Photocatalytic Water Splitting Activity Research Articles

Interface Regulation of Bi3TiNbO9/Rh Photocatalysts by Introducing Ultrathin Heterolayer to Enhance Overall Water Splitting

AbstractAurivillius compounds‐based photocatalysts have attracted extensive interest largely due to their ferroelectric properties and modifiable characteristics arising from the alternate stacking of structural units. However, the interfacial Schottky barrier between such semiconducting compounds as light absorbers and metallic cocatalysts for hydrogen evolution restrains the transfer of photogenerated electrons, resulting in low photocatalytic overall water splitting activity and stability. Here, an anions‐induced surface structure transformation strategy is employed to modulate the interface structure between Bi3TiNbO9 as a typical Aurivillius compound and the cocatalyst Rh by in situ growing ultrathin Bi2MoO6 heterolayer on the surface of Bi3TiNbO9 nanosheets. The introduction of Bi2MoO6 heterolayer lowers the energy barrier near the Bi3TiNbO9 surface, which can facilitate the transfer of the photogenerated electrons from bulk to surface and thus the reduction of Rh cocatalyst for highly active hydrogen production. Compared with the Bi3TiNbO9‐Rh photocatalyst, the proportion of low‐valence metallic Rh0 in Bi2MoO6 heterolayer‐modified Bi3TiNbO9‐Rh (Bi3TiNbO9‐Bi2MoO6‐Rh) is improved by 7.76%, giving rise to a photocatalytic overall water splitting activity enhancement by a factor of 4.74. This strategy emphasizes the importance of interface regulation in promoting the transfer of photogenerated charge carriers in Aurivillius‐type photocatalysts, providing an effective pathway for designing and fabricating high‐performance photocatalysts.

Boosting solar-light photocatalytic activity of Ni2P-decorated ZnCr2O4 nanofibers for H2 evolution and tetracycline elimination

Photocatalysis is a promising route to convert solar light into chemical energy for H2 evolution and pollutants degradation. Hence, Ni2P decorated ZnCr2O4 (Ni2P/ZnCr2O4) nanofibers were prepared for boosting solar-light induced water splitting and tetracycline hydrochloride (TCH) degradation. For comparison to ZnCr2O4 nanofibers, Ni2P/ZnCr2O4 composites exhibited the better photocatalytic activities for water splitting and TCH removal. Owing to the co-catalytic effect of Ni2P for the efficient photon capture, the charge carriers were rapidly separated and migrated at the interface between Ni2P and oxygen-defected ZnCr2O4. H2 evolution rate of optimal C–Ni2P/ZnCr2O4 with a nominal Ni2P content of 9.0 wt% was 575.89 μmol g−1 h−1, and its apparent quantum yield at 420 nm was 15.25 %, while its H2 evolution decreased from 2815.13 to 2602.51 μmol g−1 after fifth cycle. In addition, the removal efficiency of C–Ni2P/ZnCr2O4 was 93.68 % within 90 min, and declined to 88.76 % after five cycles for solar-light driven degradation of 50 mg L−1 TCH solution. The deactivation of C–Ni2P/ZnCr2O4 was due to the photo-corrosion for the change in valence states of compositions.

Research Progress of β-Ketoenamine-Linked Covalent Organic Frameworks for Photocatalytic Hydrogen Evolution

β-Ketoamide covalent organic frameworks (COFs), also named Tp-COFs, are considered to be a milestone material in the history of photocatalysts because of their excellent visible-light absorption, high crystallinity, ultra-high stability and structural diversity. In recent years, a large number of Tp-COFs and their composites have been successfully constructed based on molecular or composite engineering strategies, and exhibited splendid photocatalytic water splitting activity. In comparison with a composite strategy, the molecular engineering technique effectively avoids interface problems by designing and preparing frameworks at the molecular level. Therefore, it is necessary to timely summarize the construction of Tp-COF photocatalysts based on the molecular engineering strategy, so as to provide some theoretical basis and enlightenment for the subsequent development of high-performance Tp-COFs. Finally, the shortcomings and challenges of this technique and personal views on the further development of Tp-COFs are presented.

Dual Function of Naphthalenediimide Supramolecular Photocatalyst with Giant Internal Electric Field for Efficient Hydrogen and Oxygen Evolution.

Organic supramolecular photocatalysts have garnered widespread attention due to their adjustable structure and exceptional photocatalytic activity. Herein, a novel bis-dicarboxyphenyl-substituent naphthalenediimide self-assembly supramolecular photocatalyst (SA-NDI-BCOOH) with efficient dual-functional photocatalytic performance is successfully constructed. The large molecular dipole moment and short-range ordered stacking structure of SA-NDI-BCOOH synergistically create a giant internal electric field (IEF), resulting in a remarkable 6.7-fold increase in its charge separation efficiency. Additionally, the tetracarboxylic structure of SA-NDI-BCOOH greatly enhances its hydrophilicity. Thus, SA-NDI-BCOOH demonstrates efficient dual-functional activity for photocatalytic hydrogen and oxygen evolution, with rates of 372.8 and 3.8µmol h-1 , respectively. Meanwhile, a notable apparent quantum efficiency of 10.86% at 400nm for hydrogen evolution is achieved, prominently surpassing many reported supramolecular photocatalysts. More importantly, with the help of dual co-catalysts, it exhibits photocatalytic overall water splitting activity with H2 and O2 evolution rates of 3.2 and 1.6µmol h-1 . Briefly, this work sheds light on enhancing the IEF by controlling the molecular polarity and stacking structure to dramatically improve the photocatalytic performance of supramolecular materials.

Tailoring photocatalytic water splitting activity of boron-thiophene polymer through pore size engineering.

Taking into account the electron-rich and visible light response of thiophene, first-principles calculations have been carried out to explore the photocatalytic activity of donor-acceptor polymers incorporating thiophene and boron. Honeycomb-kagome boron-thiophene (BTP) polymers with varying numbers of thiophene units and fixed B center atoms are direct bandgap semiconductors with tunable bandgaps ranging from 2.41 to 1.88eV and show high absorption coefficients under the ultraviolet and visible regions of the solar spectrum. Fine-tuning the band edges of the BTP polymer is efficiently achieved by adjusting the pore size through the manipulation of thiophene units between the B centers. This manipulation, achieved without excessive chemical functionalization, facilitates the generation of an appropriate quantity of photoexcited electrons and/or holes to straddle the redox potential of the water. Our study demonstrates that two units between B centers of thiophene in BTP polymers enable overall photocatalytic water splitting, whereas BTP polymers with larger pores solely promote photocatalytic hydrogen reduction. Moreover, the thermodynamics of hydrogen and oxygen reduction reactions either proceed spontaneously or need small additional external biases. Our findings provide the rationale for designing metal-free and single-material polymer photocatalysts based on thiophene, specifically for achieving efficient overall water splitting.

Crystal Facet Engineering on SrTiO3 Enhances Photocatalytic Overall Water Splitting.

Single-crystal semiconductor-based photocatalysts exposing unique crystallographic facets show promising applications in energy and environmental technologies; however, crystal facet engineering through solid-state synthesis for photocatalytic overall water splitting is still challenging. Herein, we develop a novel crystal facet engineering strategy through solid-state recrystallization to synthesize uniform SrTiO3 single crystals exposing tailored {111} facets. The presynthesized low-crystalline SrTiO3 precursors enable the formation of well-defined single crystals through kinetically improved crystal structure transformation during solid-state recrystallization process. By employing subtle Al3+ ions as surface morphology modulators, the crystal surface orientation can be precisely tuned to a controlled percentage of {111} facets. The photocatalytic overall water splitting activity increases with the exposure percentage of {111} facets. Owing to the outstanding crystallinity and favorable anisotropic surface structure, the SrTiO3 single crystals with 36.6% of {111} facets lead to a 3-fold enhancement of photocatalytic hydrogen evolution rates up to 1.55 mmol·h-1 in a stoichiometric ratio of 2:1 than thermodynamically stable SrTiO3 enclosed with isotropic {100} facets.

Enhancement of photocatalytic activity for overall water splitting by inhibiting reverse reactions and photocorrosion of C3N4 via modified with TiO2 thin layer

Rapid H2 and O2 recombination on typical semiconductor photocatalysts and severe photocorrosion side reactions are two critical factors limiting the scale-up application of photocatalytic overall water splitting (OWS) in solar hydrogen generation with high efficiency and stability. In this work, we have developed a new strategy to enhance easy-corroded C3N4 stability by coating a thin TiO2 inert layer and have promoted the stability of the C3N4 catalyst significantly. In addition, this thin layer of TiO2 could suppress the H2 and O2 recombination effectively. TiO2@Pt/C3N4 catalyst showed 130% enhanced activity than that of the naked Pt/C3N4 in pure water (7.63 μmol g−1h−1) and satisfied stability without remarkable decay in the long-term test. The characterization results indicated the corrosion-led NO3− formation rate decreased about three times (1.06 ppm). Besides, a thin TiO2 layer-coated catalyst showed higher photocurrent, longer charge lifetime (6.60 ns), and less charge transfer resistance (27.48 kΩ). The calculation investigation results co-related with the characterization results well. A possible mechanism was proposed to promote photocatalytic OWS by blocking the reverse reaction and suppressing photocorrosion. This study provides new guidance for the design of stable and efficient C3N4-based catalysts for H2 generation via photocatalytic OWS.

Construction of NiS/MnCdS S-scheme heterojunction for efficient photocatalytic overall water splitting: Regulation of surface sulfur vacancy and energy band structure

The overall water splitting based on specific photocatalysts is one of the ultimate ways to solve the energy and environmental crisis facing humanity. Sulfide photocatalysts have greater potential in photocatalytic production of solar fuel. However, due to its easy photocorrosion phenomenon, it not effectively driving the water oxidation semi-reaction to produce oxygen, so how to use sulfide photocatalyst to decompose pure water to achieve stoichiometric reaction of H2/O2 production remains a quite challenging task. Herein, sulfur vacancies-rich MnCdS nanoparticles were modified with NiS nanosheets through the hydrothermal derivation method. Different concentration gradients of sulfur-vacancy in MnCdS nanoparticles (MnCdS-Vs-X) with tunable band structures were successful prepared by regulating the concentration of hydrazine hydrate, thus improves the efficiency of light energy utilization and charge separation and the existence of S defects was verified by transmission electron microscopy (TEM) and electron paramagnetic resonance (EPR). The density function theory (DFT) calculation bears out that the draw into of S vacancy adjusted the band structure of MnCdS. Moreover, the successful construction of an S-scheme heterojunction between NiS and MnCdS-Vs-3 has been strongly demonstrated by in-situ XPS and UPS, which promoted interfacial charge separation and further improved the photocatalytic hydrogen evolution efficiency. The as-obtained S-scheme 20 %NiS/MnCdS-Vs-3 heterojunction exhibit excellent visible-light H2 production activity of 4099.55 μmol g−1h−1, 6.75 times higher than pure MnCdS. Most importantly, the excellent and stable photocatalytic overall water splitting activity of H2-509.70 μmol g-1h−1/O2-254.90 μmol g-1h−1 were obtained over 20 % NiS/MnCdS-Vs-3, which further demonstrates the application value of NiS/MnCdS-Vs-3 photocatalysts. This work proposes new ideas for the application of S-vacancies and S-scheme heterojunction in addressing the stability issues associated with sulfide-based photocatalysts in photocatalytic water splitting.

Ba3SnGa10-xInxO20 (0 ≤ x ≤ 2): site-selective doping, band structure engineering and photocatalytic overall water splitting.

Developing new photocatalysts and deciphering the structure-property relationship are always the central topics in photocatalysis. In this study, a new photocatalyst Ba3SnGa10O20 containing two d10 metal cations was prepared by a high temperature solid state reaction, and its crystal structure was investigated by Rietveld refinements of monochromatic X-ray powder diffraction data for the first time. There are 2 Ba, 4 metal cations and 6 O independent atoms in a unit cell. Sn4+ and Ga3+ co-occupy the octahedral cavities named M1 and M2 sites, and the other two metal sites are fully occupied by Ga3+. Rational In3+-to-Ga3+ substitution was performed to reduce the potential of the conduction band minimum and enhance the light absorption ability, which was indeed confirmed using UV-vis diffuse reflectance spectra and Mott-Schottky plots for Ba3SnGa10-xInxO20 (0 ≤ x ≤ 2). Interestingly, In3+ exhibits site selective doping at M1 and M2 sites exclusively. With the light absorption ability enhanced, the photocatalytic overall water splitting activity was also improved, i.e. the photocatalytic H2 generation rate was 1.7(1) μmol h-1 for Ba3SnGa10O20, and the optimal catalyst Ba3SnGa8.5In1.5O20 loaded with 1.0 wt% Pd exhibited the H2 generation rate of 27.5(4) μmol h-1 and the apparent quantum yield at 254 nm was estimated to be 2.28% in pure water.

Full‐Spectrum Responsive Naphthalimide/Perylene Diimide with a Giant Internal Electric Field for Photocatalytic Overall Water Splitting

AbstractThe co‐assembly naphthalimide/perylene diimide (NDINH/PDINH) supramolecular photocatalysts were successfully synthesized via a rapid solution dispersion method. A giant internal electric field (IEF) in co‐assembly structure was built by the larger local dipole. NDINH coated on PDINH could reduce the reflected electric field over PDINH to improve its responsive activity to ultraviolet light. Resultantly, an efficient full‐spectrum photocatalytic overall water splitting activity with H2 and O2 evolution rate of 317.2 and 154.8 μmol g−1 h−1 for NDINH/PDINH together with optimized O2 evolution rate with 2.61 mmol g−1 h−1 using AgNO3 as a sacrificial reagent were achieved. Meanwhile, its solar‐to‐hydrogen efficiency was enhanced to 0.13 %. The enhanced photocatalytic activity was primarily attributed to the IEF between NDINH and PDINH, significantly accelerating transfer and separation of photogenerated carriers. Additionally, a direct Z‐Scheme pathway of carriers contributed to a high redox potential. The strategy provided a new perspective for the design of supramolecular photocatalysts.

Full-Spectrum Responsive Naphthalimide/Perylene Diimide with a Giant Internal Electric Field for Photocatalytic Overall Water Splitting.

The co-assembly naphthalimide/perylene diimide (NDINH/PDINH) supramolecular photocatalysts were successfully synthesized via a rapid solution dispersion method. A giant internal electric field (IEF) in co-assembly structure was built by the larger local dipole. NDINH coated on PDINH could reduce the reflected electric field over PDINH to improve its responsive activity to ultraviolet light. Resultantly, an efficient full-spectrum photocatalytic overall water splitting activity with H2 and O2 evolution rate of 317.2 and 154.8 μmol g-1 h-1 for NDINH/PDINH together with optimized O2 evolution rate with 2.61 mmol g-1 h-1 using AgNO3 as a sacrificial reagent were achieved. Meanwhile, its solar-to-hydrogen efficiency was enhanced to 0.13 %. The enhanced photocatalytic activity was primarily attributed to the IEF between NDINH and PDINH, significantly accelerating transfer and separation of photogenerated carriers. Additionally, a direct Z-Scheme pathway of carriers contributed to a high redox potential. The strategy provided a new perspective for the design of supramolecular photocatalysts.

Red Phosphorus Grafted High‐Index (116) Faceted Anatase TiO2 for Z‐Scheme Photocatalytic Pure Water Splitting

AbstractRed phosphorus (RP) is an emerging visible‐light‐responsive photocatalyst, yet the rapid charge recombination has limited photocatalytic hydrogen production activity. In this work, a Z‐scheme heterostructure with RP nanolayer coated on high‐index (116) faceted anatase TiO2 nanoparticles (TiO2@RP) is designed and fabricated via chemical vapor deposition. Compared with pristine TiO2 and RP, the optimized TiO2@RP Z‐scheme heterostructure exhibits a significantly boosted photocatalytic activity for pure water splitting, with hydrogen evolution rate reaching 12.9 µmol·h−1, under simulated solar light irradiation. The strong interfacial interaction and staggered band alignment between (116) faceted TiO2 and RP result in the formation of built‐in electric field, which can drive the directional charge migration from the conduction band (CB) of TiO2 to the valance band (VB) of RP under light irradiation, with photoelectrons and holes of high redox ability maintained at the CB of RP and the VB of TiO2, respectively. This well‐designed heterostructure greatly promotes photogenerated charge separation and migration via a direct Z‐scheme charge transfer pathway.

Band gap engineering of Au doping and Au – N codoping into anatase TiO2 for enhancing the visible light photocatalytic performance

We investigate anatase TiO2 doping with Au to determine the change in the band gap energy and optoelectronic properties using experimental and theoretical analysis. The structural analysis using XRD patterns revealed that the synthesized materials primarily exhibited an anatase phase of TiO2, with no impurity peaks observed. However, as the concentration of Au increased, additional diffraction peaks corresponding to Au crystalline phases were detected, indicating successful doping. Furthermore, the crystallite size was found to decrease with increasing Au concentration. We observe that the band gap reduces through substitution of Au into the TiO2 lattice from 3.09 eV to 2.78 eV, demonstrating the feasibility of bandgap tuning of the TiO2 system. A redshift for Au doped TiO2 is observed from absorption spectroscopy and optical absorption intensity using hybrid density functional theory, facilitating visible light absorption, although with potential electron-hole recombination limitations. To enhance a visible light photocatalytic activity for water splitting, we extend our work to explore the impact of N and Au codoping into TiO2 lattice. It reveals that the combination between N and Au leads to a suitable reduction in the band gap width of pure TiO2. Interestingly, Au–N codoping may decrease the effect of photogenerated carriers, produce a new optical absorption feature in the visible region, and enhance the photocatalytic performance of TiO2. This codoping configuration is also a promising photocatalyst for the decomposition of water using visible light without inducing unoccupied states.

A theoretical investigation of two-dimensional Antimonene/ZnSe heterostructure for photocatalytic water splitting: A DFT study

Two dimensional materials have achieved tremendous importance in the field of photocatalysis, optoelectronics, and in some advance fields. In this era of environmental crisis, production of non-polluting fuel like hydrogen is a main challenge which can be solved with photocatalytic water splitting. Therefore, in this study a heterostructure of antimonene and ZnSe monolayer has been investigated for its photocatalytic activity of water splitting along with its geometric, electronic, and optical properties using first principles calculations. The designed heterostructure is predicted to possess a type-II band alignment which is useful for carrier separation and hence oxidation and reduction of water. A built-in electric field was also observed which fastens this transfer of charge carriers among the conduction or valence bands and also prevents them from recombination. A good optical absorption also strengths its case for being a good photocatalyst for splitting of water and opens a path regarding future applications.

(Invited) Al-Doped SrTiO3 Photocatalyst for Selective CO2 Reduction with Water as a Reductant

Among various strategies toward finding new energy sources to replace conventional fossil fuels, photocatalytic conversion of CO2 into useful chemicals using water as an electron donor and a proton source has been increasingly attracting attentions because it can convert light energy to chemical energy. The photocatalytic conversion of CO2 involves photoabsorption (light harvesting), charge separation, and H2O oxidation, as well as the consumption of the photogenerated electrons for reduction. The evolution of H2 rather than CO is preferred when H2O is used as the electron donor, since the redox potential of CO2/CO (−0.51 V vs NHE, at pH 7) is more negative than that of H+/H2 (−0.41 V vs NHE, at pH 7) in an aqueous solution. Therefore, “high selectivity toward CO2 reduction” is one of the most important requirements for the photocatalytic system to achieve the CO2 conversion in water. Various mixed oxides including tantalum-based NaTaO3, Ta2O5, Sr2KTa5O15, titanium-based BaLa4Ti4O15, CaTiO3, Na2Ti3O7, K2Ti6O13, La2Ti2O7, SrTiO3, gallium-based Ga2O3, and ZnGa2O4, and SrNb2O6 were found to be excellent photocatalysts for the conversion of CO2 into CO in the presence of Ag cocatalyst. However, these materials show significant activity for CO evolution only when irradiated with deep region UV-light (λ < 300 nm). Therefore, a series of photocatalysts which can drive under the photoirradiation with UV-light at λ > 300 nm is strongly desired in the research area of the photocatalytic conversion of CO2. Recently, Domen et al. reported that Al-doped SrTiO3 (Al-STO) with suitable cocatalysts such as MoOy/RhCrOx exhibited a promising photocatalytic activity for overall water splitting under photoirradiation at λ > 300 nm; the apparent quantum efficiency of the MoOy/RhCrOx/Al-STO reached close to 69% at 365 nm. Thus, we herein describe our investigation into the photocatalytic conversion of CO2 by H2O using the Al-STO photocatalyst with Ag and AgCo cocatalysts under photoirradiation with λ > 300 nm. Al-STO photocatalyst was fabricated by a flux method using anhydrous SrCl2 as a flux reagent. A mixture of SrTiO3, Al2O3, and anhydrous SrCl2 was ground for 10 min, and then transferred into an alumina crucible and calcined at 1423 K under air for 15 h. Ag cocatalyst was employed to modify the surface of the as-prepared Al-STO photocatalyst via a chemical reduction method. Al-STO was dispersed in deionized ultrapure water, and AgNO3 and NaH2PO2 were added to the suspension, then it was maintained at 353 K for 1.5 h. Photocatalytic activity of Ag/Al-STO photocatalyst was evaluated by using an inner-irradiation type reaction vessel in a quasi-flowing batch system. The photocatalyst powder was dispersed in 1.0 L of an aqueous NaHCO3 solution, and the dissolved air in this suspension was degassed by a flow of high-purity CO2 gas. CO2 was continuously bubbled into the reaction solution at a flow rate of 30 mL/min. The suspension was irradiated using a 400 W high-pressure Hg lamp with a Pyrex filter equipped with a cooling water system (irradiation wavelength: λ > 300 nm). The gaseous products in the outlet gas, e.g., CO, H2, and O2, were analyzed by gas chromatography (FID-GC with methanizer: CO, TCD-GC: H2 and O2). The attached figure shows a formation rate of CO (red), H2 (blue), and O2 (green) in the photocatalytic conversion of CO2 with H2O using undoped STO and Al-STO in the presence of Ag cocatalyst. It should be noted that the formation rate of CO is significantly increased by the doping of Al into STO. Moreover, the addition of Co species as a second cocatalyst to Ag/Al-STO drastically improved its activity for the photocatalytic conversion of CO2 by H2O as the electron donor, with extremely high selectivity toward CO evolution (99.8%), in which Ag and Co might enable CO2 reduction and H2O oxidation on the Al-STO surface, respectively. The formation rate of CO up to 52.7 μmol/h was observed over AgCo/Al-STO when irradiated with the UV light at wavelengths above 300 nm, which is ten-times higher than that over Ag/Al-STO (4.7 μmol/h). Furthermore, isotope-experiments using 13C-labeled CO2 gas revealed that 13C-labeled CO (m/z = 29) is selectively observed by GC-MS analysis, indicating that the CO evolved over the Ag/Al-STO and AgCo/Al-STO photocatalysts should be derived from the introduced CO2 gas. Figure caption Formation rates of products and selectivity toward CO evolution in the photocatalytic conversion of CO2 using bare Ag/STO (undoped STO), Ag/ and AgCo/Al-STO. Red: CO, green: O2, blue: H2, black circle: selectivity. Loading amount of Ag: 1wt%, Ag : Co = 2 : 1 (mol). Figure 1

Ultra-fast Proton Conduction and Photocatalytic Water Splitting in a Pillared Metal-Organic Framework.

Proton-exchange membrane fuel cells enable the portable utilization of hydrogen (H2) as an energy resource. Current electrolytic materials have limitation, and there is an urgent need to develop new materials showing especially high proton conductivity. Here, we report the ultra-fast proton conduction in a novel metal-organic framework, MFM-808, which adopts an unprecedented topology and a unique structure consisting of two-dimensional layers of {Zr6}-clusters. By replacing the bridging formate with sulfate ligands within {Zr6}-layers, the modified MFM-808-SO4 exhibits an exceptional proton conductivity of 0.21 S·cm-1 at 85 °C and 99% relative humidity. Modeling by molecular dynamics confirms that proton transfer is promoted by an efficient two-dimensional conducting network assembled by sulfate-{Zr6}-layers. MFM-808-SO4 also possesses excellent photocatalytic activity for water splitting to produce H2, paving a new pathway to achieve a renewable hydrogen-energy cycle.

A Review on Oxygen-Deficient Titanium Oxide for Photocatalytic Hydrogen Production

Photocatalytic technology based on the specific band structure of semiconductors offers a promising way to solve the urgent energy and environmental issues in modern society. In particular, hydrogen production from water splitting over semiconductor photocatalysts attracts great attention owing to the clean source and application of energy, which highly depends on the performance of photocatalysts. Among the various photocatalysts, TiO2 has been intensively investigated and used extensively due to its outstanding photocatalytic activity, high chemical stability, non-toxicity, and low cost. However, pure TiO2 has a wide band gap of approximately 3.2 eV, which limits its photocatalytic activity for water splitting to generate hydrogen only under ultraviolet light, excluding most of the inexhaustible sunlight for human beings. Fortunately, the band gap of semiconductors can be manipulated, in which introducing oxygen defects is one of the most effective measures to narrow the band gap of titanium oxides. This review considers the fundamentals of photocatalytic water splitting for hydrogen production over TiO2, discusses the latest progress in this field, and summarizes the various methods and strategies to induce oxygen defects in TiO2 crystals. Then, the next section outlines the modification approaches of oxygen-deficient titanium oxide (TiO2−δ) to further improve its photocatalytic performance. Finally, a brief summary and outlook of the studies on TiO2−δ photocatalysts for water splitting to produce hydrogen are presented.

Interfacial Affinity Determined Photocatalytic Activity: A Comparison between Defective and Bulk Polymeric Carbon Nitride.

The photoexcited charge separation efficiency of photocatalysts is generally considered as the key factor for enhancement of their photocatalytic activity, and sometimes, their photoabsorption capability and interfacial reaction kinetics play a key part, but the role of interfacial affinity of photocatalysts with substrates was rarely researched systematically. Herein, nitrogen vacancy-modified polymeric carbon nitride porous nanotubes (PCNpts) were simply synthesized, using tartaric acid as a crosslinking and corrosion agent, and exhibit a remarkable increment in surface area, wettability, photoabsorption and charge separation capability, and photocatalytic activity in water splitting to produce H2, but, interestingly, exhibit substrate-dependent variation of photoactivity in contaminant degradation, compared with bulk PCN. More interestingly, the interfacial affinity of PCNpts and PCN with contaminants and H2O, rather than photoabsorption and charge separation capability, is confirmed to dominate their photoactivity.

Boron-doped polymeric carbon nitride co-modified with carbon-ring and carboxyl for efficient photocatalytic overall water-splitting

The photocatalytic overall water-splitting to generation hydrogen and oxygen over polymeric carbon nitrides is highly crucial in green energy usages and industry applications. To promote the separation efficiency of photoinduced charge carriers and adjust the band structure of polymeric carbon nitrides (PCNs), boron doped PCN modified with carbon-ring (CR) and carbonyl was synthesized via a polymerization of melamine with boric acid and benzoic acid. The combination of doped boron atom into the heptazine structure by displacing a carbon atom with the introduction of CR conjugated to the carbon atom in heptazine unit regulated the band structure, electronic structure, and delocalized the distribution of electrons, leading to improved separation and transfer of charge carriers photoinduced. Moreover, experimental results combined with density functional theory (DFT) calculations proved that the introduced polar groups, carboxyl, facilitated the adsorption of water molecules and promoted the photocatalytic hydrogen evolution. Importantly, the CR in-situ formed promoted the photocatalytic dioxygen evolution. The combined effect of doped boron, CR modification and carboxyl group endowed the improved the photocatalytic overall water-splitting activity. This work provides an avenue for the synthesis of metal-free photocatalysts with improved overall water-splitting activity via the combination of heterogeneous atom doping and construction of delocalized π-conjugated structures and polar group modification.

Boost of solar water splitting on SrTiO3 by designing V-ions center for localizing defect charge to suppress deep trap

The deep trap such as Ti3+ in SrTiO3 has a great influence on its photocatalytic performance in overall water splitting, acting as a recombination center of carriers. The applied defect engineering is focused on decreasing the Ti3+ concentration for enhancing solar water splitting mainly by doping low-valence metal ions, while the high-valence metal ions are noneffective. In this work, we doped trace V ions into SrTiO3 accompanied by the Al3+ from the Al crucible, promoting the photocatalytic activity in overall water splitting based on the photocatalysis evaluation. A series of advanced techniques are used to characterize the chemical and physical property changes in SrTiO3 aroused by V ions. Combined with the density functional theory, we found that trace V ions can cooperate with Al3+ to localize the defect charge around it, suppressing the formation of deep trap and increasing the amount of potential delocalized charge at shallow state, responsible for the boost of activity. This work provides another effective method to modulate SrTiO3-based photocatalysts in defect engineering.