Letter pubs.acs.org/NanoLett
Engineered Hematite Mesoporous Single Crystals Drive Drastic Enhancement in Solar Water Splitting Chong Wu Wang,† Shuang Yang,† Wen Qi Fang,† Porun Liu,‡ Huijun Zhao,*,‡ and Hua Gui Yang*,† †
Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China ‡ Centre for Clean Environment and Energy, Griffith School of Environment, Griffith University, Gold Coast Campus, Nathan, QLD 4222, Australia S Supporting Information *
ABSTRACT: Mesoporous single crystals (MSCs) rendering highly accessible surface area and long-range electron conductivity are extremely significant in many fields, including catalyst, solar fuel, and electrical energy storage technologies. Hematite semiconductor, whose performance has been crucially limited by its pristine poor charge separation efficiency in solar water splitting, should benefit from this strategy. Despite successful synthesis of many metal oxide MSCs, the fabrication of hematite MSCs remains to be a great challenge due to its quite slow hydrolysis rate in water. Herein, for the first time, we have developed a synthetic strategy to prepare hematite MSCs and systematically investigated their growth mechanism. The electrode fabricated with these crystals is able to achieve a photocurrent density of 0.61 mA/cm2 at 1.23 V vs RHE under AM 1.5G simulated sunlight, which is 20 times higher than that of electrodes made of solid single crystals. The enhancement is ascribed to the superior light absorption and enhanced charges separation. Our results demonstrate the advantage of incorporation of nanopores into the large-sized hematite single crystals and provide a valuable insight for the development of high performance photoelectrodes in PEC application. KEYWORDS: Mesoporous single crystals, hematite, hydrogen, photoanode, water splitting
R
water.16 Hematite has emerged as a standard material for photoelectrode in PEC water splitting, mainly due to its chemical stability, earth abundancy and favorable optical bandgap (2.1 eV) for light harvesting (to absorb approximately 40% of the solar spectrum).17−19 However, its performance has been severely limited by the conflict between light harvesting (α−1 = 0.12 μm at 550 nm) and charge separation (short diffusion length of 2−4 nm).20,21 Therefore, the synthesis of MSCs is expected to be extremely significant for the optimization of hematite and further improvement of its performance in PEC water splitting. Herein, we developed a facial crystallization and etching method for the preparation of hematite MSCs (Figure 1). Then, we demonstrated their superior performance in PEC water splitting. Photoelectrodes prepared with hematite MSCs exhibited 20 times larger photocurrent at standard condition (AM 1.5, 1.23 V vs RHE) than that with solid hematite single crystals. Further experimental results illustrated that the photoanode made from hematite MSCs not only displayed stronger light harvesting ability in the visible wavelength range but also allowed higher charge separation efficiency compared
ational design and synthesis of metal oxide nanocrystals with tunable morphology and properties have drawn considerable research interest.1−6 In many fields, well-defined single crystals exposing specific facets are demonstrated to be extremely significant owning to the highly reactive surfaces and excellent electron mobility.7−10 However, the relatively low specific surface area of single crystals crucially limits their application, especially when their size exceeds micrometer scale. Meanwhile, sintered or compacted nanoparticle thin films, which render high specific surface area, also suffer from major drawbacks: drastic decrease in electron mobility, abundant grain boundaries or defects, and lack of directional charge transfer to the back contact. Therefore, the approach that introducing nanopores into large single crystals should be desirable as mesoporous single crystals (MSCs) can offer not only high surface areas but also fast charge transfer across the crystal framework.11 Recently, anatase TiO2 MSCs have been prepared successfully via a heterogeneous hydrothermal method in the presence of seeded silica templates.12 Very soon, this strategy has been demonstrated for efficient TiO2 photoelectrode in PEC water oxidation and applied for the fabrication of many other crystals.13−16 Despite successful synthesis of many metal oxide MSCs, the preparation of hematite MSCs remains to be a great challenge due to the quite slow hydrolysis rate of Fe3+ in © XXXX American Chemical Society
Received: October 5, 2015 Revised: November 25, 2015
A
DOI: 10.1021/acs.nanolett.5b04059 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 1. Morphology analysis for typical hematite MSCs prepared with 18 mM NaF and 80 mM silica. (A) Low magnification SEM image of hematite MSCs. The inset shows the particle size distribution of the as-synthesized hematite MSCs. (B) High magnification SEM image of hematite MSCs. The inset exhibits the pore size distribution of the crystals. (C) TEM image of hematite MSCs with a hexagonal shape. (D) SAED pattern of a hematite mesoporous crystal with [001] beam incidence, suggesting the single crystal property of the obtained sample. (E) High-resolution TEM image recorded for hematite MSCs. (F) TEM images for a selected area and (G) stained ultrathin slices indicate that pores can be observed both on the surface and inside the crystals.
The inner crystal structures of the hematite MSCs were investigated by transmission electron microscopy (TEM). The as-prepared crystals exhibit hexagonal profiles with size of around 720 nm, which is consistent with the SEM results (Figure 1C). Selected area electron diffraction (SAED, Figure 1D) and high-resolution TEM (HRTEM) images given in Figure 1E exhibit the lattice spacing of 0.25 nm, and the value can be assigned to the lattice fringe of (110), (−210), and (−120) atomic planes of α-Fe2O3 crystal structure. Meanwhile, the angle of 120° between two lattice fringes agrees well with the angle of the (110) and (−210) planes. From a selected region of the hematite MSCs, the crystal surface displays foamlike structure with a pore diameter of ca. 20 nm (Figure 1F). We then prepared stained ultrathin slices to directly visualize the nanoscale porous structure inside the crystal as shown in Figure 1G. Holes with diameter of around 20 nm can be observed on these slices, which confirm that nanopores are uniformly distributed within the whole single crystal. On the basis of these investigations, it is demonstrated that highly porous hematite single crystals can be obtained through such a convenient hydrothermal method and the mesoporous configurations should be highly desirable for an efficient mass transfer (reactants and products) within the mesoporous single crystalline structure for water splitting.
to electrodes composed of solid hematite single crystals; these improvements finally lead to the enhanced PEC performance of hematite MSCs. The morphology of typical hematite MSCs were investigated by scanning electrons microscopy (SEM) measurement. As shown in Figure 1A, hexagonal hematite MSCs, with an average length of around 720 ± 100 nm and thickness of around 400 nm, have been successfully obtained via this synthetic route. Obviously, large numbers of nanosized pores can be observed on the surface of these crystals. SEM image at higher magnification (Figure 1B) reveals spherical shaped pores with a mean size of 18 nm. Existence of large holes may result from the aggregation of SiO2 during the crystallization of hematite single crystals. Meanwhile, the thickness of the wall between adjacent pores is around 15 nm, which should be favorable for separation of photogenerated charges in hematite crystals. N2 adsorption−desorption isotherms were measured for solid and mesoporous hematite single crystals (Figure S1). It is found that the BET surface area of hematite MSCs is 18.02 m2/g, almost 6 times larger than that of solid sample (3.33 m2/g). The broad hysteresis loop of hematite MSCs at high relatively pressures exhibited typical type IV isotherms curve, indicating the nature of porous materials. B
DOI: 10.1021/acs.nanolett.5b04059 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 2. SEM images of hematite crystals synthesized with different concentrations of F− anions and silica templates. X axis indicates the concentration of F− anions and Y axis displays the concentration of silica. (A−C) Hematite crystals prepared with 18, 30, and 60 mM F− anions as well as 0 mM silica. Preferential adsorption of F− on (100) planes slows down the growth speed along the {100} direction, which eliminates the (001) planes at high concentration of F− anions. (D−F) Hematite crystals prepared with 18, 30, and 60 mM F− anions as well as 20 mM silica. Dissolved silicate etches the hematite crystals along the {001} direction and adsorption of F− anions can preserve the (001) plane. (G−I) Hematite crystals prepared with 18, 30, and 60 mM F− anions as well as 80 mM templates. Adsorption of F− anions prevents the nucleation of hematite on silica templates and hematite MSCs can only be obtained at low concentration of F− anions.
It is worth noting that concentrations of F− anions and silica play important roles in tuning the particle shape. Therefore, serial concentration-dependent experiments were conducted with the same reaction duration time of 48 h to understand the influence of F− and silica. Figure 2 shows SEM images of hematite crystals synthesized with 18−60 mM NaF and 0−80 mM silica, in which the sizes of the crystals are found to increase with the concentration of F−. Meanwhile, as shown in Figure 2A−C, the hematite crystals prepared without silica templates are polyhedral, octodecahedron, and hexagonal bipyramidal crystals enclosed by 12 equiv (101) planes for 18, 30, and 60 mM of F− anions, respectively; it is because of the preferential adsorption of F− anions on (100) planes, which slows down the growth speed along [100] direction.19 However, fast growth along {001} direction leads to the exposure of (101) planes and absence of (001) planes. When trace of silica was introduced into the system (
