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Enhanced photocatalytic activity of Co doped ZnO nanodisks and nanorods prepared by a facile wet chemical method Sini Kuriakose,a Biswarup Satpatib and Satyabrata Mohapatra*a Cobalt doped ZnO nanodisks and nanorods were synthesized by a facile wet chemical method and well characterized by X-ray diffraction, field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM) with energy dispersive X-ray spectroscopy, photoluminescence spectroscopy, Raman spectroscopy and UV-visible absorption spectroscopy. The photocatalytic activities were evaluated for sunlight driven degradation of an aqueous methylene blue (MB) solution. The results showed that Co doped ZnO nanodisks and nanorods exhibit highly enhanced photocatalytic activity, as
Received 26th March 2014, Accepted 9th May 2014
compared to pure ZnO nanodisks and nanorods. The enhanced photocatalytic activities of Co doped ZnO
DOI: 10.1039/c4cp01315h
improved charge separation efficiency due to optimal Co doping which inhibit recombination of
nanostructures were attributed to the combined effects of enhanced surface area of ZnO nanodisks and photogenerated charge carriers. The possible mechanism for the enhanced photocatalytic activity of Co
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doped ZnO nanostructures is tentatively proposed.
1. Introduction Semiconductor photocatalysis has attracted significant attention as a potential solution for environmental remediation, especially for the degradation of toxic organic pollutants.1,2 Among the various semiconductor photocatalysts, ZnO has been widely used for degradation of organic pollutants. ZnO with large excitonic binding energy (60 meV) and wide band gap (3.37 eV)3–6 is suitable for a wide range of applications7 including UV lasers,8 UV detectors,9 field effect transistors,10 dye sensitized solar cells,11–14 surface enhanced Raman spectroscopy (SERS),15 lighting applications,16 gas sensors,17 and photocatalysis.18–23 The potential use of ZnO nanostructures with various morphologies such as nanoparticles, nano/microrods, nanoballs, nanoflowers for photocatalytic degradation of organic dyes has attracted increased attention in recent years.24–27 This is mainly due to their high photocatalytic efficiency, non-toxic nature, low cost and their ability to utilize abundantly available sunlight.28,29 However, ZnO suffers from the drawback of rapid recombination of photoexcited electrons and holes which deteriorates its photocatalytic efficiency.30,31 Since the photocatalytic efficiency depends on the efficient transfer and separation of photogenerated charge carriers, it is important to suppress their recombination for achieving higher efficiency. This can be done by either modifying a
School of Basic and Applied Sciences, Guru Gobind Singh Indraprastha University, Dwarka, New Delhi 110078, India. E-mail: [email protected] b Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India
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the surface of ZnO nanostructures or by doping with transition metals which could modify the electronic energy band structure of ZnO. Doping of ZnO nanostructures with transition metals has found applications in spin valves, magnetic sensors, spin light emitting diodes and non-volatile memory devices,32,33 optoelectronics,34 and photocatalysis. Doping of ZnO nanostructures with transition metals improves their photocatalytic activity as the metals act as trap sites by capturing the photogenerated electrons or holes from ZnO and thus restrain their recombination.35 Various transition metals such as Mn, Fe, Ni, Cu have been used for doping of ZnO.36–39 Among these Co is considered to be the most effective dopant for tuning the electronic and optical properties of ZnO.40 Lu et al.31 have synthesized photocatalytically active Co doped ZnO nanorods by a facile hydrothermal process which could degrade 93% of alizarin red dye in 60 minutes. They have shown that the incorporation of Co not only promoted the charge separation and enhanced the charge transfer ability but also effectively inhibited the recombination of photogenerated charge carriers in ZnO, resulting in high visible light photocatalytic activity. Xiao et al.41 synthesised Co doped ZnO by a hydrothermal method with enhanced visible light photocatalytic activity as compared to undoped ZnO, degrading 100% methylene blue (MB) dye in 300 minutes. They showed that the larger the content of oxygen vacancies and defects in ZnO, the higher is the photocatalytic activity. In this paper, we report the facile synthesis of Co doped ZnO nanostructures (nanorods and nanodisks) with highly enhanced photocatalytic activity for sunlight driven degradation of MB
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dye, degrading 99% of the dye in just 8 minutes. Even though some work has been done on Co doped ZnO photocatalysis, such high photocatalytic efficiency has not been reported till now. We have demonstrated that the photocatalytic efficiency of Co doped ZnO nanostructures depends on their crystallinity and morphology and is highest for an optimum Co doping.
2. Experimental section 2.1.
Materials
Zinc chloride (ZnCl2) and hexamethylenetetramine (HMTA) were used as the starting materials for synthesis of ZnO nanostructures. Cobalt chloride hexahydrate (CoCl26H2O) was used as the precursor for doping of Co into ZnO nanostructures. Zinc chloride, cobalt chloride and HMTA were purchased from Merck, while methylene blue (MB) was purchased from SRL, India. All the chemicals were of analytical grade and were used as received without any further purification. 2.2.
Synthesis of ZnO and Co doped ZnO nanostructures
ZnO nanostructures were synthesized by a simple wet chemical method using aqueous solutions of ZnCl2 and HMTA. In a typical synthesis, a calculated amount of ZnCl2 was dissolved in 50 mL double distilled water under stirring. HMTA taken in 1 : 1 ratio with Zn2+ ions was dissolved in 50 mL double distilled water and added to the aqueous ZnCl2 solution to attain a molarity of 0.02 M. The mixture was continuously stirred and heated at 95 1C for 3 h, after which it was allowed to cool down and left undisturbed overnight. The white precipitates formed were centrifuged and thoroughly washed by repeated centrifugation– redispersion with double distilled water. The white precipitates collected were dried in an air oven maintained at 80 1C for 20 h. The solid white powder obtained was used for characterization and photocatalytic studies. For synthesis of Co doped ZnO nanostructures, appropriate amounts of CoCl26H2O were added into the aqueous ZnCl2 solution, followed by addition of HMTA in a 1 : 1 ratio with Zn2+ ions and the above procedures were repeated. The relative concentration of CoCl26H2O in the mixture was varied for different levels of Co doping in ZnO nanostructures. The undoped ZnO sample and samples prepared with 2.5%, 5% and 10% Co in ZnO are hereafter referred to as PZ, CZ1, CZ2 and CZ3, respectively.
Fischione (model 3000). The compositional analysis was performed by energy dispersive X-ray spectroscopy (EDS, EDAX Instruments) using a Tecnai G2 F30. Energy-filtered TEM (EFTEM) measurements were carried out using a GIF Quantum SE (model 963). The sample was dispersed in ethanol using an ultrasonic bath, coated on a carbon coated Cu grid, dried, and used for TEM measurements. Raman spectra were recorded using a Horiba Jobin Yvon LabRAM using argon laser (488 nm) of spot size 1 mm. The optical properties of the samples were studied by UV-visible absorption spectroscopy and photoluminescence (PL) spectroscopy at room temperature. The powder samples were dispersed in double distilled water by sonication and their optical properties were studied by UV-visible absorption spectroscopy in the wavelength range of 200–800 nm using a HITACHI U-3300 spectrophotometer, with double distilled water as the reference medium. PL studies were performed on the powder samples using a xenon lamp (lex = 325 nm). 2.4.
Photocatalytic measurements
The photocatalytic performance of ZnO nanostructures and ZnO:Co nanostructures were evaluated by the degradation of methylene blue (MB) dye under sunlight irradiation. For the photocatalytic studies, photocatalysts typically 5 mg of as-synthesized ZnO and ZnO:Co nanostructures were ultrasonically dispersed in 5 mL distilled water in glass vials, which were used as the reactors. A 10 mM MB dye solution was added to the photocatalysts and thoroughly mixed and kept in the dark for 30 minutes to reach the adsorption–desorption equilibrium. The reaction suspensions containing MB and nanostructured ZnO and ZnO:Co photocatalysts were irradiated with sunlight for different times (2, 4, 6 and 8 minutes) with intermittent shaking for uniform mixing of the photocatalysts with the MB dye solutions. These experiments were carried out at mid day (between 12.00 and 1.00 pm) during the peak summer season ensuring irradiation with sunlight of maximum luminosity. Then the suspensions were centrifuged and the photocatalysts were removed from the suspension. The concentrations of the MB dye in the resultant solutions were monitored by UV-visible absorption spectroscopy in the wavelength range of 200–800 nm, with double distilled water as the reference medium. The photocatalytic efficiency of the photocatalysts for the degradation of the MB dye was calculated using the following formula: Z¼
2.3.
Characterization of photocatalysts
The structural properties of the samples were determined by powder X-ray diffraction (XRD) at room temperature using a Panalytical X’pert Pro diffractometer with Cu Ka radiation (l = 0.1542 nm). The surface morphology of the samples was studied using field emission scanning electron microscopy (FESEM) [MIRA, TESCAN]. Transmission electron microscopy (TEM) investigation was carried out using a FEI Tecnai G2 F30, S-TWIN microscope operating at 300 kV. High-angle annular dark field scanning transmission electron microscopy (STEM-HAADF) was employed here using the same microscope, which is equipped with a scanning unit and a HAADF detector from
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C0 C C0
(1)
where C0 is the absorbance of aqueous 10 mM MB dye solution before addition of any photocatalyst and sunlight exposure and C is the absorbance of MB in reaction suspensions with the photocatalyst following sunlight exposure for time t.
3. Results and discussion 3.1.
Structural properties
Fig. 1 shows the XRD patterns of as-synthesized samples PZ, CZ1, CZ2 and CZ3 on a log scale. The observed peaks in the spectra can be well indexed to the hexagonal wurtzite structure
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Fig. 1 XRD peaks of undoped ZnO and Co doped ZnO shown on a log scale.
of crystalline ZnO [JCPDS no. 36-1451]. No extra peaks related to any impurity or cobalt oxide were observed within the detection limit of the instrument, which confirms that the as-synthesized products are pure wurtzite ZnO. Addition of a Co dopant shows no significant shift in the (101) peak because the ionic radii of Co2+ and Zn2+ are similar (Zn2+ E 0.60 Å, Co2+ E 0.56 Å) and any expected shift in the peak would be beyond the resolution of our diffractometer.42 The micro-strain acting on the nanostructure was analysed using the Williamson–Hall method.43 The Williamson–Hall equation is given as b cosðyÞ 1 sinðyÞ ¼ þ ehkl l Dhkl l
(2)
where y is the diffraction angle, b is the full width at half maxima (FWHM) of the XRD peak, l is the wavelength of the X-rays, Dhkl is the effective crystallite size and ehkl is the microstrain. The strain obtained for the samples are 0.00039, 0.00057, 0.0012, 0.00125 for PZ, CZ1, CZ2 and CZ3 respectively. Positive values indicate tensile strain in ZnO lattice. The surface morphology of the as-synthesized samples was investigated using FESEM. Fig. 2(a–d) show FESEM images of samples PZ, CZ1, CZ2 and CZ3, respectively. The FESEM images clearly reveal the presence of a large number of nanodisk structures along with few nanorods. Fig. 3(a) presents the TEM image of Co-doped nanorods and nonodisks. The selected area electron diffraction (SAED) pattern from a region marked by a dotted circle is shown in Fig. 3(b), which indicates the single crystal nature of the ZnO nanodisk. It was indexed using the lattice parameters of hexagonal close packed (hcp) ZnO (lattice parameters, a = 3.253 Å and c = 5.213 Å). Some of the measured inter-planar spacings (d-spacing) from the SAED pattern are 2.87 Å, 2.61 Å, and 2.48 Å. These measured d-spacings are very close to the (010), (002) and (011) inter-planar spacings of hexagonal ZnO (JCPDS # 36-1451). The HRTEM image in Fig. 3(c) from a dotted yellow box region or the Fourier-filtered image in the inset from a dotted box in Fig. 3(c) clearly showing lattice fringes indicates the crystalline phase with a d-spacing of 2.48 Å. To investigate the chemical
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Fig. 2 FESEM images of (a) undoped ZnO and Co doped ZnO samples (b) CZ1, (c) CZ2 (d) CZ3.
Fig. 3 (a) Low magnification TEM image, (b) selected area electron diffraction (SAED) from a region marked by a dotted circle in (a), (c) HRTEM image showing lattice fringes and in the inset Fourier filtered image from a region marked by a dotted box in (c). (d) STEM-HAADF image and EDX spectra in the inset from a region marked by area 1, (e) relative thickness map, (f and g) profile of thickness from area 1 and 2 in (e) indicating the presence of nanorod with round morphology and nanodisk with flat top morphology.
composition of the nanorods and nanodisks, we have performed high-angle annular dark field (HAADF) analysis. Fig. 3(d) shows the STEM-HAADF image, and in the inset showing EDX spectra indicating the presence of Zn and O, Co was not detectable.
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Table 1
Fig. 4 UV-visible absorption spectra of undoped ZnO and Co doped ZnO samples.
The observed Cu and C signal is due to the C-coated Cu grid. The thicknesses of the nanorods and nanodisks were measured using a log-ratio method (Fig. 3(e)). We obtained the relative thickness (thickness/mean free path) map using EFTEM and the corresponding area profile for a nanorod and a nanodisk are shown in Fig. 3(f) and (g), respectively. Nanorods have nearly round morphology whereas nanodisks show flat-top morphology, consistent with the FESEM study. 3.2.
Optical properties
The optical absorption spectra are shown in Fig. 4, which show a small peak in the UV region for all the samples. For the Co doped ZnO, we observe a UV peak and a broad absorption band in the visible region between 400 nm and 520 nm, which can be assigned to the incorporation of Co into the ZnO lattice. The band gaps obtained from the absorption spectra are 3.36 eV, 3.375 eV, 3.377 eV and 3.377 eV for PZ, CZ1, CZ2 and CZ3 respectively.
Fig. 5
Deconvoluted PL peaks for samples PZ, CZ1, CZ2 and CZ3
Sample
Peak I (nm)
Peak II (nm)
Peak III (nm)
Peak IV (nm)
PZ CZ1 CZ2 CZ3
379.6 373.6 374.5 374.3
387.1 398.1 389.3 387.7
412.5 420.6 414.9 414.8
531.9 543.1 518.8 513.2
Fig. 5(a) shows the PL spectra of undoped ZnO and Co doped ZnO nanostructures. The PL spectra are deconvoluted into four main peaks (summarized in Table 1). The first peak as shown in Fig. 5(a) is assigned to the near band edge emission. It is clear that due to Co doping the near band edge emission is blue shifted and the band gap increases. The combined effect of optical transition to the excitonic state of ZnO and electronic transitions involving crystal-field split 3d levels in Co2+ ions substituting Zn2+ ions is responsible for the observed blue shift.44 This further proves that doped Co2+ ions have substituted Zn2+ ions in the ZnO lattice. By comparing the UV-visible and PL spectra, it can be seen that the band gap obtained from PL is lower than that obtained from UV visible spectra. This shift is defined as Stokes shift and has been ascribed to lattice vibration, exciton binding energy, localization of charge carriers due to interface defects or point defects and electron–phonon coupling.45,46 The peaks at around 390 nm and 415 nm correspond to the emission due to Zn interstitial (Zni) to valence band transition.47,48 In the case of sample CZ1, the peak at 420 nm is due to transition between radiation defects, related to the interface traps excited at grain boundaries and the valence band.49 The PL peak at around 521 nm mainly results from oxygen vacancies (VO ).41 It can also be noted that the peak at 544 nm is generated by the conduction band to interstitial oxygen transition (Oi) (assuming that the interstitial oxygen level is 1.09 eV above the VB).50 It can be observed that as compared to the undoped sample, the intensity of the defect peak increases with Co doping which
(a) PL spectra of undoped ZnO and Co doped ZnO samples and (b) schematic band diagram based on PL data.
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has already been reported in some earlier studies.41,44 An increase in the intensity of the defect peak (521 nm and 544 nm) indicates the increase in the density of oxygen vacancies and oxygen interstitials. In the photocatalytic degradation process, the increase in efficiency is associated with efficient separation of photogenerated electrons and holes. If a surface defect state is able to trap electrons or holes, recombination can be restricted. Two impurity levels which are generated due to oxygen vacancies and oxygen interstitials can act as scavengers for electrons and holes. Oxygen vacancies act as electron accepters to trap electrons and interstitial oxygen act as shallow trappers for holes, both of which prevent the recombination of photogenerated electrons and holes, thereby increasing the efficiency. Thus in our case Co doped ZnO is more efficient than undoped ZnO.51,52 Xiao et al.41 have shown the dependence of photocatalytic degradation efficiency on the surface defects and that an optimum concentration of Co doping leads to the highest photocatalytic degradation efficiency. In our case also, CZ2 with 5% Co doping shows the highest photocatalytic degradation efficiency. The schematic band diagram of the emission as obtained from the data is given in Fig. 5(b). The band edge emission from the conduction band to the valence band is 3.26 eV. The transition from the trap state to the valence band is 2.95 eV and from zinc interstitial to the valence band is 2.98 eV. The 2.38 eV emission mainly results from oxygen vacancies. The oxygen interstitial level is located at 2.28 eV below the conduction band.5 Fig. 6 shows the Raman spectra for ZnO and Co doped ZnO nanostructures. The zone centre optical phonon of wurtzite type ZnO which belongs to the C6v space group has an irreducible representation A1 + 2B1 + E1 + 2E2.53 The A1 and E1 modes are polar and hence exhibit two frequencies for transverse optical (TO) and longitudinal optical (LO) phonon modes. The B1 modes are silent in Raman scattering.54 The non-polar E2 mode has two frequencies E(high) associated with the motion of oxygen atoms 2 and E(low) associated with the motion of the Zn sub-lattice.55 The 2 backscattering Raman spectra for all the samples ranging from 200 to 800 cm1 are shown in Fig. 6. For ZnO the strongest peak at B437 cm1 is attributed to E(high) . The peak at 330 cm1 is 2 (high) (low) assigned to E2 E2 which is a second order mode caused by multi-phonon processes. The peak at 383 cm1 is attributed
Table 2
Raman spectral peaks of PZ, CZ1, CZ2 and CZ3 with reference
Peak identity
Ref. 57
PZ
CZ1
CZ2
CZ3
E(high) 2
E(low) 2
333 378 410
330.7 384.4 412.5
330.3 383.7 413.2
329.9 383.5 412.1
326.1 384 416.9
E(high) 2 A1(LO)/E1(LO) A1(LO) + E(low) 2
438 574 657
438 573.3 657.3
438.3 574.5 661.7
438.5 573.7 656.8
437.7 568.2 657
A1(TO) E1(TO)
to A1(TO). The band at 660 cm1 can be assigned to the A1(LO) + E(low) .55 The peaks at 384 cm1 and 574 cm1 are due to A1(TO) 2 and A1(LO)/E1(LO) modes, respectively. The Raman peaks observed are summarized in Table 2. The E(high) phonon mode 2 of Co doped ZnO samples does not change position considerably compared to the undoped ZnO because the E(high) mode of ZnO 2 is mainly due to the vibration of oxygen atoms and is insensitive to the mass substitution on the cation site.56 Due to Co doping, the changes in mass and stress is compensated at the core and so the presence of internal stress at the surface layer due to defects does not produce a considerable peak shift on E(high) . 2 3.3.
Growth mechanism of nanodisk like structures
In the present study, the hydroxide anions (OH) for the alkali precipitation of Zn2+ ions are provided by the hydrolysis of HMTA. HMTA decomposes to formaldehyde and ammonia upon heating (eqn (3)). Ammonia reacts with water to give OH (eqn (4)), which forms a Zn(OH)2 precipitate upon reacting with Zn2+ (eqn (5)). Zn(OH)2 upon further heating produces ZnO (eqn (6)). In the presence of excess OH ions [Zn(OH)4]2 ions are formed (eqn (7) and (8)), which act as the growth unit for the formation of ZnO nanostructures. The chemical reactions governing the growth of ZnO nanostructures are as follows: C6H12N4 + 6H2O - 4NH3 + 6HCHO
(3)
NH3 + H2O - NH4 + OH
(4)
Zn2 + 2OH - Zn(OH)2
(5)
Zn(OH)2 - ZnO + H2O
(6)
+
Zn(OH)2 + 2OH 2 [Zn(OH)4]2 ZnO + H2O + 2OH 2 [Zn(OH)4]
Fig. 6
Raman spectra for the pristine and Co doped ZnO.
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2
(7) (8)
In general, the preferential growth of the polar crystal of ZnO nanostructure is along the [0001] direction (c axis terminated by Zn) because the (0001) facet has the lowest surface energy. Normally nanorod morphology is obtained because the growth velocity along h1010i is slower than that along the [0001] direction. Nanodisks can be produced in a certain direction by suppressing the growth along the c axis.58 When OH is adsorbed on the (0001) surface, more Zn+ ions attach to the end which gives rise to the formation of nanorods. If highly electronegative Cl ions are adsorbed on the (0001) surface, it hinders the attachment of growth units of Zn(OH)42. Thus growth along (0001) is suppressed and crystal growth then proceeds along sideways leading to the formation of ZnO nanodisks.59 Li et al.60 have also reported that when Cl anions are adsorbed on the
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3.4.
Fig. 7 Formation mechanism for ZnO nanorods and nanodisks.
polar facet of ZnO, nanosheets can be fabricated by suppressing the crystal growth along its h0001i direction. Schematic diagrams depicting the underlying mechanisms of growth for ZnO nanorods and nanodisks are shown in Fig. 7.
Photocatalytic studies
Fig. 8 shows the UV-visible absorption spectra of an aqueous solution of 10 mM MB dye with photocatalysts PZ, CZ1, CZ2 and CZ3 under irradiation with sunlight for different durations of time. The characteristic absorption peak of MB at 664 nm is monitored as a function of sunlight exposure time. From the UV-visible absorption spectra it can be seen that MB is completely degraded in only 8 minutes for sample CZ2. The processes underlying photocatalytic degradation of an organic dye can be understood as follows. The first step involves adsorption of the dye onto the surface of ZnO nanostructures. Irradiation of MB adsorbed ZnO nanostructures with sunlight leads to generation of electron–hole (e–h+) pairs in ZnO (eqn (9)). These photogenerated electrons in the conduction band of ZnO interact with the oxygen molecules adsorbed on ZnO to form superoxide anion radicals ( O2) (eqn (10)). The holes generated in the valence band of ZnO react with surface hydroxyl groups to produce highly reactive hydroxyl radicals ( OH) (eqn (11)). These photogenerated holes can lead to dissociation of water molecules in the aqueous solution, producing radicals (eqn (12)). The highly
Fig. 8 UV-visible absorption spectra showing temporal evolution of photocatalytic degradation of MB upon irradiation with sunlight using samples PZ, CZ1, CZ2 and CZ3 as photocatalysts.
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reactive hydroxyl radicals ( OH) and superoxide radicals ( O2) react with MB dye adsorbed on ZnO nanostructures and lead to its degradation/decoloration resulting in its colourless form (eqn (13) and (14)). The processes underlying underlying photocatalytic degradation of MB dye can be summarized by the following equations.61 ZnO + hn - e(CB) + h+(VB)
(9)
e + O2 - O2
(10)
h+ + OH - OH
(11)
h + H2O - H + OH
(12)
OH + organic dye - degradation products
(13)
O2 + organic dye - degradation products
(14)
+
+
The molecular structure of MB dye is given in Fig. 9. The molecular interactions of MB with the OH radicals leading to its degradation can be understood as follows.62,63 The first step is the cleavage of bonds of the C–S+QC functional group in MB by OH radicals to produce C–S(QO)–C along with the opening of the central aromatic ring containing both S and N in order to conserve the double bond conjugation. The sulfoxide group can further react with an OH radical two more times, producing the sulfone and sulfonic acid respectively. The final release of SO42 ions are due to a further reaction with OH radicals. As mentioned above cleavage of the NQC double bond takes place along with the cleavage of bonds of the C–S+QC functional group. The saturation of the two amino bonds is due to H radicals which are obtained from the proton reduction by photogenerated electrons. This amino group forms the corresponding phenol and releases an NH2 radical which generates ammonia and ammonium ions, upon substitution by an OH radical. For the degradation of the two symmetrical dimethylphenyl-amino groups, one methyl group reacts with the OH radical producing an alcohol and then an aldehyde which ultimately produces CO2. By the successive reaction with OH radicals, the phenyl-methyl-amine radical is also degraded leading to the complete degradation of MB dye. The variation of MB concentration with sunlight exposure time for different photocatalysts is shown in the C/C0 graph in Fig. 10(a). It can be seen that both undoped ZnO and Co doped ZnO nanostructures are highly efficient photocatalysts. However, optimum Co doping leads to enhancement in the photocatalytic activity of ZnO nanostructures. Repetitive tests for photocatalytic degradation were also performed and the results of three runs of degradation of MB dye by CZ2 are shown
Fig. 10 (a) Variation of MB concentration with sunlight exposure time for different photocatalysts. (b) Efficiency of sunlight driven degradation of MB by CZ2 for three runs with each run spanning 8 minutes.
Molecular structure of MB dye.
in Fig. 10(b). It can be clearly seen that the photocatalytic efficiency of CZ2 remains almost unchanged even after three runs. In an earlier study, Xiao et al.41 showed that Co doped ZnO nanostructures take 300 minutes for visible light driven complete degradation of 31 mM MB dye. It can be clearly seen that the photocatalytic degradation efficiency in our case is very high as compared to earlier studies on photocatalytic degradation of MB by metal doped ZnO nanostructures. The dye degradation rate depends on the morphology and crystallinity of the photocatalysts. Nanomaterials with large surface area and higher crystallinity can increase the number of active sites and promote the separation efficiency of the electron–hole pairs in the photocatalytic reactions. Zeng et al.64 have hydrothermally synthesized single crystalline ZnO nanodisks and nanowires and have shown that ZnO nanodisks with a high population of (0001) facets show better catalytic activity for photodegradation of rhodamine B dye as compared to ZnO nanowires. Zhai et al.65 synthesized ZnO nanodisks, consisting of nanocrystals by a simple chemical hydrolysis method, which exhibited high photocatalytic activity towards degradation of methyl orange. Roy et al.66 reported the size and facet controlled synthesis of anatase TiO2 nanocrystals. They explained that due to preferential flow of photogenerated carriers to the specific facets, the exposed facets play a crucial role in the photocatalytic activity. Our samples consist of nanodisks along with nanorods of high crystalline quality as seen from FESEM and XRD. Nanodisks have higher surface area as compared to other morphologies, which increases the number of active sites.
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Our results suggest that optimum Co doping enhances the photocatalytic activity of ZnO nanodisks and nanorods since it improves the separation efficiency of the electron–hole pairs and hence they exhibit superior photocatalytic efficiency. Beyond that doping level, the photocatalytic efficiency tends to decrease. Doping with Co facilitates the charge separation and transport in ZnO, which restrains the recombination of photogenerated charge carriers. Along with this Co doping also extends the photoresponse to the visible region and improves the photocatalytic activity of ZnO nanostructures. However, increase in Co doping beyond a certain level leads to a decrease in the photocatalytic activity. Lu et al.31 have reported that such a decrease can be due to the accumulation of Co and formation of a new phase on the surface of ZnO which decreases the effective surface active sites. The new phase formed on the surface of ZnO resists the separation and transport of photogenerated charge carriers, which results in a decrease of the photocatalytic activity. Xiao et al.41 have shown that there is an optimum amount of doping for which highest photocatalytic activity was achieved because the recombination of photoinduced electrons and holes could be effectively inhibited. In our case also, it can be seen that CZ2, with an optimum amount of Co doping, is the most efficient photocatalyst for degradation of MB.
4. Conclusions In summary, we have synthesized Co doped ZnO nanodisks and nanorods by a facile wet chemical method. The structural, optical and photocatalytic properties of ZnO and Co doped ZnO nanostructures have been investigated. Both ZnO and Co doped ZnO nanostructures exhibited enhanced photocatalytic efficiency towards degradation of MB dye under sunlight irradiation due to the enhanced surface area and high crystallinity of nanodisk structures. We have demonstrated that higher surface area and optimal Co doping result in enhancement of the photocatalytic activity of ZnO nanodisks and nanorods as optimal Co doping facilitates the separation of photogenerated charge carriers in ZnO, which restricts their recombination and higher surface area helps in increased adsorption of the dye for efficient degradation.
Acknowledgements The authors are thankful to Prof. S. Annapoorni and Prof. Vinay Gupta for extending facilities for PL and Raman studies, Hemant for his help in sample preparation, Sunil and Srikanth for their help in FESEM and XRD measurements, respectively. SM is thankful to University Grants Commission (UGC), New Delhi, for funding under Major Research Project (F. No: 41-865/ 2012 (SR)). SM is thankful to Department of Science and Technology (DST), New Delhi, for providing XRD facility under the Nano Mission program. SK is thankful to Guru Gobind Singh Indraprastha University, New Delhi, for providing financial assistance through IP Fellowship.
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Phys. Chem. Chem. Phys., 2014, 16, 12741--12749 | 12749
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Enhanced photocatalytic activity of Co doped ZnO nanodisks and nanorods prepared by a facile wet chemical method Sini Kuriakose,a Biswarup Satpatib and Satyabrata Mohapatra*a Cobalt doped ZnO nanodisks and nanorods were synthesized by a facile wet chemical method and well characterized by X-ray diffraction, field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM) with energy dispersive X-ray spectroscopy, photoluminescence spectroscopy, Raman spectroscopy and UV-visible absorption spectroscopy. The photocatalytic activities were evaluated for sunlight driven degradation of an aqueous methylene blue (MB) solution. The results showed that Co doped ZnO nanodisks and nanorods exhibit highly enhanced photocatalytic activity, as
Received 26th March 2014, Accepted 9th May 2014
compared to pure ZnO nanodisks and nanorods. The enhanced photocatalytic activities of Co doped ZnO
DOI: 10.1039/c4cp01315h
improved charge separation efficiency due to optimal Co doping which inhibit recombination of
nanostructures were attributed to the combined effects of enhanced surface area of ZnO nanodisks and photogenerated charge carriers. The possible mechanism for the enhanced photocatalytic activity of Co
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doped ZnO nanostructures is tentatively proposed.
1. Introduction Semiconductor photocatalysis has attracted significant attention as a potential solution for environmental remediation, especially for the degradation of toxic organic pollutants.1,2 Among the various semiconductor photocatalysts, ZnO has been widely used for degradation of organic pollutants. ZnO with large excitonic binding energy (60 meV) and wide band gap (3.37 eV)3–6 is suitable for a wide range of applications7 including UV lasers,8 UV detectors,9 field effect transistors,10 dye sensitized solar cells,11–14 surface enhanced Raman spectroscopy (SERS),15 lighting applications,16 gas sensors,17 and photocatalysis.18–23 The potential use of ZnO nanostructures with various morphologies such as nanoparticles, nano/microrods, nanoballs, nanoflowers for photocatalytic degradation of organic dyes has attracted increased attention in recent years.24–27 This is mainly due to their high photocatalytic efficiency, non-toxic nature, low cost and their ability to utilize abundantly available sunlight.28,29 However, ZnO suffers from the drawback of rapid recombination of photoexcited electrons and holes which deteriorates its photocatalytic efficiency.30,31 Since the photocatalytic efficiency depends on the efficient transfer and separation of photogenerated charge carriers, it is important to suppress their recombination for achieving higher efficiency. This can be done by either modifying a
School of Basic and Applied Sciences, Guru Gobind Singh Indraprastha University, Dwarka, New Delhi 110078, India. E-mail: [email protected] b Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India
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the surface of ZnO nanostructures or by doping with transition metals which could modify the electronic energy band structure of ZnO. Doping of ZnO nanostructures with transition metals has found applications in spin valves, magnetic sensors, spin light emitting diodes and non-volatile memory devices,32,33 optoelectronics,34 and photocatalysis. Doping of ZnO nanostructures with transition metals improves their photocatalytic activity as the metals act as trap sites by capturing the photogenerated electrons or holes from ZnO and thus restrain their recombination.35 Various transition metals such as Mn, Fe, Ni, Cu have been used for doping of ZnO.36–39 Among these Co is considered to be the most effective dopant for tuning the electronic and optical properties of ZnO.40 Lu et al.31 have synthesized photocatalytically active Co doped ZnO nanorods by a facile hydrothermal process which could degrade 93% of alizarin red dye in 60 minutes. They have shown that the incorporation of Co not only promoted the charge separation and enhanced the charge transfer ability but also effectively inhibited the recombination of photogenerated charge carriers in ZnO, resulting in high visible light photocatalytic activity. Xiao et al.41 synthesised Co doped ZnO by a hydrothermal method with enhanced visible light photocatalytic activity as compared to undoped ZnO, degrading 100% methylene blue (MB) dye in 300 minutes. They showed that the larger the content of oxygen vacancies and defects in ZnO, the higher is the photocatalytic activity. In this paper, we report the facile synthesis of Co doped ZnO nanostructures (nanorods and nanodisks) with highly enhanced photocatalytic activity for sunlight driven degradation of MB
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dye, degrading 99% of the dye in just 8 minutes. Even though some work has been done on Co doped ZnO photocatalysis, such high photocatalytic efficiency has not been reported till now. We have demonstrated that the photocatalytic efficiency of Co doped ZnO nanostructures depends on their crystallinity and morphology and is highest for an optimum Co doping.
2. Experimental section 2.1.
Materials
Zinc chloride (ZnCl2) and hexamethylenetetramine (HMTA) were used as the starting materials for synthesis of ZnO nanostructures. Cobalt chloride hexahydrate (CoCl26H2O) was used as the precursor for doping of Co into ZnO nanostructures. Zinc chloride, cobalt chloride and HMTA were purchased from Merck, while methylene blue (MB) was purchased from SRL, India. All the chemicals were of analytical grade and were used as received without any further purification. 2.2.
Synthesis of ZnO and Co doped ZnO nanostructures
ZnO nanostructures were synthesized by a simple wet chemical method using aqueous solutions of ZnCl2 and HMTA. In a typical synthesis, a calculated amount of ZnCl2 was dissolved in 50 mL double distilled water under stirring. HMTA taken in 1 : 1 ratio with Zn2+ ions was dissolved in 50 mL double distilled water and added to the aqueous ZnCl2 solution to attain a molarity of 0.02 M. The mixture was continuously stirred and heated at 95 1C for 3 h, after which it was allowed to cool down and left undisturbed overnight. The white precipitates formed were centrifuged and thoroughly washed by repeated centrifugation– redispersion with double distilled water. The white precipitates collected were dried in an air oven maintained at 80 1C for 20 h. The solid white powder obtained was used for characterization and photocatalytic studies. For synthesis of Co doped ZnO nanostructures, appropriate amounts of CoCl26H2O were added into the aqueous ZnCl2 solution, followed by addition of HMTA in a 1 : 1 ratio with Zn2+ ions and the above procedures were repeated. The relative concentration of CoCl26H2O in the mixture was varied for different levels of Co doping in ZnO nanostructures. The undoped ZnO sample and samples prepared with 2.5%, 5% and 10% Co in ZnO are hereafter referred to as PZ, CZ1, CZ2 and CZ3, respectively.
Fischione (model 3000). The compositional analysis was performed by energy dispersive X-ray spectroscopy (EDS, EDAX Instruments) using a Tecnai G2 F30. Energy-filtered TEM (EFTEM) measurements were carried out using a GIF Quantum SE (model 963). The sample was dispersed in ethanol using an ultrasonic bath, coated on a carbon coated Cu grid, dried, and used for TEM measurements. Raman spectra were recorded using a Horiba Jobin Yvon LabRAM using argon laser (488 nm) of spot size 1 mm. The optical properties of the samples were studied by UV-visible absorption spectroscopy and photoluminescence (PL) spectroscopy at room temperature. The powder samples were dispersed in double distilled water by sonication and their optical properties were studied by UV-visible absorption spectroscopy in the wavelength range of 200–800 nm using a HITACHI U-3300 spectrophotometer, with double distilled water as the reference medium. PL studies were performed on the powder samples using a xenon lamp (lex = 325 nm). 2.4.
Photocatalytic measurements
The photocatalytic performance of ZnO nanostructures and ZnO:Co nanostructures were evaluated by the degradation of methylene blue (MB) dye under sunlight irradiation. For the photocatalytic studies, photocatalysts typically 5 mg of as-synthesized ZnO and ZnO:Co nanostructures were ultrasonically dispersed in 5 mL distilled water in glass vials, which were used as the reactors. A 10 mM MB dye solution was added to the photocatalysts and thoroughly mixed and kept in the dark for 30 minutes to reach the adsorption–desorption equilibrium. The reaction suspensions containing MB and nanostructured ZnO and ZnO:Co photocatalysts were irradiated with sunlight for different times (2, 4, 6 and 8 minutes) with intermittent shaking for uniform mixing of the photocatalysts with the MB dye solutions. These experiments were carried out at mid day (between 12.00 and 1.00 pm) during the peak summer season ensuring irradiation with sunlight of maximum luminosity. Then the suspensions were centrifuged and the photocatalysts were removed from the suspension. The concentrations of the MB dye in the resultant solutions were monitored by UV-visible absorption spectroscopy in the wavelength range of 200–800 nm, with double distilled water as the reference medium. The photocatalytic efficiency of the photocatalysts for the degradation of the MB dye was calculated using the following formula: Z¼
2.3.
Characterization of photocatalysts
The structural properties of the samples were determined by powder X-ray diffraction (XRD) at room temperature using a Panalytical X’pert Pro diffractometer with Cu Ka radiation (l = 0.1542 nm). The surface morphology of the samples was studied using field emission scanning electron microscopy (FESEM) [MIRA, TESCAN]. Transmission electron microscopy (TEM) investigation was carried out using a FEI Tecnai G2 F30, S-TWIN microscope operating at 300 kV. High-angle annular dark field scanning transmission electron microscopy (STEM-HAADF) was employed here using the same microscope, which is equipped with a scanning unit and a HAADF detector from
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C0 C C0
(1)
where C0 is the absorbance of aqueous 10 mM MB dye solution before addition of any photocatalyst and sunlight exposure and C is the absorbance of MB in reaction suspensions with the photocatalyst following sunlight exposure for time t.
3. Results and discussion 3.1.
Structural properties
Fig. 1 shows the XRD patterns of as-synthesized samples PZ, CZ1, CZ2 and CZ3 on a log scale. The observed peaks in the spectra can be well indexed to the hexagonal wurtzite structure
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Fig. 1 XRD peaks of undoped ZnO and Co doped ZnO shown on a log scale.
of crystalline ZnO [JCPDS no. 36-1451]. No extra peaks related to any impurity or cobalt oxide were observed within the detection limit of the instrument, which confirms that the as-synthesized products are pure wurtzite ZnO. Addition of a Co dopant shows no significant shift in the (101) peak because the ionic radii of Co2+ and Zn2+ are similar (Zn2+ E 0.60 Å, Co2+ E 0.56 Å) and any expected shift in the peak would be beyond the resolution of our diffractometer.42 The micro-strain acting on the nanostructure was analysed using the Williamson–Hall method.43 The Williamson–Hall equation is given as b cosðyÞ 1 sinðyÞ ¼ þ ehkl l Dhkl l
(2)
where y is the diffraction angle, b is the full width at half maxima (FWHM) of the XRD peak, l is the wavelength of the X-rays, Dhkl is the effective crystallite size and ehkl is the microstrain. The strain obtained for the samples are 0.00039, 0.00057, 0.0012, 0.00125 for PZ, CZ1, CZ2 and CZ3 respectively. Positive values indicate tensile strain in ZnO lattice. The surface morphology of the as-synthesized samples was investigated using FESEM. Fig. 2(a–d) show FESEM images of samples PZ, CZ1, CZ2 and CZ3, respectively. The FESEM images clearly reveal the presence of a large number of nanodisk structures along with few nanorods. Fig. 3(a) presents the TEM image of Co-doped nanorods and nonodisks. The selected area electron diffraction (SAED) pattern from a region marked by a dotted circle is shown in Fig. 3(b), which indicates the single crystal nature of the ZnO nanodisk. It was indexed using the lattice parameters of hexagonal close packed (hcp) ZnO (lattice parameters, a = 3.253 Å and c = 5.213 Å). Some of the measured inter-planar spacings (d-spacing) from the SAED pattern are 2.87 Å, 2.61 Å, and 2.48 Å. These measured d-spacings are very close to the (010), (002) and (011) inter-planar spacings of hexagonal ZnO (JCPDS # 36-1451). The HRTEM image in Fig. 3(c) from a dotted yellow box region or the Fourier-filtered image in the inset from a dotted box in Fig. 3(c) clearly showing lattice fringes indicates the crystalline phase with a d-spacing of 2.48 Å. To investigate the chemical
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Fig. 2 FESEM images of (a) undoped ZnO and Co doped ZnO samples (b) CZ1, (c) CZ2 (d) CZ3.
Fig. 3 (a) Low magnification TEM image, (b) selected area electron diffraction (SAED) from a region marked by a dotted circle in (a), (c) HRTEM image showing lattice fringes and in the inset Fourier filtered image from a region marked by a dotted box in (c). (d) STEM-HAADF image and EDX spectra in the inset from a region marked by area 1, (e) relative thickness map, (f and g) profile of thickness from area 1 and 2 in (e) indicating the presence of nanorod with round morphology and nanodisk with flat top morphology.
composition of the nanorods and nanodisks, we have performed high-angle annular dark field (HAADF) analysis. Fig. 3(d) shows the STEM-HAADF image, and in the inset showing EDX spectra indicating the presence of Zn and O, Co was not detectable.
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Table 1
Fig. 4 UV-visible absorption spectra of undoped ZnO and Co doped ZnO samples.
The observed Cu and C signal is due to the C-coated Cu grid. The thicknesses of the nanorods and nanodisks were measured using a log-ratio method (Fig. 3(e)). We obtained the relative thickness (thickness/mean free path) map using EFTEM and the corresponding area profile for a nanorod and a nanodisk are shown in Fig. 3(f) and (g), respectively. Nanorods have nearly round morphology whereas nanodisks show flat-top morphology, consistent with the FESEM study. 3.2.
Optical properties
The optical absorption spectra are shown in Fig. 4, which show a small peak in the UV region for all the samples. For the Co doped ZnO, we observe a UV peak and a broad absorption band in the visible region between 400 nm and 520 nm, which can be assigned to the incorporation of Co into the ZnO lattice. The band gaps obtained from the absorption spectra are 3.36 eV, 3.375 eV, 3.377 eV and 3.377 eV for PZ, CZ1, CZ2 and CZ3 respectively.
Fig. 5
Deconvoluted PL peaks for samples PZ, CZ1, CZ2 and CZ3
Sample
Peak I (nm)
Peak II (nm)
Peak III (nm)
Peak IV (nm)
PZ CZ1 CZ2 CZ3
379.6 373.6 374.5 374.3
387.1 398.1 389.3 387.7
412.5 420.6 414.9 414.8
531.9 543.1 518.8 513.2
Fig. 5(a) shows the PL spectra of undoped ZnO and Co doped ZnO nanostructures. The PL spectra are deconvoluted into four main peaks (summarized in Table 1). The first peak as shown in Fig. 5(a) is assigned to the near band edge emission. It is clear that due to Co doping the near band edge emission is blue shifted and the band gap increases. The combined effect of optical transition to the excitonic state of ZnO and electronic transitions involving crystal-field split 3d levels in Co2+ ions substituting Zn2+ ions is responsible for the observed blue shift.44 This further proves that doped Co2+ ions have substituted Zn2+ ions in the ZnO lattice. By comparing the UV-visible and PL spectra, it can be seen that the band gap obtained from PL is lower than that obtained from UV visible spectra. This shift is defined as Stokes shift and has been ascribed to lattice vibration, exciton binding energy, localization of charge carriers due to interface defects or point defects and electron–phonon coupling.45,46 The peaks at around 390 nm and 415 nm correspond to the emission due to Zn interstitial (Zni) to valence band transition.47,48 In the case of sample CZ1, the peak at 420 nm is due to transition between radiation defects, related to the interface traps excited at grain boundaries and the valence band.49 The PL peak at around 521 nm mainly results from oxygen vacancies (VO ).41 It can also be noted that the peak at 544 nm is generated by the conduction band to interstitial oxygen transition (Oi) (assuming that the interstitial oxygen level is 1.09 eV above the VB).50 It can be observed that as compared to the undoped sample, the intensity of the defect peak increases with Co doping which
(a) PL spectra of undoped ZnO and Co doped ZnO samples and (b) schematic band diagram based on PL data.
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has already been reported in some earlier studies.41,44 An increase in the intensity of the defect peak (521 nm and 544 nm) indicates the increase in the density of oxygen vacancies and oxygen interstitials. In the photocatalytic degradation process, the increase in efficiency is associated with efficient separation of photogenerated electrons and holes. If a surface defect state is able to trap electrons or holes, recombination can be restricted. Two impurity levels which are generated due to oxygen vacancies and oxygen interstitials can act as scavengers for electrons and holes. Oxygen vacancies act as electron accepters to trap electrons and interstitial oxygen act as shallow trappers for holes, both of which prevent the recombination of photogenerated electrons and holes, thereby increasing the efficiency. Thus in our case Co doped ZnO is more efficient than undoped ZnO.51,52 Xiao et al.41 have shown the dependence of photocatalytic degradation efficiency on the surface defects and that an optimum concentration of Co doping leads to the highest photocatalytic degradation efficiency. In our case also, CZ2 with 5% Co doping shows the highest photocatalytic degradation efficiency. The schematic band diagram of the emission as obtained from the data is given in Fig. 5(b). The band edge emission from the conduction band to the valence band is 3.26 eV. The transition from the trap state to the valence band is 2.95 eV and from zinc interstitial to the valence band is 2.98 eV. The 2.38 eV emission mainly results from oxygen vacancies. The oxygen interstitial level is located at 2.28 eV below the conduction band.5 Fig. 6 shows the Raman spectra for ZnO and Co doped ZnO nanostructures. The zone centre optical phonon of wurtzite type ZnO which belongs to the C6v space group has an irreducible representation A1 + 2B1 + E1 + 2E2.53 The A1 and E1 modes are polar and hence exhibit two frequencies for transverse optical (TO) and longitudinal optical (LO) phonon modes. The B1 modes are silent in Raman scattering.54 The non-polar E2 mode has two frequencies E(high) associated with the motion of oxygen atoms 2 and E(low) associated with the motion of the Zn sub-lattice.55 The 2 backscattering Raman spectra for all the samples ranging from 200 to 800 cm1 are shown in Fig. 6. For ZnO the strongest peak at B437 cm1 is attributed to E(high) . The peak at 330 cm1 is 2 (high) (low) assigned to E2 E2 which is a second order mode caused by multi-phonon processes. The peak at 383 cm1 is attributed
Table 2
Raman spectral peaks of PZ, CZ1, CZ2 and CZ3 with reference
Peak identity
Ref. 57
PZ
CZ1
CZ2
CZ3
E(high) 2
E(low) 2
333 378 410
330.7 384.4 412.5
330.3 383.7 413.2
329.9 383.5 412.1
326.1 384 416.9
E(high) 2 A1(LO)/E1(LO) A1(LO) + E(low) 2
438 574 657
438 573.3 657.3
438.3 574.5 661.7
438.5 573.7 656.8
437.7 568.2 657
A1(TO) E1(TO)
to A1(TO). The band at 660 cm1 can be assigned to the A1(LO) + E(low) .55 The peaks at 384 cm1 and 574 cm1 are due to A1(TO) 2 and A1(LO)/E1(LO) modes, respectively. The Raman peaks observed are summarized in Table 2. The E(high) phonon mode 2 of Co doped ZnO samples does not change position considerably compared to the undoped ZnO because the E(high) mode of ZnO 2 is mainly due to the vibration of oxygen atoms and is insensitive to the mass substitution on the cation site.56 Due to Co doping, the changes in mass and stress is compensated at the core and so the presence of internal stress at the surface layer due to defects does not produce a considerable peak shift on E(high) . 2 3.3.
Growth mechanism of nanodisk like structures
In the present study, the hydroxide anions (OH) for the alkali precipitation of Zn2+ ions are provided by the hydrolysis of HMTA. HMTA decomposes to formaldehyde and ammonia upon heating (eqn (3)). Ammonia reacts with water to give OH (eqn (4)), which forms a Zn(OH)2 precipitate upon reacting with Zn2+ (eqn (5)). Zn(OH)2 upon further heating produces ZnO (eqn (6)). In the presence of excess OH ions [Zn(OH)4]2 ions are formed (eqn (7) and (8)), which act as the growth unit for the formation of ZnO nanostructures. The chemical reactions governing the growth of ZnO nanostructures are as follows: C6H12N4 + 6H2O - 4NH3 + 6HCHO
(3)
NH3 + H2O - NH4 + OH
(4)
Zn2 + 2OH - Zn(OH)2
(5)
Zn(OH)2 - ZnO + H2O
(6)
+
Zn(OH)2 + 2OH 2 [Zn(OH)4]2 ZnO + H2O + 2OH 2 [Zn(OH)4]
Fig. 6
Raman spectra for the pristine and Co doped ZnO.
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2
(7) (8)
In general, the preferential growth of the polar crystal of ZnO nanostructure is along the [0001] direction (c axis terminated by Zn) because the (0001) facet has the lowest surface energy. Normally nanorod morphology is obtained because the growth velocity along h1010i is slower than that along the [0001] direction. Nanodisks can be produced in a certain direction by suppressing the growth along the c axis.58 When OH is adsorbed on the (0001) surface, more Zn+ ions attach to the end which gives rise to the formation of nanorods. If highly electronegative Cl ions are adsorbed on the (0001) surface, it hinders the attachment of growth units of Zn(OH)42. Thus growth along (0001) is suppressed and crystal growth then proceeds along sideways leading to the formation of ZnO nanodisks.59 Li et al.60 have also reported that when Cl anions are adsorbed on the
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3.4.
Fig. 7 Formation mechanism for ZnO nanorods and nanodisks.
polar facet of ZnO, nanosheets can be fabricated by suppressing the crystal growth along its h0001i direction. Schematic diagrams depicting the underlying mechanisms of growth for ZnO nanorods and nanodisks are shown in Fig. 7.
Photocatalytic studies
Fig. 8 shows the UV-visible absorption spectra of an aqueous solution of 10 mM MB dye with photocatalysts PZ, CZ1, CZ2 and CZ3 under irradiation with sunlight for different durations of time. The characteristic absorption peak of MB at 664 nm is monitored as a function of sunlight exposure time. From the UV-visible absorption spectra it can be seen that MB is completely degraded in only 8 minutes for sample CZ2. The processes underlying photocatalytic degradation of an organic dye can be understood as follows. The first step involves adsorption of the dye onto the surface of ZnO nanostructures. Irradiation of MB adsorbed ZnO nanostructures with sunlight leads to generation of electron–hole (e–h+) pairs in ZnO (eqn (9)). These photogenerated electrons in the conduction band of ZnO interact with the oxygen molecules adsorbed on ZnO to form superoxide anion radicals ( O2) (eqn (10)). The holes generated in the valence band of ZnO react with surface hydroxyl groups to produce highly reactive hydroxyl radicals ( OH) (eqn (11)). These photogenerated holes can lead to dissociation of water molecules in the aqueous solution, producing radicals (eqn (12)). The highly
Fig. 8 UV-visible absorption spectra showing temporal evolution of photocatalytic degradation of MB upon irradiation with sunlight using samples PZ, CZ1, CZ2 and CZ3 as photocatalysts.
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reactive hydroxyl radicals ( OH) and superoxide radicals ( O2) react with MB dye adsorbed on ZnO nanostructures and lead to its degradation/decoloration resulting in its colourless form (eqn (13) and (14)). The processes underlying underlying photocatalytic degradation of MB dye can be summarized by the following equations.61 ZnO + hn - e(CB) + h+(VB)
(9)
e + O2 - O2
(10)
h+ + OH - OH
(11)
h + H2O - H + OH
(12)
OH + organic dye - degradation products
(13)
O2 + organic dye - degradation products
(14)
+
+
The molecular structure of MB dye is given in Fig. 9. The molecular interactions of MB with the OH radicals leading to its degradation can be understood as follows.62,63 The first step is the cleavage of bonds of the C–S+QC functional group in MB by OH radicals to produce C–S(QO)–C along with the opening of the central aromatic ring containing both S and N in order to conserve the double bond conjugation. The sulfoxide group can further react with an OH radical two more times, producing the sulfone and sulfonic acid respectively. The final release of SO42 ions are due to a further reaction with OH radicals. As mentioned above cleavage of the NQC double bond takes place along with the cleavage of bonds of the C–S+QC functional group. The saturation of the two amino bonds is due to H radicals which are obtained from the proton reduction by photogenerated electrons. This amino group forms the corresponding phenol and releases an NH2 radical which generates ammonia and ammonium ions, upon substitution by an OH radical. For the degradation of the two symmetrical dimethylphenyl-amino groups, one methyl group reacts with the OH radical producing an alcohol and then an aldehyde which ultimately produces CO2. By the successive reaction with OH radicals, the phenyl-methyl-amine radical is also degraded leading to the complete degradation of MB dye. The variation of MB concentration with sunlight exposure time for different photocatalysts is shown in the C/C0 graph in Fig. 10(a). It can be seen that both undoped ZnO and Co doped ZnO nanostructures are highly efficient photocatalysts. However, optimum Co doping leads to enhancement in the photocatalytic activity of ZnO nanostructures. Repetitive tests for photocatalytic degradation were also performed and the results of three runs of degradation of MB dye by CZ2 are shown
Fig. 10 (a) Variation of MB concentration with sunlight exposure time for different photocatalysts. (b) Efficiency of sunlight driven degradation of MB by CZ2 for three runs with each run spanning 8 minutes.
Molecular structure of MB dye.
in Fig. 10(b). It can be clearly seen that the photocatalytic efficiency of CZ2 remains almost unchanged even after three runs. In an earlier study, Xiao et al.41 showed that Co doped ZnO nanostructures take 300 minutes for visible light driven complete degradation of 31 mM MB dye. It can be clearly seen that the photocatalytic degradation efficiency in our case is very high as compared to earlier studies on photocatalytic degradation of MB by metal doped ZnO nanostructures. The dye degradation rate depends on the morphology and crystallinity of the photocatalysts. Nanomaterials with large surface area and higher crystallinity can increase the number of active sites and promote the separation efficiency of the electron–hole pairs in the photocatalytic reactions. Zeng et al.64 have hydrothermally synthesized single crystalline ZnO nanodisks and nanowires and have shown that ZnO nanodisks with a high population of (0001) facets show better catalytic activity for photodegradation of rhodamine B dye as compared to ZnO nanowires. Zhai et al.65 synthesized ZnO nanodisks, consisting of nanocrystals by a simple chemical hydrolysis method, which exhibited high photocatalytic activity towards degradation of methyl orange. Roy et al.66 reported the size and facet controlled synthesis of anatase TiO2 nanocrystals. They explained that due to preferential flow of photogenerated carriers to the specific facets, the exposed facets play a crucial role in the photocatalytic activity. Our samples consist of nanodisks along with nanorods of high crystalline quality as seen from FESEM and XRD. Nanodisks have higher surface area as compared to other morphologies, which increases the number of active sites.
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Our results suggest that optimum Co doping enhances the photocatalytic activity of ZnO nanodisks and nanorods since it improves the separation efficiency of the electron–hole pairs and hence they exhibit superior photocatalytic efficiency. Beyond that doping level, the photocatalytic efficiency tends to decrease. Doping with Co facilitates the charge separation and transport in ZnO, which restrains the recombination of photogenerated charge carriers. Along with this Co doping also extends the photoresponse to the visible region and improves the photocatalytic activity of ZnO nanostructures. However, increase in Co doping beyond a certain level leads to a decrease in the photocatalytic activity. Lu et al.31 have reported that such a decrease can be due to the accumulation of Co and formation of a new phase on the surface of ZnO which decreases the effective surface active sites. The new phase formed on the surface of ZnO resists the separation and transport of photogenerated charge carriers, which results in a decrease of the photocatalytic activity. Xiao et al.41 have shown that there is an optimum amount of doping for which highest photocatalytic activity was achieved because the recombination of photoinduced electrons and holes could be effectively inhibited. In our case also, it can be seen that CZ2, with an optimum amount of Co doping, is the most efficient photocatalyst for degradation of MB.
4. Conclusions In summary, we have synthesized Co doped ZnO nanodisks and nanorods by a facile wet chemical method. The structural, optical and photocatalytic properties of ZnO and Co doped ZnO nanostructures have been investigated. Both ZnO and Co doped ZnO nanostructures exhibited enhanced photocatalytic efficiency towards degradation of MB dye under sunlight irradiation due to the enhanced surface area and high crystallinity of nanodisk structures. We have demonstrated that higher surface area and optimal Co doping result in enhancement of the photocatalytic activity of ZnO nanodisks and nanorods as optimal Co doping facilitates the separation of photogenerated charge carriers in ZnO, which restricts their recombination and higher surface area helps in increased adsorption of the dye for efficient degradation.
Acknowledgements The authors are thankful to Prof. S. Annapoorni and Prof. Vinay Gupta for extending facilities for PL and Raman studies, Hemant for his help in sample preparation, Sunil and Srikanth for their help in FESEM and XRD measurements, respectively. SM is thankful to University Grants Commission (UGC), New Delhi, for funding under Major Research Project (F. No: 41-865/ 2012 (SR)). SM is thankful to Department of Science and Technology (DST), New Delhi, for providing XRD facility under the Nano Mission program. SK is thankful to Guru Gobind Singh Indraprastha University, New Delhi, for providing financial assistance through IP Fellowship.
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