Room-temperature synthesis of zinc oxide nanoparticles in different media and their application in cyanide photodegradation
© Bagabas et al.; licensee Springer. 2013
Received: 29 July 2013
Accepted: 21 November 2013
Published: 6 December 2013
Cyanide is an extreme hazard and extensively found in the wastes of refinery, coke plant, and metal plating industries. A simple, fast, cost-effective, room-temperature wet chemical route, based on cyclohexylamine, for synthesizing zinc oxide nanoparticles in aqueous and enthanolic media was established and tested for the photodegradation of cyanide ions. Particles of polyhedra morphology were obtained for zinc oxide, prepared in ethanol (ZnOE), while spherical and some chunky particles were observed for zinc oxide, prepared in water (ZnOW). The morphology was crucial in enhancing the cyanide ion photocatalytic degradation efficiency of ZnOE by a factor of 1.5 in comparison to the efficiency of ZnOW at an equivalent concentration of 0.02 wt.% ZnO. Increasing the concentration wt.% of ZnOE from 0.01 to 0.09 led to an increase in the photocatalytic degradation efficiency from 85% to almost 100% after 180 min and a doubling of the first-order rate constant (k).
Cyanide has numerous applications in industry such as chelating agent, electroplating, pharmaceuticals, and mining[1, 2]. This extensive use of cyanide results in the generation of a huge amount of cyanide waste and increases the cyanide spill risk to the environment[3, 4]. Thus, cyanide must be treated before discharging. Different protocols such as adsorption, complexation, and oxidation are used for abating cyanides[1, 2, 5–7]. The procedures other than oxidation give highly concentrated products in which toxic cyanides still exist[8, 9].
Highly powerful, economically method is the photocatalytic oxidation of cyanide, which has been demonstrated in several studies[10–17]. However, an inexpensive photocatalyst is needed for the economical removal of large quantities of cyanide. ZnO is one of the most promising materials for executing this task, as an alternative to the widely used, relatively expensive titania (TiO2). Although researchers recognized comparable photodegradation mechanisms with both ZnO and TiO2, they proved that ZnO was the superior photocatalyst in degrading pesticide carbetamide, herbicide triclopyr, pulp mill bleaching wastewater, 2-phenylphenol, phenol, blue 19, and acid red 14. This superiority of ZnO photocatalytic activity is because it has more active sites, higher reaction rates, and is more effective in generating hydrogen peroxide.
Due to its direct, wide bandgap of 3.37 eV, ZnO has a wide range of applications in optoelectronic devices such as light-emitting diodes, photodetectors, and p-n homojunctions. The large exciton binding energy of 60 meV, compared to that of GaN (approximately 25 meV), enhances the luminescence efficiency of the emitted light even at room temperature and higher. The visible photoluminescence (PL) emission at approximately 2.5 eV (approximately 495 nm), originated from intrinsic defects, makes ZnO suitable for applications in field emission and vacuum fluorescent displays.
Many techniques including chemical vapor deposition, pulsed laser deposition, molecular beam epitaxy, sputtering, hydrothermal synthesis, and oxidation of metallic zinc powder[27, 28] have been used to prepare ZnO in different forms and structures for various applications. Nanoparticulate form enhances the catalytic activity due to its large surface area and the presence of vacancies and uncoordinated atoms at corners and edges. The photocatalytic activity is also improved by bandgap engineering, as a result of the quantum confinement effect[29–31].
A well-controlled synthesis process at room temperature is needed for the economical use of ZnO in catalytic applications such as water treatment and other environmental applications. Herein, we are reporting, for the first time to the best of our knowledge, a direct, simple, room-temperature synthesis method for ZnO nanoparticles using cyclohexylamine (CHA), as a precipitating agent, and zinc nitrate hexahydrate, as a source of zinc, in both aqueous and ethanolic media. The synthesized ZnO nanoparticles were examined as a photocatalyst for the degradation of the highly toxic cyanide anion [CN-(aq)] in the aqueous medium at room temperature. The kinetics for cyanide photodegradation were investigated with respect to ZnO concentration of weight percentage.
Zinc nitrate hexahydrate (pure, POCH), cyclohexylamine (GC >99%, Merck, Whitehouse Station, NJ, USA), absolute ethanol (EtOH, 99.9%, Scharlau, Sentmenat, Barcelona, Spain), potassium cyanide (≥97%, Sigma-Aldrich, St. Louis, MO, USA), potassium iodide (≥99.5%, Sigma-Aldrich), and ammonia solution (28-30% NH3 basis, Sigma-Aldrich) were commercially available and were used as received. Deionized water (18.2 MΩ.cm)was obtained from a Milli-Q water purification system (Millipore, Billerica, MA, USA).
Synthesis of ZnO nanoparticles in water (ZnOW) and in ethanol (ZnOE)
Thirty millimoles of zinc nitrate hexahydrate was dissolved in 60 ml of water at room temperature, under continuous magnetic stirring. In a separate beaker, 60 mmol of CHA was dissolved in 20 ml water at room temperature. The CHA solution was poured into the zinc solution, resulting in a white precipitate upon magnetic stirring. An extra amount of 80 ml water was added to the reaction mixture, which was left stirring for 4 days. The precipitate was filtered off through an F-size fritted filter and then was washed with 100 ml water. The precipitate was dried at room temperature under vacuum for 1 day. After drying, the precipitate was mixed with 300 ml water and was magnetically stirred for 1 day for the removal of any impurity. The precipitate was filtered off and was dried room temperature under vacuum to give 2.43 g (yield% = 89.7). This dried sample was then calcined at 500°C under air for 3 h. The temperature was ramped from room temperature to the target temperature by 1°C/min. Inductively coupled plasma (ICP) elemental analysis was carried out for the uncalcined sample, which proved the formation of zinc oxide at room temperature with a formula of ZnO · 1/2H2O [Zn (cal. 72.3%, exp. 72.9%)].
In addition, the same procedure was carried out to prepare ZnO nanoparticles in ethanolic medium instead of water. The precipitate gave 2.572 g (yield% = 98.1) of ZnO · 1/3H2O, as proven by ICP elemental analysis [Zn (cal. 74.8%, exp. 74.2%)]. Both of uncalcined ZnO nanoparticles in water (ZnOW) and in ethanol (ZnOE) were found to be soluble in HCl and NaOH, evidencing the chemical identity of ZnO.
Inductively coupled plasma (ICP) was used to determine the percentage of the zinc component in uncalcined ZnO samples, obtained at room temperature. Brunauer, Emmett, and Teller surface areas (BET-SA) and pore size distribution of the catalysts were obtained on Micrometrics Gemini III-2375 (Norcross, GA, USA) instrument by N2 physisorption at 77 K. Prior to the measurements, the known amount of the catalyst was evacuated for 2 h at 150°C. Diffuse reflectance infrared Fourier transform (DRIFT) spectra of ground, uncalcined ZnO powder samples, diluted with IR-grade potassium bromide (KBr), were recorded on a Perkin Elmer FTIR system spectrum GX (Waltham, MA, USA) in the range of 400 to 4,000 cm-1 at room temperature. X-ray diffraction (XRD) patterns were recorded for phase analysis and crystallite size measurement on a Philips X pert pro diffractometer (Eindhoven, Netherlands), operated at 40 mA and 40 kV by using CuKα radiation and a nickel filter, in the 2-theta range from 2° to 80° in steps of 0.02°, with a sampling time of 1 s per step. The crystallite size was estimated using Scherer's equation. XRD patterns were recorded for uncalcined and calcined (500°C) ZnO materials. The morphology was investigated using a field-emission scanning electron microscope (FE-SEM model: FEI-200NNL, Hillsboro, OR, USA), equipped with an energy-dispersive X-ray (EDX) spectrometer for elemental analysis, and a high-resolution transmission electron microscope (HRTEM model: JEM-2100 F JEOL, Akishima-shi, Tokyo, Japan). Carbon-coated copper grids were used for mounting the samples for HRTEM analysis. Solid-state ultraviolet-visible (UV-vis) absorption spectra for calcined ZnO powder samples were recorded on a Perkin Elmer Lambda 950 UV/Vis/NIR spectrophotometer, equipped with a 150-mm snap-in integrating sphere for capturing diffuse and specular reflectance.
The photocatalytic evaluation was carried out using a horizontal cylinder annular batch reactor. A black light-blue florescent bulb (F18W-BLB) was positioned at the axis of the reactor to supply UV illumination. Reaction suspension was irradiated by UV light of 365 nm at a power of 18 W. The experiments were performed by suspending 0.01, 0.02, 0.03, 0.05, 0.07, or 0.09 wt.% of calcined ZnO into a 300-ml, 100 ppm potassium cyanide (KCN) solution, with its pH adjusted to 8.5 by ammonia solution. The reaction was carried out isothermally at 25°C, and samples of the reaction mixture were taken at different intervals for a total reaction time of 360 min. The CN-(aq) concentration in the samples was estimated by volumetric titration with AgNO3, using potassium iodide to determine the titration end-point. The percentage of degradation of CN-(aq) has been measured by applying the following equation: %Degradation = (Co - C)/Co × 100, where Co is the initial concentration of CN-(aq) and C is the concentration of uncomplexed CN-(aq) in solution.
Results and discussion
Formation of ZnO nanoparticles in an aqueous and ethanolic media
The formation of zinc hydroxide complexes and oxide ions shifts the equilibrium in Equation 2 forward, causing further protonation of CHA and the formation of more hydroxide ions.
Equation 4 shows that the construction of ZnO crystal takes place via the interaction between the surface hydroxide of the growing crystals and the hydroxide ligands of the zinc complexes. Therefore, the formation of ZnO, according to the above proposed mechanism, is due to the high basicity of the reaction medium, which causes an increase in the concentration of the precursors (zinc hydroxide complexes) and an increase in the chemical potential of hydroxide ions.
BET surface area
BET surface area and pore volume of calcined ZnO nanoparticles, prepared either in EtOH or H 2 O
Pore volume (cm3/g)
Average crystallite size of uncalcined [a] and calcined [b] ZnO E and ZnO W
Miller indices (hkl)
Average crystallite size (nm)
The inter planar spacing and diffraction planes of un- and calcined ZnO E and ZnO W
d-spacing calculated from HRTEM (nm)
d-spacing in bulk ZnO (nm)
Miller indices (hkl) assignment
Photocatalytic degradation of cyanide
Synthesis medium effect on photocatalytic oxidation
where h is Planck's constant and ν is the frequency of UV light.
Effect of the synthesis medium on photocatalytic activity
ZnO loading (wt.%)
CN‾ degradation (%)
The superiority of ZnOE photocatalytic activity can be correlated to its particle size and shape, as it is reported in the literature[42–45]. However, the effect of ZnO particle shape on the photocatalytic activity is rarely studied in the literature. In this context, the edges and corners of ZnOE hexagonal particles have many coordinatively unsaturated sites, which usually are active in catalysis. On the other hand, the spherical shape of ZnOW particles would have much less active sites due to the lack of edges and corners. Aligning with our interpretation of ZnOE photocatalytic activity, El-sayed and his coworkers, for instance, showed that the influence of the particle shape on the catalytic activity is very important toward better activity[42, 45]. In addition, the photocatalytic activity of acetaldehyde decomposition using ZnO powder depended on several factors including the morphology of the particles. Finally, we believe that the morphology of our ZnOE particles is crucial in photocatalytic activity and our present findings will provide a hint about the role of morphology in the ZnOE photocatalytic performance.
Based on the obtained results, ZnOE nanoparticles were used in further investigation for improving the cyanide degradation efficiency.
Photocatalytic degradation of CN- using different concentrations wt.% of calcined ZnOE
Kinetic photocatalytic degradation of CN- using calcined ZnOE
Apparent rate constant ( k ) at different concentration wt.% of calcined ZnO E
k(min × 10-3)
Zinc oxide nanoparticles were readily prepared at room temperature from zinc nitrate hexahydrate and cyclohexylamine either in aqueous or ethanolic medium. The calcined ZnOE had a regular, polyhedra morphology while the calcined ZnOW had irregular spherical morphology, mixed with some chunky particles. The morphology was a key factor in the superior photocatalytic behavior of ZnOE over that of ZnOW. The differences in morphology and photocatalytic behavior are strongly influenced by the physicochemical properties of the synthesis medium.
The authors gratefully thank King Abdulaziz City for Science and Technology (KACST) for financing this work through project No. 29–280. We also thank Dr. Mohamad Mokhtar and Reda Mohammed for their useful discussion, Mr. Emad Addurihem for his technical assistance, Mr. Abdulrahman AL-Ghihab for SEM analysis, and Mr. Muath Ababtain for TEM analysis.
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