NiCo2O4 Truncated Nanocages with PdO Catalyst

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Functional Inorganic Materials and Devices

NiO/NiCo2O4 Truncated Nanocages with PdO Catalyst Functionalization as Sensing Layers for Acetone Detection Tingting Zhou, Xiupeng Liu, Rui Zhang, Yubing Wang, and Tong Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12981 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018

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ACS Applied Materials & Interfaces

NiO/NiCo2O4 Truncated Nanocages with PdO Catalyst Functionalization as Sensing Layers for Acetone Detection Tingting Zhou, Xiupeng Liu, Rui Zhang, Yubing Wang, Tong Zhang* State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, PR China

E-mail address: [email protected] *Corresponding author: E-mail address: [email protected]

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ABSTRACT:

Achieving a novel structural construction and adopting appropriate catalyst materials are key to overcoming inherent limitations of gas sensors in terms of designing sensing layers. This work introduces NiO/NiCo2O4 truncated nanocages functionalized with PdO nanoparticles, which were proved to possess the ability of the effective acetone detection. The device realized an enhanced acetone-sensing sensitivity, together with excellent selectivity and longterm stability. The sensing performance is far better than sensors based on NiO/NiCo2O4 solid nanocubes and NiO/NiCo2O4 truncated nanocages without PdO decorating, which is relation to cooperative effects of the high specific surface area and efficient catalytic activity. The results provide a promising metal−organic frameworks (MOFs) derived material with the optimization of catalytic performance, demonstrating the remarkable potential for acetone sensors.

KEYWORDS: NiO/NiCo2O4, PdO catalyst, nanocages, MOFs, acetone sensor


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1.

INTRODUCTION

Chemical sensors (CSs) have occupied an extremely important position for their practical applications in patient monitoring, environmental protection and diagnostics.[1-4] Considering the increasing demand of sensing devices with high performances, advanced sensing materials have attracted growing research interest in recent years. Particularly, since Seiyama et al. first reported the gas sensing properties of metal oxide semiconductors (MOSs),[5] more attention is paid to the synthesis and design of various nanostructured MOSs. Notwithstanding many available MOSs based sensors, researchers still continue to seek sensitive materials with high response, lower detection limitation, great selectivity and fast response process. Among them, mixed transition metal oxides are proven to be outstanding candidates because of their tunable valence states, complex composition, abundant reactive sites and the synergetic effects of different metal cations.[6-8] Ni-Co mixed metal oxides are one of typical transition metal oxides with excellent electrochemical properties and applied in different areas.[9] However, the gas sensing properties are rarely reported compared to the common MOSs such as SnO2, ZnO, α-Fe2O3, TiO2 and so on.

[10-14]

Thus, it is necessary to further

explore sensing functions of Ni-Co mixed metal oxides. To improve performances of CSs, two key factors are usually considered for sensing layers, that is, the large specific surface area and the optimal component. Hollow nanostructured materials have been extensively studied for their intriguing structural features. Complex nanostructured metal−organic frameworks (MOFs) are ideal templates to prepare novel hollow structured metal oxides. After the remove of the organics, walls of the metal oxides are always mesoporous and conductive. The sensing materials with hollow structure can provide large specific surface area, well-aligned pores and rich electroactive sites, which is in favour of the adsorption and reaction of target gas.[15-19] In addition to the construction of hollow structure,


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introducing catalytic materials (Au, Ag, Pt, and Pd/PdO) can effectively promote the surface reactions though the sensitization of catalyst.[20-25] However, the catalytic activities of nanoscale catalysts are always limited due to the decrease of surface energy by the aggregation.[2] Thus, hollow structured transition metal oxides with well-dispersed, effective catalyst loading should be further developed for the superior gas sensors. With this aim, NiO/NiCo2O4 truncated nanocages decorated with PdO nanoparticles were synthesized through a facile route combined with sacrificial MOF templating and catalyst functionalization. The unique hollow architectures and tunable compositions endow the PdONiO/NiCo2O4 truncated nanocages with remarkable performances when evaluated as a new acetone sensing platform. The acetone sensing abilities of PdO-NiO/NiCo2O4 truncated nanocages were compared systematically with their counterparts (NiO/NiCo2O4 solid nanocubes

and

NiO/NiCo2O4

truncated

nanocages).

Effectively

improved

sensing

performances were discussed in relation to the functionalization of nanoscale PdO catalyst loaded on the hollow structure. 2.

EXPERIMENTAL SECTION

2.1. Materials. Nickel nitrate hexahydrate (Ni(NO3)2·6H2O), trisodium citrate dihydrate (Na3C6H5O7·2H2O) potassium hexacyanocobaltate (III) (K3[Co(CN)6]), palladium chloride (PdCl2), methanol (CH3OH) and ammonia solution (NH3·H2O) were of analytical grade and purchased from Shanghai Chemical Corp. 2.2. Synthesis Process. Synthesis of Ni-Co PBA solid nanocubes: Ni-Co PBA solid nanocubes were synthesized by a coprecipitation method as reported elsewhere.[26] 0.174g of Ni(NO3)2·6H2O and 0.264 g of Na3C6H5O7·2H2O were dissolved in 20 mL of deionized water. Different contents of


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K3[Co(CN)6] (molar ratio of Ni/Co=1, 1.5 and 2) were dispersed in 20 mL of deionized water with continuously stirring. Then, the above-mentioned solution was mixed and vigorously stirred for 5 min to form light green solution. After reacting for 7 days at room temperature, the green product was collected and washed with ethanol and deionized water at least five times. NiO/NiCo2O4 solid nanocubes were obtained by calcining at 350 °C (heating rate of 2 °C min-1) for 2 h. Synthesis of Ni-Co PBA truncated nanocages: Typically, the as-obtained solid nanocubes were uniformly dispersed in 10 mL of ethanol. 5 mL NH3·H2O was injected into 20 mL of deionized water. Then, the ammonia-water mixture was dropped slowly in the Ni-Co PBA solution and stirred for 1 h at room temperature. The as-prepared precipitate was collected by five washing−centrifugation cycles. Different NiO/NiCo2O4 truncated nanocages with Ni/Co molar ratio of 1, 1.5 and 2 were obtained by calcining at 350 °C (heating rate of 2 °C min-1) for 2 h and defined as Ni-Co-O-1, Ni-Co-O-2 and Ni-Co-O-3. Synthesis of PdO-NiO/NiCo2O4 truncated nanocages: 3 mg of NiO/NiCo2O4 truncated nanocages was well-dispersed in 5 mL methanol. Different weight percent of PdCl2 (0.5wt%, 1.5wt%, and 2.5%) was dissolved in the above solution with ultrasonic treatment, respectively. Finally, the black products were calcined at 400 °C for 1 h in air to obtain PdO-NiO/NiCo2O4 truncated nanocages. 2.3. Fabrication of Gas Sensors 30 mg of each sample (NiO/NiCo2O4 solid nanocubes, NiO/NiCo2O4 truncated nanocages, PdO-NiO/NiCo2O4 truncated nanocages) was mixed with a few drops of water and ground for 10 min. Then, the suspensions were coated on the ceramic tube, which were equipped with platinum wires and gold electrodes. A Ni-Cr alloy heat wire was in the ceramic tube to control the working temperature. Gas sensing performances were evaluated using a homemade


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measurement in different vapor conditions (C3H6O, C2H6O, CH4O, CH2O, C7H8, NH3, NO2 and CO). Before the measurement, all of the sensors were aged at 200 °C for 2 days to ensure the stability of the sensing layers. The sensor response was calculated as Rg/Ra for p-type semiconductor, where, Rg is the sensor resistance upon exposure to target vapor and Ra is the baseline resistance in air. The response/recovery time was defined as the time to reach/fall from 90% of the total resistance change. 2.4. Characterization. The X-ray diffraction (XRD) pattern and crystallographic phases of the samples were measured by Rigaku D/Max-2550 diffractometer with Cu Kα radiation. The microstructures, morphology and elemental analyses were characterized by a field emission scanning electron microscope (FESEM, JEOL JSM-7500F) with accelerating voltage of 15 kV and transmission electron microscopy (TEM, JEOL JEM-2100F) with accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) patterns were performed on an X-ray photoelectron spectrograph (XPS, ESCALAB MKK II), equipped with Mg exciting source. A CGS-8 series Intelligent Test Meter (China, ELITE TECH) was utilized to determine the gas sensing performance. The pore volumes and specific surface areas of the different products were determined by JW-BK132F analyzer. 3.

RESULTS AND DISCUSSIONS 3.1. Material Synthesis and Structural Characterization.

As the synthesis schematic illustration demonstrates (Figure 1a), the design of PdONiO/NiCo2O4 truncated nanocages is based on ‘coprecipitation-etching-wet impregnation’ strategy. First, Ni-Co PBA nanocubes were easily formed by binding of Ni ions, Co ions and organic linkers in the process of dissolution and re-deposition at room temperature (rt). The XRD pattern in Figure S1 shows that the products obtained were PBAs of Ni3[Co(CN)6]2·xH2O


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(JCPDS card no. 89-3738). Then, as etching agent, ammonia solution was added to convert solid nanocubes into hollow truncated nanocages. The ―CN ― group is closely associated with stable [Co(CN)6]3- complex. This indicates that the ammonia etching takes place differently at the Ni Ⅱ ―N≡C―Co Ⅲ sites and Ni Ⅱ ―N≡C―Co Ⅱ sites. Thus, the chemical etching of Ni–Co PBA involves the complexation between NH3 from ammonia solution and Ni cations from weaker bonding sites, which leads to the different reactivity of corner and plane surfaces of Ni–Co PBA and the formation of truncated nanocages.[27-30] After removing sacrificial templates at the high temperature, Pd ions are infiltrated into Co-Ni mixed oxides in methanol solution. Finally, NiO/NiCo2O4 functionalized wih PdO without obvious morphological changes was successfully prepared by subsequent calcination treatment. The gas sensor based on PdO-NiO/NiCo2O4 is shown in Figure 1b. FESEM and TEM images of different samples are shown in Figure 2a-e. Figure 2a1-a2 show the Ni-Co PBA solid nanocubes before calcining in air. The Ni-Co PBA solid nanocubes with sharp edges are highly uniform in size, and the mean size is ca. 300 nm. After calcining for 2 h, it is obvious that the synthesized NiO/NiCo2O4 maintains the cubic morphology of NiCo PBA solid nanocubes except for a little shrinkage and roughness of the surfaces (Figure 2b1). The image in Figure 2b2 further clearly shows the well-defined cubic shape and dense structure for NiO/NiCo2O4 solid nanocubes. More interestingly, the Ni-Co PBA particles underwent structural evolution from nanocubes to nanocages via the simple alkali-etching route. Meanwhile, the eight corners of the cubes were obvioustly etched and truncated because of the higher surface energy compared with the six flat planes (Figure 2c1). Observation in Figure 2c2, indicates the typical morphologies and hollow interior along the diagonal of nanocubes. As for NiO/NiCo2O4 truncated nanocages, the surfaces become sunk


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and rougher (Figure 2d1-d2). After loading with PdO, NiO/NiCo2O4 nanocages are homogeneously encapsulated by large-scaled, well-dispersed 0D nanoparticles, indicating the formation of PdO-NiO/NiCo2O4 (Figure 2e1-e2). The PdO-NiO/NiCo2O4 nanocages with mesoporous shells of ca. 10 nm in the thickness inherite the original hollow shape. Rich voids and interspaces can be observed from the view of TEM images, also revealing the porous property that are potentially beneficial for applications of CSs. The HRTEM images in Figure 2f-g show two kinds of distinct lattice fringes, which are close to the spacing of the (311) and (111) planes of NiCo2O4 and NiO. For the PdO-NiO/NiCo2O4, the lattice spacing of 0.26 nm agrees well with the (101) plane of PdO phase (Figure 2h). The corresponding SAED patterns in the inset of Figure 2f-h confirm polycrystalline characteristics of NiO/NiCo2O4 solid nanocubes, NiO/NiCo2O4 truncated nanocages and PdO-NiO/NiCo2O4 truncated nanocages. The specific surface area (SSA) and pore size distribution (PSD) of different Ni-Co mixed oxides were analyzed by the nitrogen adsorption/desorption isotherm (Figure 2i-k). Accordingly, the SSA of NiO/NiCo2O4 truncated nanocages and PdO-NiO/NiCo2O4 truncated nanocages are 143.8 m2/g and 134.2 m2/g, which are significantly higher than that of the NiO/NiCo2O4 solid nanocubes (52.5 m2/g) (Figure 2i1−k1). As a result, NiO/NiCo2O4 sensing materials with higher SSA were obtained by the construction of hollow structures. The peaks of PSD of the NiO/NiCo2O4 solid nanocubes, NiO/NiCo2O4 truncated nanocages and PdONiO/NiCo2O4 truncated nanocages centered at 5.1, 3.0, and 3.8 nm, respectively (Figure 2i2−k2). The crystal structures of NiO/NiCo2O4 solid nanocubes, NiO/NiCo2O4 truncated nanocages (Ni-Co-O-1, Ni-Co-O-2 and Ni-Co-O-3) and PdO-NiO/NiCo2O4 truncated nanocages were investigated by XRD analysis (Figure S2 and Figure 3), which reveals the form of NiO (JCPDS no. 47-1049) and the NiCo2O4 (JCPDS no. 20-0781). However, the diffraction peaks of PdO


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were not observed in the XRD parttern because of the small amount of PdO, which is below the detection limit of XRD. To further investigate the element types and distribution, EDS elemental mapping results were given, as revealed in Figure 4a-c. Figure 4a shows the coprecipitation process to synthetize NiO/NiCo2O4 solid nanocubes. The TEM images and elemental mapping of an individual NiO/NiCo2O4 solid nanocube are shown in Figure 4a1-a4, which confirm the homogeneous distribution of Co, Ni and O and the solid inner structure. Figure 4b describes the evolution from solid to hollow structure of NiO/NiCo2O4 materials.. Figure 4b1-b4 also prove the coexistence of Co, Ni and O in the prepared hollow nanocages. For PdO-NiO/NiCo2O4 nanocages, elements of Co, Ni, Pd and O were distributed along asformed cubic nanocages. Particularly, uniform dispersion of active Pd component is very important for its sensitization and catalytic activity. In addition, EDS elemental spectra also confirms the PdO doped NiO/NiCo2O4 components and the final mass contents of Pd for different samples are about 0.31, 1.37 and 2.26 wt% (C, Si and Pt elements are derived from the substrate) (Figure S3). The samples were marked as S1, S2 and S3, respectively. XPS was performed to investigate the chemical binding state of PdO-NiO/NiCo2O4 samples as shown in Figure 5a-d. The Co 2p spectra (Figure 5a) can be well-fitted with two spin−orbit doublets indexing to Co2+ (781.7 and 796.7 eV) and Co3+ (780.1 and 795.2 eV), respectively.[31-33] Similarly, in XPS spectra of Ni 2p (Figure 5b), four peaks belonging to Ni2+ and Ni3+ and two shakeup satellites (denoted as “sat” ) were divided. Among them, the two fitting peaks of Ni 2p at 854.5 and 872.1 eV are indexed to Ni2+, and the peaks at 856.1 eV and 873.5 eV are related to Ni3+.[7] Pd 3d spectrum (Figure 5c) revealed two characteristic peaks of 3d5/2 and 3d3/2 at bending energies of 336.9 eV and 342.2 eV, which corresponds to the Pd2+ state. [34-35] A minor peak of Pd4+ of 3d 5/2 represented Pd4+.[36] Figure 5d shows the O 1 s spectra, which can be best fitted into three peaks at 529.9 eV (lattice oxygen,


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denoted as Olatt), 531.7 eV (deficient oxygen, denoted as Odef) and 532.1 eV (adsorbed oxygen, denoted as Oads), respectively. 3.2. Measurement of Gas Sensor. Acetone is colorless liquid which is used widely in laboratories and industries as a common reagent. It is poisonous and harmful to human health. The inhalation of 300–500 ppm of acetone for 5 min can cause slight irritation for the human body. The exposure to high concentrations of acetone could cause some symptoms like headache, muscle weakness, nausea and so on. Thus, it is very necessary for the sensitive and selective detection of acetone gas to ensure the people’s health and security.[37] To investigate the effect of the components, hollow structures and PdO doping, acetone-sensing responses of the sensors were measured in the temperature range of 180−220 °C (Figure S4 and Figure 6a). The sensor based on NiO/NiCo2O4 nanocubes (Ni/Co=1.5) displayed negligibly low responses (Rg/Ra =1.2−2.2) to 100 ppm acetone. After hollowing the nanostructure, the response of NiO/NiCo2O4 truncated nanocages was slightly improved. In particular, the sensors based on Ni-Co-O-2 sample showed the highest acetone response (Figure S4). Thus, Ni-Co-O-2 was chosen to be the best sample to investigate the effects of PdO loading. Then, we optimized loading amounts of PdO catalyst, because insufficient or excess catalyst can impede the further improvement of sensing performances. As shown in Figure S5, The S-2 sample exhibited the highest acetone-sensing responses (Rg/Ra = 6.7 ) at 210 °C, which clearly verify the acetone responses can be markedly enhanced by the combining effects of hollow nanostructure and the catalyst functionalization. The response and recovery time were calculated for the PdO-NiO/NiCo2O4 (S-2) sensor (τres < 20 s, τrecov < 30 s) (Figure S6). A comparison of the acetone-sensing performances including the response and response time between PdO-NiO/NiCo2O4 truncated nanocages and other MOSs reported in literatures is


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summarized in Table 1.

[38-41]

Obviously, the acetone response was higher and the response

time was much shorter, indicating the great potential of the sensing materials. The dynamic acetone-sensing characteristics were investigated at 210 °C for NiO/NiCo2O4 solid nanocubes, NiO/NiCo2O4 truncated nanocages (Ni-Co-O-2) and PdO-NiO/NiCo2O4 truncated nanocages (S-2) (Figure 6b). The 2.1/3.4-fold improved acetone-response of PdO-NiO/NiCo2O4 compared to NiO/NiCo2O4 nanocages without PdO decoration and NiO/NiCo2O4 nanocubes was observed. The baseline resistance is increased (NiO/NiCo2O4 solid nanocubes: 1.2 kΩ, NiO/NiCo2O4 truncated nanocages: 3.1 kΩ, PdO-NiO/NiCo2O4 truncated nanocages: 4.0 kΩ). That is because the hollow nanocages can provide the larger SSA for charge transport. Meanwhile, when PdO was introduced, a mass of heterointerfaces (PdO/NiO, PdO/NiCo2O4, NiO/NiCo2O4) are created in the sensing layer, resulting in the formation of the potential barriers and increase of resistance in air. It is worthwhile noting that the PdO-NiO/NiCo2O4 based sensors shows the lower resistance compared with the PdO-doped MOSs reported in the literatures in spite of a slight increase in sensor resistance by PdO loading.[24,42-46] The very low resistance and working temperature suggest that the PdO-NiO/NiCo2O4 based sensors is a promising material for low-power gas sensors. To further compare the acetone sensing characteristics, responses were measured in the range from 10 ppm to 1000 ppm. In stark contrast, the PdO-NiO/NiCo2O4 sensor exhibited the high acetone sensing ability (Figure 6c). Furthermore, from 10-100 ppm, the response to acetone shows a linearly increasing trend, as shown in Figure S7. The detection range (10– 100 ppm) satisfies the threshold limit value (TLV) and immediately dangerous to life or health (IDLH) detection limit (750 ppm /20000 ppm) for acetone according to the guidelines issued by the American Conference of Governmental Industrial Hygienists (ACGIH) and National Institute of Occupational Safety and Health (NIOSH).[37] The selectivity of the different sensors


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was investigated by examining various analytes including acetone (C3H6O), ethanol (C2H6O), methanol (CH4O), formaldehyde (CH2O), toluene (C7H8), ammonia (NH3), nitrogen dioxide (NO2) and carbon monoxide (CO) (Figure 6d). The response to 100 ppm acetone is higher than that of other gases with minor cross-sensitivities (Rg/Ra

NiCo2O4 Truncated Nanocages with PdO Catalyst

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