J Mater. Sci. Technol., Vol 24 No 4, 2008Invited Research articlePorous Cube-like In2O3 Nanoparticles and Their SensingCharacteristics toward EthanolChih-Chia HUANG and Chen-Sheng YEHManuscript received September 22, 2007, in revised form February 20, 2008Chih-Chia Huang was born in Kaohsiung, Taiwan, in 1981He received his B.S. degree in chemistry from NationalCheng Kung University. He is currently working as a Ph. Daduate student with Professor Chen-Sheng Yeh at the Department of Chemistry, National Cheng Kung UniversityTaiwan. His research interests include synthesis and characterization of hollow/porous nanomaterials as well as bio-system applications.Chen-Sheng Yeh was born in Taiwan in 1961. He received aB S. degree in chemistry from Tamkang University in 1984,Ms degree in chemistry from National Tsing Hua University in 1986 and a Ph. D. degree in Physical Chemistry fromUniversity of Georgia, USA, in 1998. He then worked aspostdoctoral fellow(1994)at Department of Chemistry inPurdue Universitg, USA. He was appointed as an AssistantProfessor at the Department of Chemistry, National ChengKung University, Taiwan, in 1995 and was promoted toProfessor in 2001. His research interests include the de-ration, characterization and application ofnanometer materials as well as the biomedical studies of thePorous cube-like crystalline In2 O3 nanoparticles with an average diagonal length of 34. 8 nm were fab.ricated by a laser ablation reflux process to form In(OH)3, followed by a calcination treatment to yieldporous In2 O3. HRTEM(high-resolution transmission electronic microscopy), XRD(X-ray diffraction),BET( Brunauer-Emmett- Teller), and XPS(X-ray photoelectron spectroscopy) analysis were used to characterizetheir crystalline structures, grain sizes, surface areas and surface compositions. the as-prepared porous indium oxides were tested for their sensing properties toward ethanol. Non-porous In2 O3 nanopowder(about0 nm) was also examined in order to compare the results with the as-prepared porous In O3 nanomaterialsThe porous In2 O3 exhibited much better performance than that of non-porosus In2 O3, and showed enhancedsensitivity with a lower detection limit than other reported In O3-based materials when exposed to ethanolGood gas sensitivity and linear behavior as a function of ethanol concentration were observed in the porousKEY WORDS: In2 O3; Porosity; Nanoparticles;yH中国煤化工CNMHGProf, Ph. D, to whomJ. Mater. Sci. Technol., vol24 No 4, 20081. Introductionlaser ablation was completed, the nascent colloid so-lutions were transferred into a bottle vessel for furtherIndium oxide(In2O3)is one of the most impor- reflux. The solutions were heated to 121C for 23 h.tant n-type semiconductors and is widely used in so- After the completion of the reflux, the resulting prod-flat-panel displays(2-4 optoelectronucts consisted of In(OH)3 nanocubes and In microm-devices(5, 6, and gas detectors/7-91.In2 O3 nanomateri eter spheres. Pure In(OH)3 nanocubes were obtainedals have been fabricated in various shapes, including by a simple centrifugation(2500 r/min, 2 min)to re-spheresfibersl18).However, the fabrication of porous In20 move In spheres. For the preparation or vellow In2O3the crystalline In(OH)3 nanocubes were calcined atnanomaterials with a high surface area remains a con-400°Cfor2hsiderable challenge. Porous materials have intriguingproperties of a large area and pore volume, with Excel- 2.2 Sensing testslent promise in gas sensing. The possibility of usingTo characterize the sensing property of the as-orous nanostructure In2O3 as a gas sensor is there- obtained In2 O3, the sensors were prepared by formingfore very appealing. Interestingly, there have been no thin films of the Inn O3 powders onto blank aluminalated reports of porous In2O3 nanostructure. In this substrates(2 mmx4 mm). The substrates were fit-work, we present porous cube-like In2O3 nanomate-ted on the front side with interdigitated contacts ofrials. a two-step process using laser ablation of an a gold electrode, and with a heater on the rear sideIn metal plate followed by a solution reflux was em- These sensors were introduced into the test chamberployed to fabricate cube-like In(OH)3. A subsequent to evaluate their detection sensitivity to the reduc-calcination treatment was used to yield porous cube- ing gas(ethanol). All the measurements of the gaslike crystalline In2O3. The resulting porous indium sensing properties were carried out at various work-oxides were then used as sensors for ethanol. In addi- ing temperatures. Concentrated gas was introducedtion, we examined the non-porous In2 O3 powder pur- by an injection needle into the chamber and homoge-chased from Alfa Aesar Inc. to study its gas sensing nenized by a fan installed inside the chamber. The re-properties and compared them with the as-prepared ducing gas concentration inside the test chamber wasporous In]O34. Practical gas sensors must have the capability of periments were recorded by measuring the electricalrgeting gases at a low detection concentration and current of the sensor devices under a voltage of 5 Vlow working temperatures. To detect ethanol, Lising an electrometeret al. 19 fabricated In2O3 hollow microspheres(about 2.3 Chara1 um)assembled by about 100 nm In2O3 cube-likeparticles using the vesicles, formed from a so-callednih. Electron micrographs using transmission electronroscopes JEOL 2010, at 200 KV and PhILIPSormamide-resorcinol-water system, as the templatesCM-200, at 200 KV)were taken by placing a drop ofparticles were exposed to 10 ppm ethanol with a sensi- phous carbon film, followed by evaporation of the sol-tivity(Rair/Rgas )of less than 2 at room temperature. vent in a vacuum desiccator. Additionally, the ultra-Indium oxide nanowires(0.5 to a few micrometers in thin film samples were prepared for TEM(transmis-length and 60 to 160 nm in diameter) reported by sion electron microscopy)measurements by embedChu et al. B showed a sensor signal at less than 2 ding the In2 O3 powders in resin and ultramicrotom-for 100 ppm of ethanol at 370 C. Kim et al. 20 syn- ing them to form 30-50 nm thick slices using a Leicathesized tin doped indium oxide(ito) nanoparticles ultracut UCT ultramicrotome. All XP(X-ray photo-ith a size of 15 nm exhibiting sensing characteristics electron)spectra(VG Scientific 210)were recordewhen exposed to 50 ppm ethanol with a sensor signal using an AlKa source(1v and7 A)at 10of less than 5 at 3500C. Our porous In2 Oa nanomate- 9 Torr(1.33x10-7 Pa). The binding energy scaleials showed a detection limit to ethanol at concentra- was calibrated to 285 ev for the main(Cls)peaktions as low as 1.7 ppm at a working temperature of x-ray diffraction(XRD) results were collected on a220C. The as-prepared porous Iny O3 nanoparticles Rigaku D/Max IlIv diffractometer using Cuka ra-have shown good gas response and a sensing property diation(=0.154056 nm)at 30 kV and 30 mA.Nwith a linear dependence of sensitivity on ethanol,adsorption measurements were performed at 77 Kwhere the extrapolated lowest detection limit could using a Micromeritics ASAP 2010 system utilizingbe down to 0. 1 ppm.Brunauer-Emmett-Teller(BET) calculations for sur-2. Experimentalface area and Barret-Joyner-Halenda(BJH)calcula-tions for pore size distribution from the desorption2.1 Preparing porous cube-like In2O3 nanoparticles branch of the isothermIn a typical synthesis, a 4 mL aqueous solu- 3. Results and Discussiontion containing 2.2 mmol of cetryltrimethylammonium bromide(CTAB >96%, Fluka) was mixed withFor the preparation of porous cube-like In201 mmol of poly(vinyl pyrrolidone)(PVP, Mw=40,000, hibitiy articles, laser ablation yielded In particles ex-1 mL of 2-propanol(99.9%, Mallinckrodt)containing中国煤化 Amorphous aggre-Sigma). An unfocused Nd: YAG laser(Quantel Bril- gates-meration. Followliant)operated at 10 Hz(5 ns pulse width) with a ingCN MH Gs and amorphouswavelength of 1064 nm was utilized to irradiate an aggregates converted to In(UH)3, revealing a cubeIn plate immersed in as-prepared 5 mL of solution. shape accompanied by very large In spheres up toIn general, a laser intensity of 100 mJ/pulse was em- about 3 um in size. The exclusive In(OH)a cube-likeployed to ablate a metal plate for 20 min. Once the nanoparticles were collected by a simple centrifuga-J. Mater. Sci. Technol., vol 24 No 4, 2008Table 1 Properties of porous In2O3 and non-porous In2O3(Alfa Aesar) nanomaterialsLatticeBETGrainXPS/evconstants/nm sizesxRD/nm/(m2.g"-)sizesBET/nmPorous In2o1.01844.2,451.6,Ols:5296In]O3(Alfa Aesar) 1.01127.2169.1444.2,451.6,O1s:5294Aesar). The as-prepared In2 O3 displayed a cube-like morphology with an average diagonal length of34.8 nm(Fig 3(a)). The inset in Fig3(a)shows thesingle particle TEM image indicating the porous mor-E AApepwed n.o,phology for the as-prepared In2O3. In2O3(Alfa Ae-sar) had cube-like polydispersed shapes with a diagonal length of around 40.3 nm and some irregularshapes(Fig 3(b)). High-resolution TEM(HRTEM)was performed to gain an insight into the detailedstructure. Figure 3(c)(left)refers to the porousIn2 O3 image, showing a crystalline structure witha lattice fringe of 0. 29 nm corresponding to (111)planes from bixbyite-type In2O3. The porosity of thFig1 XRD patterns of porous In2O3 and non-porousas-prepared In2 O3 nanoparticles was further investi-n2O3(Alfa Aesar) nanomaterialsgated by the ultramicrotoming technique, as shownn Fig 3(c)(right). The 2D cross section shows thatthe porous structures were channel-like pores with di-ameters of 2-4 nm, as marked by white lines. Onthe other hand, the crystalline of the In2O3(Alfa Ae-sar)nanopowder had(200)facets with 0. 50 nm latticeplanes and without a porous structure(Fig 3(d))The In2O3 samples were investigated by measur-ing the pore size distribution using N2 adsorption-desorption isotherm and Barrett-Joyner-Halenda(BJH) methods. The isotherm characterization of the(004)porous In2 O3 nanoparticles showed a type IV meso-(006)porosity with a distinct hysteresis loop in the range(222)of 0.8-1.0 P/Po, indicating a H2-type of hysteresis(Fig 4 ) The presence of H-type hysteresis repre-sents the disordered and inhomogeneous distributionof pore sizes, and indicates an interconnected network10of pores. The Bjh analysis shows the inhomogeneity with two dominant pore sizes, where a bimodalmesopore distribution with sizes located around 2 andFig2 Electron diffraction patterns of porous In2O3 13 nm are observed for the porous In2 O3. The porousIn2O3 nanocrystals have a Brunauer-Emmett-Teller(BET) surface area of 80.4 mhile the In2O3tion to remove micrometer In particles(see Support- nanopowder(Alfa Aesar)has a low surface area ofing Information). Subsequently, the thermal treat- 4.9 m2g-1, which is consistent with its non-porousment of as-synthesized In(OH)3 nanocubes led to property as shown by HRTEM. Using BET method,Iny O3 nanostructures. Figure 1 shows XRD pat- the grain size was derived as 10.4 nm for the porousterns of as-prepared In]O3 nanomaterials indicating as the cubic phase (JCPDS 71-2194). In2 O3 measurements. However, the bet size of the In2 O3nanopowder purchased from Alfa Aesar was also in- (Alfa Aesar)has a size of up to 169.1 nm, exhibitingcluded to compare it with the as-prepared cube-like a great deviation from XRDsize(27. 2 nm).This canIn2O3 nanocrystals. Based on the XRD measure- be attributed to the non-porosity of In2O3(Alfa Ae-ents, both In2O3 nanomaterials have approximately sar)and its low surface area for BEtsizeestimationhe same lattice values in unit cells. However, thepurchased In2 Oa (Alfa Aesar)has a bigger grain size ticipation of surface oxygen species adsorption, O(27. 2 nm)than the as-prepared In2 O3(9.6 nm)(Table 0, and 02-121-24), XPS(X-ray photoelectron spec-tion pattern was observed for the as-prepared In 2O3中国煤化工 for both In203as shown Fig.2. However, the presence of the elon-gated diffraction spots suggests the characteristic of the ICN MH Gls core-level bind-non-single crystalline, indicating that the as-prepared energies are shown in Table 1. The XPS detectedIn203 consists of multiple nanocrystallites with pre- In3ds/2 and In3d3/2 at the positions of around 444ferred alignment in orientationand (b)show the TEMof theand 452 eV(Fig. 5(a)), respecely, which is consis-obtainedind um oxide nanoparticles and In2 O3(Alfatent with the binding energies of the In3+ state/25J. Mater. Sci. Technol. Vol 24 No 4, 2008国65nmFig 3 TEM images of (a)porous In2 O3 and(b)non-porous In2 O3 nanopowder(Alfa Aesar),(c) HrTEM(left)and ultramicrotomed HRTEM images(right)of porous In2O3,(d)HRTEM image of In2O3 nanopowder(Alfa aesar).(The inset in Fig.1(a)shows a single particle of porous In2O3Figure 5(b)shows that the Ols binding energies andthe oxygen ls peaks were deconvoluted into two spec-tral bands at 529.6 and 531.4 ev for porous In20- Ag-prepared的°and 529.4 and 531.2 ev for In2O3(Alfa Aesarmost intense peaks at 529 eV can be attributed to thelattice oxygen in crystalline and the 531 ev of bindingenergy is assigned as the adsorbed surface OHIn the course of the formation of cube-like porousIn2O3 nanees, the cube-like In(OH)3 nanopar-ticles were first generated by laser ablation processSubsequently, the porous In2O3 nanomaterials wereformed by a calcination of In(OH)3 nanoparticles at0204400°cfor2h. In this appoach,PⅤ P and CTAB surfactants were introduced to yield cube-like In(oh)3nanoparticles. We have conducted separate experFig 4 N2 adsorption-desorption isotherm of as-obtained iments to understand the fate of PVP and CTABIn2 O3 nanomaterials(b)In , O, (Alfa Aesar)528530532526530532中国煤化工Fig5 XPS spectra of(a)the In3ds/2 and 3d3 /2 binding enerAesar)nanopowders, and(b)ols binding energies ofCNMHG203 (Alfa Aesar)The oxygen Is peaks were deconvoluted by the Gaussian function with an FWHM of 1.7 evJ Mater. Sci. Technol., Vol 24 No 4, 2008added for shape and size control. As mentioned ear- composition of In(OH)3, accompanied by the elimina-er, the laser ablation process produced In particles tion of PvP and CtAB from the surface of cube-likeexhibiting a spherical-like shape and amorphous ag. particles as evidenced by TGA analysis(see Support-gregates in the presence of PVP and CTAB. Then, ing Information), converted the non-porous In(oH)3the In particles and amorphous aggregates converted to porous In2O3 and shrunk the particle sizes fromto cube-like In(oH)3 accompanied by large In spheres 46.5 nm(In(oH)3)to 34.8 nm(In2 O3after a reflux procedure. The exclusive In(OH)3 cube-The test of the sensing characteristics of the In2O3like nanoparticles(46.5*5.7 nm)were collected by a nanoparticles toward 25 ppm ethanol was carried outsimple centrifugation to remove micrometer In spher- in a temperature range between 100 and 400oC inical particles(see Supporting Information). When order to determine the optimum temperature. The/P and CTAB were removed from the preparation, sensor signal is given as the ratio of electrical resis-the significant aggregates consisting of various mor- tance in air(rair, at a relative humidity of 48%)tophologies including sphere-like, rod-like, and cube-like that in the testing gas(Rgas), Rair/rgas. As can beshapes, as well as irregular aggregate structures, were seen in Fig. 6, the response to ethanol indicated thatobserved after going through the reflux and centrifu- the sensitivity peaked at the maximum value of 18gation processes(see Supporting Information). The for porous IngO3 at 220 C. Conversely, the responseesulting nanomaterials had a random distribution in curve for In2O3(Alfa Aesar) changed slowly acrossshape and size. When CTAB was used alone, similar varying temperatures and the sensitivity was only 3.7products consisting of sphere-like, rod-like, and cube- at 220 C, which was indeed a peak in the range ofaggregates appeared. On the other hand, we found lowed at the optimal temperature of 22 tion forlike particles were obtained, but much fewer irregular measured temperatures. Thus, further detedthat similar results to those yielded from the presenceFigure 7(a)shows the sensor signal of bothof PVP and CTAB were observed when experiments ensors at 220C as the concentration varied fromwith PVP alone were conducted, where the exclusive 1.7 to 100 ppm. Porous nanoparticles exhibited goodcube-like In(OH)3 particles could be obtained after sensing performance toward ethanol as compared withthe reflux and centrifugation operation, but the size non-porous In2O3(Alfa Aesar). At an operating tem-was distributed in a broader range(32.5+8.3 nm).: perature of 220 C, the ethanol concentrations can beThe formation of the cubic shape is believed to bedetermined by the intrinsic cubic crystal nature ofIn(OH)3 26, 27). Although the mechanisms by whichboth PVP and CTAB assist in the growth of cube-like In(OH)3 nanoparticles are not yet understood, itis apparent that the size and morphology control ofthese nanoparticles are established by cooperative ef-fects of PVP and CTAB, where PVP predominatelydetermines the formation of cube-like structures anCTAB leads to achieving more uniform size. ManyIn, o,(alfa Aesar)studies have successfully employed Pvp polymer asa structure-directing agent in the preparation of theanisotropic nanostructues 28, 29. For example, PVPis believed to strongly confine (100) planes and re-Working temp /Csults in the reduction of the R value (ratio of thegrowth rate between <100> and <111> directions)ig.6 Response to 25 ppm of ethanol at various op-leading to the cubic shapes in fcc nanocrystals/(30, 31Jerating temperatures for porous In2O3 and non-Additionally, the calcination process involving the de-porous In]O3(Alfa Aesar) nanomaterialsIn o, (Alfa Aesar)中国煤化工55011001650CNMHGmFig.7(a)Dynamic response of the porous In]Oa and In]O3(Alfa Aesar)sensors at 220C to ethanol gas withconcentrations of 1.7-100 ppm, (b )sensitivity(Rair/Rgas)of the porous In2O3 and, non-porous In O3(AlfeAesar)sensors to ethanol gas at 220C672J. Mater. Sci. TechnoL Vol 24 No 4, 2008Table 2 Comparison of sensing characteristics of various In]O3 Gag Sensorshanol/ppm Working temperature/Cr signal( rair rgasIn2O3 hollow microspheres spRoom tempera0∞0Room temperatureRoom temperatureRoom temperatureIn2O3 nanowires25ITO(tin doped indium oxide)200350ITO thin flm( 32)34.6porous In]O3(this work)10022028In2O3(Alfa Aesar00detected as low as 1.7 ppm with 2.8 of sensor signals trapped by adsorbed oxygen species, a space-chargefor the porous In2O3, while In2O3(Alfa Aesar)ex- region is formed on the surface of the metal oxideshibited virtually no sensitivity to 1.7 ppm ethanol. Ethanol reacts with ionic oxygen species, and thenThe sensitivity was up to 58.2 as the sensors were ex- the electrons trapped by the oxygen adsorbents areposed to 100 ppm ethanol for porous In2O3. Once released to the metal oxide, leading to the increasedagain, In2O3(Alfa Aesar) displayed a low response, conductivity of the oxideswith only a sensor signal of 8.9 upon the additionof 100 ppm ethanol. As shown in Table 2, the as- 4. Conclusionprepared cube-like porous In2 O3 products exhibitedmuch better sensitivity with a lower detection limitPorous cube-like crystalline In2O3 nanoparticlesand working temperatures when exposed to ethanol in were synthesized by a laser ablation-reflux processcomparison with other reported In2 O3-based materi- and have a diagonal length of less than 50 nm. Theals, except for those in Li et al. 's test of Iny O3 hollowa8-prepaled porous Ing O3 displayed improved sensi-microspheres and nanocubes at room temperatureli9l. tivity with a lower detection limit toward ethanolFigure 7(b) shows the variation in sensitivity as a than those of other reported In2 O3 based materialsfunction of ethanol concentration. The linear depen- Porous In2 O3 with a larger surface area exhibited bet-dence of the sensitivity on ethanol is observed for ter sensing performance and showed more linear be-porous In2O3 and In2O3 (Alfa Aesar). The linear de- havior as a function of ethanol concentration com-pendence results usually refect the relative sensitivity pared to commercial In2O3(Alfa Aesar).The extrap-among sensor materials. It is evident that the sensi- olated low detection limit could be down to 0. 1 ppmtivity of the porous In2 O3 is greater than that of the Porous In2 O3 has potential for practical applicationsnon-porous In2O3(Alfa Aesar)at each corresponding individual testing ethanol concentration. ItisAcknowledgementknown that gas-sensing behavior can be attributed toWe thank the National Science Cof Taiwan forporosity, grain size, morphology, microscopic or sub- financially supporting this workmicroscopic structure, and interaction between grains.The relative sensitivity between porous In2O3 andREFERENCESnon-porous In2 O3(Alfa Aesar)can be understoodfrom the point of view of surface area. Generally, [1]H Kobayashi, T. Ishida, Y Nakato, and H.Tsubomurasa large BEt surface area leads to good sensing prop-J.Appl,Phys,1991,69,1736.erties.comparingtheBetsurfaceareaofporous[21x.Li,M.W.wanlaSs,T.a.gEsserT,K.a.EmeryandIn2O3(80.4 m2.g")to that of In2O3(Alfa Aesar)T,J.Coutts: Appl. Phys. Lett., 1989, 54, 2674(4.9 m2g-1), porous In2O3 with a higher surface area 3]Y Shigesato, S Takaki and T.Haranob: J Appl. Physis expected to obtain high gas response, since a largersurface area provides more adsorption sites for surface [4] D HZhang. ZQ Liu, C Li, TTang, X.L. 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