Band-Gap Engineering of NaNbO 3 for Photocatalytic H 2 Evolution with Visible Light

1 Catalytic Materials Group, Environmental Remediation Materials Unit, National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan 2 TU-NIMS Joint Research Center, School of Materials Science and Engineering, Tianjin University, 92 Weijin Road, Nankai District, Tianjin 300072, China 3 PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan 4 International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan


Introduction
Because of the current energy crisis and environmental pollution from the consumption of fossil fuels, new source which can provide a big amount of maintainable energy must be developed in hurry.H 2 is considered as a candidate of the next generation energy source because of its renewable, unlimited, and environmental friendly performances [1,2].However, there are still several barriers to realize the practical utilization of H 2 energy, and the produce of H 2 is the most serious one.As the present H 2 is mostly generated from the reformation of fossil fuel, a new method which can produce H 2 with clean energy should be developed [3].Photocatalysis has been developed as a candidate that can satisfy the demand of supplying H 2 by splitting water with solar energy.In the past decades, a lot of photocatalysts were developed for producing H 2 with high efficiency.But most of the photocatalysts, such as TiO 2 , SrTiO 3 , and NaTaO 3 , have only UV light responsibility, and the low visible light utilization limited the practical use of photocatalysis with solar light [4][5][6].To improve the visible light absorption, the common method is doping with cations to adjust the electronic structures of photocatalysts [7].When the cation dopants replace the positions of lattice cations or occupy the interstices in the crystal lattice, impurity energy levels might be generated within the band gap of the photocatalyst, which can extend the responsive region of photocatalytic reactions into visible light [8,9].NaNbO 3 is a typical nontoxic and highly stable semiconductor which has abundant applications in photocatalysis.In many reports, NaNbO 3 has been demonstrated to be a high efficiency photocatalyst for H 2 generation [10][11][12][13][14][15][16][17].Under the irradiation of UV light, NaNbO 3 nanoparticles could reduce H 2 O to H 2 with quite high efficiency with sacrificial agents [12].Fiber-structured NaNbO 3 was also verified to be useful in splitting pure H 2 O into H 2 and O 2 [10].However, almost all the reported NaNbO 3 photocatalysts are only sensitive to the UV light.Although iridium doped NaNbO 3 was proved to be active in water splitting under visible light irradiation, 2 International Journal of Photoenergy the efficiency is still low and this method needs precious metal [18].To achieve visible light photoactivity of NaNbO 3 without previous metal dopant is still a big challenge.Cobalt, which is a typical transition element with partially occupied  state, is commonly used as dopant to improve the visible light responsibility of wide band-gap photocatalysts [19][20][21][22].However, simply doping binary oxide with cobalt may increase the defect concentration and negatively affect the photocatalytic performance.Thus, codoping is more popular to balance the charge state and decrease the defects [23,24].In this work, we developed a series of NaNbO 3 doped with lanthanum and cobalt with H 2 evolution activity under visible light irradiation.The further theoretical study indicates that the cobalt dopant creates new states in the band gap of NaNbO 3 and provides the visible light absorption.

Experimental Section
2.1.Material Preparation.The samples were synthesized via a hydrothermal method [12].In a typical synthesis of NaNbO 3 , 1.0 g of (C 2 H 5 O) 5 Nb and 0.24 g of C 2 H 5 ONa were added into 10 mL of 2-methoxyethanol and stirred at room temperature to form a clear colloid.Next, the mixture was stirred for 30 minutes and then heated to 120 ∘ C with a rate of 1 ∘ C⋅min −1 and maintained at this temperature until a dry gel was obtained.After that, 40 mL of 6 M NaOH solution was added to the powdered dry gel and stirred at room temperature to form a uniform precursor.Then, the mixture was transferred into a 50 mL Teflon sealed autoclave and heated at 180 ∘ C for 24 h.Finally, the product was washed with distilled water until pH was lower than 8.0 and the obtained powder was dried at 70 ∘ C overnight.To synthesize La, Co codoped NaNbO 3 , the dopant reagent La(CH 3 COO) 3 , and Co(CH 3 COO) 2 were added in the first step and all the other procedures were the same.

Sample Characterization.
The crystal structure of NaNbO 3 powder was determined by an X-ray diffractometer (Rint-2000, Rigaku Co., Japan) with Cu-Ka radiation.The optical absorption spectra were measured with a UV-visible spectrophotometer (UV-2500PC, Shimadzu Co., Japan) using a BaSO 4 reference.Scanning electron microscopy images were recorded with a field emission scanning electron microscopy (JSM-6701F, JEOL Co., Japan) operated at 15 kV.

Photocatalytic H 2
Evolution.The H 2 evolution experiments were carried out in a gas closed circulation system.In a typical experiment, 0.3 g catalyst was dispersed by a magnetic stirrer in a CH 3 OH solution (220 mL distilled water and 50 mL CH 3 OH) in a Pyrex cell with a side window.Calculated amount of H 2 PtCl 6 solution (0.5 wt%) was added into the solution.The light source used for cocatalyst deposition was a 300 W Xe arc lamp without filter ( > 300 nm).After the H 2 evolution rate became constant, the system was evacuated and an L-42 cutoff filter was added to the 300 W Xe arc lamp ( > 420 nm).The H 2 evolution was measured by an in situ gas chromatograph (GC-8A, Shimadzu Co., Japan) with a thermal conductivity detector (TCD).

Theoretical Calculation.
The band structures, densities of state (DOS), and partial densities of state (PDOS) of NaNbO 3 and codoped NaNbO 3 were calculated using the plane-wave density functional theory (DFT) with the CASTEP program package [25].The doping concentration was set to 3.125% by, respectively, replacing a Na atom and a Nb atom by a La and a Co atom in a 2 × 2 × 1 supercell.The electronic state of Co was [Ar]3 6 and high spin.The core electrons were replaced by ultrasoft pseudopotentials with a planewave basis cutoff energy of 410 eV, and the interactions of exchange and correlation were treated with Perdew-Burke-Ernzerhof parameterization (PBE) of the generalized gradient approximation (GGA).The FFT grids of basis in all the models were 40 × 40 × 108 and the -point sets of 3 × 3 × 1 were used.

Results and Discussions
The crystallographic structures of all the synthesized NaNbO 3 samples were determined by X-ray diffraction (XRD) measurement (as shown in Figure 1(a)).All the observed diffraction peaks in the XRD patterns of NaNbO 3 and doped NaNbO 3 present good agreement with the reference data from the standard diffraction database (JCPDS-073-0803), showing that every sample was well crystalized in a single phase with the space group of Pbcm, which is the common phase of NaNbO 3 .However, slight shifts could be found when focusing on the particular diffraction peaks.Figure 1(b) gives the enlarged diffraction peaks with the highest intensity of NaNbO 3 and doped NaNbO 3 .When doping NaNbO 3 with La and Co, the diffraction peak shifts to the smaller diffraction angle, suggesting that the unit cell of NanbO 3 has a slight expansion.As the radius changes from Na + (102 pm) and Nb 5+ (64 pm) to La 3+ (103.2 pm) and Co 3+ (61 pm), such expansion of cell volume is understandable [26].The XPS measurement (as shown in Figure S1 in Supplementary Material available online at http://dx.doi.org/10.1155/2014/380421) gives obvious evidence that the valance state of Co is +3 as no evident peak of Co 2+ is observed [27].The detailed lattice parameters of the asprepared doped and undoped NaNbO 3 samples are shown in Table 1.
Since the morphology is an important factor which can greatly affect the photocatalytic performance, the scanning electron microscope (SEM) was further used to observe the morphology of the as-prepared samples and the SEM images of NaNbO 3 and (LaCo) 0.05 (NaNb) 0.95 O 3 are shown in Figure 2. The NaNbO 3 sample is constituted by particles with the cubic morphology, and the cubic particles are generally 300∼1000 nm in length.The obtained NanbO 3 has the similar morphology as the sample synthesized by hydrothermal reaction in the previous report [12].Although the crystal structure changes a little after doping with La and Co, the crystal growth process has almost no change.The doped sample has the same morphology as the pure NaNbO 3 .
UV-visible absorption spectra of NaNbO 3 and La, Co codoped NaNbO 3 powder samples are shown in Figure 3(a).The pure NabO 3 sample only has an intense absorption  with steep edges in the UV region.Different from the pure NaNbO 3 , the samples have evident absorptions in the visible light region.The optical band gaps   of the as-prepared NaNbO 3 samples were determined according to the following equation: in which , ], , and   are absorption coefficient, light frequency, proportionality constant, and optical band gap, respectively [28].The value of index  depends on the property of materials, whereas  = 2 for the direct transition

Figure 1 :Figure 2 :
Figure 1: (a) XRD patterns of the as-prepared NaNbO 3 and La, Co codoped NaNbO 3 compared with the standard NaNbO 3 XRD pattern.(b) The enlarged XRD patterns of the highest diffraction peak of NaNbO 3 .

Table 1 :
Crystal structures of the as-prepared doped and undoped NaNbO 3 samples.
3were synthesized to realize the visible light response photocatalytic H 2 evolution.The doped NaNbO 3 samples showed narrower optical band gaps (2.65, 2.70, and 2.74 eV for (LaCo) 0.05 (NaNb) 0.95 O 3 , (LaCo) 0.03 (NaNb) 0.97 O 3 , and (LaCo) 0.01 (NaNb) 0.99 O 3 , resp.) than the pure NaNbO 3 (3.42eV).In photocatalytic H 2 evolution experiments, the doped NaNbO 3 samples showed activity under the visible light irradiation, while the undoped Figure 6: The schematic band structures of NaNbO 3 and La, Co codoped NaNbO 3 .NaNbO 3 was not active.According to the theoretical calculation, the visible light activity of La, Co codoped NaNbO 3 could be attributed to the new impurity electronic states of Co dopant.Therefore, this work presented a new material for visible light photocatalytic H 2 evolution.