Synthesis, characterization, and hydrogen gas sensing properties of AuNs-catalyzed ZnO sputtered thin films

Accepted Manuscript Title: Synthesis, characterization, and hydrogen gas sensing properties of AuNs-catalyzed ZnO sputtered thin films Author: Q.A. Drmosh Z.H. Yamani PII: DOI: Reference:

S0169-4332(16)30426-3 http://dx.doi.org/doi:10.1016/j.apsusc.2016.02.238 APSUSC 32756

To appear in:

APSUSC

Received date: Revised date: Accepted date:

21-9-2015 2-1-2016 27-2-2016

Please cite this article as: Q.A.Drmosh, Z.H.Yamani, Synthesis, characterization, and hydrogen gas sensing properties of AuNs-catalyzed ZnO sputtered thin films, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.02.238 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 proof before it is published in its final 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 hydrogen gas sensing properties of AuNs-catalyzed ZnO sputtered thin films Q.A. Drmosh and Z.H. Yamani*

Physics Department and Center of Research Excellence in Nanotechnology King Fahd University of Petroleum and Minerals, Dhahran, 31261, Kingdom of Saudi Arabia

*

Corresponding author:

E-mail address: [email protected]

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Graphical abstract

Highlights -

Modifying the surface of c-axis oriented ZnO thin film prepared by DC reactive sputtering with gold nanostructures by post annealing of thin layer of gold on ZnO. The structural, optical and compositional were studied. The gas sensing properties of the prepared sample toward hydrogen at different temperatures and concentrations were investigated and compared with as deposited and heated ZnO thin films. Au@ZnO thin film showed high response compared with as deposited and heated ZnO. A plausible mechanism for the observed enhancement is proposed.

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Abstract Hydrogen present in concentration up to 4 vol. % forms an explosive mixture with air. Its propensity to escape in the event of leak, could lead to quick build-up and formation of an explosive mixture with air in confined spaces, such as an automobile. This necessitates its detection at very low concentration. Zinc oxide (ZnO) is a wellknown wide band gap (~ 3.37 eV) semiconducting oxide that has been widely used for gas sensing applications. This work reports on the fabrication, charactrization and gas sensing performance of nanogold decorated ZnO thin films made by DC reactive sputtering. The sensor films were fabricated by depositing a very thin layer of gold on the sputtered ZnO thin film. The as deposited Au@ZnO films were converted into highly crystalline ZnO film covered with gold nanostructures (AuNs@ZnO) by mild heat treatment. The structural and morphological as well as the compositional homogeneity of the as-deposited and heat-treated ZnO, Au@ZnO and AuNs@ZnO thin films were ascertained. The gas sensing behaviour of the AuNs@ZnO thin films towards hydrogen as a function of temperature at different H2 concentrations was investigated and compared with that of pure and heat-treated ZnO films. The effect of the presence of gold nanoparticles on imparting improvement (in terms of higher response signal, high reproducibility and complete reversibility) was established; the optimal operating temperature was about 400ºC. A plausible mechanism for the observed enhancement in the sensing behavior of AuNs@ZnO films towards H2 is proposed.

Keywords: ZnO, thin films, Au nanostructures, H2 sensing

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Introduction Hydrogen is the lightest gas used extensively in a large number of industrial processes. It is also the fuel of choice for the low temperature proton exchange membrane (PEM) and the high temperature solid oxide fuel cells (SOFCs) towards future technology with lower carbon footprints [1-3]. H2-based proton exchange membrane fuel cell (PEMFCs) are projected to be used in automobiles for higher efficiency, better mileage and practically no exhaust pollution. A H2-rich reformate would be suitable for SOFCs. There are already hydrogen filling stations in the US, Europe and Japan for the fuel cell-based buses and cars [4]. However, hydrogen when present in concentration up to 4 vol. % (40, 000 ppm) forms an explosive mixture with air [5]. It’s propensity to escape in the event of leak, could lead to quick buildup and formation of an explosive mixture with air in confined spaces, such as an automobile, and necessitates its detection at concentrations far below the (lower explosive limit) LEL (4%). Thus, there is a genuine and urgent need for sensing H2 at extremely low (~500th-1000th of the LEL) concentrations. Amongst several options, hydrogen sensors based on semiconducting metal oxides are promising. ZnO is one of the most widely used semiconducting metal oxides. It has been exploited in a range of applications, such as, solar cell [6, 7] and photocatalysis [8,9] beside in gas sensing [10-13]. For hydrogen detection, the sensing attributes of ZnO in different formats, such as, thick and thin films [14,15], nanowires [16,17], nanorods [18-20], and nanotubes [21,22] have been synthesized and examined. ZnO was used for the first time in 1962 for ethanol detection [23]. Even though the films made with ZnO were one of the earliest materials that showed sensing capability towards reducing gases, they suffered and still do with long response time, slow and incomplete recovery time and exhibit low sensitivity. Several strategies have been employed to enhance their performance, such as, by annealing [24, 25], doping [26-28], or compositing [29, 30]. Recently, it has been reported that the addition of uniformly dispersed precious metals (gold, platinum or palladium) nanoparticles on the ZnO surface enhanced the interaction between gas of interest and absorbed oxygen on the film surface. For instance, Tian et al. [31] showed that the response of Au@ flower-like ZnO was much higher than that of pure ZnO towards a number of reducing gases. Guo et al. [32] found that ZnO nanowires decorated with Au nanoparticles displayed enhanced 4

resistivity and, response and recovery time towards ethanol. Mun et al. [33] investigated the performance of porous ZnO nanosheets functionalized with Au nanoparticles for NO2 detection under UV illumination, and showed that the response was higher than that of pure ZnO. In this paper, we report the development of a solid state sensor capable of detecting as low as ~75 ppm of H2 by sputtered ZnO film by modifying its surface through incorporation of Au nanostructures, using a simple and low cost method that ensures a contamination-free formulation.

Experimental Details 1. Preparation of AuNs@ZnO films An automated sputter coater (model NSC-4000, Nanomaster, USA) was used to prepare pure as well as Au-decorated ZnO thin films. The substrates were sonicated in acetone for 15 min. prior to sputtering. The zinc (99.999 %) and gold (99.99 %) targets received from Semiconductor Wafer, Inc. China, were cleaned by presputtering a blank for 3 and 1 min., respectively. The base pressure in the chamber was kept below 5 torr while the working pressure was set at 5 milli-torr. The sequential sensor fabrication stages are shown in Fig. 1. ZnO films were fabricated by DC reactive sputtering of metallic zinc at 100 W in oxygenfor 40 min. on Al2O3 substrates with pre-interdigitated Au electrodes (200 nm thick with 250 m interspace distance) supplied by the Electronic Design Center at Case Western Reserve University, Cleveland, USA. A very thin layer of gold was deposited on the sputtered ZnO surface at 30 W for 30s. The Au@ZnO thin film was heated at 600°C for 3h in argon. This yielded films with nanostructured morphology of the deposited Au particles on ZnO.

2. Characterization A variety of techniques were employed to characterize the pure and Au-decorated ZnO thin films. X-ray diffraction (XRD, Shimadzu 6000, Japan) with monochromatic 5

Cu Kα radiation (λ = 1.5406 Å) was used to identify the phases, ascertain the crystallinity and, estimate the average crystallite size. A field emission scanning electron microscope (FESEM, Tescan Lyra 3, Czech Republic) was used to examine the morphological features of the films, while X-ray photoelectron spectroscopy (XPS ESCALAB 250Xi, Thermo Scientific, USA) was used to estimate the chemical composition semi-quantitatively. Atomic Force Microscopy (AFM, Bruker, USA) was used to assess the surface roughness and the morphology. 3. Gas sensing System The sensor system includes a miniaturized small-volume test-stage, fully equipped with sample pad and gold-tipped tungsten contacts, procured from Linkam Scientific instruments (Model HFS-600E-PB4, UK) was used as the test chamber. The unit could be used to temperatures as high as 600°C and cooled very quickly. Commercial grade dry air was used as the background/reference gas. The film resistance and change in resistance in the presence of various concentrations of hydrogen were measured relative to those in the background air. A calibrated gas tank (1%H2 - bal. N2) was used as the test gas source; the concentration of hydrogen in the test chamber was varied by mixing air with this appropriately via digital mass flow controllers (MFCs, Horiba, USA), controlled through an external X PH-100 power hub supply. The H2 concentration over the sample was varied by varying the flow rates of respective gases through the MFCs. For evaluation, the sensor film deposited on the interdigitated alumina substrate was placed in the Linkam stage. Prior to introducing hydrogen, the chamber was purged with dry air for 1h at a flow rate of 40 standard cubic centimeter per minute to attain steady-state baseline film resistance at all temperatures of measurements. The signal in the form of film resistance was measured as a function of time and H2 concentrations at different temperatures, using An Agilent B1500A Semiconductor Device Analyzer (SDA). The response of the sensor is defined and expressed as [34]: 𝑅𝑒𝑠𝑝𝑜𝑛𝑠𝑒 = (

𝑅0 − 𝑅𝑔 ) ∙ 100 𝑅𝑔

where R0 and Rg are the resistance of the sensor in the background (dry air) gas and in the gas of interest, respectively.

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RESULTS AND DISCUSSIONS 1. Structural and microstructural characterization of the fabricated films To investigate the structural aspects of the as-prepared ZnO and Au@ZnO thin films, heat-treated ZnO and AuNs@ZnO thin films, systematic analysis of their XRD patterns was carried out. The results are summarized in Fig. 2. As can be seen, all the films showed strong preferential growth along c-axis (002 plane), indicating their formation and growth in the wurtzite structure of ZnO. Fujimura et al. [35] have suggestion that the preferential growth of ZnO in thin films in the 002 direction is due to the lowest surface energy of the (002) plane compared with others. No diffraction peaks could be ascribed to Au in Au@ZnO and AuNs@ZnO film which is likely due to the presence of too small amounts of Au in them. The average crystallite size of ZnO was calculated using Debye-Scherrer’s equation [36]: 𝑑ℎ𝑘𝑙 =

𝑘∙𝜆 𝛽 ∙ 𝑐𝑜𝑠𝜃

where, k is the shape factor (~ 0.9-1.0),  is the

wavelength (1.5406Å for

monochromatic CuK1 radiations),  is the full width at half maximum (FWHM) and  is the Bragg diffraction angle in radian. The computed crystallite size of ZnO in various films is summarized in Table 1. As expected, the calculated crystallite size in the heat-treated films are higher (almost 3 times) than those in the pristine samples, which could be attributed to growth at higher temperatures due to Ostwald ripening [37]. There was also a slight shift in the position of the most prominent 002 peak towards higher angle in the heat-treated samples. This has been attributed [38-40] to the increased stress in thin films brought about by heat treatment. It has been reported that the residual stress in ZnO film is a combination of thermal and intrinsic stress components [41-42]. The thermal stress which is tensile in nature, is due to the difference in the thermal expansion coefficients of the thin film material and the substrate, whereas the intrinsic stress is caused by imperfections and defects introduced during growth stage. The residual stress in hexagonal structures having highly preferred orientation along the c-axis (as in the case in our ZnO thin films) could be estimated by using a biaxial strain model [43]:

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𝜎 = 4.5 × 1011

𝑐0 − 𝑐 𝑐0

where, c0 is the unstrained lattice parameter of ZnO (0.52066 nm) and c is the lattice parameter calculated from the XRD pattern. The positive values in Table 1 represent tensile stress while compressive stress is indicated by negative values. Consequently, the residual stress in the as-made films is compressive in nature, whereas the stress in the heat-treated films was tensile in nature. Tensile stresses are related to grain growth as a result of heat-treatment at higher temperatures, resulting in the coalescence of individually nucleated islands with respect to the substrate [41]. Moreover, the intensity of the (002) reflection in the heated films increased sharply compared to that of the as-deposited counterparts. This could be ascribed to the improvement in crystallinity. In the case of Au-enriched films, no significant change in the peak position could be discerned between the as-made and heat-treated Au decorated-ZnO films, thereby obviating the possibility of Au diffusion in the ZnO lattice. The surface morphology and roughness of the thin films were investigated by FESEM and AFM, respectively. Fig. 3 shows the FESEM micrographs of the asprepared (a-b) and heat-treated (c-d) pure and Au-decorated ZnO thin films. As can be seen, the as-grown thin films show dense, smooth and homogenous morphology, with grains ranging from about 15 to 35 nm in size. Surface of heat-treated ZnO film (Fig. 3(c)) possessed smooth morphology without any aggregation or agglomeration. However, isolated Au nanostructures decorating the surface of the ZnO films were observed on the Au@ZnO thin films (Fig. 3(d)) with Au nanostructures varying in size between 20 and 90 nm. The surface roughness of the Au@ZnO (as-prepared) and AuNs@ZnO (heat-treated) films was assessed by collecting the AFM topography on a 5µm × 5µm scan area, as shown in Fig. 4. The 3D image of the Au@ZnO film (Fig. 4(a)) revealed the presence of smooth and continuous surface with dense, fine and uniform columnar structure while that of the AuNs@ZnO film (Fig. 4 (b)) shows the growth of separated and larger columnar structures. The surface composition of the two films was investigated using XPS technique with energy resolution of ± 0.1 eV. The data obtained were fitted using Gauss/Lorentz product function and incorporating appropriate background corrections. The binding

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energies of various components were corrected for the charge shift with respect to the 1s peak of carbon (BE = 284.60 eV) as reference [44]. The XPS depth profiling of Au@ZnO was used to discern the compositional uniformity across the exposed surface of the film. The surface was systematically and sequentially etched by a low energy (~ 2 keV) ion gun for 10s, prior to collecting the XPS spectra at that depth. Fig. 5 shows the concentration profiles (at. %) for Au (3f), C (1s), O (1s) and Zn (2p) at twelve depths from the exposed film surface. As can be seen, the Au concentration declined sharply (ca. ~ 27 at. % at the surface to ~ 10 at. %) within 30s of etching indicating the formation of a very thin layer of gold. In contrast, the Zn and O concentrations increased steadily and became steady beyond the depth corresponding to 30s of etching. XPS of the as deposited ZnO, Au@ZnO, heated ZnO and AuNs@ZO thin films was investigated. The binding energy, full width at half maximum (FWHM) and atomic percent are calculated and listed in Table 2. The XPS survey spectrum of as grown ZnO, and heated ZnO (not shown) confirms the presence of Zn, C and O only. The XPS survey of Au@ZnO thin film confirms the presence of Au with 20.3 % atomic weight concentration. The global XPS scan of AuNs@ZnO film shown in Fig. 6 (a) clearly identifies peaks due to Zn, Au, C and O. The de-convoluted Au4f spectra of Au@ZnO and AuNs@ZnO films confirm the presence of two core levels corresponded to Au4f5/2 and Au4f7/2. The binding energy difference (∆) between the two core levels in Au@ZnO and AuNs@ZnO samples (e.g., Fig. 6(b)) is 3.62 eV and 3.96 eV. This is in excellent agreement with 3.63 eV as reported for the spin-orbit splitting in metallic Au [45]. The Au concentration on the surface of AuNs@ZnO film was computed to be 7.93 % atomic weight concentration which is about 1/3 of that on Au@ZnO film surface. This difference could be attributed to the conversion of Au layer, in the case of Au@ZnO, to Au nanostructures as a result of heat treatment that yielded AuNs@ZnO; this is corroborated by the AFM and FESEM results shown above. The O1s spectra of the prepared samples were resolved into two or three components (e.g., Fig 6(c)). The lowest binding energy (O1s A) component of the O1s spectra is attributed to O2− ions on the wurtzite structure of hexagonal Zn2+ ion array [41] while 9

the highest binding energy (O1s C) is attributed either to water vapor component or adsorbed oxygen. The middle binding energy (O1s B) is attributed to the presence of partially reduced ZnO (ZnOx) [47,48]. The presence of the water content (O1s C) in pure ZnO and heated ZnO could be attributed to the long term exposure of the film to air prior to XPS analysis. The fitted results of Zn2p confirms the presence of doublet peaks corresponding to Zn2p1/2 and Zn2p3/2. The stoichiometric composition (Zn/O) for heated samples (ZnO and AuNs@ZnO) is relatively higher than unheated samples. It seems that the oxygen re-evaporates from the surface of the ZnO thin film during the annealing process in argon atmosphere leading to oxygen deficiencies or zinc interstitials. The binding energy differences (∆) between Zn 2p3/2 and Zn 2p1/2 are calculated to be about 23 eV which is a characteristic of the spin-orbit splitting of Zn in ZnO matrix [49]. 2. Gas sensing behaviour The gas sensing performance of the AuNs@ZnO films was qualified and quantified in terms of resistance change when exposed to hydrogen-containing mixtures at a given temperature. Fig. 7(a) shows the response of the film exposed to hydrogen over a wide range of concentrations (75-1200 ppm) at 400ºC. Clearly, the film responded to hydrogen rather quickly; the recovery upon removal of hydrogen was also swift and complete. The change of resistance even for the lowest H2 concentration in air (75 ppm; limitation of experimental set-up and flow meters) as about 21%, indicating that the sad film could detect even lower H2 concentrations. The change in resistance was proportional to the amount of hydrogen in the gas stream, which makes these films useful in H2 sensing as well as metering. Reproducibility of data over a large number of cycles is an important aspect in evaluating the suitability of a sensor device. Fig. 7(b) shows the highly repeatable response of the AuNs@ZnO film over four cycles of exposure to 600 ppm H2 at 400°C. The sensor was capable of complete recovery upon removal of hydrogen from the stream. The sensing behavior of the AuNs@ZnO films towards H2 at different temperatures (25-400ºC) was also investigated in order to identify the most optimal temperature window of operation. As expected, the performance of the AuNs@ZnO 10

films at room temperature was very poor at all levels of hydrogen (viz., 75 to 1200 ppm); this could be ascribed to rather high film resistance and slow kinetics of interaction between the oncoming hydrogen and the adsorbed oxygen species at the surface. The film became responsive above 200ºC and the sensitivity increased with increasing temperature as shown in Fig. 8. This could be attributed to the contribution of thermal energy (higher at higher temperature) towards overcoming the activation energy barrier of the interaction at the surface. Fig. 9 shows the rather excellent reversibility aspect (the so-called staircase behavior) of the sensor film in the ascending and descending mode of hydrogen level @ 400°C . The sensing behaviour of the AuNs@ZnO films at 300, 600 and 1200 ppm @ 400°C was compared with that of as-made and heat-treated ZnO sensors. As can be seen from Fig. 10, the response of the AuNs@ZnO sensor @ 400ºC was the highest at all concentrations, in comparison to that of other formulations. For example, the response of as-grown and heated ZnO films to 1200 ppm of H2 at 400°C was ~7 and 28%, respectively, while that of AuNs@ZnO film was found as high as 73% for 1200 ppm of H2 at 400°C.

3. Sensing mechanism The normally and widely accepted sensing mechanism for an n-type semiconducting oxide-based is based on the premise that oxygen adsorbed from ambient air on its surface extracts electrons from the conduction band of the oxide thereby increasing its resistance/resistivity. When a reducing gas (such as H2) comes into contact with this resistive surface, it interacts with this electronated oxygen, releasing electrons into the conduction band leading to resistance/resistivity decrease [50]. In the present case, we envisage two plausible models that would explain the observed enhancement in the response signal of the AuNs@ZnO films compared to pure ZnO films. One is based on the spillover mechanism and the second invokes modulation and control of Fermi energy level in the case of Au-decorated samples. At the temperatures employed in this work, the gold atoms in the AuNs@ZnO film are first bonded (though rather weakly) to the oxygen adsorbed from the ambient: 2 𝐴𝑢 + 𝑂2 ↔ 2 𝐴𝑢: 𝑂

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The Au:O complex thus formed is labile and subsequently dissociates releasing oxygen singlets [51], which remain attached to the gold nanostructures: 𝐴𝑢: 𝑂 ↔ 𝐴𝑢 + 𝑂 These oxygen atoms spillover forming negatively charged surface ions by extracting electrons from the conduction band of the ZnO molecule, yielding a high electrostatic potential energy in the vicinity of contact area [52-53]. The other model that explains the observed Au-mediated enhancement in the sensing performance of AuNs@ZnO compared to that of pure ZnO films involves modulation and control of Fermi energy levels in the presence of nanostructured gold. The energy band diagram is shown schematically in Fig. 11. Since, the work function of gold (qφ = 5.1 eV) is greater than that of ZnO (qχ = 4.09 eV), there is a natural flow of free electrons from the conduction band of ZnO to Au. The main difference between the two models is that in the second case, charged oxygen species adsorbed directly on the ZnO surface are neglected compared to those on Au [54]. The oxygen on the surface of Au first traps electrons from Au itself, which in turn, extracts electrons from the conduction band of ZnO. As a result of these sequential processes which are almost synchronous, a depletion layer equal to x0 and Schottky barrier height equal to qVs is created, leading to band bending (Fig. 11(b)). When H2 is adsorbed on such AuNs@ZnO surface and react with the adsorbed O2, more electrons are likely to be released and fed back into ZnO’s conduction band, leading to a larger decrease in the film resistance. As can be easily imagined, this process will be more favorable and facile if the Au were uniformly dispersed over the ZnO surface and intimately close to one another, which was the case in the present work.

CONCLUSIONS AuNs-decorated ZnO sputtered thin films were fabricated using a sequential process. ZnO thin films were fabricated by DC reactive sputtering at room temperature, followed by the deposition of a very thin layer of Au and heating the Au@ZnO films to 600°C for 3h in argon. This yielded highly crystalline ZnO films covered with uniformly distributed nanostructured gold particles (AuNs@ZnO). All films were 12

highly crystalline (conforming to the wurtzite structure of ZnO) and exhibited preferential growth along the c-axis (002 plane). The FESEM and AFM imaging confirmed the nucleation of Au NPs in the heat-treated films. Sensing characteristics of the AuNs@ZnO films toward H2 established that presence of nanostructured gold particles on the surface of sputtered ZnO improved the performance compared to those of pure ZnO films. The optimum temperature for H2-sensing was about 400ºC and as low as 75 ppm of H2 could be detected with high sensitivity. Moreover, the performance of AuNs@ZnO thin films was highly reproducible with complete recovery characteristics.

Acknowledgement The authors acknowledge the support provided by King Abdulaziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. 09-NAN772-04 as part of the National Science, Technology and Innovation Plan. The support of CENT at KFUPM is gratefully acknowledged.

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18

Figure 1. Schematic of the steps involved in the fabrication of AuNs@ZnO films

19

Figure 2. XRD patterns of: (a) as-grown ZnO, (b) Au@ZnO, (c) heat-treated ZnO and AuNs@ZnO thin films

20

(a)

(b)

500 nm

(c)

500 nm

(d)

500 nm

Figure 3. FESEM micrograph of: (a) as-grown ZnO, (b) Au@ZnO, (c) heat-treated ZnO and (d) heat-treated AuNs@ZnO thin films

21

500 nm

Figure 4. .AFM 3D images of: (a) Au@ZnO, and (b) AuNs@ZnO thin films

22

Figure 5. The XPS depth profile of the Au@ZnO thin film as a function of etching time

23

(a)

(c) (d)

(b)

(d)

Figure 6. XPS spectra of AuNs@ZnO thin film: (a) global survey, (b) Au4f, (c) O1s and (d) Zn2p

24

Figure 7. (a) The dynamic response of AuNs@ZnO film to various concentrations of H2 at 400°C. (b) Evidence of reversibility to 600 ppm H2 at 400°C

25

Figure 8. Effect of temperature on the response of the AuNs@ZnO sensor to different H2 concentrations

26

Figure 9. Evidence of complete recovery of the sensor behavior in cyclic operation at 400°C

27

Figure 10. Comparison of the response of as-deposited ZnO, heated ZnO and AuNs@ZnO films to 300, 600, 1200 ppm of H2 at 400°C

28

Figure 11. Energy band diagram of the Au/ZnO: (a) before and (b) after equilibrium

29

Table:1 Structural information of ZnO, heat-treated ZnO, Au@ZnO and AuNs@ZnO thin films Sample

2 (°)

FWHM

Crystalline

Stress (GPa)

size (nm)

Lattice parameters (nm)

(rad) pure ZnO

34.14

0.84

((???)(nm) 11

0.5248

-3.61

Au@ZnO

34.21

0.796

12

0.5237

-2.71

Heated ZnO

34.83

0.251

37

0.5147

5.12

AuNs@ZnO

34.90

0.246

38

0.5137

5.97

30

Table:2 XPS analysis of pure ZnO, as deposited Au@ZnO and heat-treated ZnO and AuNs@ZnO

Atomic %

∆ (eV)

1.92

45.68

23.10

529.5

1.66

46.08

O1s B

531.3

1.42

7.50

O1s C

532.1

0.73

0.74

Zn2p

1021.3

2.02

37.11

23.09

Au@ZnO

Au4f

84.0

0.97

20.3

3.62

(RT)

O1s A

529.7

1.65

36.71

O1s B

530.9

1.75

5.88

Zn2p

1021.0

1.72

46.48

Heated ZnO

O1s A

529.7

1.27

42.57

(600 °C)

O1s B

531.3

1.74

9.95

O1s C

532.3

1.99

1.28

Zn2p

1021.2

1.79

46.66

23.06

AuNs@ZnO

Au4f

84.1

1.02

7.93

3.69

(600 °C)

O1s A

529.8

1.38

40.72

O1s B

531.6

1.45

4.69

Sample

Name

Peak BE (eV)

FWHM (eV)

Zn2p

1021.1

Pure ZnO

O1s A

(RT)

31

23.09