Unveiling the Co2+ Ion Doping-Induced ... - ACS Publications

Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9981-9992

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Unveiling the Co2+ Ion Doping-Induced Hierarchical Shape Evolution of ZnO: In Correlation with Magnetic and Photovoltaic Performance R. Krishnapriya, S. Praneetha, S. Kannan, and A. Vadivel Murugan* Advanced Functional Nanostructured Materials Laboratory, Centre for Nanoscience and Technology, Madanjeet School of Green Energy Technologies, Pondicherry University (A Central University), Kalapet, Puducherry 605014, India

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S Supporting Information *

ABSTRACT: A sustainable, rapid microwave-assisted glycothermal (MW-GT) method has been adopted for the synthesis of pristine ZnO and a series of Zn1−xCoxO (x = 0, 0.02, 0.03, 0.05, 0.07, 0.10) within 15 min at 180 °C using ethylene glycol (EG) as solvent. The XRD results reveal that the altering of lattice parameters of ZnO by introduction of Co2+ ions and crystalline sizes of Co2+ doped ZnO samples decreased with increasing Co2+ ion content. A spectacular morphological change of ZnO from well-defined hexagonal prismoid to hierarchical flower-like 1-D nanorods-assembly upon increasing Co2+ ion concentration was perceived using FE-SEM and TEM analyses. After Co2+ ion inclusion into pristine ZnO, the width of the M−H loop significantly changes, where the diamagnetic behavior of ZnO changes from ferromagnetic to paramagnetic upon further increase in Co2+ ion content. Particularly, 5 mol % Co2+ ion doped ZnO sample shows enhanced photovoltaic performance in dyesensitized solar cells (DSSCs) due to nanoscale level intermingling of two different 1-D nanorod-like morphology with particlelike morphology, resulting in size-mismatched combination-induced light-scattering effect, photoinduced charge-carrier formation by charge-transfer transitions of high spin Co2+ ions, and lower recombination resistance together with extended electron lifetime, which were deduced from UV−vis and impedance spectroscopy analysis, respectively. KEYWORDS: Cobalt doping, Microwave glycothermal, Zinc oxide nanocrystals, Hierarchical morphology, Dye-sensitized solar cells

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INTRODUCTION Doping is the process of incorporating intentional impurities into the semiconductor nanocrystals (NCs) to modify their unique structural, optical, electrical, and magnetic properties. This approach is more promising yet challenging in semiconductor-based technologies and an effective way to tune the band gap to broaden its spectral absorption.1−3 Recently, ZnO has become an attractive wider band gap semiconducting metal oxide and exhibits unique properties that provide both technological as well as eco-friendly benefits.4−8 Various devices functioning with ZnO NCs include solar cells, field effect transistors, and photodetectors, which all affirmed their potential reliability in the development of electronics and clean energy systems. Significant efforts were made to improve ZnO materials as DSSC photoanode, because this material unveils higher electron mobility than TiO2 and enables rapid transport of photoinjected electrons. The incident light absorption of ZnO occurs mainly in the UV region with spectral wavelength equal to or less than 385 nm. However, the visible light (400−700 nm) is mainly accountable for 45% of the total solar energy absorption, and the UV region accounts for less than 10%.9 In this regard, there is a necessity to broaden the spectral absorption range to boost the solar cell performance.10,11 On the other hand, morphology tuning as © 2017 American Chemical Society

well as ZnO doped with appropriate metal ions have been demonstrated with considerable photovoltaic (PV) enhancement in the DSSCs.12−15 Moreover, when the doped NCs are applied as photoanode materials for DSSCs applications, the dopants prevent the photo-oxidation and promote effective charge transfer of absorbed photons to solar energy conversion by the appropriate dopant concentration. In addition, dopants cause effective quick localization of photoexcitation followed by suppression of undesirable reactions at NCs surface, which synergistically contribute to enhanced PV performance in solar cells.16 Doping of ZnO with various metal and nonmetal elements such as Li, B, and I, etc., were widely carried out to tailor the optical properties of photoanode in DSSCs.17−19 Transition metal (TM) doped ZnO NCs have also been proven as suitable candidates in yielding extensive photoresponse in the visible region and are hence manifested as efficient photocatalysts for solar energy conversion and degradation of toxic water pollutants.20−25 The photochemistry behind all of these processes is the excitation of a sub-band gap absorption by a d−d transition. Among various TM doped ZnO, Co2+ Received: June 13, 2017 Revised: September 20, 2017 Published: October 3, 2017 9981

DOI: 10.1021/acssuschemeng.7b01918 ACS Sustainable Chem. Eng. 2017, 5, 9981−9992

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic illustration of rapid, sustainable microwave-glycothermal (MW-GT) synthesis of Co doped ZnO with different reaction parameters.

(EG) as a solvent at 180 °C within 15 min as shown in Figure 1. A series of synthesis reactions with different microwave parameters (temperature, power, and time) have been systematically carried out to study the effect of Co2+ ion incorporation in ZnO. To the best of our knowledge, this is the first attempt to study the various content of Co2+ ion doping in ZnO and to correlate their doping-induced morphological changes, optical, magnetic, and PV properties in DSSC applications. The MW-GT method is an effective process for the inclusion of Co2+ ion in ZnO host lattice with spectacular morphological changes upon doping. Significantly, the MW-GT method is a greener, energy efficient, and sustainable chemistry approach, because the chemicals used for these reactions are nontoxic, pretty much stable in air, and inexpensive.

dopants are widely studied for photocatalytic applications and have been reported by several scientific groups.26 The incorporation of Co2+ into the ZnO lattice is quite feasible as they have ionic radii comparable to that of Zn2+ ions. The incorporation of Co2+ ions in the ZnO lattice develops significant optoelectronic properties, which is further applied in several electronic, magnetic, and clean energy devices. However, the effective Co2+ ions doping into the ZnO lattice is still a challenge. Systematic investigations related to the effect of dopants on the growth and morphology of ZnO in correlation with PV performance in DSSCs is scarce. There have been several studies on the synthesis of doped ZnO NCs by various physical and chemical routes, which include hydrothermal (HT), solvothermal (ST), sol−gel synthesis, coprecipitation method, etc. Jang et al. reported a generalized sol−gel method for the synthesis of different semiconductor oxide NCs with appropriate TM dopants for an efficient electrocatalytic oxygen evolution reaction.27 However, liquidphase synthesis methods, especially HT/ST methods, are more facile for controlling the morphology, crystal size, and shape, and also to provide compositional homogeneity.28 Nevertheless, these methods involve multisteps and time-consuming processes. The combination of liquid-phase synthesis with microwave reaction chemistry is an innovative eco-friendly process to accelerate the reaction rates with improved product yield.29,30 Microwave-assisted solvothermal method (MW-ST) is one of the most appealing approaches for the synthesis of high yield NCs.31−33 The preference of the MW-ST approach over conventional solvothermal and hydrothermal processes was due to certain advantageous factors such as minimal reaction time, uniform reaction conditions, and low-cost energy efficient process.34−36 Recently, Markus Niederberger’s group successfully synthesized various TM doped ZnO in benzyl alcohol with rapid microwave-assisted nonaqueous sol−gel route within a few minutes for diluted magnetic semiconductor property studies.37 However, their protocol required Aratmosphere using a glovebox for the precursor preparation. In this regard, we successfully demonstrate the synthesis of pristine ZnO and Co2+ ions doped ZnO NCs by a microwaveassisted glycothermal (MW-GT) synthesis in ethylene glycol

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EXPERIMENTAL SECTION

Zn(CH3COO)2·2H2O (Loba Chemi), Co(CH3COO)2·4H2O (Fisher Scientific), and the solvent ethylene glycol (EG) (Fisher Scientific) were purchased and used as received without further purification. For the synthesis of both pristine and Co2+ ions doped ZnO, the precursor solution contained 0.495 mol L−1 Zn(CH3COO)2·2H2O and 5 M NaOH in EG (volume ratio, v/v 1:1, pH ≈ 14). Co2+ ion doping was achieved by adding 2, 3, 5, 7, and 10 mol % of Co2+ precursor to Zn(CH3COO)2·2H2O solution and transferring into quartz vessels (50 mL capacity) with a magnetic stirrer sealed with a Teflon cap at the top. The microwave reaction system SOLV (Multiwave PRO), Anton Paar, GmbH, Austria, was operated at a frequency of 2.45 GHz under MW-GT conditions. The microwave reaction time was preset for 15 min at 180 °C, and autogenous pressure that developed (maximum of 20 bar) inside the reaction vessel was controlled by the system. After the completion of the reaction, the precipitates were separated by centrifugation and subsequent washing with acetone. The pristine and doped ZnO samples were finally dried at 60 °C in a vacuum oven. Characterization and Fabrication of DSSCs. The crystal phase identification of samples was investigated using a powder X-ray diffractometer (Rigaku, Ultima IV, Japan) with Cu Kα radiation (λ = 1.54 Å) at 40 kV and 30 mA to scan the diffraction angles (2θ) from 20−80° with the step size of 0.02° per second and was compared to standard ICDD (International Centre for Diffraction Data). The surface morphology of the samples was studied by using a field emission scanning electron microscope (FESEM-SUPRA-55-Carlzeiss, Germany) and high-resolution transmission electron microscopy (HR9982


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ACS Sustainable Chemistry & Engineering TEM) (JEOL/JEM 2100USA). Raman spectroscopic analysis was carried out by Confocal Raman microscope (Renishaw, UK) using an excitation energy of 514 nm by a semiconductor diode laser. The emission spectrum was recorded with an excitation wavelength of 340 nm using a fluorescence spectrofluorometer (Fluoromax-4, Horiba Scientific). UV−visible spectra were obtained using a UV-2550, Shimadzu, Japan, spectrometer operated in diffused reflection mode. The room-temperature magnetic properties of the doped samples were analyzed by a vibrating sample magnetometer (Lake Shore7410, VSM). XPS measurements were performed using the Thermo Scientific K-Alpha+ X-ray photoelectron spectrometer (XPS) system, UK, using a photon energy range 0−1350 eV with 400-μm area with 1 eV step size. The prototype DSSCs fabrication and its device characterization techniques were followed according to our earlier reports38 and were provided in the Supporting Information.

related to impurities of either metallic cobalt or cobalt oxides were identified within the detection limit of the XRD instrument. This result revealed the purity of as-synthesized samples with successful inclusion of Co2+ ions to the Zn2+ position of hexagonal wurtzite ZnO lattice. From the XRD patterns, a closer observation into the (100), (002), and (101) peaks revealed that the peak intensities, position, and linewidths changed after the incorporation of Co2+ ions in the ZnO host lattice.40 In addition, the peaks were slightly shifted toward the higher 2θ angles when the concentration of Co2+ ions increased (Figure S1). This was due to smaller ionic radius of Co2+ (0.56 Å) with respect to Zn2+ (0.60 Å), confirming that the Co2+ ion is being doped into the ZnO lattice.41 Figure 3 shows the Rietveld refinement patterns, which were deduced by using the Fullprof Program to understand precisely the lattice parameter changes of ZnO after different contents (mol %) of Co2+ ion doping. Table 1 summarizes the structural parameters and the crystallite sizes for all Co2+ ion doped ZnO samples determined quantitatively through refinement according to the previous work reported by Kannan and co-workers.42,43 From Table 1, it was found that the ZnO crystal lattice contracted with an increase in Co2+ ion doping concentration, which was attributed to the substitutional doping of Co2+ ion into the Zn2+ tetrahedral lattice. Figure S2 also shows the variation of lattice parameter value as a function of Co2+ ion concentration. When the Co2+ ion dopant concentration increased from 2 to 5 mol %, the lattice constant values also increased, which is accordance with Vegard’s law.44 A further increase of Co2+ ion content decreased the lattice constant values linearly, which is due to the oxygen stoichiometry. It was apparently show that variations in cell volume induced by Co2+ ion dopants further substantiated the successful incorporation of Co2+ ion into ZnO host lattice even at a smaller content, 2 mol %, of Co2+ ion as shown in Table 1. The morphological changes of ZnO with and without Co2+ ion doping under various MW-GT experimental conditions were investigated from the FE-SEM analysis. In the growth mechanism, five different parameters, temperature, pressure, time, kinetic energy barrier, and solvent, influence the growth pattern of crystals under nonequilibrium kinetic growth conditions in solution-based methods.45 In our experiments, temperature, pressure, solvent, and the amount of Co2+ ion content were the key parameters for the shape evolution of ZnO under MW-GT reaction condition. Figure 4a−e shows FE-SEM images and a schematic illustration of crystal growth patterns of pristine ZnO sample and the shape evolution of ZnO upon Co2+ ion doping. Figure 4a shows lower and higher magnified FE-SEM images of pristine ZnO sample, which revealed the uniform distribution of the products with distinct hexagonal prismoid like morphology. In our experiment, initially Zn2+ ions react with the EG molecules to form coordination complexes such as EG(Zn2+).45 At an elevated temperature of 180 °C and autogenous pressure (20 bar) that developed inside the reaction vessels, these complexes break up to produce ZnO crystals in the presence of dissolved oxygen in the solvent. There is another possibility that at aforementioned reaction conditions, the Zn(CH3COO)2·2H2O might also dissociate to form Zn(OH)2 in the presence of OH− ion from NaOH in the solvent, followed by formation of larger hexagonal prismoid ZnO.45 Usually a larger ZnO crystal has a Zn-rich positive polar plane and O-rich negative polar plane. During the crystal growth, the OH− group from EG gets adsorbed to the positive Zn-rich polar plane (001) of ZnO by a

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RESULTS AND DISCUSSION A schematic diagram illustrates the detailed experimental parameters for MW-GT synthesis, and photographic images of pristine ZnO and Co2+ ions doped ZnO were exhibited with a perceived color change from white to Persian-green of liquid suspensions as well as dried powder samples, respectively, shown in Figure 1. The glycothermal method using ethylene glycol (EG) as solvent, combined with microwave irradiation, rapidly forms nanoparticles at a temperature of 180 °C with a respective pressure of 20 bar.39 During the MW-GT synthesis reaction, the precursor solution gets heated as the molecules rotate due to the electromagnetic fields created inside microwave synthesizer. As a result, the nucleation process and rate of particle growth get subsequently increased. Furthermore, particle velocity increases, and thus there is sufficient thermal energy to facilitate incorporation of Co2+ ions into the host ZnO lattice. The detailed structural and morphological studies of Co2+ doped ZnO were investigated with different concentrations of Co2+ at 2, 3, 5, 7, and 10 mol % samples and named as Z-C2, ZC3, Z-C5, Z-C7, and Z-C10, respectively. Figure 2 shows the XRD pattern of the synthesized Co2+ ion doped ZnO and pristine ZnO (ZP) samples. The observed spectra can be well indexed to the hexagonal wurtzite structure of crystalline ZnO (JCPDS no. 36-1451). The sharpness and intensity of the peaks indicated high crystallinity of the obtained samples. No peaks

Figure 2. XRD pattern of pristine and Co doped ZnO prepared by MW-GT method at different doping concentrations: (a) for pristine ZnO, (b) 2 mol %, (c) 3 mol %, (d) 5 mol %, (e) 7 mol %, and (f) 10 mol %. 9983


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Figure 3. Rietveld refinement profiles of X-ray diffraction data of the cobalt doped ZnO at different Co2+ ion concentrations.

Table 1. Structural Parameters of Pristine and Co2+ Ion Doped ZnO Nanostructures (at 2, 5, 7, and 10 mol %) Obtained from XRD Refined Rietveld Pattern sample code 2 mol % 5 mol % 7 mol % 10 mol %

a = b axis 3.2491 3.2513 3.2481 3.2450

(2) (4) (2) (2)

c axis 5.2043 5.2069 5.2020 5.1967

(3) (6) (4) (4)

volume 47.6 47.7 47.6 47.4

(1) (1) (1) (1)

Rwp

Rp

χ2

RBragg

crystalline size (nm)

10.78 10.90 11.72 11.66

07.05 07.08 07.95 07.98

1.041 1.040 1.410 3.773

02.66 03.24 02.99 03.10

40.0 31.4 30.7 32.8

Coulombic interaction, thus slowing the c-axis growth, which facilitates the formation of hexagonal prismoid ZnO shown in Figure 4a. In general, the Zn-rich positive (001) and O-rich negative (001̅) surfaces being more reactive attract the opposite ionic species toward its surface. When we synthesized Co2+ ion doped ZnO at lower Co2+ ion concentration (2 mol %), no significant change in hexagonal prismoid morphology was observed as shown in Figure 4b. Recently, Gamelin’s group reported that dopant metal ions (Co2+ and Ni2+)-induced solution growth of ZnO hinders the formation of 1D-nanorods due to the thermodynamical barrier.46 In contrast, in our reaction conditions, when the Co2+ ion concentration was increased from 2 to 5 mol %, a morphological change from microsize hexagonal prismoid ZnO to 1D-nanorod-like morphology intermingling with particle-like morphology at nanoscale level was observed. It was attributed that the adsorbed Co2+ ion on the O-rich negative (001̅) surface of ZnO seeds promotes the crystal growth along the (001) direction, which leads to the formation of 1D-nanorod like morphology, that overcomes the aforementioned thermodynamic barrier. Moreover, high boiling point solvent EG provides enough thermal energy, which also promotes effective inclusion of Co2+ ion in ZnO host lattice followed by

anisotropic crystal growth along the (001) plane leading to preferential growth of 1D-nanorod like morphology under the MW-GT condition shown in Figure 4c. Interestingly, further increasing the Co2+ ions concentration to 7 mol % leads to selforganization of multiple 1D-nanorods that undergo subsequent oriented attachment with head-to-head aggregation as well as side-by side coalescence resulting in the formation of spectacular highly intense Persian-green colored hierarchial 3D flower-like ZnO as shown in Figure 4d. The schematic illustration of the growth mechanism is depicted in Figure 4e. The Persian-green colored appearance of Co2+ doped ZnO was due to Co2+ ions incorporated in the wurtzite lattice on Zn sites located in a tetrahedral environment of oxygen anions. The ligand field splitting in the tetrahedral geometry is about onehalf that of the octahedral one by which the absorption onset is red-shifted as seen from the green complement color of Co2+ doped ZnO.47 Figure 5 shows HR-TEM analysis of the selected sample Z-C5. The lattice fringes obtained for the sample indicated highly crystalline phase with d-spacing of 2.6 Å, which corresponds to growth along the (002) plane. The as-obtained ZnO nanorods have flat hexagonal crystallographic planes due to hexagonal wurtzite crystal lattice, specifying that ZnO nanorods preferentially orient toward the c-axis direction.48 9984


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Figure 4. FESEM images of Co2+ ion dopant-induced morphological changes of ZnO: (a) hexagonal plate morphology of pristine ZnO, (b) hexagonal prismoid morphology of 3 mol % Co2+ ion doped ZnO, (c) mixed hexagonal rod and particle morphology of 5 mol % Co2+ ion doped ZnO, (d) thick rod morphology of 7 mol % Co2+ ion doped ZnO, with inset figures showing the magnified FESEM images, and (e) schematic illustration of crystal growth mechanism of pristine and different Co2+ ZnO samples.

Figure 5. HR-TEM images of 5 mol % Co2+ ion doped ZnO: (a) hexagonal particle morphology, (b) thick rod morphology, (c) the SAED pattern, and (d) lattice spacing 2.6 Å between the fringes sowing the growth along the (002) plane.

chemical bonding and chemical state of the excited-state electrons in the selected Co2+ ion doped ZnO sample (Z-C5). The obtained survey scan spectrum of Zn 2p, Co 2p, along with O 1s were shown in Figure 6a. The valence state of Zn in ZnO host lattice was confirmed, and it is found that the dopant ion inclusion does not affect the ZnO wurtizite structure. Strong

Moreover, there are no indications of secondary phases or impurities visible in the HR-TEM images, signifying that all Co2+ ions are homogeneously incorporated into the ZnO nanorods and that no Co clustering had occurred. X-ray photoelectron spectroscopic (XPS) studies were carried out to probe the detailed information about the 9985


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Figure 6. High-resolution XPS spectra of 5 mol % cobalt doped ZnO: (a) survey spectrum, (b) 2p spectra of Zn, (c) 2p spectra of Co, and (d) 1s spectra of O.

Figure 7. (a) Diffuse reflectance spectra of pristine and Co2+ ion doped ZnO prepared by MW/GT method at different doping concentrations. (b) Photoluminescence spectra of pristine and Co2+ ion doped ZnO. (c) Room-temperature Raman spectra of pristine and doped ZnO.

that at least two oxygen species exist in the nearby region. The deconvoluted O 1s spectra revealed the presence of two types of oxygen species such as O 1s (1) and O 1s (2). The occurrence of the O 1s (1) peak in the lower energy region was due to the O2− anion species that bound to the metal cations (Zn2+ or Co2+) preferably in the tetrahedral sites of hexagonal wurtzite structure.51 The formation of the O 1s (2) peak at higher binding energy was attributed to weakly bound −OH species at the surface of sample and also to oxygen defects or variation in oxygen defects, thus revealing chemisorbed −OH species at the sample surface. Figure 7a shows the diffuse reflectance spectra of the prepared samples covering the entire UV−vis spectral range. No indication of quantum confinement was observed from the UV spectra. As compared to the pristine sample, Co2+ ion doped ZnO samples showed three additional peaks at 565, 608, and 655 nm (visible range), respectively, in the sub-band gap region 1.88−3.0 eV. These peaks were recognized as 4A2 (F) → 2 A1 (G), 4A2 (F) → 4T1 (P), and 4A2 (F) → 2E1 (G) interatomic d−d transitions related to tetrahedral crystal field

peaks from oxygen and zinc ions were observed along with a weak intense Co signal. The peaks observed at 1025.08 and 1048 eV in Figure 6b correspond to Zn 2P3/2 and Zn 2P1/2 states, respectively. Four peaks were observed in Figure 6c; a binding energy of Co 2p3/2 is located at 784.7 eV, while Co 2p1/2 peak is obtained at 800.7 eV along with corresponding shakeup satellites at higher energies. The presence of shakeup satellites and binding energy shift of 16 eV between Co 2p3/2 and Co 2p1/2 indicated that Co2+ ions are incorporated into Zn2+ sites and prefer to be in the +2 oxidation state.49 It was again confirmed that the Co ions in the +2 oxidation state were surrounded by an oxygen atom with tetrahedral coordination in the ZnO sample. The presence of two shakeup satellites on the higher binding energy is characteristic of high spin Co2+ originating from the charge-transfer band structure typical for 3d TM-based oxide. Hence, the chemical oxidation state of cobalt in Co doped ZnO was confirmed as +2. Moreover, the high spin divalent state of Co (3d7; S = 3/2) in the sample is expected for substituted Co ions in the Zn sites.50 The obtained O 1s profile in Figure 6d of sample is asymmetric, specifying 9986


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Figure 8. (a) Room-temperature field-dependent magnetization loop of pristine and Co2+ ion doped ZnO showing ferro- to paramagnetic ordering on increasing the dopant concentration. (b) A comparative current density versus voltage curve (J−V) for Co2+ ion doped and pristine ZnO-based photoanode DSSCs. (c) Nyquist and (d) Bode impedance plot for different Co2+ ion doped ZnO photoanode-based DSSCs. (e−h) Schematic representation of DSSC photoanode with (e) pristine ZnO and (f−h) 2, 5, and 7 mol % Co2+ doped ZnO showing dopant-induced morphological evolution and the size-mismatching effect.

exchange interaction between the “sp” and “d” orbitals that resulted in negative or positive correlation with conduction band or valence band edge. This correction factor increases with Co2+ dopant concentration and also with exchange energy for the “sp−d” interaction, which accounts for the observed band gap reduction.53 It is noteworthy to mention that the sample Z-C7 exhibited reduced intensity peak among other samples, which is possibly due to the additional Co2+ ions being incorporated into the ZnO lattice. Moreover, the positions of the triple transition were not changed for varied doping concentrations of Co2+, suggesting that the dopant environment remained the same and is thermodynamically stable as obtained from the XRD. The luminescence properties of pristine and Co2+ ion doped ZnO nanocrystalline samples were analyzed by photoluminescence spectroscopy (PL) illustrated in Figure 7b. For all samples, a strong UV emission peak was observed at 383 nm, which is obtained due to the near band edge emission (NBE)

splitting as well as charge transfer absorptions of Co2+ incorporated in ZnO host lattice and that confirm the presence of a high spin Co2+ (d7, S = 3/2) state in ZnO.52 Here, the charge transfer reaction takes place between the conduction and valence bands formed by atomic orbitals of oxygen from the ZnO and empty orbitals of Co, respectively. The obtained peak is triplet due to the L−S Russel−Sanders coupling. Moreover, the allowed d−d transitions of Co2+ ion having 3d7 high spin configuration in a tetrahedral crystal field formed by neighboring O2− ions evidently suggested that the tetrahedral coordinated Co2+ ions get substituted in the place of Zn2+ ions in the ZnO hexagonal wurtzite structure. The band gap energy of all samples was determined by plotting (αhν)2 versus hν. The linear dependence of (αhν)2 on hν in the range of 3.17− 3.25 eV was obtained, and extrapolation to (αhν)2 = 0 gave the precise optical band gap energy value. The band gap energy decreases from 3.22 eV (pristine ZnO) to 3.17 eV due to increased doping concentration (Figure S3) caused by an 9987


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Table 2. Detailed Photovoltaic Parameters Such as Jsc, Voc, FF, and PCE Derived from J−V Curve and Electron Lifetime Calculated from the Bode Plot of Pristine and Co2+ Ion Doped ZnO Photoanode-Based DSSCs cell Z-P Z-C2 Z-C3 Z-C5 Z-C7 Z-C10

Jsc (mA/cm2) 3.60 3.05 7.92 11.98 6.33 4.38

(±0.02) (±0.01) (±0.02) (±0.02) (±0.04) (±0.01)

Voc (V) 0.52 0.51 0.72 0.61 0.59 0.71

(±0.02) (±0.01) (±0.01) (±0.02) (±0.01) (±0.01)

Jmax (mA/cm2) 2.88 2.19 6.54 9.48 4.85 3.63

(±0.02) (±0.02) (±0.02) (±0.01) (±0.01) (±0.02)

Vmax (V) 0.37 0.39 0.54 0.46 0.46 0.52

(±0.02) (±0.02) (±0.02) (±0.02) (±0.01) (±0.02)

FF 56.56 55.10 61.93 59.40 59.99 60.69

(±0.01) (±0.02) (±0.02) (±0.02) (±0.01) (±0.01)

PCE (%) 1.05 0.86 3.53 4.36 2.23 1.89

(±0.01) (±0.01) (±0.01) (±0.01) (±0.01) (±0.02)

τe (ms) 5.57 (±0.01) 6.1 (±0.02) 13.92 (±0.01) 19.44 (±0.07) 21.29 (±0.05)

lattice. The absence of extra peaks in all of the doped samples demonstrated the phase purity and perfect crystallization, thus drawing a parallel relationship with the XRD data as no secondary phases were observed within the detection limit of the measurement. In general, TM doping in ZnO induces a magnetic nature, which was investigated at 300 K with a maximum applied field of ±1.5 T. Figure 8a illustrates the plot of magnetization as a function of magnetic field for pristine and different Co2+ doping concentrations. Here, we observed that pristine ZnO sample exhibited diamagnetic behavior; however, 2 and 5 mol % Co2+ ion doped ZnO samples exhibited ferromagnetism as was evident from the well-defined hysteresis loop.60−62 The width of the loop decreases as the Co2+ concentration in the sample increases, which implies the presence of soft magnets. The coercive field (HC) and the retentivity (MR) were found to decrease with increase in doping concentration, and the saturation magnetization (MS) of the sample varied in the range of 0.279−0.032 emu/g. The gradual decrease of MS upon increasing doping concentration and subsequent inverse Raman frequency shift and broadening of E 2 mode can be corroborated, and it was concluded that the substitution of Co preferably occurs at the Zn site.63,64 It is also observed that there is shifting from ferromagnetic to paramagnetic nature as the doping concentration increases, and no indication of magnetism was observed for the pristine ZnO sample. As the magnetic property is highly dependent on the synthesis condition, the observed magnetic anisotropy for the samples can be explained as a result of drastic structural changes, defects, and morphological evolution (micro- to nanosize effect). Moreover, no peaks were observed for the formation of metallic Co from the XRD analysis, suggesting that the magnetism arises merely due to doping and not because of Co cluster formation. The residual impurities such as oxygen vacancy and zinc interstitials in Co2+ doped ZnO can act as ntype shallow donors, thereby forming long-range coupling between localized magnetic moments of Co through the conduction band. Ferro to paramagnetic ordering associated with quenching of photoluminescence emission suggests that magnetism in Co2+doped ZnO NC is a defect-induced phenomenon. Fabricated dye-sensitized solar cells with pristine ZnO as well as a series of Co2+ doped ZnO (2, 5, and 7 mol %) were studied under simulated sunlight AM 1.5 with power 100 mW/cm2. Figure 8b illustrates the J−V curves of the DSSCs, and their characteristic performances comprising short circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF), and efficiency (η) are shown in Table 2. For the pristine ZnO sample, the obtained short circuit current and open circuit voltage were Jsc = 3.6 mA/cm2 and Voc = 0.52 V with a PCE of 1.05%. For cobalt doping concentration at 2 mol % (Z-C2), both Jsc and Voc decreased, and hence a lower PCE of 0.88%

originating from the direct exciton recombination. The visible luminescence from surface defects typically observed for the synthesized ZnO-NCs around 550−570 nm was found to be absent or had been quenched. This could be due to good crystallinity of the as-synthesized samples. Another possible reason is that lesser defect sites of the highly crystalline wurtzite lattice could act as electron trap centers, capable of shifting the PL intensity into the visible region. Furthermore, the defect state energies are close to the photon energy, thereby enabling one to excite the transitions in Co2+ and providing an ample pathway for quenching the absorption of energy of defect emission.54 The green yellow emission band obtained around 450−470 nm is attributed to the lattice oxygen vacancies.55 It is also noted that, under the same conditions of excitation, the UV emission intensity was gradually increased for samples ZC2 and Z-C5 followed by a broad peak with decreased intensity for sample Z-C7. This quenching of PL intensity is attributed to the competing superposition of 4A2 (F) → 2A1 (G), 4A2 (F) → 4 T1 (P), and 4A2 (F) → 2E1 (G) absorptions due to the presence of isolated Co2+ ions in ZnO host lattice at higher doping concentrations. Figure 7c indicates the room-temperature Raman spectra of pristine ZnO and Co2+ doped ZnO samples. For all of the samples, the obtained Raman shift was well matched with the wurtzite ZnO vibration modes without any impurity peaks arising from Co2+ substitution. The Co2+ doping weakened the intensity of E2 peaks, but the peak width was increased. Because E2 high mode is related to the vibration of oxygen atoms alone, the changes in its intensity and width are a direct measure of the origin of intrinsic defects associated with oxygen atoms. The shift in peak positions toward lower frequency value and the increment in fwhm were due to Co2+ inclusion, which is in well accordance with the previous reports by Phan et al.56 Thus, it can be inferred that for doped samples, the atomic substitution in ZnO lattice induces the structural disorder that breaks the translational symmetry of allowed Raman vibrations. The oxygen vacancies are produced with a regular decrease in average atomic mass, which caused the shifting of peak position toward lower wavenumber. It is also noted that as the doping concentration increases, the intensity of band centered at 580 cm−1 of multiphonon band increases due to disorder activated Raman scattering, arising from the breakdown of the crystal symmetry. The increase in intensity of E1(LO) can also be explained by the resonance Raman effect at sub-band gap absorption due to the d−d transition of Co.57−59 These observations revealed that, although the local symmetry in Co2+ ion doped ZnO varied with respect to pristine ZnO, the original wurtzite crystal structure is retained. Another interesting observation was the steady increase in intensity and subsequent broadening of the E1 (LO) peaks for the doped ZnO NCs, which was attributed to structural deformations as well as local lattice distortions by Co doping into the ZnO host 9988


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frequency region of Nyquist plot was attributed to charge transport and charge recombination resistance of photoanode semiconductor (ZnO/Co-ZnO) to triiodide of redox-electrolyte. However, under illumination, the conduction band electron density of ZnO/Co-ZnO becomes relatively very high, resulting in small frequency of middle circle dominated by capacitance of ZnO/Co-ZnO paralleled with impedance by charge recombination. Consequently, the middle (second) semicircle chiefly symbolizes the impedance by charge recombination. If the recombination resistance is substantially large, the recombination rate of photo injected electrons in photoanode with electrons of tri-iodide becomes smaller, which is beneficial to accomplish enhanced cell performance. Here, Nyquist plots exhibited three well-defined semicircles at low frequency, middle frequency, and higher frequency regions. Co2+ doping into ZnO results in a decrease of f max values. The calculated electron lifetime value (τe) for doped samples was found to be increased, which demonstrated the easy transfer of electrons over larger distances. Besides, the results also implicated that the transfer of electrons was unblocked to a larger span across the Co2+ doped ZnO/dye/electrolyte interface, resulting in improved electron collection and efficient capture. Thus, the reduction in charge recombination and increase in τe was obtained for Co2+ doped ZnO, which was attributed to Co2+ doping that could act as a charge trapping site ensuing in electron−hole pair separation. The formation of trapping sites increases with an increase in Co2+ dopant concentration, but at higher doping concentration, the fabricated DSSCs exhibited high resistance at the semiconductor oxide/electrolyte interface, which was found to hamper the device performance by seriously affecting the electron transport. At lower doping concentration (2 mol % of Co), the obtained τe was less (6.1 ms) as compared to 13.9 ms of 5 mol % Co2+ doped sample. In addition, a higher concentration of Co (7 and 10 mol %) increases τe to 19.44 and 21.29, respectively, but provides greater resistance for the effective interfacial electron transport as understood from the Nyquist impedance plot (Figure 8c). So optimized device performance was obtained for 5 mol % Co2+doped ZnO sample. The schematic representation of DSSC photoanode with pristine ZnO and Co2+ doped ZnO showing dopant-induced morphological evolution and the sizemismatching effect is shown in Figure 8e−h. Figure 9 demonstrated the optical absorbance containing N719 dye desorbed from different sensitized ZnO films of Co2+ doped ZnO and pristine ZnO. The amount of dye loading was

was obtained. Remarkably, an enhanced PCE of 4.36% was achieved for the sample Z-C5 with higher cobalt doping concentration at 5 mol %, and higher Jsc = 11.98 mA/cm2 and Voc = 0.61 V were obtained. Upon further continuous increase of doping ratio to 7 mol % (Z-C7), PCE decreases to 2.23%, but the PV performance was still higher as compared to the pristine ZnO sample. The Jsc obtained for the Z-C7 sample was 6.33 mA/cm2, which was much lower as compared to DSSCs made with Z-C5, but the Voc value was not much affected (0.59 V) due to additional dopant incorporation in photoanode films, thus preventing effective photoinduced electron−hole pairs separation by acting as recombination centers. Raj et al. also demonstrated the positive influence of Mg2+ ions on the PV performance of ZnO, which is influenced by the morphology and surface chemistry of doped semiconductor film.65 They found that the dopant ions strengthened the surface chemistry of ZnO-based photoanode film. The outcome of our study also corroborated with their results. Here, the increment in Voc for DSSCs with identical electrolyte was due to the accumulation of more electrons in the photoanode and upward movement of the Fermi level. One of the reasons for enhanced performance is the size-mismatched structure of the Co2+ doped ZnO particularly at 5 mol % of doping concentration as observed from FESEM analysis. Such morphologies illustrate advantages of both particle and rod-like structure with high surface area with enhanced scattering effect and better particle interconnection, which offers effective utilization of incident light and fast electron transport significantly through one-dimensional rod structures when applied as photoanode for DSSC. Zhang et al. reported the effect of lithium salt during the synthesis of ZnO aggregates, which proved to improve the surface stability and significant improvement in photocurrent. Their results also corroborate our results.19 Electron lifetime (τe) and recombination resistance of DSSCs were investigated using electrochemical impedance spectroscopic technique at a frequency range from 10−1 to 105 Hz with an alternative current amplitude of 10 mV. Figure 8c and d illustrates the Nyquist and Bode-phase impedance plots of DSSCs with pristine and Co2+ doped ZnO photoanodes under AM 1.5 illumination. Standard fitting software (Z fit) was used to fit the impedance data with equivalent circuit containing circuit elements series resistance Rs, the internal resistance (R), and constant phase element (CPE) or capacitance. The electron lifetimes were calculated from Bode-phase plots, and the obtained results were shown in Table 2. Generally, the Nyquist plot of DSSCs shows three semicircles at three different frequency regions (high, middle, and low) and also three characteristic frequency peaks in the Bode-phase plot.37 The short semicircle present in the high frequency region is associated with electron transfer as well as redox reactions at the interface of counter electrode (Pt) and redox electrolyte. The semicircle at the middle frequency region corresponds to the charge transfer and electron recombination resistance. However, the low frequency semicircle is related to the ionic resistance for diffusion in electrolyte. Figure 8c shows the Nyquist plots of DSSCs based on doped and pristine ZnO photoanodes. The observed impedance components of different interfaces in DSSCs at frequency regions of 103−105 Hz, 1−103 Hz, and 10−1−1 Hz were related to various charge transport at FTO/(ZnO/Co-ZnO) (Rs) and interfaces of counter electrode−electrolyte (R1), (ZnO/Co-ZnO)/N719 dye/electrolyte interfaces (R2), and the electrolytic Nernstian diffusion (R3), respectively.66,67 The impedance at the middle

Figure 9. UV−visible absorption spectra of N719 dye desorbed from different photoanode films of pristine and Co2+ ion doped ZnO. 9989



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CONCLUSIONS In summary, we have successfully demonstrated a sustainable, rapid, and energy-efficient microwave-glycothermal (MW-GT) synthesis strategy for effective doping of Co2+ ion in ZnO host lattice within 15 min by using environmentally benign EG as solvent at temperature as low as 180 °C. In correlation with structural properties, optical and photovoltaic properties caused by Co2+ inclusion into the ZnO host lattice were also systematically investigated. It was observed that the EG solvent facilitated effective incorporation of Co2+ ions into the ZnO host lattice during the MW-GT conditions. The highest PCE of 4.36% in DSSC was achieved by 5 mol % Co2+ ion doped ZnO as compared to undoped ZnO (1.05%). The enhanced PCE was attributed to several factors. First, dopant induced morphological changes from hexagonal prismoid to 1-D nanorod like intermingling with particle-like morphology resulting in size-mismatched combination induced a lightscattering effect. Second, better surface stability provided by Co2+ ion with less dye aggregation suppressed the recombination loss of photoinjected electrons. Third was photoinduced charge carrier formation by charge transfer transition of Co2+ ions and finally anisotropic magnetic behavior from ferromagnetic to paramagnetic nature. EIS studies also concluded the suppression of charge recombination along with an increase in lifetime of photoinjected electrons.

determined using the molar extinction coefficient of the dye. It is observed that the dye loading on the film of pristine ZnO is 4.35 × 10−7 mol cm−2 and for Co2+ doped ZnO is 4.69 × 10−7, 3.95 × 10−7, and 5.78 × 10−7 mol cm−2, respectively, for Co2+ doped ZnO with 2, 5, and 7 mol % Co2+. In our studies, maximum absorption was obtained for doped sample with 7 mol % of Co2+, which was significantly attributed to increased surface roughness of the sample after the incorporation of cobalt ions of higher concentration. However, the Jsc for the sample was very low as inferred from the J−V characterization of Figure 8b. The least dye absorption was obtained for 5 mol % cobalt doped ZnO sample for which the obtained Jsc was minimum. Thus, dye desorption studies suggested that no direct correlation could be drawn between the amount of dye absorbed and enhancement of Jsc and PCE. This view strongly favors the significance of doping-induced morphological variations on PV performance. Our results suggest that 5 mol % concentration of Co2+ in ZnO favors improved PV performance of DSSCs. This is benefited from several factors as discussed earlier, the sizemismatched hierarchical morphology, photoinduced carrier formation by charge transfer transitions of Co2+ ions, blue-shift in the UV region of photoluminescence spectra deprived of any significant peak associated with intrinsic crystal defects, and lowered recombination resistance together with extended electron lifetime, etc. Moreover, we strongly believe the transition stage between the ferro and paramagnetic nature that occurs at the 5 mol % Co2+ dopant concentration is also mainly responsible for structural and optical property enhancement resulting in better PV performance. Further in-depth investigation must be performed on this new aspect to understand clearly the correlation between the magnetic property and the PV performance. Figure 10 summarizes the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01918. Detailed DSSC fabrication procedure, figures showing magnified (100), (002), and (101) XRD peaks, and changes in lattice parameters and band gap as a function of cobalt content (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 91-413-2654975. E-mail: [email protected]. in, [email protected]. ORCID

R. Krishnapriya: 0000-0002-9275-5878 S. Kannan: 0000-0003-2285-4907 A. Vadivel Murugan: 0000-0003-4601-9615 Author Contributions

R.K. performed the experiments, analyzed the data, and prepared the manuscript. S.P. helped to perform MW-GT synthesis, and S.K. helped with XRD refinement. A.V.M. developed the concept, supervised, discussed the results, and critically evaluated the work.

Figure 10. Schematic illustration of possible electron transfer mechanism in Co2+ ion doped ZnO-based photoanode in DSSC.

Notes

The authors declare no competing financial interest.

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possible electron transfer mechanism occurring in the doped ZnO DSSCs. The Co2+ doping in ZnO significantly shifted the conduction band positively and also caused suppressed charge recombination as evident from the UV−vis and impedance spectroscopic studies. As a result, a significant improvement in Jsc was achieved particularly for 5 mol % Co2+ doped ZnO NCs. This enhancement was attributed to several factors such as improved electron lifetime and the enlarged driving force for electron injection from the sufficiently high LUMO of the N719 dye relative to the conduction band of ZnO.

ACKNOWLEDGMENTS We thank the funding agencies University Grants Commission (UGC), Government of India, New Delhi, under the development of dye-sensitized solar cells major project no. 41-376/2012, Pondicherry University Start-up grant no. PU/ PC/Start-up Grant/2010-12/309, and DST-TSD grant nos. PT/2011/178-G and DST/TM/CERI/C-264(G) for financial support. We also acknowledge the Central Instrumentation 9990


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Facility (CIF), Pondicherry University, for material characterization.

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