Synthesis, Characterization, and Cyanide ... - ACS Publications

Chapter 18

Downloaded by UNIV OF OKLAHOMA on April 18, 2013 | http://pubs.acs.org Publication Date (Web): April 17, 2013 | doi: 10.1021/bk-2013-1124.ch018

Synthesis, Characterization, and Cyanide Photodegradation Over Cupric Oxide-Doped Zinc Oxide Nanoparticles Abdulaziz Bagabas,*,1 Mohamed F. A. Aboud,2,3 Reda M. Mohamed,4 Zeid AL-Othman,2 Ahmad S. Alshammari,1 and Emad S. Addurihem1 1Petrochemcials

Research Institute, King Abdulaziz City for Science and Technology (KACST), P.O. Box 6086, Riyadh 11442, Saudi Arabia 2Department of Chemistry, College of Science, King Saud University (KSU), P.O. Box 2455, Riyadh 11451, Saudi Arabia 3Department of Mining, Metallurgy, and Petroleum Engineering, Al-Azhar University, Cairo 11371, Egypt 4Department of Chemistry, Faculty of Science, King Abdulaziz University (KAU), P.O. Box 80203, Jeddah 21589, Saudi Arabia *E-mail: [email protected]

A simple, rapid, inexpensive, room-temperature wet chemical route for synthesizing CuO-doped ZnO nanoparticles was established. Undoped and CuO-doped ZnO photocatalysts, with 1, 2, 3, and 4 wt.% of CuO, were prepared by using cyclohexylammine for coprecipitation of the metals from their aqueous solutions, followed by calcination at 500°C. The photocatalysts were characterized by ICP elemental analysis, UV-Vis absorption spectroscopy, XRD, SEM, EDX, and TEM. The synthesized materials were tested for oxidative photodegradation of cyanide in aqueous medium at room temperature. The undoped material exhibited an activity of 56% cyanide removal efficiency. Doping of ZnO with 1 and 2 wt.% CuO dramatically enhanced the photocatalytic activity to 89% and 93 %, respectiviely, while to 97% for both 3 and 4 wt.%. This improvement in photocatalytic activity can be attributed to the reduction of ZnO band gap and the increase in charge separation on the surface of photocatalyst particles.

© 2013 American Chemical Society In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Introduction Zinc oxide (ZnO) is a II-VI semiconductor with broad range of applications due to its unique properties. It can be also used as a model for applying quantum mechanics principles in the nanometric regime (1). Its direct, wide band gap energy (Eg) of 3.37 eV (2) enables its use in optoelectronic devices. The high sensitivity of the ZnO surface conductivity with adsorbed species nominates its utilization for gas detection (3). The abnormal radiation hardness makes it applicable in space applications (4). The ZnO large exciton binding energy of 60 meV (2), compared with that of GaN of ~ 25 meV (5), enhances its luminescence efficiency of the emitted light even at room temperature and higher. The visible photo-luminescence (PL) emission around ~ 2.5 eV, originated from intrinsic defects (6), makes ZnO suitable for applications in field emission and vacuum fluorescent displays. Its reported room temperature ferromagnetism upon transition metal doping motivates its applications for spintronic devices (7). In addition, its relatively low cost, superior chemical and mechanical stability (8), the availability of large-area substrates with desirable c-axis preferential growth nature and technological compatibility with the conventional silicon process (9) make it very strong competitor for many applications. Alteration of ZnO specifications, electronic and optical properties in particular, can be made by its doping with transition metals such as manganese, iron, cobalt, nickel, copper and lanthanides such as europium, erbium, and terbium (1). The Cu-doped ZnO semiconductor research was mainly directed to catalytic applications such as methanol synthesis (10), production of hydrogen by partial oxidation of methanol (POM) (11), carbon monoxide oxidation (12), degradation of textile dye pollutants within aqueous solutions (13) and dilute magnetic semiconductors for spintronic devices (14, 15). Copper forms in different bonding states within ZnO lattice such as metallic (Cu0), monovalent (CuI2O) and divalent (CuIIO), depending on the annealing conditions (temperature and oxygen pressure), where the fully oxidized divalent state Cu2+ is favored when the above conditions are promoted, otherwise other states would be present. Various methods for doping ZnO with CuO have been reported in the literature such as hydrothermal (1, 16), sol-gel (17, 18), and co-precipitation (19). Hydrothermal method applies both pressure (150 kPa) (1) and temperature (190°C) (16). Furthermore, it resulted in secondary phase of CuO above 3% (1) and maximum solubility of 1% for CuO (16). It also led to surface-enrichment with copper, and hence, inhomogeneous distribution of CuO within ZnO (16). The sol-gel method, on the other hand, uses a template of organic material, which requires high temperature 430°C for its removal from the desired oxide product (18). However, this organic sometimes could not be removed and resulted in presence of mixed states of copper (17). The co-precipitation uses a strong base such as sodium hydroxide at moderate temperature 80°C and results in segregated phases of CuO and ZnO (19). 328 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

We are reporting herein a co-precipitation method, avoiding the drawbacks of the above reported procedures, using a moderate base of cyclohexylamine (CHA) without using any organic template or surfactant at ambient temperature and pressure. Our method, in addition, resulted in copper-substituted ZnO wurtzite lattice structure nanoparticles, regardless of doping concentration (1, 2, 3 and 4 weight percents of copper oxide).

Experimental


Materials Copper nitrate trihydrate (98-103%, Fluka), zinc nitrate hexahydrate (pure, POCH), and cyclohexylamine (GC>99%, Merck) were commercially available and were used without further purification. Deionized water (18.2 MΩ.cm) was obtained from a Milli-Q water purification system (Millipore).

Preparation Method Undoped ZnO As described previously (20), zinc nitrate hexahydrate was mixed with cyclohexylamine in water in 1:2 mol ratio at room temperature to prepare undoped ZnO precipitate, which was calcined at 500°C for three hours.

CuO-Doped ZnO Calculated amounts of copper nitrate trihydrate, zinc nitrate hexahydrate, and cyclohexylamine were mixed according to the mol ratios as shown in Table I. For each mixture, metal nitrate precursors were first mixed and dissolved in 500 ml of deionized water at room temperature, under continuous magnetic stirring. The addition of cylcohexylamine resulted in a very light bule precipitate. Depth of blue color increased with increasing the copper content (Figure 1). The reaction mixtures were left stirring for one week. The precipitates were filtered off through F-size fritted filters, and then were copiously washed with deionized water. The precipitates were dried under vacuum for one day. After drying, the precipitates were mixed with 300 ml water and were magnetically stirred for one day for the removal of any impurity. The precipitates were filtered off, air-dried, and then calcined at 500°C for three hours. Brown solids were obtained after calcination. The depth of brown color increased with increasing the copper content (Figure 1).

329 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 1. Catalyst color depth change of the Cu2+-doped ZnO samples before (F) and after calcination (C).

Table I. Mole Ratio of Copper Nitrate to Zinc Nitrate to CHA for Preparing the Catalyst Precursors Precursor (wt %)

Cu2+:Zn2+:CHA mol ratio

1

1:117:236

2

1:47:97

3

1:31:65

4

1:23:48

Materials Characterization Inductively-coupled plasma (ICP, Varian Vista-MPX) was used to determine the copper and zinc component in the clacined CuO-doped ZnO catalysts, obtained at 500°C. X-ray diffraction (XRD) patterns were recorded for phase analysis using Philips X pert pro diffractometer, 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 1s per step. XRD patterns were recorded for Cu2+-doped ZnO materials before and after calcination. The morphology (size and shape) was investigated using a field emission scanning electron microscope (FE-SEM model: FEI-200NNL) and a high resulotion transmission electron microscope (HRTEM model: JEM-2100F JEOL). Carbon-coated copper grids were used for mounting the samples for HRTEM analysis. Elemental microanalysis of the surface was performed by energy dispersive X-ray spectroscopy (EDX), which is coupled to FE-SEM. 330 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

UV-Vis absorption spectra for Cu2+-doped ZnO materials before and after calcination were recorded on a Perkin Elmer Lambda 950 UV/Vis/NIR spectrophotometer, equiped with 150 mm snap-in integrating sphere for capturing diffuse and specular reflectance.


Photocatalytic Evaluation All the experiments were 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 at room temperature by suspending 0.02 wt% of CuO-doped ZnO catalyst sample into 300 ml, 100 ppm potassium cyanide at pH 8.5, adjusted by ammonia solution. This specific pH value was chosen on the basis of previous investigation, revealed the preferred adsorption of OH- ion over CN- ion at higher pH values (21), while hydrogen cyanide, HCN, elevates at pH≤7 according to the following equation:

The reaction was carried out isothermally at 25 °C and a sample of the reaction mixture was taken after 120 minutes. The CN- content in the solution after reaction time was analyzed by volumetric titration with AgNO3 . The removal efficiency of CN- has been measured by applying the following equation: % Removal efficiency = (Co − C)/Co × 100, where Co the original cyanide content and C the retained cyanide in solution.

Results and Discussion Catalyst Characterization The physical observation of the Cu2+-doped ZnO samples before and after calcination (Figure 1) was evident for increasing the copper content through the increase in color depth. Table II confirms that the theoretical and experimental, obtained by ICP, CuO content in the calcined samples were in good agreement. Figure 2 shows the XRD spectra of the CuO/ZnO samples with varying the copper concentration from 1 up to 4 weight percent before and after calcination. No indication of any copper secondary phases were observed upon incorporation of copper for all the samples. The absence of secondary phases could be attributed either to the complete solubility of copper within ZnO, which is higher than the reported solubility limit value of 1% (16) and the 3% (1). The low solubility was attributed to the high covalent character in Cu-O bonding, resulted from the high localization of 3d state on Cu (15). It is also likely that the high dispersion of copper phases within ZnO phase prevented its detection by XRD technique (22). However, this possibility was excluded on the basis of XRD results, explained below, and on the basis of the TEM results. The other possibility of not detecting any copper phase due to its amorphous nature was also ruled out on the basis of the TEM results, which showed the crystal planes. 331 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Table II. Theoretical and Experimental Results of CuO Content in the CuO-Doped ZnO Catalysts Sample

Theoretical (wt.%)

Experimental (wt.%)

1% CuO/ZnO

1

0.7

2% CuO/ZnO

2

1.9

3% CuO/ZnO

3

2.9

4% CuO/ZnO

4

3.9

Figure 2. XRD patterns of the Cu2+-doped ZnO samples with varying the copper concentration from 1 to 4 weight percent before and after calcination.

The shift of the ZnO peaks (for instance 100, 002, and 101) to higher 2θ is a result of replacing Zn2+ (0.06 nm) by the smaller Cu2+ ions (0.057 nm) (23) in the wurtzite lattice . The shift in the uncalcined samples was more pronounced than the calcined samples due to the decrease in defects, resulting from copper substitution upon calcinations (Figure 2). No copper phases were detected by XRD for the uncalcined or the calcined samples which confirms complete substitution of Cu2+ in the ZnO wurtzite lattice even at room temperature. The importance of having complete solubility in our CuO-doped ZnO catalyst systems is to produce a coupled system instead of having two independent ZnO and CuO composits such as those systems used in hetrojunctions (24) that may not be in harmony with each other and minimize the charge transfer from one to another. 332 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 3 (a-d) shows the SEM micrographs for the calcined CuO-doped ZnO samples. The same morphology was observed in all samples irrespective of CuO wt%. The particles are agglomerated in rice-like shape. The elemental microanalysis of the surface by EDX (Figure 4) confirmed the purity of the calcined samples and the presence of copper, zinc, and oxygen on their surfaces. However, the surfaces are rich in zinc, which is consistent with the oxygen-deficiency for the n-type ZnO.

Figure 3. SEM micrographs for the calcined CuO-doped ZnO samples a) 1 wt.%, b) 2 wt.%, c) 3 wt.%, and d) 4 wt.%.

Figure 5 shows high resolution TEM micrographs for the calcined CuO-doped ZnO samples. All the samples show similar morphology and particle size irrespective of CuO wt% doping. The particles have different shapes such as rectangular- and round-like. The average size of the particles ranges from 5-20 nm. The lattice fringes, in addition, match those of ZnO only, which supports the results obtained from XRD, indicating the replacement of Zn2+ ions by Cu2+ ions in the wurtzite lattice of ZnO. 333 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 4. EDX for calcined CuO-doped ZnO samples a) 1 wt.%, b) 2 wt.%, c) 3 wt.%, and d) 4 wt.%.

Figure 5. high resolution TEM micrographs for the calcined CuO-doped ZnO samples a) 1 wt.%, b) 2 wt.%, c) 3 wt.%, and d) 4 wt.%. 334 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 6 shows the UV-Vis absorption spectra for the copper oxide doped ZnO samples before and after calcination. The incorporation of Cu2+ is responsible for ZnO Eg reduction, i.e. red shift. As shown in Table III, a slight shift in the Eg of ZnO with increasing the content of CuO from 1 to 4 wt.% was obsereved.

Figure 6. UV-Vis absorption spectra for the copper oxide doped ZnO samples before and after calcination a) 1 wt.%, b) 2 wt.%, c) 3 wt.%, and d) 4 wt.%.

Table III. Band Gap Energy of the Cu2+-Doped ZnO Samples before and after Calcination Sample

Eg / eV (Uncalcined)

Eg / eV (Calcined)

1% CuO/ZnO

3.24

3.21

2% CuO/ZnO

3.22

3.20

3% CuO/ZnO

3.22

3.19

4% CuO/ZnO

3.21

3.19

The Egs of the unclacined and calcined samples were almost comparable. This result might imply that Cu2+ substituted Zn2+ in ZnO wurtzite lattice at room temperature, which was also supported by X-ray results. However, the small red shift in the Egs of the calcinced samples could be due to the enhancement of copper substitution upon calcinations. 335 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

The red shift in the calcined samples was less compared to the reported Eg (2.9 eV) for CuO/ZnO nanocomposite, which was prepared physically by wet impregnation method and attributed to the stoichiometry deficiency of ZnO due to impregnated CuO (13). The Eg reduction (band offsets) in our solid solution system may be attributed to the following effects (25):


1) The strong d-p coupling between copper and oxygen moves O 2p orbital up, that narrows the direct fundamental Eg of ZnO. 2) Creation of impurity energy band, especially at higher concentrations, above the ZnO valance band maximum (VBM) which creates a mixture of direct and indirect transitions. Effect of CuO Doping on Photocatalytic Activity Table IV shows that increasing the CuO content from 0 to 3 wt% led to increase in the photocatalytic activity of cyanide degradation. However, increasing the CuO content to 4 wt% did not enhance the photocatalytic activity. This finding might be due to the identical Eg of both 3% and 4 wt% CuO-doped ZnO catalysts (Table III). The enhancement in photocatalytic degradaion of cyanide ion with increasing the CuO wt.% content could be attributed to the inhibition of electron-hole pair recombination and efficient separation of the charges (26). Such easy transfer of electrons from CuO to ZnO is due to the close match of work function between CuO and ZnO (5.3eV) (13).

Table IV. Effect of CuO wt.% on Photocatalytic Activity Photocatalyst

% of cyanide degradation

ZnO

56

1% CuO/ZnO

89

2% CuO/ZnO

93

3% CuO/ZnO

97

4% CuO/ZnO

97

Conclusion Our CuO-doped ZnO solid-solution system showed enhancement in the photocatalytic performance due to Eg reduction. The narrow Eg of CuO (1.7, 1.33 eV) (26, 27) results in efficient separation of charges, which were photo-generated in the copper oxide/zinc oxide under UV-vis light illumination and suppressed their recombination. The mechanism of the photo-generated charges separation is due to their transfer between the two semiconductor materials (p-type copper oxide/n-type zinc oxide) as follows; the photogenerated electrons transfer from the conduction band (CB) of CuO to that of ZnO, while the photogenerated holes immigrate in the opposite direction from the valance band (VB) of ZnO to that 336 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

of CuO. Consequently, more electrons are accumulated in (CB) of ZnO and consumed for reduction of pollutants.

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Synthesis, Characterization, and Cyanide ... - ACS Publications

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