Visible-Light Driven Heterojunction Photocatalysts for Water Splitting – A Critical Review

Solar driven catalysis on semiconductors to produce clean chemical fuels, such as hydrogen, is widely considered as a promising route to mitigate environmental issues caused by the combustion of fossil fuels and to meet increasing worldwide demands for energy. The major limiting factors affecting the efficiency of solar fuel synthesis include; (i) light absorption, (ii) charge separation and transport and (iii) surface chemical reaction; therefore substantial efforts have been put into solving these problems. In particular, the loading of co-catalysts or secondary semiconductors that can act as either electron or hole acceptors for improved charge separation is a promising strategy, leading to the adaptation of a junction architecture. Research related to semiconductor junction photocatalysts has developed very rapidly and there are a few comprehensive reviews in which the strategy is discussed (A. Kudo and Y. Miseki, Chemical Society Reviews, 2009, 38, 253–278, K. Li, D. Martin, and J. Tang, Chinese Journal of Catalysis, 2011, 32, 879–890, R. Marschall, Advanced Functional Materials, 2014, 24, 2421–2440). This critical review seeks to give an overview of the concept of heterojunction construction and more importantly, the current state-of-the art for the efficient, visible-light driven junction water splitting photo(electro)catalysts reported over the past ten years. For water splitting, these include BiVO4, Fe2O3, Cu2O and C3N4, which have attracted increasing attention. Experimental observations of the proposed charge transfer mechanism across the semiconductor/semiconductor/metal junctions and the resultant activity enhancement are discussed. In parallel, recent successes in the theoretical modelling of semiconductor electronic structures at interfaces and how these explain the functionality of the junction structures is highlighted

Growth of Bismuth Oxide and Bismuth Ferrite Thin Films via CVD

This thesis describes the growth of bismuth oxide (Bi2O3) and multiferroic bismuth ferrite (BiFeO3) films via chemical vapour deposition (CVD). The synthesis of a range of bismuth(III) β-diketonate complexes was carried out via a ligand-exchange reaction between [Bi(N(SiMe3)2)3] and the respective free ligand, and crystal structures of [Bi(dbm)3]2 and [Bi(acac)3] are reported. The decomposition of these complexes was studied via DSC-TGA to assess their potential as single-source precursors to Bi2O3, and the mass transport characteristics of the volatile complexes [Bi(mmp)3], [Bi(thd)3] and [Bi(OtBu)3] were studied. Bi2O3 films were grown via the LPCVD reaction of the single-source precursor [Bi(OtBu)3]; the crystalline phase (and band-gap) of the resultant films depended strongly upon the reactor conditions. Films were tested for photo-oxidation of water under UV-light, revealing high activities comparable to those of TiO2 films described previously. [Bi(dbm)3]2 was utilised as a single-source precursor to β-Bi2O3 films via AACVD, together with the growth of Pt(0) films using H2PtCl6.6H2O as a precursor. Pt-nanoparticle Bi2O3 films were grown via a ‘one-pot’ AACVD reaction of both precursors; composite Pt-Bi2O3 films were able to evolve hydrogen via the photo-reduction of water, a property not observed for films containing either Pt or Bi2O3 alone. BiFeO3 films were grown via a multi-source LPCVD reaction between [Fe(acac)3], [Bi(OtBu)3] and air, as well as via the dual-source reaction of [Bi(OtBu)3] and [Fe(OtBu)3]2 without oxidising gas, and, furthermore, via the single-source precursor [{Cp(CO)2Fe}BiCl2] using AACVD. Magnetometry revealed low temperature ferromagnetism and spin-glass behaviour, characteristic of larger particle sizes. Ferroelectric measurements revealed low polarisation but nevertheless indicated films were multiferroic at room temperature. A selection of these films were tested for photo-oxidation of water under visible-light; films displayed high photoactivities with rates in excess of those from optimised TiO2 films measured under UV-light, highlighting the potential of BiFeO3 films as strong visible-light active photocatalysts

Gram-scale production of nitrogen doped graphene using a 1,3-dipolar organic precursor and their utilisation as stable, metal free oxygen evolution reaction catalysts

For the first time, a one-step scalable synthesis of a few-layer ∼10% nitrogen doped (N-doped) graphene nanosheets (GNSs) from a stable but highly reactive 1,3-dipolar organic precursor is reported. The utilization of these N-doped GNSs as metal-free electrocatalysts for the oxygen evolution reaction (OER) is also demonstrated. This process may open the path for the scalable production of other heteroatom doped GNSs by using the broad library of well-known, stable 1,3-dipolar organic compounds

MOCVD of crystalline Bi2O3 thin films using a single-source bismuth alkoxide precursor and their use in photodegradation of water

Bismuth(III) tert-butoxide [Bi((OBu)-Bu-t)(3)] was utilised as a single-source precursor to controllably deposit thin films of different phases of bismuth oxide (Bi2O3) on glass substrates via low-pressure chemical vapour deposition (LPCVD). Band gaps for the different phases have been measured (E-g = 2.3-3.0 eV) and the films displayed excellent photodegradation of water under near-UV irradiation

Photocatalytic Oxygen Evolution from Cobalt-Modified Nanocrystalline BiFeO3 Films Grown via Low-Pressure Chemical Vapor Deposition from beta-Diketonate Precursors

BiFeO3 is an interesting multifunctional narrow band gap semiconductor that exhibits simultaneous multiferroic, photovoltaic, and photocatalytic behavior. Hence there is much interest in the growth of thin films of BiFeO3 via chemical vapor deposition (CVD); however, the number of suitable bismuth precursors is severely limited. A series of homoleptic bismuth(III) β-diketonate complexes were synthesized via simple room temperature ligand-exchange reactions from [Bi(N(SiMe3)2)3] and free diketonate ligands, which yielded the crystal structure of [Bi(acac)3] as a 1-D polymer. We attempted to use these complexes for low pressure CVD (LPCVD) growth of BiFeO3 films with [Fe(acac)3]; however, all bismuth complexes exhibited poor volatilities and decomposition characteristics, and as a result film growth was unsuccessful. Subsequently, the volatile alkoxide [Bi(OtBu)3], with [Fe(acac)3], was used to grow dense BiFeO3 films via low pressure CVD. The BiFeO3 films possessed multiferroic properties at room temperature and exhibited activity for visible light-driven water oxidation in the presence of a Ag+ electron scavenger, which improved significantly when modified with a cobalt surface cocatalyst. The increase in activity, probed by time-resolved photoluminescence spectroscopy, was attributed to improved charge carrier separation arising from the in-built internal electric field of BiFeO3 in addition to the presence of an efficient cobalt oxygen evolution catalyst

Photocatalytic mineralisation of herbicide 2,4,5-trichlorophenoxyacetic acid: enhanced performance by triple junction Cu-TiO2-Cu2O and the underlying reaction mechanism

A mild and facile photodeposition method was used to fabricate novel Cu–TiO2–Cu2O composite photocatalysts. Due to the in situ rectifying charge carrier separation and enhanced conductivity, the composites present superior photocatalytic activity, leading to more than 90% mineralisation of the toxic 2,4,5-trichlorophenoxyacetic acid herbicide. This result was confirmed by both TOC and UV-vis absorption measurements. The effect of active radicals on the photodegradation of the herbicide was further investigated in order to clarify the underlying mechanism, based on which a hole-dominated photooxidation mechanism was proposed. These results not only offer a green and economical method for constructing triple junction photocatalyst materials, but also shed new insight on the rational design of a low cost and high-efficiency photocatalyst for environmental remediation

A novel route to Pt-Bi2O3 composite thin films and their application in photo-reduction of water

A novel homoleptic bismuth(III) β-diketonate (dibenzoylmethane – dbm) complex [Bi(dbm)3]2 has been used as a precursor to thin films of crystalline β-Bi2O3, and hexachloroplatinic acid (H2PtCl6·6H2O) has been demonstrated as a suitable precursor for deposition of platinum nanoparticles, both deposited via aerosol-assisted chemical vapour deposition (AACVD). Thin films of Pt–Bi2O3 were co-deposited from a mixture of [Bi(dbm)3]2 and H2PtCl6·6H2O; the introduction of Pt particles into β-Bi2O3 causes hydrogen to be evolved during photolysis of water over the composite material, a property not found for Pt particles or β-Bi2O3 alone

Visible Light-Driven Pure Water Splitting by a Nature-Inspired Organic Semiconductor-Based System

For the first time, it is demonstrated that the robust organic semiconductor g-C3N4 can be integrated into a nature-inspired water splitting system, analogous to PSII and PSI in natural photosynthesis. Two parallel systems have been developed for overall water splitting under visible light involving graphitic carbon nitride with two different metal oxides, BiVO4 and WO3. Consequently, both hydrogen and oxygen can be evolved in an ideal ratio of 2:1, and evolution rates in both systems have been found to be dependent on pH, redox mediator concentration, and mass ratio between the two photocatalysts, leading to a stable and reproducible H2 and O2 evolution rate at 36 and 18 μmol h–1 g–1 from water over 14 h. Our findings demonstrate g-C3N4 can serve as a multifunctional robust photocatalyst, which could also be used in other systems such as PEC cells or coupled solar cell systems

Defect-Free Single-Layer Graphene by 10 s Microwave Solid Exfoliation and Its Application for Catalytic Water Splitting

Mass production of defect-free single-layer graphene flakes (SLGFs) by a cost-effective approach is still very challenging. Here, we report such single-layer graphene flakes (SLGFs) (>90%) prepared by a nondestructive, energy-efficient, and easy up-scalable physical approach. These high-quality graphene flakes are attributed to a novel 10 s microwave-modulated solid-state approach, which not only fast exfoliates graphite in air but also self-heals the surface of graphite to remove the impurities. The fabricated high-quality graphene films (∼200 nm) exhibit a sheet resistance of ∼280 Ω/sq without any chemical or physical post-treatment. Furthermore, graphene-incorporated Ni-Fe electrodes represent a remarkable ∼140 mA/cm2 current for the catalytic water oxidation reaction compared with the pristine Ni-Fe electrode (∼10 mA/cm2) and a 120 mV cathodic shift in onset potential under identical experimental conditions, together with a faradic efficiency of >90% for an ideal ratio of H2 and O2 production from water. All these excellent performances are attributed to extremely high conductivity of the defect-free graphene flakes