Progress on transition metal-doped ZnO nanoparticles and its

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Progress on transition metal-doped ZnO nanoparticles and its application Puspendra Singh, Ranveer Kumar, and Rajan Kumar Singh Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01561 • Publication Date (Web): 19 Aug 2019 Downloaded from pubs.acs.org on August 19, 2019

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Progress on transition metal doped ZnO nanoparticles and its application Puspendra Singha, Ranveer Kumara*, Rajan Kumar Singh*a,b aDepartment

of Physics, Dr. Harisingh Gour Central University, Sagar, 470003,

M. P., 470003 India bDepartment

of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC

------------------------*To whom all correspondence should be addressed. Email: [email protected],

[email protected]

Tel: +91 9425635731

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Abstract ZnO nano particle is still a hot area for research after ample of work done by researchers across the world. It is the versatile material for doping of different transition metals among other Metal oxides. Having a large family of morphological structures, high electron mobility and n type carrier defects it is suitable for a number of applications such as memory devices, spintronics, optoelectronic devices, solar cell, sensors. Here, we have emphasised to review the effect of transition metal (TM) doping on the different physical properties of ZnO nanoparticles and their applications. We have gone through some theoretical models which were used by most of the researchers for explaining the variations in physical properties. Explanation of the sensing, photocatalytic and optoelectronic processes in different classes of devices is briefly described. Lastly the comparative studies of different devices are also made available for clear understanding.

1. Introduction Nanotechnology deals with the particles (nanoparticles) in the range of 1- 100 nm. The word nano comes from Latin word nanus which means dwarf. As the size of particles decreases, number of atoms or molecules available at the surface to react in comparision to number of atoms or molecule inside the bulk increases. In such condition we have to consider the contribution of surface particles in different physical properties. At such a small scale quantum size effect1-3 comes into effect, which alters catalytic activity, chemical reactivity, optical properties, electrical and thermalconductivity, heat capacity4 etc. Zinc oxide is mainly an inorganic compound found in powder form which is white in color and insoluble in water. It is found in the form of zincite mineral inside the earth crust. In 2



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powder form it is mainly used in lubricants, paints, rubber, ointments, batteries, glass, cosmetic, paper etc. Zinc oxide is an II–VI binary semiconducting material having pyroelectric and piezoelectric properties. In nano range it has band gap of 3.37 eV5, large exciton binding energy (60 meV)6 and Hall mobility of the order of 200 cm2 V-1s-17,8 at room temperature are useful for making optoelectronic devices, laser diodes9-11, LEDs (light emitting diodes)12, 13, Ultra-Violet photodetectors14, 15 and electronic devices such as varisters16-18, diodes, transistor19. Piezoelectric property20, large electromechanical coupling and lack of centre of symmetry, makes it a promising material for piezoelectric transducers21, 22 sensors and mechanical actuators23. The unique nanostructure shows that ZnO probably has the richest family of nanostructures, as shown in figure 1 and figure 2, among all materials, both in structures and in properties.

Nanocombs

(figure

2c),

nanorings,

hollow

spheres

(figure

2e),

nanohelixes/nanosprings, nanorods (figure 2g), nanobelts (figure 1f), nanowires (figure 2d) and nanocages of ZnO can be synthesized under specific growth conditions. These nanostructures have novel applications in optoelectronics, sensors24-26, transducers and biomedical sciences. Having high surface to volume ratio these different morphological structures of ZnO are very useful to prepare different gas sensors. The sensing abalitiy of these sensors enhances significantly on doping ZnO with different transition metlas.

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Figure 1: SEM images of different morphological structures of undoped ZnO nanostructures (a, b) adapted with permission from (27). Copyright (2012) American Chemical Society, (c) Reprinted (adapted) with permission from (14). Copyright (2006) nanotechnology, (d) adapted with permission from (28). Copyright (2010) American Chemical Society, (e)adapted with permission from (29). Copyright (2012) American Chemical Society, (f)adapted with permission from (30). Copyright (2001) science, (g, h)adapted with permission from (31). Copyright (2010) journal of material science, (i) adapted with permission from (32). Copyright (2012) crystal growth and design.

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Figure 2, SEM (scanning electron microscopy) images of TM doped ZnO nanoparticles (a) adapted with permission from (33). Copyright (2014) physica B: condensed matter, (b, c) adapted with permission from (34). Copyright (2016) journal of material science, (d) adapted with permission from (35). Copyright (2004) nanotechnology, (e) adapted with permission from (36). Copyright (2010) crystal growth and design, (f), adapted with permission from (37). Copyright (2010) journal of physical chemistry c, (g) adapted with permission from (38). Copyright (2013) jpurnal of physical chemistry c, (h) adapted with permission from (39). Copyright (2010) journal of physical chemistry c, (i) adapted with permission from (40). Copyright (2011) nanoscale. In comparison to other metal oxide semiconductors ZnO has high redox potential, 5


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superior physical and chemical stability, and nontoxicity. Photocatalytic activity41-43 of ZnO depends on various factors such as phase purity, surface area, crystallite size, nature of dopants, and method of preparation. The sensing ability is mainly depending upon the variation of carrier defects with and without the presence of sensing gases. These carrier defects are mainly oxygen vacancies. It was found that the oxygen vacancies can be significantly increased by doping and thus performance of devices can be further enhanced. Performance of optoelectronic devices, which is mainly depends upon the electron or hole mobility44 in different type of semiconductor (p- or n-type) can be improved by increasing mobility. Oxygen vacancies, mobility, conductivity, refractive index, direct band gap etc are found to be varying significantly with the doping of different elements such as earth metals, rare earth, transition or semi metals. From all the above class of metals rare earth and transition metal is the largest class and have a vast option of doping elements. However, having large ionic size as compared to Zn+2 rare earths have very low solubility on the other hand transition metals having comparable ionic size are highly soluble. A lot of work done on transition metal doped ZnO nanoparticles shows tremendous potential of metals in different fields such as photocatalyst, sensors, optoelectonics, medical, spintronics. Thus doping with transition metal can highly increase the performance of above devices. ZnO is considered as a good host for doping different elements. Doping with transition metals further enhances its band gap tuning for desired level, mobility, donor and acceptor defects conductivity and different optical and magnetic properties. The band gap of MnO and Cu2O is 4 and 2.05 eV respectively by varying the Mn or Cu-content, the band gap can be tuned from 3.3 to 4 or 2.05 eV for wurtzite and cubic structured MnxZn1-xO. This results in the extension/reduction of absorption wavelengths from UV (ultra-violet) (320-400 nm) to UV 6



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(280-320 nm) and UV (200-280 nm) regions or up to visible region. The shift of PL spectra of UV emission toward shorter wavelengths caused by the band gap expansion and the rapid quenching of visible luminescence shows optical properties of Mn dopped ZnO45. The doping of Fe in an optimal ratio increases photocatalytic activity of doped ZnO mesocrystals by 145%. Co-dopping of TM shows that a multivalent ionic state plays a crucial role in the ferromagnetism properties46. The TM doped ZnO nanorods shows room temperature feromagnetism which makes ZnO useful for device applications, such as high performance read heads, nonvolatile memories, and other state of-the-art storage devices47-48. ROS (reactive oxygen species) generation is the main mechanism for its anti bacterial, anti-ceptic and cytotoxic ability. Doping further improves its selectivity and efficiency so that it can be used as an effective material in medicinal treatments. Aditionally, EPR (electron paramagnetic resonance) is a strong tool to identify doped element and its oxidation state. It is very useful in analyzing the effect of different state on various physical properties. We have also discussed about EPR spectra of different transition metal doped ZnO nanoparticles in this review. In situ EPR is an effective method to moniter the synthesis mechanism. In this review we have mainly focused on the transition metal doped ZnO nanoparticles, covering effect of some important individual elements, synthesis processes, different properties and finally applications. As we are dealing with TM doping in ZnO, effect of doping on structure properties of ZnO comes in the very begning. In this section, help of TEM (transmission electron microscopy), RAMAN and EPR studies of different TM doped ZnO have been taken for describing variation in structural properties. Then comes the next important section dealing with different synthesis methods and mechanism for development of different morphological structures. Effect of these different synthesis methods on the doping of transition metal has also 7


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been discussed in synthesis section. We have also discussed which method will be effective for synthesis of transition metal doped thin film, nanostructure (nanorods, helix, tube, wire etc.) or nanopowder. After synthesis we have stated to enlighten the effect of doping step by step, firstly we have studied the doping effect of individual TM. Thermal solubity of different transition metal in ZnO will be helpful in selecting concentration and temperature limit. After describing individual doping effect we comes to the effect of doping on different properties such as magnetic, optical, electrical, photocatalytical, antibectirial. We have also taken the help of various theoretical models (BMP (bound magnetic polaron), indirect exchange interactions, SIB (spin-split impurity band), CBH (correlated barrier hopping), contact model etc.) for describing variation in different physical properties briefly described in this work. Lastly we have concluded this review with the most important section i.e. application part. In this section we have tried to cover most of the fields in which TM doped ZnO is a hope of better performance of devices. Which type of particles will be helpful in detecting reducing or oxydising gases is given sensor part. This part is mainly focused on the gas sensors. It also contains various sensor parameters such as response time, recovery time, detection limit and sensitivity. Then comes the photodiode part containing some homojunction diodes and mostly heterojuncion diodes. We have compared different photodiodes on the basis of their responsibity, recovery time, photocurrent etc. We have also described factors affecting these parameters. We deal with heterojuncion, hybrid and DSSCs (dye synthesized solar cells) in solar cell part. Medical field includes mostly antibacterial and anticarcenogenic properties of ZnO. LED part include heterojunction, hybrid heterojunction, MIM (metal insulator metal), quantum dot LEDs etc. Lastly application section is concluded by application in paper industry.

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2. Structure of Transition Metal (TM) doped ZnO Zinc oxide has a wurtzite hexagonal structure with lattice parameters a = 0.3296 and c = 0.520 65 nm. It belongs to the space group C6v or P63mc49-50. Its crystal structure does not show any alteration when doped with transition metal which shows perfect integration of transition metal into the ZnO crystal lattice. However, the lattice parameters and unit cell volume increaseson doping with transition metal having ionic radius of +2 state greater than 0.60 Å which is tetrahedral ionic radius of Zn+251-52. Hongfen et al.53 have found a slow variation in lattice parameter (decrease in c) because of very small difference in tetragonal ionic radius of Co+2 (0.58 Å) and Zn+2 (0.6 Å). The grain size reduces with increase in doping concentration because of decrease in nucleation and growth. Kumar et al54 have also found decrease in d-value from 0.280 nm to 0.274 nm for undoped and 2% Fe doped ZnO along the (100) plane as shown in figure 3(b, c, e and f). TEM images (figure 3a and d) shows that agglomeration also occurred after doping with Fe. Further, grain size may increase with the increase in doping concentration which is due to the formation of more defects and deformed lattice structure that degenerate the grain size and the shape. Doping with different element is the result in development of stress within the crystal structure that can be easily detected by shift in XRD (x-ray diffractrometer) spectra.

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Figure 3:(a) TEM images (b) HRTEM (high resolution transmission electron microscopy) image (c) SAED (selected area electron diffraction) pattern of ZnO and (d) TEM image (e) HRTEM image (f) SAED pattern of Fe doped ZnO nanoparticles adapted with permission from (54). Copyright (2015) royal society of chemistry, rsc advance.

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Figure 4: (a-c) TEM image of pure, 5% and 10% Co doped ZnO nanorods respectively (d, e) HRTEM image of 5% and 10% Co doped ZnO (f-h) SAED pattern of pure 5% and 10% Co doped ZnO nanorods respectively, adapted with permission from (55). Copyright (2017) rsc advance. TEM images of Co doped ZnO shows that with the increase in concentration (from figure 4) the density of nanorods also increases. HRTEM images shows that the c-axis is the preferential direction for rod growth as shown in figure 4. Size of particles varies with the concentration of dopent impurities. With the variation in concentration defects related to oxygen and impurities, development of grain boundary, accumulation of impurities on surface and over-aggregation of impurities results in variation of size. Lattice parameters are affected by three main parameters namely size of doping impurities, thermal and electronic effect. According 11


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to Azab et al56 electronic effect is composed of two factors, first one is the free charges of transition metal in 3d state and second one is the deformation potential of energy gap between valence band and conduction band. With the increase in energy gap lattice parameter and cell volume also increases. Their comparative study of ZnO nanoparticles doped with different transition metal (Mn, Fe, Co and Ni) shows that with the increase in atomic no. lattice parameters also increase.

Figure: 5 (a) E2 (high) peak adapted with permission from (57). Copyright (2014) rsc physical chemistry chemical physics and (b) all other peaks of Raman spectra of Co doped ZnO nanorods adapted with permission from (58). Copyright (2009) applied physics letter. The irreducible representation of phonon modes at the centre of brillouin zone are represented as Γopt = A1 + E1 +2B1 + 2E2.59-61 Where A1 and E1 modes are polar modes and both are raman and IR active modes, B1 modes are inactive modes and E2 modes are non-polar but are raman active.62 Polar modes A1 and E1 have two frequencies i.e. transverse optical (TO) and longitudinal optical (LO), for optical phonon modes.63 Cusco et al64 had observed the frequencies of different first order polar optical modes i.e. A1(LO), A1(TO), E1(LO) and E1(TO) to be around 12



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574,65 378,66 590 and 410 cm-1 respectively. Another peak around 9867 and 436cm-168 corresponds to non polar optical mode E2 low and high respectively, which is the characteristic peak of ZnO. This peak is mainly related to the vibration of oxygen atoms thus it is thought to be unaffected by the substitution of doping elements at cationic sites. Mostly, Raman spectra of transition metal doped ZnO nanostructures has found to shows no major changes in E2 (high) (figure 5a) peak of ZnO. However, some researchers had found a slight shift of the order of 1 to 2 cm-1 in E2 high mode57, 69 for doped ZnO nanoparticles. Figure 5(a) also shows that there is only a small shift in 438 cm-1 peak. From figure 5(b) we can see that with the doping of TM other secondary peaks also comes into existence.

2.1. EPR (Electron Paramagnetic Resonance) The main cause of variation in various physical properties of doped ZnO nanoparticles is the creation of carrier and vacancy defects. EPR is a sensitive technique to detect such defects (paramagnetic species) created by doping elements in the ZnO nanoparticles. Mostly there are three types of Oxygen vacancies𝑉0𝑜, 𝑉𝑜∗ and 𝑉𝑜∗∗ 70 (neutral oxygen vacancy, singly charged oxygen vacancy and doubly charged oxygen vacancy respectively). However, only singly charged oxygen vacancy being paramagnetic is EPR active71. It gives resonance peak corresponding to g value 1.9672. X band EPR spectra of ZnO give signals at g value 1.960 and 2.006 (as shown in figure 6 a), which are related to oxygen vacancy, zinc vacancy, oxygen interstitials and surface defects respectively. Many researchers have used it as an effective tool for analysing magnetic interactions in transition metal doped ZnO and in other metal oxides73, 74, 75, 76.

Shallow donor77, 78, 79 and deep acceptor levels originated due to the substitution of doping

elements can be visualised with the help of pulsed high frequency (95 or 275 Hz) EPR 13


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spectroscopy80. EPR spectroscopy significantly depends on the size of particles, EPR spectra of nanoparticles having particle size greater than 40 nm consists EPR spectra mainly related to core regin, for particle having size less than 40 nm but greater than the size of quantum dots consists spectra related to both core and surface region81. Erdem et al.82 found that with the decrease in particle size, from bulk to 5 nm, the peak at g = 2.006 starts appearing, which is due to surface defects, and peak at g = 1.960 becomes weaken (as shown in 6a). EPR spectra of colloidal quantum dots of ZnO shows that the electrons are delocalized as shallow traps inside the quantum dots83. It is widely used for determining the type of doping elements84 as EPR spectra of different transition elements in different oxidation state is different. As EPR spectra of Mn ZnO have 6 lines and of Ni doped ZnO have only 1 as shown in Figure 6 b and c85,86.

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Figure: 6 (a) X-band EPR spectra of bulk nano size and quantum size ZnO showing the origin of EPR line corresponding to surface defects adapted with permission from (82). Copyright (2011) physica status solidi rrl, (b) Q

nn-band EPR spectra of Mn doped ZnO nanoparticles having 6

EPR lines(black) and simulated spectra (red) adapted with permission from (85). Copyright (2014) acs applied material interfaces, (c) X- band EPR spectra of Nickel doped ZnO nanoparticles with different concentration of Nickel, adapted with permission from (86). Copyright (2012) American journal of materials science. Norberg87 and his group monitored the synthesis process using EPR spectroscopy. The EPR spectra were measured at different reaction intervals. They found six EPR signals characteristic of the hyperfine coupling of Mn2+ (I=5/2) and signals for MnO, which shows Mn sutitution at Zn place. Similarly, Co2+ ion gives eight lines88 due to hiperfine coupling of spin ½ and (I=7/2) nuclear spin. Iron doped ZnO gives three signals with g value 4.3, 2.20 and 2.16 corresponding to isolated Fe3+, ferromagnetic exchanges between Fe3+ ions and Fe3+ ion at distorted octahedral site respectively. These spectra get broaden with the increase in Fe concentration89. Misra90 and his group have also found a group of three EPR lines (figure 7 a and b) for Iron doped ZnO nanoparticles. The Q band EPR spectra give more details as compared to X band spectra. The EPR spectra (figure 7 d) of Cobalt doped ZnO shows broad line which consists of four spectra: intense high-spin Co2+ spectrum, oxygen vacancy spectrum, weak high-spin Co2+ spectrum and FMR (ferromagnetic resonance)91.

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Figure: 7 (a and b) broadening of X-band EPR spectra of Fe doped ZnO nanoparticles with the increase of concentration from 0.1 and 1% respectively, adapted with permission from (90). Copyright (2014) journal of magnetism and magnetic materials, (c) hyperfine splitting of X-band EPR spectra of Cu doped ZnO which diminishes with the concentration of Cu, adapted with permission from (92). Copyright (2006) journal of physical chemistry b, (d) X-band EPR spectra of Co doped ZnO nanoparticles with varying concentration at 5K temperature, adapted with permission from (91). Copyright (2015) journal of magnetism and magnetic materials. Generally, the EPR spectra of transition metal doped ZnO shows sharp lines at lower doping concentration, which gets broaden with increasing doping concentration. This is due to the interaction between transition metal ions which are doped. Figure 7 shows this broadening in EPR spectra of various transition metals doped samples at different doping concentrations. 16



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Manivannan et al.93 studied the EPR spectra of Co, Cr, Fe, Mn & Ni doped ZnO. They have found the hyperfine six-line splitting of EPR corresponding to Mn2+, broad line with g value 4.3 for Co2+, sharp line with g value 2.067 for Ni2+, sharp line with g value 2 for Cr2+, Fe ion gives both sharp and broad line. These result shows substitution of doping elements in place of Zinc.

3. Synthesis techniques for TM doped ZnO nanoparticles Various physical and chemical methods such as magnetron sputtering, pulsed laser deposition, sol-gel, spray pyrolysis, solution processing etc have been employed for the synthesis of wide band gap ZnO solid solution. Here some methods are given for the synthesis of ZnO nanoparticles doped with different transition metal and having various morphological structures.

3.1 Sol-gel method: This is one of the simplest and widely used methods for the preparation of doped ZnO nanoparticles (nanorods94) and thin film on various substrates, using dip coating95-96 or spin coating97-98 methods. Schematic diagram of synthesis procedure is shown in figure 8. This method can also be used for preparation of ceramics.

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Figure 8: preparation of film, ceramics from sol via sol-gel method, adapted with permission from (99). Copyright (2010) elsevier, materials science and engineering b. In sol-gel method pH is an important parameter that affects the morphology, particle-size, number of growth units and nuclei, etc. of the nanomaterial. Basically particle size decreases with the increase in pH100, as shown in figure 9. The pH of sol is controlled by zinc precursor and aqueous pH is controlled by basic solutions (NaOH, KOH, NH4OH etc.). Besides pH type of 18



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solvent, annealing time and temperature, aging etc. are also major parameters affecting final product. Mostly aging has been preferred for thin film preparation for better results in optical properties.

Figure 9, effect of pH on particle size, adapted with permission from (100). Copyright (2010) journal of alloy and compound. Kumar et al.101 prepared Co doped ZnO using Zinc Nitrate and Cobalt Nitrate as precursor and Citric acid as fuel. They dissolve them in aqueous Citric acid solution and dried the solution for 3 hour at 80°C the swelled xerogel was again heated for 12 hour at 140°C. This reaction was exothermic and is expressed as: M(NO3)2 + C6H8O7 + 4O2 → MO + 2NO2 + 6CO2 + 4H2O

(1) 19


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Sol-gel auto-combustion method102, a variation in sol-gel method provides good control over shape and size produces doped nanoparticles. This method is cost effective, simple and easy for synthesis of doped ZnO nanoparticles. However, for the preparation of thin film it is time consuming as for better results it requires aging of sol. This method is widely used to prepare transition metal doped ZnO nanopowder, which can be used to fabricate thin film via various deposition methods.

3.2 Pechini Method: A modification in sol-gel Pechini process is extensively used in preparation of co-doped ZnO with transition metal103. In this method the starting sol is prepared by the use of poly hydrated metal nitrate, citric acid (CA), ethylene glycol (EG), ammonia solution and deionized water. This is a complex process which needs acqurate temperature and pH of the solution for the process to take place in forward direction. Choppali et al104 have showed the effect of annealing temperature. They have used EDTA (Ethylenediaminetetraacetic Acid) instead of EG (ethylene glycol) or CA (citric acid) as a chealating agent and prepared nanoparticles of size smaller than those prepared by the earlier ones. Beltran et al.103 have used modified Pechini method for the preparation of Fe doped ZnO. They dissolved Nitrates of both metal and Citric acid in deionized water such as the ratio of metal nitrates to Citric acid was 1:3 to obtain sol, which was then heated at 70 ˚C maintaining pH at 5 (dropwise addition of ammonium solution). Ethylene glycol was added to stabilize the sol in the ratio of EG/CA at 5. Finally, for polyesterification temperature was slowly raised from 100 to 110°C. Wet gel was then dried for 4 hours at 200°C and then annealed to get nanopowder.

3.3 Co-precipitation method: This method is used to prepare both undoped and doped ZnO 20



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NPs. These can be prepared in ethanol as a solvent for homogenous and stoichiometric solution Chemical co-precipitation is widely used in industry and research because of its simplicity and perfect stoichiometry. Atomic mixing of the constituents by chemical co-precipitation yields a final product of nearly perfect stoichiometry without high temperature. This begins by dissolving the starting oxides in an acid solution or salts in aqueous medium. This solution is then mixed with a solution of dissolved precipitating agent, such as oxalic acid, ammonium oxalic or ammonium hydroxide, to precipitate the mixed oxalate or hydroxide out of solution. The final crystalline oxide is obtained by firing the precipitates at a higher temperature. Different parameters are used in this technique are: starting materials such as oxides or salts, dissolving acids such as nitric acid, acetic acid or hydrochloric acid; precipitation temperature may be in the range of 0 to 80˚C; solvent such as water or alcohol or a combination of water and alcohol. Gandhi and his group105 prepared Co doped ZnO using ZnAc (zinc acetate) as precursor and CoAc (cobalt acetate) as source, NaOH was used for precipitation. They kept sintered ppt for 5 hours at 80°C for the growth of nanoparticles. Nanoparticles were obtained after drying for 1 hour at 120°C followed by annealing for 2 hours at 300°C. Particles size, from 33 nm to 29 nm for 5% to 15%, was decreasing with the increase in Co concentration. Atomic level mixing of dopant makes this method suitable for preparation of doped ZnO nanoparticles. It requires low heating temperature. However, this method is not suitable for preparation of different morphological structures. It also requires after washing of prepared sample to remove impurities. This is also the most popular method for transition metal doped ZnO nanopowder.

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3.4 Solid State reaction method: This is a simple method and produces perfectly stoichiometric product. It generally requires homogeneous mixing of Oxides followed by annealing at high temperature. However, Zhou et al.106 have used Zinc sulphate and NaOH as reagents, presence of Zn(OH)2 in final product was detected by analyzing DTA results. They found, from XRD spectra of reagent mixture, that Zn(OH)2 was originated from solid state reaction itself rather than washing. The main reaction may be: ZnSO4·7H2O + 2NaOH ↔ Zn(OH)2 + Na2SO4 + 7H2O.

(2)

Heat release during the grinding process, measured by weight loss at different time interval, was the main finding which is one of the factors for formation of ZnO during grinding, confirmed by XRD spectra of reaction mixture. This method is superior then earlier one as it needs very less annealing temperature. Many other researchers also followed this method using different precursor of Zinc. It is environment friendly, requires high annealing temperature and large milling time. Non uniform and bigger particle size makes it unsuitable for synthesis of doped ZnO. Some time it produces particles having phase impurity and poor stoichiometry.

3.5 Hydrothermal method: The hydrothermal method does not require the use of organic solvents or additional processing of the product (grinding and calcination), which makes it a simple and environment friendly technique. The synthesis takes place in an autoclave, where the mixture of substrates is heated gradually to a temperature of 100–300°C and left for several days. As a result of heating followed by cooling, crystal nuclei are formed, which then grow. This process has many advantages, including the possibility of carrying out the synthesis at low temperatures, the diverse shapes and dimensions of the resulting crystals depending on the 22



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composition of the starting mixture, the process temperature and pressure, the high degree of crystallinity of the product, and the high purity of the material obtained.

Figure 10, Pictorial representation of synthesis mechanism of hexabranched structure of single-crystalline ZnO, adapted with permission from (29). Copyright (2012) acs nano. Kozhummal and his group29 have prepared hexabranched structure of single –crystalline ZnO by hydrothermal method using PEG as capping agent. The mechanism proposed by them tells the formation of facet structured ZnO nuclei at an initial stage. Capping of PEG suppresses the growth along (0001).

High concentration of precursor at initial stage result in slow growth

rate and thus thick hexagonal structures were obtained. Due to this process comes to growth-dissolution equilibrium. At this stage faster dissolution of ZnO at six prismatic edges, growth of prism faces along six m-planes results in pointed tips, as shown in figure 10. Marin107 and his group has prepared Fe doped ZnO nano flower-like structures and other group of Mihalache108 has also synthesised Fe doped ZnO nano flower-like structures. Furthermore, Turkylmaz109 and his group have prepared Ni, Fe, Mn and Ag doped ZnO nanoplates through hydrothermal approach. The use of microwave reactors in hydrothermal synthesis processes brings significant benefits. Microwaves make it possible to heat the solutions from which the synthesized products are obtained, while avoiding loss of energy on heating the entire vessel. 23


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Many syntheses methods proceed with greater speed and high yield, when microwaves are used as compaired to the traditional methods. This method is beneficial for the synthesis of doped ZnO having various morphological structures. This is generally used when we have to study the effect of doping and morphological structure on various physical properties. But it also requires specified apparatus for synthesis, knowledge of solvents.

3.6 Solvothermal Synthesis: Huang41 and his group have used this method for the synthesis of various, mainly {0001}, {10-11}, and {10-10}, kinds of ZnO facets to investigated dependance of photocatalytic performance of ZnO on the facet orientation41. In this method organic solvent (alcohol, amines, n-alkanes, glycols, aromatic compounds etc.)110 are used for thermal decomposition of organometallic compound. The alcohols having longer molecular length produces longer nanorods111 Tonto et al112 have shown the effect of temperature, solvent and concentration on the aspect ratio of ZnO nanorods. This method was also found to be effective in the synthesis of double shell ZnO hollow microstructures113 similarly; yolk-shell structure of Pd/ZnO was synthesized by Hao Tian114 and his group, which shows to be highly selective catalytic property towards the hydrogenation of phenylacetylene to phenylethylene. Sutka et al115 has found this method to be very effective in the synthesis of magnetic and plasmonic Co-Ga co-doped ZnO nanocrystals. The saturation magnetization was found to be 4.88 emu/g. Assisted by microwave radiation this method results in less synthesis time and more uniformity in particle size116-117 Xu et al118 had prepared Co doped ZnO by microwave assisted solvothermal sponge-like micro-spheres which can sense even 10 ppm ethanol.

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3.7 Successive ionic layer adsorption and reaction (SILAR) method: This method can produce a variety of nanostructures with good crystallinity. In this method, wastage of chemical as precipitate can be avoided. This method is simple, cost-effective and scalable and allows low temperature deposition on any complex shape substrate for the synthesis of different morphologies such as nanoflakes, nanoflowers and branched nanorods119.

3.8 Chemical bath deposition: This is one of the simplest and easiest methods for the preparation of well ordered and aligned120 doped121 and undoped ZnO nanorods within few hours. This does not require any specific substrate (Si, SiO2 etc.) instead we can grow well aligned nanorods by first growing seed layer even on the glass substrate using SILAR method as prepared by Singh et al120. Besides using rigid substrate polymer substrates122 can also be used for nanorod preparation using different seed layer growing techniques. SILAR method can also be used for the preparation of seed layer, which is easiest and less time consuming, followed by growing doped ZnO nanorods on polymer substrate. This can produce ordered and aligned nanorods even on polymeric substrate which is the future of optoelectronic and electronic devices. Besides nanorods Sharma et al123 have prepared a nanocomposite thin film of CdO and ZnO on glass substrate for ethanol gas sensing. This was able to sense 24 ppm ethanol gas with a response and recovery time of 54 and 59 second respectively. The growth mechanism is very simple and all chemical reactins are uni-directional124 in which colloidal cluster of Zn(OH)2 is formed followed by nanorods after heat treatment. Length and diameter of nanorods is depend upon the time for which the sample is kept in the furnace, both increases with the time but the growth rate decreases with time125, which is due to the decrease in concentration of Zn2+ and (OH)− ions. 25


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Bidiere et al125 have studied the effect of reaction time on the morphology of Ti-doped ZnO nanorods grown on Si substrate. They found that as the reaction time increases length and diameter of rods increases but due to revearse reaction (ZnO → Zn2+ + 2(OH)−1) they also start to dissolve, however, its rate along lateral side is greater than along axial side because of large surface area. These results shown by FESEM (Field Emission Scanning Electron Microscope) are confirmed by XRD studies which shows a decrease in (100) and (002) peaks after 3.5 and 5 hr respectively. This also confirms that in this method with the growth time prefered growth along c-axis takes place, thus we can prepare nanorods (on Si substrate or on any other substrate using SILAR method for seed layer preparation) preferrencialy along c-axis if we are able to keep the solution in furnace for appropriately long period of time. Surface energy may also be one of the factors for such kind of difference in reaction rate. However, this method requires prior knowledge of specific organic compounds for shape controlling and surface grafting.

3.9 Plasma Enhanced-Chemical Vapor Deposition (PE-CVD) method: PE-CVD is an advanced versition of chemical vapor deposition method. Bekermann et al126 elaborate the effect of, main process parameters, value of RF-power, deposition temperature, total pressure, O2 flow rate, auxiliary Ar flow rate, carrier Ar flow rate, precursor vaporization temperature etc on the system morphology, thickness, growth mechanism, growth rate etc of ZnO nanoparticles127. This method can be successfully used for the synthesis of nanoporus ZnO thin film and nanorods on various crystalline substrates under appropriate condition128.

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Figure 11: Schematic representation of the growth of the ZnO/Ni core/shell nanostructures using rapid thermal chemical vapor deposition and e-beam evaporation methods, adapted with permission from (129). Copyright (2016) scientific report. Mudusu et al129 had prepared ZnO/Ni core-shell NRs on sapphire substrate. As shown in figure 11 initially they had coated Au layer on substrate then grown ZnO nanorods using rapid thermal chemical vapour deposition (RTCVD). Finally, a layer of Ni was coated using e-beam evaporator. This method is useful for synthesis of ZnO nanorods doped with different transition metal in perfect stoichiometric ratio as compaired to CVD (chemical vapour deposition) method for nanorods. CVD method also has some limitation related to solubility of different transition metal.

3.10 Spray Pyrolysis: Low cost, high efficiency, easy to operate, simple, non-contaminating and controllable deposition environment are major advantages, for thin film formation, of this method. In this method different solvent such as deionized water or alcohol, depending upon solubility of precursor, are used. Schematic representation of process is given figure 12(i).

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Figure12: (i) schematic diagram for deposition of thin film, (ii) XRD and (iii) PL (photoluminescence) spectra of Ag doped ZnO nanorods deposited for 3, 5 and 10 min, adapted with permission from (33). Copyright (2014) physica b condensed matter. Lozada32 and his group have grown Ag doped ZnO nanorods on glass substrate. From this figure 12 (ii and iii) we can see that for large deposition time we get batter crystallinity and high PL intensity.

3.11 Pulsed laser deposition method: This technique is used for deposition of thin film, even for high doping concentration, from powder source. Ramachandran et al130 had prepared thin film of Mn doped ZnO even for 10% doping without formation of any secondary phase, confirmed by 28



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XRD spectra, using KrF excimer laser of 248 nm wavelength. They had prepared two set of samples, 1%-10% Mn, one for high concentration of Oxygen and other for low at 3×10-3 and 4×10-7torr respectively. Finally, they annealed the first set of sample in presence of Oxygen flow at 600°C for 30 min.

3.12 Ball-milling: It is considered as a promising method due to low cost, environmental friendly and quite simple process. Besides these merits this method needs high anneling temperature (900 to1200˚C) and consumes large amount of time (nearly 20 hr.), but with the increase in milling time the average grain size decreases causing X-ray line broadening, increases rms atomic level strain131. This is the easiest method to prepare transition metal doped ZnO nanopowder.

3.13 Sonochemical method: This method is used for preparation of doped and undoped ZnO nanoparticles at very low annealing temperature132. Wang et al132 prepared Cd doped ZnO nanoparticles using Acetate of Zinc and Cadmium in different molar ratios. The two separate solutions in diethanolamine were mixed and then ultrasonicated for 3 hr. Precipitate was obtained by adding water, which was finally dried at 70°C to obtain nanoparticles.

4. Transition metal doping in ZnO Doping is an effective way for controlling the properties of ZnO such as band gap, electrical conductivity, lattice parameter, magnetic properties, sensing ability132. All these properties depend on the type of defects, n-type defects or p-type defects, produces by TM doping. Mostly Zn interstitial and Zn entisites produce p-type defects and O vacancies produce 29


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n-type defects. Mandal et al133 studied the dependence of thermal solubility on growth temperature. Co, Mn, Fe and Ni were the elements over which their study was focused. They found that solubility of these elements depends strongly upon the concentration and growth temperature. 3%, 20%, 30% and 30% were the solubility limit of Ni, Fe, Mn and Co respectively. It was a difficult task for them to obtain a function that could fit the data for whole temperature, the only function, X= A - BTg, here, A and B are fitting parameter and Tg is temperature, was found to fit the data separately for low and high temperature region. In this work, a very intresting finding was that the initial slope, of graph between concentration and temperature, for all the elements (except Ni) were same at low temperature region (nearly 1×10-3/°C)133.

4.1 Ni doped ZnO: The Ni: ZnO can be synthesyse in a number of morphologies with different concentration of Ni such as nanowall, nanorods and nano-rings36. The lattice constants increases and the wurtzite crystalline structure of ZnO becomes weak134 due to the higher tetrahedral ionic size of Ni2+ (0.69Å)46. Besides keeping hexagonal wurtzite crystal structure Raman study of Ni doped ZnO nanowires by Zhang et al135 have shown that it also produces shallow donor defects (Oxygen vacancies, zinc interstitials etc.). Ni-doped ZnO (Zn1–xNixO, for 0

Progress on transition metal-doped ZnO nanoparticles and its

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