Ping Niu

Unique Electronic Structure Induced High Photoreactivity of Sulfur-Doped Graphitic C3N4

Electronic structure intrinsically controls the light absorbance, redox potential, charge-carrier mobility, and consequently, photoreactivity of semiconductor photocatalysts. The conventional approach of modifying the electronic structure of a semiconductor photocatalyst for a wider absorption range by anion doping operates at the cost of reduced redox potentials and/or charge-carrier mobility, so that its photoreactivity is usually limited and some important reactions may not occur at all. Here, we report sulfur-doped graphitic C(3)N(4) (C(3)N(4-x)S(x)) with a unique electronic structure that displays an increased valence bandwidth in combination with an elevated conduction band minimum and a slightly reduced absorbance. The C(3)N(4-x)S(x) shows a photoreactivity of H(2) evolution 7.2 and 8.0 times higher than C(3)N(4) under lambda > 300 and 420 nm, respectively. More strikingly, the complete oxidation process of phenol under lambda > 400 nm can occur for sulfur-doped C(3)N(4), which is impossible for C(3)N(4) even under lambda > 300 nm. The homogeneous substitution of sulfur for lattice nitrogen and a concomitant quantum confinement effect are identified as the cause of this unique electronic structure and, consequently, the excellent photoreactivity of C(3)N(4-x)S(x). The results acquired may shed light on general doping strategies for designing potentially efficient photocatalysts.

α-Sulfur Crystals as a Visible-Light-Active Photocatalyst

We show that in contrast to conventional compound photocatalysts, α-sulfur crystals of cyclooctasulfur (S(8)) are a visible-light-active elemental photocatalyst. The α-S crystals were found to have the ability not only to generate ·OH radicals but also to split water in a photoelectrochemical process under both UV-vis and visible-light irradiation. Although the absolute activity obtained was low because of the large particle size and poor hydrophilicity of the α-S crystals studied, there is great potential for increasing the activity with the assistance of known strategies such as surface modification, nanoscaling, doping, and coupling with other photocatalysts.

Increasing the Visible Light Absorption of Graphitic Carbon Nitride (Melon) Photocatalysts by Homogeneous Self-Modification with Nitrogen Vacancies

A novel reduced melon photocatalyst with a bandgap of 2.03 eV developed here has a widened visible light absorption range and suppressed radiative recombination of photo-excited charge carriers due to the homogeneous self-modification with nitrogen vacancies. As a consequence, the reduced melon shows a much superior photocatalytic activity compared to the pristine melon in generating •OH radicals and degrading the organic pollutant Rhodamine B.

A red anatase TiO2 photocatalyst for solar energy conversion

Narrowing the bandgap of wide-bandgap semiconductor photocatalysts (for instance, anatase TiO2) by introducing suitable heteroatoms has been actively pursued for increasing solar absorption, but usually suffers from a limited thermodynamic/kinetic solubility of substitutional dopants in bulk and/or dopant-induced recombination centres. Here we report a red anatase TiO2 microsphere with a bandgap gradient varying from 1.94 eV on its surface to 3.22 eV in its core by a conceptually different doping approach for harvesting the full spectrum of visible light. This approach uses a pre-doped interstitial boron gradient to weaken nearby Ti-O bonds for the easy substitution of oxygen by nitrogen, and consequently it substantially improves the nitrogen solubility. Furthermore, no nitrogen-related Ti3+ was formed in the red TiO2 due to a charge compensation effect by boron, which inevitably occurs in common nitrogen doped TiO2. The red anatase TiO2 exhibits photoelectrochemical water splitting activity under visible light irradiation. The results obtained may shed light on how to increase high visible light absorbance of wide-bandgap photocatalysts.

Visible‐Light‐Active Elemental Photocatalysts

Seeking visible-light-active photocatalysts for efficient solar-energy conversion has become an intensifying endeavor worldwide. In this concept paper, general requirements for finding new visible-light-active photocatalysts are briefly introduced, and recent progress in exploring elemental photocatalysts for clean-energy generation and environmental remediation are reviewed. Finally, opportunities and challenges facing elemental photocatalysts are discussed.

Visible-Light-Responsive beta-Rhombohedral Boron Photocatalysts

Photocatalytic solar-energy conversion has been attracting worldwide attention owing to its great significance in the provision of renewable energy and protection of the environment. As important as the tailoring of well-known photocatalysts, such as TiO2, for high photocatalytic efficiency [4–7] is the investigation of unknown semiconductor photocatalysts. So far, hundreds of photocatalysts have been examined, most of which have been compounds. Recently, elemental semiconductors (Si, Se, P, S) have emerged as an attractive class of photocatalysts owing to their visible-light response and suitable band edges for targeted photocatalysis reactions. It is logical to also anticipate the use of elemental boron in photocatalysis because of its semiconducting properties. When we investigated b-rhombohedral boron crystals with and without an amorphous oxide layer on their surface, we discovered that the crystals were indeed photocatalytically active under visible light, and that the existence of a surface amorphous oxide layer substantially impaired their photocatalytic activity. The findings in this study may open a door to the development boron-based photocatalysts. Boron has aroused wide interest owing to its fascinating properties (light weight, high strength, high hardness, high melting point, high chemical resistance, typical semiconductivity, and superconductivity at high pressure), although a pure phase was not obtained until 1909. It has at least 17 polymorphs (or more precisely, boron-rich compounds) as a result of electron-deficient bonding. All polymorphs contain B12 icosahedral clusters as a basic building block (see the inset in Figure 1a). Among these polymorphs, a-tetragonal, a-rhombohedral, b-tetragonal, and b-rhombohedral boron are the four main forms under ambient conditions. Experimentally, b-rhombohedral boron is the most thermodynamically stable form, although its superior stability was not supported by theoretical investigations until 2007. This long-term discrepancy stems from the difference between the idealized structural model of 105 boron atoms (B105) that is used to describe the b-rhombohedral form and the real structure, which contains partially occupied sites. As a consequence, the theoretically predicted metallic property does not agree with the experimentally derived p-type semiconducting behavior of b-rhombohedral boron with a proposed bandgap of 1.5–1.6 eV. Improved crystallographic studies and optimized theoretical structure models have reduced the gap between experiment and theory to some extent and provided a strong background for the further investigation of structure–property relationships and the exploration of new uses for b-rhombohedral boron. The bandgap of 1.5–1.6 eV indicates that b-rhombohedral boron should respond to visible light over a wide range of wavelengths. It was also found in quasi-four-electrode measurements that b-rhombohedral boron exhibited strong photoconductivity under illumination by a halogen lamp or argonion laser (at 488 nm). Encouraged by these favorable properties, we investigated the photocatalytic activity of two kinds of commercially available b-rhombohedral boron crystals (sub-micrometer-sized and micrometer-sized) by structural characterization, measurement of their optical properties, and theoretical calculation of their electronic structures. Figure 1a shows the X-ray diffraction (XRD) pattern of sub-micrometer-sized boron powder. All diffraction peaks can be assigned to b-rhombohedral boron (JCPDS: 11-0618; space group: R 3m (166); a= 10.952 , c= 23.824 ). Besides the sharp peaks, there are several diffuse peaks in the background, which indicate the existence of amorphous Figure 1. Structure characterization: a) XRD pattern of sub-micrometer-sized b-rhombohedral boron. b) High-resolution TEM image recorded at the edge of a sub-micrometer-sized b-rhombohedral boron particle. The inset in (a) shows the structure of an icosahedral B12 cluster.

Achieving maximum photo-oxidation reactivity of Cs(0.68)Ti(1.83)O(4-x)N(x) photocatalysts through valence band fine-tuning

Does a wider absorption range of the photocatalyst always result in a higher photoreactivity? By investigating a set of Cs0.68Ti1.83O4−xNx (x = 0–0.31) photocatalysts with a continuously tuned absorption edge, we found that although the absorption edge of Cs0.68Ti1.83O4−xNx is gradually shifted to the low-energy region with the increase of the nitrogen dopant, the photoreactivity of the photocatalyst in terms of generating the ˙OH radical does not correspondingly increase under the irradiation of monochromatic light of 254 nm and continuous spectrum of >300 nm. The fundamental mechanism of the anion dopant enhanced photocatalytic activity through the interplay of three key factors—absorbance, oxidative potential and mobility of charge carriers—is proposed to explain the observed photocatalytic activity change.

An Unusual Strong Visible‐Light Absorption Band in Red Anatase TiO2 Photocatalyst Induced by Atomic Hydrogen‐Occupied Oxygen Vacancies

Increasing visible light absorption of classic wide-bandgap photocatalysts like TiO2 has long been pursued in order to promote solar energy conversion. Modulating the composition and/or stoichiometry of these photocatalysts is essential to narrow their bandgap for a strong visible-light absorption band. However, the bands obtained so far normally suffer from a low absorbance and/or narrow range. Herein, in contrast to the common tail-like absorption band in hydrogen-free oxygen-deficient TiO2 , an unusual strong absorption band spanning the full spectrum of visible light is achieved in anatase TiO2 by intentionally introducing atomic hydrogen-mediated oxygen vacancies. Combining experimental characterizations with theoretical calculations reveals the excitation of a new subvalence band associated with atomic hydrogen filled oxygen vacancies as the origin of such band, which subsequently leads to active photo-electrochemical water oxidation under visible light. These findings could provide a powerful way of tailoring wide-bandgap semiconductors to fully capture solar light.

Substitutional Carbon‐Modified Anatase TiO2 Decahedral Plates Directly Derived from Titanium Oxalate Crystals via Topotactic Transition

Changing the composition and/or structure of some metal oxides at the atomic level can significantly improve their performance in different applications. Although many strategies have been developed, the introduction of heteroatoms, particularly anions to the internal part of metal oxide particles, is still not adequate. Here, an effective strategy is demonstrated for directly preparing polycrystalline decahedral plates of substitutional carbon-doped anatase TiO2 from titanium (IV) oxalate by a thermally induced topotactic transition in an inert atmosphere. Because of the carbon concentration gradient introduced in side of the plates, the carbon-doped TiO2 (TiO2-x Cx ) shows an increased visible light absorption and a two orders of magnitude higher electrical conductivity than pure TiO2 . Consequently, it can be used as a photocatalyst and an active material for lithium storage and shows much superior activity in generating hydroxyl radicals under visible light and greatly increased electrical-specific capacity at high charge-discharge rates. The strategy developed could also be applicable to the atomic-scale modification of other metal oxides.