Study on the interfacial structures of Tin oxide/multiwalled carbon nanotube heterojunctions

Abstract

Tin oxide/multiwalled carbon nanotube (SnO₂/MWCNT) heterojunctions were synthesized using the SnCl₂ solution method. The interfacial structures, especially the interactions between the SnO₂ and MWCNTs, were investigated by field emission scanning electron microscopy, transmission electron microscopy, ultrasonic destruction experiments, Raman spectroscopy, and thermal analysis. At the initial reaction stage, chemical bonds were the predominant interactions between the MWCNTs and SnO₂. Physical interactions, including Coulomb interactions and van der Waals forces, play key roles under an increasing reaction time. Raman spectroscopy and thermal analysis proved to be two valid methods for the characterization of the interactions between SnO₂ and MWCNTs. The strong interactions result in a low G/D strength ratio, as well as a low thermal stability of the MWCNTs in the SnO₂/MWCNT heterojunctions. The binding energy of the MWCNTs and SnO₂ was calculated based on differential thermal analysis and Hess's law and the results agreed with the interfacial morphological characteristics. This work presents a facile and low-cost approach to study the interfacial structures of carbon-based nanomaterials and opens a new possibility for investigating structural property relationships.

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1. Introduction

In the past few decades, carbon based nanomaterials have attracted great interest due to their unique properties. Among them, carbon nanotubes (CNTs)¹ and their aggregations exhibit p-type semiconductor characteristics and have shown excellent sensing properties.^2,3 CNT based nanocomposites are also attractive owing to their special properties, which to a great extent result from the synergistic reactions between the component species. Tin oxide (SnO₂) is a typical n-type wide band gap semiconductor, and has been a widely utilized gas-sensing material.^4–8 For polycrystalline SnO₂ nanomaterials, the stability and repeatability of the sensors often suffered degradation owing to the aggregation and growth of the nanoparticles. The formation of SnO₂/MWCNTs heterojunctions can avoid aggregation, which will improve the stability and repeatability of the sensors.⁹ In addition, the Schottky barrier between SnO₂ and CNTs is very low,^10,11 which is preferable for electron transport between SnO₂ and MWCNTs. So, SnO₂/MWCNT heterojunctions exhibit good sensing properties,^11–15 as well as increased specific mass capacity and extended durability as anodes of lithium-ion batteries .^16,17

Several strategies have been performed to prepare SnO₂/MWCNT heterojunctions, including chemical vapor deposition,¹⁸ the microwave-polyol process,¹⁹ the hydrothermal method,¹⁶ the vapor phase method,¹⁰ the solution mixing method,²⁰ and the SnCl₂ solution method.^21,22 The SnCl₂ solution method is the simplest, and the obtained interfacial structures are strongly dependent on the preparation conditions, such as the amount of HCl and the reaction temperature.^17,20 For the characterization, some special methods, such as X-ray absorption near-edge structures,^23–26 and atomic scanning transmission electron microscopy²⁷ have been utilized for the study of the interfacial structures, especially the interactions between the component species of the carbon based materials. However, it is still a challenge to investigate the interfacial structures using common methods. In addition, the interaction mechanism has never been clearly understood.

In our previous work,²⁸ using SnO₂/MWCNT heterojunctions as a precursor, porous SnO₂ nanotubes were prepared. The structures of the obtained SnO₂ nanotubes were somewhat dependent on the SnO₂/MWCNT heterojunctions, which were prepared with different reaction times. Thus indicating that the reaction time has some influence on the interfacial structures of the SnO₂/MWCNT heterojunctions. However, few report systemic studies on the influences of the reaction time on their interfacial structures. Herein we report a facile approach to control the interfacial structures of SnO₂/MWCNT heterojunctions by controlling the reaction time. In addition, chemical interactions, Coulomb interactions, and van der Waals forces were taken into account to explain the reaction time-controlled interfacial structures. Raman spectra and thermal analysis were employed to characterize the interactions between the SnO₂ and MWCNTs. The binding energies of the SnO₂/MWCNT heterojunctions were investigated based on Hess's law and differential thermal analysis.

2. Experimental

2.1. Preparation of SnO₂/MWCNT heterojunctions

MWCNTs with a 20–30 nm diameter were purchased from Shenzhen Nanotech Port Co. Ltd. The purification processes of the MWCNTs are the same as in our previous report.²⁸ In a typical synthesis of SnO₂/MWCNT heterojunctions, 1 g of tin(II) chloride (SnCl₂·2H₂O, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) was dissolved in 40 mL of distilled H₂O, and then 0.25 mL of HCl (38%, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) was added. Subsequently, 10 mg of the purified MWCNTs was dispersed in the above solution. The mixture was sonicated for 20 min and then stirred at room temperature for a certain time. The SnO₂/MWCNT heterojunctions were then separated by centrifugation and washed with distilled water several times until the pH of the solution was neutral. The obtained SnO₂/CNT heterojunctions were denoted as SnO₂/CNT-X, where X is the reaction time (hr).

2.2. Characterization

The prepared SnO₂/MWCNT heterojunctions were characterized by field emission scanning electron microscopy (FE-SEM, Sirion 200, operated at 5 kV), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HR-TEM, JEOL-2010, operated at 120 kV), and Raman spectroscopy (Raman spectrum, LABRAM-HR, He–Ne laser excitation line at 514.5 nm). The weight content of SnO₂ and the released heat was measured by thermal gravimetric analysis and differential thermal analysis (TGA and DTA, Pyris 1, heating rate 10 deg min⁻¹ in flow air).

3. Results and discussion

3.1. Influence of the reaction time on the interfacial structures

Fig. 1 presents the SEM images of the SnO₂/MWCNT heterojunctions synthesized by controlling the reaction time from 1 to 4 h. With the increased reaction time, the coated particles tend to congregate together. TGA curves shown in Fig. 2 present the weight contents of SnO₂ in the SnO₂/MWCNT-1 and SnO₂/MWCNT-2 were 39.0% and 42.3%, respectively. In SnO₂/MWCNT-3, the weight content of SnO₂ was only slightly increased to 44.1%. Further prolonging the reaction time to 4 h, the weight content decreased to 39.6%. It should be mentioned that all of the SnO₂/MWCNT heterojunctions were prepared by using the same methods, except for the reaction time. The results imply that some SnO₂ particles desquamated from the MWCNT surface in the fourth hour. However, from Fig. 1, it is clear that the diameters of the obtained heterojunctions increased with the increase in reaction time. The results suggest that SnO₂/MWCNT heterojunctions with different structures were obtained.

Fig. 1 FE-SEM images of SnO₂/MWCNT heterojunctions prepared with 1 h (a), 2 h (b), 3 h (c), and 4 h (d) of reaction time.

Fig. 2 TGA curves of the SnO₂/MWCNT heterojunctions prepared with different reaction times.

TEM and HRTEM images give more convincing evidence, and the results are shown in Fig. 3 and 4. Fig. 3 present the interfacial structures of SnO₂/MWCNT-2. The surface of the MWCNTs was coated with uniform and dense SnO₂ nanoparticles, and the average crystallite size of the SnO₂ nanoparticles was about 3–4 nm. The thickness of the SnO₂ layer was less than 10 nm. Fig. 4 presents different interfacial structures of SnO₂/MWCNT-4. The crystallite size was about 4 nm. However, though the weight content of the SnO₂ is low, the SnO₂ layer is obviously thicker than the ones of SnO₂/MWCNT-2. From Fig. 4a–c, is clear that many SnO₂ particles congregated together, and did not evenly coat the surface of the MWCNTs. Furthermore, some SnO₂ particles left the MWCNTs completely uncovered, as shown in Fig. 4d. The results suggest the interactions between the SnO₂ and MWCNTs were weaker than those of SnO₂/MWCNT-2. Therefore some SnO₂ particles were lost during the centrifugation and wash process.

Fig. 3 TEM (a, b) and HRTEM (c, d) images of SnO₂/MWCNT-2. The white dashed line in (c) presents the sidewall of a CNT.

Fig. 4 TEM (a, b, d) and HRTEM (c) images of the SnO₂/MWCNT-4. The red circles present the congregated SnO₂ particles, and the blue ones present the desquamated parts. The white dashed line in (c) presents the sidewall of a CNT.

In order to test the above conjectures, the MWCNTs in SnO₂/MWCNT-2 and SnO₂/MWCNT-4 were removed by thermal treatment at 650 °C for 5 min in air.²⁸SEM images of the obtained porous SnO₂ nanotubes before and after thermal treatment are shown in Fig. 5a and b, respectively. The diameter of the SnO₂ nanotubes obviously increased with the increase in reaction time. In Fig. 5b, some aggregated SnO₂ particles were observed. The results are consistent with the morphology of the SnO₂/MWCNT heterojunctions shown in Fig. 1. In addition, the above two heterojunctions were dispersed in water, and then treated by vigorous sonication for 1 h. After that, the MWCNTs were removed and the obtained SnO₂ nanotubes are shown in Fig. 5c, d, respectively. In Fig. 5c, large numbers of unabridged SnO₂ nanotubes and only a few aggregated SnO₂ particles were observed. Compared with Fig. 5a, the results confirm the high stability of SnO₂/MWCNT-2. However, after vigorous sonication, a great many of the aggregated SnO₂ particles and only a few short SnO₂ nanotubes were observed in Fig. 5d. The results mean some SnO₂ particles were detached from the tubes during the sonication process due to the weak interactions between the SnO₂ and MWCNTs.

Fig. 5 FE-SEM images of the SnO₂ nanotubes obtained by calcination the SnO₂/MWCNT-2 before (a) and after sonication (c). FE–SEM images of the SnO₂ nanotubes obtained by calcination the SnO₂/MWCNT-4 before (b) and after sonication (d).

3.2. Characterization of the interactions between SnO₂ and CNTs

In the SnO₂/MWCNT heterojunctions, the interactions between the SnO₂ and MWCNTs will change the structures and thermal stabilities of the MWCNTs. The SnO₂/MWCNT heterojunctions were characterized by Raman spectrum, and the results are shown in Fig. 6a. The corresponding G/D intensity values are shown in Fig. 6b. The G/D values of the pure MWCNTs, the SnO₂/MWCNT-1, and the SnO₂/MWCNT-2 are 1.12, 1.10, and 1.07, respectively. With a longer reaction time, the G/D values increased. Though the change in the G/D values was not considerable, the results imply the change of the interactions between them. The D-band of MWCNTs is known to be related to defects, and the increase of the D band is often attributed to the direct sidewall functionalization of the MWCNTs.^29,30 In this case, the aromatic carbons were converted from an sp² to an sp³ hybridization, which resulted in an increase in the average size of the sp³ domains. Before the preparation of SnO₂/MWCNT heterojunctions, the MWCNTs were calcined at 350 °C for 2 h in air, and then refluxed in 7.0 mol L⁻¹ of HNO₃ at 120 °C for 12 h. In the preparation process of SnO₂/MWCNT heterojunctions, no more defects were formed because HCl was a non-oxidizing acid. Therefore , the decrease of the G/D values resulted from the increase in sp³ hybridization. The results also imply that the SnO₂ coating promoted the formation of C–O bonds using the lattice oxygen of SnO₂,³¹ which suggests the formation of chemical interactions between the SnO₂ and MWCNTs. For SnO₂/MWCNT-4, the G/D values were similar to the ones of pure MWCNTs. So, the chemical interactions weakened with the prolonged reaction time.

Fig. 6 Raman spectra (a) and the G/D values (b) of the SnO₂/MWCNT heterojunctions prepared with different reaction times.

Fig. 7a presented the DTG curves of pure MWCNTs and the SnO₂/MWCNT heterojunctions. The onset oxidation temperatures (T_o) and the oxidation peak temperatures (T_p) are shown in Fig. 7b. The T_o of the pure MWCNTs is 477 °C. The T_o of the MWCNTs in the heterojunctions from SnO₂/MWCNT-1 to SnO₂/MWCNT-4 were 421 °C, 383 °C, 422 °C, and 446 °C, respectively. The change in T_p is similar to the change in T_o. The results indicate that the MWCNTs became unstable in the heterojunctions. Considering the diameter of MWCNTs, the lattice strain primarily determines the thermal stability, overruling the effect of defects of MWCNTs.³² The interactions between the SnO₂ and MWCNTs result in a compressive stress along a direction perpendicular to the tube surface.³³ So, the lattice strain resulting from the compressive stress determines the thermal stability of MWCNTs. In addition, the low G/D values, resulting from the strong interactions, also imply that the apparent activation energy for the oxidation of MWCNTs was decreased.³⁴ As a result, the strong interactions will favor the oxidation of the MWCNTs by SnO₂ particles in a similar manner to the Mars–van Krevelen mechanism.³⁵ In Fig. 7a, b, the T_o and T_p of MWCNTs in SnO₂/MWCNT-2 are the lowest, which further suggests that the interactions in SnO₂/MWCNT-2 were the strongest.

Fig. 7 DTG (a) and DTA curves (c) of the SnO₂/MWCNT heterojunctions prepared with different reaction times. (b) The relationships between the reaction time and the onset oxidation temperature (T_o), and the oxidation peak temperature (T_p). (d) The relationships between the reaction time and the released heat (H_d) of the heterojunctions.

The strong interactions also have influence on the released heat by MWCNT decomposition (H_d). Fig. 7c presents the differential thermal analysis (DTA) curves of the SnO₂/MWCNT heterojunctions, and the corresponding H_d values are shown in Fig. 7d. The H_d value of pure MWCNTs is −66.84 kJ mol⁻¹. The H_d values of the MWCNTs in SnO₂/MWCNT-1 and SnO₂/MWCNT-2 increased to −89.71 and −92.21 kJ mol⁻¹, respectively. Further increasing the reaction time to 3 h and 4 h, the H_d values were decreased to −81.78 and −68.28 kJ mol⁻¹, respectively. Based on Hess's law, the different H_d values should be attributed to the different initial state of MWCNTs because the final oxidation states of the MWCNTs were the same. Fig. 8 presents a Bonn Haber cycle designed for the preparation and decomposition of SnO₂/MWCNT heterojunctions. The binding energy (H_b) of SnO₂ and MWCNTs in the heterojunctions can be calculated from the H_d values. The H_b values from the SnO₂/MWCNT-1 to the SnO₂/MWCNT-4 were 19.35, 25.37, 15.03, and 1.98 kJ mol⁻¹, respectively, as shown in Fig. 8. The H_b values are much lower than the common chemical bonds, and similar to van der Waals forces. The results suggest that the chemical interactions between the SnO₂ and the MWCNTs are very weak, which may be attributed to the inadequate contact between the particles and tubes. The high H_b value means the strong interactions. So, the interactions in SnO₂/MWCNT-2 are the strongest, which consistent with the interfacial structural characters and the Raman analysis.

Fig. 8 Bonn Haber cycle presents the relationships between the released heat (H_d) and the binding energy (H_b) of the SnO₂/MWCNT heterojunctions. The relationship between the reaction time and H_b of the heterojunctions.

3.3. Influence of the reaction time on the interactions

It is well known that the addition of HCl can suppress the hydrolysis of SnCl₂. Under the same experimental conditions, the minimum amount of HCl is 0.13 ml.²¹ The formation of SnO₂ can be ascribed to the reaction between Sn²⁺ cations and dissolved oxygen in water. In the present work, 0.25 ml HCl was added, which is about 2 times greater than that of the minimum amount. At the initial reaction stage, the oxygen-containing functional groups on the side wall of MWCNTs act as active sites for the SnO₂ coating.^13,22 So, the chemical interactions between the SnO₂ and MWCNTs are the predominant interactions. After 2 h of reaction, more SnO₂ particles were formed and the MWCNTs were coated with close-packed multilayer SnO₂ particles, as shown in Fig. 3. The outer particles could not form chemical bonds with MWCNTs so the coating of these SnO₂ nanoparticles was attributed to physical adsorption via physical interactions.

Coulomb interactions and van der Waals forces are two major physical interactions between the SnO₂ particles and MWCNTs. In the present work, the strong acidity resulted in the formation of C–OH₂⁺ and Sn–OH₂⁺ on the MWCNT and SnO₂ surface, respectively.^36,37 As a result, the repulsive electrostatic force between the MWCNTs and the outer SnO₂ particles was very strong. In addition, with the oxidation of Sn²⁺ to Sn⁴⁺ by the dissolved oxygen in water, the ionic strength of the solution was increased which resulted in the decrease of the repulsive electrostatic force between the particles.³⁸ At the same time, the repulsive electrostatic force between the SnO₂ and MWCNTs were decreased simultaneously. However, the radius vector between the outer SnO₂ particles and MWCNTs was larger than for the SnO₂ particles. The decreased repulsive electrostatic force between the SnO₂ and MWCNTs can not avoid the congregation of the outer SnO₂ particles. So, the physically adsorbed SnO₂ particles were unstable, and tended to congregate together.

Van der Waals forces have been used to investigate the interactions between CNTs and CNT based nanocomposites.^39–41 Herein, van der Waals forces also have great influence on the interactions of between the SnO₂ and MWCNTs. Van der Waals forces between CNTs and other nanoscale species are likely to be much more long-range (∼1/r³) than those typically observed between colloidal species (∼1/r⁶).⁴¹ So, with the decrease of the repulsive electrostatic force between the SnO₂ particles, the distance between them also decreased. As a result, the attractive van der Waals forces between the SnO₂ particles increased rapidly. So, the surface of the SnO₂/MWCNT heterojunctions turned appreciably coarser with the prolonged reaction time, which can be attributed to the increased attractions between the outer SnO₂ nanoparticles.

By changing the acidity of the solution, SnO_x/MWCNT heterojunctions with different morphology and oxidation state were prepared by Fang et al.²¹ The strong acidity is disadvantageous to the formation of the SnO₂, as well as the attractive physical interactions. In our previous work,⁴² in the process of SnO₂/MWCNT-2 preparation, small amount of iron cations (Fe³⁺) were added to the SnCl₂ solution. Using the Fe³⁺ doped SnO₂/MWCNT heterojunctions (Fe-SnO₂/MWCNT-2) as a precursor, SnO₂ nanowires were prepared though thermal treatment in an inert atmosphere. Here, the interfacial structures of the Fe–SnO₂/MWCNT-2 were investigated in order to test the influence of ionic strength discussed above. TEM and HRTEM images of the Fe–SnO₂/MWCNT-2 are shown in Fig. 9a, b. The atomic content of iron in the product was 0.34%. From Fig. 9a, b, it is clear that only a few SnO₂ particles coated onto the surface of MWCNTs. More importantly, these SnO₂ particles were attached on the MWCNTs surface in a monolayer fashion, which suggests that the interactions between them are chemical interactions. Fig. 9c shows that the weight content of SnO₂ in Fe–SnO₂/MWCNT-2 was 43.5%, which is similar to the SnO₂/MWCNT-2. The results mean that the added Fe³⁺ can not suppress the formation of SnO₂. However, most of the SnO₂ particles could not form the multilayer SnO₂ coating, which means that the attractive physical interactions between the SnO₂ and MWCNTs were very weak. The possible reason may be attributed to the repulsive electrostatic force between the Sn–OH₂⁺ and the Fe³⁺, which was concentrated on the surface of SnO₂ and the MWCNTs, respectively. On the contrary, the increased ionic strength lowers the repulsive electrostatic force between the SnO₂ particles, and as a result, more SnO₂ particles congregated together.

Fig. 9 TEM (a), HRTEM (b), TGA, DTG (c), and DTA curve (d) of the Fe–SnO₂/MWCNT-2 heterojunctions.

In addition, the thermal stability of the MWCNTs in Fe–SnO₂/MWCNT-2 was also studied, as shown in Fig. 9c, d. The T_o and the T_p of Fe–SnO₂/MWCNT-2 were 423 and 587 °C, respectively. The H_d and the H_b values were −77.98 and 11.14 kJ mol⁻¹, respectively. The results indicate that the lattice strain of the MWCNTs in Fe–SnO₂/MWCNT-2 was lower than the ones of SnO₂/MWCNT-2, which can be attributed to the weak interactions between the SnO₂ and MWCNTs. In combination with the interfacial morphologies, the results further confirm that Raman spectra and thermal analysis are two valid methods to evaluate the interactions between the SnO₂ and MWCNTs.

4. Conclusions

SnO₂/MWCNT heterojunctions were synthesized using the SnCl₂ solution method. The chemical bonds were the predominant interactions between the MWCNTs and the inner layer SnO₂ nanoparticles. The physical interactions, including Coulomb interactions and van der Waals forces, between the outer layer SnO₂ and MWCNTs were determined by the acidity and the ionic strength of the solution. Raman spectrum and thermal analysis were chosen and proved to be two effect methods to evaluate the interactions between the SnO₂ and MWCNTs. The binding energy of the MWCNTs and SnO₂ was calculated based on differential thermal analysis and Hess's law.

Acknowledgements

This work was supported by the One Hundred Person Project of the Chinese Academy of Sciences, the Natural Science Foundation of Anhui University of Traditional Chinese Medicine (Grant No. 2011zr017B), the China Postdoctoral Science Foundation (Grant No. 2011M501073), the National Key Scientific Program, Nanoscience and Nanotechnology (Grant No. 2011CB933700), and the National Natural Science Foundation of China (Grant Nos. 90923033, 60801021, 61071054, and 20907035).

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