Arrays of Ultrathin CdS Nanoflakes with High-Energy Surface for

Subscriber access provided by NEW YORK UNIV

Article

Arrays of Ultrathin CdS Nanoflakes with Highenergy Surface for Efficient Gas Detection Xiao-Hua Liu, Peng-Fei Yin, Sergei A. Kulinich, Yu-Zhu Zhou, Jing Mao, Tao Ling, and Xi-Wen Du ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13601 • Publication Date (Web): 16 Dec 2016 Downloaded from http://pubs.acs.org on December 18, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Arrays of Ultrathin CdS Nanoflakes with High-energy Surface for Efficient Gas Detection

Xiao-Hua Liu,†, ‡ Peng-Fei Yin,† Sergei A. Kulinich,§, ¶ Yu-Zhu Zhou,† Jing Mao,† Tao Ling,*,† and Xi-Wen Du*,†, ‡ †

Institute of New-Energy Materials, School of Materials Science and Engineering, Tianjin University, Tianjin,

300072, China ‡

Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of Education, Tianjin University,

Tianjin, 300072, China §Institute

of Innovative Science and Technology, Tokai University, 4-1-1 Kitakaname, Hiratsuka, Kanagawa

259-1292, Japan ¶Aston

Institute of Photonic Technologies, Aston University, Aston Triangle, Birmingham B4 7ET, United

Kingdom E-mail: [email protected]; [email protected]

ABSTRACT: It is fascinating and challenging to endow conventional materials with unprecedented properties. For instance, cadmium sulfide (CdS) is an important semiconductor with excellent light response; however, its potential in gas-sensing was underestimated owing to relatively low chemical activity and poor electrical conductivity. Herein, we demonstrate that an ideal architecture, ultrathin nanoflake arrays (NFAs), can improve significantly gas-sensing properties of CdS material. The CdS NFAs are grown directly on the interdigitated electrode to expose large surface area. Their thickness is reduced below the double Debye length of CdS, permitting to achieve a full depletion of carriers. Particularly, the prepared CdS nanoflakes are enclosed with high-energy {0001} facets exposed, which provides more active sites for gas adsorption. Moreover, the NFAs exhibit the light-trapping effect, which further enhances their gas sensitivity. As a result, the as-prepared CdS NFAs demonstrate excellent gas-sensing and light-response properties, thus being 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

capable of dual gas and light detection.

KEYWORDS: gas sensing; cadmium sulfide; nanoflakes; ultrathin; high-energy surface

INTRODUCTION Highly sensitive gas sensors play a key role in daily life, contributing actively to toxic- and explosive-gas alarms, automotive cabin air-quality control, health care, houshold appliances and so on.1-3 Therefore, tremendous efforts have been recently made to design and fabricate reliable and durable sensors with high sensitivity and selectivity toward various gases.2, 4 Pioneering works in the field indicate that the performance of gas sensors can be significantly enhanced through reducing the dimension of gas-sensing materials and engineering their surface states.5-11 Specifically, fully-depleted nanocrystals (NCs), whose radius is smaller than the Debye length, can exhibit gas-sensing properties superior to their bulk counterparts owing to the so-called small-size effect.12-16 Besides, unique surface states, such as high-energy facets, dangling bonds, and unsaturated atoms, are favorable for the chemical activity of sensing materials, which is known as surface effect.8-10,17 Very recently, theoretical studies predicted that light irradiation of photo-active materials can enhance their gas-sensing properties through the photo-excitation effect, which opens new avenues toward highly-sensitive gas detection.18-20 Although numerous efforts have been made to exploit the small-size effect or high-energy facets of nanostructures based on SnO2,6,12 ZnO9,21 or Cu2O,5 no sensing material combining these two virtues was reported so far. In comparison with conventional NC-based materials, nanoflakes display certain merits due to their structure.22 First, because of their large surface area, nanoflakes provide more intimate contact with target gas, facilitating gas diffusion and mass transport.11,23 Second, they can be thinned down to several nanometers and shaped to expose a sole high-energy surface, thus permitting fully to take advantage of both the size and surface effects during gas sensing. Third, the 3D architecture of nanoflakes exhibits strong light-trapping effect which facilitates light absorption and then electron transition,24 further enhancing gas-sensing response through the photo-excitation effect. This makes devices based on nanoflakes very promising for the realization of highly sensitive gas detection. Cadmium sulfide (CdS) is well-known as an important semiconductor with excellent light response and has been widely employed as a material for light detection.25-27 Although its application in gas sensing has 2


Page 2 of 19

Page 3 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


been reported, the performance was relatively poor due to the low chemical activity and poor electrical conductivity of sulfide semiconductors as well.28-33 Given that a proper CdS-based material with high gas response is designed, a highly efficient device combining both gas and light detection will be realized. In the present work, we demonstrate that an ideal architecture, ultrathin nanoflake arrays (NFAs) with high-energy surface, can surpass the limited gas-sensing properties of existing CdS materials. The CdS NFAs prepared by us possess several features making them attractive for efficient gas sensing: (i) They are grown directly and properly oriented on the interdigitated electrode (Scheme 1a and Figure S1), so that the issue of nanoflake stacking is overcome and large surface area of nanoflakes is exposed. (ii) Their thickness is reduced below the double Debye length of CdS, permitting to achieve a full depletion of carriers (Scheme 1b). (iii) The prepared CdS nanoflakes are enclosed with high-energy {0001} facets exposed, which provides more active sites for gas adsorption. (iv) The NFAs exhibit the light-trapping effect, which further enhances their gas sensitivity. As a result, the prepared CdS NFAs demonstrate excellent gas-sensing properties when exposed to volatile organic compounds, their sensitivity being 12 times that of conventional CdS nanoparticles and similar to that of oxide nanomaterials.10-12,34-37 At the same time, the as-prepared gas-sensing device is also responsive to visible light, thus being capable of dual gas and light detection. Our work thus illustrates that properly designed nanostructures can attain properties superior to those of their conventional counterparts, which opens up new avenues toward effective brand-new devices.

Scheme 1. Schematic illustration of structure of as-prepared CdS NFAs (a) and gas-sensing mechanism 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(b) of sensor on their basis. RESULTS AND DISCUSSION The CdS NFAs were prepared by sulfurizing Cd NFAs (see Figures S2 and S3) preliminarily grown on interdigital electrode via chemical vapor deposition.24,38 The precursor Cd NFAs showed uniform morphology and regular distribution, with each nanoflake being single-crystalline and exposed surface being {0001} (Figure S2). Importantly, the thickness of Cd nanoflakes was found to increase with growth time, which allowed us to control the thickness of the product CdS nanoflakes. For example, CdS nanoflakes with thickness of 7, 15 and 28 nm were obtained using Cd nanoflakes grown for 10, 30 and 100 min, respectively (Figures 1a-1c and Figure S4). EDS and XPS spectra of the final product illustrate that the as-prepared nanoflakes consist of Cd and S atoms with the atomic ratio close to 1:1 (Figure 1d and Table S1). XRD pattern reveals that CdS nanoflakes present a hexagonal wurtzite crystal structure. An intense (0002) peak is seen in Figure 1e, implying strong texturing of the flakes. Indeed, the calculated orientation index of the flakes (see Table S2) indicated their exposed facets being mainly {0002} planes (for details, see Supporting Information, section 2). Moreover, both XPS (Figure S5) and FTIR (Figure S6) results suggest the existence of oxo-sulfur species on the surface of the CdS NFAs (for details, see Supporting Information, section 1).

4


Page 4 of 19

Page 5 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. (a)-(c) CdS NFAs with different flake thickness, with insets presenting AFM profiles of corresponding flakes. (d) EDS spectrum and (e) XRD pattern of as-prepared product.

The microstructure of CdS nanoflakes was further examined by transmission electron microscopy (TEM). Each nanoflake was found to consist of numerous nanoplates with sizes of 50-100 nm (Figure 2a). The selected-area electron diffraction (SAED) pattern in the inset of Figure 2a exhibits polycrystalline rings corresponding to the [0001] crystal zone of hexagonal structure, which is consistent with the XRD pattern in Figure 1e. The high-resolution TEM (HRTEM) images of randomly selected grains presented in Figures 2b-2f and their fast Fourier transform (FFT) patterns (see corresponding insets) confirm that each nanoflake is built of highly-textured NCs, with exposed surfaces being {0001}. This is well-supported by the FFT patterns in the insets of Figures 2b-2f.

Figure 2. TEM characterization of as-fabricated CdS nanoflakes. (a) TEM image of CdS nanoflake consisting of highly crystalline NCs, with SAED pattern (inset) corresponding to hexagonal CdS. (b)-(f) HRTEM images of individual CdS grains indicated by markers ① to ⑤ in panel (a), respectively. Insets are corresponding FFT patterns.

To demonstrate the advantages of the as-produced CdS NFAs, we compared them with CdS nanoparticles synthesized via a conventional wet-chemistry route adopted after Bao and coworkers.39 As 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

seen in Figure S7, the CdS nanoparticles also possess a wurtzite structure. At the same time, they are somewhat smaller, being ~10 nm on average (Figure S7). Importantly, their XRD pattern is different from that of CdS NFAs and similar to the standard pattern well-known for the hexagonal CdS. More specifically, —

—

their three main peaks, (101 0), (101 1) and (0002), show very similar intensities, suggesting that the nanoparticles are enclosed with random low-index planes. The random orientation of the nanoparticles is also confirmed by their HRTEM images in which facets with different indexes were directly observed (Figure S8). In addition, the surface area of both the CdS nanoparticles and NFAs was measured by means of N2 adsorption/desorption approach. As expected, the surface area of the nanoparticles (23.8 m2/g) calculated through the Brunauer–Emmett–Teller theory was larger than that of the NFAs with 7-nm-thick flakes (22.6 m2/g) (Figure 3).

Figure 3. Typical N2 adsorption and desorption isotherms of CdS NFAs and CdS nanoparticles.

Keeping this in mind, we then compared the gas-sensing performance of the CdS NFAs and CdS nanoparticles as reference. Unlike the NFAs, which were grown directly on the interdigitated electrode, the CdS nanoparticles were dip-coated onto a similar electrode, after which annealed at 180 oC for 48 h to form a stable thick layer. The optimum working temperature of the NFAs was determined as 250 and 225 oC for ethanol and isopropanol, respectively (Figure S9), while the nanoparticles showed the highest response to isopropanol at 275 oC (Figure S10). Therefore, all further measurements were carried out at these temperatures. Figure 4a compares gas-sensing performance of the CdS NFAs (with different flake thickness) and nanoparticles when exposed to different concentration of isopropanol. It is clearly seen that 10-nm-sized CdS 6


Page 6 of 19

Page 7 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanoparticles (with larger surface area) show much weaker response to isopropanol, which is in agreement with previous reports on other CdS nanostructures found in the literature.29,30 At the same time, the CdS nanoflakes are seen in Figure 4a to respond much stronger toward isopropanol, with their sensitivity increasing inversely with their thickness. For example, at 200 ppm of isopropanol, the response of the sensor fabricated with 7-nm-thick CdS flakes reaches 76, which is 12 times that of its counterpart with CdS nanoparticles. This sensitivity is the highest ever reported for CdS nanomaterials, 28,29,40 being comparable to that of ZnO , a common oxide forgas sensors (summarized in Table S3).11,17,35,41

Figure 4. Gas-sensing performance of CdS NFAs fabricated on interdigitated electrode and comparison with that of CdS nanoparticles. (a) Response of CdS NFAs with different nanoflake thickness and CdS nanoparticles versus different concentration of isopropanol. (b) Real-time response of 7-nm-thick CdS NFAs to different concentrations of isopropanol. (c) Response of 7-nm-thick nanoflakes toward isopropanol, methanol, ethanol, acetone and methylbenzene with different concentrations. (d) Long-term stability of sensor with 7-nm-thick flakes evaluated over 20 days.

As a next step, the detection limit of the as-fabricated CdS NFAs was evaluated. Figure 4b presents the real-time response of 7-nm-thick CdS nanoflakes with isopropanol concentration varying from 50 ppb to 5 ppm. The electric resistance of the sensor was observed to decrease abruptly in presence of isopropanol and 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

recovered completely to its initial value once the target gas was pumped out. As the detection limit of gas sensors is defined as a gas concentration at which the signal intensity is three times that of noise,42 the detection limit of the CdS NFAs for isopropanol is seen in Figure 4b to be 100 ppb, which is among the best results ever reported for CdS-based sensors.28,30,40 We also investigated the selectivity of the novel NFA-based sensor. Of several volatile organic compounds tested (benzene, alcohol, and ketone), the sensor only responded to isopropanol and ethanol (Figure 4c). Thus, the selectivity is defined by the ratio of the response sensitivity for isopropanol to that for ethanol. Based on this, the selectivity value around 5 can be derived from the graphs in Figure 4c, being high enough to distinguish isopropanol from the other gases. We further tested other compounds including 1-propanol, 1-butanol, 2-butanol and tert-butanol. The results shown in Figures S11 and S12 indicate that the sensor responds selectively to secondary alcohols. The long-term stability of the sensor was evaluated over a period of 20 days. As shown in Figure 4d, the response toward 200 ppm of isopropanol only slightly decreased from 76 to 71, suggesting sensor’s high stability. Similarly, the cyclic test conducted under same air flow suggests very good cyclic stability of the CdS NFAs (Figure S13). Last, but not least, the sensor’s response time was found to be much shorter than that of nanoparticles (Figure S14). More specifically, in case of 7-nm-thick NFAs, it was almost twice shorter than that of 10-nm-sized nanoparticles (12 s vs. 21 s). Next, we discuss the gas-sensing mechanisms and reasons for the observed high performance of the CdS NFAs. The classical sensing theory for semiconductors is based on the space-charge-layer model, in which oxygen molecules from air adhere to semiconductor surface and capture electrons, forming oxygen species ( O -2 , O2-, O-) and an electron-depleted layer.43,44 When exposed to a reductive gas, the depletion layer can be compensated by electrons from the adsorbed gas molecules, which leads to the recovery of electron resistance. For a gas sensor complying with the space-charge-layer model, there must exist a linear relationship between the logarithm of sensitivity and the logarithm of gas concentration, which is the case for the CdS NFAs prepared in this study (see Figure 5a and Supporting Information, section 4). On the other hand, the fitted values of charge parameter were found to be close to 1 for all target gases (Table S4), implying that the oxygen species on the CdS nanoflake surface are mainly O- ions.17,28,29 This suggests that the gas-sensing performance of the prepared NFAs depends on the space-charge layer which, in turn, varies with the structural parameters (morphology, size, surface area, exposed surface, etc.). 8


Page 8 of 19

Page 9 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The CdS NFAs prepared in this study possess ultrathin thickness, high energy surface, large surface area, and 3D configuration, the combination of these parameters being advantageous for light trapping. All these features can cause tremendous changes in the electrical resistance of the sensor before and after its exposure to target gas, leading to high sensitivity of gas response (Rair/Rgas, where Rair and Rgas denote the sensor’s resistance in air and in gas, respectively). First of all, we significantly reduced the nanoflake thickness, which led to the realization of fully electron-depleted nanoflakes with higher Rair. Theoretically, a semiconductor material can be fully-depleted when its dimension is close to or below its double Debye length,5,6,12,13 the latter Debye length (D) being expressed by the following equation (1):45,46

D = (εkT/q2n)1/2

(1)

where ε is the static dielectric constant, k the Boltzmann’s constant, T absolute temperature, q the electrical charge of the carrier, and n is the carrier concentration which can be determined by the Mott-Schottky (M-S) analysis (Figure S15). Accordingly, the Debye length of CdS was calculated as 6.7 nm. As the thickness of the CdS nanoflakes prepared in this study decreases from 28 to 15 (close to the double Debye length, 13.4 nm) and then to 7 nm (i.e. below the double Debye length), the volume fraction of the depletion layer in the nanoflakes increases and finally approaches 100%. Correspondingly, the Rair values of thinner flakes increase (Figure 5b). The average radius of the CdS nanoparticles (~5nm) was also shorter than the Debye length, however, the large number of interfaces between them was believed to be responsible for their highest Rair value observed in Figure 5b.

9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. (a) Linear correlation between log (Sg-1) and log Cg measured for the CdS NFAs in presence of different gases at optimum working temperature. (b) Schematic model presenting sensing by CdS nanoflakes with different flake thickness and that of CdS nanoparticles. (c) Surface energy values calculated for low-index planes of wurtzite CdS, with corresponding surface atom arrangements for each surface (cations being shown as blue and anions as yellow balls). (d) Light response of CdS NFAs with flake thickness of 7 nm at room temperature.

Second, the uncommonly exposed {0001} planes with high surfaces energy are also expected to affect both gas adsorption and the overall sensing performance of the CdS NFAs. We applied the density functional theory calculations to determine the surface energy values of several planes (see Supporting Information, section 5 and Figure S16). As shown in Figure 5c, among various low-index planes of CdS, the {0001} plane has the highest surface energy, which is consistent with previous reports.47,48 High-energy planes are known to be prone to gas molecule adsorption. Moreover, polar {0001} planes of wurtzite CdS are terminated by a —

layer of either Cd or S atoms. The S-terminated (0001) surface usually contains many sulfur vacancies and dangling bonds due to their relatively low formation energy.49,50 Such defects further enhance the adsorption of both oxygen and target gases,9,10 leading to higher Rair and lower Rgas, and eventually to a high sensitivity 10


Page 10 of 19

Page 11 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of the gas sensor. In contrast, the CdS nanoparticles used in this study were found to be enclosed with low-energy facets (see Figure S8),39,51 which implies a weak affinity of such surfaces to reducing gas. This explains why the nanoparticles only showed a moderate decrease in Rgas (and low sensitivity) when exposed to isopropanol. Third, because of their large surface area (reaching several microns in size, Figure 1), as well as their appropriate angles (20-40°) between neighbor flakes, the prepared CdS material is very efficient in light trapping via the multi-reflection effect.24 Therefore, as well seen in Figure 5d, the CdS NFAs are predictably responsive to visible light, thus showing great potential in both gas and light detection. More intriguingly, the gas sensitivity of 7-nm-sized nanoflakes was found to be enhanced when the sensor was excited by visible light (Figures S17). In general, visible light can produce extra carriers via electron transitions. The light-generated carriers can be depleted in air due to the low thickness of the CdS nanoflakes, still being preserved in reductive gas and contributing to electric conduction. As a result, the Rair is expected to remain constant while Rgas decreases under light irradiation, which results in an increase in gas sensitivity. Since most of gas sensors are intended for work in visible light, their light-enhanced gas sensitivity makes them much more attractive for gas detection. In addition to high sensitivity, the sensors prepared in this study demonstrate long-term stability and short response time. The long-term stability can be partially attributed to the tough 3D skeleton of the NFAs as-grown on the electrode (Figure 1). Unlike nanoparticles deposited onto electrode as substrate and usually suffering the Ostwald ripening at high working temperatures, the reported nanoflakes are well-separated and contact each other only at intersections (Figure 1). Their high-energy surfaces can be stabilized by absorbed gas molecules, thus helping the NFAs maintain their morphology and high performance during high-temperature tests and long-term exposure to air (Figure S18). As for the quick response of such flakes to target gases, it is believed to be related to the open trumpet-shaped structure of the NFAs (well-seen in Figs. 1a-1c), which allows for a quick infusion and adsorption of target-gas molecules. The high selectivity may arise from the synergic effect of two factors: (1) the affinity between the sensing material (CdS NFAs) and target gas and (2) the reducibility of the target gas. Alcohols with polar hydroxyl group can adsorb easily on CdS nanoflakes, while the affinity of ketones and benzene to CdS NFAs is much weaker. At the same time, target gases with high reducibility will provide many electrons to compensate the depletion layer, leading to the fast recovery of the electron conductivity. The high affinity and reducibility of secondary alcohols are believed to contribute jointly to the high response of the CdS 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

NFAs to these compounds. In comparison, other volatile organic compounds with low reducibility (such as methanol) cannot inject electrons into depletion layers efficiently,28,52 As a result, the CdS NFAs do not respond noticeably to them.

CONCLUSIONS In summary, we present a highly-sensitive and durable gas sensor consisting of ultrathin CdS nanoflake arrays that are as-grown directly on the interdigitated electrode. The elaborately designed nanoflakes allow for their thickness control, demonstrating the highest gas-sensing sensitivity when their thickness reaches the Debye length for CdS. The prepared nanoflakes possess exposed high-energy surfaces and strong light-trapping effect, which helps them demonstrate the best gas-sensing performance ever reported for sulfide semiconductors. The nanoflakes also show excellent response to visible light, thus being capable of both light and gas detection. This work provides a good example of how a proper structural design can endow conventional materials with improved and unique properties.

EXPERIMENTAL METHODS Synthesis of CdS nanostructure First, Cd nanoflake arrays (NFAs) were grown on a comb-shaped gold electrode (Figure S1) in a tube furnace by a thermal evaporation process reported in our previous work.24,38 Second, the Cd NFAs and 0.05 g of sublimed sulfur were closely placed in the middle of the tube furnace, after which heated to 280 oC and kept at this temperature for 0.5 h. As a result, the Cd NFAs were transformed into CdS NFAs. In order to improve the crystallinity, the CdS NFAs were further annealed at 400 oC for 1h. As-synthesized CdS NFAs on comb-shaped gold electrode was directly used as the working electrode in gas-sensing (Figure S1). Gas-sensing measurements The gas-sensing device based on CdS nanoparticles was fabricated via a dip-coating process. Firstly, the as-prepared nanoparticles were dispersed in 10 mL of ethanol through sonication for 10 min. Then, a comb-like silver-palladium electrode, with geometry identical to that used to host the CdS NFAs, was dipped into the dispersion and lifted with a speed of 1 mm/s. The dip-coating action was repeated three times to obtain a film thick enough for gas-sensing measurements. All the sensors were aged at 180 oC for 48 h before testing. The sensitivity of gas sensors was defined by the resistance ratio Rair/Rgas, where Rair and Rgas stand for the electrical resistance of the sensor in atmospheric 12


Page 12 of 19

Page 13 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


air and in the presence of target gas, respectively. The response and recovery times are determined as time needed for the sensor to achieve 90% of the total resistance change following gas injection and discharge. The humidity in the test chamber was 30% relative humidity. Characterization of Morphology and Performances The as-prepared materials were examined using X-ray diffraction (XRD, Bruker-D8 advanced diffractometer with Cu Kα radiation), scanning electron microscopy (SEM, Hitachi S-4800), transmission electron microscopy (TEM, FEI Technai G2 F20 TEM), and energy-dispersive X-ray spectroscopy (EDS, Genesis XM2). For the preparation of TEM samples, the products were ultrasonically dispersed in ethanol, and the solution was dropped on a Cu grid coated with a holey carbon film and dried with air. The surface area of the CdS nanoflakes and nanoparticles were tested in a BET tool (Quantachrome ASiQwin automated gas sorption analyzer) by using N2 adsorption and desorption. The gas-sensing properties were detected in an intelligent gas sensing analysis system (CGS-1TP, Beijing Elite Tech Co., Ltd) that offered an external temperature control (from room temperature to 500 °C) with a precision of 1 °C.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

.

The digital photos of sensor device and the schematic diagram of the electrode. Calculation of orientation index of Cd and CdS NFAs. Surface chemistry analysis of CdS NFAs. Gas sensing mechanism analysis of CdS NFAs. Calculation of surface energies for CdS. Characterization of compared samples. Comparisons and supplements of the sensing performances.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected] AUTHOR CONTRIBUTIONS The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENTS 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

This work was supported by the National Basic Research Program of China (2014CB931703), the Natural Science Foundation of China (51571149 and 51471115), and the Natural Science Foundation of Tianjin city (15JCYBJC18200). REFERENCES 1.

Gurlo, A. Nanosensors: Towards Morphological Control of Gas Sensing Activity. SnO2, In2O3, ZnO and WO3 Case Studies. Nanoscale 2011, 3, 154-165.

2.

Jia, L. C.; Cai, W. P. Micro/Nanostructured Ordered Porous Films and Their Structurally Induced Control of the Gas Sensing Performances. Adv. Funct. Mater. 2010, 20, 3765-3773.

3.

Shaymurat, T.; Tang, Q. X.; Tong, Y. H.; Dong, L.; Liu, Y. C. Gas Dielectric Transistor of CuPc Single Crystalline Nanowire for SO2 Detection Down to Sub-ppm Levels at Room Temperature. Adv. Mater. 2013, 25, 2269-2273.

4.

Pan, X.; Liu, X.; Bermak, A.; Fan, Z. Self-gating Effect Induced Large Performance Improvement of ZnO Nanocomb Gas Sensors. Acs Nano 2013, 7, 9318-9324.

5.

Wan, X. J.; Wang, J. L.; Zhu, L. F.; Tang, J. N. Gas Sensing Properties of Cu2O and Its Particle Size and Morphology-dependent Gas-detection Sensitivity. J. Mater. Chem. A 2014, 2, 13641-13647.

6.

Yang, D. J.; Kamienchick, I.; Youn, D. Y.; Rothschild, A.; Kim, I. D. Ultrasensitive and Highly Selective Gas Sensors Based on Electrospun SnO2 Nanofibers Modified by Pd Loading. Adv. Funct. Mater. 2010, 20, 4258-4264.

7.

Franke, M. E.; Koplin, T. J.; Simon, U. Metal and Metal Oxide Nanoparticles in Chemiresistors: Does the Nanoscale Matter? (vol 2, pg 36, 2006). Small 2006, 2, 301-301.

8.

Alenezi, M. R.; Alshammari, A. S.; Jayawardena, K.; Beliatis, M. J.; Henley, S. J.; Silva, S. R. P. Role of the Exposed Polar Facets in the Performance of Thermally and UV Activated ZnO Nanostructured Gas Sensors. J. Phys. Chem. C 2013, 117, 17850-17858.

9.

Tian, S. Q.; Yang, F.; Zeng, D. W.; Xie, C. S. Solution-processed Gas Sensors Based on ZnO Nanorods Array with an Exposed (0001) Facet for Enhanced Gas-sensing Properties. J. Phys. Chem. C 2012, 116, 10586-10591.

10. Han, X. G.; Jin, M. S.; Xie, S. F.; Kuang, Q.; Jiang, Z. Y.; Jiang, Y. Q.; Xie, Z. X.; Zheng, L. S. Synthesis of Tin Dioxide Octahedral Nanoparticles with Exposed High-energy {221} Facets and Enhanced Gas-sensing Properties. Angew. Chem.-Int. Edit. 2009, 48, 9180-9183. 11. Zhao, Q.; Shen, Q.; Yang, F.; Zhao, H.; Liu, B.; Liang, Q.; Wei, A. H.; Yang, H. Q.; Liu, S. Z. Direct 14


Page 14 of 19

Page 15 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Growth of ZnO Nanodisk Networks with an Exposed (0001) Facet on Au Comb-shaped Interdigitating Electrodes and the Enhanced Gas-sensing Property of Polar {0001} Surfaces. Sensors and Actuators B-Chemical 2014, 195, 71-79. 12. Kida, T.; Fujiyama, S.; Suematsu, K.; Yuasa, M.; Shimanoe, K. Pore and Particle Size Control of Gas Sensing Films Using SnO2 Nanoparticles Synthesized by Seed-mediated Growth: Design of Highly Sensitive Gas Sensors. J. Phys. Chem. C 2013, 117, 17574-17582. 13. Banerjee, S.; Bumajdad, A.; Devi, P. S. Nanoparticles of Antimony Doped Tin Dioxide as a Liquid Petroleum Gas Sensor: Effect of Size on Sensitivity. Nanotechnology 2011, 22. 14. Rothschild, A.; Komem, Y. On the Relationship between the Grain Size and Gas-sensitivity of Chemo-resistive Metal-oxide Gas Sensors with Nanosized Grains. Journal of Electroceramics 2004, 13, 697-701. 15. Rothschild, A.; Komem, Y. The Effect of Grain Size on the Sensitivity of Nanocrystalline Metal-oxide Gas Sensors. Journal of Applied Physics 2004, 95, 6374-6380. 16. Xu,C., Tamaki, J., Miura, N.,& Yamazoe, N. Grain Size Effects on Gas Sensitivity of Porous SnO2-based Elements. Sensors and Actuators B-Chemical, 1991, 3, 147-155. 17. Kaneti, Y. V.; Yue, J.; Jiang, X. C.; Yu, A. B. Controllable Synthesis of ZnO Nanoflakes with Exposed (10(1)over-bar0) for Enhanced Gas Sensing Performance. J. Phys. Chem. C 2013, 117, 13153-13162. 18. Nasiri, N.; Bo, R. H.; Wang, F.; Fu, L.; Tricoli, A. Ultraporous Electron-depleted ZnO Nanoparticle Networks for Highly Sensitive Portable Visible-blind UV Photodetectors. Adv. Mater. 2015, 27, 4336-4343. 19. Gogurla, N.; Sinha, A. K.; Santra, S.; Manna, S.; Ray, S. K. Multifunctional Au-ZnO Plasmonic Nanostructures for Enhanced UV Photodetector and Room Temperature NO Sensing Devices. Scientific Reports 2014, 4. 20. Zhai, J. L.; Wang, L. L.; Wang, D. J.; Li, H. Y.; Zhang, Y.; He, D. Q.; Xie, T. F. Enhancement of Gas Sensing Properties of CdS Nanowire/ZnO Nanosphere Composite Materials at Room Temperature by Visible-light Activation. ACS Appl. Mater. Interfaces 2011, 3, 2253-2258. 21. Li, C. C.; Du, Z. F.; Yu, H. C.; Wang, T. H. Low-temperature Sensing and High Sensitivity of ZnO Nanoneedles due to Small Size Effect. Thin Solid Films 2009, 517, 5931-5934. 22. Ling, T.; Wang, J.-J.; Zhang, H.; Song, S.-T.; Zhou, Y.-Z.; Zhao, J.; Du, X.-W. Freestanding Ultrathin Metallic Nanosheets: Materials, Synthesis, and Applications. Adv. Mater. 2015, 27, 5396-5402. 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

23. Chen, M.; Wang, Z. H.; Han, D. M.; Gu, F. B.; Guo, G. S. Porous ZnO Polygonal Nanoflakes: Synthesis, Use in High-sensitivity NO2 Gas Sensor, and Proposed Mechanism of Gas Sensing. J. Phys. Chem. C 2011, 115, 12763-12773. 24. Yin, P.-F.; Ling, T.; Lu, Y.-R.; Xu, Z.-W.; Qiao, S.-Z.; Du, X.-W. CdS Nanoflake Arrays for Highly Efficient Light Trapping. Adv. Mater. 2015, 27, 740-745. 25. Sun, Z. J.; Zheng, H. F.; Li, J. S.; Du, P. W. Extraordinarily Efficient Photocatalytic Hydrogen Evolution in Water Using Semiconductor Nanorods Integrated with Crystalline Ni2P Cocatalysts. Energy & Environmental Science 2015, 8, 2668-2676. 26. Simon, T.; Bouchonville, N.; Berr, M. J.; Vaneski, A.; Adrovic, A.; Volbers, D.; Wyrwich, R.; Doblinger, M.; Susha, A. S.; Rogach, A. L.; Jackel, F.; Stolarczyk, J. K.; Feldmann, J. Redox Shuttle Mechanism Enhances Photocatalytic H2 Generation on Ni-decorated CdS Nanorods. Nat. Mater. 2014, 13, 1013-1018. 27. Li, L. D.; Lou, Z.; Shen, G. Z. Hierarchical CdS Nanowires Based Rigid and Flexible Photodetectors with Ultrahigh Sensitivity. ACS Appl. Mater. Interfaces 2015, 7, 23507-23514. 28. Guo, W.; Ma, J. M.; Pang, G. S.; Wei, C. Y.; Zheng, W. J. Synergistic Effect of the Reducing Ability and Hydrogen Bonds of Tested Gases: Highly Orientational CdS Dendrite Sensors. J. Mater. Chem. A 2014, 2, 1032-1038. 29. Fu, X. Q.; Liu, J. Y.; Wan, Y. T.; Zhang, X. M.; Meng, F. L.; Liu, J. H. Preparation of a Leaf-like CdS Micro-/Nanostructure and Its Enhanced Gas-sensing Properties for Detecting Volatile Organic Compounds. J. Mater. Chem. 2012, 22, 17782-17791. 30. Zhu, L. H.; Wang, Y.; Zhang, D. Z.; Li, C.; Sun, D. M.; Wen, S. P.; Chen, Y.; Ruan, S. P. Gas Sensors Based on Metal Sulfide Zn1-xCdxS Nanowires with Excellent Performance. ACS Appl. Mater. Interfaces 2015, 7, 20793-20800. 31. Gaiardo, A.; Fabbri, B.; Guidi, V.; Bellutti, P.; Giberti, A.; Gherardi, S.; Vanzetti, L.; Malagu, C.; Zonta, G. Metal Sulfides as Sensing Materials for Chemoresistive Gas Sensors. Sensors 2016, 16. 32. Navale, S. T.; Mane, A. T.; Chougule, M. A.; Shinde, N. M.; Kim, J.; Patil, V. B. Highly Selective and Sensitive CdS Thin Film Sensors for Detection of NO2 Gas. Rsc Advances 2014, 4, 44547-44554. 33. Calestani, D.; Villani, M.; Mosca, R.; Lazzarini, L.; Coppede, N.; Dhanabalan, S. C.; Zappettini, A. Selective Response Inversion to NO2 and Acetic Acid in ZnO and CdS Nanocomposite Gas Sensor. Nanotechnology 2014, 25. 16


Page 16 of 19

Page 17 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


34. Fua, X. Q.; Liu, J. Y.; Han, T. L.; Zhang, X. M.; Meng, F. L.; Liu, J. H. A Three-dimensional Hierarchical CdO Nanostructure: Preparation and Its Improved Gas-diffusing Performance in Gas Sensor. Sensors and Actuators B-Chemical 2013, 184, 260-267. 35. Li, S. H.; Chu, Z.; Meng, F. F.; Luo, T.; Hu, X. Y.; Huang, S. Z.; Jin, Z. Highly Sensitive Gas Sensor Based on SnO2 Nanorings for Detection of Isopropanol. J. Alloy. Compd. 2016, 688, 712-717. 36. Hu, D.; Han, B.; Han, R.; Deng, S.; Wang, Y.; Li, Q.; Wang, Y. SnO2 Nanorods Based Sensing Material as an Isopropanol Vapor Sensor. New Journal of Chemistry 2014, 38, 2443-2450. 37. Cai, X.; Hu, D.; Deng, S.; Han, B.; Wang, Y.; Wu, J.; Wang, Y. Isopropanol Sensing Properties of Coral-like ZnO-CdO Composites by Flash Preparation via Self-sustained Decomposition of Metalorganic Complexes. Sensors and Actuators B-Chemical 2014, 198, 402-410. 38. Zhu, Z. L.; Cui, L.; Ling, T.; Qiao, S. Z.; Du, X. W. CdTe Nanoflake Arrays on a Conductive Substrate: Template Synthesis and Photoresponse Property. J. Mater. Chem. A 2014, 2, 957-961. 39. Bao, N.; Shen, L.; Takata, T.; Domen, K.; Gupta, A.; Yanagisawa, K.; Grimes, C. A. Facile Cd-thiourea Complex Thermolysis Synthesis of Phase-controlled CdS Nanocrystals for Photocatalytic Hydrogen Production under Visible Light. J. Phys. Chem. C 2007, 111, 17527-17534. 40. Jiao, X.; Zhang, L. C.; Lv, Y.; Su, Y. Y. A New Alcohols Sensor Based on Cataluminescence on Nano-CdS. Sensors and Actuators B-Chemical 2013, 186, 750-754. 41. Li, J.; Fan, H. Q.; Jia, X. H. Multi Layered ZnO Nanosheets with 3D Porous Architectures: Synthesis and Gas Sensing Application. J. Phys. Chem. C 2010, 114, 14684-14691. 42. Liu, H.; Li, M.; Voznyy, O.; Hu, L.; Fu, Q. Y.; Zhou, D. X.; Xia, Z.; Sargent, E. H.; Tang, J. Physically Flexible, Rapid-response Gas Colloidal Quantum Dot Solids. Adv. Mater. 2014, 26, 2718-2724. 43. Zhang, J.; Liu, X.; Neri, G.; Pinna, N. Nanostructured Materials for Room-temperature Gas Sensors. Adv. Mater. 2016, 28, 795-831. 44. Wang, C. X.; Yin, L. W.; Zhang, L. Y.; Xiang, D.; Gao, R. Metal Oxide Gas Sensors: Sensitivity and Influencing Factors. Sensors 2010, 10, 2088-2106. 45. Sun, Y. F.; Liu, S. B.; Meng, F. L.; Liu, J. Y.; Jin, Z.; Kong, L. T.; Liu, J. H. Metal Oxide Nanostructures and Their Gas Sensing Properties: A Review. Sensors 2012, 12, 2610-2631. 46. Yin, X. M.; Li, C. C.; Zhang, M.; Hao, Q. Y.; Liu, S.; Li, Q. H.; Chen, L. B.; Wang, T. H. SnO2 Monolayer Porous Hollow Spheres as a Gas Sensor. Nanotechnology 2009, 20. 47. Liu, L. P.; Zhuang, Z. B.; Xie, T.; Wang, Y. G.; Li, J.; Peng, Q.; Li, Y. D. Shape Control of CdSe 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nanocrystals with Zinc Blende Structure. J. Am. Chem. Soc. 2009, 131, 16423-16429. 48. Barnard, A. S.; Xu, H. F. First Principles and Thermodynamic Modeling of CdS Surfaces and Nanorods. J. Phys. Chem. C 2007, 111, 18112-18117. 49. Shanavas, K. V.; Sharma, S. M.; Dasgupta, I.; Nag, A.; Hazarika, A.; Sarma, D. D. First-Principles Study of the Effect of Organic Ligands on the Crystal Structure of CdS Nanoparticles. J. Phys. Chem. C 2012, 116, 6507-6511. 50. Nag, A.; Hazarika, A.; Shanavas, K. V.; Sharma, S. M.; Dasgupta, I.; Sarma, D. D. Crystal Structure Engineering by Fine-tuning the Surface Energy: The Case of CdE (E = S/Se) Nanocrystals. J. Phys. Chem. Lett. 2011, 2, 706-712. 51. Li, Y. X.; Tang, L. F.; Peng, S. Q.; Li, Z. C.; Lu, G. X. Phosphate-assisted Hydrothermal Synthesis of Hexagonal CdS for Efficient Photocatalytic Hydrogen Evolution. Crystengcomm 2012, 14, 6974-6982. 52. Giberti, A.; Casotti, D.; Cruciani, G.; Fabbri, B.; Gaiardo, A.; Guidi, V.; Malagu, C.; Zonta, G.; Gherardi, S. Electrical Conductivity of CdS Films for Gas Sensing: Selectivity Roperties to Alcoholic Chains. Sensors and Actuators B-Chemical 2015, 207, 504-510.

18


Page 18 of 19

Page 19 of 19

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table Of Contents

19


Arrays of Ultrathin CdS Nanoflakes with High-Energy Surface for

Recommend Documents