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Synthesis, photophysical properties, and photocatalytic applications of Bi doped NaTaO3 and Bi doped Na2Ta2O6 nanoparticles
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Synthesis, photophysical properties, and photocatalytic applications of Bi doped NaTaO3 and Bi doped Na2Ta2O6 nanoparticles Pushkar Kanhere, Yuxin Tang, Jianwei Zheng, Zhong Chen
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Received date: 19 October 2012 Revised date: 20 May 2013 Accepted date: 25 June 2013 Cite this article as: Pushkar Kanhere, Yuxin Tang, Jianwei Zheng, Zhong Chen, Synthesis, photophysical properties, and photocatalytic applications of Bi doped NaTaO3 and Bi doped Na2Ta2O6 nanoparticles, Journal of Physics and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2013.06.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis, photophysical properties, and photocatalytic applications of Bi doped NaTaO3 and Bi doped Na2Ta2O6 nanoparticles
Authors: Pushkar Kanhere1, Yuxin Tang1, Jianwei Zheng2, and Zhong Chen1* Affiliations: 1School of Materials Science and Engineering, Nanyang Technological University, Block N4.1, 50 Nanyang Avenue, Singapore 639798 2
Institute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632
[email protected] (PUSHKAR D. KANHERE) [email protected] (YUXIN TANG)
[email protected] (JIANWEI ZHENG) [email protected] (ZHONG CHEN) *
CorrespondingAuthor:ZhongChen
Email:[email protected] Telephone:+6567904256 Fax:+6567909081
Abstract
Phase formation and photophysical properties of bismuth doped sodium tantalum oxide (Perovskite, defect Pyrochlore) nanoparticles, prepared by hydrothermal method were studied in detail. It was revealed that the synthesis conditions like NaOH concentration and bismuth precursor (NaBiO32H2O) markedly affect the crystal structure of sodium tantalum oxide. At lower NaOH concentration and higher bismuth precursor (NaBiO32H2O) content, Bi doped Na2Ta2O6 (defect pyrochlore) phase was predominantly formed, while at higher NaOH concentration, Bi doped NaTaO3 (perovskite) phase was formed. It was observed that the defect pyrochlore (Bi doped Na2Ta2O6) phase was formed and stabilized by the presence of dopant precursor (NaBiO32H2O). The chemical analysis of the samples confirmed the doping of Bi3+ cations in both phases. Doping of bismuth enabled visible light absorption up to 500 nm in perovskite and defect pyrochlore type sodium tantalum oxide. Bi doped NaTaO3 samples showed excellent performance for the photocatalytic degradation of Rhodamine B than that of Bi doped Na2Ta2O6, under visible light irritation ( > 420 nm). The present results shed light on phase formation of sodium tantalate and these results are useful in understanding properties of NaTaO3 based compounds, synthesized by hydrothermal method.
Keywords: Na2Ta2O6, sodium tantalum oxide, Bi doped NaTaO3, visible light photocatalyst
Introduction
In the recent years, alkali tantalates (ATaO3 A=Na, K) have gained significant attention due to their superior photocatalytic properties [1, 2]. Pristine NaTaO3 has shown highly efficient splitting of pure water under Ultra-Violet (UV) radiation [3, 4]. The photocatalytic activity of this phase has been further improved by doping of La ions and by loading of NiO co-catalyst, leading to a dramatic increment in the efficiency of the water splitting reaction. [5-7]. Due to its attractive photophysical properties, NaTaO3 it is considered as a good host material to develop visible light photoctalysts. The visible light response in NaTaO3 is achieved by doping of certain ions such as Bi, N, Co, and Cr [8-13]. Among these dopants, bismuth doping in NaTaO3 shows markedly enhanced photocatalytic hydrogen evolution under the visible radiation [11, 14]. Bi doped NaTaO3 photocatalayst, produced by hydrothermal synthesis has shown promising results for utilizing solar radiation and thus detailed investigations on its photophysical properties is necessary[11]. The studies on the correlation between synthesis conditions and photophysical properties of Bi doped NaTaO3 nanoparticles in the hydrothermal synthesis are not reported till date. Such studies are useful for understanding and optimizing the properties of this system for efficient photocatalytic applications. In this work, the effect of synthesis conditions on the phase and the photocatalytic activities of Bi doped NaTaO3 powders are studied in detail. It is shown that synthesis conditions like NaOH concentration and NaBiO32H2O affect the phases of the final products. At lower NaOH concentration and higher precursor (NaBiO32H2O) content, Bi doped Na2Ta2O6 (defect pyrochlore) phase is predominantly formed while at the higher concentration of NaOH, Bi doped NaTaO3 (Perovskite) phase is formed. Among these two
phases, Bi doped NaTaO3 nanoparticles showed excellent performance for the photocatalytic degradation of Rhodamine B under visible light radiation ( > 420 nm).
2 Materials and Methods: In a typical synthesis, 0.441g of Ta2O5 (Alpha Aaser) was mixed with 50 mL of NaOH solution with concentration ranging from 0.1 to 6M. Appropriate amount of sodium bismuth oxide (NaBiO32H2O, Sigma Aldrich) (0 to 7.5% by moles) was added to the mixture as bismuth precursor. The mixture was stirred for 2 hours and transferred to a Teflon cup (120 ml) with steel casing. This autoclave was heated at 240º C for 24 hours in an oven. The products so formed, were thoroughly washed with de-ionized water and dried below 100 ºC. The chemical bonding of bismuth in NaBiO32H2O and Bi doped NaTaO3 was studied by XPS analysis. The XPS spectra were recorded using XPS machine and the fitting of XPS data was done using the software CASA. The reference energy of 284.5 eV was chosen as calibration standard. Crystal structure of the samples was studied by XRD patterns recorded using Bruker D8 Advance; Cu K=1.54 Å. The samples were scanned within 2 range of 10° to 120° with the steps of 0.01° at a scan rate of 1 second per step. The optical absorption properties of the powders were studied by Diffused Reflectance Spectroscopy (DRS). The morphology of the nanoparticles was studied by field emission scanning electron microscope (JEOL 6400F). The surface area of the samples was estimated by BET technique using Micromeritics ASAP 2020. The photocatalytic performance of the doped powders was tested by degradation of Rhodamine B under visible light. In a typical experiment, 50 mg of catalyst was suspended in 50 mL of 10 ppm solution of Rhodamine B. The suspension was kept in the dark for 30 min to achieve adsorption equilibrium. The suspension was then irradiated with a 300 W Xe lamp (Asahi Spectra Inc). The UV component and IR component in the spectra were removed by inserting a
420 nm cut-off filter and a super-cold (Asahi Spectra Inc.) filter, respectively. The experiments were also performed without catalyst to confirm that the degradation occurred by the photocatalytic process.
3 Results and Discussion 3.1 Formation of Bi doped NaTaO3 Figure 1 shows XRD patterns of the samples prepared with NaOH concentration of 6.0 M along with the standard pattern of NaTaO3. All the XRD patterns match well with the reported pattern of NaTaO3 (Space Group No. 62; Pbnm; JCPDS card# 025-0863). In addition to NaTaO3, a small amount of un-reacted Ta2O5 impurity was observed. The impurity peaks have low intensity and thus are not seen in the present XRD patterns in figure 1. However, no other phase was detected in the XRD patterns. It is known that high concentration of NaOH results in highly crystalline nanoparticles of NaTaO3 therefore, such high concentration was selected in the present experiments [2]. Figure 2 shows the variation in the unit cell volume of Bi doped NaTaO3 samples along with Bi content. It was seen that the unit cell volume increased with increment in bismuth content up to 7.5% Bi, and for 10% doping it reached a saturation value. Systematic increment in the unit cell volume and impurity free XRD indicate that Bi ions were accommodated in the lattice, successfully. The increments in the unit cell volume are consistent with the ionic radii of Bi3+ (1.02 Å) and Ta5+ (0.64Å). Alternatively, accommodation of Bi5+ (0.76 Å) at Ta5+ (0.64 Å) could also be possible [15]. Further, the Vegard’s law was followed up to 7.5% Bi doping, indicating that Bi ions were uniformly doped in the lattice. It was confirmed that the XRD pattern of the precursor of bismuth belonged to the hydrated phase of NaBiO3·2H2O and no peaks belonging to NaBiO2 or Bi2O3 were present.
3.2 Formation of Bi doped Na2Ta2O6 In order to understand the effects of NaOH concentration on the crystal structure of the final products, the doped samples were prepared with varying NaOH concentrations. Figure 3 shows the XRD patterns of 7.5% Bi doped samples, prepared with NaOH concentration ranging from 0.1 to 6.0 M. Additionally, standard XRD pattern of Na2Ta2O6 is also shown in figure 3. For higher concentrations of NaOH, i.e. 6.0 M, NaTaO3 and traces of un-reacted Ta2O5 were seen in XRD. As NaOH concentrations decreased below 0.5 M, a defect pyrochlore structure, Na2Ta2O6 (cubic; a=10.526 Å; JCPDS card No. 70-1155) was formed along with NaTaO3 and Ta2O5. It was observed that the amount of Na2Ta2O6 phase increased with a decrease in NaOH concentration. Na2Ta2O6 phase was also formed in absence of NaOH, i.e. when Ta2O5 and NaBiO32H2O were used as starting materials. The quantitative analysis of the phases was carried out by the Rietveld refinement of XRD patterns. The composition of the phases in the final products, along with changes in NaOH concentration is summarized in Table 1. It was further observed that the Na2Ta2O6 phase was formed only when dopant precursor, NaBiO32H2O was present in the starting materials. In absence of NaBiO32H2O, at low NaOH concentration (0.1 M) only Ta2O5 and traces of NaTaO3 were detected in XRD patterns. Therefore, the effect of precursor (NaBiO32H2O) content on the phase composition was studied at low (0.1 M) NaOH concentration. It was seen that the amount of Na2Ta2O6 increased with an increase in NaBiO32H2O content, reaching a saturation level at 7.5% precursor content. The composition of the phases in the final products along with changes in NaBiO32H2O content is summarized in Table 2. Results in Table 1 and 2 show that at lower NaOH concentration and
higher NaBiO32H2O content, the growth of defect pyrochlore phase (Na2Ta2O6) was promoted and the growth of perovskite phase (NaTaO3) was suppressed. The synthesis and photophysical properties of Na2Ta2O6 are seldom reported in the literature, as this is the metastable phase [7]. The crystal structures of these phases differ significantly in terms of their packing factors and lattice parameters (Figure 4). NaTaO3 (Space group No. 62, Pcmn, Pbnm) has a close packed orthorhombic structure, while Na2Ta2O6 (Space group No. 227, Fd-3m) has a cubic lattice with hexagonal interstitial framework [16].
Studies on hydrothermal synthesis of NaTaO3 suggest that Na2Ta2O6 could be formed as an intermediate product and it could be further transformed into NaTaO3 [17]. Thus the pyrochlore structure is considered as a meta-stable phase. In the present experiments, this metastable phase is stabilized and its growth is promoted by bismuth precursor, NaBiO3·2H2O. The crystal structure of NaBiO3·2H2O is a layered type structure, with corner connected BiO6 octahedra framework. It was seen that in absence of NaBiO32H2O, the defect pyrochlore phase was not formed. Thus it is proposed that the nucleation of Na2Ta2O6 phase has occurred from NaBiO3·2H2O seeds. The layered structure could have been responsible in preventing the formation of close packed structure (NaTaO3) and promoting the growth of structure with lower packing factor. The phase analysis of the samples prepared under various conditions shows that the crystal structure and composition of the final products are highly sensitive to the concentration of NaOH as well as amount of bismuth precursor in the initial reactants.
3.3 Microstructure and optical properties Figure 5a shows the FESEM image of pristine NaTaO3 synthesized using 6.0 of NaOH. Figure 5b show and 5c show FESEM images of 7.5% Bi doped NaTaO3 and Bi doped Na2Ta2O6 (predominant phase), respectively. Both the pristine NaTaO3 and Bi doped NaTaO3 showed cubic shaped particles. On the other hand, the particles shapes are deviated from cubic to irregular shapes in the case of Bi doped Na2Ta2O6. As compared to the perovskite powders, the defect pyrochlore phase (Na2Ta2O6) showed agglomerated growth and irregular shapes, indicating reduced crystallinity. Pristine NaTaO3 showed absorption in the UV region with a band gap value of 4.01 eV. All the bismuth doped samples showed extension of absorption spectra to the visible region (Figure 6a-6b). Further, the absorption in the visible region increased with increase in the bismuth content, indicating bismuth doping caused the band gap narrowing of tantalate phases. For a doping concentration of 7.5% bismuth, the absorption spectra extended up to 500 nm in both the phases. The present DRS spectra suggest that Bi doping enables visible light absorption in NaTaO3 as well as Na2Ta2O6. Earlier work on Bi doped NaTaO3 suggests that occupancy of Bi ions at Ta site would induce Bi 6s induced mid-gap energy states that are responsible for visible light absorption [12]. Therefore, in the present samples, doping of Bi at Ta site in NaTaO3 and Na2Ta2O6 is considered as a main reason for the visible light absorption.
3.4 Chemical analysis Considering the values of ionic radii and electronegativity of bismuth and tantalum, bismuth could be doped in NaTaO3 lattice in Bi3+ or Bi5+ chemical state. The oxidation state of bismuth could have a marked effect on the photocatalytic properties of doped NaTaO3. Therefore, in order to study the oxidation state of bismuth, the X-ray photoemission spectra (XPS) of the samples were investigated. Figure 7a-c shows XPS spectra of Bi 4f peaks in Bi doped NaTaO3 samples with increasing bismuth content. All the doped samples showed presence of bismuth ions and Bi 4f peaks located at around 158.5 eV and 163.7 eV, were assigned to Bi3+ ions [18]. Similarly, the XPS spectra of Bi doped Na2Ta2O6 showed presence of Bi3+ cations. Bi 4f peaks in doped samples showed lower energy components, when fitting of the peaks was carried out. The two peaks correspond to different binding energies of Bi 4f. The differences in the binding energy values are attributed to the different chemical environment or surface vs. bulk location of the Bi ions. The XPS spectra of 4f peaks of NaBiO32H2O showed similar binding energy values (158.71 and 163.91 eV) as compared to those of Bi doped NaTaO3 samples (Figure 7d). This result is similar to the previously reported studies, where binding energy values of Bi5+ cations were close to those of Bi3+ cations [19]. The reported values of the binding energies of Bi 4f peak of Bi5+ cations range between 158.8 to 159.4 eV (or 164.0 to 164.6 eV) [18, 20]. Therefore the presence of Bi5+ ions could not be conclusively confirmed from the present XPS analysis. However, it is known that NaBiO32H2O is a metastable compound and dissociates when exposed to moisture and heat. NaBiO32H2O undergoes phase transition to compounds with lower oxidation states when heated above 200ͼC [21, 22]. In the present experiments, the heat treatment of the hydrothermal vessel was done at 250ºC in aqueous medium. This temperature is sufficient to transform NaBiO32H2O into compounds with lower oxidation state such as Bi2O3
or Bi2O4. Thus it is reasonable to conclude that the final products mainly contain Bi3+ cations. The quantitative analysis XPS spectra of the samples show that the Bi content in samples is close to the starting stoichiometry (Table 3). The quantitative analysis of the XPS data shows that Bismuth content in the final products is close to the initial stoichiometry. A small amount of discrepancy in the bismuth content could be attributed to the surface sensitivity of the XPS technique. It is further noted that the, atomic fraction of Na, Ta, and O are close to the stoichiometry of 1:1:3 in the doped NaTaO3 samples.
3.5 Photocatalytic Activity: The photocatalytic activity of the samples was tested by studying degradation of Rhodhamine B (Rh B) under visible light radiation ( > 420 nm). Figure 8 shows the reduction in concentration of Rh B when different photocatalyst powders were used. It was seen that the concentration change under the dark conditions (30 mins) was negligible, indicating that the adsorption of the dye on the surface was insignificant. The pristine NaTaO3 sample did not show any significant photocatalytic activity as it did not absorb any visible light. Bi doped Na2Ta2O6 (predominant defect pyrochlore) phase showed photocatalytic degradation of Rh B, degrading around 40% of the dye in four hours. On the other hand, Bi doped NaTaO3 showed nearly complete degradation of the dye in four hours, under similar conditions. The best performance was shown by 7.5% Bi doped NaTaO3 sample, which could be attributed to higher visible light absorption, as seen in DRS spectra (Figure 6a). Further, we have compared the surface area values of the two different types of powders. The surface area of the powders with 2.5, 5.0, 7.5% Bi doped NaTaO3 were found to be 2.15, 2.26 and 2.21 m2/g. While the surface area of the 2.5, 5.0, and 7.5% Bi doped Na2Ta2O6 (major phase) were estimated to be 1.95, 2.42, 2.20 m2/g. The surface area values of Perovskite and Pyrochlore phase did not show any
significant difference. Thus the contribution of surface area to the photocatalytic activity is considered to be the same. The lower performance of Na2Ta2O6 phase may be due to its poor crystallinity. Further, it is known that pristine NaTaO3 shows better photocatalytic performance for water splitting reaction than Na2Ta2O6 [23]. Therefore, Bi doped perovskite samples showed higher photocatalytic activity than corresponding pyrochlore phases.
4 Conclusions In summary, this work studies the effects of synthesis conditions on the phase and optical and photocatalytic properties of sodium tantalum oxide nanoparticles doped with bismuth. Further, it sheds light on the precursor induced formation of defect pyrochlore type sodium tantalate. The main conclusions drawn from this study are listed as follows. 1. Bi3+ doped NaTaO3 (orthorhombic) and Bi3+ doped Na2Ta2O6 (cubic) nanoparticles were successfully synthesized by using NaBiO32H2O as bismuth precursor in the hydrothermal synthesis. 2. The crystal structure of the final products is sensitive to the concentration of the dopant precursor (NaBiO32H2O) as well as concentration of NaOH. At lower NaOH concentration (< 0.5 M) and higher NaBiO32H2O content, the formation of defect Pyrochlore (Na2Ta2O6) is highly favorable while at higher NaOH concentration and lower NaBiO3 content, perovskite type NaTaO3 is highly favorable. 3. Bismuth doping in both phases i.e. NaTaO3 and Na2Ta2O6 induces visible light absorption up to 500 nm and these compounds show visible light driven photocatalytic degradation of
Rhodamine B. The perovskite phase (Bi doped NaTaO3) has a better photocatalytic performance than that of the defect pyrochlore phase (Bi doped Na2Ta2O6).
List of Tables
Table 1 The phase composition of the final products with 7.5% Bi doping and varying NaOH concentration Table 2 The phase composition of the final products prepared with 0.1M NaOH concentration and varying NaBiO3·2H2O content Table 3 Quantitative analysis of XPS spectra showing percentage atomic content in Bi doped NaTaO3.
List of Figures
Figure 1 X-ray diffraction patterns of pristine and Bi doped NaTaO3 prepared with 6.0 M of NaOH, along with the reference pattern of NaTaO3. Figure 2 The variation of the unit cell volume along with bismuth content Figure 3 XRD patterns of 7.5% Bi doped NaTaO3 powders with varying concentration of NaOH, along with the reference pattern of Na2Ta2O6. Figure 4 Crystal structures of sodium tantalum oxide, (a) NaTaO3 Perovskite (b) Na2Ta2O6 defect Pyrochlore Figure 5 FESEM images of (a) Pristine NaTaO3, (b) 7.5% Bi doped NaTaO3 (6.0 M NaOH), and (c) 7.5% Bi doped Na2Ta2O6 (0.1 M) Figure 6a Diffused reflectance spectra of Bi doped NaTaO3 samples prepared with 6.0 M NaOH and varying bismuth content Figure 6b Diffused reflectance spectra of doped samples prepared with 0.1M NaOH and varying bismuth content (Na2Ta2O6 as predominant phase) Figure 7 XPS spectra of Bi 4f peaks in Bi doped NaTaO3 with bismuth content of (a) 2.5%, (b) 5.0%, (c) 7.5% and XPS spectra of Bi 4f peaks in (d) NaBiO32H2O Figure 8 Photocatalytic degradation of Rhodamine B over pristine NaTaO3 and Bi doped NaTaO3 and Bi doped Na2Ta2O6 (predominant phases) under visible light radiation ( > 420 nm Iavg = 150 mW/cm2); ST is perovskite; PR is defect pyrochlore.
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Table 1 The phase composition of the final products with 7.5% Bi doping and varying NaOH concentration NaOH (M) for 7.5%
Na2Ta2O6 (%)
NaTaO3 (%)
Ta2O5 (%)
6.0
0.00
94.39
5.61
2.0
11.85
80.30
7.85
0.5
45.58
49.51
4.91
0.1
81.59
1.44
16.97
0.0
77.80
1.06
21.14
Bi doping
Table 2 The phase composition of the final products prepared with 0.1M NaOH concentration and varying NaBiO3·2H2O content NaBiO32H2O content
Na2Ta2O6 (%)
NaTaO3 (%)
Ta2O5 (%)
0.0
0.00
93.60
6.30
2.5
70.17
18.13
11.70
5.0
89.25
5.56
5.19
7.5
83.11
1.19
15.70
(%) for 0.1 M NaOH
Table 3: Quantitative analysis of XPS results showing percentage atomic content in Bi doped NaTaO3.
Bi content/ Atomic Fraction
2.5
5.0
7.5
10.0
Na
25.99
19.76
16.12
18.59
Ta
17.17
14.29
17.50
20.71
O
56.02
62.79
59.78
53.19
Bi
0.82
3.16
6.60
07.51
Highlighhts
x
N 2Ta2O6 naanoparticles with w visible light absorpption Bi doped NaTaO3 andd Bi doped Na
x
The crysttal structure of the finall products (N NaTaO3 or Na N 2Ta2O6) iss sensitive to t the concentraation of the dopant preccursor (NaB BiO32H2O) as well as concentratioon of NaOH
x
NaBiO32 2H2O promootes the grow wth of defectt Pyrochloree (Na2Ta2O6) at lower NaOH N concentraation and supppresses the growth of Perovskite (N NaTaO3)
x
Visible lig ght driven phhotocatalyticc degradatioon of Rhodam mine B over Bi doped tantalates
Graphiical Abstrract
Figure(s)
Figure 1 X-ray diffraction patterns of pristine and Bi doped NaTaO3 prepared with 6.0 M of NaOH, along with the reference pattern of NaTaO3.
Figure 2 The variation of the unit cell volume along with bismuth content
Figure 3 XRD patterns of 7.5% Bi doped NaTaO3 powders with varying concentration of NaOH, along with the reference pattern of Na2Ta2O6.
Figure 4 Crystal structures of sodium tantalum oxide, (a) NaTaO3 Perovskite (b) Na2Ta2O6 defect Pyrochlore
Figure 5 FESEM images of (a) Pristine NaTaO3, (b) 7.5% Bi doped NaTaO3 (6.0 M NaOH), and (c) 7.5% Bi doped Na2Ta2O6 (0.1 M)
Figure 6a Diffused reflectance spectra of Bi doped NaTaO3 samples prepared with 6.0 M NaOH and varying bismuth content
Figure 6b Diffused reflectance spectra of doped samples prepared with 0.1M NaOH and varying bismuth content (Na2Ta2O6 as predominant phase)
Figure 7 XPS spectra of Bi 4f peaks in Bi doped NaTaO3 with bismuth content of (a) 2.5%, (b) 5.0%, (c) 7.5% and XPS spectra of Bi 4f peaks in (d) NaBiO3·2H2O
Figure 8 Photocatalytic degradation of Rhodamine B over pristine NaTaO3 and Bi doped NaTaO3 and Bi doped Na2Ta2O6 (predominant phases) under visible light radiation (λ > 420 nm Iavg = 150 mW/cm2); ST is perovskite; PR is defect pyrochlore.
Synthesis, photophysical properties, and photocatalytic applications of Bi doped NaTaO3 and Bi doped Na2Ta2O6 nanoparticles Pushkar Kanhere, Yuxin Tang, Jianwei Zheng, Zhong Chen
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S0022-3697(13)00247-3 http://dx.doi.org/10.1016/j.jpcs.2013.06.013 PCS7119
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Journal of Physics and Chemistry of Solids
Received date: 19 October 2012 Revised date: 20 May 2013 Accepted date: 25 June 2013 Cite this article as: Pushkar Kanhere, Yuxin Tang, Jianwei Zheng, Zhong Chen, Synthesis, photophysical properties, and photocatalytic applications of Bi doped NaTaO3 and Bi doped Na2Ta2O6 nanoparticles, Journal of Physics and Chemistry of Solids, http://dx.doi.org/10.1016/j.jpcs.2013.06.013 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Synthesis, photophysical properties, and photocatalytic applications of Bi doped NaTaO3 and Bi doped Na2Ta2O6 nanoparticles
Authors: Pushkar Kanhere1, Yuxin Tang1, Jianwei Zheng2, and Zhong Chen1* Affiliations: 1School of Materials Science and Engineering, Nanyang Technological University, Block N4.1, 50 Nanyang Avenue, Singapore 639798 2
Institute of High Performance Computing, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632
[email protected] (PUSHKAR D. KANHERE) [email protected] (YUXIN TANG)
[email protected] (JIANWEI ZHENG) [email protected] (ZHONG CHEN) *
CorrespondingAuthor:ZhongChen
Email:[email protected] Telephone:+6567904256 Fax:+6567909081
Abstract
Phase formation and photophysical properties of bismuth doped sodium tantalum oxide (Perovskite, defect Pyrochlore) nanoparticles, prepared by hydrothermal method were studied in detail. It was revealed that the synthesis conditions like NaOH concentration and bismuth precursor (NaBiO32H2O) markedly affect the crystal structure of sodium tantalum oxide. At lower NaOH concentration and higher bismuth precursor (NaBiO32H2O) content, Bi doped Na2Ta2O6 (defect pyrochlore) phase was predominantly formed, while at higher NaOH concentration, Bi doped NaTaO3 (perovskite) phase was formed. It was observed that the defect pyrochlore (Bi doped Na2Ta2O6) phase was formed and stabilized by the presence of dopant precursor (NaBiO32H2O). The chemical analysis of the samples confirmed the doping of Bi3+ cations in both phases. Doping of bismuth enabled visible light absorption up to 500 nm in perovskite and defect pyrochlore type sodium tantalum oxide. Bi doped NaTaO3 samples showed excellent performance for the photocatalytic degradation of Rhodamine B than that of Bi doped Na2Ta2O6, under visible light irritation ( > 420 nm). The present results shed light on phase formation of sodium tantalate and these results are useful in understanding properties of NaTaO3 based compounds, synthesized by hydrothermal method.
Keywords: Na2Ta2O6, sodium tantalum oxide, Bi doped NaTaO3, visible light photocatalyst
Introduction
In the recent years, alkali tantalates (ATaO3 A=Na, K) have gained significant attention due to their superior photocatalytic properties [1, 2]. Pristine NaTaO3 has shown highly efficient splitting of pure water under Ultra-Violet (UV) radiation [3, 4]. The photocatalytic activity of this phase has been further improved by doping of La ions and by loading of NiO co-catalyst, leading to a dramatic increment in the efficiency of the water splitting reaction. [5-7]. Due to its attractive photophysical properties, NaTaO3 it is considered as a good host material to develop visible light photoctalysts. The visible light response in NaTaO3 is achieved by doping of certain ions such as Bi, N, Co, and Cr [8-13]. Among these dopants, bismuth doping in NaTaO3 shows markedly enhanced photocatalytic hydrogen evolution under the visible radiation [11, 14]. Bi doped NaTaO3 photocatalayst, produced by hydrothermal synthesis has shown promising results for utilizing solar radiation and thus detailed investigations on its photophysical properties is necessary[11]. The studies on the correlation between synthesis conditions and photophysical properties of Bi doped NaTaO3 nanoparticles in the hydrothermal synthesis are not reported till date. Such studies are useful for understanding and optimizing the properties of this system for efficient photocatalytic applications. In this work, the effect of synthesis conditions on the phase and the photocatalytic activities of Bi doped NaTaO3 powders are studied in detail. It is shown that synthesis conditions like NaOH concentration and NaBiO32H2O affect the phases of the final products. At lower NaOH concentration and higher precursor (NaBiO32H2O) content, Bi doped Na2Ta2O6 (defect pyrochlore) phase is predominantly formed while at the higher concentration of NaOH, Bi doped NaTaO3 (Perovskite) phase is formed. Among these two
phases, Bi doped NaTaO3 nanoparticles showed excellent performance for the photocatalytic degradation of Rhodamine B under visible light radiation ( > 420 nm).
2 Materials and Methods: In a typical synthesis, 0.441g of Ta2O5 (Alpha Aaser) was mixed with 50 mL of NaOH solution with concentration ranging from 0.1 to 6M. Appropriate amount of sodium bismuth oxide (NaBiO32H2O, Sigma Aldrich) (0 to 7.5% by moles) was added to the mixture as bismuth precursor. The mixture was stirred for 2 hours and transferred to a Teflon cup (120 ml) with steel casing. This autoclave was heated at 240º C for 24 hours in an oven. The products so formed, were thoroughly washed with de-ionized water and dried below 100 ºC. The chemical bonding of bismuth in NaBiO32H2O and Bi doped NaTaO3 was studied by XPS analysis. The XPS spectra were recorded using XPS machine and the fitting of XPS data was done using the software CASA. The reference energy of 284.5 eV was chosen as calibration standard. Crystal structure of the samples was studied by XRD patterns recorded using Bruker D8 Advance; Cu K=1.54 Å. The samples were scanned within 2 range of 10° to 120° with the steps of 0.01° at a scan rate of 1 second per step. The optical absorption properties of the powders were studied by Diffused Reflectance Spectroscopy (DRS). The morphology of the nanoparticles was studied by field emission scanning electron microscope (JEOL 6400F). The surface area of the samples was estimated by BET technique using Micromeritics ASAP 2020. The photocatalytic performance of the doped powders was tested by degradation of Rhodamine B under visible light. In a typical experiment, 50 mg of catalyst was suspended in 50 mL of 10 ppm solution of Rhodamine B. The suspension was kept in the dark for 30 min to achieve adsorption equilibrium. The suspension was then irradiated with a 300 W Xe lamp (Asahi Spectra Inc). The UV component and IR component in the spectra were removed by inserting a
420 nm cut-off filter and a super-cold (Asahi Spectra Inc.) filter, respectively. The experiments were also performed without catalyst to confirm that the degradation occurred by the photocatalytic process.
3 Results and Discussion 3.1 Formation of Bi doped NaTaO3 Figure 1 shows XRD patterns of the samples prepared with NaOH concentration of 6.0 M along with the standard pattern of NaTaO3. All the XRD patterns match well with the reported pattern of NaTaO3 (Space Group No. 62; Pbnm; JCPDS card# 025-0863). In addition to NaTaO3, a small amount of un-reacted Ta2O5 impurity was observed. The impurity peaks have low intensity and thus are not seen in the present XRD patterns in figure 1. However, no other phase was detected in the XRD patterns. It is known that high concentration of NaOH results in highly crystalline nanoparticles of NaTaO3 therefore, such high concentration was selected in the present experiments [2]. Figure 2 shows the variation in the unit cell volume of Bi doped NaTaO3 samples along with Bi content. It was seen that the unit cell volume increased with increment in bismuth content up to 7.5% Bi, and for 10% doping it reached a saturation value. Systematic increment in the unit cell volume and impurity free XRD indicate that Bi ions were accommodated in the lattice, successfully. The increments in the unit cell volume are consistent with the ionic radii of Bi3+ (1.02 Å) and Ta5+ (0.64Å). Alternatively, accommodation of Bi5+ (0.76 Å) at Ta5+ (0.64 Å) could also be possible [15]. Further, the Vegard’s law was followed up to 7.5% Bi doping, indicating that Bi ions were uniformly doped in the lattice. It was confirmed that the XRD pattern of the precursor of bismuth belonged to the hydrated phase of NaBiO3·2H2O and no peaks belonging to NaBiO2 or Bi2O3 were present.
3.2 Formation of Bi doped Na2Ta2O6 In order to understand the effects of NaOH concentration on the crystal structure of the final products, the doped samples were prepared with varying NaOH concentrations. Figure 3 shows the XRD patterns of 7.5% Bi doped samples, prepared with NaOH concentration ranging from 0.1 to 6.0 M. Additionally, standard XRD pattern of Na2Ta2O6 is also shown in figure 3. For higher concentrations of NaOH, i.e. 6.0 M, NaTaO3 and traces of un-reacted Ta2O5 were seen in XRD. As NaOH concentrations decreased below 0.5 M, a defect pyrochlore structure, Na2Ta2O6 (cubic; a=10.526 Å; JCPDS card No. 70-1155) was formed along with NaTaO3 and Ta2O5. It was observed that the amount of Na2Ta2O6 phase increased with a decrease in NaOH concentration. Na2Ta2O6 phase was also formed in absence of NaOH, i.e. when Ta2O5 and NaBiO32H2O were used as starting materials. The quantitative analysis of the phases was carried out by the Rietveld refinement of XRD patterns. The composition of the phases in the final products, along with changes in NaOH concentration is summarized in Table 1. It was further observed that the Na2Ta2O6 phase was formed only when dopant precursor, NaBiO32H2O was present in the starting materials. In absence of NaBiO32H2O, at low NaOH concentration (0.1 M) only Ta2O5 and traces of NaTaO3 were detected in XRD patterns. Therefore, the effect of precursor (NaBiO32H2O) content on the phase composition was studied at low (0.1 M) NaOH concentration. It was seen that the amount of Na2Ta2O6 increased with an increase in NaBiO32H2O content, reaching a saturation level at 7.5% precursor content. The composition of the phases in the final products along with changes in NaBiO32H2O content is summarized in Table 2. Results in Table 1 and 2 show that at lower NaOH concentration and
higher NaBiO32H2O content, the growth of defect pyrochlore phase (Na2Ta2O6) was promoted and the growth of perovskite phase (NaTaO3) was suppressed. The synthesis and photophysical properties of Na2Ta2O6 are seldom reported in the literature, as this is the metastable phase [7]. The crystal structures of these phases differ significantly in terms of their packing factors and lattice parameters (Figure 4). NaTaO3 (Space group No. 62, Pcmn, Pbnm) has a close packed orthorhombic structure, while Na2Ta2O6 (Space group No. 227, Fd-3m) has a cubic lattice with hexagonal interstitial framework [16].
Studies on hydrothermal synthesis of NaTaO3 suggest that Na2Ta2O6 could be formed as an intermediate product and it could be further transformed into NaTaO3 [17]. Thus the pyrochlore structure is considered as a meta-stable phase. In the present experiments, this metastable phase is stabilized and its growth is promoted by bismuth precursor, NaBiO3·2H2O. The crystal structure of NaBiO3·2H2O is a layered type structure, with corner connected BiO6 octahedra framework. It was seen that in absence of NaBiO32H2O, the defect pyrochlore phase was not formed. Thus it is proposed that the nucleation of Na2Ta2O6 phase has occurred from NaBiO3·2H2O seeds. The layered structure could have been responsible in preventing the formation of close packed structure (NaTaO3) and promoting the growth of structure with lower packing factor. The phase analysis of the samples prepared under various conditions shows that the crystal structure and composition of the final products are highly sensitive to the concentration of NaOH as well as amount of bismuth precursor in the initial reactants.
3.3 Microstructure and optical properties Figure 5a shows the FESEM image of pristine NaTaO3 synthesized using 6.0 of NaOH. Figure 5b show and 5c show FESEM images of 7.5% Bi doped NaTaO3 and Bi doped Na2Ta2O6 (predominant phase), respectively. Both the pristine NaTaO3 and Bi doped NaTaO3 showed cubic shaped particles. On the other hand, the particles shapes are deviated from cubic to irregular shapes in the case of Bi doped Na2Ta2O6. As compared to the perovskite powders, the defect pyrochlore phase (Na2Ta2O6) showed agglomerated growth and irregular shapes, indicating reduced crystallinity. Pristine NaTaO3 showed absorption in the UV region with a band gap value of 4.01 eV. All the bismuth doped samples showed extension of absorption spectra to the visible region (Figure 6a-6b). Further, the absorption in the visible region increased with increase in the bismuth content, indicating bismuth doping caused the band gap narrowing of tantalate phases. For a doping concentration of 7.5% bismuth, the absorption spectra extended up to 500 nm in both the phases. The present DRS spectra suggest that Bi doping enables visible light absorption in NaTaO3 as well as Na2Ta2O6. Earlier work on Bi doped NaTaO3 suggests that occupancy of Bi ions at Ta site would induce Bi 6s induced mid-gap energy states that are responsible for visible light absorption [12]. Therefore, in the present samples, doping of Bi at Ta site in NaTaO3 and Na2Ta2O6 is considered as a main reason for the visible light absorption.
3.4 Chemical analysis Considering the values of ionic radii and electronegativity of bismuth and tantalum, bismuth could be doped in NaTaO3 lattice in Bi3+ or Bi5+ chemical state. The oxidation state of bismuth could have a marked effect on the photocatalytic properties of doped NaTaO3. Therefore, in order to study the oxidation state of bismuth, the X-ray photoemission spectra (XPS) of the samples were investigated. Figure 7a-c shows XPS spectra of Bi 4f peaks in Bi doped NaTaO3 samples with increasing bismuth content. All the doped samples showed presence of bismuth ions and Bi 4f peaks located at around 158.5 eV and 163.7 eV, were assigned to Bi3+ ions [18]. Similarly, the XPS spectra of Bi doped Na2Ta2O6 showed presence of Bi3+ cations. Bi 4f peaks in doped samples showed lower energy components, when fitting of the peaks was carried out. The two peaks correspond to different binding energies of Bi 4f. The differences in the binding energy values are attributed to the different chemical environment or surface vs. bulk location of the Bi ions. The XPS spectra of 4f peaks of NaBiO32H2O showed similar binding energy values (158.71 and 163.91 eV) as compared to those of Bi doped NaTaO3 samples (Figure 7d). This result is similar to the previously reported studies, where binding energy values of Bi5+ cations were close to those of Bi3+ cations [19]. The reported values of the binding energies of Bi 4f peak of Bi5+ cations range between 158.8 to 159.4 eV (or 164.0 to 164.6 eV) [18, 20]. Therefore the presence of Bi5+ ions could not be conclusively confirmed from the present XPS analysis. However, it is known that NaBiO32H2O is a metastable compound and dissociates when exposed to moisture and heat. NaBiO32H2O undergoes phase transition to compounds with lower oxidation states when heated above 200ͼC [21, 22]. In the present experiments, the heat treatment of the hydrothermal vessel was done at 250ºC in aqueous medium. This temperature is sufficient to transform NaBiO32H2O into compounds with lower oxidation state such as Bi2O3
or Bi2O4. Thus it is reasonable to conclude that the final products mainly contain Bi3+ cations. The quantitative analysis XPS spectra of the samples show that the Bi content in samples is close to the starting stoichiometry (Table 3). The quantitative analysis of the XPS data shows that Bismuth content in the final products is close to the initial stoichiometry. A small amount of discrepancy in the bismuth content could be attributed to the surface sensitivity of the XPS technique. It is further noted that the, atomic fraction of Na, Ta, and O are close to the stoichiometry of 1:1:3 in the doped NaTaO3 samples.
3.5 Photocatalytic Activity: The photocatalytic activity of the samples was tested by studying degradation of Rhodhamine B (Rh B) under visible light radiation ( > 420 nm). Figure 8 shows the reduction in concentration of Rh B when different photocatalyst powders were used. It was seen that the concentration change under the dark conditions (30 mins) was negligible, indicating that the adsorption of the dye on the surface was insignificant. The pristine NaTaO3 sample did not show any significant photocatalytic activity as it did not absorb any visible light. Bi doped Na2Ta2O6 (predominant defect pyrochlore) phase showed photocatalytic degradation of Rh B, degrading around 40% of the dye in four hours. On the other hand, Bi doped NaTaO3 showed nearly complete degradation of the dye in four hours, under similar conditions. The best performance was shown by 7.5% Bi doped NaTaO3 sample, which could be attributed to higher visible light absorption, as seen in DRS spectra (Figure 6a). Further, we have compared the surface area values of the two different types of powders. The surface area of the powders with 2.5, 5.0, 7.5% Bi doped NaTaO3 were found to be 2.15, 2.26 and 2.21 m2/g. While the surface area of the 2.5, 5.0, and 7.5% Bi doped Na2Ta2O6 (major phase) were estimated to be 1.95, 2.42, 2.20 m2/g. The surface area values of Perovskite and Pyrochlore phase did not show any
significant difference. Thus the contribution of surface area to the photocatalytic activity is considered to be the same. The lower performance of Na2Ta2O6 phase may be due to its poor crystallinity. Further, it is known that pristine NaTaO3 shows better photocatalytic performance for water splitting reaction than Na2Ta2O6 [23]. Therefore, Bi doped perovskite samples showed higher photocatalytic activity than corresponding pyrochlore phases.
4 Conclusions In summary, this work studies the effects of synthesis conditions on the phase and optical and photocatalytic properties of sodium tantalum oxide nanoparticles doped with bismuth. Further, it sheds light on the precursor induced formation of defect pyrochlore type sodium tantalate. The main conclusions drawn from this study are listed as follows. 1. Bi3+ doped NaTaO3 (orthorhombic) and Bi3+ doped Na2Ta2O6 (cubic) nanoparticles were successfully synthesized by using NaBiO32H2O as bismuth precursor in the hydrothermal synthesis. 2. The crystal structure of the final products is sensitive to the concentration of the dopant precursor (NaBiO32H2O) as well as concentration of NaOH. At lower NaOH concentration (< 0.5 M) and higher NaBiO32H2O content, the formation of defect Pyrochlore (Na2Ta2O6) is highly favorable while at higher NaOH concentration and lower NaBiO3 content, perovskite type NaTaO3 is highly favorable. 3. Bismuth doping in both phases i.e. NaTaO3 and Na2Ta2O6 induces visible light absorption up to 500 nm and these compounds show visible light driven photocatalytic degradation of
Rhodamine B. The perovskite phase (Bi doped NaTaO3) has a better photocatalytic performance than that of the defect pyrochlore phase (Bi doped Na2Ta2O6).
List of Tables
Table 1 The phase composition of the final products with 7.5% Bi doping and varying NaOH concentration Table 2 The phase composition of the final products prepared with 0.1M NaOH concentration and varying NaBiO3·2H2O content Table 3 Quantitative analysis of XPS spectra showing percentage atomic content in Bi doped NaTaO3.
List of Figures
Figure 1 X-ray diffraction patterns of pristine and Bi doped NaTaO3 prepared with 6.0 M of NaOH, along with the reference pattern of NaTaO3. Figure 2 The variation of the unit cell volume along with bismuth content Figure 3 XRD patterns of 7.5% Bi doped NaTaO3 powders with varying concentration of NaOH, along with the reference pattern of Na2Ta2O6. Figure 4 Crystal structures of sodium tantalum oxide, (a) NaTaO3 Perovskite (b) Na2Ta2O6 defect Pyrochlore Figure 5 FESEM images of (a) Pristine NaTaO3, (b) 7.5% Bi doped NaTaO3 (6.0 M NaOH), and (c) 7.5% Bi doped Na2Ta2O6 (0.1 M) Figure 6a Diffused reflectance spectra of Bi doped NaTaO3 samples prepared with 6.0 M NaOH and varying bismuth content Figure 6b Diffused reflectance spectra of doped samples prepared with 0.1M NaOH and varying bismuth content (Na2Ta2O6 as predominant phase) Figure 7 XPS spectra of Bi 4f peaks in Bi doped NaTaO3 with bismuth content of (a) 2.5%, (b) 5.0%, (c) 7.5% and XPS spectra of Bi 4f peaks in (d) NaBiO32H2O Figure 8 Photocatalytic degradation of Rhodamine B over pristine NaTaO3 and Bi doped NaTaO3 and Bi doped Na2Ta2O6 (predominant phases) under visible light radiation ( > 420 nm Iavg = 150 mW/cm2); ST is perovskite; PR is defect pyrochlore.
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Table 1 The phase composition of the final products with 7.5% Bi doping and varying NaOH concentration NaOH (M) for 7.5%
Na2Ta2O6 (%)
NaTaO3 (%)
Ta2O5 (%)
6.0
0.00
94.39
5.61
2.0
11.85
80.30
7.85
0.5
45.58
49.51
4.91
0.1
81.59
1.44
16.97
0.0
77.80
1.06
21.14
Bi doping
Table 2 The phase composition of the final products prepared with 0.1M NaOH concentration and varying NaBiO3·2H2O content NaBiO32H2O content
Na2Ta2O6 (%)
NaTaO3 (%)
Ta2O5 (%)
0.0
0.00
93.60
6.30
2.5
70.17
18.13
11.70
5.0
89.25
5.56
5.19
7.5
83.11
1.19
15.70
(%) for 0.1 M NaOH
Table 3: Quantitative analysis of XPS results showing percentage atomic content in Bi doped NaTaO3.
Bi content/ Atomic Fraction
2.5
5.0
7.5
10.0
Na
25.99
19.76
16.12
18.59
Ta
17.17
14.29
17.50
20.71
O
56.02
62.79
59.78
53.19
Bi
0.82
3.16
6.60
07.51
Highlighhts
x
N 2Ta2O6 naanoparticles with w visible light absorpption Bi doped NaTaO3 andd Bi doped Na
x
The crysttal structure of the finall products (N NaTaO3 or Na N 2Ta2O6) iss sensitive to t the concentraation of the dopant preccursor (NaB BiO32H2O) as well as concentratioon of NaOH
x
NaBiO32 2H2O promootes the grow wth of defectt Pyrochloree (Na2Ta2O6) at lower NaOH N concentraation and supppresses the growth of Perovskite (N NaTaO3)
x
Visible lig ght driven phhotocatalyticc degradatioon of Rhodam mine B over Bi doped tantalates
Graphiical Abstrract
Figure(s)
Figure 1 X-ray diffraction patterns of pristine and Bi doped NaTaO3 prepared with 6.0 M of NaOH, along with the reference pattern of NaTaO3.
Figure 2 The variation of the unit cell volume along with bismuth content
Figure 3 XRD patterns of 7.5% Bi doped NaTaO3 powders with varying concentration of NaOH, along with the reference pattern of Na2Ta2O6.
Figure 4 Crystal structures of sodium tantalum oxide, (a) NaTaO3 Perovskite (b) Na2Ta2O6 defect Pyrochlore
Figure 5 FESEM images of (a) Pristine NaTaO3, (b) 7.5% Bi doped NaTaO3 (6.0 M NaOH), and (c) 7.5% Bi doped Na2Ta2O6 (0.1 M)
Figure 6a Diffused reflectance spectra of Bi doped NaTaO3 samples prepared with 6.0 M NaOH and varying bismuth content
Figure 6b Diffused reflectance spectra of doped samples prepared with 0.1M NaOH and varying bismuth content (Na2Ta2O6 as predominant phase)
Figure 7 XPS spectra of Bi 4f peaks in Bi doped NaTaO3 with bismuth content of (a) 2.5%, (b) 5.0%, (c) 7.5% and XPS spectra of Bi 4f peaks in (d) NaBiO3·2H2O
Figure 8 Photocatalytic degradation of Rhodamine B over pristine NaTaO3 and Bi doped NaTaO3 and Bi doped Na2Ta2O6 (predominant phases) under visible light radiation (λ > 420 nm Iavg = 150 mW/cm2); ST is perovskite; PR is defect pyrochlore.