Formation of orthorhombic SnO₂ originated from lattice distortion by Mn-doped tetragonal SnO₂

Abstract

Tin dioxide (SnO₂) is an n-type semiconductor material with a tetragonal rutile crystal structure under normal conditions and displays many interesting physical and chemical properties. Another form of SnO₂ with an orthorhombic crystal structure is known to be stable only at high pressures and temperatures. However, there are limited reports on the effects of Mn-doped tetragonal phase SnO₂ on micro/nanostructured characteristics. In this article, micro/nanostructures of Mn-doped tetragonal phase SnO₂ have been successfully prepared by the chemical co-precipitation method. The micro/nanostructural evolution of Mn-doped tetragonal phase SnO₂ under different heat treatment temperatures is evaluated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy. It is surprisingly found that the orthorhombic phase SnO₂ is formed in Mn-doped tetragonal phase SnO₂. The obvious diffraction peaks and clear lattice fringes confirmed that the orthorhombic phase SnO₂ nanocrystals exist in Mn-doped SnO₂ samples. Experimental results indicated that the XRD peak intensities and crystal planes of the orthorhombic phase SnO₂ decrease with increasing heat treatment temperatures. The formation of orthorhombic phase SnO₂ is attributed to the lattice distortion of tetragonal phase SnO₂ due to the Mn-doped tetragonal phase SnO₂.

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Introduction

Tin dioxide (SnO₂) is a unique material with widespread technological applications, particularly in the field of strategic functional materials. SnO₂, as a kind of n-type wide-band-gap semiconductor material (E_g = 3.64 eV at 300 K), has been extensively and intensively studied in the past few years,^1–4 and exhibits superior properties such as transparency, and remarkable chemical and thermal stabilities, and is used in solar cells,^5,6 gas sensors,^7,8 electrode materials,^9,10 catalysts,¹¹ and optoelectronic devices.^12,13 It has been noted that the most important form of SnO₂ is cassiterite, a phase of SnO₂ with the tetragonal rutile crystal structure. In recent years, the micro/nanostructural characteristics and prosperities of tetragonal phase SnO₂ have been extensively studied by our and other research groups,^14–17 motivated in part by many technological applications in gas sensing, optical and electrical properties.^18–22

In addition to the stable tetragonal phase SnO₂, it existed another form and was called orthorhombic phase crystal structure (a = 0.4714 nm, b = 0.5727 nm, and c = 0.5214 nm).¹⁴ However, orthorhombic phase SnO₂ has been seldom investigated in the past few years because it is metastable structure. It is known that the orthorhombic phase SnO₂ was usually found in high pressure and temperature experiments. For example, Suito's research group has first synthesized SnO₂ powders with an orthorhombic phase structure at a high pressure of 15.8 GPa and a temperature of 800 °C.²³ The orthorhombic phase SnO₂ has also been formed in diamond-anvil SnO₂ experiments by Liu,²⁴ who found that it was formed from a higher-pressure fluorite-type phase upon release of pressure. Joseph Lai and Shek's research group detected the orthorhombic phase in X-ray scattering measurements of SnO₂ powders.²⁵ Ultrafine oxidized tin particles with particle size about 6 nm have been prepared by inert gas condensation deposition under low oxygen pressure. They believed that the orthorhombic phase SnO₂ may be an intermediate product when disordered tin oxide (amorphous or nanoparticle) transforms to stable tin oxide (rutile phase) on annealing under oxygen deficiency conditions. Kaplan also reported that both tetragonal and orthorhombic SnO₂ phases were found in Sn–O films deposited at substrate temperatures in the range 350–500 °C.²⁶ The high-pressure orthorhombic phase SnO₂ is believed to have the same crystal structure as α-PbO₂, which was refined by Kong et al. using a high vacuum metal organic chemical vapor deposition (MOCVD) system.²⁷ A similar sequence of orthorhombic phase SnO₂ was reported by our research group in studies of SnO₂ thin films using pulsed laser deposition (PLD).^28,29 Above experimental results indicated that the orthorhombic phase SnO₂ can be synthesized by a variety of technological routes. It can be reasonable to extrapolate that the high pressures/temperatures or strain may be the vital factors for the formation of the orthorhombic phase SnO₂.

Transition metal Mn-doped tetragonal SnO₂ may induce their lattice distortion since the Mn³⁺ (0.65 Å) or Mn⁴⁺ (0.54 Å) ionic radius is smaller than that of Sn⁴⁺ (0.69 Å),³⁰ which will result in high compressive stresses or high pressures on the SnO₂ and generate the orthorhombic phase SnO₂. However, this strategy has not been reported so far. Sangaletti and co-workers have reported SnO₂ multilayer thin film grown by the rheotaxial growth and thermal oxidation method on Al₂O₃ substrates.³¹ Their results indicated that, in addition to the SnO₂ cassiterite phase, a contribution from another SnO₂ orthorhombic phase was present, which can be related to cassiterite by introducing micro-twinning effects. This SnO₂ multilayer thin film showed a higher sensitivity towards CO with respect to the conventional single layer SnO₂ sensors. The formation of orthorhombic-phase SnO₂ is intimately tied to a number of important synthesis parameters such as high pressures and temperatures.^23,24,32 Müller found an unknown epitaxial interface phase of SnO₂ on α-quartz (101̄0), which indicated that different octahedra stacking in the case of SnO₂ may give rise to different orthorhombic possibilities.³³ Arbiol and co-workers have reported the synthesis of pure monocrystalline orthorhombic SnO₂ nanowires and pure monocrystalline orthorhombic SnO₂ nanowires decorated with cassiterite SnO₂ nanoclusters.³⁴ In fact, when previous experiments are examined in detail, it is often difficult to rule out the possibility of the presence of high compressive stress.

In this article, the doping of tetragonal phase SnO₂ particles with Mn ions will be carried out using a simple chemical co-precipitation method. The micro/nanostructures of Mn-doped tetragonal phase SnO₂ under different heat treatment temperatures are evaluated in detail by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). We corroborated the coexistence of both tetragonal and orthorhombic SnO₂ phases in Mn-doped SnO₂ samples under different heat treatment temperatures. The obvious XRD peaks and clear lattice fringes observed by HRTEM confirmed that the orthorhombic phase SnO₂ nanocrystals evidently exist in Mn-doped SnO₂ samples. Experimental results indicated that the XRD peak intensities and crystal planes of the orthorhombic phase SnO₂ decrease with increasing of heat treatment temperatures. The micro/nanostructural analyses certificated that the crystallographic structure and lattice mismatch at the interface between tetragonal and orthorhombic SnO₂ phases is very important in defining the micro/nanostructure characteristics. The strain originated by the different lattice parameters of the tetragonal and orthorhombic SnO₂ in relation to the doping of tetragonal phase SnO₂ particles with Mn ions may play a critical role in formation of the orthorhombic phase SnO₂.

Experimental

All chemicals used in this experiment were analytical grade without further purification. A typical procedure to synthesize Mn-doped SnO₂ particles was performed as follows: the Mn-doped tetragonal phase SnO₂ particles were prepared by a simple chemical co-precipitation method using SnCl₂·2H₂O and MnCl₂·6H₂O as the sources of Sn and Mn ions. The SnCl₂·2H₂O was added in de-ionized water (0.2 mol L⁻¹) and mixed with MnCl₂·6H₂O (5 mol%) homogeneously. The above mixed solution was refluxed at 130 °C for 36 h under air atmosphere. The precipitation was carried out using aqueous ammonia (1 mol L⁻¹) after cooling the refluxed solution. The primal sample was washed several times with de-ionized water to remove the water-soluble impurities and free reactants and dried at 80 °C for 10 h. In order to obtain the better crystalline SnO₂ particles, the as-precursor was calcined in air at four different temperatures: 250, 350, 450, and 550 °C for 3 h, respectively. In order to better understand the effects of the Mn-doped SnO₂ particles, a pure tetragonal phase SnO₂ sample was also prepared using a similar method and calcined in the same conditions. The micro/nanostructural evolution of the resultant powders was characterized by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) techniques.

XRD patterns were obtained from Japan Rigaku D/max-2500 using Cu Kα radiation in reflection geometry. A proportional counter with an operating voltage of 40 kV and a current of 40 mA was used. XRD patterns were recorded at a scanning rate of 0.08° s⁻¹ in the 2θ ranges from 20 to 60°. HRTEM observations were performed on a JEOL JEM-2010F transmission electron microscope operating at 200 kV.

Results and discussion

The crystalline evolution of samples from the as-synthesized powders during heat treatment in air for different temperatures was investigated by XRD techniques. Fig. 1 shows the typical XRD patterns of the undoped samples which were taken from the as-synthesized sample (Fig. 1A) and heat treatment in air for 250 °C (Fig. 1B), 350 °C (Fig. 1C), 450 °C (Fig. 1D), and 550 °C (Fig. 1E). The XRD patterns at various heat treatment temperatures show that the peak intensities and sharpnesses are enhanced with compared to the as-synthesized sample, indicating the undoped samples at various heat treatment temperatures are well-crystallized. The presence of the broad and weak peaks as shown in Fig. 1A indicated that the nanoparticles of the as-synthesized sample are smaller than heat treatment one because the width of the XRD peaks is related to the particle size through Scherrer's formula: D = Kλ/β cos θ, where D is the diameter of the nanoparticles, K = 0.9, λ (Cu Kα) = 1.5406 Å, and β is the full width at half-maximum of the diffraction peak. The major diffraction peaks of all undoped samples corresponded to the tetragonal unit cell structure of SnO₂ with lattice constants a = b = 4.738 Å and c = 3.187 Å, which are consistent with the standard values for bulk SnO₂ (International Center for Diffraction Data, PDF file no. 41-1445). The observed (hkl) peaks are (110), (101), (200), (211), (220), and (002). Comparing with Fig. 1A and Fig. 1B–E, it can be confirmed that the growth of the SnO₂ nanoparticles is influenced significantly by heat treatment temperatures. Experimental results indicated that the grain size of the undoped SnO₂ nanoparticles increases with increasing of heat treatment temperatures. All peaks as shown in Fig. 1B–E became sharp and stronger, proving that the heat treatment temperature is a potentially powerful technique to improve the growth of SnO₂ nanocrystals. However, it is not able to find the surprise changes in the undoped SnO₂ samples. We will investigate in detail the effects of Mn-doping of tetragonal phase SnO₂ on micro/nanostructural characteristics.

Fig. 1 The typical XRD patterns of pure tetragonal phase SnO₂ nanoparticles after heat treatment in air for (A) as-synthesized, (B) 250 °C, (C) 350 °C, (D) 450 °C, and (E) 550 °C.

In order to search of novel micro/nanostructured materials with orthorhombic features along with precise controllable physical and chemical properties, the wide band gap oxides such as SnO₂, TiO₂, ZnO, and HfO₂ doped with transition metal ions such as Mn, Co, Ni, Fe, Cr, etc. have attracted considerable attention due to a number of distinctive optical and electronic properties originating from large sp–d exchange interaction between the transition metal ions and the band electrons.³⁵ Fig. 2 represents the typical XRD patterns of the Mn-doped tetragonal phase SnO₂ with the Mn = 5 mol% which were taken from the as-synthesized sample (Fig. 2A) and heat treatment in air for 250 °C (Fig. 2B), 350 °C (Fig. 2C), 450 °C (Fig. 2D), and 550 °C (Fig. 2E). It was surprised to find that the Mn-doped samples of the as-synthesized and heat treatment at various temperatures were composed of orthorhombic and tetragonal SnO₂. The diffraction peaks at (110), (101), (200), (211), (220), and (002) planes can be indexed to the tetragonal phase SnO₂ and the reflections at (110), (111), (021), (022), (130), and (113) planes are attributed to the orthorhombic unit cell of SnO₂. The higher heat treatment temperature gave rise to an increase in the peak intensity of tetragonal phase SnO₂ at the expense of orthorhombic phase SnO₂. However, the Mn-doped sample heated even up to 550 °C was still composed of a mixture of orthorhombic and tetragonal SnO₂. The experimental results indicated that there were no extra peaks of manganese oxides such as MnO, Mn₂O₃, MnO₂, Mn₃O₄, and any Sn/Mn ternary oxides, implying that the transition metal ions have substituted at the Sn⁴⁺ sites. Furthermore, the peak position shifted to larger angle at the Mn-doped samples, revealing possible changes in lattice parameters. As the heat-treatment temperature increasing, the peaks of the Mn-doped samples have no obvious changes, suggesting that a portion of the metal oxide ions formed stable solid solutions with SnO₂ and occupied in the regular lattice sites of SnO₂.

Fig. 2 The typical XRD patterns of Mn-doped tetragonal phase SnO₂ (Mn = 5 mol%) after heat treatment in air for (A) as-synthesized, (B) 250 °C, (C) 350 °C, (D) 450 °C, and (E) 550 °C.

In order to understand the precise formation processes of the orthorhombic phase SnO₂ and their relation to micro/nanostructure with tetragonal phase SnO₂ by the Mn-doping, we will investigate the possible mechanism to explain the crystallographic behavior. HRTEM observations in relation to the Mn-doping of tetragonal phase SnO₂ particles at different heat treatment temperatures can give useful information about local composition and lattice mismatch at dislocation cores. A more detailed analysis can be made based on the highly magnified HRTEM images as shown in Fig. 3, which were taken from the as-synthesized undoped SnO₂ nanoparticles as shown in Fig. 3a and as-synthesized Mn-doped SnO₂ (Mn = 5 mol%) as shown in Fig. 3b. It can be found that the vague lattice fringes were formed in the as-synthesized undoped and Mn-doped samples. It appeared that the surface layer of the as-synthesized undoped and Mn-doped samples is covered with the amorphous oxides since a long-range ordering of lattice planes is not observed. However, the detailed crystallographic analysis indicated that some of the observed lattice fringes may originate from different crystal structure. The lattice fringes observed in Fig. 3a demonstrated that the as-synthesized undoped sample is composed of ultrafine nanoparticles with a diameter below 5 nm, even if it is greater than one of Fig. 3b. On closer inspection, the recurrent values of separation distance between lattice layers are found (in particular, 0.33 nm), which corresponds to the lattice parameters of the tetragonal structure of SnO₂ (evidenced in the inset of Fig. 3a). Simultaneously, it is found from the as-synthesized Mn-doped sample (Fig. 3b) that, besides the tetragonal phase SnO₂ related to the crystal plane (110), the (111) crystal plane also was observed, which corresponds to the interplanar spacing (0.29 nm) of the orthorhombic phase SnO₂ as shown in the inset of Fig. 3b. The average nanoparticle size was less than 3 nm. A more detailed analysis from Fig. 3b indicated that the slight misorientations are visible in the HRTEM image of a nanocluster composed of several primary SnO₂ nanocrystallines. These misorientations or defects originated from imperfect attachment among several nanocrystallines, resulting in the edge and screw dislocations. The strain originated by the different lattice parameters of the tetragonal and orthorhombic SnO₂ in relation to the Mn-doping of tetragonal SnO₂ particles may play a critical role in formation of the orthorhombic phase SnO₂. Above experimental results indicated that both tetragonal and orthorhombic SnO₂ phases are coexisted in the as-synthesized Mn-doped SnO₂ sample, which is consistent with the XRD results as shown in Fig. 2A.

Fig. 3 The typical HRTEM micrographs of (a) as-synthesized undoped SnO₂ nanoparticles and (b) as-synthesized Mn-doped SnO₂ (Mn = 5 mol%).

It is known that the SnO₂ with an orthorhombic structure was stable only at high pressures and temperatures, and was a metastable phase under normal conditions. Further advancement of the formation processes for this orthorhombic phase SnO₂ requires a clear understanding of its thermal stabilities. Fig. 4 shows the typical HRTEM micrographs of the Mn-doped SnO₂ (Mn = 5 mol%) after being heated at (a) 250 °C (Fig. 4a) and (b) 350 °C (Fig. 4b). It can be seen that the crystallinity properties of the Mn-doped SnO₂ heated at 250 °C and 350 °C increased with increasing of the heat treatment temperatures, comparing to the as-synthesized Mn-doped SnO₂ as shown in Fig. 3b. Many long-range ordered lattice fringes can be clearly observed in the Mn-doped SnO₂ and the average nanoparticle sizes increased with increasing of the heat treatment temperatures as shown in Fig. 4. When the Mn-doped SnO₂ sample heated to 250 °C, the (110) crystal plane related to the tetragonal SnO₂ and (111) crystal plane connected with the orthorhombic SnO₂ were clearly observed. Moreover, the average nanoparticle size increased to about 5 nm as shown in Fig. 4a. When the Mn-doped SnO₂ sample heated to 350 °C, the lattice firings became much clearer, and the average nanoparticle size increased to about 7 nm as shown in Fig. 4b. The experimental results indicated that the heat-treatment effects could improve the growth of the SnO₂ nanoparticles.

Fig. 4 The typical HRTEM micrographs of Mn-doped SnO₂ (Mn = 5 mol%) after being heated at (a) 250 °C and (b) 350 °C.

When the Mn-doped SnO₂ (Mn = 5 mol%) samples were heated up to higher temperatures, e.g. 450 °C and 550 °C, the careful XRD examinations revealed that the Mn-doped SnO₂ samples displayed the coexistence of the tetragonal and orthorhombic SnO₂ as shown in Fig. 2D and E, respectively. In order to examine the micro/nanostructure evolution of the Mn-doped SnO₂ (Mn = 5 mol%) samples at the higher heat-treatment temperatures, further HRTEM investigations are presented in Fig. 5, which shows the typical HRTEM micrographs of the Mn-doped SnO₂ (Mn = 5 mol%) after being heated at (a) 450 °C (Fig. 5a) and (b) 550 °C (Fig. 5b). It can be seen that both SnO₂ phases, tetragonal and orthorhombic structures, were still randomly distributed in the higher temperature heat treatment samples. The gradual growth of the tetragonal SnO₂ at the expense of the orthorhombic SnO₂ was observed in the micro/nanostructure evolution. The (110) crystal plane related to the tetragonal SnO₂ and (111) crystal plane connected with the orthorhombic SnO₂ can be more clearly observed in the insets of Fig. 5a and b. The misorientations and defects reduced and the lattice fringes were more perfect with increasing of the heat treatment temperatures. The average nanoparticle size increased to about 8 nm and 10 nm as shown in Fig. 5a and b, respectively. All the above experimental results proved that the strain originated by the different lattice parameters of the tetragonal and orthorhombic SnO₂ in relation to the Mn-doping of the tetragonal phase SnO₂ may play a critical role in formation of the orthorhombic phase SnO₂.

Fig. 5 The typical HRTEM micrographs of Mn-doped SnO₂ (Mn = 5 mol%) after being heated at (a) 450 °C and (b) 550 °C.

A detailed process of the transformation mechanism must take into account the instability of the Mn-doped SnO₂ samples in the temperature range in which the orthorhombic SnO₂ phase begins to form. For the as-synthesized Mn-doped SnO₂ (Mn = 5 mol%) sample, the nonstoichiometric oxides underwent a disproportionation reaction forming tetragonal SnO₂ and perhaps intermediate oxides. If the intermediate oxides serve as the matrix in which the orthorhombic SnO₂ phase nucleates, then the nucleation must obviously occur before the disproportionation reaction takes place. The formation of the orthorhombic phase SnO₂ is favored by a nucleation barrier which is lower than that for the tetragonal phase SnO₂. The enhancement of the metastable phase could be steric, that is, the formation of the orthorhombic phase SnO₂ may require less atomic rearrangement than the formation of the tetragonal phase SnO₂. As shown by Fig. 2A, the Mn-ions are inclined to incorporate into the tetragonal phase SnO₂ lattice in the form of Mn³⁺ (0.65 Å) or Mn⁴⁺ (0.54 Å) since their ionic radius is smaller than that of Sn⁴⁺ (0.69 Å),³⁰ which led to the shrinkage of the lattice constant due to smaller ionic radius of Mn³⁺ or Mn⁴⁺ substituted in place of Sn⁴⁺ sites. When the Mn-ions replaced the Sn⁴⁺, the production of the orthorhombic phase SnO₂ increased since the grain size was reduced, perhaps, the reduced grain size inhibited the disproportionation reaction. Such an effect has also been previously detected in a high-temperature X-ray scattering experiment of tin oxide powders with varying particle sizes.³⁶ In this way, it is therefore appeared more crystal planes of the orthorhombic phase SnO₂ in the as-synthesized Mn-doped sample. On the other hand, the XRD results as shown in Fig. 2B–E indicated that the initial heat treatment sample began to transform into the tetragonal SnO₂ after being heated at 250 °C, 350 °C, 450 °C, and 550 °C, respectively. It was again clearly revealed by the HRTEM observations as shown in Fig. 4 and 5. This research indicated that the strain originated by the different lattice parameters of the tetragonal and orthorhombic SnO₂ in relation to the Mn-doping of the tetragonal phase SnO₂ may play a critical role in formation of the orthorhombic phase SnO₂. The formation of the orthorhombic phase SnO₂ could be attributed to the lattice distortion of the tetragonal phase SnO₂ due to the Mn-doping.

Conclusions

In summary, the micro/nanostructures of the Mn-doped tetragonal phase SnO₂ have been successfully prepared by a chemical co-precipitation method. Their micro/nanostructural evolution under different heat treatment temperatures could be reasonably evaluated by the XRD and HRTEM techniques. It was surprisingly found that the orthorhombic phase SnO₂ could be formed in the Mn-doped tetragonal phase SnO₂. The obvious diffraction peaks and clear lattice fringes confirmed that the orthorhombic phase SnO₂ nanocrystals evidently exist in the Mn-doped SnO₂ samples. Experimental results indicated that the XRD peak intensities and crystal planes of the orthorhombic phase SnO₂ decrease with increasing of the heat treatment temperatures. The formation of orthorhombic phase SnO₂ could be therefore attributed to the lattice distortion of the tetragonal phase SnO₂ due to the Mn-doping. Our findings may enable this novel functional material with the orthorhombic phase SnO₂ to be tailor-made for a large number of applications such as optoelectronic devices and gas sensors.

Acknowledgements

The work described in this article was financially supported by the National Natural Science Foundation of China (Project Numbers: 11375111, 11428410, and 11074161), the Research Fund for the Doctoral Program of Higher Education of China (Project Number: 20133108110021), the Key Innovation Fund of Shanghai Municipal Education Commission (Project Numbers: 14ZZ098 and 10ZZ64), the Science and Technology Commission of Shanghai Municipality (Project Numbers: 14JC1402000 and 10JC1405400), the Shanghai Pujiang Program (Project Number: 10PJ1404100), and the Program for Changjiang Scholars and Innovative Research Team in University (Project Number: IRT13078). This work was also supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China [Project no. (RGC Ref. no.), CityU 119212].

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