-coated glass are studied. Substrate film and NR density have a strong influence on the preferred growth of the Cu₂O film. The X-ray diffractometer (XRD) analysis results show that highly (2 0 0)-preferred Cu₂O film was formed when plating on plain ITO substrate. However, a highly (1 1 1)-preferred Cu₂O film was obtained when plating on sparse ZnO NRs. SEM, TEM and XRD studies on sparse NR samples indicate that the Cu₂O nano-crystallites mostly initiate its nucleation on the peripheral surfaces of the ZnO NRs, and are also highly (1 1 1)-oriented. Solar cells with ZnO NRs yielded much higher efficiency than those without. In addition, ZnO NRs plated on a ZnO-coated ITO glass significantly improve the shunt resistance and open-circuit voltage (V_oc) of the devices, with consistently much higher efficiency obtained than when ZnO NRs are directly plated on ITO film. However, longer NRs do not improve the efficiency due to low short-circuit current (J_sc) and slightly higher series resistance. The best conversion efficiency of 0.56% was obtained from a Cu₂O/ZnO NRs heterojunction solar cell fabricated on a 80 nm ZnO-coated ITO glass with V_oc=0.514 V, J_sc=2.64 mA/cm² and 41.5% fill factor.     

Photoelectrocatalytic hydrolysis of ammonia borane by electrochemical deposited cuprous oxide on titanium dioxide nanotube arrays

Photoelectrocatalytic hydrolysis of ammonia borane (AB) is a promising technique for producing hydrogen gas (H₂) in the presence of an appropriate photocatalyst under light irradiation and bias. The application of cuprous oxide on titanium dioxide nanotube arrays (Cu₂O/TNA) for the photoelectrocatalytic hydrolysis of AB was studied. Cu₂O/TNA exhibited a spectral response in the ultraviolet–visible region and an onset wavelength of 600 nm. With AB in an electrolyte, Cu₂O/TNA exhibited a significant increase in its photocurrent spectral response at a bias of 0.1 V versus Ag/AgCl. The H₂ generation rate by photoelectrocatalysis (under 5-mW cm^?2 irradiation at 455 nm; bias of 0.1 V vs. Ag/AgCl) was 0.018 μmol s^?1 cm^?2, which was twice that by photocatalysis and four times those by catalysis and electrocatalysis; a Faradaic efficiency of 77% (corresponding to the oxidation reaction of AB) was also observed. Hence, Cu₂O/TNA is an efficient photoanode for photoelectrocatalytic hydrolysis of AB.