ang Jiang

Surface tuning for promoted charge transfer in hematite nanorod arrays as water-splitting photoanodes

AbstractHematite (α-Fe2O3) nanorod films with their surface tuned by W6+ doping have been investigated as oxygen-evolving photoanodes in photoelectrochemical cells. X-ray diffraction, field emission scanning electron microscopy, UV-visible absorption spectroscopy, and photoelectrochemical (PEC) measurements have been performed on the undoped and W6+-doped α-Fe2O3 nanorod films. W6+ doping is found to primarily affect the photoluminescence properties of α-Fe2O3 nanorod films. Comparisons are drawn between undoped and W6+-doped α-Fe2O3 nanorod films, WO3 films, and α-Fe2O3-modified WO3 composite electrodes. A close correlation between dopant concentration, photoluminescence intensity, and anodic photocurrent was observed. It is suggested that W6+ surface doping promotes charge transfer in α-Fe2O3 nanorods, giving rise to the enhanced PEC performance. These results suggest surface tuning via ion doping should represent a viable strategy to further improve the efficiency of α-Fe2O3 photoanodes.

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.

CdS/CdSe core-shell nanorod arrays: energy level alignment and enhanced photoelectrochemical performance.

Novel CdS/CdSe core-shell nanorod arrays were fabricated by a chemical bath deposition of CdSe on hydrothermally synthesized CdS nanorods. The CdS rods were hexagonal phase faced and the top of the rod was subulate. After the chemical bath deposition approach, CdS nanorod arrays were encapsulated by a uniform CdSe layer resulting enhanced absorbance and extended absorption edges of the films. A tandem structure of the energy bands of CdS/CdSe was also formed as a result of the Fermi level alignment, which is a benefit to the efficient separation of photogenerated charges. CdS/CdSe core-shell arrays gave a maximum photocurrent of 5.3 mA/cm(2), which was 4 and 11 times as large as bare CdS and CdSe, respectively.

Surface passivation of undoped hematite nanorod arrays via aqueous solution growth for improved photoelectrochemical water splitting

A facile solution-based strategy was found to be effective for surface passivation of undoped hematite nanorod photoanodes by adding noble-metal chlorides such as HAuCl4 and H2PtCl6 in the Fe(3+) precursor solution. XPS and Raman spectra revealed that noble-metal ions would not be doped into, but lead to surface disorder of hematite. Incident photon to current efficiency (IPCE) of the hematite photoanode grown in HAuCl4 and H2PtCl6 contained precursor solution was increased from 3.6% to 11.5% and 12.9%, respectively, at 365 nm and 1.23 V vs. RHE (reversible hydrogen electrode). This photoelectrochemical (PEC) enhancement was ascribed to the surface passivation, which resulted in a reduced recombination of photogenerated carriers, as confirmed by ultrafast transient absorption spectroscopy.

Tin(II) Antimonates with Adjustable Compositions: Effects of Band‐Gaps and Nanostructures on Visible‐Light‐Driven Photocatalytic H2 Evolution

A series of tin(II)–antimonate photocatalysts with varied Sn content were prepared by altering the ion‐exchange time and reaction temperature to control their physicochemical properties, especially their band‐gaps and nanostructures. Furthermore, the effect of these catalysts on visible‐light‐driven photocatalytic H2‐evolution was also investigated. With an increase in Sn content, the narrowed band‐gaps enhanced the absorption of photons to excite the photogenerated charge carriers. A decrease in nanocrystal size approaching stoichiometric compositions impeded the recombination of the photogenerated charge carriers; the increased surface areas and pore volumes, owing to the nanostructural transformation, accelerated the redox reactions. Consequently, the photocatalytic activities gradually improved and the highest rate was observed for stoichiometric Sn2Sb2O7. As a result, the as‐prepared tin(II) antimonates—especially Sn2Sb2O7—were confirmed to be stable and efficient photocatalysts for visible‐light‐driven H2 evolution. Moreover, the activities of these photocatalysts could be improved by tuning their physicochemical properties to jointly optimize all of the processes in the photocatalytic reaction.

Synthesis and characterization of nanoporous Bi3NbO7 films: application to photoelectrochemical water splitting

Nanostructured Bi3NbO7 films were successfully prepared via an ultrasonic spray pyrolysis method by using Bi(NO3)3·5H2O and Nb2O5 as precursors. The as-prepared films were systematically characterized by X-ray diffraction, Atomic Force Microscopy (AFM), field emission scanning electron microscopy (FESEM), UV-vis absorption spectra and X-ray photoelectron spectroscopy. The characterization results revealed that the nanostructured Bi3NbO7 possessed a cubic structure, nanoporous morphology and a visible light absorption with an optical band gap of about 2.8 eV. Moreover, electrochemical and photoelectrochemical measurements were carried out and a maximum photocurrent density of 2.8 μA cm−2 at the potential of 0.7 V vs. SCE was obtained for the Bi3NbO7 film deposited at 350 °C. The improvement of the film photoelectrochemical properties was contributed by novel nanoporous morphology that supply sufficient electrode–solution contact area. By addition of methanol into the solution, the photocurrent increased by 60%. The photoelectrochemical results reveal that the prepared films have the potential for hydrogen production via splitting water.

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.