Polypyrrole Nanospheres with

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Cu2+-Doped SnO2 Nanograin/Polypyrrole Nanospheres with Synergic Enhanced Properties for Ultrasensitive Room-Temperature H2S Gas Sensing Jian Shu, Zhenli Qiu, Shuzhen Lv, Kangyao Zhang, and Dianping Tang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b03491 • Publication Date (Web): 26 Sep 2017 Downloaded from http://pubs.acs.org on September 30, 2017

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Analytical Chemistry

Cu2+-Doped SnO2 Nanograin/Polypyrrole Nanospheres with Synergic Enhanced Properties for Ultrasensitive Room-Temperature H2S Gas Sensing Jian Shu, Zhenli Qiu, Shuzhen Lv, Kangyao Zhang, and Dianping Tang* Key Laboratory of Analytical Science for Food Safety and Biology (MOE & Fujian Province), State Key Laboratory of Photocatalysis on Energy and Environment, Department of Chemistry, Fuzhou University, Fuzhou 350116, People's Republic of China *Corresponding Author: Phone: +86-591-2286 6125. Fax: +86-591-2286 6135. E-mail: [email protected]. ABSTRACT: The organic-inorganic nanohybrids are emerging as one of the most attractive sensing materials in the area of gas sensors and usually exhibit some advanced properties because of synergetic/complementary effects between organic molecules and inorganic components. This work demonstrates a novel class of organic-inorganic nanohybrids, Cu2+-doped SnO2 nanograin/poly pyrrole nanospheres, for the sensitive room-temperature H2S gas sensing. Doping Cu2+ in SnO2 nanograins remarkably enhances the surface potential barrier by tailoring surface defects. After polymerizing pyrrole surrounded nanograins in aqueous media to form the organic-inorganic nanohybrids, the resulting nanoheterojunctions further improve the sensitivity. Additionally, the nanohybridsbased sensor provides high surface area and abounding reaction sites to accelerate gas diffusion and adsorption as well as the electron transfer. Compare with pristine SnO2 nanograins alone, the sensitivity of using the nanohybrids increases 7 times for the detection of 50-ppm H2S. The response and recovery rate can increase 27 and 22 times at room temperature, respectively. Significantly, this work provides an attractive material for the real-time monitoring of H2S, whereas the insights into organic-inorganic composite interactions within the sensing mechanism may pave the way for designing functional materials with tailored properties.

Hydrogen sulfide (H2S; a typical toxic, corrosive and flammable gas) widely exists in industrial production, the agricultural activity and people's daily life.1,2 As a highly toxic gas that affects the nervous system and poisons multiple metabolic systems in the human body, H2S is limited below 10 parts per million (ppm) according to Occupational Safety and Health Administration (OSHA) permissible exposure limits.3,4 Additionally, H2S is also recognized as an endogenously produced gasotransmitter in living organisms because it exists in the tissues with low concentrations to fulfill normal physiological functions.5-7 So, sensitive monitoring of trace H2S is particularly imperative to both human production and life. Nowadays, great attention has been paid to the development of H2S gas sensors owing to their prospect for sensitive and real-time monitoring at a low cost. As concepts of gas sensing platforms are relatively simple, ongoing progress in this field has been scored over the last several decades. It is well-recognized that the sensing properties (e.g., sensitivity, selectivity, stability and recovery time) are highly dependent on the sensing materials.8,9 In this regard, the high-performance sensing materials are the core element for the excellent gas sensor development, especially for the trace-amount gas detection and the real-time monitoring.10,11 Thanks to the advantages over the low cost, good reliability and high compatibility with microelectronic processing, SnO2 (a typical n-type metal oxide semiconductor with a bandgap of 3.6 eV) has been recognized as the representative material and intensively explored with various morphologies and modifications for gas sensing.12,13 Particularly, SnO2 matrixes incorporated with Cu exhibit the most promising for H2S gas sensing

due to their strong interaction.14 Lee and co-workers found the response of CuO-doped SnO2 nanowires toward H2S (20 ppm) at a ratio of as high as 809 at 300 °C and an increase of up to 74-fold response after doping CuO with SiO2 nanowires.15 Just like traditional metal oxide semiconductors-based gas sensors, however, SnO2 also suffers from poor selectivity and recoverability at room temperature (RT) because adsorption, desorption and chemical reactions of gas molecules only occur quickly and efficiently on the surface of the sensing materials at high temperatures (normally above 150 °C).16 Obviously, the high-operating temperature not only decreases stability and service life, but severely hinders their practical applications in many areas, such as low temperature and explosive environment. Moreover, the heating undoubtedly increases the energy consumption and complicates the sensing devices. Developing reliable gas sensors with the satisfactory performance that can be operated at or near room temperature is still a big challenge. Compared with the most inorganic compounds, the conducting polymers including polypyrrole (PPy), polyaniline, polythiophene and their derivatives exhibit the improved sensing characteristics such as rapid response and recovery ability at relative low temperature and are recognized as the next-generation sensing materials.17-19 Among these conducting polymers, PPy with high electrical conductivity, redox properties and surface charge tunable characteristics has attracted considerable attention.20 Furthermore, PPy with relatively good chemical stability against atmospheric conditions is easily prepared by using routine methods.21 Even so, the poor mechanical strength and sensitivity are its inherent disadvantages. The drawbacks of the inorganic or organic compounds alone that work as the

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Figure 1. SEM images of (a) Cu2+-doped SnO2 nanograins (dNGs) (inset: HRTEM image), (b) polypyrrole nanospheres (SP) (inset: magnification image) and (c) Cu2+-doped SnO2 nanograin/polypyrrole nanospheres (dNG@SP) (inset: EDS data); (d) Raman spectra of SnO2 nanograins (NGs) (black) and dNGs (red) (inset: Raman spectra of SnO2 doped with various-amount copper); (e) XPS spectra of dNG@SP (inset: XPS spectra of Cu 2p); and (f) XRD patterns of NGs, dNGs and dNG@SP (inset: XRD pattern of polypyrrole, PPy).

sensing materials restrict their development and application. To overcome these problems, researchers have proposed the organic-inorganic nanohybrids with different combinations of two/more components. Gas sensors based on nanohybrids usually exhibit the enhanced overall performance than those of single component.22,23 The yearly increased researches showed that rational hybridization of various components could erode or eliminate particular drawbacks ascribed to their synergetic or complementary effects between the different components.2426 Followed by this research idea, the general strategies for the exploitation of organic-inorganic hybrids employ the inorganic component as the protective matrix while the organic moiety as the sensing component to improve stability and selectivity. In this work, an overwheling strategy on the basis of inorganic and organic compounds to cooperate the sensing mechanism and generate the synergetic effects is proposed for essentially improving the gas sensing performance. The Cu2+-doped SnO2 nanograin/polypyrrole nanospheres (dNG@SP) is prepared by in-situ chemical oxidative polymerization of pyrrole in Cu2+doped SnO2 nanograins (dNGs). Use of dNG@SP is expected as a promising candidate to monitor the low-concentration H2S.

EXPERIMENTAL SECTION Fabrication of Gas Sensor and Performance Evaluation. Synthesis and characterization of Cu2+-doped SnO2 nanograin/ polypyrrole nanospheres (dNG@SP) were described detailedly in the Supporting Information. Sensor device was fabricated by coating homogeneous slurry of the corresponding sensing materials on the gold (Au) interdigital electrode, followed by keeping for 2 h at 60 °C to form a porous solid sensing film (note: The uniformly covered film was connected with gold electrode to provide conductive path). Thereafter, the sensor connected circuit was inserted into the self-made test chamber. Air was used as a reference gas and all operations were carried

out at RT. The real-time precise concentration of H2S was calibrated through a high-accuracy SGA-500B-H2S detector (Detectable range: 0-50 ppm; Resolution: 0.01 ppm; Singoan Electron. Technol. Inc., Shenzhen, China; www.singoan.com). The electric current was recorded as the signal of sensor by AutoLab electrochemical workstation (µAUTIII.FRA2.v, The Netherlands). The sensor response (S) is defined as S = (Ig Ia)/Ia, where Ig and Ia are the corresponding steady-state currents of the sensor collected in H2S and air, respectively. The response and recovery time are defined as the period in which the sensor current increase to 90% and decrease to 10% of the equilibrium response value toward H2S gas, respectively.

RESULTS AND DISCUSSION Characterization of the Nanohybrids. Under the structure direction effect of cetyltrimethylammonium bromide, pyrrole monomers are aggregated around the dNGs in aqueous dispersion. Upon addition of ammonium persulfate, pyrrole is rapidly oxidized to polypyrrole (PPy), and numerous dNGs are encapsulated on the surface or interior of PPy nanospheres. The successful doping of Cu2+ into SnO2 was the first critical step for the dNG@SP formation, which was proved by Raman spectra. As shown in Figure 1d, SnO2 nanograins (NGs) exhibited four peaks at 314, 478, 634 and 777 cm-1 that were fundamental Raman peaks of rutile SnO2. Without variation of Raman scattering was observed between dNGs and NGs except a shift of the peak from 634 to 631 cm-1 with a slight increase of band width. With the further increment of Cu2+doped amount up to 10%, the peak shifted to 629 cm-1, which was an important character of Cu2+ successful doping into SnO2.27,28 The oxidative polymerization of pyrrole was also carefully investigated and proved by IR spectra (Figure S-1). The scanning electron microscopy (SEM) images of dNG, PPy and dNG@SP are shown in Figure 1a-c. The dNGs were dens-


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er nanograins with legible outline and the average diameter was about 10 nm. PPy appeared irregular shape with highly smooth and clean surface, while most dNG@SP nanohybrids were regular nanosphere (50 – 200 nm) with bumpy surface that contained numerous white rough dots of dNGs. The dNGs on the surface of the PPy matrix with different covering thicknesses were proved preliminarily by energy dispersive X-ray spectroscopy (EDS) analysis. The major elements of C, O, N, Sn and a little of Cu were appeared as expected in dNG@SP (Figure 1c, inset). Additional, X-ray photoelectron spectroscopy (XPS), a typical surface analysis technique, further confirmed the surface component and chemical states of dNG@SP (Figure 1e). Consistent with the EDS analysis, five major elements were observed in survey scan spectrum. The existence of Cu2+ in the dNG@SP was evidenced by the binding energy of Cu 2p spectrum (Figure 1e, inset). The characteristic peaks appeared at 932.3 and 952.5 eV should be ascribed to Cu 2P3/2 and Cu 2P1/2, respectively. The satellite peaks around 941.6 and 961.0 eV indicated that Cu element existed in the form of Cu2+.29 The O1s peak was decomposed into four peaks and analyzed in detail later. The attribution and analysis of Sn 3d, C 1s and N 1s spectrum are shown in Figure S-1. To examine the effect of Cu2+ doping and PPy hybridizing on the crystal phase of SnO2, XRD analysis was conducted. As show in Figure 1f, all the distinct diffraction peaks of NGs, dNGs and dNG@SP could be indexed as rutile structure (JCPDS No. 41-1445 a0 = 4.738 Å, c0 = 3.178 Å) and no impurity phase was observed.30 Considering ionic radius of Cu2+ (0.69 Å) and Sn4+ (0.71 Å) is close and bond length of Cu-O bond and Sn-O bond is also comparable, the Cu2+ entrance into SnO2 crystal lattice by replacing Sn4+.31,32 According to the XPS and EDS results, the actual amount of Cu2+ in SnO2 bulk was far less than its concentration in precursor solution. The very low proportion of Cu2+ (~0.74 wt % analyzed by the EDS for dNGS prepared with 5% of Cu2+ doping in precursor solution) was not sufficient to change the lattice structure. Due to the week diffraction intensity of PPy (Figure 1f, inset), no character peak of PPy was observed in dNG@SP. Additionally, nearly identical peak shapes of dNGs and dNG@SP demonstrated that oxidative polymerization process had no influence on the structure of inorganic component. Since the dNG@SP was prepared by in-situ chemical oxidative polymerization of pyrrole instead of simply mechanical mixing and FT-IR spectra results also demonstrated the interaction between inorganic and organic component, it was probable that the organic and inorganic component within the nanohybrids could enhance or decorate each other with new properties from molecular level comparing with single component. Sensing Performance of the Nanohybrids. The H2S gas sensors were fabricated by a simple coating method. The schematic diagram of the sensing device and mechanism are shown in Figure 2a. The photographs and surface micrographs of Au interdigital electrode before and after coated with dNG@SP film are shown in Figure 2b-e. To preliminarily explore the sensing characteristics of the dNG@SP-based sensor and electrical contact properties between dNG@SP film and Au electrode, the current-voltage (I–V) curves were measured at RT in different atmospheres and plotted in Figure 2f. The distinctly S-shaped I-V curves suggested that the Schottky contact instead of Ohmic behavior existed at the contact interface.33 Current changed slowly with the potential variation in

air while it changed dramatically in H2S atmosphere. This I-V characteristic was attributed to the reduction of potential barrier in H2S gas. It also clearly proved that dNG@SP acting as the active sensing element had response to H2S at RT and might be well employed to detect H2S. One of the current challenges in gas sensing field is real-time monitoring at a relatively low temperature. Here, the dynamic response characteristics of different materials toward low (0.6 ppm), middle (10 ppm) and high (50 ppm) concentration of H2S gas were fully assessed at RT to explore the sensing behavior under realistic operating conditions as well as the effects of Cu2+ doping and PPy hybridizing. The detailed statistics about response amplitude, response time and recovery time are summarized in Figure 2g and Figure S-2, respectively. The sensors exhibited switch like characteristics with significantly different response amplitude when alternately exposure in H2S and air. The sensors’ resistances decreased upon exposure in H2S and recover to initial value for some time in air. According to the sensing properties of NGs, NG@SP and dNGs (Cu2+doped amount is preferably 5% for discussion) observed form the testing, we can conclude that hybridizing with PPy could significantly shorten the time for response as well as recovery and Cu2+ doping could enhance the sensitivity. On the whole, NG@SP and dNGs-based sensors were insensitive to low level of H2S and the respond or recovery were slow to high level of H2S, but they were superior to the NGs, either from sensitivity or from recovery rate. With the amount of Cu2+ doping increasing from 0 to 5% in dNG@SP, the time needed for response and recovery shortened slightly and the sensitivity increased remarkably. The performance gradually decreased with the further increment of Cu2+ doping level in the range of 5-10%. Apparently, the dNG@SP with 5% Cu2+ doping was an excellent sensing material with the most satisfactory dynamic response in all cases and it was defined as dNG@SP for the following discussion. Especially, the dNG@SP exhibited remarkably enhanced overall sensing performance (Figure 2h). Comparing with that of NGs, the sensitivity of dNG@SP was improved over 7 times for the 50 ppm of H2S detection at RT and the response and recovery rate are increased nearly 27 and 22 times, respectively. The dynamic responses of dNG@SP toward various concentrations of H2S are shown in Figure 2i. The NG@SP and dNGs were also evaluated for comparison (the corresponding dynamic response curves are shown in Figure S-3), but single component (NGs and PPy) were not considered as their low sensitivity. Consistence with the above result, the dNG@SP showed highest sensitivity while NG@SP has lowest sensitivity to all concentrations of H2S. Inspiringly, the response of dNG@SP toward H2S is still appreciable (about 4) even concentration as low as 0.05 ppm. The dependence of the sensor’s response on H2S concentration was approximately linear in the range from 0.3 to 50 ppm and the fitted linear regression equation for the calibration curve was S = 11.50 + 11.47 × C[H2S] (ppm, R2 = 0.97) with maximum relative standard deviations 6.5% (Figure 2j). Throughout the entire concentration range, the response and recovery were fast and reversible at RT, indicating its excellent dynamic response properties to ensure the real-time detection of H2S at RT.



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Figure 2. (a) Schematic diagram of the gas sensor; (b, c) Photographs and (d, e) surface micrographs of Au interdigital electrode (b, d) before and (c,e) after coating with dNG@SP; (f) I-V characteristic curves of dNG@SP-based sensor exposured in air and H2S; (g) Sensing responses of different materials (x% dNG means dNGs synthesized by x% Cu2+ doping amount in precursor solution) to three concentrations of H2S and (h) dynamic response curves for 50 ppm of H2S; (i) Sensor response of different nanocomposites-based sensors versus H2S concentration; (j) Dependence of the response upon H2S concentration of dNG@SP-based sensor (inset: the corresponding dynamic response curve); (k) Long term stability and (l) selectivity testing of dNG@SP-based sensor.

To investigate the thermal stability of dNG@SP, it was heated to 80 °C in air for 24 h. The ignorable structural change before and after heat treatment proved by XRD patterns and FT-IR spectra (Figure S-4a,b) revealed the good thermal stability and a wide operating temperature range of the [email protected] The response of the dNG@SP-based sensor was almost the same and could quickly recover to the initial value during the successive exposure to H2S and air (Figure S-4c). The relative standard deviation (RSD) of response to 2 ppm and 50 ppm of H2S in cyclic sensing tests were 3.5% and 2.3%, respectively. Similar results were also obtained from the other two sensors that fabricated with different batches of dNG@SP under the same conditions. The variation coefficients of inter-assay towards H2S with concerntrations of 0.6, 10 and 50 ppm were less than 13% (RSD). Additionally, the sensor exhibited stable response for one month without the obvious attenuation (Figure 2k). These results suggested the high stability and repeatability of the dNG@SP-based sensor.35 Last but not least, selectivity, a crucial parameter for gas sensors should be evaluated to consider the distraction from other components in practical application. As shown in Figure 2l, the response of dNG@SP-based sensor toward water vapour, CH3OH, HCl, NH3 SO2 and NO2 at RT were small in comparison with the response toward 10 ppm H2S, suggesting the good selectivity against these gases. To investigate H2S sensing performance, dNG@SP-based sensor was compared comprehensively with other recently reported chemiresistive sensors operated at a relatively low temperature and the results are summarized in Table S-1. Apparently, the proposed dNG@SP-based sensor was comparable with (even superior to) the other sensors in overall performance. The dNG@SP was an excellent sensing material for RT H2S detection with high sensitivity, fast response and recovery rate, satisfactory selectivity and long term stability.

These results also intuitively demonstrated that Cu2+ doping and PPy hybridizing could be a promising strategy to tailor the SnO2 sensing properties, but the important roles of Cu2+ and PPy in the organic-inorganic nanohybrid of dNG@SP needed further investigation. Sensing Mechanism of the Nanohybrids. According to the sensing tests, the electrical resistance of the nanohybrids was closely linked to the surface properties associated with adsorbed gas molecules. TEM images showed no variation in sizes and morphology between the NGs and dNGs (Figure 3a). Cu2+ doping had a negligible effect on BET specific surface area (SBET) (Table S-2). XRD results also demonstrated the Cu2+ doping caused no lattice structure modification of SnO2 or no new phase creation of Cu as no characteristic peak variation observed. Thus, the improved sensing performance of dNG might be attributed to the Cu2+ doping itself. From the mainstream understanding to the semiconductors-based chemiresistive gas sensors, sensing properties are dominantly dependent on the width of charge depletion layer (w) that resulting from gas adsorption and it could be expressed as w = LD (2eVs / kT)1/2, where LD, e, k and T represent the Debye length, electron charge, Boltz-mann constant and absolute temperature, respectively.36 The eVs is the adsorbate induced surface potential barrier and could be classically expressed as eVs = (eNt)2 (2ε0εrNd)-1, where Nt, ε0, εr and Nd represent the density of the adsorbed oxygen ions, permittivity of free space, dielectric constant and charge carrier density, respectively.37,38 Obviously, the Vs is a function of temperature and pressure and depends on the density of the adsorbed oxygen species and carrier concentration, which influences w and thus sensitivity of the sensors. Therefore, exploring the difference of adsorbed oxygen species and charge carrier density before and after Cu2+ doping is the breakthrough of understanding improved sensing mechanism.


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Figure 3. (a) The size statistics of NGs and dNGs (inset: the corresponding HRTEM images; Bar scale: 20 nm); XPS spectra of O 1s for (b) NGs and (c) dNGs; (d) EPR spectra of NGs and dNGs at 123 K; (e) Structural model of dNGs in air; and (f) Charge depletion layer and band model of NGs (left) and dNGs (right).

As mentioned above, Cu2+ substituted for Sn4+ in SnO2 matrix. It is well-known that appropriate vacancies will be compensated in the host lattice when ions are replaced by foreign atoms with different charges.32 Usually, oxygen vacancies are recognized as the most common defects in metal oxide.39 According to the previous theoretical research, the formation energy of oxygen vacancies is substantially reduced in the Cu2+-doped SnO2 under the same conditions and Cu2+ tends to doping in a very shallow region near the surface of SnO2.32,40 Thus, substituting Cu for Sn promotes the generation of oxygen vacancies in the SnO2 host. Since surface oxygen vacancies of metal oxides could act as O2 adsorption sites and the adsorbed oxygen species played a decisive role in the gas sensing performance,41 the chemical states of Sn and O in the NGs and dNGs were investigated by XPS. As shown in Figure 3b-c, different levels of asymmetry of O1s peaks located around 530 eV demonstrate them containing multiple valence states and the O1s peaks were deconvoluted into three symmetrical peaks by Gaussian distribution. The lowest (530.4 ± 0.1 eV) and highest (532.5 ± 0.1 eV) binding energy components were usually attributed to the lattice oxygen and surface adsorbed oxygen, respectively. The medium binding energy (531.6 ± 0.1 eV) was associated with oxygen vacancies of the host and the relative peak area could reflect the oxygen vacancies concentration.42,43 The remarkably different relative peak areas with binding energy around 531.6 eV obtained from NGs (27.8%) and dNGs (36.4%) demonstrated that Cu2+ doping effectively created more oxygen vacancies. According to the chemical composition quantitative analysis of XPS, the ratio of Sn/O in the dNGs was approximately 1.19 times over that of NGs, indicating the existence of more abundant nonstoichiometric oxygen vacancies in dNGs.44 Since oxygen vacancy center easily traps an electron to form a singly ionized oxygen vacancies (Vo•), which causes electron paramagnetic resonance (EPR) signal, EPR spectra of NGs and dNGs were evaluated to intuitionally evidenced the increase of oxygen vacancies. As expected, NGs and dNGs exhibitd different EPR signal intensity with g = 1.999, which implied they possessing

the same type of electron paramagnetic resonance source with different amounts. According to the g value and considering the fact that there were no unpaired electrons in the structure of Sn4+, oxygen vacancy should be responsible for EPR signals.28,45 Apparently, introduction of Cu2+ into NGs significantly increased the amount of oxygen vacancy because the EPR signal intensity was multiplied in dNGs. It should be worth mentioning, UV-vis diffuse reflectance spectrum (DRS) results showed that Cu2+ doping expanded the absorption spectrum and the corresponding energy band gaps for NGs and dNGs were 3.49 and 3.32 eV, respectively (Figure S-5). This result implied the increased oxygen vacancies not only affected local density of states, but also generated new energy level in the matrix.39 The oxygen vacancies acted as gas adsorption or reaction sites to form numerous negatively charged oxygen adsorbates (O2- and O22-) by trapping electrons from the metal oxide and decreased the charge carrier density, which induced an electron depletion layer and blocked the electronic transport (Figure 3e).36,46,47 Appropriate Cu2+ doping provided more gas adsorption sites and the adsorbed O2 molecules more easily trapped electrons from the metal oxide. Thus, the electron depletion layer of the dNGs (Figure 3f, right) was larger than that of the NGs (Figure 3f, left). The relative change of current during dNGs exposure to air and H2S was dramatic due to the initial higher potential barrier height, which remarkably enhanced the sensing response. However, overly doping Cu2+ into SnO2 would cause heavy compensation and extremely low carrier concentration, manifesting as high isolation even transform conduction from ntype into p-type.48 With the Cu2+ doping concentration increased from 1% to 5%, Vs and w increased and resultant sensing performance improved. Further increasing Cu2+ concentration from 5% to 10%, Vs and w gradually disappeared, which badly affected the sensing performance. The increase of adsorbed oxygen species at surface and decrease of the charge carriers density resulted by Cu2+ doping were the pivotal elements in the sensing performance enhancement. Thanks to their extremely small dimensions, over a dozen orders of mag-



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nitude surface oxygen sites per cubic centimeter existed in semiconducting oxide nanograins.49 Thus, even an incredibly small concentration variation in adsorbed oxygen species, significant change in conductivity of sensing material could be obtained. Also, NG@SP, especially dNG@SP exhibited higher sensitive toward H2S than those of PPy and NGs. Different from other reducing gas to decrease the conductivity of p-type semiconductor, H2S exhibits proton acid doping effects to some polymer, such as polyaniline and PPy.50-54 On the surface of nanohybrids with the Cu2+ doping, the partially absorbed H2S easily dissociated into H+ and HS- (S2-) and the H+ subsequently protonated the PPy, which caused ionic conduction to increase the conductivity. Proton acids also might cause the inter-molecular and inner-molecular delocalization in the PPy to improve the conductivity. This is in agreement with the weak interference from HCl (Figure 2l). On the other hand, the dissociated S2- with high reactivity would directly react with the metallic or semiconductive portions.55-56 This chemical conversion changed the conductivity is considered to be an important sensing mechanism, which might be a reasonable reason for high selectivity to H2S.57-58 Since charge carrier transfer between the gas molecules and sensing materials resulting electric resistance changes mainly occurs on the surface, a large surface to volume ratio is crucial to achieving a high response. The dNG@SP exhibited a highest SBET (243.5 m2 g-1) compare with NGs (37.8 m2 g-1) and PPy (105.7 m2 g-1). The high surface area had more chances to adsorb and desorb gas molecules which accelerated gas diffusion as well as the electron transfer at a relatively low temperature.

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interface provided additional potential barrier. In such organicinorganic heterostructures, two different-type depletion layers would coexist (Figure 4b). The one depletion layer (І) that caused by adsorption of oxygen species located at the surface of NGs, while another depletion layer (П) associated with organic-inorganic heterojunction existed at the contact interface of PPy and NGs.59 These depletion layers were both modulated by the surrounding atmosphere. In air, the O2 molecules adsorbed at surface sites trap electrons from the junction and raise surface potential barrier. Thus, the electronic interaction between PPy and NGs was difficult and the very low current of the sensors was observed. Upon exposure sensor to H2S atmosphere, the reactions between H2S and the adsorbed oxygen species can be simplified as follow: H2S (gas) + O2- (ads) → SO2 + e-

(1)

H2S (gas) + O22- (ads) → SO2 + 2e-

(2)

The electrons were released back to the sensing material surface, which shifted the Fermi level away from the VB toward the CB and drastically decreased potential barrier in the depletion region.60 The adsorbed gas molecules extracted or donated electrons determining the depth of the surface depletion layer, which in turn effected the heterostructure depletion layer.61 Coupling with the effect of Cu2+ doping on oxygen vacancies which adsorbs more oxygen species to significantly increase the surface potential barrier, the two types of potential barrier height of dNG@SP (Figure 4c) were amplified compared with that of NG@SP (Figure 4b). Namely, there was a synergetic effect between Cu2+ doping and PPy hybridizing within dNG@SP. For NGs, an ignorable electron depletion layer was formed because chemisorption of oxygen species hardly occurs on the surface of NGs at RT. The electron injection from H2S into NGs was not apparent, which caused low and slow response toward H2S (Figure 4d, curve 'a'). Additional potential barrier formed at p-n heterojunction in NG@SP augmented resistance change and improved the performance to a certain extent (Figure 4d, curve 'b'). The Cu2+ doping improved the response by increasing surface potential barrier of NGs (Figure 4d, curve 'c'). In addition, the improved surface potential barrier further affected the heterojunction potential barriers between the n-type dNGs and the p-type PPy. Eventually, a series of interactions in the multi-barrier system altered resistance of dNGs@SP significantly upon exposure to oxidizing and reducing gases (Figure 4d, curve 'd'). The synergetic effects within the dNG@SP was the root cause of the improved comprehensive sensing performance for H2S gas.

CONCLUSIONS Figure 4. (a) The energy band diagram in dNG@SP nanohybrids; (b,c) Schematic diagrams of two-type depletion regions and threepotential barriers within (b) NG@SP and (c) dNG@SP in air (Ec and Ef represent the conduction band and Fermi energy, respectively); and (d) Schematic illustration of the response sensitivity to H2S for different materials.

Above all, only the homojunction was formed in the pure PPy and NGs-based sensors. After n-type inorganic NGs hybridizing with p-type organic PPy, the holes within the PPy and electrons within the NGs migrated to each other due to the difference in work function (Figure 4a). The band bent on both side of interface to accommodate the equalization of their Fermi levels. Consequentially, the space charge regions (depletion layer) associated with the p-n heterojunction at the

In summary, Cu2+-doped SnO2 nanograin/polypyrrole nanospheres (an organic-inorganic nanohybrid) are synthesized by two steps for H2S sensing. Results revealed that the dNG@SP showed the remarkably improved performance compared with single component. The sensitivity of dNG@SP was improved 7 times for 50 ppm of H2S detection at RT. Moreover, the response and recovery rate increase nearly 27 and 22 times, respectively. The organic-inorganic nanohybrids also showed superior performance in comparison with the sensors reported recently. Different from other organic-inorganic nanohybrids, the organic and inorganic components of dNG@SP cooperated in the sensing mechanisms and generated synergetic effects. Cu2+ doping created more shallow surface oxygen vacancies and provided extra active site, thereby efficiently improving


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the adsorption of oxygen species and surface potential barrier of SnO2 nanograins. Cu2+-doped SnO2 nanograin hybridizing with PPy nanosphere formed organic-inorganic nanostructures and additional electron depletion layers were created at the heterostructure interface. All of these greatly manipulated the variation of charge depletion layer exposed different gaseous environment and thus significantly improved comprehensive sensing performance. This work not only provides a low cost, excellent material for H2S gas sensing, but paves the way for designing new materials with enhanced properties for various applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.0000. Experimental section, FT-IR and XPS spectra (Figure S-1), response and recovery time of various materials (Figure S-2), dynamic responses of NG@SP and dNGs (Figure S-3), stability test (Figure S-4), comparison of different H2S gas sensors (Table S-1), BET specific surface areas (Table S-2) and DRS spectra (Figure S-5) (PDF)

AUTHOR INFORMATION Corresponding Author * Phone: +86-591-2286 6125. Fax: +86-591-2286 6135. E-mail: [email protected] (D. Tang)

Notes The authors declare no competing financial interest

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21675029 & 21475025), the National Science Foundation of Fujian Province (2014J07001), and the Program for Changjiang Scholars and Innovative Research Team in University (IRT15R11).

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Polypyrrole Nanospheres with

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