Zijun Sun

Extraordinarily efficient photocatalytic hydrogen evolution in water using semiconductor nanorods integrated with crystalline Ni2P cocatalysts

Photocatalytic hydrogen evolution via water splitting is an attractive scientific and technological goal to address the increasing global demand for clean energy and to reduce the climate change impact of CO2 emission. Although tremendous efforts have been made, hydrogen production by a robust and highly efficient system driven by visible light still remains a significant challenge. Herein we report that nickel phosphide, as a cocatalyst to form a well-designed integrated photocatalyst with one-dimensional semiconductor nanorods, highly improves the efficiency and durability for photogeneration of hydrogen in water. The highest rate for hydrogen production reached ∼1200 μmol h−1 mg−1 based on the photocatalyst. The turnover number (TON) reached ∼3 270 000 in 90 hours with a turnover frequency (TOF) of 36 400 for Ni2P, and the apparent quantum yield was ∼41% at 450 nm. The photoinduced charge transfer process was further confirmed by steady-state photoluminescence spectra and time-resolved photoluminescence spectra. Such extraordinary performance of a noble-metal-free artificial photosynthetic hydrogen production system has, to our knowledge, not been reported to date.

Black Phosphorus Revisited: A Missing Metal-Free Elemental Photocatalyst for Visible Light Hydrogen Evolution.

Metal-free elemental photocatalysts for hydrogen (H2 ) evolution are more advantageous than the traditional metal-based inorganic photocatalysts since the nonmetal elements are generally cheaper, more earth-abundant, and environmentally friendly. Black phosphorus (BP) has been attracting increasing attention in recent years based on its anisotropic 2D layered structure with tunable bandgap in the range of 0.3-2.0 eV; however, the application of BP for photocatalytic H2 evolution has been scarcely reported experimentally although being theoretically predicted. Herein, for the first time, the visible light photocatalytic H2 evolution of BP nanosheets prepared via a facile solid-state mechanochemical method by ball-milling bulk BP is reported. Without using any noble metal cocatalyst, the visible light photocatalytic hydrogen evolution rate of BP nanosheets reaches 512 µmol h-1 g-1 , which is ≈18 times higher than that of the bulk BP, and is comparable or even higher than that of graphitic carbon nitrides (g-C3 N4 ).

A robust hydrogen evolution catalyst based on crystalline nickel phosphide nanoflakes on three-dimensional graphene/nickel foam: high performance for electrocatalytic hydrogen production from pH 0–14

The design and preparation of highly active catalysts for the hydrogen evolution reaction (HER) is very important for water splitting. Herein, we report a highly active HER catalyst, which is synthesized by loading nanostructured nickel phosphide (Ni2P) on three-dimensional few-layer graphene/nickel foam (G@NF). G@NF was successfully prepared by a chemical vapor deposition process in the presence of methane at high temperature. Compared with nickel phosphide, G@NF, as well as commercial platinum, the Ni2P–G@NF catalyst exhibited very high activity in electrocatalytic H2 production from water (∼7 mV overpotential in alkaline solutions, pH ∼ 14; and ∼30 mV overpotential in acidic solutions, pH ∼ 0). The high catalytic activity of Ni2P–G@NF is attributed to the excellent performance of Ni2P, the large 3D framework which facilitates proton accessibility and electron transfer, and the high surface area.

A cocatalyst-free CdS nanorod/ZnS nanoparticle composite for high-performance visible-light-driven hydrogen production from water

Highly efficient, visible-light-induced hydrogen (H2) production via water splitting can be achieved without the help of a cocatalyst by using a noble-metal-free core–shell photocatalyst, in which zinc sulfide (ZnS) nanoparticles as the protective shell are anchored on the surface of cadmium sulfide nanorods (CdS NRs). Due to the close interfacial contact of component semiconductors, the electronic structure of CdS is strongly coupled with that of ZnS nanoparticles, leading to efficient transfer of charge carriers between them and the improvement of the CdS photostability. The CdS/ZnS NR photocatalyst showed much higher catalytic activity for H2 production than CdS NRs and ZnS under visible light irradiation (λ > 420 nm), which is probably due to fast transfer of the photogenerated charge carriers and/or electron tunneling in the one-dimensional core–shell nanorod structure. Under optimal conditions, the highest hydrogen evolution rate reached 239 μmol h−1 mg−1, which is much greater than ZnS and CdS NRs and also among the best cocatalyst-free photocatalysts for H2 production. The average apparent quantum yield can be achieved as ∼16.8% after 8 h of irradiation (monochromatic light at 420 nm ± 5 nm). A possible mechanism for the photocatalytic reaction based on CdS/ZnS NRs is also discussed.

MoP is a novel, noble-metal-free cocatalyst for enhanced photocatalytic hydrogen production from water under visible light

The generation of hydrogen (H2) through photocatalytic water splitting by employing various cocatalysts has attracted much attention. Herein we report for the first time that metallic molybdenum phosphide (MoP), as a highly active cocatalyst, can significantly enhance photocatalytic H2 production from water. A series of MoP/CdS nanorod (NR) hybrids were facilely prepared. The optimal amount of MoP led to a maximal H2 evolution rate of 163.2 μmol h−1 mg−1 under visible light illumination (λ > 420 nm), which is more than 20 times higher than that of freshly prepared CdS NRs. This work demonstrated that the suitable Fermi level alignment of MoP and CdS is responsible for the high photocatalytic activity of H2 production under visible light in the present system, as evidenced by both experimental and theoretical results.

Copper phosphide modified cadmium sulfide nanorods as a novel p–n heterojunction for highly efficient visible-light-driven hydrogen production in water

Developing efficient photocatalysts made of earth-abundant elements for hydrogen (H2) production from water is considered to be a key pathway for future clean energy supply. Herein we report for the first time that p-type copper phosphide (Cu3P) can be an efficient promoter to improve photocatalytic H2 production from water when loaded on n-type cadmium sulphide nanorods (CdS NRs). The formation of a p–n junction in Cu3P/CdS NRs leads to fast charge transfer and enhanced photocatalytic activity under visible light irradiation. Under optimal conditions, the H2 evolution rate was as high as ∼200 μmol h−1 mg−1 (λ > 420 nm) and the apparent quantum yield at λ = 450 nm was ∼25% in water.

Direct growth of porous crystalline NiCo2O4 nanowire arrays on a conductive electrode for high-performance electrocatalytic water oxidation

Herein we report a facile and direct synthesis of porous NiCo2O4 nanowire arrays (NWAs) with robust mechanical adhesion to conductive electrodes by a simple two-step method. Upon complete pyrolysis of the cobalt–nickel-hydroxide precursor, high-quality crystalline NiCo2O4 is achieved. The porous NiCo2O4 nanowires were found to be highly active for catalytic water oxidation when serving as the working electrodes without any external materials (binder and/or carbon black), as evidenced by exhibiting higher catalytic current density for water oxidation compared to precious metal oxide catalysts such as iridium oxide (IrO2) under the same conditions and appreciable catalytic wave at ∼1.52 V (vs. RHE). The optimal performance of the as-synthesized NiCo2O4 nanowires showed a current density of 10 mA cm−2 under an overpotential of only 0.46 V and 20 mA cm−2 under an overpotential of 0.72 V, corresponding to a Faradaic efficiency of nearly 100%. The atomic-scale analysis of the NiCo2O4 nanowires was further conducted by spherical-aberration-corrected transmission electron microscopy (TEM). The highly exposed high-index facets and one-dimensional (1D) configuration of the as-synthesized porous NiCo2O4 nanowires may be responsible for the high catalytic performance of water oxidation, which exhibit excellent activity and unique advantages for catalytic water splitting.

Green Cobalt Oxide (CoOx) Film with Nanoribbon Structures Electrodeposited from the BF2-Annulated Cobaloxime Precursor for Efficient Water Oxidation

In this study, we report for the first time on the use of a water-soluble BF2-annulated cobaloxime, Co(dmgBF2)2(OH2)2 (Co-DMB, dmgBF2 = difluoroboryl-dimethylglyoxime), as a catalyst precursor for electrocatalytic water oxidation. Oxygen gas bubbles were clearly produced on the FTO electrode at a low overpotential under neutral pH conditions containing Co-DMB. Interestingly, stable green films were produced under these conditions. The current densities can reach to >5 mA/cm(2) at 1.1 V and >10 mA/cm(2) at 1.5 V (vs Ag/AgCl). The morphologies of the films showed nanoribbon structures, which were characterized by scanning electron microscope (SEM), energy-dispersive X-ray analysis (EDX), and X-ray photoelectron spectroscopy (XPS).

Core–shell amorphous cobalt phosphide/cadmium sulfide semiconductor nanorods for exceptional photocatalytic hydrogen production under visible light

Efficient hydrogen (H2) production is considered to be a key pathway for future clean energy supply. Herein we report that a photocatalyst made of core–shell amorphous cobalt phosphide (CoPx) integrated with cadmium sulfide nanorods (CdS NRs) gives exceptional performance of photocatalytic H2 production under visible light. Under optimal conditions, the CoPx/CdS NRs photocatalyst allows an H2 evolution rate of ∼500 μmol h−1 mg−1 based on the photocatalyst (λ > 420 nm) and the apparent quantum yield was ∼35% in aqueous solutions (λ = 450 nm). The turnover numbers (TONs) reached ∼630 000 per mole of cobalt in 70 hours with a TOF of ∼9000 h−1. Such high performance of an artificial photosynthetic H2 production system using a cobalt-based cocatalyst has, to the best of our knowledge, not been reported to date.

Enhanced photocatalytic H2 production on cadmium sulfide photocatalysts using nickel nitride as a novel cocatalyst

Photocatalytic hydrogen production using solar energy offers a clean and sustainable pathway for future energy supply. Herein, we report nickel nitride (Ni3N) as a novel cocatalyst on cadmium sulfide nanorod (CdS NR) semiconductors to enhance photocatalytic hydrogen production in water. The Ni3N cocatalyst was grown by a facile in situ growth method. Under optimal conditions, the hydrogen production rate can be improved more than 10 times by introducing an appropriate amount of Ni3N on CdS NRs. PL spectra and photoelectrochemical measurements suggest that faster interfacial charge transfer between Ni3N and CdS may be the key factor for the enhanced photocatalytic activity.