Inte rnatio nal Jo urnal o f Sc ie ntific & Eng inee ring Re se arc h, Vo lume 3, Issue 2, February -2012 1
ISSN 2229-5518
Synthesis and Characterization of In2O3
Nanomaterials
Gwen B. Castillon, Gil Nonato C. Santos
Abs tract — Indium (III) Oxide (In2O3) nanomaterials w ere grow n on glass and Si (100) substrates using the horizontal vapor phase crystal grow th technique. A greater yield of nanomaterials w as retrieved on the glass substrate than on the Si (100) substrate. Nanopyramids, nanooctahedrons, nanotriangles, and f aceted nanoparticles w ere f ound at temperatures of 1200°C,
1000°C, and 800°C. EDX results of representative structures revealed an atomic composition of ~40% indium and ~60% oxygen. XRD results revealed that the nanomaterials produced w ere indeed indium oxide and that the sample grow n on Si (100) has better crystallinity than those f ormed on glass. Transmission measurements conf irm that the samples grow n on the glass substrate w ere transparent to visible light w ith w avelengths of 528, 586, and 673 nm.
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In2O3 is an n-type s emiconductor belonging to a s pecial clas s of rare materia ls known as trans parent conductive
oxides (T CO) – materia ls with high op tical trans parency and high electronic conductivity at the s ame t ime. Th is type of materia ls has become es s ential co mponents for optoelectro n- ic applications and photoelectrochemica l devices . The e x- cellent ca rrier mobilities (>100c m2/ Vs ) o f In2O3 and its opti- cal band gap outs ide the vis ible range (>3.2 e V) has been unmatched by other TCO’s like Zn O and SnO2 [1].
Among the applications of In 2O3 nano materia ls is in the fie ld of che mical s ens ing. In fact, s everal authors have s t u- died its s ens itivity to different materia ls like is opropene [3], nitrogen dio xide [2, 3], ethanol [2, 4, 5], a mmonia [3, 4 ], benzene, toluene, LPG, 90 # gas oline, 97 # gas oline, and methane [4], hydrogen sulphide [6], ozone [7 ], and redo x proteins , s pecifica lly, low dens ity lipoprote in (LDL) and Cytochrome C [8].
The relat ively low e lectron affinity (~3.5 e V), conven i-
ence of n-type doping, high che mical in ertnes s , and s putter res is tance of In2O3 als o ma kes it one of the mos t attractive conductive oxides for fie ld e mis s ion [9]. Als o, becaus e of its high re fractive index of 2.0, 1D nanos tructure s of In2O3 are e xpected to function as waveguides for guiding and ma- nipulating light on the micro meter s cale [10 ].
In this s tudy, the res earcher attempted to grow In 2O3 na- nomateria ls us ing a method developed in De La Sa lle Un i- vers ity called Horizontal Vapor Phas e Crys tal Gro wth Tec h- nique. The s aid technique has s uccess fully produced nan o-
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Gil Nonato C. Santos, Doctor of Philosophy in Materials Science and En-
gineering, De La Salle University-Manila, Philippines. E-m ail: san-
tosg@dlsu.edu.ph
s tructures of wurtzite (Z inc Oxide, Zn O) and rutile (Tin Oxide, SnO2) crys tal s tructures . This s tudy is an attempt to produce nanostructures of In2O3 whos e crys tal s tructure is that of a body centered cubic crys tal s tructure.
Unlike the other methods us ed in the synthes is of In 2O3 nanomateria ls by other res earchers , the s aid method a t- tempted to grow the nano materia ls in a s ealed and evacuated glas s tube instead of growing the nanomateria ls in the pre s- ence of carrier gas es . Als o, no other catalys ts or re agents were us ed x in the s ynthes is of In2O3 nanos tructures . Syn- thes is was carried out a te mperature of 800 °C, 1000° C, and
1200° C while the growth time was s et at 8 hours and the ra mp time was s et to 80 minutes .
Thirty-five milligra ms of h igh purity In2O3 powder (99.99%) obtained from Aldrich Corporat ion was were loaded into a number of clean and dry clos ed -end glass tubes . Silicon s ubtrates were ins erted into half of the clos ed end glass tubes . The clos ed -end glass tubes were then v a- cuum-s ealed us ing a Thermionics High Vacuu m Sys tem at a
vacuum pres sure of ~106 Torr. The vacuum-s ealed glas s
tubes were then baked in a Thermo lyne horizontal tube fu r- nace. To achieve a temperature gradient necess ary for the migrat ion of the growth s pecies and formation of the nan o- s tructures , half of the length of a glas s tube was allo wed to protrude from the tube furnace. The morphology of the s tructures formed on the s ubstrates were s tudied us ing a JEOL 5310 Scanning Electron Micros cope , the ele mental compos ition were determined us ing an Energy Dis pers ive X-ray Spectros copy (Oxford with Link Is is ), the crystal s tructure were verified us ing an XRD (Bede Scientific D3
Sys tem), and the optical trans mis s ion s pectrum were taken
us ing an Oly mpus BX61 upright fluores cence mic ros cope.
IJSER © 2012
Inte rnatio nal Jo urnal o f Sc ie ntific & Eng inee ring Re se arc h Vo lume 2, Issue 5, May -2011 2
ISSN 2229-5518
A. SEM Results
A 1-2 c m depos ition band was found on the inner wall of the glass tubes as well as on the Si (100) s ubs trate located a few centimeters fro m the heated end of the tube, particularly at the s ite were the temperature g radient e xis ted.
Figure 1 s hows the depos ition bands formed on the inner
wall o f the glas s tube as well as on the Si (100) s u bs trate. It could be s een that the dens ity of the nanomateria ls in the depos ition band on the Si (100) s ubstrate is s malle r than that of the dens ity of nanomaterials in the depos ition band on the glas s wall. This is due to the fact that the flat s urface of the Si (100) s ubs trate has a much higher s urface energy than the concave inner wall of the glas s tubes [11].
Fig. 1 Deposit ion band on Glass subst rat e and on Si(100) Subst rat e
Scanning electron mic ros copy revealed that mic ron s ized crys tals were fo rmed on the left -mos t (hot) end of the depos ition band, thin films we re formed o n the middle of the depos ition band, while nanomaterials we re formed on the right-mos t (cold) end of the depos ition band.
Fig. 2 a, c, and e s how the different s tructures formed
on the glass s ubs trate while Fig. 2 b, d, and f s hows the s tructures formed on the Si (100) s ubs trate at varying tem- peratures and at growth time of 8 hours . Pyra mida l s tru c- tures were the main s tructures formed at diffe rent tempera- tures on different s ubs trates . Some octahedrons were found on the glass subs trate at temperatures of 1200° C and
1000° C. Triangular nanostructures as well as faceteted n a-
noparticles were the dominant structures formed at a te m- perature of 800° C. Nanopyramids , nanoctahedrons , and nanotriangles having the (111) facets were frequently found becaus e for a materia l with a cubic crys tal s tructure like In2O3, the s urface energy is cons idered to pursue the s e- quence γ {111} < γ {100} < γ {110} [12].
The obs ervation that octahedrons were cons istently found at temperatures of 1000° C and 1200° C is cons is tent with Quras hi et. al.’s findings . The octahedrons were formed due to the high s upers aturation ratio of In 2O3 vapors at the depos ition region that made the growth rates perpe n- dicular to {111}, {100}, and {110} facets quite clos e [13].
The s tuctures formed on the glas s s ubstrate were found to be s malle r than that formed on the Si (100) s u bs trate. This is poss ibly due to the lack of defect in the surface of the Si (100) s ubs trate. There were no cracks or dents on the s urface of the s ubstrate that could have kept the n uclei fro m coales cing with other nuclei during the in itia l s tage of the growth, and thus producing particles of dimens ions in the micro meter s cale.
It was als o found that the s ize dis tribution was less un i-
form on the glass subs trate than that on the Si (100) s ub- s trate. This could be due to the curvature of the glass tube as well as the rough surface of the unpolis hed glass wall that led to the uneven dis tribution of the growth species within the depos ition region. On the other hand, the more unifo rm s ize dis tribution on the Si (100) s ubs trate could be attributed to the flatness of its s urface that allows a uniform dis trib u- tion of the growth s pecies along the s urface of the Si (100) substrate.
Fig. 2 (a) Oct ahedrons and pyramids with arrises lengths o f ~1.4 -1.8 m formed at 1200°C on glass, (b) pyramids, triangles, and faceted part icles of lengt hs of ~1-100m formed at 1200°C on Si (100), (c) Oct ahedrons and pyramids wit h arrises lengths of ~ 400 -800 nm formed at 1000°C on glass, (d) pyramids, triangles, and facet ed part icles of lengths of ~3 -40m formed at 1000°C on Si (100), (e) pyramids, t riangles, and facet ed particles of lengt hs of ~50-900 nm formed at 800°C on glass, and (f) triangles, and faceted part icles of lengt hs of ~300-600 nm formed at 800 °C on Si (100)
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Inte rnatio nal Jo urnal o f Sc ie ntific & Eng inee ring Re se arc h Vo lume 2, Issue 5, May -2011 3
ISSN 2229-5518
Fig. 3 EDX Spectrum of (a) a pyramid and (b) an oct ahedron formed at 1200°C, and (c) a t riangle formed at 800°C
B. E DX Results
Fig. 3 a, b and c be low a re repres entative EDX s pectra of an octahedron, a pyramid, and a triangle, res pe ctively. The unlabeled peaks corres pond to gold (Au), which was us ed in coating the s amples for EDX characterizat ion while the Si peaks are attributed to the glass s ubstrate (SiO) or the Si (100) s ubs trate.
Table 1 s hows that the atomic ratios of Indiu m to Oxy-
gen are ~41:59 for an octahedron, ~45:55 for a pyramid, and
~35:65 for a triangle. The s aid values are clos e to the theo- retical va lue of the compos ition of In 2O3 which is 40% in- dium and 60% o xygen. Similar res ults were found for other faceted particles although no dis cernable pattern was found regarding the effect of variation in growth t emperature and s ubstrate material.
TABLE 1. CALCULATED OXYGEN AND INDIUM CONTENT ST RUCTURES
S TRUCTURE | COMPOS ITION (Atomic %) | |
S TRUCTURE | Indium | Oxygen |
Octahedron | 41.12 | 58.88 |
Py ramid | 45.31 | 54.69 |
Triangle | 34.92 | 65.08 |
C. XR D Results
Fig. 4 X-ray Diffract ion patt ern of the bulk In2 O3 powder and nanomat erials grown on glass and on Si (100) subst rat e at 800°C for 8 hours
The X-ray Diffraction pattern of the bulk In2O3 powder as well as the XRD pattern of the nanomaterials grown at a temperature 800° C and growth time of 8 hours on glas s and on Si (100) is s hown in the Fig . 4 be low. The co mputed Miller indices of the lattice planes were found to be cons is tent with the s tandard values in literature for In2O3 with cubic c -type rare earth s tructure. [ 14]
The XRD patterns of the nanomateria ls grown on glas s and on s ilicon s ubs trate were found to have s imilar XRD peaks with that of the XRD pea ks of the s ource powder co n- firming the fact that the s ynthes ized nanomaterials a re in- deed In2O3. The intens ity of the prominent peaks at (222) and (400) fo r the nanomaterials grown at 800 ° C and 8 hours on Si (100) s ubs trate were found to be greater than the inten- s ities of the s ame peaks for the s amp les grown at the s ame parameters on glas s subs trate.
Va lues of the lattice para meter a of In2O3 powder and that of In2O3 nanomateria ls on glas s and on Si (100) sub- s trate were ca lculated fro m Bragg’s Law and are s hown on Table 2. The average Grain Sizes of the nanomaterials on glas s and Si (100) a re a ls o s hown in the table.
TABLE 2. LATTICE PARAMETER AND GRAIN SIZE OF IN 2 O3 POWDER, IN2O3
NANOMATERIALS ON GLASS, AND IN2 O3 ON SILICON
S ample | In2O3 powder | In2O3 on glass | In2O3 on silicon |
Lattice p arameter (nm) | 1.0110 | 1.0192 | 1.0118 |
Grain Size (nm) | - | 20.5895 | 41.1887 |
The values obtained were found to be cons iste nt with the standard lattice parameter a for cubic In2O3 which is
1.0117n m [15]. The lattice para meter of the nanomateria ls
grown on Si (100) was found to be clos er to the s tandard while the lattice para mete r of the nanomaterials grown on glas s was found to be greater than the s tandard. The average grain s ize, D, of the nanomateria ls were ca lculated us ing Scherre r’s Formu la. It was found that the average grain s ize of the nanomateria ls grown on Si (100) is greater than that of the nanomaterials grown on glas s . This may be attributed to the reduction of the c rys tal defects originated fro m the s tructural s imilarity that the Si (100) have with In 2O3 [28]. Both Si (100) and In 2O3 have cubic s tructures .
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Inte rnatio nal Jo urnal o f Sc ie ntific & Eng inee ring Re se arc h Vo lume 2, Issue 5, May -2011 4
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The relat ively h igh intens ity of the diffraction pea ks as well as the larger gra in s ize of the s amples grown on Si (100) s ubs trate indicates that the s ample grown on Si (100) had better crystallin ity than that of the s ample gro wn on glas s .
D. Optical Characterization Results
Figure 5a below s hows the s pectrum of the light s ource us ed in the optical characterization of the s amples . The s ource had peaks at 528, 586, and 673 n m. Figure 5b, on the other hand shows the trans mis s ion s pectrum of the s amples grown for 8 hours at a temperature of 1200° C (red), 1000° C (green), and 800° C (p ink). It could be eas ily s een that the s amples on glas s were optically trans parent to the wav e- lengths of the light s ource.
Optical characterization of the s amples grown on the Si
(100) s ubs trate was not done, however, becaus e th e Si (100)
s ubstrate interferes with the trans mis s ion meas ure ments .
Fig. 5 Transmission spectrum of (a) t he light source used for opt ical cha- ract erizat ion and (b) t ransmission spect rum of In2O3 nanomaterials grown on glass – red for sample at 1200°C, Green for sample at 1000°C, and P ink for sample at 800°C
In2O3 nanomaterials we re s ynthes ized through the Hori- zontal Vapor Phas e Crys tal Growth depos ition without a catalys t at a tempe rature of 1200° C, 1000° C, and 800° C and at growth times of 8 hours on Glas s and on Silicon (100) Subs trates .
SEM characterization revea led that the s ynthes ized n a-
nostructures had the morphology cons istent to that of the cubic crys tal s tructure: pyra midal, octahedral, and triangula r.
It was obs erved that octahedrons were cons is tently found at temperatures of 1000° C and 1200° C. It was ob- s erved that there was little a mount of nanostructures formed on the Si (100) s ubs trate as compared to the amount of s tru c- tures formed on glas s . Als o, the s tru ctures found on the Si (100) s ubs trate were in the mic ro meter s cale. The s ize dis- tribution on the Si (100) s ubs trate was als o obs erved to be more uniform than that of the s ize d is tribution on the glas s wall owing to the flat and uniform s urface of the Si (100) s ubstrate.
EDX characterizat ion revealed that the s tructures had
atomic co mpos itions of ~40% indiu m and ~60% o xygen. XRD res ults revealed that the nanomaterials produced were indeed indiu m o xide and that the s ample grown on Si (100) had better crystallin ity than thos e formed on glas s .
Optical characterization of the s amples grown on glas s s ubstrate confirmed that the s amples are trans parent to vis i- ble light.
The aouthor would like to acknowledge the Depa rtment of Science and Technology-Science Education Institute for the financial aid and her advisers, Dr. Gil Nonato Santos and Dr. Reuben Quiroga for all the support
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