Photocatalysis efficiency is intrinsically determined by three aspects of photocatalysts, namely light absorption, separation and transfer of photogenerated charge carriers, and surface catalysis of materials (_{photocatalysis} = _absorption × _{separation/transfer} × _catalysis). Deep understanding and smart controlling each step plays a pivotal role in providing scientific basis and technologic support for designing and constructing high-efficiency photocatalysts. Systematic investigations on scientific and technologic challenges associated with key steps were conducted. A series of progress in wide light spectrum absorption, promoting separation and transfer of photogenerated charge carriers, and surface catalysis has been achieved.  

Achieving wide-spectrum absorption of solar light

The aim is to increase visible light absorption of wide-bandgap semiconductors by introducing heteroatoms or defects so that solar light can be fully harvested. Our major concern is how to realize the band-to-band redshift of light absorption edge by controlling the spatial distribution of dopants. Meanwhile, unknown visible light responsive photocatalysts consisting of earth-abundant elements will be explored. 

　　Figure 1  Homogeneous N doping in Cs_0.68Ti_1.83O₄. The left panel: UV-visible absorption spectra of (1) homogeneous N doped Cs_0.68Ti_1.83O₄ and (2) surface N doped TiO₂. The right panel: optical photograph of Cs_0.68Ti_1.83O₄ samples before and after homogeneous N doping. (Band-to-band visible-light photon excitation and photoactivity induced by homogeneous nitrogen doping in layered titanates, Chem Mater 2009, 21, 1266-1274)

　　Figure 2  Homogeneous S doping in g-C₃N₄. The left panel: schematic of two lattice N sites for substitutional S in perfect graphitic carbon nitride. The right panel: a typical time course of hydrogen evolution from water containing 10 vol% triethanolamine scavenger by Pt-deposited g-C₃N₄ (a) and g-C₃N_4-xS_x (b) under λ > 300 and 420 nm. (Unique electronic structure induced high photoreactivity of sulfur-doped graphitic C₃N₄, J Am Chem Soc 2010, 132, 11642-11648)

　　Figure 3  A red anatase TiO₂ with a gradient B/N doping. The left panel: optical photograph of the prepared red TiO₂ sample. The right panel: UV-visible absorption of the white TiO₂ and red TiO₂. (A red anatase TiO₂ photocatalyst for solar energy conversion, Energy Environ Sci 2012, 5, 9603-9610)

　　Figure 4  Homogeneous modification with nitrogen vacancies in g-C₃N₄. The left panel: schematic of the two dimensional sheets of pristine g-C₃N₄ (melon). The right panel: UV-visible absorption spectra of g-C₃N₄ and g-C₃N_4-x (obtained by reducing g-C₃N₄ in a hydrogen atmosphere). (Increasing the visible light absorption of graphitic carbon nitride (melon) photocatalysts by homogeneous self-modification with nitrogen vacancies, Adv Mater 2014, 26, 8046)

Figure 5 Homogeneous amorphization of g-C₃N₄. The left panel: schematic of the two dimensional sheets of disordered pristine g-C₃N₄. The right panel: UV-visible absorption spectra of g-C₃N₄ and amorphous C₃N₄ (obtained by heating g-C₃N₄ in an argon atmosphere). (An amorphous carbon nitride photocatalyst with greatly extended visible-light-responsive range for photocatalytic hydrogen generation, Adv. Mater., 2015, 27, 4572)

Figure 6 α-S photocatalyst. The left panel: UV-visible absorption spectrum of α-sulfur. The inset is a photograph of the α-S crystal powder. The right panel: Applied potential bias dependence of the photocurrent generated by the photoanode of α-S crystals under UV-visible and visible light irradiation. (α-sulfur crystals as a visible light active photocatalyst, J Am Chem Soc 2012, 134, 9070-9073)

Figure 7 β-boron photocatalyst. The left panel: schematic of atomic structure of β-boron. The right panel: UV-visible absorption spectra of boron powder with and without surface amorphous layer (Visible-light-responsive β-rhombohedral boron photocatalysts, Angew Chem Int Ed 2013, 52, 6242-6245)

Promoted separation and transfer of photogenerated charge carriers

To promote the separation and trasnfer of photogenerated electrons and holes in semiconductor based photocatalysts, two strategies are used. One is shortening the diffusion length of the charge carriers from the bulk to surface by decreasing the size along one or two dimensions of photocatalysts to nanometers so that the recombination of photoexcited charge carriers will be greatly lowered. The other is to enable spatial distribution of photogenerated electrons and holes between two components or regions of one component in heterostructured photocatalysts.

　　Figure 8  g-C₃N₄ nanosheets. SEM images of bulk g-C₃N₄ and g-C₃N₄ nanosheets (Graphene-like carbon nitride nanosheets for improved photocatalytic activities, Adv Funct Mater 2012, 22, 4763-4770)

　　Figure 9  Photoanode of Ta₃N₅ nanorod arrays. SEM image of Ta₃N₅ nanorod arrays supported on Ta substrate and photoelectrochemical water oxidation activity of Co(OH)x modified Ta₃N₅ (Template-free synthesis of Ta₃N₅ nanorod arrays for efficient photoelectrochemical water splitting, Chem Commun 2013, 49, 3019-3021)

Figure 10 TEM images of (a) pristine g-C3N4 and (b) porous g-C3N4 photocatalysts after loading Au particles (black particles) via a photodeposition method. The spatial distribution of Au particles on photocatalysts shows the abundance of reductive sites. Scale bars are 50 nm. (Selective breaking of hydrogen bonds of layered carbon nitride towards greatly enhanced visible light photocatalysis, Adv. Mater., 2016, 28, 6471–6477)

　　Figure 11  CdS/ZnS core-shell particles. The left panel: schematic of CdS-mesoporous ZnS core-shell particles with the separation of charge carriers. The middle and right panels: photocatalytic hydrogen generation with ZnS, CdS, and the core-shell particles from the aqueous solution of Na₂S/Na₂SO₃ under visible light. (CdS-mesoporous ZnS core-shell particles for efficient and stable photocatalytic hydrogen evolution under visible light, Energy Environ Sci 2014, 7, 1895–1901)

Figure 13 Comparison of photocatalytic hydrogen generation from mixture of water/methanol with pristine rutile TiO₂ and Ti³⁺/Ti⁴⁺ core/shell rutile TiO₂ particles after loading 1 wt% Pt co-catalyst. (Enhanced photocatalytic H2 production in core-shell engineered rutile TiO₂, Adv. Mate., 2016, 28, 5850-5856)

Faceting mediated catalysis

The aim is to modulate surface atomic structures of photocatalysts by controlling the selective exposure of different crystal facets during the growth of crystals. These faceted photocatalysts provide an excellent platform to reveal surface structure dependent photocatalytic activity.

Figure 14  N doped anatase TiO₂ crystal with dominant {001} facets. UV-visible absorption spectrum of nitrogen doped anatase TiO₂ crystals with dominant {001}. The insets are optical photograph and SEM image of nitrogen doped anatase TiO₂ crystals with dominant {001}. (Visible light responsive nitrogen doped anatase TiO₂ sheets with dominant {001} facets derived from TiN, J Am Chem Soc 2009, 131, 12868-12869)

Figure 15  Anatase TiO₂ crystals with a predominance of low index facets. Schematic (A) and SEM images (B-D) of anatase TiO₂ single crystals with different percentages of {001}, {101}, and {010} facets. (On the true photoreactivity order of {001}, {010} and {101} facets of anatase TiO₂ crystals, Angew Chem Int Ed 2011, 50, 2133-2137)

Figure 16  {001} dominated Anatase TiO₂ microspheres with tunable spatial distribution of boron. The left panel: SEM images of anatase TiO₂ microsphere with nearly 100% {001} surface. The right panel: schematic of boron distribution in the microsphere before and after heating. (Heteroatom-modulated switching of photocatalytic hydrogen and oxygen evolution preferences of anatase TiO₂ microspheres, Adv Funct Mater 2012, 22, 3233–3238)

Figure 17 Crystal facet dependent interfacial electric conductivity in faceted anatase TiO₂ crystal. I-V curves along different crystallography orientations were measured by contacting one TiO₂ particle with two tungsten probes in SEM microscope. (Greatly enhanced electronic conduction and lithium storage of faceted TiO₂ crystals supported on metallic substrates by tuning crystallographic orientation of TiO₂, Adv. Mater., 2015, 27 3507–3512)

　　Figure 18  Ferroelectric field assisted selective deposition of co-catalysts on different sides of facet. (a) Schematic of single-domain & single crystalline ferroelectric material with in-built electric field; (b) SEM image of PbTiO₃ nanoplates with dominant {001} facets; (c) SEM image of PbTiO₃ nanoplates with a selective depositionof Au and MnOx on different sides; (d) Comparison of photocatalytic hydrogen generation between the PbTiO₃ with the selective deposition of reducing co-catalyst Pt and the PbTiO₃ with the nonselective deposition of reducing co-catalyst Pt. (Selective Deposition of Redox Co-catalysts to Improve the Photocatalytic Activity of Single-Domain Ferroelectric PbTiO₃ Nanoplates, Chemical Communications 2014, 50, 10416 -10419)