Effect of Deposition Temperature on The Structural and Optical properties of ZnO /Au Nanocomposite for Photocatalytic Application
L. Roza 1,2 , Annisa Citra Dwicahya 1 , Vivi Fauzia 1*
1. EXPERIMENTAL DETAIL
ZnO nanorods were synthesized via seed mediated and hydrothermal procesess as previously reported (REF). In seeding process, 0.2 M zinc acetate dihydrate dissolved in ultrapure water and then placed in a container of a commercial ultrasonic nebulizer. The seed solution droplets were sprayed using the ultrasonic wave (1,7 MHz) onto preheated glass substrates on a hotplate at 450 °C for 15 minutes. Samples continued to be annealed at 450 °C for 1 hour. Furthermore, growth process started by immersing the samples in a mixture of 30 mL 0.05 M zinc nitrate tetrahydrate and 30 mL 0.01 M hexamethylenetetramine and heated in an oven at 95 °C for 6 hours. After that, the samples were rinsed several times in ultrapure water, dried using hot air and then annealed at 200 °C for 1 h.
For the synthesis and deposition of Au nanoparticles directly on ZnO nanorods surface, the ZnO nanorods coated on glass substrate were firstly immersed in 0.1% of Poly-L-Lysine solution for 15 minutes. Furthermore, the sample was immersed into gold precursor solution,
which contains 0.1 mL of 0.01 M HAuCl4 aqueous solutions, 0.5 mL of 0.01 M Trisodium Citrate following by addition of 18.5 mL of deionized water. The mixture was shaken for several minute and then allowed for 30 minutes in room temperature for reduction process of Au3++3e- Au. Finally, 0.5 mL ice-cooled 0.04 of Sodium Borohydrate was added and then left undisturbed at various growth temperature namely 90, 100, 110 and 120 ° C for 90 minute inside an electronic oven. After the reaction, the sample was cooled down naturally to room temperature in order to avoid thermal shock. The preparation of the Au/ZnO heterostructure was completed by rinsing the sample with adequate amount of deionized water and subjected to an annealing process at 200°C for 30 min in air.
The ZnO nanorods and Au/ZnO heterostructure sample were characterized through TEM FE Tecnai G20 S Twin 200 kV for particle size analysis, observed the morphological and compositional of the sample. The sample for TEM measurement were prepared by dropping the treated solution on the carbon coated copper grids (200-mesh) and air dried for about 2 h.
Morphological characterization of the Au/ZnO heterostructure was observed using Zeiss Supra 55VP Field-emission SEM (FESEM). The microstructures of the Au/ZnO heterostructure sample were examined by x-ray diffraction (XRD) Bruker D8 Advance equipment. An optical spectrophotometer UV-Vis Lambda 900 Perkin Elmer, UV-Vis Diffuse Reflectance U-3900H Spectrophotometer and FLS920 photoluminescence spectrometer Edinburgh instruments were employed to study the optical properties of the ZnO samples. All the characterizations are performed at room temperature.
The photocatalytic activity of Au/ZnO heterostructure was observed by the photodegradation of MB dye in an aqueous medium. The samples were placed in 20 ml of 10- mM MB solution and irradiated with UV light (40 W). At certain time intervals, the samples were simply removed from the solution, and the optical absorption spectra were monitored using a UV-Vis spectrophotometer at the characteristic absorption peak wavelength of the MB dye at 596 nm.
2 RESULT AND DISCUSSION
The XRD patterns of the ZnO nanorods and Au/ZnO nanocomposite prepared with various Au deposition temperatures are shown in Figure 1. The diffraction peaks of the sample could be indexed to hexagonal wurtzite (ICSD file no 98-018-0868). The strong and sharp diffraction peaks suggested that the ZnO nanorods and Au/ZnO nanocomposite sample is polycrystalline.
The XRD patterns from all sample had eight noticeable peaks at 2θ 31.75o, 34.53o, 36.24o, 47.61o, 63.45 o, 67.79 o and 73.69 o corresponding to the crystal planes (100), (002), (101), 102, 110, 103, 200 and 112, respectively. All samples demonstrate similar patterns however differ from the peaks intensity at (002) plane. The extensive higher intensity of the (002) diffraction peaks showed that sample were preferentially oriented in the c-axis direction. Au/ZnO heterostructure with deposition temperature 100oC possesses the highest peaks intensity than other sample at (002) plane. Meanwhile, pure ZnO nanorods exhibit the lowest peaks intensity at the plane. The height of peaks intensity at other plane for all samples is almost same while peaks position and width 2θ of these peaks slightly changed. It is indicated that interesting variation in the lattice parameter of sample.
The peaks intensity from Au nanostructure (Au NPs) was only found detected at Au/ZnO heterostructure prepared with deposition temperature 100°C and 110°C. These sample exhibit lower diffraction peaks intensity of Au face centered cubic (fcc) structure corresponding to hkl (002) and (111), respectively. These planes are confirmed by ICSD standard file no. 98-018- 0868. The absence of Au diffraction peaks in other samples is probably be due to the small amount of Au nanospherical are formed and attached on ZnO nanorods surface. It was also observed that increasing the deposition temperature from 100oC to 110oC decrease the peaks intensity at (111) crystal plane.
Fig 1. XRD patterns of ZnO nanorod and Au/ZnO heterostructure with various Au deposition temperature
The prominent peak of (002) crystal plane demonstrate the preferred orientation of the ZnO nanorods and Au/ZnO heterostructure is perpendicular to the substrate. The analysis of
preferred orientation of ZnO nanorods and Au/ZnO heterostructure sample can be evaluated by calculating the texture coefficient (TC) following equation [6]. The data is presented in Table 1.
Table 1. Texture Coefficient (TC) value Deposition
Temperature (oC)
Crystal Plane (hkl)
(002) (101) (102) (103)
- 0,93 0,15 0,49 0,30
90 0,93 0,05 0,44 0,51
100 1,84 0,11 0,30 0,29
110 1,77 0,10 0,42 0,35
120 0,83 0,07 0,24 0,18
Commonly, the highest TC value will result in better orientation along c-axis. As can be seen from Table 1 it was discovered that the (002) crystal plane have the highest TC value which indicates the one-dimensional nature of the ZnO sample in this plane. It was found that TC value slightly increased with deposition temperature up to 100 oC (from 0.93 to 1.84). It indicated that ZnO sample exhibit better orientation which increasing of deposition temperature. However, the TC value significantly reduces from 1.77 to 0.83 when the deposition temperature further increased to 120 oC. The result revealed that attachment of Au nanospherical on ZnO nanorods surface with different deposition temperature seemingly does not significantly affect to preferred crystal orientation of the sample.
The average crystallite size was estimated by measuring the full width at half maximum (FWHM) of the most intense diffraction peak using the Debye Scherer calculation. The calculated values of average crystallite size are given in Table 2. It can be realized that the average crystallite size of pristine ZnO nanorods is 82,745 greater than that of Au/ZnO heterostructure (20,909 ). Decrease in the values of average crystallite size upon doping is due to re-crystallization process during annealing process at 400oC following the deposition process of Au nanospherical into ZnO nanorods surface.
Table 2. Microstructure data of Au/ZnO heterostructure hkl (002) crystal plane Deposition
Temperature (°C)
2 thetha a=b (Å) c (Å) Cristallite Size (Å)
d-spacing (Å)
- 34,50 3,25 5,20 82,74 2,60
90 34,43 3,24 5,20 39,51 2,60
100 34,41 3,24 5,20 23,58 2,60
110 34,41 3,24 5,20 23,11 2,60
120 34,42 3,24 5,20 20,90 2,60
As mentioned above the peaks intensity from Au nanostructure was only found detected at Au/ZnO heterostructure prepared with deposition temperature 100°C and 110°C at (002) and (111) crystal plane. The XRD analysis from structural parameter of Au NPs peaks including 2θ peak position, lattice parameters, and d-spacing are presented in Table 3. The measured lattice parameter of Au nanocrystal at the (002) and (111) plane are similar, namely a= 4.07 Å and c = 4.07 Å. However, the d-spacing of (002) plane is 2.03 Å and (111) plane is 2.81 Å. It was observed that peaks intensity from Au/ZnO heterostructure with deposition temperature up to 100oC possesses the highest peaks intensity at these planes. Furthermore, the peaks intensity decreased when the deposition temperature increase to 110 oC. The variation in lattice parameters is influenced by the concentration of foreign atoms, crystal defects, external strains, and temperature.
Figure 2. XRD patterns of Au fcc structure at (002) and (111) crystal plane prepared with deposition temperature 100 and 110°C
Table 3. Microstructure data of Au/ZnO heterostructure hkl (111) crystal plane Deposition
Temperature (°C)
Crystal Plane (hkl)
2 theta a=b (Å) c (Å) Cristallite Size (Å
d-spacing (Å)
100 002 44,38 4,07 4,07 245,59 2,03
111 38,15 4,07 4,07 189,35 2,81
110 002 44,38 4,07 4,07 182,09 2,03
111 38,18 4,07 4,07 144,52 2,35
Au Au
Figure 3 shows the high resolution TEM image of Au/ZnO heterostructure with Au deposition temperature is 110 °C. The HRTEM image clearly highlights the existence of Au NPs and can be observed as dark spot with spherical shape on ZnO nanorod surface. The Au NPs seemingly attached in each ZnO nanorods grains and almost covering all ZnO nanorods surface. Furthermore, the TEM image confirmed the successful formation of Au nanoparticles with average diameter of 5-15 nm.
Fig 3. HRTEM image of Au/ZnO heterostructure with Au deposition temperature is 110 ° C
The Selected Area Electron Diffraction (SAED) pattern of ZnO nanorods and Au NPs are shown in Figure 4. In Fig. (a) it is clearly observed that there is a white ring pattern on the ZnO SAED image and indicated that hexagonal structure of ZnO nanorods are polycrystalline.
Meanwhile, SAED patterns in Fig. (b) was found the existence of other points thought to be derived from the polycrystalline nature of the gold nanoparticles (F & Qin, 2015).
Fig 4. SAED Pattern of (a) ZnO nanorods and (b) Au/ZnO heterostructure with Au deposition temperature is 110 ° C
Figure 5 shows the morphological observation results of Au/ZnO nanocomposite prepared with deposition temperature at 110 °C. The FESEM image revealed that hexagonal-shaped of Au/ZnO heterostructure growing perpendicular to the substrate and compactly covering the glass substrate. The morphology of the sample is not homogenous since the diameter of the sample varied from ca. 90-120 nm. From the cross-sectional image it is clearly seen that ZnO nanorods grown perpendicular to substrate and having average length 1.4 μm. Meanwhile, the Au nanospherical were not clearly observed in the FESEM images. This could be attributed to the combined effect of the lower concentration and smaller scattering ability of Co2+.
Fig 5. FESEM images of Au/ZnO heterostructure (a) magnification 10kx, (b) 50kx (c) 100kx and (d.) Cross-
section image of Au/ZnO heterostructure with Au deposition temperature is 110 ° C
Figure 6. Shows the optical absorption spectra of ZnO nanorods and Au/ZnO heterostructure taken at room temperature. The figure indicates the presence of strong absorption peaks in UV light region and the absorption edge is shifted to higher wavelength upon addition of Au NPS with various deposition temperatures. The absorption spectra UV light region is ascribed to the intrinsic nature of the wide band gap ZnO semiconductor which absorb more photon in the UV region to exited the electrons in the valence band into the conduction band (Yu et al., 2012). The absorption of ZnO in the presence of Au at different deposition temperature shows slightly shift to the visible light region, compared to that ZnO
nanorods. Particularly, the combination of Au nanospherical and ZnO nanorods can extend the absorption spectra to visible to near infra-red region. This absorption peaks is assigned to the contribution of the Localized Surface Plasmon Resonance (LSPR) effect from Au nanoparticles. The UV-Vis extinction spectra of Au/ZnO samples proven the formation of gold nanoparticles on ZnO nanorods surface (Bora, Myint, Al-Harthi, & Dutta, 2015; Wang et al., 2015). It was revealed that Au/ZnO heterostructure sample with deposition temperature of 110
°C exhibits the highest absorption spectrum at 520 nm. The increased in the absorption peaks of the sample is due to increasing the number of gold nanoparticles deposited on ZnO nanorods surface. This result is good agreement with the XRD patterns as shown in Figure 1 which exhibits the presence of Au phases only in Au/ZnO heterostructure sample with deposition temperature of 110 °C.
Fig 6. UV-Vis absorption Spectra of Au/ZnO heterostructure
The UV–Vis diffuse-reflectance spectra of Au/ZnO heterostructure with various Au deposition temperatures is presented in Figure 7. It was found that each sample exhibit sharp edge at wavelength about 390 nm related to the common absorption spectra of ZnO nanostructure. The ZnO sample exhibits the highest reflectance spectra. Meanwhile, Au/ZnO heterostructure with Au deposition temperature 100oC possesses the lowest reflectance spectra.
In other words, sample with higher absorption spectra retains lower reflectance spectra and vice versa.
Fig 7. Diffuse Reflectance Spectra of Au/ZnO heterostructure
The reflectance spectra all sample were then used to calculate the bandgaps energy of the sample using the Kubelka Munk equation (Ghoul & El, 2015). Meanwhile bandgap energy of the sample was determined using the Touc plot, which is the plot between ( ) and the photon energy ( ) as shown in Figure 8. By extrapolating the linear part of the plot, the band gap is defined when ( ) or at the intersection of the linear slope with the photon energy axis. The estimated bandgap of ZnO nanorods and Au/ZnO heterostructure with deposition temperature 90, 100, 110 and 120 oC is 3.22, 3.21, 3.21 and 3.20 eV, respectively.
The narrowing of optical band gap of ZnO sample upon addition of Au NPs at different deposition temperature can be due to the modification of the local lattice symmetry and introduce defect centers into the ZnO lattice (Huang, Feng, Xu, & Liu, 2016). The narrowing of optical band gap might result from the interaction of Au NPs and ZnO nanorods that presents the intra-gap energy levels inside the band gap of ZnO.
Fig 8. Band gap energy of ZnO nanorods and Au/ZnO heterostructure with different deposition temperature
Figure 9. Shows photoluminescence (PL) spectra of Au/ZnO sample prepared with various deposition temperatures taken at room temperature. The figure indicated that all sample exhibit three prominent emission peaks at wavelength of 390 nm, 440 nm and 620 nm. The high emission peaks in the UV region is physically represents the radiative transition from excitation of free electron at valence band to conduction band combined with the electron-hole recombination of ZnO (Sun et al., 2011), which commonly called the near band edge emission (NBE). The pure ZnO nanorods sample possesses the highest photoluminescence peak, followed by Au/ZnO heterostructure prepared with deposition temperature 90, 110, 100 and 120 oC. These results indicate that pure ZnO nanorods sample has the highest rate of recombination between electron and hole, followed by Au/ZnO heterostructure prepared with deposition temperature 90, 110, 100 and 120 oC.
The emission peaks centered at wavelength of 440 nm is assumed to come from the glass substrate. Meanwhile, very broad emission peaks in the visible light region centered at wavelength 620 nm is due to the emergence of natural defects in ZnO nanorod samples, such as zinc-interstitial (Zi) with violet-blue emission (405-440 nm), zinc-vacancy (VZn) with blue emission (450-460 nm), oxygen-interstitial (Oi), and oxygen vacancy (VO) with green emission
(490-650 nm). These emission peaks commonly called deep level emission (DLE). A similar phenomena has been observed by Fageria, Gangopadhyay, & Pande, 2014; Ghosh et al., 2015;
Hou, 2014. It can be noticed that ZnO nanorods possesses the highest emission peak meanwhile, Au/ZnO heterostructure prepared with deposition temperature 120 oC possesses the lowest emission peaks. The decrease emission intensity of ZnO sample at all spectrums is due to the fact that this radiative recombination process has been greatly reduced as the excited electrons have been surrendered by Au NPs. Because of its strong UV emission at room temperature and having high electron concentration it’s believed that ZnO sample was potential in photocatalytic application.
Fig 9. Photoluminesence Spectra of Au/ZnO heterostructure
Figure 10. shows the absorption spectra of an aqueous solution of methyl blue in the presence of ZnO nanorods and Au/ZnO heterostructure catalyst under UV light illumination. It was found that the maximum absorbance of methy blue dyes solution tends to decrease as the illumination time decrease. These results indicate that the MB dye was steadily degradedunder illumination of UV light.
Fig 10. Absorbance spectra of methyl blue (a) without ZnO photocatalyst (b) with pure ZnO nanorods and (c) Au/ZnO heterostructure with various Au deposition temperature
The absorption spectra was then used to calculate the degradation efficiency of ZnO nanorods and Au/ZnO heterostructure. Figure 11 show the photodegradation efficiency of methyl blue under UV light illumination for 42.5 minute. The photodegradation efficiency of methyl blue without ZnO photocatalyts is 9.63 %. Meanwhile photodegradation efficiency of methyl blue in the presence of Au/ZnO heterostructure catalyst prepared with deposition temperature 90, 100, 110 and 120 oC are 65.11%, 69.36%, 75.19% and 68.07%, respectively.
These results indicated that the Au/ZnO heterostructure prepared with deposition temperature 110 oC exhibit the fastest degradation rate. However photodegradation efficiency of methyl blue with pure ZnO nanorods catalyst is 72.26% which clearly indicated that photodegradation efficiency pure ZnO nanorods much lower compared to Au/ZnO heterostructure catalyst prepared with deposition temperature 110 oC. The result show that Au/ZnO heterostructure prepared with deposition temperature 110 oC possesses the fastest degradation rate compared to other sample.
As can be observed from XRD result Au/ZnO heterosructure with Au deposition temperature 110 oC surrounded with high amount of Au NPs that expected to harvest higher number of photon. Furthermore, since the Au NPS on ZnO nanorods surface believed act as electron sink which promote to larger number of electron-hole pair with an efficient
charge separation. Meanwhile, ZnO-metal interface suppress the electrons-hole recombination and improve performance of Au/ZnO heterosructure. This also supported by the PL result presented in Fig. 9 for which the 110 oC sample and accelerate the process of adsorption of oxygen molecule and OH- ion to produce H2O2 and hydroxyl radical (*OH).
which would absorb more photosexcited electrons.
Fig 11. Photodegradation efficiency of methyl blue under UV light by ZnO nanorods and Au/ZnO heterostructure with various Au deposition temperature
CONCLUSIONS
Au nanospherical was successfully growth and deposited directly onto ZnO nanorods surface. The deposition process of Au nanospherical on the ZnO nanorods achieved via hydrothermal method at various deposition temperatures for 1 hour in an electronic oven.
Numerous amount of Au nanospherical was found grow on ZnO nanorods surface with diameter ca 5-15 nm nm. The Au modification at various deposition temperatures was found effectively changed the lattice parameter and decrease the crystallite size of Au/ZnO heterostructure due to re-crystallizations process of ZnO nanorods during deposition of Au nanospherical. The temperature of 110°C was considered as the optimum temperature to deposited Au nanospherical on the ZnO nanorods surface which exhibit enhance photocatalytic
degradation rate of methyl blue solution than that of pure ZnO nanorods. The presence of Au nanospherical believed facilitates the interfacial charge transfer process. Furthermore, high amount of Au nanospherical on ZnO nanorods surface would enhance the absorption process the photosexcited electrons since the Au NPS on ZnO nanorods surface believed act as electron sink and the interfacial between ZnO-metal surfaces would be suppress the electron-hole recombination.
ACKNOWLEDGEMENTS
This research was financially supported by Hibah Penelitian Unggulan Perguruan Tinggi 2019 (No. 21 / AKM/PNT/ 2019) from the Ministry of Research, Technology and Higher Education Indonesia.
REFERENCES
Bora, T., Myint, M. T. Z., Al-Harthi, S. H., & Dutta, J. (2015). Role of surface defects on visible light enabled plasmonic photocatalysis in Au-ZnO nanocatalysts. RSC Advances, 5(117), 96670–96680.
Bremner, S. P., Yi, C., Almansouri, I., Ho-Baillie, A., & Green, M. A. (2016). Optimum band gap combinations to make best use of new photovoltaic materials. Solar Energy, 135, 750–757.
Chen, Y., Zeng, D., Zhang, K., Lu, a, Wang, L., & Peng, D. L. (2014). Au-ZnO hybrid nanoflowers, nanomultipods and nanopyramids: one-pot reaction synthesis and photocatalytic properties. Nanoscale, 6(2), 874–881.
Fageria, P., Gangopadhyay, S., & Pande, S. (2014). Synthesis of ZnO/Au and ZnO/Ag nanoparticles and their photocatalytic application using UV and visible light. RSC Advances, 4(48), 24962–24972.
Ghosh, A., Guha, P., Samantara, A. K., Jena, B. K., Bar, R., Ray, S., & Satyam, P. V. (2015).
Simple growth of faceted Au-ZnO hetero-nanostructures on silicon substrates (nanowires and triangular nanoflakes): A shape and defect driven enhanced photocatalytic performance under visible light. ACS Applied Materials and Interfaces, 7(18), 9486–
9496.
Hou, X. (2014). Nonaqueous fabrication of ZnO/Au nanohybrids with enhanced photocatalytic activity. Materials Letters, 137, 319–322.
Hou, X., & Wang, L. (2014). Controllable fabrication and photocatalysis of ZnO/Au nanohybrids via regenerative ion exchange and reduction cycles. RSC Adv., 4(100), 56945–56951.