Molybdenum sulfides are very attractive noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER) from water. The atomic structure and identity of the catalytically active sites ...have been well established for crystalline molybdenum disulfide (c-MoS2) but not for amorphous molybdenum sulfide (a-MoSx), which exhibits significantly higher HER activity compared to its crystalline counterpart. Here we show that HER-active a-MoSx, prepared either as nanoparticles or as films, is a molecular-based coordination polymer consisting of discrete Mo3S13(2-) building blocks. Of the three terminal disulfide (S2(2-)) ligands within these clusters, two are shared to form the polymer chain. The third one remains free and generates molybdenum hydride moieties as the active site under H2 evolution conditions. Such a molecular structure therefore provides a basis for revisiting the mechanism of a-MoSx catalytic activity, as well as explaining some of its special properties such as reductive activation and corrosion. Our findings open up new avenues for the rational optimization of this HER electrocatalyst as an alternative to platinum.
Effective ion intercalation nanomaterials provide tremendous opportunities to various deionization systems such as capacitive deionization (CDI) to significantly improve the removal capacity of ...brackish water desalination. However, the asymmetric design of CDI devices causes a low removal rate due to the indispensable regeneration half-cycle. Furthermore, choices of chloride selective electrodes for such devices are limited. This imposes a big challenge on further improvement of CDI systems. Herein, we report a cation-selective CDI system using a single bi-functional Na
2
VTi(PO
4
)
3
@carbon nanomaterial with redox couples of V
4+
/V
3+
and Ti
3+
/Ti
4+
as an advanced symmetric electrode. The as-prepared continuous desalination set-up shows a superior removal rate of 0.022 mg g
−1
s
−1
(1.32 mg g
−1
min
−1
) with a high half-cycle removal capacity of 35 mg g
−1
, and extremely low energy consumption of 0.14 W h g
−1
(at a current density of 100 mA g
−1
). In addition, an extremely high cycle-stability of at least 50 cycles is achieved. The bi-functional intercalation mechanism is investigated by
in situ
XRD and
ex situ
XPS. The symmetric device yields a simplified and low-cost configuration with improved energy efficiency and high removal capacity. This opens a new horizon towards the commercialization of CDI technologies.
A high desalination rate with low energy consumption was enabled using a bi-functional Na
2
VTi(PO
4
)
3
@C electrode in symmetrical cation-selective CDI.
Solar water oxidation is considered as a promising method for efficient utilization of solar energy and bismuth vanadate (BiVO4) is a potential photoanode. Catalyst loading on BiVO4 is often used to ...tackle the limitations of charge recombination and sluggish kinetics. In this study, amorphous nickel oxide (NiOx) is loaded onto Mo‐doped BiVO4 by photochemical metal–organic deposition method. The resulting NiOx/Mo:BiVO4 photoanodes demonstrate a two‐fold improvement in photocurrent density (2.44 mA cm−2) at 1.23 V versus reversible hydrogen electrode (RHE) compared with the uncatalyzed samples. After NiOx modification the charge‐separation and charge‐transfer efficiencies improve significantly across the entire potential range. It is further elucidated by open‐circuit photovoltage (OCP), time‐resolved‐microwave conductivity (TRMC), and rapid‐scan voltammetry (RSV) measurements that NiOx modification induces larger band bending and promotes efficient charge transfer on the surface of BiVO4. This work provides insight into designing BiVO4‐catalyst assemblies by using a simple surface‐modification route for efficient solar water oxidation.
Bending the rules: Nickel oxide (NiOx) loading on BiVO4 photoanode demonstrates a twofold improvement in photocurrent density as compared with the uncatalyzed one. NiOx modification increases both the charge separation and the charge transfer efficiency significantly, which is attributed to larger band bending and promoted charge transfer at the surface of BiVO4.
Cu
2
ZnSnS
4
(CZTS) is a promising photocathode in water splitting systems due to its appropriate conduction band position with the water reduction potential, suitable band gap and high absorption ...coefficient. However, CZTS has yet to demonstrate unbiased solar to hydrogen efficiency above 1% in a photocathode-photoanode tandem setup unlike its CuInGaSe
2
chalcogenide counterpart due to its low onset potential and photocurrent. This low onset potential and photocurrent is believed to be due to the high density of defects in CZTS and at the CZTS/CdS interface which limits the open-circuit voltage in CZTS solar cells. In this work (Ag
x
Cu
1−
x
)
2
ZnSnS
4
(ACZTS) with Ag
+
partially replacing Cu
+
is fabricated by a solution process and investigated as a photocathode. Our ACZTS/CdS/Pt photocathode yields a maximum photocurrent of 17.7 mA cm
−2
at 0 V
RHE
with 4% Ag (
x
= 0.04) and a maximum onset potential of 0.85 V
RHE
with 8% Ag (
x
= 0.08), which is a substantial improvement from our CZTS/CdS/Pt photocathode that has a photocurrent of 13 mA cm
−2
and an onset potential of 0.65 V
RHE
. A combination of incident photon to current efficiency (IPCE) measurements done in a photoelectrochemical (PEC) and photovoltaic (PV) setup attributes the improvement to the interface properties. Other PV measurements such as capacitance-voltage profiling (CV) and Mott-Schottky measurements reveal a lower apparent carrier concentration and higher built-in voltage of ACZTS.
Ag substitution improves the photocurrent and onset potential of CZTS/CdS/Pt photocathode to 17 mA cm
−2
and 0.85 V
RHE
due to reduction of bulk and interface defects as well as increased depletion width.
Hematite is a promising photoanode for solar water splitting by photoelectrochemical (PEC) cells, but its performance is limited by the slow kinetics of water oxidation reaction or oxygen evolution ...reaction (OER). Surface modification of hematite photoanodes with a suitable water oxidation cocatalyst is a key strategy for improving the kinetics of water oxidation. In this study, a CeOx overlayer is deposited on the surface of the hematite photoanode by a water‐based solution method with ceric ammonium nitrate (CAN) followed by heat treatment. The photocurrent of CeOx‐modified hematite is 3 times higher than that of pristine hematite (at 1.23 V vs. RHE) under AM 1.5G, 1 sun conditions. Through hole‐scavenger measurements, Tafel plot analysis, and electrochemical impedance spectroscopy, it is concluded that CeOx overlayer increases the hole injection efficiency, improves the surface catalytic activity, and enhances charge transfer across the photoanode/electrolyte interface. These observations are attributed to the synergistic effects of Ce3+/Ce4+ redox species in CeOx and the oxygen vacancies. This work elucidates the role of CeOx as an efficient cocatalyst overlayer to improve the OER kinetics of photoanodes.
Think thrice: CeOx deposited on the surface of hematite photoanode improves the surface kinetics for water oxidation. The photocurrent is increased 3 times that of pristine hematite. It is assumed that oxygen vacancies in CeOx increase the adsorption of hydroxy groups to form intermediates and also facilitate hole transfer for O2 evolution.
Nickel oxide (NiO) is the most common low-cost high-performance anodic electrochromic material that is widely used in the applications for smart windows. The coloration mechanism is, however, still ...not well understood and we show that this is due to the evolving chemical nature of the film during the electrochromic process. Chemical bath deposited (CBD) NiO was studied using the near-grazing incidence angle Fourier transform infrared spectroscopy (NGIA FTIR) and endurance potential cycling. We will show that the initial hydration of NiO films toward Ni(OH)2 proceeds gradually through a combination of coloration from hydroxyl (OH−) ions and bleaching through H+ ions. This process increases the optical modulation of the deposited film. However, when the OH− ion diffusion is significantly enhanced, OH− ion incorporation during coloration will lead to water incorporation. The extensive intercalated networks can then isolate NiOOH grains that results in irreversible coloration and this is commonly reported as degradation. We will propose a model to show that an isolation process can explain this degradation and can be easily reversible by annealing. This understanding of the coloration and degradation mechanisms suggests that an optimum control of hydroxyl ions is critical for both efficiency and durability of NiO electrochromic devices.
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•Showed that the coloration of NiO occurs through two redox reactions.•Explained how the coupled reaction leads to gradual activation.•Explained degradation of NiO through an isolation process.•Demonstrated that degraded NiO can be regenerated.
Cu
2
ZnSnS
4
(CZTS) is considered as one of the most promising photocathodes for photoelectrochemical (PEC) water splitting due to its suitable optoelectronic properties. However, its PEC performance ...and stability degrade due to the poor interface between buffer layer CdS and catalyst Pt. In this work, indium tin oxide (ITO) was investigated as a charge transfer and protective layer for the CZTS/CdS/Pt photocathode. The solution-processed CZTS thin film coated with a CdS/ITO double layer and Pt catalyst (CZTS/CdS/ITO/Pt) yielded a photocurrent of 29 mA cm
−2
at 0 V
RHE
and an onset potential of 0.75 V
RHE
, which is significantly higher than that of the pristine CZTS/CdS/Pt photocathode. More importantly, the addition of the ITO layer was found to have a "recovery" effect that enables the CZTS/CdS/ITO/Pt photocathode to remain stable under photo-reducing conditions, as confirmed by dark linear sweep voltammetry runs after a stability test. Our results suggest that the improved photocurrent, onset potential, and recovery effect are probably attributed to the removal of phosphate ions adhering to the surface of the ITO layer and higher catalytic activity at the semiconductor/electrolyte surface by forming In-Pt and Sn-Pt interactions due to partial reduction of In and Sn on the ITO surface.
The study employs an ITO layer to enhance CZTS photocathodes for water splitting, resulting in improved photocurrent, onset potential, and stability through phosphate ion removal and In-Pt/Sn-Pt interactions on the ITO surface.
We report the synthesis of iron based titanate (Fe2TiO5) thin films using a simple low cost hydrothermal technique. We show that this Fe2TiO5 works well as a photoanode for the photoelectrochemical ...splitting of water due to favorable band energetic. Further characterization of thin films including band positions with respect to water redox levels has been investigated. We conclude that Fe2TiO5 is a promising material comparable to hematite for constructing PEC cells.
In this work, we identified the problem faced by conventional successive ionic layer adsorption and reaction (SILAR) process in producing uniform nickel hydroxide films as the homogeneous ...precipitation reaction in water. Subsequently we proposed a novel SILAR recipe with a modified rinsing step and successfully demonstrated, for the first time, a layer-by-layer coating of uniform nickel hydroxide thin film without surface structures or agglomerated precipitates. In addition, we explored and demonstrated the capability of the proposed SILAR process in incorporating additional element such as aluminum into the nickel hydroxide film. The aluminum incorporated nickel hydroxide film shows improved electrochromic stability evidenced by reducing the degradation in coloration efficiency from 46% to 6%. The average coloration efficient is reported as 22 cm2/C and the response time is 4.5 s for bleaching and 4 s for coloration.
Display omitted
•Novel layer-by-layer dip coating recipe for producing uniform thin film of metal hydroxide.•Capability of incorporating additional metal through simple modification of the process.•Deposited thin film shows improved electrochromic performance.
Producing densely packed high aspect ratio In0.53Ga0.47As nanostructures without surface damage is critical for beyond Si-CMOS nanoelectronic and optoelectronic devices. However, conventional dry ...etching methods are known to produce irreversible damage to III−V compound semiconductors because of the inherent high-energy ion-driven process. In this work, we demonstrate the realization of ordered, uniform, array-based In0.53Ga0.47As pillars with diameters as small as 200 nm using the damage-free metal-assisted chemical etching (MacEtch) technology combined with the post-MacEtch digital etching smoothing. The etching mechanism of In x Ga1−x As is explored through the characterization of pillar morphology and porosity as a function of etching condition and indium composition. The etching behavior of In0.53Ga0.47As, in contrast to higher bandgap semiconductors (e.g., Si or GaAs), can be interpreted by a Schottky barrier height model that dictates the etching mechanism constantly in the mass transport limited regime because of the low barrier height. A broader impact of this work relates to the complete elimination of surface roughness or porosity related defects, which can be prevalent byproducts of MacEtch, by post-MacEtch digital etching. Side-by-side comparison of the midgap interface state density and flat-band capacitance hysteresis of both the unprocessed planar and MacEtched pillar In0.53Ga0.47As metal-oxide-semiconductor capacitors further confirms that the surface of the resultant pillars is as smooth and defect-free as before etching. MacEtch combined with digital etching offers a simple, room-temperature, and low-cost method for the formation of high-quality In0.53Ga0.47As nanostructures that will potentially enable large-volume production of In0.53Ga0.47As-based devices including three-dimensional transistors and high-efficiency infrared photodetectors.
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