Tantalum doped BaZrO3 for efficient photocatalytic

Catalysis Communications 28 (2012) 82–85

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Short Communication

Tantalum doped BaZrO3 for efficient photocatalytic hydrogen generation by water splitting Ziyauddin Khan, Mohammad Qureshi ⁎ Materials Science Laboratory, Department of Chemistry, Indian Institute of Technology Guwahati, Assam-781039, India

a r t i c l e

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Article history: Received 21 June 2012 Received in revised form 27 July 2012 Accepted 3 August 2012 Available online 14 August 2012 Keywords: BaZrO3 Photocatalytic activity Hydrogen generation Water splitting

a b s t r a c t Ta doped BaZrO3 photocatalyst with a cubic perovskite structure was synthesized using precipitation from homogenous solution method. After calcination, the obtained BaZr1 − xTaxO3 (x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05) samples were characterized by powder X-ray diffraction, UV–vis diffuse reflectance spectroscopy, surface area analyses and photocatalytic activity. As-prepared BaZr1 − xTaxO3 samples show efficient hydrogen generation by water splitting without addition of any co-catalyst under UV–vis light irradiation. The highest hydrogen production rate (180 μmol/h) was achieved by BaZr0.96Ta0.04O3 which could be attributed to extended absorption in visible region and surface area. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Numerous metal oxides have been recognized as active photocatalyst for water splitting after the breakthrough work by Honda et al. [1–4]. In essence, the process of water splitting by semiconductor metal oxide involves an electronic transition from the valence band (VB) to the conduction band (CB) with generation of electron–hole pairs. The separation and transfer of these photoinduced charge carriers take place followed by the redox reactions on the photocatalytic surface which generates hydrogen and oxygen from water. Thus, band engineering of any material is a vital and effective way to design novel photocatalyst which can be efficiently used for water splitting. In the last few decades several efforts have been made to modify the electronic structure via photo-sensitizer or cation/anion substitution [5–8]. Recently, perovskite oxides (ABO3) have been found to be an active material for H2 production due to their robust and tunable electronic structures due to scope of dopants it can take due to its structural diversity [9–13]. Perovskite metal oxide is an ideal host material for chemical substitution due to its liability for fractional substitution in both A-site and B-site [14–16]. Recently several perovskite structured based photocatalyst captivated the scientific community due to their electronic structure and liability for substitution [3,15–20]. For example, Kato et al. demonstrated that La doped NaTaO3 photocatalyst showed highly efficient catalytic activity for water splitting [3]. Kanhere et al. converted the UV driven NaTaO3

⁎ Corresponding author. Tel.: + 91 361 2582320; fax: + 91 361 2582349. E-mail address: [email protected] (M. Qureshi). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2012.08.002

into visible light driven photocatalyst by doping with Bi 3 + at both A-site and B-site [17]. Recently, it was found that barium zirconate (BaZrO3) is one of the most explored perovskite materials with ideal cubic structure, presenting wide range of technological applications such as thermal coating material for aerospace industries. Previously, Yuan et al. demonstrated that BaZrO3 is good photocatalyst for hydrogen generation by water splitting upon UV light illumination without using any co-catalyst [21]. High hydrogen production rate of BaZrO3 was ascribed to the high negative potential of photoinduced electrons and the large dispersion of its CB. Thereafter, the same group extended their work and elucidated the effect of Sn4 + substitution on photocatalytic activity BaZrO3 without co-catalyst [22]. Herein, we have synthesized BaZrO3 with Ta substitution (BaZr1 − xTaxO3) in an attempt to examine the effect of Ta substitution on photocatalytic water splitting under light irradiation. 2. Experimental section Barium chloride dihydrate (99.8% pure), zirconia oxychloride octahydrate (99.0% pure) and urea (99.5% pure) were purchased from Merck, India while tantalum ethoxide (99.98% pure) was purchased from Sigma Aldrich and all chemicals were used without further purification. 2.1. Photocatalyst synthesis BaZr1 − xTaxO3 photocatalyst with x = 0, 0.01, 0.02, 0.03, 0.04 and 0.05 was synthesized by the precipitation from homogenous solution method [23]. In a typical synthesis, calculated amount of ZrOCl2·8H2O and BaCl2·2H2O was dissolved in 25 mL water followed by the addition of Ta(OEt)5 with constant stirring. 30 g urea (in 25 mL water) solution

Z. Khan, M. Qureshi / Catalysis Communications 28 (2012) 82–85

03-0632) [21,22]. From diffractogram it is clear that diffraction peaks were successively shifted toward high angles (2θ) with increased Ta concentration. The shift in 2θ values is attributed to different ionic radii of Zr4 + (0.72 Å) and Ta5 + (0.64 Å) cation. The ionic size of Zr4 + is higher than that of Ta5 + in an octahedral environment thereby leading to a distortion in the crystalline lattice structure of BaZr1 − xTaxO3. On Ta substitution from x=0 to x=0.05, the tolerance factor (t) [24], of BaZr1 − xTaxO3 is still close to unity, indicating that the structure of these samples is cubic. The decreased lattice parameters for these cubic samples, along with unit cell volumes with the increasing addition of Ta are shown in Table S1.

(110) (211)

(200)

x = 0.05 (220)

(310)

(222)

Intensity (a.u.)

(111)

83

x=0

3.2. Materials morphology

20

30

40

50

60

70

80

2θ (degree) Fig. 1. Powder XRD patterns for BaZr1 − xTaxO3 photocatalyst from x = 0.00 to 0.05 after the calcination at 1000 °C.

was added to the reaction mixture and stirred the solution for 30 min to make homogenous solution. The solution was then slowly heated to 90 °C for 24 h under constant stirring to allow the urea decomposition. A white precipitate appeared slowly but in gradual manner. The suspension containing white solid phase was then centrifuged, washed several time with deionized water and dried at 100 °C for 12 h. The resulting powders were ground using an agate mortar and pestle then calcined at 1000 °C for 12 h to give BaZr1 − xTaxO3 products. 3. Result and discussion

The morphology of the calcined BaZr1 − xTaxO3 samples was observed by SEM (Fig. 2). Particles showed similar morphology with size ranging from 200 nm to 50 μm. From SEM it is found that aggregation and clumping are observed in the samples, which is in agreement with the previous observations in the same type of system [25]. EDX results showed that BaZr1 − xTaxO3 contained only Ba, Zr, Ta and O elements (Fig. S2). In the insets of Fig. S2(a)–(f), it should be noted that the doping levels in the prepared photocatalyst of BaZr1 − xTaxO3 were practically same having some deviation, with the variation of x. The representative transmission electron microscopic (TEM) images of BaZrO3 and BaZr0.96Ta0.04O3 are shown in Fig. 3a and b, respectively. TEM images suggest that BaZrO3 and BaZr0.96Ta0.04O3 are cubic in shape and sizes of particles are in range of 200 nm for both samples (Fig. 3a and b) [22]. High resolution TEM of BaZrO3 obtained from the (110) cubic plane confirms the formation of pure BaZrO3 with the lattice spacing of 2.99 Å (Fig. 3c). HR-TEM image of BaZr0.96Ta0.04O3 reveals that the lattice spacing between two fringes is 2.45 Å associated with (111) plane of the cubic phase (Fig. 3d). Inset of Fig. 3d shows selected area electron diffraction pattern (SAED) showing the cubic nature of BaZr0.96Ta0.04O3.

3.1. Crystal structure 3.3. UV–vis diffuse reflectance spectra The purity and crystal structure of BaZr1 − xTaxO3 were determined by powder X-ray diffractometer. The diffractogram patterns of all the samples revealed a typical cubic perovskite pattern (Fig. 1) and all peaks are indexable, belonging to diffraction peaks of BaZrO3 (JCPDS

a

1μ m

b

EHT = 10.00 kV

Mag = 2.5 KX

WD = 15 mm

SignalA = SE1

d

1μ m

The UV–vis diffuse reflectance spectra of BaZr1 − xTaxO3 photocatalysts are shown in Fig. 4. It is observed that the absorption edge is progressively shifted to longer wavelength with the increase of Ta concentration up

1μ m

c

EHT = 10.00 kV

Mag = 2.5 KX

WD = 15 mm

SignalA = SE1

e

EHT = 10.00 kV

Mag = 2.5 KX

WD = 15 mm

SignalA = SE1

1μ m

1μ m

EHT = 10.00 kV

Mag = 2.5 KX

WD = 15 mm

SignalA = SE1

EHT = 10.00 kV

Mag = 2.5 KX

WD = 15 mm

SignalA = SE1

f

EHT = 10.00 kV

Mag = 2.5 KX

WD = 15 mm

SignalA = SE1

1μ m

Fig. 2. SEM images of calcined BaZr1 − xTaxO3 at 1000 °C for 12 h: (a) x = 0, (b) x = 0.01, (c) x = 0.02, (d) x = 0.03, (e) x = 0.04, (f) x = 0.05.

84


a

b

c

d (111)

(110)

(110) d = 2.99 Å BaZrO 3

(111) d = 2.45 Å BaZr 0.96Ta0.04O3

Fig. 3. TEM micrographs of (a) BaZrO3 and (b) BaZr0.96Ta0.04O3; HR-TEM of (c) BaZrO3 and (d) BaZr0.96Ta0.04O3. Inset of (d) represents SAED pattern of BaZr0.96Ta0.04O3.

to 4% indicating the red shift in the optical absorption caused by the Ta doping in BaZrO3 lattice. Please note that for 4% Ta doped BaZrO3 has extended absorption in visible region in comparison to all other doped samples. From the literature, the CB of BaZrO3 [16,22] is mainly composed of Zr 4d orbital, while after the addition of Ta the CB is made up Ta 5d. On Ta substitution decrease in the band gap is observed with increasing Ta content for all the doped samples except BaZr0.95Ta0.05O3. The decrease in the band gap of BaZr1 − xTaxO3 (x=0.0 to 0.04) with increase in Ta concentration could possibly be attributed to the sp–d exchange interactions [26,27]. Reduction in the band gap increases the number of photon absorbed [21], which enhances the charge carrier density in the CB of BaZr1 − xTaxO3. The increased band gap of BaZr0.95Ta0.05O3 could be explained by the Burstein–Moss (B–M) shift because of that

1.0

BaZrO3 BaZr0.99Ta0.01O3 BaZr0.98Ta0.02O3

Absorbance

0.8

BaZr0.97Ta0.03O3 BaZr0.96Ta0.04O3

0.6

BaZr0.95Ta0.05O3

0.4

the absorption edge shifts toward higher energy with an increase of carrier density [28–30]. When a semiconductor material is doped, Fermi level lies just below the conduction band edge and above the donor states. With increase in the dopant concentration, more donor states are produced which shifts the Fermi level to a higher value in energy, causes increase in the band gap of such semiconductor material. This Fermi level shift is known as Burstein–Moss shift. 3.4. Photocatalytic hydrogen production The hydrogen and oxygen generation ability of BaZr1 − xTaxO3 samples was determined by performing photocatalytic water splitting experiment. The comparative hydrogen and oxygen evaluation rate along with surface area of BaZr1 − xTaxO3 is shown in Table 1. Fig. 5 represents the effect of Ta concentration on the photocatalytic hydrogen generation abilities of BaZrO3 under UV–vis light irradiation, wherein the surface area of each BaZr1 − xTaxO3 sample is also given. With increase in the Ta doping up to 4%, there is a regular increase in the hydrogen production in accordance with the bandgap movement and beyond 4% there is a change in bandgap movement according to the B–M shift, contributing to the decrease in the hydrogen Table 1 H2 and O2 evaluation rates of BaZr1 − xTaxO3 (x = 0 to 0.05) along with surface area.

0.2

0.0 250

300

350

400

450

500

550

Wavelength (nm) Fig. 4. UV–vis diffuse reflectance spectra for BaZr1 − xTaxO3 samples.

Photocatalyst

Surface area (m2/g)

H2 evaluation (μmol/h)

O2 evaluation (μmol/h)

BaZrO3 BaZr0.99Ta0.01O3 BaZr0.98Ta0.02O3 BaZr0.97Ta0.03O3 BaZr0.96Ta0.04O3 BaZr0.95Ta0.05O3

18 21 30 32 36 25

65 80 128 152 180 116

33 40 64 76 90 58


85

200 180 180

160 140

152 128

120 100

116

80

80

65

60 40 18

20

21

30

32

36

0

25

BaZrO3

BaZr0.99Ta0.01O3

BaZr0.98Ta0.02O3

Hydrogen production (μmol/h/0.2 g)

BaZr0.97Ta0.03O3

BaZr0.96Ta0.04O3

Surface Area (m2/g)

BaZr0.95Ta0.05O3

Fig. 5. Rate of hydrogen evolution along with the surface area of BaZr1 − xTaxO3 samples under UV–vis light irradiation.

production rate. The observed phenomenon is similar to that of TiO2 and other metal oxide as demonstrated earlier [22,31,32]. From Fig. 5, the rate of hydrogen evolution noticeably increased with an increase in Ta concentration and highest gas evolution rate (180 μmol/h) was observed on the sample with Ta concentration x = 0.04. The production of hydrogen gas was decreased with the Ta concentration at x = 0.05. The possible reason for these results is discussed below. The photocatalytic activity of a material depends on the number of photon absorbed by the catalyst. From UV–vis diffused reflectance spectra (Fig. 4) it is clear that with increase of x value up to x = 0.04 in BaZr1 − xTaxO3, the amount of absorbed photons should increase due to suitable band gap in-terms of the source excitation. In addition to this, increase in surface area of the photocatalyst also contributes to the enhanced photocatalytic activity [33–35]. From Fig. 5, the surface area value of BaZr1 − xTaxO3 photocatalysts is improving with the increase of Ta concentration up to x = 0.04, while decreased in surface area was observed at higher Ta concentration. 4. Conclusions We have demonstrated the synthesis of Ta doped BaZrO3 by precipitation from homogenous solution method. After the calcination, the BaZr1 − xTaxO3 samples were characterized by various analytical methods which act as a highly efficient photocatalyst for hydrogen generation under UV–vis light irradiation without addition of any cocatalyst. High hydrogen production rate of BaZr1 − xTaxO3 is attributed to reduce the band gap, including the high surface area. Present study can be extended for the doping of various metal ions to cubic perovskite materials which can be used as an efficient photocatalyst for water splitting. Acknowledgment The research was supported by the Department of Science and Technology (DST), India (SR/S1/IC-25/2009) and Department of Atomic Energy, BRNS (2010/37P/11/BRNS). We thank Dr. M. De and Mr. Vinoth for providing photocatalytic experimentation facility. Assistance from CIF, IIT Guwahati is also acknowledged. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.catcom.2012.08.002.

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