Photocatalytic Hydrogen Evolution Rate Research Articles

Isolated Electron Trap‐Induced Charge Accumulation for Efficient Photocatalytic Hydrogen Production

AbstractThe solar‐driven evolution of hydrogen from water using particulate photocatalysts is considered one of the most economical and promising protocols for achieving a stable supply of renewable energy. However, the efficiency of photocatalytic water splitting is far from satisfactory due to the sluggish electron‐hole pair separation kinetics. Herein, isolated Mo atoms in a high oxidation state have been incorporated into the lattice of Cd0.5Zn0.5S (CZS@Mo) nanorods, which exhibit photocatalytic hydrogen evolution rate of 11.32 mmol g−1 h−1 (226.4 μmol h−1; catalyst dosage 20 mg). Experimental and theoretical simulation results imply that the highly oxidized Mo species lead to mobile‐charge imbalances in CZS and induce the directional photogenerated electrons transfer, resulting in effectively inhibited electron‐hole recombination and greatly enhanced photocatalytic efficiency.

Aqueous Processable Two-Dimensional Triazine Polymers with Superior Photocatalytic Properties.

Efficient and scalable production of high-quality and processable two-dimensional (2D) polymers are highly desired but have not yet been reported. Herein, we demonstrate a convenient noncovalent functionalization strategy for producing highly uniform, aqueous processable and semiconducting 2D triazine polymers. Experimental and theoretical analysis reveal that the aromatic amphiphilic 1-pyrenebutyrate can adsorb and intercalate into the interlayer of bulk crystalline covalent triazine framework (CTF) through noncovalent π-π stacking interaction between the pyrene moiety and the porous basal plane of 2D triazine polymer layer, which greatly facilitate the exfoliation of CTF in water in large scale. The as-prepared highly water-dispersible single-layer/few-layer 2D triazine polymer nanosheets can be easily processed into ultralight aerogels with a density of 5-15 mg cm-3, which can be further shaped into mechanically strong films upon simple compression. This noncovalent functionalization not only improve the dispersibility and processability of 2D triazine polymer, but also optimize its band structure and promote the photogenerated carrier separation via an interesting surface molecule doping effect, thus resulting in a remarkable photocatalytic hydrogen evolution rate of 1249 μmol h-1 (24980 μmol g-1 h-1) and apparent quantum efficiency up to 27.2% at 420 nm for the 2D triazine polymer, outperforming most metal-free photocatalysts ever reported.

Aqueous Processable Two‐Dimensional Triazine Polymers with Superior Photocatalytic Properties

AbstractEfficient and scalable production of high‐quality and processable two‐dimensional (2D) polymers are highly desired but have not yet been reported. Herein, we demonstrate a convenient noncovalent functionalization strategy for producing highly uniform, aqueous processable and semiconducting 2D triazine polymers. Experimental and theoretical analysis reveal that the aromatic amphiphilic 1‐pyrenebutyrate can adsorb and intercalate into the interlayer of bulk crystalline covalent triazine framework (CTF) through noncovalent π‐π stacking interaction between the pyrene moiety and the porous basal plane of 2D triazine polymer layer, which greatly facilitate the exfoliation of CTF in water in large scale. The as‐prepared highly water‐dispersible single‐layer/few‐layer 2D triazine polymer nanosheets can be easily processed into ultralight aerogels with a density of 5–15 mg cm−3, which can be further shaped into mechanically strong films upon simple compression. This noncovalent functionalization not only improve the dispersibility and processability of 2D triazine polymer, but also optimize its band structure and promote the photogenerated carrier separation via an interesting surface molecule doping effect, thus resulting in a remarkable photocatalytic hydrogen evolution rate of 1249 μmol h−1 (24980 μmol g−1 h−1) and apparent quantum efficiency up to 27.2 % at 420 nm for the 2D triazine polymer, outperforming most metal‐free photocatalysts ever reported.

Mn-doped CdS/Cu2O: An S-scheme heterojunction for photocatalytic hydrogen production

This study aimed to regulate the band structure and enhance the reduction ability of CdS by substituting it with Mn2+. Subsequently, this nano MnCdS solid solution was anchored on the surface of truncated octahedral Cu2O through a simple hydrothermal process, forming an S-scheme heterojunction at the interface between MnCdS and Cu2O. Equilibrating the Fermi level at the MnCdS/Cu2O heterojunction led to spontaneous diffusion of electrons in MnCdS to Cu2O, generating an internal electric field that drove the recombination of electrons in the conduction band (CB) of Cu2O and holes in the valence band (VB) of MnCdS. Consequently, this process preserved the powerful photogenerated electrons in the CB of MnCdS, facilitating robust photocatalytic hydrogen evolution. The MnCdS/Cu2O exhibited a photocatalytic hydrogen evolution rate of 66.3 mmol g−1 h−1, which was 3.4- and 54.3-times higher than that of MnCdS (19.4 mmol g−1 h−1) and Cu2O (1.2 mmol g−1 h−1), respectively. This study elucidates advanced S-scheme heterojunction catalyst architectures for photocatalytic hydrogen production.

A self-assembled 2D nanosheet II-type heterojunction photocatalyst for photocatalytic hydrogen evolution

The exploration of highly efficient and low-cost catalysts for photocatalytic hydrogen evolution is of great increasing interest under the dual pressure of energy shortage and environment pollution. In this work, flake-like SrCO3 was in-situ self-assembled at the surface of SrTiO3 nanosheet to fabricate a novel 2D/2D heterojunction nanocomposite catalyst with strong interaction inside these two semiconductors by a simple hydrothermal method. An accurate loading amount of SCO was responsible for the high photoactivity of 10SCO-STO-Pt, which leads to a 12.9-fold improvement of photocatalytic hydrogen evolution rate (from 0.22 to 2.83 mmol [Formula: see text][Formula: see text]) compared to that of pure STO-Pt. Meanwhile, the apparent quantum efficiency (AQE) of 10SCO-STO-Pt composite with the optimum SCO loading (10 wt.%) reached up to 16.9% under the irradiation of 313 nm light. The introduction of SCO can form a II-type heterojunction in close contact with STO, which can significantly improve the photogenerated electron transfer and reduce the recombination of photogenerated carrier in STO. As a result, the charge separation efficiency and photocatalytic activity of STO were obviously enhanced.

Kinetic Evidence for Type-II Heterojunction and Z-Scheme Interactions in g-C3N4/TiO2 Nanotube-Based Photocatalysts in Photocatalytic Hydrogen Evolution

Composite materials based on g-C3N4 and TiO2 nanotubes have been synthesized as environmentally friendly photocatalysts with heterojunctions suitable for enhanced photocatalytic hydrogen production. Composites were prepared with various ratios (x = 0–1) of g-C3N4 and after chemical modification and exfoliation of bulk g-C3N4. Contact formation between g-C3N4 and TiO2 generally enhanced photoactivity, which caused the x-dependent changes in the photocatalytic hydrogen evolution rates of the g-C3N4/TiO2 compounds to vary following a volcano-shaped curve with the maximum rate at x ∼ 0.6 for all the compounds regardless of the pretreatment (bulk or modified) of g-C3N4. The modified g-C3N4-based composites showed higher photoactivities than the unmodified bulk g-C3N4 due to the high surface area. The major reason for the enhanced photoactivity with the volcano shape was attributed to the Z-scheme interaction at the heterojunction. Interestingly, detailed analysis of the kinetic H2 evolution rates of the composites with Pt cocatalysts only on TiO2 nanotubes further showed that the dominant type of interaction at the heterojunctions changed from the type-II heterojunction to the Z-scheme at x ∼ 0.1. It is inferred that structural diversity at the g-C3N4/TiO2 interfaces is the origin of the changes in the dominant type of interaction in the composites with increasing ratios of g-C3N4.

Replacing CC Unit with B←N Unit in Isoelectronic Conjugated Polymers for Enhanced Photocatalytic Hydrogen Evolution.

Three linear isoelectronic conjugated polymers PCC, PBC, and PBN are synthesized by Suzuki-Miyaura polycondensation for photocatalytic hydrogen (H2 )production from water. PBN presented an excellent photocatalytic hydrogen evolution rate (HER) of 223.5µmolh-1 (AQY420 =23.3%) under visible light irradiation, which is 7 times that of PBC and 31 times that of PCC. The enhanced photocatalytic activity of PBN is due to the improved charge separation and transport of photo-induced electrons/holes originating from the lower exciton binding energy (Eb ), longer fluorescence lifetime, and stronger built-in electric field, caused by the introduction of the polar B←N unit into the polymer backbone. Moreover, the extension of the visible light absorption region and the enhancement of surface catalytic ability further increase the activity of PBN. This work reveals the potential of B←N fused structures as building blocks as well as proposes a rational design strategy for achieving high photocatalytic performance.

Impact of Postprocessing Approaches and Interface Cocatalysts Regulation on Photocatalytic Hydrogen Evolution of Protonic Titanate Derived TiO2 Nanostructures

TiO2–based photocatalysis system for splitting water into hydrogen offers a sustainable and green technology to produce clean hydrogen energy. However, pristine TiO2 still exists inherent shortcomings restricting its practical applications. Herein, the impact of postprocessing approaches of protonic titanate on engineering of oxygen vacancy and photocatalytic hydrogen evolution of TiO2−x is studied. Subsequently, interfacial cocatalysts are successfully involved in the optimized TiO2−x for enhanced photocatalytic hydrogen evolution. TiO2−x with the highest photocatalytic hydrogen evolution performance of 3112.09 μmol g−1 h−1, denoted as TiO2–C, is selected to adjust the interface with Cu and MoS2 respectively. Cu–TiO2–C and MoS2–TiO2–C composites are synthesized to enhance the separation ability of photogenerated electron‐hole pairs and significantly improve the photocatalytic hydrogen evolution performance. The photocatalytic hydrogen evolution rates of 5 wt% Cu–TiO2–C and 40 wt% MoS2–TiO2–C are 9225.75 and 5765.48 μmol g−1 h−1, respectively. It is proved that different postprocessing methods can tune the content of oxygen vacancy in TiO2−x and regulate the photocatalytic hydrogen evolution performance of TiO2−x materials. The interface regulation of the cocatalyst also contributes to the separation of photogenerated electron‐hole pairs and serves as active sites to enhance hydrogen evolution performance.

Cation exchange synthesis of hollow-structured cadmium sulfide for efficient visible-light-driven photocatalytic hydrogen evolution

Efficient solar absorption and photoinduced charge separation are extremely important for solar-energy conversion on semiconductor photocatalysts. To advance the photocatalytic performance, we developed a general templated-assisted reverse cation exchange strategy to successfully synthesize hollow-structured CdS semiconductors with the textile structural surface. The crystal phase, particle morphology, optical/electrical properties, and photocatalytic performance of the as-syntheszied sample are investiagted by XRD, SEM, TEM, XPS, DSR, PL, ESR photoelectrochemical measurements, and Photocatalytic H2 evolution test. The final CdS sample exhibits an enhanced photocatalytic hydrogen evolution rate of up to 965 μmol·g−1 h−1, 2.8 times higher than the reported CdS nanorods. Based on the experimental and characterization results, the improved photocatalytic activity of the cadmium sulfide semiconductor can be ascribed to the special hollow cubic structure with a thin shell, which can enhance the light-harvesting ability and provide abundant photocatalytic active sites for facilitating the separation of photogenerated electron/hole pairs. This synthetic strategy may pave a new path for the rational design of efficient sulfur-based semiconductor photocatalysts for solar driven H2 production.

Dual charge-accepting engineering modified AgIn5S8/CdS quantum dots for efficient photocatalytic hydrogen evolution overall H2S splitting

Low charge carrier separation and transfer efficiency continues to be the major obstacle to harvest solar energy to split hydrogen sulfide (H2S) as hydrogen source for H2 production with value-added utilization for waste by-products. Herein, we designed a Type-II core/shell AgIn5S8/CdS (AIS/CdS) quantum dots (QDs) photocatalyst capped with short-chain inorganic sulfide ion (S2−) ligand with dual charge-accepting engineering to promote charge carrier extraction and faster photogenerated electron transfer. Transient absorption spectroscopy analysis demonstrates that the excited electrons are fast injected into CdS shell from AIS core within 1.7 ps, instead of transferring to sub-bandgap states. Consequently, the highest photocatalytic hydrogen evolution rate of AIS/CdS QDs with S2- ligand is 12.74 mmol g−1 h−1 that is more than four times of pristine AIS core. This work offers insightful guidance on the rational design of charge-accepting engineering QDs-based photocatalysts, thereby stimulating solar to hydrogen generation and resource utilization of pollutants.

Metal-organic framework derived hierarchical ZnO nanosheets/CdS composites for high photocatalytic activity under solar radiation

A novel ZnO nanosheets (NSs) was successfully fabricated by a two-step process involving a successive hydrothermal transformation and calcination. The ZnO NSs was further hybridized with CdS to form a hierarchical CdS/ZnO NSs heterostructure, of which, the ZnO NSs can provide abundant active sites, and the CdS broaden the range of light absorption and prohibit the recombination of photogenerated carriers. Notably, the 18 wt% hierarchical CdS/ZnO NSs exhibited the highest photocatalytic degradation activity and hydrogen evolution rate of 1835 μmol g−1h−1. Furthermore, the activity of the catalyst did not decrease obviously after successive five cycles, indicating its excellent stability.

Solvent-exfoliated D-A π-polymer @ ZnS heterojunction for efficient photocatalytic hydrogen evolution

Constructing organic/inorganic soft-hard semiconductor heterojunctions has emerged as a promising strategy to design high-performance photocatalysts. Herein, N-methyl pyrrolidone-assisted solvent exfoliation is employed for the first time to the in-situ construction of benzothidiazole (BT) and bithiophene (TT)-based donor-acceptor (D-A) π-polymeric-inorganic heterojunction (i.e., BT-TT@ZnS) for Pt-free photocatalytic hydrogen production (PHP). Despite that the pristine polymer BT-TT has no photocatalytic activity, the BT-TT@ZnS heterostructure, however, shows a photocatalytic hydrogen evolution rate up to 15.8 mmol g−1 h−1 in the absence of Pt co-catalyst, which is also 10.5 times as high as that of pristine ZnS, and outperforms the majority of the previously reported π-polymers photocatalysts. Systematical studies reveal that BT-TT@ZnS heterostructures simultaneously possess the broadened light absorption and the improved charge separation compared to the single components of BT-TT or ZnS. This work develops a facile and generally applicable strategy to construct π-polymer colloids @ inorganic heterostructure for PHP application.

Tailoring high-index-facet and oxygen defect of black In2O3−x/In2O3 as highly photothermal catalyst for boosting photocatalytic hydrogen evolution and contaminant degradation

Defect engineering and the high index surface (HIF) can be regarded as two effective methods to tuning the microstructure of catalysts and photocatalytic activities. So far, the integration of the synergistic effects of oxygen vacancies and HIF advantages into semiconductors to enhance photocatalytic performance has received little attention. Herein, Black In2O3−x/In2O3 with oxygen vacancy and exposed (321) surface active crystal plane has been synthesized as excellent photocatalysts via a facile method. The catalysts of Black In2O3−x/In2O3 can reach the highest photocatalytic hydrogen evolution rate (1046 μmol h−1 g−1) and degradation efficiency of MB (0.017 min−1) with 120 min, respectively. As a special phase junction, Black In2O3−x/In2O3 could effectively boost the separation and transfer of photogenerated charges because of unique defect homojunction microstructure resulting in exposing abundant photocatalyst redox active sites. The experiment characterization and density functional theory (DFT) calculations can further employ to unveil mechanism of enhanced hydrogen evolution reaction (HER) activity. It is obvious that enhanced HER activity was attributed to synergy of oxygen defect and exposed (321) surface active crystal planes due to electron distribution and lower Gibbs free energy of H adsorption. This work might open up new avenues to integrate exposed facets and oxygen vacancy for enhancing the photocatalytic hydrogen evolution with renewable energy sources and environmental remediation.

Unraveling the role of phase engineering in tuning photocatalytic hydrogen evolution activity and stability

In this work, taking NiSe2 as a prototype to be used as cocatalyst in photocatalytic hydrogen evolution, we demonstrate that the crystal phase of NiSe2 plays a vital role in determining the catalytic stability, rather than activity. Theoretical and experimental results indicate that the phase structure shows negligible influence to the charge transport and hydrogen adsorption capacity. When integrating with carbon nitride (CN) photocatalyst forming hybrids (m-NiSe2/CN and p-NiSe2/CN), the hybrids show comparable photocatalytic hydrogen evolution rates (3.26 µmol/h and 3.75 µmol/h). Unlike the comparable catalytic activity, we found that phase-engineered NiSe2 exhibits distinct stability, i.e., m-NiSe2 can evolve H2 steadily, but p-NiSe2 shows a significant decrease in catalytic process (∼57.1% decrease in 25 h). The factor leading to different catalytic stability can be ascribed to the different surface conversion behavior during photocatalytic process, i.e., chemical structure of m-NiSe2 can be well preserved in catalytic process, but partial p-NiSe2 tends to be converted to NiOOH.

Highly efficient flower-like ZnIn2S4/CoFe2O4 photocatalyst with p-n type heterojunction for enhanced hydrogen evolution under visible light irradiation

The construction of a p-n heterojunction structure is considered to be an effective method to improve the separation of electron-hole pairs in photocatalysts. A series of ZnIn2S4/CoFe2O4 (ZIS/CFO) photocatalysts with p-n heterojunctions were prepared via a method involving ultrasonication and calcination. The synthesized photocatalysts were tested and analyzed via various testing techniques, and their hydrogen evolution rates were evaluated. Compared with pure ZIS, ZIS/CFO with different mass ratios of CFO to ZIS showed improved photocatalytic hydrogen production performance, and the optimal photoactivity showed a nearly 12-fold increase, which can be attributed to the formation of p-n junctions and the formed internal electric field, accelerating the separation of electron-hole pairs and effectively improving the photocatalytic hydrogen evolution rate. The excellent stability of the ZIS/CFO composite was proven by three cycle experiments. In addition, the ZIS/CFO composite also possessed excellent magnetic properties to realize facial magnetic recoverability. This work paves the way for the design and preparation of magnetically recoverable p-n heterojunction photocatalysts.

Perovskite-like Oxide KCuTa3O9 Decorated with Ultra-Thin Carbon Layers as a Visible-Light-Response Photocatalyst for Hydrogen Evolution

Development of a perovskite-like metal oxide (PLMO) photocatalyst with a visible light response has long been pursued in the field of photocatalysis. Herein, a PLMO KCuTa3O9 (KCTO) photocatalyst with a narrow band gap of 2.68 eV and a suitable energetic position that straddles the water redox potential has been developed. Subsequently, a 3 nm-thick carbon layer was coated uniformly on the surface of KCuTa3O9 (KCTO@C). The optimized KCTO@C-10 photocatalyst exhibits an excellent photocatalytic hydrogen evolution rate under visible light irradiation (λ > 420 nm), which is 2.87 times higher than that of pure KCTO. Systematic investigations reveal that carbon layer coating on the surface of KCTO not only expands visible light absorption but also promotes electron transfer, facilitating photoinduced electron–hole separation. In addition, an unobvious structural change of the KCTO@C-10 photocatalyst after four cycle tests demonstrates its good photochemical stability. This work offers a perspective for exploiting carbon-coated perovskite-like metal oxide photocatalysts to achieve high-efficiency photocatalytic hydrogen evolution.

Three-dimensional S-scheme heterojunction by integration of purple tungsten oxide nanowires and cadmium sulfide nanospheres for effective photocatalytic hydrogen generation

The practical photocatalytic application of cadmium sulfide (CdS) has been significantly constrained by fast carrier recombination and significant photocorrosion. Therefore, we developed a three-dimensional (3D) step-by-step (S-scheme) heterojunction using the coupling interface between purple tungsten oxide (W18O49) nanowires and CdS nanospheres. The photocatalytic hydrogen evolution rate of optimized W18O49/CdS 3D S-scheme heterojunction can reach 9.7 mmol·h−1·g−1, 7.5 and 16.2 times greater than pure CdS (1.3 mmol·h−1·g−1) and 10 wt%-W18O49/CdS (mechanical mixing, 0.6 mmol·h−1·g−1), proving that the tight S-scheme heterojunction constructed by the hydrothermal method can efficiently enhance the carrier separation. Notably, the apparent quantum efficiency (AQE) of W18O49/CdS 3D S-scheme heterojunction approaches 7.5% and 3.5% at 370 nm and 456 nm, respectively, which is 7.5 and 8.8 times than pure CdS (1.0% and 0.4%). The produced W18O49/CdS catalyst also has relative stability of structure and hydrogen production. Additionally, the H2 evolution rate of W18O49/CdS 3D S-scheme heterojunction is 1.2 times greater than 1 wt%-platinum (Pt)/CdS (8.2 mmol·h−1·g−1), which indicates that the W18O49 can effectively replace the precious metal for boosting the hydrogen production rate.

Nanoscaling and Heterojunction for Photocatalytic Hydrogen Evolution by Bimetallic Metal‐Organic Frameworks

AbstractUsing sunlight to manufacture hydrogen offers promising access to renewable clean energy. For this, low‐cost photocatalyst with effective light absorption and charge transfer are crucial, as current top‐performing systems often involve precious metals like Pd and Pt. An integrated organic–inorganic photocatalyst based on the cheap metals of iron and nickel are reported, wherein the metal ions form strong metal‐sulfur bonds with the organic linker molecules (2,5‐dimercapto‐1,4‐benzenedicarboxylic acid, H4DMBD) to generate 2D coordination sheets for promoting light absorption and charge transport. The 2D sheets are further modified through ionic metal‐carboxylate moieties to allow for functional flexibility. Thus, high‐surface‐area thin nanosheets of this 2D material, with an optimized Fe/Ni ratio (0.25:1.75), and in heterojunction with CdS nanosheet, achieve a stable photocatalytic hydrogen evolution rate of 12.15 µmol mg−1 h−1. This work synergizes coordination network design and nano‐assembly as a versatile platform for catalyzing hydrogen production and other sustainable processes.

Benzotrithiophene‐based Covalent Organic Framework Photocatalysts with Controlled Conjugation of Building Blocks for Charge Stabilization

AbstractCovalent organic frameworks have recently shown high potential for photocatalytic hydrogen production. However, their structure‐property‐activity relationship has not been sufficiently explored to identify a research direction for structural design. Herein, we report the design and synthesis of four benzotrithiophene (BTT)‐based covalent organic frameworks (COFs) with different conjugations of building units, and their photocatalytic activity for hydrogen production. All four BTT‐COFs had slipped parallel stacking patterns with high crystallinity and specific surface areas. The change in the degree of conjugation was found to rationally tune the rate of photocatalytic hydrogen evolution. Based on the experimental and calculation results, the tunable photocatalytic performance could be mainly attributed to the electron affinity and charge trapping of the electron accepting units. This study provides important insights for designing covalent organic frameworks for efficient photocatalysts.

Benzotrithiophene-based Covalent Organic Framework Photocatalysts with Controlled Conjugation of Building Blocks for Charge Stabilization.

Covalent organic frameworks have recently shown high potential for photocatalytic hydrogen production. However, their structure-property-activity relationship has not been sufficiently explored to identify a research direction for structural design. Herein, we report the design and synthesis of four benzotrithiophene (BTT)-based covalent organic frameworks (COFs) with different conjugations of building units, and their photocatalytic activity for hydrogen production. All four BTT-COFs had slipped parallel stacking patterns with high crystallinity and specific surface areas. The change in the degree of conjugation was found to rationally tune the rate of photocatalytic hydrogen evolution. Based on the experimental and calculation results, the tunable photocatalytic performance could be mainly attributed to the electron affinity and charge trapping of the electron accepting units. This study provides important insights for designing covalent organic frameworks for efficient photocatalysts.