Zhaoming Xia

One-step synthesis of multi-walled carbon nanotubes/ultra-thin Ni(OH)2 nanoplate composite as efficient catalysts for water oxidation

We report a novel approach to synthesize ultra-thin β-Ni(OH)2 nanoplates with a thickness of 1.5–3.0 nm and their composites with multi-walled carbon nanotubes (MWCNTs) by one-step hydrothermal in the absence of surfactants. Ultra-thin β-Ni(OH)2 nanoplates have a large surface area of 139.0 m2 g−1, associated with more exposed surface Ni species, and exhibit better catalytic activity for oxygen evolution reaction (OER) than that of thick β-Ni(OH)2 nanoplates previously reported. Compared to β-Ni(OH)2 nanoplates alone and MWCNTs + Ni(OH)2 nanoplate physical mixture, the composite exhibits much higher electrocatalytic OER activity in terms of low onset overpotential, small Tafel slope, large exchange current density and high OER catalytic current densities at specific applied potentials. The Tafel slope of 87 mV dec−1 for the composite in pH 13 KOH is much smaller than that of β-Ni(OH)2 nanoplates (165 mV dec−1) and their physical mixture (140 mV dec−1). The enhanced catalytic activity of the MWCNTs/Ni(OH)2 composite could be ascribed to the synergic interface of MWCNTs and ultra-thin β-Ni(OH)2 nanoplates for improved conductivity, efficient chemical transfer and high oxidation state of Ni species in the composite electrodes by introducing MWCNTs. No obvious degradation of the OER catalytic current density of the composite electrode over a period of six hours was observed.

Highly Efficient and Robust Nickel Phosphides as Bifunctional Electrocatalysts for Overall Water-Splitting

To search for the efficient non-noble metal based and/or earth-abundant electrocatalysts for overall water-splitting is critical to promote the clean-energy technologies for hydrogen economy. Herein, we report nickel phosphide (NixPy) catalysts with the controllable phases as the efficient bifunctional catalysts for water electrolysis. The phases of NixPy were determined by the temperatures of the solid-phase reaction between the ultrathin Ni(OH)2 plates and NaH2PO2·H2O. The NixPy with the richest Ni5P4 phase synthesized at 325 °C (NixPy-325) delivered efficient and robust catalytic performance for hydrogen evolution reaction (HER) in the electrolytes with a wide pH range. The NixPy-325 catalysts also exhibited a remarkable performance for oxygen evolution reaction (OER) in a strong alkaline electrolyte (1.0 M KOH) due to the formation of surface NiOOH species. Furthermore, the bifunctional NixPy-325 catalysts enabled a highly performed overall water-splitting with ∼100% Faradaic efficiency in 1.0 M KOH electrolyte, in which a low applied external potential of 1.57 V led to a stabilized catalytic current density of 10 mA/cm(2) over 60 h.

Hollow Fluffy Co3O4 Cages as Efficient Electroactive Materials for Supercapacitors and Oxygen Evolution Reaction

Nano-/micrometer multiscale hierarchical structures not only provide large surface areas for surface redox reactions but also ensure efficient charge conductivity, which is of benefit for utilization in areas of electrochemical energy conversion and storage. Herein, hollow fluffy cages (HFC) of Co3O4, constructed of ultrathin nanosheets, were synthesized by the formation of Co(OH)2 hollow cages and subsequent calcination at 250 °C. The large surface area (245.5 m2 g(-1)) of HFC Co3O4 annealed at 250 °C ensures the efficient interaction between electrolytes and electroactive components and provides more active sites for the surface redox reactions. The hierarchical structures minimize amount of the grain boundaries and facilitate the charge transfer process. Thin thickness of nanosheets (2-3 nm) ensures the highly active sites for the surface redox reactions. As a consequence, HFC Co3O4 as the supercapacitor electrode exhibits a superior rate capability, shows an excellent cycliability of 10,000 cycles at 10 A g(-1), and delivers large specific capacitances of 948.9 and 536.8 F g(-1) at 1 and 40 A g(-1), respectively. Catalytic studies of HFC Co3O4 for oxygen evolution reaction display a much higher turnover frequency of 1.67×10(-2) s(-1) in pH 14.0 KOH electrolyte at 400 mV overpotential and a lower Tafel slope of 70 mV dec(-1). HFC Co3O4 with the efficient electrochemical activity and good stability can remain a promising candidate for the electrochemical energy conversion and storage.

Ultrathin porous Co3O4 nanoplates as highly efficient oxygen evolution catalysts

Oxygen evolution reaction (OER) catalysts are of central importance for electrocatalytic/photocatalytic water oxidation and fuel generation. Here we report a new type of ultrathin porous Co3O4 nanoplate as a highly efficient OER catalyst. The porous Co3O4 nanoplates annealed at 250 °C can be readily synthesized in large quantities with a large surface area of 160.9 m2 g−1, a very small crystalline size of ∼3.0 nm, and a thin thickness of ∼10 nm. The large surface area provides more surface active sites for the OER. The structural features of the porous nanoplates significantly enrich the amount of surface abundant catalytic sites, provide more active edge and corner cobalt species with low coordination numbers, and subsequently enhance their OER activity. Meanwhile, the thin thickness facilitates efficient diffusion of chemicals and the escape of the generated O2 within the electrode. Taken all together, the porous Co3O4 nanoplates annealed at 250 °C deliver a high OER activity with an overpotential as low as 258 mV at 1 mA cm−2, a turnover frequency of 0.0042 s−1 (low bound), a Tafel slope of 71 mV dec−1 in 1.0 M KOH solution and an excellent electrochemical stability.

Facile synthesis of CoX (X = S, P) as an efficient electrocatalyst for hydrogen evolution reaction

Developing environmentally-friendly and earth-abundant electrocatalysts is desirable for an efficient hydrogen evolution reaction (HER). Herein, we report a facile and controllable synthesis of CoX (X = S, P) nanocatalysts by the chemical conversion of thin Co(OH)2 nanoplates under mild conditions. Both catalysts delivered high catalytic activity for HER. Small onset potentials of 59 and 32 mV, along with low Tafel slopes of 56.2 and 54.8 mV dec−1, were observed for CoS and CoP, respectively. Analyses suggest that the better HER performance of CoP nanocatalysts could be attributed to the intrinsically more positively charged nature of the Co metal center, the longer Co–P bond length, and more catalytic active sites due to the smaller size of the CoP nanocatalysts. High catalytic stability in acidic media was also observed for both CoS and CoP catalysts for a duration of 18 hours.

Quaternary pyrite-structured nickel/cobalt phosphosulfide nanowires on carbon cloth as efficient and robust electrodes for water electrolysis

The strategy of element substitution is an effective way to tune the electronic structures of the active sites in catalysts, thereby leading to improvements in both the catalytic activity and stability. Herein, we design and synthesize pyrite-type nickel/phosphorus co-doped CoS2 nanowires on carbon cloth (NiCoPS/CC) as efficient and durable electrodes for water electrolysis. Introduction of nickel and phosphorus produced stepwise and superb enhancement of the performance of the electrodes in the hydrogen evolution reaction due to regulation of the electronic structures of the active sites of the catalyst and accelerated charge transfer over a wide pH range (0−14). The NiCoPS/CC electrodes also delivered a nearly undecayed catalytic current density of 10 mA·cm−2 at a low overpotential of 230 mV for oxygen evolution due to in situ formation of surficial Ni–Co oxo/hydroxide in 1.0 M KOH. Thus, the NiCoPS/CC electrodes gave rise to a catalytic current density of 10 mA·cm−2 for overall water splitting at potentials as low as 1.54 V during operation over 100 h in 1.0 M KOH with a Faradic efficiency of ~100%.

CsPbBr3 perovskite nanocrystals as highly selective and sensitive spectrochemical probes for gaseous HCl detection

A rapid and facile gaseous anion-exchange reaction between CsPbBr3 perovskite nanocrystals and HCl vapor was carried out under the ambient conditions. The resultant CsPb(Br/Cl)3 nanocrystals preserved the morphological features and crystal structure of the original CsPbBr3 nanocrystals and exhibited a significant blue-shift in the ultraviolet-visible light absorption and photoluminescence spectra. Inspired by the visual observations in the fluorimetry upon gaseous anion-exchange reaction, the CsPbBr3 nanocrystals were developed as highly sensitive and selective spectrochemical probes for the detection of toxic and corrosive HCl vapor with a concentration as low as 5 ppm.

Tuning chemical compositions of bimetallic AuPd catalysts for selective catalytic hydrogenation of halogenated quinolines

Catalytic hydrogenation of halogenated quinolines is a longstanding challenge due to the harsh reaction conditions and disillusionary chemoselectivity owing to dehalogenation. Exploration of novel catalytic materials is still a big challenge. Herein, density functional theory calculations indicate that halogenated quinolines are selectively adsorbed on the Au surface via the nitrogen atom in the tilted orientation and on Pd via the quinoline ring in the flat orientation. In the tilted orientation, the C–Cl bond is away from the surface of catalysts, which can avoid the hydrogenation of the C–Cl bond by the surface activated hydrogen species. A series of Au1−xPdx bimetallic catalysts were deposited on CeO2 nanorods by a facile electroless chemical deposition method. The Au1−xPdx catalysts with low Pd content delivered enhanced activity and improved chemoselectivity for the hydrogenation of halogenated quinolines. Highly dispersed Pd in the Au matrix of bimetallic catalysts with low Pd content triggers hydrogen activation on Pd sites and leads to the selective adsorption of halogenated quinolines on Au sites in the tilted orientation. The generated active hydrogen species can diffuse from Pd to Au sites for the hydrogenation of the tilted halogenated quinolines, resulting in suppressed dehalogenation and high chemoselectivity to the expected products.

Thermally stable sandwich-type catalysts of Pt nanoparticles encapsulated in CeO2 nanorod/CeO2 nanoparticle core/shell supports for methane oxidation at high temperatures

The thermal stability of nanocatalysts is of great importance to develop high performing catalysts in terms of high activity and robust catalytic stability, especially for high-temperature catalysis. Herein, we report a sandwich-type Pt nanocatalyst encapsulated in ceria-based core/shell supports (CNR@Pt@CNP), which consists of CeO2 nanorods as core, CeO2 nanoparticles as shell and Pt nanoparticles (PtNPs) embedded between the CeO2 nanorods and CeO2 nanoparticles. The catalysts exhibited remarkable thermal stability at high temperature by effectively preventing PtNPs from thermal sintering. Methane combustion was carried out on the CNR@Pt@CNP catalysts at 400–700 °C to evaluate their catalytic activity and stability. By comparing to the same amount of PtNPs supported on CeO2 nanorods (CNR@Pt), CNR@Pt@CNP delivered higher catalytic activity at high temperatures (>500 °C). The methane conversion catalyzed by CNR@Pt@CNP slightly decreased from 82.3% to 80.0% after 12 hours at 650 °C. The improved performance of CNR@Pt@CNP originated from the CeO2 nanoparticles as stabilizer, which can prevent the thermal sintering of PtNPs, strengthen the thermal stability of the catalyst and enhance the metal-support interaction.

Turbostratic carbon nitride enhances the performance and stability of cadmium sulfide nanorod hydrogen evolution photocatalysts

We report CdS nanorods encapsulated in a thin amorphous t-phased CNx layer modified with Pt nanoparticles (CdS/CNx-Pt) as photocatalysts for hydrogen production in 0.35 M Na2S and 0.25 M Na2SO3 solution. Compared to CdS/Pt counterparts, the CdS/CNx-Pt catalysts deliver more efficient and extraordinary stabilized catalytic activity. Herein, the t-CNx layer functions as (1) a protection layer to isolate CdS from the electrolyte and suppress the anodic photocorrosion of CdS and (2) as a tunnel junction to promote charge separation and electron transport to PtNPs for proton reduction. Surface photovoltage spectra (SPS) confirm 2.25 eV as the effective bandgap and the n-type character of the CdS nanorods. Photoelectron injection into the PtNPs and photohole injection into the sacrificial donors can be directly observed.