Structure-directing property and growth mechanism induced by capping agents in nanostructured ZnO during hydrothermal synthesis—A systematic review

Nano-Structures & Nano-Objects 22 (2020) 100426

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Structure-directing property and growth mechanism induced by capping agents in nanostructured ZnO during hydrothermal synthesis—A systematic review Parita Basnet, Somenath Chatterjee

∗

Centre for Materials Science and Nanotechnology, Sikkim Manipal Institute of Technology, Sikkim Manipal University, Sikkim, India

article

info

Article history: Received 10 July 2019 Received in revised form 14 January 2020 Accepted 20 January 2020 Keywords: Zinc oxide nanoparticles Hydrothermal synthesis Capping agents Growth mechanism Applications

a b s t r a c t Zinc oxide nanoparticles (ZnO NPs) have become a topic of comprehensive research from the past many years due to their excellent optical, physical and chemical properties, which gives rise to numerous applications. The challenge, however, in ZnO NPs synthesis is the agglomeration of particles along with the design of their desired morphology and size. To overcome such challenges and to obtain application specific size and morphology of ZnO NPs, organic capping ligands are extensively used. The current review focuses on this aspect, wherein, a detailed analyses on the role of common capping agents during ZnO NPs synthesis has been provided. Keeping in view the effect of synthetic processes which contribute to size and morphology alteration of ZnO NPs, focus is particularly paid on hydrothermal synthesis because of its several advantages. Through this review, readers may get an idea about the type of ZnO NPs’ morphology with a particular capping agent. For instance, most surfactants and amino acids direct the formation of 2-D rod-like morphology of ZnO NPs, while polymers and polysaccharides may lead to the formation of various ZnO NPs’ morphologies, such as, nanospikes, microstars, etc. Further, the theoretical basis governing the rational design of NPs possessing modelled and controlled size, morphology and microstructure has also been discussed. To the best of Author’s knowledge, no previous reports have focused on these aspects. Moreover, the various applications of ZnO NPs have also been briefly discussed. © 2020 Published by Elsevier B.V.

Contents 1.

2.

3.

Introduction......................................................................................................................................................................................................................... 1.1. Importance of the review ..................................................................................................................................................................................... 1.2. Implication of hydrothermal synthesis ............................................................................................................................................................... 1.3. ZnO NPs and the role of capping agents ............................................................................................................................................................ 1.4. Theoretical basis of size and shape controlled NPs’ synthesis......................................................................................................................... Types of capping agents .................................................................................................................................................................................................... 2.1. Surfactants: Growth, structure and formation mechanism of surfactant capped ZnO NPs ......................................................................... 2.2. Amino acids: Growth, structure and formation mechanism of amino acid capped ZnO NPs ..................................................................... 2.3. Polymers: Growth, structure and formation mechanism of polymer capped ZnO NPs................................................................................ 2.4. Polysaccharides: Growth, structure and formation mechanism of polysaccharide capped ZnO NPs ......................................................... 2.5. Bioextracts: Growth, structure and formation mechanism of plant extracts and microbes functionalized ZnO NPs .............................. Applications of ZnO NPs .................................................................................................................................................................................................... 3.1. Rubber industry ..................................................................................................................................................................................................... 3.2. Textile industry ...................................................................................................................................................................................................... 3.3. Electronic and electro-technological industries ................................................................................................................................................. 3.4. Photocatalysis ......................................................................................................................................................................................................... 3.4.1. Photocatalytic dye degradation ............................................................................................................................................................ 3.4.2. Photocatalytic water-splitting............................................................................................................................................................... 3.4.3. Photocatalytic anti-microbial activity ..................................................................................................................................................

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (S. Chatterjee). https://doi.org/10.1016/j.nanoso.2020.100426 2352-507X/© 2020 Published by Elsevier B.V.

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3.5. Other applications.................................................................................................................................................................................................. Conclusions and outlook.................................................................................................................................................................................................... Declaration of competing interest.................................................................................................................................................................................... Acknowledgement .............................................................................................................................................................................................................. References ...........................................................................................................................................................................................................................

1. Introduction 1.1. Importance of the review Over the past many years, it has been well established that the nanostructured materials are superior to their bulk counterparts [1]. Primary reason is their high specific surface area, which allow the reactions to proceed much faster from a greater number of reaction sites [2]. However, nanoparticles (NPs) are fairly unstable due to their small size corresponding to high surface energy. This causes agglomeration of the individual particles, wherein, tuning the size, morphology and microstructure becomes difficult. Agglomeration of NPs is facilitated by Ostwald ripening and sintering processes [3]. In case of powder NPs, heat treatment (calcination or roasting) imparts further energy to the particles, causing them to combine non-uniformly, which is undesirable. The stabilization of NPs may be brought about through the immobilization of particles on a support or by covering their surfaces with organic ligand shell (i.e. by using capping agents) [4–7]. For the capping purpose, surfactants, polymers, dendrimers, polysaccharides, etc. have been extensively investigated and used [7]. Additional advantage of using capping agent is the fact that due to organic nature of the capping material present on the NP’s surface, compatibility of the NPs with another phase may be increased or certain functionalities may be improved [8]. For example, the introduction of polyaniline (PANI) on the NP’s surface is known to enhance the conductivity of the material [9]. Therefore, capping approach ensures stability of the NPs with value-added properties. Further, while the basis of this Review is the capped ZnO NPs in the powdered form, ZnO thin films are also a subject of great interest with potential application in diverse fields [10,11] and hence, have also been discussed. 1.2. Implication of hydrothermal synthesis Tuning the morphology and size of the NPs depend upon the type of synthetic methods employed. Several synthetic approaches of NPs are reported in Literature such as coprecipitation, hydrothermal, solvothermal, sol–gel, laser ablation, and others [1]. Amongst these methods, hydrothermal method is comparatively an ease and less-steps involved process, costeffective, eco-friendly and tunable, wherein, the NPs may be grown in a controlled environment. Although, Green nanochemistry (also called biosynthesis) has attracted substantial interest for producing NPs [12], factors like stability and aggregation, control of crystal growth, morphology and size tuning of the NPs, etc. are yet to be fully addressed [13,14]. Further, the competency of the NPs synthesized through Green nanochemistry route, compared to physio-chemical synthetic approaches, is not fully explored [15]. Contrariwise, it has been well established that the hydrothermal method of NPs synthesis is a feasible process in controlling the particle size, morphology, crystal growth, etc. The main purpose of utilizing hydrothermal method for producing ZnO NPs involve various factors, such as, the inherent absence of ligand-field stabilization energy, inadequate effect of pH, and ionic strength on the octahedral–tetrahedral equilibrium of the Zn (II) aqua complex, in addition to the inability of water to deprotonate hydrated divalent Zn cations at ambient pressure [10].

18 18 18 18 18

It is a technique which employs homogeneous or heterogeneous phase reactions in water medium at elevated temperature and pressure of >25 ◦ C and >100 kPa, respectively, to crystallize NPs directly from the reaction solution [16]. In hydrothermal process, autogenous pressure corresponding to the saturated vapour pressure of the solution at specified temperature and composition are employed [17]. An advantage of hydrothermal approach for NPs synthesis is the regulation of rate and uniformity of nucleation, growth and ageing, resulting in the control of particle size and morphology of the nanocrystallites. This is due to the ability of this method to precipitate the already crystallized powders straight from the solution. Consequently, the aggregation levels of the individual crystals are highly reduced. This phenomenon may not be possible in many of the conventional synthetic methods [18]. Additionally, the purity of hydrothermally crystallized NPs surpasses that of the precursor materials because hydrothermal crystallization is a self-purifying process, wherein, the growing units tend to cast-off impurities present in the reaction medium [19]. The removal of impurities occur together with the crystallizing solution in hydrothermal method, which is not possible in other synthetic methods. Likewise, there are several other advantages of hydrothermal synthesis as compared to other solution, gas-phase or solid-phase methods [16] as has been tabulated and presented in Table 1. Therefore, generalizing a standard method of size and morphology tuning through hydrothermal process is of urgent requirement and the basis of this review. 1.3. ZnO NPs and the role of capping agents Over the last decade, studies related to zinc based nanostructures such as zinc oxide (ZnO) [20,21], zinc sulphide [22– 24], etc. have been increasing [25]. Amongst these nanostructures, ZnO NPs are multifunctional material with unique physical and chemical properties, such as high chemical stability, high electrochemical coupling co-efficient, high photostability, doping amenability, and others [26,27]. Moreover, Food and Drug Administration (FDA, USA) has categorized ZnO as ‘‘generally recognized as safe’’ (GRAS) (21CFR182.8991) [28]. As was discussed previously, the stability of ZnO NPs depend on the type of capping agents used during its synthesis. Moreover, capping agents also act as structure-directing agents [29] and hence, play a significant role towards the formation of various ZnO nanostructures. However, it is important to understand the kinetics of particle growth if the control in morphology of ZnO NPs is to be properly executed. According to Fall et al. [30], in situ grazing-incidence diffraction (GID) experiments may be utilized to study both the kinetics of growth and sedimentation of particles present in very close vicinity to the surface of the substrate. They observed that the crystalline hexagonal phase of ZnO did not necessarily result from the drying process but existed in the solution phase. Further, they were also able to detect the critical concentration required for properly understanding the growth kinetics of ZnO NPs. They concluded that the particles of ZnO nucleated in solution, and in time, the aggregated particles grew in size. At the critical concentration, the growth kinetics was able to be followed and it was also possible to confirm that the growth progressed even during the sedimentation process. To inhibit the growth of particular facets, so to obtain ZnO NPs with specific morphology, capping

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agents may be used. ZnO NPs of various morphologies and nanostructures may be synthesized owing to their chemically active polar and inert non-polar faces [31–33]. The most stable structure of ZnO is hexagonal wurtzite form, which consists of two polar facets and six side facets, wherein: (001), (001) are chemically active polar planes, (010), (100) and (110) are chemically stable non-polar planes and (100) is the most stable chemically inert plane [31–34]. The growth rate of these facets may be controlled via capping mechanism to form various nanostructures of ZnO. It is also to be noted that although, different capping agents direct the orientation of particles in different ways, the type of synthesis involved is one of the imperative factors that equally govern the final morphology, microstructure and size of ZnO NPs [35,36]. 1.4. Theoretical basis of size and shape controlled NPs’ synthesis The rational design of NPs possessing modelled and controlled size, morphology and microstructure, is governed by a certain theoretical basis [40] which is discussed in this section. According to Vayssieres [41], such controlled design of NPs is largely dependent upon the interfacial free energy of the system, i.e. by experimentally controlling its interfacial tension, it is possible to tune the thermodynamics and kinetics of nucleation, growth and ageing of the NPs. In case of metal oxide semiconductor NPs, such as ZnO NPs, alteration in the pH of precipitation from the typical point of zero charge (PZC) causes an increase in the surface charge density due to the adsorption of hydroxyl ions. Thus, the chemical composition of the interface will change, consequently leading to a change in the interfacial tension of the system. Moreover, high ionic strength contributes towards a further increase of the surface charge density, since with an increase in the ionic strength of the system, surface sites will develop more charge due to the higher screening effect of the interfacial charged sites. In such a scenario, the surface charge may reach a maximum charge density, which eventually depends upon the chemical composition of the interface. Further, at maximum charge density, interfacial tension of the system will become minimum and hence, the thermodynamic colloidal stability may be reached. This ultimately results in an appreciable lowering of the secondary ageing processes and thereby, constrains the NPs’ size and their overall size distribution from increasing while at the same time, preventing crystal phase and morphological transformations. For precipitation occurring between PZC and the point of zero interfacial tension (PZIT), the surface charge of the system does not reach maximum and therefore, the interfacial tension will remain positive causing the system to undergo secondary growth and ageing processes. Consequently, NPs will evolve in the solution and their size will increase with time, along with the simultaneous phase transformation of their crystal structure. When the precipitation occurs at pH ≥ PZIT, the NPs will be thermodynamically stabilized, and hence, secondary growth phenomenon such as Ostwald ripening will not occur. Therefore, the NPs’ size is directly related to the precipitation conditions such as pH and ionic strength at a given temperature and reaction parameters. Further, on considering the nucleation and growth processes that rule the formation of solid phases from solutions, a maximum is present for the first derivative of the free enthalpy of nucleation with respect to the number of precursors [41]. According to this, reduction in the interfacial tension leads to a lowering of the nucleation energy barrier, which ultimately leads to a decrease in the NPs’ size. Additionally, the thermodynamic stabilization leads to a tailoring of the NPs’ morphology as well as the crystallographic structure. When the precipitation conditions are thermodynamically favourable, the shape of the crystallites are governed by the symmetry of the crystal structure as well as by the chemical

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environment of the system, which subsequently, leads to the formation of various morphologies of the NPs. Therefore, for the production of application specific morphology and size of NPs, suitable reaction parameters should be applied to the transition metal ion under study along with the natural crystal symmetry and anisotropy or by compelling the particles to grow along a specific crystallographic direction by carefully controlling the specific interfacial adsorptions of ions and/or capping agents and crystal-field stabilization energy [11]. Thus, this review focuses on the significant interaction of capping agents with the nuclei of ZnO NPs during its hydrothermal synthesis and understanding the role of capping agents as reductants and structure-directing agents. Further, this study aims to correlate the possible morphology of ZnO NPs with a specific capping agent during the hydrothermal synthesis. Moreover, a brief insight into the various applications of ZnO NPs has also been provided in Section 3 of this review paper. 2. Types of capping agents The capping agents are, basically, organic surface ligands employed for selectively binding the specific planes of nanocrystals through oriented attachment process, consequently allowing self-organization of the nanocrystals along the less capped crystal plane(s) [42]. Depending upon previous reports [43–51], common types of capping agents may be broadly categorized as surfactants, polymers, amino acids, polysaccharides, and bioextracts. Flowchart 1 represents each of these categories with sub-categories and examples. 2.1. Surfactants: Growth, structure and formation mechanism of surfactant capped ZnO NPs Surfactants play a key role in preventing rapid flocculation and aggregation of the particles [52]. Surfactants cap (or passivate) the surface of NPs and exert a strong influence on its morphology by controlling the growth rate of various crystallographic surfaces and generating orientations in crystal formation. This means that the growth of NPs is controlled by the diffusion and attachment rates of surfactants onto the NP’s surface. The most common type of surfactants used for controlling the morphology of ZnO NPs are sodium dodecyl sulphate (SDS) and cetyl trimethyl ammonium bromide (CTAB). Yin et al. [53] explained the formation of rod-like ZnO NPs, as shown in Fig. 1(a), employing a surfactant mediated hydrothermal synthesis approach. The growth behaviour of crystals primarily depend upon its internal structure and external conditions [54–57]. In case of wurtzite ZnO, the polar axis is the c-axis, wherein, Zn2+ and O2− are tetragonally coordinated in the unit cell of the crystal [58]. Therefore, the maximum growth rate in ZnO crystal will occur on the [0001] direction compared to [1010] and [1011] directions. In the initial stage, the formation of ZnO nuclei takes place according to the following equations Equilibrium

Zn (OH)2 ←−−−−→ Zn2+ + Zn (OH)24− + OH− 2+

Zn

− Hydrothermal Treatment

+ 2OH ←−−−−−−−−−−→ ZnO (nuclei) + H2 O

(1) (2)

In the first reaction, under equilibrium condition, Zn(OH)2 precursor dissociates into Zn2+ and Zn(OH)4 2− ions [55]. In the second reaction, when the concentration of Zn2+ and OH− reaches the supersaturation degree, ZnO nuclei forms. Finally, after nucleation, excess Zn(OH)4 2− may grow into ZnO nanorods [55] via elongation along the c-axis of the ZnO crystallite nuclei, according to the following equation Zn (OH)24− ↔ ZnO (nanorods) + H2 O + 2OH−

(3)

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Table 1 Tabulation of advantages and disadvantages of various NPs synthetic approaches. S.N. 1.

2.

3.

Approach

Method

Advantage(s)

Disadvantage(s)

Ref.

Metallurgical

Large-scale production

Rapid agglomeration; Contamination

[1]

Laser ablation

High purity NPs

Requires high energy; Difficult to control: size distribution, agglomeration & crystal structure

[37]

Solid-state

Rapid synthesis; No reagent/solvent waste/excess

Contamination; Neat milling may lead to unwanted product amorphization

[38]

Co-precipitation

Simple & rapid preparation; Easy control of particle size & composition; Requires low temperature & is Energy efficient

Not applicable to uncharged species; Presence of trace impurities; Time consuming; May not be reproducible; Not application to reactants having different precipitation rates

[39]

Hydrothermal

Ambient reaction conditions; Regulation of rate & uniformity of nucleation, crystal growth & ageing; Control over morphology & particle size; Reduced aggregation level; Self-purifying process

Requires autoclave; Closed reaction system

[16,18,39]

Solvothermal

Material may be made soluble in a suitable solvent through heat & pressure in the system close to its critical point

Involves the use of solvents: non-environment friendly & costly; Requires autoclave; Closed reaction system

[39]

Sol–gel

High purity products; Controllable porosity degree; Control over particle size; Operable at low temperatures

Longer reaction time; Use of organic solvents

[39]

Plant extracts

Eco-friendly; Cost-effective; Non-toxic; Ambient reaction condition; Stable product; Does not require solvent or perilous chemicals

Requires low temperature (∼5 ◦ C) for extract storage

[1]

Microbial extracts

Eco-friendly; Low thermal budget; Does not require solvent or perilous chemicals

Time consuming; Screening of the microbe requires; Contamination susceptible from surrounding microbes; Requires specialized laboratory for operation

[1]

Physical

Chemical

Biological

Flowchart 1. Various type of capping agents as employed during ZnO NPs synthesis for stabilization and surface modification purposes.

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The concentration of SDS used greatly affects the rate of nanorods formation. On account of low concentration of SDS (lower than the critical micelle concentration (CMC)), SDS will exist in its ionic form. Co-ordination of SDS with ZnO NPs occur through the attachment of hydrophobic (methyl) group with the hydrophilic (sulfonic) group of SDS pointing towards the surface [55]. The interaction of SDS with Zn ions result into the formation of micro-spherical capsules in the first step provided the concentration of SDS is higher than CMC. Owing to the electrostatic interaction occurring at the interface between the sulfonic group and Zn2+ , exterior portion of the microcapsule will attract several Zn ions. As a result, these adsorbed ions will convert into ZnO nuclei during the course of hydrothermal reaction. Further, these SDS capped ZnO nuclei will serve as active growth sites for ZnO nanorods formation, wherein, anisotropic growth behaviour of the ZnO crystal and the effect of SDS capping on ZnO nuclei will restrict the growth direction on the capped site. This ensures growth of the crystal towards only one direction. Yin et al. [53] demonstrated that without SDS, rod-like structure of ZnO did not form, indicating the role of SDS towards ZnO nanorods structural formation. The Transmission Electron Microscopy (TEM) image of individual ZnO nanorod is shown in Fig. 1(b), with the inset figure depicting the schematic diagram of ideal growth habit of ZnO nanorods. Fig. 1(c) represents the corresponding High Resolution TEM (HRTEM) image of the corresponding ZnO nanorods, with the inset showing the Selected Area Electron Diffraction (SAED) pattern of the individual ZnO nanorod. Likewise, Lu et al. [59] reported a surfactant-assisted self-assembly and growth of 3D-ZnO hierarchical nanostructures. Fig. 2(a) explains the formation mechanism of 3D hierarchical nanostructures and Fig. 2(b) represents the corresponding Scanning Electron Microscopy (SEM) images of ZnO nanostructures synthesized hydrothermally using various capping agents. Similar to the previous discussion, Lu et al. have also explained the faster growth rate along the c-axis as compared to the other axes. The addition of a surfactant above CMC results into the formation of spherical micelles which then function as a nano-reactor [60]. Thereafter, Zn(OH)4 2− spreads towards the inner surface of the micelle with the water molecules, present in the reaction medium, lying in the spherical micelles. Under hydrothermal treatment, reaction between Zn2+ and OH− takes place and forms the precursor nuclei (Zn(OH)4 2− ) when their concentration exceeds the threshold point, following which, the supersaturated nuclei will transform into ZnO clusters. The presence of surfactant hinders growth of the crystals in all directions through their capping property and only allow the growth through certain specific directions (for e.g. along the c-axis to form nanorods). Juabrum et al. [61] have proposed a similar mechanism about the functionalization of the surfactant SDS as a capping and structure directing agent. Similar observations and explanations have been provided in several other Literature reports [62,63]. Another highly investigated surfactant is CTAB. According to Sun et al. [64], the growth process of ZnO NPs in the presence of CTAB occurs through a mechanism different than without CTAB. In the reaction medium, CTAB reduces the surface tension, thereby, lowering the energy required to create a new phase. Thus, ZnO crystals form in a lower supersaturation condition. Additionally, CTAB also influences the erosion process of Zn and the growth mechanism of ZnO NPs through electrostatic and stereochemical effects, as shown in Fig. 3(a). Since CTAB is a cationic surfactant, it has a tetrahedron structure with a long hydrophobic tail. On the other hand, the growth unit of ZnO (i.e. Zn(OH)4 2− ) [64] is although a tetrahedron, it is an anionic moiety. Thus, CTA+ and Zn(OH)4 2− may interact electrostatically, wherein, during the interaction between the two grants, CTAB

5

has the ability to function as an ion carrier [65], as illustrated in Fig. 3(b). Based on the ionic difference, CTAB adsorbs on the surface of Zn particles to form a film. Erosion process is then facilitated when CTAB leaves the surface and the barrier layer (i.e. film) becomes thinner. Therefore, through this process, CTAB serves as a growth as well as an agglomeration controller via forming a film, covering the newly formed ZnO crystal during the crystallization process. Moreover, the growth rate and orientation of the crystals depend upon the adsorption of growth units on crystal surfaces [66]. During the growth of ZnO crystals, CTAB floating film may form at the interphase between the solution and the crystal in order to reduce the interfacial energy. This floating film will then be released at the surface of ZnO single crystal by CTAB molecules carrying the ZnO growth units. Since CTAB preferentially forms a film, wherein, the molecules incline to be perpendicular to the adsorbed surface, ZnO growth units will tend to face-land onto the growing interface. This phenomenon will, thus, result into the formation of Zn-O-Zn bonds, making the landing mode more prominent in this fashion as compared to vertex- or edge-landing. Consequently, ZnO crystal will then grow along the c-axis [0001] preferentially. Thus, it may be concluded that CTAB not only accelerates the erosion process by functioning as a transporter of ZnO growth units but also, acts as a structure directing-agent by orientating the growth of ZnO nanorods. According to Cheon et al. [67], under non-equilibrium kinetic growth conditions in the solution-based synthesis method, the growth pattern of nanocrystals depend upon four parameters, namely, kinetic energy barrier, temperature, time and capping molecules. Under hydrothermal conditions, the effect of temperature and nature of capping molecules play an important role to impart changes in the formation mechanism. Zhang et al. [68] reported the influence of CTAB for ZnO formation. According to their report, capsules of CTAB generates in the saturated solution and due to the coulombic force of interaction between the growth unit and CTAB capsules, complexing agent forms which are then adsorbed at the circumference of ZnO nuclei. This adsorption mechanism decreases the overall surface energy of the ZnO nuclei and generates active sites on its surface. Therefore, even under low temperature hydrothermal synthesis, ZnO nanorods with pointed ends may grow on those active sites, as illustrated in Fig. 4(a). The SEM image of flower-like ZnO NPs is shown in Fig. 4(b), while HRTEM image of an individual swordlike ZnO nanorod, which composes the flower-like morphology of ZnO is presented in Fig. 4(c). In general, due to the long chain nature and binding property, formation of ZnO nanorods have mostly been reported in the presence of surfactants. Although, many Literature also reports about the formation of flower-like 3D ZnO nanostructures using surfactant as the capping agent, these flower-like structures are actually considered to be the agglomeration of individual ZnO nanorods [53]. In case of NPs other than ZnO such as cerium dioxide (CeO2 ), a similar microstructure was obtained using CTAB as the capping agent, i.e. hexagonal rods or nanofibers [69]. Therefore, it may be concluded that provided the method of synthesis is similar, not much difference in the function of the capping agent may be expected. The influence of various surfactants on the morphology and size of ZnO NPs synthesized via hydrothermal technique has been given in Table 2. 2.2. Amino acids: Growth, structure and formation mechanism of amino acid capped ZnO NPs Amino acids are the organic compounds called the ‘‘building blocks of proteins’’. Amino acids have a basic skeletal structure which comprises of an amino group (–NH2 ), carboxyl group (COO− ) and a side chain (–R group). Based on this structure,

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Fig. 1. (a) Schematic illustration for the formation mechanism of chrysanthemum-like ZnO nanocrystals, (b) TEM image of individual ZnO nanorod, with the inset figure depicting the schematic diagram of ideal growth habit of ZnO nanorods, and (c) HRTEM of the corresponding ZnO nanorods, with the inset showing the SAED pattern of the individual ZnO nanorod [53].

Fig. 2. (a) Schematic depicting the formation mechanism of 3D hierarchical nanostructures, and (b) Corresponding SEM images of ZnO nanostructures synthesized hydrothermally using various capping agents [59].

Fig. 3. (a) Schematic illustration of the erosion process: (I) in the absence of CTAB; (II) in the presence of CTAB. Black colour, Zn; grey colour, ZnO, wherein, the wave-like patterns indicate the CTA+ ions. A CTA+ ion carrying a zincate ion is shown, and (b) (i) Schematic illustration of ion-pair formed between CTA+ and Zn(OH)4 2− , and (ii) Schematic illustration of landing process on the surface of ZnO [0 0 0 1] crystal face [64].

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Fig. 4. (a) (i) Schematic of larger ZnO crystal growth, (ii) Illustration of ZnO nanostructures growth by the hydrothermal process without (1) and with CTAB (2), (b) SEM image of the corresponding flower-like ZnO nanostructures, and (c) HRTEM image of an individual sword-like ZnO nanorod, which aggregates to form flower-like ZnO nanostructures [68]. Table 2 Influence of surfactant-type on the morphology and size of ZnO NPs synthesized by hydrothermal method. S.N.

Capping agent (surfactant)

Morphology

Size/Diameter (nm)

Reference

1. 2. 3.

Carbamide CTAB CTAB

10–20 100 80–120

[70] [71] [72]

4.

CTAB

Nanobelts Nanorods Tapered Nanorods Elongated Triangular Bipyramidal Irregular rod-like Sword-like & Flower-like Nanorods

56

[64]

5. 6. 7.

CTAB CTAB

8. 9.

Ethylene diamine Ethylene diamine SDS SDS

10.

SDS

11. 12. 13. 14. 15. 16.

Sodium dodecyl sulfonate SDS SDS SDS SDS SDS

17. 18.

SDS SDS

100 100 125–170

[71] [73] [74]

molecule has been put forward by Wu et al. [83]. Fig. 5 represents the formation mechanisms of various ZnO nanostructures with their corresponding FESEM images. According to them, the histidine molecules and hydroxyl ions act as competitive ligands for Zn2+ owing to the different formation constants of the complex [84]. They have thoroughly discussed the formation process through a series of Eqs. (4)–(9) given below log K1 = 6.52

(4)

→ Zn(C6 H9 N3 O2 )2 2+

log K2 = 12.11

(5)

2+

log K1 = 4.4.

(6)

Zn(OH) + OH → Zn(OH)2

log K2 = 11.30

(7)

Zn(OH)2 + OH− → Zn(OH)3 −

log K3 = 14.14

(8)

log K4 = 17.66

(9)

Zn2+ + C6 H9 N3 O2 → Zn(C6 H9 N3 O2 )2+ Zn(C6 H9 N3 O2 )2+ + C6 H9 N3 O2

Zn

−

+ OH → Zn(OH) +

Nanorods Nanorods Plate-like & Nanorods Self-assembly of Nanorods into Chrysanthemumlike crystals Ultra-long nanowires Nanorods Irregular rod-like Nanowires Flower-like Needle-like & Flower-like Nanowhiskers 1-D Needle-like

110 ± 40

[75]

100 22–26

[71] [76]

200

[53]

8

[42]

50–100 100 140–160 150 200

[77] [71] [78] [79] [80]

80–150 6.98

[81] [82]

an amino acid, when used as a capping agent, may attack ZnO NPs from either of the two groups, –NH2 or COO− [83]. An important point to consider here is the interaction between active surfaces of the particles during growth and positively charged (e.g. histidine), negatively charged (e.g. aspartic acid) or neutral (e.g. glycine) amino acids. Amongst the amino acids used for ZnO NPs synthesis via the hydrothermal process, histidine is comparatively more common. A detailed explanation about the hydrothermal formation mechanism of ZnO NPs with histidine (C6 H9 N3 O2 ) as the capping

+

−

−

−

Zn(OH)3 + OH → Zn(OH)4

2−

Accordingly, when NaOH molar ratio is many times greater than that of Zn2+ (here, 1:22), OH− , being the predominant species in the reaction medium, plays the critical role in regulating the growth mechanism of the various crystal faces owing to the increased number of Zn(OH)n complex formation as compared to that of Zn(C6 H9 N3 O2 )n complex. Consequently, the formation of anisotropic prism-like ZnO takes place [73,83,85– 88]. On decreasing the NaOH molar concentration (here, 1:10 and 1:2.5), the competitive ligand, histidine molecules, adsorb on the surface of ZnO nuclei and direct the formation of flower-like ZnO NPs. Additionally, ZnO NPs may self-assemble to form hollowmicrospheres through hydrogen-bonding interactions between the histidine molecules adsorbed at the surface of ZnO nuclei. In a scenario, when the molar ratio of histidine to Zn2+ is increased (i.e. 1:1 to 1:2), the histidine molecules adequately cover the ZnO nuclei, thereby, preventing further crystallization and leading to the formation of non-crystalline hollow-microspheres. Hence, different morphologies and nanostructures of ZnO may be obtained through the adjustment in molar ratios of the competing ligands. Moreover, an interesting report by Yu and co-workers [73,85– 88] suggest that, histidine was not necessary for the formation of prism-like ZnO, while flower-like and hollow-microspheres required the presence of histidine molecules for their formation. Therefore, the manipulation in the structures of ZnO NPs is possible by tuning the amount of capping agent as being used. Similar observations have been reported in other Literatures [33,89–94].

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Fig. 5. Formation mechanisms of the ZnO Hierarchical Architectures and their corresponding FESEM images [83].

Fig. 6. (a) Schematic representations for the ZnO complex formation, wherein, (i) a faceted ZnO crystal bounded with top and bottom planes and six pyramidal planes, (ii) hourglass-like ZnO nanostructure formed by joining the [0001] planes, the yellow part in the centre shows vacancy, (iii) linear assembly of ZnO nanostructures, (iv) dotted purple frames: symmetric planes within the building unit and inside the resultant linear assemblies, and (b) FESEM image of a linear assembly of ZnO nanostructures; the white arrow indicates an open slit along the juncture of two [0001] planes; Inset figures represent a proposed growth mechanism of twined ZnO nanocrystals at the interfacial region [91].

A number of Zn complexes interact with amino acid ligands [95–98], leading to the modification in morphologies of the growing particles, i.e. ranging from low-dimensional grain-like to high definition 3-D nanostructures. The potential of Zn2+ to chemically bond with amino acids has been established through coordination chemistry [99]. In case of thin films, although the concentration and substrate choice are important parameters in structural determination of ZnO NPs [100], reports suggest that increase in the amino acid (capping agent) concentration

increases the film thickness while decrease in the concentration might cause inhibition of the thin film formation [33]. Yao et al. [91] have demonstrated a facile hydrothermal synthesis of hourglass-like ZnO NPs using polyoxyethylene sorbitan trioleate (Tween-85) as the capping agent. According to them, the twinned crystal structures have self-assembled into straight linear aggregates with a central vacant space in each twined building block, as shown in Fig. 6(a).

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Since transition metal ions may promote cleavage of a carbonyl ester [101–103], they inferred that Tween-85 did not adsorb at the surface of ZnO NPs but originated from its hydrophobic head-group, i.e. alkylated oleate group, while the carboxylate anion was bound to the surface of Zn2+ to form a compact ligand shell. Therefore, Zn2+ served as an active catalytic centre for ester cleavage, forming the alkylated oleate capping groups. From all their observations and analysis, Yao et al. [91] formulated the formation mechanism of the as-observed ZnO nanostructure. The bilayer formation (as shown in Fig. 6(b)) occurred from alkylated oleate molecules which served as a molecular template for nucleation and growth of the ZnO nanostructure [81]. Zn2+ gets attracted towards the negatively charged layer of carboxylate anions, subsequently, forming two (0001) planes of ZnO on each side, as shown in Fig. 6. The gap existing between these two planes may further be filled up by interring the organic molecules. The final complex of hourglass-like ZnO nanostructure then forms through the continuous growth and a subsequent solid evacuation processes. Thus, herein, capping agents have proven to act as efficient growth controllers during ZnO NPs formation. Therefore, on considering amino acids as capping agents in general, it may be concluded that, for zinc ion (Zn2+ ), two competing groups, i.e. amino acid and hydroxyl group, exists. Depending upon the higher or lower concentration of these groups, structure of the ZnO NPs may be determined. A tabulation of organic capping molecules, influencing the morphology and size of ZnO NPs, during hydrothermal synthesis has been presented in Table 3. 2.3. Polymers: Growth, structure and formation mechanism of polymer capped ZnO NPs The most common example of polymers used as binders during the synthesis of ZnO NPs are polyethylene glycol (PEG) and polyvinyl pyrrolidone (PVP), amongst others [48,110–112]. It has been well established from previous reports that polymers may enhance the optical, electronic and physical properties of pure ZnO NPs [49,113]. Polymers interact with the metal ions through complexation or ion-pair formation. Hence, they may be used to impart various physical properties to ZnO NPs [114,115]. The polymers, to function as to effective binders, must have low toxicity, excellent water solubility, biocompatible lubricity, and thermal stability [116,117]. Further, it is well known that ZnO is almost insoluble and agglomerates immediately in water during synthesis due to the high polarity of water leading to deposition [118]. Several concerns, therefore, arises such as aggregation, re-precipitation, settling, or non-dissolution, during the time of ZnO NPs synthesis [119]. Hence, to overcome these problems, the use of polymers as capping agents is considered. Adhyapak et al. [120] obtained hierarchical flower-like morphology of ZnO NPs using PEG as the capping agent by the hydrothermal synthesis of ZnO NPs. The role of PEG, as a structure directing agent, was confirmed based on the comparative study of ZnO NPs synthesis with and without its use. In the absence of PEG, irregular small sheet-like structures, clinging together in an irregular fashion was recorded through FESEM while, ZnO inherited the morphology of nanoflower in the presence of PEG. The schematic illustration for the formation mechanism of flowerlike ZnO is given in Fig. 7(a), while Fig. 7(b) and (c) present the FESEM images of ZnO nanostructures hydrothermally prepared at reaction durations of 8 h and 12 h, respectively [120]. Further, through XRD analysis, it was observed that the ratio of PEG influences the preferential growth orientation of ZnO. Another such study has been reported by Stankovic et al. [121], wherein, they have used different polymers, such as PVP, polyvinyl alcohol (PVA) and poly α , γ , l-glutamic acid (PGA), as

9

capping agents during the hydrothermal synthesis of ZnO NPs. The as-obtained ZnO NPs exhibited nanorods of hexagonal prismatic and hexagonal pyramid-like nanostructures, with certain spherical and ellipsoid morphologies. The Authors’ concluded that such differences in morphology and dimensions of the particles were due to different nature of the capping agents used in their synthesis [121–123]. The differences in the structures of asobtained ZnO NPs, employing various capping agents, and their comparison with commercial ZnO NPs have been presented in Fig. 8 based on FESEM images, wherein, (a) is for PVP capped ZnO NPs, (b) for PVA capped ZnO, (c) for PGA capped ZnO, and (d) is for commercially available ZnO NPs. PVP, a non-ionic polymer, when used as a capping agent, prevents crystal growth along the (1010) plane, while directing the growth towards the (0001) plane. This results into the formation of elongated crystal structures [122]. Stankovic et al. [121], have also analysed the result of crystal growth as-obtained after hydrothermal treatment. Since the growth velocities under hydrothermal treatment are V[0001] > V[1011] > V[1010] , the different growth rates of particular crystal planes result into the formation of rod-like morphology of ZnO. Further, the plane with a greater growth velocity disappear comparatively more quickly than the other crystal planes, which may be the reason for the appearance of pointed shape at the ends of the c-axis [57,124], as shown in the Fig. 8. However, in case of an amphiphilic polymer, such as PVA, the above explanation does not suffice the formation mechanism of ZnO NPs [121]. PVA adsorbs at the interface, wherein, plenty of free OH− groups are present [121]. PVA has the ability to adsorb onto the substrate, through hydrogen-bond formation, creating a physical barrier to the preferred growth crystal planes. Therefore, nature of the polymer used as capping agent strongly determines the morphology and crystal orientation of ZnO NPs. Additionally, morphology and action of capping agents also depend upon the pH of the reaction medium since the growth units vary at different pH levels; ZnO growth unit is Zn(OH)2 at pH 6–9, while Zn(OH)4 2− at 9–13 is the dominant species [125]. Since the latter is a growth unit bearing a negative charge, the crystal growth process, hence, strongly depends on the charge or neutral characteristics of the capping agent used during the synthetic process [123]. The intrinsic crystal structure and external conditions, including kinetic energy barrier, temperature, time, capping agents, etc. influence the growth habit and growth rate of the crystals [67,126]. On considering PEG as the capping agent, it has been found that PEG may bind to the positively charged Zn2+ and terminate (0001) polar plane more strongly and readily as compared to other nonpolar crystal surfaces, which may be attributed to the coulombic force of interaction [120]. Subsequently, the anisotropy will then slow down the crystal growth along [0001] plane. Therefore, it is understandable that due to the ( use of ) PEG, orientation of the ZnO NPs changed from (0001) to 1010 and hence, the morphology designed into nanosheets built microflowers. According to Adhyapak et (al. [120], ) such type of protuberances preferably grow along the 1010 direction with lagging direction {0001} within the [2 110] plane, leading to the formation of ZnO nanosheets on the lateral surface. Additionally, some outshoots are also present on the growing nanosheets which, consequently, will result into the of secondary branched nanosheets with terminated ( formation ) 2 110 facets and inter-planar angles of ∼60◦ . With the maturity of hydrothermal reaction, third or fourth branched nanosheets may also be formed, on the already grown nanosheets, resulting into the creation of flower-like ZnO nanostructure, as shown in Fig. 8(a). A very interesting nanostructure of ZnO, as shown in Fig. 9(a), synthesized by hydrothermal method employing PVP as the capping agent, has been put forward by Krishnakumar et al. [127],

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P. Basnet and S. Chatterjee / Nano-Structures & Nano-Objects 22 (2020) 100426 Table 3 Compilation of organic molecule-type, morphology and size of ZnO NPs synthesized by hydrothermal method based on Literature. S.N.

Capping agents (amino acids/organic molecules)

Morphology

Size (nm)

References

1. 2.

Citric acid Histidine

NA NA

[104] [83]

3. 4. 5.

4–7 NA NA

[105] [94] [94]

Tertiary Nanobranches

NA

[94]

7. 8.

Histidine Hexamethylenetetramine Hexamethylenetetramine & Diaminopropane Hexamethylenetetramine & Sodium citrate L-Lysine Lysine

Disk-like Hexahedral prism-like, flower-like, crystalline & non-crystalline hollow microspheres Nanocrystals Nanorods Needle-like nanobranches

400–650 >500

[106] [107]

9. 10. 11.

L-Cysteine L-Arginine Sodium malate

50–90 200–450 16 nm

[106] [106] [93]

12.

Sodium citrate

NA

[94]

13.

Sodium succinate hexahydrate Tween-85 Thiol Urea Urea

Hexagonal rod & Cubic-like Dumbbells, twin-prisms, whiskers, and nanorod bundles Sphere, brain-like & Pussy willow-like Quasi-spherical, Nanopowders & Cubic-like Hexagonal shaped micro-particles constructed with orderly arranged nanoplates Secondary nanoplates formed on columnar facets of the primary nanorods Cluster whiskers & rod-like whiskers

100–800

[5]

Hourglass-like Nanowires & nanotubes Nanobelts Hollow nanorods assembled into microflowers

400–600 150, 294 10–20 10–50

[91] [108] [70] [109]

6.

14. 15. 16. 17.

Fig. 7. (a) Schematic representation for the formation of flower-like ZnO nanostructures, FESEM images of ZnO obtained at, (b) 9 h and (c) 12 h, of hydrothermal treatment in the presence of PEG [120].

wherein, they have termed PVP as the ‘‘shape modifier’’. They obtained star-like ZnO nanostructure in their synthesis within only 10 min of microwave-assisted hydrothermal synthesis approach. Another remarkable synthesis approach has been reported by Parra et al. [128], wherein, they have modified the structure of ZnO using PVP as the capping agent, the obtained ZnO nanostructure has been shown in Fig. 9(b). According to their report, the proposed formation mechanism involves, reaction of Zn2+ with PVP to form a coordination compound, which reacts with OH− ions and dehydrates to form ZnO. A proportional relation exists between the low temperature crystallization of ZnO and PVP concentration, which increases to a critical value or above (∼0.2 g). To obtain uniformity in grain size and reduce surface roughness,

overnight ageing of the material may prove to be beneficial due to the development of denser nucleation sites. Several reports are available in Literature which offers similar results and explanations (Table 4). A list of commonly used polymers as capping and surface directing agents during the hydrothermal synthesis of ZnO NPs, with reference to previously reported Literature, is provided in Table 4. It should be noted that the polymer’s charge and length of its hydrophobic tail, to be used for the capping purpose, needs to be considered before synthesis since these parameters ultimately decide and design the nanostructure of the material.

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Fig. 8. FESEM images of: (a) PVP capped ZnO, (b) PVA capped ZnO, (c) PGA capped ZnO, and (d) commercial ZnO, nanostructures, respectively [121].

Fig. 9. (a) & (b) TEM micrographs showing star-shaped ZnO nanostructures [127], and (c) & (d) SEM morphologies of PVP modified ZnO NPs at different magnifications, respectively [128].

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Table 4 List of polymers used as capping agents for controlling the morphology and size of ZnO NPs, as reported in Literature. S.N.

Capping agent/ composite (polymer)

Morphology

Size (nm)

Reference

1.

Polyvinyl pyrrolidone (PVP) Polyvinyl alcohol (PVA) Poly (α , γ , L-glutamic acid) Polyethylene glycol (PEG)

Hexagonal prismatic rods Spherical

100

[121]

30

[121]

Ellipsoid

100

[121]

Assembly of nanosheets into Flower-like structure Star-like Nano-rods Dumbbell-like Pineal-type, Flower-type & Sea-urchin-type Nanorods Spherical to wall-like

10–25

[120]

19 30 NA NA

[127] [128] [104] [129]

35–60 50

[130] [131]

Nanorods

35–60

[130]

Quasi-spherical

20

[132]

Nanospikes Mulberry-like

80–100 30–47

[133] [134]

2. 3. 4.

5. 6. 7. 8.

PVP PVP PVA Poly acrylonitrile

9. 10.

Triton X 100 Tri-noctylphosphine oxide Polyethylene glycol-6000 Living Block Copolymer latex PEG TEA

11. 12. 13. 14.

2.4. Polysaccharides: Growth, structure and formation mechanism of polysaccharide capped ZnO NPs It is a well-known fact that capping of inorganic nanostructures with organic molecules establish unique photophysical and photochemical properties as a result of the synergistic or combinatorial effects of the inorganic and organic phases [135,136]. In case of ZnO NPs, its conjugated nanostructure with an organic binder, such as a polysaccharide, may extend the absorption band up to the visible range and thereby, modify the photophysical properties of ZnO [137,138]. According to the previously reported studies [141,142], starch is one of the best candidates used as a green capping agent since it is bio-compatible, biodegradable, and a renewable polysaccharide [136]. Starch contains two basic biopolymers, namely, amylose (∼20%) and amylopectin (∼80%), which exhibits interesting dynamic supramolecular associations facilitated by both inter- and intra-molecular hydrogen bonding. This leads to the formation of molecular-level capsules that may act as templates for NPs synthesis [139,143,144]. Starch adopts a right-handed helical conformation in aqueous solution, wherein, the large number of hydroxyl groups may facilitate the complexation process of Zn2+ to the molecular matrix [145,146]. Hence, starch provides a stable surface passivation to prevent agglomeration of the NPs so-formed [147,148]. Buazar et al. [139] reported a noble method for the synthesis of starch capped ZnO NPs using starch-rich potato extract. According to their investigation, the amylose and amylopectin content of starch are linear and branched molecules, respectively, with a large number of hydroxyl groups. These hydroxyl groups facilitate the complexation of Zn2+ to the molecular matrix, whereas, the aldehyde terminals function as reductants of the cation to form Zn0 NPs [140]. An advantage of using starch as the capping agent is the fact that the bond between starch and metal NPs is comparatively weak than that between NPs and conventional thiol-based binders [149]. This means that the capping agent, attached to the surface of

as-formed ZnO NPs, may be removed by annealing at higher temperature. Based on this discussion, Buazar et al. [139] devised a mechanism of organic phase synthesis, as shown in Fig. 10(a). The long alkyl chain of starch causes the easy dissolution of ZnO NPs, in addition, to preventing its agglomeration [151]. This indicates that starch exhibits a high dispersing ability in encouraging the dispersion of modified-ZnO NPs, leading to an increase in monodispersity and decrease in the average particle size of the as-formed ZnO NPs [151]. Another starch mediated hydrothermal synthesis of ZnO NPs has been reported by Ramimoghadam et al. [140], wherein, they have employed rice as a soft biotemplate. Since the main component of rice is carbohydrate, it can play multiple roles during the synthesis of ZnO NPs, such as coating/capping, functionalizing, stabilizing, poring and/or coordinating [140]. According to their report, the polymeric part of starch, consisting of helical-shaped carbonaceous matrix bearing multiple polyol groups, form a functionalized protecting shield for Zn2+ , thereby, acting as a structure-directing agent. The hydroxyl groups of amylopectin has the ability to coordinate transition metal ions and may be involved in both intra- and/or intermolecular associations [153,154]. Herein, the rice granules, used as starch source, swell-up in aqueous medium and change their semi-crystalline structure into smaller amylose molecules, which consequently, leaches out of the granule. These small molecules may form complexes with Zn2+ owing to their high number of coordinating functional groups. In such case, it is preferable that the majority of Zn2+ remain closely linked with the starch molecules, therefore, nucleation and initial growth of the crystals may favourably occur within the area comprising of high starch and Zn2+ concentration [155]. Moreover, depending upon the concentration of the uncooked rice, used as a biotemplate/capping agent, the formation of various nanostructures of ZnO is possible, as depicted in Fig. 10(b). Similar to starch, cellulose is one such highly investigated polysaccharide, which has been used as a capping agent during NPs synthesis [156]. Yin et al. [157] reported the self-assembly of hierarchical cage-like one-dimensional ZnO nanostructures employing sodium carboxymethyl cellulose, as the capping and structure-directing agent, under hydrothermal conditions. They proposed that, initially, hexagonal ZnO microplates or microrods may form, followed by the growth of nanorods in channels on the polar top and bottom faces of (0001) plane of the microplates or microrods. As the reaction proceeds, a number of ZnO nanorods form, which then aggregate to form cages like ‘‘broccoli’’, as shown in Fig. 11, in order to reduce the specific surface energy. Finally, due to the high concentration of ammonia solution (aqueous) in combination with the gas–liquid equilibrium provided by the autoclave, the formation of hollow structure takes place, wherein, the bundled nanorods erode from the centre of the as formed broccoli-like structure [157]. Another commonly used polysaccharide for its capping property is chitosan [158–160,165], which is a linear polysaccharide composed of randomly distributed β -(1→4)-linked Dglucosamine and N-acetyl-D-glucosamine. Chitosan, a bifunctional material, may capture the metal ions in aqueous solution through coordination/chelation bonding of the hydroxo- or the more active amino- groups [154,166–168]. According to Abiraman et al. [152], the growth mechanism of ZnO NPs in the presence of chitosan may be explained as: Zn2+ forms chelation with the chitosan biopolymer, which then reacts with NaOH, leading to the formation of colloidal Zn(OH)2 solution. This solution then converts into ZnO nanorods. The synthesis procedure, growth mechanism and the as-obtained TEM image of chitosan-ZnO NPs is given in Fig. 12 [152]. Herein, they have demonstrated the role of capping agent by confirming that in the absence of chitosan,

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Fig. 10. (a) Schematic for the proposed mechanism of ZnO NPs hydrothermal synthesis using starch-rich potato extract as both reducing and stabilizing agent, with the corresponding TEM image of as-obtained ZnO NPs [139], and (b) FESEM images of ZnO NPs prepared using various concentrations of uncooked rice [140], respectively.

Fig. 11. SEM images of the double-cage nanorod-assembled superstructures at magnifications of: (a) 10 µm, (b) 5 µm, (c) 3 µm, and (d) 1 µm, respectively [150].

the ZnO NPs aggregated. Likewise, Magesh et al. [161], confirmed through HRTEM analysis that the polysaccharide chitosan enhanced the Ostwald ripening kinetics, leading to inhibited growth by forming the bond between Zn2+ and amine or hydroxyl groups of chitosan. Likewise, numerous other polysaccharides have been

used for their efficient capping properties such as glucose, fructose, and others [141,156,169,170]. Based on previous Literature reports, common polysaccharides used in ZnO NPs hydrothermal synthesis and the corresponding morphology and size of ZnO NPs have been provided in Table 5.

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Fig. 12. Schematic displaying the synthesis procedure, probable growth mechanism and TEM image of the as-synthesized chitosan-ZnO NPs, (where, CS-chitosan) [152]. Table 5 List of Polysaccharides used during ZnO NPs hydrothermal synthesis for structure directing and capping properties, as reported in Literature. S.N.

Capping agent (polysaccharide)

Morphology

Size (nm)

Reference

1. 2. 3. 4.

Chitosan Chitosan Chitosan Chitosan

Hexagonal Nanorods Nanoflower Well aligned nanorods Flake-, flower- and 3-D star-like Flake-, flower-, rose-, star- & rod-like structures Nanorods Hollow cage-like superstructures assembled by nanorods Microstars Spherical

23.5 60–70 NA 100

[158] [159] [160] [161]

200–1000

[162]

100

[140]

50 50–100

[142] [157]

358 39

[163] [164]

Palm olein 5.

Rice

6. 7.

Starch Sodium carboxymethyl cellulose

8. 9.

Xanthan Gum Honey

Therefore, in general, a polysaccharide interferes in different synthesis steps of ZnO NPs. This biopolymer, when dispersed in liquid medium, acts as an organic matrix and bind through their functional groups, such as hydroxylic, carboxylic, or amino groups, with the metal cations (Zn2+ ). The preliminary interaction of Zn2+ with polysaccharide regulates a homogeneous dispersion of the cation in well confined spaces. Afterwards, in the presence of a precipitating agent (such as NaOH), the binding positions regulate nucleation and the growth process for the hydrolysed metal ions owing to the high degree of supersaturation. The conversion of the oxide precursor into metal oxide further requires heat treatment. 2.5. Bioextracts: Growth, structure and formation mechanism of plant extracts and microbes functionalized ZnO NPs Generally, the hydrothermal synthesis of ZnO NPs involve the use of various chemicals, such as NaOH, NH4 OH, etc., which may

be harmful and perilous to the environment. Moreover, the synthesis involved may not be cost-efficient [171–173]. Therefore, in an attempt to modify the existing hydrothermal synthesis method, the naturally occurring plant and microbial extracts are now being realized as safe reducing and capping agents. Basnet et al. [1] have elaborately discussed biosynthesis of ZnO NPs using these naturally occurring plant and microbial extracts, including their systematic growth mechanism and application. In case of ZnO NPs synthesis using plant extracts, different parts of the plant may be used, such as leaves, roots, rhizomes, bark, fruits, flowers, etc. [1]. Since the plant extracts act as both reductants and stabilizing agents due to the presence of biochemicals, no extra chemical reducing and capping agents are required. According to Ahmed et al. [171], the major phenolic compounds present in the plant extracts are: (i) flavones, which include orientin, isoorientin, vitexin, isovitexin, luteolin, and chrysoeriol, (ii) flavonols, which comprises of quercetin, hyperoside, isoquercitrin, and rutin, and (iii) flavanones composed of dihydroorientin, dihydro-isoorientin, and hemiphlorin. Singh et al. [174] synthesized ZnO NPs employing the hydrothermal-biosynthesis method with Solanum lycopersicum flower extract. They concluded that the process of formation involved the reaction between Zn2+ from the zinc precursor (zinc acetate dihydrate) and the phytochemicals of the flower extract including a precipitating agent, NaOH. With reference from the FTIR analysis, they inferred that biomolecules, such as polyphenols, carboxylic acid, polysaccharide, amino acids and proteins, present in the flower extract, acted as reducing agent and were present at the surface of ZnO NPs, thereby, functioning as capping agents. Herein, the aforementioned biomolecules not only operated as the growth terminator of ZnO NPs but also acted as a linker molecule between two or more ZnO NPs making their self-assembly [175]. The mechanism involved, reduction of zinc acetate by the phytochemicals of the flower extract and conversion of the as-formed complex into Zn(OH)2 under the influence of NaOH at an optimum pH. This complex then acted as primary growth units for the synthesis of ZnO NPs [174,176], wherein, the Zn(OH)2 particles converted into ZnO NPs under hydrothermal condition. Similar observation and formation mechanism have been reported in other studies as well [175,177].

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Fig. 13. (a) Hypothetical mechanism of ZnO synthesis, with the step-wise reactions involved, and (b), (c), (d) SEM images of ZnO NPs with different Thyme leaf extract concentration, respectively [178].

Zare et al. [178] reported the bio-hydrothermal synthesis of ZnO NPs using Thymus vulgaris leaf extract. From the phytochemical screening test, they observed that phenol derivatives, such as thymol and flavonoid were the major components present in the leaf extract and these acted as capping agents during ZnO NPs formation. Through FTIR spectra, they confirmed the presence of OH/COOH groups in the phenol derivatives attached to the surface of ZnO NPs, thereby, functioning as capping agents. Consequently, these groups avoids over-reaction, formation of other complexes, and agglomeration of the as-formed particles. The hypothetical formation mechanism of ZnO NPs with the stepwise reaction is provided in Fig. 13 [178]. A type of synthesis, involving the use of living organisms, is the microbial synthesis method. In this, the organism used for synthesis may be a bacterium, fungus or an algae. However, as compared to biosynthesis using plant extracts, microbial biosynthesis has several disadvantages, for example, it is a timeconsuming process since screening of the microbe is required, the culture broth has to be carefully monitored during the entire synthetic process and contamination much be strictly prevented. Further, control over the size and morphology of the NPs may be difficult and also, the cost linked with the medium (wherein, the microbial growth takes place) is very high [1] However, despite these drawbacks, the microbial biosynthesis is gaining attention, as compared to physiochemical synthesis methods, because this approach is not only eco-friendly but it also avoids the use of harsh chemicals and high energy [183]. To cite the advantage of bacterial biosynthesis, the bacteria, Rhodococcus [184] has the ability to withstand adverse conditions and may metabolize hydrophobic compounds, thereby, favouring biodegradation. Microorganisms have various reductase enzymes, which are capable of reducing metal salts to metal NPs. Further, metal-resistant genes, proteins, enzymes, cofactors, and organic materials present in the microbes act as capping agents, and thereby, prevent agglomeration while imparting stability towards the as-synthesized NPs. As far as the microbial biosynthesis is concerned, there are basically two methods: extracellular synthesis and intracellular synthesis. A detailed explanation of the synthesis processes involved in these two methods

Table 6 Compilation of previous Literature reports on the hydrothermal bio-synthesis of ZnO NPs using plant/microbial extracts as capping and surface-directing agents. S.N.

Capping agent (Plant/microbial extracts)

Morphology

Size (nm)

Reference

1.

Ceropegia candelabrum (plant) Eryngium foetidum L. (plant) Sapindus rarak DC (plant) Streptococcus thermophiles (microbe) Thymus vulgaris (plant)

Hexagonal

12–35

[192]

Spherical

8

[193]

Agglomerated & spherical Hollow spheres

NA

[194]

200–400

[195]

Nanoparticles

50–60

[178]

2. 3. 4.

5.

has been provided by Singh et al. [183]. With the growing need for environmental remediation, several reports on the microbial biosynthesis have been reported in Literatures [43,180,183,185– 191]. Fig. 14(a) represents the schematic mechanism for the formation of Rhamnolipids@ZnO NPs and the step-wise chemical reactions [179]. The possible role of Rhamnolipids (RLs) detrived from Pseudomonas aeruginosa bacterium for encapsulation and stabilization of ZnO NPs has been shown. Fig. 14(b) provides the FESEM image of ZnO NPs obtained through biosynthesis using Aeromonas hydrophila extract [180], 14(c) represents TEM image of Sargassam myriocystum mediated biosynthesized ZnO NPs [181], and 14(d) is the TEM image of ZnO NPs synthesized using Aspergillus fumigatus extracts [182]. From Fig. 14(b), 14(c) and 14(d), it may be seen that different micro-organisms gives rise to different morphologies of ZnO NPs. Table 6 provides an insight into the type of plant/microbial extract used during the hydrothermal bio-synthesis of ZnO NPs with their corresponding size and morphology. Therefore, through these discussions, it may be concluded that the method of biosynthesis is much environmentally benign than the common physiochemical processes, although, time-constrain

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Fig. 14. (a) Schematic formation mechanism of RL@ZnO NPs with the corresponding step-wise chemical reactions [179], (b) FESEM micrograph of ZnO NPs biosynthesized using Aeromonas hydrophila [180], (c) TEM image of S. myriocystum mediated green synthesis of ZnO NPs [181], and (d) ZnO NPs biosynthesized employing Aspergillus fumigatus extract as observed under TEM [182], respectively.

and screening of the microbes may put some restrictions for Research in a small platform. 3. Applications of ZnO NPs The diverse physical and chemical properties of ZnO NPs make it a potential candidate for numerous applications, ranging from tyres to ceramics, from agriculture to pharmaceuticals, and from chemicals to paints. In this section, the common applications of ZnO NPs, as given in Scheme 1, have been discussed in brief. 3.1. Rubber industry A major portion of ZnO NPs produced each year is consumed by the rubber industry, wherein, the NPs are used to manufacture different cross-linked rubber products [196]. ZnO NPs, as nanoscale conductivity fillers, are mainly employed to improve the thermal conductivity (while retaining the high electrical resistance) of typical pure silicone rubber, which otherwise is relatively low [197]. Since the high energy ZnO NPs may agglomerate in the polymer matrix to form larger particles, consequently losing its productivity, surface modification/passivation is a helpful technique to overcome this challenge. Research articles, related to this type of work, may be found plenty in Literature. For example, in the work reported by Yuan et al. [100], high conductivity silicone rubber was prepared by the incorporation of vinylsilane group capped ZnO NPs into the silicone rubber through the process of hydrosilylation. On comparison of conventional silicone rubber with the modified one, better mechanical property and higher thermal conductivity of the latter were observed, attributed to the formation of a cross-linking structure with the silicone rubber matrix and better dispersion in that matrix. To investigate the relationship between the characteristics of ZnO NPs and its role in the cross-linking process for carboxylated nitrile elastomer, Przybyszewska et al. [198] utilized ZnO NPs having various particle sizes and morphologies. They reported that ZnO NPs resulted into vulcanizates with better mechanical properties and higher crosslink density when compared to the vulcanizates obtained via crosslinking between micro-sized ZnO (commercially used cross-linking agent).

3.2. Textile industry Innovations to improve the quality of textile materials in order to meet the growing need of today’s lifestyle and considering environmental factors, have led to the synthesis of water-repellent and self-cleansing textiles with UV-absorbing properties [199– 205]. In this aspect, ZnO NPs are widely used as textile coatings owing to their biocompatibility, air-permeability and efficient UV-blocking properties [206]. Ates et al. [207] reported the selfcleaning, super-hydrophobicity and UV-blocking application of steric acid functionalized ZnO nanowires grown on cotton fabric. The efficiency of ZnO NPs in polyester fibres were evaluated in Poznan University of Technology and Textile Institute in Lodz [208]. The results indicated that the textile material modified with ZnO NPs exhibited UV-shielding and anti-bacterial properties, unlike the unmodified fabric. Therefore, on considering the advantages of ZnO NPs in water-repellence, self-cleansing and UV-blocking properties, the as-modified textile materials may be considered to be promising candidates for: military applications, where laundering in severe conditions may be avoided and in the business domain, where unwanted stains in the clothes may be prevented. 3.3. Electronic and electro-technological industries ZnO NPs have a variety of applications in electronics and electro-technological industries [209–211]. The direct wide bandgap (≥3.37 eV) and high binding energies (60 meV) of ZnO NPs [212,213] makes them a promising candidate in photoelectronics [214], electronic equipment [215], surface acoustic wave devices [216], field emitters [217], sensors [212,214–222], UV LASERS [223], and in solar cells [224]. Moreover, due to the superior luminescence, UV-resistant and higher electrical conductivity of ZnO NPs as compared to sulphur and phosphorus compounds, they are used in field emission display equipment, like television. The thin-films of ZnO NPs display high conductivity and enhanced permeability of visible rays, making them extremely useful material for the construction of light permeable electrodes in solar batteries. Additionally, these films may be

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17

Scheme 1. Various applications of ZnO NPs.

employed as transparent electrodes in photovoltaics, electroluminescent, and UV-emitting devices [225]. Another interesting application of ZnO NPs in the electro-technology industry is as gas sensors [226–229]. ZnO NPs are commonly used to detect CO, CO2 , H2 , SF6 , C4 H10 , C2 H5 OH, NOx , CH4 , etc. ZnO NPs are also employed in the manufacture of varistors (resistors with a non-linear current–voltage characteristic) [230,231]. 3.4. Photocatalysis Over the past many years, several studies disclosed the importance of photocatalysis using ZnO NPs [20,21,83,138,143]. Photocatalysis involves the absorption of photons by ZnO NPs and subsequent formation of electron–hole pairs through oxidation and reduction reactions occurring at the surface of the catalyst. Photocatalysis comprises of application towards organic pollutant degradation [20,138], water-splitting [232], anti-microbial [233], photocatalytic conversions and/or synthesis of organic compounds [234]. 3.4.1. Photocatalytic dye degradation Photocatalytic degradation of organic pollutants (methylene blue, rhodamine B, paracetamol drug, and p-Nitro phenol reduction) have been previously explained by the Authors using ZnO NPs capped by tea-phytochemicals and polyethylene glycol [1, 20,21]. The mechanism involves: generation of reactive oxidative species (ROS), particularly, hydroxyl radicals (• OH), superoxide radicals (O2 −• ) and holes (h+ ) in the reaction medium [20,21]. These ROS then undergoes redox-reactions with the adsorbed organic pollutants at the surface of ZnO NPs to convert them into negligibly toxic, simple inorganic compounds. The photocatalytic efficiency is greatly affected by the surface area, size, morphology and crystal defects of ZnO NPs, wherein, higher surface area, smaller size and greater number of crystal defects largely enhances the photocatalytic performance probably due to the creation of higher number of active reaction sites, respectively [235]. Moreover, surface-passivation of NPs by capping agents has shown to largely affect the photocatalytic performance, whereby, the presence of capping agents may have an

adverse impact due to the hindered access of reactants to the catalyst’s surface [236,237]. To overcome this drawback, various strategies have been proposed for the removal of capping agent from the NPs’ surface, such as thermal annealing [238], solvent washing [239], and UV–ozone treatment [240]. However, alteration in the particle size, morphology and stability of the NPs resulted through these treatments [236]. Hence, capping approach, to control the size, shape and stability of ZnO NPs, may be done in such a fashion that the capping material does not encapsulate the entire particle’s surface but pose restrictions only to control agglomeration and to direct specific growth pattern [20]. For example, the study reported by Sudha et al. [137] compared the photocatalytic efficiency of bare and PEG capped ZnO NPs towards the degradation of rhodamine B dye. They concluded that PEG capped ZnO NPs exhibited reduced photocatalytic activity compared to bare ZnO NPs through the analysis based on reduced chemical oxygen demand and total organic carbon. On the other hand, Authors [20] found a higher photocatalytic degradation activity of PEG capped ZnO NPs compared to uncapped ZnO NPs towards RhB as well as MB dyes. Through these, it may be concluded that the amount of capping agent (with respect to other precursors), utilized during ZnO NPs synthesis, largely determines the extent of ZnO NPs surface passivation. 3.4.2. Photocatalytic water-splitting In case of photocatalytic water-splitting systems, briefly, ZnO NPs is immersed into an aqueous electrolyte with a set potential difference and thereby, the transfer of charge takes place between the electrode and electrolyte until equilibrium Fermi level is reached [241]. Since ZnO is an n-type semiconductor, the redox potential of the electrolyte is typically lower than the Fermi level, therefore, electrons will move from the electrode into the solution. This subsequently generates a positive charge related with the space charge region, which is reflected in an upward bending of the band edges [242]. Hence, the photoexcitation of an electron causes the simultaneous unidirectional flow of photoexcited electrons towards the photocathode. Holes, at the surface of ZnO NPs, function as active sites for the subsequent water oxidation steps. In order to improve the photon-to-current conversion efficiency,

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one of the proposed methods is to modify the electronic structure of ZnO and extend its light harvesting property towards the visible region of the electromagnetic spectrum. This, in turn, may be enhanced by controlling the morphology and size of ZnO NPs via the utilization of various capping agents [243]. 3.4.3. Photocatalytic anti-microbial activity The mechanism of photocatalytic antimicrobial studies is yet to be fully understood, however, certain distinctive pathways have been put forward in Literature, such as: direct-contact of ZnO NPs with the bacterial cell walls, which ultimately leads to destruction of the bacterial cell integrity [146,244,245]; liberation of Zn2+ ions, which are considered to be toxic towards microbe [246,247]; and ROS generation [248–250]. Although, the mechanism needs to be explored fully, the potential of ZnO NPs towards bacterial and fungal inhibitions have been explored in a number of scientific reports [233].

2. Rigorous analysis related to differentiate and independently regulate the contributions towards electronic and geometrical properties on the capping agent behaviour. 3. Theoretical modelling exactly supporting the experimental studies is not fully developed. 4. Susceptible to changes in reproducibility of the experimental results. Overcoming these challenges may prove to be beneficial for producing NPs with value-added properties and more widely ready-to-use application. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

3.5. Other applications

Acknowledgement

ZnO NPs have adsorbent properties, which is helpful for the removal of pollutants such as hydrogen sulphide liberated from petroleum products [233], heavy metal ions present in waste water [251], and others. In agriculture, ZnO NPs are known to enhance the soil microbial population as well as the soil exoenzyme activities. This consequently results into an increased growth activity of the plant (peanut plant) [252]. Since ZnO NPs are excellent UV-light absorbers, ZnO NPs are used in crèmes and lotions as sunscreen/UV-shields [253]. Owing to the outstanding drying, antimicrobial, wound healing properties of ZnO NPs, they are widely employed for the manufacture of medicine and dermatological substances [254,255]. ZnO NPs are also used in dietary supplements and nutritional product [256] to address the requirement of Zn ion content in the human body.

Ms. Parita Basnet is obliged for scholarship from Pai Endowment fund, India, Ref SMU/VC/2015-70, dated 17/11/2016.

4. Conclusions and outlook Keeping in view the role of various capping agents, benefit of the hydrothermal synthesis approach and multi-functional ZnO NPs, exact growth mechanism of the ZnO nuclei in the presence of respective capping agents, have been provided with reference from the Literature. The simplicity of controlling the various growth parameters during ZnO synthesis, hydrothermal method was chosen as the subject of interest here. Further, separate sections for polymers and surfactants (which are often confused to be identical) used as ZnO capping agents have been provided since not all polymers may be considered as surfactants; only those polymers possessing the properties of surfactants, such as acrylate copolymers (e.g. 2-acrylamide-2-methyl-1propanesulfonic acid and alkyl methacrylamide, alkyl methacrylate or alkyl acrylate, poly (allylamine)-supported phases, poly (ethyleneimine)) are surfactants, often called as polymeric surfactant. This review also provides a summary on the versatile nature, namely, optical, physical, and chemical, etc. of ZnO NPs in diverse application fields. Therefore, concisely, this review provides the basis for the formation mechanism of ZnO NPs during hydrothermal process in the presence of various capping agents. Although, improvement in Research related to ZnO NPs synthesis involving capping agents is developing rapidly to meet the growing demand of environmental safety and industrial requirements, certain challenges still remain to address, which may be summarized as follows1. Identification of exact involvement of capping agents require a further in-depth study.

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