Wen Qi Fang

Rational screening low-cost counter electrodes for dye-sensitized solar cells

Dye-sensitized solar cells have attracted intense research attention owing to their ease of fabrication, cost-effectiveness and high efficiency in converting solar energy. Noble platinum is generally used as catalytic counter electrode for redox mediators in electrolyte solution. Unfortunately, platinum is expensive and non-sustainable for long-term applications. Therefore, researchers are facing with the challenge of developing low-cost and earth-abundant alternatives. So far, rational screening of non-platinum counter electrodes has been hamstrung by the lack of understanding about the electrocatalytic process of redox mediators on various counter electrodes. Here, using first-principle quantum chemical calculations, we studied the electrocatalytic process of redox mediators and predicted electrocatalytic activity of potential semiconductor counter electrodes. On the basis of theoretical predictions, we successfully used rust (α-Fe2O3) as a new counter electrode catalyst, which demonstrates promising electrocatalytic activity towards triiodide reduction at a rate comparable to platinum.

Yolk@shell anatase TiO2 hierarchical microspheres with exposed {001} facets for high-performance dye sensitized solar cells

We report a facile, template-free and nontoxic one-pot solvothermal route for synthesizing submicrometer-sized yolk@shell hierarchical spheres, which possess a permeable shell self-assembled by ultrathin anatase TiO2 nanosheets (NSs) with nearly 90% of exposed {001} facets and mesoporous inner sphere with a high specific surface area. Compared to the {001} faceted TiO2 NSs and standard Degussa P25, the anatase TiO2 yolk@shell hierarchical spheres (TiO2 YSHSs) were obtained with surface area up to 245.1 m2 g−1 and their submicrometer scale simultaneously promoted light scattering in the visible region. A light to electricity conversion efficiency (η) of 6.01% was achieved for the DSSCs with TiO2 YSHSs as its photoanode, under 100 mW cm−2 illumination, indicating 49.9% and 34.8% increases compared to the DSSCs with TiO2 NSs (4.01%) and the standard Degussa P25 (4.46%) as photoanodes, respectively. The enhancement can be mainly attributed to the higher dye loading on TiO2 YSHSs (4.35 × 10−5 mol cm−2) than that of TiO2 NSs (3.14 × 10−5 mol cm−2) and P25 (3.32 × 10−5 mol cm−2); longer lifetime of the injected electrons in TiO2 YSHSs film (65.79 ms) than that of in TiO2 NSs film (57.90 ms); and the good capability of light scattering of TiO2 YSHSs in visible light region, which are confirmed by UV-vis spectrophotometer and electrochemical impedance spectroscopy (EIS). The growth mechanism of the TiO2 YSHSs has also been investigated in detail.

Fluorine‐Doped Porous Single‐Crystal Rutile TiO2 Nanorods for Enhancing Photoelectrochemical Water Splitting

Fluorine-doped hierarchical porous single-crystal rutile TiO(2) nanorods have been synthesized through a silica template method, in which F(-) ions acts as both n-type dopants and capping agents to make the isotropic growth of the nanorods. The combination of high crystallinity, abundant surface reactive sites, large porosity, and improved electronic conductivity leads to an excellent photoelectrochemical activity. The photoanode made of F-doped porous single crystals displays a remarkably enhanced solar-to-hydrogen conversion efficiency (≈0.35 % at -0.33 V vs. Ag/AgCl) under 100 mW cm(-2) of AM=1.5 solar simulator illumination that is ten times of the pristine solid TiO(2) single crystals.

Engineered Hematite Mesoporous Single Crystals Drive Drastic Enhancement in Solar Water Splitting

Mesoporous single crystals (MSCs) rendering highly accessible surface area and long-range electron conductivity are extremely significant in many fields, including catalyst, solar fuel, and electrical energy storage technologies. Hematite semiconductor, whose performance has been crucially limited by its pristine poor charge separation efficiency in solar water splitting, should benefit from this strategy. Despite successful synthesis of many metal oxide MSCs, the fabrication of hematite MSCs remains to be a great challenge due to its quite slow hydrolysis rate in water. Herein, for the first time, we have developed a synthetic strategy to prepare hematite MSCs and systematically investigated their growth mechanism. The electrode fabricated with these crystals is able to achieve a photocurrent density of 0.61 mA/cm(2) at 1.23 V vs RHE under AM 1.5G simulated sunlight, which is 20 times higher than that of electrodes made of solid single crystals. The enhancement is ascribed to the superior light absorption and enhanced charges separation. Our results demonstrate the advantage of incorporation of nanopores into the large-sized hematite single crystals and provide a valuable insight for the development of high performance photoelectrodes in PEC application.

Manipulating solar absorption and electron transport properties of rutile TiO2 photocatalysts via highly n-type F-doping

In this work, we report a facile and nontoxic one-pot hydrothermal method for synthesizing F-doped rutile single crystalline TiO2 with tuneable solar absorption. The optical band gap of the catalyst can be easily manipulated from 3.05 to 2.58 eV via altering the initial F : Ti molar ratio of reaction precursors. The photoanodes made of rutile TiO2 single crystals with appropriate F-doping concentration show excellent photoelectrocatalytic activity towards water oxidation under ultraviolet and visible light illumination. The best photoelectrocatalytic performance under UV irradiation can be obtained by F-doped TiO2 with an initial F : Ti molar ratio of 0.1, which is almost 15 times higher than that of un-doped TiO2. Further, the F-doped TiO2 photoanodes also exhibit superior photoelectrocatalytic activity under visible irradiation, and the best performance can be achieved by F-doped TiO2 photoanode with an initial F : Ti molar ratio of 0.05. The superior photoelectrocatalytic activity could be attributed to the highly n-type dopant introduced by fluorine, which significantly tunes the electrical conductivities and band structures of the resulting TiO2 photoanodes, and thus the photoelectrocatalytic activities under both UV and visible irradiation. Different techniques have been employed to characterize the electrical conductivity, charge carrier density and band structures of the F-doped rutile TiO2 films, such as photoelectrochemical method, electrical impedance spectroscopy (EIS) measurements, Mott–Schottky plots and XPS valence band spectra.

Highly efficient overlayer derived from peroxotitanium for dye-sensitized solar cells

Low-cost and high-efficiency dye-sensitized solar cells (DSSCs) have attracted intense research attention recently, especially the key component of sensitized titanium dioxide (TiO2) photoanode. Owing to the short electron diffusion length of sintered TiO2 nanoparticles and the charge recombination near the FTO substrate, crucial structures of overlayers on the FTO substrate and the whole TiO2 film have been applied. However, the generally accepted TiCl4 overlayer cannot work effectively due to its loose structure and the presence of chloride ions. Therefore, researchers are still faced with the challenge of developing new efficient overlayers on TiO2 photoanodes. In this study, by employing super-pure peroxotitanium solution (PTS), we have succeeded in significantly improving DSSC performance with simple dip coating. The uniform and compact structure of PTS overlayers can be ascribed to a polymerization mechanism. Overlayers derived from TiCl4, PTS and other materials such as Nb2O5, Al2O3, SiO2 or ZrO2 are probed. Among these coatings, an energy conversion efficiency as high as 7.33% has been achieved by applying PTS overlayers on the FTO substrate and the whole TiO2 film.

Operando NMR spectroscopic analysis of proton transfer in heterogeneous photocatalytic reactions

Proton transfer (PT) processes in solid–liquid phases play central roles throughout chemistry, biology and materials science. Identification of PT routes deep into the realistic catalytic process is experimentally challenging, thus leaving a gap in our understanding. Here we demonstrate an approach using operando nuclear magnetic resonance (NMR) spectroscopy that allows to quantitatively describe the complex species dynamics of generated H2/HD gases and liquid intermediates in pmol resolution during photocatalytic hydrogen evolution reaction (HER). In this system, the effective protons for HER are mainly from H2O, and CH3OH evidently serves as an outstanding sacrificial agent reacting with holes, further supported by our density functional theory calculations. This results rule out controversy about the complicated proton sources for HER. The operando NMR method provides a direct molecular-level insight with the methodology offering exciting possibilities for the quantitative studies of mechanisms of proton-involved catalytic reactions in solid–liquid phases.

Structure disorder of graphitic carbon nitride induced by liquid-assisted grinding for enhanced photocatalytic conversion

Graphitic-C3N4 with a disordered structure was processed for the first time by a liquid-assisted planetary ball milling approach. Through this simple and effective mechanochemistry method, the milled samples displayed outstanding visible-light photoactivity and the optimized one showed 7-fold higher H2 evolution rate than the bulk one.

Switching the photocatalytic activity of g-C3N4 by homogenous surface chemical modification with nitrogen residues and vacancies

A facile two-step homogenous approach is established to produce and control the nitrogen vacancies on g-C3N4 photocatalysts. The g-C3N4 undergoes a solvothermal N2H4·H2O reduction inactivation and subsequent thermal reduction process to reactivate and achieve an enhanced photocatalytic activity toward hydrogen evolution.

Brønsted base site engineering of graphitic carbon nitride for enhanced photocatalytic activity

Graphitic carbon nitride (g-C3N4) is a promising two-dimensional polymeric photocatalyst in the field of solar energy conversion. In the past few years many modifications of g-C3N4 have been studied extensively; however, the difficulty in obtaining detailed structural information both on its intrinsic covalent interactions and surrounding bonding environments largely restricts the rational design and development of inherent structure-controlled g-C3N4 based photocatalysts and fundamental understanding of their mechanistic operations. Herein, we demonstrate a high-pressure hydrogenation treatment method for g-C3N4 and introduce 1D 13C and 15N and 2D 15N Radio Frequency-driven Dipolar Recoupling (RFDR) solid-state nuclear magnetic resonance spectroscopy for identifying the structural information and surrounding hydrogen-bonding environment of treated g-C3N4 samples. The surface Bronsted base sites of g-C3N4 samples can be tuned systematically through changing the treatment conditions. We find that the terminal isolated –NH2 and the hydrogenated nitrogen species in treated g-C3N4 samples seem to be the origin of their improved activities for photocatalytic hydrogen evolution and favor the enhancement of light harvesting and carrier transport. The as-prepared HCN400-4-2 sample treated at a pressure of 4 MPa and a temperature of 400 °C for 2 h in a hydrogen atmosphere displays the highest H2 evolution reaction (HER) activity, which is over 26 times higher than that of pristine g-C3N4.