Synthesis, characterization and acetone gas sensing applications of Ag-doped ZnO nanoneedles

Author’s Accepted Manuscript Synthesis, characterization and acetone gas sensing applications of Ag-doped ZnO nanoneedles Yas Al-Hadeethi, Ahmad Umar, Ahmed A. Ibrahim, Saleh. H. Al-Heniti, Rajesh Kumar, S. Baskoutas, Bahaaudin M. Raffah www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(17)30279-1 http://dx.doi.org/10.1016/j.ceramint.2017.02.088 CERI14704

To appear in: Ceramics International Received date: 28 January 2017 Revised date: 19 February 2017 Accepted date: 20 February 2017 Cite this article as: Yas Al-Hadeethi, Ahmad Umar, Ahmed A. Ibrahim, Saleh. H. Al-Heniti, Rajesh Kumar, S. Baskoutas and Bahaaudin M. Raffah, Synthesis, characterization and acetone gas sensing applications of Ag-doped ZnO n a n o n e e d l e s , Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2017.02.088 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis, characterization and acetone gas sensing applications of Ag-doped ZnO nanoneedles Yas Al-Hadeethi1,2, Ahmad Umar3,4*, Ahmed A. Ibrahim,3,4,5, Saleh. H. Al-Heniti1, Rajesh Kumar6, S. Baskoutas5, Bahaaudin M. Raffah3

1

2

Department of Physics, Faculty of Sciences, King Abdulaziz University, Jeddah-21589, Kingdom of Saudi Arabia

Lithography in Devices Fabrication and Development Research Group, Deanship of Scientific Research, King Abdulaziz University, Jeddah 21589, Kingdom of Saudi Arabia 3

4

Department of Chemistry, Faculty of Science and Arts, Najran University, P.O. Box 1988, Najran-11001, Kingdom of Saudi Arabia

Promising Centre for Sensors and Electronic Devices (PCSED), Najran University, P.O. Box 1988, Najran-11001, Kingdom of Saudi Arabia. 5

6

Department of Materials Science, University of Patras, Greece

PG Department of Chemistry, JCDAV College, Dasuya-144205, Punjab, India [email protected] [email protected]

*

Corresponding author:

Abstract Herein, we report the simple synthesis, characterization and acetone gas sensing applications of Ag-doped ZnO nanoneedles prepared by facile hydrothermal method. The synthesized nanoneedles were characterized through different characterization techniques to examine its crystallinity, phase structure, morphological, compositional, optical, vibrational and gas sensing properties. The detailed morphological studies revealed that the Ag-doped ZnO nanoneedles are assembled into non-homogeneously distributed flower-shaped structures which 1

are grown in high density. Further characterizations confirmed that the synthesized nanoneedles are pure, possessing well-crystallinity and exhibiting good optical and vibrational properties. The synthesized Ag-doped ZnO nanoneedles were used as functional material to fabricate high sensitive acetone gas sensors. The effect of operating temperature and concentration of the acetone were analyzed for detailed sensing performance of synthesized nanoneedles. By detailed sensing experiments, the response and recovery times of 10 s and 21 s, respectively were calculated at acetone concentration of 100 ppm at an optimized operating temperature of 370 °C. A maximum sensitivity of 30.233 was recorded at 370 °C operating temperature for 200 ppm of acetone for the fabricated acetone sensor based on Ag-doped ZnO nanoneedles. Keywords: Ag-doped ZnO; Nanoneedles; Acetone; Gas Sensor 1. Introduction Zinc oxide (ZnO), a II-VI n-type semiconductor material with a wide band gap of 3.37 eV and large exciton binding energy of ~ 60 meV, has received a considerable attention due to its multifunctional properties and wide applications in electronics, optoelectronics, sensors and actuators, solar cells and so on [1-7]. Recently, developments in the field of sensors based on multifunctional ZnO nanomaterials have led to the fabrication of highly sensitive and selective sensors and number of reports confirmed that ZnO and based nanomaterials are potential candidates for the fabrication of electrochemical, biochemical and gas sensor [3–7]. Thus, for enhanced sensing performance, a number of strategies, such as engineering the morphology of ZnO nanostructures, doping with noble metals and metal ions to sensitize the ZnO electronically, and loading other n- or p-type metal oxide semiconducting materials for chemical sensitization, are suggested in the literature [8]. Amongst these, doping of the ZnO nanomaterials with metal

2

ions is considered to be the best strategy that not only improves the electrical, semiconducting, catalytic, magnetic, thermal, optical, photoluminescence and mechanical properties but also has a remarkable effect on the particle size, crystallinity, and surface to volume ratio [9–11]. Important sensor parameters such as sensitivity, limit of detection, selectivity, linear dynamic range, fast response and rapid recovery time etc. are highly influenced by these improved characteristics of the doped ZnO nanomaterials[12,13]. Silver doped zinc oxide (Ag-doped ZnO) nanomaterials represent a special class of noble metal ion-doped ZnO nanomaterials. Ag-doped ZnO nanostructures with various morphologies such as nanoflowers[14], nanofibers[15], nano-needles[16], nanocones[12], nanowires[17], nanorods[18], 3D hollow micro/nanospheres[19,20] etc. have been synthesized through a variety of methods and reported in the literature [14-20]. Chen et al.[21] observed a gas response of 22 for 10 ppm of ethanol using 1 wt% Ag-doped sea-urchin-like ZnO nanostructures with high sensitivity and quick response as compared to ZnO nanospheres at an optimized working temperature of 260 oC. For a flexible acetylene gas sensor comprising Ag-loaded vertical ZnO nanorods, supported on a polyimide/polytetrafluoroethylene substrate, a high response magnitude of 27.2 for 1000 ppm, short response and recovery time of 62 s and 39 s, respectively at a low operating temperature of 200oC, were recorded [22]. A composite of 3 wt% Ag-doped ZnO-reduced graphene oxide showed a low detection limit of 1 ppm with very fast response and recovery times of 25 s and 80 s, respectively toward acetylene gas at 150oC temperature[23]. Thus a synergic effect of the Ag doping and n-type semiconducting nature of ZnO plays a vital role in enhancing the electrochemical properties and hence the sensing performances. In this paper, a facile hydrothermal synthesis and detailed characterization of Ag-doped 3

ZnO nanoneedles are reported. Further, gas sensing behavior of synthesized Ag-doped ZnO nanoneedles toward acetone gas was studied by fabricating a specially designed sensor. Finally, the possible acetone sensing mechanism of Ag-doped ZnO nanoneedles based on different acetone concentrations and operating temperatures are also discussed.

2. Materials and methods 2.1. Materials Zinc nitratehexahydrate (Zn(NO3)2.6H2O), hexamethylenetetramine (HMTA; (CH2)6N4), silver nitrate (AgNO3), ammonium hydroxide (NH4OH) were obtained from Sigma-Aldrich and used as received without further purification. For the synthesis of Agdoped ZnO material, de-ionized (DI) water was used as a solvent.

2.2. Synthesis of Ag-doped ZnO nanoneedles Ag-doped ZnO nanoneedles were synthesized by the facile hydrothermal process. In a typical experimental process, 0.1M Zn(NO3)2.6H2O and 0.05M AgNO3 were dissolved in DI water under vigorous stirring for 30 min. After stirring, an aqueous solution of 0.1M HMTA, made in 25 ml DI water, was added to the previous solution and the resultant solution was again stirred for 30 min. The pH of the solution was adjusted to 10 by adding few drops of NH4OH. After adjusting the pH, the resultant solution was transferred to Teflon lined autoclave which was then sealed and heated to 4

150ºC for 4h. After completion of the reaction, the autoclave was cooled to roomtemperature and finally yellowish colored precipitate was obtained which was washed with DI water and ethanol and finally dried at 60 ºC for 3h.

2.3. Characterizations of Ag-doped ZnO nanoneedles The morphology of as-synthesized material was characterized by field emission scanning electron microscopy (FESEM; JEOL-JSM-7600F) attached with energy dispersive spectroscopy (EDS) for elemental analysis. Crystallinity and crystal phases of the synthesized materials were examined by X-ray powder diffraction (XRD; PAN analytical Xpert Pro.), measured with Cu-Kα radiations (λ = 1.54178 Å) in the angular range of 10-80o with the scan speed of 8o/min. The optical properties were studied by obtaining UV-Vis spectrum ((Perkin Elmer-UV-VIS-Lambda 950) at room temperature in the range of 250-800 nm.. To measure the UV-Vis. spectrum, a homogeneous suspension of Ag-doped ZnO nanoneedles was prepared in DI water and then examined. For proper homogenization, the aqueous suspension was ultra-sonicated for 30 min. To examine the functional bonds and chemical structures, the synthesized Ag-doped ZnO nanoneedles were examined by FTIR spectroscopy (FTIR; Perkin Elmer-FTIR Spectrum-100) with KBr dispersion and palletization. The palletization was done by homogeneously mixing the Ag-doped ZnO nanoneedle powder with KBr which was later pressed at a pressure of 5 tons using a pellet-forming die. The obtained pellet was subjected to FTIR analysis. The FTIR analysis was done in the range of 450-4000 cm-1 at room temperature. To study the vibrational properties, the Ag- doped ZnO nanoneedles were examined by Raman-scattering spectroscopy in

5

the scan range of 200–1200 cm−1 using Raman spectrophotometer (Perkin Elmer-Raman Station 400 series).

2.3. Fabrication of acetone gas sensors based on hydrothermally synthesized Ag-doped ZnO nanoneedles In order to increase the crystallinity and to introduce the surface defects Ag-doped ZnO nanoneedles were calcined at 400 oC for 3 h. After that, a homogeneous slurry of the calcined Ag-doped ZnO nanoneedles was prepared in DI water for the fabrication of acetone gas sensor. A thin coating of the slurry was coated on the surface of the pre-cleaned alumina sheet with a surface area of 1.5 cm x 1.5 cm and thickness of 0.25 mm. In order to design a suitable electrical circuit for detecting the sensor parameters two Au electrodes were connected on the top surface and a temperature controlled micro-heater underneath was used as a substrate. IR temperature sensor (Rayomatic14814-2, Euroton IRtec Co.) was used for the control and to measure the operating temperature. The fabricated gas sensor was further air dried followed by heat-treatment at 400C for 3h. The sensitivities (S= Ra/Rg, Ra: resistance in air, Rg: resistance in the presence of acetone) toward acetone were recorded at various (270, 320 and 370C) operating temperatures using a DC 2-probe electrometer interfaced with a computer. The response time (τres) is the time required to attain the 90% steady response value, whereas the recovery time (τrec) corresponds to the time required to attain the 10% of the initial response value [24,25].

3. Results and discussion 6

3.1. Characterization of Ag-doped ZnO nanoneedles. To examine the morphologies, the synthesized Ag-doped ZnO materials were characterized by field emission scanning electron microscopy (FESEM). As can be seen from the low magnification FESEM images, the synthesized Ag-doped ZnO nanomaterials possess flower-shaped morphologies which are grown in very high density (Fig. 1(a) and (b)). The high resolution FESEM images confirmed that the flower-shaped structures are composed of highaspect ratio Ag-doped ZnO nanoneedles (Fig. 1(c) and (d)). The nanoneedles are assembled into non-homogeneously distributed flower-shaped structures. The bases of the nanoneedles are loosely held in position in the flower shapes are pointed outward. The average diameter of each flower structure is ~8-10 μm. The nanoneedles are wider at their bases with sharper and blunt ends. The typical length of each needle is ~5 – 6 μm with an average width of ~150 nm at the middle (Fig.1 (c-d)). Additionally, some Ag-doped ZnO needles are randomly distributed and are not assembled to form flower-shaped structures (Fig.1 (a)). According to Lv et al.[26], the nonhomogeneous and random distribution of flowers and nanoneedles, respectively is due to the high concentration of the HO- ions in the growth solution. It is well reported in the literature that the hydrothermal, alkaline growth of ZnO occurs through the formation of [Zn(OH)4]2intermediate species[27–29]. Thus the presence of sufficient concentration of HO- ions in the growth solution is essential for ZnO growth. However, at higher concentration of these HO- ions, partial hydrolysis of amphoteric ZnO structures may take place due to corrosive nature of these ions[26]. Hence, as reported hydrothermal alkaline method is an effective way of synthesizing dense ZnO non-homogenous flower shaped morphologies with high surface to volume ratio

7

which is quite imperative for the fabrication of highly sensitive and selective acetone gas sensors.

Fig.1. (a-b) Low-magnification and (c-d) high-resolution FESEM images of Ag-doped ZnO nanoneedles assembled into flower-shaped structures.

To examine the elemental composition and purity of hydrothermally synthesized Agdoped ZnO nanoneedles assembled into flower-shaped structures was determined by energy dispersive spectroscopy (EDS) attached with FESEM. Fig 2 (a) clearly revealed that the Ag8

doped ZnO nanoneedles assembled into flower-shaped structures are made of zinc, silver, and oxygen only. Thus these Ag-doped ZnO nanostructures are highly pure without any impurities or contaminations. To examine the crystallinity and crystal phases, the synthesized Ag-doped ZnO nanoneedles were characterized by X-ray diffraction. Fig.2 (b) represents the typical X-ray diffraction pattern for Ag-doped ZnO nanoneedles. Several well-defined diffractions peaks appearing at 2θ= 31.75°, 34.40°, 36.25°, 47.50°, 56.55°, 62.80°, 66.3°, 67.95°, 69.05°, 72.55° and 77.35° correspond to the diffraction planes (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202), respectively are observed which are well matched with the wurtzite hexagonal phase of ZnO. The observed diffraction peaks for ZnO is well matched with the JCPDS data card no. 36-1451 and other reported literature [27,30]. In addition, there are other three diffraction peaks situated at 38.10°, 44.25° and 64.40° which are assigned to the (111), (200), (220) plane of face-centered cubic (fcc) lattice for Ag (JCPDS data card no. 04– 0783) [31,32]. No other peak in the X-ray diffraction pattern for Ag-doped ZnO nanoneedles was observed which further confirmed that the synthesized material is Ag-doped ZnO without any significant impurity. Typical FTIR spectrum for Ag-doped ZnO nanoneedles is shown in fig. 3 (a). Various peaks at 917, 1006, 1409, 1597 and 3445 cm-1 were observed in the FTIR spectrum. The band at 917 cm-1 may be assigned to the in-plane vibrational modes of nitrate ( NO3 ) ions [33]. The Very low-intensity band at 3445 cm-1 and additional sharp band at 1409 cm−1 may be due to O-H stretching vibrational modes and deformations, respectively of the physisorbed water molecules on the surface of the Ag-doped ZnO nanoneedles [34]. 9

1000

500

1500

10 20 30

2 (degree) 40

10 50

ZnO (002)

60 70 ZnO (202)

ZnO (103) Ag (220) ZnO (200) ZnO (112) ZnO (201) ZnO (004)

ZnO (110)

Ag (200) ZnO (102)

Ag (111)

2000

ZnO (100)

3000 ZnO (101)

Intensity (Arb. units)

O Zn Zn

Ag Zn

3500

(b)

2500

0

80

Fig.2. (a) EDS spectrum and (b) XRD patterns for Ag-doped ZnO nanoneedles.

In order to study the optical properties and to be familiar with the band gap energy of the Ag-doped ZnO nanoneedles, UV-Vis. spectrum was recorded in the scan range of 250-800 nm (Fig. 3(b)). A single and sharp absorption peak, characteristics to the Wurtzite hexagonal phase of ZnO, at 380 nm can be clearly seen. Eq. 1 was used to calculate the band gap energy in electron volt and the wavelength in nanometer which was found to be 3.26 eV [35–37]. A single and sharp absorption peak in the UV-Vis. spectrum further, confirms the excellent optical properties and purity of the Ag-doped ZnO nanoneedles.

Eg 

1240 ……………..(1)  max

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12

(a)

8

4

0

1597

2

4000

917

1006

6

1409

% Transmittance

10

3445

3500

3000

2500

2000

1500 -1

1000

500

Wavenumber (cm ) 0.50

380 nm

(b)

0.48

Absorbance

0.46 0.44 0.42 0.40 0.38 0.36 0.34 0.32

300

400

500

600

700

800

Wavelength (nm)

Fig.3. (a) FTIR and (b) UV-Vis. spectrums of Ag-doped ZnO nanoneedles. Raman spectrum for Ag-doped ZnO nanoneedles is shown in fig.4. ZnO belongs to 4 typical wurtzite hexagonal phase with C 6v (P63mc) space group with Zn and O atoms occupying

the C3v sites in crystal lattice[38]. The Raman-scattering spectrum shows a prominent phonon peak at 464 cm-1 attributed to the E High mode and corresponds to the ZnO non-polar optical 2

12

phonon. Normally this peak is situated at 435-440 cm-1 for pristine ZnO. A Large shift of about 20-25 cm-1 may be due to the doping of the Ag into the ZnO space lattice[39,40].

900

-1

950

464 cm (E2)

Intensity (Arb. units)

1000

850 800 750 700 200 300 400 500 600 700 800 900 1000 1100 1200 -1

Raman shift (cm ) Fig.4. Raman spectrum of Ag-doped ZnO nanoneedles.

3.2. Sensing behavior of Ag-doped ZnO nanoneedles towards acetone

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In order to extract the best results for the acetone sensing parameters, influence of the operating temperature which has a significant role was considered in this investigation. For the excitation of the electrons from valence band to the conduction band, generation of oxygenated anions from adsorbed oxygen, and surface activation of and Ag-doped ZnO nanoneedles for the adsorption of the acetone molecules, optimization of operating temperature is quite important[21]. Operating temperature also significantly affects the response and recovery times of the sensor materials [41,42]. Fig.5 shows the variations of sensitivity for the Ag-doped ZnO nanoneedles based gas sensors when exposed to 100 ppm of acetone at 270, 320 and 370oC operating temperatures. The maximum sensitivity of 18.112 was observed for Ag-doped ZnO nanoneedles based acetone sensors at 370 °C. In contrast at 270 and 320 °C operating temperatures, sensitivities of 5.388 and 4.311, respectively were found.

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Sensitivity (Ra/Rg)

16 14 12 10 8 6 4 2 0

320

270

370 o

Temperature ( C) Fig.5. Variations of the sensitivity of Ag-doped ZnO nanoneedles based gas sensors against 100 ppm of acetone as a function of operating temperature. 14

In fig. 6, the real-time dynamic voltage-time response transients for 10-200 ppm of acetone at optimized 370C operating temperature are represented. A significant rise in the voltage for the very addition of acetone, confirms the sensing ability of the as fabricated Agdoped ZnO nanoneedles based gas sensors. Further, the sensor showed a significant voltage rise for the even very low concentration of 10 ppm of acetone. Response (res= 10 s) and recovery (rec = 21 s) times were determined from the voltage response transient for 100 ppm acetone, shown in the inset of fig.6. It was suggested by Chen et al. [43] and Khan et al. [44], that Ag+ ions doping into the ZnO space lattice increase the surface defects, mainly in the form of oxygen vacancies, which further improves the charge carrier density, and hence better sensing responses.

5.5 2.5

5.0 2.0

1.0 0.5 0.0

2.5

0

20

40

60

80

100

120

1.5 1.0

50 ppm

20 ppm

2.0

140

70 ppm

Time (s)

100 ppm

3.0

200 ppm

3.5

1.5

10 ppm

Voltage (V)

4.0

rec=21s

res=10s

150 ppm

Voltage (V)

4.5

0.5 0.0 0

200

400

600

800

Time (s)

15

1000

1200

Fig.6. Real-time dynamic V-T response curves for acetone for 10-200 ppm concentrations at 370oC temperature. The inset represents the response curves for 100 ppm of acetone with response and recovery times.

The concentration of the analyte gas is yet another crucial factor that affects sensing responses of the pure and doped metal oxide based gas sensors [45]. At higher concentration, a greater number of gas molecules are adsorbed and oxidized on the surface of the gas sensors. The electrons released during this oxidation are responsible for the notable rise in the sensitivities. In fig.7 the effect of the concentration of the acetone on the sensitivities is shown. The concentration range for the acetone was from 10-200 ppm and the experiment was conducted at an optimized temperature of 370oC. A positive correlation, though not fully linear, between the sensitivities and the concentrations of the acetone can be seen from the results. A maximum sensitivity of 30.233 was recorded at 370oC operating temperatures for 200 ppm of acetone.

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35

Sensitivity (Ra/Rg)

30 25 20 15 10 5 0 0

25

50

75

100

125

150

175

200

Concentration (ppm) Fig.7. Effect of acetone concentration (10-200 ppm) on the sensitivities of Ag-doped ZnO nanoneedles based sensors at 370oC. 3.3. Sensing mechanism Needle-shaped morphologies of the as-synthesized Ag-doped ZnO nanostructures, not only provide the large surface area for the adsorption of acetone and oxygen but also have substantial amount of surface defects due to Ag doping. At an optimized temperature of 370 oC, chemisorbed atmospheric O2 molecules from the air are converted into negatively charged oxygenated species like, O22 , O 2 and O  etc. with the aid of conduction band electrons (Eq. 2, 3)[46,47].

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( )

( )

→

(

→

)

→

)→

(

(

(

)

)

……….(2)

…………..(3)

Acetone on interaction with these oxygenated anionic species is oxidized to different byproducts. The electrons released during this oxidation process are transferred back to the conduction band thereby decreasing the resistance of the Ag-doped ZnO nanoneedles (Fig.8) (Eq. 4-6) [48, 49]

(

…………….(4)

)

(

…………….(5)

)

(

…………….(6)

)

18

Fig.8. Sensing mechanism of Ag-doped ZnO nanoneedles shaped structures for acetone.

4. Conclusion In summary, Ag-doped ZnO nanoneedles were synthesized by simple and facile hydrothermal process and characterized in detail in terms of morphological, structural, compositional, optical and vibrational properties. The detailed morphological characterizations

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confirmed that the nanoneedles are grown in high density and possessing high aspect ratio. The structural, compositional, optical and vibrational properties revealed that the synthesized nanoneedles are pure Ag-doped ZnO, possessing well-crystallinity and good optical and vibration properties. Further, the synthesized Ag-doped ZnO nanoneedles were used as functional nanomaterial to develop nanosensor device by coating it on the proper substrate with suitable electrodes. The fabricated sensors based on nanoneedles were tested towards acetone gas which exhibited fast response with the very low response and recovery times. The optimum sensitivity of the fabricated sensor was found to be 30.3 at acetone concentration of 200 ppm and operating temperature of 370oC. The observed results clearly confirmed that the synthesized Agdoped ZnO nanoneedles are efficient functional material for the fabrication of excellent, low cost gas sensors. Thus, Ag-doped ZnO nanomaterials are potential candidates for variety of toxic and harmful gases.

Acknowledgements: This project was funded by the Deanship of Scientific Research (DSR) at King Abdulaziz University, Jeddah, under grant no. (RG/1/130/37). The authors, therefore, acknowledge with thanks DSR for technical and financial support.

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