Xiao Hua Yang

Top-Down Fabrication of α-Fe2O3 Single-Crystal Nanodiscs and Microparticles with Tunable Porosity for Largely Improved Lithium Storage Properties

In this work, we report a facile top-down approach to fabricate uniform single-crystal α-Fe(2)O(3) nanodiscs via selective oxalic acid etching. Phosphate ions are employed as a capping agent to control the etching to along the [001] direction. We also show that α-Fe(2)O(3) melon-like microparticles with contrasting textural properties can be generated using the same approach. The etched particles exhibit a much larger total pore volume and average pore size compared to the pristine ones, thus serving as the possible origin for their greatly enhanced capacity retention when tested as potential anode materials for lithium-ion batteries.

Rational screening low-cost counter electrodes for dye-sensitized solar cells

Dye-sensitized solar cells have attracted intense research attention owing to their ease of fabrication, cost-effectiveness and high efficiency in converting solar energy. Noble platinum is generally used as catalytic counter electrode for redox mediators in electrolyte solution. Unfortunately, platinum is expensive and non-sustainable for long-term applications. Therefore, researchers are facing with the challenge of developing low-cost and earth-abundant alternatives. So far, rational screening of non-platinum counter electrodes has been hamstrung by the lack of understanding about the electrocatalytic process of redox mediators on various counter electrodes. Here, using first-principle quantum chemical calculations, we studied the electrocatalytic process of redox mediators and predicted electrocatalytic activity of potential semiconductor counter electrodes. On the basis of theoretical predictions, we successfully used rust (α-Fe2O3) as a new counter electrode catalyst, which demonstrates promising electrocatalytic activity towards triiodide reduction at a rate comparable to platinum.

Ultra-thin anatase TiO2 nanosheets dominated with {001} facets: thickness-controlled synthesis, growth mechanism and water-splitting properties

Ultra-thin anatase TiO2 nanosheets with dominant {001} facets (∼82%) and controllable thickness (1.6–2.7 nm) were synthesized by using a modified one-pot hydrothermal route. As a morphology controlling agent, the concentration of hydrofluoric acid has a significant impact on the thickness of the as-synthesized TiO2 nanosheets. In addition, according to the XRD patterns and TEM images of the products on different reaction stages, the growth process of TiO2 nanosheets was clarified for the first time. We further measured the efficiency for H2 evolution of the ultra-thin anatase TiO2 nanosheets loaded with 1 wt% Pt from photochemical reduction of water in the presence of methanol as a scavenger. The TiO2 nanosheets exhibited a H2 evolution rate as high as 7381 μmol h−1 g−1 under UV-vis light irradiation, attributing to their exposed reactive {001} facets and high crystallinity.

Facet-dependent catalytic activity of platinum nanocrystals for triiodide reduction in dye-sensitized solar cells

Platinum (Pt) nanocrystals have demonstrated to be an effective catalyst in many heterogeneous catalytic processes. However, pioneer facets with highest activity have been reported differently for various reaction systems. Although Pt has been the most important counter electrode material for dye-sensitized solar cells (DSCs), suitable atomic arrangement on the exposed crystal facet of Pt for triiodide reduction is still inexplicable. Using density functional theory, we have investigated the catalytic reaction processes of triiodide reduction over {100}, {111} and {411} facets, indicating that the activity follows the order of Pt(111) > Pt(411) > Pt(100). Further, Pt nanocrystals mainly bounded by {100}, {111} and {411} facets were synthesized and used as counter electrode materials for DSCs. The highest photovoltaic conversion efficiency of Pt(111) in DSCs confirms the predictions of the theoretical study. These findings have deepened the understanding of the mechanism of triiodide reduction at Pt surfaces and further screened the best facet for DSCs successfully.

Low-cost SnS(x) counter electrodes for dye-sensitized solar cells.

SnS nanosheets (NSs), SnS nanowires (NWs) and SnS2 nanosheets were synthesized and investigated as counter electrode (CE) catalysts in a I3(-)/I(-) based dye-sensitized solar cell (DSC) system for the first time. It is found that the SnS NS based DSCs show comparable power-conversion efficiency (E(ff) = 6.56%) to Pt (7.56%), while the E(ff) of SnS NW and SnS2 NS based DSCs are 5.00% and 5.14% respectively, indicating the excellent catalytic activity of SnS(x) for the reduction of triiodide to iodide.

Yolk@shell anatase TiO2 hierarchical microspheres with exposed {001} facets for high-performance dye sensitized solar cells

We report a facile, template-free and nontoxic one-pot solvothermal route for synthesizing submicrometer-sized yolk@shell hierarchical spheres, which possess a permeable shell self-assembled by ultrathin anatase TiO2 nanosheets (NSs) with nearly 90% of exposed {001} facets and mesoporous inner sphere with a high specific surface area. Compared to the {001} faceted TiO2 NSs and standard Degussa P25, the anatase TiO2 yolk@shell hierarchical spheres (TiO2 YSHSs) were obtained with surface area up to 245.1 m2 g−1 and their submicrometer scale simultaneously promoted light scattering in the visible region. A light to electricity conversion efficiency (η) of 6.01% was achieved for the DSSCs with TiO2 YSHSs as its photoanode, under 100 mW cm−2 illumination, indicating 49.9% and 34.8% increases compared to the DSSCs with TiO2 NSs (4.01%) and the standard Degussa P25 (4.46%) as photoanodes, respectively. The enhancement can be mainly attributed to the higher dye loading on TiO2 YSHSs (4.35 × 10−5 mol cm−2) than that of TiO2 NSs (3.14 × 10−5 mol cm−2) and P25 (3.32 × 10−5 mol cm−2); longer lifetime of the injected electrons in TiO2 YSHSs film (65.79 ms) than that of in TiO2 NSs film (57.90 ms); and the good capability of light scattering of TiO2 YSHSs in visible light region, which are confirmed by UV-vis spectrophotometer and electrochemical impedance spectroscopy (EIS). The growth mechanism of the TiO2 YSHSs has also been investigated in detail.

A sulfur-assisted strategy to decorate MWCNTs with highly dispersed Pt nanoparticles for counter electrode in dye-sensitized solar cells

A novel strategy was introduced to prepare hybrid nanomaterials of platinum nanoparticles and multi-walled carbon nanotubes (Pt–MWCNTs). A thin layer of sulfur on MWCNTs could absorb and bind the Pt precursors from solution on the MWCNTs’ surface and also restrict their particle growth during reduction which resulted in ultrafine dispersions of metal nanocrystallites due to the strong affinity of sulfur for noble metals. The composite material was applied as the counter electrode (Pt–MWCNTs CE) for dye-sensitized solar cells (DSCs) and the fill factor (FF) and power-conversion efficiency (η) of the DSC with Pt–MWCNTs CE were 0.63 and 7.69%, respectively. The corresponding values of the DSC with bare Pt CE were 0.55 and 6.31%. The morphology and elemental composition of Pt–MWCNTs were characterized by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The dispersion and attachment of Pt nanoparticles on the surface of MWCNTs were observed by transmission electron microscopy (TEM). Electrochemical impedance spectra (EIS), Tafel polarization measurement and cyclic voltammetry (CV) were performed to characterize the catalytic activities of this hybrid CE.

Controllable Nanocarving of Anatase TiO2 Single Crystals with Reactive {001} Facets

Owing to its wide applications in photocatalysis, photo/ electrochromics, solar cells, sensors, and smart surface coatings, anatase titanium dioxide (TiO2) has attracted intensive research interests during the past decades. Most available and stable anatase TiO2 single crystals have truncated octahedral-bipyramid shapes, in which the majority of the surface area is normally surrounded by thermodynamically stable {101} facets, rather than the more reactive {001} facets. For a long period, tailored synthesis of {001}-faceted anatase TiO2 single crystals has been a great challenge. Recently, we made a breakthrough in the hydrothermal synthesis of anatase TiO2 single crystals with a large percentage (35–47%) of highly reactive {001} facets by using hydrofluoric acid (HF) as a specific morphology-controlling agent. Later, with the assistant of isopropanol (iPrOH)—a synergistic capping agent together with HF, anatase TiO2 single crystals with a higher ratio of {001} facets were prepared through a water–isopropanol solvothermal synthetic route. During these processes, HF was reported to act as a capping agent by reversing the relative stability of {001} and {101} facets and therefore promoting the growth of {001}faceted surfaces of anatase TiO2. By using similar strategies, a series of studies by research groups worldwide have demonstrated the syntheses and applications of anatase TiO2 with exposed reactive {001} facets. Unfortunately, few researchers have explored the etching effect of HF, which was traditionally considered a strong etching agent for metal oxides. Thus studying the chemical stability of {001}-faceted anatase TiO2 in the presence of HF is quite important and can help to understand the real function of HF during these important chemical processes. Herein, the dual role of HF—as capping agent and etching agent—is revealed through the controllable nanocarving of {001}-faceted anatase TiO2 single crystals. This process was realized through thermal treatment of anatase TiO2 single crystals with 35% of {001} facets by using HF-containing solutions or just by one-step solvothermal reactions of aqueous titanium tetrafluoride (TiF4) solutions in mixed solvents containing HF. In addition, a feasible mechanism is proposed to clarify the etching process involving HF and the growth process of the anatase TiO2 products with diverse morphologies. Figure 1a presents a typical SEM image of anatase TiO2 single crystals with 35% of reactive {001} facets, which was synthesized according to the method described in our earlier

A novel strategy to prepare a Pt–SnO2 nanocomposite as a highly efficient counter electrode for dye-sensitized solar cells

A novel strategy was introduced to prepare a Pt–SnO2 nanocomposite, in which the reduction of Pt4+ and the exfoliation of SnS2 were performed in one step. The Pt–SnO2 nanocomposite was applied as the counter electrode (CE) for dye-sensitized solar cells (DSCs). When compared with the energy conversion efficiency (Eff) of SnO2 CE based DSCs, the DSCs with Pt–SnO2 CE showed an overall Eff of 8.83%, giving an improvement of 198%. Meanwhile, better electrocatalytic activity towards I3−/I− redox pairs than Pt CE indicated that the Pt–SnO2 nanocomposite was a promising electrocatalyst for DSCs. Moreover, the low Pt content of the Pt–SnO2 nanocomposite would accelerate the large-scale applications of DSCs in the future.

A free radical assisted strategy for preparing ultra-small Pt decorated CNTs as a highly efficient counter electrode for dye-sensitized solar cells

In this paper, a free radical assisted strategy was introduced into the functionalization of counter electrode materials for the first time. With the assistance of a radical initiator, short-chained thiol groups can be attached on a multi-walled carbon nanotube (MWCNT) surface covalently, which was confirmed to be beneficial for both controlling the particle growth of platinum (Pt) on MWCNTs and improving the electron transfer between counter electrode materials and the substrate. The obtained Pt–MWCNT composite has been applied as a counter electrode material for dye-sensitized solar cells, and the power conversion efficiency (η) was detected to be 8.62%, surpassing that of the cell with a conventional Pt counter electrode (η = 7.56%). Especially, the fill factor improved considerably from 0.57 to 0.65.