Penghui Guo

Surface Engineered Doping of Hematite Nanorod Arrays for Improved Photoelectrochemical Water Splitting

Given the narrow band gap enabling excellent optical absorption, increased charge carrier density and accelerated surface oxidation reaction kinetics become the key points for improved photoelectrochemical performances for water splitting over hematite (α-Fe2O3) photoanodes. In this study, a facile and inexpensive method was demonstrated to develop core/shell structured α-Fe2O3 nanorod arrays. A thin, Ag-doped overlayer of ~2–3 nm thickness was formed along α-Fe2O3 nanorods via ultrasonication treatment of solution-based β-FeOOH nanorods in Ag precursor solution followed by high temperature annealing. The obtained α-Fe2O3/AgxFe2−xO3 core/shell nanorod films demonstrated much higher photoelectrochemical performances as photoanodes than the pristine α-Fe2O3 nanorod film, especially in the visible light region; the incident photon-to-current efficiency (IPCE) at 400 nm was increased from 2.2% to 8.4% at 1.23 V vs. RHE (Reversible hydrogen electrode). Mott-Schottky analysis and X-ray absorption spectra revealed that the Ag-doped overlayer not only increased the carrier density in the near-surface region but also accelerated the surface oxidation reaction kinetics, synergistically contributing to the improved photoelectrochemical performances. These findings provide guidance for the design and optimization of nanostructured photoelectrodes for efficient solar water splitting.

Visible-light-driven photocatalytic water splitting on nanostructured semiconducting materials

In view of the unlimited resource of solar energy and the abundance of water on earth, producing hydrogen through photocatalytic splitting of water under solar irradiation has the great potential to offer a low cost, environmentally friendly green fuel that does not contribute to greenhouse gas emissions. Since the pioneering work of Fujisima and Honda in 1972, tremendous research on semiconductor photocatalysis has yielded better understanding of the processes involved in photocatalytic water splitting, as well as notable enhancement of energy conversion efficiency for solar hydrogen generation. However, the solar-to-hydrogen energy conversion efficiency is still too low to be viable for practical applications, primarily due to the limitation of electronic band structure of semiconductor photocatalysts and rapid recombination of photogenerated charges. Thus, ideal photocatalysts are the key for the realisation of high efficiency hydrogen generation system. Fortunately, various kinds of effective semiconductors have been developed as good candidates for photocatalytic hydrogen generation, and high-efficiency photocatalysis systems in lab scale have also been constructed. This paper provides an overview of the common approaches that have been used in the search for high efficiency photocatalysts (i.e., photocatalyst itself and co-catalyst) and matched reaction systems of sacrificial reagents for water splitting, especially under visible light irradiation. From this review, one can also observe that nanotechnology plays an important role in the design of novel nanostructured or heterogeneous photocatalysts for the establishment of high-efficiency photocatalytic solar hydrogen system. These may offer a general guide to those who are interested in tackling the challenges.

Spatial engineering of photo-active sites on g-C3N4 for efficient solar hydrogen generation

ZnFe2O4 modified g-C3N4 was successfully synthesized by a simple one-pot method. The visible-light-driven photocatalytic hydrogen production activity of g-C3N4 was significantly enhanced due to spatial engineering of the photo-active sites via ZnFe2O4 modification and Pt loading. It is proposed that ZnFe2O4 does not function as visible light sensitizer but as oxidation active sites. In the present ZnFe2O4/g-C3N4 photocatalysts, the photo-induced holes in g-C3N4 tend to transfer to ZnFe2O4 due to the straddling band structures (Type I band alignment), while the photo-induced electrons in g-C3N4 prefer to transfer to the loaded Pt cocatalysts, which can function as reduction active sites for hydrogen production. As a result, the photoinduced electrons and holes in g-C3N4 are efficiently separated by spatial engineering of the photo-active sites, and hence enhanced photocatalytic hydrogen generation activity is obtained.

Physical and photoelectrochemical characterization of Ti-doped hematite photoanodes prepared by solution growth

We present the fabrication and characterization of Ti-doped hematite (α-Fe2O3) films for application as photoanodes in photoelectrochemical (PEC) cells for water splitting. It is demonstrated that Ti doping significantly improves the PEC activity as the photocurrent at 1.0 V vs. Ag/AgCl electrode for a 400 nm thick Ti-doped film (0.66 mA cm−2) was found to be ∼14 times higher than that of an undoped film (0.045 mA cm−2). The films were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and ultrafast transient absorption spectroscopy to obtain information about their structural, electronic, and charge carrier dynamic properties. Based on characterization of the chemical states of the involved elements as well as the charge carrier dynamics of the films with Ti doping, it appears that the photocurrent enhancement is related to an increase in charge carrier density or reduced electron–hole recombination. The highest incident photon conversion efficiency (IPCE) measured for this system was 27.0% at 360 nm at a potential of 1.23 V vs. reversible hydrogen electrode (RHE), which was obtained on a 400 nm thick Ti-doped α-Fe2O3 film.

A novel Sn2Sb2O7 nanophotocatalyst for visible-light-driven H2 evolution

AbstractA novel pure cubic-phase pyrochlore structure tin(II) antimonate nanophotocatalyst, stoichiometric Sn2Sb2O7, has been prepared by a modified ion-exchange process using an antimonic acid precursor, and employed in visible-light-driven photocatalytic H2 evolution for the first time. The physicochemical properties (crystal phase, chemical composition and state, textural properties, and optical properties) of the material were investigated by different instrumental techniques. Compared with the antimonic acid precursor, the as-prepared Sn2Sb2O7 had a narrower bandgap, smaller crystal size, and larger BET surface area. The as-prepared Sn2Sb2O7 was validated as a promising candidate for visible-light-driven photocatalytic H2 evolution with a constant rate of 40.10 μmol·h−1·gcat−1.

Facile Growth of Porous Hematite Films for Photoelectrochemical Water Splitting

We introduced a simple fabrication method of porous hematite films with tunable thickness in an aqueous solution containing FeCl3 as the single precursor. We demonstrated that the optimized thickness was necessary for high performance photoelectrochemical water splitting, by balancing photon absorption and charge carrier transport. The highest photocurrent of ca. 0.15 mA cm−2 at 1.0 V versus Ag/AgCl was achieved on the 300 nm thick porous hematite film as photoanode, with IPCE at 370 nm and 0.65 V versus Ag/AgCl to be 9.0%. This simple method allows the facile fabrication of hematite films with porous nanostructure for enabling high photon harvesting efficiency and maximized interfacial charge transfer. These porous hematite films fabricated by this simple solution-based method could be easily modified by metal doping for further enhanced photoelectrochemical activity for water splitting.

Continuous solid solutions of Na0.5La0.5TiO3–LaCrO3 for photocatalytic H2 evolution under visible-light irradiation

A series of continuous (Na0.5La0.5TiO3)1−x(LaCrO3)x solid solutions with orthorhombic-phase perovskite structure and with LaCrO3 contents (x) in the range from 0 to 1.0 were synthesized by a facile polymerized complex method, and were employed as photocatalysts for H2 evolution under visible-light irradiation. It was found that photocatalytic activities of the solid solutions significantly increased with the increase of x to 0.3, and reached the highest H2-evolution rate of 238.2 μmol h−1 gcat−1 on (Na0.5La0.5TiO3)0.7(LaCrO3)0.3, because the narrowed bandgaps of samples enhanced the generation of charge carriers and the increased lattice distortion of samples could promote the separation and migration of charge carriers. Nevertheless, photocatalytic activities of the solid solutions gradually decreased with the further increase of x from 0.3, since both the bandgaps and lattice distortion rarely changed but Cr6+ and defects gradually increased and thus accelerated the recombination of charge carriers.

The effect of cation doping on the morphology, optical and structural properties of highly oriented wurtzite ZnO-nanorod arrays grown by a hydrothermal method.

Undoped and C-doped (C: Mg2+, Ni2+, Mn2+, Co2+, Cu2+, Cr3+) ZnO nanorods were synthesized by a hydrothermal method at temperatures as low as 60 °C. The effect of doping on the morphology of the ZnO nanorods was visualized by taking their cross section and top SEM images. The results show that the size of nanorods was increased in both height and diameter by cation doping. The crystallinity change of the ZnO nanorods due to each doping element was thoroughly investigated by an x-ray diffraction (XRD). The XRD patterns show that the wurtzite crystal structure of ZnO nanorods was maintained after cation addition. The optical Raman-active modes of undoped and cation-doped nanorods were measured with a micro-Raman setup at room temperature. The surface chemistry of samples was investigated by x-ray photoelectron spectroscopy and energy-dispersive x-ray spectroscopy. Finally, the effect of each cation dopant on band-gap shift of the ZnO nanorods was investigated by a photoluminescence setup at room temperature. Although the amount of dopants (Mg2+, Ni2+, and Co2+) was smaller than the amount of Mn2+, Cu2+, and Cr3+ in the nanorods, their effect on the band structure of the ZnO nanorods was profound. The highest band-gap shift was achieved for a Co-doped sample, and the best crystal orientation was for Mn-doped ZnO nanorods. Our results can be used as a comprehensive reference for engineering of the morphological, structural and optical properties of cation-doped ZnO nanorods by using a low-temperature synthesis as an economical mass-production approach.

Facile Synthesis of Ultrafine Hematite Nanowire Arrays in Mixed Water–Ethanol–Acetic Acid Solution for Enhanced Charge Transport and Separation

Nanostructure engineering is of great significance for semiconductor electrode to achieve high photoelectrochemical performance. Herein, we report a novel strategy to fabricate ultrafine hematite (α-Fe2O3) nanowire arrays in a mixed water-ethanol-acetic acid (WEA) solvent. To the best of our knowledge, this is the first report on direct growth of ultrafine (∼10 nm) α-Fe2O3 nanowire arrays on fluorine-doped tin oxide substrates through solution-based fabrication process. The effect of WEA ratio on the morphology of nanowires has been systematically studied to understand the formation mechanism. Photoelectrochemical measurements were conducted on both Ti-treated α-Fe2O3 nanowire and nanorod photoelectrodes. It reveals that α-Fe2O3 nanowire electrode has higher photocurrent and charge separation efficiencies than nanorod electrode if the carrier concentration and space-charge carrier width are in the same order of magnitude. Normalized by electrochemically active surface area, the Ti-treated α-Fe2O3 nanowire electrode obtains 6.4 times higher specific photocurrent density than nanorod electrode. This superiority of nanowires arises from the higher bulk and surface charge separation efficiencies, which could be partly attributed to reduced distance that holes must transfer to reach the semiconductor-liquid junction.