A family of photocatalysts for water splitting into hydrogen was prepared by distributing TiO(6) units in an MTi-layered double hydroxide matrix (M = Ni, Zn, Mg) that displays largely enhanced photocatalytic activity with an H(2)-production rate of 31.4 μmol h(-1) as well as excellent recyclable performance. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) mapping and XPS measurement reveal that a high dispersion of TiO(6) octahedra in the layered doubled hydroxide (LDH) matrix was obtained by the formation of an M(2+)-O-Ti network, rather different from the aggregation state of TiO(6) in the inorganic layered material K(2)Ti(4)O(9). Both transient absorption and photoluminescence spectra demonstrate that the electron-hole recombination process was significantly depressed in the Ti-containing LDH materials relative to bulk Ti oxide, which is attributed to the abundant surface defects that serve as trapping sites for photogenerated electrons verified by positron annihilation and extended X-ray absorption fine structure (EXAFS) techniques. In addition, a theoretical study on the basis of DFT calculations demonstrates that the electronic structure of the TiO(6) units was modified by the adjacent MO(6) octahedron by means of covalent interactions, with a much decreased bandgap of 2.1 eV, which accounts for its superior water-splitting behavior. Therefore, the dispersion strategy for TiO(6) units within a 2D inorganic matrix can be extended to fabricate other oxide or hydroxide catalysts with greatly enhanced performance in photocatalysis and energy conversion.
Materials: Chemical reagents including Ni(NO3)2·3H2O, Mg-
(NO3)2·6H2O, Al(NO3)2·9H2O, Zn(NO3)2·6H2O, K2CO3, NaOH, urea,
Na2CO3, NaNO3, and HCl were obtained from Beijing Chemical Co.,
Ltd. a-Ni(OH)2, TiO2, and TiCl4 were purchased from Sigma–Aldrich
Co. Deionized and decarbonated water were used in all experimental
Synthesis of MTi–LDHs (M=Ni, Zn, MgAl): MTi–LDHs (M=Ni, Zn,
MgAl) catalysts were prepared by using the coprecipitation method. The
synthesis of NiTi–LDHs was similar to that described by He et al. In
brief, a TiCl4 solution (0.5 mL; the solution was prepared from TiCl4 and
HCl with a volume ratio of 1:1, in which TiCl4 is 0.002 mol), Ni(NO3)2
(0.008 mol), and urea (0.1 mol) were dissolved in deionized water
(100 mL) under vigorous stirring for 27 h at reflux temperature (90 8C).
The resulting solid was filtered, washed with deionized water and anhydrous ethanol, and dried at 608C. The NiTi–LDHs with different molar
ratios of Ni2+/Ti4+ were prepared by using the same procedure but by
varying the Ni2+ dosage.
ZnTi–LDH was prepared by the coprecipitation of zinc and titanium
salts from a homogeneous solution according to the literature. TiCl4
(0.22 mL), Zn(NO3)2·6H2O (1.19 g) and urea (3.0 g) were dissolved in deionized water (100 mL) under vigorous stirring. The resulting reactant
was aged in an autoclave at 130 8C for 48 h. The precipitate was centrifuged, washed thoroughly with water, and finally dried at 60 8C for 24 h.
MgAlTi–LDH was prepared by the coprecipitation method with low supersaturation. A solution that contained Mg(NO3)2·6H2O, Al-
(NO3)3·9H2O, and TiCl4, and another solution that contained NaOH
(2.0m) and Na2CO3 (0.50m) were simultaneously added to a solution of
Na2CO3 solution (100 mL, 0.01m), and the pH of the resulting slurry was
maintained at approximately 10. The mixture was held at 708C for 27 h.
The final precipitate was filtered, washed thoroughly with deionized
water, and dried at 60 8C for 24 h.
For comparison, the layer structure K2Ti4O9 was synthesized according to
a solid-state reaction by heating the mixture of TiO2 and K2CO3 (with a
10% stoichiometric excess amount) at 960 8C for 10 h.[6, 15]
Photocatalytic reaction of splitting water into hydrogen: The photocatalytic reaction was carried out using a Splitting water systemCEL-SPH2N
(Beijing AuLight Co., Ltd.); the setup is shown in Figure S1 in the Supporting Information. The photocatalytic reaction was performed in a
Pyrex glass cell with a stationary temperature at 508C, which was connected with a closed gas circulation system. LDH material (0.10 g) was
suspended in an aqueous solution (150 mL) that contained lactic acid
(LA; 0.20 mL) as a sacrificial agent. Pt (0.1 wt%) was loaded onto the
surface of LDH in situ by the photoreduction method using H2PtCl6
aqueous solution. The suspension was then thoroughly degassed and irradiated using an Xe lamp (300 W). The amount of H2 produced was analyzed at given time intervals using an online gas chromatograph (GC7890II; Techcomp. Co., Ltd). The activity of different catalysts was determined on the basis of the average rate of H2 evolution in the first 5 h.
Characterization: Powder X-ray diffraction (XRD) patterns of the samples were collected using a Shimadzu XRD-6000 diffractometer using a
CuKa source, with a scan step of 0.028 and a scan range between 7 and
708. Fourier transform infrared spectra were recorded using a Vector22
(Bruker) spectrophotometer in the range 4000–400 cm1 with 2 cm1 of resolution. The standard KBr disk method (1 mg of sample in 100 mg of
KBr) was used. UV/Vis spectra were recorded using a Beijing PGENERAL TU-1901 spectrometer in the 350–800 nm wavelength range. Laser
flash photolysis experiments were carried out using an Edinburgh LP920
spectrophotometer (Edinburgh Instruments) to record the transient absorption spectra. Sample was excited by using 355 nm output with pulse
energies of 1.5 mJ per pulse from an OPO pumped by an Nd:YAG laser
(10 Hz, 8 ns) (Continuum Surelite). Data were analyzed by using the
online software of the LP920 spectrophotometer. The fitting quality was
judged by weighted residuals and a reduced c2 value. Fluorescence emission spectra were recorded using an RF-5301PC fluorophotometer
(1.5 nm resolution) in the range of 430–500 nm with the excitation wavelength of 366 nm and slit widths of 3 nm. The morphology of the film
samples was investigated using a Zeiss Supra 55 scanning electron microscope (SEM) with an accelerating voltage of 20 kV. The morphology of
the layered double hydroxide (LDH) flake was characterized by using a
JEOL JEM-2010 high-resolution transmission electron microscope with
an accelerating voltage of 200 kV. HAADF-STEM measurements were
obtained using a Titan 80–300 at 80 kV and an FEI Tecnal G2 F20 UTWIN instrument at an operating voltage of 200 kV. The specific surface
area determination was performed by Brunaer–Emmett–Teller (BET)
methods using a Quantachrome Autosorb-1C-VP analyzer. Analyzer Xray photoelectron spectra (XPS) were recorded using a PHIQ2000 X-ray
photoelectron spectrometer equipped with a monochromatized AlKa Xray source. Positron annihilation experiments were carried out using a
fast/slow coincidence ORTEC system with a time resolution of 187 ps full
width at half-maximum. A 5 105 Bq source of 22Na was sandwiched between two identical samples. The Ni K-edge X-ray absorption near-edge
structure (XANES) measurements of the samples were performed at the
beam line 1W1B of the Beijing Synchrotron Radiation Facility (BSRF),
Institute of High Energy Physics (IHEP), Chinese Academy of Sciences
(CAS) at room temperature in the transmission mode.
See the Supporting Information for details on the density functional calculations.