Optical studies of crystalline ZnO–SiO₂ developed from pyrolysis of coconut husk

Waste disposal is a complicated methodical process of removing nonessential materials including burial, burning, discharge and recycling. Waste materials were produced from various sources of operations such as factories, industries, mining operation, and agricultural sectors. These waste materials can be hazardous, toxic, corrosive and sometimes can be fatal to human. Thus, these obsolete items require appropriate waste management and disposal technique so that the problems can be contained since most countries have limited number of suitable disposal sites [1]. However, finding the best and innovative solution for effective management of waste require lots of time, energy and expense. Despite that, methods of recycling have been developed to minimize the harmful effects caused by landfill disposal and to convert the waste into variety of useful products.

Among the waste materials being recycled today, bio-waste is one of the most heavily studied because of its potential to be converted into raw materials. Bio-waste is defined as waste from living organisms, animals or plants. It is believed that major problems can arise if it is not disposed properly. Various researches have been conducted to 'recycle' bio-waste into materials that have potential to be used in certain applications. Among them were rice husk [2–6], rice straw[7–10], banana peel [11], corn cob [12, 13], palm waste [14, 15], sugarcane [16], turmeric [17], and coconut husk [18, 19]. Raw materials silica (SiO₂) is one of the elements presents in most agricultural waste and commonly used in numerous researches for optical application especially in phosphors materials production. Therefore, production of phosphors materials from waste materials indeed solved the landfill problems at the same time provided a different route with less production cost.

One of the most promising candidates for phosphors materials is zinc silicate. Zinc silicate exhibits good luminescence properties and often used as a phosphors materials in numerous applications including fluorescent lamp, television, and displays materials or lighting appliances [20–22]. There are many advantages associated with the use of zinc silicates as optical host materials since they are able to acquire a wide range of multi-colors from luminescent activators [23]. Recent studies on zinc silicate showed that its absorbance and optical band gap value varied with the method of preparation. In addition, its optical properties can be altered by adjusting few parameters such as heat treatment, sintering temperature and dopant [2, 4, 24–26]. Commonly, zinc silicate was synthesized using sol-gel method or solution precipitation method, which has the limitation due to the complicated process and higher cost of production. To overcome these issues, zinc silicate was synthesized by using solid-state reaction method, where waste coconut husk was used as the alternative silica source. In the nutshell, this research will greatly contribute the society by providing an alternative path of green synthesis method of producing zinc silicate phosphors from waste coconut husk. The optical properties of this novel ZnO–SiO₂ product from the green synthesis will be explored.

2.1. Starting materials and synthesis

2.2. Characterization

FESEM analysis of the morphological structure of sample before and after heat treatment was performed with magnification level of 10 000 and 25 000. The micrograph in figure 1 clearly revealed that the surface morphology of the sample was not consistent and the shape of grains was irregular before applying the heat treatment. The sample size distribution was sporadic that probably due to the different size of starting materials used in the work. Contrary to sample before treatment, FESEM images of the samples after the heat treatment at 1000 °C (figure 2) have well-distinct boundaries. Undoubtedly, the heat treatment caused the sample to increase in size and produced larger crystal. The increased in grain size following the heat treatment was according to kinetic grain growth phenomenon [27]. With the increased in sintering temperature at constant holding time, the migration of crystal boundaries became more rapid thus increased the average grain size [28]. The sample was also no longer in irregular shape with no porosity observed in the figure due to the increase in the grain growth. This caused the particles to form neck with each other and agglomerated into bigger particles [24, 29]. Based on figures 1 and 2, the grain size was measured, where the average grain size for ZnO–SiO₂ was 198 nm for sample before heat treatment, and it increased to 460 nm after heat treatment at 1000 °C.

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The XRD pattern of ZnO–SiO₂ samples is shown in figure 3. All the diffraction peaks were matched with the standard value from Joint Committee on Powder Diffractions Standards (JCPDS). Before heat treatment, ZnO (JCPDS 14-0653) and SiO₂ (JCPDS 82-0511) were identified by the diffraction peaks emerged on the spectrum. On the other hand, different peaks started to emerge after heat treatment that can be assigned to zinc silicate (Zn₂SiO₄) (JCPDS 37-1485). This result is in good agreement with previous reported studies [30, 31]. Also, the peaks with higher diffraction intensity that can be observed after heat treatment depicted as high crystallinity of the zinc silicate. This is due to the high heat treatment temperature increased the atomic mobility that cause the grain growth and thus results in better crystallinity [32, 33].

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The optical characterization was conducted to investigate the performance of ZnO–SiO₂ as potential phosphors materials. The optical properties of a metal nanostructures are determined by the collective oscillation of free electrons within the metal nanostructures, or known as plasmon. Its corresponding phenomenon is called surface plasmon resonance (SPR) [34]. The absorbance spectra of ZnO–SiO₂ before and after heat treatment are shown in figure 4. From the spectra, it can be seen that ZnO–SiO₂ showed the characteristic of SPR peak at around 364 nm and also a broad band at wavelength around 550–600 nm [35, 36]. This result is in good agreement with thus obtained by Bajpai et al [34] who found that ZnO nanoparticles exhibit a SPR peak around 364 nm. Apart from this, the absorption edge observed at wavelength 400 nm suggested that the absorption properties of the sample occurred in the UV region. It can also be observed that the absorbance intensity decreased and the absorbance edge was blue-shifted after the heat treatment at 1000 °C. The decrease in absorbance intensity at high temperature 1000 °C was due to the deformation of ZnO that leads to the formation of zinc silicate or ratio of crystallized/disordered ZnO [37]. It is known that the intensity of absorbance band was highly dependent on ZnO content [38]. The blue-shifted on the spectrum occurred due to high crystallinity of the zinc silicate sample when at high temperature of 1000 °C.

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Based on the absorbance spectrum, the relation between the absorption coefficient, α and photon energy, hv can be obtained by using the equation [39, 40]:

$$\begin{eqnarray}&&\alpha \left(v\right)=\displaystyle \frac{{(hv-{E}_{g})}^{2}}{hv}\end{eqnarray} \tag{ 1 }$$

The equation (1) were rearranged into (2)

$$\begin{eqnarray}&&{(\alpha hv)}^{1/n}=hv-{E}_{g}\end{eqnarray} \tag{ 2 }$$

$$\begin{eqnarray}&&{(\alpha hv)}^{2}=hv-{E}_{g}\end{eqnarray} \tag{ 3 }$$

Where n, is the type of transition, h is the Planck's constant, ν is the frequency and E_g is the optical band gap. Different studies used different value of n, e.g. n = 2 used for amorphous semiconductor and for n = 1/2 was used in most crystalline semiconductor [41]. In this work, n = 1/2 was used in equations (2) and (3) to determine the optical band gap of the sample based on the direct allowed transition. The band gap can be found by extrapolating the linear portion of the absorption curve until the x-intercept as shown in figure 5. From the figure, the optical band gap of untreated sample was found to be 3.22 eV. However, the band gap value of heat-treated sample at 1000 °C were calculated at 4.05 eV. The improvement of the sample crystallinity as proven in the XRD analysis caused the reduction of delocalized states and thus making the sample with less defects resulting in increase of the optical band gap value [42].

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In this work, ZnO–SiO₂ crystal system was synthesized and investigated. Effect on heat treatment on the sample was discussed. The particle size of the sample measured from the FESEM analysis was 198 nm but increased to 460 nm after the heat treatment. In addition, the grains boundaries become more distinct after being sintered at 1000°C. Formation of zinc silicate crystalline system was determined by the diffraction peaks found on XRD spectrum after the sample was heat-treated. Lastly, the absorbance intensity of the sample decreased after the heat treatment. The optical band gap of untreated sample was 3.22 eV but increased significantly to 4.05 eV after the heat treatment.

The authors gratefully acknowledge the financial support for this study from the Malaysian Ministry of Education (MOE) and the Universiti Putra Malaysia (UPM) through the Putra Grant. The laboratory facilities provided by the Institute of Advanced Technology and Faculty of Science, Universiti Putra Malaysia, are also acknowledged.

The work was funded by Malaysian Ministry of Education (MOE) and the Universiti Putra Malaysia (UPM) through the Putra Grant 9642800. The fund was used for purchasing materials, apparatus and the characterization process fee.

The authors declare no conflict of interest.