A ternary Ag/TiO2/CNT photoanode for efficient photoelectrochemical water splitting under visible light irradiation

A ternary Ag/TiO₂/CNT photoanode was prepared by grafting Ag nanoparticles on the surface of as-synthesized TiO₂/CNT nanocomposite for the photoelectrochemical (PEC) water splitting under visible light irradiance. The ternary composite photoanode was observed to generate four times higher photocurrent density compared to binary TiO₂/CNT nanocomposite under visible light irradiance. The Ag nanoparticles on the surface of nanocomposite act as a surface plasmon resonance (SPR) photosensitizer under visible light. The enhanced photocurrent density of Ag/TiO₂/CNT ternary photoanode is attributed to the increased light absorption in the visible region, decrease in band-bending and effective interfacial electron transfer due to the synergetic effect of Ag nanoparticles and CNTs. The enhanced charge transfer within the Ag/TiO₂/CNT was also confirmed by the electrochemical impedance spectroscopy. This work demonstrates a feasible route to improve the PEC performance of TiO₂ towards water splitting under sunlight irradiation.     

Metal-organic framework derived Co3O4/TiO2 heterostructure nanoarrays for promote photoelectrochemical water splitting

In this work, we proposed a simple and new method to fabricate Metal-organic frameworks (MOFs) derived Co₃O₄ modified TiO₂ nanorods (NRs) photoelectrode by immersion and anneal treatment. The positively charged Co-MOF (ZIF-67) was adsorbed on the negatively charged TiO₂ NRs by electrostatic interaction, and then annealed in air to obtain the Co₃O₄/TiO₂ photoelectrodes. The photoelectrochemical (PEC) performance of the Co₃O₄/TiO₂ photoelectrodes has been significantly improved compared with the pure TiO₂, the best photocurrent density of Co₃O₄/TiO₂ photoelectrode could reach 1.04 mA/cm² (1.23 V vs RHE) which was almost 1.65 times than that of pure TiO₂. On the Co₃O₄/TiO₂ photoelectrodes, the significant improvement in PEC performance could be attributed to the constructed p-n heterostructure, which can promote charge transfer within the system and improve the efficiency of electron/hole separation. Meanwhile, under the action of the MOFs-derived Co₃O₄, the number of active sites increases significantly and visibly improve the photoresponse performance.     

The co-decorated TiO2 nanorod array photoanodes by CdS/CdSe to promote photoelectrochemical water splitting

A cascade structure of TiO₂/CdS/CdSe semiconductor heterojunction is synthesized using a three-step technique of facile hydrothermal growth for the enhancement of the photoelectrochemical performances. The optical and photoelectrochemical properties controlled by the deposition processing parameters have been investigated. It is shown that the patterns of semiconductor heterojunction enlarge the absorption range of solar spectra, and improve the properties of the photogenerated charge carriers describing separation and transportation, and reduce the interface resistance between the photoelectrode and electrolyte comparing with the pure TiO₂ and CdS-decorated TiO₂ nanorod array photoanodes. The higher photocurrent density and photoconversion efficiency are up to 4.23 mA cm⁻² and 4.2%, which are the 4.1 and 25.3 times superior than that of the pure TiO₂ photoanode. The hydrothermal growth time increment of CdSe yields greater photoelectrochemical water splitting performances. The underlying physics mechanisms have been discussed based on forming a type-Ⅱ energy band alignment structure.     

TiO2/CeO2 core/shell heterojunction nanoarrays for highly efficient photoelectrochemical water splitting

In this paper, novel TiO₂/CeO₂ core/shell heterojunction nanorod (NR) arrays were synthesized as photoanode for photoelectrochemical (PEC) water splitting via a simple and facial two-step hydrothermal approach. This synthesis route can obtain different amount of CeO₂ nanoparticles by controlling the hydrothermal time and eventually achieve uniform TiO₂/CeO₂ core/shell nanostructures. The uniform TiO₂/CeO₂ core/shell heterojunction nanoarrays exhibit a markedly enhanced photocurrent density of 5.30 mA·cm^?2 compared to that of pristine TiO₂ NR 1.79 mA·cm^?2 at 1.23 V vs. RHE in 1 M KOH solution. The superior PEC performance of the TiO₂/CeO₂ core/shell heterojunction is primarily due to much enhanced visible light absorption and appropriate gradient energy gap structure. This work not only offers the synthesis route for the novel TiO₂/CeO₂ core/shell heterojunction, but also suggests that this new core/shell heterojunction has a great potential application for efficient PEC water splitting devices.     

Photoelectrode nanomaterials for photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting is among the most promising approaches for energy conversion due to its practical efficiency. Unfortunately, many works simply test typical cases without profound insight, and this does not lead us very far. Two concepts are usually neglected: (i) the rate-determining step is usually the electrocatalytic process conducting the water splitting, and thus, the interfacial reaction at the electrode surface can be practically more important that the absorption of photons by the semiconductors, and (ii) the architecture of nanomaterials can directly control both photon absorption and electrochemical catalysis. While narrating the importance of the first concept the primary focus of the present review focuses on the second concept to summarize the general effects of nanostructures on the PEC performance. The nano-architecture has a larger impact on the electrocatalytic properties of the photoelectrode rather than light harvesting capability, but this feature is usually neglected. In fact, designing a nano-architecture is a tuning process to balance a series of different processes, which are in competition in a complicated system, for optimizing the PEC performance. The most important task is to maximize the number of electrocatalytic sites and forming a more effective electrode/electrolyte interface while reducing the number of charge recombination centers. Nanostructuring is normally in favor of the former while unfavorably supporting the latter.     

TiO2-rutile/anatase homojunction with enhanced charge separation for photoelectrochemical water splitting

The mismatched interfaces of heterojunction usually have lots of defects, deriving in recombination of generated electron-hole pairs. On the other hand, homojunction interfaces are considered to be beneficial to the separation of charge carriers due to the similar characteristics in two sides of homojunction. TiO₂ have rutile and anatase two typical photoactive phases in nature. In this work, TiO₂-rutile/anatase (TiO₂-R/A) homojunction photoanode is fabricated by in situ growth of anatase TiO₂ on TiO₂-R surface. By contrast with TiO₂-rutile/rutile (TiO₂-R/R) photoanode, TiO₂-R/A displays higher photocurrent density (1.70 mA cm^?2 at 0.6 V vs. SCE). Deep insight into the mechanism suggests that TiO₂-R/A homojunction has intense band bending and enhanced surface area, which facilitate the charge separation and transmission. This study offers some novel insights to design and fabricate semiconductors photoanodes for highly efficient photocatalytic reactions.     

Hierarchically SrTiO3@TiO2@Fe2O3 nanorod heterostructures for enhanced photoelectrochemical water splitting

In this particular work, the fabrication of SrTiO₃@TiO₂@ Fe₂O₃ nanorod heterostructure has been demonstrated via hydrothermal growth of SrTiO₃ cubic on the rutile TiO₂ nanorod as a template and later sensitized with Fe₂O₃ for photocatalytic solar hydrogen production in a tandem photoelectrochemical cell and dye-sensitized solar cell (DSSC) module. The photocatalytic solar hydrogen production of this heterostructure was optimized by controlling the amount of Sr and Fe on the surface of photocatalyst. The details of the influencing parameters on the physicochemical and photoelectrochemical properties are discussed. It was found that the morphology and quality of the fabricated materials were greatly manipulated by the concentration of Sr and Fe. The optimized 0.025 M SrTiO₃@TiO₂@ Fe₂O₃ heterostructure exhibited a higher photoconversion efficiency with a long electron lifetime, low charge transfer resistance and large donor density at the electrode and electrolyte interface. This composite has significantly improved the photocatalytic hydrogen production, yielding 716 μmol/cm² of maximum accumulative hydrogen. These results show that morphology rendering and manipulation of energy band alignment is crucial in creating efficient heterojunctions for excellent contributions in photocatalytic applications.     

An overview of photocells and photoreactors for photoelectrochemical water splitting

Solar hydrogen production from direct photoelectrochemical (PEC) water splitting is the ultimate goal for a sustainable, renewable and clean hydrogen economy. While there are numerous studies on solving the two main photoelectrode (PE) material issues i.e. efficiency and stability, there is no standard photocell or photoreactor used in the study. The main requirement for the photocell or photoreactor is to allow maximum light to reach the PE. This paper presents an overview of the PE configurations and the possible photocell and photoreactor design for hydrogen production by PEC water splitting.     

Boosting the visible-light photoelectrochemical performance of C3N4 by coupling with TiO2 and carbon nanotubes: An organic/inorganic hybrid photocatalyst nanocomposite for photoelectrochemical water spitting

Developing photoanode with proficient sunlight harvesting, stability as well as enhancing the electron injection across the interface remains a major challenge in the photoelectrochemical water splitting strategy to generate hydrogen. Herein, we design and fabricate an organic/inorganic TiO₂/C₃N₄/CNT photoanode by a hydrothermal technique which exhibits much enhanced photoelectrochemical properties. The TiO₂/C₃N₄/CNT photoanode exhibits a photocurrent density of 2.94 mA/cm², which is ~6.4 time higher than pristine graphitic carbon nitride (C₃N₄) at an applied bias potential of 0.6 V vs. Ag/AgCl. The excellent photoelectrochemical performance benefits from the impactful migration of photo-induced electrons at the TiO₂/C₃N₄ interface from C₃N₄ to TiO₂ and their intimate interface contact with CNT. Kelvin probe force microscopy result shows a smaller interface barrier height (~10 meV) between TiO₂ and C₃N₄, suggesting that electrons transport is favored through TiO₂/C₃N₄ interfaces in a ternary photoanode. The TiO₂/C₃N₄/CNT photoanode exhibited an onset potential of 0.25 V vs. Ag/AgCl which is much lower compared to pristine C₃N_4. The electrochemical impedance spectroscopy results also confirmed the enhanced electron injection across the interface in a ternary photoanode. These results demonstrate a promising approach to develop a highly proficient and visible light active photoanode with excellent stability for renewable energy applications.     

Novel CoOx/CdS/TiO2 with multi-shelled porous heterostructure for enhanced charge separation efficiency of photoelectrochemical water splitting

Hydrogen production by solar energy is an efficient and clean approach to fulfill the future energy demand. Herein, a novel multi-shelled porous heterostructure CoO_x/CdS/TiO₂ photoanode was fabricated by the hydrothermal and chemical method. There were more active sites, suitable surface defects and heterojunction structures in the homogeneous-porous-multi-shelled CoO_x/CdS/TiO₂ photoanode. It showed a photocurrent density of 2.89 mA/cm² at 1.23V vs. RHE, which is 2.22 fold of the original TiO₂ photoanode. The heterostructure fabrication of the CdS/TiO₂ could broaden the visible light absorption and enhance the charge separation efficiency. The multi-shelled homogeneous porous structure of the CoO_x/CdS/TiO₂ further enhanced the charge separation efficiency and accelerated the interfacial oxygen evolution kinetics. The mechanism for the enhanced photoelectrochemical water splitting of favorable CoO_x/CdS/TiO₂ photoanode is proposed.     

Electrochemical co-deposition of cobalt and graphene,produced from recycled polypropylene,on TiO2 nanotube as a new catalyst for photoelectrochemical water splitting

In this study, to improve the photoelectrochemical properties of TiO₂ nanotubes (TNT), the hybrid TNT-graphene-Co(OH)₂ was prepared using electrodeposition of graphene and cobalt salt. The employed graphene was obtained from the waste polypropylene using chemical vapor deposition method. The hybrid material showed high thermal stability and promising visible light absorption. Moreover, its XRD pattern and Raman spectrum are in accordance with the precursors. The photoelectrochemical performance of this hybrid material was investigated using linear sweep voltammetry, transient open circuit potential, and chronoamperometry. Because of the existence of graphene, the prepared hybrid material exhibited excellent photoelectrochemical properties and the results were more appropriate than those of TNT and cobalt deposited TNT. Among the various samples with different amounts of graphene, the sample with the highest graphene content showed the highest photoelectrochemical abilities. This improvement is mainly related to the visible-light harvesting ability of graphene and facile recombination of photogenerated electron-hole pairs due to the synergistic effect of the involved materials.     

Photocatalytic water splitting hydrogen production via environmental benign carbon based nanomaterials

Environmentally benign hydrogen production via photochemical and photoelectrochemical processes by water splitting using carbon-based nanomaterials utilizes sunlight as the source of energy. Owing to their large surface area, pore volume, chemical and thermal stability, and favorable morphology, the carbon-based nanomaterials are quite effective in photocatalytic water splitting. The present review elucidates the photocatalytic nature of carbon materials such as graphene, graphene oxide, carbon nanotubes, graphitic carbon nitride, and fullerenes as they have the tendency to narrow the band gap, allocate electrons, and act as semiconductors, co-catalysts, photosensitizers, and support materials. The production methods, advantages as well as shortcomings of carbon-based materials and their applications in hydrogen production are critically discussed.     

Unveiling the role of carbon defects in the exceptional narrowing of m-ZrO2 wide-bandgap for enhanced photoelectrochemical water splitting

The development of efficient photoelectrodes via defect engineering of wide-band gap metal oxides has been the prime focus for many years. Specifically, the effect of carbon defects in wide-band gap metal oxides on their performance in photoelectrochemical (PEC) applications raised numerous controversies and still elusive. Herein, the effect of various carbon defects in m-ZrO₂ was investigated using the density functional theory to probe the thermodynamic, electronic, and optical properties of the defective structures against pristine m-ZrO₂. The defect formation energies revealed that elevating the temperature promotes and facilitates the formation of carbon defects. Moreover, the binding energies confirmed the stability of all studied complex carbon defects. Furthermore, the band edge positions against the redox potentials of water species revealed that all the studied defective structures can serve as photoanodes for water splitting. Additionally, C_O3c (carbon atom substituted O_3c site) was the only defective structure that exhibited slight straddling of the redox potentials of water. Importantly, all investigated defective structures enhanced light absorption with different optical activities. It is worth mentioning that our results showed exceptional reduction in the bandgap energy compared to those reported experimentally for ZrO₂-based materials. Finally, C_O3cV_O3c (carbon atom substituted O_3c associated with O_3c vacancy) defective m-ZrO₂ enjoyed lowest sub-bandgap (1.9 eV), low defect formation energy, low exciton binding energy, high mobility of charge carriers, fast charge transfer, and low recombination rate. Concurrently, its optical properties were exceptional in terms of high absorption, low reflectivity and improved static dielectric constant. Hence, the study recommends C_O3cV_O3c defective m-ZrO₂ as the leading candidate to serve as a photoanode for PEC applications.     

Ti3C2 MXene nanoparticles modified metal oxide composites for enhanced photoelectrochemical water splitting

MXene, an emerging family of two-dimensional (2-D) material, has shown outstanding electronic properties and promise for the applications on energy storage and conversion. In this paper, Ti₃C₂ MXene nanoparticles were synthesized by a facile solvent exfoliation method and used to construct metal oxide/Ti₃C₂ heterostructures. When these heterostructures were used as photoanodes for photoelectrochemical water splitting, significantly improved photoactivity and stability were achieved. Compared to pristine TiO₂, 6-fold enhanced applied bias photon-to-current efficiency (ABPE) was achieved for TiO₂/Ti₃C₂ heterostructures. According to the electron spin resonance, electrical impedance spectroscopic and Mott-Schottky measurements, the enhanced photoelectrochemical performance was ascribed to the presence of Ti₃C₂ as oxygen evolution cocatalysts and the strong interfacial interactions between metal oxide and Ti₃C₂. Therefore, our research provides a new way to design MXene-based heterostructures for solar energy conversion applications.     

Efficient solar water splitting using a CdS quantum dot decorated TiO2/Ag2Se photoanode

Metal sulfide photoanodes emerge into an efficient platform for converting light energy into hydrogen by water splitting. Herein, we demonstrate the facile fabrication of a ternary photoanode (TiO₂/Ag₂Se/CdS) by decorating a TiO₂/Ag₂Se electrode with CdS quantum dots. The ternary electrode exhibits low charge transfer resistance, and high-density photocurrent (24.6 mAcm⁻² at 1.23 V). We estimate the photon-to-hydrogen conversion efficiency at 14% (at 0.43 V) due to sulfite oxidation. Also, Ag₂Se improves the electrode stability, highlighting the promising nature of the electrode for practical applications. These excellent photocatalytic properties of TiO₂/Ag₂Se/CdS are achieved by the favorable band-edges positions of CdS and Ag₂Se quantum dots, the broad absorption of solar photons in the UV-to-NIR region, and the separation and transport of charge carriers.     

Branched multiphase TiO2 with enhanced photoelectrochemical water splitting activity

An efficient hierarchical structure, nano-branch containing anatase TiO₂ nanofibers and rutile nanorods, was prepared via the combination of the electrospinning and hydrothermal processes. This novel configuration of TiO₂ multiphase possessed higher surface area, roughness, and fill factors compared with each single phase component prepared in the same condition, which significantly enhanced its light absorption. Our experimental results showed that within the interface of multiphase TiO₂, the heterojunction promoted the charge separation and improved the charge transfer rate, leading to higher efficiency for photoelectrochemical water splitting. The photocurrent density of the nano-branched TiO₂ electrode could reach 0.95 mA/cm², which was almost twice as large as that of the pristine TiO₂ nanorod. Our work provides a simple and feasible routine to synthesize complex TiO₂ nanoarchitectures, which lays a foundation for improving energy storage and conversion efficiency of TiO₂-based photoelectrodes.     

Enhancement of photoelectrochemical activity of Fe2O3 nanowires decorated with carbon quantum dots

Owing to its up-conversion photoluminescence, photo-induced electron transfer property, and excellent conductivity, carbon quantum dots (CQDs) have been established as effective sensitizers in combination with Fe₂O₃ nanowires for enhancing the catalytic activity of photoelectrochemical water oxidation. In comparison to pristine Fe₂O₃ nanowires, Fe₂O₃ nanowires decorated with CQDs demonstrate 27 orders of magnitude increase in photocurrent density at 0.23 V vs. Ag/AgCl. The mechanism of enhanced photoelectrochemical activity of CQDs/Fe₂O₃ composite was also investigated. Thereby, it is confirmed that the enhanced optical absorption, accelerated interfacial charge carrier transfer and effective separation of photogenerated electron-hole pairs induced by CQDs decoration account for the enhancement of CQDs/Fe₂O₃ nanowire arrays in photoelectrochemical application.     

Influence of the morphology of ZnO nanowires on the photoelectrochemical water splitting efficiency

Defect-free ZnO nanowire arrays were synthesized and assessed as photoanodes in photoelectrochemical cells. Several tens of samples classified into five different average diameters in the range 40–260 nm were prepared to explore the role of morphology and polar surfaces. The photoelectrochemical performance of the NWs was studied in basic aqueous electrolytes (pH 12.7). A non-monotonic behavior of the performance was demonstrated, which maximizes for nanowires with diameter ～120 nm. The maximum applied bias photoconversion efficiency is ～6.3% upon irradiation with 11.5 mW at 365 nm. The photoanodes exhibit a rather stable performance for ～10 h, while they stabilize at ～60% of the initial photocurrent after 20 h of continuous bias illumination. The degradation of their performance was attributed to partial detachment of the NWs from the supporting conductive film. The enhanced stability is attributed to the decrease of the pH at the electrochemical interface to values that inhibit the dissolution of ZnO.