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Controllable Evolution of Dual Defects Zni and Vo Associatesrich ZnO Nanodishes with (0001) Exposed Facet and Its Multiple Sensitization Effect for Ethanol Detection Zhenggang Xue, Zhixuan Cheng, Jin Xu, Qun Xiang, Xiaohong Wang, and Jiaqiang Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13370 • Publication Date (Web): 08 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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ACS Applied Materials & Interfaces

Controllable Evolution of Dual Defects Zni and VO Associates-rich ZnO Nanodishes with (0001) Exposed Facet and Its Multiple Sensitization Effect for Ethanol Detection Zhenggang Xue †, Zhixuan Cheng †, Jin Xu ‡, Qun Xiang †, Xiaohong Wang* †, Jiaqiang Xu * † †

NEST Lab, Department of Chemistry, College of Science, Shanghai University, Shanghai,

200444, China. ‡

School of Industrial Engineering, Purdue University, 315 N. Grant St, West Lafayette, IN

47907, USA. * Corresponding author Tel.: +86 21 66132701 E-mail: [email protected] E-mail: [email protected] KEYWORDS: ZnO nanodishes, surface defects, mechanism, crystal facet, gas sensing.

ACS Paragon Plus Environment

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Abstract Building an effective way for finding the role of surface defects in gas sensing property remains a big challenge. In the present work, we synthesized the ZnO nanodishes (NDs) and first explored the forming process of rich electron donor surface defects by means of studying mechanism for the ZnO NDs synthesis. The test results revealed that, ZnO-6, added by 6 mmol Zn powder, had the best gas sensing properties with the excellent selectivity to ethanol than the others. Specially, the ZnO-6 sensor exhibited the best response (about 49) to 100 ppm ethanol at 230 °C among four as-synthesized samples, while non-customized ZnO was only 28. It was mainly caused by the following two reasons: the exposure of target (0001) crystal facet and rich electron donor surface defects zinc interstitial (Zni) and oxygen vacancy (VO). As a guide, the forming process of surface defects was revealed by an ideal defect model. By the small-angle XRD and TEM patterns, we could conclude that ZnO NDs, changing stoichiometric ratio, increased the content of Zni by adding Zn powder, while excessive Zn powder promoted the growth of c axis of ZnO NDs in the self-assembly engineering. Besides, a depletion model has been provided to explain how the surface defects work on the sensors and the complex mechanism of gas sensing performance. These findings will develop the application of ZnO based gas sensor in health and security.


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1. Introduction In response to the frequent accidents in public and production, the increasingly serious air and indoor pollution, the traffic accidents caused by drunk or drugged driving, the demands of semiconductor gas sensors have been increased greatly1. Especially, gas sensors have been playing an irreplaceable role in detecting harmful gases and disease diagnoses2. Thus, numerous materials have been employed and explored as sensor materials, such as ZnO, SnO2, WO3, In2O3 and so on3. Thereinto, ZnO is one of the most popular materials due to its good stability and high electrons mobility4. Over the past few years, ZnO nanomaterials with various nanomorphologies, including nanorods5, nanowires6, nanotubes7, nanosheets8, 3D-flowerlike nanostructures9 and hollow microspheres10, have been studied to improve the gas-sensing properties. In principle, gas sensors will have an abrupt resistance change when exposing in detected gases. It is mainly due to the oxidation-reduction reactions occurring on the surface of sensors11. Therefore, the sensing performance is strongly related to the exposed crystal facets of the sensing materials12. Many previous references have reported the crystal-facets-dependent gas sensing properties of metal oxide nanocrystal materials, such as ZnO13, SnO214, and WO315. For example, it was reported that WO3 nanorods with exposed (002) facets showed a better sensing property to acetone compared with those with exposed (100) facets15. Besides the exposed crystal facets, crystal defects also play an important role in gas-sensing properties of ZnO nanomaterials16. Especially, the oxygen vacancy (VO) are proved to be linearly proportional to the gas sensing properties13. The reason for the formation of defects is that it is unavoidable for the formation of lattice creep13. Besides, as a same performance defect of electron donor with VO, zinc interstitial (Zni) also effects the forming process of VO and the promotion of gas sensing properties17. So far, there are numerous researches on the optimization


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of the catalytic VO by means of controlling sintered temperature18, adjusting concentrations of partial oxygen19, adding ultraviolet radiation20 and so on. Especially, wet-chemical preparation was an important way to introduce defects on ZnO surface21. For example, Xu et al. have prepared rich-oxygen-vacancy ZnO nanosheets by introducing reactive Zn power22. On the other hand, no matter the exposure of target crystal facets or the formation of defects, they both greatly depend on the forming mechanism of semiconductors. However, few studies can be enforced for the forming mechanism, particularly in aqueous phase. Enlightened by the studies of Xie group about “the formation of VO-rich In2O3 sheets”23, we made a deep analysis in the forming process of ZnO NDs. By considering its complex forming mechanism at a nanometer level, we could prepare ZnO NDs with especial crystal facets and rich electron donor defects. Here, we successfully fabricated ZnO NDs gas sensors exposed with a (0001) facet. Meanwhile, contents and types of surface defects were studied to reveal the effects on gassensing performance. Especially, the dual defects of Zni and VO acted a positive role in enhancing the gas-sensing performance. In addition, the sensing mechanism of ZnO NDs and controllable defects were discussed via building flexible crystal models. Thus, the exposed facets, effects of defects and forming mechanism of ZnO NDs were established to further investigate the sensing performance.

2. EXPERIMENTAL SECTION All chemicals were analytical grade reagents from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). 2.1 Synthesis of ZnO nanodishes. ZnO NDs samples were prepared by a flexible hydrothermal method. In a typical synthesis, 1.92 g hexamethylenetetramine (HMT) was


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dissolved into 24 mL deionized water. After stirring it for 10 min, 3.00 g zinc acetate dehydrate [Zn(CH3COO)2·2H2O] and a certain amount of metal zinc powder were added into the solution. The generated solution was stirred for another 10 min with a magnetic stirrer and then transferred into a 50 mL Teflon-lined autoclave and maintained at 97 °C for 12 h. Then, the precipitates were thoroughly washed with distilled water and ethanol three times and then dried at 60 °C in a vacuum oven, followed by calcinating them at 200 °C for 2 h. The products synthesized with the addition of 0 mmol, 2 mmol, 6 mmol, and 10 mmol metal zinc powder were denoted as ZnO, ZnO-2, ZnO-6, ZnO-10, respectively. 2.2. Characterization and Response Test of the Gas Sensor. The characterization instruments of ZnO NDs were displayed in Note S1. The sensor we used was side-heated resistive type. And the fabrication and test process were described detailedly in Figure S1 and Note S2.

3. RESULTS AND DISCUSSION 3.1. Microstructure Characterizations of ZnO NDs. The XRD patterns were revealed in Figure 1. All of the diffraction peaks match to the hexagonal wurtzite phase (PDF No. 36-1451) with lattice constants of a = b = 3.25 Å and c = 5.2 Å. No characteristic peaks of other materials are found, indicating the high purity of single-phase ZnO. However, the peaks of (0002) and (101ത 0) revealed obvious differences. Compared with ZnO, ZnO-2 and ZnO-10, the ZnO-6 sample obviously shows a lager intensity ratio of (0001) facet to (101ത0) facet (I(0002)/I(10 1ത 0)), which means more exposure of (0001) crystal facet11. As thus, the visible evidence extracted by XRD patterns demonstrates the different rates of exposed crystal facets corresponding to ZnO NDs. Figure 2 shows the SEM images of all products. Among them, ZnO, ZnO-2 and ZnO-6 are


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respectively presented in Figure 2a, 3b and 3c. They are 400 nm in diameter and 200 nm thickness with a similar uniform hexagonal morphology. However, ZnO-10 displays a 300-400 nm thickness with an obvious growth along the c axis, as seen in Figure 2d. As seen in Figure 3a and 3b, the TEM images of ZnO-6 and ZnO-10 clearly show the dish-like morphology. The HRTEM image of ZnO (Figure 3c) shows the crystal fringes with distance of about 0.26 nm, corresponding to the (0002) plane of ZnO crystal. It is noted that the SAED pattern (inset in Figure 3c) displays a spot pattern, indicating as-synthesized ZnO NDs were single crystal. Meanwhile, Figure 3d gives further details of ZnO-6 materials: the HRTEM image exhibits many discontinued crystal fringes (marked by the yellow circles), indicating a great number of crystal defects in the ZnO NDs24.

Figure 1. XRD patterns for all products.


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Figure 2. Dish-like ZnO. STM topography images of (a) ZnO, (b) ZnO-2, (c) ZnO-6, (d) ZnO-10.

Figure 3. Dish-like ZnO. TEM topography images of (a) ZnO-6, (b) ZnO-10. HRTEM of (c) ZnO and (d) ZnO-6. Corresponding SAED pattern image (inset in Figure 3c).


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3.2. Gas Sensing Properties of the ZnO NDs. It is well-accepted that working temperature has a great influence on sensing performance25. Therefore, the responses of ZnO NDs sensors toward 100 ppm of ethanol were tested at different working temperature. As seen in Figure 4a, when the temperature was changed from 150 to 260 ℃, we can find a response maximum of the sensor at 230°C. So, the temperature of 230°C was selected as an optimal working temperature. Meanwhile, we could conclude that ZnO-6 had the best gas-sensing response toward 100 ppm ethanol, and the response value was about 49. Generally speaking, the operating temperature of ZnO products is between 300 to 400 °C, as shown in Table 1. The purpose of working at a high temperature of semiconducting gas sensor is for avoiding interference of humidity and endowing semiconducting sensor a suitable resistance. But to address thoroughly the safety detection of combustible gas, working at room temperature is a desired idea for a semiconducting gas sensor. But it is still a challenge owing to the limits as above. Actually, our sensors have successfully reduced the optimum operating temperature to 230 °C. We will try our best to develop semiconducting gas sensors with acceptable gas sensing properties and ability to work at room temperature, especially in detection of flammable and explosive gas. Furthermore, selectivity was considered as another important factor for sensors design. Figure 4b exhibited the responses of sensors to different gases. Clearly, the gas response to ethanol was better than the others, indicating a satisfactory selectivity for detecting ethanol. Figure 4c showed the response-recovery curves of ZnO-6. The response time was defined as taking time of reaching 90% of the total resistance change, while the recovery time as taking time of returning to 10% of resistance change. The response time of ZnO-6 was calculated to be 6s while the recovery time was 13s. According to the related studies shown in Table 1, the


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response/recovery time of ZnO-6 exhibited satisfied results. Figure 4d demonstrated the dynamic response-recovery curves of the sensors. It could be observed that the response was promoted when increasing gas concentration. This demonstrated that it was an n-type semiconductor sensor. Figure 4e showed the effect of humidity on ZnO-6 sensors. As we can see, there was a stable response when the humidity is below 53%. This meant that water didn’t have an obvious influence on ZnO-6 at lower humidity. With the raise of humidity, the response of ZnO-6 to ethanol showed a decreasing trend. When the humidity reached to 71%, the response of ZnO-6 was still 30.2, which indicated that ZnO-6 had a satisfied performance at higher humidity. The long-term stability of the sensor was investigated, as shown in Figure 4f. After running for 35 days, the response of ZnO-6 retained at almost the same level, which confirmed the good stability. Furthermore, ZnO-6 exhibited a higher response to ethanol compared with previous works, as shown in Table 1.


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Figure 4. Gas sensing properties of the ZnO NDs. (a) ZnO NDs sensors toward 100 ppm ethanol at different temperature. (b) Gas response towards 100 ppm NO2, CO, NH3, H2S, HCHO, CH2OH, and C2H5OH. (c) response-recovery results. (d) Sensors upon exposure to various concentrations of ethanol. (e) The response under different humidity conditions. (f) The longterm stability of the sensors.

Table 1. Ethanol sensing properties in the literatures and this work. Sample

Temperature (℃)

Concentration (ppm)

Humidity (%)

Response

Response/recovery time (s)

Reference

ZnO nanowires

300

100

AIR

33.0

11/38

26

ZnO nanoplates

380

100

AIR

8.9

32/17

27

Flower-like ZnO

420

100

AIR

10.0

-

28

ZnO nanorods

332

200

AIR

18.9

10/30

29

Flower-like ZnO

320

100

AIR

25.4

2/15

30

Flower-like ZnO

370

100

AIR

31.0

-

31

ZnO nanopyramids

260

250

AIR

5.7

8/48

32

ZnO nanorods

320

100

30

26.0

-

33

ZnO nanodishes

230

100

53

49.0

6/13

Present work


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3.3.

Defects

Characterizations

of

ZnO

Nanodishes.

Photoluminescence (PL)

spectroscopy method was used to study each defect structure in ZnO NDs, as shown in Figure S2. In detail, according to the original curves, the whole PL spectrum in the range 370−600 nm can be Gaussian deconvoluted into eight parts34, as shown in Figure 5. These parts can be attributed to different defects35. Detailedly, the contents of these defects are shown in Table 2. As a result, Zni and VO can be regarded as electron donors in ZnO samples, and the more donors are, the higher responses are13. From the Table 2, ZnO-6 has the most electron donors (Zni and VO)， which can provide more electrons to react with oxygen and absorb more detection gas to improve sensing performance36. This leads to a best sensing performance. As for ZnO-10, it exhibits a decreased ingredient of Zni and VO, which will lead to a low sensing performance.


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Figure 5. Characterizations. Photoluminescence Spectroscopy of different products.

Table 2. The Contents of Defects of the PL Spectra. Origination

Peak(nm)

ZnO

ZnO-2

ZnO-6

ZnO-10

385

63.7

63.8

48.7

55.0

Zni ( 2.90 eV )

435, 450

16.3

17.5

22.2

18.9

VZn ( 2.70 eV )

465

13.8

11.4

13.1

13.0

VO ( 2.53 eV )

485, 500

3.3

4.7

5.5

4.5

OZn ( 2.38 eV )

520

1.3

2.2

8.1

6.9

Oi ( 2.23 eV )

540

1.5

0.5

2.4

1.5

19.6

22.2

27.7

23.4

(CB-VB)combination

Zni+VO


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Furthermore, X-ray photoelectron spectroscopy (XPS) spectra were given to describe the contents of the oxygen species of ZnO NDs7. The survey scan spectra (Figure S3) showed obvious peaks for zinc and oxygen and negligible carbon detected from the substrate, suggesting that the products were single ZnO. The O 1s XPS peaks of ZnO, ZnO-2, ZnO-6 and ZnO-10 can be decomposed into three component peaks centered at 529.11 ± 0.15 eV (OI), 531.06 ± 0.15 eV (OII) and 533.94 ± 0.15 eV (OIII), respectively, as shown in Figure 6. The peak on the low binding energy (OI) is attributed to O2- ions in wurtzite structure of hexagonal Zn2+ ions array37. The medium peak (OII) is associated with oxygen-deficient regions in ZnO crystal38, while the peak with higher binding-energy component (OIII) is assigned to the dissociated or chemisorbed oxygen or OH species on the ZnO surface39. The contents of these oxygen species are displayed in Table 3. The absorbed oxygen species are benefit to improve sensing performance. So the most OII and OIII implies an excellent response. The significant differences of OII and OIII further confirm the differences of these products’ oxygen-deficient.

Figure 6. O 1s XPS peaks of (a) ZnO, (b) ZnO-2, (c) ZnO-6 and (d) ZnO-10.


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Table 3. Summarized Percentages of the O 1s in XPS. Species

Peak(eV)

ZnO

ZnO-2

ZnO-6

ZnO-10

OI

529.11

0.63

0.57

0.41

0.55

OII

531.06

0.34

0.36

0.51

0.38

OIII

533.94

0.03

0.07

0.08

0.07

Raman scattering has been employed to characterize the defect types of the as-grown ZnO NDs. The spectra of all ZnO NDs are shown in Figure 7. Three common prominent vibration peaks of these samples can be observed at about 330, 380, 437 cm-1. A sharp peak at 437.1 cm-1 is attributed to the oxygen vibration, which can reflect the crystallinity. As we see, ZnO-6 has the best crystallinity among these four samples. The peaks located at ∼330 and ∼380 cm−1 are related to the modes E2H−E2L (E2High-E2Low) and A1T of the ZnO crystal, respectively40. Especially, the peak centered at ~585cm-1 is assigned as the mode E1L (E1Low), which is correlated to surface defects in the ZnO NDs. Detailedly, the intensity of this peak is more strongly affected by the electron donor defects such as VO and Zni41. Furthermore, the ZnO-6 exhibits the highest peak at ∼585 cm−1 compared with other samples, indicating that the ZnO-6 will have the most VO and Zni. This notable and indubitable evidence extracted by Raman spectra demonstrates the success of introducing VO and Zni.


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Figure 7. Characterizations. Raman spectra of the different products of ZnO NDs. 3.4. Synthesis Mechanism of ZnO Nanodishes. In this work, we highlight the rather complicated intermediate during the synthetic process of ZnO nanodishes (Figure 8a): initially, a HMT molecule hydrolyzed to NH4+ and coordinated with Zn2+ to form [Zn(NH3)4]2+42, then two acetate ions interacted with one [Zn(NH3)4]2+ to form a [Zn(NH3)4]2+-acetate complex by electrostatic interaction, and all the [Zn(NH3)4]2+ ions were uniformly separated by two acetate ions rapidly. After a reaction of 2 h, the acetate ions tended to move along one direction, which was mainly due to the reduction of surface energy19. Correspondingly, the homogeneously dispersed acetate ions on the surface of [Zn(NH3)4]2+ ions exactly led the complex to take on a stratified structure called “side-by-side”. As seen in Figure S4, the small-angle XRD pattern demonstrated the presence of a phase with a = 13.3 Å, which agrees with the length of two acetate ions and two [Zn(NH3)4]2+ radii43, indicating the uniform dispersion of acetate ions. As the hydrothermal reaction going, the growth structure seems to reach the limit. At the same time, the phenomenon of “layer stretch” seems more inevitable. As seen in Figure S4, it shows the appearance of new layer spacing of ca. 21.6 Å at 6 h. Finally, after a reaction of 8 h, only weak


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peaks could be found (Figure S4), which suggested that stratified structures were gradually disappear and fused to ZnO NDs. Especially, intermediate [Zn(NH3)4]2+-acetate complex could be demenstrated by FT-IR (Figure S5 and Note S3).

Figure 8. (a) Schematic illustration for the formation of ZnO NDs. (b) Evolution of dual defects Zni and VO. The plane is ZnO (0001) crystal facet. Defect redistribution and the relationship of Zni and VO are exhibited. (c) Schematic illustration for a growth of c axis in the ZnO crystals. 3.5. Sensing Mechanism of Sensors. From the above, notably, the superior visible gas sensing properties of ZnO-6 could be contributed to the exposure of target crystal facet and the more contents of electron donors VO and Zni. In our previous studies, experimental and computational study demonstrated that dominating exposed (0001) crystal facet of ZnO nanomaterial had a better gas response and an excellent selectivity than that with a similar morphology exposed (10 10) crystal facet owing to the dominate growth of electron donor defects


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in (0001) crystal facet44. On account of the addition of Zn powder will change the vacancy contents of ZnO semiconductor, one model can be used to describe this process vividly in Figure 8b. For a typical illustration, ZnO nanodishes are composed of the (0001) crystal lattice. To express intuitively, the crystal facet was built as a perfect surface. In this system, the existent of Zni could be exhibited mainly by two ways: on the one hand, the addition of Zn powder will alter the stoichiometric ratio of the products. A part of “effective” Zn powder can be converted into Zn2+ and mingle into the lattice of ZnO to form Zni, as can be seen in Figure 8b①. As for ZnO-10, with more adding contents of Zn powder, it exhibits a decreased ingredient of Zni. This is mainly due to the control of synthetic conditions. In the case of constant temperature, the synthesis of products relies on dynamic influences primarily (here chiefly refers to the amount of raw materials)45. Rather than higher reactive fringe region controlled by thermodynamics, this leads to a growth along c axis. That means more amounts of Zn powder turn into no longer the Zni but an extension of c axis in the crystals2b. This result is mainly depended on the stratified growth concluded by small-angle XRD. Additionally, this is in agreement with the SEM and TEM characterizations. A specifical model has been built to exhibit this process, as shown in Figure 8c. On the other hand, the formation of Zni also depends on the defects of the crystal itself, as seen in Figure 8b②. The lattice oxygen will inevitably transform into vacancy oxygen in the calcinating process. Specifically, oxygen anions (O2−) will be removed from surface and react to form oxygen molecules18. Then, nonbonding Zn atoms combine with Zni to form the Zn-Zn bond, for the lower formation energy compared with other defects46. As for the formation of VO, there are a few differences in morphology among ZnO, ZnO-2 and ZnO-6. With the increase of Zn powder, ZnO-6 has more vacancy oxygen. On the contrary, the decrease of surface area of


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ZnO-10 makes it a lower concentration of VO. Furthermore, a well-accepted fact is that the sensing performance of semiconductor material can be promoted by its rich electron donors VO and Zni. Such a complicated process can be described easily in Figure 9. As a result, oxygen molecules capture electrons to form O-2 , O-, O2on the ZnO surface47. It can produce a depletion layer on the surface of ZnO48. The depletion thicknesses (LD) is thought to potential barriers of the contacts between the ZnO NDs49. Thus, models of depletion layer could be exhibited in Figure 9. Besides, the related reactions are expressed as follows: O2 (gas) ↔ O2 (ads)

(1)

O2 (ads) + e- ↔ O-2 (ads)

(2)

O-2 (ads) + e- ↔ 2O- (ads)

(3)

O- (ads) + e- ↔ O2- (ads)

(4)

At an operating temperature of 230 °C, O- will be chemisorbed on the ZnO surface50. Furthermore, the reduction of electrons in conduction band will bring a larger resistance and a thicker depletion layer. ZnO-6, with more electron donors, could adsorb more oxygen molecules on its surface when it was exposed in the air at working temperature. When ethanol gas is absorbed on the sensor surface, it can be oxidized to acetaldehyde, and then reacts with oxygen ions51. This results in the electrons return into the conduction band, further giving rise to a thinner potential barrier and a smaller electrical resistance of the sensor. The reaction is following: C2H5OH →CH3CHO + H2

(5)

CH3CHO (ads) + 5O- (ads) → 2CO2+ 2H2O + 5e-

(6)


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As a result, ZnO-6 has a larger Ra and a smaller Rg, leading to a large Sg. In conclusion, the bigger changes of LD are, the higher gas response of the sensors will be. Thus, ZnO-6 shows the best gas response.

Figure 9. The illustrations of sensing processes of ZnO NDs at 230 °C. The depletion layer LD (the gray region) changed in air and in ethanol. The purple region is the conducting area. 4. CONCLUSION. In conclusion, the ZnO nanodishes with different exposed crystal facets and defects contents have been synthesized for studying the role of electron donor defects in sensing performance. Adding Zn powder intrinsically changes Zn/O dual defects in the forming process of ZnO nanodishes. Particularly, with the amount of zinc powder increasing, Zni and VO have a


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promotion; when adding more Zn powder, it is more likely to grow along with c axis as a raw material. All the PL, XPS, and Raman spectra confirm the different Zni and VO concentrations in all the products. Derived from defects evolution and exposed crystal facets, the ZnO-6 exhibits the best gas response and selectivity, which is attributed to the exposure of target (0001) crystal facet and rich electron donor surface defects zinc interstitial (Zni) and oxygen vacancy (VO). Besides, our work demonstrates the role of semiconductors’ forming mechanism in aqueous phase and sheds lights on the crucial role of defects playing in the sensing performance.


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ASSOCIATED CONTENT Supporting Information Figure of gas sensing test system; PL, XPS, small-angle XRD, and FT-IR spectra of samples; the characterization instruments and the process of gas sensors fabrication and response test; and FTIR spectra feature analysis. (PDF) AUTHOR INFORMATION Corresponding Author Tel.: +86 21 66132701 E-mail: [email protected] E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors thank the supports of National Natural Science Foundation of China (61671284). The authors thank the help of Instrumental Analysis and Research Center in Shanghai University for material characterization. REFERENCES

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(0001) Exposed Facet and Its Multiple ... - ACS Publications

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