ZnO 1D Fibrous

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Multi-Level Effective Heterojunctions Based on SnO2/ZnO 1D Fibrous Hierarchical Structure with Unique Interface Electronic Effects Hui Li, Shushu Chu, Qian Ma, Hang Li, Quande Che, Junpeng Wang, Gang Wang, and Ping Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10410 • Publication Date (Web): 02 Aug 2019 Downloaded from pubs.acs.org on August 3, 2019

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ACS Applied Materials & Interfaces

Multi-Level Effective Heterojunctions Based on SnO2/ZnO 1D Fibrous Hierarchical Structure with Unique Interface Electronic Effects Hui Li, Shushu Chu, Qian Ma*, Hang Li, Quande Che, Junpeng Wang, Gang Wang and Ping Yang*

School of Material Science and Engineering, University of Jinan, 250022 Jinan, P. R. China

KEYWORDS: SnO2 and SnO2/ZnO fibrous hierarchical structures, multi-level effective heterojunctions, electrospinning, gas sensor, trace detection in cosmetics.

ABSTRACT

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One-step single-spinneret electrospinning synthesis of 1D fibrous hierarchical structure can not only prevent the agglomeration or restacking of fibers or particles and enlarge surface active area, but also promote the directional migration of electrons in materials and achieve effective regulation of resistances. Herein, tunable SnO2 and SnO2/ZnO fibrous hierarchical structures with in-situ growth of monodisperse sphericallike particles on surface provide a new sight for adjusting component distribution, surface absorption and chemical reaction, electronic transmission path, and electron transfer efficiency. Compared with SnO2 porous fibers and SnO2 hierarchical structures, the optimal SnO2/ZnO sensors exhibit superior gas-sensing response value of 366 to 100 ppm ethanol at 260 oC as well as excellent gas selectivity and long-term stability, in which the enhanced gas-sensing mechanism is primarily derived from multi-level effective heterojunctions with unique interface electronic effects. Especially, these SnO2-based sensors can achieve favourable linear relationship of the response and gas concentration for sensitive trace detection in cosmetics for the first time, providing a new strategy to design composite materials for quantitative analysis of volatiles in cosmetics evaluation process. 2 ACS Paragon Plus Environment

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INTRODUCTION

Until now, ingredients in cosmetics have been only detected by high performance liquid chromatography (HPLC), gas chromatography (GC), and liquid chromatography (LC), which require a mass of harmful organic solvents and fussy pretreatment, and hence it is necessary to develop a new and facile method for quantitative analysis of volatiles in cosmetics evaluation process.1 Ethanol is the only solvent other than water that is allowed to be used in cosmetics. Due to some ingredients are difficult to dissolve in water, the addition of ethanol is beneficial for avoiding the thickening and greasiness of product texture and removing grease on skin surface. However, ethanol with excellent volatility is easy to drain water of skin away and markedly reduce water content of corneum, hardly recovered in a short time even if the elimination of ethanol. In addition, ethanol removes a lot of sebum that leads to a weak skin barrier, easily causing allergies. Therefore, the effective detecting and monitoring of targeted ethanol in cosmetics is essential for the quality of products and human's skin care.



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Gas sensors play an important role in detecting the leakage of a variety of flammable, explosive, volatile, and toxic gases which often exist under a complex condition and hence sensors should be highly upgraded in sensitivity and selectivity.2,3 SnO2, a typical metal oxide semiconductors, is paid much attention to the field of gas-sensing because of its high response value, low production cost, and good chemical stability. However, for the practical application of SnO2 gas sensors, its gas-sensing performances still need to be further improved from the perspective of response value, selectivity, and operating temperature.4-6 It is believed as an effective technique for enhancing the properties to design one-dimensional (1D) hierarchical structure sensors with high specific surface area, more active sites, and limited electronic transmission along axial direction which may be greatly beneficial to gas adsorption, diffusion, and desorption, resulting in a lager change in resistance, thereby improving gas sensitivity.7-9 For example, Wang et al. prepared SnO2 nanofiber/nanosheets with hierarchical nanostructures by electrospinning and hydrothermal method, promoting the ability to detect formaldehyde due to synergistic eﬀect of nanofibers and nanosheets, hierarchical

structures,

and

larger

specific

surface


areas.10

Khoang

et

al.

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synthesized

SnO2/ZnO hierarchical nanostructures via thermal evaporation and

hydrothermal processes with good gas-sensing performance, mainly resulting from homogenous and heterogeneous contacts.11 At present, most of 1D hierarchical structures of inorganic compound adopt a two-step fabrication process, in which matrix material is first synthesized and then the second phase is loaded on surface. However, the obtained heterointerfaces by two-step method usually have obvious disadvantages of weak bonding force along with the difficulty to regulate the modulation of electron transfer and potential barriers, then resulting in slightly improving gas-sensing performance.12-14 Although several hierarchical structures of semiconductor composites have been prepared by one-step method, the branches and backbones are composed of different components, forming the single heterojunction with the similar interface electron transfer behavior. For instance, Yang et al. obtained a novel branched SnO2/ZnO composite composed of ZnO backbones and SnO2 branches using one-step hydrothermal method, presenting the enhanced electron transfer efficiency because of heterojunction effects.15 But, the species and number of



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heterojunctions based on the interface structure consisting of independent component practically limit more improvement potential of gas-sensing properties. Electrospinning is widely used to synthesize continuous polymer or metal oxide semiconductors fibers owing to its low cost and simplicity in fabrication. Numerous studies have demonstrated that electrospun metal oxide semiconductors fibers have highly gas-sensitive behaviors due to their high specific surface area and high porosity compared with other materials such as nanosheets, nanowires, and nanoparticles.16-18 However, the introduction of nanoparticles onto the surface of fibers obtained by conventional electrospinning methods often require an additional synthesis procedure. For example, Shen et al. synthesized 1D SnO2 fibers decorated with In2O3 nanoparticles

employing

a

combination

of

electrospinning

and

hydrothermal

approaches.19 Lee et al. prepared SnO2 fibers decorated with N-doped ZnO nanonodules using single-nozzle co-electrospinning with a phase-separated, mixed polymer composite solution that still exist interface connection problem.20 Clearly, unique hierarchical structures with multi-level effective heterojunctions achieved by onestep electrospinning process have rarely been reported up to now. 6 ACS Paragon Plus Environment

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In this work, tunable SnO2 fibrous hierarchical structures with in-situ growth of monodisperse spherical-like particles on surface are synthesized by one-step singlespinneret electrospinning technique. Both the strong bonding force of interface and multiple transmission paths are conducive to the effective electron transfer. SnO2-based 1D hierarchical structure can efficiently prevent the agglomeration or restacking of fibers or particles and enlarge surface active area. The response of SnO2-600 sensors to 100 ppm ethanol at 260 oC is 119, superior to SnO2 porous fibers with a response of 13.2. SnO2/ZnO fibrous hierarchical structures with multi-level effective heterojunctions are constructed by introducing Zn component into precursor spinning solution. Both the backbones and branches of hierarchical structure are composed of SnO2/ZnO heterojunctions. The sensitivity of 7 mol% Zn sensors with novel interface effects can reach up to 366 to 100 ppm ethanol, 3 times higher than SnO2-600 sensors. More importantly, 7 mol% Zn sensors with excellent gas selectivity and long-term stability are employed for availably trace detection in cosmetics, revealing the potential of new detecting approach. The enhanced gas-sensing mechanism can be attributed to multi-



level effective heterojunctions of hierarchical microstructures with novel interface electronic effects.

EXPERIMENTAL SECTION

Materials. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) (Sinopharm Chemical Reagent Company), Polyvinylpyrrolidone (MW=1 300 000) (Aladdin Reagent Company), Tin(IV) chloride (SnCl4·5H2O) (Tianjin Chemical Reagent Institute), ethanol (anhydrous, AR) (Tianjin Chemical Reagent Institute), and dimethylformamide (DMF, anhydrous, 99.8%) (Tianjin Chemical Reagent Institute) are used as received without any further processing or refining. Preparation of SnO2 hierarchical structures. Firstly, 1 mmol SnCl4.5H2O are dissolved in the mixed solvent which contain 1 mL of ethanol and 4 mL of DMF. Secondly, 0.7 g PVP is added and the solution is stirring for 12 h. Thirdly, the spinning solution is electrospun: positive voltage is 18 kV, negative voltage is -0.5 kV, the distance of needle tip and collector is 18 cm, and 7 μL/min is selected as the optimal feed rate. Relative humidity and ambient temperature are maintained at 40 % and 35 oC during 8 ACS Paragon Plus Environment

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electrospinning process. Finally, PVP/Sn precursor fibers are placed in muffle furnace and heated to different calcination temperatures for 2 h at a heating rate of 1 oC/min, and cooled to room temperature. According to the calcination temperature, we name samples as SnO2-400, SnO2-500, SnO2-600, and SnO2-700, respectively. And samples are named as SnO2-600-0.5, SnO2-600, and SnO2-600-2 by adjusting the addition amounts of SnCl4.5H2O with 0.5, 1, and 2 mmol, respectively. Preparation of SnO2/ZnO hierarchical structures. The synthetic process of SnO2/ZnO hierarchical structures is similar to SnO2-600 sample procedure except adding various amounts of Zn(NO3)2·6H2O ( 3, 5, 7, and 10 mol%) in the spinning solution. PVP/Sn/Zn precursor fibers are calcined at 600oC for 2 h at a heating rate of 1 oC/min. Based on adding amount of Zn ingredients, samples are named as 3 mol% Zn, 5 mol% Zn, 7 mol% Zn, and 10 mol% Zn, respectively. Materials preparation and characterization. Samples are obtained by electrostatic spinning machine (FM-1206, Beijing Future Material Sci-tech Co., Ltd). A field-emission scanning electron microscope (FESEM, QUANTA 260 FEG, FEI, USA) and energy dispersive spectrometer (EDS) are used to investigate the morphology and composition 9 ACS Paragon Plus Environment


of samples. X-ray diffraction (XRD, D8-ADVANCE of Bruker Corporation, CuK radiation source) and transmission electron microscopy (TEM/HRTEM, Tecnai F20, FEI) are observed to examine crystal structures and phase composition of samples. High resolution Raman spectrometer (LabRAM HR Evolution, HORIBA JOBIN YVON SAS), UV-Vis spectrometer (Hitachi U-4100), multifunction adsorption instrument (MFA-140, Builder Company, Beijing), and X-ray photoelectron spectroscopy (XPS, ESCALAB 260) are employed to measure Raman spectra, UV-Vis diffuse reflectance spectra (DRS), the specific surface area and pore size distribution, and X-ray photoelectron spectroscopy (XPS) spectra, respectively. Gas-sensing measurement. CGS-4TPs (Beijing Elite Tech Co., Ltd) is used to test gas-sensing performances of samples. Several drops of water are mixed with samples to form a paste which brushes to Ag-Pd interdigital electrodes. The sensor is placed at room temperature for 12 h. Then, four sensors are located in a test chamber with a volume of 1.8 L, and then heated at the working temperature aging for 3 h. The resistances of sensors in air are obtained and defined as Ra. As closing test chamber


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and injecting different gases, the resistances of sensors in gases are obtained and defined as Rg. The gas-sensing response value of sensors is defined as Ra/Rg.

RESULTS AND DISCUSSION

Figure 1. (a) SEM image of as-spun PVP/Sn precursor fibers. (b) and (c) SEM images of SnO2-600. (d) and (e) TEM and HRTEM images of SnO2-600. (f)-(h) SEM image and the corresponding EDS elemental mappings of SnO2-600. (i) and (j) SAED patterns of fibers and particles based on SnO2-600.



SEM image of as-spun PVP/Sn precursor fibers is shown in Figure 1a. 1D fibrous structure with uniform diameters in the range of 550-650 nm can be observed. SnO2600 sample shows a typical hierarchical structure with reduced diameters due to the decomposition, densification, and crystallization of precursor during calcinations process (Figure 1b and c). The average diameters of fibers and particles dispersed on the surface of SnO2 fibers are about 90-120 nm and 220-310 nm, respectively. Figure 1d presents a hierarchical structure: monodisperse SnO2 particles are tightly attached to the surface of fibers, of which both structural units are composed of numerous small well-crystallized SnO2 nanocrystals. The sizes of fiber and particle are 95 and 230 nm, respectively, which are consistent with SEM results. As shown in Figure 1e, HRTEM image reveals that lattice spacings of 0.334 and 0.264 nm match reasonably with the (110) and (101) planes of tetragonal rutile SnO2, indicating that SnO2 fibers and particles possess good crystallinity. And it is clear that the interface between fibers and particles presents low energy plane (110) of SnO2 with good interface bonding owing to low energy stability. EDS elemental mappings in Figure 1f-h confirm that Sn and O elements are evenly distributed throughout the targeted area, suggesting the successful 12 ACS Paragon Plus Environment

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formation of SnO2 hierarchical structure. Meanwhile, SAED patterns in Figure 1i and j clearly demonstrate the good crystallinity nature of SnO2 fibers and particles, due to the observation of crystal planes of (110), (101), (211), and (301) for SnO2 fibers, and (110) and (101) for SnO2 particles.21



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Figure 2. SEM images: (a) and (e) 3 mol% Zn. (b) and (f) 5 mol% Zn. (c) and (g) 7 mol% Zn. (d) and (h) 10 mol% Zn. (i) Schematic formation of SnO2 and SnO2/ZnO hierarchical structures. Figure 2a depicts the low-magnification SEM image of 3 mol% Zn, of which a number of particles decorated on fibers with uniform size and good monodispersity are observed. The enlarged SEM image in Figure 2e provides the clear and typical SnO2/ZnO hierarchical structure, in which well-distributed fibers are composed of many small grains and monodisperse particles are closely contacted with the surface of fibers. As shown in Figure S1a, the diameters of fibers and particles are about 95-120 and 160180 nm, respectively, indicating the significant effect on the morphological evolution by introducing Zn component. As the adding amount of Zn component increasing from 3 to 10 mol%, it is obvious from Figure 2a-h that fiber diameters slightly decrease, and the average particle sizes distinctly reduce along with the amounts of particles increasing. For example, 7 mol% Zn sample still maintains hierarchical structure with lots of monodisperse and uniform nanoparticles covered on surface, providing an opportunity for

modifying

the

composition

distribution

of

inorganic


elements

and

the

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surface/interface electron transfer process. Figure 2i reveals the formation mechanism of SnO2 and SnO2/ZnO hierarchical structures. Figure S2 presents SEM images and XRD result of SnO2 sample obtained by lid covering during the calcination process, confirming that the product belongs to welldispersed SnO2 porous fibers. In fact, precursor fibers calcined without lid covering are evenly heated at sufficient O2 atmosphere under slow heating rate, which allows the preferential nucleation of nanocrystals on surface of matrix fibers. Unlike particles migration, Ostwald ripening process can be employed for understanding the crystal nucleation and growth process that occur in the liquid phase as well as the gas-solid phase. Although Ostwald ripening is considered to be hardly carried out in gas-solid phase reaction because of the disorientation of atoms, the formation process of SnO2 hierarchical structures might be attributed to the mechanism under the accurate fabrication parameters. It means that small particles can emit atoms or tiny clusters at high temperature to the adjacent larger particles, and these two particles eventually grow up and mature. Based on Ostwald ripening mechanism, smaller SnO2 nanoparticles on surface disappear in the initial crystal growth stage, together gathering 15 ACS Paragon Plus Environment


and forming larger particles attached into the matrix fibers, thereby obtaining SnO2 hierarchical structure. Figure S3 shows SEM images of the samples obtained at different temperatures. As shown in Figure S3a and d, a large number of nanoparticles are tightly attached on surface of fibers, proving the effect of Ostwald ripening on the crystal growth habit as the temperature increasing to 400 oC. As the temperature increasing to 500 oC, the diameter of matrix fibers is further contracted and most crystal grains on the surface grow up in Figure S3b and e, along with the enlarging of spacings between adjacent particles and the residual of nanoparticles. Morphological evolution of SnO2-400, SnO2500, and SnO2-600 completely accords with Ostwald ripening mechanism of small nanoparticles disappearing and big ones growing. Figure S3c and f demonstrate the destroyed hierarchical structure of SnO2-700 due to over-high calcination temperature. As shown in Figure S4a and b, the diameter of matrix fibers is uniform but the particles on surface are disordered and aggregated as Sn component in precursor spinning solution decreasing to 0.5 mmol. While Sn component is 2.0 mmol, the competition effect is greatly enhanced and lots of SnO2 grains tend to encase the surface of each 16 ACS Paragon Plus Environment

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fiber in Figure S4c and d. Thus, the adding amount of Sn source plays an important role in the adjustment of 1D fibrous microstructure. Additionally, Figure S5 indicates that hierarchical structure can be barely achieved as heating rate increasing to 5 oC/min, mainly resulting from over-fast kinetic reaction of the nucleation and growth of crystals. The introduction of Zn ingredient has been verified as an available approach for modifying composition distribution and surface/interface structures of SnO2/ZnO composites. The size and dispersion of both fibers and particles based on SnO2-600 are regulated as the increasing of Zn component, in which Ostwald ripening process apparently slow down and more reactive crystallization sites and particles attached on surface have opportunity to emerge and grow. Furthermore, One-step synthesis of SnO2/ZnO hierarchical structures featured by in-situ growth of particles on surface of fibers can dramatically improve electronic transmission path and electron transfer efficiency, contributing to the breakthrough in physico-chemical performance.



Figure 3. (a)-(c) TEM and HRTEM images of 7 mol% Zn. (d)-(g) TEM image and corresponding EDS elemental mappings of 7 mol% Zn. (h) SEM image and corresponding EDS line scanning (h), and EDS spectrum of 7 mol% Zn (i). Figure 3a presents TEM image of 7 mol% Zn that contains well-distributed and uniform particles with average size of 35-45 nm closely attached on surfaces of fibers. The lattice spacings in Figure 3b and c are detected to be 0.247, 0.334, and 0.264 nm, which are assigned to (101) plane of ZnO, and (110) and (101) planes of SnO2 phase, 18 ACS Paragon Plus Environment

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respectively. Compared with 3 mol% Zn, Figure S6 reveals the successful formation of SnO2/ZnO heterojunction at the interface for 7 mol% Zn, further demonstrating the occurrence of more probability and quantity of heterojunction as the introducing of Zn component

increasing.

Especially,

a

large

number

of

multi-level

effective

heterojunctions located simultaneously in various places referring of fibers, particles, and interfaces have great promising for accelerating electronic transfer and resistance regulation of composites. Moreover, lattice planes of ZnO (101), SnO2 (110), SnO2 (101), SnO2 (211), and SnO2 (301) identified from SAED patterns in Figure S7a and b also manifest tunable phase distribution and good crystallinity of hierarchical structures. Figure 3d-g show TEM image and corresponding EDS elemental mappings of 7 mol% Zn. Clearly, Sn, Zn, and O elements are evenly distributed throughout the area, further confirming that fibers and particles are composed of SnO2 and ZnO. EDS line scanning in Figure 3h shows the scanning route (pink line), the element distribution of Sn (blue line), oxygen (red line), and Zn (green line). EDS spectrum in Figure 3i also reveals the presence of Sn, Zn, and O elements.



Figure 4. (a) XRD pattern and (b) Raman spectra of samples. (c) N2 adsorptiondesorption isotherm and pore size distribution (the inset) of SnO2-600 and 7 mol% Zn. XPS spectra of (d) Sn 3d spectra of SnO2-600 and 7 mol% Zn. (e) O 1s spectrum of SnO2-600. (f) O 1s spectrum of 7 mol% Zn. XRD is performed to determine the composition and crystal structure of materials as seen in Figure 4a and Figure S8. All peaks of SnO2-600 are characterized as tetragonal rutile SnO2 which is consistent with the reported JCPDS card (No.41-1445). With the increasing of Zn source, a characteristic peak is gradually observed at the location of 2θ = 36.2 ° that corresponds to (101) lattice plane of ZnO (JCPDS 89-1397), implying the


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formation of SnO2/ZnO composite. Because of low calcination temperature of SnO2-400, there are two weak peaks of SnO (JCPDS No. 07-0195) as shown in Figure S8. Figure 4b shows Raman spectra of SnO2-600 and SnO2/ZnO hierarchical fibers. Raman peaks at 322, 643, and 781 cm-1 are identified, corresponding to the vibration mode 2E2 (M) of wurtzite ZnO, and the vibration modes A1g and B2g of SnO2, respectively.22,23 The everincreasing of Raman peaks at 643 and 781 cm-1 confirms that introducing Zn component can adjust the size, crystallinity, and structural order of materials. Besides, the peak at 598 cm-1 related to the effect of nano-sized particles for local lattice disordering and oxygen vacancies can be detected as feature in Raman spectra. Figure S9 shows the transformed Kubelka-Munk function vs energy of light. Band gaps of SnO2-600 and 7 mol% Zn are calculated to be 3.65 and 3.58 eV, respectively.24 The reduction of band gap is beneficial to transiting more available electrons from valence band to conduction band. Figure 4c reveals N2 adsorption-desorption isotherm and pore size distribution of SnO2-600 and 7 mol% Zn. Typical IV type isotherm of hysteresis loop can be obtained from N2 adsorption-desorption isotherms. Specific surface area and single point average pore radius of SnO2 porous fibers (Figure S10), SnO2-600, and 7 21 ACS Paragon Plus Environment


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mol% Zn samples are 31.8-3.0, 55.8-4.6, and 115.9-6.7 m2/g-nm, respectively. Highest specific

surface

area

of

SnO2/ZnO

hierarchical

structures

with

mesoporous

characteristic provides more active sites for enhancing the surface adsorption behavior. XPS analysis has been employed for identifying the phase composition and valence states of SnO2-600 and 7 mol% Zn. As shown in Figure 4d, Sn 3d spectrum each sample displays two typical peaks around 485.2-485.4 and 493.5-493.7 eV, strictly belonging to Sn 3d5/2 and Sn 3d3/2, respectively. No obvious peaks of Sn2+ can be detected in any of samples, demonstrating the production of SnO2 phase. In general, small variation in oxidation state of Sn can prominently change the intensity and position of peaks at Sn 3d, and the shift to higher binding energy direction represents the stronger bonding capacity of the material.25 Compared with SnO2-600, all of Sn 3d peaks of 7 mol% Zn fibers are slightly shifted toward higher binding energy, meaning the enhanced bonding strength from adjacent atoms in SnO2/ZnO hierarchical structures as the introducing of Zn component. As depicted in Figure S11, XPS spectrum of Zn 2p contains two peaks at 1021.9 and 1045.0 eV, corresponding to Zn 2p3/2 and Zn 2p1/2, respectively, indicating the formation of ZnO composition.26 On 22 ACS Paragon Plus Environment

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account of O 1s spectra of SnO2-600 and 7 mol% Zn, the significant difference of peak shape is observed. In Figure 4e and f, O 1s can be divided into three peaks in the range of 525-535 eV, assigned to lattice oxygen species (OL), oxygen vacancy species (OV) and chemisorbed and dissociated oxygen species (Oc), respectively. The positions of corresponding central peaks and relative percentages of each oxygen species are listed in Table S1. As for 7 mol% Zn, although there is no obvious change for the main peak positions compared with SnO2-600, the actual content of OV and OC species are markedly increased by introducing Zn composition. For example, the relative percentages of OL, OV, and OC components are approximately 92.04, 5.01, and 2.95 % for SnO2-600, while the corresponding values are 71.08, 15.81, and 13.10 % for 7 mol% Zn. It is known that high levels of OV and OC can provide more active sites for gas adsorption and reaction process on surface of sensing materials.27-29 Therefore, it is reasonably assumed that 7 mol% Zn sensors can exhibit superior and unique response performance for the practical detection due to their higher Ov and Oc components.



Figure 5. (a) Responses of sensors to 100 ppm for different gases at 260 oC. (b) Successive response curves of sensors to different ethanol concentrations and (c) corresponding linear fit of log(s-1)-log(c). Response transient resistance and the corresponding response value of SnO2-600 (d) and 7 mol% Zn (e) to different ethanol concentrations at 260 oC. (f) Response and recovery time curves of SnO2-600 and 7 mol% Zn to 100 ppm ethanol at 260 oC. (g) Six-cycle response curve of SnO2-600 and 7 mol% Zn to 100 ppm ethanol at 260 oC. (h) Response curves of SnO2-600 and 7 mol% 24 ACS Paragon Plus Environment

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Zn to different cosmetics (injecting 10 μL) at 260 oC. (i) The corresponding ethanol content of different cosmetics (injecting 10 μL). Gas selectivity, optimal operating temperature, response value, and corresponding linear fit of different SnO2 hierarchical sensors are shown in Figure S12. All sensors have the similar characteristics of preferable sensitivity to ethanol than triethylamine, methanol, acetone, toluene, benzene, and ammonia, and the response of SnO2-600 sensors to ethanol is about 7.7-15.8 times higher than other gases. Relationship between the response and operating temperature is evaluated in Figure S12a. The responses of sensors firstly increase and then decrease with the operating temperature increasing in the range of 200-320 oC. The optimal temperature can be selected as 260 oC.

As temperature is below 260 °C, surface activity is lower and surface reaction is

poor. The temperature of 260 oC can be employed to facilitate production of oxygen species and surface chemical reaction between HCHO and oxygen species, then enhancing gas-sensing performance. When the operating temperature exceeds 260 oC, reactants (including oxygen species and HCHO) tend to escape, resulting in the decrease of performance. The response value of all sensors increases with the 25 ACS Paragon Plus Environment


concentration of ethanol increasing, and the detailed responses of SnO2-400, SnO2-500, SnO2-600, and SnO2-700 sensors are 30.9, 85.2, 119, and 54.5 to 100 ppm ethanol at 260 oC, respectively. Each sensor has a good linear relationship between log(s-1) and log(c): s represents response value and c represents the concentration of ethanol. Figure S13 shows the responses of two sensors obtained by adding different amounts of Sn source to 100 ppm ethanol at 260 oC. Compared with other sensors, it is distinct that SnO2-600 sensors exhibit the highest gas-sensing performance with 119 response value to 100 ppm ethanol at 260 oC, which may be attributed to the construction of hierarchical microstructure associated with good crystallinity. Gas-sensing performance of SnO2/ZnO composites obtained by introducing different contents of Zn component into SnO2-600 has been investigated. Figure 5a shows the responses of 3 mol% Zn, 5 mol% Zn, 7 mol% Zn, and 10 mol% Zn sensors to 100 ppm ethanol at 260 oC are about 7.6-17.5, 7.8-22.0, 9.1-24.4, and 8.4-22.5 times higher than other gases, indicating the highest gas selectivity based on 7 mol% Zn sensors and the enhanced gas selectivity behavior derived from the addition of Zn component. The optimal operating temperature of SnO2/ZnO sensors is 260 oC, in agreement with SnO2 26 ACS Paragon Plus Environment

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sensors, as shown in Figure S14a. Figure 5b reveals that responses of SnO2/ZnO sensors increase with the concentration of ethanol increasing, much higher than that of SnO2-600 sensors in different concentration ranges. As the adding amount of Zn component is too low, such as 3 and 5 mol%, there is not enough SnO2/ZnO heterojunctions at the interface between SnO2 and ZnO crystals. However, as the adding amount of Zn component is too high, such as 10 mol%, the microstructure of SnO2/ZnO composites might wrap surface active sites that are responsible for sensing reactions. Thus, 7 mol% Zn sensors are considered as the optimal construction of multilevel effective SnO2/ZnO heterojunctions with excellent gas-sensing properties for potential application. All sensors present the superior linear relationship and provide strong support for actual quantitative detection of ethanol in Figure 5c. Figure 5d and e display the response transient resistance and corresponding response value based on SnO2-600 and 7 mol% Zn sensors at 260 oC as exposed to ethanol in concentration range of 5-500 ppm. The resistance instantly decreases and the response increases once injected to ethanol, reflecting the gas-sensing characteristics of n-type semiconductor of two sensors. Regardless of ethanol gas concentration, the resistances 27 ACS Paragon Plus Environment


can return to the baseline level after stopping ethanol exposure, revealing good ability of reversibility.30 The response/recovery times of SnO2-600 and 7 mol% Zn sensors are calculated as 9/75 s and 8/45 s, respectively, as shown in Figure 5f. The responses of SnO2-600 and 7 mol% Zn sensors under different RH conditions including 20, 35, 50, 65, and 80 % are depicted in Figure S14b. There is no significant degradation of response even at high environmental humidity, which demonstrates good stability and the potential application in the harsh conditions. Gas circulation stability and long-term stability of both samples to 100 ppm ethanol at 260 oC are tested in Figure 5g and Figure S14c. It is clear that the no obvious change of the response (especially for 7 mol% Zn) can be detected in spite of carrying out six-cycle or 30 days measurements, proving the excellent reproducibility and stability of the constructed hierarchical structures. Figure S14d shows response value of SnO2-600 and 7 mol% Zn sensors to different ethanol concentrations (5-500 ppm) at 260 oC. Vertical error bars presents standard deviations based on three exposures to ethanol, revealing good repeatability. Besides, the gas-sensing properties of SnO2 porous fibers referring of the low response value


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(13.2 to 100 ppm ethanol at 260 oC) and poor linear relationship between log (s-1) and log (c), are also provided as shown in Figure S15. To further investigate the pioneering application value of hierarchical structures in industrial field, five cosmetics from the general market are selected for evaluating the quality of products by precisely detecting ethanol components. Figure 5h displays that the responses of five cosmetics (10 μL) given by SnO2-600 sensors are 52, 74, 30, 37, and 44, while the ones of 7 mol% Zn sensors are 120, 194, 64, 77, and 92, respectively. Linear relationship formulas of y = 0.356 x + 1.325 and y = 0.527 x + 1.485 as shown in Figure 5i are employed to evaluate the actual content of ethanol. The obtained concentrations of different cosmetics based on SnO2-600 and 7 mol% Zn have approximate values of 11.9/13.2, 32.5/33.0, 2.4/3.9, 4.4/5.6, and 7.3/7.9 ppm, respectively. When the amount of ethanol in cosmetics is small, testing results of two sensors are slightly different due to inherent disadvantages of linear relationship, selectivity, and response of SnO2-600 sensor. By undergoing the similar test procedure, however, the response results of SnO2 porous fibers in Figure S16 can be hardly applied for the quantitative analysis for 10 μL five cosmetics. Therefore, 7 mol% Zn 29 ACS Paragon Plus Environment


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sensors with high response, high selectivity and good linear relationship have broad prospects of practical application. Figure S17 and Table S2 present the definite ingredients of different cosmetics by using GC-MS, and Figure S18 displays the excellent selectivity of 7 mol% Zn sensors for other components in cosmetics, further determining the available approach for effective trace detection of ethanol in cosmetics based on multi-level effective heterojunctions. Moreover, Table S3 shows the comparison of gas-sensing performances of SnO2-based sensors for detecting ethanol in

recent

reports,

also

demonstrating

the

superior

response/recovery times of 7 mol% Zn sensors.


response

and

faster

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Figure 6. (a) I-V polarization curves of different sensors within a bias from -4 V to +4 V at 260 oC in air and the corresponding insert represents fit diagram of voltage-current. (b) Impedance plot of different samples. As shown in Figure 6a, I-V polarization curves of different sensors display the excellent linearity ranging from -4 to 4 V, indicating good Ohmic contact between sensing film and interdigitated electrodes. The fit slope in inset of Figure 6a represents the resistance value of sensor at 260 oC in air, approximately consistent with Ra obtained by CGS-4TPs (Figure 5d and e and Figure S15). It is clearly seen from the radius of the semicircle of different samples in the impedance plot in Figure 6b that 7 mol% Zn have the smallest radius compared with SnO2-600 and SnO2 porous fibers, indicating that the construction of hierarchical structures consisting of multi-level effective heterojunctions can intensively improve charge transfer characteristics and regulate the resistance variation.



Figure 7. Schematic illustrations of gas-sensing mechanism of SnO2-600 sensor: (a)-(c) and (g)-(i), and 7 mol% Zn sensor: (d)-(f) and (j)-(l). For SnO2-600 sensors in Figure 7a, both matrix fibers and particles in-situ growth on surface of fibers can simultaneously adsorb oxygen molecules and targeted gas molecules, thereby increasing the amount of molecules participated in surface reaction


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process due to the tight interface bonding connection. When sensors are exposed to air in Figure 7b, oxygen molecules are preferentially adsorbed to the surfaces of fibers and particles, and then trap electrons from conduction band to form oxygen species (O2-, or O- at 260 oC), resulting in thick electron depletion layer and less conductivity of materials. At the same time, the conduction band near boundaries can be bend upward, forming potential barriers (Φ1) and restricting flow of electrons through nanograins.31 Thus, the resistance is higher in the air, meaning the higher Ra value. As sensors are exposed to ethanol in Figure 7c, ethanol molecules interact with chemisorbed oxygen species and release electrons with concomitant generation of CO2 and H2O, leading to the thin electronic depletion layer and the reduced height of potential barrier (Φ2), and then allowing flow of electrons through the well-dispersed nanograins for resistance modulation. Therefore, the resistance is lower in the ethanol, that is, Rg value is lower. It is confirmed that the Ra/Rg value extremely reflects the response of gas-sensitive sensors. Generally, unique heterogeneous hierarchical structure can greatly improve specific surface area with the modified porosity of materials, and hence 7 mol% Zn sensor 33 ACS Paragon Plus Environment


(115.9 m2/g) has the highest specific surface area compared with SnO2 porous fibers (31.8 m2/g) and SnO2-600 sensors (55.8 m2/g).

More active centers and more

adsorbed molecules on surface consequently promote the effective surface/interface reaction which can signally increase Ra value and reduce Rg value. In addition to the increase of specific surface area, the introduction of Zn ingredients into inorganic compound is beneficial to regulate surface structures and chemical states and produce more electron donor states and oxygen vacancies corresponding to high levels of OV and OC in Figure 3f, which can provide more active sites for surface adsorption of sensing materials, as shown in Figure 7d.32 It is reported that the heterojunction between ZnO and SnO2 at grain boundary is easy to achieve electronic interaction, which is conducive to enhance surfaces reaction between adsorbed oxygen and current-carrying electrons.33 Since work functions of n-SnO2 and n-ZnO are 4.55 and 5.20 eV, respectively, the electronic flow occurs from SnO2 to ZnO, ultimately equating the Fermi levels of both materials in Figure 7l.31 Thus, in the air, hierarchical 7 mol% Zn sensors have more electrons involved in reaction and transport, thicker electronic depletion layer, and higher barrier height compared with SnO2-600 sensors, 34 ACS Paragon Plus Environment

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resulting in higher Ra value as shown in Figure 7e. When 7 mol% Zn sensors are exposed in the ethanol in Figure 7f, more electrons are released back into materials along with thinner electron depletion layer and lower barrier height, readily leading to lower Rg compared with SnO2-600 sensors. It is worth mentioning that hierarchical structure with in-situ growth of nanoparticles on surface obtained by one-step electrospinning techniques are more advantageous for the transfer and flow of electrons due to the matching of crystal faces involved in formation process, eventually facilitating the achievement of multi-level effective heterojunctions for excellent gas-sensing performance. The height of energy bend is closely related to surface state and surface reaction, concretely including the extracted and donated electrons from adsorbed gas molecules.34-36 Gas-sensing mechanisms of various sensors have been proposed in Figure 7g-l and Figure S19. As for SnO2 porous fibers, the similar regulation of potential barrier and surface physicochemical characteristics can be observed as shown in Figure S19, moderately resulting in the poor gas-sensing performance. Figure 7g-i represent that the enhanced gas-sensing mechanism of SnO2-600 sensors can be 35 ACS Paragon Plus Environment


mainly attributed to the effective regulation of potential barriers situated in various structural units due to in-situ growth of particles on the surface of matrix fibers. Potential barrier regulation of particles and interfaces contributes to significantly enhance gassensing performance due to limited regulation of potential barrier of fibers. Thus, a large amount of O2 can be absorbed by particles on surface and effectively participate in surface reactions, depleting enough electrons to form oxygen species. At the same time, ethanol molecules react with more oxygen species returning more electrons back to the materials which leads to effective regulation of potential barriers at the interface and particles on surface improving gas-sensing properties. It is clear that the absorption and transfer efficiency at the interface and particles on surface are much higher compared with matrix fibers based on the structural features as depicted in Figure 7g-i. As for 7 mol% Zn sensors, hierarchical structure consisting of two structural units (fibers and particles) contribute for the construction of multi-channel and efficient electron transfer process, such as in fibers, in particles, between fibers and particles, even between particles and particles, as shown in Figure 7j and k. In addition, compared with SnO2600 sensors, the monodisperse particles with high dispersion density of 7 mol% Zn 36 ACS Paragon Plus Environment

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sensors can be inevitably connected. Figure 7j and k also presents multi-level effective heterojunctions derived from four different areas of matrix fibers, particles on surface, and the interfaces, which can not only greatly affect the density and mobility of electrons, but also efficiently promote potential barriers throughout heterojunctions regions.37 The effective transfer of electrons leads to a sharp increase in Ra and decrease in Rg, and hence the response (Ra/Rg) significantly increases because of the synergistic regulation of potential barriers for entire hierarchical structures in Figure 7j-l.

CONCLUSIONS

Tunable SnO2 and SnO2/ZnO fibrous hierarchical structures with in-situ growth of monodisperse spherical-like particles on surface are fabricated by a facile one-step electrospinning technique for the first time. Compared with SnO2 porous fibers and SnO2-600, 7 mol% Zn sensors exhibit the enhanced gas-sensing performance with high sensitivity and selectivity, fast response/recovery times, and excellent stability, predominantly attributed to the novel construction of multi-level effective heterojunctions based on 1D fibrous hierarchical structure with unique interface electronic effects. 37 ACS Paragon Plus Environment


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ASSOCIATED CONTENT

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. TEM and HRTEM images of 3 mol% Zn; SEM images and XRD pattern of SnO2 porous fibers; SEM images of SnO2-400, SnO2-500, and SnO2-700; SEM images of SnO2-600-0.5 and SnO2-600-2; SEM images: PVP/Sn precursor fibers are calcined at 600 oC for 2 h at a heating rate of 5 oC/min; HRTEM images of 7 mol% Zn; SAED patterns of fibers and particles based on 7 mol% Zn; XRD pattern of different hierarchical SnO2 fibers; The plot of transformed KubelkaMunk function vs energy and corresponding UV-vis diffuse reflectance spectra (the inset) of SnO2-600 and 7 mol% Zn; N2 adsorption-desorption isotherm and pore size distribution of SnO2 porous fibers; XPS spectrum of Zn 2p of 7 mol% Zn; Gas-sensing performances of SnO2-400, SnO2-500, SnO2-600,and SnO2-700 sensors; Gas-sensing performances of SnO2-600-0.5 and SnO2-600-2;

Gas-sensing

performances

of

different

SnO2/ZnO

sesnors:

temperature

characteristics, humidity characteristics, long-term stability, and concentration characteristics; Gas-sensing performances of SnO2 porous fibers; Response curve of SnO2 porous fibers to different lotions ( injecting 10 μL) at 260 oC; GC-MS results of different lotions; Selectivity of sensors to other components in lotions; Schematic illustration of the sensing mechanism of SnO2 porous fibers; Fitting results of O 1s XPS spectra of SnO2-600 and 7 mol% Zn; The GC-MS results of different lotions; Comparison of gas-sensing performances of SnO2-based sensors for detecting ethanol. (PDF)


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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +86-531-87974453. Tel: +86-531-89736225 (Q.M.). *E-mail: [email protected] (P.Y.).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was supported by the projects from the National Natural Science Foundation of China (51402123) and the project from Shenzhen Gangchuang Building Material Co., Ltd..

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189x161mm (150 x 150 DPI)

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ZnO 1D Fibrous

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