Metal doped ZnO nanorods decorated with carbon nanodots as an efficient photocatalyst for degradeation of Methyl Blue Solution
2. EXPERIMENTAL DETAIL 1. Materials
Analytical grade of zinc acetate dehydrate (Zn(CH3COO)2.2H2O), zinc nitrate tetrahydrate (Zn(NO3).6H2O) and hexamethylenetetramine (C6H12N4) powder were purchased from Merck and used as raw materials without further purification. Methyl blue was purchased from Sigma- Aldrich. Deionized water with 18.2 Ω was produced by a Millipore Direct-Q system. Meanwhile, fresh cassava peels was used as carbon source to synthesize carbon quantum dots (C-dots) and purchased from a traditional market in Kramatjati, Jakarta Timur, Indonesia. Absolute ethanol was used as solvent to disperse C-dots and was purchased from Emsure. Glass substrate purchased from Gea Medical is cleaned using acetone and isopropyl alcohol in an ultrasonic bath for 15 minute.
2.2 Preparation of carbon dot (C-dots)
The carbon dots were synthesized through simple carbonization process of cassava peel in an electronic oven at low temperature followed by immersion in absolute ethanol for 6 days.
Briefly, 100 gram of cassava peels as the raw material was washed with distillated water to remove soluble impurities. These procedures were repeated until the last rinse of water becomes clear and colorless. A cleaned cassava peel was then dried in an oven and maintained at 120 oC for 12 hours for carbonization process. Dried cassava peels were ground into a fine powder using a blender. Subsequently, the powder was filtered using 40 mesh sieve (hence, the grain size of dried cassava peels becomes ±425µm). Ethanolic solution of C-dots with concentration of 0.1 mg/mL was prepared by adding 1 gram cassava peel powder into 10 mL of absolute ethanol. The resulting solution was kept under stirring for 4 h at 1500 rpm. After being stirred, the dispersion was continuously aged at room temperature for 6 days until the dispersed solution was found visually to separate from other large agglomerated particles. In order to purify C-dot, the
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dispersed solutions of C-dots was carefully pipetted to eliminate the impurities and undispersed large agglomeration particles. Finally, the residual undispersed particles were removed by centrifugation at 10000 rpm for 30 minutes. Hence, homogeneous clear and transparent yellowish-brown dispersed C-dots solutions were obtained.
2.3 Preparation of ZnO nanorods
An analytical grade zinc acetate dehydrate with 4.389 grams was dissolved in 100 mL of deionized water to obtain 0.2 M of seed solution. The seed solution was atomized to form a small aerosol droplet and sprayed directly onto heated glass substrate using ultrasonic nebulizer (1.7 MHz) for 15 minutes at 450 °C. The prepared sample was then annealed at 450oC for 1 hour and cooled to room temperature. The sample was continuously subjected to hydrothermal treatment by dipping the glass substrate covered with ZnO nanoseed in equimolar of 0.05 M Zn(NO3).6H2O and HMT and kept at 95 oC for 6 h inside an oven. The prepared sample was cooled down to room temperature followed with rinsing with deionized water, dried with hairdryer and then annealed for 1 h at 450 °C. Meanwhile, the metal doped ZnO nanorods used in this work were prepared with similar procedure with addition of element Co and Mn as previously described in [18].
2.4 Synthesis of zinc oxide/ carbon dots (ZnO/C-dots)
In a typical synthesis process, heterostructure ZnO/C-dots composite were prepared by two- steps spin-coating process for 6 seconds at 500 rpm and followed for 30 seconds at 1000 rpm by dropping 20 µl of C-dots dispersed solution onto 1 × 1.5 cm2 the surface of ZnO nanorods sample. The sample was then dried at room temperature for slowly evaporating the solvent, annealed at 50 oC on a hot-plate for 15 minutes and then cooled down to room temperature.
These procedures were repeated with various numbers of layers, namely, 2, 4 and 6 layers in order to get appropriate thickness of C-dots onto ZnO nanorods surface. The samples were annealed at 70 oC for 1 hour in air condition, using a hotplate and subsequently used as photocatalys to degrade methyl blue solution.
2.5 Characterization of ZnO nanorods/C-dots
The surface morphology and structural properties of ZnO/C-dots were determined using scanning electron microscopy (SEM) (JSM-6510LA) and x-ray diffraction (XRD) Bruker D8
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Advance equipment, respectively. 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/C-dots samples. Particle size analyzer (PSA) was performed using Microtrac type Sync instrument. The transmission electron microscope (TEM) images of C-dots dispersed solution were recorded using a FEI Tecnai G2 20S-Twin, 200Kv electron microscope.
All the characterizations were performed at room temperature.
2.6 Photocatalytic activity
To investigate the photocatalytic activity of ZnO nanorods, ZnO nanorods/C-dots with variation of C-dots layer and metal doped ZnO/C-dots heterostructure, photodegradation experiment of methyl blue (MB) in aqueous aqueous solution were performed under ultraviolet light exposure in a photocatalytic chamber. Prior to the irradiation, all samples were immersed in 20 mL with concentration 10 mM MB solution and kept under darkness for 30 min for adsorption-desorption equilibrium of the MB molecules on the sample surface. The photo degradation of MB with ZnO/C-dots was recorded using UV-Vis spectrophotometer after being irradiated with a UV lamp (20 watt). After desired time interval, an aliquot of the solution was isolated and its absorbance was measured using UV-visible spectrophotometer to calculate the degradation. The degradation of MB under UV irradiation without ZnO photocatalyst was also observed as a baseline.
3 RESULT AND DISCUSSION
The physical appearance of C-dots dispersed solution prepared from cassava peels under visible light and irradiated with UV 405 nm laser can be seen in Figure 1. As can be observed from Figure 1, as prepared C-dots in absolute ethanol solution demonstrates a transparent yellowish-brownish coloration and has no fluorescence under visible light exposure. On the other hand, C-dots dispersed solutions clearly show fluorescence under UV light exposure by 420 nm laser, which indicates the formation of carbon dots. The C-dots sample emits intense blue-green fluorescence under UV light exposure due to their small grain size and different surface state of emission trap on the surface of carbon nanosized that could be called as quantum confinement
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size effect. This is in good agreement with other published data on ZnO/C-dots nanocomposite [19].
Figure 1. Digital photographs captured (a) under visible, (b and c) UV light exposure of the cuvette containing C-dots sample prepared from cassava peels in daylight and dark condition
The absorption and photoluminescence emission spectrum of C-dots from cassava peels in absolute ethanol is shown in Figure 2(a) and (b), respectively. Meanwhile, the TEM image of C- dots solution is shown in Figure 2(c). Commonly, C-dots dispersed solution exhibits the absorbance peaks at the wavelength from 260-360 nm in the range of UV light spectrum. It is found that absorbance peaks of C-dots dispersed solution appears in broad absorption peaks at the wavelength from 260 to 550 nm. The absorption peaks at 345 nm originates from electronic transition n-π* and corresponds to the C = O bond from the carbon core of C-dots sample and the transition from π-π* of the C = C conjugated bond from carbon surface of C-dots sample [20].
The present result indicates the successful formation of C-dots. The photoluminescence spectrum (PL) of the C-dots dispersed solution under excitation by 420 nm laser is given in Figure. 2(b). It has been observed that the emission peak intensity of C-dots dispersed solution centered at wavelength around 488 nm and indicates that the emission spectrum of C-dots dispersed solution shows strong blue-green emission. This result further confirms that C-dots has been successfully formed in as prepared sample which is in good agreement with physical appearance from C-dots dispersed solution under UV light exposure in dark as shown in Figure 1(c). The blue emission
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from C-dots dispersed solution can be attributed from defect states related to intrinsic defects from the structure of C-dots sample [21]. It was also could be from impurity containing functional groups on the surface of C-dots sample [22] . The green emission is further recognized to sp2 site from carbon networks which may shift due to size effects [23]. The TEM image (Figure 2(c)) reveals that the distance of the lattice fringes of the C-dots sample is determined below 10 nm with good crystallinity.
Figure 2. (a) UV–Vis absorption spectrum (b) photoluminescence emission spectrum (λ of excitation 420 nm) and (c) TEM image of colloidal C-dots
The particles size distribution of C-dots dispersed solution is shown in Figure 3.
According to Fig. 3, it is found that the C-dots particles demonstrate high dispersion, high homogeneity dimension with an approximate diameter of 4.8 nm. Furthermore, the poly-
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dispersity index (PdI) of the as prepared C-dots dispersed solution is about 0.346 nm. The lower of PdI indicates the particle size distribution of C-dots dispersed solution almost identical. This result proved the formation of C-dots since the material can be classified as carbon dot nanomaterial if their particle size in the range of 2-10 nm and demonstrates unique photoluminescence properties under UV light.
Figure 3. Estimation grain size distribution of C-dots dispersed solution
The optical absorption spectra of ZnO/C-dots heterostructure with various C-dots layer can be seen in Figure 4. The ZnO sample exhibit almost similar absorption characteristics with strong absorption in UV light region with sharp absorption edge at the wavelength 320 to 390 nm and weak absorption in the visible light region with wide absorption window area at the wavelength from 400 to 600 nm. The absorption spectra at the wavelength about 320 to 390 nm is related to the characteristics of ZnO band gap energy where the sample absorb more photons in UV light region and then excites the electrons from the valence band to the conduction band of ZnO. The absorption spectra of all samples in the UV region are almost indistinguishable.
Meanwhile, the absorption peaks of ZnO sample coated with 4 layers of C-dots have continuous wide absorption in visible light region. The continuous wide absorption in visible light originally
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