In2O3 surface

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Functional Nanostructured Materials (including low-D carbon)

Pd-catalyzed reaction producing intermediate S on Pd/In2O3 surface: a key to achieve the enhanced CS2 sensing performances Bo Liu, Ying-Ming Xu, Ke Li, Hong Wang, Lei Gao, Yuanyuan Luo, and Guotao Duan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b01638 • Publication Date (Web): 02 Apr 2019 Downloaded from http://pubs.acs.org on April 2, 2019

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ACS Applied Materials & Interfaces

Pd-catalyzed reaction producing intermediate S on Pd/In2O3 surface: a key to achieve the enhanced CS2 sensing performances Bo Liua,b,c, Yingming Xud, Ke Lib,c, Hong Wangb,c, Lei Gaob,c, Yuanyuan Luob,c, Guotao Duana,* a

School of Optical and Electronic Information, Huazhong University of Science and

Technology, Wuhan 430074, P. R. China b

Key Lab of Materials Physics, Anhui Key Lab of Nanomaterials and

Nanotechnology, Institute of Solid State Physics, Chinese Academy of Science, Hefei 230031, P. R. China c

University of Science and Technology of China, Hefei, 230026, P. R. China

d

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education,

School of Chemistry and Materials Science, Heilongjiang University, Harbin,150080, China. * Email: [email protected], telephone number: +865515595380

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Abstract Although chemiresistive gas sensors, based on metal-oxide semiconductors, have exhibited particular promise for the monitoring of air pollution, they are often limited to the poor selectivity. In the case, to overcome this issue, according to the essence of gas sensing process, the method of reforming the surface reaction path on the surface of sensing materials was used. Here we report that Pd nanoparticles supported over the In2O3 composites, featured with a yolk-shell structure, enable the trace detection of carbon disulfide (CS2) gas molecules, which are immensely dangerous to human and animals. Moreover, the prominent enhancement of gas response and ultraselective CS2 sensing characteristic were acquired in comparison with pristine In2O3 sensors. Significantly, density functional theory calculations revealed that the Pd supported on In2O3 greatly facilitates the adsorption capacity to CS2, and the intermediate S, produced by Pd-catalyzed desulfurization reaction, on the Pd/In2O3 surface during the sensing process is a key to achieve high CS2 gas response as well as ultra-selectivity, which is well agreement with the XPS analysis results. On the basis of these results, a new sensing mechanism model for the CS2 sensing process was put forward. Key words. Carbon disulfide; Pd/In2O3 composite; ultra-selectivity; density functional theory; intermediate S.

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1. Introduction Effective monitoring means for air pollution, especially with real-time and onsite features, are of great important for practical applications in the environment protection.1-4 However, typical analytical approaches that are capable of detecting noxious gases, such as the gas chromatography-mass spectrometry5, cataluminescence detection6-7, and optical spectroscopy8, will be confronted with the tremendous limitation of either expensive large-sale instruments or intricate and time-consuming testing process, thus it is no doubt that both on-site and real-time detection are very difficult to be realized for these means mentioned above. Currently, chemiresistive gas sensors, based on the resistance change of metal-oxide semiconductors (MOSs),9 has received enormous concerns due to the advantages of miniaturization and portability, easy integration, and low cost.10 Furthermore, various MOSs sensing materials, such as SnO211-12, ZnO13-14, WO315-16, etc, have been widely used to monitor harmful gas. However, despite great progress has been made in the development of gas sensors, there is still a vital issue, that is the po or selectivity, that need to be addressed. For chemiresistive gas sensors, the sensing nature was attributed to the resistance variation induced by the occurrence of surface redox reaction.17-18 Thereby, to achieve high selectivity of gas sensors, the key step is how to modulate the path of surface reaction. Given that nano-catalysts supported on MOSs can offer the high catalytic activity,19 which originated from strong metal-support interaction,20-22 that enables to reform the absorption and surface reaction process of gas molecules on the surfaces, thus the metal-catalyst doping is an alternative option to regulate gas sensing characteristics, especially for selectivity. For instance, Jong-Heun Lee et al. achieved the ultra-selective detection of benzene by a Pd-loaded SnO2 sensing film 3



coated with a thin Co3O4 catalytic layer.23 Also, Duan group anchored Pt nanoparticles into SnO2/α-Fe2O3 nanoheterojunction to realize the highly selective detection of styrene.24 Further, Sang-Joon Kim and co-workers utilized Pt-based bimetallic decorated WO3 nanofibers to acquire the excellent selectivity toward acetone or hydrogen sulfide.25 Even if these reports revealed the improved gas sensing performances, the sensing mechanism was still a matter of discussion, and there was no in-depth exploration on the reason why it boosted the sensing properties. Additionally, considering that the adequate understanding of intrinsic essence enables to guide and design rational sensing materials, it is, therefore, of vital importance to find out what reaction happened actually on the surface during the gas sensing process, so as to solve the problem of selectivity as soon as possible. Herein, chosen the case of the detection of carbon disulfide (CS2), a volatile sulfur compound, which not only widely existed in artificial viscose film and pesticides, but also used as raw materials to produce organic reagents and soften rubber, and it is immensely dangerous to human and animals due to the fact that it can trigger coronary artery as well as accelerated atherosclerosis even in the low concentration ( the threshold limit value is 6.7 ppm according to the U. S. Environmental Protection Agency (EPA) ),26-27 we showed that the optimization of the surface reaction path has a great effect on the gas sensing properties. The sensing material made from Pd nanoparticles supported by In2O3 scaffold was primarily prepared by combining a MOF-templated method with the subsequent annealing treatment. Moreover, the as-prepared Pd/In2O3 sensors exhibited excellent sensing properties to CS2 gas molecules. Particularly, it was found that Pd nanoparticles confined over the In2O3 significantly enhanced the sensing performances toward CS2, especially selectivity and gas response value, in comparison with pure In2O3 4


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materials. For this case, density functional theory (DFT) calculations demonstrated that intermediate S, produced by Pd-catalyzed desulfurization reaction, on the Pd/In2O3 surface during the sensing process is a key to achieve high CS2 gas response as well as ultra-selectivity. More importantly, the XPS analysis results further confirmed the presence of such S on the Pd/In2O3 surface during the CS2 sensing process. On the basis of these results, a new sensing mechanism model for the CS2 sensing process was put forward. 2. Experiment Section 2.1. Chemicals and materials: all chemicals were used as received without further purification. Indium nitrate (In(NO3)3·2H2O), isophthalic acid (IPA), palladium nitrate (Pd(NO3)2·2H2O, Pd ≥ 39%), sodium borohydride (NaBH4, 98%), N,N-dimethylformamide (DMF, 99.5%), acetone and ethanol. Deionized water was used for all experiments. 2.2. Synthesis of amorphous In-MOF spheres. 0.33 mmol In(NO3)3·2H2O and 0.33 mmol IPA were added into the 40 mL mixture of DMF/acetone (V/V=1:1), followed by stirring for 3 h and the mixture was transferred into a Teflon-lined stainless steel autoclave. After reacting at 160 oC for 6 h, the products were centrifuged and washed three times by ethanol and deionized water, respectively. Then, the obtained products were dried at 60 oC for 4 h. 2.3. Synthesis of Pd/In-MOF composites. 50 mg In-MOF spheres were dispersed into 30 mL acetone and ultrasounication for 5 min. Subsequently, 15 mL acetone solutions containing 1.5 mg Pd(NO3)2 · 2H2O were injected into abovedispersion and stirred for 5 h at room temperature. Then, the products were centrifuged and washed three times by deionized water. The washed products were re5



dispersed into 30 mL ethanol, followed by adding the 3mL, 1M NaBH4 aqueous solution. After stirring 5 h at room temperature, the products were centrifuged and washed three times by deionized water. The as-synthesized sample was dried at 80 oC for 2h. The Pd loading on the products were tuning by adjusting the Pd(NO3)2 ·2H2O concentrations from 0.05 to 0.5 mg/mL. 2.4. Synthesis of yolk-shell Pd/In2O3 composites. The as-synthesized samples were calcinated at 500 oC in air for 60 min with a heating rate of 5 oC /min, followed by treating in H2/Ar (1:10) at 250 oC for 90 min with a heating rate of 2 oC /min to achieve the yolk-shell Pd/In2O3 composites. 2.5. Measurement and characterization: The XRD with a Philips X’Pert powder X-ray diffractometer using Cu Kα radiation (λ=0.15419 nm) was used to investigate the phase of the obtained samples. The X-ray photoelectron spectroscopy (XPS) was conducted through a PREVAC system. Field-emission transmission scanning electron microscopy (TEM), the energy dispersive X-ray spectrometry (EDS), mapping high-resolution TEM (HRTEM) and field-emission scanning electron microscopy (FE-STM) were applied to analyze the construction and morphology of as-synthesized products. The loading amount of Pd NPs were evaluated by an inductively coupled plasma source mass spectrometer (ICP-MS, Icap Qc, USA). The detailed computing method was described in Supporting Information. 2.6. Gas sensing measurements The gas sensors were gained by primarily dispersing samples to ethanol, followed by brushing onto the testing electrodes, which is composed of the ceramic substrate, heating wires and gold electrodes (Figure S1). Then, the aging process was performed to the obtained sensing layer for five days at 160 oC. Followed by injecting 6


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the desirable amount of target gas into testing chamber and the actual concentration of target gas is calculated according to the equation: Vinjection × 20000 ppm = X ppm × 20 L, which the Vinjection is the injected volume of the standard gas in the gas cylinder, and the 20000 ppm is the standard gas concentration of target gas in the gas cylinder, purchased by Hefei Ningte Gas Managenment Co., Ltd. The X represents the required concentration of target gas in the testing chamber, and the 20 L is the true volume of the testing chamber. As a result, a series of sensing measurements for the aged sensors were carried out with a relative humidity of 45% and a chamber temperature of 25 oC in a static measurement system with a multimeter/DC power supply as well as a test chamber (~20 L).28 Moreover, the gas response toward target gas for the sensor is defined as the S= Rair / Rgas, where the Rair and Rgas are the resistance of sensors in air and testing gas, respectively, and the response/recovery time is the defined time, which the gas response value reached 90% of the final equilibrium value according to the previous reports.28 3. Results and Discussion 3.1. Synthesis and characterization of as-prepared products The whole preparation procedure for the Pd/In2O3 composites, featured with yolk-shell architecture, was depicted in Scheme 1. Due to metal-organic frameworks (MOFs) can function as an excellent scaffold for the growth of metal catalysts.29 Thereby the amorphous In-MOF were synthesized by a simple solvothermal method. As indicated in Figure S2, the yielding products exhibited the spherical configuration with an average diameter of around 0.8-1μm and a uniform size distribution. Moreover, the size of as-prepared In-MOF spheres could be tuned by changing the concentration of indium ions and organic ligands in the solvent (Figure S3). Isophthalic acid (H2IPA), the carboxylate ligands, endowed In-MOF spheres with 7



abundant negative charges in surfaces and cavities, which facilitated the adsorption of palladium ions by electrostatic attraction. Subsequently, the Pd NPs were formed over the In-MOF by an in-situ reduction process. By means of the thermal instability of MOF materials,30 the calcination treatment in air and the subsequent reduction in H2/Ar flow were implemented in sequence, generating the Pd/In2O3 composites. As shown in Figure 1a, the as-synthesized Pd/In-MOF composites exhibited no diffraction peaks, indicating the size of the Pd NPs was very small and the amorphous structure of In-MOF. However, after calcination in air, the amorphous In-MOF absolutely converted into cubic phase In2O3 (JCPDS 01-071-2195), and a diffraction peak located on 33.8°, which matched well with the (101) plane of tetragonal phase PdO (JCPDS 00-006-0515), appeared. After the reduction with H2/Ar, the cubic phase In2O3 still retained while the tetragonal phase PdO switched to cubic phase Pd NPs, suggesting the acquisition of Pd/In2O3 composites. XPS measurement was also performed to further investigate the chemical states of the as-fabricated samples. Evidently, as shown in Figure 1b, two peaks located on 451.9 and 444.3 eV were revealed, which was attributed to In3+ 3d3/2 and In3+ 3d5/2, respectively.31 Furthermore, the characteristic binding energy of Pd 3d3/2 (340.8 eV) and Pd 3d5/2 (335.5 eV) was also clearly observed (Figure 1c),32 and there was no other peaks, which was in agreement with the XRD results, testifying the existence of Pd NPs on the In2O3 particles. Additionally, three oxygen states featured with hydroxyl oxygen, O2-,and O2- were presented (Figure 1d), in response to hydroxyl oxygen, lattice oxygen (O2-) in the In2O3 and chemisorbed oxygen species (O2-).32 On the basis of above results, it is confirmed that the In2O3/Pd composites were acquired. The morphology and construction of the as-fabricated products were further investigated. As seen in Figure S4, obviously, when Pd NPs were anchored on the 8


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whole In-MOF particles, the Pd/In-MOF particles still maintained the well-defined spherical morphology, nevertheless, its surfaces become very rough in contrast with pure In-MOF particles, as demonstrated by SEM observation (Figure S4 a, b, d). Moreover, a lattice spacing of 0.228 nm was observed in HR-TEM image (Figure S4 e), which is consistent with the (111) plane of cubic phase Pd NPs. Before the calcination in air, the Pd/In-MOF particles showed the solid architecture, as indicated through TEM study (Figure S4 c). Likewise, EDX element mapping was utilized to evaluate the elementary composition of such samples, it could be observed that the obtained products solely consisted of In, O, and Pd (Figure S4 f), which was consistent with the XPS results. It was also noted that Pd NPs were dispersed uniformly all over the whole In-MOF sphere, further confirming the encapsulation of ultra-small Pd NPs in the In-MOFs spheres. Furthermore, the calcination process has a great influence on the interior structure of as-synthesized products. As shown in Figure 2 a, b, c, after complete pyrolysis in air and subsequent reduction in H2/Ar, it could be clearly seen that Pd/In2O3 composites featured with a well-defined yolk-shell construction via broken spheres, which further confirmed by TEM images (Figure 2d, e). Because of the decomposition of the existent organic ligands during the pyrolysis process, thus yielding vast of CO2 or H2O to diffuse outward, leading to the formation of porous shell surface and ultra-small In2O3 nanoparticle subunits. This special structure is beneficial to gas adsorption and desorption, promoting the gas sensing response. Additionally, except for the lattice fringe of 0.228nm, in accordance with (111) plane of Pd NPs, another lattice fringe of 0.294 nm was observed, corresponding to the (222) plane of In2O3 (inset Figure 2e), indicating the occurrence of phase transition from

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amorphous In-MOF to cubic In2O3. The EDX element mapping (Figure 2f) indicated that Pd NPs were uniformly supported on the whole yolk-shell In2O3 sphere. To probe the formation mechanism of such yolk-shell architecture, we primarily evaluated the effect of annealing temperature on this structure. As shown in Figure S5, the Pd/In-MOF composites displayed a solid sphere without heat treatment. As the annealing temperature increased to 350 oC with a heating rate of 5 oC/min, the product began to emerge a small portion of the cavity as well as the thin shell. Whereas, once boosted the temperature to 500 oC, the inner cavity became larger in volume, and the distinct separation between the yolk and shell appeared. Thereby, with regard to such structural evolution process, the so-called self-templated heterogeneous contraction plays a key role.33 Owing to the presence of a large temperature gradient (ΔT)34 along the radical direction during the heating process, which produces two kind of force, that is contraction force and adhesion force, inducing the generation of shell of Pd/In2O3 on the surface of Pd/In-MOF core at the initial stage of pyrolysis. As extended heating, the temperature gradient decreases gradually, prompting an equilibrium between contraction force and adhesion force, resulting in the termination of inward shrinkage. Therefore, a small cavity appeared at 350 oC. Crucially, the larger the temperature gradient, the more dominant the contraction force. The temperature raised to 500 oC, the temperature gradient is larger than that of 350 oC, which causes the stronger inward shrinkage, thus creating the larger cavity and the separation between the yolk and shell. It is noted that the loading of Pd NPs is another factor in the production of this architecture. As indicated in Figure S6 c and d, it is evident that neat In2O3 particles, directly derived from pristine In-MOF spheres by annealing at 500 oC, exhibited loose porous construction rather than yolk-shell. Under the same condition, Pd/In2O3 composites, however, possessed the yolk-shell structure. 10


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Considering acetone was used as dispersing agent during the Pd2+ adsorption over the In-MOF spheres, we speculated that outer organic ligands of In-MOF may be partially dissolved in acetone, evidenced by the rough surface of Pd/In-MOF (Figure S4e), causing the exterior of In-MOF scaffold to be loose. Eventually, these two factors resulted in the achievement of such yolk-shell Pd/In2O3 composites. 3.2. Gas sensing properties of the as-synthesized Pd/In2O3 composites Owing to the introduction of metal catalysts might affect the gas sensing performances of MOS materials,35-37 thereby, in order to evaluate the potential feasibility of these yolk-shell Pd/In2O3 composites as sensing platforms for the trace detection of target gas molecule, a series of gas sensing measurements towards CS2 gas molecules were carried out. In the case of chemiresistive MOS-based sensing materials, due to the gas response is greatly dependent on the working temperature, thus we firstly explored its optimum operating temperature. As shown in Figure 3a, the gas response value for 10 ppm CS2 gas molecules based on Pd/In2O3 sensors increased initially and decreased afterwards with the working temperature changed from 67 to 226 oC. The highest response value (Rair / Rgas = 23.4) was obtained at the working temperature of 135 oC, indicating its optimum operating temperature was 135 oC.

Notably, it was found that the optimum operating temperature of such Pd/In2O3

sensors is much lower than that of pure In2O3 sensors (198 oC), indicating the Pd NPs loading played a key role to tune the working temperature. As for this case, the DFT calculation displayed that the first-step dissociation of CS2 molecules on the Pd(111)/In2O3(222) surface only overcame a barrier of 0.46 eV (Figure 6), which is much less than that on the In2O3(222) surface (3.65 eV). This indicates that the Pd NPs supported on the In2O3 surface greatly reduced the surface reaction energy compared with pure In2O3, thus loading to the lower operating temperature toward CS2 gas. This will benefit to reduce 11



energy consumption and ensure safety in practical application. Besides, we found that the Pd loading amount also has great impacts on gas sensing characteristics. As shown in Figure 3c, we investigated the gas response value for 10 ppm CS2 based on pristine In2O3 sensor and Pd/In2O3 composites with different Pd loading amount, respectively, it is distinctly observed that the 4.26% of Pd loading over the In2O3 particles revealed the highest response value. Therefore, according to above results, it ascertained that the 4.26% of Pd loading and the working temperature of 135 oC were optimal for CS2 sensing on the Pd/In2O3 sensing materials. The response/recovery time of these Pd/In2O3 sensing layers toward 5 ppm CS2 gas was examined, however, a relatively long response/recovery time (132.3 and 112.1s, respectively) was exhibited in Figure 3b, which was probably caused by the relatively low working temperature. Likewise, there is little change about the response value in continuous five tests to 10 ppm CS2 for Pd/In2O3 sensing platform, determining its excellent reproducibility. The detection limit of gas sensors is another significant criteria for actual gas monitoring. Hence, in order to identify the detection limit of as-prepared Pd/In2O3 sensing platform, the test toward different concentration of CS2 gas molecules was conducted at 135 oC. To be noted, with the concentration reduced, the gas response value decreased (Figure S7 a). Although the concentration lowed to 1 ppm, which is lower than its threshold limit value (6.7 ppm), the obvious response value (Rair / Rgas = 1.31) was still acquired (Figure S7 b). Afterwards, considering the fact that an outstanding stability is of great significant for a gas sensor, the time-dependent gas response for 10 ppm CS2 based on as-synthesized Pd/In2O3 sensors was investigated at optimum operating temperature. As expected, no obvious variation of gas response was revealed (Figure S7 c), demonstrating its superior long-term stability and will be promising in the detection of CS2 gas in practical application. 12


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To further estimate the effect of Pd catalyst on the sensing features of In2O3 particles, we implemented comparison tests based on pristine In2O3 particles and Pd/In2O3 composites, respectively, toward CS2 at 135 oC. At first, it is apparently seen that the Pd/In2O3 sensors showed the higher CS2 gas response compared with pure In2O3 sensors (Figure 3e). More importantly, although the CS2 concentration boosted to 50 ppm, very small response (Rair / Rgas = 4.32, 50 ppm) was displayed for the pure In2O3 sensors. However, in contrast, the Pd/In2O3 sensor showed very high response (Rair / Rgas = 135.3, 50 ppm), which is about 31.3 times higher than that of In2O3 sensors, indicating that the Pd NPs supported over the In2O3 particles greatly improved the CS2 gas response. Accordingly, the linear relationship between response value and CS2 concentration was also traced in Figure 3f. It was found that a good linear relation for such Pd/In2O3 sensing layers was shown. This benefits to the realization of real-time detection of CS2 gas. The selectivity of sensing materials is also of extremely vital for the practical application. In this study, we utilized eight interfering gases, that is, dimethyl disulfide ( (CH3)2S2 ), dimethyl sulfide ( CH3SCH3 ), styrene, methanthiol (CH3SH), ammonia gas (NH3), hydrogen sulfide (H2S), ethanol, acetone and NO2, to evaluate the selectivity for as-fabricated Pd/In2O3 sensors at 135 oC. As revealed in Figure S8 b, with regard to 10 ppm of other interfering gases, there is very small response to be exhibited for such Pd/In2O3 based sensor, instead, it only showed the highest CS2 sensing response. Moreover, for NO2 gas, almost no response was observed (Figure S 9). In contrast, pure In2O3 based sensors, however, displayed approximate response value for both dimethyl disulfide and dimethyl sulfide (Figure S8 a), suggesting its poor selectivity. More significantly, the Pd NPs supported on In2O3 particles only tremendously enhanced CS2 gas response, but lessened the response of other 13



interfering gases compared with pristine In2O3 sensors (Figure 4), certifying Pd NPs supported on In2O3 greatly improved the CS2 selectivity of this sensing layer. As for the superior selectivity toward CS2 gas, we calculated the molecular-surface affinity, that is adsorption energy (Eads), on the Pd/In2O3 surface model by density function theory (DTF). Here we mainly compared the five gas that contained sulfur element. As shown in Figure S12, the Eads of CS2 on the Pd/In2O3 surface model, which is the optimized active site, was calculated to be -4.26 eV, which is much larger than those of H2S (-2.23 eV), CH3SH (-2.85 eV), CH3SCH3 (-2.14 eV), while (CH3)2S2 was nonbinding to the surface and spontaneously decomposed into two CH3S molecules. These calculated results indicated CS2 molecules bound strongly to Pd/In2O3 surface, and the outstanding selective CS2 gas response for Pd/In2O3 sensors possibly rooted in the special strong affinity to CS2 molecules. 3.3. The enhanced sensing mechanism of as-synthesized Pd/In2O3 composites toward CS2 gas molecules As reported in previous studies, for the n-type chemiresistive MOS-based sensors, the sensing mechanism generally involved the traditional theory of electron depletion layer, that is, the surface reaction between analyte and adsorbed oxygen species induced the electrons change during the gas sensing process, resulting in the resistance variation of sensing layers. Accordingly, for the metal catalyst doped MOS sensing materials, although the most accepted sensing mechanism involved two factors: Fermi-level control mechanism38 induced by the difference in work functions or oxygen spillover effect39 caused by metal catalysis, nevertheless, it was still a matter of discussion, and did not have an insight into how and what happed actually during the gas sensing process. Hence, in order to penetrate what the role of Pd NPs

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supported over In2O3 played in the CS2 gas sensing process, a series of quasi in-situ XPS analysis and DFT computations were carried out. Initially, to evaluate whether the Pd NPs supported on In2O3 particles motivated the oxygen spillover effect, we made a contrast about the amount of chemisorbed oxygen species (O2-) between pure In2O3 and Pd/In2O3 sensors before contacting with CS2. As revealed in Figure S10, without the exposure to CS2 gas molecules, the O 1s high-resolution XPS spectra of these two sensors exhibited almost no difference about the percentage of O2- (16.1% for pure In2O3 and 16.74% for Pd/In2O3, Figure S10), which was also evidenced by with the actual baseline resistance comparison between the pure In2O3 and Pd/In2O3 sensor in air (Figure S11). This result means that the Pd NPs supported on the In2O3 particles did not promote the dissociation of oxygen molecules into O2-, confirmed that the improved CS2 sensing features for Pd/In2O3 sensors are uncorrelated to the so-called oxygen spillover effect induced by metal catalyst. Now that the inexistence of such oxygen spillover effect, what does the factor cause the enhanced sensing performances during CS2 sensing process? We emphatically focus on the intricate surface reaction for the CS2 sensing on the Pd/In2O3 sensors. To evidence this viewpoint, we implemented another comparison about reaction products between Pd/In2O3 sensors and pristine In2O3 after exposing to CS2 gas by using XPS. Particularly, due to the recovery process is relatively slow (Figure 3b), these two sensors firstly conducted the CS2 sensing test, and were immediately taken out to perform XPS measurements. Thus, it should be pointed out that, strictly speaking, this XPS measurement was not an in situ analysis and only identified reaction products. As shown in Figure 5b, for the pure In2O3 sensors, only one peak of 167.6 eV, which was attributed to SO2, arose. However, distinctively, as the Pd/In2O3 sensors exposed into CS2 gas molecules, two remarkable peaks, located 15



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on 168.4 and 164.3 eV (Figure 5c), appeared, which were in accordance with SO2 and S, respectively. Likewise, the intensity of S peak was much stronger than that of SO2 peak. These results suggested that the presence of Pd NPs over In2O3 particles reformed indeed the surface reaction path in comparison with pure In2O3 sensors. Well, in this case, another key issue emerged: why does so intense S characteristic peak appeared, and how does the surface reaction proceed? To understand deeply this issue, we performed the DFT calculations to trace the surface reaction path for CS2 sensing on the Pd/In2O3 sensors. Two surface models (Figure S12) were built to stand for pure In2O3(222) and Pd(111)/In2O3(222) based on the HRTEM observation (Figure 2e). As seen from Figure S14, the affinity for CS2 on the Pd(111)/In2O3(222) surface is much stronger than that on the In2O3(222) surface, indicating the Pd NPs loading greatly facilitated the adsorption capacity for CS2 molecules. Moreover, the first-step

dissociation

of

CS2

molecules

on

the

In2O3(222)

surface

is

thermodynamically hampered because of the very high energy barrier of 3.65 eV (Figure 6). However, in contrast, when CS2 adsorbed to Pd(111)/In2O3(222) surfaces, we found that it was very easy to dissociate into CS and S with a very low barrier of 0.46 eV (CS2→CS + S) (Figure 6). More significantly, the further dissociation of the CS species on the Pd(111)/In2O3(222) surface was a spontaneous process (without a barrier), and directly generated C and S (CS→C + S), which was in accordance with XPS results (Figure 5 c). Furthermore, it has been evidenced that the Pd nanocrystals are capable of exciting inert O2 molecules to yield the highly reactive singlet O2 rather than O2-.40 Thus, on the basis of above results, it is speculated that the surface reactions for the CS2 sensing process on such Pd/In2O3 sensors occurred as follows: Pd

CS2 C + 2S Pd

O2(gas) O2(singlet) 16


(1) (2)

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C + O2(singlet)→CO2(gas) S + O2― →SO2(gas) + e -

(3) (4)

Therefore, as depicted in Figure 5a, the enhanced CS2 sensing mechanism for Pd/In2O3 sensors could be interpreted below: on the surface of Pd NPs supported over the In2O3, CS2 gas molecules were easily bound to the surface, dissociating into S and C caused by Pd-catalyzed desulfurization reaction. Then the formed C reacted with singlet O2, which was induced by the O2 activation process on the surface of Pd NPs,40 to generate CO2. Simultaneously, the produced intermediate S on the surface of Pd NPs would slowly diffuse to interfaces between In2O3 and Pd NPs, which is a reason for the long response time (Figure 3b), and reacted with the self-existent O2- on the In2O3 surface. This would produce SO2 and a great deal of electrons. Ultimately, the electrons returned back into the conduction band of In2O3, resulting in the resistance reduction of Pd/In2O3 sensors and achieving the enhanced CS2 sensing performances. 4. Conclusion Generally, we successfully synthesized the Pd/In2O3 composites with yolk-shell structure by incorporating a MOF-templated route with a heat treatment. The assynthesized Pd/In2O3 composites as sensing platform exhibited superior CS2 sensing features with a detection limitation of 1ppm as well as a good long-term stability. Moreover, in contrast with pristine In2O3 particles, this Pd/In2O3 sensor not only showed the greatly enhanced gas response, but also provided the ultra-selectivity for CS2 gas molecules. Specifically, density functional theory calculations revealed that intermediate S, produced by Pd-catalyzed desulfurization reaction, on the Pd/In2O3 surface is a key to achieve high CS2 gas response as well as ultra-selectivity, which is well agreement with the XPS analysis results, during such CS2 sensing process. On 17



the basis of these results, we believe that this work will offer a new idea for fabricating gas sensors with excellent sensing properties, especially ultra-selectivity. Acknowledge The authors acknowledge the financial supports from National Key R&D Program of China (2016YFC0201103), the financial supports from Natural Science Foundation of China (Grant No. 11674320 and 51471161), Youth Innovation Promotion Association CAS, and Key Research Projects of the Frontier Science CAS ( QYZDB-SSW-JSC017). Supporting Information Detailed computational method and the used model, the typical schematic figure of the plate electrode, SEM images of the In-MOF spheres, SEM images of the InMOF at different concentration of In(NO3)3 and IPA, SEM, TEM and EDX images of Pd/In-MOF, schematic illustration of the formation procedure of the yolk/shell Pd/In2O3, SEM images of In-MOF, In2O3 and Pd/In2O3, and the corresponding TEM images, the CS2detection limit and the stability testing of Pd/In2O3 sensors, the selectivity comparison between In2O3 and Pd/In2O3, the NO2 gas response of Pd/In2O3 sensor, O1s high-resolution XPS spectra of In2O3 and Pd/In2O3, respectively, before exposure to CS2 gas molecules, the baseline resistance of In2O3 and Pd/In2O3 toward CS2 gas, the top and side view of In2O3 (222) and Pd (111)/In2O3 (222) structure, the Eads of CS2, H2S, CH3SH, CH3SCH3, (CH3)2S2 in the Pd/In2O3 surface model, the Eads of CS2 on the In2O3 (222) and Pd (111)/In2O3 (222), respectively. Reference 1. Chen, Y.; Ebenstein, A.; Greenstone, M.; Li, H., Evidence on The Impact of Sustained Exposure to Air Pollution on Life Expectancy from China’s Huai River Policy. PANS, 2013, 110 (32), 12936-12941. 18


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Scheme 1. Schematic illustrations of the synthesis procedure of yolk-shell Pd/In2O3 composites. 24


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Figure 1. (a) XRD patterns of the synthesized Pd/In-MOF (black curve), PdO/In2O3 (red curve), Pd/In2O3 (blue curve), respectively. XPS high-resolution spectra of Pd/In2O3 composites: (b) In 3d, (c) Pd 3d, and (d) O 1s.

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Figure 2. (a), (b), (c) SEM images of Pd/In2O3 composites; (d), (e) the corresponding TEM images and (inset) HRTEM image; (f) EDX element mapping images of Pd/In2O3 composites.

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Figure 3. (a) Relationship between gas responses based on pristine In2O3 and yolk-shell Pd/In2O3, respectively, to 10 ppm carbon disulfide (CS2) gas and different working temperatures, (b) the respond/recovery time of Pd/In2O3 based sensors to 5ppm carbon disulfide at 135 oC, (c) gas response value toward 10 ppm CS2 at 135 oC for the pure In2O3 particles and yolk-shell Pd/In2O3 with different Pd loading amount, (d) reproducibility of Pd/In2O3 based sensors exposed to 10 ppm CS2 for five-cycle testing at135 oC, (e) sensing response vs. different concentrations of CS2 gas based on pure In2O3 sensors (black curve) and Pd/In2O3 sensors (pink curve); (f) the corresponding linear relationship. 28


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Figure 4. The selectivity comparison between In2O3 sensors and Pd/In2O3 based sensors toward 10 ppm different target gas at 135 oC.

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Figure 5. (a) Schematic illustration of the possible mechanism of Pd/In2O3 composites toward CS2 at 135 oC. The S 2p high-resolution XPS spectra of (b) pure In2O3 and (c) Pd/In2O3 after exposure to 10 ppm CS2 gas molecules at 135 oC.

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Figure 6. The calculated energy profiles for the surface reaction on Pd/In2O3 composites and pristine In2O3, respectively, during CS2 sensing process. The x axis represents the reaction intermediates and transition states (TSs), and the y axis represents the relative energy of each state.

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In2O3 surface

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