A high-sensitivity hydrogen gas sensor based on Ag nanoparticle-decorated porous Co 3 O 4 nanorods

. In this report, Ag nanoparticle-decorated cobalt carbonate hydroxide microflowers were synthesized using a simple hydrothermal approach without using surfactants, which were used as precursors to prepare Ag nanoparticle-decorated porous Co 3 O 4 nanorods through an annealing process. The porous Co 3 O 4 nanorods were composed of small primary nanoparticles with a size of ~ 10 nm, and their surface was decorated with uniform Ag nanoparticles (~ 10 nm). Sensors based on the porous Co 3 O 4 nanorods decorated with Ag nanoparticles have higher sensitivity and selectivity to H 2 gas than other reduced gases, as well as rapid response-recovery times. The enhanced H 2 sensing properties of the sample may be attributed to the excellent catalytic features of Ag nanoparticles and unique porous Co 3 O 4 nanorods. The results demonstrated the potential of Ag nanoparticle-decorated porous Co 3 O 4 nanorods as sensing materials for the detection of hydrogen gas at low temperatures.


INTRODUCTION
Metal oxide semiconductors (MOS), which include p-type and n-type semiconductors, have been considered promising sensing materials for the detection of toxics and flammable gases.This is owing to their fascinating advantages such as low cost, good stability, good repeatability and high response, rapid response-recovery times, and easy integration [1].Until now, n-type metal oxide semiconductor-based sensors have been widely investigated, whereas the gas sensing properties of p-type metal oxide semiconductors have received less attention [2].Recent reports indicated that p-type metal oxide semiconductor-based sensors have distinct merits such as low working temperature, good selectivity, good anti-humidity feature, and high thermal stability [3].Therefore, the development of p-type MOSs-based sensing materials to explore their new gas sensing performance is very necessary.Among many p-type MOSs, cobalt oxide (Co 3 O 4 ) with an indirect band gap of about 1.6-2.2eV is of special interest owing to its potential applications such as catalysts, magnetic materials, in particular gas sensors, due to their excellent catalytic properties, multivalent characteristics and abundant adsorbed oxygen [4].Wen et al. [5] synthesized rhombus-shaped Co 3 O 4 nanorod arrays-based sensors that showed the highest sensitivity to ethanol because of good contact, porous structure, good crystallinity, high surface-to-volume ratio and open space.Geng et al. [6] reported that the Co 3 O 4 nanosheets showed superior sensitivity to CO and CH 4 at room temperature, as well as good reproducibility and fast response/recovery times.Hoa et al. [7] indicated that the meso-and macroporous Co 3 O 4 nanorods-based sensors showed high response and selectivity to acetone with fast response-recovery times.Many synthetic strategies have been developed to prepare Co 3 O 4 nanostructures.Among them, the Co(CO 3 ) 0.5 (OH) .0.11H 2 O nanostructure, which was an important candidate for various areas [8], has been considered a potential precursor for designing porous Co 3 O 4 nanostructures.Recently, we have synthesized Co 3 O 4 nanochains that showed good detection of H 2 S with high sensitivity and rapid response time [9].However, there are few reports on the H 2 gas sensing properties of Co 3 O 4 nanostructures-based sensors.
Hydrogen is being investigated as a sustainable energy carrier because of its high energy density and environmental benignity.In addition, hydrogen is employed in numerous industrial fields, such as metallurgy, glass, electronics, chemicals, and the production of textile fibers [10].However, H 2 is a flammable and explosive gas, which becomes explosive when its concentration increases beyond 4 %.[7].Therefore, to avoid an explosion, it is essential to develop novel sensors with high sensitivity and rapid response-recovery times to monitor and detect hydrogen at low concentrations.
The use of noble metal nanoparticles such as Pd, Au and Ag decoration on the MOS nanostructures is a potential route to improving their H 2 gas sensing performance due to the catalytic effect of noble metal nanoparticles [11].For example, the homogeneous and optimal distribution of the p-type PdO nanoparticles in n-type WO 3 film sensors demonstrated high sensitivity and low detection limit toward H 2 gas at 160 o C [12].The Pd modified SnO 2 nanoparticles sensors remarkably upgraded the H 2 sensing performances compared with the pristine SnO 2 gas sensor, which could originate from the catalytic effect and resistance modulation [13].The Pd-WO 3 nanoparticle sensors could detect H 2 at room temperature with a remarkably high response and selectivity [14].The Ag-decorated ZnO nanocrystallines possessed excellent H 2 gas-sensing properties with a high gas response, fast response-recovery times, good repeatability and stability owing to the catalytic activity of Ag nanoparticles, which led to more active sites for the reaction of hydrogen on the surface of sensors [15].Among noble metals, silver is the cheapest.Therefore, the Ag nanoparticle-decorated MOSs have been considered an effective method for designing novel sensing materials for detecting H 2 gas at lower cost.
In this report, we used a template-free hydrothermal approach for fabricating Co(CO 3 ) 0.5 (OH) .0.11H 2 O microflowers, which were assembled by many nanorods.The microflowers obtained were decorated with Ag nanoparticles by the chemical reduction of Ag + ions using NaBH 4 .As a result, the Ag-decorated Co(CO 3 ) 0.5 (OH) .0.11H 2 O microflowers were annealed in air to form Ag-loaded porous Co 3 O 4 nanorods.Gas sensing characteristics of assynthesized nanomaterials showed that the Ag-decorated porous Co 3 O 4 nanorods exhibited high response toward H 2 , with selectivity and fast response-recovery times.

Materials
Cobalt nitrate (Co(NO 3 ) 2 .6H 2 O, silver nitrate (AgNO 3 ) and urea (CO(NH 2 ) 2 were purchased from Sigma-Aldrich.All reagents were of analytical grade and used without further purification.

Synthesis of Ag nanoparticles-decorated porous Co 3 O 4 nanorods
Ag-decorated porous Co 3 O 4 nanorods were fabricated via a simple and scalable hydrothermal method without using templates.First, Co(CO 3 ) 0.5 (OH) .0.11H 2 O microflowers were synthesized using a slightly modified protocol of our recent report [9].In a typical experiment, urea (0.075 mol) and Co(No 3 ) 2 •6H 2 O (0.001 mol) were added to 70 mL of deionized water, followed by stirring for 20 min to form a clear solution.This solution was transferred to a 100 mL Teflon-lined stainless steel autoclave.The autoclave was aged at 60 •C for 12 h, and then the reaction temperature was raised to and kept at 90 •C for 8 h.The Co(CO 3 ) 0.5 (OH) .0.11H 2 O microflower sample was collected by centrifugation, and washed with distilled water and ethanol several times, and finally dried at 60 •C for 24 h.To decorate the Co(CO 3 ) 0.5 (OH) .0.11H 2 O microflowers with Ag nanoparticles, 1 g of cobalt carbonate hydroxide microflowers was dissolved in 70 mL of DI.Then, 2 mL of 1 % AgNO 3 solution was added to this suspension, followed by stirring for 30 min.After that, 2 mL of 0.2 % NaBH 4 solution was slowly added to the above mixture, which was then stirred for 3 h.The products were collected by centrifugation and washed with distilled water and ethanol several times.The Ag nanoparticles-decorated Co(CO 3 ) 0.5 (OH) .0.11H 2 O microflower sample was dried at 60 o C for 24 h and subsequently annealed at 600 o C for 3 h with a heating rate of 5 •C min -1 to form Ag nanoparticles-decorated porous Co 3 O 4 nanorods.

Instrument and characterization of the samples
Transmission electron microscopy (TEM) images and high-resolution TEM (HRTEM) images were measured on a JEOL and JEM 1230, respectively.Scanning electron microscopy (SEM) images were performed on a JSM-5300LV.Phases of the samples were characterized by X-ray diffraction (XRD, D8 Advance, Brucker, Germany) with a Cu-Kα line source (λ ~ 1.5406 Å).Infrared spectra were recorded using a Nicolet 6700 FTIR Spectrometer.Nitrogen adsorption/desorption isotherms of Ag nanoparticles-decorated porous Co 3 O 4 nanorods were obtained using a Micromeritics at 77 K.The specific surface area (S BET ) of the samples was determined by the N 2 adsorption-desorption isotherm using the Brunauer-Emmett-Teller (BET) equation, and the desorption isotherm was used to determine pore size distribution using the Barret-Yoyner-Halender (BJH) method.

Gas sensing testing
The Ag nanoparticle-decorated Co(CO 3 ) 0.5 (OH) .0.11H 2 O microflower precursors were dispersed in ethanol and then drop-casted onto an interdigitated electrode substrate using a micropipette.Thereafter, sensors were heat-treated at 600 •C for 3 h to obtain Ag nanoparticledecorated porous Co 3 O 4 nanorods as well as to increase adhesion between sensing materials and substrates.Gas sensing properties of nanocomposites were tested with several reduced gases including H 2 , CO, C 2 H 5 OH and NH 3 at different operating temperatures (150 -250 o C) using a homemade setup with high-speed switching gas flow (from/to air to/from balance gas).The air or target gases were directly exposed to the surface of the sensors through a quart tube at a constant flow rate of 200 sccm".Balance gases (0.1 % in air) were purchased from Air Liquid Group (Singapore).During sensing measurements, resistance of sensors was automatically recorded through a Keithley controlled by a computer via a software program.Sensor response was calculated as S = R gas /R air , where R gas and R air are the form of sensor resistance in the presence of analytic gas and in air, respectively.The response time (τ resp ) and recovery time (τ recov ) were defined as the times required to achieve 90 % resistance to transformation when exposed to gas and air, respectively.The phase purity and surface stages of as-synthesized nanomaterials were characterized by XRD and FTIR, as shown in Figure 1.For Ag nanoparticle-decorated Co(CO 3 ) 0.5 (OH) .0.11H 2 O microflowers, the XRD patterns of the precursor sample correspond to typical diffraction peaks of orthorhombic Co(CO 3 ) 0.5 (OH) .0.11H 2 O phase (JCPDS card No. 48-0083) (Figure 1a).Meanwhile, the FTIR of the sample (Figure 1b) shows bands at 1498, 834, 746, and 657 cm -1 , which may be related to the stretching vibrations of (OCO 2 ), δ(CO 3 ), δ(OCO), and ρ(OCO), respectively.The peak at 1351 cm -1 is related to the symmetric stretching of  s (COO -).The bands centered at 576 and 972 cm -1 are attributed to the vibrations of ρ w (Co-OH) and δ(Co-OH), respectively.The broad band at 3488 cm -1 indicates the presence of water in the sample [16].After the annealing process, the XRD pattern of the calcined sample shows that all peaks match well with the cubic spinel Co 3 O 4 phase (JCPDS card No. 43-1003).Additionally, there are two diffraction peaks at 38.6 o and 77.5 o that are representative of the (111) and (311) planes of silver crystals [17].The FTIR of the calcined sample indicates that these characteristic peaks of Co(CO 3 ) 0.5 (OH) .0.11H 2 O disappear, and in the meantime, two characteristic peaks of Co 3 O 4 are clearly observed (Figure 1d).The band at 673 cm -1 originates from the stretching vibration mode of the tetrahedrally coordinated Co 2+ -O bound, and the peak at 567 cm -1 is assigned to the octahedrally coordinated Co 3+ -O bound [18].Meanwhile, the bands at 1628 and 3432 cm -1 are attributed to the bending mode of the H-O-H and O-H of water in the sample [19].These results confirm that Ag-decorated Co(CO 3 ) 0.5 (OH) .0.11H 2 O was thoroughly transformed to Co 3 O 4 with a great purity by annealing at 600 o C for 5 h.To determine the morphology and structure of the precursor sample, the SEM, TEM and HRTEM analyses were performed and the results obtained are shown in Figure 2. The low magnification SEM image (Figure 2a) of Ag nanoparticle-decorated Co(CO 3 ) 0.5 (OH) .0.11H 2 O composites shows that the sample consists of flower-like microstructures assembled by many nanorods.The length and diameter of a primary nanorod are about 2 µm and 30 nm, respectively.From the higher magnification SEM image (Figure 2b), uniform Ag nanoparticles with a size of ~ 10 nm decorated on the surface of cobalt carbonate hydroxide hydrate nanorods are clearly observed.The TEM image (Figure 2c) of the precursor sample further demonstrates that the Ag nanoparticles are homogeneously distributed on the surface of cobalt carbonate hydroxide hydrate nanorod.The HRTEM image of the sample (Figure 2d) reveals well-arranged lattice fringes with an interplanar spacing of 0.30 nm which corresponds well to the (300) crystal plane of Co(CO 3 )0.5(OH)0.11H 2 O, whereas the lattice spacing of 0.23 nm coincides with the (111) plane of Ag.

RESULTS AND DISCUSSION
The morphology of the calcined sample changed significantly when compared to the Agdecorated Co(CO 3 )0.5(OH)0.11H 2 O precursor, as shown in Figure 3.The Co(CO 3 )0.5(OH)0.11H 2 O flower-like microsphere architecture is broken, and the primary Co(CO 3 )0.5(OH)0.The average primary Co 3 O 4 particle size is about 10 nm, which is the same particle size as Ag nanoparticles.So it is difficult to distinguish Co 3 O 4 nanoparticles and Ag nanoparticles in a porous Co 3 O 4 nanorod decorated Ag nanoparticles (Figure 3c).However, comparing the TEM image of the precursor (Figure 2c) with the TEM image of the calcined sample, the silver nanoparticles are the ones coated on the surface of porous nanorods, as shown in Figure 3c.As can be seen, the dark region and bright region in the TEM image confirm the formation of highly porous structures in an individual Ag-decorated Co 3 O 4 nanorod.The HRTEM image (Figure 3d) of the sample clearly exhibits a lattice spacing of 0.46 nm that matches the (111) crystal plane of cubic spinel CO 3 O 4 phase, whereas the lattice spacing of 0.23 nm corresponds to the (111) plane of metallic Ag.The formation mechanism of Co(CO 3 )0.5(OH)0.11H 2 O microflowers, porous Co 3 O 4 nanorods and Ag nanoparticles can be explained according to the following reactions [20,21]:  The highly porous architecture of Ag nanoparticle-decorated Co 3 O 4 nanorods was further characterized by nitrogen adsorption-desorption isotherm (Figure 4). Figure 4a shows the N 2 adsorption-desorption isotherm of the sample, which indicates the H3 hysteresis loop with a type IV isotherm according to the IUPAC classification, demonstrating the presence of a mesoporous structure.Such a highly porous nanorod gives rise to a high BET specific surface area of 43 m 2 /g.The BJH pore size distribution plot of as-synthesized Ag-decorated porous Co 3 O 4 nanorods exhibits a pore size range of 3-80 nm and a maximum peak pore diameter of about 17 nm (Figure 4b).The highly porous structure of Ag-decorated Co 3 O 4 nanorods can be attributed to the porosity of interparticles.As-fabricated porous Ag nanoparticle-decorated Co 3 O 4 nanorods with a large surface area may favor their new gas sensing properties.To evaluate the potential applicability in the gas sensing field of Ag nanoparticle-decorated porous Co 3 O 4 nanorods, four reduced gases, including H 2 , CO, C 2 H 5 OH and NH 3 were used.The H 2 gas sensing properties of the sensors are given in Figure 5, which shows the test results with various H 2 concentrations (25 -500 pm) at different working temperatures to find out the optimized operating temperature.As shown in Figure 5(a-c), the sensors respond and recover quickly to H 2 gas at all working temperatures ranging from 150 to 250 o C. At 200 o C, the sensors exhibit the highest response to H 2 gas, which increases from 2.2 to 4.8 with an increment of the H 2 concentration from 25 to 500 ppm (Figure 5d).The influence of operating temperature on the H 2 response of the Ag nanoparticle-decorated porous Co 3 O 4 nanorod sensor can be explained according to the kinetics and mechanism of gas adsorption and desorption on the sensor's surface.When the operating temperature is lower than the optimal temperature, chemical activation of sensing materials is weak, leading to a low response.By increasing the temperature, the optimal temperature can enhance the oxygen ionization and the reaction between oxygen species and hydrogen molecules, contributing to an increase in the response.When the operating temperature is higher than the optimal working temperature, the adsorbed O 2 and H 2 molecules on the sensor's surface may quickly escape before their reaction, thus the response will decrease correspondingly [22].Figure 5e shows the logarithm profile of the sensor response and H 2 concentration (25 -500 ppm).The results show good linearity, indicating that Ag nanoparticle-decorated porous Co 3 O 4 nanorods are promising sensing materials for detecting H 2 gas with a large dynamic range.Additionally, the sensors display rapid response and recovery times.At 200 o C, their response and recovery times for 500 ppm H 2 are 6 and 12 s, respectively (Figure 5f).Compared to other sensors, the sensor in this report shows higher response and faster response-recovery times (Table 1).
The Ag nanoparticle-decorated porous Co 3 O 4 nanorod sensors were also tested with CO, C 2 H 5 OH and NH 3 gases.Figure 6(a-d) presents the dynamic CO sensing graphs of the nanosensors as a function of CO concentration (1-200 ppm) at different operating temperatures.The Ag nanoparticle-decorated porous Co 3 O 4 -based sensors show the best detection of CO gas at 200 o C, which is similar to that of H 2 gas.At the optimal working temperature, their sensitivities increase from 1.4 to 2.8 with a range of CO concentration from 1 to 200 ppm; these values are lower than those of H 2 gas.The sensors also show insensitivity to C 2 H 5 OH and NH 3 gases, which increase slightly from 1.4 to 2.6 and from 1.3 to 2.1 for C 2 H 5 OH (10-250 ppm) and NH 3 (5-100 pm), respectively.The sensitivity of sensors to H 2 gas is significantly higher than that of other gases, which indicates that the Ag nanoparticles-decorated porous Co 3 O 4 nanorods present good selectivity and potential as a candidate sensor for H 2 gas detection.The superior sensing performance of the Ag nanoparticle-decorated porous Co 3 O 4 nanorods for H 2 over other gases is possibly due to the excellent catalytic feature of Ag nanoparticles, which promotes the dissociation of H 2 molecules into atoms, which is beneficial for the oxidation of H 2 molecular gas at low temperatures [29].Furthermore, the highly porous Co 3 O 4 nanorods have a large exposure of surface atoms, providing more active sites for the absorption of hydrogen molecules and hence promoting the surface reaction [30].

CONCLUSIONS
In summary, Ag nanoparticle-decorated porous Co 3 O 4 nanorods were synthesized by a simple method using Ag nanoparticle-decorated Co(CO 3 ) 0.5 (OH) .0.11H 2 O microflower precursors.The Co 3 O 4 nanorods with a highly porous structure were assembled by many primary Co 3 O 4 nanoparticles, and their surface was decorated by uniform Ag nanoparticles.The Ag nanoparticle-decorated porous Co 3 O 4 nanorod-based sensors showed promising detection of H 2 gas with high response, good selectivity and fast response and recovery times.The combination of the excellent catalytic features of Ag nanoparticles and highly porous Co 3 O 4 nanorods may be attributed to the enhanced H 2 gas sensing performance of the sensors.
11H 2 O nanorod precursor with a smooth surface are transferred into roughsurfaced porous Co 3 O 4 nanorods (Figure 3a-b).At high temperatures, the phase transition from Co(CO 3 )0.5(OH)0.11H 2 O to Co 3 O 4 may cause the flower-like structure to collapse.The TEM image of an individual porous Co 3 O 4 nanorod indicates that the nanorods are formed by interconnecting nanoparticles (Figure 3c).

Figure 4 .
Figure 4.The nitrogen adsorption/desorption isotherms (a) and BJH pore size distribution profiles (b) of the Ag nanoparticle-decorated porous Co 3 O 4 nanorods.

Figure 5 .
Figure 5. H 2 gas sensing properties of Ag nanoparticles-decorated porous Co 3 O 4 nanorods sensors: Response transients to various H 2 concentrations at different working temperatures: (a) 150 o C, (b) 200 o C and (c) 250 o C; Response as a function of H 2 gas concentration (d); Relationship between sensitivity and H 2 concentration (e), and response-recovery times for 500 ppm H 2 at 200 o C (f).

Table 1 .
Comparison of hydrogen gas sensor performance with reported data.