Hierarchical NiO hollow microspheres assembled from nanosheet-stacked nanoparticles and their application in a gas sensor

Abstract

A facile and robust route for the mass preparation of hollow NiO microspheres assembled from nanosheet-stacked nanoparticles is developed. The Ni(HCO₃)₂ precursor with a hollow spherical structure was firstly prepared by a hydrothermal reaction without any surfactants or organic additives. The reaction generated gas bubble may act as a template for the formation of Ni(HCO₃)₂ hollow microspheres. These are converted into hierarchical NiO hollow microspheres assembled from nanoparticles (with diameter of ∼25 nm) upon calcination, which are further assembled by the stacking of ultrathin nanosheets. The hierarchical NiO hollow structures are attractive for catalyst, sensor and environmental applications, benefiting from their large surfaces. The sensing properties of the hierarchical NiO hollow microspheres were evaluated. They show high sensitivity, short response and recovery times, and good response and recovery characteristics to n-butanol. As compared with NiO nanoparticles with similar dimensions (∼20–35 nm), the nanoparticle assembled NiO hollow microspheres exhibit enhanced gas sensing properties. The effective integration of several nanostructures in one microunit would provide a novel way to design new materials for nanodevices.

---

Introduction

Nanosheets are a new class of two-dimensional (2D) nanostructure that are characterized by a thickness on the level of nanometres (often smaller than 5 nm) and lateral dimensions up to the micrometre scale. Since the discovery of graphene, such two-dimensional nanostructures have attracted great attention in the past few years.^1–3 They are fascinating not only owing to their unique structure and structure-induced properties but also to their numerous promising applications.^4,5 These 2D nanosheets offer very interesting features. First, in contrast to other low dimensional nanostructures, the 2D structure facilitates their processability. They can be assembled into highly oriented films by employing Langmuir–Blodgett (LB) or layer-by-layer (LbL) techniques, which offer the possibility to precisely control the thickness of the formed films.^6–8 Furthermore, the ability of the nanosheets to self-assemble allows the formation of restacked lamellar aggregates with a disordered and porous nature,⁹ leading to enhanced properties as found in H₄Nb₆O₁₇¹⁰ and HTiNbO₅¹¹ for photocatalysis, Li/MnO₂¹² for electrodes of lithium-ion batteries, and H/RuO₂ for double-layer capacitors.¹³ Second, based on these 2D nanosheets, multifunctional hybrid structures with improved physical properties may be constructed through rational design and choice of constituting nanoblocks and control of their arrangement in the solid state.^14,15 So far, some transition-metal oxide nanosheets such as Ca₂Nb₃O₁₀,¹⁶ CeO₂,¹⁷ Nb₆O₁₇,¹⁸ Ti_1−δO₂,^19,20 and MnO₂^21,22 have been investigated due to the rich chemistry and physics of their bulk counterparts. However, the present transition-metal oxide nanosheets are only limited to several materials, mostly layered compounds. Moreover, most research has focused on the preparation of nanosheets, limited reports have been on their assembly.

Among the oxide materials, we are particularly interested in NiO, which is an important p-type semiconductor with a cubic structure. It has been extensively investigated because of its myriad applications in catalysts,²³ gas sensors,^24,25 battery materials,²⁶ electrochromic coatings, active optical fibers, fuel cell electrodes,²⁷ and so on. NiO is also used in magnetic multilayer devices,²⁸ and has potential applications in nonvolatile memories due to its excellent resistive switching behavior.^29,30 Up to now, a variety of methods have been used to achieve nanostructured NiO, such as hollow spheres,^31,32 nanorods/wires,^33–35 nanoflowers,³⁶ polyhedrons,³⁷ mesoporous NiO walls,³⁸ and palates.³⁹

In this study, we present a new hierarchical NiO hollow structure obtained by a facile method, that is, ultrathin NiO nanosheets are assembled layer-by-layer into nanoparticles, which then further aggregated into hollow microspheres. The route involves the controlled precipitation of Ni(HCO₃)₂ through a hydrothermal process followed by calcination. This simple, salt dicarbonate-mediated method may be utilized to fabricate a wide variety of nanosheet-based hierarchical nanoarchitectures. Furthermore, we show that the as-synthesized NiO nanostructure has enhanced gas sensing performance as compared with NiO nanoparticles of similar size.

Experimental

Preparation of hierarchical NiO hollow microspheres

All the reagents of analytical grade (A. R.) were purchased from Shanghai Chemical Reagent Co., Ltd. and used without further purification. In a typical experimental procedure, 2.8 g of Ni(NO₃)₂·6H₂O was dissolved in 25 mL of deionized water. The solution was then added to 3.5 g of urea with stirring. After complete dissolution, the obtained solution was then transferred into an autoclave. The subsequent hydrothermal reaction was conducted in an electric oven at 120 °C for 5 h. The autoclave was then allowed to cool down naturally to room temperature; the as-obtained solid precursor was separated by centrifugation and thoroughly washed with ethanol and deionized water five times. For the generation of nanostructured NiO, an appropriate amount of the as-synthesized precursor was calcined in air at 500 °C for 1 h. After cooling down naturally to room temperature, hierarchical NiO hollow microspheres were obtained.

Control experiment for the preparation of NiO nanoparticles

For comparison of gas sensing properties, NiO nanoparticles were prepared. In a typical procedure, 0.94 g of NiCl₂·6H₂O was dissolved in 32 mL of ethylene glycol. The solution was then added to 2.8 g of CH₃COONa and 0.75 mL of polyethylene glycol (PEG-400) with stirring, which were then transferred to an autoclave, and the reaction was conducted in an electric oven at 180 °C for 8 h. After cooling naturally to room temperature, the as-obtained solid was separated by centrifugation and thoroughly washed with ethanol and deionized water. NiO nanoparticles were generated by the subsequent calcination of the obtained solid in air at 500 °C for 1 h.

Characterization

For crystal phase identification, the samples were examined with a X-ray powder diffractometer (XRD; Cu-Kα radiation; Shimaduzu; λ = 1.5406 Å). The morphologies and dimensions of the samples were investigated using a Hitachi S-4800 field emission scanning electron microscope (FE-SEM) and a JEOL-2100 high-resolution transmission electron microscope (HR-TEM), which was also used for selected area electron diffraction (SAED) analysis (accelerating voltage of 200 kV). Raman spectra were recorded with a JY HR-800 Raman spectrometer with a 453 nm laser excitation. The gas sensing tests were carried out on a commercial HW-30 gas sensing measurement system (HanWei Electronics Co., Ltd, Henan, China) at a relative humidity of 20–35%. The structure, fabrication, and testing principle of our gas sensor based on the as-prepared NiO are similar to those of our reported In₂O₃ sensor.⁴⁰ The sensing response, S, was determined as the ratio, R_g/R_a, where R_a is the resistance in ambient air and R_g is the resistance in the tested gas atmosphere.

Results and discussion

The crystal structure and purity of the as-synthesized precursor sample were firstly examined by powder XRD. As shown in Fig. 1a, all the peaks observed can be easily indexed to Ni(HCO₃)₂ with body-centered cubic structure (JCPDS No. 1507-82). No peaks corresponding to other substances such as Ni(OH)₂ or Ni₂(OH)₂CO₃ were observed. (Fig. 1a). After thermal treatment of the as-obtained Ni(HCO₃)₂ precursor sample at 500 °C in air for 1 h, face-centered cubic NiO (JCPDS No. 78-0643) is generated, as indicated by the XRD pattern shown in Fig. 1b, in which the peaks correspond to the (111), (200), (220), (311) and (222) crystal planes of face-centered cubic NiO. It can be concluded that there has been complete conversion of Ni(HCO₃)₂ into NiO during the calcination process since no peaks other than those of NiO are detected in the XRD pattern. The sharp diffraction peaks suggest the high crystallinity of NiO.

Fig. 1 XRD patterns of the as-prepared (a) Ni(HCO₃)₂ precursor and (b) NiO product; the standard data of cubic Ni(HCO₃)₂ (JCPDS No. 1507-82) are also shown for comparison.

The size and morphology of the precursor product were examined by FE-SEM. As shown in Fig. 2a–b, the SEM images of Ni(HCO₃)₂ show nearly spherical units with diameter of 0.5–1.5 μm. A hole can be observed in some Ni(HCO₃)₂ sub-microspheres (as shown by arrows in Fig. 2a), which suggests the hollow structure feature.

Fig. 2 (a, b) SEM images of the as-synthesized Ni(HCO₃)₂.

The hydrothermal reaction of transition metal ions and urea often produces carbonate hydroxide salts, which are often used as precursors for the fabrication of oxide micro-/nanostructures.^41,42 Here nickel dicarbonate is formed. It was found that which salt is formed is dependent on the concentration of urea. A lower concentration of urea would cause the formation of carbonate hydroxide salts. The formation of Ni(HCO₃)₂ could be described by the following reactions:

CO(NH₂)₂ + 3H₂O → 2NH₃·H₂O + CO₂ (1)

NH₃·H₂O → NH₄⁺ + OH⁻ (2)

CO₂ + 2OH⁻ → CO₃²⁻ + H₂O (3)

CO₃²⁻ + H₂O + CO₂ → 2HCO₃⁻ (4)

Ni²⁺ + 2HCO₃⁻ → Ni(HCO₃)₂ (5)

When the precursor solution is heated to 120 °C, urea undergoes hydrolysis and releases OH⁻ through reactions (1) and (2). The generated CO₂ will transform into CO₃²⁻ ions in alkali solution according to reaction (3). When the concentration of urea is high, the large amount of CO₂ produced from reaction (1) would further react with CO₃²⁻ to form HCO₃⁻ ions (reaction (4)), which then react with Ni²⁺ forming Ni(HCO₃)₂ monomers (reaction (5)). If the concentration of urea is low, the amount of CO₂ will be low, and not enough CO₂ will react with CO₃²⁻ forming HCO₃⁻, so the formed products are carbonate hydroxide salts. As the reaction proceeds, the concentration of Ni(HCO₃)₂ monomers in the solution will reach saturation, form Ni(HCO₃)₂ crystal nuclei, and subsequently grow into nanostructures.

It is interesting that the Ni(HCO₃)₂ crystals grow into hollow structures in such a reaction system without surfactants or additive templates. The formation of the hollow structures would relate with the abundant CO₂ bubbles produced from the hydrolysis of urea. After the initial nucleation of the monomers, Ni(HCO₃)₂ nuclei will grow and have a tendency to aggregate. At the same time, lots of CO₂ produced from reaction (1) owing to the high concentration of urea would cause the generation of microbubbles and provide an aggregation center. Driven by the minimization of interfacial energy, the Ni(HCO₃)₂ may aggregate at the gas–liquid interface between CO₂ and solution, finally forming hollow Ni(HCO₃)₂ microspheres. The use of gas bubbles for the fabrication of hollow microspheres has also been observed for ZnSe.⁴³ Compared with other template methods, using reaction generated gas bubbles as soft-templates to fabricate hollow microspheres has many advantages, such as convenience, no need to remove templates, and avoiding the introduction of impurities, and is therefore suitable for modern chemical synthesis.

To obtain NiO hollow microspheres, the as-synthesized Ni(HCO₃)₂ products were calcined at 500 °C in air for 1 h. Fig. 3 shows SEM images of the prepared NiO product. It is apparent that the spherical morphology of Ni(HCO₃)₂ is somewhat preserved, while the size has shrunk from 0.5–1.5 μm to 0.3–1.2 μm. One can see that the surfaces of the NiO microspheres are coarse and some of them have small holes (as shown by the arrows in Fig. 3). It seems that the microspheres are loose and porous, which would relate to the release of water and carbon dioxide during the thermal treatment process.

Fig. 3 (a, b) SEM images of the as-synthesized NiO hollow microspheres.

Further structural characterization comes from TEM observations. The TEM image displayed in Fig. 4a clearly shows the hollow structural feature. According to the TEM image with higher magnification (Fig. 4b), it is observed that the NiO microspheres are composed of NiO nanoparticles with size around 25 nm. Due to the close aggregation and packing of these NiO nanoparticles, the hollow structure is fabricated. The corresponding SAED pattern (inset of Fig. 4b) recorded on one NiO hollow microsphere suggests its polycrystalline nature. The diffraction rings can be easily indexed to the (311), (222), (220), (111), and (200) lattice planes of cubic NiO. Interestingly, these NiO nanoparticles have further microstructures. Careful observation by HRTEM found that they are actually composed by the stacking of nanosheet-like building blocks, as shown in Fig. 4c. The observation on at least ten NiO nanoparticles confirms the same structural feature (more TEM images are shown in Fig. S1, See ESI†). As shown in the noted area of Fig. 4c, the sheet building blocks are ultrathin with thickness of ∼2 nm. The scheme shown in the inset of Fig. 4c shows the structural feature of these unique NiO units. It is proposed that the formation of these lamellar NiO units is related to the intrinsic crystal structure of Ni(HCO₃)₂.

Fig. 4 (a–d) TEM and HRTEM images of the as-synthesized NiO hollow microspheres. The inset of (b) is a SAED pattern recorded on one hollow microsphere. The scheme shown in the inset of (c) shows the structural feature of these unique NiO units. The inset of (d) is a SAED pattern recorded on one NiO nanoparticle.

A microstructure analysis of these sheet-like units was also conducted (shown in Fig. 4d). The observed lattice fringe spacing of 0.21 nm is consistent with the d-spacing of the (200) planes in cubic NiO. The inset of Fig. 4d shows a SAED pattern recorded on one NiO nanoparticle with [001] axes. From this the surface lattice planes of {001} are concluded. Owing to the coarse and porous structural feature, the synthesized NiO hollow microspheres fabricated by the aggregation of NiO nanoparticles could be used as sensing materials and as photoanodes for the low-cost manufacture of high-performance solar cells.

Fig. 5 shows the Raman spectrum of the as-synthesized hierarchical NiO hollow microspheres. There are three Raman peaks at 536, 688, and 1057 cm⁻¹. The peak at 536 cm⁻¹ can be assigned to the longitudinal optical (LO) one-phonon (1P) modes of NiO. The peaks at 688 and 1057 cm⁻¹ can be assigned as two-phonon (2P) modes of 2TO and 2LO, respectively.⁴⁴ Compared with NiO single-crystal, the 1P band is pronounced in our prepared NiO nanostructures due to the presence of defects or surface effects. The relatively weaker band of the 1P mode suggests a low concentration of defects.⁴⁵ However, the two-magnon band usually located at 1490 cm⁻¹ is undetectable for our sample, which agrees well with the previous report,⁴⁵ suggesting a decrease of antiferromagnetic spin correlation and the antiferromagnetic-to-paramagnetic transition in the nanosized NiO.

Fig. 5 Raman spectrum of the as-synthesized NiO hollow microspheres.

Hollow nanostructures with thin shell layers are very attractive in sensing and catalyst fields due to their high surface area and low density. Enhanced gas responses have been demonstrated with hollow oxide nanostructures as gas sensor materials.^46,47 Here, the gas sensing properties of the as-prepared hierarchical NiO hollow microspheres were investigated. For comparison, another gas sensor based on NiO nanoparticles with a diameter of 20∼35 nm was also fabricated and tested (see Experimental section and Fig. S2 in the ESI†).

The sensing properties of the two sensors were measured at a working temperature of 350 °C with air as the reference gas. n-Butanol was used as a model gas for the sensor property tests. The obtained response and recovery characteristic curves against n-butanol concentration (5–800 ppm) of the two sensors are shown in Fig. 6a. At the measured temperature, the sensor resistance increases upon exposure to n-butanol and recovers approximately to the initial value when the test chamber is refreshed with air, which is expected for a p-type semiconductor (such as NiO) upon exposure to reducing gas.⁴⁸ The results clearly indicate that even the lowest concentration of n-butanol, i.e. 5 ppm, can be detected. The sensing response of the as-synthesized NiO hollow microspheres is higher than that of the NiO nanoparticles. This means that the unique hierarchical hollow nanostructure has a better sensing performance to n-butanol than that of the NiO nanoparticles. After eight cycles between the test gas and fresh air, the resistances of the sensors could recover to their initial state, which indicates that the sensors have good reversibility. For an effective sensor, short response and recovery times are needed in the rapid detection of flammable and toxic gases. For our sensors, the response time and recovery time (defined as the time required to reach 90% of the final equilibrium value) are less than 60 s.

Fig. 6 (a) Response and recovery characteristic curves of the two sensors based on the prepared hierarchical NiO hollow microspheres and NiO nanoparticles to butanol, respectively. (b) The gas concentration-dependent responses.

The sensing responses as a function of n-butanol vapor concentration from 5 ppm to 800 ppm are shown in Fig. 6b. It can be seen that the responses R_g/R_a of the two sensors are dramatically enhanced with the increasing concentration of the test gas at first (below 200 ppm). Above 200 ppm of n-butanol, the responses increase slowly and tend to saturation. This may be attributed to many reasons such as the insufficiency of negatively charged oxygen ions on the sensor which cannot oxidize all the analyte gases near the sensor surface, and the full covering of gas molecules on the sensor.^49,50 The response of oxide semiconductor sensor to gas is usually empirically depicted as S = 1+AP_g^β, where P_g is the target gas partial pressure, and so the response is characterized by the prefactor A and exponent β.^51,52 The value of β depends mainly on the charge of the surface species and stoichiometry of the elementary reactions on the surface and usually takes 1 or 1/2. As shown in Fig. 6b, the responses increase exponentially for both sensors, suggesting the value of β is 1/2 in these cases (Fig. S3, See ESI†).

The sensing response properties of the sensors towards other gases, propanol, acetone, formaldehyde, ethanol, heptanes, were also tested. Fig. 7 shows the responses of the two sensors to 100 ppm of test gases. Obviously, the sensor based on NiO hollow microspheres has a higher response than that fabricated from NiO nanoparticles for all tested gases. For the NiO hollow microsphere sensor, the response is highest for butanol, followed by propanol, acetone, formaldehyde, ethanol, and heptane.

Fig. 7 The responses of the two sensors based on the prepared NiO hollow microspheres and NiO nanoparticles to 100 ppm of butanol, propanol, acetone, formaldehyde, ethanol, and heptane.

The change in resistance is primarily caused by the adsorption and desorption of the target gas moleculars on the surface of the sensing material. With air as reference gas, the oxygen molecules in the air are adsorbed on the surface of the NiO (according to eqn (1) to (4)) and capture electrons from them.⁵³ NiO is a typical p-type semiconductor with hole carriers; the adsorption of the oxygen molecules results in an increase in the carrier density and the formation of a hole-rich layer on the surface of NiO; this is in contrast with an n-type semiconductor.

O₂ (gas) ↔ O₂ (ads) (1)

O₂ (ads) + e⁻ → O₂⁻ (ads) (2)

O₂⁻ (ads) + e⁻ →2O⁻ (ads) (3)

O⁻ (ads) + e⁻ →O²⁻ (ads) (4)

In general, below 150 °C O₂⁻ dominates, while above this temperature the atomic species (O²⁻, O⁻) are the main forms.⁵⁴ At the beginning, the air supplies enough oxygen molecules adsorbed on the NiO surface. When n-butanol or a similar reducing gas is injected in the test chamber, the test molecules interact with pre-adsorbed oxygen on the surface of NiO, generating water and carbon dioxide molecules according to equ (5). At the same time, the captured electrons are released, which neutralizes the hole carriers in the NiO surface layer and the sensor resistance increases until a new equilibrium is obtained. When the test chamber is charged with fresh air, oxygen molecules (in the air) will again adsorb on the surface of the NiO and the sensor resistances decrease to their initial values.

C₄H₉OH + 12O⁻ (ads) → 4CO₂ (gas) + 5H₂O (gas) + 12e⁻ (5)

The gas sensing superiority of hierarchical NiO hollow spheres over NiO nanoparticles can be understood. Firstly, the hierarchical NiO microspheres with a hollow structure have a relatively larger accessible surface area, thus more gas molecules including O₂ and test gas would adsorb on the surface and cause a higher gas sensing response. In contrast, the NiO nanoparticles easily aggregate when the samples are coated on supports for sensor fabrication. Secondly, from theoretical simulations and experimental results, sensor response can remarkably increase as the average crystallite size of the sensing material decreases to about 20 nm, which is about twice the thickness of the electron depletion layer.^55,56 The diameter of the NiO building blocks (about 25 nm) in the NiO hollow spheres is near this value, which is obviously beneficial for the enhancement of sensing performance. Thirdly, the unique 2D sheet-like structures would also contribute to the sensing. It is well known that the resistance of a sensing film is mainly controlled by the inter-nanocrystal contact barrier, the modulation of which by gas is also responsible for the sensing response.^40,57 A distinct characteristic of the sensor based on our prepared NiO hierarchical structures is that besides the contacts between particles and particles, there are face-to-face contacts between nanosheets. Both of them will contribute to the sensing properties. This is in contrast to the NiO nanoparticles without hierarchical structure.

Conclusions

Large-scale hierarchical NiO hollow microspheres have been successfully synthesized via the Ni(HCO₃)₂ precursor. The in situ generated CO₂ bubbles assist the formation of hollow Ni(HCO₃)₂ microspheres, which are converted into NiO by calcination. The obtained NiO hollow microspheres are composed of NiO nanoparticles, which are further assembled from ultrathin nanosheets. Enhanced gas sensing properties have been demonstrated to a series of volatile organic compounds. The synthesized NiO hollow microspheres were found to be effective in the detection of butanol, propanol, and acetone. The sensing response is higher than that of NiO nanoparticles with similar size. The present results imply that the inorganic salt precursor route would be an effective way to achieve higher structural control and property improvement for oxide nanostructures.

Acknowledgements

The authors are grateful for financial support from the Startup Fund for Distinguished Scholars (No. 10JDG132), China Postdoctoral Science Foundation (2011M500085), Jiangsu Postdoctoral Science Foundation (1102001C) and the National Natural Science Foundation of China (No. 51102117, No. 51072071).

References

1. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666.
2. R. Z. Ma, K. Takada, K. Fukuda, N. Iyi, Y. Bando and T. Sasaki, Angew. Chem., Int. Ed., 2008, 47, 86.
3. M. J. Allen, V. C. Tung and R. B. Kaner, Chem. Rev., 2010, 110, 132.
4. C. N. R. Rao, A. K. Sood, K. S. Subrahmanyam and A. Govindaraj, Angew. Chem., Int. Ed., 2009, 48, 7752.
5. P. V. Kamat, J. Phys. Chem. Lett., 2010, 1, 520.
6. S. Bai and X. P. Shen, RSC Adv., 2012, 2, 64.
7. Z. Liu, R. Ma, M. Osada, N. Iyi, Y. Ebina, K. Takada and T. Sasaki, J. Am. Chem. Soc., 2006, 128, 4872.
8. J. X. He, K. Kobayashi, M. Takahashi, G. Villemure and A. Yamagishi, Thin Solid Films, 2001, 397, 255.
9. Y. Ebina, T. Sasaki, M. Harada and M. Watanabe, Chem. Mater., 2002, 14, 4390.
10. K. Domen, Y. Ebina, S. Ikeda, A. Tanaka, J. N. Kondo and K. Maruya, Catal. Today, 1996, 28, 167.
11. A. Takagaki, M. Sugisawa, D. Lu, J. N. Kondo, M. Hara, K. Domen and S. Hayashi, J. Am. Chem. Soc., 2003, 125, 5479.
12. L. Wang, K. Takada, A. Kajiyama, M. Onoda, Y. Michiue, L. Zhang, M. Watanabe and T. Sasaki, Chem. Mater., 2003, 15, 4508.
13. W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami and Y. Takasu, Angew. Chem., Int. Ed., 2003, 42, 4092.
14. D. M. Kaschak, J. T. Lean, C. C. Waraksa, G. B. Saupe, H. Usami and T. E. Mallouk, J. Am. Chem. Soc., 1999, 121, 3435.
15. M. Osada, Y. Ebina, K. Takada and T. Sasaki, Adv. Mater., 2006, 18, 1023.
16. M. Fang, C. H. Kim, G. B. Saupe, H.-N. Kim, C. C. Waraksa, T. Miwa, A. Fujishima and T. E. Mallouk, Chem. Mater., 1999, 11, 1526.
17. T. Yu, B. Lim and Y. Xia, Angew. Chem., Int. Ed., 2010, 49, 4484.
18. R. Abe, K. Shinohara, A. Tanaka, M. Hara, J. N. Kondo and K. Domen, J. Mater. Res., 1998, 13, 861.
19. T. Sasaki, M. Watanabe, H. Hashizume, H. Yamada and H. Nakazawa, J. Am. Chem. Soc., 1996, 118, 8329.
20. X. H. Yang, Z. Li, G. Liu, J. Xing, C. H. Sun, H. G. Yang and C. Z. Li, CrystEngComm, 2011, 13, 1378.
21. Z.-H. Liu, K. Ooi, H. Kanoh, W.-P. Tang and T. Tomida, Langmuir, 2000, 16, 4154.
22. K. Kai, Y. Yoshida, H. Kageyama, G. Saito, T. Ishigaki, Y. Furukawa and J. Kawamata, J. Am. Chem. Soc., 2008, 130, 15938.
23. Y. Li, B. C. Zhang, X. W. Xie, J. L. Liu, Y. D. Xu and W. J. Shen, J. Catal., 2006, 238, 412.
24. N. G. Cho, I.-S. Hwang, H.-G. Kim, J.-H. Lee and I.-D. Kim, Sens. Actuators, B, 2011, 155, 366.
25. C. W. Na, H. S. Woo and J. H. Lee, RSC Adv., 2012, 2, 414.
26. P. Poizot, S. Laruelle, S. Grugeon, L. Dupont and J. M. Tarascon, Nature, 2000, 407, 496.
27. R. C. Makkus, K. Hemmes and J. H. W. D. Wir, J. Electrochem. Soc., 1994, 141, 3429.
28. H. Kumagai, M. Matsumoto, K. Toyoda and M. Obara, J. Mater. Sci. Lett., 1996, 15, 1081.
29. S. C. Chae, J. S. Lee, S. Kim, S. B. Lee, S. H. Chang, C. Liu, B. Kahng, H. Shin, D. W. Kim, C. U. Jung, S. Seo, M. J. Lee and T. W. Noh, Adv. Mater., 2008, 20, 1154.
30. K. Oka, T. Yanagida, K. Nagashima, T. Kawai, J. S. Kim and B. H. Park, J. Am. Chem. Soc., 2010, 132, 6634.
31. C. Li, Y. Liu, L. Li, Z. Du, S. Xu, M. Zhang, X. Yin and T. Wang, Talanta, 2008, 77, 455.
32. X. Song and L. Gao, J. Phys. Chem. C, 2008, 112, 15299.
33. B. Liu, H. Yang, H. Zhao, L. An, L. Zhang, R. Shi, L. Wang, L. Bao and Y. Chen, Sens. Actuators, B, 2011, 156, 251.
34. Z. P. Wei, M. Arredondo, H. Y. Peng, Z. Zhang, D. L. Guo, G. Z. Xing, Y. F. Li, L. M. Wong, S. J. Wang, N. Valanoor and T. Wu, ACS Nano, 2010, 4, 4785.
35. H. Q. Cao, X. Q. Qiu, Y. Liang, L. Zhang, M. J. Zhao and Q. M. Zhu, ChemPhysChem, 2006, 7, 497.
36. L. Bai, F. Yuan, P. Hu, S. Yan, X. Wang and S. Li, Mater. Lett., 2007, 61, 1698.
37. W. Zhou, M. Yao, L. Guo, Y. M. Li, J. H. Li and S. H. Yang, J. Am. Chem. Soc., 2009, 131, 2959.
38. F. Jiao, A. H. Hill, A. Harrison, A. Berko, A. V. Chadwick and P. G. Bruce, J. Am. Chem. Soc., 2008, 130, 5262.
39. J. Hu, K. Zhu, L. F. Chen, H. J. Yang, Z. Li, A. Suchopar and R. Richards, Adv. Mater., 2008, 20, 267.
40. L. J. Guo, X. P. Shen, G. X. Zhu and K. M. Chen, Sens. Actuators, B, 2011, 155, 752.
41. G. X. Zhu, Y. J. Liu, C. Zhang, Z. L. Zhu and Zheng Xu, Chem. Lett., 2010, 39, 994.
42. S. L. Wanga, L. Q. Qiana, H. Xua, G. L. Lüa, W. J. Donga and W. H. Tang, J. Alloys Compd., 2009, 476, 739.
43. Q. Peng, Y. Dong and Y. D. Li, Angew. Chem., Int. Ed., 2003, 42, 3027.
44. W. Z. Wang, Y. K. Liu, C. K. Xu, C. L. Zheng and G. H. Wang, Chem. Phys. Lett., 2002, 362, 119.
45. N. Mironova-Ulmane, A. Kuzmin1, I. Steins, J. Grabis, I. Sildos and M. Pärs, J. Phys. Conf. Ser., 2007, 93, 012039.
46. J. H. Lee, Sens. Actuators, B, 2009, 140, 319.
47. J. Park, X. P. Shen and G. X. Wang, Sens. Actuators, B, 2009, 136, 494.
48. T. He, D. Chen, X. Jiao, Y. Wang and Y. Duan, Chem. Mater., 2005, 17, 4023.
49. Z. J. Wang, Z. Y. Li, J. H. Sun, H. N. Zhang, W. Wang, W. Zheng and C. Wang, J. Phys. Chem. C, 2010, 114, 6100.
50. S. B. Patil, P. P. Patil and M. A. More, Sens. Actuators, B, 2007, 125, 126.
51. R.W. J. Scott, S. M. Yang, G. Chabanis, N. Coombs, D. E. Williams and G. A. Ozin, Adv. Mater., 2001, 13, 1468.
52. Q. Yu, C. Yu, W. Fu, M. Yuan, J. Guo, M. Li, S. Liu, G. Zou and H. Yang, J. Phys. Chem. C, 2009, 113, 12016.
53. A. Tricoli, M. Righettoni and A. Teleki, Angew. Chem., Int. Ed., 2010, 49, 7632.
54. N. Barsan and U. Weimar, J. Electroceram., 2001, 7, 143.
55. G. Zhang and M. L. Liu, Sens. Actuators, B, 2000, 69, 144.
56. F. Hernandez-Ramirez and J. D. Prades, Adv. Funct. Mater., 2008, 18, 2990.
57. P. Feng, X. Y. Xue, Y. G. Liu and T. H. Wang, Appl. Phys. Lett., 2006, 89, 243514.

---

Footnote

† Electronic supplementary information (ESI) available: Additional HRTEM images of hierarchical NiO microspheres, SEM, TEM, HRTEM images, plots of log(S − 1) vs. log(c), and XRD pattern of NiO nanoparticles. See DOI: 10.1039/c2ra01307j

---