Hydrogenated TiO2 Nanorod Arrays Decorated with Carbon Quantum Dots toward Efficient Photoelectrochemical Water Splitting

Particle-Based Photoelectrodes for PEC Water Splitting: Concepts and Perspectives

This comprehensive review delves into the intricacies of the photoelectrochemical (PEC) water splitting process, specifically focusing on the design, fabrication, and optimization of particle-based photoelectrodes for efficient green hydrogen production. These photoelectrodes, composed of semiconductor materials, potentially harness light energy and generate charge carriers, driving water oxidation and reduction reactions. The versatility of particle-based photoelectrodes as a platform for investigating and enhancing various semiconductor candidates is explored, particularly the emerging complex oxides with compelling charge transfer properties. However, the challenges presented by many factors influencing the performance and stability of these photoelectrodes, including particle size, shape, composition, morphology, surface modification, and electrode configuration, are highlighted. The review introduces the fundamental principles of semiconductor photoelectrodes for PEC water splitting, presents an exhaustive overview of different synthesis methods for semiconductor powders and their assembly into photoelectrodes, and discusses recent advances and challenges in photoelectrode material development. It concludes by offering promising strategies for improving photoelectrode performance and stability, such as the adoption of novel architectures and heterojunctions.

Untangling the Effect of Carbonaceous Materials on the Photoelectrochemical Performance of BaTaO2N

The water oxidation reaction is a rate-determining step in solar water splitting. The number of surviving photoexcited holes is one of the most influencing factors affecting the photoelectrochemical water oxidation efficiency of photocatalysts. The solar-to-hydrogen energy conversion efficiency of BaTaO2N is still far below the benchmark efficiency set for practical applications, notwithstanding its potential as a 600 nm-class photocatalyst in solar water splitting. To improve its efficiency in photoelectrochemical water splitting, this study offers a straightforward route to develop photocatalytic materials based on the combination of BaTaO2N and carbonaceous materials with different dimensions. The impact of diverse carbonaceous materials, such as fullerene, g-C3N4, graphene, carbon nanohorns, and carbon nanotubes, on the photoelectrochemical behavior of BaTaO2N has been examined. Notably, the use of graphene and g-C3N4 remarkably improves the photoelectrochemical performance of the composite photocatalysts through a higher photocurrent and acting as electron reservoirs. Consequently, a marked reduction in recombination rates, even at low overpotentials, leads to a higher accumulation of photoexcited holes, resulting in 2.6- and 1.7-fold increased BaTaO2N photocurrent densities using graphene and g-C3N4, respectively. The observed trends in the dark for the oxygen reduction reaction (ORR) potential align with the increase in the photocurrent density, revealing a good correlation between opposite phenomena. Importantly, the enhancement observed implies an underlying accumulation phenomenon. The verification of this concept lies in the evidence provided by oxygen reduction and is in line with photoredox flux matching during photocatalysis. This research underscores the intricate interplay between carbonaceous materials and oxynitride photocatalysts, offering a strategic approach to enhancing various photocatalytic capabilities.

How to Survive at Point Nemo? Fischer-Tropsch, Artificial Photosynthesis, and Plasma Catalysis for Sustainable Energy at Isolated Habitats

Inhospitable, inaccessible, and extremely remote alike the famed pole of inaccessibility, aka Point Nemo, the isolated locations in deserts, at sea, or in outer space are difficult for humans to settle, let alone to thrive in. Yet, they present a unique set of opportunities for science, economy, and geopolitics that are difficult to ignore. One of the critical challenges for settlers is the stable supply of energy both to sustain a reasonable quality of life, as well as to take advantage of the local opportunities presented by the remote environment, e.g., abundance of a particular resource. The possible solutions to this challenge are heavily constrained by the difficulty and prohibitive cost of transportation to and from such a habitat (e.g., a lunar or Martian base). In this essay, the advantages and possible challenges of integrating Fischer-Tropsch, artificial photosynthesis, and plasma catalysis into a robust, scalable, and efficient self-contained system for energy harvesting, storage, and utilization are explored.

TiO2 nanorod/nanotube interface reconstruction and synergistic role of oxygen vacancies and gold in H-Au-TiO2 NR/NT for photoelectrochemical bacterial inactivation and water splitting

Photoelectrochemical (PEC) water splitting by semiconductor photoanodes is limited by sluggish water oxidation kinetics coupled with serious charge recombinations. In this paper, an effective strategy of TiO2 nanorod/nanotube nanostructured interface reconstruction, oxygen vacancies and surface modification were employed for stability and efficient charge transport in the photoanodes. Successive anodization and hydrothermal routes were adopted for the TiO2 NR/NT photoanodes interface reconstruction, followed by Au nanoparticles/clusters (Au NP) loading and hydrogen treatment. This resulted in H-Au-TiO2 NR/NT photoanodes. A three-dimensional structure of TiO2 NR on TiO2 NT/Ti foil nanotubes achieved the highest photocurrent density (1.42 mA cm-2 at 0.3 V vs. Ag/AgCl). The optimal oxygen vacancies and Au NP loading on TiO2 NR/NT exhibited 1.62 mA cm-2 photocurrent density at 0.3 V vs. Ag/AgCl in H-Au-TiO2 NR/NT photoelectrode, which is eight times higher than the TiO2 NT/Ti foil. TRPL analyses confirm the hydrogen treatments to TiO2 exhibited the emission lifetime (46 ns) in the H-Au-TiO2 NR/NT photoanodes due to newly formed lower Ti3+-related trapped electron states and Au NP. The optimum H-Au (4)-TiO2 NR/NT photoanodes achieved 95% photoelectrochemical (PEC) bacterial inactivation and effective PEC water splitting with (278 and 135.4) μmol of hydrogen and oxygen generation, respectively. In this study, oxygen vacancies combined with gold particles and interface reconstruction provide an innovative way to design effective photoelectrodes.

Tailoring Eco-Friendly Colloidal Quantum Dots for Photoelectrochemical Hydrogen Generation

A photoelectrochemical (PEC) cell is able to realize effective solar-to-hydrogen energy conversion from water by using the semiconductor photoelectrode. Semiconducting colloidal quantum dots (QDs) with captivating features of size-tunable optoelectronic properties and broad light absorption are regarded as promising photosensitizers in solar-driven PEC systems. Up to now, different types of QDs have been developed to achieve high-efficiency PEC H2 generation, while the majority of state-of-the-art QDs-PEC systems are still fabricated from QDs consisting of heavy metals (e.g., Cd and Pb), which are extremely harmful to the human health and natural environment. In this context, substantial efforts have been made to mitigate the usage of highly toxic heavy metals and concurrently promote the development of alternative environment-friendly QDs with comparable features. This review presents recent advances of solar-driven PEC devices based on several typical environment-friendly QDs (e.g., carbon QDs, I-III-VI QDs and III-V QDs). A variety of techniques (e.g., shell thickness tuning, alloying/doping, and ligands exchange, etc.) to engineer these QD's optoelectronic properties and achieve high-efficiency PEC H2 production are thoroughly discussed. Furthermore, the critical challenges and future perspectives of advanced eco-friendly QDs-PEC systems in terms of QDs' synthesis, photo-induced charge kinetics, and operation stability/efficiency are briefly proposed.

Driving a NiTiO3 photocatalyst for the oxygen evolution reaction with near-infrared light

A nickel titanate (NTO) photocatalyst has been developed for the oxygen evolution reaction (OER) with an exceptionally broad light wavelength excitation ranging from visible to infrared. Specifically, by loading CoOx as the co-catalyst, the apparent quantum yields for the OER were ca. 2.2%, 1.0%, and 0.8% at wavelengths of 470, 760, and 850 nm, respectively. The achievements reveal that the NTO photocatalyst is highly efficient even under illumination with near-infrared (NIR) light, which confers the potential for highly efficient solar-driven oxidation reactions.

Enhanced Photoelectrochemical Water Oxidation Using TiO2-Co3O4 p-n Heterostructures Derived from in Situ-Loaded ZIF-67

Exposing catalytically active metal sites in metal-organic frameworks (MOFs) while maintaining porosity is beneficial for increasing electron transport to achieve better electrochemical energy conversion performance. Herein, we propose an in situ method for MOF formation and loading onto TiO2 nanorods (NR) using a simple solution-processable method followed by annealing to obtain TiO2-Co3O4. The as-prepared TiO2-ZIF-67 based photoanodes were annealed at 350, 450, and 550 °C to study the effect of carbonization on photo-electrochemical water oxidation. The successful loading of ZIF-67 on TiO2 and the formation of TiO2-Co3O4 heterojunction were confirmed by XRD, XPS, FE-SEM, and HRTEM analyses. TiO2-Co3O4-450 (the sample annealed at 450 °C) showed an enhanced photocurrent of 2.4 mA/cm2, which was 2.6 times larger than that of pristine TiO2. The improved photocurrent might be ascribed to the prepared p-n heterostructures (Co3O4 and TiO2), which promote electron-hole separation and charge transfer within the system and improve the photoelectrochemical performance. Moreover, the preparation of Co3O4 from the MOF carbonization process improved the electrical conductivity and significantly increased the number of exposed active sites and enhanced the photoresponse performance. The as-prepared ZIF-67 derived TiO2-Co3O4 based photoanodes demonstrate high PEC water oxidation, and the controlled carbonization method paves the way toward the synthesis of low-cost and efficient electrocatalysts.

Effects of activation overpotential in photoelectrochemical cells considering electrical and optical configurations

Photoelectrochemical cells (PECs) are a promising option for directly converting solar energy into chemical energy by producing hydrogen (H₂) gas, thus providing a clean alternative to consuming fossil fuels. H₂ as fuel is free from any carbon footprints and negative environmental impacts. Therefore, the H₂ production, especially directly using sunlight in PECs, is critically important for the rapidly growing energy demand of the world. Although promising, PECs are inefficient and must overcome a few inherent losses in producing H₂-the most important being the activation overpotential (ηa) required for splitting water. This work analyzes the impact of ηa on solar-to-fuel efficiency (ηSTF) and H₂ production rate (HPR). This work also discusses choosing appropriate photo-absorbing materials based on their energy bandgaps and suitable electrode pairs to achieve desired ηSTF and HPR for different electrical and optical PEC configurations. Significant changes are observed in ηSTF and HPR when ηa is considered in water splitting.

Quantum Dots, Passivation Layer and Cocatalysts for Enhanced Photoelectrochemical Hydrogen Production

Solar-driven photoelectrochemical (PEC) hydrogen production is one potential pathway to establish a carbon-neutral society. Nowadays, quantum dots (QDs)-sensitized semiconductors have emerged as promising materials for PEC hydrogen production due to their tunable bandgap by size or morphology control, displaying excellent optical and electrical properties. Nevertheless, they still suffer from anodic corrosion during long-term cycling, offering poor stability. This Review discussed advancements to improve long-term stability of QDs particularly in terms of cocatalysts and passivation layers. The working principle of PEC cells was reviewed, along with all important configurations adopted over recent years. The equations to assess PEC performance were also described. A greater emphasized was placed on QDs and incorporation of cocatalysts or passivation layers that could enhance the PEC performance by influencing the charge transfer and surface recombination processes.

Bifunctional photoelectrochemical aptasensor based on heterostructured Ag3PO4/Ag/TiO2 nanorod array for determination of two tumor markers

Constructing of heterostructures can significantly improve the photoelectrical (PEC) response signal by promoting the migration and suppressing the recombination of photogenerated carries. A bifunctional PEC sensing platform was designed for simultaneous high-performance detection of mucin-1 (MUC1) and carcinoembryonic antigen (CEA), which was based on generated Z-scheme heterostructured Ag₃PO₄/Ag/TiO₂ nanorod arrays (NAs) and enzyme-mediated catalytic precipitation by alkaline phosphatase (ALP) and Au/hollow Co₃O₄ polyhedron. The proposed aptasensor displayed linear ranges of 1.0-100 ng mL^-1 and 0.1-50 ng mL^-1 for MUC1 and CEA with limit of detections of 0.430 and 0.058 ng mL^-1, respectively. This strategy offers potential applications for early diagnosis, monitoring progression, and even evaluating the prognosis of breast cancer in practice.

Self-Doped Carbon Dots Decorated TiO2 Nanorods: A Novel Synthesis Route for Enhanced Photoelectrochemical Water Splitting

Herein, we have successfully prepared self-doped carbon dots with nitrogen elements (NCD) in a simple one-pot hydrothermal carbonization method, using L-histidine as a new precursor. The effect of as-prepared carbon dots was studied for photoelectrochemical (PEC) water splitting by decorating NCDs upon TiO2 nanorods systematically by changing the loading time from 2 h to 8 h (TiO2@NCD2h, TiO2@NCD4h, TiO2@NCD6h, and TiO2@NCD8h). The successful decorating of NCDs on TiO2 was confirmed by FE-TEM and Raman spectroscopy. The TiO2@NCD4h has shown a photocurrent density of 2.51 mA.cm−2, 3.4 times higher than the pristine TiO2. Moreover, TiO2@NCD4h exhibited 12% higher applied bias photon-to-current efficiency (ABPE) than the pristine TiO2. The detailed IPCE, Mott–Schottky, and impedance (EIS) analyses have revealed the enhanced light harvesting property, free carrier concentration, charge separation, and transportation upon introduction of the NCDs on TiO2. The obtained results clearly portray the key role of NCDs in improving the PEC performance, providing a new insight into the development of highly competent TiO2 and NCDs based photoanodes for PEC water splitting.

Synthesis of Fe3O4@CdS@CQDs ternary core-shell heterostructures as a magnetically recoverable photocatalyst for selective alcohol oxidation coupled with H2O2 production

Photocatalytic aerobic oxidation of aromatic alcohols to corresponding aldehydes coupled with producing hydrogen peroxide (H₂O₂) represents one of the most efficient strategies for converting solar energy into chemical energy. In this work, a magnetically recoverable photocatalyst of Fe₃O₄@CdS@CQDs ternary core-shell heterostructures is elaborately fabricated through the hydrothermal growth of CdS on Fe₃O₄ nanospheres with in-situ incorporation of carbon quantum dots (CQDs) and used for selective alcohol oxidation coupled with H₂O₂ production. The Fe₃O₄@CdS@CQDs photocatalyst possess distinct advantages of full solar spectral absorption, efficient charge separation, and high stability. The Fe₃O₄-nanosphere cores not only endow photocatalyst with the characteristics of magnetic recovery but also form Fe₃O₄@CdS Z-scheme heterojunction to prevent CdS from photocorrosion. The in-situ modified CQDs act as charge mediators to accelerate the photogenerated electron-hole separation and afford active sites to facilitate H₂O₂ production. As a result, the Fe₃O₄@CdS@CQDs photocatalyst exhibits excellent performance in selectively converting benzyl alcohol to benzaldehyde accompanied with H₂O₂ production. The generation rates of benzaldehyde and H₂O₂ reach up to 57.22 and 27.06 mmol·g_CdS^-1·h^-1, respectively. This work highlights a rational construction of magnetic heterostructure photocatalyst and its application in the photo-redox coupling reactions.

Surface-Terminated Hydroxyl Groups for Deciphering the Facet-Dependent Photocatalysis of Anatase TiO2

Understanding the relation between a crystal facet and photocatalytic performance is of great importance for the development of effective catalysts. In this work, we focus on anatase TiO₂ with controllable exposed facets toward photocatalytic hydrogen evolution by water splitting. By combining temperature-programmed desorption (TPD) and diffuse reflectance infrared spectroscopy (DRIFTS), we obtain that the adsorption of hydroxyl groups and the photo-driven breaking of hydroxyl groups depend strongly on the exposed facets. As a result, the higher catalytic hydrogen evolution activity of TiO₂ enclosed with (101) facets than that of (001) facets should be ascribed to the more favorable depletion of hydroxyl groups. Moreover, graphene quantum dots (GQDs) with rich surface functional groups are deliberately deposited on the TiO₂ surface. The determination of the states and dynamics of surface hydroxyl groups suggests that GQDs facilitate the reaction of hydroxyl groups on (001)TiO₂, thus leading to the activity enhancement. By contrast, the already active (101)TiO₂ become apparently less efficient after GQD deposition due to the restricted reaction of hydroxyl groups. Overall, our findings not only provide a unique guidance for understanding the crystal-plane-dependent photocatalysis but also present a powerful approach by which to tailor the photocatalytic performance.

Strategy to Achieve Augmented Photoelectrochemical Water Oxidation via Heteroband Structure Engineering and In Situ Interface Activation

A simple strategy to achieve remarkable activity of photoelectrochemical (PEC) water oxidation is both a critical requirement prior to accelerating the deployment of hydrogen and an important pursuit for renewable solar energy utilization. Despite various ways to improve the PEC performance, laborious fabrication and a statically driven process for water oxidation are still the most challenging tasks. We, herein, demonstrated an extremely simple but extraordinary effective strategy to boost PEC water splitting in a three-dimensional (3D) network structure (Ni foam, i.e., NF)-supported ZnO nanowire (NW)/CdS nanoparticle (NP) (NF/ZnO/CdS) photoelectrode. On the one hand, the establishment and optimization of a heteroband junction between ZnO NWs and CdS NPs substantially reduce the solar energy loss and increase the photocarrier utilization rate at the interface of two phases. On the other hand, the solution flow rotation in the unique macroporous 3D network structure gives rise to obvious piezoelectric (PE) polarization of ZnO located on the skeleton of Ni foam, which can dynamically enforce directional charge transfer. This real-time self-built PE polarization, assisted by the heteroband junction, enables the NF/ZnO/CdS photoanode system to obtain an improved photocurrent density by 12.2-fold compared with pure ZnO (at 1.23 V vs RHE). This work sheds light on an ingenious design strategy for fabricating photoelectrodes that combines a simplified structure and an effectively enhanced water oxidation performance.

Carbon dots dominated photoelectric surface in titanium dioxide nanotube/nitrogen-doped carbon dot/gold nanocomposites for improved photoelectrochemical water splitting

The dynamic behavior of electron-hole pairs at the interface of the nanocomposites is important for photoelectrochemical catalysis, but it is difficult to characterize. Here we construct a ternary titanium dioxide/nitrogen-doped carbon dot/gold (TiO₂/NCD/Au) complex as the model catalyst to investigate the kinetic indexes at their interfaces. Under irradiation (200 mW cm^-2), the photocurrent density of TiO₂/NCD/Au is 10.26 mA cm^-2, which is higher than those of TiO₂/Au (4.34 mA cm^-2), TiO₂/NCD (7.55 mA cm^-2) and TiO₂ (3.34 mA cm^-2). The evolved oxygen of TiO₂/NCD/Au reaches 125.8 μmol after 5000 s test. The energy bands of complexes are very similar to that of the unmodified TiO₂ catalyst due to the low content modification of NCDs and Au. In addition, the transient photovoltage (TPV) tests with a series of control samples show differences about the carriers' separation and transfer process, which verify that Au can increase the separation quantity of electron-hole pairs while NCDs play a more important role on the increase of the separation quantity and separation rate simultaneously. This work quantifies the function of each component in a composite catalyst and deepens the understanding of the catalyst interface design.

Assembling γ-graphyne surrounding TiO2 nanotube arrays: an efficient p-n heterojunction for boosting photoelectrochemical water splitting

Photoelectrochemical water splitting is an excellent strategy for hydrogen generation and it is pivotal to the development of photoanodes with sufficient sunlight harvesting, rapid charge separation, and enhanced electron injection efficiency. In this work, we rationally constructed a γ-graphyne/TiO₂ (GY/TiO₂) p-n heterojunction in which p-type γ-graphyne nanosheets were distributed in the three-dimensional space surrounding TiO₂ nanotube arrays. The GY/TiO₂ photoanode achieves a photocurrent density of 0.75 mA cm^-2 at 1.23 V (vs. RHE), 1.7 times that of bare TiO₂, and extends the electron lifetime of TiO₂ from 0.51 to 1.16 ms. The improvement arises from moderate γ-graphyne modification, contributing to broadened light absorption, the suppressed recombination of electron-hole pairs, an increase in charge transfer, and a higher injection efficiency of surface electrons. This work provides a reliable approach for the utilization and conversion of sustainable solar energy.

The design of high performance photoanode of CQDs/TiO2/WO3 based on DFT alignment of lattice parameter and energy band, and charge distribution

Photoanode is the key issue for photoelectrocatalytic (PEC) water splitting and organics degradation. However, it always faces several restrictions including severe photocorrosion, low charge separation and transfer efficiencies, poor visible light harvesting, and sluggish interfacial reaction kinetics, which often required a variety of modifications with only low improvements achieved. Herein, a high performance CQDs/TiO₂/WO₃ photoanode was designed on the basis of density function theory (DFT) alignment of lattice parameters and energy band, and charge distribution. The TiO₂/WO₃ heterojunction can abate photocorrosion through the hetero-epitaxial growth of TiO₂ (001) on WO₃ (002) for the lattice mismatch <3% eliminating dangling bonds, with high corrosion resistance and photostability of TiO₂. As the built-in field constructed by a staggered band alignment structure with the valence band offset (VBO) of 0.51 eV, the photogenerated carriers transfer and separation are promoted dramatically. Through the DFT calculations, the sunlight absorption wavelength can be extended, and the interfacial reaction kinetics can be expedited with the modification of carbon quantum dots (CQDs) on TiO₂/WO₃, due to the narrower bandgap (E_g) and the accumulation of electrons at TiO₂ side. The DFT designed CQDs/TiO₂/WO₃ photoanode significantly increase photocurrent density from 0.90 to 2.03 mA cm^-2 at 1.23 V, charge separation efficiency from 56.3 to 79.2% and charge injection efficiency from 51.2 to 70.4%, and extend light absorption edge from 455 to 463 nm over pristine WO₃, with better photostability and lower holes-to-water resistance.

Interfacial Charge Transport in 1D TiO2 Based Photoelectrodes for Photoelectrochemical Water Splitting

1D nanostructured photoelectrodes are promising for application as photoelectrochemical (PEC) devices for solar energy conversion into hydrogen (H₂ ) owing to the optical, structural, and electronic advantages. Titanium dioxide (TiO₂ ) is the most investigated candidate as a photoelectrode due to its good photostability, low production cost, and eco-friendliness. The obstacle for TiO₂ 's practical application is the inherent wide bandgap (UV-lights response), poor conductivity, and limited hole diffusion length. Here, a comprehensive review of the current research efforts toward the development of 1D TiO₂ based photoelectrodes for heterogeneous PEC water splitting is provided along with a discussion of nanoarchitectures and energy band engineering influences on interfacial charge transfer and separation of 1D TiO₂ composited with different dimensional photoactive materials. The key focus of this review is to understand the charge transfer processes at interfaces and the relationship between photogenerated charge separation and photoelectrochemical performance. It is anticipated that this review will afford enriched information on the rational designs of nanoarchitectures, doping, and heterojunction interfaces for 1D TiO₂ based photoelectrodes to achieve highly efficient solar energy conversion.

Carbon quantum dots-TiO2 nanocomposite as an efficient photocatalyst for the photodegradation of aromatic ring-containing mixed VOCs: An experimental and DFT studies of adsorption and electronic structure of the interface

In this work, we have developed and optimized TiO₂ nanoparticles decorated with carbon quantum dots to examine its potential use in the photocatalytic oxidation of aromatic ring containing gas-phase mixed volatile organic compounds, e.g., benzene, toluene, and p-xylene. Carbon quantum dots decorated TiO₂ demonstrated good photodegradation efficiency in contrast to pure TiO₂ under UV-vis light illumination. For example, with 0.5 wt% carbon quantum dots decorated on TiO₂, 64 % of the mixed volatile organic compounds were photodegraded, while pure TiO₂ only exhibited 44 % of the photodegradation efficiency. Also, the carbon quantum dots (0.5 wt%)/TiO₂ nanocomposite demonstrated considerable photocatalytic activity within the visible region. On the other hand, pure TiO₂ remained inactive within the visible region. The density functional theory study of the carbon quantum dots/TiO₂ interface revealed that C 2p states of carbon quantum dots incorporated new energy states around the Fermi level near the lowest conduction band. This might be accountable for the improved charge separation process and better conductivity of the photogenerated electrons. The improved photocatalytic performance of the carbon quantum dots/TiO₂ nanocomposites can be attributed to good light harvesting within the UV-vis region, charge separation, and adsorption capability.

Surface oxygen vacancies of TiO2 nanorods by electron beam irradiation for efficient photoelectrochemical water splitting

TiO₂ nanorods with oxygen vacancies are produced via a facial, controllable and scalable high energy electron beam irradiation treatment, resulting in a remarkable enhancement of photocurrent density of about 85.4% at 1.23 V_RHE.

The role of carbon dots - derived underlayer in hematite photoanodes

Hematite is a promising candidate as photoanode for solar-driven water splitting, with a theoretically predicted maximum solar-to-hydrogen conversion efficiency of ∼16%. However, the interfacial charge transfer and recombination greatly limits its activity for photoelectrochemical water splitting. Carbon dots exhibit great potential in photoelectrochemical water splitting for solar to hydrogen conversion as photosensitisers and co-catalysts. Here we developed a novel carbon underlayer from low-cost and environmental-friendly carbon dots through a facile hydrothermal process, introduced between the fluorine-doped tin oxide conducting substrate and hematite photoanodes. This led to a remarkable enhancement in the photocurrent density. Owing to the triple functional role of carbon dots underlayer in improving the interfacial properties of FTO/hematite and providing carbon source for the overlayer as well as the change in the iron oxidation state, the bulk and interfacial charge transfer dynamics of hematite are significantly enhanced, and consequently led to a remarkable enhancement in the photocurrent density. The results revealed a substantial improvement in the charge transfer rate, yielding a charge transfer efficiency of up to 80% at 1.25 V vs. RHE. In addition, a significant enhancement in the lifetime of photogenerated electrons and an increased carrier density were observed for the hematite photoanodes modified with a carbon underlayer, confirming that the use of sustainable carbon nanomaterials is an effective strategy to boost the photoelectrochemical performance of semiconductors for energy conversion.

The Effect of Laser Re-Solidification on Microstructure and Photo-Electrochemical Properties of Fe-Decorated TiO2 Nanotubes

Fossil fuels became increasingly unpleasant energy source due to their negative impact on the environment; thus, attractiveness of renewable, and especially solar energy, is growing worldwide. Among others, the research is focused on smart combination of simple compounds towards formation of the photoactive materials. Following that, our work concerns the optimized manipulation of laser light coupled with the iron sputtering to transform titania that is mostly UV-active, as well as exhibiting poor oxygen evolution reaction to the material responding to solar light, and that can be further used in water splitting process. The preparation route of the material was based on anodization providing well organized system of nanotubes, while magnetron sputtering ensures formation of thin iron films. The last step covering pulsed laser treatment of 355 nm wavelength significantly changes the material morphology and structure, inducing partial melting and formation of oxygen vacancies in the elementary cell. Depending on the applied fluence, anatase, rutile, and hematite phases were recognized in the final product. The formation of a re-solidified layer on the surface of the nanotubes, in which thickness depends on the laser fluence, was shown by microstructure studies. Although a drastic decrement of light absorption was recorded especially in UV range, laser-annealed samples have shown activity under visible light even 20 times higher than bare titania. Electrochemical analysis has shown that the improvement of photoresponse originates mainly from over an order of magnitude higher charge carrier density as revealed by Mott-Schottky analysis. The results show that intense laser light can modulate the semiconductor properties significantly and can be considered as a promising tool towards activation of initially inactive material for the visible light harvesting.

Surface Plasmon-Enhanced Carbon Dot-Embellished Multifaceted Si(111) Nanoheterostructure for Photoelectrochemical Water Splitting

Because of the excellent electronic properties, Si is a well-established semiconducting material for PV technology. However, slow kinetics and a fast corroding nature make Si inefficient for the hydrogen evolution reaction (HER) in photoelectrochemical (PEC) applications. Herein, we demonstrate a multifacet Si nanowire (SiNW) decorated with surface plasmon-enhanced carbon quantum dots (AuCQDs) as efficient, stable, economical, and scalable photocathodes (PCs) for HER. The PEC performance of SiNW_AuCQDs has more than a fourfold efficiency enhancement than the pristine SiNW, which we have attributed to the combined effect of enhanced solar absorption and efficient carrier transport. The optimized PC SiNW_AuCQDs results in the highest photocurrent ∼1.7 mA/cm², an applied bias photon-to-current conversion efficiency of ∼0.8%, and H₂ gas evolution rate of ∼182.93 μmol·h^-1. Furthermore, these SiNW_AuCQDs PCs provide extraordinary stability under continuous operating conditions with 1 sun illumination (100 mW/cm²). The process-line compatible fabrication process of these PCs will open a new direction at the wafer-level designing of a heterostructure for large-scale solar-fuel conversion.

Revealing adsorption and the photodegradation mechanism of gas phase o-xylene on carbon quantum dots modified TiO2 nanoparticles

Here, we report the photocatalytic oxidation (PCO) of o-xylene on carbon quantum dots (CQDs) modified TiO₂ nanoparticles. The results demonstrated that with 1 wt% CQDs loading, 87 % of o-xylene (50 ppm) can be photodegraded, which is 55.3 % higher than pure TiO₂ (56 %) under UV/visible light. This improved photocatalytic activity is associated with the important role of CQDs on TiO₂ surface, which increased the o-xylene adsorption and facilitated the photogenerated hole-electron separation process. Also, the 1 wt%CQDs/TiO₂ nanocomposite showed photocatalytic activity in the visible region (λ > 400 nm) compared to pure TiO₂ (inactive). The DFT study revealed that o-xylene strongly adsorb on TiO₂ (001) surface than (101) through π electrons of the aromatic ring. The in situ DRIFTS study showed that free OH groups on the photocatalyst surface could act as effective Lewis sides for the o-xylene adsorption. The interaction of π electrons of the aromatic ring and isolated OH groups was also observed. The FTIR peaks for CO₂ increased in the case of CQDs/TiO₂ nanocomposite contrasted to pure TiO₂, which suggested that the presence of CQDs improved the mineralization potency of TiO₂. These findings should affect the quest for a better photocatalyst to photodegrade VOCs.

Morphology-dependent photocatalytic and gas-sensing functions of three-dimensional TiO2–ZnO nanoarchitectures

Nanocomposites consisting of three-dimensional ZnO nanorods-decorated TiO₂ nanorod templates (TiO₂–ZnO) have been prepared by combining sputtering and hydrothermal growth strategies.

Ni/Si-Codoped TiO2 Nanostructure Photoanode for Enhanced Photoelectrochemical Water Splitting

We synthesized Ni/Si-codoped TiO₂ nanostructures for photoelectrochemical (PEC) water splitting, by electrochemical anodization of Ti-1Ni-5Si alloy foils in ethylene glycol/glycerol solutions containing a small amount of water. The effects of annealing temperature on PEC properties of Ni/Si-codoped TiO₂ photoanode were investigated. We found that the Ni/Si-codoped TiO₂ photoanode annealed at 700 °C had an anatase-rutile mixed phase and exhibited the highest photocurrent density of 1.15 mA/cm² at 0 V (vs. Ag/AgCl), corresponding to a photoconversion efficiency of 0.70%, which was superior to Ni-doped and Si-doped TiO₂. This improvement in PEC water splitting could be attributed to the extended light absorption, faster charge transfer, possibly lower charge recombination, and longer lifetime.

Solar Hydrogen Generation from Ambient Humidity Using Functionalized Porous Photoanodes

Solar hydrogen is a promising sustainable energy vector, and steady progress has been made in the development of photoelectrochemical (PEC) cells. Most research in this field has focused on using acidic or alkaline liquid electrolytes for ionic transfer. However, the performance is limited by (i) scattering of light and blocking of catalytic sites by gas bubbles and (ii) mass transport limitations. An attractive alternative to a liquid water feedstock is to use the water vapor present as humidity in ambient air, which has been demonstrated to mitigate the above problems and can expand the geographical range where these devices can be utilized. Here, we show how the functionalization of porous TiO₂ and WO₃ photoanodes with solid electrolytes-proton conducting Aquivion and Nafion ionomers-enables the capture of water from ambient air and allows subsequent PEC hydrogen production. The optimization strategy of photoanode functionalization was examined through testing the effect of ionomer loading and the ionomer composition. Optimized functionalized photoanodes operating at 60% relative humidity (RH) and T_cell = 30-70 °C were able to recover up to 90% of the performance obtained at 1.23 V versus reverse hydrogen electrode (RHE) when water is introduced in the liquid phase (i.e., conventional PEC operation). Full performance recovery is achieved at a higher applied potential. In addition, long-term experiments have shown remarkable stability at 60% RH for 64 h of cycling (8 h continuous illumination-8 h dark), demonstrating that the concept can be applicable outdoors.