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Multiphase TiO2 nanostructures: a review of
eﬃcient synthesis, growth mechanism, probing
capabilities, and applications in bio-safety andhealthRajni Verma,ab Jitendra Gangwarbc and Avanish K. Srivastava*abThis review article provides an exhaustive overview of the eﬃcient synthesis, growth mechanism andresearch activities that mainly concern an assortment of multiphase TiO2 nanostructures and theirstructural, morphological, optical and biological properties co-relations. First, a systematic review isprovided for the state-of-art and synthetic mechanisms utilized to generate these nanostructured
systems. A range of remarkable photoactive characteristics with respect to the eﬀects of the structural
Received 21st June 2017
Accepted 27th August 2017phases as well as morphogenesis are then brieﬂy covered and organized into sections presenting thenanostructures' photo-luminescence and photo-catalysis properties. A plausible mechanism for theirnano–bio interaction is also reviewed, and clearly shows their potential in bio-safety and health

DOI: 10.1039/c7ra06925a

applications. These studies should inspire researchers involved in both fundamental and application

rsc.li/rsc-advances

based research to pursue further developments in multiphase TiO2 nanostructures.

Academy of Scientic and Innovative Research, CSIR – National PhysicalLaboratory, Dr. K. S. Krishnan Road, New Delhi – 110 012, India. E-mail: aks@
nplindia.org; avanish.aks555@gmail.com

aDepartment of Physics, RPS Degree College, Balana, Mohindergarh, Haryana – 123029, India

c

Sophisticated and Analytical Equipments Division, CSIR – National PhysicalLaboratory, Dr. K. S. Krishnan Road, New Delhi – 110 012, Indiab

Dr Avanish Kumar Srivastava is a Scientist at CSIR – National Physical Laboratory, New Delhi and an
adjunct faculty member of the Academy of Scientic and Innovative Research (AcSIR) at the NPL campus.
He received his Master's degree in Physics from the Indian Institute of Technology Roorkee(1986), M.Tech. in Materials Science from the Indian Institute of Technology Kanpur (1988), and Ph.D.
in Metallurgy from the Indian Institute of Science, Bangalore (1996). He works in the eld of the processing, characterization, properties and possible applications of metals and alloys, composites, semiconductors and various nanostructures. His prolic research to understand the nucleation-growthmechanisms, phase transformations, microstructures and defects of materials is well known. Dr Srivastava has solved complex crystalline structures with the help of reciprocal space analysis employing
electron microscopy. He has carried out collaborations with many institutions, including the Indian
Institute of Science Bangalore, Banaras Hindu University Varanasi, Indian Institute of Technologies
Delhi, Kanpur & Hyderabad, University of Paris & University of Reims France, Technical University
Darmstadt Germany; Okayama University of Science Japan, University of Arkansas USA, Pohang
University of Science and Technology South Korea, Department of Energy, Center for NanoEngineered Materials and Surfaces Milano Italy
and Institute for Technical Physics and Materials Science, Hungarian Academy of Sciences Hungary and Argonne National Laboratory
USA. He has more than 200 publications in reputed international journals and more than 200 papers and invited talks at diﬀerent
scientic meetings in India and abroad. He has edited a book on Oxide Nanostructures: Growth, Microstructure and Properties, published
by PanStanford Singapore and printed by Taylor and Francis, USA. He is the recipient of many awards and positions, including: (i) DSTBOYSCAST fellowship of Government of India, (ii) Kwan Im Thong Travel Fellowship Singapore, (iii) INSA-KOSEF fellowship of theGovernments of India & South Korea, (iv) Materials Research Society of India (MRSI) Medal-2011 and (v) Metallurgist of the Year Award –2011 by the Ministry of Steel, Government of India. He is an elected Fellow of The Indian Institute of Metals and Electron Microscope
Society of India.

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1. Introduction
Photoactive potential is dened as the capacity of a material toundergo any chemical or physical alteration when a number ofphotons fall on it. It is well known that when the size of materials or grains approaches the nanometre (nm) scale, many
physical, chemical and biological characteristics are strongly
inuenced and diﬀer signicantly from those of bulk materials
and from those of an isolated atom/ion/molecule.1–7 The
possible fundamental causes for this are the following: (i)interatomic interactions, (ii) cations with valence states, (iii)anions with deciencies, (iv) anisotropy and (v) high surfacearea-to-volume ratio.8 Semiconductor metal oxide nanomaterials (SMONs) are amongst the most widely studied oxidematerials in recent years as they have the capability to generate
photo-excited electrons and holes when photons of a wavelength equal to or greater than the band gap of the SMON fall on
it.9 The noteworthy and signicant physico-chemical characteristics of SMON can be attributed to their reduction in crystal
size as well as their surface structure that would substantially
increase the catalysis performance and the ability to tune theband gap by possible modications of SMON, which is one of
the most exciting research areas. In addition, the band gap of
SMON can be easily tuned as per the requirement for inhibiting
charge-carrier recombination, which is important for photoactive applications. In addition, nanometre-sized SMONs haveattracted much interest as a component of electronic devices
depending on their structural phases. The intriguing characteristics of several SMONs, together with their electronic andstructural phases, that possess photoactive potential are listed
in Table 1.
Amongst the various SMONs,10–29 TiO2-nanostructured
systems (particularly nano-particles,23,30 -wires,31 -needles,32

Table 1-belts33 and -tubes34,35) are the most enthralling classes of SMON
with a variety of physico-chemical properties. The dominantphysicochemical properties of TiO2 nanostructures are classied in terms of their crystallinity, purity, structure, chemicalcomposition, size- and/or shape-distribution, dimensionalityand defect centres, which can be easily modied by changing
the parameters during various synthetic methods. TiO2 nanostructures have been widely explored owing to their extensive
applications across a myriad of areas, particularly in photocatalysis, including those for the decomposition of organic/
inorganic dyes,16,23,36–40 hydrogen production from water,41–44
removal of pollutants and plastics from the environment,45–48
dye-sensitized solar cells,49 sensors,50 sunscreens,51–54 paints,55,56
rechargeable batteries,57–59 supercapacitors,60 food colouring,61–64 environmental remediation65–94 and biomedicalelds.95,96 Over the past few decades, extensive research eﬀort
has been dedicated to the engineering of TiO2 nanostructures
synthesized via chemical routes, such as sol–gel,97–107 hydrothermal,108–126 solvothermal,8 wet-chemical,6,8,23,127 thermolysis128,129 and electrodeposition techniques.130 TiO2 is eﬀectively
used in the aforementioned applications based on its various
structural phases, morphologies, exceptional low-cost, nontoxicity, high abundance in nature and slow rate of chargecarrier recombination,131–139 and therefore is the main concernof this review article. Moreover, the excellent photoactive
performances of multiphase TiO2 nanostructures mainly
depend on the various synthesis routes. Due to the outburst of
publications in this eld, the authors do not claim that thisdominantly multiphase TiO2 nanostructures review includes allof the published work, but rather an exposure of the eﬃcientsyntheses, probing capabilities, photoactive potential andapplication of these TiO2 nanostructures in biosafety andhealth. We apologize to the authors of the excellent work that,

Summary of diﬀerent binary and ternary SMONs based on photoactive potential with their band gap, structural phase and melting pointa

Metal oxides

Band gap (eV)

Structural phases

Melting point ( C)

Ref.

Binary oxides
CuO: cupric oxide
Fe2O3: iron(III) oxide
NiO: nickel oxide
Sb2O3: antimony trioxide
Sb2O4: antimony tetroxide
SnO2: tin dioxide
TiO2: titanium dioxide
VO2: vanadium oxide
WO3: tungsten trioxide
ZnO: zinc oxide

1.2
1.7–2.7
3.6–4.2
2.8–3.1
3.26
3.5–3.9
2.8–3.3
0.5–0.7
2.5–3.7
3.3–3.5

Monoclinic
Rhombohedral (a), cubic (g)Cubic (rock salt)
Cubic
Orthorhombic (a), monoclinic (b)Tetragonal (R)Tetragonal (R, A); orthorhombic (B)Monoclinic, tetragonal
Triclinic, tetragonal, monoclinic, orthorhombic
Hexagonal (wurtzite), cubic (zinc blend)

1326
1566
1955
656
>930
1630
1843
1967
1473
1975

10 and 157
11 and 12
13
14
15
11
16
8
17 and 18
19–21

Ternary oxides
BiVO4: bismuth vanadate
SrTiO3: strontium titanate
ZnAl2O4: zinc aluminate
ZnFe2O4: zinc iron oxide
Zn2SnO4: zinc stannate
Zn2SiO4: zinc silicate

2.2–2.8
3.2–3.4
4.1
1.9
3.6
5.5

Tetragonal, monoclinic
Cubic
Cubic
Cubic
Cubic
Hexagonal

2700
2080
NA
NA
570
1509

22
23 and 24
25
25 and 26
25
27–29

a

Symbols introduced: a / alpha, b / beta, g / gamma, R / rutile, A / anatase and B / brookite.

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Review

due to the large activity in this area, we have inadvertentlyle out.
Thus, in order to modify the structural, morphological,
optical, antimicrobial, physico-chemical and photocatalytic
properties of various multiphase-TiO2 nanostructures, bynecessity, it is essential to develop and emphasize the design ofcost-eﬃcient, high yield and long charge-carrier lifetime materials with desirable controlled morphology as well as specicmultifunctional properties based on basic chemistry andphysics concepts/models.

2. Properties of multiphase TiO2
nanostructures
Titanium dioxide is an alkaline oxide of titanium (Ti) containing the elements Ti and oxygen (O) with the chemical formula
TiO2 and is commonly referred to as titania. Moreover, TiO2 is
one of the most abundant and nontoxic compounds on ourplanet and is a very appealing material for a variety of applications in catalysis, UV sunscreens, cosmetics, medical implants
and self-cleaning and antifogging surfaces.97 TiO2 nanostructures are conventional wide band gap (Eg ¼ 2.8–3.3 eV)semiconductor materials with tunable properties, which haveattracted growing interest in recent decades from the scienticand engineering community.
2.1. Structural phases (multiphase TiO2)
Structurally, TiO2 nanomaterials are a kind of versatile polymorph material (a material with the same composition but
a diﬀerent crystal structure) and exist in both crystalline andnoncrystalline or amorphous (a-TiO2) forms.141–143 Up to now, 14
structural phases of TiO2 have been identied in which 11
phases are crystalline and 3 are noncrystalline. The crystalline
phases of TiO2 are rutile, anatase, brookite, TiO2-B (bronze),
TiO2-R (ramsdellite), TiO2-H (hollandite), TiO2-II (columbite),
TiO2-III (baddeleyite), TiO2-OI (uorite), TiO2-OII (cotunnite)
and TiO2-cubic,144 whereas the noncrystalline-TiO2 phases area low-density a-TiO2 and two high-density a-TiO2 forms.142
Mesoporous a-TiO2 with a periodic ordering of the pores wasrst reported in 1995 via a modied sol–gel process applying analkyl phosphate surfactant.144
To date, the structural characteristics, particularly the natureof phase formation as well as transformations in TiO2, have been
inadequately established. The three main naturally existing polymorphs of TiO2 (especially rutile, anatase and brookite) are stable
at ambient or low-pressure but diﬀer in how they share theadjacent TiO6 octahedra coordination such that the overall stoichiometry remains TiO2. The relative stability of the TiO2 phases
depends mainly on the particle size, with rutile-TiO2 being thethermodynamically most stable form in bulk TiO2. The anataseTiO2 phase is the most stable phase of TiO2 at sizes below 14 nm.
Moreover, brookite-TiO2 is the metastable form, which is notcommonly observed in minerals and is diﬃcult to synthesize ina pure form.140,145 Their densities vary in range from 4.24 (TiO2rutile)146 to 3.89 (TiO2-anatase)146,147 and 4.12 (TiO2-brookite)148 g
cm3. Anatase- and brookite-TiO2 are regarded amongst the mostThis journal is © The Royal Society of Chemistry 2017

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widely studied phases with the thermodynamically stable rutileTiO2 phase.149,150 However, based on the diﬀerent arrangements ofoxide anions (O2) adjoining the titanium cations (Ti4+), thesetransition/multiphase TiO2 polymorphs can be divided into two
main categories: (i) a tetragonal structure (in rutile- and anataseTiO2), (ii) an orthorhombic structure (in brookite-TiO2).151 On thebasis of the available literature reviews and the experimental Ti–O
phase diagram, anatase-TiO2 is more stable than rutile152 at room
temperature and atmospheric pressure, in contrast to the thermodynamic data, which indicates anatase-TiO2 is metastable withregard to rutile-TiO2.145 The rutile phase of TiO2 consists of TiO6
units with both edge as well as corner sharing, whereas theanatase phase of TiO2 contains TiO6 units only with edge sharing.
Additionally, in the brookite phase of TiO2, the TiO6 units areconnected through the edges of neighbouring units.153–156
In the rutile phase structure, two opposite edges of eachoctahedra are united forming a linear chain along the (001)
direction. These linear chains are then linked to each other bysharing corner oxygen atoms. However, zigzag chains of octahedra are linked through sharing edges, as no corner sharing isobserved in the crystal structure of anatase. Four edges areshared per octahedron in anatase. Interstitial spaces between
octahedra are larger because of there being more edge sharing
and thus this phase structure is less dense than rutile.145 The
orthorhombic crystal structure of brookite is even more
complex, wherein, it comprises an orthorhombic unit constructed by an octahedron of oxygen ions arranged abouta single titanium ion. Three edges with adjoining octahedra areshared per octahedron. Moreover, the remarkable properties
and excellent performance in various potential applications forthe diﬀerent polymorphs of multiphase TiO2 are outlined inTable 2. Fig. 1(a–d3) illustrate the various applications: (a), balland-stick models (b1–b3), Wulﬀ structures (c1–c3) andpolyhedron models (d1–d3) of diﬀerent polymorph TiO2, inparticular anatase-, brookite- and rutile-TiO2.142

2.2. Morphogenesis characteristics (TiO2 nanostructures)
Accumulating all the available reviews from the early 2000s,
a variety of nanostructured materials with fascinating morphologies have piqued the interests of material scientists because of
their higher surface-to-volume ratio, which is benecial toenhancing the eﬃciency of oxide surface reactions. Based on theorientation of nanocrystals, nanostructured materials or nanomaterials are divided into three main categories within a size
range of 1–100 nm: (i) spherical (nano-particles,23 -spheres,158-granules,159 -platelets,160 -pores161), (ii) elongated (nano-rods,162-tubes,34,35 -wires,31 -bres,98,163 -needles,164 -petals,165 -whiskers,166,167 -trees,55 -laments168,169) and (iii) planar (nanoplates,170,171 -sheets,172 -lms,173 -coatings174,175).
Spherical nanomaterials show a diverse range of applications, such as in electronics,176 photodynamic therapy,177,178
sensors,50 probes,179 catalysis23,180 and antimicrobial.51,181,182
Elongated nanomaterials, on the other hand, also demonstratevarious potential applications, such as in display technologies,183,184 microelectromechanical systems (MEMS),185 opticalsensors175
andbiologicalareas,
suchascancer

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Table 2

Review

Polymorphs-TiO2 structure with their possible properties and applications

Phase-TiO2

Properties

Advanced applications

Anatase-TiO2

Brown to black, metastable polymorph,lustrous, lower absorption rate, hard, highdensity, larger band gap
Dark brown to greenish black, brittle, very raremetastable polymorph, non-uorescent
Dark red, high refractive index, hard, chemical
resistant, UV-ray absorbent, obtained at hightemperatureGems, paints, papers, ceramics, photocatalysis

Brookite-TiO2
Rutile-TiO2

therapeutics,186,187 sensing,188,189 imaging186,187 and drug delivery.
Planar nanomaterials are utilized for packaging purposes, as
anti-corrosives,190,191 -bacterial,192,193 -abrasion,191 -reection,194,195 -graﬃti coatings191 and in surgical procedures.196 suchas wound dressing materials and in tissue engineering,197,198
such as cellular scaﬀolds.199 Amongst various morphogenesis,
nano-particles,23,30 -bres,98,163 -tubes,34,35 -rods162 and layers ofmultiphase-TiO2 are the most desired nanostructures for manyindustrial, scientic and technological applications (Fig. 2).

2.3. Impact of multiphase TiO2 nanostructures onphotoactive and biological responses
The photoactive potential states the capability of a substance toundergo any physical or chemical changes in response to illumination. Herein, the photoactive potential of multiphase TiO2

Fig. 1 (a) Various applications, (b1–b3) ball-and-stick models, (c1–c3)
Wulﬀ structures and (d1–d3) polyhedron models of diﬀerent polymorphs TiO2 for (b1, c1, d1) anatase-TiO2, (b2, c2, d2) brookite-TiO2 and(b3, c3, d3) rutile-TiO2. The unit cells in polyhedron models are outlined
using thin lines.142 By courtesy of https://www.google.co.in/search?
q¼wulﬀ+structure+of+tio2&tbm¼isch&tbo¼u&source¼univ&sa¼X&
ved¼0ahUKEwiYgZiDnYXUAhUFT48KHWRAfQQsAQIJg&biw¼1280&bih¼591#imgrc¼X5z468XQyZyqOM: Reprinted with permission
from ref. 142. Copyright 2014, American Chemical Society.

44202 | RSC Adv., 2017, 7, 44199–44224

Photocatalysis, photovoltaic devices, jewellery
Paint, paper, plastic, food, sunscreens,photocatalysis, interference applications

nanostructures stands for: (i) UV-Vis characteristics, (ii) photoluminescence and (iii) photocatalysis properties.
2.3.1. Inuence on photoactive potential. Generally,ultraviolet-visible (UV-Vis or UV/Vis) spectroscopy is aimed atabsorption or reectance ranging from 190 nm (6.52 eV) to
800 nm (1.55 eV). When an atom or molecule absorbs UV orvisible light, electrons are promoted from their ground state(bonding: s-, and p- or non-bonding: n-electrons) to an excitedstate (anti-bonding: s*- or p*-electrons).23,200 Atomic vibrations/rotations also have discrete energy levels, which are considered
as being packed on top of each electronic level and appear in theform of a continuous absorption band.130 In the luminescenceprocess, an energy source stimulates an electron withina material to rise from a ground state to an excited state, and theelectron is then released to the ground energy state by liberating
the energy in the form of light. In the photoluminescenceprocess, photons are the primary energy source and can be used
to analyse the PL spectra of nanomaterials. The separation ofphotogenerated electron–hole pairs (excitons) when UV rays are
illuminated provides a platform for photodegradation by
generating reactive oxygen species (ROS).201–205 A mechanism
related to this photodegradation is depicted in Fig. 3.
Multiphase TiO2 nanostructures have been widely exploredin terms of photocatalysis owing to their high surface area, highabsorption capability and slow rate of charge-carrier recombination.23,180 The photocatalysis property is useful in many
applications that provide a therapeutic action for human healthby decontaminating water/air206,207 and also by degrading toxic
organic/inorganic dyes.23,208 Thus, the ability to manipulate thesurface property is an extremely vital factor to optimize multiphase TiO2 nanostructures for photo-related phenomena.
However, the existing recent reviews scrutinized that the surface
modication of multiphase TiO2 nanostructures can be achieved either by organic/inorganic surface coating or by adding
dopants. However, surface coating on TiO2 nanostructures with

Fig. 2 Illustration showing the orientation of nanomaterials: (a1)spherical, (a2) elongated and (a3) planar.130 Reprinted with permission
from ref. 130. Copyright 2015, The Royal Society of Chemistry.

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Fig. 3

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Photocatalysis mechanism in multiphase TiO2 nanostructures.

Schematic representation of nano–bio interactions and: (a)factors inﬂuencing the nano–bio interactions,60 (b) physico-chemical
properties of nanomaterials essential to establish nano–bio relations,239 (c) eﬀects of the protein corona surrounding a nanoparticle;
the corona constitutes a primary nano–bio interface that determines
the fate of the nanoparticle and can cause deleterious eﬀects on theinteractive proteins,226 (d) interactions between nanoparticles. Traditional forces for colloidal fabrication (for example, electrostatic, VDW,
covalent) and other important interactions (for example, solvation,
solvophobic, biomolecular, depletion) that occur when particles are
suspended in biological media and come into contact with cells.226 (a)is reprinted with permission from ref. 60. Copyright 2014, American
Chemical Society. (b) is reprinted with permission from ref. 239.
Copyright 2014, Wiley Online Library. (c and d) is reprinted withpermission from ref. 226. Copyright 2009, Nature Publishing Group.
Scheme 1

209

210

numerous inorganic oxide materials, such as SiO2, Al2O3,
ZrO2,211 WO3 (ref. 212) and MoO3 (ref. 213) can lose theirintrinsic property, thus, organic surface coatings are preferred
to circumvent this property. In addition, coating with anorganic moiety can provide various functional groups, such ashydroxyl-, aldehydic- and carboxyl-groups, as well as the benetfrom organic species being cheaper as well as generally moreabundant in nature.214,215
2.3.2. Inuence on bio-safety and human health. Whennanostructures enter into the human body, they are distributed
into various organs and thus interact with biological moieties,
such as proteins and cells. They can either remain in the samestructure or become altered.216 Nanomaterials may also get
absorbed via inhalation,217 dermal exposure218,219 and could
potentially cause some toxic characteristics220–225 in the humanbody. Thus, the pharmacokinetics of nanomaterials depends on
various physico-chemical properties of both nanomaterials aswell as biological systems, which are elaborated in detail herein.
2.3.2.1. Physico-chemical inuences of the nanostructures.
The driving forces for nano–bio interactions mainly include thephysico-chemical properties of: (i) nanomaterials, such as the
structural aspects (size and shape), surface properties (crystallinity, hydrophobicity, coating, stereochemical interactions)and chemical composition (atomic/molecular features, polymorphism); (ii) biological systems, such as the surface charge,
conformation, H- and p-bonds and (iii) environmental prospects, such as the polarity, pH, viscosity, temperature and ionicstrength, as shown in Scheme 1(a–c).226–228 The available excitingand existing reviews clearly demonstrate the interaction ofmultiphase TiO2 nanostructures with biological systems
through various plausible mechanisms. It is an imperative task,
therefore, to present a review to assist composing a novel
mechanism emphasizing the eﬀect of multiphase TiO2 nanostructures on human health using fundamental evidence.
2.3.2.2. Pharmacokinetics of nanostructures in the humanbody. Pharmacokinetics (e.g. absorption, distribution, metabolism and excretion) is dened as the study of the mobility ofnanomaterials in a biological system.229–233 Nanostructures canbe administered in vivo by major routes, such as inhalation,
ingestion, transdermal and intravenous injection. Once nanostructures enter into the human body, they are distributed into

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various organs, where they may remain in the same structure or
become amended or metabolised before leaving towards otherorgans or to be excreted.234,235 The study to date on the inuenceof multiphase TiO2 nanostructures on human health has been
focused and debated extensively over the past decade. Several
participating factors, including the partial absorption of nanostructures, and parameters associated with the particles (suchas the material characteristics, shape, size and concentration)
have been considered to account for the anomalous inuenceon biosafety/human health.236–239 The present review brieyoutlines the issues around biosafety by taking into account themultiphase TiO2 nanostructures, employing date reported in
the various review articles in this area.

3. Eﬃcient synthesis routes andgrowth mechanisms
Synthetic methods of multiphase TiO2 play a crucial role in thedevelopment of nanostructures from zero-dimensional (0D) tothree-dimensional (3D) structures. The synthesis of multiphase
TiO2 nanostructures in a variety of shapes with controlled
morphologies, sizes (100 nm) and more complex nanostructures are of substantial interest due to their exceptional
properties and prevalent application possibilities straddlingfrom physico-chemical to biological elds.240,241 With a highdegree of synthetic control, the size and shape of multiphase
TiO2 nanostructures can be easily tuned from the micro- and

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nanometre to the molecular level and from spherical nanoparticles to nano-rods/-tubes/-wires. Moreover, owing to its
excellent stoichiometry (Ti/O: 1/2), multiphase TiO2 nanostructures have always been a subject of enormous researchinterest and there is emerging attention paid now to thecontrolled synthesis or fabrication of structurally stoichiometric
TiO2 with diverse shapes, with results reported on single andmixed phase TiO2 polymorphs. The synthesis of multiphase
TiO2 nanostructures, therefore, is very interesting in terms of: (i)the structural phases (anatase-, brookite- and rutile-TiO2), (ii)the well-controlled morphogenesis (nano-particles,23,158-tubes,34,35 -wires,31 -rods,162 -crystals242,243 and -owers144,244), and(iii) the particle size (regime 1–100 nm) for a variety of technological and biosafety applications.
With the emergence of new synthetic techniques that are
inexpensive, highly scalable and have a high yield, developmentof the large-scale cost-eﬀective production of SMON is one of
the key challenges for today's nanoscience and nanotechnology.245 Each fabrication method of SMON has its own
advantages and strategy for the synthesis of SMON in 0D to 3D
systems. In this section, we briey describe the numerous
synthesis methods for producing SMON, especially multiphase
TiO2 nanostructures. Generally, two fundamental strategies
have been developed to broadly prepare multiphase TiO2
nanostructures: the top-down approach and the bottom-up
approach.
3.1. Top-down approach
Basically, the top-down approach is a miniaturization technique, in which small structures are fabricated from larger
ones. The key advantageous of this approach are that thecomponents, e.g. in the production of integrated circuits, areboth patterned and assembled in position, so that no separate
fabrication step is needed. Lithography, attrition or milling,
quenching, etching and successive cutting are all typicalexamples of the top-down approach for the synthesis of metaloxide nanomaterials.108,246

Reviewmaterial.252,253 Thus, the technological challenge has moved to
the facile synthesis of multiphase TiO2 nanostructures via
chemical routes and that could maintain their activity, selectivity and suitability over long-term operation in various
potential technological and biosafety applications. In addition,
the chemical method is an easy way to produce multiphase-TiO2nanostructures using hazardless precursors in a short reaction
time and at low cost for high scale-up with high purity in highthroughput and good yield in industrial production. Due tonumerous advantages such as simplicity, inexpensive instrumental set-up, easy operation and good control over the experimental conditions, the chemical routes are encouraged fromthe view-point of practical application. The chemical synthesis
processes to obtain multiphase crystalline TiO2 nanostructures,
including amongst others sol–gel or even hydrothermal
approaches, wet chemical solution and precipitation methods,
have become increasingly popular in recent years to achieve thegoals of identically shaped, uniformly sized, perfect crystals,
defect-free and high-purity materials. The entire synthetic
mechanism, which provides exquisite control over many of theparameters cited above for the synthesis of nanostructures, isdemonstrated in Scheme 2.
Using these approaches, various morphologies of multiphase TiO2 have been synthesized and can be easily scaled up
for large-scale production. The synthesis methods via a topdown approach normally require harmful organic/inorganic
surfactants complex technologies as well as sophisticatedinstruments. Consequently, the bottom-up approach ispreferred and has attracted key interest due to its versatility andpotential to produce SMONs for many applications. Moreover,the chemical synthesis of multiphase TiO2 nanostructures via
the sol–gel, hydrothermal and co-precipitation methods
provides numerous signicant advantages, such as:(i) precise control of the structural phases and stoichiometry,(ii) control of the size, morphology and homogeneity,(iii) reproducible process suitable for the large-scale, costeﬀective and high-purity production of nanomaterials,(iv) short reaction conditions and inexpensive procedures.

3.2. Bottom-up approach
In this context, synthesis through a bottom-up approach, inwhich molecular precursors react to form the nal larger
structures, is a unique approach for the preparation of SMONs
owing to its exceptional versatility.157,247–249 In the bottom-up
approach, the molecular precursors self-associate, interact
and combine with each other in a coordinated way to give rise tosimple and more complex nanostructures. The bottom-up
approach has four major branches: (i) physical, (ii) chemical,(iii) hybrid and (iv) biological. Nowadays, these synthetic routes
have been established for the engineering of emerging multiphase TiO2 nanostructures (spherical, elongated and planarwith sizes smaller than 100 nm).103,109,250,251 The existing physical, hybrid and/or biological synthesis methods typicallyinvolve toxic organic/inorganic reagents and need sophisticatedinstrumentation with complicated reaction conditions to
produce the required materials, which are low in quantity as
well as quality, even with a low-yield of the required

44204 | RSC Adv., 2017, 7, 44199–44224

Scheme 2 Schematic representation of the chemical synthesis routesof SMON via (a) sol–gel, (b) hydrothermal, and (c) co-precipitation
methods8 Reprinted with permission from ref. 8. Copyright 2016,
DRDO.

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Review

3.2.1. Sol–gel method. The sol–gel method is broadly used
to synthesize oxide materials, especially SMONs. Processing by
this method involves the transformation of a sol (a colloidal
suspension of nanometre-sized solid particles in a liquid phase)
into a gel (a 3D network of metal–oxygen bonds, containing
a continuous solid skeleton attached to a continuous liquid
phase), which is then usually thermally treated to obtain thedesired SMON.98 Namely, when the liquid is removed from thesol, the sol turns into a gel. The rst gel derived from a metal
alkoxide was synthesized by Ebelmen as early as 1846.101
Amongst all the available processes, the sol–gel approach ismainly based on the controlled hydrolysis, the condensation of
appropriate precursors and inorganic polymerization reactions.254 The sol–gel process includes four steps: (i) hydrolysis,(ii) polycondensation, (iii) drying and (iv) thermal decomposition. The precursors of metal alkoxides hydrolyze with H2O oralcohols according to the hydrolysis process, followed by either
H2O or alcohol condensation. Additionally, H2O or alcohol, anacid or a base can also assist hydrolysis of the precursor. In thecase of acids, a reaction takes place between the alkoxide andacid to form a solution. Aer the sol formation, the solution iscondensed to a gel and the solvent (H2O or alcohol) must be
removed. A higher annealing/calcination temperature is needed
to decompose the organic/inorganic precursor.240 Furthermore,
the sol–gel method usually results in SMONs that are amorphous or badly crystalline because they are fabricated at or nearroom temperature. A thermal treatment (annealing or calcination) is normally needed to transform the gel into the nal
required SMON.97–107
The sol–gel processing of multiphase TiO2 nanostructures is
an interesting approach with a high level of control of the shapebased on the controlled hydrolysis and condensation of theappropriate precursors. The sol–gel method is valuable fora high-throughput production of multiphase TiO2 nanostructures and much work has been carried out to date on thisprocess, mainly involving the grain nanostructures. Varying thesynthesis reaction conditions and parameters (particularly thesolution composition, pH value and temperature) in sol–gel
processing markedly aﬀects the fabricated multiphase TiO2
nanostructures with varying sizes and diﬀerent morphologies.255 A schematic diagram of the sol–gel processing for thesynthesis of SMONs is shown in Scheme 2(a).
Multiphase TiO2 with diﬀerent types of nanostructures, suchas nano-particles, -whiskers, -brous and -spindles, have beensuccessfully synthesized using this method (as shown in Fig. 4).
The upper segment of Fig. 4 shows the sol–gel-derived sphericalnanostructures of TiO2, whereas the lower segment presents theelongated nanostructures. Fig. 4a1 elucidates anatase-TiO2 with
20–60 nm crystallites agglomerated into 2 mm spherical spheres.
Fig. 4a2 demonstrates the brookite-TiO2 nanoparticles are ina uniform spherical shape morphology with a diameter of about
400 nm.98 Fig. 4a3 shows the nano-scaled aggregates of a-TiO2
crystallites in particulate structure synthesized at ratios of
EGMT : HCl ¼ 1 : 5.4.144 Fig. 4a4 and b3 show the TEM images ofhighly crystalline anatase-TiO2 nanoparticles with diﬀerent
sizes and shapes prepared by the sol–gel method.256 Fig. 4a5

This journal is © The Royal Society of Chemistry 2017

RSC AdvancesFig. 4 Sol–gel synthesis of multiphase TiO2 nanostructures; (a1–a4)
nanoparticles, (a5) nanowhiskers, (b1) nanoﬁbrous, (b2) nanoﬁbres, (b3)nanoparticles, (b4 and b5) nanospindle. Ref: [a1, b1],102 [a3, a5],144 [a4,b3],256 [b2],163 [b4 and b5].97 (a1, b1) are reprinted with permission fromref. 102. Copyright 2012, American Chemical Society. (a3, a5) are
reprinted with permission from ref. 144. Copyright 2012, The Royal
Society of Chemistry. (a4, b3) are reprinted with permission from ref.
256. (b2) is reprinted with permission from ref. 163. Copyright 2011,
American Chemical Society. (b4, b5) are reprinted with permission fromref. 97. Copyright 2014, American Chemical Society.

shows the cauliower-like nanowhisker arrangements of rutileTiO2 with a diameter of 4–5 nm and a length of 50 nm.144
Fig. 4b1 represents the higher magnication SEM image of the
TiO2 nanobrous structure.98 The bre-like rutile-TiO2 nanostructures are illustrated in Fig. 4b2.163 Fig. 4b4 and b5 indicate
the anatase-TiO2 particles with a spindle-like morphology
depending on the pH value of the initial solution.97
3.2.2. Hydrothermal method. The hydrothermal method isa simple, versatile and universal synthesis method that employsa solvent (generally water, H2O) at a moderate temperature andhigh pressure for producing SMONs. A schematic illustration
for the formation of a SMON through the hydrothermal method
is depicted in Scheme 2(b). The hydrothermal method is very
comparable to the solvothermal method, with the main
distinction between these two methods being in the precursor
solutions. Specically, aqueous and non-aqueous precursor
solutions are used for the hydrothermal and solvothermal
methods, respectively. Besides, the solvothermal method oﬀers
a better control of the size distribution, shape and crystallinity
in SMONs than the hydrothermal method. The major chemical
reactions occur at a critical temperature (374  C) and pressure(218 atm) in a stainless steel Teon autoclave. In the hydrothermal method, the dielectric constant and viscosity of H2O
decrease with increasing the temperature and increase with the
expanding pressure with the temperature eﬀect predominating.
Due to the changes in such characteristics of H2O, the raised
temperature within a hydrothermal system has a considerable
eﬀect on the sample solubility and the transport of solids.108,109
In hydrothermal processing, mostly a lamellar phase is formed
before the hydrothermal condition, in which the anions play the
key role of surfactant to stabilize the layered organization of
metal cation species. The layer, then, rolls up to generate onedimensional (1D) nanostructures (particularly nano-tubes,-wires, -polyhedrons and -rods) in the process of crystallization under hydrothermal treatment. The crystalline/amorphous
metal–hydroxide phase is, thus, obtained. Aer heat treatment
annealing or calcination or sintering, the metal–hydroxide

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phase is, therefore, dehydrated and transformed to metal oxide
nanostructures, while the 1D morphology is preserved and theorganic/inorganic compounds or surfactants are removed. The
hydrothermally synthesized 1D SMONs are expected to be
identical in crystalline structure, uniform in size distribution,
have a regular chemical composition, be perfect in shape andhave no morphological defects or dislocations (particle–particle
interface), which are exponentially useful for the applications of
SMONs in photocatalysis, biosafety and human health.109 By
using suitable parameters, such as precursors, solvent, surfactants, working temperature and reaction time, the structuralphases, shape and size distribution of SMONs can be precisely
controlled.
Numerous morphologies of multiphase TiO2 nanostructures (nano-particles, -cuboid shape, -wires, -tubes,-belts and many others) have been eﬀectively fabricated inthis way (as represented in Fig. 5). A typical SEM micrographof TiO2 nanowires made with the hydrothermal method isshown in Fig. 5a1.256 Fig. 5a2 and a5 correspond to the SEM
and TEM images of TiO2 nanotubes, indicating a broustexture with lengths up to 1–3 mm, outer diameters between
7 nm and 10 nm, inner diameters of 5 nm and an inter-wall
spacing of about 0.5 nm.257 Fig. 5a3 and a4 illustrates therepresentative SEM micrographs of anatase-TiO2 nanopolyhedrons (possessing diameters in the range of 300–
600 nm) and nano–belts (approximately 130 nm in width,
50 nm in thickness and several micrometres in length),respectively.33 In addition to elongated TiO2 nanostructures,
spherical TiO2 nanostructures (nanoparticles and cuboid
shaped or thin sheets, as depicted in Fig. 5b1–b3) have also
been synthesized with this method.172 Fig. 5b1 clearlyshows uniform ultra-ne anatase-TiO2 nanoparticles (about
5–15 nm). Fig. 5b2 elucidates either a cuboid shaped or thinsheets of these anatase-TiO2 nanoparticles. One of these thinsheets with an edge length of 15 nm is marked in the micrograph (Fig. 5b2) and a set of Moiré fringes are also marked(encircled in Fig. 5b2). Fig. 5b4 and b5 represent the TEM
images of TiO2 nano-tubes and -rods, respectively.108,110–126

Fig. 5 Hydrothermal synthesis of multiphase TiO2 nanostructures: (a1)nanowires,256 (a2, a5) nanotubes,257 (a3) nano-polyhedrons,33 (a4)nanobelts,33 (b1) ultra-ﬁne nanoparticles,172 (b2) nano-cuboid shapedor thin sheets,172 (b3) nanoparticles,256 (b4) nanotubes,256 (b5) nanorods.256 (a1, b3–b5) are reprinted with permission from ref. 256. (a2, a5)
are reprinted with permission from ref. 257. Copyright 2012. Elsevier.
(a3–a4) are reprinted with permission from ref. 33. Copyright 2012.
Springer. (b1 and b2) are reprinted with permission from ref. 172.
Copyright 2012. Elsevier.

44206 | RSC Adv., 2017, 7, 44199–44224

Review

3.2.3. Co-precipitation method. As described above, many
physical/chemical methods have been utilized for producing
multiphase TiO2 nanostructures. Amongst all the synthetic
approaches, the co-precipitation chemical strategy is based on
the condition of supersaturation from aqueous solutions followed by thermal decomposition. Co-precipitation is also
known as the wet chemical solution method and also a common
method to synthesize SMONs with largely scalable production.8,240 In the co-precipitation method, the micro- or nanostructures of multiphase TiO2 are greatly aﬀected by the reaction conditions, but also by the reactivity of the starting/intermediate materials, pH value of the reaction medium andthe reaction parameters, such as temperature and time. The
variations mentioned above present numerous parameters toconstruct diﬀerent high-quality nanoparticles and other nanostructures. A schematic illustration for the formation of oxide
nanostructures through the co-precipitation method is depicted
in Scheme 2(c).
Various shapes of multiphase TiO2 nanostructures havebeen synthesized, including nanorods and nanoparticles.
Fig. 6a1–b4 elucidate the multiphase TiO2 nanostructures
prepared via this method. Fig. 6a1 represents the SEM
morphology of anatase-TiO2 nanorods,256 while Fig. 6a2 illustrates the rutile-TiO2 nanoparticles with a diameter of about
400 nm synthesized by the co-precipitation method.258 Fig. 6a3
and a4 present the SEM images of anatase-TiO2 and rutile-TiO2
nanoparticles,23 while Fig. 6b3 and b4 indicate the corresponding TEM images.23,259 The morphology of anatase-TiO2 nanoparticles monitored by SEM is depicted in Fig. 6b1.327 The TEM
micrograph of TiO2 nanoparticles synthesized by this method isshown in Fig. 6b2, clearly indicating the uniform nanoparticles(size about 5–10 nm) distributed throughout the sample. Even
though the method of preparing TiO2 nanoparticles via coprecipitation is very easy and simple, enormously complicatednanostructures can also be successfully fabricated in this way.
Here, we review all the literature reports, to the best of ourknowledge, concerning an easy way to produce multiphase TiO2

Fig. 6 Co-precipitation synthesis of multiphase TiO2 nanostructures;(a1) nanorods,256 (a2, a3, b3) rutile nanoparticles,23,258 (a4, b4) anatasenanoparticles,23 (b1 and b2) nanoparticles.327 (a1) is reprinted withpermission from ref. 256. (a2) is reprinted with permission from ref.
258. Copyright 2017. Elsevier. (a3, a4 and b3) are reprinted withpermission from ref. 23. Copyright 2016. Elsevier. (b1) is reprinted withpermission from ref. 327. Copyright 2016. Thai Journal of Nanoscienceand Nanotechnology.

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Reviewnanostructures in a short reaction time, using hazardlessstarting ingredients and that are inexpensive for feasible scaleup and to produce a high yield with high purity in industrial
production via the mainly sol–gel, hydrothermal and coprecipitation methods. Table 3 presents an overview of the
facile chemical synthesis approaches for multiphase-TiO2nanostructures that are usually available through these routes.

3.3. Growth mechanism
One of the most demanding concerns in the eﬃcient processingof these multiphase TiO2 nanostructures is the control of theirsize, morphology and microstructure. An apparent interpretation of the growth mechanism for the formation of multiphase
TiO2 nanostructures is mandatory in this respect and is shownin Scheme 3. It is well known that the synthesis process for theformation of nanostructures is classically categorized into two
events: (i) nucleation and (ii) growth.260–262 In the nucleation
process, small clusters composed of a few atoms (nuclei) areformed in the solution starting from monomers; while in thegrowth regime, existing nuclei grow to form larger particles.97
The mechanistic understanding of various aspects of thesynthesis procedure can be established from a combination of
crystallography and kinetics. Interestingly, the detailed mechanism still needs further investigation of the phase, structureand shape formation of TiO2. The chemical growth of nanostructured materials certainly entails the process of gelation or
precipitation of a solid phase from the solution depending on
the synthetic routes. An excellent perceptive of the processes
and parameters ensuring the gel or precipitate formation helps
to enhance the growth of nanoparticles to obtain the desired
size and morphology.127 As the recent literature search indicated, there is immense interest in explaining the preliminary
mechanisms, such as Finke–Watzky two step mechanisms,
coalescence and oriented attachment as well as the LaMer burst
nucleation followed by Ostwald ripening,137 aggregation and

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many more.262–265 The study of the nucleation and growth
process, the tailored morphology and crystalline structure andannealing/calcination treatments, has thus, become an exciting
concern to produce multiphase nanostructures. With that inmind, the present review expresses the description of phasetransformation, nucleation, growth, coarsening and agglomeration processes, including the change in temperature andchemical environment of multiphase TiO2 nanostructures
synthesized via facile chemical routes.
Herein, the present review elaborates a brief description on thephase formation/transformation and diﬀerent morphologicalcharacteristics of multiphase TiO2 nanostructures. The optimized
procedures and growth mechanisms for the sol–gel, hydrothermaland co-precipitation methods of multiphase TiO2 nanostructures
are also reviewed. In addition, the impacts of these nanostructures on the photoactive and biological responses are also
described. Moreover, the review discusses the phase-, size- andmorphology-dependent diﬀerent transitions relating to the defectsites on multiphase TiO2 nanostructures as assessed by UV-Vis
and PL spectroscopy. Furthermore, a comparative study
assessed photocatalysis by modifying the surface of multiphase
TiO2 nanostructures with inorganic (SiO2) to that of an organic
coating derived from green tea (Camellia sinensis), independently
with a comprehensive mechanism given. Additionally, it covers
the fundamental concepts on eﬃcient formation via facile
chemical routes and the growth mechanism of multiphase TiO2
nanostructures. Finally, the present review composes a novel
mechanism emphasizing the eﬀect of multiphase TiO2 nanostructures on biosafety/human health by using essential evidence
available in the literature.

4. Crystallographic structures
X-ray diﬀraction (XRD) analysis is essential to identify the
crystallographic characteristics, such as gross structural analysis, phase transition, phase identication and phase purity
determination of multiphase TiO2 nanostructures. Illustrationsof the XRD patterns of the anatase-, -brookite and -rutile
structural phases of TiO2 nanostructures are depicted in Fig. 7–9, which reveal the crystallinity, phase identication and phase
purity of multiphase TiO2 nanostructures. In specic 2q ranges(20–80 ), the XRD patterns are suﬃciently distinct, creating
a clear classication of the multiphase TiO2 nanostructures.
4.1. Anatase-TiO2

Diﬀerent possible growth mechanisms for the synthesis ofnanostructured TiO2 materials. (a) is reprinted with permission fromref. 325. Copyright 2014. American Chemical Society. (c) is reprinted
with permission from ref. 326. Copyright 2015. The Royal Society ofChemistry.
Scheme 3

This journal is © The Royal Society of Chemistry 2017

Fig. 7a1 provides the XRD pattern of anatase-TiO2 nanostructures. The Bragg's position of the diﬀraction peaks and therelative intensity of the diﬀraction peaks accord perfectly withthe standard JCPDF card no. 73-1764, corresponding to thetetragonal crystal system of anatase-TiO2 (space group: I41/amd(141); a ¼ b ¼ 0.377 nm, c ¼ 0.948 nm).23 All of the diﬀraction
peaks were identied as characteristic of the XRD pattern ofanatase-TiO2. The presence of diﬀraction peaks at 25.06 ,36.9 , 37.8 , 38.6 , 48.1 , 54.1 , 55.1 , 62.7 , 68.7 , 70.1 and75.1 indicate the (101), (103), (004), (112), (200), (105), (211),(204), (116), (220) and (215) lattice planes, respectively, indexing

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Review

Fig. 7 Structural characteristics of anatase-TiO2; (a1) XRD pattern,275
142(b1) ball-and-stick structure, (c1) the tetragonal system with a dipyramidal habit.256 Colour code; O atoms: red (small balls), grey (large
balls) and Ti atoms: blue (large balls), black (small balls). Unit cells are
outlined using thin lines. (a1) is reprinted with permission from ref. 275.
Copyright 2015. VBRI Press. (b1) is reprinted with permission from ref.
142. Copyright 2014. American Chemical Society. (c1) is reprinted withpermission from ref. 256.

to a pure and highly crystalline anatase-TiO2 phase. No additional peaks corresponding to impurities and other structuralphases were detected, which indicates the phase purity andhigh crystallinity synthesis of the anatase-TiO2 phase. The
representative ball-and-stick structure of anatase-TiO2 is illustrated in Fig. 7b1. The anatase-TiO2 has a crystalline structurethat agrees with the tetragonal system with a dipyramidal habit(as shown in Fig. 7c1).142,256
4.2. Brookite-TiO2
The XRD pattern of brookite-TiO2 nanostructures is shown in
Fig. 8a1. The Bragg's position of the diﬀraction peaks and therelative intensity of the diﬀraction peaks accord perfectly with thestandard JCPDF card no. 76-1936, corresponding to an orthorhombic crystal system of brookite-TiO2 (space group: Pbca (61);a ¼ 0.919 nm, b ¼ 0.546 nm, c ¼ 0.515 nm). All of the diﬀraction
peaks were identied as characteristic of the XRD pattern ofbrookite-TiO2. The presence of diﬀraction peaks at 25.26 , 37.9 ,Structural characteristics of brookite-TiO2; (a1) XRD pattern,275(b1) ball-and-stick structure,142 (c1) an orthorhombic crystalline structure.256 Colour code; O atoms: red (small balls), grey (large balls) and Ti
atoms: blue (large balls), black (small balls). Unit cells are outlined using
thin lines. (a1) is reprinted with permission from ref. 275. Copyright2015. VBRI Press. (b1) is reprinted with permission from ref. 142.
Copyright 2014. American Chemical Society. (c1) is reprinted withpermission from ref. 256.
Fig. 844208 | RSC Adv., 2017, 7, 44199–44224

Fig. 9 Structural characteristics of rutile-TiO2; (a1) XRD pattern,23 (b1)ball-and-stick structure,142 (c1) the tetragonal system with a prismatic
habit.256 Colour code; O atoms: red (small balls), grey (large balls) andTi atoms: blue (large balls), black (small balls). Unit cells are outlined
using thin lines. (a1) is reprinted with permission from ref. 23. Copyright2016. Elsevier. (b1) is reprinted with permission from ref. 142. Copyright2014. American Chemical Society. (c1) is reprinted with permission
from ref. 256.

47.9 , 54.1 , 55.1 , 62.7 , 68.9 , 70.3 and 75.3 indicate the (210),(021), (321), (230), (421), (331), (040), (332) and (531) lattice planes,
respectively, indexing to a pure and highly crystalline brookiteTiO2 phase. No additional peaks corresponding to impurities andother structural phases were detected, which indicates the phase
purity and high crystallinity synthesis of the anatase-TiO2 phase.
The representative ball-and-stick structure of brookite-TiO2 isillustrated in Fig. 8b1. The brookite-TiO2 phase has a crystalline
structure that agrees with the orthorhombic crystal system (asshown in Fig. 8c1).142,256
4.3. Rutile-TiO2
Fig. 9a1 provides the XRD pattern of TiO2 nanostructures. The
Bragg's position of the diﬀraction peaks and the relative intensityof the diﬀraction peaks accord perfectly with the standard JCPDF
card no. 73-1765, also corresponding to the tetragonal crystal
system of rutile-TiO2 (space group: P42/mnm (136); a ¼ b ¼
0.458 nm, c ¼ 0.295 nm). All of the diﬀraction peaks were identied as characteristic of the XRD pattern of rutile-TiO2. The
presence of diﬀraction peaks at 27.5 , 36.2 , 39.3 , 41.4 , 44.0 ,54.3 , 56.5 , 62.7 , 63.9 , 69.1 , 69.8 and 76.7 indicate the (110),(101), (200), (111), (210), (211), (220), (002), (310), (301), (311) and(202) lattice planes, respectively, indexing to a pure and highly
crystalline rutile-TiO2 phase. No additional peaks corresponding
to impurities and other structural phases were detected, whichindicates the phase purity and high crystallinity synthesis of theanatase-TiO2 phase. The representative ball-and-stick structure ofrutile-TiO2 is illustrated in Fig. 9b1. The rutile-TiO2 has a crystalline structure that agrees with the tetragonal system with a prismatic habit (as shown in Fig. 9c1).142,256

5. Morphological and microstructuralfeatures
Electron microscopy (EM) studies using scanning EM (SEM), eld
emission scanning EM (FE-SEM) and high-resolution transmission EM (HR-TEM) were utilized to investigate the diﬀerent
morphologies and for microstructural identication in real as wellThis journal is © The Royal Society of Chemistry 2017

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Review

SEM micrographs of a-TiO2 nanostructures: (a–d) nanotube
arrays, (e) nanoparticles, (f) nanotube, (g) thin ﬁlm. Ref: [a],266 [b, c, f],267[d],268 [e],328 [g].173 (a) is reprinted with permission from ref. 266.
Copyright 2011. American Scientiﬁc Publisher. (b, c and f) are reprinted
with permission from ref. 267. Copyright 2012. John Wiley and Sons.
(d) is reprinted with permission from ref. 268. Copyright 2014. The
Chemical Society of Japan. (e) is reprinted with permission from ref.
328. Copyright 2012. John Wiley and Sons. (g) is reprinted withpermission from ref. 173. Copyright 2005. Elsevier.
Fig. 10as in reciprocal spaces for assessing the phase-dependentmorphology of amorphous- and crystalline-TiO2 nanostructures.
5.1. SEM aspects
The representative micrographs of amorphous- and multiphase-TiO2 (anatase-, brookite- and rutile-TiO2) nanostructures

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using SEM and FE-SEM images are shown in Fig. 10–13. Thesegures reveal the general/surface morphology of the TiO2
nanostructures.
5.1.1. Amorphous-TiO2. Fig. 10 depicts the SEM micrographs of amorphous-TiO2 (a-TiO2) nanostructures synthesized
via diﬀerent fabrication routes. Fig. 10(a) shows a SEM image of
a-TiO2 nanotube arrays synthesized aer electrochemical
cycling at a current density of 400 mA cm2.266 Fig. 10(b, c and f)
represent the FE-SEM images of a-TiO2 nanotube arrays
synthesized via the hydrothermal method under diﬀerent
reaction conditions.267 Fig. 10(d) illustrates the bottom surfaceof double-wall a-TiO2 nanotube arrays produced via anodization.268 Uniform a-TiO2 nanoparticles with an average diameterof 500 nm obtained through a sol–gel process is represented inFig. 10(e). Fig. 10(g) elucidates the electrodeposited thin lm of
a-TiO2.173
5.1.2. Multiphase-TiO2. When atoms/ions/molecules arearranged in an innite, periodic, regular and ordered way, thenanomaterials become crystalline and have diﬀerent structuralphases (polymorph materials). The SEM/FE-SEM images ofmultiphase TiO2, in particular anatase-, brookite- and rutileTiO2, nanostructures, are represented in Fig. 11–13.
5.1.2.1. Anatase-TiO2. Fig. 11(a1–c6) show various morphological features of anatase-TiO2 nanostructures (elongated andspherical) synthesized via numerous methods. Fig. 11(a1 and a2)
clearly demonstrate the SEM images of nanotubes synthesized

Fig. 11 SEM micrographs of anatase-TiO2 nanostructures: (a1 and a2) nanotubes, (a3 and a4) nanoﬁbres, (a5) nanorods, (a6) nanorods with
nanoﬂakes, (b1) nanorod arrays, (b2) nanoribbons, (b3) nanowires, (b4–b6) nanosheets, (c1) 3D network, (c2) nanospheres, (c3) nanocups, (c4 andc5) nanospheres and (c6) nanoparticles. Insets (a1, a4, a6, b5 and c1) provide the corresponding higher magniﬁcation images. Ref: [a1, a2],50 [a3],329[a4],50 [a5],269 [a6],270 [b1],330 [b2],271 [b3],50 [b4],292 [b5],41 [b6],331 [c1],50 [c3],274 [c2, c4 and c5],273 [c6].275 (a1, a2, a4, b3 and c1) are reprinted with permission
from ref. 50. Copyright 2014. American Chemical Society. (a3) is reprinted with permission from ref. 329. Copyright 2005. Elsevier. (a5) is reprinted
with permission from ref. 269. Copyright 2013. The Royal Society of Chemistry. (a6) is reprinted with permission from ref. 270. Copyright 2012.
The Royal Society of Chemistry. (b1) is reprinted with permission from ref. 330. Copyright 2004. The Royal Society of Chemistry. (b2) is reprinted
with permission from ref. 271. Copyright 2011. American Chemical Society. (b4) is reprinted with permission from ref. 292. Copyright 2009.
American Chemical Society. (b5) is reprinted with permission from ref. 41. Copyright 2009. American Chemical Society. (b6) is reprinted withpermission from ref. 331. Copyright 2011. John Wiley and Sons. (c2, c4 and c5) are reprinted with permission from ref. 273. Copyright 2007.
American Chemical Society. (c3) is reprinted with permission from ref. 274. Copyright 2013. The Royal Society of Chemistry. (c6) is reprinted withpermission from ref. 275. VBRI Press.

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SEM micrographs of brookite-TiO2 nanostructures: (a1 and a2) nanorices,276 (a3) nanoparticles,275 (a4) nanobullets,293 (b1) nanorices,276 (b2)eggshell,277 (b3) nanotubes,278 (b4) nanoﬂowers, (c1 and c2) nanorods,279,280 (c3) nanotubes,278 (c4) nanorods.281 Inset (a4) illustrates the corresponding tilt-view of nanobullets. (a1, a2 and b1) are reprinted with permission from ref. 276. Copyright 2014. The Royal Society of Chemistry. (a3)is reprinted with permission from ref. 275. Copyright 2015. VBRI Press. (a4) is reprinted with permission from ref. 293. Copyright 2016. Nature
Publishing Group. (b2) is reprinted with permission from ref. 277. (b3, c3) are reprinted with permission from ref. 278. Copyright 2008. The Royal
Society of Chemistry. (c1) is reprinted with permission from ref. 279. Copyright 2013. The Royal Society of Chemistry. (c2) is reprinted withpermission from ref. 280. Copyright 2012. Elsevier. (c4) is reprinted with permission from ref. 281. Copyright 2013. The Royal Society ofChemistry.
Fig. 12Fig. 13 SEM micrographs of rutile-TiO2 nanostructures: (a1) nanorods,282 (a2–a5) nanorod arrays,283 (a6) nanorod spheres,284 (b1, b3 and b4)
nanorod bundles,283,285 (b2) ﬂower-like super architectures,286 (b5) urchin-like,286 (b6) nanospheres,286 (b7) nanotrees,286 (b8) a single nanoparticle,287 (c1) self-assembled nanowires,288 (c2) microspheres,286 (c3) nanorod arrays,286 (c4) quasi spheres,286 (c5) microspheres286 and (c6)nanoparticles.275 Insets: (lower inset in a1) nanorods at higher magniﬁcation, (upper inset in c2) top view of the nanorods with square-shapedends. (a1) is reprinted with permission from ref. 282. Copyright 2011. The Royal Society of Chemistry. (a2–a5) are reprinted with permission fromref. 283. Copyright 2014. Nature Publishing Group. (a6) is reprinted with permission from ref. 284. Copyright 2014. American Chemical Society.
(b1) is reprinted with permission from ref. 285. Copyright 2014. Nature Publishing Group. (b2, b5–b7, c2–c5) are reprinted with permission from ref.
286. Copyright 2012. The Royal Society of Chemistry. (b3 and b4) are reprinted with permission from ref. 283. Copyright 2014. Nature PublishingGroup. (b8) is reprinted with permission from ref. 287. Copyright 2002. The Royal Society of Chemistry. (c1) is reprinted with permission from ref.
288. Copyright 2010. The Royal Society of Chemistry. (c6) is reprinted with permission from ref. 275. Copyright 2015. VBRI Press.

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via a liquid-phase deposition and ten cycles of TiCl4 followed by
the H2O deposition method, respectively.50 Fig. 11(a3 and a4)
illustrate nanobres of anatase-TiO2 synthesized by calcination
at 700  C for 4 h (ref. 268) and an atomic layer deposition
process, respectively.50 Fig. 11(a5 and a6) represent the FE-SEM
images of nanorods,269 and nanorods with a nanoake lm,
respectively.270 In addition, Fig. 11(b1–b6) show the SEM
micrographs of elongated anatase-TiO2 nanostructures: (b1)nanorod arrays,271 (b2) nanoribbons,271 (b3) nanobelts,50 (b4–b6)
nanosheets.41,272,292,331 Moreover, the spherical morphologies ofanatase-TiO2 nanostructures are depicted in Fig. 11(c1–c6), suchas (c1) a 3D network grown on the silicon through a surfacereaction-limited pulsed chemical vapour deposition process,50(c2) uniform, smooth, solid spherical particles,273 (c3) nanocups,274 (c4 and c5) nanospheres273 (c6) nanoparticles.275
5.1.2.2. Brookite-TiO2. Fig. 12(a1–c4) represent the sphericaland elongated nanostructures of brookite-TiO2 obtained atvarious reaction conditions. Fig. 12(a1 and a2) elucidate the SEM
images of nanorices obtained hydrothermally with prolonging
the reaction time to 24 h and 48 h.276 The SEM images ofbrookite-TiO2 nanostructures: (a3) nanoparticles synthesized by
the sol–gel method,275 (a4) top-view SEM image of nanobullets
grown hydrothermally showing sharpened tips, with the upper
inset providing the corresponding tilt-view,277 (b1) a single ricelike particle,276 (b2) eggshell morphology obtained with thethermolysis method.277 Fig. 12(b3 and c3) show the SEM imagesof hydrothermally derived nanotubes,278 (b4) nanoowers obtained by a one-step hydrothermal synthesis route,181 (c1, c2 andc4) nanorods279–281 of brookite-TiO2 synthesized via facile
chemical routes.
5.1.2.3. Rutile-TiO2. Fig. 13(a1–c6) depict the microstructuralfeatures in the elongated and spherical forms of rutile-TiO2nanostructures. Fig. 13(a1) represents the SEM micrographs ofrutile-TiO2 nanorods synthesized under diﬀerent reaction
conditions in the sol–gel process under 365 nm UV light illumination. The inset in Fig. 13(a1) elucidates the nanorods at
a higher magnication.282 Fig. 13(a2–a5) show bundles ofnanorods under diﬀerent reaction conditions.283 Fig. 13(a6)demonstrates a nanorod sphere.284 Fig. 13(b1, b3 and b4) show
nanorod bundles grown hydrothermally.283,285 Fig. 13(b2 and b5)
represent the ower-like super architecture and urchin-likestructure composed of loose nanorods.286 Fig. 13(b6 and b7)
represent the SEM micrographs of nano-spheres and -trees,
respectively.286 Fig. 13(b8) depicts a rutile-TiO2 particle with anaverage size of 1 mm showing a tetragonal prism structurewith four planes, which are assigned to the {110} faces. Eachend of the prism is capped by four planes, which are assigned tothe {011} faces.287 Fig. 13(c1) represents the self-assembled
nanowires synthesized via microwave heating.288 Fig. 13(c2)displays domain like arrangements of the TiO2 quasi spheres
leading apparently to a at surface. The upper inset in
Fig. 13(c2) provides the top view of the nanorod with a squareshaped end.286 Fig. 12(c3) shows the microspheres connectedto each other but not merging together.286 Fig. 13(c4, c5 and c6)
demonstrate the SEM micrographs of microspheres composedof nanorods, spherical architectures286 and nanoparticles,275
respectively.

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5.2. TEM/HR-TEM aspects
The detailed microstructural evidence, ne structure arrangement at the atomic level and selected area electron diﬀraction(SAED) and fast Fourier transform (FFT) patterns of amorphousTiO2 (a-TiO2) and crystalline-TiO2 (anatase-, brookite- andrutile-TiO2) nanostructures can be realized in real and reciprocal spaces through taking and analyzing the TEM and HRTEM images, which are discussed in Fig. 14–17.
5.2.1. Amorphous-TiO2. Amorphous solids do exhibit
short-range order in their structures. Fig. 14(a1–a6) provide theTEM and HR-TEM images and SAED and FFT patterns of a-TiO2nanostructures. Fig. 14(a1) represents the TEM micrograph of aTiO2 nanoparticles having a narrowly distributed size of about
10–15 nm.259 Fig. 14(a2 and a3) show the morphological changesof the nanotube arrays with the introduction (as shown in
Fig. 14(a2)) and without the introduction (as depicted inFig. 14(a3)) of oxygen (O2) gas.268 Fig. 14(a4 and a5) elucidate thetypical HR-TEM micrograph at low and high magnications,
respectively. The FFT pattern of a-TiO2 (as illustrated in theupper inset of Fig. 14(a5)) represents the semicircle-like diﬀuse
spots centred at 3.1  0.1 Å. The FFT pattern reveals that thecontrast patterns in the micrograph can be assigned to thechains of octahedral species linked through Ti–O–Ti bridges at
a distance that is: (i) lower than that related to the cornersharing (generally 3.5 Å), but (ii) higher than the characteristic
distance for Ti–O–Ti octahedrons in an edge-sharing conguration (normally 3.0 Å).289 Fig. 14(a6) shows the TEM image of
a-TiO2 nanotube arrays with a gradually thickened wall withdepth. The upper inset of Fig. 14(a6) reveals the diﬀuse ring
SAED pattern of the single nanotube and indicates the amorphous nature of nanotube arrays.267
5.2.2. Multiphase TiO2. Most solids are crystalline, with theatoms, ions, or molecules of which they are composed falling
into regular, repeated 3D patterns. The presence of long-rangeorder is thus the dening property of a crystal. The

Fig. 14 TEM/HR-TEM micrographs of a-TiO2 nanostructures: (a1) TEM
image of nanoparticles,259 (a2) nanotubes with the introduction of O2
gas,268 (a3) nanotubes without the introduction of O2 gas,268 HR-TEM
image of nanoparticles at low (a4) and high (a5) magniﬁcation,289 (a6)TEM images of a single nanotube.267 Insets provide: (a5) corresponding
FFT pattern and (a6) corresponding SAED pattern. (a1) is reprinted withpermission from ref. 259. Copyright 2013. Indian Journal of Engineering & Materials Sciences. (a2 and a3) are reprinted with permission
from ref. 268. Copyright 2014. The Chemical Society of Japan. (a4 anda5) are reprinted with permission from ref. 289. (a6) is reprinted withpermission from ref. 267. Copyright 2012. John Wiley and Sons.

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TEM/HR-TEM micrographs of rutile-TiO2 nanostructures: (a1and a2) TEM image and corresponding HR-TEM image of a nanobelt,290(b1 and c1) TEM images of one individual nanowire and nanorod,271 (b2and c2) corresponding HR-TEM images of the nanowire and nanorod,271 (d1 and d2) low magniﬁcation TEM and HR-TEM images ofmultilevel branched architectures,286 (e1) the lattice image and (e2)reconstructed image from the square region in (e1).294 Insets (in a1, b1
and c1) illustrate the corresponding SAED patterns. (a1 and a2) are
reprinted with permission from ref. 290. Copyright 2015. John Wileyand Sons. (b1, b2, c1 and c2) are reprinted with permission from ref. 271.
Copyright 2011. American Chemical Society. (d1 and d2) are reprinted
with permission from ref. 286. Copyright 2012. The Royal Society ofChemistry. (e1 and e2) are reprinted with permission from ref. 294.
Copyright 2004. Nature Publishing Group.
Fig. 17

Fig. 15 TEM/HR-TEM micrographs of anatase-TiO2 nanostructures:(a1) bright-ﬁeld TEM image of a nanobelt and the corresponding HRTEM image,290 (a2) taken along the [100] direction,290 (b1) low- and (b2)high-magniﬁcation TEM images of nanorods,291 (c1) low- and (c2) highmagniﬁcation TEM images of nanoﬂakes,291 (d1) HR-TEM image ofa nanocrystal and its corresponding lattice fringe image (d2),292 (e1)
TEM and (e2) HR-TEM images of the nanoﬁbres329 and (f1) TEM andcorresponding HR-TEM image (f2) of nanoparticles.23 Insets in (a1, e1, f1)
provide their corresponding SAED pattern. (a1 and a2) are reprinted
with permission from ref. 290. Copyright 2015. John Wiley and Sons.
(b1, b2, c1 and c2) are reprinted with permission from ref. 291. Copyright2012. The Royal Society of Chemistry. (d1 and d2) are reprinted withpermission from ref. 292. Copyright 2009. American Chemical Society.
(e1 and e2) are reprinted with permission from ref. 329. Copyright2005. Elsevier. (f1 and f2) are reprinted with permission from ref. 23.
Copyright 2016. Elsevier.

Fig. 16 TEM/HR-TEM micrographs of brookite-TiO2 nanostructures:(a1, b1, c1, d1) TEM images of nanotubes,244,278,289,293 (a2, b2, c2, d2) HRTEM images of nano-tubes, -ﬂowers, -particles, -bullets, respectively.244,278,289,293 Insets (in b2, c1, d1) correspond to the FFT patterns. (a1and a2) are reprinted with permission from ref. 278. Copyright 2008.
The Royal Society of Chemistry. (b1 and b2) are reprinted withpermission from ref. 244. Copyright 2009. The Royal Society ofChemistry. (c1 and c2) are reprinted with permission from ref. 289. (d1and d2) are reprinted with permission from ref. 293. Copyright 2016.
Nature Publishing Group.

observation of multiphase TiO2 (especially anatase-, brookite-,and rutile-TiO2) nanostructures by means of TEM/HR-TEM arecommonly overlooked, but here the results are displayed as well
as discussed in Fig. 15–17.
5.2.2.1. Anatase-TiO2. Fig. 15(a1–f2) provide the TEM/HRTEM micrographs at low and high magnications as well as
the SAED and FFT patterns of anatase-TiO2 nanostructures.
Fig. 15(a1 and a2) represent the bright-eld TEM and HR-TEM
micrographs of nanobelts obtained from an alkalihydrothermal process with the growth direction along the44212 | RSC Adv., 2017, 7, 44199–44224[010] plane.290 Fig. 15(b1 and b2) show the low- and highmagnication TEM images of nanorods, respectively. Fromthe gures, it can clearly be observed that the crystalline
behaviour is along their entire lengths. The calculated interplanar spacing d(101) ¼ 0.35 nm, which aﬃrms the presence ofanatase-TiO2.291 Fig. 15(c1 and c2) elucidate the low- and highmagnication TEM images of nanoakes. The lattice fringes
with interplanar spacings of d(101) ¼ 0.35 nm are consistent withthe anatase-TiO2.291 Fig. 15(d1 and d2) show the continuous(200) atomic planes with a lattice spacing of 0.19 nm.292
Fig. 15(e1 and e2) indicate the TEM and HR-TEM images of thenanobres, respectively.268 Moreover, the TEM and HR-TEM
images of nanoparticles synthesized by a wet chemical
method are displayed in Fig. 15(f1 and f2), respectively.23
5.2.2.2. Brookite-TiO2. Fig. 16(a1–d2) display the TEM/HRTEM micrographs and SAED and FFT patterns of brookiteTiO2 nanostructures. Fig. 16(a1 and a2) elucidate the TEM
images of a nanotube adapting the tube-like morphology witha crystalline multiwall structure.278 Fig. 16(b1) represents the
TEM images of nanorods, revealing that the length of a single
nanorod is about 1000 nm, and a single cluster of owers canalso be observed due to agglomerated nanorods. Fig. 16(b2)shows the HR-TEM image of a nanorod with a diameter of about
40 nm. The SAED pattern represented in the upper inset of
Fig. 16(b2) also conrms that the nanorods are a single crystal ofbrookite-TiO2.244 Fig. 16(c1 and c2) depict the TEM and HR-TEM
images of agglomerated nanoparticles.289 Fig. 16(d1 and d2) also
elucidate the TEM and HR-TEM images of nanobullets,
respectively.293
5.2.2.3. Rutile-TiO2. Fig. 17(a1–e2) illustrate the TEM/HRTEM micrographs and SAED pattern of rutile-TiO2 nanostructures. Fig. 17(a1 and a2) show the TEM and HR-TEM
images of a nanobelt prepared by the hydrothermal
method.290 Fig. 17(b1) represents the TEM image of nanowires,
indicating the nanowires are narrow with diameters of around

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5 nm. The observed lattice fringes in Fig. 17(b2) demonstratethat the growth of the nanowires is parallel to the (110) planes
but perpendicular to the (001) planes.271 Fig. 17(c1) elucidates
the TEM image of one individual nanorod. Fig. 17(c2) demonstrates the distance between adjacent lattice fringes was around0.325 nm, corresponding to the interplanar distance of (110)planes in the rutile-TiO2. The distance between the fringes
perpendicular to the wall, d ¼ 0.296 nm, corresponds to theinterplanar distance of (001) planes.271 Fig. 17(d1 and d2) display
the TEM images of the multi-trunk and branch structure, where
the red arrowhead indicates the primary growth, whereas therst and second secondary growths are indicated by green andyellow arrowheads, respectively. Fig. 17(d1) also reveals that thetips of the nanorods contain many small nanorods witha diameter of 20–30 nm. The lattice fringes, taken from theregion labelled by white squares (as marked with 1 and 2 inFig. 17(d1)) are shown in Fig. 17(d2), which reveal that thenanorods are highly crystalline in nature. The clean latticefringes with a spacing of 0.252 nm correspond to the (101)plane.286 Fig. 17(e1) depicts the lattice image of two rutile-TiO2
nanoparticles coalesced over the (110) face into a single crystal
in the [110] zone axis. Fig. 17(e2) is the reconstructed image
from the square region in Fig. 17(e1) showing the edge dislocations (denoted by T symbols) at the interface and within thesmaller particle.294

6. Spectroscopic and bandcharacteristics
The optical properties of SMONs have been studied via
numerous spectroscopic techniques, such as Raman, Fouriertransform infrared (FT-IR), UV-Visible (UV-Vis) and photoluminescence (PL) spectroscopy.322 Raman spectroscopy isa spectroscopic technique that is mainly based on an inelastic
scattering of monochromatic light, normally from a laser sourcein the visible, near IR or ultraviolet range. Raman peaks initiate
mainly from the vibrational modes (Eg and A1g), which are
related to diﬀerent crystal planes. This provides, thus, a highaccuracy and sensitivity for measuring the percentage of the
exposed facets from the micro perspective of molecular bonding
together with the benet of fewer measured errors. FT-IR
spectroscopy is an absorption spectroscopic technique that
establishes the vibrational transitions in molecules generated
by various stretching or bending motions in the chemicalbonds. It is well known that an optical transition occurs whena photon is absorbed or emitted by a defect; and because thistransition takes place essentially instantaneously on the scale oflattice relaxations, it arises at xed atomic coordinates. The
absorption band characteristics of photons have been widely
observed using UV-Vis spectroscopy.13,275,299,300,313,314,321–324
6.1. Raman spectroscopy
Fig. 18 shows the Raman spectra of multiphase TiO2 (anatase-,brookite- and rutile-TiO2) nanoparticles synthesized by the sol–gel method. Raman spectroscopy is analyzed in the Raman shiranging from 200 cm1 to 800 cm1 at the excitation line ofThis journal is © The Royal Society of Chemistry 2017

RSC AdvancesFig. 18 Raman spectra of brookite-, anatase- and rutile-TiO2 nanoparticles.275 Reprinted with permission from ref. 275. Copyright 2015.
VBRI Press.

514 nm at room temperature. When Raman spectroscopy wasapplied to characterize the nanoparticles, it was observed that:(i) the Eg peak is mainly caused by symmetric stretching, (ii) theB1g peak is caused by symmetric bending and (iii) the A1g peak iscaused by antisymmetric bending vibrations of O–Ti–O in the
TiO2 nanoparticles. The major Raman bands were observed at
398, 517 and 640 cm1 and verify the anatase phase of TiO2.
These bands can be attributed to Raman active modes of theanatase structural phase with the symmetries of B1g, A1g and Eg,
respectively. The strong Raman peaks of brookite-TiO2 wereseen at 395, 446 and 637 cm1, where the peaks at 395 cm1 and637 cm1 correspond to B2g and A1g, respectively.295 The thermodynamically stable rutile-TiO2 exhibited major peaks at 397,
515 and 638 cm1 with a minor peak at 247 cm1.296–298
6.2. FT-IR spectroscopy
FT-IR spectroscopy has been used to investigate the nature ofchemical bonding present in multiphase TiO2 nanostructures.
Fig. 19 represents the FT-IR spectra in the region 4000–
400 cm1 versus the transmittance for anatase-, brookite-, andrutile-TiO2 nanoparticles synthesized by facile chemical routes.
The observed transmittance bands in the region 800–400 cm1
are attributed to Ti–O and/or Ti–O–Ti bonds, as represented inFig. 19. The peaks, therefore, at 545, 745 and 770 cm1 can beassociated with n(Ti–O) vibrations in multiphase-TiO2 nanoparticles.23 Also, transmittance bands are found at 1431, 1451,
1637, 1648 and 1655 cm1 and were assigned to an O–H
bending mode. Two transmittance bands were noticed at 2514
and 2922 cm1, representing the combination bands in TiO2
nanoparticles.23,299,300 Additionally, broad transmittance bands
were ascertained in the wavenumber region of 3650–3000 cm1and could be ascribed to physically adsorbed water on thesurface of TiO2 nanoparticles.301,302

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Summary of the obtained parameters from UV-Vis spectra
and the indirect and direct optical band gaps of multiphase-TiO2nanostructures

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Table 4

Optical band gap values (Eg in eV)
Multiphase-TiO2

Extrapolation

Indirect

DirectAnatase-TiO2
Brookite-TiO2
Rutile-TiO2

3.21
3.13
3.00

3.20
3.13
3.01

3.53
3.56
3.37

TiO2 nanostructures. The corresponding optical band gaps ofthe multiphase TiO2 nanostructures are calculated from the UVVis reectance spectra using the Tauc equation:(ahn)n ¼ B(hn  Eg)

Fig. 19 FT-IR spectra of brookite-, anatase- and rutile-TiO2 nanoparticles.23,301 Reprinted with permission from ref. 23. Copyright 2016.
Elsevier and ref. 301. Copyright 2000. Springer.

Fig. 20 Optical characteristics: (a) UV-Vis reﬂectance spectra, (b)optical band gap evaluation of multiphase TiO2 nanoparticles, where
the symbols A, B and R correspond to anatase-, brookite and rutileTiO2.289 Reprinted with permission from ref. 289.

6.3. UV-Vis spectroscopy
The optical properties and related band gap characteristics havebeen studied using UV-Vis spectroscopy and the results are
displayed in Fig. 20(a and b). Fig. 20(a) shows the results fromreectance measurements of anatase-, brookite-, and rutile-

Table 3where a is the absorption coeﬃcient, hn is the energy of theincident photon, B is a constant (characteristic of the SMON), n
has two diﬀerent values, either 2 for direct or 1/2 for indirect
band gap transitions, and Eg is the optical band gap of SMON.
The intercept on the abscissa (X-axis) of the Tauc plot (ahn)nversus hn gives the optical band gap energy of the SMON
(Fig. 20(b)). The extrapolation of the reectance versus the
wavelength curves determines the optical band gap energy ofmultiphase TiO2 nanoparticles: (i) 3.21 eV for anatase-TiO2, (ii)3.13 eV for brookite-TiO2 and (iii) 3.00 eV for rutile-TiO2 nanoparticles. The obtained reectance spectra were converted toequivalent absorption coeﬃcients289 providing the same results
as those obtained from extrapolation for indirect transitions (asprovided in the upper segment in Fig. 20(b)). Furthermore,
analysis for the direct transitions (as shown in the lower
segment in Fig. 20(b)) resulted in transitions at higher energies.204 A summary of the obtained parameters from the UV-Vis
spectra and the indirect and direct optical band gaps are displayed in Table 4.

7. Applications
7.1. Photocatalysis
Photocatalysis is the surface phenomenon that is dened as thecatalytic property of a SMON when photons are irradiated on it.
The surface property of multiphase-TiO2 nanoparticles (inparticular anatase- and rutile-TiO2) can be manipulated by coating
their surfaces with either an inorganic (silica) or organic (greentea) layer via a chemical approach. In general, the main constituent of green tea extract is polyphenols, predominantly the catechins.303–305 Out of the four major catechins [(i) epicatechin (EC)

Summary of various synthesis routes for multiphase-TiO2 nanostructures

Phase-TiO2

Nanostructures

Synthesis routes

Anatase-TiO2

Nano-particles, -wires, -tubes, -cuboid, -belts,-tubes, -bres, -cups, -spheres, -sheets
Nano-particles, -bullets, -rices, -tubes, -owers
Nano-particles, -rods, -tree, -spheres, -rod arrays

Sol–gel, hydrothermal and co-precipitation

Brookite-TiO2
Rutile-TiO2

44214 | RSC Adv., 2017, 7, 44199–44224

Sol–gel, hydrothermal and co-precipitation
Sol–gel, hydrothermal and co-precipitation

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RSC AdvancesFig. 21 Photocatalytic and photoluminescence spectra of anataseand rutile-TiO2 nanoparticles. UV-Vis spectra of the photodegradationof methyl orange dye by anatase (a20) and rutile (b20), respectively, atvarious time intervals. (a21), and (b21) show the PL characteristics ofanatase and rutile, respectively at lex ¼ 300 nm.23 Reprinted withpermission from ref. 23. Copyright 2016. Elsevier.

(ii) epicatechin gallate (ECG) (iii) epigallocatechin (EGC) and (iv)epigallocatechin gallate (EGCG)], EGCG is the main component
found in green tea.306 The antioxidant action of polyphenols foundin green tea is known to increase the antioxidant capacity ofblood.307 Thus, green tea extract was expected to be an attractive
material for modifying the surface of photocatalysts.227,235 Herein,
anatase- and rutile-TiO2 were selected in the present context inregard to their photocatalytic applications. In addition, these two
structural phases were coated with: (i) silica (TiO2_silica; anatase/rutile_silica) and (ii) green tea (TiO2_polyphenol; anatase/rutile_polyphenol).23
7.1.1. Bare anatase- and rutile-TiO2 nanoparticles. The
photocatalytic activity of bare anatase- and rutile-TiO2 nanoparticles was analyzed by studying the photodegradation of
methyl orange (MO) solution and then conrmed by the PL
spectra, as depicted in Fig. 21(a10 and b11). PL is an important
tool for exploring the recombination phenomenon occurring in
the excitons analysis of SMON. The UV-Vis absorption spectrumof MO solution with an initial concentration 0.153 mg l1established as a broad band at 464 nm, which corresponds to p
/ p* transition,308 as depicted in Fig. 21(a10 and b10). In the PL

spectrum (inset in Fig. 21(b11)), three peaks can be observed at334, 394 and 524 nm in the case of the bare MO solution. A
summary of the photodegradation and photoluminescencecharacteristics of methyl orange dye by the catalysts, bareanatase- and rutile-TiO2 nanoparticles at various UV irradiation
time is represented in Table 5. However, the trend in photocatalytic activity as evidenced by UV-Vis absorption was found tobe peculiar aer 60 min and 30 min when anatase-TiO2 andrutile-TiO2, respectively, were used as the catalyst. The reason
behind this is that deactivation of the catalyst occurs to someextent due to the formation of a number of degradation fragments of MO at the catalyst surface that block the active sites of
the catalyst and also as the solution becomes concentrated,which scatters the UV light instead of transmitting it
through the solution. These cause hindrance for further
photodegradation.23,309,310
7.1.2. Coated anatase- and rutile-TiO2 nanoparticles
7.1.2.1. Anatase- and rutile-TiO2_silica. The photocatalytic
activity of anatase- and rutile-TiO2_silica nanoparticles was also
assessed by studying the photodegradation of methyl orange

Fig. 22 Photocatalytic and photoluminescence spectra of silicacoated anatase- and rutile-TiO2 nanoparticles. UV-Vis spectra of thephotodegradation of methyl orange dye by anatase_silica (a20) andrutile_silica (b20), respectively, at various time intervals. (a21) and (b21)show the PL characteristics of anatase_silica and rutile_silica,
respectively.23 Reprinted with permission from ref. 23. Copyright 2016.
Elsevier.

Summary of the photodegradation and photoluminescence (PL) characteristics of methyl orange dye by bare anatase- and rutile-TiO2
nanoparticlesa
Table 5Characteristics

Remarks

Bare TiO2

UV irradiation time (minutes)

UV absorbancePL intensity

Photodegradation

Recombination

Anatase TiO2

30
60
120
30
60
120

Decreases
Again decreases
Increases
Decreases
Increases
Decreases

Least peakModerate peakHighest peak
Lower peakHighest peakModerate peak

3
3
7
3
7
3

7
3
3
7
3
3

Rutile TiO2

a

Symbols introduced: 3 / yes, 7 / no.

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Fig. 23 Photocatalytic and photoluminescence spectra of polyphenol-coated-anatase and -rutile-TiO2 nanoparticles. UV-Vis
spectra of the photodegradation of methyl orange dye by anatase_polyphenol (a30) and rutile_polyphenol (b30), respectively, at various
time intervals. (a31) and (b31) show the PL characteristics of anatase_polyphenol and rutile_polyphenol, respectively.23 Reprinted withpermission from ref. 23. Copyright 2016. Elsevier.

(MO) solution, and then its corresponding PL spectra was also
investigated, as depicted in Fig. 22(a20 and b21). The photocatalytic degradation of MO dye by anatase_silica and rutile_silica show a frequent decrease in absorbance at 0 min as compared
to pristine MO solution, which may be due to inter/intra-chaininteractions311 of the catalyst with the MO dye as shown in
Fig. 22(a20 and b20). As UV irradiation is exposed at various timeintervals, overlapping and diminished intensity absorbancecurves are observed revealing the uniform nature of the silica
coating helps to inhibit the photocatalytic activity of the catalyst.
Silica, being a good radical scavenger, engulfs all the radicals
produced by TiO2. To understand the fate of the photogenerated
charge carriers, PL spectrum analysis was also employed at lex ¼
300 nm, which showed a decreasing trend with increasing irradiation time, indicating that the electrons and holes generated
are involved in degradation and not in exciton generation, thus
leading to a drop in PL emission, as shown in Fig. 22(a21 and b21).
The reduction in the photodegradation of MO dye by coating thesurface of rutile with silica correlates with the already reported
results in the literature.312

7.1.2.2. Anatase- and rutile-TiO2_polyphenol. The photocatalytic activity of anatase- and rutile-TiO2_polyphenol nanoparticles was also evaluated by studying the photodegradationof methyl orange (MO) solution, and then its corresponding PL
spectra was also analyzed, as depicted in Fig. 23(a30 and b31). On
30 min exposure of UV irradiation, the absorbance increases,
leading to the reduced photocatalytic degradation of MO dye byanatase_polyphenol, as shown in Fig. 23(b30). However, at
60 min UV exposure, the photocatalysis is observed to beenhanced, which is accounted for as due to its microstructuralcharacteristics, showing that a prolonged UV irradiation time(around 60 min) is required to increase photodegradation. A
summary of the photodegradation and photoluminescencecharacteristics of methyl orange dye by anatase- and rutileTiO2_polyphenol nanoparticles is represented in Table 6.
However, in the case of rutile_polyphenol as a photocatalyst,
dye degradation was found to be higher up to the initial 60 minof UV irradiation, as shown in Fig. 23(b30). The generation of
excitons supporting e–h+ separation (photocatalysis) as well as
mild recombination are evidenced in the PL emission spectra.
At 120 min of prolonged UV irradiation, PL emission is favoured. Thus, it can be inferred that the improved texturalproperties and eﬃcient excitonic separation are responsible forthe enhanced light photocatalytic activities.23,306,313,314
7.2. Concomitant summary
On comparing the coated forms of anatase- and rutile-TiO2
nanoparticles, the polyphenol-coated form shows a higher
photocatalytic activity than silica-coated forms, which isaccounted for by some microstructural alterations and by thenature and thickness of the coating moiety. In summary, (asillustrated in Scheme 4), the results show that anatase- andrutile-TiO2_silica were found to be least photocatalytically
active, ensuring their cosmetic application in sunscreens,whereas rutile-TiO2_polyphenol and bare anatase-TiO2 had thehighest photocatalytic activity amongst their corresponding
forms when reviewed in a broad research area including thephotodegradation of toxic organic dyes.
7.3. Bio-safety and human healthTiO2 nanoparticles are utilized to inhibit bacterial growth owing
to their broad antibacterial characteristics. The inuence ofTable 6 Summary of the photodegradation and photoluminescence characteristics of methyl orange dye by anatase- and rutile-TiO2_polyphenol nanoparticlesa

Characteristics

Remarks

Phase-TiO2_polyphenol

UV irradiation time (minutes)

UV absorbancePL intensity

Photodegradation

Recombination

Anatase-TiO2_polyphenol

30
60
120
30
60
120

Increases
Decreases
Increases
Decreases
Again decreases
Increases

Reduced peakHighest intensity
Least intensity
Lower intensity
Least intensity
Highest intensity

7
3
7
3
3
7

7
3
7
7
7
3

Rutile-TiO2_polyphenolaSymbols introduced: 3 / yes, 7 / no.

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Scheme 4 Pictorial representation of the photodegradation of methyl
orange by TiO2 nanoparticles.23 Reprinted with permission from ref. 23.
Copyright 2016. Elsevier.

Fig. 25 (a)–(c) SEM micrographs of TiO2 nanoparticles (700 mg ml1)
interaction with Enterobacter sp. for 72 h incubation, TEM images of(d)–(f) bare Enterobacter sp. and (g)–(i) interaction of TiO2 nanoparticles with bacterial bioﬁlm. Blue squares (in (g) and (i)) represent
a ruptured bioﬁlm.51 Reprinted with permission from ref. 51. Copyright2017. Taylor and Francis.

Fig. 24 Growth of bacterial species versus incubation time (h) forEnterobacter sp.51 Reprinted with permission from ref. 51. Copyright2017. Taylor and Francis.

nano-sized anatase-TiO2 particles on biolms formed by Enterobacter sp. (1.25 mm)315 was evaluated to treat wound infection.
Diﬀerent concentrations of anatase-TiO2 nanoparticles were
utilized to assist gaining an in-depth understanding. In ourcase, in the presence of nanoparticles, it was noticed thatbacteria using their appendages expressed surface adhesion
proteins, became adhered to the solid surface and formeda biolm via a defence mechanism.
These nanoparticles showed antibacterial characteristics asevidenced by the serial dilution plate method, as displayed inFig. 24, which clearly demonstrate that cell viability could beobserved to be completely arrested at 700 mg ml1 of nanoparticle concentration and at 72 h incubation time.
The obtained results from the serial dilution plate method
were supported by electron microscopy analysis, SEM and
TEM, as represented in Fig. 25(a–i). EM results revealed theinteraction of the bacterial biolm with nanoparticles. SEM

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micrographs, as shown in Fig. 25(a–c), revealed the fact that
nanoparticles were spread all over the bacterial lump and thus
the biolms were encrusted with nanoparticles. To orchestrate
the bio-distribution of nanoparticles into the biolm and to
detect the size of bacterium, HR-TEM was performed, as elucidated in Fig. 25(d–i). The formation of a bacterial biolm wasagain conrmed by TEM, as depicted in Fig. 25(d–e). The size of
single Enterobacter sp. bacterium was observed to be 1.25 mm,
which was found to be in agreement with Bergey's Manual of
Determinative Bacteriology.315 Moreover, complete entrapment of
agglomerated TiO2 nanoparticles into the biological mass wasaﬃrmed by HR-TEM, as portrayed in Fig. 25(g–i). The nanoparticles also aﬀected the morphology of bacterial cells as represented by blue squares, shown in Fig. 25(g and i). Thus, theantibacterial activity of anatase-TiO2 nanoparticles against
a biolm can be used to treat human wound infection.51,316–320

8. Conclusions
Thanks to the eﬃcient eﬀorts of experienced scientists andengineers throughout the world working on multiphase TiO2
nanomaterials over the past decades, the advances made in
research in this eld are enormously signicant and noteworthy. The present review article has presented a widespread
analysis of the state-of-art research into the growth mechanism
behind the eﬃcient synthesis of multiphase TiO2 nanostructures through chemical routes. More exploration on a wide
variety of unique nanostructures has been analyzed according
to their various structural, morphological and microstructuralfeatures.

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In brief, the present review mainly provides key information
to unveil more about the multiphase TiO2 nanostructures,ranging from their eﬃcient synthesis to control themorphology, growth mechanism and properties to their photoactive potential characteristics. Additionally, the optobiological properties are strongly inuenced by the tunablemorphologies, particle sizes and phase transformations ofmultiphase TiO2 nanostructures.
Ultimately, we hope that this review article not only
summarizes the recent progress but will also help to inspire
research activity in this eld on understanding the intriguingcharacteristics of multiphase TiO2 nanostructures. The present
review has mainly focused on anatase, brookite and rutile;
however, the research focus on the structural, morphologicaland microstructural features of the remaining crystalline andnoncrystalline phases of TiO2 should also be interesting in thenear future. Moreover, their nano–bio assessment also deserves
more attention in the future.

Conﬂicts of interest
There are no conicts to declare.

Acknowledgements
The authors sincerely thank the Director, NPL New Delhi, India
for providing the necessary experimental facilities. We would
like to acknowledge CSIR network project, BSC-0112 (NanoSHE). We also acknowledge to all the authors whose publishedarticles help us as a resource for completion of this review
article. There is no nancial concern to publish this review
article.

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