Chapter 3 Li-recyclability of ternary tin oxides with
3.4.1 Characterization by XRD, SEM and XPS 96
The compounds CaSnO3, SrSnO3 and Ca2SnO4 are white whereas BaSnO3 is light yellow colored powder. While CaSnO3 and SrSnO3 (sol-gel) could be prepared at 800oC, we found that single phase BaSnO3 forms only after heating at 1200oC. The XRD patterns of CaSnO3, SrSnO3 and BaSnO3 prepared by solid-state and sol-gel techniques were found to be identical. Fig. 3.1(a-d) shows the XRD powder patterns of all the synthesized compounds. It is known from the literature that CaSnO3 and SrSnO3 have orthorhombically-distorted perovskite structure [27] whereas BaSnO3
adopts the cubic perovskite structure [28]. This is due to the difference in the tolerance factors, which is related to the ionic radii of the ions for the formation of the perovskite structure even though all the compounds possess SnO6 octahedra [29]*. The patterns were indexed and the lattice parameters were calculated by the least square fitting method. These are also given in Fig. 3.1. The a, b and c are in very good agreement with those reported for CaSnO3 (JCPDS card no. 03-0756), SrSnO3 ( [27]
and the JCPDS card no. 77-1798) and the a value for BaSnO3 ( [28] and the JCPDS card no. 74-1300). The XRD pattern of Ca2SnO4 is shown in Fig. 3.1(d) and the lattice parameters are in agreement with the JCPDS card no. 74-1493.
__________________________________________
*The tolerance factor is given by, t= rA+ro/%2(rB+ro), where rA,rB and ro are the ionic radii of the ions A, B and O respectively in the compound with the formula ABO3
[29]. If t=1, the structure will be cubic and for t<1 a distortion of the cubic structure to tetragonal or orthorhombic will occur. For BaSnO3, t=1.02 whereas for SrSnO3 and CaSnO3 t=0.90 and 0.86 respectively (ionic radii for Ba2+(XII), Ca2+ (VIII), Sr2+(VIII), Sn4+(VI) and O2-(VI) are 1.61, 1.12, 1.26, 0.69 and 1.40 Å respectively.
The r values taken from R. D. Shannon, Acta Crystallogr. 32A(1976)751).
It has a crystal structure analogous to Sr2PbO4 [30,31], having an orthorhombic unit cell, comprising infinite chains of edge-shared SnO6 octahedra connected by a perpendicular network of CaO7 monocapped trigonal prisms, the Ca-ion being in seven-fold oxygen coordination.
Fig. 3.1 Powder X-ray diffraction (XRD) patterns of CaSnO3, SrSnO3, BaSnO3, and Ca2SnO4. CuK, radiation. Miller indices (hkl) and lattice parameters (a,b and c) are shown.
1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0
0 5 0 0 1 0 0 0 1 5 0 0 2 0 0 0 2 5 0 0 3 0 0 0
Å
Å Å
a= 1 1 . 0 6 ( + 0 . 0 4 ) ; b= 7 . 9 0 ( + 0 . 0 1 ) ; c= 1 1 . 3 1 ( + 0 . 0 1 )
( a ) C a S n O 3
!"#$% !"&$%
!'"(%
!$"(%!"#(% !#''%
!&#$%
!))&% !)#&%
!)')%!)"&%
!"'"%!"#*%!#)'%!")'%!'#)%
!""'%!)))%
!")"%
10 20 30 40 50 60 70 80
0 400 800
1200 a= 5.76 (+0.01) ; b= 9.70 (+0.03) ; c= 3.27 (+ 0.01)
241 440421142420132331340112250
002330320310041221211201140040220111130200
110
(d) Ca2SnO4
2+, degrees
10 20 30 40 50 60 70 80
0 1500 3000 4500 6000
Å Å
Å
a= 4.12 (+0.05)
013
220
211
200
111
110
(c) BaSnO3
10 20 30 40 50 60 70 80
0 1000 2000 3000 4000 5000
Å Å Å
a= 5.71 (+0.01) ; b= 5.69 (+0.01) ;Å
c= 8.05 (+ 0.01)
332
314
400
312131222
220
200
002
(b) SrSnO3
In te ns ity , C ou nt s
The SEM pictures of the compounds shown in Fig. 3.2 differ from each other revealing that the morphology is a function of the composition and the synthetic procedure. The SEM photographs of the powders of CaSnO3 (Figs.3.2 a and b) reveal the formation of nano-crystallites with average size 200-300 nm (sol-gel method) and bigger agglomerated particles (solid-state method). SrSnO3 (solid-state) (Fig. 3.2c) showed well-separated smooth particles with the average size, 0.5-1.0 #m. A considerable size reduction was achieved by the sol-gel synthesis of SrSnO3 (Fig.
3.2d), yielding aggregates of nano-particles with size, 200-300 nm. The morphology of BaSnO3 (solid-state) (Fig. 3.2e) shows agglomerates of submicron particles and this is due to the higher synthesis temperature (1200oC) needed for the compound formation. The Ca2SnO4 is comprised of oblong particles (1-5 #m) as shown in Fig.
3.2f.
Fig.3.2 SEM photographs of the powders of : (a) CaSnO3 (sol-gel), (b) CaSnO3 solid- state), (c) SrSnO3 (solid-state), (d) SrSnO3 (sol-gel) (e ) BaSnO3 (solid- state), (f) Ca2SnO4. The bar scale is 1#m.
It is well-known that X-ray photoelectron spectroscopy (XPS) is a powerful and non-destructive technique to ascertain the valence states of the metal ions in the simple and complex compounds [32]. The core level binding energies (BE) of the metal or non-metal ions are characteristic of the oxidation state and their coordination in the crystal lattice. Hence, XPS has been used extensively to characterize the LIB- electrode materials [33, 34]. The XPS of only BaSnO3 has been reported in the literature [35]. Presently, we have studied the XPS of ASnO3, A= Ca, Sr and Ba, and Ca2SnO4. The BE of the various ions have been evaluated and are compared with those reported for BaSnO3, SnO2 and related perovskite compounds. Figs. 3.3-3.5 (a- d) show the XPS core level spectra of the Sn 3d, O 1s, Ca 2p/ Sr 3d /Ba 3d regions for the compounds, ASnO3, A=Ca,Sr,Ba and Ca2SnO4 respectively. The binding energies (BEs) for all these and related compounds from the literature are listed in Table 3.1.
The Sn 3d spectra of ASnO3 and Ca2SnO4 (Fig. 3.3a-d) show two peaks in the energy range, 485-495 eV corresponding to Sn 3d5/2 and 3d3/2. The peak separation, $= 3d5/2- 3d3/2= 8.4 eV in all the cases indicating the correctness of the assignment. Manorama et. al. [35] have reported only the Sn 3d5/2 peak with BE of 486.9 eV in BaSnO3
whereas Ayouchi et al. [36] reported BEs of 487.1 eV for 3d5/2 and 495.5 eV for 3d3/2
for a film comprising both SnO and SnO2. The XPS spectra of the core levels in the Sn 3d region of SnO2 film deposited on SiO2/Si(100) given in ref. [37] showed the BEs : 486.8 and 495.2 eV for Sn 3d5/2 and Sn 3d3/2 respectively. It is clear from Fig.
3.3 and Table 3.1 that tin is in 4+ valency in the compounds presently studied. Since BaSn4+O3 has cubic structure, it has perfect SnO6 octahedra and Ba2+ ions adopt a 12- fold O-coordination. The compounds ASnO3 (A=Ca,Sr), Ca2SnO4 and SnO2 which have distorted SnO6 octahedra show higher BE values for both 3d5/2 and 3d3/2 as compared to BaSnO3. This is more evident in the case of Ca2SnO4 and SnO2.
482 484 486 488 490 492 494 496 20
40
60 (d)Ca2SnO4
Binding energy (BE), eV 0
40 80 120
Sn (3d3/2)
Sn (3d5/2) (a)CaSnO3
C ou nt s pe r se co nd ( x 102 ) 0 40 80
120 (b) SrSnO3
0 10 20 30
40 (c)BaSnO3
Fig.3.3 XPS spectra in the Sn-3d region of (a) CaSnO3, (b) SrSnO3, (c ) BaSnO3 and (d) Ca2SnO4. Base line and curve fitting of the raw data are shown. The 3d5/2 and 3d3/2 regions are indicated.
30 40
50 (c) BaSnO3
C ou nt s pe r se co nd ( x1 02 )
30 40 50
60 (b)SrSnO3
40 50 60
70 O (1 s) (a)CaSnO3
526 528 530 532 534 536
40 60
80 (d)Ca2SnO4
Binding energy (BE), eV
Fig.3.4 XPS spectra in the O-1s region of (a) CaSnO3, (b) SrSnO3, (c ) BaSnO3, and (d) Ca2SnO4. Base line and curve fitting of the raw data are shown.
342 344 346 348 350 352 354 10
20 30 40 50
Ca(2p1/2)
Ca(2p3/2) (d) Ca2SnO4
Binding energy(BE), eV
342 344 346 348 350 352 354
10 15 20 25 30 35
Ca(2p1/2)
Ca(2p3/2) (a) CaSnO3
130 132 134 136 138 140
5 10 15 20 25
30 Sr(3d3/2)
Sr(3d5/2)
(b) SrSnO3
775 780 785 790 795 800
80 100 120
140 Ba(3d5/2) Ba(3d3/2) (c) BaSnO3 C ou nt s pe r se co nd ( x1 02 )
Fig.3.5 XPS spectra in (a) Ca-2p region of CaSnO3, (b) Sr 3d region of SrSnO3, (c) Ba 3d region of BaSnO3 and (d) Ca 2p region of Ca2SnO4. Base line and curve fitting of the raw data are shown.
Table 3.1 XPS binding energies (BE, +0.1 eV) of Sn, O, Ca, Sr and Ba in the compounds, ASnO3 (A=Ca, Sr, Ba), Ca2SnO4, SnO, SnO2 and other compounds with perovskite structure. $ is the difference in BEs.
Compound Element (region) BE(eV) Reference
CaSnO3
SrSnO3
BaSnO3
Ca2SnO4
BaSnO3
SnO/SnO2
SnO2
Sn (3d5/2; 3d3/2) 485.7; 494.1 ($=8.4) 485.9; 494.3 ($=8.4) 485.4; 493.8 ($=8.4) 486.3; 494.7 ($=8.4) 486.9
487.1;495.5 ($=8.4) 486.8; 495.2 ($=8.4)
This study This study This study This study
Manorama et.al. [35]
Ayouchi et. al. [36]
Barreca et al. [37]
CaSnO3
SrSnO3
BaSnO3
Ca2SnO4
BaSnO3
Bulk SnO Bulk SnO2
LaMO3 (M= Cr, Mn, Fe, Co, Ni)
(La0.4Sr0.6)(Co0.8Fe0.2)O3-)
O (1s) 529.3, 531.4
529.4, 531.4 528.7, 530.4 531.2
530.2, 531.7, 533.2 530.4
530.5
528.3-528.9, 530.5- 531.5
529.2,532.2
This study This study This study This study
Manorama et al. [35]
Jimenez et al.[40]
Barreca et al. [37]
Yokoi et al. [38]
Machkova et al. [39]
CaSnO3
Ca2SnO4
Ca (2p3/2; 2p1/2) 346.2; 349.7($=3.5) 346.9; 350.4($=3.5)
This study This study SrSnO3
(La0.4Sr0.6)(Co0.8Fe0.2)O3-)
Sr (3d5/2; 3d3/2) 132.7; 134.4 132.8; 134.2
This study
Machkova et al. [39]
BaSnO3
BaSnO3
Ba (3d5/2; 3d3/2) 778.1, 779.8; 793.8 781.5
This study
Manorama et al. [35]
The O 1s spectra for ASnO3 (Fig. 3.4a-d) compounds show broad and asymmetric peaks and have been fitted to two overlapped peaks with BEs in the 529 and 531eV region. The BEs are listed in Table 3.1. The overlapped peak is indicative of two different oxygen contributions. Similar such asymmetric peak in the O 1s region was observed in other perovskite oxide compounds [35,38,39]. These are generally ascribed to the lattice oxygen and adsorbed oxygen. Therefore, the peak at 528.7 eV in BaSnO3 is attributed to the O2- contribution from the perovskite anionic network. The other peak, lower in intensity, occurring at a higher BE, 530.4 eV, is ascribed to OH- and/or surface adsorbed oxygen. As expected, the O1s BEs including that due to the adsorbed oxygen of ASnO3, A=Ca,Sr and other perovskite oxides with A=La [38] (Table 3.1), which have distorted SnO6 oxygen octahedra and 9- or 8-fold O-coordination for the A-ions, show higher values than that of BaSnO3. The O 1s spectrum for Ca2SnO4 (Fig. 3.4d) comprises only one peak at 531.2 eV attributable to the lattice oxygen. It does not show any contribution from the adsorbed oxygen. We note that the ASnO3 compounds with the perovskite structures have been found to be conducive for the adsorption of gases and OH- species [35]. The O 1s peak for bulk SnO2 and SnO occurs at 530.4 eV and 530.5 eV respectively [37,40] (Table 3.1).
The best fit of XPS spectra of Ca 2p in CaSnO3 and Ca2SnO4 show that the resultant curve is a mixture of two overlapping peaks (Fig. 3.5a and d). These are attributed to Ca 2p3/2 and 2p1/2 with a BE separation of 3.5 eV. This result is in good agreement with the BE values in literature [32] with Ca 2p3/2 at 346.6 eV and a 2p3/2
and 2p1/2 peak separation of 3.55 eV for CaS, CaCl2, CaO, CaCO3 and Ca(NO3)2. The spectrum of Sr 3d in SrSnO3 (Fig. 3.5b) also comprises two peaks at 132.7 and 134.4 eV with a peak separation of 1.7 eV between the 3d5/2 and 3d3/2. This assignment is in agreement with the reported Sr 3d BE values, 132.8 and 134.2 eV in the oxide with
the perovskite structure, (La0.4Sr0.6)(Co0.8Fe0.2)O3-) [39] (Table 3.1). The XPS of Ba 3d in BaSnO3 (Fig. 3.5c) shows an asymmetric peak, that can be fitted into two peaks with BEs, 778.1 (main) and 779.8 (small intensity) eV. The second peak is symmetric with BE of 793.8 eV. The peaks at 778.1 and 793.8 eV are attributed to the Ba 3d5/2
and 3d3/2 in BaSnO3 with a peak separation of 15.7 eV. This is different from the result of Manorama et al. [35], who reported only one peak in BaSnO3 with a BE of 781.5 eV. According to literature [32], the Ba 3d5/2 peak in Ba-compounds (BaS, BaSO4, BaCO3, Ba(NO3)2 etc.) occurs at BE of 780.6 eV with 3d5/2- 3d3/2= 15.33 eV.
The latter value is comparable to that observed by us in BaSnO3. The minor peak at 779.8 eV may be due to the Ba 3d in BaO present as an impurity in BaSnO3.
In summary, the XPS data clearly show that Sn4+ is present in ASnO3, in an octahedral oxygen coordination (A=Ca,Sr,Ba) and Ca2SnO4. The A ions are bivalent.
The ASnO3 has adsorbed oxygen on the surface of the particles.