Highly sensitive C2H5OH sensors using Fe-doped NiO hollow spheres

Sensors and Actuators B 171–172 (2012) 1029–1037

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Highly sensitive C2 H5 OH sensors using Fe-doped NiO hollow spheres Hyo-Joong Kim, Kwon-Il Choi, Kang-Min Kim, Chan Woong Na, Jong-Heun Lee ∗ Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea

a r t i c l e

i n f o

Article history: Received 16 March 2012 Received in revised form 3 June 2012 Accepted 12 June 2012 Available online 20 June 2012 Keywords: NiO Gas sensors Fe doping Hollow spheres p-Type semiconductors

a b s t r a c t NiO and Fe-doped NiO hollow spheres with the shell thickness of ∼12 nm have been prepared by applying uniform coatings of Ni- and Fe-precursors onto Ni spheres, partial oxidation of Ni spheres near their surfaces at 300 ◦ C, the dissolution of core Ni using dilute HCl aqueous solution, and subsequent heat treatment at 500 ◦ C, and their gas sensing characteristics at 300–400 ◦ C were compared. The response to 100 ppm C2 H5 OH of Fe-doped NiO hollow spheres at 350 ◦ C (Rg /Ra = 172.5; Rg , resistance in gas; Ra , resistance in air) was 31.4 times higher than that of NiO hollow spheres (Rg /Ra = 5.5). The reasons for the significant enhancement of C2 H5 OH response by doping Fe into NiO hollow spheres were discussed in relation to the incorporation of Fe components into the NiO lattice and their consequent impact on the gas sensing reaction. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Chemoresistive oxide semiconductors have been used to detect trace concentrations of harmful, environmental, explosive, and toxic gases [1–3]. Oxide semiconductors for gas sensors can be classified into two different groups according to the major charge carrier type: n-type and p-type semiconductors. In n-type oxide semiconductors such as SnO2 , ZnO, TiO2 , In2 O3 , and WO3 , the adsorption of negatively charged oxygen ions (O− or O2− ) forms the electron depletion layer near the particle surface. When the particles make point contacts or the conduction across the neck is negligible, the sensor resistance is determined by the serial connection between the semiconducting core and the resistive inter-particle contacts [4]. The reducing gases are oxidized through reactions with negatively charged oxygen and the consequent release of electrons leads to the decrease of sensor resistance in proportion to the gas concentration. In this case, the chemoresistive variation is dominated by resistive contacts between particles. The adsorption of negatively charged oxygen ions in p-type oxide semiconductors, in contrast, forms hole accumulation layers near the particle surfaces. The conduction is explained by the competition between parallel contributions along the narrow conductive shells and the broad resistive particle cores [5]. The release of electrons by the reaction between reducing gases and ionized oxygens at the surface decreases the charge carrier concentration near the

∗ Corresponding author at: Department of Materials Science and Engineering, Korea University, Anam-Dong, Sungbuk-Gu, Seoul 136-713, Republic of Korea. Tel.: +82 2 3290 3282; fax: +82 2 928 3584. E-mail addresses: [email protected], [email protected] (J.-H. Lee). 0925-4005/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2012.06.029

surface by electron–hole recombination, which in turn increases the sensor resistance. In n-type oxide semiconductors, the gas response can be increased dramatically as the particle sizes become comparable to or smaller than twice the electron depletion layer thickness [6]. By contrast, as suggested by Pokhrel et al. [7], in p-type oxide semiconductors, a high response to reducing gas is relatively difficult to achieve. For example, the responses of undoped p-type NiO nanostructures to 100 ppm C2 H5 OH (Rg /Ra ; Rg , resistance in gas; Ra , resistance in air) in the literature range from 1.9 to 4.1 [8–10], while responses of Ra /Rg > 50 to 100 ppm C2 H5 OH have been frequently reported in various n-type oxide semiconductors [11–14]. This implies that the chemoresistive variation of p-type oxide semiconductor gas sensors that results form thinning of the hole accumulation layer under the parallel equivalent circuit model is relatively small compared to that of n-type oxide semiconductor gas sensors due to thinning of the electron depletion layers near the particle contacts under a serial equivalent circuit model. It should be noted that, under the parallel equivalent circuit model of p-type semiconductor gas sensors, the gas responses are less dependent on the particle sizes if the neck configuration remains similar [15]. This indicates that the enhancement of gas response in p-type oxide semiconductors for the real application of gas sensors is a challenging issue. In the literature, it has been reported that the gas responses of NiO gas sensors can be increased either by employing a hemispherical hollow morphology [10] or by loading noble metal catalysts such as Pt and Au [8,16,17]. The control of morphology is related to the increase of gas accessibility and/or gas sensing area, while the noble metal catalysts are generally loaded in order to achieve the chemical sensitization of gas detection reaction. The electronic sensitization, that is, the

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enhancement of gas response by the control of sensor resistance via doping aliovalent materials can be considered as another promising approach. From the studies of dilute magnetic semiconductors [18–20], it is known that a few at% of Fe-component can incorporate into the lattice of NiO. This indicates that the doping of Fe components can be used as a useful approach to control the resistance or response of NiO sensor. However, to the best of author’s knowledge, the enhancement of the gas response and selectivity in NiO gas sensors by doping with Fe components has been barely investigated. In this study, we report that the doping of Fe can significantly increase the C2 H5 OH response of NiO sensors. In order to verify the role of Fe components in the gas sensing reaction, well-defined NiOand Fe-doped NiO hollow structures were prepared by applying uniform coatings of Ni- and Fe-precursors onto Ni spheres, followed by partial oxidation of precursor-coated Ni spheres and subsequent removal of core Ni, and their gas sensing characteristics were compared. The main focus of the study was the design of highly sensitive and selective C2 H5 OH sensors using p-type NiO hollow spheres.

2. Experimental 2.1. Preparation of NiO and Fe-doped NiO hollow spheres The Ni spheres (NF32, Toho Titanium Co., Ltd., Japan, average diameters: 300 nm) were heat-treated at 300 ◦ C for 1 h for the partial oxidation of Ni spheres near the surfaces. Then, the Ni–NiO core–shell spheres were immersed in dilute HCl aqueous solution (pH = 1.27) for 3 days to remove the core Ni parts. After washing and drying, the powders were heat-treated at 500 ◦ C for 2 h. However, NiO hollow spheres were significantly aggregated into large secondary particles, which hampered the preparation of gas sensors. In order to prepare NiO hollow spheres that are highly dispersed even after high-temperature calcination, the Ni spheres were uniformly coated with Ni-precursors. In this process, 0.145 g of nickel(ll) nitrate hexahydrate (Ni(NO3 )2 ·6H2 O, Sigma–Aldrich Co., USA), 0.073 g of l(+)-lysine (C6 H14 N2 O2 , ≥98%, Sigma–Aldrich Co., USA) and 0.630 g of oxalic acid dihydrate ((COOH)2 ·2H2 O, Cica-reagent, KANTO chemical Co., Japan) were dissolved in 100 ml of deionized water and the stock solution was homogenized for 10 min by stirring. Subsequently, hydrazine monohydrate (NH2 NH2 ·H2 O, 80.0%, Samchun Chemical Co.) was added gradually under magnetic stirring until the pH of the stock solution became 7.4. After the addition of 0.4 g of Ni spheres, the solution was stirred for 5 h. During the process, thin Ni-precursor layers were uniformly coated on Ni spheres through a controlled hydrolysis reaction. The Ni-precursor-coated Ni spheres were heat-treated at 300 ◦ C for 1 h to decompose the Ni precursors and to oxidize the surfaces of the Ni spheres. After the removal of core Ni parts using dilute HCl aqueous solution and subsequent heat treatment at 500 ◦ C for 2 h, well-dispersed configurations of NiO hollow spheres with high porosity were prepared. To investigate the role of Fe doping on the gas sensing characteristics of NiO hollow spheres, the Fe-doped NiO hollow spheres were also prepared by the following procedures. First, 0.202 g of iron(lll) nitrate nonahydrate (Fe(NO3 )3 ·9H2 O, ACS reagent ≥ 98%, Sigma–Aldrich Co., USA), 0.073 g of l(+)-lysine and 0.630 g of oxalic acid dihydrate were dissolved in 100 ml of de-ionized water and the pH of the stock solution was tuned to 7.4 by gradual addition of hydrazine. After the addition of 0.4 g of Ni spheres, the solution was stirred for 5 h to coat the Fe precursors onto the Ni spheres. The Fedoped NiO hollow spheres have been prepared by heat treatment of Fe-precursor-coated Ni spheres at 300 ◦ C for 1 h, dissolution of core Ni, and subsequent heat treatment of hollow spheres at 500 ◦ C for 2 h, which will be named ‘Fe–NiO hollow spheres’ for simplicity.

2.2. Characterization The phase and crystallinity of the powders were analyzed by X-ray diffraction (XRD, Rigaku D/MAX-2500V/PC), while the morphologies of the precursors and hollow powders were investigated by field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi Co. Ltd., Japan) and transmission electron microscopy (TEM, FEI TITAN 80-300TM , FEI Co.). We carried out inductively coupled plasma (ICP-AES, OPTIMA 4300 DV, Perkin Elmer Instruments, USA) analysis to determine the doping concentration of Fe in NiO hollow spheres. 2.3. Gas sensing characteristics An alumina substrate (area: 1.5 mm × 1.5 mm, thickness: 0.25 mm) with two Au electrodes on its top surface (electrode widths: 1 mm, separation: 0.2 mm) and a micro-heater on its bottom surface was used. The NiO or Fe-doped NiO hollow spheres were dispersed in deionized water and the slurry was coated on the Au electrodes by drop-coating using a micro-pipette. The sensor temperatures were controlled using the micro-heater underneath the substrate and were measured using an IR temperature sensor (Rayomatic 14814-2, Euroton IR tec Co.). Heater powers of 225, 300, 375 and 450 mW heated the substrates to 250, 300, 350 and 400 ◦ C, respectively. The sensors were contained in a specially designed, low-volume (1.5 cm3 ), quartz tube to minimize delays in changing their surrounding atmosphere. After the removal of residual solvent by heating the sensor at 400 ◦ C for 6 h, the gas responses (S = Rg /Ra ; Rg , resistance in gas; Ra , resistance in air) to 5 ppm CO, H2 and C2 H5 OH were measured at 250–400 ◦ C. Gas concentrations were controlled by changing the mixing ratio of the parent gases (100 ppm CO, 100 ppm H2 and 100 ppm C2 H5 OH, all air balance) and dry synthetic air. The DC 2-probe resistances were measured using an electrometer interfaced with a computer. 3. Results and discussion 3.1. Phase analysis As-received Ni spheres (average diameter: 300 nm) were identified as cubic phase. After coating Ni-precursors on the surfaces of Ni spheres, the phase did not change. The absence of a Ni precursor peak can be explained by its small amount and low crystallinity (Fig. 1a). After heat treatment at 300 ◦ C for 1 h, most of the Ni phase remained the same (Fig. 1b), while the (1 1 1), (2 0 0) and (2 2 0) peaks of the cubic NiO phase with low intensities (Fig. 1b, insets) were found, indicating the transformation of Ni-precursor layers into NiO and/or partial oxidation of Ni near the surface. This forms Ni–NiO core–shell structures. Most of the metallic core Ni parts were removed by dissolution in dilute HCl aqueous solution for 3 days, which led to the NiO hollow spheres containing residual Ni (Fig. 1c). To oxidize the residual Ni and increase the thermal stability of NiO hollow spheres at the sensing temperature (300–400 ◦ C), the NiO hollow spheres after Ni dissolution were heat-treated further at 500 ◦ C for 2 h (Fig. 1d). No Ni peak was found, indicating the complete oxidation of residual Ni into NiO. In the Fe-precursor-coated Ni spheres, the second phase was not found (Fig. 2a). The partial oxidation of the Ni surface by heat treatment at 300 ◦ C was confirmed again from the low intensity of NiO peaks (Fig. 2b, insets). Similar to the Ni-precursor-coated specimen, the incomplete dissolution of core Ni parts led to the formation of Fe-doped NiO hollow spheres with residual Ni (Fig. 2c). Finally, the specimen was completely oxidized into Fe–NiO hollow spheres by a second heat treatment at 500 ◦ C for 2 h (Fig. 2d). No metallic Fe or Fe-containing second phase was found because of the oxidation of Fe-component. Throughout

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the experiment, no second phase containing Fe components such as FeO, Fe2 O3 and Fe3 O4 was detected. In the literature, the solubility limits of Fe components in NiO specimens in the literature prepared by carbonate co-precipitation [18], solid state reaction [19], and electrospinning [20] are >2 at%, ∼2 at%, and >5 at%, respectively. Wang et al. [18] prepared Fe-doped NiO powders by carbonate coprecipitation and subsequent heat treatment at 600 ◦ C. Single NiO phase was attained at [Fe]doping < 2 at%, while the second phase of Fe2 O3 was found at Fe doping concentrations ([Fe]doping ) higher than 5 at%. The Fe doping concentration in Fe–NiO hollow spheres in the present study was determined to be 0.3 at% by inductively coupled plasma mass spectroscopy. Accordingly, the Fe component is regarded to be incorporated into the NiO lattice. 3.2. SEM and TEM analyses

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2θ (deg, CuKα) Fig. 1. X-ray diffraction patterns of (a) Ni-precursor-coated Ni spheres, (b) the specimen after heat treatment of the sample in Fig. 1a at 300 ◦ C for 1 h, (c) the specimen after dissolution of metallic Ni parts from the sample in Fig. 1b using dilute HCl aqueous solution, and (d) the NiO hollow spheres after heat treatment of the sample in Fig. 1c at 500 ◦ C for 2 h.

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2θ(deg, CuKα) Fig. 2. X-ray diffraction patterns of (a) Fe-precursor-coated Ni spheres, (b) the specimen after heat treatment of sample in Fig. 2a at 300 ◦ C for 1 h, (c) the specimen after dissolution of metallic Ni parts from the sample in Fig. 2b using dilute HCl aqueous solution, and (d) the Fe–NiO hollow spheres after heat treatment of the sample in Fig. 2c at 500 ◦ C for 2 h.

Ni spheres (Fig. 3a) (average diameter: 300 nm) were used as the templates for NiO hollow spheres. The surfaces of Ni spheres were oxidized into NiO by heat treatment at 300 ◦ C for 1 h, while the core parts remained metallic. The rough surface morphologies reflect the oxidation of Ni (Fig. 3b). The NiO shells are relatively stable in a weakly acidic environment. Accordingly, the metallic core Ni parts were dissolved out in a selective manner using dilute HCl aqueous solution in order to prepare NiO hollow spheres (Fig. 3c). However, small amounts of Ni still remained, as shown in X-ray diffraction patterns (Fig. 1c), probably due to an incomplete dissolution procedure. To oxidize the residual Ni and increase the thermal stability during sensor operation (300–400 ◦ C), the NiO hollow spheres after Ni dissolution were heat-treated further at 500 ◦ C for 2 h (Fig. 3d). After heat treatment, large aggregates of NiO hollow spheres (size: 40–100 ␮m) were formed although the residual Ni phase was successfully converted into NiO and the hollow morphology of individual spheres was maintained (inset in Fig. 3d). To prevent the agglomeration of NiO hollow spheres, Niprecursors were coated on the Ni spheres by a controlled hydrolysis reaction (Fig. 4a). After heat treatment at 300 ◦ C for 1 h, the surfaces of the spheres became rougher (Fig. 4b). The NiO hollow spheres were prepared by dissolution of the core Ni using dilute HCl aqueous solution (Fig. 4c) and a second heat treatment at 500 ◦ C for 2 h (Fig. 4d). Note that NiO hollow spheres showed good dispersion without agglomeration. One of the present authors reported that bare Ni spheres with the average size of ∼350 nm begin shrinking at ∼300 ◦ C and the process is completed at ∼500 ◦ C in air atmosphere, while the Ni spheres coated with thin TiO2 and BaTiO3 layers begin to shrink at ∼500 ◦ C and ∼1000 ◦ C, respectively [21,22]. Considering the oxidation of Ni spheres above 300 ◦ C, the shrinkage at 300–500 ◦ C can be attributed to that between Ni-NiO core–shell spheres or that between NiO spheres. Accordingly, the large agglomerates formed by the heat treatment at 500 ◦ C in the present study can be attributed to the connection between NiO hollow spheres due to sintering. The increase of shrinkage temperature that results from coating TiO2 or BaTiO3 layers indicates that the passivation of Ni spheres using thin oxide shell layers is a very effective way to prevent necking or sintering between Ni or NiO spheres. In the present study, if the Ni-precursor layers were completely converted into crystalline NiO by the heat treatment at 300 ◦ C for 1 h, significant aggregation should occur after the second heat treatment of NiO hollow spheres at 500 ◦ C for 2 h. However, a highly dispersed configuration of NiO hollow spheres was maintained even after heat treatment at 500 ◦ C for 2 h. This suggests that the shell layers formed by heat treatment of Ni-precursors are not identical to NiO layers formed by surface oxidation of bare Ni spheres, and they are very effective passivation layers. In order to prepare Fe-doped NiO hollow spheres, Fe-precursors were coated on Ni spheres by a controlled hydrolysis reaction (Fig. 5a). After heat treatment at 300 ◦ C for 1 h, the surfaces of

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Fig. 3. SEM images of (a) as-received Ni spheres, (b) Ni–NiO core–shell spheres after heat treatment at 300 ◦ C for 1 h, (c) NiO hollow spheres with residual Ni components prepared by the dissolution of core Ni parts in the samples in Fig. 3b using dilute HCl aqueous solution, and (d) large aggregates of NiO hollow spheres prepared by heat treatment of the samples in Fig. 3c at 500 ◦ C for 2 h.

spheres became rougher (Fig. 5b). The hollow spheres were prepared by dissolution of core Ni (Fig. 6a). The hollow morphologies could be observed from the broken shells. A closer look showed that each hollow sphere consists of small primary particles (size: ∼20 nm) (Fig. 6b). The hollow morphologies were confirmed again from the bright contours at the inner parts of the spheres in the TEM image (Fig. 6c). The average thickness of shells measured from 40 different regions was 12.0 ± 3.0 nm. The high magnification image, lattice-resolved image, and fast Fourier transformation (FFT) pattern (Fig. 6d–f) suggest that the hollow spheres are polycrystalline NiO. The doping of Fe in the NiO hollow spheres was confirmed by the uniform distributions of Ni, Fe, and O in TEM elemental mapping (Fig. 6g–i). The hollow morphology of Fe-doped Ni spheres was maintained after heat treatment at 500 ◦ C for 2 h (Fig. 7a and b). No significant agglomeration between Fe and NiO hollow spheres indicates that the coating of Fe-precursors is also very effective at preventing necking and sintering between Ni or NiO spheres. After heat treatment at 500 ◦ C for 2 h, the hollow spheres became more nanoporous, while the average thickness of shells remained similar (13.7 ± 2.8 nm) (Fig. 7c and d). The increase of porosity can be explained by the oxidation, crystallization, and sintering of primary particles in Fe–NiO hollow spheres. The phase of the hollow spheres was identified as NiO from the HR-TEM image (Fig. 7e) and the FFT pattern (Fig. 7f). Furthermore, the TEM elemental mapping image (Fig. 7g–i) suggested that uniform distributions of Ni, Fe, and O

within Fe–NiO hollow spheres are maintained after heat treatment at 500 ◦ C for 2 h. 3.3. Gas sensing characteristics Gas responses to 100 ppm C2 H5 OH, H2 , and CO were measured at 250–400 ◦ C (Fig. 8). In NiO hollow spheres, the responses (S = Rg /Ra ) to C2 H5 OH were significantly higher than those to H2 and CO regardless of sensor temperature (Fig. 8a), indicating that selective detection of C2 H5 OH is possible. The response to 100 ppm C2 H5 OH was highest (5.5) at 350 ◦ C. In Fe–NiO hollow spheres, the response to C2 H5 OH increased from 11.3 to 172.5 as the sensor temperature increased from 250 to 350 ◦ C, and then decreased to 53.9 at 400 ◦ C (Fig. 8b). Note that the y-axis scales in Fig. 8 differ by a factor of 10. Although both the responses to H2 and CO of Fe–NiO hollow spheres showed maxima (4.9 and 16.2) at 350 ◦ C, those are negligibly small compared to the response to C2 H5 OH at the same temperature. The maximum C2 H5 OH response of Fe–NiO hollow spheres (172.5) was 31.4 times higher than that of NiO hollow spheres (5.5), suggesting that the doping of Fe to NiO hollow spheres is a very efficient way to enhance the C2 H5 OH response. The ratios between the responses to ethanol and other interference gases (Sethanol /Sgas ) were calculated as a measure of selectivity to C2 H5 OH. A higher Sethanol /Sgas value means a higher selectivity. The Sethanol /SCO and Sethanol /SH2 values of Fe–NiO hollow spheres at 350 ◦ C were 35.2 and 10.7, respectively, while those

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Fig. 4. SEM images of (a) Ni-precursor-coated Ni spheres, (b) the specimen after heat treatment of the samples in Fig. 4a at 300 ◦ C for 1 h, (c) the specimen after dissolution of metallic Ni parts from the samples in Fig. 4b using dilute HCl aqueous solution, and (d) the NiO hollow spheres after heat treatment of samples in Fig. 4c at 500 ◦ C for 2 h.

of NiO hollow spheres were 3.7 and 4.3, respectively. This clearly indicates that not only the gas response but also the selectivity to C2 H5 OH of NiO hollow spheres can be enhanced to a significant degree by Fe doping. The dynamic sensing transients to 5–40 ppm C2 H5 OH at 350 ◦ C showed stable chemoresistive variation (Fig. 9a and b). That is, the sensor resistance was increased upon exposure to C2 H5 OH and was

reversibly recovered to its original value upon exposure to air. The C2 H5 OH responses of Fe–NiO hollow spheres increased from 25.5 to 172.5 as the C2 H5 OH concentration increased from 5 to 100 ppm (Fig. 9c). By contrast, the responses to 5–100 ppm C2 H5 OH of NiO hollow spheres ranged from 2.5 to 5.5. The 90% response and 90% recovery times ( res and  recov ), the times to reach 90% variation in resistance upon exposure to gas and air, were determined from the

Fig. 5. SEM images of (a) Fe-precursor-coated Ni spheres, (b) the specimen after heat treatment of samples in Fig. 5a at 300 ◦ C for 1 h.

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Fig. 6. SEM and TEM images of Fe–NiO hollow spheres prepared by heat treatment of Fe-precursor-coated Ni spheres at 300 ◦ C for 1 h and the dissolution of the core Ni part using dilute HCl: (a and b) SEM images; (c and d) TEM images; (e) lattice-resolved image; (f) fast Fourier transform pattern on (e); (g–i) elemental mapping images of Ni, Fe, and O.

transients. The  res values of Fe–NiO hollow spheres (120–144 s) were higher than those of NiO hollow spheres (93–108 s), while the  recov values of Fe–NiO hollow spheres (7–12 s) were significantly lower than those of NiO hollow spheres (38–71 s). 3.4. Discussion The significant enhancement of the C2 H5 OH response by Fedoping should be understood or discussed in relation to the incorporation of Fe into the NiO lattice and its consequent impact on the gas sensing reaction. The Fe-doping-induced variations of hollow morphologies such as surface porosity or surface area can be considered as a possible reason. The pore size distributions of NiO and Fe–NiO hollow spheres determined by nitrogen adsorption and desorption did not show notable differences (not shown) and the specific surface area of NiO (10.2 m2 /g) and Fe–NiO hollow spheres (11.2 m2 /g) were also similar. This indicates that the significant enhancement of the C2 H5 OH response does not emanate

from the morphological variation by Fe-doping. The ionic radii of Fe3+ and Fe2+ at the coordination number (CN) of 6 are 0.64 and ˚ respectively, which are similar to that of Ni2+ (0.69 A) ˚ at 0.74 A, CN = 6 [18]. Thus, either Fe3+ or Fe2+ can be incorporated into NiO. If the Fe component exists in the state of Fe2+ and incorporates into NiO, the sensor resistance will not vary. In comparison, if the Fe component exists in the state of Fe3+ and it substitutes into the Ni2+ site in NiO, the sensor resistance will significantly change in order to compensate the local charge (FeNi • ). In air atmosphere, Fe3 O4 are known to be converted into Fe2 O3 by heat treatment at >450 ◦ C [23]. Considering that our sensors are heat-treated at 500 ◦ C in air atmosphere before operation, the incorporation of Fe3+ into the NiO lattice seems to be plausible. Indeed, the average Ra value determined from 5 different sensors of Fe–NiO hollow spheres was 428 ± 25 k, while that determined from 5 different sensors using NiO hollow spheres was 92 ± 8 k. The 4.65-fold increase of sensor resistance by Fe doping supports again the incorporation of Fe3+ into NiO lattice.

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Fig. 7. SEM and TEM images of Fe–NiO hollow spheres prepared by heat treatment of the sample in Fig. 6 at 500 ◦ C for 2 h: (a and b) SEM images; (c and d) TEM images; (e) lattice-resolved image; (f) fast Fourier transform pattern on (e); (g–i) elemental mapping images of Ni, Fe, and O.

Two different charge compensation mechanisms can be considered as follows. 2NiO

 Fe2 O3 → 2FeNi • + 2OX O + 2e + 3NiO

Fe2 O3 →

•

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1 O2 (g) 2

(1) (2)

In the electronic compensation mechanism, the recombination between holes in NiO and electrons generated by Eq. (1) will lead to the decrease of hole concentration. If ionic species like VNi  are responsible for the charge compensation, the variation of hole concentration will not be high. However, the localization of holes near the opposite charge of VNi  can be considered as a possibility to decrease the effective charge carrier concentration [24]. In either case, the incorporation of Fe3+ into NiO and its consequent decrease of effective charge carrier concentration can be regarded as the main reason for the increase of sensor resistance. The gas response in p-type oxide semiconductor is given by Rg /Ra , which can be changed into  a / g = (pa ep )/(pg ep ) ( a and  g , conductivities in air and gas; pa and pg , hole concentrations in

air and gas; e, charge, p , hole mobility). It can be assumed that the hole mobilities of sensors before and after gas exposure are the same. Then, the gas response can be rewritten as the following: S=

p pa pa = +1 = pg pa − p (pa − p)

(3)

where p = pa –pg is the variation of hole concentration by the exposure to reducing gas. Under the same morphology of sensing materials, the p value should be the same for a constant concentration of reducing gas if the surface reaction is not changed by Fe doping. Because the pa value is always larger than p, Eq. (3) represents a hyperbola in the region pa > p. Thus, the gas response, S, will increase significantly as pa decreases to the p value. This can explain the 31-fold increase of gas response by Fe doping. Jinkawa et al. [25] reported that the C2 H5 OH responses of oxide semiconductors are closely dependent upon the acid–base properties of sensing and additive materials. They suggested that C2 H5 OH can be dehydrogenated into CH3 CHO (g) + H2 (g) at the surface of a basic oxide and can be dehydrated into C2 H4 (g) + H2 O (g) at the

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Fig. 8. Gas responses of undoped NiO and Fe–NiO hollow spheres to 100 ppm C2 H5 OH, 100 ppm CO, and 100 ppm H2 at 250–400 ◦ C: (a) undoped NiO hollow spheres and (b) Fe–NiO hollow spheres.

surface of an acidic oxide. That is, the additions of basic additives are advantageous to enhance the C2 H5 OH response. According to the electro-negativity data [26], Fe3+ is known to be more acidic than Ni2+ . This shows that the enhancement of C2 H5 OH in the present

study did not emanate from changes in the acid–base properties of the sensing materials. 4. Conclusion The C2 H5 OH response of NiO hollow spheres was significantly enhanced by doping with Fe. NiO and Fe-doped NiO hollow spheres in highly dispersed configurations have been prepared by the coating of Ni- or Fe-precursors onto Ni spheres, the surface oxidation of Ni spheres at 300 ◦ C, dissolution of core Ni, and heat treatment at 500 ◦ C. In contrast, the surface oxidation of bare Ni spheres at 300 ◦ C, dissolution of core Ni, and subsequent heat treatment at 500 ◦ C formed very large agglomerates of NiO hollow spheres. This indicates that the coatings of Ni- and Fe-precursors on Ni spheres play the roles of passivation layers to prevent necking and sintering between NiO hollow spheres. The doping of Fe onto NiO hollow spheres enhanced the response (Rg /Ra ) to 100 ppm at 350 ◦ C from 5.5 to 172.5. The 31.4-fold increase of gas response was explained by the electronic sensitization mechanism due to the incorporation of Fe3+ into the NiO lattice and consequent decrease of the effective hole concentration. Acknowledgement This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (No. R0A-2008-000-20032-0). References

Fig. 9. Gas sensing characteristics of undoped NiO and Fe–NiO hollow spheres at 350 ◦ C: (a) gas concentration profile; (b) sensing transients to 5–10 ppm C2 H5 OH and (c) gas responses to 5–100 ppm C2 H5 OH.

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Biographies Hyo-Joong Kim studied materials science and engineering and received his BS and MS degrees in 2009 and 2011, respectively, from Korea University. He is currently studying for a Ph.D. at Korea University. His research topic is oxide semiconductor gas sensors using hollow nanostructures. Kwon-Il Choi studied materials science and engineering and received his BS and MS degrees from Korea University in 2008 and 2010, respectively. He is currently studying for a Ph.D. at Korea University. His research topic is the use of oxide nanostructures for chemical sensor applications. Kang-Min Kim studied materials science and engineering and received his BS degree from Kangwon National University, Korea, in 2004. In 2006, he received his MS degree from Korea University. He has worked for Samsung Corning and Samsung Corning Precision Glass Companies for 3 years. He is currently studying for a Ph.D. at Korea University. His research interests are oxide nanostructure-based gas sensors and solar cells. Chan Woong Na studied material chemistry and received his BS and MS degree from Korea University, Korea, in 2004 and 2006. He has worked for OCI Company from 2006 to 2009. He is currently studying for a Ph.D. at Korea University. His research interests are nanostructure-based gas sensors and light emitting diodes. Jong-Heun Lee joined the Department of Materials Science and Engineering at Korea University as an associate professor in 2003, where he is currently professor. He received his BS, MS, and Ph.D. degrees from Seoul National University in 1987, 1989, and 1993, respectively. Between 1993 and 1999, he developed automotive air–fuelratio sensors at the Samsung Advanced Institute of Technology. He was a Science and Technology Agency of Japan (STA) fellow at the National Institute for Research in Inorganic Materials (currently NIMS, Japan) from 1999 to 2000 and a research professor at Seoul National University from 2000 to 2003. His current research interests include chemical sensors, functional nanostructures, and solid oxide electrolytes.