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Static and Dynamic Disorder in Metastable Phases of Tin Oxide Feng Zhang, Yadong Lian, Min Gu, Ji Yu, and Tong Bor Tang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04477 • Publication Date (Web): 12 Jul 2017 Downloaded from http://pubs.acs.org on July 13, 2017

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The Journal of Physical Chemistry

Static and Dynamic Disorder in Metastable Phases of Tin Oxide

Feng Zhang, † Yadong Lian, † Min Gu, †,* Ji Yu, † Tong Bor Tang ‡

†

National Laboratory of Solid State Microstructures and Department of Physics,

Collaborative Innovation Center of Advanced Microstructures, Nanjing University,

Nanjing 210093, P.R. China ‡

Department of Physics, Hong Kong Baptist University, Kowloon, Hong Kong SAR,

P.R. China; now at Asia Power Development (Group), 2502 Win Plaza, San Po Kong,

Hong Kong SAR, P.R. China

*

Corresponding author. Tel.: +86 25 83593508, Fax: +86 25 83595535, E-mail

address: [email protected] (Min Gu) 1

ACS Paragon Plus Environment


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ABSTRACT

The multi-stage transformation of Sn3O4 to SnO2 in oxygen-free atmosphere has been studied using hydrothermally synthesised samples, both pristine and heated, with in-situ X-ray diffractometry, Raman spectroscopy, differential scanning calorimetry, dielectric spectroscopy, and ex-situ X-ray photoelectron and nuclei magnetic resonance spectroscopies. Experimental evidence consistently shows that, beginning from 250 oC, Sn3O4 converts to Sn2O3 and from 350 oC, to SnO2, accompanied by the formation of metallic tin. These two stages are associated with frequency-dependent dielectric losses, as revealed by impedance measurements, which also indicate a significant conductivity of 1.78 eV in activation energy. It is speculated that the relaxation-like processes relate to the transition from static to dynamic disorder, then to order, in the oxygen sublattice.

2


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1. INTRODUCTION

Stannic oxide, SnO2, is an n-type semiconductor with a wide band gap of 3.6 eV. A key functional material, it has many technological roles, e.g. as base materials in gas sensors

1-2

and photovoltaic devices 3, as electrode materials in batteries 4, solar

cells 5 and flat-panel displays 6, and as catalysts 7. Its preparations in both bulk and film forms entail diverse processing techniques but all follow either of two chemical routes: direct synthesis from compounds containing Sn4+ ions, or oxidation of divalent tin compounds. For the latter group, the most commonly used precursor is stannous oxide, SnO. Whereas SnO2 has a rutile crystal structure with tetragonal unit cells of space-group symmetry P42/mnm, in which each quadrivalent Sn ion is surrounded by six anions 8, SnO has litharge structure with a tetragonal unit cell of P4/nmm symmetry and the divalent cation in 4-fold coordination 9. SnO is metastable due to its oxygen deficiency 10. It transforms rapidly to SnO2 at temperature above 200 oC in an oxidative environment. This transformation proceeds in two steps: first from SnO to some intermediate oxide(s), thence to SnO2 11-14. Uncertainty remains on the identity of the intermediate(s), which may be Sn3O4

2, 14-15

, Sn2O3

10, 16-17

, or both 13,

18-19

. However, we have noticed that our Sn3O4 samples, after storage at room

temperature inside a desiccator, exhibited nuclear magnetic resonance (NMR) peaks identical to those of SnO2. Our subsequent study on the change of Sn3O4 to SnO2, as described here, show that Sn2O3 itself is an intermediate phase in this change. The uncertainty may therefore have arisen because Sn2O3 does later appear even if SnO first oxidises to Sn3O4. Sn3O4 adopts a triclinic structure with space group P-1(2). Theoretically, it forms when two layers oxygen on each (101) plane are removed from rutile SnO2, so that it consist of SnO2-like local structures in which Sn valance is quadrivalent together with SnO-like local structures with divalent Sn

18

. Sn2O3 is also triclinic

20-21

, and

similarly has oxygen vacancies layered on (101) planes, but with different 3



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proportionation. Both consist of two kinds of Sn ions, namely 3-coordinated Sn2+ and 6-coordinated Sn4+ 20. To obtain insight into the atomic mechanisms in the conversion of SnO into SnO2, we synthesised Sn3O4 by a hydrothermal method, and undertook a series of spectroscopic and other measurements on the as-prepared powder as well as samples heated in- or ex-situ. Our conclusion is that Sn3O4 first transforms to Sn2O3, then into SnO2, involving at each stage the cooperative jump of oxygen ions clusters away from divalent tin ions after their reduction to metal. The conversion of tin (II) to tin (IV) oxide therefore goes through the homologous series Snn+1O2n.

2. EXPERIMENTAL DETAILS

2.1 Preparation of Sn3O4 A solution of tin (II) chloride dihydrate in distilled water (8.12 g in 60 ml) had its pH value adjusted to 2.0 by slow admixing with NaOH. After having been stirred continually for 2 h, it was transferred into a Teflon-lined autoclave and kept at 150 oC for 24 h. Residues were then washed with ethanol three times, before being dried overnight in vacuo at 50 oC. 2.2 Characterisation In situ variable-temperature X-ray diffractograms (XRD) were produced from the residue held at various temperatures between 25 and 420 oC in vacuum environment, in a RIGAKU D/max 2500VL/PC diffractometer, that employed Cu Ka1 radiation of λ = 1.5406 A. Another sample powder was studied after undergoing three heating-cooling cycles between room temperature and 420 oC in vacuum. Temperature-resolved Raman spectroscopy was next performed in a JY Horiba HR800 micro-Raman spectroscope, using 633 nm excitation and with the sample purges with nitrogen. Differential scanning calorimetry (DSC) proceeded under the same anoxic atmosphere, from 25 to 600 oC at the heating rate of 10 K/min, in a 4


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NETZSCH STA 409 PC/PG calorimeter. SnO2 of 99% purity, sourced from Alfa Aesar, was also examined for comparison purpose. Impedance spectra were recorded of pellets, 8 mm in diameter and 0.6 mm in thickness made by uniaxial compaction at 6 MPa, as an Agilent E4980A meter applied a working voltage of 1.0 V in three-terminal configuration, at nine chosen frequencies within the range of 100 to 1000 kHz. Sandwiched between silicon-plated platinum plates, the pressed disc was dried in dynamic vacuum of 30 Pa for at least 1 h, before being heated in vacuo at 2 K/min to 425 oC, with its temperature monitored by an adjacent Cu−CuNi thermocouple calibrated to the accuracy of 0.1 K. It would then return to room temperature under natural cooling. For a second batch of pellets, this thermal cycling was repeated three times whilst their permittivities were measured. X-ray photoelectron spectra (XPS), if needed, were acquired in a PHI Versa II microprobe using micro-focused Al Kα X-ray of 1486.7 eV. Their binding energy scale was calibrated by reference to C1s, and they are analyzed with XPSPEAK41 software. Finally, solid-state nuclear magnetic resonance (NMR) measurements were undertaken in a Bruker AV-300 spectrometer, whose rotor spun at 10 kHz during accumulation of 119Sn spectra over 128 scans, utilizing hahn-echo pulse sequence with a delay of 50 s.

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3. RESULTS

Figure 1.

XRD at (a) 25 (b) 90 (c) 200 (d) 250 (e) 300 (f) 360 and (g) 420 oC, and

(h) at room-temperature after three thermal cycles. In (g) the peaks marked ‘*’ arose from the substrate, platinum, and an arrow points to an enlargement of the part enclosed by dotted lines. Room-temperature XRD for SnO2, Sn3O4 and Sn2O3 from data base is reproduced at top and bottom. Figure 1(a-g) show XRD profiles during heat treatments, and Figure 1(h), the diffractogram after three thermal cycles between 25 and 420 oC. The pattern 1(a) matches JCPDS 16-0737, the standard for triclinic Sn3O4, and is free from any characteristic peaks of tin. After treatment at increasing temperature, the (101) and (312) peaks gradually decreased in intensity and the two peaks near 50o broadened and weakened. The latter was completely gone in 1(d), a profile that resembles the JCPDS 25-1259 for triclinic Sn2O3. This indicates the transformation of Sn3O4 into Sn2O3 above 250 oC, as previously observed

22-23

. Finally, at 420 oC, the (101) peak

disappeared totally and those around 50o merged: profile 1(g) tends towards 1(h), which matches the reference XRD of SnO2. A finer point concerns the appearance of substrate peak in the last XRD profile. 6


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That was due to the depletion of sample as temperature was raised. We shall see that Sn3O4 partially decomposed to metallic Sn, which vaporized.

Figure 2. (a) Raman spectra; (b) magnified views of selected parts. Inset: thermal dependence of peak intensity at 170 cm-1. In Figure 2(a), the Raman peaks at 140, 170 and 240 cm-1 are characteristic phonon-modes of Sn3O4 24-26, and the enlargements in 2(b) reveal no additional peaks from impurities. The highest peak at 170 cm-1 gradually disappeared as temperature rose. At 450 oC, peaks were discernible at 472, 570, 632 and 692 cm-1, which are characteristic of SnO2. These changes again demonstrate the transformation of Sn3O4 to SnO2 even in an anoxic atmosphere 27-28.

Figure 3. DSC thermographs of SnO2 and as-prepared Sn3O4; inset: enlargement. Figure 3 depicts representative DSC results. Endothermic peaks with onsets at 170 oC appeared in both substances, corresponding to the desorption of absorbed 7



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water 24. In heated Sn3O4, an exothermal step with onset at 297 oC (insert) suggests a relaxation process, so dielectric spectroscopy was next conducted.

Figure 4. Complex permittivity ε of a typical pristine Sn3O4 pellet. Inset: respective Arrhenius plot for relaxation time of dielectric loss peaks P2 and P3. The thermal dependence of dielectric loss presented in Figure 4 exhibits three peaks. The first was always located at 200 oC and can obviously be ascribed to dehydration

29

. The other two peaks P2 and P3 shifted to higher temperature with

increasing frequency, and appear to originate in some relaxation processes. The relaxation time, obtained as the inverse of angular frequency, does follow the Arrhenius relationship when plotted against the inverse of the peak temperature: τT exp ⁄ where ω denotes angular frequency; Ea, an activation energy; kB, Boltzmann’s constant; and , pre-exponential factor. However, the Arrhenius parameters evaluated from least-squares fits for P2 and P3 have the respective values of 3.45 eV, 10-34 s, 9.85 eV and 10-79 s, whose orders of magnitude are anomalous. In the next section we shall speculate on the origins of the loss peaks. The permittivity data indicates fairly significant dielectric loss. AC conductivity 8


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can be evaluated as ε0ωε″, where ε0 stands for vacuum permittivity, and Figure 5 illustrates the example for ω/2π = 31.6 kHz; its inset shows that the Arrhenius plot of conductivity is linear from room temperature to 390 oC, with an Ea of 1.78 eV that stay unchanged in Sn3O4 and Sn2O3.

Figure 5. Imaginary part of permittivity at 31.6 kHz, as a function of inverse temperature. Blue line is fitted background with the two peaks subtracted. Inset plots derived conductivity.

Another batch of pellets were subjected to repeated temperature cycling between 25 and 425 oC. Figure 6 gives an example of their dielectric data, measured at 31.6 kHz. It was found that the peak P2 disappeared in the second cycle, and likewise P3, in the third. The same happened at other frequencies.

9



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Figure 6. Permittivity measured during three successive thermal cycles. After thermal cycling a pellet always left behind, on the electrodes, a little black substance, which we characterised with the help of XPS. The core levels Sn3d5/2 and Sn3d3/2 each consists of two components (Figure 7). The deconvoluted peaks of 485.2 and 493.6 eV in binding energy belong to Sn, and those of 487.1 and 495.5 eV, to Sn4+ 30-32

. Thus metallic Sn had formed during heating in vacuo 10-11, 33.

Figure 7. High-resolution XPS of Sn3d peaks.

Figure 8. Ex-situ Sn3d and O1s peaks in XPS of as-prepared Sn3O4, untreated or previously heated to 350 oC or 420 oC under a vacuum of 10 Pa.

To provide further information on the thermal decomposition, Sn3O4 powder was heated at 2 K/min to various temperatures, and, after cooling down, had their XPS 10


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scanned at high resolution in ranges of interest. Pertinent results are displayed in Figure 8. There, each Sn 3d5/2 peak can be resolved into the same three components at 486.1, 486.8 and 485.0 eV; the first of which is attributable to Sn2+, the second to Sn4+, and the third to metallic Sn 34. Likewise, the O1s peak has two components at 530 and 532 eV, assigned respectively to lattice and chemisorbed oxygen

35

. After

heating Sn3O4 exhibited the Sn component due to its decomposition. Table 1 and Figure 10 summarise our deductions from XPS analysis. The molar ratio O/Sn was 1.41, and Sn2+/(Sn2++Sn4+), 0.66 in as-prepared sample, confirming its Sn3O4 identity. After it had once been heated to 350 OC, O/Sn became 1.53 and the latter ratio, 0.5, which suggest the Sn2O3 composition

18

. When the temperature had

been 420 OC, the predominance of O increased further but that of Sn2+ decreased, indicating the conversion of the latter to Sn4+/Sn0.

Table 1. Binding energy EB and the relative peak area S of the components in Sn3d and O1s XPS, after Sn3O4 has previously been heated to temperature T. Components of O1s peak

T

Components of Sn3d5/2 peak Sn4+

Sn2+

Sn0

chemisorbed oxygen

lattice oxygen

EB (eV) S (%)

532.0 11

530.11 89

486.8 34

486.1

350 C

EB (eV)

531.9

530.1

486.9

486.3

S (%)

22

78

45

45

10

420 oC

EB (eV)

532.2

530.4

486.9

486.4

485.1

S (%)

13

87

60

30

10

25 oC o

11


66 485.0


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Figure 9. 119Sn NMR spectra of Sn3O4 and heated Sn3O4. 119

Sn NMR spectroscopy was also performed on the aforesaid XPS samples.

As depicted in Figure 9, each shows three resonance peaks at -569, -585 and -603 ppm, which are respectively assigned to 3, 4 and 6-coordinated Sn ions36-39. The change in their relative concentrations with heat treatment temperature again support our deduction that 3-coordinated Sn2+ in heated Sn3O4 are oxidised into 6-coordinated ions of SnO2. The persistence of 4-fold coordination will be discussed below.

Figure 10. Bottom: relative concentrations of Sn0, Sn2+ and Sn4+; Top: molar ratios Sn2+/(Sn2++Sn4+) and O/Sn.

12


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4. DISCUSION

XRD confirms that we have prepared Sn3O4 with a triclinic structure, and this and other experimental evidence, its thermal instability. It transforms to Sn2O3 above 250 oC, and to SnO2 above 420 oC in oxygen-free atmosphere, accompanied by the formation of metallic Sn. DSC reveals step-like enthalpy change, and impedance spectroscopy, relaxation-like processes that accompany the transformations. An examination of Figure 11 shows the (101)-layered structures of Sn3O4, Sn2O3 and SnO2. Compared to the last, Sn2O3 is short of two layers of oxygen over every four tin layers, whereas Sn3O4 is short of two layers for every three. These oxygen deficient layers on the (101) planes connect to 3-coordinated Sn2+; elsewhere tin ions are 6-coordinated Sn4+. At elevated temperature, electron transfers presumably occur at divalent ions situated on surfaces or interfaces, so that they are reduced to metallic Sn, while remaining 3-coordinated Sn2+ ions are oxidised to 6-coordinated Sn4+. The overall transformation in chemical composition is from Sn3O4 to Sn2O3 and finally to SnO2, plus a heterogeneous phase of Sn that nucleates and grows as islands. XRD shows that this chemical transformation merely causes slight shifts in diffraction peaks, apart from (101) and (312), which disappear. Apparently the tin sublattice is little affected all along. Deductions from NMR (Figure 10) indicate, however, the continual presence of 3-, 4- and 6-coordinated tin ions, though the first kind drops drastically in concentration at the end. The persistence of 4-coordinated Sn requires that the schematic structures of Figure 11 need to incorporate oxygen vacancies at unspecified sites. It is possible that those vacancies produce a static disorder, which, above 300 oC, becomes a dynamical disordered state, as in the case of a relaxor ferroelectric

40

. Such a diffuse phase transition involves the hopping of

O2- and corresponds to dielectric loss peak P2. As temperature further increases, P3 is detected when the ordered phase of SnO2 takes over.

13



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Figure 11. Schematic crystal structures of the three tin oxides; dotted lines represent (101) planes.

5. CONCLUSION

Sn3O4 is metastable and transforms to SnO2 in stages, with Sn2O3 as an intermediate, as divalent tin converts to quadrivalent ions and atoms, and changes from being 3-to 6-coordinated. However, 4-fold coordination persists and therefore oxygen vacancies exist. The two stages are associated with relaxation-like processes, associated probably with the transitions from static to dynamic disorder, and then to order, in the oxygen sublattice.

ACKNOWLEDGMENTS

This work was supported by the National Basic Research Program of China (under Grants 2016YFA0201604 and 2012CB934000).

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Figure 1. XRD at (a) 25 (b) 90 (c) 200 (d) 250 (e) 300 (f) 360 and (g) 420 oC, and (h) at roomtemperature after three thermal cycles. In (g) the peaks marked ‘*’ arose from the substrate, platinum, and an arrow points to an enlargement of the part enclosed by dotted lines. Room-temperature XRD for SnO2, Sn3O4 and Sn2O3 from data base is reproduced at top and bottom. 79x79mm (300 x 300 DPI)


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Figure 2. (a) Raman spectra; (b) magnified views of selected parts. Inset: thermal dependence of peak intensity at 170 cm-1. 63x50mm (300 x 300 DPI)



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Figure 3. DSC thermographs of SnO2 and as-prepared Sn3O4; inset: enlargement. 56x40mm (300 x 300 DPI)


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Figure 4. Complex permittivity ε of a typical pristine Sn3O4 pellet. Inset: respective Arrhenius plot for relaxation time of dielectric loss peaks P2 and P3. 87x95mm (300 x 300 DPI)



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Figure 5. Imaginary part of permittivity at 31.6 kHz, as a function of inverse temperature. Blue line is fitted background with the two peaks subtracted. Inset plots derived conductivity. 59x44mm (300 x 300 DPI)


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Figure 6. Permittivity measured during three successive thermal cycles. 77x74mm (300 x 300 DPI)



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Figure 7. High-resolution XPS of Sn3d peaks. 65x53mm (300 x 300 DPI)


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Figure 8. Ex-situ Sn3d and O1s peaks in XPS of as-prepared Sn3O4, untreated or previously heated to 350 oC or 420 oC under a vacuum of 10 Pa. 67x57mm (300 x 300 DPI)



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Figure 9.

119

Sn NMR spectra of Sn3O4 and heated Sn3O4. 64x51mm (300 x 300 DPI)


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Figure 10. Bottom: relative concentrations of Sn0, Sn2+ and Sn4+; Top: molar ratios Sn2+/(Sn2++Sn4+) and O/Sn. 87x94mm (300 x 300 DPI)



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Figure 11. Schematic crystal structures of the three tin oxides; dotted lines represent (101) planes. 58x42mm (300 x 300 DPI)


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Static and Dynamic Disorder in Metastable Phases of Tin Oxide - The

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