In2O3 NANOSTRUCTURES AND THEIR CHEMICAL ... - UM Repository

‫‪Ahsanulhaq Qurashi, Muhammad Faisal Irfan and Mir Wakas Alam‬‬

‫‪In2O3 NANOSTRUCTURES AND THEIR CHEMICAL AND‬‬ ‫‪BIOSENSOR APPLICATIONS‬‬ ‫* ‪Ahsanulhaq Qurashi‬‬ ‫‪Center of Excellence in Nanotechnology and Chemistry Department‬‬ ‫‪King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia‬‬

‫‪Muhammad Faisal Irfan‬‬ ‫‪Department of Chemical Engineering, Faculty of Engineering, University of Malaya‬‬ ‫‪50603, Kuala Lumpur, Malaysia‬‬

‫‪and Mir Wakas Alam‬‬ ‫‪Department of Physics, Fergusson College, Pune 411004, Maharashtra, India‬‬

‫اﻟﺨﻼﺻـﺔ‪:‬‬ ‫إن ﺗﺮاآﻴﺐ أآﺎﺳﻴﺪ اﻟﻤﻌﺪن اﻟﻨﺎﻧﻮﻧﻴﺔ ﺗﺴﺘﺠﻠﺐ اﻻهﺘﻤﺎم اﻟﻤﺘﺰاﻳﺪ ﻟﻜﻞ ﻣﻦ اﻟﻌﻠﻮم اﻷﺳﺎﺳﻴﺔ و اﻟﺘﻄﺒﻴﻘﻴﺔ ‪ ،‬وﻣﻨﻬﺎ ﻣﺎدة ‪ In2O3‬ﺷﺒﻪ اﻟﻤﻮﺻﻠﺔ اﻟﻜﻔﺆة‪ .‬ﻳﺰود هﺬا‬ ‫اﻟﻤﻘﺎل اﻟﻤﺮاﺟﻊ اﻟﺘﻔﺎﺻﻴﻞ ﺣﻮل اﻟﻨﺸﺎﻃﺎت اﻟﺒﺤﺜﻴﺔ اﻟﺘﻲ ﺗﻢ إﺟﺮاؤهﺎ ﺣﺘﻰ اﻵن ﻓﻲ ﺗﺤﻀﻴﺮ و وﺻﻒ ﺗﺮاآﻴﺐ ‪ In2O3‬اﻟﻨﺎﻧﻮﻧﻴﺔ‪ ,‬ﺣﻴﺚ ﺗﻢ ﻓﻲ هﺬﻩ اﻟﻤﺮاﺟﻌﺔ‬ ‫ﺑﺤﺚ ﻣﺨﺘﻠﻒ اﻟﺘﻘﻨﻴﺎت اﻟﻤﺴﺘﺨﺪﻣﺔ ﻓﻲ ﻋﺪد واﺳﻊ ﻣﻦ أﺷﻜﺎل ﺗﺮاآﻴﺐ ‪ In2O3‬اﻟﻨﺎﻧﻮﻧﻴﺔ آﺎﻟﺠﺰﻳﺌﺎت اﻟﻨﺎﻧﻮﻧﻴﺔ‪ ,‬واﻟﻘﻀﺒﺎن اﻟﻨﺎﻧﻮﻧﻴﺔ‪ ,‬واﻷﺳﻼك اﻟﻨﺎﻧﻮﻧﻴﺔ‪,‬‬ ‫واﻟﺪﺑﺎﺑﻴﺲ اﻟﻨﺎﻧﻮﻧﻴﺔ‪ ,‬واﻹﺑﺮ اﻟﻨﺎﻧﻮﻧﻴﺔ‪ ,‬واﻷﺑﺮاج اﻟﻨﺎﻧﻮﻧﻴﺔ و أﻗﻼم اﻟﺮﺻﺎص اﻟﻨﺎﻧﻮﻧﻴﺔ اﻟﺦ‪ .‬ﻟﻘﺪ ﺗﻢ ﻣﻨﺎﻗﺸﺘﺔ ﺗﺤﻠﻴﻞ اﻟﺘﺮآﻴﺐ اﻟﻤﻔﺼﻠﺔ ﻟﺘﺮاآﻴﺐ ‪ In2O3‬اﻟﻨﺎﻧﻮﻧﻴﺔ‬ ‫اﻟﺘﻲ أﻋﻄﺖ ﻃﺮﻳﻘﺔ ﺑﺪﻳﻠﺔ ﻟﻔﻬﻢ و ﺗﻮﻗﻊ اﻟﺨﻮاص اﻟﻔﻴﺰﻳﻮآﻴﻤﻴﺎﺋﻴﺔ اﻟﻜﺎﻣﻨﺔ ﻟﺘﺮاآﻴﺐ أآﺎﺳﻴﺪ اﻟﻤﻌﺪن اﻟﻨﺎﻧﻮﻧﻴﺔ‪ .‬وﻳﻌﻄﻲ هﺬا اﻟﻤﻘﺎل أﻳﻀ ًﺎ ﺗﺒﺼﺮًا ﻋﻤﻴﻘًﺎ و ﻣﺮاﺟﻌﺔ‬ ‫ﻣﺤﺪﺛﺔ ﻟﺘﺮاآﻴﺐ ‪ In2O3‬اﻟﻨﺎﻧﻮﻧﻴﺔ أﺣﺎدﻳﺔ اﻟﺒﻌﺪ ﻋﺎﻟﻴﺔ اﻟﺪﻗﺔ آﺤﺴﺎﺳﺎت آﻴﻤﻴﺎﺋﻴﺔ و ﺑﻴﻮﻟﻮﺟﻴﺔ‪ .‬وﺗﻢ أﻳﻀ ًﺎ ﻣﻨﺎﻗﺸﺔ ﺗﺄﺛﻴﺮ ﺣﺴﺎﺳﺎت ﻏﺎزﻳﺔ ﻋﺎﻟﻴﺔ اﻟﺪﻗﺔ و ﺳﺮﻳﻌﺔ‬ ‫اﻻﺳﺘﺠﺎﺑﺔ ﺑﺎﺳﺘﺨﺪام ﺗﺮاآﻴﺐ ‪ In2O3‬اﻟﻨﺎﻧﻮﻧﻴﺔ‪.‬‬

‫_______________‬ ‫‪∗Corresponding Author:‬‬ ‫‪E-mail: [email protected]‬‬ ‫‪Fax +966-3-860-726‬‬

‫______________________________________________________________________________‬ ‫‪Paper Received February 7, 2010; Paper Revised June 13, 2010; Paper Accepted June 19, 2010‬‬

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‫‪The Arabian Journal for Science and Engineering, Volume 35, Number 1C‬‬

‫‪June 2010‬‬

Ahsanulhaq Qurashi, Muhammad Faisal Irfan and Mir Wakas Alam

ABSTRACT Metal oxide nanostructures are of increasing interest in both fundamental and applied sciences, and among them In2O3 is one of the capable semiconductor materials. This review provides details of research carried out so far for the synthesis and characterization of In2O3 nanostructures. In this review, various techniques have been investigated for the fabrication of diverse and fascinating shapes of In2O3 nanostructures, for instance, nanoparticles, nanorods, nanowires, nanopushpins, nanoneedles, nanotowers, nanopencils, etc. The detailed structural analysis of In2O3 nanostructures has been discussed, which gives an alternative way to understand or predict the potential physicochemical properties of metal oxide nanostructures. This article also gives a profound insight and an up-todate review of high-quality 1-D In2O3 nanostructures as chemical sensors and biosensors. The impact of highly sensitive and fast responding gas sensors using In2O3 nanostructures is also discussed Key words: In2O3, nanostructures, FESEM, structural properties, biosensors and gas sensors

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In2O3 NANOSTRUCTURES AND THEIR CHEMICAL AND BIOSENSOR APPLICATIONS 1. INTRODUCTION Advancements in material science have focused attention on the properties and behavior of materials on the scale of nanometers. Recently, science has provided the means to manipulate materials on an atom-by-atom basis and established procedures for material characterization on the small size scales. These developments are having an almost immediate impact on technology and serious efforts have been made to explore them for various applications. In the last two decades, there has been considerable interest in the synthesis of nanoscale materials [1–12]. One of the most attractive classes of materials for functional nanodevices is the metal oxide semiconductor. Various methods have been developed for the synthesis of metal-oxide semiconductor nanostructures [13–29]. A great deal of effort has focused on the catalyst supported synthesis of nanomaterials using approaches that are not substrate specific but compatible with the basic device configuration. Moreover, the nanodevice functionality has been demonstrated with these materials in the form of various device structures typically for one-dimenisional (1-D) semiconducting nanostructures [30–40]. In this part, the well studied and most common are metal-oxide semiconductor nanostructures. Among the metal-oxide nanostructures, a unique and particularly interesting material is indium oxide (In2O3). In2O3 has also attracted considerable research. It is known to have a body centered cubic structure (a = 10.12 A°) [41]. In2O3 has been widely used in the microelectronic field as gas detectors, window heaters, solar cells, memory devices, and flat panel display materials [42–57]. The applications of the In2O3 nanostructures depend on controlled synthesis of these materials with specific morphologies. Different In2O3 morphologies such as nanowires (including nanowire arrays), nanobelts, nanoparticles (including nanocubes, nanoflowers and octahedra), nanotowers, microarrows, nanotubes, In/In2O3 nanocables, lotus-root-like In2O3 nanorods and nanocolumns, nanocrystal chains, and hollow spheres have been prepared by chemical vapor deposition (CVD), pulsed laser deposition (PLD), the alumina or mesoporous silica template method, and wet chemical methods, and so on. 1-D has attracted tremendous attention since the revolutionary discovery of carbon nanotubes in 1991. These 1-D nanostructures with high aspect-ratios suggest better crystallinity, smooth surface, higher integration density, and lower power consumption. Moreover, owing to a large surface-to-volume ratio and a Debye length comparable to the small size, they exhibit superior sensitivity to surface chemical processes. Also, their size internment provides tunable band gap, higher optical gain, and faster operation speed. 1-D In2O3 nanostructures have been employed to fabricate field-effect transistors, nanoscale chemical sensors for the detection of NO2, NH3, acetone, DNA, and biosensing devices, etc. In this review, we report on the synthesis of In2O3 nanostructures. The ability to control the synthesis of highquality In2O3 nanowires leads to potential applications in photodetectors, gas sensing, and transparent electronics. The detailed biosensing and gas sensing properties of In2O3 nanostructures are highlighted in this review. 2. GENERAL PROPERTIES IN2O3 Indium (III) oxide (In2O3) is an amphoteric oxide of indium. It is an n-type semi-conductor and it forms bixbyitetype cubic crystals. It is known to have a body centered cubic structure (a = 10.12 A°). Its band gap has recently been to 2.9 eV from the old value of about 3.7 eV [58]. Indium oxide is used in some types of batteries, thin film infrared reflectors transparent for visible light (hot mirrors), optical coatings, antistatic coatings, photocatalyst sensors, etc. It is also used in resistive elements and integrated circuits. When is combined with tin oxide, it forms indium tin oxide, traditionally known as ITO, and used for transparent conductive coatings. In histology, indium oxide is used as a part of some stain formulations. Thin films of chromium-doped indium oxide (In2-xCrxO3) were recently reported to be magnetic semiconductors displaying high-temperature ferromagnetism, single-phase crystal structure, and semiconductor behavior with a high concentration of charge carriers. It has possible applications in spintronics as a material for spin injectors. Recent theoretical simulation predicted that the oxygen-depleted indium oxide (100) surface could be ferromagnetic [59]. To characterize the electrical property of In2O3 nanowires, individual nanowires had been configured as field-effect transistors using the photolithography technique [60]. 3. FABRICATION OF IN2O3 NANOSTRUCTURES Various methods have been adopted for the growth of In2O3 nanostructures. On the basis of their techniques, they can be divided into two major types: dry methods and wet chemical synthesis methods. The dry methods are again divided into various types based on the growth mechanism and processes. Generally, metal-oxide nanostructures are synthesized by the vapor transport process where the reaction between metal vapor and oxygen gas occurs and,

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finally, the material is deposited on the surface of the substrate. The two most familiar growth mechanisms which facilitate the formation of nanostructures are vapor-liquid-solid (VLS) and vapor-solid (VS) [13,14]. The chemical synthesis methods provide another alternative to the fabrication of In2O3 nanostructures. The chemical synthesis methods are high reproducible, easier, low cost, less hazardous, straightforward, and environmentally benign. 3.1. Vapor Transport Method Vapor transport growth of In2O3 nanostructures is generally carried out in a tubular furnace in which a reaction between metal and oxygen gas occurs and the product is deposited on the surface of the substrate. The growth of In2O3 nanostructures by these techniques is based on the vapor-solid and vapor-solid-liquid mechanism. 3.1.1. Vapor solid growth mechanism Vapor solid is a catalyst-free technique for the fabrication of metal or metal-oxide nanostructures [56]. Indium metal or indium oxide powder with active carbon can be used as source materials. Substrates like silicon, silicon oxide, sapphire, etc. can be used for the growth of the final product. The nanostructures formed by the vapor solid growth mechanism are of high purity because of the catalytic-free approach. During the anisotropic growth, nanostructures formed due to the competition in growth rate among different crystallographic planes. 3.1.2. Vapor-solid-liquid mechanism This method was first discovered in 1967 by Wagner and Ellis [57]. In this method, a thin layer of metal is deposited on the substrate which acts as catalyst. A metal coated substrate is used for the growth of In2O3 nanostructures in the tubular furnace. Indium metal or In2O3 and activated carbon can be used as source materials. Oxygen gas and some time carrier gas is usually passed in the furnace for the growth of In2O3 nanostructures [61]. 3.2. Chemical Synthesis Methods Chemical synthesis has been a powerful method for producing nanomaterials since the manifestation of 1-D nanomaterial solution-based chemical synthesis methods have been extensively explored [30,31]. This growth process generally entails ambient temperature in order to reduce the complexity of the fabrication process. From the cost point of view, a low-cost chemical synthesis with a high yield is undoubtedly superlative for future commercialization of 1-D nanostructures. In order to expand strategies that can guide and confine the growth of nanostructures, researchers have used various approaches, which may be grouped into various sub-categories. 3.2.1. Aqueous solution method An aqueous solution method is one of the prominent illustrations of anisotropic crystal growth [62]. It has become a subject of interest in the fabrication of 1-D-oriented arrays of nanostructures because of its simple, lowtemperature, and low-cost features. Due to the decomposition of metal salt and base at an elevated temperature, the concentrations of metal ions+ and hydroxyl ions increase correspondingly, and nuclei start to form when the degree of supersaturation exceeds the critical value. The concentration of building units in solution increases until the critical super-saturation level is reached and the nucleation occurs. This growth process is influenced by several interesting mechanisms like ostwald ripening, kirkendall effect, etc. 3.2.2. Hydrothermal synthesis Hydrothermal techniques have been used for the fabrication of highly crystalline nanostructures. This process occurs when the metal salt is mixed in aqueous solution. The mixed solution is placed in an autoclave under elevated temperature and relatively high-pressure conditions. Generally, the temperature range between 100–280°C and pressure exceeds 1 atm. When the autoclave is heated, it generates pressure and, subsequently, interesting morphologies of nanostructures can be formed. By controlling the parameters, one can control the shape and crystallinity of nanostructures [63]. 3.2.3. Solvothermal synthesis Solvothermal synthesis is a particularly versatile low-temperature route, in which polar solvents, under pressure and at temperatures above their boiling point, are employed. Under solvothermal conditions, the solubility of the reactants increases significantly, enabling reaction to take place at considerably lower temperatures compared to conventional synthetic techniques. This type of soft-chemistry approach is mild enough to allow “molecular” building blocks such as chains and rings to form and participate in the formation of nanostructures [64]. 3.2.4. Electrochemical deposition Electrochemical deposition has been extensively used to fabricate metal and metal oxide nanowires in porous structures. When a metallic salt is dissolved in water, it dissociates to form positively charged ions. The solution that contains these charged ions is referred to as an electrolyte or a plating solution. By passing a sufficient amount of

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electric current through this electrolyte, one can reduce the quantity of metal ions to form solid metal In2O3 nanowires [65]. 3.2.5. Sol-gel deposition The sol-gel process usually consists of four steps: (1) the desired colloidal particles once dispersed in a liquid to form a sol; (2) the deposition of sol solution produces the coatings on the substrates by spraying, dipping, or spinning; (3) the particles in the sol are polymerized through the removal of the stabilizing components and produce a gel in a state of a continuous network; and (4) the final heat treatments pyrolyze the remaining organic or inorganic components and form an amorphous or crystalline coating [66]. 3.2.6. Surfactant-assisted growth Surfactant-aided anisotropic crystal growth has been regarded as an expedient way to synthesize oxide nanostructures. The growth is carried out in a micro-emulsion system composed of three phases: oil phase, surfactant phase, and aqueous phase. To obtain the desired nanostructures, there is a need to standardize the stoichiometry of reactant species and type of surfactant (anionic, cationic, and ionic), and set the parameters, such as pH, temperature, etc. This process is based on a trial-and-error procedure and requires many efforts to choose appropriate capping agents and reaction environments [67]. 3.2.7. Vertical growth of In2O3 nanostructures The properties of nanomaterials strongly depend on their dimensions and morphologies. Therefore, investigation of anisotropic 1-D nanostructure in highly oriented, aligned, and ordered arrays is of critical importance for the development of novel devices [68]. Various methods have been adopted to grow vertically oriented nanorod arrays on different substrates. For the growth of highly aligned nanostructures, gas-phase deposition [VS and VLS] is one of the principal technologies and VLS is one of the most productive techniques to grow vertical nanostructures from catalytic seeds. Eptitaxial growth of well-aligned nanowires can be achieved by a single crystalline substrate. The oriented growth on the single crystalline substrate can be controlled by crystallographic directions. In this case, the nature of the substrate and reaction conditions play significant roles for the aligned growth of nanostructures. 4. MORPHOLOGY OF In2O3 NANOSTRUCTURES It is well documented that diverse morphologies have a significant impact on the performance of nanostructure materials. Various researchers successfully synthesized different shapes of In2O3 nanostructures. Recently, we investigated the vapor transport growth of In2O3 nanostructures [69–72]. A variety of In2O3 nanostructures like pyramids, octahedrons, nanorods, nanoparticles, nanowires, nanoneedles, nanopushpins, nanotowers, nanopencils, etc. are grown through VS or VLS mechanism. There are various reaction parameters which affect the morphology of nanostructures. These include the reaction temperature, flow of carrier gas, nature of substrate, position of substrate, time, and so on. We obtained different morphologies under different experimental conditions. In2O3 triangular structures were grown at 700°C using indium as source materials. Indium particles were placed under the substrate by adjusting the distance. Figures 1(a) and (b) show Field Emission Scanning Electron Microscopy (FESEM) images of triangular-shaped In2O3 nanostructures [69]. When the temperature was raised to 800°C, highdensity pyramids were grown on the entire surface of the substrate, as shown in Figure 1(c). A single pyramid is shown in the magnified image of Figure 1(d). The pyramids have four equilateral triangles and a square. Figure 1(e) shows the formation of In2O3 octahedrons, when the reaction temperature is further increased to 900°C. Figure 1(f) shows a high-magnification FESEM image of an octahedron. An In2O3 octahedron is made up of eight well-faceted equilateral triangles, four of which meet at each nano-vertex. Each side of triangle has a length of c.a. ~1.5 µm. A tremendous change was observed in the morphology of In2O3 nanostructures when the argon gas (carrier gas) was mixed with oxygen with an appropriate ratio. Figures 2(a) and (b) display the morphology of the products revealing In2O3 nanopushpin-like nanorods [70]. The magnified FESEM image in Figure 2(b) revealed that each nanopushpin consisted of a nanorod stem and a tetrahedral tip. The diameter of each nanorod was between 80–120 nm and the length of each In2O3 nanorod was about 500 nm–1µm. Each nanorod showed a tetrahedral tip-like structure. Each tip-like structure has four equilateral triangles. We observed a profound effect of flow rate of carrier gas and Au coating of silicon substrate (10 nm) on the morphology of In2O3 nanostructures. Figures 3(a) and (b) show low and high-magnification FESEM images of In2O3 nanoparticles grown on an Au-coated silicon substrate when the flow of argon gas was set to about 50 mL.min-1 [71]. These In2O3 nanoparticles have a size of 60 ± 10 nm. The nanoparticles were grown consistently on the substrate. When the flow of argon was increased to 100 mL.min -1, In2O3 nanowires were formed as shown in Figures 3(c) and (d). The In2O3 nanowires were of 1–5 µm in length and 100 ± 20 nm in diameter. The high-magnification image in Figure 3(d) reveals that each nanowire demonstrates a tetrahedral nanorpyramid on its tips. When the argon flow rate was further increased to 150 mL.min-1, In2O3 microrods were formed, as shown in Figures 3(e) and (f). The length of the rod-like structure was about 1µm and their diameter was about 300 ± 100 nm. The flow rate of the argon gas was also affected on the shape of the nanostructures. June 2010


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Figure 1: Low and high magnification FESEM images of In2O3 crystals (a and b) pyramids, (c and d) and octahedrons (e and f). Reprinted from Ref. [69], with permission from Copyright 2010 Elsevier.

Figure 2: (a) and (b) low and high-magnification FESEM images of In2O3 nanopushpins. Reprinted from Ref. [70], with permission from Copyright 2010 American Institute of Physics.

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Figure 3: Low and high magnification FESEM images of In2O3 nanoparticles (a) and (b), nanowires (c) and (d) and microrods, (e) and (f). Reprinted from Ref. [71], with permission from Copyright 2010 Elsevier.

The catalyst plays a very important role in the growth of 1-D metals and metal-oxide nanostructures. The catalyst facilitates the preliminary nucleation to generate active sites for the precipitation of 1-D nanostructures. By modulating the reaction temperature, we observed three different types of 1-D In2O3 nanostructures on Au-coated silicon substrate. Figures 4(a) and (b) show low and high magnification FESEM images of In2O3 nanostructures grown at 700°C [72]. Figures 4(c) and (d) illustrate the formation of In2O3 nanowires at 800°C. The diameter of each In2O3 nanowire was between 70–80 nm and the lengths were several micrometers. These nanowires were terminated at their growing ends by Au nanoparticles. The presence of Au nanoparticles at the end of nanowires indicated the vapor-liquid-solid (VLS) growth process. When the reaction temperature was further elevated to 900°C, In2O3 nanoneedles were formed as shown in Figures 4(e) and (f). The diameter of each nanoneedle was about 150–200 nm and the length was about 4–5 µm. The stems of the nanoneedles confirmed their identical diameter and their tips exhibited extremely sharp morphology. The diameter of each nanotip was about 50 nm. The In2O3 nanoneedles showed a low aspect-ratio compared with the nanowires grown at 800°C.

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Figure 4: Low and high magnification FESEM images of In2O3 nanorods (a) and (b) nanowires, (c) and, (d) nanoneedles, (e) and (f). Reprinted from Ref. [72], with permission from Copyright 2010 Elsevier.

The shape of the nanostructure was also found to be reliant on the position of the source materials. Three different experiments were carried out by placing the source in front of the substrate [73]. Figure 5(a) illustrates a low-magnification FESEM image of the nanostructures grown at 700°C in the center of the furnace at position A. The shape of the nanostructures looks like nanowires. However, a high-magnification FESEM image noticeably designates an attractive capped monument-like nanostructure. An individual monument is shown in Figure 5(b). It is noteworthy to mention that each monument-like structure is grown with a uniform stem, pyramidal tip, and truncated base. Figure 5(c) shows a typical low-magnification FESEM image of the samples grown at 800°C. Nanotower-like In2O3 nanostructures were formed at position A. A high-magnification FESEM illustrates a single In2O3 nanotower with a stacking arrangement of nanosheets with a bulging appearance. The diameter of the rectangular base of the nanowtower was about 1 µm. From bottom to top, the diameter of the base is decreasing dramatically and the tip has a diameter of about 100 nm. The stepwise reduction of diameter and successive arrangement of nanosheets has resulted in a nanotower-like structure shape.

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An FESEM image of the samples grown at 900°C is shown in Figures 5(e) and (f). Figure 5(e) depicts the dense nanopencil structures with high crystallinity. We can see from Figure 5(f) that the faceted stem of each nanopencillike structure has an average diameter of 2–3 µm and the tips on the top have a diameter of about 120–150 nm. Fascinatingly, all In2O3 nanopencils are well-faceted with edges and corners. Different shapes of In2O3 nanostructures were obtained by our group by modulating the reaction conditions. Various other research groups fabricated the variety of In2O3 nanostructures by various methods [74–80].

Figure 5: (a-e) Low and high magnification of FESEM images of In2O3 nanostructures grown at 700°C, 800°C, and 900°C. Reprinted from Ref. [73], with permission from Copyright 2010 American Scientific Publishers.

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5. STRUCTURE OF In2O3 NANOSTRUCTURES Figure 6 shows the x-ray diffraction (XRD) pattern and its Miller indices on each diffraction peak. All the observed peaks can be perfectly indexed to the crystal planes of the cubic In2O3 (JCPDS card no. 06-0416). The nanostructures are highly crystalline in nature [69].

Figure 6: XRD spectra of In2O3 pyramids. Reprinted from Ref. [69], with permission from Copyright 2010 Elsevier.

The transmission electron microscopy (TEM) has its primary uses in metals, metal oxides, metal chalcogenides, and the biological sciences, especially in the study of cells at the molecular level. TEMs have been particularly useful in terms of developing images of crystals and metals at the molecular level― allowing scientists to study their structure, interactions, and identify flaws. To obtain a better understanding of the formation and shape evolution of In2O3 nanostructures, TEM investigations were carried out as shown in Figure 7. Thus, the properties of nanostructures can be directly correlated with their well-defined structures by this technique. We studied the TEM analysis of In2O3 nanowires indicated in Figure 7(a). The selected electron area diffraction pattern (SAED) indicated the single crystal nature. The SAED pattern in Figure 7(b) shows single crystalline In2O3 with a cubic structure and grown along [100] direction [69]. We have investigated the growth direction using the camera constant as shown in Figure 7. However, HRTEM analysis, shown in Figure 7(c), revealed the distance between lattice fringes was 0.50 nm. Also, TEM attached EDX analysis was also carried for a single nanowire to learn the exact chemical composition, as indicated in Figure 7(d). Bixibite In2O3 nanobelts structure (C-rare earth crystal structure) was investigated by Kong et al. [74], as shown in Figure 8. They calculated the projection of [100] type nanobelts from [001]. The structural model of [120] type nanobelts with a similar (001) top surface is elaborated on in Figure 8(c).

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Figure 7: (a) TEM images of single nanowire, (b) shows SAED patterns, (c) is a HRTEM image, and (d) shows an EDX spectrum for single nanowire. Reprinted from Ref. [71], with permission from Copyright 2010 Elsevier.

Figure 8: (a) Unit cell of In2O3. (b) [001] Projected model of the [100] type nanobelts. (c) [001] Projected model of the [120] type nanobelts. Reprinted from Ref. [74], with permission from Copyright 2010 Elsevier. (image courtesy of Z. L. Wang, Georgia Institute of Technology)

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6. In2O3 NANOSTRUCTURES AS SENSORS Sensors are devices that can measure a parameter of interest, typically non-electrical, and generate an (usually) electrical signal that can be processed by computers. They are very prevalent in modern technology and all types of industries as strain sensors, temperature sensors, chemical sensors, optical sensors, biosensors, etc. The core of sensor structure is materials. Materials are, therefore, pivotally important for the enhancement of sensors in terms of sensitivity, selectivity, reversibility, sustainability, dynamic range, etc. Nanomaterials develop new phenomena that have the potential to revolutionize the field of sensor technology, resulting in new generations of sensors with enhanced performance and properties. This is due to the huge specific surface area which provides higher recognition probability/increased sensitivity. In addition, functionalization of 1-D nanostructures further enhanced the overall performance of these nanosensors. Various kinds of metals and metal-oxide nanostructures have been investigated for gas sensors recently. Among metal-oxide semiconductors, 1-D In2O3 nanostructures have received considerable attention for their technological applications in optoelectronic devices and gas sensors owing to their high electric conductance, high transparency to visible light, and the strong interaction between certain poisonous gas molecules and In2O3 surfaces. Curreli et al. [81] reported for the first time the biofunctionalization of In2O3 nanowires by electrochemically activating their surfaces and then immobilizing single-strand DNA. In2O3 nanowire surfaces were functionalized by generating a self-assembled monolayer (SAM) of 4-(1,4-dihydroxybenzene)butyl phosphonic acid (HQ-PA). An electrochemically active HQ-PA molecule containing a hydroquinone group going through reversible oxidation/reduction at low potentials reacts with a range of functional groups, which can be easily incorporated into biomolecules and other materials, such as thiols, azides, cyclopentadienes, and primary amines. In this way, thiolterminated DNA molecules were functionalized by the desired nanowires. Figure 9 shows a photographic image of a In2O3 nanowires device and florescent microscopic images to confirm selective functionalization. A dominant electrostatic interaction mechanism of In2O3 nanowire using streptavidin as a model analyte was revealed by Ishikawa et al. [82].

Figure 9: (a) A photograph of a NW mat sample contacted by two groups of electrodes. Only the HQ-PA attached to the NWs between the upper electrodes was converted to Q-PA. (b) An SEM image of the In2O3 NWs before functionalization. The brighter stripes are gold electrodes covering the NW mat. (c) The same sample imaged at higher magnification, where the NW mat is clearly visible. (d) A fluorescence image of the NWs with Q-PA taken after DNA attachment. The gold electrodes, passivated with an alkanethiol, appear dark under the fluorescence microscope. (e) A fluorescence image of the NWs with HQ-PA after DNA incubation. The NWs appear dark, indicating no DNA attached to HQ-PA. Reprinted from Ref. [81], with permission from copyright 2010 American Chemical Society.

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Figure 10(a) shows a photograph of a complete 3 inch wafer with multiple biosensor chips typical device with interdigitated electrodes, schematic steps of sensing diagram of FET set up, and IV characteristics and typical plots of change of current vs times for three typical devices exposed to streptavidin [82]. They intensely studied the correlation between gate dependence (dIds/dVg) and the absolute response (absolute change in current, ∆I). Ishikawa et al. also successfully functionalized In2O3 nanowires with antibody mimic proteins (AMPs) to detect nucleocapsid (N) protein, a biomarker for severe acute respiratory syndrome (SARS) [83]. Various research groups studied chemical sensing properties of In2O3 nanostructures.

Figure 10: (a) Optical micrograph of a 3 in. wafer with multiple biosensor chips; (b) photograph of one chip with an inset showing an optical image of the interdigitated electrodes; (c) SEM image of multiple In2O3 nanowires between the source and drain electrodes. The inset is a magnified image of an individual nanowire; (d) Ids_Vds plots under different Vg; (e) schematic diagram of the sensing setup illustrating an FET biosensor device operated by the liquid gate; (f) typical plots of the change in current vs time for three devices which were exposed to streptavidin (S-Av) of 100 nM at t = 100 s in 0.01xPBS. Vds of 0.2 V and Vg of 0.6 V were used for the measurement. Reprinted from Ref. [82], with permission from Copyright 2010 American Chemical Society.

Zhang et al., [84] reported the ammonia (NH3) gas sensing properties of semiconducting In2O3 nanowires with different dopant concentrations. They found that oxygen has a reasonably inconsequential effect on the nanowire conductance, while moisture can reduce the nanowire conductance considerably. Li et al. [85] investigated the ammonia sensing of In2O3 nanowires under UV exposure. Figure 11(a) shows current-voltage (I-V) curves recorded before and after exposing a sample with UV initialization to 1% NH3 in dry air for 5 minutes with the gate bias Vg 1.5 V [85]. Typical I-V characteristics of In2O3 nanowires before exposure are elaborated on. However, after the exposure of 1% NH3, the device demonstrated a 5-time increase in electrical conductance. They also investigated the effect of O2 by introducing the dry air, which showed limited effect in the decrease of electrical conductance, as shown in Figure 11(b).

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Figure 11: NH3 sensing properties of sample #1. (a) I-V curves taken before and after exposure to 1% NH3 with UV initialization. Inset: SEM image of the device. (b) I-Vg curves taken with the device residing in a vacuum, in dry air, and after the NH3 exposure. Inset: band diagram of this device. Reprinted from Ref. [84], with permission from Copyright 2010 American Chemical Society.

Du et al., [86] also investigated the ammonia sensor response from different morphologies of In2O3 nanostructures, like broken nanotubes, nanowires, and nanoparticles. However, they found that broken In2O3 nanotubes showed a high response [86]. Figure 12 shows the response of In2O3 nanostructures. Figure 12 shows the response of In2O3 broken nanotubes with 20 ppm concentration of NH3 vs time and the sensor shows a very high sensitivity of about 2500. Furthermore, the gas sensor device based on porous In2O3 nanotubes offered a better response and recovery time of less than 20 seconds, as well as good reliability with relatively negligible deviations. The gas sensors based on In2O3 nanowires, regular In2O3 nanotubes, and In2O3 nanoparticles showed a comparatively poor performance.

Figure 12: (a) Sensitivity response vs ammonia concentration [5–25 ppm] at room temperature for four types of gas sensors based on In2O3 nanostructures including broken In2O3 nanotubes, regular In2O3 nanotubes, In2O3 nanowires, and In2O3 nanoparticles: (b) Sensitivity response of the gas sensor based on broken In2O3 nanotubes vs the time for a concentration of NH3 of 20 ppm at room temperature. Reprinted with permission from Ref. [86].

Choi et al., [87] investigated the CO sensing relatively at a higher temperature, i.e., 400◦C as shown in Figure 13. The sensing behavior was extremely reliant upon the morphology of the In2O3 powder. The gas responses (Ra/Rg) of the hierarchical and hollow spheres to 10–50 ppm CO were 2.16–3.81 and 1.99–2.79, respectively. Moreover, the recovery of the sensor resistance was reproducible in the hollow and hierarchical spheres (Figures 13(a) and (c)). The sensor resistance fluctuates slightly in agglomerated In2O3 powder (Figure 13(e)). They also investigated the reliability of hollow and hierarchical spheres-based sensors.

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Figure 13: Dynamic CO sensing transients of (a and b) the hollow In2O3 microspheres prepared by HT of the ISL precursors, (c and d) the urchin-like In2O3 microspheres prepared by HT of the IL precursors, and (e and f) the agglomerated In2O3 powder prepared by HT of the IS precursors (measurement at 400 ◦C). Reprinted from Ref. [87], with permission from Copyright 2010 Elsevier.

Recently, we studied the hydrogen gas sensing response of In2O3 nanopushpins at different operating temperatures. Figure 14 shows the dynamic and quick response of In2O3 nanopushins toward hydrogen gas (reducing gas) at 250°C [71]. The electrical resistance decreases by introducing hydrogen gas. It can be seen from Figure 14 that by increasing the concentration of hydrogen gas from 500 to 1500 ppm, the resistance of In2O3 nanostructures decreases considerably. The response time for 500 ppm H2 gas was about 35 seconds. The rate of recovery was slower, i.e., 60 seconds.

Figure 14: (a) Dynamic responses of the In2O3 nanopushpins to H2 pulses at 250°C and (b) sensitivity at various operating temperatures. Reprinted from Ref. [71], with permission from Copyright 2010 American Institute of Physics.

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Also, recently, we investigated the response of In2O3 nanowires and nanoneedles towards H2. Figure 15(a) shows the gas sensor response of In2O3 nanowires at 200°C [72]. The In2O3 nanowire sensor shows a reversible response to hydrogen gas. The resistance decreases when the hydrogen gas was introduced into the chamber. By changing the concentration of hydrogen gas from 500 (0.1%) to 1500 ppm, the resistance of In2O3 nanowires decreases drastically. The response time for 500 ppm H2 gas is about 31 seconds and the recovery time of In2O3 nanowires is 80 seconds. Figure 15(b) demonstrates the response of In2O3 nanowires and nanoneedles at different operating temperatures [72]. Here the response is defined as the ratio of change in the conductance of the sensing material between gas and air to the initial conductance in air per unit concentration. S = ( Rair-Rgas/ Rgas)

(3)

The overall response increased by increasing the operating temperature. Due to a high aspect ratio and excellent crystal quality, nanowires showed better performance compared to the nanoneedles.

Figure15: (a) Response of a gas sensor made of In2O3 nanowires at 200°C to H2 gas at different concentrations and (b) temperature vs sensitivity of In2O3 nanowires and nanoneedles [72], (accepted for publication in Sensors Actuators B)

Recently, Vomiero et al., [88] investigated the electrical response of the In2O3 nanowires and thick wires toward acetone and NO2. The influence of various acetone concentrations on the electrical conductance of nanowires or thick wires are shown in Figure 16. They found that the electrical conductance was higher for the thick wires. They proposed that such an effect can be attributed to the surface space charge region extending on the lateral sides of the wires and exposed to ambient atmosphere. They also investigated various concentrations of NO2 with respect to different temperatures. Figure 17(a) demonstrates that the sensitivity of acetone increases with the operating temperature up to a maximum located at about 400°C. Also, thin nanowires showed 7 times better response compared to thick nanowires. Figure 17(b) shows the response to 500 ppb and it indicates that the response decreases upon increasing the working temperature. The exceptional properties of In2O3 nanostructures with diverse morphologies made them interesting candidates for bio and chemical sensors. Here we included a few interesting examples of In2O3 nanostructures as a chemical or bio sensor. However, there is still a need to investigate the extensive chemical sensing properties of In2O3 nanostructures.

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Figure 16: (a) Kinetic variation of current of In2O3 nanowires (red) and thick wires (black) towards 25, 50, and 100 ppm of acetone (dotted line) at 40% relative humidity, 20 °C ambient temperature, and 400°C operating temperature; (b) values of steady-state electrical conductance of nanowires (red) and thick wires (black) as a function of temperature in synthetic air (solid symbols) or after the introduction of 100 ppm of acetone (open symbols); and (c) response of nanowires (red) and thick wires (black) as a function of acetone concentration at 40% relative humidity, 20°C ambient temperature, and 400°C operating temperature. Reprinted from Ref. [88], with permission from Copyright 2010 American Chemical Society.

Figure 17: Response of In2O3 nanowires and thick wires as a function of the operating temperature toward (a) 25 ppm of acetone and (b) 500 ppb of NO2 . Reprinted from Ref. [88], with permission from Copyright 2010 American Chemical Society.

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7. CONCLUSIONS This article reviewed the extensive research on various shapes of nanostructures of In2O3 for sensing applications. Although we could not cover all the interesting works reported due to the great amount of research in the field, we did choose a few significant reports which can contribute to the further development of this field. Various types of In2O3 nanostructures, for instance, nanoparticles, nanorods, nanowires, nanopushpins, nanoneedles, nanotowers, nanopencils, etc., which are fabricated by vapor transport and other potential synthesis methods, are discussed in this review paper. Effective biofunctionalization and biosensing properties of In2O3 nanowires are reviewed in detail. Substantial efforts and development have been made to express the advantages of 1-D In2O3 nanostructures as building blocks for bio and chemical sensing applications in the recent past. However, there is still a need to focus on where 1-D In2O3 nanostructure sensors can be applied beyond current commercial chemical sensors. The most significant area is still the development of high-quality 1-D In2O3 nanostructures to be used as chemical sensors and biosensors with very high sensitivity, selectivity, and stability. The other one is medical diagnosis of human breath by electronic nose system. REFERENCES

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