Hematite is recognized as a promising photocatalyst for solar water oxidation. However, its practical performance at a low bias is still very low due to large onset potential. Here we report a combination of F-treatment and Rh-treatment on Fe2TiO5/Fe2O3 to lower the onset potential with a large value of 230 mV. The final onset potential is 0.63 V versus RHE comparable to the lowest value ever reported for hematite. The F- and Rh-cotreated photoanode yields a high photocurrent of 1.47 mA/cm2 at 1.0 V versus RHE, which is more than 3 times that of the pristine sample. X-ray photoelectron spectroscopy reveals the existence of surface Ti–F bonds after F-treatment. Moreover, the surface groups can be further modified after immersion in the working NaOH solution, which can form an interfacial hydrogen-bond network to accelerate the hole transfer. The surface Fe atoms are also partly reduced to accelerate the hole transport. Rh-treatment can further lower the onset potential with a good stability. The enhanced performance can be attributed to a synergetic effect of F- and Rh-based treatments, in which the F-based interface structure accelerates the hole transfer while the Rh-based material improves the catalytic performance.Keywords: F-treatment; hematite; interface structure; lowering the onset potential; Rh-treatment; solar water splitting;

Oxidized Pt–Ni nanoparticles were deposited on N-doped graphene oxide (NGO) for the hydrolysis of ammonia borane (AB). The optimized Pt3Ni7O–NGO sample shows a high total turnover frequency (TOF) value of 709.6 mol H2 per mol Cat-Pt per min in the hydrolysis of AB, which is one of the best values for Pt-based catalysts to date. Moreover, when calculating all metal contents (mainly Ni, Pt : Ni = 1 : 5) in the hybrid, the total TOF value is still as high as 120.7 mol H2 per mol Cat-metal per min, better than that of pure Pt/C or Pt on NGO. The hybrid also exhibits a good stability with more than 76% activity (TOF = 544.9) after 9 cycles. Synchrotron radiation X-ray absorption spectroscopy reveals that Pt and Ni in the catalyst exhibit a strong interaction through oxygen bonds and the bimetallic structure shows further interaction with the support material. All the components in the hybrid can thus be connected to show a synergetic effect for enhanced catalytic performance. The excellent performance can be related to the unique electronic structure of the hybrid with the synergetic effect, which may also shed light on the design of high-efficiency catalysts for other energy-related applications.

Aligned one-dimensional carbon nanostructures with different morphologies such as carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have been synthesized by the plasma-enhanced chemical vapor deposition method with different catalyst/underlayer combinations. The electronic structures of CNTs and CNFs have been studied by X-ray absorption near-edge structure spectroscopy (XANES), which reveals that CNTs have much fewer oxidized groups than CNFs. Moreover, the electrical transport properties of a single CNT or CNF have been measured in situ under transmission electron microscopy observation and the results show that CNTs have 2 orders lower resistivity than that of CNFs. A single CNT can be applied with higher voltage and larger current before thermal breakdown compared to a single CNF, which can be related to the electronic structure as revealed by XANES. Our results offer a good example of examining the relationship between morphological structure, electronic structure and electrical transport properties in carbon nanomaterials, which will certainly be useful in the applications of nano-devices.

A FeNiOOH-decorated hematite photoanode has been prepared using a facile electrodeposition method, with a significant cathodic shift of the onset potential (up to 190 mV) compared to the pristine sample. Synchrotron radiation based techniques have been used to identify the composition of the catalyst indicating the presence of FeOOH and NiOOH (FeNiOOH). The enhanced performance can be attributed to the better oxidation evolution reaction kinetics with the FeNiOOH cocatalyst. The FeNiOOH-decorated hematite is very stable for a long time. Moreover, the cocatalyst can be well coupled to the Pt-modified hematite photoanode achieving a high photocurrent of 2.21 mA cm−2 at 1.23 V vs. RHE. The good catalytic properties and the facile preparation method suggest that the decoration of FeNiOOH is a favorable strategy to improve the performance of hematite.

Carbon-coated α-Fe2O3 nanostructures, as the anode of Li-ion battery, have been deposited on the stainless steel substrate by a facile pyrolysis of ferrocene. The anode shows a high reversible capacity of 1138 mA h g−1 after 300 cycles at the current density of 500 mA g−1 and maintains a good capacity of 458.8 mA h g−1 even when cycled at the high current density of 10000 mA g−1. This high capacity can be associated to the nanostructure and the carbon layer coated on hematite. Moreover, the mechanism for the capacity evolution with cycling has been investigated by scanning transmission X-ray microscopy (STXM). The results reveal that the detailed composition and electronic structure change in the cycling process. Fe chemical state plays a critical role in the capacity evolution and a low oxidation state of Fe (such as Fe2+) might reduce the capacity by trapping Li+ ions, and the recovery of Fe2+ to hematite (Fe3+) significantly enhances the capacity. Data also show the growth and inhomogeneous distribution of a solid electrolyte interphase (SEI) layer containing carbon-based film, Li2O and Li2CO3. The facile synthesis of carbon-coated α-Fe2O3 opens an efficient way for large-scale anode production of Li-ion batteries, and the STXM study provides new insights into the mechanism of hematite-based Li-ion battery.

The hybrids of carbon nanotubes (CNTs) and the supported Ni nanoparticles (NPs) have been studied by scanning transmission X-ray microscopy (STXM) and tested by the hydrolysis reaction of ammonia borane (AB, NH3BH3). Data clearly showed the existence of a strong interaction between Ni NPs and thin CNTs (C–O–Ni bonds), which favored the tunable (buffer) electronic structure of Ni NPs facilitating the catalytic process. The hydrolysis process of AB confirmed the hypothesis that the hybrids with a strong interfacial interaction would show superior catalytic performance, while the hybrids with a weak interfacial interaction show poor performance. Our results provide a wealth of detailed information regarding the electronic structure of the NP–CNT hybrids and provide guidance towards the rational design of high-performance catalysts for energy applications.

We have stabilized the iron oxide nanoparticles (NPs) of various sizes on layered carbon materials (Fe-oxide/C) that show excellent catalytic performance. From the characterization of X-ray absorption spectroscopy (XAS), X-ray emission spectroscopy (XES), scanning transmission X-ray microscopy (STXM) and X-ray magnetic circular dichroism spectroscopy (XMCD), a strong interfacial interaction in the Fe-oxide/C hybrids has been observed between the small iron oxide NPs and layered carbon in contrast to the weak interaction in the large iron oxide NPs. The interfacial interaction between the NPs and layered carbon is found to link with the improved catalytic performance. In addition, the Fe L-edge XMCD spectra show that the large iron oxide NPs are mainly γ-Fe2O3 with a strong ferromagnetic property, whereas the small iron oxide NPs with strong interfacial interaction are mainly α-Fe2O3 or amorphous Fe2O3 with a nonmagnetic property. The results strongly suggest that the interfacial interaction plays a key role for the catalytic performance, and the experimental findings may provide guidance toward rational design of high-performance catalysts.Keywords: catalysts; interfacial interaction; iron oxide; layered carbon; magnetic property; soft X-ray spectroscopy;

Scanning transmission X-ray microscopy (STXM) combined with an identical region in transmission electron microscopy (TEM) image has been used to detect the interior structure of individual thick multi-walled carbon nanotubes (MWCNTs). Two individual MWCNTs show similar morphology and diameters from the TEM images, while from the STXM spectra their interior structures have significantly different hollow parts. Moreover, the different hollow structures result in obviously different electronic structures which can be detected by X-ray absorption spectroscopy. Our results show an effective way to measure the inside hollow structure of thick MWCNTs.

The reduction of hematite nanostructures was achieved by the pyrolysis of NH3BH3 (AB) solution, which introduced oxygen vacancies in hematite and significantly improved the photocurrent for solar water oxidation. Moreover, by covering the sample with a crucible in the annealing process, a significant cathodic shift of the onset potential (up to 70 mV) was observed. Data suggested that a higher inside pressure was introduced by the cover, which led to a further reduction of hematite in the near surface region. The depth-reduction resulted in a cathodic shift compared to the sample treated in the surface region. The cathodic shift was also confirmed at various concentrations of AB and annealing temperatures. Our results suggest that the treating depth could be a key role for the low onset potential of hematite for solar water oxidation.

A thin Fe2TiO5 layer was produced on hematite either by evaporating a TiCl4 solution on FeOOH or by a simple HF-assisted Ti treatment of FeOOH, both followed by annealing. The prepared Fe2TiO5-hematite heterostructure showed a significant enhancement in photocurrent density compared to that of the pristine hematite. For example, the sample after HF-assisted Ti treatment exhibited a significantly enhanced photocurrent of 2.0 mA/cm2 at 1.23 V vs RHE. Moreover, the performance of the Fe2TiO5-hematite heterostructure can be further improved by coupling with Co-Pi catalysts, achieving a higher photocurrent of 2.6 mA/cm2 at 1.23 V vs RHE. Synchrotron-based soft X-ray absorption spectroscopy analyses clearly revealed the existence of an Fe2TiO5 structure on hematite forming a heterojunction, which reduced the photogenerated hole accumulation and then improved the performance.Keywords: hematite nanostructures; solar water oxidation; Ti treatment;

By coupling Ti-doping and oxygen vacancies in hematite nanostructures, an efficient photoelectrode for solar water oxidation was prepared which showed a high photocurrent of 2.25 mA cm−2 at 1.23 V vs. RHE and a remarkable maximum value of 4.56 mA cm−2 at 1.6 V vs. RHE at a relatively low activation temperature of 550 °C. In addition, the partial oxygen pressure range suitable to produce oxygen vacancies in Ti-doped hematite could be expanded to a wide region compared to that in undoped hematite, which was critical to the photoelectrode production in practical applications. The facile way by coupling independently developed methods with the cumulative effect stands for an effective strategy for efficient solar water oxidation.

Hydrogen-treated hematite nanostructures were prepared by a simple pyrolysis of NaBH4 in a crucible. The H2-treated hematite photoelectrode showed high efficiency for solar water oxidation with a photocurrent of 2.28 mA cm−2 at 1.23 V vs. RHE, which was over 2.5 times higher than that for pristine hematite (0.88 mA cm−2). The significant improvement of the photocurrent can be attributed to increased oxygen vacancies after the H2 treatment. Moreover, the onset potential for H2-treated hematite was low and when compared to the hematite photoelectrode treated in an oxygen-deficient atmosphere to produce oxygen vacancies, a cathodic shift of the onset potential was observed by about 120 mV (from 0.99 to 0.87 V vs. RHE). The cathodic shift of the onset potential was attributed to the surface effect of H2 treatment while the oxygen-deficiency treatment mainly affected the bulk, which was confirmed by X-ray absorption spectroscopy. The results also suggest that the presence of surface defect states of Fe2+ in hematite is not the reason for high onset potential described in the literature. The H2-treated hematite with high efficiency could be used as a good starting material to achieve better performance for practical applications with further modifications such as surface catalysts or elemental doping.

Carbon-coated hematite nanostructures for solar water splitting were prepared by a simple pyrolysis of ferrocene which showed a remarkable photocurrent of 2.1 mA cm−2 at 1.23 V vs. RHE, compared to a value of 0.5 mA cm−2 for hematite without the carbon layer. The carbon layer is a few nm thick covering the surface of hematite nanostructures. X-Ray photoelectron spectroscopy and X-ray absorption spectroscopy revealed that the electronic structure of hematite was significantly modified with the existence of oxygen vacancy, which was responsible for the remarkable photocurrent. The carbon layer plays an important role for the appearance of oxygen vacancy. The simple and cheap method could be scaled up easily which may pave the way for the practical application for efficient solar water splitting.

The formation of a carbon coating on carbon nanotubes (CNTs) was observed in X-ray microscopy experiments. X-ray absorption near-edge structure (XANES) spectroscopy showed that the coating originated from the lacey carbon on the substrate, which was heated by the X-ray beam and then deposited on CNTs. The coating shows some fingerprint features around the π* peak of the C 1s XANES spectrum, which were widely observed in literature but attributed to chemical modification or interfacial interaction in carbon nanomaterials. Our results suggest that the X-ray induced coating could be a possible reason for those XANES features.

The electronic structure of individual carbon nanotubes (CNTs) with different diameters has been investigated by scanning transmission X-ray microscopy (STXM). Various factors that may alter the STXM spectral features in CNT bundles have been discussed. Tube diameter has been found to be proportional to the spectral intensity, whereas it has negligible effect on the spectral shape. Disordered structure in CNTs introduced in the synthesis process was observed, which significantly affected the X-ray absorption spectrum by a reduction of the π* excitation. Moreover, tube–tube interaction in CNT bundles was detected, which can enhance the π* excitation. Insights into the nature of CNT bundles with various electronic structures may provide a new vision to understand the performance of CNT-based devices.

The coating of nanoscaled carboxylated carbonaceous fragments on carbon nanotubes (CNTs) has been directly observed in chemical imaging with a concurrent identification of their electronic structure by scanning transmission X-ray microscopy. The coating also shields the detection of the CNT/nanoparticle interaction.

•We show a discrete Fe2TiO5-incorporation in hematite to improve the performance.•It can be well coupled with surface P-modification with a synergetic effect.•It shows a high photocurrent of 2.90 mA/cm2 at 1.23 VRHE with Co-Pi catalysts.•It provides a good insight to understand other Ti-based treatments of hematite.Hematite is a promising photocatalyst for solar water splitting while its performance has been severely limited by various factors. Recently surface Fe2TiO5 layer was widely reported to enhance the performance of hematite with a favorable band position to facilitate hole transport. Here we further show that the Fe2TiO5-incorporation in bulk hematite can also improve the performance with faster charge separation. Moreover, it can be well coupled with surface P-modification to simultaneously improve charge separation and hole transfer with a synergetic effect. The Ti and P co-modified hematite shows a significantly enhanced photocurrent of 2.37 mA/cm2 at 1.23 V vs. RHE when compared to the pristine value of 0.85 mA/cm2. After coupling with Co-Pi catalysts, the hematite sample can even achieve a stable, high photocurrent of 2.90 mA/cm2 at 1.23 V vs. RHE. The design of Ti and P co-modified hematite hollow nanostructures can be used as a promising candidate for solar water splitting applications. The discrete Fe2TiO5-incorporation also provides a good insight on the mechanism to understand other Ti-based treatments of hematite.

The coating of nanoscaled carboxylated carbonaceous fragments on carbon nanotubes (CNTs) has been directly observed in chemical imaging with a concurrent identification of their electronic structure by scanning transmission X-ray microscopy. The coating also shields the detection of the CNT/nanoparticle interaction.

Carbon-coated α-Fe2O3 nanostructures, as the anode of Li-ion battery, have been deposited on the stainless steel substrate by a facile pyrolysis of ferrocene. The anode shows a high reversible capacity of 1138 mA h g−1 after 300 cycles at the current density of 500 mA g−1 and maintains a good capacity of 458.8 mA h g−1 even when cycled at the high current density of 10000 mA g−1. This high capacity can be associated to the nanostructure and the carbon layer coated on hematite. Moreover, the mechanism for the capacity evolution with cycling has been investigated by scanning transmission X-ray microscopy (STXM). The results reveal that the detailed composition and electronic structure change in the cycling process. Fe chemical state plays a critical role in the capacity evolution and a low oxidation state of Fe (such as Fe2+) might reduce the capacity by trapping Li+ ions, and the recovery of Fe2+ to hematite (Fe3+) significantly enhances the capacity. Data also show the growth and inhomogeneous distribution of a solid electrolyte interphase (SEI) layer containing carbon-based film, Li2O and Li2CO3. The facile synthesis of carbon-coated α-Fe2O3 opens an efficient way for large-scale anode production of Li-ion batteries, and the STXM study provides new insights into the mechanism of hematite-based Li-ion battery.

By coupling Ti-doping and oxygen vacancies in hematite nanostructures, an efficient photoelectrode for solar water oxidation was prepared which showed a high photocurrent of 2.25 mA cm−2 at 1.23 V vs. RHE and a remarkable maximum value of 4.56 mA cm−2 at 1.6 V vs. RHE at a relatively low activation temperature of 550 °C. In addition, the partial oxygen pressure range suitable to produce oxygen vacancies in Ti-doped hematite could be expanded to a wide region compared to that in undoped hematite, which was critical to the photoelectrode production in practical applications. The facile way by coupling independently developed methods with the cumulative effect stands for an effective strategy for efficient solar water oxidation.

Hydrogen-treated hematite nanostructures were prepared by a simple pyrolysis of NaBH4 in a crucible. The H2-treated hematite photoelectrode showed high efficiency for solar water oxidation with a photocurrent of 2.28 mA cm−2 at 1.23 V vs. RHE, which was over 2.5 times higher than that for pristine hematite (0.88 mA cm−2). The significant improvement of the photocurrent can be attributed to increased oxygen vacancies after the H2 treatment. Moreover, the onset potential for H2-treated hematite was low and when compared to the hematite photoelectrode treated in an oxygen-deficient atmosphere to produce oxygen vacancies, a cathodic shift of the onset potential was observed by about 120 mV (from 0.99 to 0.87 V vs. RHE). The cathodic shift of the onset potential was attributed to the surface effect of H2 treatment while the oxygen-deficiency treatment mainly affected the bulk, which was confirmed by X-ray absorption spectroscopy. The results also suggest that the presence of surface defect states of Fe2+ in hematite is not the reason for high onset potential described in the literature. The H2-treated hematite with high efficiency could be used as a good starting material to achieve better performance for practical applications with further modifications such as surface catalysts or elemental doping.

A FeNiOOH-decorated hematite photoanode has been prepared using a facile electrodeposition method, with a significant cathodic shift of the onset potential (up to 190 mV) compared to the pristine sample. Synchrotron radiation based techniques have been used to identify the composition of the catalyst indicating the presence of FeOOH and NiOOH (FeNiOOH). The enhanced performance can be attributed to the better oxidation evolution reaction kinetics with the FeNiOOH cocatalyst. The FeNiOOH-decorated hematite is very stable for a long time. Moreover, the cocatalyst can be well coupled to the Pt-modified hematite photoanode achieving a high photocurrent of 2.21 mA cm−2 at 1.23 V vs. RHE. The good catalytic properties and the facile preparation method suggest that the decoration of FeNiOOH is a favorable strategy to improve the performance of hematite.