Accepted Manuscript Transition-metal-doped ZnO nanoparticles: synthesis, characterization and photocatalytic activity under UV light Rosari Saleh, Nadia Febiana Djaja PII: DOI: Reference:
S1386-1425(14)00500-9 http://dx.doi.org/10.1016/j.saa.2014.03.089 SAA 11922
To appear in:
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Received Date: Revised Date: Accepted Date:
27 December 2013 22 March 2014 24 March 2014
Please cite this article as: R. Saleh, N.F. Djaja, Transition-metal-doped ZnO nanoparticles: synthesis, characterization and photocatalytic activity under UV light, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.03.089
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Transition-metal-doped ZnO nanoparticles: synthesis, characterization and photocatalytic activity under UV light
Rosari Saleh1 and Nadia Febiana Djaja Departemen Fisika, Fakultas MIPA-Universitas Indonesia, 16424 Depok, Indonesia
Abstract ZnO nanoparticles doped with transition metals (Mn and Co) were prepared by a co-precipitation method. The synthesized nanoparticles were characterized using X-ray diffraction, scanning electron microscopy, energy dispersive X-rays, Fourier transform infrared spectroscopy, electron spin resonance spectroscopy and diffuse reflectance spectroscopy. The photocatalytic activities of the transition-metal-doped ZnO nanoparticles were evaluated in the degradation of methyl orange under UV irradiation. ZnO nanoparticles doped with 12 at.% of Mn and Co ions exhibited the maximum photodegradation efficiency. The experiment also demonstrated that the photodegradation efficiency of Mn-doped ZnO nanoparticles was higher than that of Co-doped ZnO nanoparticles. These results indicate that charge trapping states due to the doping were the decisive factor rather than the average particle size and energy gap. Moreover the effect of pH values on the degradation efficiency was discussed in the photocatalytic experiments using 12 at.% Mn- and Co-doped ZnO nanoparticles.
KEYWORDS: Transition-metal-doped ZnO nanoparticles, characterization, photocatalytic activity, methyl orange, UV irradiation 1
Author to whom correspondence should be addressed:
Electronic mail: [email protected]
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Tel: +62217694975; Fax: +62217515171
1. Introduction Water pollution, such as wastewater from textile or other dyestuff industries is one of the primary problems that affect the environment. The pollution is normally due to the persistent and non-biodegradable organic substances and can cause damage to the ecosystem and human health [1-4]. Traditional wastewater treatment methods [5-7] have been found to be ineffective for removing the above organic substance from textile or other industries that used dyestuff. Photocatalysis is an advanced oxidation process that is used for photodegradation of various pollutants and has gained significant attention in recent years for solving the problems of water pollution. The degradation of organic pollutants in water using a semiconductor as a photocatalyst has attracted extensive attention in the last decade. Among the various semiconductors, ZnO is widely used as an excellent material for the photocatalytic process due to the photosensitivity, the strongly oxidizing, non-toxic nature, the favorable band gap energy and the excellent chemical- and mechanical –stabilities of the material [8-9]. Photocatalytic activity occurs when the ZnO absorbs a photon with an energy equal or greater than the material’s band gap energy that results in the formation of electron and hole pairs, which can subsequently migrate to the ZnO surface and react with adsorbed molecules to generate such reactives species as H2O2, superoxide anion radicals (O2−) and hydroxyl radicals (OH) [10-11]. These species are very strongly oxidizing and highly reactive agents that can degrade an organic pollutant into harmless compound. However, ZnO has several weaknesses such as the fast recombination rate of the photogenerated electron and hole pairs and a low quantum yield in photocatalytic reactions in aqueous solutions, which can obstruct the photocatalytic degradation process . Significant effort has been focused to enhance photocatalytic activity such as the rate of electron and hole pairs induced redox reaction and the rate of electron and hole recombination. It 2
is known that the surface charge transfer processes and strong electron and hole recombination is strongly related to the structure and optical properties of the photocatalysis. Therefore to enhance the photocatalytic activity the surface charge transfer processes should be increased and the recombination rate of electron and hole should be decreased. Various methods have been developed to reduce electron−hole recombination and increased the surface charge transfer. One of the interesting approach is to doped ZnO photocatalysis with transition metal ions, which have been shown to reduce band gap energy and improve charge separation between electron and hole by forming electron traps [9, 10, 12-13]. The hole will be able to migrate towards the surface of the photocatalysis and adsorbed organic compound. Fu et al.  have confirmed that Cu-doped ZnO nanoparticles exhibited better photodegradation efficiency than undoped ZnO. Ba-Abbad et al.  have investigated the doping of Fe into ZnO matrix prepared by sol-gel technique and found that the presence of small amount of Fe ions influenced the photocatalytic activity. Xiao et al.  and Ekambaram et al.  reported the enhanced photocatalytic activity for Mn and Co dopants compared to undoped ZnO. This work deals with the synthesis of ZnO doped with transition metals (Co and Mn) by using co-precipitation method. Such method is a non-expensive and versatile approach that allows us to synthesize a great variety of crystalline metal oxides at low temperature. The doping of Co and Mn were chosen due to their comparable ionic radii to the ionic radius of Zn2+ and ease to synthesize without change the ZnO crystalline structure over a wide range of dopant concentrations. Moreover, it is known than Co and Mn incorporated into ZnO lattices can form an impurity energy level and can facilitate mobility of charge carrier. Here we reported a systematic study on the influence of Co and Mn ions doping degree on the structure, morphology and optical properties of ZnO nanoparticles. The photocatalytic oxidation of methyl orange and
methylene blue in water was observed to study the influence of transition metal ions doping degree on the performance of ZnO.
2. Experimental details 2.1 Materials Zinc (II) sulfate hepta hydrate (ZnSO4.7H2O, 99%), manganese (II) sulfate monohydrate (MnSO4.H2O, 99%), cobalt (II) chloride hexahydrate (CoCl2.6H2O, 99%), methyl orange and NaOH were all purchased from Merck.
2.2 Synthesis of Mn- and Co-doped ZnO nanoparticles Transition-metal-doped ZnO nanoparticles were synthesized as described previously . Briefly, specific amounts of ZnSO4.7H2O were mixed in distilled water with MnSO4.5H2O and CoCl2.6H2O. This solution was designated as solution A. Solution A was placed in an ultrasonic cleaner operating at 57 kHz for 2 h. Solution B was obtained by adding 44 mmol of NaOH to 440 mL of de-ionized water. After sonication, solution A was mixed with a magnetic stirrer at room temperature, and solution B was added to solution A until a pH of 12 was reached. The resulting solution was magnetically stirred for 0.5 h and then allowed to stand at room temperature for 18 h. Subsequently, the solution was centrifuged and washed several times with ethanol and distilled water to remove residual and unwanted impurities. The final product was dried in a vacuum oven at 200°C for 1 h to obtain the transition metal doped ZnO powder.
2.3 Characterization of Mn- and Co-doped ZnO nanoparticles Morphology and elemental analyses of the samples were performed by energy-dispersive X-ray (EDX) spectroscopy using a scanning electron microscope. To evaluate the phase purity of the samples, X-ray diffraction (XRD) measurements were performed using a Philips PW 1710 and monochromatic Cu-K ( = 1.54060 Å) radiation operated at 40 kV and 20 mA in the range of 10o to 80o. The optical band gap was obtained from diffuse reflectance UV-Vis measurements using a Shimadzu UV-Vis spectrophotometer with an integrating sphere and a spectral reflectance standard over a wavelength range of 200-800 nm. The infrared absorption spectra of the samples were recorded using a Shimadzu FTIR spectrophotometer in the range of 400−4000 cm-1 with a resolution of 4 cm-1. The samples were mixed with KBr powder before being pressed to form optically clear pellets. Electron spin resonance (ESR) was performed using an X-band JEOL JES-RE1X at room temperature and an X-band spectrometer equipped with a 9.1 GHz field modulation unit. The resonance was optimized for the modulation amplitude, receiver gain, time-constant and scan time. The amount of powder used in all measurements was the same. DPPH was used as the standard. The shape and area of the ESR spectra were analyzed using standard numerical methods.
2.4 Photocatalytic degradation study Analytical grade methyl orange (MO) was used as the model dye chemical. The degradation of MO in the presence of transition-metal-doped nanoparticles was performed under UV light irradiation. Approximately 20 mg of nanoparticles as a photocatalyst was added into 100 mL of an aqueous solution of MO at a constant initial concentrations of 20 mg/L. Prior to irradiation, the suspensions were sonicated in the dark for 30 min to obtain a colloidal solution.
The photocatalytic experiment was conducted at room temperature in a cylindrical glass vessel equipped with a magnetic stirrer under a UV tube light positioned horizontally 30 cm above the colloid surface. The glass vessel was illuminated by two 20 W UV lamp. The entire arrangement was put in a box sealed with aluminum foil to avoid the passage of other lights into the box. The aqueous solution of MO was subjected to a series of experimental conditions ; (i) without a catalyst in the dark (hydrolysis), (ii) without a catalyst under UV irradiation (photolysis), (iii) immersing Mn- and Co-doped ZnO nanoparticles in the dark (adsorption) and (iv) immersing Mn- and Co-doped ZnO nanoparticles under UV light (photocatalytic). The experimental conditions (i) and (ii) were performed to confirm that no reaction take place in the absence of the UV-light or in the absence of a catalyst. In the experimental condition (iii), the colloidal solutions were magnetically stirred in the dark to ensure the establishment of an absorption equilibrium of MO in the solution. Subsequently, in the experimental condition (iv), the solution was irradiated for 2 h in regular intervals. The obtained solutions were analyzed using a Dynamica UV-visible spectrophotometer with a quartz cuvette having an optical length of 10 mm. The effect of pH and catalyst dosage were also studied by treating the batches of MO solutions at pH values of 4, 7 and 13, and catalyst dosages of 0.2 − 1.0 gr/L.
3. Experimental Results 3.1 Sturctural studies The chemical compositions of Mn- and Co-doped ZnO nanoparticles were studied by EDX spectra. Representative EDX spectra of Mn- and Co-doped ZnO nanoparticles (12 at.%) are illustrated in inset Fig.1. The EDX spectra analysis confirmed the presence of doping elements in the samples. Table 1 provides the elemental analysis obtained from the EDX measurements. Quantitative characterization of the Mn/Zn and Co/Zn ratios was performed by 6
calculating the area of the corresponding spectral K lines. The amounts of Mn and Co in the ZnO nanoparticles were found to vary between 6 and 25 at.% and between 3 and 18 at.%, respectively. The results were obtained by averaging four values from different regions of a sample. Table 1 shows that the chemical compositions of the constituents obtained from EDX deviated from the target compositions within ± 2−4 at.% which is very small. The information about the microstructure and morphology of Mn- and Co-doped ZnO was obtained from SEM measurements. Fig. 1 shows SEM images of both of the samples. As shown in the figures, the morphology of the doped samples has a spherical form with a diameter that ranges from 50 to 70 nm. Typical XRD patterns of Mn- and Co- doped ZnO nanoparticles are shown in Fig. 2. In the present case all the diffraction peaks at angles 2θ of 31.4, 34.0, 35.9, 47.2, 56.3, 62.5, 67.6, and 68.8 correspond to the reflection from the (100), (002), (101), (102), (110), (103), (200) and (112) crystal planes of the hexagonal wurtzite zinc oxide structure. The XRD patterns were analyzed using MAUD programs by employing the Rietveld refinement method. The XRD patterns for all samples can be refined using the space group P63mc. Within the detection limits of this technique, the subsequent doping does not introduce any additional phase such as ZnMn2O4, MnO, MnO2, Mn2O3, Mn3O4, ZnCo2O4, CoO, and Co3O4 [20-23], which indicates that the wurtzite structure is not disturbed by doping. However, the results of these data do not mean there is an absence of Mn and Co clusters because we do not exclude the possibility of cluster formation below the detection limit of the X-ray diffractometer. Moreover the intensity and width of these peaks changed, which revealed interesting variations in the lattice parameters (i.e., a and c) and the lattice volume V. The lattice constants, calculated from Rietveld refinement using MAUD programs, are summarized in Table 1. As the doping concentration increased, the
lattice constants, which were proportional to the ionic radii, changed slightly [24-25]. The lattice constants of the doped ZnO samples are comparable with those of undoped ZnO . The variation of the aspect ratio (c/a) with the dopant concentration is also shown in Table 1. The ratio is approximately constant for all samples and has a good correlation with the ratio of undoped ZnO sample. This result suggested again that dopant atoms are incorporated in the ZnO lattices with little or no effect on the overall crystal structure. The measured d-spacings of ≈ 2.81, 2.60, 2.47, 1.91, 1.62, 1.47, 1.40, and 1.37 correspond to the reflection from the (100), (002), (101), (102), (110), (103), (200) and (112) crystal planes of the hexagonal wurtzite zinc oxide structure. The d-spacing of doped ZnO particles are found to be relatively constant with increasing doping concentrations as presented in Table 2. To study the change in the electron density due to the presence of dopants substituted into the Zn atoms, the distance between the Zn and O atoms was calculated . The results were tabulated in Table 2. It is observed that the values of the distance between the Zn and O atoms have a good correlation with the lattice parameters. The breadth of the XRD peak can be linked to the average crystallite size, microstrain and defects or dislocations. The average crystallite size using XRD measurements is not generally the same as the particle size due to powder aggregates. The average crystallite size as related to the line broadening can be calculated using Scherrer’s equation, as given in Eq. (1):
where = the volume-weighted crystallite size, = the shape factor (close to unity in our work, this value was set to 0.9), = the wavelength of Cu-Khkl[2hkl measured−2instrumental] 1/2 = instrumental corrected integral breadth of the reflection located at 2, and = the angle of
reflection. The average crystallite size for the Mn- and Co- doped ZnO calculated using Scherrer’s equation is presented in Table 2. The formation of ZnO wurtzite structures in the Mn- and Co-doped ZnO samples was further supported by FTIR measurements. Similar spectra were obtained for the undoped ZnO as well as for the Mn- and Co-doped ZnO samples (Fig.3). For all doped ZnO samples the absorption peaks in the range of 400 −700 cm-1 could be attributed to the ZnO stretching modes . Also observed were weak absorption peaks in the range of 1100 − 1600 cm-1 corresponding to the OH bending mode , C-OH plane bending and C-OH out-of-plane bending . In general, the hydrogen in oxide crystals prefers to form strong OH bonds with lattice oxygen. In ZnO, first-principles theoretical calculations and experimental studies revealed that hydrogen usually forms strong OH bonds with lattice oxygen, and the stretching vibrational modes OH bonds are responsible for the IR absorption peak observed. In this study, we observed a broad band in the 2900 − 3700 cm-1 region which can be explained as overlapping O-H stretching modes and C-H stretching modes. C-OH and C-H local vibration modes in the range of 1100 − 1600 cm-1 and 2900 − 3700 cm-1, are probably due to the atmospheric water and carbon dioxide adsorbed on nanoparticle surface . Although a similar local vibration mode have been observed in numerous semiconductors [31-33] such as GaAs  and GaN , since in some cases, carbon atoms are unintentionally incorporate during crystal growth because carbon is readily available in the growth environment .
3.2 Electron spin resonance study ESR spectroscopy is a sensitive technique for examining paramagnetic species that can provide information about the nature of the species and their coordination symmetries in the
solid. Moreover, this technique also provides valuable information about the lattice site in which a paramagnetic dopant ion is located [35-36]. Typical ESR spectra of Mn- doped ZnO nanoparticles recorded at room temperature are illustrated in Fig. 4a. A broad resonance appears in all samples at a lower field and is attributed to ferromagnetic-resonance that arises from transition within the ground state of the ferromagnetic domain [37-38] and a spectrum of hyperfine and fine lines arising from the paramagnetic states of Mn2 ions at room temperature is detected at a higher field. At lower doping concentrations, an isotropic hyperfine spectrum with six lines is clearly observed. The electronic configuration of a Mn2+ ion is 3d5 , and the electronic ground state is 6S5/2, which splits into three Kramers doublets ( 5/2, 3/2 and 1/2). In the presence of a magnetic field, the spin degeneracy of the Mn2+ ion can be completely removed by Zeeman splitting, which results in five fine-structure transitions, each split into six hyperfine lines. Due to hyperfine splitting, each of these transitions will be split into six hyperfine sublevels. As the Mn concentration increases, the contribution of the hyperfine spectrum of the isolated Mn2+ will become weaker. To determine the g-value and the number of spins associated with each line, the ESR signals were fit with two Lorentzian functions as depicted in the inset of Fig.4a. The g-values, line widths and corresponding numbers of spins participating in the production of the S1 and S2 signals are provided in Table 3. In Fig. 4b, the ESR spectra of Co-doped ZnO particles are shown. It is known that the Co2+ ion has an electronic configuration of 3d7 and that the electronic ground state is S7/2. The only natural isotope is
Co, and the ESR spectrum is expected to show eight fine transitions.
However, the ESR spectra of the Co-doped samples in the present investigation showed only a broad spectrum without hyperfine or fine structures. The ESR spectrum, which is characterized by ΔHpp of several hundreds of Gauss and a characteristic shift of the resonance field with an
increase of the concentration of doping, could be assigned to the ferromagnetic resonance of the Co ions. The broad spectrum obtained from the samples in present investigation is most likely due to the magnetic interaction between a Co2+ ion and another Co2+ ion in the nearest neighbor . It is believed that a shift of the spectra to the lower resonance field is due to the presence of an inhomogeneous local magnetic field in the long-range ferromagnetic exchange interaction. By fitting all spectra in the entire range of the magnetic field, the g-value, ΔHpp and the number of spin contributing to the signal can be found. The results are illustrated in Table 3.
3.3 Optical properties To study the electronic interactions near the optical band gap region due to the presence of dopants and to obtain the energy gap, diffuse-reflectance measurements were performed on the samples in the UV-Vis region at room temperature. The band gap energies of all doped ZnO samples were calculated from the diffuse-reflectance spectra by performing a Kubelka-Munk analysis using the following equation: F(R) = (1-R)2/ 2 R
with R as the diffuse reflectance. According to this function the band gap energy can be obtained by plotting the F(R)2 vs. the energy in electron volts. The linear part of the curve was extrapolated to F(R)2 = 0 to calculate the direct band gap energy. Fig. 5 shows the band gap for all samples as a function of the doping concentrations. The absorption edge shifts to lower energies/longer wavelength. In this case a red shift upon increasing the doping concentration has been attributed to sp−d exchange interactions between the band electrons and the localized d electrons of the substituted dopant ions [40-43].
3.4 Photocatalytic activity 3.4.1 Effect of the doping concentration The photocatalytic activity of Mn- and Co-doped ZnO nanoparticles has been evaluated by photocatalytic degradation of MO in aqueous solutions under UV irradiation. The absorption spectra of irradiated samples were recorded at various time intervals and the rate of decolorization of MO was observed in terms of the change in the intensity at 464 nm. To study the enhancement of the photoactivity, comparative experiments were deployed. The experimental results demonstrated that the concentrations of MO revealed no degradation or reduction after 2h in the absence of doped nanoparticles or light irradiation. It is suggested that the hydrolysis and photochemical processes can be neglected and that the photocatalytic experiment occurred in a pure photocatalytic regime . To understand the response of the immersed Mn- and Co-doped ZnO nanoparticles on the percentage of degradation of MO and MB, the degradation rate of MO has been followed under UV irradiation light at various time intervals with a UV-Vis spectrophotometer. The results were compared with the MO degradation result with immersed Mn- and Co-doped ZnO nanoparticles in the dark. Fig. 6 shows the degradation of the MO efficiency as a function of irradiation time for the presence of Mn- and Co-doped ZnO nanoparticles with different doping concentrations. In the two figures, C0 and Ct are the initial concentration of MO before and after light irradiation, respectively. Several researchers have reported that the kinetic behavior of a photocatalytic reaction can be described by a pseudo-first order model. The photocatalytic activity of all doped ZnO nanoparticles in this study obeys the pseudo-first-order reaction kinetics. The apparent reaction rate constants k of the MO degradation using different doped ZnO nanoparticles were summarized in Table 4. As seen from Table 4 there is no significant different in the apparent reaction rate constants of MO. The
apparent reaction constants follow the order 12 at.% Mn (0.0139 min-1) > 9 at.% Mn (0.0116 min-1) > 6 at.% Mn (0.0099 min-1) > 25 at.% Mn (0.0078 min-1) and 12 at.% Co (0.0087 min-1) > 6 at.% Co ( 0.0070 min-1) > 3 at.% Co (0.0063 min-1) > 18 at.% Co ( 0.0043 min-1). It is clearly seen from Fig. 6 that for different doping concentrations of ZnO nanoparticles the photocatalytic activity firstly increases with increasing dopant content and then trends downtoward with further increases of doping concentrations. The optimal doping level to obtaining the highest photocatalytic activity for the degradation of MO is approximately 12 at.% for all doped ZnO nanoparticles. Compared to undoped ZnO, the doped ZnO at an optimum dopant concentration showed considerably enhanced activity (Fig. 7). These results indicated that the localized electronic states of the dopant served as charge carrier traps for photogenerated charge carriers under UV irradiation . Comparing the activity of the doped ZnO samples, the maximum photodegradation efficiencies were obtained as follows: Mn-doped ZnO > Co-doped ZnO, which indicated that different types of transition metal ions offer a different way to trap charge carriers and extend the lifetime of one or both of the charge carriers because the incorporation of dopant in the ZnO matrix may not have the same position . On the basis of the ESR results, it was confirmed that the incorporation of transition metal ions in the ZnO matrix is not in the same position. Therefore, it is understandable if the photocatalytic efficiency is different for different type of dopants [45-47]. Among the dopants in this study, the Mn2+ ion decolorizes the MO most efficiently. It is known that dopant incorporated in a ZnO matrix can serve as a trap for an electron if the electronic state or energy of that electron is just below the conduction band or for a hole if the electronic state or energy of that hole is just above the valence band [44, 48-49]. According to the literature, the trapped electron or hole will be migrated to the catalyst surfaces where it
will participate in a redox reaction with the dye molecules, thereby suppressing the electron and hole recombination and hence substantially increasing the photodegradation efficiency [12, 44, 50]. From the photocatalytic activity results, it is evident that the presence of an appropriate amount of dopant ions can indeed enhance the photocatalytic activity; however, excessive dopant ions are detrimental. Chauhan et al.  explained that intrinsic point defects could be induced as the doping concentration is increased, which may serve as recombination centers and lead to quenching of the photocatalytic activity. Furthermore, it is suggested that the different photocatalytic activity behaviors also depend on several factors such as the average crystallite size , the band gap  and the specific surface area [54-55]. According to the XRD and UV-Vis measurements in the present study, Mn- and Co- ions dopants in the ZnO nanoparticles led to reduce average crystallite sizes and narrower of band gaps. It is reported that the electron and hole recombinations could be divided into volume and surface recombinations. In large crystallite size particles, volume recombination is dominant and can be inhibited by decreasing the particle size . In addition, a decrease in the crystallite size will raise the specific surface area and increases the surface active site, which consequently leads to a higher interfacial charge carrier transfer for photocatalysis . However, in the present study, the photodegradation constant rate k decreased as the dopant concentration rose higher than the optimal values although the average crystallite size is further decreased. Furthermore, the variation of the average crystallite size between the optimal doping concentrations (12 at.%) compared to the ZnO nanoparticles doped with higher doping concentrations (> 12 at.%) is relatively unchanged. Therefore, the average crystallite size may play an important role but is not a decisive factor.
From the UV-Vis spectra, it was observed that the energy gap of the doped ZnO samples decreased with increasing dopant concentrations. This behavior suggested that the sp-d spin exchange interactions between the band electrons and the localized d-electrons of the transition metal ion that substitutes the cation form new electron states in the gap [57-58], and can serve as trapping sites for the charge carriers. The results of the present study show that dopant ions cause a red shift in the band gap transition of all samples. This shift can induce more photogenerated electron and hole pairs to be involved in the photocatalytic reactions. However, it is again observed that the photodegradation constant rate k decreased as the dopant concentrations exceeded 12 at.% although the energy gap is further decreased. Moreover, the degradation of the MO efficiency as a function of irradiation time for the presence of Mn- and Co-doped ZnO nanoparticles with different doping concentrations under visible light irradiation is clearly lower compared to the MO degradation efficiency under UV light irradiation (Fig. 8). These results suggested that the band gap narrowing resulting from transition metal doping may not always be a favorable factor for the enhancement of photocatalytic activity. Based on the structural and morphology characterizations and properties of the doped ZnO catalyst, a plausible mechanism for the photocatalytic activity is as follows: under UV light irradiation, the doped ZnO catalyst absorbed UV light and was excited. Photogenerated electrons and holes accumulated on the conduction and valence band tended to transfer to the adsorbed MO molecule on the particle surface, enter the molecular structure of MO and completely decompose the MO [12, 44]. However, not all photogenerated electrons will be excited to the conduction band; some of the electrons will be transferred to electron-trapping states of the dopant, which effectively separate the electrons and holes and suppress the probability of electron-hole recombination. In this process, the electrons collected at the trapping states can
later react with the absorbed molecular oxygen to yield O2−. It is believed that the leftover holes interacting with surface-bound OH or water can generate OH radicals and might be used for oxidation of other MO molecules. In this study, it is apparent from the ESR spectra that there is an increase in total number of paramagnetic centers with increasing doping concentrations; consequently, the probability that photogenerated electrons are trapped in these centers was higher, and the separation of electrons is more efficient. This phenomenon might be one of the reasons why the photocatalytic activity increases with increasing doping concentrations in this study. The other possible reasons were the crystallite size and the narrowing of the band gap. These three factors might be synergized and give the best combination at doping concentrations of 12 at.%. According to Devi et al. , for highly doping concentrations, the trap centers might be close to one another so that the trapped charge carriers can recombine through quantum tunneling; as a result the electron hole recombination can occur, and the photocatalytic activity will be decreased. Therefore, we believed that charge trapping states due to the doping were the decisive factor rather than the average particle size and the energy gap.
3.4.2 Effect of the photocatalyst dosage on the degradation of MO The influence of the photocatalyst dosage on the degradation of MO using Mn and Co was studied by varying the amount of photocatalyst between 0.2 and 1.0 g/L. In this study the effect of the dosage on the degradation efficiency was studied over 12 at.% of Mn- and Codoped ZnO photocatalyst with an irradiation time of 120 min, for pH value and at an initial MO concentration of 7 and 20 mg/L, respectively. The results shown in Fig. 9 suggested that increasing the amount of photocatalyst from 0.2 gr/L to 0.6 gr/L results in an increase in the degradation efficiency of MO from 84% to 91% and from 67% to 71% for Mn- and Co-doped
ZnO. However, as the photocatalyst dosage was further increased, the degradation efficiency tends to decrease. The limit of the photocatalyst concentration that must be used for the optimum photodegradation was also observed by Konstantinou and Albanis , Akpan and Hameed  and Alwash et al. . The reason generally advanced for this result is that increasing the amount of photocatalyst increases in the available surface area or the number of active sites on the photocatalysist surface. Consequently, the number of hydroxyl and superoxide radicals will be increased. However, when the amount of photocatalyst is increased above the optimum value, the degradation efficiency decreased due to the increased the opacity of the suspension; therefore, the interception of the light by the suspension or the light scattering will be increased. As a result, the illumination will be prevented, and part of the photocatalyst surface becomes unavailable for light absorption .
3.4.3 Effect of pH on the degradation of MO Heterogeneous catalysts, such as ZnO have been found to be pH dependent and consequently could affect the adsorption and degradation of dyes [63-65]. Therefore, it is important to study the role of pH on the degradation of dyes. ZnO is an amphoteric oxide and is therefore chemically stable in the pH range of 4 – 14 and can be dissolved both in acidic and alkaline environments. Therefore, in this study, the effect of pH values on the degradation efficiency was studied for the pH values of 4, 7 and 13 at initial MO concentrations of 20 mg/L. The degradation of MO was studied the optimal dopant concentration, namely 12 at.%. The influence of the pH value on photocatalytic degradation is depicted in Fig. 10. The photocatalytic degradation curve of MO is shown as a function of the time of irradiation with a UV light in the solution with different pH values. Note that all curves are following a pseudo-first-order reaction
kinetics model. The apparent reaction rate constants, k, are shown in Table 5. It is found that the k of MO decreased with increasing pH values. The inset of Fig. 10 shows the degradation efficiency of MO. The degradation efficiency of MO is strongly favored in acidic pH values. The degradation efficienies of 93 and 73% were obtained at pH 4.0 followed by 84 and 67% at pH 7.0 and 23 and 13% at pH 11 for Mn- and Co-doped ZnO catalysts, respectively. The degradation efficiency of MO is strongly favored in acidic pHs. A similar result was also found by Nam et al.  and Zhang et al. . These researchers reported that the acidic solutions produced the highest degradation rate of MO and gave the best photocatalytic activities. It is known [60, 68] that the interpretation of the influence of pH on the photodegradation efficiency of dyes is a very difficult task because it is related to multiple factors such as the ionization state of the surface, the acid base property of the surface particles and reactant dyes and their products. As shown in Fig. 10, an acidic pH alone under irradiated with UV light resulted in degradation efficiencies of approximately 13%, 10%, and 3% for pH 4, 7 and 13, respectively. Zhang et al.  reported that acid solutions were more effective in degrading MO than neutral solutions. These researchers argued that sodium ions that dissociated from MO could react with adsorbed OH− ions and generate ONa which can reduce the amount of OH. They also explained that with increasing pH, the MO structure could change from quinone to azo that is more difficult to degrade. In ZnO base materials, it was reported that the zero point charge is 9.0 ± 0.3 [65, 68]; the surface of the particles is negatively charged by adsorbed OH− ions above this value and is positively charged below that value. Furthermore, because MO is an anionic dye, it was conceivable that the surface becomes negatively charged at lower pH value. These opposite charges between the surface of MO and the photocatalyst lead to an enhancement of the
efficiency of the photocatalytic activity due to the increase in their induced interaction. In addition, positively charged surface of particles assist the migration of photoinduced electrons, which could react with adsorbed O2 to produce O2−. Simultaneously, this process could also inhibit electron hole recombination and generate more OH through reactions of the holes with water. Therefore, both O2− and OH radical ions could be responsible for the enhancement of the photocatalytic activity. In the alkaline solution, the Coulombic repulsion emerges between the negatively charged surface of MO and the negatively charged photocatalyst surface, which limits the diffusion of the surface generated OH radicals towards the MO anions for the subsequent reaction [59, 69-70]. This result is one of the possible explanations for the optimum pH value for the photocatalytic reaction in this study.
Conclusions Zinc oxide nanoparticles doped with manganese and cobalt were synthesized by a coprecipitation method and were used as a catalyst in the process of photodegradation of methyl orange as a dye model. Doping of ZnO with manganese and cobalt results in an enhanced photodegradation efficiency. Based on the experimental results obtained in this study, the photodegradation efficiency was influenced by different reaction parameters such as the type of dopant, the amount of dopant and the pH values. The maximum photodegradation efficiency for methyl orange was obtained with a catalyst ZnO loading of 12 at.% of Mn and a pH value of 4.
Acknowlegments This work was financially supported in part by funds of Hibah Riset Utama Universitas Indonesia under contract 0975/H2.R12/HKP.05.00/2013 (Indonesia)
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Figure Caption Fig. 1.a FESEM Image of Co doped ZnO nanoparticles. The inset shows the related EDX spectra. Fig. 1.b FESEM Image of Mn doped ZnO nanoparticles. The inset shows the related EDX spectra. Fig. 2 XRD patterns of the Co- and Mn-doped ZnO nanoparticles synthesized with different concentrations of Co and Mn. The XRD pattern of undoped ZnO nanoparticles was also shown. Fig. 3 FTIR spectra of the Co- and Mn-doped ZnO nanoparticles at various doping concentrations. The spectra are shifted vertically for clarity. The FTIR spectra of undoped ZnO nanoparticles was also shown. Fig. 4.a Experimental and simulated ESR spectra of the Mn-doped ZnO nanoparticles prepared at various Mn concentrations. The spectra are deconvoluted into two signals: S1 signal (dotted line) and S2 (dashed line) signal Fig. 4.b Experimental ESR spectra of the Co-doped ZnO nanoparticles prepared at various Co concentrations. Fig. 5 Diffuse reflectance spectra of Co- and Mn -doped ZnO nanoparticles synthesized with various doping concentrations. The diffuse reflectance spectra of undoped ZnO nanoparticles was also shown. The inset shows the correlated optical gap of Co- and Mn-doped ZnO nanoparticles as a function of doping concentration. Fig. 6 Photodegradation of methyl orange with respect to the irradiation time using the Co- and Mn- doped ZnO nanopartilces exposed to the UV light Fig. 7 Photodegradation of methyl orange by Co- and Mn-doped ZnO 12 at.% nanoparticles. For comparison, photodegradation of undoped ZnO was also shown.
Fig. 8 Photodegradation of methyl orange with respect to the irradiation time using the Co- and Mn- doped ZnO nanopartilces exposed to the visible light Fig. 9 Photodegradation of methyl orange by Co- and Mn-doped ZnO nanoparticles as a function of UV light irradiation time with different loadings of catalyts. Fig. 10 Photodegradation of methyl orange by Co- and Mn-doped ZnO nanoparticles as a function of UV light irradiation time in acid, neutral, and alkaline medium. The inset shows the decolorization rate (%) of methyl orange by both of samples after UV irradiation for 120 minutes.
Table Caption Table 1. Rietveld refined XRD data, the lattice volume V, and the aspect ratio (c/a) of Co- and Mn- doped ZnO nanoparticles Table 2. The average crystallite size, d-spacing, and bond length of Co- and Mn- doped ZnO nanoparticles Table 3. The optical band gap, g-values, line width, and peak area of the deconvoluted ESR signals as a function of doping concentration of Co- and Mn- doped ZnO nanoparticles Table 4. The apparent reaction rate constants of Co- and Mn-doped ZnO nanoparticles at various doping concentrations Table 5. The apparent reaction rate constants of Co- and Mn-doped ZnO 12 at.% nanoparticles in acid, neutral, and alkaline medium
Table. 1. Saleh et al.
Co doped ZnO
Mn doped ZnO
Table. 2. Saleh et al.
Mn doped ZnO
Table. 3. Saleh et al.
Signal 1 Sample
Co doped ZnO
Mn doped ZnO
at.% Eg (eV)
ΔHPP Area (x106)
Table. 4. Saleh et al.
Co doped ZnO
Mn doped ZnO
3 6 12 18 6 9 12 25
0.0063 0.0070 0.0087 0.0043 0.0099 0.0116 0.0139 0.0078
Table. 5. Saleh et al.
Mn doped ZnO 12 at.%
Co doped ZnO 12 at.%
0.0009 0.0008 0.0002
0.0185 0.0139 0.0019
0.0100 0.0087 0.0013
Acidic Neutral Alkaline
►We synthesized Co-and Mn-doped ZnO nanoparticles with wurtzite structure using coprecipitation. ►Effects of dopant contents on the structural, optical properties, spin resonance study and photocatalytic activity under UV irradiation of ZnO particles were investigated and correlated. ►The results show that the degradation efficiency of Mndoped ZnO higher than that of Co-doped ZnO.