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Twin Crystal induced Near Zero Thermal Expansion in SnO2 Nanowires He Zhu, Qiang Li, Chao Yang, Qinghua Zhang, Yang Ren, Qilong Gao, Na Wang, Kun Lin, Jinxia Deng, Jun Chen, Lin Gu, Jiawang Hong, and Xianran Xing J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03232 • Publication Date (Web): 05 Jun 2018 Downloaded from http://pubs.acs.org on June 5, 2018

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Journal of the American Chemical Society

Twin Crystal Induced near Zero Thermal Expansion in SnO2 Nanowires He Zhu,† Qiang Li,† Chao Yang, ⊥ Qinghua Zhang,§ Yang Ren,‡ Qilong Gao,† Na Wang,† Kun Lin,† Jinxia Deng,† Jun Chen,† Lin Gu,§ Jiawang Hong,⊥ Xianran Xing†,* † Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China ⊥ School of Aerospace Engineering, Beijing Institute of Technology, Beijing 100081, China § Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ‡ X-Ray Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States

Supporting Information Placeholder ABSTRACT: Knowledge of controllable thermal expansion is a fundamental issue in the field of material science and engineering. Direct blocking of the thermal expansions in positive thermal expansion (PTE) materials is a challenging but fascinating task. Here we report a near zero thermal expansion (ZTE) of SnO2 achieved from twin crystal nanowires, which is highly correlated to the twin boundaries. Local structural evolutions followed by pair distribution function (PDF) revealed a remarkable thermal local distortion along the twin boundary. Lattice dynamics investigated by Raman scattering evidenced the hardening of phonon frequency induced by the twin crystal compressing, giving rise to the ZTE of SnO2 nanowires. Further DFT calculation of Grüneisen parameters convinces the key role of compressive stress on ZTE. Our results provide an insight into the thermal expansion behavior regarding to twin crystal boundaries, which could be beneficial to the applications.

It is well known that most materials expand upon heating, derived intrinsically from the anharmonicity of the intera1 tomic potential. However, materials with ZTE property are much desired for the applications such as precise instruments, micro-devices or systems in the environment of fre1 quent thermal shock. To date, two strategies are well studied to reach the goal of ZTE: combining PTE materials with negative thermal expansion (NTE) components to build compo2 sites, and tailoring the structural features of NTE materials chemically by doping to tune their coefficients of thermal 3 expansions (CTEs). For the ZTE materials within the first strategy, the internal stress induced from thermal expansion 4 mismatch for different components is a major concern. On the other hand, the second strategy is hindered by the very 5 few number of NTE materials. The ability to approach single-phase ZTE materials from PTE materials, i.e., directly weakening the thermal expansion without chemical constituent alteration, would therefore offer a significant step for-

ward. Recently, it has been shown that materials with shortrange coherence could exhibit much different CTEs in contrast to bulk, due to the defects and disorders that surely 6 affect the lattice thermal evolutions. This motivates us to modify the thermal expansion of materials by nanostructured designs. As a typical n-type semiconductor (3.6 eV, 300 K), SnO2 has attracted much attention due to its wide range of appli7 8 9 cations, including gas sensor, catalysis, solar cell, field 10 emitter, etc. The success of these applications requires a sound understanding of the short-range structure (the so 11 called structure-property relationship), and the thermal expansion property governs the deactivation mechanism in temperature fluctuate. Twin crystal boundaries, as one kind of planar defects, has been observed in single-crystal and 10, 12 13 14 poly-crystal nanowires, films, nanobelts and nanocrys14 15 tals in SnO2, which benefits the gas sensitivity and field10 emission property . Although mirror symmetric arrangements could be identified from transmission electron microscopy (TEM), the related local distortions regarding to the twin boundaries are far from clear. As a result, a fundamental study on the local distortion as well as thermal expansion property regarding to twin crystal boundaries deserves to be undertaken. Here we report a near ZTE in SnO2 nanowires, which composed of entirely twin nanocrystals. Pair distribution function (PDF) of synchrotron X-ray scattering was carried out to study the evolutions of local thermal distortions. The direct weakening of the thermal expansion is correlated to the stressed twin boundaries, where the sandwiched atoms vibrate at a high and constant frequency as the temperature rises. The present study provides a good example of tunable thermal expansion with nanostructured modifications. SnO2 twin crystal nanowires were prepared by hydrothermal-hydrolysis of SnCl4 in the presence of ethanol and NaOH (detailed synthesis are described in SI). According to the TEM image (inset of Figure 1a), flexural nanowires with diameter around 5 nm were achieved (see the histogram of

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straight-section lengths in Figure S1 inset). High-resolution TEM images (HRTEM, Figure 1a and Figure S1) demonstrate that the nanowires are composed of twin-nanocrystals with the twin boundaries parallel to (1 0 1) plane. Viewed along [1 1 -1] direction, the (1 -1 0) planes in the neighboring crystals are mirror symmetric about the twin boundaries, as also shown in the inverse Fourier transform images (see Figure S1). Consequently, zigzag morphology with periodic break-angle of o 133 was observed for the nanowires.

Figure 1. (a) The HRTEM image of SnO2 twin crystal nanowires. The zigzag morphology of the nanowire has been sketched. The inset is the TEM image of the nanowires. (b) ABF image of atomic arrangement around twin boundary viewed along [1 1 -1]. The left inset is the zoom-in image emphasizing the atomic arrangement in the yellow box. The right inset is the schematic diagram of the mirror symmetric structure regarding to the (1 0 1) twin boundary.

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(1.20 ± 0.08) × 10 / C for a-axis and (0.84 ± 0.07) × 10 / C for c-axis. This makes the volume CTE weaken to (3.23 ± 0.16) -6 o × 10 / C (Figure S4). The only difference between the twin crystal nanowires and nanoparticles is the oriented attachment of the grains, which proves the key role of twin boundaries in weakening the thermal expansion of SnO2 nanowires. SnO2 with rutile-type structure possesses two kinds of SnO bonds to form edge-shared SnO6 octahedron chains along c-axis (see Figure 1b inset). Each octahedron is constituted by four longer Sn-O bonds (marked as 1) in the rectangle section, as well as two shorter Sn-O bonds (marked as 2) along its symmetric axis (Figure 3a). The thermal evolutions of local structure in nanowires, nanoparticles and bulk were clearly revealed by full profile refinements of X-ray pair distribution function (PDF, Figure S5, details in Table S4-S9). From the view of short-range structure, the ZTE of the nanowires could be attributed to two aspects. On one hand, the longer Sn-O bonds keep almost constant with temperature (Figure 3b), which gives rise to the ZTE in the c direction. On the other hand, the obtuse O-Sn-O angles (θ in Figure 3a) increase upon heating (Figure 3c), compensating the slight expansion of shorter Sn-O bonds in the ab plane (Figure 3d). These PDF results could reflect the lattice thermal distortions influenced by the twin boundaries, because the atoms away from the twin boundaries possess the same coordination environment as nanoparticles, whose local thermal evolutions are similar to the bulk (see Figure 3b-d). The general size effects on the ZTE of nanowires could be eliminated.

Annular bright-field (ABF) scanning TEM shows the atomic-scaled image of the twin boundary, in which dark spots represent the atomic arrangement (Figure 1b). The adjacent twin-crystals share Sn atoms without mismatch, and both Sn and O atoms present mirror symmetry with respect to the twin boundaries. Unlike the nonstoichiometric twin bounda16 ry generally observed (in TiO2 for example ), the oxygen vacancies are not essential for the construction of the present SnO2 twin crystals. The occupancy of the oxygen atoms sited specularly across the twin boundary is comparable to that in the lattice, which results in a significant contraction of the O-O distance (shown in the inset of Figure 1b). This closely packed oxygen sublattice certainly indicates local lattice distortions in the twin crystal boundaries of SnO2 nanowires. For comparison, SnO2 nanoparticles with similar grain size to the nanowires (~ 4 nm) were also prepared (synthetic method in SI, TEM images shown in Figure S2). The tetragonal lattice parameters (space group, P42/mnm) of SnO2 twin nanowires along with nanoparticles and bulk SnO2 under variable temperatures were investigated by X-ray diffraction (XRD, Figure S3). Because of the broadening and overlapping of the XRD peaks, Lebail method was carried out to extract 6a, the lattice parameters instead of the exact atomic locations 17 (see detailed lattice parameters and CTEs in Table S1-3), and all the XRD measurements were carried out from high to low temperature to eliminate the possible water effect on 18 lattice . As seen in Figure 2, bulk SnO2 shows a positive -6 o thermal expansion with the CTEs of (4.90 ± 0.10) × 10 / C -6 o for a-axis and (5.42 ± 0.11) × 10 / C for c-axis. For the SnO2 nanoparticles, the CTEs for both a- and c-axis are close to -6 o those of bulk, with the values of (3.45 ± 0.11) × 10 / C and -6 o (4.70 ± 0.12) × 10 / C, respectively. Remarkably, the twin crystal nanowires show a near ZTE behavior, with the CTE of

Figure 2. Thermal expansions of SnO2 nanowires, nanoparticles and bulk sample for (a) a-axis and (b) c-axis.

We performed density-functional theory (DFT), as written 19 into VASP code , to provide a more specific view of the distorted lattice around (1 0 1) twin boundary (computational details are also given in SI). The completely relaxed unit cell of rutile SnO2 was regarded as the benchmark and the building block. The twin boundary was modelled by periodic supercell within mirror symmetric arrangement about (1 0 1) plane, and only three layers of Sn-O octahedra across the


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Journal of the American Chemical Society twin boundary were allowed to relax during the structural optimization (Figure S6). In purely geometrical terms of mirror symmetry, the O-O distance across a twin boundary has to be substantially reduced from bulk spacing of 3.186 Å towards twin spacing of 1.619 Å (see Figure S6, also observed in the ABF image, Figure 1b). Upon relaxation, the O-O distance reduces to 2.035 Å, which is still much shorter than the bulk one. This compressed oxygen sublattice causes a strong repulsion at the twin boundary, which consequently drives the distortion of the Sn-O octahedra to avoid the electronic dispersion overlapping (Figure S7). In short, the local structures that are sandwiched between two twinned crystals are distorted driven by the interatomic repulsion. Such internal stress could affect more atoms than the ones just sandwiched between the twin crystals, and also act on their atomic vibrations (phonon), which governs the thermal expansion property.

It is known that the atomic thermal vibration is highly cou21 pled with crystal structure. The observed invariable phonon frequencies indicate that the local structures, which correspond to the vibrational displacements, remain unchanged dimensions, driving the near ZTE behavior of SnO2 nanowires. From our DFT calculation, the repulsion caused from the short O-O distance cannot be completely released through lattice distortion, since the relaxed O-O atom pairs is still compressed. The phonon stiffness, as also observed in 22 graphene with compressive stress, provides experimental evidence of internal stress from the compressed twin boundaries. Anisotropic Grüneisen parameters were calculated to investigate the stress effect on phonon (details are given in 23 SI). We found that the stressed lattice could lead to decreased Grüneisen parameters along both a and c directions. Such trend indicates that the compressive stress from the twin boundaries could weaken the thermal expansion property, which further supports the proposed mechanism on 24 ZTE. From energetics point of view, the non-expanded Sn25 O bond, which is very rare unless coupled with magnetism, 26 27 ferroelectricity, or charge transfer, is a clear sign of the more harmonic interatomic potential. From the above, a conclusion could be drawn that the near ZTE of the SnO2 nanowires is driven by the phonon hardening, induced from the stressed twin boundaries.

Figure 3. (a) Schematic diagram of SnO2 unit cell. The pink lines depict the longer Sn-O bonds and the blue lines depict the shorter Sn-O bonds. The obtuse O-Sn-O angles are marked as θ. The PDF results of thermal evolution of (b) the longer Sn-O bond, (c) the angle θ and (d) the shorter Sn-O bond for nanowires, nanoparticles and bulk.

Raman spectra study, which provides a direct observation of phonon mode, was carried out to further evidence the ZTE behavior of SnO2 twin crystal nanowires. The typical Raman spectrum of bulk contained three bands, Eg, A1g, B2g, from -1 400 to 800 cm (Figure S8a). For the nanoparticles and the twin crystal nanowires, the two newly appeared broad fea-1 -1 tures, approximately at ~437 cm and ~565 cm respectively 19 (Figure S8b-8c), could be ascribed to surface disorders. These newly emerged features conceal the Eg band of nanowires, and the Eg and B2g bands of nanoparticles, so only the frequency shifts of the other bands as a function of temperature were taken into account (Figure 4a-b, insets are the related atomic vibrational modes). Redshifts of the Raman peaks for bulk and nanoparticle samples were observed with temperature rising, which is the result of PTE within quasi20 harmonic approximation. In addition, the invariable phonon frequencies of twin-crystal nanowires could be relevant to the ZTE property. Remarkably, the phonon modes of nanowires exhibit a trend of stiffness in contrast to the bulk ones. This means the atoms near the twin boundary vibrate at a high and constant frequency as the temperature rises.

Figure 4. (a) Frequency shifts as a function of temperature for A1g and (d) B2g. The insets in (a) and (b) show the vibration modes corresponding to the Raman features.

In summary, the SnO2 nanowires composed entirely of twin crystals were prepared. A near ZTE property was observed for the nanowires, which is highly correlated to the twin boundaries. The lattice near the twin boundary has been evidenced to be compressed by the specular matching, which not only distorts the local structure, but also hardens the phonon to give ZTE. In this study, we have shown that the thermal expansion could be weakened for a stressed lattice of twin crystal, which might provide a new route of controlling thermal expansion property.

ASSOCIATED CONTENT Supporting Information



The synthesis of SnO2 nanowires and nanoparticles, details of ab initio calculations, characterization, and other experimental results mentioned in this work. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author [email protected].

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21590793 and 21731001), the Program for Changjiang Scholars, the Innovative Research Team in University (IRT1207), and the Program of Introducing Talents of Discipline to Universities (B14003). This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.

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Journal of the American Chemical Society SYNOPSIS TOC


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Figure 1. (a) The HRTEM image of SnO2 twin crystal nanowires. The zigzag morphology of the nanowire has been sketched. The inset is the TEM image of the nanowires. (b) ABF image of atomic arrangement around twin boundary viewed along [1 1 -1]. The left inset is the zoom-in image emphasizing the atomic arrangement in the yellow box. The right inset is the schematic diagram of the mirror symmetric structure regarding to the (1 0 1) twin boundary. 150x72mm (300 x 300 DPI)


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Figure 2. Thermal expansions of SnO2 nanowires, nanoparticles and bulk sample for (a) a-axis and (b) caxis. 103x154mm (300 x 300 DPI)



Figure 3. (a) Schematic diagram of SnO2 unit cell. The pink lines depict the longer Sn-O bonds and the blue lines depict the shorter Sn-O bonds. The obtuse O-Sn-O angles are marked as θ. The PDF results of thermal evolution of (b) the longer Sn-O bond, (c) the angle θ and (d) the shorter Sn-O bond for nanowires, nanoparticles and bulk. 112x90mm (300 x 300 DPI)


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Figure 4. (a) Frequency shifts as a function of temperature for A1g and (d) B2g. The insets in (a) and (b) show the vibration modes cor-responding to the Raman features. 138x183mm (300 x 300 DPI)


Twin Crystal Induced near Zero Thermal ... - ACS Publications

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