Enhanced Gas-Sensing Performance of Fe-Doped Ordered

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Enhanced Gas-Sensing Performance of Fe-Doped Ordered Mesoporous NiO with Long-Range Periodicity Xiaohong Sun,† Xudong Hu,† Yongchao Wang,† Rui Xiong,† Xin Li,† Jing Liu,† Huiming Ji,† Xiaolei Li,*,† Shu Cai,† and Chunming Zheng*,‡ †

School of Materials Science and Engineering, Key Lab of Advanced Ceramics and Machining Technology, Tianjin University, Tianjin 300072, PR China ‡ State Key Laboratory of Hollow-Fiber Membrane Materials and Membrane Processes, School of Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387, PR China ABSTRACT: This paper reports the effect of mesoporous morphology and metal doping on the gas-sensing behavior of ordered mesoporous nickel oxide (NiO) materials. NiO samples with varied porous periodicity have been successfully synthesized via an improved nanocasting method. TEM, SEM, SAXRD, XRD, and nitrogen physisorption techniques are used to prove the presence of different mesostructured periodicity, as well as crystallinity, particle size, and pore size distribution of the as-synthesized NiO. The gas-sensing characterization indicates that the ordered mesoporous NiO with a long-range mesoporous periodicity based gas sensor exhibits a better ethanol sensing property than that of the ordered mesoporous NiO with short-range mesoporous periodicity. This result is due to the large specific surface area with sufficient sensing active sites, appropriate pore size distribution for enough gas diffusion, and proper particle size for effective charge accumulation of ordered mesoporous NiO with long-range periodicity. Furthermore, metal element doping, including Fe and Co, is used to adjust the sensitivity of the ordered mesoporous NiO based sensor. By doping of the Fe element on the optimizing mesoporous NiO, the gas-sensing property was obviously improved. The reason for that is attributed to the mesoporous structure and surface chemical state changed by the doping of Fe, resulting in more gas adsorption, diffusion, and reaction and attaining the enhanced gas-sensing performance. sensors.9,11 Ordered mesoporous metal oxides with large specific surface area and high surface-to-volume ratio are perspective nanostructures for improving gas-sensing performances because these structural and morphology features provide improved surface sensing activities, such as sufficient surface reactions and fast gas diffusion, based on their porous structure and high surface permeability, which allow more gas exposures and easy gas sensing.12,13 In our previous work, we reported an improved and effective “container effect” nanocasting method for synthesis of ordered mesoporous metal oxides, including Fe2O3, Cr2O3, In2O3, CeO2, Co3O4, Mn3O4, and NiO.14 By utilizing this method, the mesostructured periodicity, crystallinity, and particle diameter of the ordered mesoporous metal oxides could be controlled in a large range. This “container effect” nanocasting method provides an effective approach to study the effect mechanism of mesostructured periodicity of ordered mesoporous metal oxides on their corresponding application properties, especially gas-sensing performance. However, by far, the mesoporous structures strategy, which is effective to produce a thin surface charge layer and improve gas-sensing properties, has been only based on n-type metal

1. INTRODUCTION Chemical gas sensors have found wide applications in industrial production, medical diagnosis, environmental monitoring, and air quality control.1,2 Increasing requirements for accurate detection of explosive, toxic, and biomarker gases have led to growing interest in high-performance gas sensors with high sensitivity, rapid response and recovery, and selective detection.3,4 Among them, resistive metal oxide semiconductor gas sensors based on the change in conductivity upon exposure to gas molecules are the most beneficial because of their simple, low cost, real-time monitoring, and easy application.5,6 Different metal oxides, both n-type (SnO2, ZnO, TiO2, WO3, Fe2O3, and In2O3) and p-type (CuO, Co3O4, and NiO), with different phase structures, geometrical morphologies, and hybrid compositions, have been developed for resistive gas sensor applications.3,7,8 However, due to the poor sensitivity or unsatisfying selectivity, it is still urgent to find an effective method to prepare gas-sensing materials with high sensing response and excellent selectivity.9 Three important aspects are known to be involved in the response of gas sensors, including diffusion of gas molecules, recognition by the reactive surface, and transducting by a sensing layer.5,10 Therefore, the improvement of effective adsorption and diffusion of detecting gas and carrier mobility would be a good way to promote the sensing property of gas © 2015 American Chemical Society

Received: December 15, 2014 Revised: January 17, 2015 Published: January 21, 2015 3228

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The Journal of Physical Chemistry C oxide semiconducting materials,15,16 including our recent work on the sensitivity of mesoporous In2O3 with appropriate mesostructured periodicity and particle size.17,18 The p-type metal oxide gas sensors with ordered mesoporous structures are rarely studied. Thus, there are great demands for the preparation of mesoporous p-type metal oxides with optimum mesostructure and gas-sensing properties. In addition, doping of metal elements, especially noble metals, such as Pt, Au, and, Pd has been used to functionalize the semiconducting metal oxides (e.g., SnO2, Fe2O3, WO3, and NiO) for enhanced detection of different gases.9,19,20 However, the high cost of noble metals reduces the potential applications of this strategy. The doping of other metal elements for metal oxide gas sensors is also of advantage for their gas-sensing enhancement, which makes it possible to obtain an ideal gas sensor with outstanding sensitivity, good selectivity, and low cost. Bloor et al. reported tantalum- and titanium-doped In2O3 thin films with superior gas-sensing response.21 Lee and coworkers found that ultraselective and sensitive detection of xylene and toluene can be achieved using Cr-doped NiO hierarchical nanostructures.4 Pati et al. synthesized the indiumdoped ZnO gas sensors and found the n- to p-type carrier reversal.22 Nickel oxide (NiO), a wide bandgap (3.6−4.0 eV) p-type semiconductor with high chemical and thermal stability and unique magnetic, optical, electrical, and catalytic properties, was also found to be a good candidate for gas sensor applications. Le Thuy and co-workers reported the fabrication of 2Dgraphene/2D-NiO nanosheet based hybrid nanostructure sensors for detecting of NO2.23 Soleimanpour et al. researched the enhancement of hydrogen gas sensing of nanocrystalline NiO synthesized by the pulsed-laser irradiation method.24 However, most NiO gas-sensing materials with nanostructured morphology will kind of suffer from degradation because of the aggregation growth among nanoparticles/nanopowders during high-temperature sensing progress. To the best of our knowledge, for reducing gas sensing, there is no report on the study of the mesoporous periodicity effect and metal element doping in ordered mesoporous p-type NiO materials synthesized by the improved nanocasting technique. Therefore, it is still a gap needing to be bridged to synthesize NiO gassensing materials with novel structure and morphology for effective and reliable gas sensing. In this work, we research the “container effect” nanocasting synthesis and gas-sensing properties of ordered mesoporous NiO with varied mesoporous morphology and doping elements. The mesoporous morphology, metal doping effect, and gas-sensing properties have been systemically studied. Furthermore, the sensing mechanisms of ordered mesoporous NiO with different mesoporous periodicity and doping structures have been explored in detail.

tetraethylorthosilicate (TEOS, 98%, 14 mL) was added and stirred at 35 °C for another 24 h. The obtained gel was transferred to a Teflon-lined autoclave and maintained in the hydrothermal process for 24 h at 110 °C. The resulting sample was filtered, washed, and dried at 60 °C. Finally, the block copolymer P-123 was removed by calcination of the sample at 550 °C for 6 h in air (heating rate of 2 °C min−1). In the second step, nanocasting synthesis of ordered mesoporous NiO using KIT-6 as a hard template was previously reported by Tian et al.26 The porous periodicity, crystallinity, and particle size control of ordered mesoporous NiO was carried out by our previously improved “container effect” nanocasting method.14 Typically, the KIT-6 silica template (1.5 g) was mixed with toluene (60 mL) and heated to 65 °C. Then, nickel nitrate hexahydrate (Ni(NO3)2·6H2O, 3 g) was added into the mixture under vigorous stirring. The nickel nitrate hexahydrate was melted and liquefied, and automatically moved into the template pores because of the capillary condensation effect due to the strong interaction between the nitrate and silica inner pore surface. The obtained precursor@silica composites by filtration were dried at room temperature in air overnight. Then, the precursor@silica composites were calcined in a muffle furnace (chamber dimensions (width × height × depth): 11.2 × 15.9 × 11.2 cm) with a heating rate of 2 °C min−1 from room temperature to 500 °C and kept at that temperature for 5 h to convert the nickel nitrate precursors to NiO. The furnace has a smog tube with a diameter of 1.6 mm on its top. During calcination, different container effects (closed system and open system) were used for the synthesis of NiO with different mesoporous periodicity but the calcination temperature program was kept the same. For the synthesis of NiO with long-range mesoporous periodicity, the glass bottle (diameter of 12 mm and depth of 45 mm) as a thermal-treatment container was covered with a glass strip with 100% coverage (called closed system). For the synthesis of NiO with shortrange mesoporous periodicity, the Petri dish (called open system) with a diameter of 60 mm and depth of 12 mm was used as the thermal-treatment container. Such differences in the container conditions during calcination are sufficient to change the morphology of the NiO and have a profound influence on their gas-sensing behavior, as demonstrated later. After calcination, the resulting NiO@silica samples were treated with 2 M NaOH solution twice to remove the silica template. Final products were recovered by centrifuging and washing with water and ethanol several times. 2.2. Synthesis of Ordered Mesoporous Fe- or CoDoped NiO with Long-Range Periodicity. Ordered mesoporous NiO materials with Fe or Co element doping (5 mol % Fe or Co doping concentration) were synthesized by doping the Fe or Co element into the ordered mesoporous NiO intermediate with long-range periodicity. The Fe or Co precursor used here is ferric nitrate nonahydrate (Fe(NO3)3· 9H2O) or cobalt nitrate hexahydrate (Co(NO3)2·6H2O). In a typical synthesis, ordered mesoporous NiO with long-range periodicity (0.5 g) was mixed with ethanol (80 mL) and stirred for 1 h. Then, Fe(NO3)3·9H2O (0.14 g) or Co(NO3)2·6H2O (0.1 g) was dissolved in another 80 mL of ethanol and stirred with the preformed NiO/ethanol mixture for 30 min. Then, ethanol was evaporated off at 60 °C in air. After that, the asprepared composite was calcined in air at 300 °C for 3 h to form the ordered mesoporous Fe- or Co-doped NiO with longrange periodicity. The calcination temperature was increased from room temperature with a heating rate of 5 °C/min.

2. EXPERIMENTAL SECTION 2.1. “Container Effect” Nanocasting Synthesis of Pure Ordered Mesoporous NiO. To synthesize ordered mesoporous NiO with varied mesoporous morphology, a two-step method was adapted as described below. First, ordered mesoporous silica hard template KIT-6 with Ia3d symmetry was synthesized according to the work reported by Ryoo et al.25 Typically, block copolymer P-123 (6 g) was dissolved in a mixture of deionized water (217 mL) and hydrochloric acid (36%, 10 mL) at a temperature of 35 °C. Then, n-butanol (6 g) was added into the mixture and stirred for 2 h. After that, 3229

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The Journal of Physical Chemistry C 2.3. Characterization. The wide-angle X-ray powder diffraction (XRD) data of the powder samples were carried out on a Philips X’pert powder diffractometer with a graphite monochromator and Fe Kα1 source (λ = 0.193 nm). The XRD patterns were collected from 20 to 90° with a resolution of 0.2°. The Debye−Scherrer equation, D = Kλ/(β cos θ), was used to estimate the average crystallite size of the samples. Here, D stands for the average crystallite size, β denotes the corrected peak width (full width at half-maximum), K is a constant corresponding to the shape of the crystallites (K = 0.94), λ is the wavelength of the employed X-rays, and θ is the diffraction angle. The (200) diffraction peak with the highest peak intensity was selected for the crystallite size calculation. The small-angle X-ray diffraction (SAXRD) patterns were performed on a Philips X’pert MPD thin film XRD using Cu Kα radiation (λ = 0.154 nm). Transmission electron microscopy (TEM) and selected area electron diffraction (ED) measurements were taken on a FEI Tecnai G2 F20 microscope. All testing samples were ultrasonically dispersed in alcohol before the TEM measurements and drop-cast onto copper grids. The energy dispersive X-ray spectrometer (EDS) attached to the TEM apparatus was used to determine the chemical compositions of the samples. The SEM images were performed on a FEI XL40 device. The SEM elemental mapping was measured at the same time using EDS attached to the SEM. Nitrogen physisorption was performed on a Micromeritics TriStar porosimiter apparatus at 77 K. The NiO samples were outgassed overnight at 150 °C before the measurements. The specific surface area data of samples were obtained using the Brunauer−Emmett−Teller (BET) method. The pore size distribution results were calculated using the adsorption branch of the isotherm by the Barrett−Joyner−Halanda (BJH) method. The metal element analysis of the doped NiO samples was carried out by inductively coupled plasma optical emission spectroscopy (ICP-OES, Thermo Jarrel Ash model IRIS Intrepid II XDL, USA) to confirm the respective concentrations of metal elements in the system. Surface elemental analysis of samples was taken using the ESCALAB250 X-ray photoelectron spectroscopy (XPS). 2.4. Gas Sensor Fabrication and Measurement. The NiO based gas sensors were fabricated by dip-coating the water paste of ordered mesoporous NiO or doped NiO onto an alumina ceramic tube with gold electrodes. The operating temperature of the gas sensors was performed through varying the heating current of the inserted Ni−Cr heating wire inside the ceramic tube. The NiO based water paste was synthesized by mixing of NiO or doped NiO (50 mg) and deionized water (0.5 mL). The obtained sensors were dried under ambient conditions overnight before use. Four platinum wires attached to two gold electrodes were used to realize the electrical contacts. The gas-sensing properties of the sensor were measured by a CGS-8 series sensing measurement device (Beijing Elite Tech Co., LTD, China). The relative humidity (RH) during the gas sensing was monitored and kept at about 40%. The gas-sensing performances of the NiO based gas sensors were performed under a steady-state condition in an organic glass chamber (20 L). The required amount of liquid gas was injected by a microinjector into the closed chamber to form the prospective gas concentration (200 ppb, 500 ppb, 1 ppm, 2 ppm, 5 ppm, 15 ppm, 50 ppm, and 100 ppm). During all the sensing tests, air was used as the diluting and reference gas for the different detecting concentrations. When the gassensing test was completed, the sensor was exposed to air again

by keeping the chamber total open. The gas response (sensitivity, S = Rg/Ra) of the sensor was defined as the resistance ratio in the target gas (Rg) to that in air (Ra). The response and recovery times were defined as the times required for resistance change to reach 90% of the equilibrium value when the detected gas was injected or removed, respectively. Ethanol, formaldehyde, acetone, methanol, and toluene were used to measure the selectivity of the NiO based gas sensor as the detecting gas.

3. RESULTS AND DISCUSSION In this paper, the first research aim was to find the particular relation between the mesostructured periodicity as well as the particle size, crystallinity, and porous distribution of ordered mesoporous NiO materials and their gas-sensing performance. The control synthesis of ordered mesoporous metal oxides including NiO with different mesoporous periodicity has been realized in our previous work using the improved “container effect” nanocasting method.14 The calcination container size and shape in conjunction with opening accessibility could be used to control the escape rate of water and other byproducts for the calcination process, and subsequently affect the structure and morphology of the metal oxide materials inside the mesopore space of the hard template. In this way, the crystallinity, particle diameter, and mesostructured periodicity of the final samples can be systemically controlled. In this work, two kinds of container conditions (closed system and open system) are used to prepared the mesoporous NiO with different mesoporous periodicity. The TEM images (Figure 1a

Figure 1. TEM (a and b) and SEM (c and d) images of ordered mesoporous NiO with long-range mesoporous periodicity (a and c) and short-range mesoporous periodicity (b and d).

and b) reveal that both NiO samples exhibit nearly spherical particles with periodic cubic (Ia3d) mesostructure and their morphology is definitely different from their parent silica template KIT-6, which is with irregular shape and much larger size than the NiO replica.25 The particle sizes and mesoporous periodicity of ordered mesoporous NiO synthesized in a closed system are much larger than those of ordered mesoporous NiO 3230

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The Journal of Physical Chemistry C synthesized in an open system. Hence, the ordered mesoporous NiO synthesized in a closed system with long-range mesoporous periodicity would be called L-NiO and the other one with short-range mesoporous periodicity synthesized in an open system would be called S-NiO. The SAED pattern of LNiO (inset in Figure 1a) reveals that L-NiO is composed of single crystallites, whereas that of S-NiO (inset in Figure 1b) demonstrates that S-NiO has a polycrystalline structure. For LNiO, one can see that the particle size is almost equal with the mesoporous periodicity range. For S-NiO, lots of isolated mesoporous nanoparticles with a size less than 50 nm and without long-range mesoporous periodicity can be found, which arise from the “solid decomposition” of the nickel precursor during the calcination process for the open system nanocasting way.14 The container effect for the synthesis of ordered mesoporous NiO with varied mesostructured periodicity and particle size can also be clearly observed from the SEM images (Figure 1c and d). The average particle sizes of LNiO and S-NiO are about 216 and 47 nm. They were obtained by measuring the diameter of about 30 particles for each NiO sample. Obviously, the mesoporous periodicity and particle sizes of the as-synthesized NiO samples can be controlled easily and in a large range by the improved “container effect” nanocasting method. The SAXRD patterns (Figure 2a) of both NiO samples show one intense peak at 2θ of around 0.90°, attributed to the 211 diffraction peak of Ia3d symmetry, which implies that the mesostructured ordering of the template KIT-6 is retained in both NiO replicas. Comparing to the SAXRD pattern of KIT-6, the disappearance of the shoulder peak at 2θ = 1.03° means that the mesoporous structures of NiO are lower than those of KIT-6. At the same time, the 211 diffraction peak intensity of ordered mesoporous NiO synthesized in a closed system (LNiO) is higher than that of NiO synthesized in an open system (S-NiO), which can be interpreted as an increment in the X-ray scattering contrast between the mesopore and the framework, revealing that the mesoporous ordering of NiO synthesized in a closed system is in a longer range than that of NiO synthesized in an open system. This phenomenon is consistent with the above TEM and SEM images. Figure 2b shows the XRD patterns of both NiO samples. Well-defined diffraction peaks suggesting the crystalline nature can be attributed well to the face-centered cubic phase of NiO (JCPDS card No. 47-1049). Meanwhile, each XRD peak width of ordered mesoporous NiO synthesized in a closed system is narrower than that of NiO synthesized in an open system, which can be assigned to the increase in crystallite size. By using the Debye−Scherrer equation, the average crystallite sizes of L-NiO and S-NiO were calculated to be 12.2 and 9.6 nm, respectively. One can see that the crystal domain sizes of S-NiO are lower than those of LNiO; nevertheless, the average crystallite size of even S-NiO (9.6 nm) is still more than the repeat distance of the mesopores. In other words, the presence of regular arranged mesopores does not seem to disturb the atomic-scale crystallinity of NiO. This is different from many other mesoporous metal oxides, such as MgO27 and ZnO,28 which have been prepared by hard template replication and where the single-crystalline grains turned out to be hardly larger than the pore wall thickness. The above results indicate that the particle size, mesoporous periodicity, and crystallite sizes of ordered mesoporous NiO can be controlled easily by using the “container effect” nanocasting method.

Figure 2. (a) SAXRD patterns of KIT-6, ordered mesoporous NiO with long-range mesoporous periodicity (L-NiO), and ordered mesoporous NiO with short-range mesoporous periodicity (S-NiO). (b) XRD patterns of ordered mesoporous NiO with long-range mesoporous periodicity (L-NiO) and ordered mesoporous NiO with short-range mesoporous periodicity (S-NiO).

The N2 physisorption results of both NiO samples are shown in Figure 3. Both samples gave a typical IV isotherm with a clear H1-type hysteresis loop, which is characteristic for mesoporous materials. Clearly, the hysteresis loop of S-NiO synthesized in an open system occurs at a much higher relative pressure, revealing a much larger mesoporous diameter, which is also substantiated by the pore size distributions (Figure 3b) calculated by the BJH method.39 For both samples, two welldefined steps of capillary condensation corresponding to two pore size distributions (1.7 and 7.8 nm for L-NiO and 2.0 and 12.8 nm for S-NiO) can be observed. The former one is caused by the hard template replica, and a narrow porous size distribution confirms the ordered uniform pore structure. The latter piled porosity with wide pore size distribution and porous diameter arises from the aggregation of mesoporous particles.29 From the above SEM and TEM results, it is clear that the mesoporous particle sizes of both L-NiO and S-NiO are in a wide range which makes the piled porous diameters of L-NiO and S-NiO also in a broad distribution. At the same time, since the particle size of S-NiO is much smaller than that of L-NiO, the pore size distribution of piled larger mesopores of S-NiO is much wider than that of L-NiO, which was also observed by the previous nanocasting synthesis of other metal oxides.17,31 The 3231

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Figure 3. Nitrogen physisorption isotherms (a) and pore size distributions (b) of ordered mesoporous NiO with long-range mesoporous periodicity (L-NiO) and ordered mesoporous NiO with short-range mesoporous periodicity (S-NiO). (Pore size distributions are shifted for clarity.)

specific surface areas of L-NiO and S-NiO samples are 101 and 118 m2 g−1, respectively, which are lower than that of the parent KIT-6 template. This phenomenon has been observed by previously reported work and may be due to the lower structural ordering and larger density of metal oxide replicas.30,31 As discussed above, the container conditions for nanocasting synthesis have a significant effect on the mesoporous periodicity and pore size distribution of the ordered mesoporous NiO samples. This method makes it possible to research the relationship between the mesoporous periodicity morphology of ordered mesoporous NiO and their corresponding gas-sensing properties, which to the best of our knowledge has never been researched. The structure and morphology of metal oxide materials were reported to have a large effect on their physical, chemical, and application performance. For example, in our previous work, the gas-sensing properties of metal oxide, such as Fe2O3 and In2O3, vary a lot with the different mesoporous morphology.18,31 In this work, the mesoporous periodicity and particle size control of NiO are realized by the improved nanocasting method, which is expected to bring about more efficient gassensing performance as compared to conventionally prepared NiO materials with unique structure. The temperaturedependence behavior of the as-prepared L-NiO and S-NiO sensors to 50 ppm of ethanol was studied by varying the heating current to achieve the highest sensitivity and the optimum operating temperature (Figure 4a). It can be seen that the sensitivity of the sensors increases with the increase of the operating temperature in the beginning and reaches a maximum value at an optimal temperature, and then decreases with further increase of the operation temperature. L-NiO was found to be more sensitive than the S-NiO sensor with the highest sensitivity of 2.40 and the optimum operating temperature of 300 °C. In contrast, the S-NiO sensor exhibited a lower response of 2.12 at the optimum operating temperature of 270 °C. The “increase-maximum-decay” sensitivity trend for the metal oxide semiconductor sensors with the increase of the operating temperature was often observed.32,33 As the temperature increases, a higher response is observed because of the activation of adsorbed molecular oxygen and lattice oxygen to

Figure 4. (a) Sensitivity versus operating temperature of ordered mesoporous NiO with long-range mesoporous periodicity (L-NiO) and ordered mesoporous NiO with short-range mesoporous periodicity (S-NiO) exposed to 50 ppm of ethanol. (b) Dynamic ethanol sensing transient of ordered mesoporous NiO with long-range mesoporous periodicity (L-NiO) to 50 ppm of ethanol at 300 °C and ordered mesoporous NiO with short-range mesoporous periodicity (SNiO) to 50 ppm of ethanol at 270 °C. (c) Sensitivity reproducibility of the L-NiO sample to 20 ppm of ethanol at 300 °C.

form active O2− and O− and mobile O2− species, respectively.1 This feature continues up to a certain optimum temperature, beyond which exothermic gas adsorption turns out to be difficult and gas molecules begin to desorb in large quantities, 3232

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The Journal of Physical Chemistry C leading to a decrease in sensor response.34 Hence, the optimum operating temperature is a balance point between two conflicting mechanisms. It can be concluded that the mesoporous structure and morphology influenced the gassensing performances significantly. The response and recovery times are also important parameters for a gas sensor, which were defined as the time for the resistance change to reach 90% of the final value. The dynamic ethanol sensing transients of both NiO samples at their respective optimum operating temperatures are shown in Figure 4b to 50 ppm of ethanol. The response time of L-NiO is 7 s, which is faster than that of S-NiO (12 s). The recovery times of L-NiO and S-NiO are almost the same (18 s). Stability (repeatability) is a significant factor for commercial gas sensors, which is the ability to retain performance characteristics over time without the need for replacement and recalibration.24 In order to investigate the stability of the L-NiO sample, the asfabricated sensor was performed upon five successive sensing tests to 20 ppm of ethanol. Figure 4c shows the reproducibility data of L-NiO, indicating that the sensor maintains its initial sensitivity without a clear decrease and has a satisfying longterm stability and reproducibility (S = 2.1). From the above results, ordered mesoporous NiO with longrange periodicity (L-NiO) has better gas sensing than S-NiO. The obvious difference in gas-sensing properties between two NiO samples confirms that the improved “container effect” nanocasting method is efficient to optimize the gas sensitivity of ordered mesoporous NiO materials by controlling the mesostructured periodicity and particle diameter. It is known that, for porous metal oxides, specific surface areas, particle sizes, and porous morphology are all important factors for their gas-sensing performance to greatly affect the adsorption and diffusion of gas molecules and carrier mobility.35,36 In this work, since the same KIT-6 were used as the replication template, the mesopore size, mesopore wall thickness, and mesopore interconnectivity of the as-synthesized NiO should be similar. Hence, the effective charge accumulation area should arise from the effective diffusion of target gases within the NiO sensing layer. The BET specific surface area of L-NiO samples (101 m2 g−1) is a little lower than that of S-NiO (118 m2 g−1), which should not be beneficial for their gas sensing. The average particle sizes of L-NiO and S-NiO are 216.4 and 46.7 nm, respectively. L-NiO has a highly ordered mesoporous structure, whereas S-NiO is composed of less ordered nanoparticles. During the calcination treatment and high-temperature operation of the NiO sensor, S-NiO with nanoscale morphology suffers from the particle growth easily and tends to form more compact nanoscale particle aggregation, while LNiO is more stable and prefers to keep its mesoporous morphology, which has been proved by the above BJH average pore size distributions (Figure 3b), since most pore distribution for S-NiO comes from the larger piled pores instead of the ordered replica pores for L-NiO. Hence, for compact S-NiO nanoparticles, it is theoretically difficult for target detecting gas to diffuse into the interior of the NiO sensing layer and only a small charge accumulation area can form (Figure 5).10 As a result, poor gas sensitivity is expectable, even though its BET specific surface area is a little higher than that of L-NiO. On the other hand, L-NiO with a larger particle size, more ordered mesostructures, and an appropriate proportion of larger piled pores (Figure 3b) makes the diffusion of target gases within the sensing layer comparably easy, and results in a long effective

Figure 5. Illustration of respective sensing factors of L-NiO and S-NiO samples.

diffusion distance and thick electron depletion area, which leads to the higher gas-sensing performance (Figure 5).37 Metal element doping has long been proven to be a facile and efficient way to enhance the sensing property of semiconductor metal oxide sensors, because doping can effectively modulate the parameters of the crystal cell and the adsorption of oxygen.32,38 Doping of Fe and Co elements is usually recommended for catalyzing various reactions due to unique catalytic and electronic activities. Lee et al. doped the Fe element to p-type NiO nanofibers and attained an enhanced ethanol gas-sensing property in relation to electronic sensitization.39 However, there has been little exploration of the Fe- or Co-doped ordered mesoporous NiO in gas-sensing reactions. In this work, we synthesized the Fe- or Co-doped ordered mesoporous NiO with long-range periodicity (named Fe-doped L-NiO or Co-doped L-NiO) and researched the metal element doping effect on their gas-sensing properties. The doped Fe or Co element concentrations in L-NiO were analyzed by ICP-OES, which shows that the molar ratios of Fe and Co in L-NiO are 4.83 and 4.86 mol %, respectively. It can be concluded that the results are close to the nominal values (5 mol %), which subsequently confirm the totally doping of Fe or Co in the NiO samples. Figure 6a displays the sensitivity of the as-synthesized Fe-doped and Co-doped L-NiO sensors toward 50 ppm of ethanol as a function of the operating temperature. One can see that the response of the Fe-doped L-NiO sensor presents a rapid increase and reaches its maximum value of 3.9 at 240 °C, and then gradually decreases with further increase of the operating temperature. The Co-doped L-NiO sensor displays a similar response tendency, and its sensitivity reaches the maximum value of 1.8 at the same operating temperature of 240 °C. This “increase-maximum-decay” phenomenon is commonly observed for many semiconducting metal oxide based sensors and can be explained by the balance between the speed of chemical reaction and the speed of gas diffusion.33 From the above results, we can see that Fe element doping has been proved to be a better way to improve the sensitivity of LNiO than Co element doping, and at the same time, doping with Fe element makes the optimum operating temperature of 3233

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Figure 6. (a) Sensitivity versus operating temperature of Fe-doped LNiO and Co-doped L-NiO exposed to 50 ppm of ethanol. (b) Sensitivity values of L-NiO (at 300 °C), Fe-doped L-NiO (at 240 °C), and Co-doped L-NiO (at 240 °C) to 50 ppm of various gases: ethanol, acetone, methanol, formaldehyde, and toluene.

Figure 7. (a) Dynamic sensing transient to different ethanol concentrations of L-NiO (at 300 °C) and Fe-doped L-NiO (at 240 °C). (b) Sensor sensitivity of L-NiO and Fe-doped L-NiO with varied ethanol concentration.

L-NiO decrease from 300 °C to only 240 °C. Selectivity is another very important parameter for a utility-type metal oxide gas sensor, because poor selectivity will induce mistaken alarm and limit its extensive utilization.40,41 The responses of the pure L-NiO, Fe-doped L-NiO, and Co-doped L-NiO based gas sensors to five target gases (ethanol, acetone, methanol, formaldehyde, and toluene) with 50 ppm concentration at their corresponding optimum operating temperatures (300 °C for L-NiO and 240 °C for Fe-doped and Co-doped L-NiO) were further investigated, and the relevant results are shown in Figure 6b. It is seen that the responses of the Fe-doped L-NiO sensor to all test gases are significantly larger than those of the L-NiO and Co-doped L-NiO sensor, indicating that the sensing ability to all target gases of the nanocasting synthesized L-NiO has been effectively improved by the Fe element doping method. Meanwhile, three L-NiO based sensors prefer to respond to ethanol vapor, and show less sensitivity to other gases. That is to say, the present sensors display quite outstanding selectivity to ethanol, especially the Fe-doped LNiO, of which the responses reach 3.9, 3.0, 2.7, 1.8, and 2.3 to ethanol, acetone, methanol, formaldehyde, and toluene, respectively. It is known that the development of a gas sensor which can sense gas at a lower detection concentration and allow quantification of gas over a wide concentration range is of practical interest.42,43 Figure 7a shows the typical response and recovery characteristics as a function of ethanol concentration

for the L-NiO and Fe-doped L-NiO sensors at their corresponding optimum operating temperature (300 °C for L-NiO and 240 °C for Fe-doped L-NiO). It is obvious that both NiO based sensors have a wide detection range for ethanol gas from 200 ppb to 100 ppm. With the increase of ethanol gas concentration, the responses greatly increase. The sensitivities of both NiO based sensors as a function of ethanol gas concentration are provided in Figure 5b. The Fe-doped LNiO based sensor shows sensitivity of approximately 1.12, 1.31, 1.37, 1.48, 1.83, 2.41, 3.88, and 4.80 at 0.2, 0.5, 1, 2, 5, 15, 50, and 100 ppm ethanol, respectively. In comparison, the pure LNiO based sensor shows sensitivity of 1.11, 1.28, 1.33, 1.41, 1.62, 2.01, 2.40, and 2.71 at the same ethanol concentrations, respectively. In short, the sensing property of the L-NiO after the doping with the Fe element has been improved even at the lowest detection limit. It is known that, compared with the n-type metal oxide sensors, in p-type metal oxide sensors, a high response to reducing gas is relatively difficult to achieve because the chemoresistive variation of p-type metal oxide under sensing is relatively small compared to that of n-type metal oxide gas sensors.38 Therefore, the improvement of gas accessibility, sensing area, and gas reaction resulting from the change of the hole accumulation layer of p-type metal oxides would be good ways to improve the sensing performance of gas sensors.44,45 In order to understand the role of the doped Fe and Co elements on ordered mesoporous L-NiO materials and their gas-sensing 3234

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crystallite size of Co-doped L-NiO is larger than that of Fedoped L-NiO, even larger than that of the pure L-NiO (12.2 nm), which indicates that the Co doping makes the crystal growth of L-NiO during doping progress and hence decrease the mesoporous size of L-NiO accordingly (Figure 8a). The nitrogen adsorption analysis shows that the specific surface areas of Fe-doped L-NiO and Co-doped L-NiO samples are 101 and 90 m2 g−1, respectively, which confirmed the above SAXRD and XRD data from the side, since the BET specific surface area of pure L-NiO is 101 m2 g−1. The TEM images (Figure 8c and d) reveal that both doped L-NiO samples exhibit nearly spherical particles with well remained periodic cubic (Ia3d) mesostructure, which is in accordance with the SAXRD results. The EDS spectrum (Figure 8e) collected from the Fe-doped L-NiO sample shows that the main elements are Ni, O, and Fe. Figure 8f shows the corresponding elemental mappings of Fe-doped L-NiO. It is seen that elements Ni, Fe, and O are evenly distributed throughout each mesoporous particle, implying that NiO formed a uniform chemical phase and the Fe component is homogeneously dispersed and is incorporated into the NiO lattice. To illuminate the surface element composition and chemical state of the elements existing in doped and undoped ordered mesoporous NiO samples, we researched the XPS spectra of Fe-doped L-NiO, Co-doped L-NiO, as well as pure L-NiO samples. The elemental binding energies of these elements were corrected on the basis of carbon (C) 1s binding energy at 284.5 eV. The survey spectra of two doped samples (Figure 9a) confirm the presence of Ni, O, C, and the doped Fe or Co atoms. In Figure 9b, it is found that the binding energies of Ni 2p for both doped L-NiO were higher by ca. 0.3 eV than those of the pure L-NiO. It is believed that the electronic interaction between the doped elements and NiO surfaces occurs and the binding energy shifts originate from this electronic sensitization, which finally increases the Fermi level of NiO.9 In Figure 9c, the O 1s peaks of the binding energy for pure L-NiO are observed at 529.5 and 531.4 eV, respectively, corresponding to the lattice oxygen in crystalline NiO and the deficient oxygen on the sample surface.32 The atomic ratio percentage of deficient oxygen for Fe-doped L-NiO is calculated to be 66.7%, which is more than that of Co-doped L-NiO (60.6%) and pure L-NiO (65.4%). This result indicates that Fe-doped L-NiO has more acceptor content (oxygen vacancies) than undoped LNiO and Co-doped L-NiO, which is beneficial to the adsorption of oxygen and the surface reaction. That is why the gas response of ordered mesoporous L-NiO is significantly enhanced by doping with the Fe element instead of the Co element. The gas-sensing mechanism of p-type metal oxides, like NiO, is based on the change in resistance that is mainly caused by the adsorption and desorption of gas molecules on the sensor surface.7 In air, the oxygen adsorbed on the surface of NiO trapped the conduction electrons and thus induced a chargeaccumulation space charge layer. If the sensor is exposed to the reducing gases, they will react with the adsorbed oxygen molecules and release electrons, resulting in a decrease in the space charge layer. The reaction between reducing gas and negatively charged oxygen on the surface of NiO will then decrease the concentration of surface holes via electron−hole recombination, which increases the resistance of the sensor.42 Hence, the ability of the sensor material to absorb and ionize oxygen species is key to the sensor performance. From the above results, it is obvious that Fe-doped L-NiO materials have

mechanism, SAXRD, XRD, TEM, EDX, and elemental mapping were measured and shown in Figure 8. The SAXRD

Figure 8. (a) SAXRD patterns of Fe-doped and Co-doped L-NiO. (b) XRD patterns of Fe-doped and Co-doped L-NiO. TEM images of Fedoped L-NiO (c) and Co-doped L-NiO (d). EDS (e) and elemental mapping (f) data of Fe-doped L-NiO.

patterns (Figure 8a) of both doped NiO samples show one peak attributed to the 211 diffraction of Ia3d symmetry, which reveals that the doping of Fe and Co does not seem to disturb the mesostructured regularity of L-NiO replicas. Compared with the SAXRD pattern of Fe-doped L-NiO, the 211 diffraction peak of Co-doped L-NiO shifted to a little more degree indicates that the doping of the Co element makes the mesoporous size of L-NiO smaller than that of Fe-doped LNiO. Figure 8b shows the XRD patterns of both doped L-NiO samples. Well-defined diffraction peaks can be indexed well to the face-centered cubic phase of NiO (JCPDS card No. 471049). No metallic or metal-containing other phase was found, which confirms the substitutional doping of Fe or Co atoms into the NiO lattice. By using the Debye−Scherrer equation based on the (200) reflection, the average crystallite sizes of Fedoped L-NiO and Co-doped L-NiO were calculated to be about 11.5 and 13.4 nm, respectively. One can see that the 3235

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4. CONCLUSIONS In summary, ordered mesoporous NiO with various porous periodicity, particle size, and pore size distribution has been successfully constructed via the improved nanocasting strategy. An enhanced sensing property of ordered mesoporous NiO with long-range mesoporous periodicity to ethanol, including high response value, fast response characteristic, and good selectivity, was demonstrated. This is because small particle size, high surface area, and wide pore size distribution are able to supply enough sensing active sites, sufficient gas diffusion, and effective charge accumulation on the long-range mesoporous NiO surface. Furthermore, the gas-sensing performance of mesoporous NiO can be further enhanced by doping of the Fe element. It is not only related to the increase in surface area and mesoporous size compared to that of the Co-doped samples, but it is also associated with the greater acceptor content (oxygen vacancies). The present result evidently proves that the mesostrucuture periodicity control strategy by the improved nanocasting method as well as the Fe-doping method would provide a new and effective applicable strategy for enhancing the gas response of less sensitive p-type oxide semiconductors.

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AUTHOR INFORMATION

Corresponding Authors

*Phone: +86-22-27406114. Fax: +86-22-27406114. E-mail: [email protected]. *Phone: +86-22-83955661. Fax: +86-22-83955661. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by funding from the National Natural Science Foundation of China, NSFC (51202159, 51208357, 51372166, 51472179), Fund for the Doctoral Program of Higher Education, Ministry of Education of China (20120032120017), General Program of Municipal Natural Science Foundation of Tianjin (13JCYBJC16900, 13JCQNJC08200).

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REFERENCES

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