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Influence of OH− Ion Concentration on the Surface Morphology of ZnO-SiO2 Nanostructure
The influence of varying OH− ion concentration on the surface morphology of chemically deposited ZnO-SiO2 nanostructures on glass substrate was investigated. The morphological features, phase structure, and infrared characteristics were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR), respectively. Results revealed that silica significantly changes the hexagonal morphology of bare ZnO rod to “pointed tips” when using low initial OH− precursor concentration. Increasing OH− ion concentration resulted in a “flower-like” formation of ZnO-SiO2 and a remarkable change from “pointed tips” to “hemispherical tips” at the top surface of the rods. The surface capping of SiO2 to ZnO leads to the formation of these “hemispherical tips.” The infrared spectroscopic analysis showed the characteristics peaks of ZnO and SiO2 as well as the Si-O-Zn band which confirms the formation of ZnO-SiO2. Phase analysis manifested that the formed ZnO-SiO2 is of wurtzite structure. Furthermore, a possible growth mechanism is proposed based on the obtained results.
Zinc oxide (ZnO) is a widely known -type semiconductor that attracted research interest due to its properties like wide energy band gap of 3.37 eV and high exciton binding energy of 60 meV  which makes it suitable for many applications like in solar cells , photodetectors , and gas sensors . Another interesting aspect of ZnO growth is its ability to be engineered to different structural features by changing some parameters during synthesis. Tailoring the morphology, shape, and size of ZnO could be a way of expanding its potential especially in solar cell and sensor technologies where reaction at the surface is very crucial. Works have been reported for controlling size and shape of ZnO nanostructures like varying the initial zinc and hydroxyl ion concentrations [5, 6] and also by introducing silica (SiO2) to ZnO as a concept of composite material [7, 8]. With good mechanical resistance and high dielectric strength, SiO2 is likely to be an ideal component to enhance the properties of ZnO. ZnO-SiO2 is seen to have significant potential application as an antibacterial coating , luminescent material [10, 11], photocatalyst , and also as gas sensor  because of the good sensing capability of ZnO which could be enhanced more with the presence of SiO2 for size and shape control that could lead to better chemical activity at the surface. However, understanding the mechanism for the growth of ZnO-SiO2 nanostructures is still a formidable task particularly on controlling its surface morphology. Aside from these, most works related to ZnO synthesis use complex processes and sophisticated equipment like PLD , CVD [15, 16], and vapor phase transport process [17–19] among others which is not practical and economical in nature.
Recently, we reported preliminary investigations on the synthesis of ZnO-SiO2 both in powder form  and deposited on glass substrates  by using simple wet chemical deposition process. It is also worth mentioning that the SiO2 from rice husk ash, a naturally occurring material, was used as an alternative to the expensive liquid tetraethyl orthosilicate (TEOS). We found out that SiO2 modifies the surface of ZnO from hexagonal into pointed tips and confirms the attachment of SiO2 to ZnO through FTIR spectroscopy. In this current contribution, the influence of initial OH− ion concentration on the surface morphology of ZnO-SiO2 is presented. Moreover, a proposed mechanism for the growth of ZnO-SiO2 on glass substrates is also presented based on the evolution of morphology of ZnO-SiO2 with varying initial OH− ion concentration. Results of this work would pave the way for the control of the morphology of the nanostructures and for its possible use as methane gas detector.
2. Experimental Procedure
2.1. Wet Chemical Deposition Process
ZnO-SiO2 nanostructures were synthesized through low temperature chemical deposition process using zinc sulphate (ZnSO4) and ammonium hydroxide (NH4OH). First, 0.03 M ZnSO4 aqueous solution and varied ammonium hydroxide (NH4OH) concentrations (1.0 M, 2.0 M, and 3.0 M) were prepared separately in a beaker. The ammonium hydroxide was then added dropwise to the ZnSO4 solution and magnetically stirred while subsequently placing the precleaned glass substrates. After 30 minutes’ stirring of the mixed precursors, 0.1 grams of laboratory (in-house) prepared amorphous silica powder was then added and then the solution was continuously stirred for 3 hours while maintaining the bath temperature at 70°C. After 3 hours of constant stirring and heating, the substrates were washed with distilled water in order to remove loosely attached residues present and were allowed to dry at ambient room temperature. Table 1 shows the parameters considered for the synthesis of ZnO-SiO2 where fixed concentration of ZnSO4 was used at varying NH4OH concentrations. The amount of amorphous silica powder used was the same for all samples.
2.2. Characterization of Materials
The morphological characteristics of the samples were studied using JEOL JSM-6510LA analytical scanning electron microscope at low and high magnifications. Functional groups were determined by using Perkin Elmer Spectrum 100 infrared spectrometer from 4000 to 650 cm−1 over 32 scans with 4 cm−1 spectral resolution. Phase analysis of the samples was studied by X-ray diffraction analysis using Shimadzu XRD-7000 diffractometer from 5 to 40° (2θ) with step scanning mode of 0.02° (2θ).
3. Chemistry of Precursors for the Synthesis of ZnO-SiO2
To understand the mechanism for the deposition of ZnO-SiO2, it is better to look first on the growth of ZnO prior to addition of SiO2. The reaction between zinc sulfate and ammonium hydroxide upon dissociation in aqueous solution results in the formation of Zn(OH)2 which acts as seed nuclei for the growth of ZnO. The reaction is expressed in The formation of Zn(OH)2 induces rapid nucleation and further addition of ammonia dissolves the solid Zn(OH)2 to form ammonium-zincate bath shown in On further reaction with water in the active solution during deposition, ZnO forms according to this chemical reaction:Upon addition of SiO2 to the solution, the silica particles in the solution might interact with the OH− group of Zn(OH)2 because the surface of silica is composed of functional groups, namely, siloxane (Si-O-Si) bridges and silanol groups (Si-OH) where the OH− group acts as the center of molecular adsorption. Thus, a competition of attraction to OH-group of the Zn(OH)2 seed nuclei between silica and the formation of ZnO is possible to happen.
4. Results and Discussions
4.1. Characteristics of ZnO-SiO2 Using 1.0 M NH4OH
To show that indeed ZnO-SiO2 can be synthesized using silica from rice husk ash via chemical deposition process, a figure of bare ZnO is presented first to have an idea on what would be the effect of SiO2 on ZnO. Figure 1(a) presents a typical micrograph of ZnO grown on glass substrate. It was observed that dense and well-defined hexagonal nanorods were present and preferentially grow perpendicular to the glass substrate, though some were tilted. The hexagonal feature of the micrograph is a characteristic of a wurtzite structure which is the more stable phase of synthetic ZnO. Figure 1(b) meanwhile shows the corresponding IR spectra of bare ZnO. The peaks observed at 779 cm−1 and 3279 cm−1 are attributed to the stretching mode and hydroxyl group of ZnO, respectively . On the other hand, Figure 2 shows the micrograph of amorphous SiO2 and its FTIR data. It can be seen in Figure 2(a) that the SiO2 particles were agglomerated and were found to have micron size (>3 μm). Additionally, Figure 2(b) presents the characteristic infrared spectra of amorphous SiO2. Peak observed at 474 cm−1 is for the bending vibration of Si-O-Si bond while 804 and 1098 cm−1 are the vibrational bonds of Si-O. The peak at 3470 cm−1 is for both the Si-OH and the O-H vibration of physically adsorbed water .
Figure 3 on the other hand shows the micrographs using the same concentration of precursors as in Figure 1 but added with amorphous SiO2 powder grown also on glass substrates. It can be observed that the micrographs have uniform growth of dense ZnO rods. The surface morphology shows the preferential growth at -axis having pointed tips of approximately 250 nm in average diameter and 2 μm in average length which is far different compared to the morphology of bare ZnO. The modification at the surface of ZnO from hexagonal to pointed tips is due to the presence of SiO2 where it attaches to the nonpolar facets of ZnO which hinders the formation of hexagonal structure at its metastable facet. To maintain a much stable structure, the polar facets of ZnO may undergo surface reconstruction during the chemical deposition process resulting in the pointed tips of ZnO rods. Using low OH− ion concentration enables the growth of the mostly -axis oriented ZnO-SiO2 rods. This growth habit is due to the higher growth rate along the (0001) plane compared to other growth faces of ZnO.
To confirm ZnO-SiO2 formation, the infrared spectrum of ZnO-SiO2 using 1.0 M NH4OH is shown in Figure 4. Characteristic peak at 746 cm−1 indicates the presence of ZnO . The peaks at 832 cm−1 and 888 cm−1 are for the Si-O-Si asymmetric stretching vibrations while peak at 1063 cm−1 is for the Si-O-Si symmetric stretching. Peaks observed at 1500 cm−1 and 1553 cm−1 are for the Si-OH stretching vibration and the peak at 1393 cm−1 is for the Si-O-Si bending. All of these characteristic peaks indicate the presence of the functional groups of amorphous silica from RHA. Furthermore, peak at 1600 cm−1 and a broad absorption band situated at 3000 cm−1 to 3600 cm−1 are attributed to the O-H bending and O-H stretching vibrations of ZnO-SiO2, respectively, which reveals the presence of physically adsorbed water. Moreover, the peak at 1045 cm−1 is assigned for the Si-O-Zn stretching which confirms the formation of ZnO-SiO2 [24, 25].
4.2. Characteristics of ZnO-SiO2 Using Higher NH4OH Concentration
Increasing the ammonium hydroxide concentration resulted into much larger diameter but shorter rods. Flower-like formation and apparent change from pointed tips to hemispherical tips on the surface of the structures were observed as shown in Figure 5 for ZnO-SiO2 deposited on glass substrate using 2.0 M NH4OH. On the other hand, Figure 6 shows the micrographs of ZnO-SiO2 using 3.0 M NH4OH where much shorter (~0.850 μm) rods were present together with flake-like structures. The formation of flower-like structures using high NH4OH concentration was due to the sufficient amount of OH− which was necessary for the nucleation and subsequently growth of anisotropic ZnO-SiO2. The amorphous silica added to the ammonium-zincate bath solution causes the formation of hemispherical tips of the flower-like structures and is also the reason for having shorter and thicker rods. It is believed that, in higher OH− ion concentration, the SiO2 capped the surface of ZnO rods leading to the formation of these hemispherical tips. This is due to enough OH− ions that interact with the silanol group of silica in the solution. The notable flakes observed in Figure 6 (encircled) are associated with the excess OH− ions and may form Zn complexes.
Figure 7 shows the infrared spectra of the chemically deposited ZnO-SiO2 nanostructures prepared using high OH− ion concentration together with the spectra of ZnO-SiO2 prepared using 1.0 M NH4OH for comparison. It was observed that intensity of the peaks at 1500 cm−1 and 1553 cm−1 attributed to the Si-OH (silanol) stretches of silica decreases while peak at 1045–1050 associated with the Si-O-Zn band has a more pronounced intensity as OH− ion concentration is increased. This observation might suggest that the silanol group interacts with the OH− ions in the solution and that prolonged stirring resulted in the surface capping of SiO2 to ZnO resulting in the formation of flower-like ZnO-SiO2 with hemispherical tips.
The X-ray diffraction diagram of the bare ZnO and amorphous SiO2 and the chemically deposited ZnO-SiO2 nanostructures at varying OH− ion concentration are presented in Figures 8 and 9, respectively. A broad peak is observed centered at 21.6° (2θ) which is attributed to the amorphous nature of SiO2  while the diffractogram of bare ZnO showed the (100), (002), and (101) reflections indicating the formation of ZnO. Presence of sulfides and hydroxides of zinc was also observed which is due to the precursors used as depicted in Figure 8. Figure 9, on the other hand, shows the diffractogram of the prepared ZnO-SiO2 samples. Phase analysis revealed that the peaks correspond to wurtzite structure of ZnO as indexed to PDF 36–1451. The broad hump observed at 15–30° (2θ) was associated to the amorphous silica added in the ammonium-zincate bath for the synthesis of ZnO-SiO2; the same diffractogram was observed as in the work of . Presence of hydroxide of zinc was also observed.
4.3. Proposed Mechanism for the Growth of ZnO-SiO2
Following the chemistry of solution as previously discussed, the dropwise addition of NH4OH to aqueous solution of ZnSO4 produces Zn(OH)2 precipitate. For ZnO-SiO2 prepared using 1.0 M NH4OH, the Zn(OH)2 produces Zn2+ and OH− ions upon dissociation and induces the formation of ZnO nuclei. Based on the growth habit of ZnO, using low initial OH− ion concentration (1.0 M) resulted in a slow rate of ZnO nuclei formation leading to the creation of flat end-rods with hexagonal shape as observed in Figure 1. This can be explained by the high growth rate along the (0001) plane of ZnO compared to the other growth faces resulting in a rod-like structure. Upon addition of amorphous silica to the solution, the silanol functional group of silica attaches to the nonpolar facets of ZnO and, to maintain a stable structure, the metastable polar facet of ZnO exhibits surface reconstruction resulting in the pointed tips of ZnO-SiO2 rods.
However, using high initial OH− ion concentrations (2.0 M and 3.0 M) resulted in the flower-like formation of ZnO-SiO2 with hemispherical tips. Based on the micrographs, hydroxide cluster formation might be favourable for the growth of flower-like structures. Since there are sufficient OH− ions, more Zn(OH)2 clusters were formed and subsequently adhere on the glass substrates which serve as seed nuclei while some may dissociate to Zn2+ and OH− ions. Further addition of NH4OH triggers more formation of Zn(OH)2 which tends to continuously produce free Zn2+ and OH− ions and starts to form ZnO rods on the Zn(OH)2 seed nuclei. When amorphous silica was added, its silanol group attaches to the free OH− ions competing against the Zn2+ ions leaving more Zn2+ resulting in the formation of thicker and shorter ZnO-SiO2 rods. Excess OH− ions, especially using 3.0 M NH4OH, lead to the formation of zinc complexes as evidently suggested by the flake-like structures. As deposition time is prolonged, ZnO-SiO2 structures were slowly deposited on the substrate having already seed nuclei and, consequently, the neighbouring ZnO from the solution adheres on the seed nuclei that were already deposited on the substrate and forms the flower-like structure. The formation of hemispherical tips is believed to be due to enough OH− ions present in the solution that attaches to the silanol group (Si-OH) of silica and then subsequently caps the surface of ZnO rods resulting also in a much bigger but shorter rod.
In conclusion, the control of the morphology of ZnO-SiO2 was presented by changing initial OH− ion concentration. For a better understanding of the growth of ZnO-SiO2, we demonstrated first the effect of SiO2 on the morphology of ZnO followed by the influence of varying OH− ion concentration on its size and shape. It was found that bare ZnO rods exhibit hexagonal morphology having a -axis preferred growth direction. Addition of amorphous silica to the ammonium-zincate bath for ZnO-SiO2 synthesis resulted in a remarkable change of morphology from hexagonal to “pointed tip” rods. Increasing initial OH− ion concentration produces “flower-like” structure having hemispherical top surface but shorter rods due to surface capping of silica to ZnO. Infrared spectroscopic study confirms the formation of ZnO-SiO2 from the functional groups present. Phase analysis by XRD suggested a wurtzite crystal structure of ZnO-SiO2 was developed. Preliminary studies on the gas sensing capability and testing of the optical and electrical properties for solar cell application of the produced ZnO-SiO2 are currently being conducted.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank the Commission on Higher Education-Philippine Higher Education Research Network (CHED-PHERNet) through the Office of the Vice Chancellor for Research and Extension of MSU-IIT for the financial support of this research study.
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