Abstract

Mechanical exfoliation (ME) using a duct tape has been conducted upon SnO₂ thin film. The film is synthesized from direct heating of SnCl₂ powder. The SnCl₂ powder is deposited upon a special arrangement of glass slides and directly heated using an electric stove with a temperature of around 350°C. The material resulted from the heating process occurs on glass slides adjacent to the heated powder glass slides. The materials are then analyzed using scanning electron microscope (SEM) and energy dispersive X-ray (EDX) to confirm the presence of SnO₂ material. The SEM results show stacking of spherical particles with sizes in the ranges of 700 nm to 1 μm. The EDX result confirms the occurrence of 20% and 66% of Sn and O, respectively, as well as 13% of carbon and a very small percentage (0.99%) of chlorine remaining. The thin films are then mechanically exfoliated using a duct tape for as many as 5, 10, and 20 times. For each ME variation, the thin films are analyzed and compared using X-ray diffraction (XRD). The XRD results show semi-crystalline structure of SnO₂ in cubic phase. The XRD results after ME show peaks, which are characteristics to SnO₂ and tend to shift the peaks to higher 2θ. Furthermore, the intensity of the peaks is highest for 10 times ME showing crystalline improvement of the thin film after the ME treatment.

1. Introduction

Tin dioxide (SnO₂) or stannic oxide is an inorganic compound material that has been a subject of various studies especially concerning its applications as catalysts ^{1, 2}, ceramic glazes, polishing, coatings ³, antibacterial ⁴, lithium battery storage ^{5, 6, 7}, solar cells ^{8, 9, 10}, gas sensor ^{11, 12, 13}, supercapacitors ^{14, 15, 16}, and transparent thin films ^{17, 18, 19}. The various applications above use SnO₂ materials in many forms with sizes ranging from micro to nano scales ²⁰, such as nanoparticles ^{21, 22, 23}, nanowires and nanobelts ^{24, 25, 26}, nanoribbons ^{27, 28}, nanorods ²⁹, quantum dots ^{30, 31}, and nanocomposites ³². Moreover, various methods have been used in order to synthesize SnO₂, such as hydrothermal ^{33, 34, 35}, electrospinning with side-by-side spinneret ³⁶, spray pyrolysis ³⁷, chemical vapor deposition (CVD) ^{38, 39, 40}, and dip coating ⁴¹. These methods are put forward to produce good quality of SnO₂ thin films for various uses and applications.

In this study, we use a simple method in synthesizing SnO₂ materials by direct heating of SnCl₂ powder in a special arrangement of glass slides using an electric heater, which provides a temperature of around 350°C. This effort is of course acknowledged inferior to the previously mentioned methods, but, for scientific curiosity this study may give additional information in the various methods in preparing and synthesizing thin film materials. Furthermore we have added a treatment, which has not been reported before, that is a repeated mechanical exfoliation (ME) using a duct tape after the materials are printed on the appropriate glass slides.

ME using a duct tape is initially conducted to produce monocrystalline graphitic films from graphite materials ⁴². The advantage of this arguably simple method is that it produces graphene layers with extremely good physical and chemical properties. However, this method is usually limited to fundamental scientific interests and has not been used to fabricate large scale graphene for direct applications ⁴³. In this case, we investigate whether this method may be used for thin films such as SnO₂ produced by simple direct heating of SnCl₂ powder. The aim of this study is to understand the effect of repeated ME towards the synthesized SnO₂ thin films based on X-ray diffraction (XRD) patterns by varying the number of ME for as many as 5, 10, and 20 times.

2. Experimental Method

The materials utilized for this study are i) 25 grams of SnCl₂ powder, ii) 2 liters of distilled water, iii) one box of tissue, and iv) a duct tape (Goldtape brand). The equipment used in this study are i) an electrical stove, ii) a stopwatch, iii) glass slides, iv) two clamps, v) an asbestos screen, vi) a measuring cup, vii) a ruler, viii) a digital thermometer, ix) a glass cutter, x) an XRD apparatus (Rigaku Mniflex 600), and xi) an SEM-EDX apparatus (JSM 6510 series).

The procedure in obtaining SnO₂ thin films from SnCl₂ powder by direct heating may be explained as follows. Clean the glass slides using distilled water. Wipe the glass slides using tissues to dry them. Weigh a mass of SnCl₂ powder as much as 0.2 grams. Put the weighted SnCl₂ powder on a clean glass slide in a form of a circular shape with diameter of 1 cm (Figure 1a). Place another clean glass slide on top of the prepared SnCl₂ powder such that the powder is sandwiched between two glass slides (Figure 2). Lay the sandwiched SnCl₂ powder on the middle of the asbestos screen. Put two clean glass slides on the left and right sides of the sandwiched SnCl₂ as shown in Figure 1b. Turn on the electric stove with a power of 300 watts and wait around 10 minutes to get a constant temperature of around 350°C. Set the asbestos screen on top of the electric stove. Heat the SnCl₂ powder for about 10 minutes. SnO₂ films will gradually appear on the left and right glass slides of the sandwiched SnCl₂ powder. Turn off the electrical stove and let the SnO₂ films materials cool down.

Once the glass samples are cool enough, we may proceed with the repeated ME treatment and carry out XRD and SEM-EDX tests. This procedure is done as follows: i) cut some pieces of duct tape with a size matching the glass slide, ii) take a glass slide layered with SnO₂ film, iii) cut a small piece of the slide that consists of SnO₂ film and perform XRD and SEM-EDX tests, iv) stick a piece of duct tape upon the glass slide that consists of SnO₂ layer, v) slowly and firmly press the duct tape one way throughout the glass slide, vi) slowly detach the duct tape from the glass slide and discard it, vii) take a new duct tape and repeat steps iv) to vi) upon the same glass slide using as many as 5 pieces of duct tape, viii) slice a small sample of the glass slide and perform XRD measurement, ix) repeat steps iv) to viii) for 10 and 20 repeated ME.

3. Results and Discussion

Oxide materials are generally synthesized using tools and methods that are fairly complex, but in this study we use a simple method, namely direct heating using an electric stove. The heating is carried out with a temperature of 350 °C in order to exceed the melting point of SnCl₂ which is around 247 °C. Furthermore, the oxygen (O₂) in the air will be used to form the layers of SnO₂. The oxygen will react with the tin element contained in the SnCl₂ compound. This reaction will form layers of SnO₂ on the glass slides. The simple reaction takes the following form:

The SnO₂ materials produced by direct heating of SnCl₂ powder may be observed in Figure 3. Three pairs of SnO₂ layers are obtained by different time duration of the direct heating. It may be observed that all layers of the SnO₂ materials show different colors starting from white color at the outermost layer, and then changing to yellow, violet, blue, and green. The pattern is then repeated. These colored layers are produced due to the thin-film interference effect caused by different thickness of the SnO₂ layers. As the white colored layer is the thinnest layer, the farther the distance of the left and right substrates from the SnCl₂ powder, the thinner the layer produced. This might be due to less exposure of the heated SnCl₂ on farther parts of the glass slides. The SnO₂ layers are also not uniformly distributed through the glass slides. The longer the time duration of the direct heating process, the more area of the glass slides are covered by SnO₂ materials. For 5 minutes of direct heating, the layers on the right and left glass slides form a heart-like structure, whereas for 10 and 15 minutes of direct heating, the layers form leaf-like structures where the top regions are wider than the lower regions. Furthermore, for 5 minutes of direct heating, the right and left layers are quite symmetric; however, it is not the case for layers resulted from 10 and 15 minutes of direct heating.

Next, the SEM results of the SnO₂ material may be observed in Figure 4. The SEM images show spherical grains distributed throughout the glass slides. The sizes of these grains range from 700 nm to 1 micron. The grains appear to be stacked on top of each other. Figure 4a and Figure 4b show that some regions have more stacked grains than others; hence the stacking is not homogenous throughout the sample. This different stacking may contribute to different thickness of the SnO₂ layers, and hence produces different colors in Figure 3.

The EDX results may be viewed in Figure 5. From the EDX results of Figure 5, the substances that occur on the sample include 20% of tin, 66.2% of oxygen, 12.8% of carbon, and a remaining of 1% chlorine. It can be deduced that the SnO₂ layer formed on the sample has a molarity ratio of Sn : O of around 1.0: 3.3, which theoretically should be 1 : 2. This mismatch may be due to the carbon atom impurity that is high enough to eliminate places for Sn atoms in a thin layer. If the carbon atoms of 12.8% are replaced with Sn, then the Sn atoms will become 32.8%, and hence the ratio is closer to 1: 2. Moreover, the data also shows that the chlorine element contained in the sample is very small. This suggests that the compound of SnCl₂ material has been separated; the most likely elements are separated into the air in gaseous form.

Finally, the XRD results of the samples being treated using repeated ME may be perceived in Figure 6. Figure 6(a) to Figure 6(d) is XRD results of SnO₂ layers with 0, 5, 10, and 20 times repeated ME, respectively. It may be observed from the XRD data that the SnO₂ materials are semi-crystals as no sharp distinctive high peaks are found. Furthermore, the XRD patterns of SnO₂ samples show a cubical crystal phase. The result from XRD of SnO₂ thin film without ME treatment shows two peaks on (111) and (200). For 5 ME treatment the XRD result shows peaks at (200), (220), and (222). However, the peak at (111) disappears. For 10 ME treatment the peak at (111) occurs again with other peaks at (200), (220), and (222). Finally, for 20 ME the peaks at (111) and (200) disappear, but a deeper 2θ at a peak of (400) occurs. It can be observed that increasing ME treatment makes the peaks tend to shift to larger diffraction angles. ME treatment may cause the X-ray to penetrate deeper into the SnO₂ layers. Moreover, the XRD results show that the ME treatment makes the crystal phase becomes more visible. This is shown by the peak at (200) where the X-ray intensity increases from 5 to 10 times ME treatments. But, for 20 times ME the intensity drops again.

It can also be observed that the FWHM values of the XRD data at certain peaks change due to the ME treatment. The FWHM values tend to increase when the ME treatment is applied. FWHM values for 5, 10, and 20 times ME at the peak of (220) are shown in Table 1. The table shows that FWHM data increase from 5 to 10 times of ME, but then decrease when ME treatment is applied for 20 times. This might be associated to the surface stress of the SnO₂ layer caused by the ME treatments. From 5 to 10 times of ME the surface stress increases as the FWHM also increases. But after 20 times of ME, the treatments do not have any more effect towards the surface stress of the SnO₂ layer.

4. Conclusions

The SnO₂ thin film layers produced by direct heating of SnCl₂ powder show a range of different colors in certain areas of the layers indicating different thickness of the layers. This is also supported by the SEM results that show non-homogeneity of spherical particles with sizes in the range of 700 nm to 1 micron. Furthermore, EDX results show the sample consisting of tin, oxygen, carbon, and chlorine atoms with a composition of 20%, 66%, 13%, and 1%, respectively. The XRD results show that ME treatments cause the XRD peaks to shift to higher diffraction angles. The XRD intensities especially at the peak of (200) indicate that after 10 times of ME treatment the crystallinity of the SnO₂ thin layer increases, but then decreases after 20 times of ME. Finally, the FWHM values at the peak of (220) indicate an increase of surface stresses after 10 times of ME but decreases after 20 times of ME.

Acknowledgements

The authors would like to thank the Faculty of Mathematics and Natural Sciences of Universitas Negeri Yogyakarta for the funding of this study under the grant of DANA DIPA FAKULTAS 2017.

References