Photocatalytic H2 Production Activity Research Articles

Ultrasound-Assisted Synthesis of Nonmetal-Doped Titanium Dioxide Photocatalysts for Simultaneous H2 Production and Chemical Oxygen Demand Removal from Industrial Wastewater.

A series of nonmetal-doped titanium dioxide (Nmx/TiO2, where x is the weight fraction of nonmetal elements) photocatalysts was prepared via ultrasonic-assisted impregnation for simultaneous hydrogen (H2) production and chemical oxygen demand (COD) removal from industrial wastewater. Three types of Nm elements, carbon (C), silicon (Si), and phosphorus (P), were explored. The P1/TiO2 exhibited a higher photocatalytic activity for H2 production and COD removal than the C1/TiO2 and Si1/TiO2 photocatalysts. Approximately 6.43 mmol/g photocatalyst of H2 was produced, and around 26% COD removal was achieved at a P1/TiO2 loading of 4.0 g/L, a light intensity of 5.93 mW/cm2, and a radiation time of 4 h. This is because the P1/TiO2 photocatalyst exhibited lower point of zero charge values and a more appropriate band position compared with other Nmx/TiO2 photocatalysts to produce H+, which can consequently form H2, and reactive oxygen species (HO· and O2·–), which serve as oxidizing agents to degrade the organic pollutants. Increasing the content of the P element doped into the TiO2-based material up to 7.0% by weight enhanced the H2 production and COD removal up to 8.34 mmol/g photocatalyst and 50.6%, respectively. This is attributed to the combined effect of the point of zero charge value and the SBET of the prepared photocatalysts. The photocatalytic activity of the P7/TiO2 photocatalyst was still higher than the TiO2-based material after the fourth use.

Light-Induced Migration of Spin Defects in TiO2 Nanosystems and their Contribution to the H2 Evolution Catalysis from Water.

The photocatalytic activity for H2 production from water, without presence of hole scavengers, of thermally reduced TiO2 nanoparticles (H-500, H-700) and neat anatase were followed by in-situ continuous-wave light-induced electron paramagnetic resonance technique (CW-LEPR), in order to correlate the H2 evolution rates with the electronic fingerprints of the photoexcited systems. Under UV irradiation, photoexcited electrons moved from the deep lattice towards the superficially exposed Ti sites. These photogenerated redox sites mediated (e- +h+ ) recombination and were the crucial electronic factor affecting catalysis. In the best-performant system (H-500), a synergic combination of mobile electrons was observed, which dynamically created diverse types of Ti3+ sites, including interstitial Ti3+ , and singly ionized electrons trapped in oxygen vacancies (VO . ). The interplay of these species fed successfully surface exposed Ti4+ sites, which became a catalytically active, fast reacting Ti4+ ⇄Ti3+ state that was key for the H2 evolution process.

Modulating Coordination Environment of Single-Atom Catalysts and Their Proximity to Photosensitive Units for Boosting MOF Photocatalysis

Well-organized photosensitive units and catalytic sites in proximity are crucial for improving charge separation efficiency and boosting photocatalysis. Herein, a general and facile strategy for the construction of high-loading (>4 wt %) single-atom catalysts (SACs) with a tunable coordination microenvironment has been developed on the basis of metal-organic frameworks (MOFs). The neighboring -O/OHx groups from a Zr6-oxo cluster in the MOFs provide lone-pair electrons and charge balance to immobilize the extraneous single metal atoms. The well-accessible and atomically dispersed metal sites possess close proximity to the photosensitive units (i.e., linkers), which greatly accelerates charge transfer and thereby promotes the redox reaction. The coordination environment of the representative single-atom Ni sites significantly modulates the electronic state and the proton activation barrier toward hydrogen production. As a result, the optimized Ni1-S/MOF with a unique Ni(I) microenvironment presents excellent photocatalytic H2 production activity, up to 270 fold of the pristine MOF and far surpassing the other Ni1-X/MOF counterparts. This work unambiguously demonstrates the great advantage of MOFs in the fabrication of high-content SACs with variable microenvironments that are in close proximity to photosensitive linkers, thereby facilitating the electron transfer and promoting photocatalysis.

Ultrathin Fe2P nanosheet co-catalyst CdS nanorod: The promising photocatalyst with ultrahigh photocatalytic H2 production activity

Developing 2D ultra-thin materials as co-catalysts is an effective way to boost the photocatalytic activity of semiconductors, but it remains challenging to unveil the design principle. In this work, we prepared the ultrathin 2D Fe2P nanosheets with an average thickness of ca. 1.41 nm via the facile solvothermal method. Such 2D Fe2P nanosheets were used as co-catalyst to fabricate a 2D-1D type of Fe2P-CdS photocatalysts for visible-light-driven photocatalytic H2 evolution. Experimental and first-principles results show that the Fe2P nanosheets co-catalyst with metallic characteristics modified CdS nanorods to form a 2D-1D heterointerface with strong ionic interaction, which induces the electron density redistribution in Fe2P-CdS composite, resulting in a lower recombination rate of carriers. Moreover, Fe2P nanosheets also reduce the overpotential for hydrogen evolution, provide abundant active sites and enhance H2O adsorption, significantly promoting the release of H2. As a result, the 11 wt% Fe2P-CdS exhibited an extremely high photocatalytic performance (207.82 mmol·h−1·g−1), which is more than 78 times higher than that of the pristine CdS. This work demonstrates the potential of 2D Fe2P as a promising co-catalyst in photocatalysis and provides a useful guide on phosphide-based material designing.

A novel strategy for loading metal cocatalysts onto hollow nano-TiO2 inner surface with highly enhanced H2 production activity

The loading strategy of cocatalysts affects its activity exerting and atom utilization. Here, a novel strategy for loading precious metal (Pt) cocatalysts by means of ultrathin N-doped carbon layer is reported. The strategy is based on a pyrolysis process of predesigned N-containing polymers and Pt complexes on hard-template surface, during which Pt can be reduced by carbon from pyrolysis at high temperatures. Finally, the hollow TiO2 composite with stable and dispersed Pt on its inner surface was prepared. It shows an ultrahigh photocatalytic H2 production activity as high as 25.7 mmol h−1 g−1 with methanol as sacrificial regent, and displays an apparent quantum yield as 13.2%. The improved photocatalytic activity and stability can be attributed to the highly dispersed and ultrafine Pt nanoparticles, enhanced interaction between Pt-species and carbon support, fast photo-excited electron transport from the high graphitization degree of NC layers, ample oxygen vacancies/defects, as well as the manipulated local charge distribution of Pt/NC-layer configuration. Additionally, the universality of the proposed strategy was demonstrated by replacing metal sources (such as, Ru and Pd). This work presented a promising strategy for the design and development of novel photocatalysts, which shows a broad application prospect.

Ammonia-assisted fabrication of Cr1.3Fe0.7O3@amine-containing carbon quantum dot core-shell architectures with ultrathin shell for broad-spectrum-driven photocatalytic hydrogen evolution

To make the efficient spatial charge separation and exploration of broad spectrum-driven H2 evolution remains a pivot challenge. Here, a novel core@shell Cr1.3Fe0.7O3@amine-containing carbon quantum dots (S1@NCQD) composite was successfully prepared by a facile bottom-up self-assembly route. An ordinary contact structure of carbon quantum dots (CQD) modified Cr1.3Fe0.7O3 was also prepared. The photocatalytic H2 production rate of Cr1.3Fe0.7O3 composite has not been improved only through the modification of CQD. Remarkably, the as-obtained S1@NCQD composite illustrated the highest photocatalytic H2 production activity under visible light illumination, which is approximately 5.5 and 7.9 times higher than that of S1 and CQD/S1 composite, respectively. Importantly, the new class of S1@NCQD composite can also exploit broad spectrum light to trigger H2 production (3.12 μmol). Effective photoinduced electron transfer is derived from a powerful interaction between shell and core, which was characterized by using photoluminescence, time-resolved PL spectroscopy, and photocurrent measurement.

Synergistic effects of morphology control and calcination on the activity of flower-like Bi2WO6-Bi2O3 photocatalysts prepared by an ionic liquid-assisted solvothermal method

A flower-like Bi2WO6-Bi2O3 photocatalyst was prepared by a two-step process, including an ionic liquid-assisted solvothermal step and calcination. The effects of the ionic liquids and calcination on the morphology, property, and photocatalytic performance of Bi2WO6-based photocatalysts were studied. The addition of ionic liquids during the preparation of Bi2WO6 based-materials lead to increased sizes of photocatalysts and the formation of more petal sheets in the flower-like microspheres. The results of XRD, XPS, and TEM confirm that Bi2WO6-Bi2O3 photocatalysts formed after the calcination treatment of Bi2WO6 photocatalyst in an air atmosphere. The Bi2WO6-Bi2O3 flower-like photocatalyst exhibits better photocatalytic H2 production activity than the Bi2WO6 photocatalyst. The improved activity of the Bi2WO6-Bi2O3 photocatalysts was attributed to their enhanced light absorption, increased surface area, matched band structure, and the inhibited recombination of the photogenerated charge carriers.

Bridging regulation in graphitic carbon nitride for band-structure modulation and directional charge transfer towards efficient H2 evolution under visible-light irradiation

The bridging N atom in g-C3N4 structure plays a decisive role in photo-generated charge transfer because it usually confines photo-generated electrons and holes in each heptazine, thus leading to severe recombination. In this work, a kind of 2-aminoterephthalic acid-derived benzene ring group with rich π-electrons was considered to integrate with bridging N to break the above-mentioned confining effect. On the basis of density-functional theory calculations and experimental analysis, this 2-aminoterephthalic acid-derived bridging structure facilitated to draw photo-generated charge out of heptazine unit, and its polarized asymmetric structure promoted the directional transfer of photo-generated charge carriers across adjacent heptazines, thus efficiently reducing the recombination. Meanwhile, the 2-aminoterephthalic acid-derived bridging structure also reinforced the connectivity of heptazine units in g-C3N4 framework and led to high degree of polymerization, which thus extended the π-conjugated electronic system of g-C3N4 and modulated the band structure favoring photocatalytic hydrogen production. Consequently, a high photocatalytic H2-production activity of 24,595 μmol h−1 gcat-1 was achieved on the bridging regulated g-C3N4 under visible light, with an apparent quantum yield of 48.7% at 425 nm.

Novel S-scheme 2D/2D BiOBr/g-C3N4 heterojunctions with enhanced photocatalytic activity

The design and construction of heterojunction photocatalysts, which possess a staggered energy band structure and appropriate interfacial contact, is an effective way to achieve outstanding photocatalytic performance. In this study, 2D/2D BiOBr/g-C3N4 heterojunctions were successfully obtained by a convenient in situ self-assembly route. Under simulated sunlight irradiation, 99% of RhB (10 mg·L−1, 100 mL) was efficiently degraded by 1.5-BiOBr/g-C3N4 within 30 min, which is better than the performance of both BiOBr and g-C3N4, and it has superior stability. In addition, the composite also exhibits enhanced photocatalytic activity for H2 production. The enhanced activity can be attributed to the intimate interface contact, the larger surface area, and the highly efficient separation of photoinduced electron–hole pairs. Based on the experimental results, a novel S-scheme model was proposed to illuminate the transfer process of charge carriers. This study presents a simple way to develop novel step-scheme photocatalysts for environmental and related applications.

Novel NiO/RP composite with remarkably enhanced photocatalytic activity for H2 evolution from water

The low photocatalytic activity of red phosphorus (RP) for H2 production seriously restricts its wide application. In this work, we reported a facile synthesis and remarkable activity improvement of NiO/RP composite for water splitting. As a result, the 3% NiO/RP composite exhibited the highest photocatalytic activity for H2 production (57.27 μmol g−1 h−1), which is 68.56 times higher than that of pure RP (0.82 μmol g−1 h−1) under visible light (λ ≥ 420 nm) irradiation. The investigation of photocarriers separation mechanism indicated NiO/RP composite applied a Z-scheme mechanism to promote the photocarriers separation. This is a potential strategy to dramatically enhance the photocatalytic activity of RP for H2 evolution using transition metal oxide to efficiently separate the photocarriers under light irradiation.

Fabrication of S-scheme CdS-g-C3N4-graphene aerogel heterojunction for enhanced visible light driven photocatalysis

Constructing S-scheme heterojunction photocatalysts reveals a greatly improved separation efficiency of photogenerated carriers and enhanced harvesting ability of solar energy in photocatalytic field. Herein, a ternary CdS-g-C3N4-GA heterojunction has been fabricated by a facile ultrasound strategy, which behaved as a S-scheme heterojunction with an intimate interface formed, and GA played as an electronic transportation platform to promote the separation of photo-induced charge carriers, which was certified through photoelectrochemical techniques. Density functional theory calculations revealed that the different component in ternary CdS-g-C3N4-GA heterojunction demonstrated an obvious difference of work function, resulting in the charge transfer from CdS to g-C3N4 through GA with S-scheme principle. In the optimized conditions, the S-scheme CdS-g-C3N4-GA heterojunction not only displayed greatly enhanced photocatalytic performances for degradation of dye and antibiotic wastewater, but also improved photocatalytic H2 production activity. In addition, the photocatalytic mechanism and driving force of charge transfer and separation in S-scheme CdS-g-C3N4-GA heterojunction were studied. This study offers a feasible strategy to construct a ternary S-scheme heterojunction for environmental and energy photocatalysis.

Ag2S/CdS/Ni ternary nanostructures: long-range electric field to enhanced photocatalytic hydrogen production activity

Silver sulfide, cadmium sulfide and nickel (Ag2S/CdS/Ni) ternary nanostructures were fabricated by a two step process and characterized by x-ray powder diffraction, transmission electron microscopy, and UV–vis diffuse reflection spectroscopy. The photocatalytic hydrogen production activity of ternary nanostructures and reference samples were evaluated using triethanolamine (TEOA) as sacrificial reagent in water under visible-light illumination (λ ≥ 420 nm). The result shows Ag2S/CdS/Ni nanostructures exhibited a high visible light photocatalytic H2 evolution rate of 1.54 mmolh−1 g−1, which was 4.6 times and 1.4 times higher than that of 1.2% mol Ag2S/CdS and Ni/CdS. The degree of photocorrosion of CdS were employed to study the photogenerated carriers transfer route by measuring and comparing the concentration of Cd2+ in the solution of the photocorrosion experiments in a nonsacrificial system. A long-range electric field, which is similar to the p-i-n electric field structure, was proposed to be constructed by Ni nanoparticles and Ag2S nanoparticles in CdS. Most of the photogenerated electrons and holes in CdS drift to the electron donor and electron acceptor respectively under the action of long-range electric field, which greatly improves the separation efficiency of photogenerated carriers and the photocatalytic H2 production activity.

Ultrathin 2D/2D ZnIn2S4/g‐C3N4 Nanosheet Heterojunction with Atomic‐Level Intimate Interface for Photocatalytic Hydrogen Evolution under Visible Light

AbstractDesignation of high‐efficiency water splitting photocatalyst is still a challenge in converting solar energy into chemical fuels. Heterojunction can inhibit recombination of carriers which is considered to be a reliable strategy to improve photocatalytic performance on water splitting. In this work, a “face‐to‐face” 2D tight heterostructure is constructed by growing ZnIn2S4 nanosheets on g‐C3N4 nanosheets. Due to the ultrathin 2D structure and large amounts of NS bonds forming at the interface of heterojunction affording charge transferring tunnel, the synthesized 2D heterojunction photocatalysts successfully accelerate the carrier migration rate and decrease recombination probability of photogenerated electrons and holes. As a result, the optimized ZISCN‐50 sample has excellent photocatalytic H2 production activity (10.92 mmol h−1 g−1) under visible light, which is ≈5.2 times of ZnIn2S4 (2.09 mmol h−1 g−1) and 136.5 times of pure g‐C3N4 nanosheets (0.08 mmol h−1 g−1). Cycle experiments show that the composite material has excellent stability and recyclability. This work provides fresh insights into atomic‐level structure and interface design in order to synthesize high‐efficiency 2D/2D heterojunction photocatalysts.

Amino group-rich porous g-C3N4 nanosheet photocatalyst: Facile oxalic acid-induced synthesis and improved H2-evolution activity

The reasonable modulation of tri-s-triazine structure units of g-C3N4 is an effective method to optimize its intrinsic electronic and optical properties, thus boosting its photocatalytic hydrogen-evolution activity. Herein, amino groups are successfully introduced into the tri-s-triazine structure units of g-C3N4 nanosheets to improve their H2-evolution activity via a facile oxalic acid-induced supramolecular assembly strategy. In this case, the resulting amino group-rich porous g-C3N4 nanosheets display a loose and fluffy structure with a large specific surface area (70.41 m2 g−1) and pore volume (0.50 cm3⁠ g−⁠1), and enhanced visible-light absorption (450–800 nm). Photocatalytic tests reveal that the amino group-rich porous g-C3N4 nanosheets (AP-CN1.0 nanosheets) exhibit a significantly elevated photocatalytic H2-production activity (130.7 μmol h−1, AQE = 5.58%), which is much greater than that of bulk g-C3N4 by a factor of 4.9 times. The enhanced hydrogen-generation performance of amino group-rich porous g-C3N4 nanosheets can be mainly attributed to the introduction of more amino groups, which can reinforce the visible-light absorption and work as the interfacial hydrogen-generation active centers to boost the photocatalytic hydrogen production. The present facile and effective regulation of tri-s-triazine structure units may provide an ideal route for the exploitation of novel and highly efficient g-C3N4 photocatalysts.

Manipulating metal oxidation state over ultrastable metal-organic frameworks for boosting photocatalysis

The development of efficient and stable photocatalysts in drastic conditions is highly desirable yet remains challenging. Herein, we report an exceptionally stable cerium metal-organic framework (MOF), Ce-TTCA, which can achieve photocatalytic hydrogen (H2) production in both pH = 2 and 12 aqueous solutions using Pt as a cocatalyst (Pt/Ce-TTCA), with H2 production rates of 60.3 and 348.8 μmol g−1 h−1, respectively. More impressively, partial oxidation of Ce3+ to Ce4+ in Ce-TTCA not only extends the light harvesting ability, but also promotes the effective charge separation, thus greatly improving photocatalytic H2 production activity. The Pt/Ce-TTCA-65 with the content of 65 % Ce4+ shows 6-fold improvement of photocatalytic activity than that of Pt/Ce-TTCA without Ce4+, well highlighting that the crucial role of metal oxidation state in photocatalysis. This work represents the first report on regulating MOF photocatalysis for hydrogen production by manipulating the metal oxidation state.

Z-scheme 2D/2D g-C3N4/Sn3O4 heterojunction for enhanced visible-light photocatalytic H2 evolution and degradation of ciprofloxacin

Photocatalytic water splitting for H2 production and antibiotics pollution removals under wide solar spectrum illumination have been broadly investigated. Graphitic carbon nitride (CN) based binary heterogeneous composite is an important component in artificial photosynthesis. In this work, a novel all-solid-state direct Z-scheme photocatalyst based on CN/Sn3O4 heterojunction is successfully synthesized using a simple calcination strategy. Sn3O4 nanosheets were well dispersed on the surface of CN to establish 2D-2D heterostructure. Benefiting from the wide visible light response of Sn3O4 and the facilitated charge transfer from Sn3O4 to CN for the band alignment, CN/Sn3O4 composite exhibits fascinating photocatalytic H2 evolution and ciprofloxacin degradation. In particular, CN/Sn3O4-3 with 3 wt% of Sn3O4 performs the optimum photocatalytic H2 production activity (98.07 μmol·h-1) and the degradation rate constant is about 3.4 times than CN under visible light irradiation. The enhanced photocatalytic performance mainly derived from the fabrication of direct Z-scheme heterojunction, which maximizes the photo redox capacity of composite. This work will provide new insight into the design of 2D photocatalysts with considerable catalytic activity for water splitting and pollutants degradation.

Tuning the strength of built-in electric field in 2D/2D g-C3N4/SnS2 and g-C3N4/ZrS2 S-scheme heterojunctions by nonmetal doping

Single photocatalysts usually exhibit unsatisfactory performance due to the serious recombination of photogenerated electron‒hole pairs. Combining two photocatalysts to construct S-scheme heterojunction could solve this problem. In S-scheme mechanism, the interfacial built-in electric field (IEF) provides a vital driving force for efficient charge separation. Modifying the IEF is a feasible strategy to further improve the photocatalytic activity. Herein, a novel idea of tuning the strength of IEF in 2D/2D graphitic carbon nitride (g-C3N4)/MS2 (M = Sn, Zr) S-scheme heterojunctions by nonmetal doping was developed by employing density functional theory calculation. Three nonmetal elements (O, P, and S) were severally introduced into g-C3N4/MS2 composites. Charge density difference suggested that O and S doping led to increased interfacial electron transfer, while P doping had minimal influence. As expected, the calculated field strength of O- and S-doped g-C3N4/MS2 composites was significantly larger than that of pristine and P-doped g-C3N4/MS2 composites. Therefore, O and S doping endowed g-C3N4/MS2 S-scheme heterojunctions with enhanced IEF and more thorough charge transfer. Correspondingly, the experimentally synthesized O-C3N4/SnS2 composite exhibited better photocatalytic H2-production activity than g-C3N4/SnS2 composite. This work proposed an original idea of employing proper nonmetal doping to magnify the advantage of S-scheme heterojunction in accelerating charge separation.

Fructose-regulated ZnO single-crystal nanosheets with oxygen vacancies for photodegradation of high concentration pollutants and photocatalytic hydrogen evolution

ZnO-based photocatalysts have been widely applied to photocatalytic reactions. However, the pure phase ZnO is rarely used in degradation of refractory pollutants and photocatalytic hydrogen evolution due to its poor photo-redox ability. In this study, we develop a novel surface atom regulation strategy using fructose molecules containing hydroxyl groups to improve photo-oxidation ability of ZnO. Interestingly, the fructose-regulated ZnO nanosheets with oxygen vacancies exhibit more positive surface valence band maximum than that of pristine ZnO. The photogenerated holes with strong oxidation capacity can be formed on the fructose-regulated ZnO nanosheets. The fructose-regulated ZnO nanosheets can rapidly degrade high concentration of phenol and other organic pollutants, which is difficultly achieved by the pristine ZnO nanosheets. In addition, photocatalytic H2 production activity is also found on our pure phase ZnO. This work provides a new design strategy for modifying the surface structure of photocatalysts to improve photocatalytic performance.

In‐situ synthesis of defects modified mesoporous g‐C 3 N 4 with enhanced photocatalytic H 2 evolution performance

Organic polymer g-C3N4 with photocatalytic ability has become a heated discussion. Because of terrible electronic structure and small surface area, the pristine g-C3N4 (CN) exhibited weak catalytic performance. Herein, a novel strategy was applied for preparing defect modified mesoporous g-C3N4 (DMCN). Compared with CN (0.26 mmol h−1 g−1), DMCN showed remarkable enhanced visible-light photocatalytic activity for H2 production (3.29 mmol h−1 g−1) under the same conditions (Pt co-catalyst). Through a great many of characterizations, it was found that excellent photocatalytic activity of DMCN is due to synergistic influence of mesoporous structure and defects (cyano groups and N-vacancies). On the one hand, the formation of mesoporous structure provides more catalytic sites by increasing specific surface area. On the other hand, the introduction of defects benefits to promote the separation efficiency of photoexcited charges and enlarge visible-light absorption range. Thus, this work detailedly reveals the effect of defects and mesopores on the physicochemical property of g-C3N4, which might provide referred role for the preparation of other photocatalysts with defects or porous structure.

Enhancement in the photocatalytic H2 production activity of CdS NRs by Ag2S and NiS dual cocatalysts

Cocatalysts play an indispensable role in photocatalytic H2 production via water splitting for the conversion of solar energy into storable chemical energy. Herein, the hybrid catalyst CdS/Ag2S/NiS is synthesized via hydrothermal and photodeposition methods. CdS/Ag2S/NiS shows a drastically elevated hydrogen production rate of 48.3 mmol g−1 h−1 under visible light due to the combined merits of the Schottky junction between CdS and metal-like Ag2S and the constructed p–n junction between CdS and NiS. Time-resolved photoluminescence spectroscopy and photochemical tests reveal the accelerated charge transfer and significantly reduced electron–hole pair recombination. Further investigation with in-situ surface photovoltage imaging technology demonstrates that the reduction cocatalyst Ag2S and oxidation cocatalyst NiS can serve as photogenerated electron and hole traps, respectively. This research not only provides insight into designing high-efficiency photocatalyst for hydrogen production but also utilize a brand new method for the confirmation of charge–carrier migration pathways.