Development of new layer-structured materials as photocatalysts for water splitting

Abstract

To solve many environment and energy related issues, photosynthesis has been widely investigated for the past several decades as it promises to be a potential reliable method to convert the nearly unlimited solar energy to chemical energy. In particular, photocatalytic water splitting has drawn great interest, which can decompose water molecules to hydrogen and oxygen in the presence of semiconductor photocatalysts under light irradiation. Among many semiconductors investigated for photocatalysis, layered semiconductors form a unique group of photocatalysts due to their special properties such as improved charge separation and easily tuned composition. Moreover, some layered semiconductors can be exfoliated to ultrathin nanosheets (NSs), which not only exhibit distinct properties compared to the bulk materials, but also can be used to fabricate novel nanostructures. This dissertation is devoted to develop nanocomposites based on layered semiconductors for efficient photocatalytic water splitting. A series of heterogeneous photocatalysts were constructed from layered semiconductors such as graphitic carbon nitride (g-C3N4) and exfoliated semiconductors including titanium oxide (Ti0.91O2) NSs and g-C3N4 NSs. By coupling Ti0.91O2 NSs and octahedral gold nanoparticles (NPs), a nanohybrid was formed, in which Au NPs were embedded into the matrix of titanium oxide. The surface plasmon resonance (SPR) effect of Au NPs introduced a broad absorption peak in the visible region to the nanohybrid. The nanohybrid showed considerably improved photocatalytic hydrogen production performance compared to naked titanium oxide and conventional titanium oxide with photo-deposited Au due to the local light intensity enhancement and photon scattering effect. A nanocomposite photocatalyst composed of Ti0.91O2 NSs and CdTe@CdS nanocrystals (NCs) was successfully synthesized via a facile restacking approach. In the nanocomposite, the Ti0.91O2 NSs randomly stacked upon each other while the CdTe@CdS NCs were sandwiched between Ti0.91O2 NSs. The introduction of CdTe@CdS NCs not only extended the visible light absorption, but also formed p-n nanojunctions in the nanocomposite. The photocatalytic hydrogen evolution under visible light irradiation was significantly enhanced compared to pure CdTe@CdS NCs due to the effective charge separation, which was achieved by the synergetic effect of proper energy band alignment and p-n nanojunctions. A new type of polymeric composite photocatalyst was prepared by co-loading g-C3N4 with poly(3,4-ethylenedioxythiophene) (PEDOT) as hole transport pathway and Pt as electron trap. As a result of the spatial separation of reduction and oxidation reaction sites on g-C3N4 surface, the photocatalytic water splitting activity of the composite was dramatically increased compared to those of C3N4-PEDOT and C3N4-Pt. Phosphorus-doped g-C3N4 NSs were synthesized by doping bulk g-C3N4 and subsequent exfoliation in water. The obtained 1 nm-thick P-doped g-C3N4 NSs were highly stable in water suspension and carried negative surface charge, while pure g-C3N4 did not undergo effective exfoliation. The NSs suspension was able to produce hydrogen under visible light irradiation, in contrast to the visible-light-inactive undoped counterpart. More importantly, the P-doped g-C3N4 NSs were demonstrated to be used to construct layer-by-layer (LBL) assembly onto different substrates. The photoelectrode with LBL assembly on FTO glass substrate showed visible light response.