Photocatalytic water splitting of surfactant-free fabricated high surface area NaTaO3 nanocrystals

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 2 7 3 9 e1 2 7 4 6

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Photocatalytic water splitting of surfactant-free fabricated high surface area NaTaO3 nanocrystals Wei Jiang, Xiuling Jiao, Dairong Chen* School of Chemistry & Chemical Engineering, National Engineering Research Center for Colloidal Materials, Shandong University, Jinan 250100, PR China

article info

abstract

Article history:

Uniform NaTaO3 ca. 15 nm and ca. 70 nm nanocrystals were synthesized via a novel

Received 6 April 2013

surfactant-free solvothermal reaction. The acidic alkoxide hydrolyzation leads to small

Received in revised form

size and high surface area particles. The different size of the nanocrystals was controlled

28 June 2013

by altering concentration. The alkoxide-based synthetic route leads to small particle size

Accepted 20 July 2013

and high surface area, which improved the charge separation and migration of photo-

Available online 15 August 2013

generated carriers and benefited the surface chemical reaction of catalysts. As a result, the high total yield of photocatalytic water splitting hydrogen generation was obtained, which was as high as 3.106 mmol h
1 g
1.

Keywords:

Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

NaTaO3

reserved.

Nanocrystals High surface area Photocatalysis Water splitting

1.

Introduction

Photocatalysis for water splitting has been considered as one of the most promising solutions for the increasingly severe energy crisis, but the catalytic efficiency of existing photocatalyst is still far from practical application [1]. In theory, photocatalytic water splitting includes three procedures. The catalyst particles adsorb incident light and generate electrons and holes. The charge separation of photogenerated carriers occurred inside the catalyst particles and migration of the electrons and holes to the particle surface. Finally, the surface photochemical redox reaction executed. Thus an ideal catalyst should provide several characteristics to meet these requirements. A proper band structure was considered as primary factor for a catalyst to respond the external incident

light and generate electrons and holes. Small particle size and high crystallinity were essential to promote the charge migration/separation and prevent the recombination of photogenerated carriers. Suitable morphology and high surface area were also contributed to the photocatalytic reaction by supplying abundant active points. Various semiconductor materials have been researched as water splitting photocatalysts [2e4]. Semiconductor titanates and tantalates were considered as outstanding candidates for their unique electron shell structure [5]. The conduction band of tantalates was composed of Ta 5d orbital, which was in lower energy level than Ti 3d orbital in titanates, then tantalates showed prior photocatalytic water splitting efficiency, theoretically [6]. Perovskite sodium tantalate (NaTaO3) is the most efficient water splitting catalyst reported, with the band

* Corresponding author. Tel./fax: þ86 531 88364281. E-mail address: [email protected] (D. Chen). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.07.072

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gap up to 4.01 eV [7,8]. To improve its photocatalytic performance, several methods were employed to adjust the band structure, the phase structure, the particle morphology and size of NaTaO3 [9e11]. Among these methods, doping metal or non-metal with different valence from the hosts to adjust its band structure is mostly popular, and the cocatalyst is usually applied [12,13]. Kato et al. introduced La into the solid-state synthesized NiO-loaded NaTaO3, the hydrogen production rate was raised up to 19.8 mmol h
1 g
1 [14]. Phase-controlled synthesis of NaTaO3 was also reported because the charge separation and migration were affected by crystal structure, and the monoclinic NaTaO3 theoretically shows prior photocatalytic properties than the orthorhombic one [15]. Surface morphology of the NaTaO3 particle was also modified to adjust its photocatalytic property by increasing the active points. It is well known that large surface area can effectively improve the catalytic activity of the catalyst because the catalytic reaction mainly occurs on the particle surface. Differ from the photodegradation of organic molecules, absorption was not existed as an influence factor in the photocatalytic water splitting. Thus the surface area of catalyst played a more important role in the water oxidation [16,17]. The photocatalytic property of tantalates was closely related to the quantity of their surface active point, then surface area should impact on the catalytic efficiency [18e20]. However, there are few reports on the synthesis of NaTaO3 with high surface area to the best our knowledge. Both traditional solid-state reaction and solegel synthesis require followed-up calcinations, which increased the difficulty of controlling the particle size and surface area [6,21]. Hydro/solvothermal method shows potentials in synthesizing inorganic crystals with high surface area and controlled morphology for its moderate conditions. Several groups reported the hydrothermal synthesis of NaTaO3 in strong alkaline conditions from Ta2O5 [22,23]. However, due to the slow hydrothermal reaction rate of alkaline with tantalum oxides, the amount of generated nuclei in these systems was relatively small, and thus particles with large size and small surface area were collected. Recently, Kondo et al. fabricated a colloidal array of NaTaO3 nanoparticles through the template method, which showed high surface area of 34 m2 g
1 and superior catalytic property on water splitting, but the synthesis process was complicated [24]. The preparation of NaTaO3 nanoparticles with high surface area by a simple route still remains great challenge. In this article, an alkoxide-based solvothermal route was employed to fabricate NaTaO3 nanocrystals. The rapid hydrolysis of alkoxide precursors simultaneously created massive nuclei under solvothermal condition, which favored the formation of nanocrystals with small size and large surface area. As a result, the product showed the surface area as high as 73.6 m2 g
1 and the yield of photocatalytic water splitting hydrogen generation reached 3.106 mmol h
1 g
1 without doping or cocatalyst.

2.

Materials and methods

All chemicals were analytical grade and used as received without further purification. In a typical synthesis, 0.179 g (0.5 mmol) of tantalum chloride (TaCl5, Alfa-Aesar) was

dissolved into 12 mL of ethanol (Shanghai Sinopharm Chemical Co.) under nitrogen protection, and then a tantalum alkoxides contained transparent solution was formed. Then sodium ethoxide (NaOEt, Shanghai Sinopharm Chemical Co.) was added at the mole ratio of Na:Ta ¼ 20:1. The yellow sticky suspension was formed under vigorous stirring, and then it was transferred to a 15 mL Teflon-lined autoclave, sealed and heated to 240  C for 4 h in an electric oven. The autoclave was cooled to room temperature naturally. The white product was collected by centrifugation and washed by ethanol and distilled water for several times. For the synthesis of ca. 70 nm nanocrystals, 0.358 g (1.0 mmol) of TaCl5 was employed, the NaOEt:TaCl5 ratio was retained as 20:1 and other conditions were unchanged. This procedure can be scaled up to the total volume of 50 mL. X-ray powder diffraction (XRD) was applied to identify the phase of the samples using Rigaku D/Max 2200 PC diffractometer with graphite-monoromatized Cu Ka radiation (l ¼ 0.15418 nm). The morphology of samples was observed by JEOL JEM-1400 transmission electron microscopy (TEM, accelerating voltage ¼ 120 kV), and JEM-2100 high-resolution transmission electron microscopy (HR-TEM, accelerating voltage ¼ 200 kV). The UVevis diffuse reflectance spectra were recorded using an Agilent Cary-100 ultraviolet and visible spectrophotometer with a 70 mm integrating sphere (Labsphere DRA-CA-30I), using analytical grade BaSO4 (Shanghai Sinopharm Chemical Co.) as reference. The infrared (IR) spectra of as-prepared samples and activated samples were collected on a Nicolet 5DX FT-IR instrument using a KBr pellet technique. Thermogravimetric (TG) analyses of the cubic nanocrystals were conducted at a heating rate of 20.0  C/min from room temperature to 800  C in an O2 flow of 50.0 mL/min (TA’s SDT Q600 thermal-analyzer). The photocatalytic water splitting was taken in an XPAG8 closed photocatalysis system (Nanjing Xujiang Inc.). All samples were activated under 250  C for 2 h, and the activation does not affect the size and morphology of sample particles. In a typical process, 0.10 g of catalyst powder sample was dispersed in 100 mL of distilled water under ultrasonic treatment. The system was evacuated and filled with catalyst suspension previously than the irradiation. A 500 W mercury lamp with cooling water was preheated for 20 min to stabilize the light wavelength, then directly inserted into the reactor. The gas sample was measured before exported into a gas chromatograph with TCD detector (Shandong Lunan Analytic Inc.).

3.

Results and discussion

In typical experiments, ca. 15 nm and ca. 70 nm NaTaO3 nanocrystals were synthesized via a simple solvothermal reaction under acidic condition. By simply changing the reagent concentration, NaTaO3 nanocrystals in different size can be obtained and this procedure can be scaled up to total volume of 50 mL. In the synthesis, excess amount of sodium ethoxide (NaOEt) was introduced to control the phase and morphology of the product, the mole ratio was determined at NaOEt:TaCl5 ¼ 20:1. For lower Na:Ta ratio, impurity peaks were found in the XRD patterns. Fig. 1 shows the XRD pattern,

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 2 7 3 9 e1 2 7 4 6

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Fig. 1 e a) XRD pattern, b) TEM image and c) HR-TEM image of ca. 15 nm NaTaO3 cubic nanocrystals.

TEM and HRTEM images of the product with the tantalum concentration of 0.083 mol$L
1. In Fig. 1a, all the reflections can be assigned to monoclinic NaTaO3 (JCPDS No.74-2479). The three sharp diffraction peaks at 40 , 57.8 and 67.9 further confirmed the monoclinic structure (Fig. 1a), rather than conventional orthorhombic phase. The calculated particle size for the nanocrystal products was 23.8 nm according to Scherrer formula, with the FWHM of 0.006 at 2q ¼ 32.495 . TEM and HR-TEM images of the samples were examined. The products represent the morphology of well dispersed uniform nanocubes (Fig. 1b). The average size of nanocubes was ca. 15 nm. The nanocubes showed difference in their contrast, but there was no sign of structural pores with their internal structure. The interplanar spacing was 0.388 nm, which is in accordance with the (100) and (001) lattice planes of monoclinic NaTaO3 (Fig. 1c). The parallel lattice fringes were in good order, but the difference in contrast implied the imprint of the self-assembly growth of the nanocubes [25,26]. The clear lattice fringes without distort or defections showed the high crystallinity of as-prepared NaTaO3 nanocrystals, which could effectively inhibit the recombination of photogenerated carriers. The reduced particle size of NaTaO3 nanocubes could decrease the migration distance of the photogenerated carriers and lower the possibility of recombination, benefits the photochemical reaction, theoretically. The surface area of NaTaO3 nanocubes was 56.7 m2 g
1, calculated by BrunauereEmmetteTeller (BET) method, which was close to the theoretical surface area of 42.1 m2 g
1 for their corresponding-sized solid cubes, and was much larger than those in previous reports. Time dependent experiments were conducted to specify the forming mechanism of NaTaO3 nanocubes. The XRD patterns of 0.5 h, 1 h, and 2 h products were shown in Fig. 2a. The 0.5 h product was amorphous matter after removing NaCl by ethanol (Fig. 2b). EDS analysis indicates the molar contents of Ta, Na, C and O in amorphous matter were respectively 29.13%, 16.45%, 9.77% and 44.65%. Combined with the FTIR and TG characterizations (Electronic Supplementary Material, Figs. S1eS4), it is concluded that the amorphous matter should be partly hydrolyzed complex alkoxide of Na and Ta, which implies that the formation of NaTaO3 nanocubes in the present system might go through the hydrolysis of the metal

alkoxide. The diffraction peaks of monoclinic NaTaO3 appeared at 1 h, the peak intensity increased over time, indicating the improvement of the crystallinity. Meanwhile, the wide bulge at 2q ¼ 20e30 no longer existed at 1 h, indicating the transformation of amorphous precursor to monoclinic NaTaO3 accomplished (Fig. 2a). Some nanocubes were found at 1 h, incorporated with the primary nanoparticles (Fig. 2c). The amount of nanocrystals increased over time with the decrease of primary nanoparticles simultaneously, thus it is concluded that the forming of NaTaO3 nanocrystals went through the self-assembly from primary nanoparticles under surfactant-free environment. With the continuous aggregation, crystal growth and orientation adjustment, the NaTaO3 nanocrystals became more regular in shape and larger in size during the reaction stage of 1e4 h (Fig. 2bed, Fig. 1b). For the synthesis of larger nanocrystals, the concentration of TaCl5 was increased to 0.167 mol L
1 with the NaOEt:TaCl5 ratio retained the same. The FWHM of reflection was 0.007 at 2q ¼ 32.469 , thus the average particle size was calculated as 20.4 nm (Fig. 3a). TEM image showed the product was the aggregated particles with the average particle size of ca. 70 nm. The morphology and texture of ca. 70 nm particles were similar with the ca. 15 nm nanocubes (Fig. 3b). Detailed observation of single 70 nm nanocrystal was represented in Fig. 3c and d. From the difference in contrast of central and edge part, the single nanocrystal was concluded tilt-laid. The HR-TEM image showed obvious signs of nanocrystals aggregation, the primary nanoparticles were in cubic shape and less than 15 nm size. The aggregation product showed mesocrystal-like architecture, which generated numerous pore structures. The parallel lattice fringes were continuous and clear, which confirmed the oriented-attached nature of the final product. The interplanar spacings of 0.388 nm and 0.275 nm represented the (100), (001) and (110), (011) crystal planes, respectively. Thus oriented-attachment was concluded to be occurred after the forming of self-assembled nanocrystals. Comparing to the ca. 15 nm nanocubes, the aggregation-based structure possessed large numbers of pores, which favored its application in catalyst and related fields. The surface of the ca. 70 nm nanocrystals showed step structure (denoted by lines in Fig. 3d), which probably introduced large numbers of escape points for hydrogen [27].

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Fig. 2 e a) XRD patterns, and TEM images of b) 0.5 h, c) 1 h, d) 2 h products.

The N2 adsorptionedesorption test gave the BET surface area of 73.6 m2 g
1 of the larger nanocrystals with the most probable BJH (BarretteJoinereHalenda) pore size distribution of 3.46 nm. The pore structure of the product was attributed to the oriented aggregation of the primary building blocks [28]. The high surface area of product promoted the photocatalytic water splitting because the reaction rate was directly correlated with surface active point quantity. The FTIR spectra and TG curves of NaTaO3 samples were presented in Fig. 4. The absorption bands at 3450 cm
1 and 1638 cm
1 were assigned to the vibration of surface hydroxyl and the hydroxyl of adsorbed water (Fig. 4a). The strong broad band at 800e530 cm
1 was ascribed to the stretching vibrations of TaeO bond in TaO6 octahedron. The weak absorption band at 1347 cm
1 (denoted by arrow in Fig. 4a) was possibly derived from the vibration of CeOeC bond of diethyl ether compounds, which was removed after the activation. According to our previous reports, the water molecules for hydrolyzation probably derived from the dehydration of ethanol molecules under solvothermal condition. The existence of ethers was a result of the dehydration of solvent molecules [29]. TG curves showed continuous weight loss from room temperature to 800  C (Fig. 4b). The weight loss at 20e150  C was due to the removal of adsorbed water. Combined with the FTIR analysis, the weight loss at 150e300  C

was assigned to the decomposition of alkoxy, and that at 300e600  C was mainly attributed to the removal of surface hydroxyl. It is concluded the surface species of activated NaTaO3 nanocrystals was mainly hydroxyl, and there was no other capping molecules affect the photochemical reaction. The total weight loss was 4.47% and 6.51% for ca. 15 nm and 70 nm nanocrystal samples, respectively. Due to its higher surface area, the total weight loss of ca. 70 nm nanocrystals was larger than that of smaller nanocubes. The formation of ca. 15 nm NaTaO3 nanocrystals was based on self-assembly of primary nanoparticles. The formation of primary nanoparticles followed the alkoxide hydrolysis mechanism, thus the equilibrium yield and stability of alkoxide in the corresponding solvent is an important factor that affects their crystallization. Differ from the solid-state reaction, solution-based synthesis is more favorable to obtain crystalline particles with small size and uniform distribution. Due to the difference in electronegativity, the acidic tantalum chloride combined with the alkaline sodium ethoxide to generate complex alkoxides and NaCl (Electronic Supplementary Material) [30,31]. For the formation of larger aggregates, higher reactants’ concentrations resulted in higher alkoxide concentration in the solution, then kinetically massive nuclei were spontaneously generated through hydrolysis and condensation under the solvothermal conditions.

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Fig. 3 e a) XRD pattern, b) TEM image, and HR-TEM image of c) one typical ca. 70 nm NaTaO3 nanocrystal, d) edge view of one nanocrystal.

Therefore, in accordance with the classical nucleation-growth theory, the size of primary nanoparticles collected in higher concentration was relatively smaller than that of lower concentration. In the followed self-assembly process, the smaller primary nanoparticles tend to occur secondary orientedattachment under the influence of their higher surface energy,

thus larger aggregation-based architectures were formed under higher concentration [32e35]. The band structure of as-prepared NaTaO3 nanocrystals was examined by powder UVevis diffuse reflectance spectroscopy (Fig. 5). The samples showed obvious absorption at ultraviolet region. The UV absorption edge was determined at

Fig. 4 e a) IR spectra of ca. 70 nm NaTaO3 nanocrystals, b) TG curves of ca. 15 nm (dash line) and ca. 70 nm (solid line) NaTaO3 nanocrystals.

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Table 1 e BET surface areas and gas evolution of NaTaO3 photocatalysts. Nanocrystal size ca. 15 nm ca. 70 nm

Fig. 5 e UVevis diffuse reflectance spectra of ca. 15 nm (dash line) and ca. 70 nm (solid line) NaTaO3 nanocrystals. The inset shows the calculated band gap curve of nanocrystals.

305 nm and 320 nm for ca. 15 nm and 70 nm nanocrystals, respectively. The (ahn)2-hn scheme was obtained based on the UVevis spectra (Fig. 5 inset), the intersections of x-axis and the tangent of the curve at the slope mutation point were determined, which gave the band gap of 4.10 eV and 4.14 eV for ca. 15 nm and ca. 70 nm nanocrystals, respectively (denoted by straight line in Fig. 5 inset). The band gap of ca. 70 nm nanocrystals was slightly larger than that of ca. 15 nm nanocubes, and both these values were larger than reported micrometer-sized products in the literature. The larger band gap of ca. 70 nm nanocrystals was attributed to their smaller primary building blocks [36,37]. The absorption intensity of ca. 70 nm nanocrystals was also higher than that of ca. 15 nm nanocubes, which could possibly increase the absorbed photon energy in the photocatalytic reaction. The water splitting activities of NaTaO3 nanocrystals were demonstrated as the total amount of generated gas in Fig. 6.

H2 evolution rate (mmol h
1 g
1)

O2 evolution rate (mmol h
1 g
1)

56.7 73.6

1.277 3.106

0.634 1.543

The total amount of H2 production in 8 h was 1.030 mmol for ca. 15 nm nanocubes and 2.507 mmol for ca. 70 nm nanocrystals, respectively. The amount of O2 generated was approximately half as that of H2. The calculated H2 and O2 generation rate and physical properties of NaTaO3 samples were summarized in Table 1. To better evaluate the photocatalysis efficiency, the gas generation rate was calculated to the unit of mmol h
1 g
1. In the water splitting kinetics, adsorption is not an important factor, but surface area does affect the active point quantity of photochemical reaction. Thus the hydrogen yield is determined by the band structure of the catalysts, and affected by particle size and surface state. The surface adsorbed organic molecules were removed in the activation treatment to prevent the capping of surface active point (Fig. 4a). It is reported the doping of La3þ could bring in step structures on the particle surface, which significantly benefits the photocatalysis by providing more escape point for hydrogen [13]. In our experiments, the as-prepared aggregated nanocrystals showed similar surface step structure without doping, combined with their higher surface area, leads to the superior photocatalytic property than nanocubes. The catalytic efficiency of solid-state synthesized pure NaTaO3 is very low without doping or loading cocatalyst [14,38]. Comparing to the previous reports, the as-prepared pure NaTaO3 nanocrystals showed significantly increased hydrogen yield. The high catalytic efficiency is closely related to the small size and high surface area of as-prepared nanocrystals. The reducing of particle size not only expanded the band gap, but also lowered the transportation distance of photogenerated carriers, and the high crystallinity effectively reduced the possibility of their recombination. The higher surface area and structural pore expanded the contact area in photochemical reaction, and significantly increased the number of active point.

4.

Fig. 6 e Photocatalytic water splitting for H2 and O2 generation, demonstrated as generated gas amount: a) ca. 15 nm (dash line) and b) ca. 70 nm (solid line) NaTaO3 nanocrystals.

Surface area (m2 g
1)

Conclusions

In conclusion, non-doped NaTaO3 cubic nanocrystals were synthesized in surfactant-free environment by an alkoxide hydrolyzation based rapid reaction. The uniform monoclinic product particles were obtained under mild solvothermal condition. The self-assembly of primary nanoparticles was controlled by concentration adjustment, larger nanocrystals with pores were obtained under higher concentration. The well-dispersed cubic nanocrystals were in small size and high crystallinity. The products showed surface step structure and high surface area with abundant active points for water splitting photoreaction. The small-sized products show expanded band gap, which enhanced the absorption of

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 8 ( 2 0 1 3 ) 1 2 7 3 9 e1 2 7 4 6

incident light. The ca. 15 nm and ca. 70 nm nanocrystal samples show the H2 generation rate of 1.277 and 3.106 mmol h
1 g
1, respectively. The superior photocatalytic property of pure NaTaO3 without doping or cocatalyst benefited from the small particle size, large surface area and active surface structure.

[14]

Acknowledgments

[15]

This work is supported by the National Natural Science Foundation of China (Grant 21271118), the Major State Basic Research Development Program of China (973 Program) (No. 2010CB933504), and the Natural Science Foundation of Shandong Province (No. ZR2011BZ002).

[13]

[16]

[17]

[18]

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2013.07.072.

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