H2 Evolution on Lanthanum and Carbon Co-doped NaTaO3 Photocatalyst

We report a carbon-modify lanthanum doped sodium tantalum oxide powders (La-C-NaTaO3) by sol-gel process. The resultant materials are characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The X-ray diffraction of La-CNaTaO3 show a single phases with a good crystallinity and without any impurity. The sample is exactly indexed as NaTaO3 monoclinic structure with the space group P2/m. The SEM measurements give a smaller particle size of doped NaTaO3 than pure NaTaO3. The effect of dopant on the photocatalytic activity of La-C-NaTaO3 in the photocatalytic of hydrogen generation is studied and compared with pure NaTaO3. The results show that the rate of hydrogen evolution over La-C-NaTaO3 is higher as compared to that of pure NaTaO3. The enhancement of photocatalytic activity of La-C-NaTaO3 nanocrystalline is mainly due to their capability for reducing the electron hole pair recombination. The La-C-dopant is believed to play a key role in the enhancement of photocatalytic properties of La-CNaTaO3 crystalline. © 2014 BCREC UNDIP. All rights reserved


Introduction
Hydrogen has emerged as a potential energy carrier in various low greenhouse gas energy applications due to its renewability and environmentally friendly [1][2][3][4].Photocatalytic water splitting into hydrogen using solar energy, as one of the most promising ways to obtain hydrogen and has attracted great scientific interest [5][6].Much attention has been paid to find-ing ways to produce hydrogen from renewable energy sources such as the sun and wind [7].Hydrogen production from water by using semiconductors as photocatalysts provides a potential way to obtain hydrogen efficiently, due to its clean, low-cost and environmentally friendly production process by utilizing solar energy.
Sodium tantalum oxide has been proved to be a promising photocatalyst material for applications in hydrogen production.Doping rareearth or other metal oxides into the perovskite type alkali tantalates can increase their capability of trapping and transferring electron/hole pairs, which improves their photocatalytic activities [8][9].Husin et al. [10] observed that the water-splitting reaction of NaTaO3 could be improved by lanthanum doping, because the Ladoped NaTaO3 powders have a small particle size with high crystallinity.But this photocatalyst works only under UV-light irradiation.
Zhou et al. reported that Fe-doped NaTaO3 was red-shifted to the visible region, which potentially could be active for overall water splitting under visible light irradiation [11].Recently, Fu et al. synthesized N-doped NaTaO3 photocatalysts, which showed high photo activity for formaldehyde photo-degradation under visible-light irradiation [12].However, in their studies, they did not use this photocatalyst to split water.In semiconductor doping technology, co-doping can overcome some limitations of single ion doping, such as poor thermal stability and more recombination centres for electron-hole pairs.Thus, we attempt to dope carbon at La-NaTaO3 to modify its performance.To our knowledge, studies on carbon doping at La-doped NaTaO3 and its photocatalytic performance have not been reported so far.
In this work, a La-C co-doped NaTaO3 photocatalyst was synthesized by the sol-gel reaction method.The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).The hydrogen evolution was used to evaluate the photocatalytic properties of the photocatalyst.The effect of various carbon contain will be report in the future work.

Catalyst Preparation
La-C-doped NaTaO3 was synthesized by means of sol-gel procedure using ethanol as solvent system.All chemicals were analytical grade reagents and used without further purification.In a typical procedure, a TaCl5 was firstly dissolved in ethanol solution and then NaOH dissolved in deionized water.La(NO3)3.6H2Owas dissolved in deionized water and then added into the solution.The mixture was mixed with C12H22O11 solution for 2 h under magnetic stirring.Citric acid solution was employed as a chelating agent in the developed process.Under vigorous stirring, 50 ml of citric acid solution was slowly dropped into the above solution to produce sol solution.The pH was adjusted to 4 with NH3 solution.Then, a La-C-doped NaTaO3 compound was obtained by heating the mixture at constant temperature of 80 o C until a white gels formed.The obtained gel was dried in oven at 100 o C. The resulting powder precursor was sintered at 400 o C and continuous heating at 800 o C for 8 h under air flow.The sample was cooled to room temperature and underwent characterization.In this work, we also prepared the NaTaO3 sample without doping for comparison.

Catalyst Characterization
To investigate the morphology of the structure, a scanning electron microscope (SEM) images of the final nanosized of the La-C-NaTaO3 was recorded by a (SEM, Philips XL-30) apparatus.The transmission electron microscope (TEM) images of the nanosized NaTaO3 were recorded by a Philips/FEI Tecnai 20G2 S-Twin TEM apparatus.The samples were characterized by X-ray powder diffraction (XRD).The XRD measurements were performed on a XRD-7000 with Cu Kα radiation (l = 1.5418Å).The operation voltage and current were maintained at 40 kV and 40 mA, respectively.

Catalyst Testing for Photocatalytic Water Splitting
Photocatalytic hydrogen evolution reactions were carried out in an inner irradiation quartz reactor.Typically, 1 g of the catalyst was suspended in an aqueous solution of 400 ml (H2O and 10 vol.% of methanol).The suspension was degassed for 30 minute with high-purity argon prior to light irradiation in order to eliminate dissolved oxygen.The amount of H2 produced was measured by gas chromatography (Shimadzu 8A) equipped with a molecular sieve column and a TCD detector with Helium carrier gas.

XRD Measurements
The XRD patterns of the NaTaO3 without doping and La-C co-doped NaTaO3 are given in A similar phenomenon was also observed by Hu et al. [13] who synthesized NaTaO3 powder with monoclinic phase from the sol-gel methods.The powder X-ray diffraction patterns of La-C co-doped NaTaO3 shows all diffraction peaks can be readily assigned to a pure phase and no diffraction peaks from impurity phase were observed.
These behaviours confirm that the welldefined structure, high purity and good crystallinity were achieved by doping substitution.From the XRD pattern, we can see that the relative intensity of the peaks sample decrease after doping lanthanum and carbon as co-doped suggesting that the existence of doping can suppress the crystal growth of NaTaO3.
In this report, the average crystallite sizes of the catalysts was calculated from XRD diffraction peaks using Scherrer formula with the full-width at half plane (100) of NaTaO3 and La -C-NaTaO3 peaks.Data for the average crystallite sizes of both samples are listed in Table 1.
The crystallite size of the non-doped Na-TaO3 is demonstrated of 37.3 nm.The size of the samples is greatly improved after doping lanthanum and carbon (46.0 nm), where intensive diffraction peaks of La-C-NaTaO3 phase can be observed at 2θ of 22.799 o , as depicted in Figure 1b.The XRD was known to be crystallite size sensitive where the larger crystallite size within the samples would produce the narrower and stronger diffraction peaks [14].

SEM and TEM Images
Scanning electron microscopy (SEM) micrograph of the non-doped and La-C-doped Na-TaO3 photocatalysts are show in Figure 2 and 3.As demonstrated in Figure 2, the non-doped NaTaO3 grew into irregular shapes with the particle sizes of the powders were approximately 50-400 nm.
For La-C-NaTaO3 powders prepared under the same conditions, the doped photocatalysts are uniform, had a very regular shape, demonstrates the stabilizing effect of the La-C dopants on the samples.Since the substitution of the NaTaO3 lattice by dopant species could protect the NaTaO3 nanoparticles from agglomeration during calcinations [14].
The particle sizes of La-C-NaTaO3 powders were around 30-200 nm as depicted in Figure 3, which is much smaller than the pure NaTaO3 particles.
The regular shape and smaller particle sizes of doped NaTaO3 describing the enhancement in crystalline quality and stability of the samples.The inset of Figure 3 shows a magnification of monoclinic La-C-co-doped NaTaO3 photocatalyst.

Bulletin of Chemical Reaction Engineering & Catalysis, 9 (2), 2014, 83
Copyright © 2014, BCREC, ISSN 1978-2993  a Estimated by using the Scherrer equation The regular shape and smaller particle sizes of doped NaTaO3 describing the enhancement in crystalline quality and stability of the samples.The inset of Figure 3 shows a magnification of monoclinic La-C-co-doped NaTaO3 photocatalyst.
Figure 4 and 5 show the TEM morphologies of the La-C co-doped NaTaO3 samples obtained via sol-gel method.The La-C-NaTaO3 shows smaller particle sizes than either the undoped NaTaO3, indicating that dopant can prevent agglomeration.The results are also consistent with the XRD results.The La-C-NaTaO3 nanocrystal with high crystallinity is expected to have high photocatalytic activity.The picture shows a clear surface, suggesting the good crystal character of the as-synthesized La-C-NaTaO3.

Bulletin of Chemical Reaction Engineering & Catalysis, 9 (2), 2014, 84
Copyright © 2014, BCREC, ISSN 1978-2993 This crystal La-C-NaTaO3 has an advantage over the crystalline particles as photocatalyst because the smooth surface interfaces in crystal particles can effectively reduce the recombination probability of the photogenerated holes and electrons [15].

Photocatalytic Activity
The photocatalytic activity of La-C-NaTaO3 was evaluated by hydrogen evolution from aqueous methanol-solution in a closed reactor circulations system.In order to investigate the effect of doping on NaTaO3 precursor on the photocatalytic activity, the experiment was performed by comparing the non-doped and doped La-C-NaTaO3.
Figure 6 shows the photocatalytic activities   As can be seen in Figure 6, the photocatalytic of hydrogen production on non-doped NaTaO3 achieve of 66.7 (ml g -1 h -1 ).
The photocatalytic activity of La-C-NaTaO3 is increases remarkably (123.333ml g -1 h -1 or 1.85 times) after doping of lanthanum and carbon.It is clear that the La-C-doped NaTaO3 exhibited higher photocatalytic activities than that of pure NaTaO3.There are many important parameters taken into account to explain the obtained results.As can be seen from the XRD investigation (Figure 1, Table 1), that the La-C-NaTaO3 shows higher crystallinity than the non-doped sample.The reason is due to the fact that the high crystallinity of the samples could suppress recombination between photogenerated electron (e -) and hole (h + ) pairs, in order to perform the desired redox reactions, which may lead to high photocatalytic activity and stability.
The dopant makes a more uniform crystalline phase and helps to increase the crystal growth, prevent agglomeration, and smaller particle sizes of NaTaO3 photocatalyst [10], as depicted from SEM images.The particle sizes of La-C-NaTaO3 (Figure 3), was smaller than that of non-doped NaTaO3 (Figure 2).Generally speaking, higher crystallinity, stability, and smaller particle size can improve the photocatalytic activity of a La-C-NaTaO3 catalyst, associated with the distance electron-hole pairs must travel through the bulk of the catalyst particle to reach the active sites [16].This arises from the efficient separation of photogenerated carriers at the photocatalyst inter-faces and/or the promotion of catalytic performance.
This is attributed to the short distance from the bulk to surface, which is derived from small particle, so that photogenerated electrons and holes efficiently migrate onto the surface with less opportunity for recombination [17].In addition, in general catalysis, the surface area of sample plays significant role in determining the reaction activity due to the capability of adsorbing reactants on surface active sites for undertaking reaction [18].
The La-C-NaTaO3 was quite stable for repeated cycles of hydrogen production, as demonstrated in Figure 7, while the deactivation was started at the third run on the pure Na-TaO3 can be observed.The TEM images show the clear difference between La-C-NaTaO3 and the pure NaTaO3 catalysts.The La-C-NaTaO3 particles (see Figure 5), shows regular cubicshape morphology and a clear surface.Therefore, the electron transfer on La-C-NaTaO3 is well defined with enhanced and efficient charge separation.

Conclusions
Nanocrystalline La-C-NaTaO3 photocatalyst with smaller particle size was successful synthesized by a sol-gel technique.From XRD result, the La-C co-doped NaTaO3 sample should be respectively assigned to the monoclinic symmetry perovskite-type of NaTaO3.The SEM measurements give a particle size of pure and doped NaTaO3 are around 50-400 nm and 30-200 nm, respectively.The resulting La-C-NaTaO3 photocatalysts was systematically evaluated their performance via the photocatalytic H2 evolution in comparison with pure Na-TaO3.The La-C-NaTaO3 photocatalyst provided higher photocatalytic activities than that of pure NaTaO3.Dopant La-C makes the photocatalyst possess small particle sizes with a high crystallinity, which may lead to high photocatalytic activity and stability.Thus the La-C codoped NaTaO3 photocatalyst shows high activity of H2 evolution from aqueous methanol solution.

Figure 1 .
Figure 1.The patterns of NaTaO3 show a single phase with a narrow diffraction peak indicated a high crystallinity of the sample.The X-ray diffraction patterns of La-C-NaTaO3 powders (La: 1 mol %, C: 2 mol %) are shows in Figure 1b.It can be observed that the XRD analysis of the La-C-NaTaO3 and nondoped NaTaO3 have a similar crystal structure.Compare to the data documented in the powder diffraction files of the JCPDS, the La-C codoped NaTaO3 sample should be respectively assigned to the monoclinic symmetry perovskite-type of NaTaO3, in agreement with the JCPDS card no 74-2478 (P2/m with a = 3.8936 Å, b = 3.8905Å and c = 3.8936 Å).A similar phenomenon was also observed by Hu et al.[13] who synthesized NaTaO3 powder with monoclinic phase from the sol-gel methods.The powder X-ray diffraction patterns of La-C co-doped NaTaO3 shows all diffraction peaks can be readily assigned to a pure phase and no diffraction peaks from impurity phase were observed.These behaviours confirm that the welldefined structure, high purity and good crystallinity were achieved by doping substitution.From the XRD pattern, we can see that the relative intensity of the peaks sample decrease