International Journal of Scientific & Engineering ResearchVolume 2, Issue 10, October-2011 1

ISSN 2229-5518

Temperature and Deposition Time

Dependence of the Geometrical Properties of

Tin Oxide Nanostructures

Gil Nonato C. Santos, Arnel A. Salvador, Reuben V. Quiroga

Abstract—Tin Oxide nanomaterial was synthesized using the horizontal vapor phase growth (HVPG) technique. The study investi- gated the optimum growth parameters by varying the growth temperature from 900C to 1200C and growth time of 1 hour to 5

-5

hours. The SnO2 bulk powder with purity rate of 99.99% were placed in a sealed quartz tube with a vacuum pressure of 10

Torr

and baked with the desired growth parameters. The resulting nanocrystals displayed different structures ranging from nanobelts to

nanorods as confirmed by the SEM. Results from EDX and DTA showed that indeed the grown samples were congruent based on the atomic composition and thermal property of the nanomaterials. The XRD also verified that the crystal structure was rutile but with low indexed peaks. Using the same growth technique, samples were grown on Silicon (100) substrate and exhibited nanorods and nanobelts. The SnO2 nanomaterial also displayed fluorescence and photoluminescence signals. The photoluminescence spec- trum has a broad emission in the visible region with peaks at 558 nm and 666 nm. The visible light emission was known to be re- lated to defect levels within the band gap of SnO2, associated with O vacancies or Sn interstitials that have formed during the syn- thesis process.

Keyword —nanomaterial, horizontal vapor phase growth (HVPG) technique, photoluminescence

1 INTRODUCTION

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anostructured semiconducting oxide materials had received broad attention due to their distinguished performance in electronics, optics and photonics. Synthe- sis of semiconducting oxide thin films have been an ac- tive field because of their applications as sensors, trans- ducers, and catalysts. The studies of semiconducting oxides have been focused on two-dimensional films and zero-dimensional nanoparticles, which can be readily synthesized with various well-established techniques such as chemical vapor deposition, sol-gel processing, pulsed laser deposition, and solid state reaction.[1] In this study, the material of our particular interest is tin oxide, an n-type wide band-gap semiconductor and a key functional material that has been extensively used for optoelectronic devices and gas sensors. [1]-[2] For the gas-sensing applications, it has been shown that the syn- thesis of SnO2 films under a nanostructured form consi-
derably enhances their gas-sensing performance.
As stimulated by the novel properties of carbon nano- tubes, wire-like nanostructures have attracted extensive interest over the past decade because of their great po- tential for addressing some basic issues about dimensio-

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Gil Nonato C. Santos, Doctor of Philosophy in Materials Science, Asso- ciate Professor, De La Salle University-Manila, Philippines. E-mail: san- tosg@dlsu.edu.ph

Arnel A. Salvador, Doctor of Philosophy in Physics, Professor, University

of the Philippines-Diliman. E-mail: arnelsalvador@nip.upd.edu.ph

Reuben V. Quiroga, Doctor of Philosophy in Physics, De La Salle Univer-

sity-Manila. Email: quirogar@dlsu.edu,ph

nality and space-confined transport phenomena as well as applications.[5] Besides nanotubules [6]-[7], many oth- er wire-like nanomaterials, such as carbides [SiC [8]-[10] and TiC [8], nitrides (GaN [11]-[12] and Si3N4[13]), com- pound semiconductors [14]-[15], element semiconductors (Si [16]-[18]) and Ge [16], and oxide Ga2O3 [23] and MgO [24] nanowires, have been successfully fabricated. In geometrical structures, these nanostructures can be clas- sified into two main groups: hollow nanotubes and solid nanowires, which have a common characteristic of cylin- drical symmetric cross section.
In this work, SnO2 nanomaterials were grown using horizontal vapor phase growth technique. The motiva- tions to adopt this technique over the other processes are the following: 1.) High purity materials can be grown; 2.) Source material is powder in form; 3.) Greater quantity products with less source material; 4.) No gas molecules in between; 5.) Its versatility in composition of deposit;
6.) The ability to produce unusual nanostructures; 7.) The substrate is easily installed; 8.) Has an excellent bonding to the substrate; and 9.) Elimination of pollutants and effluents which is an important ecological factor [23].

2EXPERIMENTAL SECTION

A small amount of 0.035 grams of high purity Merck SnO2 (99.9%) powder was weighed and loaded in a close- end quartz tube with inner size diameter of 8.5 mm, out- er size diameter of 11 mm, and length of 220 mm. The quartz tube was attached to the THERMIONICS High Vacuum System and was sealed at a vacuum pressure of

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International Journal of Scientific & Engineering ResearchVolume 2, Issue 10, October-2011 2

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10-5 Torr. The sealed quartz tube was divided into three zones where zone 1 was set as the hottest zone based on the assigned growth temperature, while zone 2 and 3 are in deceasing temperature. The samples were grown from
900to 1200ºC with varying growth time of 1 hour to 5
hours. The temperature profile along the tube furnace was also monitored using a Type K Thermocouple Fluke Digital Thermometer. To examine the synthesized prod- ucts, the quartz tube was gently cracked and observed to be deposited at zones 2 and 3 where the fluffy white products are seen on the walls of the quartz tube using an SEM. Several samples were also collected for XRD, DTA, and Spectral Imaging measurements.

3 RESULTS AND DISCUSSION

Figure 1 presents some of the SEM images of the sil- ver Growth of SnO2 nanostructure follows the SVLS (Sol- id-Vapor-Liquid-Solid) phase growth. The nanowire growth begins after the source material becomes supersa- turated in liquid and continues as long as it remains in a liquid state. During growth, the temperature gradient directs the nanowire’s growth direction and size. Ulti- mately, the growth terminates when the temperature is below the eutectic temperature of the source material. Figure 1 shows the SEM image of synthesized SnO2 na- nomaterial.

Fig. 1 SEM Image of the SnO2 Nanomaterial grown at 1200°C for 1 hour

Also grown at 1200°C for 1 hour was the SnO2 nano- belt. Observe in Figure 2 the prominent structure of the SnO2 nanobelt with a measured width of 1.84 µm and thickness of 90 nm. Nanobelts are nanowires that have
well-defined geometrical shape and side surfaces.

Fig. 2 SEM Photo of SnO2 Nanobelt

Using a high resolution scanning electron microscope in Figure 3, the surface of the SnO2 nanomaterial was observed to be porose and composed of many uniform crystalline grains of approximately25 nm in size. This surface was observed to be suitable for fabricating gas sensors because of its porosity over large surface[32].

Fig. 3 High Resolution Image of the SnO2 Nanomaterial magnified at 250,000x

The study also found that Silicon (100) can be an ef- fective substrate for growing SnO2 nanomaterials as shown in Figures 4 and 5. In this process, the growth direction of the nanorods was led by an epitaxial orienta- tion defined by the substrate that determines the aligned growth. The nanorods tend to take the least mismatch orientation on the substrate to reduce the interface mis-
match energy.

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Fig. 4 SnO2 nanorods grown on Si (100) substrate

Fig. 5 SnO2 nanobelt grown in Silicon (100) substrate

Figure 6 illustrates the EDX analysis of the SnO2 Na- nomaterial grown at Zone 3 with a growth temperature of 1200C for 1 hour. The atomic composition of the SnO2 nanomaterial was shown to be 1:2 indicating that the growth process was congruent with the starting mate- rials.

c = 3.187 Ǻ (JCPDS File No. 41-1445) while several XRD measurements were done to determine the crystal struc- ture of the SnO2 nanomaterial. The results show that it has low indexed peaks which were observed at (110), (101), and (211).

Fig. 6 EDX Analysis of the SnO2 Nanomaterial with Au coating

All of the products including those several nanome- ters in size are found to be straight and bent as seen us- ing the SEM. The SnO2 nanomaterial has a normal rutile crystal structure wherein the SnO2 nanorods displayed a rectangular cross section enclosed by (010) and (101) fa- cet planes. The growth direction of these SnO2 nanorods are parallel to the [101] crystal direction.
Figure 8 presents the DTA analysis of the SnO2 powd- er and nanomaterial. From the resulting graph, it was shown that both materials had undergone a similar phase although there was a slight difference in its melt- ing point. Thermodynamics analysis indicates that the reaction occurs at high temperature where s represents the solid state and grepresents the gas state respectively.
SnO2(s) SnO(g) + ½ O2 (g)
Although SnO(g) was relatively stable, the following two reactions can happen spontaneously during the process
Figure 7 exhibits the XRD pattern verified from the SnO2 powder source material and the nanomaterial grown. The Miller indices are indicated on each diffrac- tion peak. The diffraction peaks of the (110), (101), (200), (211), (220), (002), (310), (112), (301), (202), and (321) planes can be readily indexed to the tetragonal rutile
structure of SnO2 with lattice constants of a = 4.738 Ǻ and
SnO(g) + ½ O2(g) SnO2(s) (1) SnO(g) ½ SnO2(s) + ½ Sn(l) (2)
Reaction 1 is a process of reoxidization of the SnO va- pors while reaction 2 is a decomposition process of gas- state SnO. It is possible for both reactions to be responsi- ble for SnO2 products. Another oxidized reaction might
also occur in the system.

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Sn(l) + O2SnO2(s) (3)

Fig. 7 XRD of the SnO2 powder (1) and SnO2 Nanomaterial (2-4)

Considering the oxidizing reactions (1) and (3), it is evident that oxygen preferably reacts with liquid-state Sn than the SnO vapors. In the present study, the metal Sn has been clearly identified existing in the SnO source powders after evaporation. This is an indication that the concentration of oxygen is limited in the present equip- ment system. The metal Sn particles are also found coex- isting with SnO2 nanobelts at the high-temperature re- gion indicating the occurrence of the decomposition of SnO vapors. Thus, the decomposition of SnO vapors is likely the dominant process to be responsible for the formation of SnO2 products in our system.

Fig. 9 Fluorescence Imaging of SnO2 Nanomaterial magnified at

20x

The PL spectrum of the nanomaterial exhibits a strong emission peak at 666.41 nm as shown in Figure 10. Visi- ble emissions with a peak wavelength positioned at around 558.39 nm are dominantly observed. This was known to be related to the defect levels within the band gap of SnO2, associated with O vacancies or Sn intersti- tials that have formed during the synthesis process [28]. A similar emission has been reported in the case of SnO2 nanoribbons synthesized by laser ablation30 and SnO2 nanorods synthesized by solution phase growth [31].

Fig. 10 Photoluminescence Imaging of SnO2 Nanomaterial magni- fied at 20x

Fig. 8 DTA Curve of SnO2 Nanomaterial (1) and SnO2 Powder (2)

Figure 9 shows the PL spectra of the SnO2 nanoma- terial measured with a fluorescence spectrophotometer using a Xe lamp with an excitation wavelength of 325 nm at room temperature.

4 CONCLUSION

The study showed that the growth of the SnO2 na- nomaterials was successful using the HVPG technique. The nanomaterials grown were dependent on the va- cuum pressure, the quartz tube used, growth tempera- ture and time. Depending on the functionality and appli- cation of the nanomaterial, it can be concluded that the
best temperature to grow SnO2 nanomaterials was ob-

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served at 1200C. It was also relevant to consider that as the growth time increases, the amount of nanomaterials produced increases. The source material used in that syn- thesis was SnO2 powder and the composition of the SnO2 nanomaterial was similar to the starting oxide. The growth was governed by a SVLS process, in which the SnO2 vapor which evaporated from the starting oxide at a higher temperature zone and directly grows on the quartz tube or on a substrate at a lower temperature re- gion that produce different nanostructures. The belt-like morphology was distinct from those of semiconductor nanowires. The XRD proved that the SnO2 nanomaterial was rutile in structure with relatively low indexed peaks. With a well-defined geometry and perfect crystallinity, the SnO2 nanobelts are likely to be a model material for a systematic experimental and theoretical understanding in the fundamental electrical, thermal, optical, and ionic transport processes in wirelike nanostructures with the absence of dislocations and defects.

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