Unveiling the Co2+ Ion Doping-Induced Hierarchical Shape Evolution

Subscriber access provided by TUFTS UNIV

Article

Unveiling the Co2+ ion Doping Induced Hierarchical Shape Evolution of ZnO: In Correlation with Magnetic and Photovoltaic Performance Ramachandran Krishnapriya, Selvarasu Praneetha, Sanjeevi Kannan, and Arumugam Vadivel Murugan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01918 • Publication Date (Web): 03 Oct 2017 Downloaded from http://pubs.acs.org on October 5, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Sustainable Chemistry & Engineering is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

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), Dr. R. V. Nagar, Kalapet, Puducherry-605014, India. Tel. +91-413-2654975, *

Email:[email protected]; [email protected]

KEYWORDS: cobalt doping, microwave glycothermal, zinc oxide nanocrystals, hierarchical morphology, dye-sensitized solar cells

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 minutes at 180 oC 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 decrease 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 in Co2+ ion concentration was perceived using FE-SEM and TEM analyses. After Co2+ ion inclusion into pristine ZnO, the width of M-H loop significantly changes, where diamagnetic behaviour 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 dyeACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 36

sensitized solar cells (DSSCs) due to nanoscale level intermingling of two different 1-D nanorod-like morphology with particle-like morphology, resulting in size-mismatched combination induced lightscattering effect, photoinduced charge-carrier formation by charge-transfer transitions of high spin Co2+ ions and lower recombination resistance together with extended electron life-time, which were deduced from UV-vis and impedance spectroscopy analysis respectively.

INTRODUCTION

Doping is the process of incorporating intentional impurities into the semiconductor nanocrystals (NCs) to modify their unique structural, optical, electric 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 which provide both technological as well as eco-friendly benefits.4-8 Various devices functioning with ZnO NCs including 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, since this material unveils higher electron mobility than TiO2 and enables rapid transport of photoinjected electrons. The incident light absorption of ZnO occurs mainly in 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 UV region accounts for less than 10%.9 In this regard, there is a necessity to broaden the spectral absorption range in order to boost the solar cell performance.10,11 On the other hand, morphology tuning as 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 ACS Paragon Plus Environment

Page 3 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


synergistically contribute to enhanced PV performance in solar cells.16 Doping of ZnO with various metal and non-metal 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 candidate 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 these processes is the excitation of a sub-band gap absorption by d-d transition. Among various TM doped ZnO, Co2+ 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 comparable ionic radii 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 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 synthesis of doped ZnO NCs by various physical and chemical routes which include hydrothermal (HT), solvothermal (ST), sol-gel synthesis, co-precipitation 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, liquid phase synthesis methods especially HT / ST methods are more facile for controlling the morphology, crystal size, shape and also to provide compositional homogeneity.28 Nevertheless, these methods involve either multi-steps and timeconsuming processes. The combination of liquid-phase synthesis combined 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 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 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 36

Niederberger group successfully synthesized various TM doped ZnO in benzyl alcohol with rapid microwave-assisted non-aqueous sol-gel route within a few minutes for diluted magnetic semiconductor property studies.37 However their protocol required Ar-atmosphere using a glove box for the precursor preparation. In this regard, we successfully demonstrate the synthesis of pristine ZnO and Co2+ ions doped ZnO NCs by a microwave-assisted glycothermal (MW-GT) synthesis in ethylene glycol (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 correlating 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, MW-GT method is greener, energy efficient and sustainable chemistry approach, because the chemicals were used for these reactions are non-toxic, pretty much stable in air and inexpensive. EXPERIMENTAL 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. Synthesis of both pristine and Co2+ ions doped ZnO, the precursor solution was containing 0.495 mol L1

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 transferred into quartz vessels (50 mL capacity) with magnetic stirrer sealed with teflon cap at the top. 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 developed (maximum of 20 bar) inside the reaction vessel was controlled by the system. After the completion of reaction, the precipitates were separated by

ACS Paragon Plus Environment

Page 5 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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= 1.54Ǻ) at 40 kV and 30mA to scan the diffraction angles (2θ) from 20 - 80° with the step size of 0.02° per second and was compared with standard ICDD (International Centre for Diffraction Data). The surface morphology of the samples was studied by using field emission scanning electron microscope (FESEM-SUPRA-55-Carlzeiss, Germany) and High Resolution transmission electron microscopy (HR-TEM) (JOEL/JEM 2100USA). Raman spectroscopic analysis was carried out by Confocal Raman microscope (Renishaw, UK) using 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). UVvisible spectra were obtained using UV-2550, Shimadzu, Japan spectrometer operated in diffused reflection mode. The room-temperature magnetic properties of the doped samples were analyzed by 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-micron 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. 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 perceived color change from white to Persian-green of liquid suspensions as well as dried powder samples respectively are shown in Figure 1. The glycothermal method using ethylene glycol (EG) as solvent, combined with microwave irradiation rapidly forms nanoparticles at temperatures of 180 °C with respective pressure of 20 bar was achieved.39 During the MW-GT synthesis reaction, the precursor solution gets heated as the ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 36

molecules rotate due to the electromagnetic fields created inside microwave synthesizer. As a result, the nucleation process and rate of particle growth gets subsequently increased. Furthermore, particle velocities increase, thus sufficient thermal energy to facilitate incorporate 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 were named as Z-C2, Z-C3, 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 related to impurities either metallic cobalt or cobalt oxides were identified within the detection limit of the XRD instrument. This results 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 (100), (002) and (101) peaks are revealed that the peak intensities, position and line-widths changed after the incorporation of Co2+ ions in the ZnO host lattice.40 In addition the peaks were slightly shifted towards the higher 2θ angles when the concentration of Co2+ ions increases (Figure S1, Supporting Information). 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 Fullprof Program to understand precisely the lattice parameters changes of ZnO after different content (mol%) of Co2+ ion doping. Further 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 ZnO crystal lattice was contracts with increase in Co2+ ion doping concentration which was attributed to the substitutional doping of Co2+ ion into 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 ACS Paragon Plus Environment

Page 7 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Vegard’s law.44 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 were further substantiating the successful incorporation of Co2+ ion into ZnO host lattice even at less 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 namely, temperature, pressure, time, kinetic energy barrier and solvents influence the growth pattern of crystals under non-equilibrium 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 4 (a-e) shows FE-SEM images and schematic illustration of crystal growth patterns of pristine ZnO sample and shape evolution of ZnO upon Co2+ ion doping. Figure 4 a 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) 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 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 Coulombic interaction, thus slowing down the c-axis growth which facilitates the formation of hexagonal prismoid ZnO shown in Figure 4 a. In general, the Zn-rich positive (001) and O-rich negative (00ī) surfaces being more reactive attract the opposite ionic species towards 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. ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 36

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 Co2+ ion concentration was increased from 2 mol % 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 (00ī) surface of ZnO seeds promotes the crystal growth along (001) direction which leads to 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 (001) plane leading to preferential growth of 1D-nanorod like morphology under the MW-GT condition shown in Figure 4 c. Interestingly, further increasing the Co2+ ions concentration to 7 mol % leads to self-organization 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 3-D flowerlike ZnO as shown in Figure 4 d. The schematic illustration of growth mechanism is depicted in Figure 4 e. The Persian-green colored appearance of Co2+ doped ZnO was owed 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 half as 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 HRTEM 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 asobtained ZnO nanorods have flat hexagonal crystallographic planes due to hexagonal wurtzite crystal lattice, specifying that ZnO nanorods preferentially orient towards the c-axis direction.48 Moreover, there are no indications of secondary phases or impurities visible in the HR-TEM images signifying that all Co2+ ions are homogenously incorporated into the ZnO nanorods and that no Co clustering had occurred. ACS Paragon Plus Environment

Page 9 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


X-ray photoelectron spectroscopic (XPS) studies were carried out to probe the detail information about the 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 O1s 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 peaks from oxygen and zinc ions were observed along with 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, 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 shake-up satellites at higher energies. The presence of shake-up satellites and binding energy shift of 16 eV between Co 2p3/2 and Co 2p1/2 indicated that Co2+ ions are incorporated in Zn2+ sites and prefer to be in the +2 oxidation state.49 It was again confirmed that the Co ions in +2 oxidation state was surrounded by oxygen atom with tetrahedral coordination in the ZnO sample. The presence of two shake-up 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 into the Zn sites.50 The obtained O 1s profile in Figure 6d of sample is asymmetric, specifying that at least two oxygen species are existing 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 O 1s (1) peak at lower energy region was owed 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 O 1s (2) peak at higher binding energy was attributed to weakly bound –OH species at the surface of sample and also due to oxygen defects or variation in oxygen defects; thus revealing chemisorbed -OH species at the sample surface. Figure 7 a 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. Compared to the pristine sample, Co2+ ion doped ZnO samples showed three additional peaks at 565, 608 and 655 nm ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 36

(visible range) respectively in the sub-band-gap region 1.88 to 3.0 eV. These peaks were recognized as 4

A2 (F)2A1 (G) , 4A2 (F)4T1 (P) and 4A2 (F)2E1 (G) interatomic d-d transitions related with

tetrahedral crystal field splitting as well as charge transfer absorptions of Co2+ incorporated in ZnO host lattice and which confirm the presence of high spin Co2+ (d7, S = 3/2) state in ZnO.52 Here the charge transfer reaction take place between the conduction and valance band formed by atomic orbitals of oxygen from the ZnO and empty orbitals of Co respectively. The obtained peak is triplet owing to the LS 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 were 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, Supporting Information) caused by exchange interaction between the ‘sp’ and ‘d’ orbitals that resulted in negative or positive correlation with conduction band or valance 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 sample which is possibly due to the additional Co2+ ions incorporated into the ZnO lattice. Moreover, the positions of the triple transition were not changed for varied doping concentration of Co2+, suggesting that 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 7 b. For all samples a strong UV emission peak was observed at 383 nm which is obtained due to the near band edge emission (NBE) originating from the direct exciton recombination. The visible luminescence from surface defects typically observed for the synthesized ZnO-NCs around 550 - 570 nm were found to be absent or have ACS Paragon Plus Environment

Page 11 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


been quenched. This can 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 centres, capable of shifting the PL intensity into the visible region. Furthermore, the defect state energies are close to the photon energy, thereby enabling 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 Z-C2 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 as 4A2 (F)2A1 (G) , 4A2 (F)4T1 (P) and 4A2 (F)2E1 (G) absorptions due to the presence of isolated Co2+ ions in ZnO host lattice at higher doping concentrations. Figure 7 c indicates the room temperature Raman spectra of pristine ZnO and Co2+ doped ZnO samples. For all the samples, the obtained Raman shift was well matching 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. Since E2 high mode is related with the vibration of oxygen atoms alone, the changes in its intensity and width is a direct measure of the origin of intrinsic defects associated with oxygen atoms. The shift in peak positions towards lower frequency value and the increment in FWHM were due to Co2+ inclusion which is 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 towards lower wave number. It is also noted that as the doping concentration increases, the intensity of band centred at 580 cm-1 of multiphonon band increases owing 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 d-d transition of Co.57-59 These observations revealed that although the local symmetry ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 36

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 lattice. The absence of extra peaks in all 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 300K with a maximum applied field of ±1.5 T. Figure 8 a 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 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 emu/g to 0.032emu/g. The gradual decrease of MS upon increasing doping concentration and subsequent inverse Raman frequency shift and broadening of E2 mode can be corroborated and concluded that the substitution of Co preferably occurs at Zn site.63,64 It is also observed that there is shifting from ferromagnetic to paramagnetic nature as doping concentration increases and no indication of magnetism was observed for pristine ZnO sample. As the magnetic property is highly depending 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 nano size 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 not because of Co cluster formation. The residual impurities such as oxygen vacancy and zinc interstitials in Co2+ doped ZnO can act as n-type shallow donors thereby forming long-range coupling between localized magnetic moments of Co through the conduction band. Ferro to para magnetic ACS Paragon Plus Environment

Page 13 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ordering associated with quenching of photoluminescence emission suggests that magnetism in Co2+doped ZnO NC is defect-induced phenomenon. Fabricated dye-sensitized solar cells with pristine ZnO as well as a series of Co2+ doped ZnO (2 mol %, 5 mol % and 7 mol %) were studied under simulated sunlight AM 1.5 with power 100 mW/cm2. Figure 8 b illustrates the J-V curves of the DSSCs and their characteristic performance comprising short circuit current density (Jsc), open-circuit voltage (Voc), fill factor (FF) and efficiency (Ƞ) are precised in Table 2. For pristine ZnO sample, the obtained short circuit current and open circuit voltage was Jsc = 3.6 mA/cm2 and Voc = 0.52 V with PCE of 1.05 %. For cobalt doping concentration at 2 mol % (Z-C2), both Jsc and Voc decreased and hence lower PCE of 0.88% 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 PV performance was still higher compared to pristine ZnO sample. The Jsc obtained for the Z-C7 sample was 6.33 mA/cm2 which was much lower 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 photo-induced electronhole 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 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 36

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 corroborating our results.19 Electron life time (τe) and recombination resistance of DSSCs were investigated using electrochemical impedance spectroscopic technique at a frequency range of 10-1 Hz to 105 Hz with an alternative current amplitude of 10 mV. Figure 8 c 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 life-times were calculated from Bode-phase plots and the obtained results were shown in Table 2. Generally, Nyquist plot of DSSCs shows three semi circles at three different frequency regions (high, middle and low) and also three characteristic frequency peaks in Bode-phase plot.37 The short semi-circle present in high frequency region is associated with electron transfer as well as redox reactions at the interface of counter electrode (Pt) and redox-electrolyte. The semi-circle at middle frequency region corresponds to the charge transfer and electron recombination resistance. However, the low frequency semi-circle is related to the ionic resistance for diffusion in electrolyte. Figure 8 c 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 to 105 Hz, 1 to 103 Hz, and 10-1 to 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 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


Page 15 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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 lesser, which is beneficial to accomplish enhanced cell performance. Here, Nyquist plots exhibited well-defined 3 semi-circles at low frequency, middle frequency and higher frequency regions. Co2+ doping into ZnO results in decrease of fmax values. The calculated electron life time value (τe) for doped samples were found to be increased, which demonstrated the easy transfer of electrons over larger distances. Besides, the results also implicated that the transfer of electrons were 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 increase in Co2+ dopant concentration, but at higher doping concentration, the fabricated DSSCs exhibited high resistance at semiconductor oxide / electrolyte interface which 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) compared to 13.9 ms of 5 mol% Co2+ doped sample. In addition, 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 Nyquist impedance plot (Figure 8 c). So optimized device performance 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 size-mismatching effect are shown in Figure 8 e – 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 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 mol cm-2, 3.95 × 10-7 mol cm-2, and 5.78 × ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 36

10-7 mol cm-2 respectively for Co2+ doped ZnO with 2 mol %, 5 mol % 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 8 b. 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; namely the size-mismatched 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 life time etc. Moreover, we strongly believe the transition stage in 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 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.


Page 17 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CONCLUSIONS In summary, we have successfully demonstrated a sustainable, rapid and energy-efficient microwaveglycothermal (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, 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 compared to undoped ZnO (1.05%). The enhanced PCE was attributed to several factors. Firstly, dopant induced morphological changes from hexagonal prismoid to 1-D nanorod like intermingling with particle-like morphology resulting in size-mismatched combination induced light-scattering effect. Secondly, better surface stability provided by Co2+ ion with less dye aggregation that suppress the recombination loss of photo-injected electrons. Thirdly, 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 increase in life time of photo-injected electrons. ASSOCIATED CONTENT Supporting Information Detailed DSSC fabrication procedure, figures showing magnified (100), (002) and (101) XRD peaks, changes in lattice parameters and band gap as a function of cobalt content. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *A. Vadivel Murugan. Email: [email protected]; [email protected]: 91-413-2654975. 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 for XRD refinement. A.V.M developed the concept, supervised, ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 36

discussed the results and critically evaluated the work. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We thank the funding agencies University Grants Commission (UGC), Government of India, New Delhi, under 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 No. PT/2011/178-G & DST/ TM/CERI/ C264(G) for the financial support. We also acknowledge Central Instrumentation Facility (CIF), Pondicherry University, for material characterization. REFERENCES 1. Buonsanti, R.; Milliron, D. J. Chemistry of Doped Colloidal Nanocrystals. Chem. Mater. 2013, 25, 1305-1317. 2. Janisch, R.; Gopal, P.; Spaldin, N. A. Transition metal-doped TiO2 and ZnO—present status of the field. J. Phys. Condens. Matter. 2005, 17, R657-R689. 3. Liu, S.; Su, X. The Synthesis and Application of Doped Semiconductor Nanocrystals. Anal. Methods 2013, 5, 4541-4548. 4. Zhang, Q.; Dandeneau, C. S.; X. Zhou, G. Cao. ZnO Nanostructures for Dye-Sensitized Solar Cells. Adv. Mater. 2009, 21, 4087-4108. 5. Djurisic, A. B.; Chen, X.; Leung, Y. H.; Ng, A. M. C. ZnO Nanostructures: Growth, Properties and Applications. J. Mater. Chem. 2012, 22, 6526-6535. 6. Mende, L. S.; Driscoll, J. L. M. ZnO – Nanostructures, Defects, and Devices. MaterialsToday 2007, 10, 40-48. 7. Wang, Z. L. Splendid One-Dimensional Nanostructures of Zinc Oxide: A New Nanomaterial Family for Nanotechnology. ACS Nano 2008, 10, 1987-1992. ACS Paragon Plus Environment

Page 19 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8. Wang, Z. L. Zinc Oxide Nanostructures: Growth, Properties and Applications. J. Phys.: Condens. Matter 2004, 16, R829-R858. 9. Djurisic, A. B.; Leung, Y. H. Optical Properties of ZnO Nanostructures. Small 2006, 2, 944961. 10. Srikant, V.; Clarke, D. R. On the Optical Band Gap of Zinc Oxide. J. Appl. Phys. 1998, 83, 5447-5451. 11. Sun, Y-H.; Liao, W-P.; Chen, D-W.; Wu, C-T.; Chang, G-J.; Wu, J-J. Room-Temperature Tailoring of Vertical ZnO Nanoarchitecture Morphology for Efficient Hybrid Polymer Solar Cells. Adv. Funct. Mater. 2012, 22, 3808-3814. 12. Guo, D.; Xiao, S.; Fan, K.; Yu, J. Hierarchical TiO2 Submicrorods Improve the Photovoltaic Performance of Dye-Sensitized Solar Cells. ACS Sustainable Chem. Eng. 2017, 5, 1315−1321. 13. Anta, J. A.; Guillen, E.; Zaera, R. T. ZnO-Based Dye-Sensitized Solar Cells. J. Phys. Chem. C 2012, 116, 11413−11425. 14. McCune, M.; Zhang, W.; Deng, Y. High Efficiency Dye-Sensitized Solar Cells Based on ThreeDimensional Multilayered ZnO Nanowire Arrays with “Caterpillar-like” Structure. Nano Lett. 2012, 12, 3656-3662. 15. Baxter, J. B.; Aydil, E. S. Dye-sensitized Solar Cells Based on Semiconductor Morphologies with ZnO Nanowires. Solar Energy Materials & Solar Cells 2006, 90, 607-622. 16. Erwin, S. C.; Zu, L.; Haftel, M. I.; Efros, A. L.; Kennedy, T. A.; Norris, D. J. Doping Semiconductor Nanocrystals. Nature 2005, 436, 91-94. 17. Zhang, Q.; Dandeneau, C. S.; Candelaria, S.; Liu, D.; Garcia, B. B.; Zhou, X.; Jeong, Y-H.; Cao, G. Effects of Lithium Ions on Dye-Sensitized ZnO Aggregate Solar Cells. Chem. Mater. 2010, 22, 2427–2433. ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 36

18. Mahmood, K.; Park, S. B. Highly Efficient Dye-sensitized Solar Cell with an Electrostatic Spray Deposited

Upright-Standing

Boron-doped

ZnO

(BZO)

Nanoporous

Nanosheet-based

Photoanode. J. Mater. Chem. A 2013, 1, 4826-4835 19. Zheng, Y-Z.; Tao, X.; Hou, Q.; Wang, D-T.; Zhou, W. -L.; Chen, J-F. Iodine-Doped ZnO Nanocrystalline Aggregates for Improved Dye-Sensitized Solar Cells. Chem. Mater. 2011, 23, 3–5. 20. Kuriakose, S.; Satpati, B.; Mohapatra, S. Highly Efficient Photocatalytic Degradation of Organic Dyes by Cu Doped ZnO Nanostructures. Phys. Chem. Chem. Phys. 2015, 17, 25172-25181. 21. Mishra, Y. K.; Modi, G.; Cretu, V.; Postica, V.; Lupan, O.; Reimer, T.; Paulowicz, I.; Hrkac, V.; Benecke, W.; Kienle, L.; Adelung, R. Direct Growth of Freestanding ZnO Tetrapod Networks for Multifunctional Applications in Photocatalysis, UV Photodetection, and Gas Sensing. ACS Appl. Mater. Interfaces 2015, 7, 14303–14316. 22. Lam, S-M.; Sin, J-C.; Abdullah, A. Z.; Mohamed, A. R. Transition Metal Oxide Loaded ZnO Nanorods: Preparation, Characterization and Their UV–vis photocatalytic activities. Sep. Purif. Technol. 2014, 132, 378-387. 23. Ma, Q.; Lv, X.; Wang, Y.; Chen, J. Optical and Photocatalytic Properties of Mn Doped Flowerlike ZnO Hierarchical Structures. Optical Materials 2016, 60, 86-93. 24. Schelonka, D.; Tolasz, J.; Stengl, V. Doping of Zinc Oxide with Selected First Row Transition Metals for Photocatalytic Applications. Photochemistry and Photobiology 2015, 9, 1071-1077. 25. Kaneva, N. V.; Dimitrov, D. T.; Dushkin, C. D. Effect of Nickel Doping on the Photocatalytic Activity of ZnO Thin Films under UV and Visible Light. Applied Surface Science 2011, 257, 8113-8120.


Page 21 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


26. Djerdj, I.; Jaglicic, Z.; Arcon, D.; Niederberger, M. Co-Doped ZnO Nanoparticles: Minireview. Nanoscale 2010, 2, 1096-1104. 27. Jang, D. M.; Kwak, I. H.; Kwon, E. L.; Jung, C. S.; Im, H. S.; K.; Park, J. Transition-Metal Doping of Oxide Nanocrystals for Enhanced Catalytic Oxygen Evolution. J. Phys. Chem. C 2015, 119, 1921-1927. 28. Xu, L.; Hu, Y-L.; Pelligra, C.; Chen, C-H.; Jin, L.; Huang, H.; Sithambaram, S.; Aindow, M.; , Joesten, R.; Suib, S. L. ZnO with Different Morphologies Synthesized by Solvothermal Methods for Enhanced Photocatalytic Activity. Chem. Mater. 2009, 21, 2875-2885. 29. Manthiram, A; Vadivel Murugan, A; Sarkar, A; Muraliganth, T. Nanostructured Electrode Materials for Electrochemical Energy Storage and Conversion. Energy Environ. Sci., 2008, 1, 621-638. 30. Baghbanzadeh, M.; Carbone, L.; Cozzoli, P. D.; Kappe, C. O. Microwave-Assisted Synthesis of Colloidal Inorganic Nanocrystals. Angew. Chem. Int. Ed. 2011, 50, 11312 – 11359. 31. Praneetha, S.; VadivelMurugan, A. Development of Sustainable Rapid Microwave Assisted Process for Extracting Nanoporous Si from Earth Abundant Agricultural Residues and their Carbon based Nanohybrids for Lithium Energy Storage. ACS Sustainable Chem. Eng. 2015, 3, 224–236. 32. Chetty, S. S.; Praneetha, S.; Basu, S.; Sachidanandan, C.; VadivelMurugan, A. Sustainable, Rapid Synthesis of Bright-Luminescent CuInS2-ZnS Alloyed Nanocrystals: Multistage Nanoxenotoxicity Assessment and Intravital Fluorescence Bioimaging in Zebrafish-Embryos. Sci. Rep. 2016, 6, 26078. 33. Krishnapriya, R.; Praneetha, S.; VadivelMurugan, A. Microwave-solvothermal Synthesis of Various TiO2 Nano-morphologies with Enhanced Efficiency by Incorporating Ni Nanoparticles ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 36

in an Electrolyte for Dye Sensitized Solar Cells. Inorg. Chem. Front., 2017, DOI: 10.1039/C7QI00329C. 34. Krishnapriya, R.; Praneetha, S.; Rabel, A. M.; VadivelMurugan, A. Energy Efficient, One-step Microwave-solvothermal Synthesis of Highly Electro Catalytic NiCo2S4 Thiospinel/graphene Nanohybrid as a Novel Sustainable Counter Electrode Material for Pt-free Dye-sensitized Solar Cells. J. Mater. Chem. C 2017, 5, 3146—3155. 35. Andjelkovic, I.; Stankovic, D.; Nesic, J.; Krstic, J.; Vulic, P.; Manojlovic, D.; Roglic, G. Fe Doped TiO2 Prepared by Microwave-Assisted Hydrothermal Process for Removal of As(III) and As(V) from Water. Ind. Eng. Chem. Res. 2014, 53, 10841–10848. 36. VadivelMurugan, A.; Muraliganth, T.; Manthiram, A. Rapid, Facile Microwave-Solvothermal Synthesis of Graphene Nanosheets and Their Polyaniline Nanocomposites for Energy Strorage. Chem. Mater. 2009, 21, 5004-5006. 37. Bilecka, I.; Niederberger, M. Microwave Chemistry for Inorganic Nanomaterials Synthesis. Nanoscale 2010, 2, 1358–1374. 38. Krishnapriya, R.; Praneetha, S.; VadivelMurugan, A. Energy-efficient, Microwave-Assisted Hydro/Solvothermal Synthesis of Hierarchical Flowers and Rice Grain-like ZnO Nanocrystals as Photoanodes for High Performance Dye-sensitized Solar Cells.

CrystEngComm. 2015, 17,

8353–8367. 39. VadivelMurugan, A.; Muraliganth T.; Ferreira, P. J.; Manthiram, A. Dimensionally Modulated, Single-Crystalline LiMPO4 (M= Mn, Fe, Co, and Ni) with Nano-Thumblike Shapes for HighPower Energy Storage. Inorg. Chem. 2009, 48, 946-952. 40. Djerdj, I.; Garnweitner, G.; Arčon, D.; Pregelj, M.; Jagličić, Z.; Niederberger, M. Diluted Magnetic Semiconductors: Mn/Co-doped ZnO Nanorods as Case Study. J. Mater. Chem. 2008, 18, 5208-5217.


Page 23 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


41. Pazhanivelu, V.; Selvadurai, A. P. B.; Murugaraj, R.; Muthuselvam, I. P.; Chou, F. C. Influence of Co Ions Doping in Structural, Vibrational, Optical and Magnetic Properties of ZnO Nanoparticles. J Mater Sci: Mater Electron. 2016, 27, 8580-8589. 42. Sudhakar, N; Singh, R. K; Mishra S. K; Kannan, S. Quantitative Studies on the Size Induced Anatase to Rutile Phase Transformation in TiO2–SiO2 Binary Oxides During Heat Treatments. RSC Adv., 2014, 4, 49752–49761 43. Vasanthavel, S; Nandha Kumar, P; Kannan S. Quantitative Analysis on the Influence of SiO2 Content on the Phase Behavior of ZrO2. J. Am. Ceram. Soc. 2014, 97, 635–642. 44. Gandhi, V; Ganesan, R. Syedahamed H. H. A; Thaiyan, M. Effect of Cobalt Doping on Structural, Optical, and Magnetic Properties of ZnO Nanoparticles Synthesized by Coprecipitation Method. J. Phys. Chem. C 2014, 118, 9715−9725. 45. Ghoshal, T; Kar, S; Chaudhuri, S. ZnO Doughnuts: Controlled Synthesis, Growth Mechanism, and Optical Properties Crystal Growth & Design, 2007, 7, 136-149. 46. Schwartz, D. A.; Norberg, N. S.; Nguyen, Q. P.; Parker, J. M.; Gamelin, D. R. Magnetic Quantum Dots: Synthesis, Spectroscopy, and Magnetism of Co2+- and Ni2+-Doped ZnO Nanocrystals. J. Am. Chem. Soc. 2003, 125, 13205-13218. 47. Busgen, T.; Hilgendorff, M.; Irsen, S.; Wilhelm, F.; Rogalev, A.; Goll, D.; Giersig, M. Colloidal Cobalt-Doped ZnO Nanorods: Synthesis, Structural and Magnetic Properties. J. Phys. Chem. C 2008, 112, 2414−2417. 48. Boppella, R.; Anjaneyulu, K.; Basak, P.; Manorama, S. V. Facile Synthesis of Face Oriented ZnO Crystals: Tunable Polar Facets and Shape Induced Enhanced Photocatalytic Performance. J. Phys. Chem. C 2013, 117, 4597-4605.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 36

49. Lu, Y.; Lin, Y.; Wang, D.; Wang, L.; Xie, T.; Jiang, T. A High Performance Cobalt-Doped ZnO Visible Light Photocatalyst and its Photogenerated Charge Transfer Properties, Nano Res. 2011, 4, 1144-1152. 50. Wang, T; Liu, Y; Xu, Y; He, G; Li, G; Lv, J; Wu, M; Sun, Z; Fang, Q; Ma, Y; Li, J. Synthesis of 1D and Heavily Doped Zn1-X Coxo Six-Prism Nanorods: Improvement of Blue–Green Emission and Room Temperature Ferromagnetism. J. Mater. Chem. 2011, 21, 18810–18816. 51. Catto, A. C.; da Silva, L. F.; Bernardi, M. I. B.; Bernardini, S.; Aguir, K.; Longo, E.; Mastelaro, V. R. Local Structure and Surface Properties of CoxZn1−xO Thin Films for Ozone Gas Sensing. Appl. Mater. Interfaces 2016, 8, 26066−26072. 52. Koidl, P. Optical Absorption of Co2+ in ZnO. Phys. Rev. B 1997, 15, 2493-2499. 53. Qiu, X.; Li, L.; Li, G. Nature of the Abnormal Band Gap Narrowing in Highly Crystalline Zn1xCoxO

Nanorods. Appl. Phys. Lett. 2006, 88, 11410.

54. Yamamoto, S. Photoluminescence Quenching in Cobalt Doped Zno Nanocrystals. Journal of Applied Physics 2012, 111, 094310. 55. Zeng, H.; Duan, G.; Li, Y.; Yang, S.; Xu, X.; Cai, W. Blue Luminescence of ZnO Nanoparticles Based on Non-Equilibrium Processes: Defect Origins and Emission Controls. Adv. Funct. Mater. 2010, 20, 561-572. 56. Phan, L; Vincent, R; Cherns, D; Nghia, N. X.; Ursaki, V. V. Raman Scattering in Me-Doped Zno Nanorods (Me=Mn, Co, Cu And Ni) Prepared by Thermal Diffusion. Nanotechnology 2008, 19, 475702. 57. Du, C. L.; Gu, Z. B.; Lu, M. H.; Wang, J.; Zhang, S. T.; Zhao, J.; Cheng, G. X.; Heng, H.; Chen, Y. F. Raman Spectroscopy of (Mn, Co)-Codoped Zno Films. J. Appl. Phys. 2006, 99, 123515. ACS Paragon Plus Environment

Page 25 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


58. Cuscó, R.; Lladó, E. A.; Ibáñez, J.; Artús, L.; Jiménez, J.; Wang, B.; Callahan, M. J. Temperature Dependence of Raman Scattering in Zno Phys. Rev. B 2007,75, 165202. 59. Bergman, L.; Chen,X.-B.; Huso, J.; Morrison, J. L.; Hoeck, H. Raman Scattering of Polar modes of ZnO Crystallites. J. Appl. Phys. 2005, 98, 093507. 60. Hays, J.; Reddy, K. M.; Graces, N. Y.; Engelhard, M. H.; Shutthanandan, V.; Luo, M.; Xu, C.; Giles, N. C.; Wang, C.; Thevuthasan, S.; Punnoose, A. Effect of Co Doping on the Structural, Optical and Magnetic Properties of Zno Nanoparticles. J. Phys.: Condens. Matter 2007, 19, 266203. 61. Tahir, N.; Karim, A.; Persson, K. A.; Hussain, S. T.; Cruz, A. G.; Usman, M.; Naeem, M.; Qiao, R.; Yang, W.; Chuang, Y-De.; Hussain, Z. Surface Defects: Possible Source of Room Temperature Ferromagnetism in Co-Doped ZnO Nanorods.

J. Phys. Chem. C 2013, 117,

8968−8973. 62. Basu, S.; Inamdar, D. Y.; Mahamuni, S.; Chakrabarti, A.; Kamal, C. G.; Kumar, R.; Jha, S. N.; Bhattacharyya, D. Local Structure Investigation of Cobalt and Manganese Doped ZnO Nanocrystals and its Correlation with Magnetic Properties.

J. Phys. Chem. C 2014, 118,

9154−9164. 63. Singhal, A.; Achary, S. N.; Manjanna, J.; Chatterjee, S.; Ayyub, P.; Tyagi, A. K. Chemical Synthesis and Structural and Magnetic Properties of Dispersible Cobalt- and Nickel-Doped ZnO Nanocrystals . J. Phys. Chem. C 2010, 114, 3422–3430. 64. Inamdar, D. Y.; Lad, A. D.; Pathak, A. K.; Dubenko, I.; Ali, N.; Mahamuni, S. Ferromagnetism in ZnO Nanocrystals: Doping and Surface Chemistry. J. Phys. Chem. C 2010, 114, 1451-1459. 65. Raj, C. J.; Prabakar, K.; Karthick, S. N.; Hemalatha, K. V.; Son, M-K.; Kim, H-J. Banyan Root Structured Mg-Doped ZnO Photoanode Dye-Sensitized Solar Cells. J. Phys. Chem. C 2013, 117, 2600-2607.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 36

66. Liberatore, M.; Decker, F.; Burtone, L.; Zardetto, V.; Brown, T. M.; Reale, A.; Di Carlo, A. Using EIS for Diagnosis of Dye-sensitized Solar Cells Performance. J. Appl. Electrochem. 2009, 39, 2291-2295. 67. Wang Q., Moser, J-E. Gra1tzel, M. Electrochemical Impedance Spectroscopic Analysis of DyeSensitized Solar Cells. J. Phys. Chem. B 2005, 109, 14945-14953. Table 1. Structural parameters of pristine and Co2+ doped ZnO nanostructures (at 2, 5, 7 and 10 mol%) obtained from XRD refined Rietveld pattern. Sample Code

a=b axis

c-axis

volume

Rwp

Rp

χ2

Rbragg

Crystalline size (nm)

2 mol%

3.2491 (2)

5.2043 (3)

47.6 (1)

10.78

07.05

1.041

02.66

40.0

5 mol%

3.2513 (4)

5.2069 (6)

47.7 (1)

10.90

07.08

1.040

03.24

31.4

7 mol %

3.2481 (2)

5.2020 (4)

47.6 (1)

11.72

07.95

1.410

02.99

30.7

10 mol %

3.2450 (2)

5.1967 (4)

47.4 (1)

11.66

07.98

3.773

03.10

32.8

Table.2 Detailed photovoltaic parameters such as Jsc, Voc, FF, PCE derived from J-V curve and electron life time calculated from the bode plot of pristine and Co2+ doped ZnO photoanode based DSSCs.

Cell

Jsc (mA/cm2)

Vmax (V)

FF

PCE (%)

τe (ms)

Z-P

3.60 (±0.02) 0.52 (±0.02) 2.88 (±0.02)

0.37 (±0.02)

56.56 (±0.01)

1.05 (±0.01)

5.57 (±0.01)

Z-C2

3.05 (±0.01) 0.51 (±0.01) 2.19 (±0.02)

0.39 (±0.02)

55.10 (±0.02)

0.86 (±0.01)

6.1 (±0.02)

Z-C3

7.92 (± 0.02) 0.72 (± 0.01) 6.54 (± 0.02)

0.54 (± 0.02)

61.93 (± 0.02)

3.53 (± 0.01)

-

Z-C5

11.98 (±0.02) 0.61 (±0.02) 9.48 (±0.01)

0.46 (±0.02)

59.40 (±0.02)

4.36 (±0.01)

13.92 (±0.01)

Z-C7

6.33 (±0.04) 0.59 (±0.01) 4.85 (±0.01)

0.46 (±0.01)

59.99 (±0.01)

2.23 (±0.01)

19.44 (±0.07)

Z-C10

4.38 (± 0.01) 0.71(± 0.01) 3.63 (± 0.02)

0.52 (± 0.02)

60.69 (± 0.01)

1.89 (± 0.02)

21.29 (± 0.05)

Voc (V)

Jmax (mA/cm2)


Page 27 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIGURE CAPTIONS Figure 1. Schematic illustration of rapid, sustainable Microwave-Glycothermal (MW-GT) synthesis of Co doped ZnO with different reaction parameters. Figure 2. XRD pattern of Pristine and Co doped ZnO pre-pared by MW-GT method at different doping concentrations. (a) for pristine ZnO (b) 2 mol% (c) 3 mol% (d) 5 mol% (e) 7 mol% (f) 10 mol% Figure 3. Rietveld refinement profiles of X-ray diffraction data of the cobalt doped ZnO at different Co2+ concentrations. 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. Inset figures showing the magnified FESEM images (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 mor-phology (c) the SAED pattern (d) lattice spacing 2.6 Å between the fringes sowing the growth along (002) plane. Figure 6. High resolution XPS spectra of 5 mol% Cobalt doped ZnO (a) survey spectrum (b) 2p spectra of Zinc (c) 2p spectra of Co (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+ doped ZnO (c) Room-temperature Raman Spectra of pristine and doped ZnO. 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 Vs. Voltage curve (J-V) for Co2+ ion doped and pristine ZnO based photoanode DSSCs. (c) Nyquist & (d) bode Impedance plot for different Co2+ ion doped ZnO photoanode based DSSCs. (e – ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 36

h) The 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. Figure 9. UV-Visible absorption spectra of N719 dye desorbed from different photoanode films of Pristine and Co2+ ion doped ZnO. Figure 10. Schematic illustration of possible electron transfer mechanism in Co2+ doped ZnO based photoanode in DSSC.


Page 29 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FIGURES

Figure.1

Figure.2



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure.3


Page 30 of 36

Page 31 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Figure.4



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure. 5

Figure.6


Page 32 of 36

Page 33 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Figure.7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Figure. 8


Page 34 of 36

Page 35 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure. 9

Figure.10


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 36

TOC Cobalt doped ZnO nanocrystals have been synthesized via a sustainable rapid microwave-assisted glycothermal method for Dye-sensitized solar cells.


36

Unveiling the Co2+ Ion Doping-Induced Hierarchical Shape Evolution

Recommend Documents