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Herein, this work constructs a three-dimensional porous graphitic carbon nitride assembled by highly crystalline and ultrathin nanosheets (3D g-C3N4NS). 3D g-C3N4NS could directly split pure water into H2and O2with high evolution rate up to 101.4 and 49.1 μmol g−1 h−1under visible light, respectively, approximately 11.8 and 5.1 times higher than bulk g-C3N4and g-C3N4NS. Besides, it achieves a notable apparent quantum yield of 1.4% at 420 nm, significantly superior to previously reported Pt/g-C3N4. The efficient activity of 3D g-C3N4NS is mainly attributed to its 3D interconnected open-framework, assembled by highly crystalline ultrathin nanosheet unit, provides a pathway for faster charge carrier transport. Moreover, benefitting from its 3D structure for preventing agglomeration of nanosheets, 3D g-C3N4NS is stable for more than 100 h of overall water splitting reaction.

Metal-free graphitic carbon nitride (g-C3N4) has triggered huge interests for converting solar energy into fuels. However, direct-calcination derived bulk g-C3N4always suffers from low surface area and high recombination of charge carriers, prompting attempts to foster g-C3N4nano/microstructures to achieve high performance. Conventional routes, like templating method, always yields g-C3N4with tedious morphology and requires post-treatment. Here we release the first report on development of horn-like hollow mesoporous ultrathin (HHMU) g-C3N4tubes via first forming a horn-like Br-containing intermediate followed by further decomposition transformation under co-pyrolysis of melamine and substantial NH4Br. The multiple-superiorities achieved here (hollow/mesoporous/ultrathin/horn-like) allows g-C3N4high surface area, drastically boosted bulk charge separation, carrier density and surface charge transfer efficiency. This advanced g-C3N4thus casts outstanding photocatalytic performance for H2evolution with an apparent quantum efficiency (AQE) of 14.3% at 420 ± 15nm, far exceeding most of reported g-C3N4. HHMU g-C3N4also delivers a strengthened photocatalytic CO2reduction activity into CO and CH4. Selective photo-deposition results provide an in-depth insight into charge movement behavior and high photo-reactivity that the photo-generated electrons migrate to the outer shell and holes prefer to transfer onto the inner shell of HHMU g-C3N4tubes, thus achieving efficient spatial anisotropic charge separation. The current study may furnish a reference towards developing efficient tactics for integrally advancing g-C3N4for renewable energy generation, and disclose a new perspective into promoting charge separation via microstructure design.

Graphitic carbon nitride (g-C3N4) has attracted enormous research attention as a promising low cost, visible-light driven semiconductor photocatalyst. However, low photoabsorption efficiencies and unsatisfactory charge separation limit the potential of g-C3N4in many applications, motivating attempts to manipulate the structure and electronic properties of g-C3N4to achieve improved performance. Here we describe a novel precursor-reforming strategy that ultimately affords 3D mesoporous ultrathin g-C3N4with superior photocatalytic performance compared to conventional calcination-derived g-C3N4. We demonstrate that during hydrothermal treatment of melamine and urea, melamine undergoes an irreversible monoclinic to orthorhombic phase transformation, and the additive urea (excess typically 3-fold) serves as an additional N source and porogen. Calcination of the orthorhombic melamine yields mesoporous g-C3N4with enhanced photoabsorption properties and an outstanding photoactivity. A 23-fold increased hydrogen evolution rate of 3579μmolh−1g−1(λ > 420nm) was achieved with an apparent quantum efficiency (AQE) of 27.8% at 420 ± 15nm, a level of performance far beyond any AQE previously reported for ultrathin/porous/doped g-C3N4photocatalyst. Our work conclusively demonstrates a new synthetic strategy towards high performance g-C3N4-based photocatalytic materials for energy applications.

Photothermal catalytic reduction of CO2with H2as the reducing agent holds great promise for the conversion of combustion-derived CO2into hydrocarbon fuels and other valuable commodity chemicals. Whilst a number of promising photothermal catalysts have been developed for CO2reduction, reaction mechanisms remain elusive, and the factors controlling product selectivity are unclear. In this work, three distinct transition metal alloy nanoparticle catalysts, each supported by amorphous alumina, were fabricated by hydrogen reduction of CoFeAl-layered double hydroxide (CoFeAl-LDH), NiFeAl-LDH and NiCoAl-LDH nanosheet precursors at 650 °C. By using the different alloy nanoparticles (CoFe, NiFe or NiCo) and tuning the reaction parameters including temperature and gas flow rate during photothermal CO2reduction with H2, different product distributions (CH4, CO, light olefins, paraffins) were obtained, with the distribution depending strongly on the alloy catalyst composition. A reaction pathway for CO2reduction consisting of multiple steps was discovered and confirmed by in-situ diffuse reflectance infrared Fourier transform spectroscopy studies and a series of control catalytic experiments. Findings provide valuable new insights about the reaction mechanism of photothermal catalytic CO2reduction to hydrocarbon products over metal catalysts and the factors regulating product selectivity.

Intrinsic oxygen vacancies at CeO2surface are known to activate thermodynamically stable CO2molecules, enhancing the reaction rate and reducing reduction energy. However, charge recombination at the ceria-based cathode surface suppresses the multi-electron transfer process required for a complete reduction of CO2molecules to generate useful hydrocarbons. To suppress this charge recombination and facilitate the multi-electron transfer process, p-type NiO and reduced graphene oxide (rGO) were hybridized with CeO2to form rGO-grafted NiO-CeO2photocatalyst, which can convert CO2to formaldehyde at a rate of 421.09 μmol g−1h−1; about 4 times higher than that of pristine CeO2. Formation of photo-induced oxygen vacancy of CeO2photocatalyst resulted in a change of Ce-O bond length at ceria surface were monitored in-situby X-ray absorption near edge structure (XANES), and X-ray absorption fine structure (EXAFS) spectroscopy. Tracking the formation of CO2anion radical (CO2•-) and its subsequent protonation with in-situelectron paramagnetic resonance spectroscopy and attenuated total reflection-infrared (ATR-IR) spectroscopy, mechanism and reaction pathway of CO2reduction into formaldehyde formation have been elucidated.