Programmed Synthesis of Sn3N4 Nanoparticles via a Soft Chemistry

Jan 3, 2017 - When innovatively utilized as sensing material for a gas sensor, Sn3N4 .... Tin nitride nanoparticles were prepared by a soft urea route...
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Designed Synthesis of Sn3N4 Nanoparticles through Soft Urea Route with Excellent Gas Sensing Properties Fengdong Qu, Yao Yuan, and Minghui Yang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03435 • Publication Date (Web): 03 Jan 2017 Downloaded from http://pubs.acs.org on January 6, 2017

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Fengdong Qu‡, Yao Yuan‡ and Minghui Yang * Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China. Email: [email protected]; Tel/Fax: +86 411 82463026 ABSTRACT: Metal nitrides are a significant class of multifunctional materials that have attracted a huge and everincreasing interest for their new structural and redox chemical, as well as physical characteristics. In this work, we present designed synthesis of Sn3N4 nanoparticles through soft urea route for the first time. The strategy includes the synthesis of gel-like tin-urea precursor and subsequent transformation to Sn3N4nanoparticles via thermal treatment of the as-prepared precursor under NH3 flow. Various techniques were employed to characterize the structure and morphology of the asprepared Sn3N4 samples. When innovatively utilized as sensing material for gas sensor, Sn3N4nanoparticles exhibited high sensitivity, excellent cyclability, and long-term stability to ethanol at the operating temperature of 120 °C, which is lower than those of metal oxides-based ethanol sensors. This research work provides an efficient method for preparing Sn3N4nanoparticles that are promising sensing materials for ethanol gas sensor.

Metal nitrides are of great interest because of their high electrical conductivity, high chemical and thermal stability, narrow band gap and high electron density.1 Recently, a significant development of new synthetic methods for new nitride materials has been carried out. However, it has been observed that; most of the adopted synthetic methods for the nitride materials are still very complex with large thermodynamic barriers based on bonding or breaking of N≡N bonds (945KJ/mol). In addition, many nitrides easily transform into oxides and hydroxides because of the air and moisture sensitivity,2 and these factors result into scarcity of nitride compounds and limited synthetic methods. Generally, the common methods use for the synthesis of metal nitrides are: (1) heating metals (mixed with carbon in some case) at high temperature under flow of nitrogen or ammonia3; (2) ammonolysis of oxides,4, 5 chlorides6 and sulfides7 with ammonia; (3) decomposition from nitrogen compounds such as azides.8, 9And there are also other ways to prepare nitride, including vapor deposition of films,10 solid state metathesis reaction,11 solvothermal synthesis8 and so on. These types of synthetic methods usually lead to only highly thermal stable nitrides, limited classes of compounds and purities of the obtained products are low even under harsh conditions.12 Soft urea pathway is an easy method to prepare metal nitrides and shows a promise for broader application and scalability because it is simple, nontoxic, flexible and adaptable. Tin nitride, Sn3N4, adopts the spinel structure and belongs to the covalent nitrides, which is semiconductor and has limited thermal stability.13 It appears to be of in-

terest because of its potential applications in the development of many fields, such as optoelectronic devices,14, 15 batteries,16 and microelectronic devices16 due to their promising semiconductive properties, electrochromic properties and electron transport properties.17, 18 However, there are few reports about the research on the synthetic method, and tin nitride was first prepared by electrical discharge method in 1909.19 Since then, a number of researches have been carried out on its properties and promising applications in both bulk and thin-film forms. The most widely studied preparation method is thin film. Crystalline Sn3N4 thin films can be prepared by chemical vapor deposition (CVD),20, 21 and reactive ion plating and reactive sputtering techniques.14, 15, 22. As far as we know, one of the only two methods of bulk preparation still in use was achieved by Maya’s synthesis and the modification of this route, which reacts SnX4 (X=Br, I) with KNH2 in liquid ammonia (-40oC), followed by thermal decomposition of the resultant polymer and purification process to remove by-produced tin metal with HCl (aq).23 However, the above synthetic strategies for Sn3N4 are complex, environmental unfriendly and highly cost. Thus, a simple and efficient strategy for synthesis of Sn3N4 with welldefined nanoparticles is highly desired. Solid-state gas sensors can widely be used in environmental monitoring, controlling chemical process and personal safety.24 Nitride semiconductors are a promising class of sensing materials for gas sensors and are expected to exhibit excellent sensing properties due to their better electrical transport properties. More importantly, nitride can provide a great opportunity for new materials discov-

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ery, due to the less explored compared to other semicon-

ductors

gas

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materials.

In

recent

years,

Scheme 1. Schematic illustration of the formation process of Sn3N4 nanoparticles. Step I: the preparation of tin-urea precursor; Step II: Transformation of tin-urea precursor into Sn3N4 by thermal annealing of the as-prepared precursor in NH3 ambient. some metal nitrides have shown promising performances for gas sensors, including work reported by Kao et al.25 on indium nitride, and by Yun et al.26 on gallium nitride. To the best of our knowledge, no study on tin nitride as gas sensing material has been reported. Herein, we demonstrate for the first time a generally applicable strategy for the efficient synthesis of Sn3N4 nanoparticles through soft urea route. The strategy includes; the synthesis of gel-like tin-urea precursor and subsequent transformation to Sn3N4 nanoparticles via thermal annealing of the as-prepared precursor under ambient NH3. As a potential application, Sn3N4 nanoparticles are applied to a gas sensor, and excitingly, the Sn3N4based sensor shows remarkable sensitive performance in detecting volatile organic compounds (VOCs), especially ethanol at a low working temperature of 120 oC.

Synthesis process All the chemical reagents were analytical grade without further purification. Tin nitride nanoparticles were prepared by a soft urea route. In this way, the precursor was obtained by adding 1 g tin (IV) chloride pentahydrate (SnCl4·5H2O, ≥ 99 %, Aladdin Industrial Corporation) powder to 2 mL ethanol in order to obtain the targeted concentration, and form a stable and clear solution. Then, 1 g urea (≥ 99 %, Aladdin Industrial Corporation) was added to the alcoholic solution. This mixture was stirred until the urea was completely dissolved and became a clear solution. And then, the solution was aged for 12 h before the heating treatment, this was done in order to ensure complete reaction of urea, and during this period the gel slowly turned from colorless transparent to deep yellow gel-like solution. The gels were then put into an oven and treated under a NH3 of flowing rate 100 ml·min-1 at 400~500 oC with a heating rate of 10 oC·min−1 for a specific period of time. It is important to increase the temperature slowly to avoid excessive foaming of the polymer-like cohesive and eruptive release of the leftovers of the solvent. Then, the sample was cooled down to the room temperature at the same flowing rate and passivated in a flow of Ar (20 ml·min-1) for 2 h. Characterization

Powder X-Ray diffraction (XRD) analysis was conducted on a Rigaku MiniFlex 600 powder X-ray diffractometer with Cu Kα radiation (λ=1.5418Å) (Japan). FT-IR spectra were recorded with a BRUKER TENSOR27 instrument (Germany). Scanning electron microscopy (SEM) images were performed on a JSM-7800F (Japan) instrument. Transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM) observations were conducted on an instrument of JEM2100 (Japan). The binding energies of the surface Sn, O, N species were determined by using X-ray photoelectron spectra (XPS) with an ESCALAB250 X-ray photoelectron spectrometer with contaminated C as internal standard (C1s=284.8 eV) Fabrication and measurement of gas sensor Typically, 50 mg·mL-1 Sn3N4-ethanol solution was deposited on the surface of the device and calcined at 100 °C for 3 h. In this study, S is defined as the relative resistance change (S=Ra/Rg, where Ra is the initial resistance of the sensor in air and Rg is the measured resistance after exposure to test gas. τres and τrecov are defined as the time taken by the sensor to achieve 90 % of the total resistance change in the case of response and recovery, respectively. A gas mixing line equipped with mass flow controllers was designed to prepare target gases at specific concentrations, and the resistance changes of sensor in air or tested gas were monitored by a high-resistance meter (Victor, 86E, China). The schematic circuit diagram of the testing system and gas mixing line equipment were shown in ESI Fig.S1 (see in Supporting Information).

Structural and morphological properties of Sn3N4 nanoparticles Sn3N4 nanoparticles were synthesized through a simple and generally applicable strategy, which includes the synthesis of gel-like tin-urea precursor and subsequent transformation to Sn3N4 nanoparticles via thermal treatment of the as-prepared precursor under ambient NH3 , as illustrated in Scheme 1. When tin (IV) chloride pentahydrate was mixed with the ethanol, the metal precursor reacts slowly with the ethanol to form the corresponding metalorthoesters. And then, urea was added into the mixture as

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nitrogen source for preparing gel-like tin-urea precursor. It is worth emphasizing that, when the metal precursor exists in the ethanol, the solubility of urea significantly increases (the solubility of urea in pure ethanol is 4.877g/100 g at 18.2 oC), and this might indicate the formation of soluble complex and coordinated polymers where urea is bonded with tin. The structure of the tinurea precursor can be expected in that urea molecules locate in the first coordination sphere near the tin atom, while the chloride anions locate in the outer coordination sphere. This is consistent with Fourier Transform Infrared spectra (FT-IR) result of tin-urea precursor which is shown in Fig. 1a, and the main bands in the FT-IR spectra are summarized in ESI Table S1. The absence of carbonyl group stretching variation (νc=o at 1675 cm-1) signifies that carbonyl group of urea bonded with tin replacing the position of the ethoxide group. The formation of intramolecular linkage between urea molecules is determined by the absence of δN-H at 1624 cm-1, and by the decrease in the wave number and peak number of νΝ-H. In this kind of structure, the urea can also act as stabilizing agent to stabilize the metal ethoxide in an ambient atmosphere. The decomposition reaction takes place to obtain the targeted product by heating this metal-urea complex at high temperature under ammonia-rich atmosphere in a tube furnace. At high temperature, urea is gradually decomposed into NH2, NH3, HNCO, and H2NCO, which have been identified by pioneer contributors.27-29 Therefore, urea can serve as the source of the nitrogen in the process of nitriding. The possible reaction is as follow: 3Sn[OC(NH2)2]nCl4→ Sn3N4+12HCl+(3n-4-x)NH3+(3n-x)HNCO+xNH2+xH2NCO (1)

Figure 1. (a) FT-IR spectra of pure urea and tin-urea precursor; (b) XRD pattern of Sn3N4 nanoparticles. The XRD pattern of Sn3N4 is shown in Fig.1b. Ammonolysis of the tin-urea polymer at 500 oC resulted into a polycrystalline phase of Sn3N4. It is very worth mentioning that, there is no solid precipitation, recrystallization and no major side product when keeping the process without any further purification. It can be seen that all the peaks can be easily indexed to the data of JCPDS document of cubic tin nitride (Sn3N4) (space group Fd-3m) with unit cell parameters a=9.029 Å (9.0370 Å in the JCPDS document). For the tin nitride, the strongest peak of Sn3N4 is the peak (311) in the spectrum located at 2θ=32.84°. A Rietveld fit to the XRD pattern of Sn3N4 produced an average crystallite size of 11 nm which estimated by the Scherrer’s equation from main peaks (311) of the cubic structure of Sn3N4. They are in good agreement with the size obtained by TEM investigation, which shows crystallites around this size with limited aggregation.

Figure 2. The N 1s (a)and Sn 3d (b) XPS spectra of Sn3N4 nanoparticles. Fig. 2 presents the N 1s and Sn 3d core level X-ray photoelectron spectra (XPS) of Sn3N4 sample. Fig. 2a is the N 1s core level spectra and it can be found that there are two peaks with different intensity at the bonding energies of 397.1 eV and 398.6 eV, respectively. Generally, the peak at 397.1 eV reflects the formation of N-Sn bonds, which suggests that most of the detected nitrogen is bonded as nitride. The XPS peak at 398.6 eV can be assigned to interstitial N or surface amide in Sn3N4.30 In addition, in Fig. 2b,the binding energy of the Sn 3d3/2 (EB=495.0 eV) and 3d5/2(EB=486.6 eV) peaks corresponds to a Sn4+ nitride species. Fig. S2 demonstrates that the O 1s peak around 531.47 eV was observed, which is related to the hydroxyl groups or water adsorption on the surfaces, and no peaks of Sn-O bonds (EB≈530 eV) in the SnO2 lattice can be seen in the spectra. Fig. 3a shows the SEM images of tin nitride nanoparticles, and from the SEM image, it can be seen that the particle size of tin nitride is less than 20 nm and is homogeneous in large scale. The typical powder texture of very small particles is found, and also, no structural side products can be seen. Other X-ray data were also confirmed with TEM and HRTEM experiments. TEM images are shown in Fig. 3b, c, and samples for TEM were prepared by dispersing a small amount of tin nitride powder in ethanol and using ultrasonic bath to ensure complete powder dispersion without addition of any further dispersing agent. The well-defined and crystalline character of the Sn3N4 nanoparticles can be reflected from the grid of reticular planes. The interesting information about other

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transition metal nitrides using the similar method is that amorphous carbon is found as an extremely defined, ca.24 nm thick coating around the particles.28 Here, HRTEM image of Sn3N4 nanoparticles shows no any carbon coating or side product in this work. The inter-planar distance of Sn3N4 calculated from the TEM images is 0.273 nm, corresponding to the d311 plane of Sn3N4 bulk. This is in good agreement with the expected value (d=2.725 Å ) for cubic Sn3N4 (ref JPCDS#70-3184). The SAED pattern (Fig. 3d) of the sample shows the polycrystalline configuration of the Sn3N4 nanoparticles.

Figure 3. (a) SEM image, (b-c) TEM and HRTEM images, and (d) SAED pattern of Sn3N4 nanoparticles. Gas sensing characteristics of Sn3N4 nanoparticles We tested the operating temperature of the Sn3N4-based sensor at room temperature to 200 oC. As shown in Fig. 4a, the sensor based on Sn3N4, during the temperature changing process, exhibited a “volcano” shape and achieved the maximum response of 49.8 at 120 oC. The phenomenon

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ance as a function of operating temperatures. (b) Responses of Sn3N4 sensor to various test gases at 120 oC. could be explained as follows: at low temperature, the relatively low response is mainly attributed to the small amount of adsorbed oxygen and minor reason of the targeted gas molecules having no enough thermal energy to react with the oxygen species adsorbed on the surface of the sensing materials. The reason for the increase in response with the increase in operating temperature is that; the increasing amount of adsorbed oxygen by more excited electrons, and the targeted gas molecules have enough energy to overcome the activation energy barrier in order to react with the surface oxygen species. Beyond the optimum operating temperature, such temperature will lead to low gas adsorption ability, resulting in the decrease of response. Besides, the response time of the Sn3N4-based sensor decreased largely within the operating temperature range (room temperature to 120 oC) and subsequently came into saturation. Thus, the optimum operating temperature of the Sn3N4 sensor was suggested to be 120 o C. In addition, in realistic applications, gas sensors should work in different ambient environments and situations, which contain multiple gases. Thus, an excellent sensor should have the ability to recognize or detect the targeted gas from interfering gases. The response of the sensor based on the as-prepared Sn3N4 nanoparticles was measured on exposure to various gases at 120 oC and the results were shown in Fig.4b. These results implied that the Sn3N4 nanoparticles-based sensor exhibited an obvious response to ethanol and lesser effects for others at 120 oC, indicating the excellent selectivity to ethanol. The excellent selectivity to ethanol should be attributed to the different optimum operating temperature to different gases. The responses of the Sn3N4-based sensor to different ethanol concentrations were measured at 120 oC to check the relation of response-concentration and estimate the low detection limit (inset of Fig. 5b). The transients sensing curves (Fig. 5a) showed the stable sensing and recovering characteristics. Fig. 5b showed the responses of the Sn3N4 sensor versus ethanol concentration, and the almost linear curve of response versus concentration trend means that the sensor endows a great potential in the quantitative gas analysis. The response to 0.5 ppm ethanol was 3.04, indicating that the present Sn3N4-based sensor can detect sub-ppm levels of ethanol. The detection limit of

Figure 4. (a) Response and response time of Sn3N4 sensor in the presence of 100 ppm ethanol in synthetic air bal-

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Figure 5. (a) Sensing transients of Sn3N4 sensor to 0.5 to 5 and 10 to 100 ppm ethanol at 120 oC. (b) Response and

response time of Sn3N4 sensor as a function of ethanol concentration at 120oC.

the Sn3N4 sensor for ethanol is calculated to be approximately 0.07 ppm based on signal-to-noise ratio of 3.

working temperature of 120 oC during the long-term stability measurement of 15 days. The results showed that the sensor exhibited a decrease of about 5.24 %, which indicates a good thermal stability.

Cycling and long-term stability is another key factor to a gas sensor, which could maintain that gas sensor shows a high reliability. The cycling performance to 100 ppm ethanol of the Sn3N4-based sensor conducted at the optimum operating temperature (120 oC) is depicted in Fig. 6a, and the response remained at 95.08% after 200 cycles. The dynamic 3-cycles response measurements (Fig.S3) to 100 ppm ethanol for Sn3N4 at 120 oC verified the excellent response and recovering characteristics of the Sn3N4 nanoparticles. Fig. 6b represented the responses of the Sn3N4-based sensor to 100 ppm ethanol at a constant

Figure 6. (a) Response of Sn3N4 sensor to 100 ppm ethanol at 120 oC versus cycle number. (b) Long-term stability of Sn3N4 sensor to 100 ppm ethanol at 120 oC.

According to the XPS analysis of the sensing materials, the N 1s, O 1s and Sn 3d core level (Fig. S4) of the Sn3N4 nanoparticles after operation as a gas sensor confirmed that Sn3N4 nanoparticles were not oxidized to SnO2, indicating Sn3N4 nanoparticles were stable during the whole measurement of gas sensing properties. The ethanol sensing characteristics based on bare semiconductors materials reported in other literatures are summarized in Table 1.31-38 Up to now, lots of semiconductors sensing materials have been applied for detecting ethanol, including; SnO2 hierarchal nanostructures,31 ZnO porous core-shell microstructures,32 In2O3 nanospheres,33 α-Fe2O3 hollow microspheres,34 CuO decahedron,35 Co3O4 nanofibers,36 NiO hollow hemispheres37 and GaN film.38 Our Sn3N4 nanoparticles sensor has a high response of about 50 to 100 ppm at 120 oC. The high response and low working temperature make our sensor competitive when compared with most other ethanol gas sensors. For example, Y. Liu et al. reported a sensor based on SnO 2 hierarchal nanostructures exhibited the same working temperature with our sensor, but with a lower response. J.-H. Lee et al. use Co3O4 nanofibers as sensing materials to detect 100 ppm ethanol, showing a response of 6.45. However, the working temperature (301 oC) is relatively high, which increase the power consumption, restricting its application in many realistic conditions. It is noteworthy that the GaN film sensor reported by D.-D. Lee and his co-operators has a response of 1.43 to ethanol (1000 ppm) in actual measurements at 350 oC.

Table 1. Gas sensing characteristics of different sensing materials to ethanol, as reported in the literature and the present study Sensing materials

Ethanol conc. (ppm)

Response (Rg/Ra or Ra/Rg)

Operating temperature (oC)

Reference

Sn3N4 Nanoparticles

100

51.3

120

Our work

SnO2hierarchal nanostructures ZnO porous core-shellmicrostructures In2O3nanospheres α-Fe2O3 hollow microspheres CuOdecahedron Co3O4 nanofibers NiO hollow hemispheres GaN film

100 100 100 100 100 100 200 1000

40 32.5* 20* 26.9 15* 51.2 5* 1.43

120 300 275 300 200 301 300 350

31 32 33 34 35 36 37 38

Note: The asterisk symbol “*” indicates that the data was extrapolated from the curve. The most popular and widely accepted sensing mechanism for semiconductor chemiresistive sensors is based on the change in resistance of the sensor in different types of atmospheres.39, 40 Typically, Sn3N4 is a n-type semiconductor, and the oxygen molecules will adsorb on the surface of Sn3N4 and ionize into species (O2-, O-, and

O2-) through capturing electrons from the conduction band of Sn3N4. Reducing gases can change the surface carrier concentration by releasing electrons to the conduction band of Sn3N4, acting as electron-donor groups. However, the reason why Sn3N4 has good sensing performance compared with traditional oxide materials is un-

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clear. A further detailed sensing mechanism of Sn3N4 nanoparticles is still under investigation by our group.

In summary, we have creatively synthesized Sn3N4 nanoparticles through soft urea route. The strategy includes the synthesis of gel-like tin-urea precursor and subsequent transformation to Sn3N4 nanoparticles via thermal treatment of the as-prepared precursor in ambient NH3 flow. When evaluated as a gas sensor for the first time, the Sn3N4 nanoparticles exhibit excellent gas sensing characteristics to ethanol. Furthermore, we expect our findings to bring up new promising gas sensing materials, and to inspire rational synthesis of Sn3N4-based sensing materials for high-performance gas sensors.

The schematic circuit diagram of the testing system and gas mixing line equipment, additional XPS spectra and detailed dynamic response measurements. The Supporting Information is available free of charge on the ACS Publications website.

* E-mail: [email protected] ‡These authors contributed equally.

This work is supported by NSF China through grant 21471147 and Liaoning NSF through grant 2014020087. M. Yang would like to thank for the National "Thousand Youth Talents" program of China.

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