Chelating-Template-Assisted in Situ Encapsulation of Zinc Ferrite

Aug 31, 2016 - Chelating-Template-Assisted in Situ Encapsulation of Zinc Ferrite Inside Silica Mesopores for Enhanced ... *E-mail: [email protected]...
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Chelating-Template-Assisted in Situ Encapsulation of Zinc Ferrite Inside Silica Mesopores for Enhanced Gas-Sensing Characteristics Kui Niu,*,† Liman Liang,† Fei Peng,† Fan Zhang,† Yao Gu,‡ and Hongyan Tian† †

Chemistry Department & College of Life Science and Technology & Center of Instrumental Analysis, Hebei Normal University of Science and Technology, Qinhuangdao 066004, PR China ‡ The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, Jiangnan University, Wuxi 214122, PR China S Supporting Information *

ABSTRACT: A facile in situ approach has been designed to synthesize zinc ferrite/mesoporous silica guest−host composites. Chelating surfactant, N-hexadecyl ethylenediamine triacetic acid, was employed as structure-directing agent to fabricate mesoporous silica skeleton and simultaneously as complexing agent to incorporate stoichiometric amounts of zinc and iron ions into silica cavities. On this basis, spinel zinc ferrite nanoparticles with grain sizes less than 3 nm were encapsulated in mesoporous channels after calcination. The silica mesostructure, meanwhile, displayed a successive transformation from hexagonal p6mm through bicontinuous cubic Ia3̅d to lamellar phase with increasing the dopant concentration in the initial template solution. In comparison with zinc ferrite nanopowder prepared without silica host, the composite with bicontinuous architecture exhibited higher sensitivity, lower detection limit, lower optimum working temperature, quicker response, and shorter recovery time in sensing performance toward hydrogen sulfide. The significant improvements are from the high surface-to-volume ratio of the guest oxides and the three-dimensional porous structure of the composite. We believe the encapsulation route presented here may pave the way for directly introducing complex metal oxide into mesoporous silica matrix with tailorable mesophases for applications in sensing or other fields. KEYWORDS: mesoporous composite, chelating template, zinc ferrite, mesophase, hydrogen sulfide sensor

1. INTRODUCTION Gas sensors based on semiconducting metal oxides have been extensively investigated owing to their sufficient sensitivity and adequately long life span, accompanied by the benefit in terms of low cost and simplicity.1−4 Tremendous efforts, including noble metal modification, peculiar microstructure fabrication, and heterogeneous metal oxides combination, have been devoted to further improving the adsorption ability, catalytic activity, sensitivity, and thermodynamic stability of simple metal oxides.5−11 As one of the most promising candidates for gas detection, spinel-type ferrites with formula MFe2O4 have been proven to display characteristic responses toward both oxidizing and reducing gas species as well as to exhibit extremely high thermal and chemical stability in these atmospheres.12−14 Although multiple methods involving coprecipitation process, solid state reaction, combustion route, and hydrothermal/ solvothermal technique have been developed for preparing © XXXX American Chemical Society

spinel ferrites during the past decades, the Pechini-type polymerized complex approach always shows incomparable advantages in accurate stoichiometric control and good compositional homogeneity since the immobilization of mixed metal−chelate complexes in a semirigid polyester net can effectively minimize segregations of particular metals during the decomposition process of the polymer at high temperatures.15−17 Another superiority of the Pechini method is the production of particles with relatively small size and high surface-to-volume ratio, both of which have a substantial impact on the design of gas sensors because the sensitivity of metal oxides can be enhanced significantly by using materials with ultrafine Received: June 4, 2016 Accepted: August 31, 2016

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DOI: 10.1021/acsami.6b06689 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces grains.18,19 Compared with thermolabile polymeric matrices, inorganic porous supports, especially for ordered mesoporous molecular sieves, have better performance in mitigating guest nanoparticle growth and hindering interparticle aggregation based on the confinement effect of the rigid mesoporous hosts.20−22 So far, sensor materials involving metal complexes, single metal oxide, and metal salt have been encapsulated in silica mesopores to obtain guest−host composites for optical, resistive, and capacitive gas-sensing applications.23−29 Such devices have been shown to significantly improve the sensing performances toward specific gases not only because the confined active sites possess a very large surface-to-volume ratio but also because the composites with high specific surface area and interconnected mesochannels substantially increase the gas accessibility. Traditional strategies for the synthesis of silica-supported nanostructured materials include wet impregnation method and grafting pathway, both of which are multistep procedures and need to establish physical or chemical driving force to bridge the metal precursor and the silica pore wall.30,31 Although various noble metals, single metal oxides, and sulfides have been successfully embedded in mesoporous silica via the above approaches, studies on multicationic metal oxides loaded onto mesoporous supports are rarely reported. This probably is ascribed to the uncontrollable adsorption capacity or grafting ability of different metal cations on the inner surface of presynthesized hosts, resulting in a difficulty of introducing these cations in mesopores with specific proportion and sufficient compositional homogeneity, which are crucial for obtaining phase-pure complex oxide with desired chemical composition. In our previous work, we have proposed an in situ encapsulation route for fabrication of single metal oxide/ mesoporous silica composites by using an amino tricarboxylic acid based surfactant, N-hexadecyl ethylenediamine triacetic acid (HED3A), both as a chelating agent and structuredirecting agent for binding guest metallic and host siliceous precursors.32,33 The outstanding chelating ability of HED3A makes it possible to completely introduce an equimolar amount of metal ions into the cavities of the porous host. Such a strategy enables guest oxides finely dispersed in silica channels or cages after aerobic calcination. Furthermore, the host frameworks of the as-synthesized composites display diverse and tailorable mesophases, retaining the distinctive characteristics of anionic-surfactant-templated mesoporous silica (AMS). Herein, we focus specifically on extending this simple approach to embedding complex metal oxide into mesoporous silica host. Zinc ferrite (ZnFe2O4), which has outstanding performances in detecting reducing gases, is selected to be the target guest within this work for further sensor application. To verify the feasibility of this design, we first used HED3A to replace citric acid in the typical Pechini process for the purpose of obtaining phase-pure ZnFe2O4 particles uncoated with silica hosts. Further attempts were also made to construct silica skeletons templated from the micellar assemblies of metal−HED3A hybrid complex, aiming for encapsulating zinc ferrite inside silica mesopores after subsequent calcination. The microstructure and composition of these mesoporous composites together with the particular relation between the mesostructured ordering and gas-sensing performance were investigated in detail.

2. EXPERIMENTAL SECTION 2.1. Materials and Characterization. HED3A was synthesized based on the procedures from Wang et al.34 The structural characterizations of the surfactant are presented in the Figure S1a− c. N-Trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (50% in methanol) (TMAPS) was obtained from Tokyo Chemical Industry Co., Ltd. All the other chemical reagents used in the experiment were analytical grade and purchased from Sinopharm Chemical Reagent Co., Ltd. Both small-angle X-ray diffraction (SAXRD) and wide-angle X-ray diffraction (WAXRD) patterns of the as-prepared products were collected using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). A JEOL JEM-2100 instrument equipped with an energy-dispersive X-ray (EDX) spectrometer was used to record the high-resolution transmission electron microscopy (HRTEM) images and examine the composition of the composites. The specimens were prepared by dispersing the powder in anhydrous ethanol with ultrasonic vibration for 5 min and then dripping a drop onto a standard holey-carbon-covered copper TEM grid. The nitrogen adsorption−desorption isotherms were measured at −196 °C with a Micromeritics ASAP-2020 surface area and porosity analyzer, and all samples were degassed at 400 °C for 24 h under vacuum prior to the adsorption measurements. The specific surface area was estimated by the Brunauer−Emmett−Teller (BET) equation based on the adsorption isotherm, and the total pore volume was assessed from the amount adsorbed at a relative pressure of 0.99. X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Thermo ESCALAB 250 spectrometer using Al Kα (hν = 1486.6 eV) as the radiation source by referencing the C 1s peak to 284.6 eV. 2.2. Preparation of ZnFe2O4 Powder. First, 0.298 g (1 mmol) of Zn(NO3)2·6H2O, 0.808 g (2 mmol) of Fe(NO3)3·9H2O, and 1.375 g (3 mmol) of HED3A were separately dissolved in a minimum amount of distilled water at temperature of 80 °C. The solutions were mixed under constant agitation, followed by addition of 0.745 g (12 mmol) of ethylene glycol and then maintained at 80 °C for 6 h to accelerate the polyesterification. The resulting resin was dried at 110 °C for 3 h, calcined at 200 °C in flowing N2 for 1 h and finally at 600 °C in air for 6 h to obtain ZnFe2O4 powder (designated as ZFP). 2.3. Preparation of ZnFe2O4/Mesoporous Silica Composites. In a typical synthesis of ZnFe2O4/mesoporous silica composite, 0.458 g (1 mmol) of HED3A was completely dissolved into 30 mL of 0.1 M NaOH at 42 °C. An aqueous solution of Zn(NO3)2·6H2O and Fe(NO3)3·9H2O mixed at molar ratio of 1:2 was then slowly injected to the system under intense stirring. Then, the resulting solution was adjusted to a pH value of 10.0 with 2.0 M NaOH and a final volume of 35 mL with distilled water. After stirring for a further 2 h, a mixture of 1.3 mL of TMAPS and 2.0 mL of tetraethoxysilane (TEOS) was subsequently injected into the medium with vigorous stirring, and a continuous stirring for 10 min was required. The suspension was retained under static conditions at 42 °C for 48 h, and then the resultant precipitates were filtered and dried in air at 50 °C overnight, followed by successive calcination at 200 °C in flowing N2 for 1 h and at 600 °C in air for 6 h to produce the final composite. Samples prepared via different HED3A/Zn2+/Fe3+ molar ratios of 1.0:0.1:0.2, 1.0:0.2:0.4, and 1.0:0.3:0.6 are denoted as 1-ZFS, 2-ZFS, and 3-ZFS, respectively. 2.4. Fabrication and Measurement of Sensor Devices. For the fabrication of sensors, each sample was mixed with an appropriate quantity of α-terpineol and ground in an agate mortar to form a homogeneous paste. The mixture was then printed onto an alumina tube substrate which was already assembled with a pair of Au electrodes and four Pt wires on both ends of the tube. A Ni−Cr alloy coil through the tube was employed as a heater to control the operating temperature by tuning the heating voltage. The schematic diagram of the sensor unit is displayed in Figure S2. The sensor elements were dried in air and annealed at 300 °C for 1 h to evaporate α-terpineol. Each device was aged at the operating temperature for 72 h to improve its stability and repeatability prior to use. For the measurement of reproducibility and long-term stability, the device was stabilized at 300 °C for 72 h. Gas-sensing tests were performed on a B

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ACS Applied Materials & Interfaces HW-30A gas-sensing measurement system (Han Wei Electronics Co., Ltd.) at a relative humidity of 30%. The gas concentrations were calculated according to the volumes of the gas chamber (15 L) on the gas-sensing measurement system and the test gas. Predetermined amounts of test gases were introduced into the test chamber by a syringe, using ambient air as the diluting and reference gas. Two electric fans installed in the chamber are used to help form a homogeneous and stable gas environment in the test system. The working principle of the gas-sensing test is illustrated in Figure S3. The response, S, is defined as S = Rair/Rgas, wherein Rair and Rgas are the electrical resistances of sensor in dry air and in tested gas, respectively. The response and recovery time are defined as the time required for the sensor to reach 90% of its maximum response and fall to 10% of its maximum response, respectively.

speculation could be proved by estimating the average grain sizes of the unencapsulated and encapsulated ferrites with the aid of Scherrer equation, which were found to be around 15 nm and below 3 nm, respectively. SAXRD measurements of the composites were also implemented in order to determine the mesoporous features of the silica hosts. The pattern of 1-ZFS shows a main diffraction accompanied by two satellite reflections in the region of 2θ = 1.5−6° that can be indexed to the (100), (110), and (200) reflections associated with a structure possessing p6mm hexagonal symmetry.37 A minimum of three wellresolved peaks were observed from the SAXRD data of 2-ZFS, corresponding to the (211), (220), and (400) reflections of a bicontinuous cubic phase with Ia3̅d symmetry.38 In addition, at least two (00l) signals in the pattern of 3-ZFS were distinguished, indicative of a lamellar mesophase.39 The intensity of the SAXRD reflections decreases sharply with increasing the doping level, which should probably be ascribed to the decrease of the scattering contrast between the pore and the silica pore wall caused by the incorporation of heterogeneous nanoparticles.40,41 HRTEM was applied to provide intuitive observations of these composites, and some of the captured images are shown in Figure 2. In accordance with the XRD results, images in

3. RESULTS AND DISCUSSION 3.1. Microstructural and Compositional Characteristics. Sample ZFP was synthesized via a modified Pechini-type polymerized complex route which was marked by the use of HED3A as complexation reagent. As shown in Figure 1, all of

Figure 1. SAXRD and WAXRD patterns of as-synthesized powder and composites.

the diffraction peaks in the WAXRD pattern of ZFP match well with the standard pattern of spinel ZnFe2O4 and the calculated lattice parameter value of 8.447 Å is in conformity with the Powder Diffraction File No. 79-1150 (Joint Committee on Powder Diffraction Standards, [year]), showing that the asprepared ferrite has a single-phase cubic spinel structure.35,36 By comparison, additional peaks assigned to wurtzite ZnO could be observed from the pattern of powder synthesized with a nonstoichiometric Zn/Fe ratio of 0.75, while hematite phase of Fe2O3 could be detected from that with a ratio of 0.40 (Figure S4). These results validate the effectiveness of HED3A in yielding zinc ferrite through Pechini process and as such confirm that the stoichiometry of the starting elements is a critical requirement for controlling over the phase purity of the final oxide materials. In the case of mesoporous composites, besides the amorphous silica humps centered at 2θ = 22°, all the other identifiable peaks in their WAXRD patterns are entirely consistent with those of ZFP, although these peaks are relative broad and weak. This comparison verifies qualitatively that HED3A has the capability of assembling spinel zinc ferrite both in resin and silica mesopores; moreover, the rigid silica framework seems to play an enhanced role in hindering particle agglomeration during the thermal treatment process. Such

Figure 2. HRTEM images of each ZnFe2O4/mesoporous silica composite recorded along (a) [001] and (b) [110] of 1-ZFS (p6mm), (c) [111], (d) [110], and (e) [311] of 2-ZFS (Ia3̅d), and (f) of 3-ZFS (lamellar), respectively. The insets on the upper-right corners of the panels show in all cases the corresponding Fourier transform diffractograms. The insets on the lower-right corners of c and f show enlarged lattice fringes of the selected regions.

Figure 2a,b indicate that the framework of 1-ZFS consists of well-organized nanochannels with long-range 2D hexagonal ordering, while this mesophase evolves to a 3D bicontinuous cubic one for 2-ZFS (Figure 2c−e) and finally transforms to lamellar structure for 3-ZFS (Figure 2f). The approximate distances between neighboring silica pore walls of these samples are in the range of 3−4 nm. Viewing perpendicular to the hexagonal axis for 1-ZFS, as shown in Figure 2b, one can see that extremely rough inner surfaces emerge from entire pore channels, revealing an attachment of tiny ZnFe2O4 particles onto the silica walls. It is clearly discernible from Figure 2c that a thickened layer of ferrite is uniformly coated on the interior of each mesopore of 2-ZFS, and further increase of doping amount leads to a continuous growth of the layer to nearly completely filled pores, as shown in Figure 2f. The C

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Scheme 1. Illustration of HED3A-Assisted Introduction of Stoichiometric Amounts of Zinc and Iron Ions into Silica Cavities and Mesophase Evolution of the Silica Matrices

Figure 3. Nitrogen adsorption−desorption isotherms (a), BJH desorption pore distribution curves (b), and nitrogen t-plot curves (c) of ZnFe2O4/ mesoporous silica composites.

resulting mesophase evolution of the silica matrices is shown in Scheme 1. In the first instance, when HED3A is mixed with nitrates in the initial solutions, this EDTA derivative chelates Zn2+ and Fe3+ (see spectroscopic analysis of Figure S7), forming a ternary surfactant combination. It has already been demonstrated that negatively charged surfactants, with similar amino acid head groups and the same carbon chain length, exhibit practically ideal mixing behavior in aqueous solution.44,45 Accordingly, these components are theorized to be alternately arranged in the mixed micelle to achieve maximum dispersion, thereby increasing the entropy to oppose the tendency toward phase separation of the mixed micellar solution. After that, electrostatically attached TMAPS condenses with TEOS to form silica frameworks and consequently enwraps the self-assembled aggregates in the host cavities. The key to pure phase formation of guest zinc ferrite should be

interplanar lattice spacing of 0.298 nm for 2-ZFS and 0.254 nm for 3-ZFS corresponds to the respective d-values of (220) and (311) of the spinel structure.42,43 In addition, the STEM images and corresponding elemental mappings of these composites recorded in Figure S5 indicate that the elements of Zn and Fe were uniformly distributed within the silica mesopores. The elemental ratios between Zn and Fe derived from EDX analysis are almost 1:2 for these composites (Figure S6), which is the proper metal ratio in view of the guest oxide material. It is worth mentioning that the combination mode of guest species and host matrix, especially in the cases of 1-ZFS and 2-ZFS, could effectively avoid the excessive blocking of the channels so as to retain the internal connectivity of the pore system to a maximum degree. An illustration of HED3A-assisted introduction of stoichiometric amounts of zinc and iron ions into silica cavities and the D

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Figure 4. XPS survey-scanned spectrum of (a) 2-ZFS, and the fitted fine-scanned spectra of (b) Zn 2p, (c) Fe 2p, and (d) O 1s.

(IUPAC classification) accompanied by a typical H1 hysteresis loop at relative pressure from 0.35 to 0.65 that is usually associated with cylindrical type pores.48 With increasing ZnFe2O4 loadings, the adsorption step arising from capillary condensation in the mesopores gradually shrinks. Furthermore, the BJH curves in Figure 3b exhibit a broader tendency for the pore size distribution, and the corresponding t-plot curves in Figure 3c show downward deviation at lower coverage. The above results suggest that the implantation of ferrites decreases the pore diameter and hence results in the formation of more micropores.49,50 The specific BET surface areas of these samples calculated from the BET tests together with microporous surface areas and volumes evaluated by the t-plot method are summarized in Table S1. It is also worth noting that ferrite−silica composites with highly ordered structure still retained large specific surface areas, which are very important to their sensitive properties. XPS analysis was applied to the determination of chemical composition and elements states of sample 2-ZFS, whose results are shown in Figure 4. The survey-scanned spectrum in Figure 4a elucidates the coexistence of Zn, Fe, O, Si, and adventageous C in 2-ZFS. As shown in Figure 4b, the doublet peaks of Zn 2p core-level with the binding energies at 1044.8 and 1021.2 eV are in agreement with Zn 2p1/2 and Zn 2p3/2, confirming the oxidation state of Zn2+ in the present sample.51 Meanwhile, Figure 4c shows two energy peaks located at 724.8

largely attributed to the molecular-level dispersion of complexes containing stoichiometric amounts of zinc and iron, similar to performing the Pechini process on the inner surface of silica walls. It should not be ignored that the coordination between metal ions and carboxyl groups of HED3A indeed causes a decrease in electronegativity of its hydrophilic headgroup. Hence, a bigger proportion of complex in each micellar aggregate will result in a continuous reduction of repulsion between the adjacent amphiphiles, therefore bringing about a decrease in the interface curvature of the micelles. On a theoretical level, this phenomenon could also be expressed by the numerical changes of the surfactant packing parameter g, defined as g = V/(a0l), where V and l respectively represent the total volume and length of the hydrophobic moiety and a0 is the effective headgroup area at the micelle surface which will be strongly affected by the charge density on the surfactant headgroup.46,47 On that account, decreasing values of a0 cause g values to increase, thereupon favoring mesophases with progressively decreasing ordering of interface curvature, triggering the symmetry changes of silica mesostructure from hexagonal p6mm through bicontinuous cubic Ia3̅d-type to lamellar. In order to investigate the change of porosity upon the incorporation of ZnFe2O4, nitrogen adsorption/desorption data of these composites were collected and shown in Figure 3. As can be seen in Figure 3a, 1-ZFS displays a type IV isotherm E

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ACS Applied Materials & Interfaces and 711.6 eV, with two shakeup satellite peaks at 733.1 and 718.7 eV, which indicate a normal state of Fe3+ in the resulting ferrite.52 In addition, the atomic ratio of Zn to Fe calculated from XPS data is very close to 1/2, agreeing with the formula of ZnFe2O4. The asymmetric O 1s peak in Figure 4d can be fitted with three Gaussian peaks at binding energies of 533.1, 531.2, and 529.2 eV, respectively. The dominant peak at 533.1 eV corresponds to the oxygen in the host silica wall, the peak at 531.2 eV is characteristics of oxygen in the surface-adsorbed H2O or carbonate species, and the peak at 529.2 eV is assigned to typical lattice oxygen in the metal (Zn/Fe)−oxygen framework.53 3.2. Gas-Sensing Properties. It has been proven that the guest semiconducting particles inside host silica pores are separated by a network, rather than wrapped by amorphous silica; hence, they build up an interconnected web that exhibits electric conductivity to create a resistive sensor even though silica inherently is an insulator.54,55 Herein, similar to sample ZFP, the as-prepared composites exhibited typical n-type conductivity as their electrical resistances decrease when exposed to reducing gas, for instance, in the presence of 100 ppm hydrogen sulfide at the working temperature of 200 °C (presented in Figure 5). It is interesting to note that although

Figure 6. (a)Correlation between working temperature and response to 100 ppm hydrogen sulfide for ZFP- and 2-ZFS-based sensors; response−recovery curves of (b) ZFP- and (c) 2-ZFS-based sensors at each Topt. Figure 5. Change of resistance in air and in 100 ppm hydrogen sulfide for each sensor at 200 °C.

those of the sensors made of ZFP (Topt = 250 °C) and other nanozinc ferrites previously reported.56,57 The response of 2ZFS-coated sensor (S = 54.3) toward hydrogen sulfide is not so high as that of other SnO2 based sensors58,59 but improves substantially with regard to the aforementioned ZnFe2O4 based sensors. Besides, the response and recovery curves in Figure 6b,c indicate that the response time (τres) and recovery time (τrec) of 2-ZFS-based sensor (13 and 11 s, respectively) are much shorter than that of ZFP-coateed one (34 and 21 s, respectively) at each Topt. In the view of working principle of oxide semiconductor gas sensors, it is acknowledged that the receptor function, transducer function, and utility factor are generally considered as key factors that determine the gassensing performance.60,61 In our cases, the intrinsically small ZnFe2O4 particles within 2-ZFS may improve oxygen vacancy concentration on their surfaces, leading to an increased adsorption of oxygen ions and thus enhancing the receptor function. However, the tiny size of guest grains may become comparable to the thickness of the depletion region, the results of which will activate the transducer function of the sensing body. In terms of utility factor, the remaining 3D interconnected porous architecture of 2-ZFS endows rapid

2-ZFS sensing layer is more resistive than that of ZFP the response of the former sensor is approximately 1 order of magnitude higher than that of the latter. Nevertheless, sensor fabricated by 1-ZFS has the lowest response when performing the same measurement. That is most probably because the very low doping level of guest semiconductor leads to insufficient grain-to-grain contact, and consequently, the long-range electron transport inside the material is limited. In contrast, 3-ZFS has a higher doping level than 2-ZFS, but the corresponding sensor shows a lower response, possibly due to the lower gas accessibility to the surface of inner oxide grains since the enlarged guest clusters may partially block the pores so that the gas diffusion would be seriously handicapped. Considering that the response of gas sensor is greatly influenced by operating temperature, the temperature-dependence behaviors of 2-ZFS- and ZFP-based sensors to 100 ppm hydrogen sulfide were investigated. Figure 6a shows a volcanoshaped correlation with an optimum working temperature (Topt) of 150 °C for 2-ZFS-coated sensor, markedly lower than F

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The correlations between response and hydrogen sulfide concentration for both 2-ZFS- and ZFP-based sensor at each Topt are shown in Figure 8. A common observation for these

gas diffusion and adsorption toward the whole sensing surface, making it possible to accelerate the reaction of the test gas molecules with surface-adsorbed oxygen species at relatively low temperature as well as to achieve a fast gas response. To evaluate the selectivity of ZFP- and 2-ZFS-based sensors, a variety of other reducing gases including acetone, ethanol, formaldehyde, ammonia, and toluene at a concentration of 100 ppm are tested at different working temperatures. As depicted in Figure 7a, the response for ZFP-coated sensor to ethanol is

Figure 8. Gas responses of ZFP- and 2-ZFS-based sensors as a function of hydrogen sulfide concentration from 1 to 800 ppm at each Topt.

two sensors is that the sensitivity increases with increasing the concentration of hydrogen sulfide. As expected, 2-ZFS sensor has much higher response and larger amplitude of increase in response over the whole range of concentrations. Impressively, the response of 2-ZFS sensor still undergoes drastic rise when exposed to higher concentrations (200−800 ppm), whereas the ZFP sensor has become more or less saturated in that range. Furthermore, the magnified inset shows the detection limit of the 2-ZFS sensor is down to 1 ppm with a sensitivity of 1.8, which confirms that the sensor is capable of detecting hydrogen sulfide in a wide range of concentrations. Measurements for reproducibility and long-term stability of both sensors toward 10 ppm hydrogen sulfide at each Topt were also carried out. The 2-ZFS-based sensor could be continuously operated over many cycles without loss of sensitivity, and the results were reproducible within the deviation limits of ±3.5% when repeated for a period of 3 months and 20 times (Figure S8). Transient resistance−time curves of 2-ZFS-based sensor toward 100 ppm hydrogen sulfide at its Topt when operated initially and repeated after 3 months are displayed in Figure S9. The baseline resistance of this sensor remained substantially unchanged, and the response still held about 97.4% of its original value. These excellent properties are thought to be attributed to the confinement of host silica matrix that might plays a key role in decreasing the surface energy and thus improving the thermal stability of guest ZnFe2O4 nanocrystals. The outstanding sensing features of 2-ZFS lie in three aspects. First, the host silica provides an isolated and confined space for orderly arranging and effectively stabilizing the ferrite nanoparticles with high surface-to-volume ratio, which is beneficial for the sensor to present high sensitivity, good selectivity, and long-term stability. Apart from that, the 3D porous structure of the composite is conducive to gas diffusion and mass transport and therefore endows the sensor relatively short response and recovery times. Last but not least, since the response was defined as the ratio between Rair and Rgas, the encapsulation of ZnFe2O4 in silica skeleton drastically increases

Figure 7. Histogram showing response for (a) ZFP- and (b) 2-ZFSbased sensors to 100 ppm of various gases at different working temperatures.

apparently higher than that to other gases, but the corresponding Topt is as high as 350 °C. The histogram in Figure 7b shows that 2-ZFS-coated sensor also exhibits an appreciable response (S = 32.8) toward ethanol vapor at a higher operating temperature up to 250 °C. Additionally, these two sensors give much lower responses to acetone and formaldehyde and almost no response to ammonia and toluene. It is noteworthy that unlike 2-ZFS-coated sensor the maximum response to ethanol for ZFP-coated one (S = 8.2) is even higher than that to hydrogen sulfide. The discrepancy of selectivity between these two sensors may be caused by the different spatial arrangements of ferrite grains in each sensing layer. Compared with disorderly stacking of ZFP nanoparticles, the agglomerate structure of guest ZnFe2O4 nanocrystals within 2-ZFS is more like nanotube arrays confined in host matrices. Similar heterostructures are regularly used for shaping nanotube or nanowire networks through hard-templating synthesis route, and the fabricated 1D and 3D nanoarchitectures for sensor applications always exhibit unusual selectivity in contrast with bulk materials.62−64 One may speculate that the cubic Ia3d̅ silica framework should function as a “nanoreactor” on the crystal growth and orientation, by which the predominantly exposed facets of generated ZnFe2O4 nanocrystals possess higher adsorption capacity or chemical activity toward hydrogen sulfide than ethanol, especially at lower temperatures around 150 °C. Since the actual sensing process is significantly complicated, further experimental investigation is underway to verify this viewpoint. G

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and the Open Project Program of the Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, Jiangnan University (No. JDSJ2014-02).

the baseline resistance so that has positive impact on the response characteristics.65−67

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4. CONCLUSIONS Phase-pure ZnFe2O4 nanopowder has been successfully prepared utilizing HED3A as complexing agent via Pechini process. Based on this, we have devised a simple but effective procedure for implanting ZnFe2O4 nanoparticles into mesoporous silica host by further exploiting the structure-directing role of the chelating surfactant. This in situ encapsulation pathway allows extremely small guest ferrites to be monodispersed and attached to the inner surface of silica walls after aerobic calcination and enables the final composites to maintain high specific surface areas. Variation of initial metal ions concentration not only changes the doping amount of the composites but also induces an evolution of the silica mesophase from hexagonal p6mm to bicontinuous cubic Ia3̅d to lamellar. Among them, the composite with 3D cubic mesostructure can be applied as a competent sensing layer for selective detection of hydrogen sulfide at low temperature. The sensor also exhibits a short response/recovery time, remarkable sensitivity, low detection limit, and good stability and reproducibility. We hope this hybrid-chelating-templated route could provide a feasible approach to encapsulating nanosized complex metal oxides, probably not limited to ferrites, into silica mesopores in view of the molecular-level distribution of dissimilar metal ions in mesostructured silica cavities. The ultrafine size of the guest material, coupled with diverse mesophases of the host matrix, may develop unique properties for these guest−host systems so as to explore various applications in different fields.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06689. Structural characterizations of HED3A; schematic diagram of the sensor unit; working principle of the gas-sensing test; WAXRD patterns of powders synthesized with different Zn/Fe ratio; STEM images and elemental mappings of ZnFe2O4/mesoporous silica composites; EDX patterns of ZnFe2O4/mesoporous silica composites; spectroscopic analysis of the template solutions for the synthesis of ZnFe2O4/mesoporous silica composites; surface and porosity characteristics of ZnFe2O4/mesoporous silica composites; characterizations for the reproducibility and long-term stability of the sensors (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86 335 8387040.Tel.: +86 335 8387040. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (No. 21503067), the Program for the Top-notch Young Talents of Hebei Province (No. BJ2016025), the Technology Foundation for Selected Overseas Chinese Scholar of Hebei Province (No. 20100505), H

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DOI: 10.1021/acsami.6b06689 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX