KONA Powder and Particle Journal No. 41 (2024) 197–220/https://doi.org/10.14356/kona.2024014

Review Paper

Synthesis of Anisotropic Metal Oxide Nanoparticles via
Non‑Aqueous and Non‑Hydrolytic Routes †Sherif Okeil 1,4, Julian Ungerer 2, Hermann Nirschl 2 and Georg Garnweitner 1,3*
1 Institute for Particle Technology, Technische Universität Braunschweig, Germany

2 Institute for Mechanical Process Engineering and Mechanics, Karlsruhe Institute of Technology (KIT), Germany
3 Laboratory for Emerging Nanometrology, Technische Universität Braunschweig, Germany

4 Pharmaceutical Analytical Chemistry Department, Faculty of Pharmacy, Ain Shams University, Egypt

Due to their low cost, high stability and low toxicity, metal oxide
nanomaterials are widely used for applications in various fields such as
electronics, cosmetics and photocatalysis. There is an increasing
demand thereby for nanoparticles with highly defined properties, inparticular a narrow particle size distribution and a well-definedmorphology. Such products can be obtained under high control via
bottom-up synthesis approaches. Although aqueous processes arelargely found in literature, they often lead to particles with low
crystallinity and broad size distribution. Thus, there has been a growing
trend towards the use of non-aqueous and non-hydrolytic synthesis
routes. Through variation of the reaction medium and the use of
adequate additives, such non-aqueous systems can be tuned to adapt the
product properties, and especially to yield anisotropic nanoparticles
with peculiar shapes and even complex architectures. Anisotropic
particle growth enables the exposure of specific facets of the oxide nanocrystal, leading to extraordinary properties such as enhanced
catalytic activity. Thus, there is an increasing demand for anisotropic nanoparticles with tailored morphologies. In this review, the nonaqueous and non-hydrolytic synthesis of anisotropic metal oxide nanoparticles is presented, with a particular focus on the different
parameters resulting in anisotropic growth to enable the rational design of specific morphologies. Furthermore, secondary phenomena
occurring during anisotropic particle growth, such as oriented attachment mechanisms, will be discussed.
Keywords: non-aqueous synthesis, non-hydrolytic synthesis, anisotropic nanoparticles, metal oxides, oriented attachment

1. Introduction

Metal oxides emerged as an indispensable class of materials due to the wide range of properties they can exhibit
according to their composition, degree of crystallinity andmorphology. Thus, they are employed in various technological fields such as catalysis (Védrine, 2017), electronics,
including transistors (Yu et al., 2016) and gas sensors(Arafat et al., 2012; Chavali and Nikolova, 2019; Yu et al.,2022), as well as energy storage (Ellis et al., 2014;
Fleischmann et al., 2022; Su et al., 2016) and solar cells(Chen et al., 2012). These diverse applications of metaloxides often require the use of particles with defined properties. In particular, due to their advantages such as highspecific surface area providing high surface activity or
small size providing a high degree of homogeneity,
nanoparticles of metal oxides are desired. Hence, synthesis
†

*
1
3

Received 7 September 2022; Accepted 4 May 2023
J-STAGE Advance published online 31 August 2023
Corresponding author: Georg Garnweitner;
Add: Volkmaroder Str. 5, 38104 Braunschweig, Germany
Add: Langer Kamp 6A, 38106 Braunschweig, Germany
E-mail: g.garnweitner@tu-braunschweig.de
TEL: +49 (531) 391-65371 FAX: +49 (531) 391-9633

routes which enable the synthesis of metal oxides with tailored properties and which at the same time are reliable,
cost-effective, versatile and scalable are important prerequisites for many of these applications.
In the past decades, scientists have always been working
on different synthesis techniques to access metal oxide
nanomaterials. Therefore, various synthesis methods exist,
ranging from top-down methods such as ball milling andlaser ablation to bottom-up methods like gas-phase synthesis and solution-based methods (Chavali and Nikolova,
2019; Parashar et al., 2020). While gas-phase synthesis isoften expensive and requires large instrumentation, thesolution-based synthesis of metal oxide nanoparticles is
simple, comparatively less expensive, easily scalable andstill enables precise control of the formed nanoparticles. To
date, the solution-based synthesis of metal oxides is carriedout mainly via aqueous synthesis, although non-aqueousand non-hydrolytic synthesis routes offer interesting advantages for nanoparticle synthesis.
In the case of aqueous synthesis, the precursor compound—which often is a metal alkoxide or other reactive
metal compound—reacts with water directly in a hydrolysis step. This usually results in very high reaction rates,

Copyright © 2024 The Authors. Published by Hosokawa Powder Technology Foundation. This is an open
access article under the CC BY license (https://creativecommons.org/licenses/by/4.0/).

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which decreases the homogeneity of the resulting productand leads to difficulty in adjusting or controlling the product properties (Chang and Doong, 2006). Many reaction
parameters such as the rate of addition of precursor or water, the type and intensity of mixing, as well as solution
parameters like pH, ionic strength, the type and concentration of chelating agents must be precisely controlled toobtain reproducible results. Concerning crystallinity of theresulting nanoparticles, the aqueous sol-gel synthesis at
low temperatures usually results in amorphous or lowcrystallinity oxides, and requires further heat treatment of
the obtained product to obtain crystallinity (Wu et al.,2007a; Zhang et al., 2005). However, calcination at hightemperatures results in an increase of crystallite size, neckformation and thus aggregate formation, and eventuallyeven decomposition or phase transformation (Satapathy etal., 2014; Wang et al., 2002). In contrast, in the nonaqueous or non-hydrolytic synthesis, highly crystalline
metal oxides are obtained directly in many cases (Wang etal., 2005). Moreover, the crystal phase can even be tuned
by varying the reaction conditions of the non-aqueous
synthesis (Wu et al., 2007a). This makes the non-aqueousand non-hydrolytic synthesis an attractive pathway for thesimplified production of certain nanomaterials (Buonsantiet al., 2008; Niederberger and Garnweitner, 2006).
In non-aqueous synthesis routes, no water is added to thereaction mixture. Usually, an organic solvent is employed
as a reaction medium but it can also act as a stabilizing
agent, which results in a simpler reaction procedure andenables the synthesis of nanomaterials with uniform properties and a lower amount of strongly bound organics dueto the absence of surfactants in the case of solvent-directedsyntheses (Mutin and Vioux, 2013; Niederberger and
Pinna, 2009). In contrast, the synthesis of nano-sized particles in aqueous media often requires the addition of various
surfactants and capping agents that limit the growth of the
inorganic particles and stabilize the resulting dispersion.
While enabling a precise control over the resulting shapeand particle size distribution, this also leads to coverage ofthe surface of the resulting particles by a large amount of
surface organics (Niederberger and Pinna, 2009). In
non-aqueous synthesis, alcohols are often used as reaction
media, and water may be formed in situ during the synthesis process. On the other hand, non-hydrolytic synthesis
proceeds in the absence of hydroxyl groups and thus requires the use of an aprotic medium.
In the past few decades, there was an immense increaseof interest in crystal engineering, where a huge spectrum of
anisotropic particle shapes has been reported for nanoparticles of different materials (Görke and Garnweitner, 2021).
A general classification of nanomaterials can be made according to their dimensionality into one-dimensional nanostructures, like nanorods and nanowires, two-dimensional
nanostructures such as nanoplatelets and sheet-like struc-198

Review Papertures, and finally more complex three-dimensional nanostructures such as nanocubes, nanopyramids, nanostars,
tetrapods or multipods. Small nanoparticles, in contrast, areoften referred to as zero-dimensional nanodots. Each ofthese morphologies can lead to distinct anisotropic properties which are in most cases unique and make them suitable
for specific applications rather than spherical nanoparticles
which lack the direction-dependent properties (Burrows etal., 2016; Sajanlal et al., 2011). For example, the exposureof specific crystal planes was found to have a drastic effecton the photocatalytic efficiency of metal oxide nanoparticles (Chang and Waclawik, 2012). Anisotropic iron oxide
nanoparticles show magnetic behavior which is highly dependent on its shape anisotropy, resulting in enhanced performance in different biomedical applications (Andrade etal., 2020). Metal oxide nanotube structures expose outerand inner surfaces as well as edges, which results in an increased surface area that proved to be highly beneficial forenergy storage applications and can be used to embed othermaterials to form nanocomposites (Ellis et al., 2014; Patzke
et al., 2002). All this shows the importance of anisotropic
metal oxide nanoparticles. Generally, anisotropic shapes
can be formed either through seed-mediated growth whichusually results in monocrystalline nanoparticles, throughoriented attachment or controlled aggregation (Burrows etal., 2016). Interestingly, non-aqueous synthesis routes canfeature chemical species formed in situ that can direct
anisotropic growth (Djerdj et al., 2008a) or even form
inorganic-organic hybrid nanostructures through selfassembly (Pinna and Niederberger, 2008). Thus, thenon-aqueous or non-hydrolytic synthesis routes show ahigh potential in obtaining anisotropic shapes in a simple
manner.
Understanding the formation mechanisms of metal oxide
nanoparticles is crucial for predicting anisotropic growthand enabling the rational design of new non-aqueous synthesis routes capable of producing such anisotropic shapes.
There are several review articles (Deshmukh andNiederberger, 2017; Jun et al., 2006; Mutin and Vioux,2013, 2009; Niederberger and Garnweitner, 2006;
Niederberger and Pinna, 2009; Pinna and Niederberger,
2008; Styskalik et al., 2017) presenting the different
non-aqueous or non-hydrolytic synthesis routes for metaloxides and discussing the reaction mechanisms leading toparticle formation. Additionally, several general reviews
have been published on the wet chemical synthesis of metaloxide nanoparticles (Jun et al., 2006; Nikam et al., 2018) or
their applications (Chavali and Nikolova, 2019; Yu et al.,2016). In contrast to these, this contribution will be limited
to the synthesis of anisotropic metal oxides in non-aqueousand non-hydrolytic systems. In this way, we aim to provide
a deeper insight into the formation of anisotropic metaloxide nanoparticles or nanostructures, and discuss the different factors and mechanisms involved in the emergenceSherif Okeil et al. / KONA Powder and Particle Journal No. 41 (2024) 197–220

of anisotropic particle shapes in the different non-aqueous
and/or non-hydrolytic reaction systems. This should help inthe design of new tailored non-aqueous synthesis routes foranisotropic metal oxide nanoparticles.
In the following paragraphs, an overview of the synthesis of anisotropic particles of the most important metal oxides via non-aqueous and/or non-hydrolytic synthesis
routes will be presented. Each oxide is discussed separately
as the interaction of the different solvents and surfactants
with the nanocrystal surface differs according to the crystal
properties of the metal oxide. This will be followed by adiscussion of the unique mechanisms, especially oriented
attachment, encountered in the formation of anisotropic
metal oxides in non-aqueous and non-hydrolytic synthesis
routes. Then, the formation of anisotropic metal oxides insome special solvents, like benzyl alcohol is reviewed. The
review is finally concluded by an outlook on possible future directions.

2. Titanium dioxide

The synthesis of titanium dioxide as the most-used photocatalytic material has been the focus of research for decades (Liu et al., 2014). Many studies were devoted to thesynthesis of TiO2 nanoparticles with distinct morphologies
due to the increased surface-to-volume ratio where the increased interface permits enhanced catalytic activity(Fattakhova-Rohlfing et al., 2014). Table S1 in the supplementary material provides a list of non-aqueous andnon-hydrolytic synthesis approaches resulting in different
anisotropic TiO2 nanocrystal morphologies in chronological order of their publication. The main factors affecting theanisotropic growth in non-aqueous systems will be discussed in the following text. Although the main factors arepresented, it must be considered that interconnections between the different factors exist, which makes it hard toisolate the effect of single factors completely.

2.1 Effect of solvent

Non-aqueous sol-gel synthesis routes were shown as
being capable of controlling the size and shape, thusachieving different anisotropic TiO2 structures with simple
one-pot synthesis by simply changing the solvent used. The
first non-aqueous synthesis procedure to well-defined
anisotropic titania particles was reported by Wang et al.,using titanium tetrachloride (TiCl4) as the precursor in asolvent mixture of absolute ethanol and acetic acid (Wanget al., 2001). In this work, different organic solvent mixtures were tested for the synthesis of titania nanoparticles,with only the combination of absolute ethanol with acetic
acid yielding anisotropic rutile titania structures (tenuous
fiber and rod-like structures, Table S1). Ethylene glycol or
glycerol as alternatives for ethanol yielded spherical
nanoparticles, which was evident from the corresponding
TEM and XRD analysis. The authors suggested that the ti-

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tania formation in these systems mainly proceeds via a hydrolytic mechanism due to the water formed in the systemfrom the esterification reaction between ethanol and acetic
acid. The use of solely ethanol as a solvent also resulted inisotropic nanoparticles (Wang et al., 2001; 2002). The difference in the obtained morphologies by changing the alcohol type was rationalized by the preferential surface
binding of the ethanol molecules to specific lattice planes,
slowing down their growth (Doxsee et al., 1998). The investigation of a series of other alcohols as solvents for the
non-aqueous synthesis of titania from TiCl4 at a low temperature showed that fibrous structures could also be obtained in the case of using isopropanol and n-butyl alcohol
as solvents (Wang et al., 2002).

2.2 Effect of precursor concentration (precursorto‑solvent ratio)

Investigating the influence of the precursor concentration, it was found that the TiCl4/butanol ratio plays an important role in determining the phase and morphology ofthe formed TiO2 nanocrystals. Only a low TiCl4/butanol
ratio resulted in the formation of rutile-phase nanorods,
while higher ratios resulted in isotropic particles (Cao etal., 2011). While the sol-gel reaction of titanium tetrachloride and benzyl alcohol normally yields spherical particles(Niederberger et al., 2002a), nanorod particles have beenobtained by Abazović et al. (Abazović et al., 2006; 2008)
using the same reaction system, which underlines the importance of the applied reaction conditions. The formationof anatase TiO2 nanorods in this reaction system has been
attributed to the prolonged aging time at an elevated temperature (75 °C for 3 days) , which is longer than that used
for spherical particles (24 h only at 75 °C). Rutile-phase
TiO2 nanofibers were also synthesized in a non-aqueous
system from TiCl4 in acetone at 110 °C, where the ratio of
the precursor to acetone was found to be crucial for tuning
the phase and morphology of the synthesized TiO2. A 1:10
ratio of precursor to acetone was required to obtain rutile-phase nanofibers, while lower precursor concentrations
resulted in anatase-phase isotropic and irregular-shaped
particles (Wu et al., 2007a; 2007b). The same behavior was
also observed with other ketones like butanone, 4methylpentanone, and acetophenone (Wu et al., 2007b). In
contrast, the reaction of titanium isopropoxide in ketones
generally results in spherical anatase nanoparticles(Garnweitner et al., 2005).

2.3 Effect of surfactants

While it is evident that changing the solvent can control
the morphology of the resulting metal oxide, the addition ofa surfactant can further tune and direct the nanoparticle
growth and can be used to obtain new particle shapes.
Surfactants can preferentially bind to certain facets of acrystal changing its surface-free energy and thus changing

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the growth rate of the different facets resulting in anisotropic
growth (Bakshi, 2016). Alivisatos et al. used two different
surfactants, namely lauric acid (LA), which selectively
binds to the anatase {001} facets, and trioctylphosphineoxide (TOPO), which nonselectively coordinates to theTiO2 surface. By variation of the ratio of these surfactants,
it is possible to obtain different anisotropic TiO2 particles
ranging from bullet-shaped and rod-shaped nanocrystals tomore complex branched nanocrystal shapes (Jun et al.,2003). At low LA concentrations, anisotropic crystal
growth along the [001] direction takes place, eliminatingthe {001} faces and exposing {101} faces forming bulletshaped (Fig. 1(a)) and diamond-shaped (Fig. 1(b)) nanocrystals. Increasing the amount of the LA surfactant slows
down the growth along the [001] direction, leading to theformation of facetted nanorods (Fig. 1(c) and (d)). At very
high LA concentrations, further inhibition of the growth in
the [001] direction favors the growth of the {101} crystal
faces and results in the branching-out of the {101} faces,
yielding branched nanorod structures (Fig. 1(e)) (Jun et al.,2003).
The use of oleic acid as a surfactant in the non-aqueous
titania synthesis in anhydrous toluene yielded dumbbellshaped nanorods at 250 °C only when the amount of titanium isopropoxide (TIP) precursor was high, using a constant surfactant/precursor ratio of 3:1. At lower precursor
concentrations, an increase of the surfactant/precursor ratio
above 3:1 was necessary to obtain the dumbbell-shaped
nanorods (Kim et al., 2003). This shows that not only thesurfactant/precursor ratio is crucial for obtaining anisotropic structures but also the relative amount of precursor to
the non-aqueous solvent used.

a

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A reaction of TIP with oleic acid at lower temperatures(80 °C) results in the derivatization of the titanium precursor forming a titanium oleate complex. This can be used as
an alternative precursor for the TiO2 synthesis due to its
lower reactivity, which can enable better control of thenanoparticle formation. Adapting the chemical reactivity ofthe precursor, further control of the nanoparticle synthesis
can be achieved. The injection of oleylamine into thenon-coordinating solvent 1-octadecene at 260 °C starts thedecomposition of the titanium oleate complex throughaminolysis, resulting in the formation of TiO2 which iscapped by carboxylate ions. The higher reactivity of the(001) planes leads to the facilitated aminolysis of the capping oleate at these planes, which enables anisotropic
growth of the titania nanocrystals in the [001] direction.
Thus, titania nanorods are formed whose length can beeasily tuned through variation of the injected oleylamine
amount. An increase in the oleylamine amount from 1 to
2 mmol increases the titania nanorod length in this system.
A further increase was found to suppress anisotropic
growth where 3 mmol oleylamine leads to a decrease in the
resulting nanorod length; finally for 4 mmol oleylamine,
isotropic titania nanoparticles were achieved. The observed
suppression of anisotropic growth could be due to the accelerated aminolysis and increased consumption of
surface-coordinated oleate by the high oleylamine concentration. Even the diameter of the titania nanorods formed
could be tuned in this system through the use of the more
weakly binding surfactant cetyltrimethylammonium bromide (CTAB) (Zhang et al., 2005). Thus, variation of theoleylamine/oleic acid ratio represents a powerful tool totune the morphology of titania nanocrystals.

b
c

d

e

Fig. 1 High-resolution TEM images showing the evolution of different shapes of TiO2 with varying relative ratio of LA and TOPO surfactants: (a)bullet-shaped, (b) diamond-shaped, (c) short nanorod, (d) long nanorod and (e) branched nanorod structures (Scale bar = 3 nm). Adapted with permission from Ref. (Jun et al., 2003). Copyright 2003 American Chemical Society.

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Through variation of the oleylamine/oleic acid ratio it isalso feasible to finely control the shape of the obtained nanorods. Buonsanti et al. observed that at a lower oleylamine/oleic acid ratio of 26, brookite-phase TiO2 nanorods
with tapered tips were obtained, whereas at much higher
ratios of about 104, rectangular endings of the TiO2 nanorods were observed (Buonsanti et al., 2008). It has to be
noted that the precursor used is TiCl4 instead of TIP and aseeded-growth technique has been adopted for the synthesis.
A similar system was used by Hyeon et al. for the largescale non-aqueous synthesis of TiO2 nanorods, where theTIP precursor was allowed to react with oleic acid at
270 °C for 2 hours, assuming that an ester elimination reaction results in the formation of the titania nanorods growing
along the [001] direction (Fig. 2(a)). The addition of controlled amounts of 1-hexadecylamine (HDA) enabled control of the resulting nanorod diameter as, according to theauthors, HDA strongly coordinates to the (101) anatasefaces slowing down its growth. Thus, an increase of the
HDA amount leads to a decrease in the titania nanorod diameter from 3.4 nm to 2.0 nm (Joo et al., 2005b). Although
it was noted by Zhang et al. that the use of oleic acid oroleylamine alone did not result in particle formation(Zhang et al., 2005), the reaction system of Joo et al. enabled the formation of particles by solely reacting TIP witholeic acid (Joo et al., 2005b).
It was also feasible to obtain ultrathin TiO2 nanosheets
upon heating of TiCl4 precursor in oleylamine alone, whichin this case serves as both the solvent and surfactant(Fig. 2(b)) (Yang et al., 2015). This shows that the concentration of the precursor and surfactant plays an importantrole in the particle formation, along with the reaction temperature and synthesis time. It is noteworthy that the synthesis of TiO2 in oleic acid results in preferential growth in
the [001] direction, resulting in nanorods (Fig. 2(a)) (Joo etal., 2005b), while the synthesis in oleylamine results in an
inhibition of the growth in the [001] direction (c-axis)
where the preferred growth along the a- and b-axis yields inabFig. 2 TEM images of (a) TiO2 nanorods synthesized in oleic acid(adapted with permission from Ref. (Joo et al., 2005b). Copyright 2005
American Chemical Society) and (b) ultrathin TiO2 nanosheets synthesized in oleylamine (adapted with permission from Ref. (Yang et al.,2015). Copyright 2015 The Royal Society of Chemistry).

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the formation of nanosheets (Fig. 2(b)) (Yang et al., 2015),
which could indicate the different roles of these surfactants.
Detailed investigations of the parameters leading to theanisotropic growth of TiO2 nanocrystals using a solvothermal synthesis in cyclohexane with titanium butoxide as aprecursor and linoleic acid as a surfactant at a low temperature (150 °C for 48 h) were performed by Li et al. The influence of surfactant amount, reaction temperature, reaction
time and type of surfactant were studied, where an increasein surfactant amount promoted the anisotropic growth. The
nanorods formed also showed spherical tips similar to thedumbbell-shaped nanorods previously obtained in the workby Kim et al. (2003). GC-MS analysis determined the sideproducts formed during the solvothermal synthesis, whichindicated an ester elimination reaction mechanism. Checking the effect of carboxylic acid surfactants revealed thatester elimination alone is not sufficient for particle formation; a long-chain organic acid is also crucial for TiO2 particle formation (Li et al., 2006).

2.4 Effect of doping and precursor addition rate

Alkyl halide elimination for the synthesis of Zr-doped
TiO2 nanocrystals, using a non-hydrolytic, non-aqueous
synthesis route, was adopted by using a mixture of a metal
halide and a metal alkoxide. TOPO was used as the coordinating solvent. The formation of anisotropic facetted Zrdoped TiO2 nanorods was only possible at high temperatures(400 °C), with an increase in Zr-doping leading to a subsequent enlargement of the nanorods (Chang and Doong,2006). The same titania precursors also yielded anisotropic
structures at a lower temperature of 300 °C in the noncoordinating solvent 1-octadecene when using oleylamine
as the surfactant. The morphology and even the phase of
the evolving TiO2 nanostructures could be precisely controlled through the slow addition of the precursors to hot
oleylamine via a syringe pump. A comparably high injection rate of 30 mL/h of the precursors yielded anatase TiO2
nanorods of 6 nm in diameter and 50 nm in length(Fig. 3(a)). Lowering the precursor addition rate to
2.5 mL/h enables the formation of larger anatase nanorodsof 9 nm in diameter and 100 nm in length together withrutile-phase star-shaped nanostructures (25 × 200 nm indimension) (Fig. 3(b)). Reducing the flow rate to 1.25 mL/hyields pure rutile star-shaped TiO2 of 25 nm × 450 nm insize (Fig. 3(c)). Consequently, a slower precursor addition
rate enabled the formation of fewer nuclei which continuously grew with continuous precursor addition. In contrast,
one-shot injection of these precursors into the hot oleylamine resulted in the formation of a mixture of polydisperse
nanoparticles and nanorods, which underlines the importance of the addition rate of the precursors in controlling
particle morphology (Koo et al., 2006).

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3. Zinc oxide

Zinc oxide, as a relatively cheap and non-toxic
semiconductor with a wide range of functional properties,
has attracted broad attention in various application fields
such as gas sensing, piezoelectric devices, photocatalysis,
optoelectronic devices and even biomedicine (Bharat et al.,2019; Kolodziejczak-Radzimska and Jesionowski, 2014;
Zhang et al., 2013). In all of these applications, shape control can open the door for better integration of ZnO nanostructures into the selected application to reach the desired
outcome. Using non-aqueous and non-hydrolytic synthesis
routes, unique morphologies such as teardrop-like shape(Zhong and Knoll, 2005), hexagonal pyramid shape (Choi
S.H. et al., 2005) and nanowires (Yuhas et al., 2006) wereachieved. Supplementary Table S2 lists the different ZnO
particle shapes reported to be obtained from non-aqueousand non-hydrolytic synthesis routes in chronological orderof their publication. In the following paragraphs, the effects
of different factors on ZnO particle morphology will be
discussed in more detail.

3.1 Effect of solvent

Through the use of solvents with different coordinatingabReview Paper

powers, it was possible to control the shape of evolving
ZnO nanoparticles. Thin zinc oxide nanorods (2 nm in diameter) were obtained in trioctylamine (TOA) with oleic
acid as a surfactant. The self-assembly of the synthesized
nanorods in stacks was observed in this system, with thenanorods being arranged parallel to each other (Fig. 4(a))(Andelman et al., 2005; Yin et al., 2004). The use of the lesscoordinating solvent 1-hexadecanol (HD) resulted in theformation of less elongated ZnO nanoparticles in the formof nanotriangles (Fig. 4(b)). Using the non-coordinating
solvent 1-octadecene (OD) results in the loss of anisotropic
growth, yielding spherical ZnO nanoparticles (Fig. 4(c))(Andelman et al., 2005). Thus, the choice of solvent, in
combination with the surfactant, played an important rolein stabilizing the different crystal planes and determining
the resulting nanoparticle morphology.
Zhang et al. also tested the influence of different noncoordinating and coordinating solvents on the morphologyof the evolving ZnO nanoparticles formed from zinc carboxylate precursors using oleylamine as reagent and surfactant. It is interesting to note that in this work, whenemploying oleylamine as both a reagent and surfactant, theuse of non-coordinating solvents such as dioctyl ether,cFig. 3 TEM images of (a) anatase TiO2 nanorods formed at 30 mL/h, (b) mixture of anatase nanorods and rutile star-shaped TiO2 structures formed
at 2.5 mL/h and (c) pure star-shaped rutile TiO2 formed at 1.25 mL/h precursor addition rate. Adapted with permission from Ref. (Koo et al., 2006).
Copyright 2006 American Chemical Society.

a

b

c

Fig. 4 TEM images with the corresponding crystal structures of (a) ultrathin ZnO nanorods self-assembled in stacks, (b) ZnO nanotriangles and (c)spherical ZnO nanoparticles formed in solvents with different coordinating power. Adapted with permission from Ref. (Andelman et al., 2005). Copyright 2005 American Chemical Society.

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diphenyl ether and OD yielded ZnO nanorods (Zhang Z. etal., 2006), which shows that the control of anisotropic
growth is a complex interplay between different factors,
including the combination of solvent and surfactant used.
In this particular system, the use of the coordinating solvent
trioctylphosphine (TOP) led to the formation of nanotetrahedrons, similar to the nanotriangles obtained in the workof Andelman et al. (Andelman et al., 2005; Zhang Z. et al.,2006). Increasing the coordinating power by using TOPO
as a solvent results in the formation of irregularly shaped
particles, which was attributed to the very strong coordination of TOPO to the zinc acetate precursor and the stericeffect of the bulky alkyl chains hindering the access foroleylamine to the nanocrystal surface, thus drastically decreasing the kinetics of the aminolytic reaction essential forparticle growth in this system (Zhang Z. et al., 2006).
It also has to be mentioned that the role of solvent andsurfactant cannot be separated as the solvent itself usuallyshows some coordination and thus surface activity in covering certain facets of the evolving nanocrystal.
A study of the effect of solvent alone was conducted by
Kunjara Na Ayudhya et al., where the influence of different
glycols, alcohols, n-alkanes and aromatic solvents on themorphology of ZnO nanoparticles was investigated(Kunjara Na Ayudhya et al., 2006). The formation of ZnOfrom the zinc acetate precursor in glycols and alcohols wasachieved at 250 °C, while in non-polar and noncoordinating solvents (n-alkanes and aromatic solvents),
the precursor decomposition temperature increased to
300 °C. The decomposition temperature of the zinc acetate
precursor in the corresponding solvent was related to thedielectric constant of the latter. When the highly polar glycols were used as a solvent, low anisotropic particles ofpolyhedral shape were obtained, while alcohols yielded
nanorods of moderate aspect ratio. The use of n-alkanes
and aromatic solvents resulted in nanorods with a highaspect ratio (Kunjara Na Ayudhya et al., 2006). Thus, theaspect ratio of the ZnO nanorods formed was found to be
tunable depending on the polarity of the solvent used: thehigher the polarity of the solvent, the less the preferential
growth of ZnO in the [001] direction. Thus, alcohols withincreasing chain length resulted in an increase of the aspectratio of the ZnO nanorods formed (Cheng and Samulski,
2004; Kunjara Na Ayudhya et al., 2006; Rai et al., 2013;
Tonto et al., 2008). Similar observations were made when
benzyl alcohol was replaced by the non-polar anisole as asolvent in ZnO synthesis from zinc acetate, where a substantial increase in the nanorod aspect ratio could be observed (Clavel et al., 2007). Other studies reported similar
but still deviating behavior using a different precursor anddifferent reaction conditions (Ramya et al., 2019; Šarić etal., 2019). The synthesis of ZnO nanoparticles through a
precipitation reaction from zinc acetate and sodium hydroxide in a series of alcohols at low temperatures resulted

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Fig. 5 TEM image of a ZnO nanostar formed from zinc acetate in a
mixture of ethanol and ethylene glycol at 80 °C. Adapted with permission from Ref. (Wang et al., 2005). Copyright 2005 Elsevier.

in isotropic nanoparticles in methanol, mixed nanoparticles
and short nanorods in ethanol and nanorods with increasing
aspect ratio from butanol to octanol. In octanol, isotropic
particles were also observed together with the highaspect-ratio nanorods. Finally, isotropic particles were obtained again in decanol. In this system, the growth behavior
was found to be related to the zeta potential of the formed
colloids, which results in different aggregation rates wherea lower zeta potential increases the aggregation rate of thecolloid and leads to an increased growth rate (Šarić et al.,2019).
Solvent mixtures are also capable of producing interesting anisotropic structures through selectively enhancing or
retarding the growth of specific crystal planes. For example, a mixture of ethanol and ethylene glycol resulted in theformation of ZnO nanostars from a zinc acetate precursor
at a low temperature (80 °C) without the need for a surfactant, as can be seen in Fig. 5 (Wang et al., 2005). In thiscase, the anisotropic structure formed could be an aggregate of smaller primary particles which possibly resulted ina hierarchical structure, which, however, was not investigated in this work.

3.2 Effect of surfactant and reaction temperature

The ester elimination reaction, similar to that used forthe large-scale synthesis of TiO2 nanorods, has also been
applied for the synthesis of different ZnO nanoparticles.
The Hyeon Group demonstrated thereby that the use ofdifferent surfactants resulted in strong changes in morphology. TOPO, OA, HDA and TDPA in dioctyl ether as solvents resulted in the formation of wedge-shaped ZnOnanocones self-assembled in spherical nanostructures(Fig. 6(a)), cone-shaped ZnO nanocrystals (Fig. 6(b)),
hexagonal cone-shaped ZnO nanocrystals (Fig. 6(c)) andZnO nanorods (Fig. 6(d)), respectively (Joo et al., 2005a).
Changing the alcohol used to 1,2-dodecanediol and thesolvent to the higher-boiling-point-solvent benzyl ether toperform the synthesis at higher temperatures while using
TOPO as the surfactant yielded similarly pyramidal orcone-shaped particles (Kachynski et al., 2008).

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a

c

b

d

Fig. 6 TEM micrographs of (a) spherical hierarchical structures composed of wedge-shaped ZnO nanocones synthesized using TOPO, (b)ZnO nanocones synthesized using oleic acid, (c) hexagonal ZnO nanocones synthesized using HAD and (d) ZnO nanorods synthesized using
TDPA as surfactant. Adapted with permission from Ref. (Joo et al.,2005a). Copyright 2005 Wiley-VCH.

Increasing the concentration of the surfactant also has an
influence on the particle shape and can even result in the
self-assembly of anisotropic particles to various complex
nanostructures. For example, a rise in the concentration ofoleylamine used in the synthesis of ZnO in TOP led to theformation of nanotetrahedrons that self-assembled into socalled nanofans. A further increase in the oleylamine concentration resulted in larger dumbbell-shaped nanostructures(Zhang Z. et al., 2006).
The decomposition of zinc acetylacetonate (Zn(acac)2)
in oleylamine, which in this case had the role of solvent andsurfactant, yielded different particle shapes by varying theoleylamine/precursor molar ratio together with the reaction
time and temperature. A ratio of 10:1 at a reaction temperature of 205 °C for 1 h yielded short nanorods arranged inbelt-like structures. A lower precursor concentration (ratio
100:1 of oleylamine/precursor) resulted in heart-shaped
nanoparticles at 205 °C for 24 h (Liu et al., 2007).
Similarly, the influence of oleylamine on the particle
morphology was also investigated by Zhang et al., however, using zinc acetate as the precursor. In this system, theamine was able to attack the precursor in an amide elimination reaction or aminolysis yielding ZnO. In that case, it
was found that increasing the molar ratio of oleylamine toprecursor decreases the anisotropic growth along the [001]direction, leading to the formation of nanorods with a lower
aspect ratio and finally nanoprisms in the case of ratios
greater than 3 (Fig. 7). A further increase of the oleylamine/precursor ratio up to 24:1 had no significant effect on the
resulting morphology (Zhang et al., 2007). It has to be
noted that in this work, the precursor used is different,

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Fig. 7 Diagram showing the relation between the molar ratio of oleylamine to zinc acetate used and the resulting aspect ratio as well as thediameter of the ZnO nanocrystals. Above, for each morphology in the
diagram (long nanorods, short nanorods, bullet-shaped nanocrystals andnanoprisms), the corresponding TEM image proving the formation ofsuch ZnO nanomaterial is shown (scale bar: 100 nm). Adapted withpermission from Ref. (Zhang et al., 2007). Copyright 2007 Wiley-VCH.

which affects the rate of monomer generation and partly
explains the difference to the shapes obtained in the work
by Liu et al. (2007). Nevertheless, both works showed adecrease in anisotropic growth with an increasing amountof oleylamine (Liu et al., 2007; Zhang et al., 2007), which
was related to an increased reaction rate, leading to rapid
nucleation and leaving fewer monomers available for further particle growth. On the other hand, a low amine-toprecursor ratio resulted in a low reaction rate, leading to theformation of fewer initial nuclei and leaving more monomers for further particle growth, where the growth offaster-growing planes becomes apparent, forming highly
anisotropic nanocrystals (Zhang et al., 2007).
The testing of other amines (hexadecylamine, dioctylamine and dodecylamine) showed that the longer the alkyl
chain of the amine used, the thinner and longer are the ZnO
nanowires or nanorods produced when all other reaction
parameters are kept constant (Zhang et al., 2007). Correlating these results with other studies where the effect of thesolvent was investigated (Clavel et al., 2007; Kunjara Na
Ayudhya et al., 2006), it can be deduced that the amines
with longer-chain lengths result in higher aspect ratios dueto their lower polarity. Using other amines like benzylamine as a reagent to initiate the aminolysis reaction of zincacetate in benzyl ether also resulted in different anisotropic
shapes such as nanocones, nanobullets, nanorods and nanoplates depending on the molar ratio of benzylamine to thezinc precursor as well as the reaction temperature used
(Ahmad et al., 2013; Chang and Waclawik, 2012). By

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adjusting the amount of benzylamine and the reaction temperature, the reaction rate was controlled which resulted indifferent nucleation and growth mechanisms. It was foundthat zinc acetate does not decompose in pure benzyl ether
at temperatures below 210 °C unless a reagent initiating anaminolysis reaction is added. Thus, performing a reaction
at 170 °C using a benzylamine/Zn ratio of 10 would be apure aminolysis reaction resulting in the formation of ZnO
nanocones. On the other hand, keeping the same amineto-Zn ratio while using a higher reaction temperature of
210 °C would result in a mixed thermal decomposition andaminolysis reaction, together with an increased reaction
rate due to the increased temperature. Thus, the reaction at
210 °C resulted in the formation of ZnO nanobullets where
the top cone shape of the nanobullets was explained by thecapping effect of the excess benzylamine. This underlines
the dual role of the amine reagent, where on the one hand it
is a reagent for the aminolysis reaction, and on the other
hand, any excess not involved in the aminolysis reaction
acts as a capping agent for the polar ZnO facets. This isfurther proved by decreasing the benzylamine/Zn ratio to 1
while keeping the reaction temperature at 210 °C, resulting
in the formation of nanorods without the cone-shaped tip
due to the absence of excess benzylamine, which would
take the function of a capping agent. An increase of the ratio to 40 resulted in nanoplate formation due to the strong
coordination of excess benzylamine to the polar Zn2+ surface, inhibiting growth along the [001] direction. It has tobe mentioned that the heating rate was also found to have apronounced effect on the resultant morphology in this system, where rapid heating resulted in the ZnO nanoplates,
while slow heating (~ 5 °C/min) to the same reaction temperature led to the formation of “uncompleted” ZnO nanocones (Chang and Waclawik, 2012).
While oleylamine has been widely used as a surfactant
and morphology-controlling agent in the synthesis of ZnOnanostructures, oleic acid would be another possible interesting surfactant for morphology control due to its strong
coordination ability with the ZnO surface. Usually, however, OA was used in combination with amines (Choi S.H.
et al., 2005; Yin et al., 2004) during the synthesis of ZnO,ab

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which makes it hard to determine the role of each in theformation of the anisotropic structure. Zhong et al. used a
mixture of OA and oleyl alcohol to synthesize ZnO nanostructures from zinc acetate by the well-known ester elimination reaction. In this system, the alcohol acts as a solvent
and reagent for the formation of ZnO from zinc acetate
through an “esterification alcoholysis reaction”, while the
OA acts as a surfactant and structure-directing agent. By
adjusting the relative amount of oleic acid to oleyl alcohol,
while keeping the total volume of the mixture constant, it
was possible to obtain a range of complex nanostructures
such as spiked ZnO nanostructures (Fig. 8(a)) and ZnO
tetrapods of various dimensions (Fig. 8(b) and (c)). By increasing the relative amount of oleyl alcohol and decreasing the relative amount of oleic acid, smaller-sized ZnO
tetrapods were obtained due to the increased reaction rateof the alcohol with the precursor, which results in a large
number of initial nuclei in a short time (Zhong et al., 2007).
Similarly, Zhang et al. also obtained Mn-doped ZnO tetrapods and multipods from bulk ZnO using OA and oleyl alcohol in the non-coordinating solvent 1-octadecene (Zhang
and Li, 2009). The use of other alcohols like 1,12dodecanediol and 1-hexadecanol with oleic acid and zincacetate yielded ZnO nanocones (Joo et al., 2005a) andnanotriangles (Andelman et al., 2005), respectively.
Oleic acid was also used as a capping agent without anyother additives in the non-polar solvent n-octadecene. In
this medium, previously prepared zinc oleate was used as aprecursor with further addition of oleic acid as the surfactant, and the mixture reacted under reflux. Interestingly,
defined triangular and hexagonal two-dimensional platelike structures were formed with prolonged reflux time
(Fig. 9(a)). The formation of these two-dimensional thinplate-like ZnO structures of about 10 nm in thickness is
attributed to the strong binding affinity of oleic acid to thechemically active Zn2+ terminated (0001) surface, which
prevents further growth in this direction (c-direction) (Chiuet al., 2008). While a longer synthesis time yielded moredefined shapes, it was also observed that a thinning of thecentral area of the ZnO platelets took place (Fig. 9(b)),
which was correlated to an etching activity of the boundcFig. 8 TEM images of (a) ZnO spiked clusters formed using a volume ratio of 1:2 oleic acid to oleyl alcohol, (b) large ZnO tetrapods formed using a
volume ratio of 1:3.5 oleic acid to oleyl alcohol and (c) small ZnO tetrapods formed using a volume ratio of 1:8 oleic acid to oleylamine while keeping
the total volume constant. Adapted with permission from Ref. (Zhong et al., 2007). Copyright 2007 Wiley-VCH.

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Sherif Okeil et al. / KONA Powder and Particle Journal No. 41 (2024) 197–220abFig. 9 TEM images of two-dimensional ZnO platelets (a) showing thedifferent triangular and hexagonal platelet structures formed after
90 min synthesis time and (b) an individual ZnO platelet formed after
120 min synthesis time showing a thinning effect in the center with increased synthesis time. Adapted with permission from Ref. (Chiu et al.,2008). Copyright 2008 Elsevier.

oleic acid. Thus, it was predicted that even longer reflux
times could yield ring-like ZnO structures (Chiu et al.,2008).
The use of ethylene diamine as a short-chain diamine
was reported to yield micrometer-long ZnO nanorods inalcohols (Panchakarla et al., 2007). Hexamethylenetetramine (HMTA) was also used as a surfactant for the synthesis of ZnO nanorods from Zn(acac)2 in the coordinating
solvent trioctylamine at a low temperature, where an increase in the surfactant-to-precursor ratio resulted inwell-defined nanorods with a higher aspect ratio. However,increasing the reaction temperature from 70 °C to 140 °C
while maintaining the HMTA to Zn(acac)2 constant resulted in a decrease in the aspect ratio, forming less anisotropic structures (Devarepally et al., 2012). Alternatively,
polyvinylpyrrolidone (PVP) in ethanol was reported to result in nanorods at a low temperature (80 °C) with good
crystallinity (Wang et al., 2005), showing that strongly coordinating ligands such as amines are not necessary fornanorod growth.
The formation of ZnO from zinc acetate and a hydroxide
is a typical precipitation reaction that can also be performed
in organic media, and as no water was added to the system,
this can still be interpreted as a non-aqueous synthesis. Alcohols like methanol and ethanol have been mainly used assolvents, where usually an alcoholic solution of NaOH or
KOH is added to a zinc salt dissolved in the same alcohol.
While ZnO nanorods have been mainly obtained at higher
temperatures (Cheng and Samulski, 2004; Pacholski et al.,
2002; Rai et al., 2013), interwoven ZnO nanosheets wereobtained when the reaction was performed at room temperature (Khokhra et al., 2015) which shows that the reaction temperature is also a crucial factor for tuning the shapeof the nanostructures obtained.

3.3 Effect of precursor type

The type of metal oxide precursor used also has a significant influence on the nanostructure obtained. Heating different zinc alkylcarboxylate precursors in TOP using

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a

c

b

d

Fig. 10 TEM micrographs of ZnO nanostructures formed from different zinc carboxylate precursors: (a, b) multiarmed ZnO nanostructures
formed of nanotetrahedrons obtained from (a) zinc acetate and (b) zinc
hexanoate, (c, d) squamous ZnO nanostructures produced from (c) zincoctanoate and (d) zinc oleate. The inset of (d) shows a schematic of thestructure of the squamous nanocrystals. Adapted with permission from
Ref. (Zhang Z. et al., 2006). Copyright 2006 American Chemical Society.

oleylamine as a surfactant results in various types of nanostructures. Zinc salts of short alkyl chain carboxylic acids,
such as zinc acetate and zinc hexanoate, resulted in theformation of multiarmed ZnO nanostructures formed of
nanotetrahedrons, while precursors containing longer-chain
carboxylates like zinc octanoate and zinc oleate produced“squama-like” (fish scale-like) nanostructures (Fig. 10)(Zhang Z. et al., 2006). In these non-aqueous synthesis
methods, the oleylamine attacks the carboxylates in anaminolytic reaction resulting in amide elimination and ZnO
formation. The change in morphology with precursor typewas attributed to the difference in the reactivity of the precursor to the oleylamine, thus modulating the rate of theaminolytic reaction. With an increasing chain length of the
carboxylate in the precursor, the steric hindrance increased,
resulting in a decrease of the aminolysis rate at the coordinated {110} planes but leaving the growth in the [001] direction unhindered. Thus, precursors containing
shorter-chain carboxylates formed nanotetrahedrons, while
the longer-chain carboxylate precursors formed squamouslike nanocrystals, where the strong binding affinity andsteric effects of the longer-chain carboxylates such as
oleate to the (110) plane leads to further suppression of thegrowth in the [110] direction, forming very thin “nanosquamas” (Zhang Z. et al., 2006).
In a separate study by Xu et al., different precursors also
resulted in a good variation of particle morphology whenused in tetrahydrofuran (THF). While zinc acetate resulted
in hexagonal-based pyramidal particles, using Zn(acac)2

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instead led to isotropic particles arranged in a cauliflowerlike morphology (Xu et al., 2009). The decomposition
temperature of each precursor in the same solvents alreadygives a clue about the different behavior of different precursors. Kunjara Na Ayudha et al. found that the solvothermal
synthesis of ZnO from zinc acetate in the non-polar solvents decane and toluene was not feasible at 250 °C, andthe synthesis temperature needed to be increased to 300 °C
to obtain ZnO nanorods (Kunjara Na Ayudhya et al., 2006).
On the other hand, it was possible to obtain ZnO from
Zn(acac)2 in decane and toluene at the much lower temperature of 120 °C. In that case, completely different morphologies were obtained where the synthesis in decane
resulted in truncated hexagonal pyramidal particles, while
in toluene hourglass-like particles were obtained (Xu et al.,2009).

3.4 Effect of precursor concentration

It is generally known from different reaction systems
that nanocrystal growth is highly dependent on the monomer concentration. A low monomer concentration usually
results in isotropic growth while a high monomer concentration results in an anisotropic growth profile due to thedifferent growth rates of the diverse crystal facets (Manna
et al., 2000; Peng and Peng, 2002). In the synthesis of ZnO
nanoparticles in a basic methanolic solution of zinc acetate
dihydrate, it was observed that by simply increasing the Zn
precursor concentration (10-fold compared to the concentration yielding spherical particles), anisotropic ZnO nanorods could be obtained (Pacholski et al., 2002). Similar
observations were also made for the microwave-assisted
synthesis of ZnO from zinc acetylacetonate in 1-butanol,
where an increase of the precursor concentration increased
the aspect ratio of the formed nanoparticles, resulting in
ZnO nanorods (Ambrožič et al., 2011).
While in some systems, no effect of the precursor concentration on the resulting morphology of the nanoparticles
was detected—although an increase of the overall particle
size was still observed (Ambrožič et al., 2010; Tonto et al.,2008)—the influence of precursor concentration was more
pronounced in other systems (Ambrožič et al., 2011;
Pacholski et al., 2002), which underlines the fact that it isan interplay of the precursor type used and overall synthesis conditions used. In this context, Ambrožič et al. observed a concentration-dependent morphology change
when using microwave irradiation, whilst a concentrationindependent behavior was observed for the same reaction
system using reflux conditions with conventional heating(Ambrožič et al., 2010; 2011).

3.5 Effect of doping

It was also observed that doping could affect the anisotropic growth of metal oxide nanoparticles. Co- andMn-doping of ZnO nanorods synthesized in benzyl alcohol

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was found to decrease the aspect ratio of the resulting ZnOnanorods, with the effect of Co-doping on the nanorod aspect ratio being more pronounced (Djerdj et al., 2008b).

3.6 Other factors

There are different non-trivial factors that can significantly affect the growth behavior of nanocrystals in the reaction system studied but they are usually overlooked.
While the effects of solvents, ligands or surfactants, typesof metal oxide precursors, solvent/surfactant/precursor ratios, reaction temperature and synthesis time are themost-studied factors, some other factors can still significantly affect anisotropic crystal growth. Zhao et al. investigated the influence of the mixing time of the precursor,
dicyclohexyl zinc, with the ligand, dodecylamine, on theanisotropic growth of ZnO particles (Zhao et al., 2021). A
mixing time of more than 6 hours before the start of theactual hydrolysis reaction was found to inhibit the anisotropic particle growth, which was ascribed to the increasing
viscosity of the reaction mixture with increasing mixing
time, subsequently hindering the anisotropic growth ofZnO through the oriented attachment mechanism (Zhao etal., 2021). Thus, whilst many different factors have been
identified that facilitate the synthesis of ZnO nanostructures with anisotropic morphologies, a particular reaction
system must always be understood in detail to enableproper control of the particle properties.

4. Iron oxide

Iron oxide is one of the most important magnetic materials, and iron oxide nanocrystals with defined properties areused in manifold applications such as magnetic storage
media and magnetic resonance imaging (MRI) (Ajinkya etal., 2020; Teja and Koh, 2009). Anisotropy can be used tomodulate the magnetic properties of such nanoparticles
where it was found that anisotropic iron oxide nanoparticles possess improved magnetic properties (Zhang et al.,2009). Tuning the shape of the iron oxide nanoparticles results in the exposure of different crystal facets that could be
used to vary the T1 and T2 proton relaxation time shortening
effects, which in turn is important for the design of efficient
MRI contrast agents (Zhou et al., 2015). Table S3 in the
supplementary material summarizes the state of the literature on the synthesis of anisotropic iron oxide nanocrystals
in non-aqueous media in chronological order.
Whilst typically, magnetite (Fe3O4) is the desired material, maghemite (γ-Fe2O3) shows similar ferromagnetic
behavior and is obtained under more oxidative conditions.
As magnetite particles may oxidize during extended storage, particularly in aqueous media, the direct synthesis ofmaghemite might thus provide a more stable product(Grabs et al., 2012; Masthoff et al., 2014). In non-aqueous
reaction systems for the synthesis of iron oxide, surfactants
are typically required to obtain particle shapes other than207

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spheres. The use of long-chain amines such as dodecylamine was found to result in the formation of large
hexagon-shaped γ-Fe2O3 particles from iron pentacarbonyl(Fe(CO)5) at a surfactant-to-precursor ratio of at least 10.
Lower ratios resulted in a mixture of spherical, triangularand diamond-shaped nanocrystals (Cheon et al., 2004).
Unique star-shaped Fe3O4 nanocrystals were obtained by
the use of the bulky 1-adamantanecarboxylic acid insteadof the linear oleic acid as the surfactant together witholeylamine (Fig. 11) (Zhang L. et al., 2006). It was proposed that the growth mechanism resulting in the anisotropic star shape is based on the bulkiness of the
1-adamantanecarboxylic acid surfactant molecules. Dueto their bulkiness, some sites were not covered by thesurfactant molecules and led to their further growth,which resulted in the observed spikes of the star-shaped
nanocrystals. Further substitution of oleylamine with thebulky 1-adamantaneamine so that the surfactant system
is composed of 1-adamantanecarboxylic acid and 1adamantaneamine instead of the traditional oleic acid/oleylamine surfactant system resulted in a certain anisotropy where irregular-shaped particles were obtained forming flower-like aggregates (Zhang L. et al., 2006). With aternary surfactant mixture of oleic acid, oleylamine andhexadecane-1,2-diol in a ratio of 3:3:5, it was even possible
to produce tetrapod-shaped maghemite nanocrystals from
Fe(CO)5 (Fig. 12) (Cozzoli et al., 2006). Increasing the
precursor concentration of resulted in an increase in anisotropic growth that led to longer tetrapod arms. It was also
found that a significant deviation from the optimized 3:3:5
ratio of the three-component surfactant system—or omitting any of these surfactants from the reaction system—
resulted in a loss of the tetrapod geometry, which shows theimportant role of all surfactants in this specified ratio in
forming the unique tetrapod geometry (Cozzoli et al.,2006). Analogous tripods, or shamrock-shaped particles, ofnickel manganese ferrite were interestingly obtained in amuch more facile synthesis without surfactants via an ori-aented attachment mechanism (Masthoff et al., 2015).
Hofmann et al. also investigated the influence of the surfactant ratio of the surfactant system of trioctylamine and oleic
acid on the morphology of iron oxide nanoparticles andfound that the nanocrystal shape is significantly influenced
by the surfactant ratio used. Increasing the trioctylamine tooleic acid ratio resulted in the formation of cubic wüstiteiron oxide nanocrystals with concave faces which were referred to as nanotetracubes (Hofmann et al., 2008).
Fatty acid surfactants such as oleic acid were used inseveral studies, often in the form of its oleate anion as partof the iron oxide precursor. Typically, iron(III) oleate is first
synthesized in a separate step and the purified precursor isthen mixed with the solvent and heated for the actual synthesis. In this case the oleate already present in the precursor can act as a surfactant and structure-directing agent.
Heating the iron oleate complex in 1-octadencene at
320 °C for one hour resulted in cubic iron oxide nanocrystals (Kwon et al., 2007). In another study, the controlled
decomposition of the iron oleate precursor at a low temperature of 150 °C in the same solvent formed iron oxide
nanowhiskers. At temperatures higher than 300 °C, spherical particles were formed, as commonly reported in the literature. Thermogravimetric analysis (TGA) and density
functional theory (DFT) calculations revealed the dissociation of two of the three oleate ligands from the complex at
150 °C, and the third bound oleate ligand was assumed toact as a structure-directing agent for the formation of the
one-dimensional nanowhiskers (Palchoudhury et al.,2011a). The addition of the weakly binding surfactant
TOPO to a system of iron oleate and oleic acid at a ratio of
6.3:1 (TOPO/OA) resulted in the formation of iron oxide
nanoworms, which was attributed to the controlled aggregation of spherical nanoparticles (Palchoudhury et al.,2011b).
A plethora of different iron oxide nanocrystal shapes
were accessed by the use of oleate salts instead of oleic acidas the surfactant. Although the oleate ligands in thebFig. 11 (a) TEM image of iron oxide nanostars obtained using thebulky 1-adamantanecarboxylic acid as a surfactant. The inset shows the
HRTEM image of a single iron oxide nanostar. (b) Schematic illustration showing the proposed formation mechanism of iron oxide nanostars. Adapted with permission from Ref. (Zhang L. et al., 2006).
Copyright 2006 Elsevier.

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Fig. 12 TEM image of tetrapod maghemite nanocrystals formed
through a 3:3:5 surfactant ratio of oleic acid, oleylamine and hexadecane-1,2-diol. The inset shows a HRTEM image of a single tetrapod
nanocrystal viewed along the <111> zone axis. Adapted with permission
from Ref. (Cozzoli et al., 2006). Copyright 2006 American Chemical
Society.

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precursor can act as a surfactant, further addition of oleic
acid and sodium oleate to the reaction mixture has a significant influence on the shape evolution of the iron oxide
nanocrystals. Kovalenko et al. observed that replacing thesurfactant oleic acid with its sodium or potassium salt in areaction system containing iron oleate as a precursor resultsin the formation of cubic instead of spherical iron oxide
nanocrystals. Decreasing the sodium oleate-to-iron oleate
precursor ratio even resulted in bipyramidal nanocrystals.
From conductivity measurements, it was inferred that theoleate salts in octadecene dissociate at temperatures above
220–230 °C, resulting in ionic oleate species, which is not
the case for oleic acid; this difference was related to thedifferent shape evolution (Kovalenko et al., 2007). Similarly, Bao et al. synthesized iron oxide nanorods from ironoleate in benzyl ether by replacing oleic acid with sodium
oleate as surfactant, whereby the aspect ratio of the productcould be carefully tuned by varying the synthesis temperature (Bao et al., 2012).
A broad range of different iron oxide nanocrystal shapes
was also synthesized by Gao’s group through variation of
the sodium oleate-to-iron oleate molar ratio and using
1-octadecene or trioctylamine as a solvent (Fig. 13) (Zhouet al., 2015). Increasing this ratio in 1-octadecene resulted
in an increased exposure of the {111} facets, which led tothe shape evolving from spherical particles (in the absenceof sodium oleate) to hexagonal plates, truncated octahedrons, and finally to tetrahedrons (Fig. 13(b)–(d)). Accordingly, it was assumed that the oleate ions preferentiallyattached to the {111} facets, thus stabilizing them and retarding their growth, which resulted in an increased exposure of the {111} facets with an increasing sodium oleate
amount. Using trioctylamine as the solvent, in which

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higher synthesis temperatures are possible, iron oxide
nanocubes (Fig. 13(e)) were obtained at low sodium
oleate-to-iron oleate ratios (NaOL/FeOL ≤ 1:10), while at aratio of 2:10, spikes started to emerge at the cube corners
forming what the authors called “concave” nanocrystals(Fig. 13(f)). A further increase of the sodium oleate amount(NaOL/FeOL = 5:10) resulted in assembled nanostructures
which were supposedly formed through the oriented attachment of smaller building blocks (Fig. 13(g)). Using
low NaOL to FeOL ratios (NaOL/FeOL = 1:10) that previously resulted in nanocubes formed multibranched ironoxide nanoparticles when higher dilutions were used (i.e.
lower concentrations of NaOL and FeOL in the system)(Fig. 13(h)) (Zhou et al., 2015).
In addition, the use of long-chain quaternary ammonium
salts such as tetraoctylammonium bromide was found to
induce anisotropic iron oxide nanocrystal growth where
nanooctahedra were obtained from the iron oleate precursor. In this system, the authors also addressed the fact thatchemical species formed in situ during the synthesis canplay an important role in determining the shape of the synthesized nanoparticles (Shavel et al., 2009).
Besides the amount of surfactant or the surfactant-toprecursor ratio, some other non-trivial factors were described to influence the anisotropic growth during the
non-aqueous and non-hydrolytic synthesis. Recently, theeffect of nitrogen purging on the anisotropic growth of ironoxide nanoparticles was investigated by AbuTalib et al.
(2021). Through testing different configurations for nitrogen flow, it was found that bubbling the reaction mixture
with nitrogen induced anisotropic growth in their system,
while applying nitrogen flow over the reaction mixture reduced the anisotropic growth, and finally the use of only a

a

b

c

d

e

f

g

h

Fig. 13 (a) Overview of the different iron oxide particle morphologies obtained using iron oleate as a precursor and sodium oleate as a surfactant;
TEM images showing iron oxide (b) nanoplates, (c) truncated octahedrons, (d) tetrahedrons obtained in 1-octadecene and (e) nanocubes, (f) concave
particles, (g) assembled structures and (h) multibranched nanocrystals obtained in trioctylamine using different sodium oleate-to-iron oleate ratios.
Adapted with permission from Ref. (Zhou et al., 2015). Copyright 2015 American Chemical Society.

209Sherif Okeil et al. / KONA Powder and Particle Journal No. 41 (2024) 197–220“nitrogen blanket” (i.e. positive pressure of nitrogen in thesystem without steady flow) favored isotropic crystal
growth. The nitrogen flow rate did not play a significant
role in the extent of anisotropic growth. It is assumed thatthe reason for this anisotropy observed in the nitrogenbubbling system arises from increased turbulence, the bubbles acting as nucleation sites and inducing aggregation, or
the improved removal of volatile by-products from thesystem which otherwise influence the particle morphology(AbuTalib et al., 2021). Additionally, a significant influenceof the amount of the oleylamine surfactant relative to theFe(acac)3 precursor on the morphology of the product was
found in this system. While at very high oleylamine-toprecursor ratios (above 40 equivalents), isotropic particles
were observed, decreasing the oleylamine amount to 25
equivalents resulted in anisotropic nuclei which formed
iron oxide nanocrystals with two branches (bipods). A further decrease in the oleylamine amount to 17 equivalents
resulted in multiply branched nanocrystals and finallybranched nanocrystals with a flower-like appearance at 15
equivalents of oleylamine. An investigation of the evolution of the branched iron oxide nanocrystals over time revealed the formation of small anisotropic nuclei in mostcases, which further aggregate in the initial stages of thesynthesis forming polycrystalline nanocrystals. The defects
and misalignments in the formed aggregates are then removed through rearrangement and ordering over the courseof the reaction, and finally monocrystalline branched ironoxide nanocrystals are obtained.
Prolongation of the synthesis time also has an influenceon the product morphology. While the decomposition ofiron oleate in 1-octadecane at 320 °C results in the formation of monodisperse spherical nanocrystals for a synthesistime up to 30 minutes (Park et al., 2004a), an increase of
the synthesis time to one hour resulted in monodisperse
cubic nanocrystals (Kwon et al., 2007). The spherical ironoxide nanocrystals are converted to cubic nanocrystals
upon prolonged aging at 320 °C, forming the thermodynamically most stable shape for the iron oxide spinel structure with only {100} planes exposed (Kwon et al., 2007). Achange of shape with reaction time was also observed
during the synthesis of iron oxide from iron(II) acetate intrioctylamine using oleic acid as the surfactant at 255 °C.
While at shorter synthesis times (10–25 min) cubic nanocrystals were formed, the particles transformed into truncated octahedrons when the reaction time was prolonged
further (Redl et al., 2004).
Two-dimensional iron oxide nanocrystals obtained from
non-aqueous and non-hydrolytic synthesis are still rare andthe parameters and conditions required for their synthesis
are still not well understood. Examples of such twodimensional iron oxide nanocrystals were presented by
Casula et al., where circular nanodisks were obtained fromiron pentacarbonyl in octyl ether using tridecanoic acid as

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the surfactant (Casula et al., 2006). It is important to notethat the disk form of the obtained nanoparticles has been
confirmed through AFM height measurements. In general,
even if TEM investigations reveal a spherical nanocrystal
appearance, it is important to investigate the true height of
the nanoparticles to prove their isotropy or anisotropy, but
this is usually not performed in most of the publishedworks so far. In another work, hexagonal and triangulariron oxide nanoplates were obtained in diethylene glycol
from iron(III) chloride without the use of any surfactant(Zhang et al., 2009).

5. Other metal oxides

A number of other metal oxides and mixed-metal oxides
have been synthesized as anisotropic nanocrystals via
non-aqueous synthesis routes. As there are only a few reports about each, they are discussed in this section in combined form. Table S4 in the supplementary material
provides an overview of the materials and morphologies
obtained and the respective synthesis conditions. In thefollowing paragraphs, some more insight will be given forsome of these reaction systems.
Zirconia nanocrystals synthesized in benzyl alcohol using sodium lauryl sulfate as a surfactant showed onedimensional growth when ZrOCl2∙8H2O was used as the
precursor, while Zr(SO4)2·H2O yielded only isotropic
nanoparticles (Siddiqui et al., 2012). The reaction of zirconium n-propoxide in benzyl alcohol was reported to lead tofractal nanostructures, in particular at lower reaction temperatures and extended reaction times (Stolzenburg et al.,2016).
The synthesis of V2O3 from VOCl3 in benzyl alcohol led
to ellipsoidal-shaped nanorods (Niederberger et al., 2002b;
Ohayon and Gedanken, 2010), whereby doping with therare earth metals Gd and Nd resulted in an alteration of themorphology to nanoflakes and nanocubes, respectively(Venkatesan et al., 2015).
Due to the great potential of tungsten oxide in different
fields, such as for batteries, gas sensing, as well as photocatalytic and electrochromic applications, different
non-aqueous synthesis procedures for tungsten oxide nanorods or nanowires have been developed (Choi H.G. etal., 2005; Lee et al., 2003; Polleux et al., 2006; Woo et al.,2005). Woo et al. conducted a systematic study which revealed that the aspect ratio of tungsten oxide nanorods
could be tuned through the coordinating power of the surfactant system used. Increasing the coordinating power was
thereby found to decrease the tungsten oxide nanorod
length, which was attributed to the increased inhibition of
growth in the [010] direction resulting from the stronger
ligand binding (Woo et al., 2005). In a different approach,
Choi et al. were able to tune the aspect ratio of tungsten
oxide nanorods by adjusting the precursor concentration inan ethanolic solution, without the need for any surfactant.

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Using lower WCl6 precursor concentrations resulted inhigher aspect ratios and even in the growth of tungsten oxide nanowires (Choi H.G. et al., 2005). To obtain more
complex structures, Zhao et al. added urea to the ethanolic
WCl5 precursor solution to act as a ligand and structuredirecting agent due to its strong hydrogen bonding ability.
This enabled the formation of tungsten oxide nanotubes
using a template-free non-aqueous synthesis for the firsttime, whereby the as-synthesized product was identified astungstic acid hydrate (H2W1.5O5.5·H2O) and was transformed to WO3 through calcination without loss of thehollow structure (Zhao and Miyauchi, 2008). Variation of
the synthesis parameters, such as the urea amount and reaction time, yielded further hollow tungstic acid hydrate
nanostructures (hollow spheres and boxes), which wereconverted to tungsten trioxide upon calcination. Interestingly, a broad range of morphologies was obtained thereby,
which was attributed to different self-assembly processesof smaller nanoparticles and nanorods due to the presenceof urea in the reaction system (Zhao and Miyauchi, 2009).
There has also been an interest in synthesizing manganese oxide due to its magnetic properties, especially at thenanoscale (Park et al., 2004b). Uniform MnO nanorods
were obtained by Park et al. through injecting a previously
prepared manganese oleylamine complex into trioctylphosphine or triphenylphosphine at high temperatures (330 °C)(Park et al., 2004b). Zitoun et al. were able to prepare MnO
multipods via the decomposition of Mn(oleate)2 in ntrioctylamine in the presence of oleic acid at 320 °C, where
the arms showed an arrow-like ending and a zigzag structure which indicated an oriented attachment process of theinitially formed cube-shaped or truncated octahedral nuclei(Zitoun et al., 2005). Similar anisotropic MnO nanoparticle
shapes termed as “dumbbell-shaped” MnO nanocrystals
were obtained by Zhong et al. through the use of manganese formate hydrate as a precursor (Zhong et al., 2006).
Anisotropic shapes for manganese cobalt oxide (MnCoOx)
ternary mixed-metal oxides were obtained through the decomposition of Mn(oleate)2 and Co(oleate)2 precursors in
1-octadecene in the presence of oleic acid (Gliech et al.,2020). Interestingly, the use of a single metal oxide precursor in this system yielded cubic or octahedral shapes for
manganese oxide and aggregated octahedral shapes forcobalt oxide, while for the mixed-metal oxide, rod-, T-,cross- and hexapod-shaped nanocrystals were obtainedwith decreasing cobalt concentrations. A new solutionsolid-solid (SSS) oxide mechanism was suggested for the
anisotropic growth of the mixed-metal oxide nanoparticles,
which is a good example of the influence of other metal
ions on the anisotropic growth of metal oxides. In the suggested SSS growth mechanism, first MnO nuclei areformed, followed by the deposition of Co monomers toform Co-rich regions which serve as sites for outward
growth in the <100> directions, thus forming the arms. At

Review Paper

low Co concentration, the Co has time to accumulate on
different facets of the MnO nuclei, thus forming several
starting points for the branching, resulting in multiarmed
structures such as hexapods. Increasing the Co concentration decreases the cobalt accumulation time and lessbranched structures occur, thus leading to nanorod structures at high concentrations (Gliech et al., 2020).
Even more exotic metal oxides such as actinide oxides
have been synthesized using the non-aqueous and nonhydrolytic synthesis route. In several cases, the use of certain surfactants and certain synthesis conditions yielded
anisotropic shapes for those oxides. For example, anisotropic thorium oxide (ThO2) has been obtained from thorium
acetylacetonate in benzyl ether using a ternary surfactant
mixture of TOPO, OA and trioctylamine at 280 °C (Hudry
et al., 2012).

6. Oriented attachment mechanisms
resulting in anisotropic structures

Different mechanisms have been reported to potentiallylead to the formation of anisotropic nanoparticles or nanostructures such as controlled aggregation (for example via
oriented attachment), etching and branching (AbuTalib etal., 2021). According to the classical crystallization model,
anisotropic growth in metal oxides can be mainly achieved
via kinetic control through the use of surfactants or by theselective formation of nuclei of specific crystal phases featuring different surface energies (Jun et al., 2006; Sajanlal
et al., 2011). Besides the classical crystallization theory,oriented attachment has been proven to represent an important pathway to result in anisotropic metal oxide nanocrystals.
The oriented attachment mechanism is based on thecontrolled aggregation of nanoparticle building blocks
showing an aligned crystal orientation towards each otherand thus, in principle, leads to one larger crystal. Thereby,
isotropic metal oxide nanoparticles can also be used asbuilding blocks for larger anisotropic and more complex
three-dimensional structures. This process can be observedin the absence of organic surfactants which delimits thisprocess from usual self-assembly phenomena that occur
due to organic building blocks. This phenomenon was
identified to represent an important non-classical crystal
growth mechanism and was found in many systems including non-aqueous and/or non-hydrolytic metal oxide synthesis (Chen et al., 2022; Pacholski et al., 2002; Xue et al.,2014). Kinetic models describing the oriented attachment
process have been developed by Ribeiro et al. (2005;
2006).
The general mechanism for the formation of anisotropic
nanoparticles or nanostructures from smaller (usually isotropic) nanoparticles was identified to consist of the following steps: 1) Collisions of nanoparticles leading to theirloose attachment where sometimes the crystal planes are211Sherif Okeil et al. / KONA Powder and Particle Journal No. 41 (2024) 197–220

already aligned, 2) Rotation of the attached nanoparticles
takes place to eliminate some misorientation, 3) Smoothening of the surface occurs due to further monomer additionand eventually Ostwald ripening (Fig. 14) (Lee et al., 2005;
Xue et al., 2014). Interestingly, this process can occur insome systems even at room temperature without the addition of specific ligands (Leite et al., 2003), while higher
temperatures usually enhance the particle coalescence andthus the oriented attachment process (Lee et al., 2005). The
presence of some crystal defects or misalignments such as
twinning, stacking faults or dislocations in the final productcan give a hint on the crystal evolution through the oriented
attachment mechanism (Chen et al., 2022). The oriented
attachment process can even give rise to anisotropic growth
directions that are generally not accessible through tradi-tional monomer deposition or classical crystal growth (Leeet al., 2005; Leite et al., 2003). The use of selective surfactants can facilitate and direct the oriented attachment process and inhibit Ostwald ripening (Xue et al., 2014). This
makes the oriented attachment process a very effective
mechanism for achieving novel and even more complex
anisotropic structures.
A good example showing the formation of anisotropic
zinc oxide nanorods from quasi-spherical ZnO nanoparticles via oriented attachment in non-aqueous systems waspresented by Pacholski et al. (2002). Thereby, a sol of theZnO nanoparticles previously prepared from a dilute basic
methanolic solution of zinc acetate dihydrate (Fig. 15(a))
was first concentrated through solvent evaporation. Refluxing of the resulting sol resulted in ZnO nanorods where thedimensions of the nanorods increased with increasing reflux time, yielding excellent monocrystalline ZnO nanorods after one day (Fig. 15(b)). Sampling at different
time intervals showed the arrangement of the quasispherical particles into chains where neck formation andcoalescence with excellent alignment of the particle crystal
planes was observed in high-resolution TEM images(Fig. 15(c)–(g)) (Pacholski et al., 2002). Similar observations were made by Peukert et al. through an in-depth investigation of ZnO nanorod formation kinetics in the same

Fig. 14 Schematic showing the collision, attachment and rotation steps
involved in the oriented attachment process. Adapted with permission
from Ref. (Leite et al., 2003). Copyright 2003 American Institute ofPhysics.

acReview Paper

b

d

e

f

g

Fig. 15 TEM images of (a) quasi-spherical ZnO nanoparticles used as starting sol for the synthesis of ZnO nanorods, (b) final monocrystalline ZnO
nanorods formed by oriented attachment of the quasi-spherical ZnO particles, (c) sample of the reaction mixture after 2 h showing some ZnO nanorods
and quasi-spherical ZnO particles arranged in shorter and longer chains, (d–g) high-resolution TEM micrographs of differently attached particles
showing neck formation and coalescence with aligned crystal planes. Adapted with permission from Ref. (Pacholski et al., 2002). Copyright 2002
Wiley-VCH.

212

Sherif Okeil et al. / KONA Powder and Particle Journal No. 41 (2024) 197–220

Review Paper

Fig. 16 Schematic showing the steps involved in the temporal evolution of ZnO fan- and bouquet-like nanostructures in benzyl alcohol. Adapted withpermission from Ref. (Ludi et al., 2012). Copyright 2012 The Royal Society of Chemistry.

system (Voigt et al., 2010). Buha et al. observed the formation of hexagonal ZnO nanoplatelets from Zn(acac)2 in
acetonitrile, which was identified in high-resolution TEM
to be a mesocrystal formed of perfectly aligned crystal domains, so that the final hexagonal nanocrystal behaves as asingle crystal (Buha et al., 2007). Similarly, the formationof hexagonal Al-doped ZnO (AZO) in benzylamine wasobserved and investigated via small-angle X-ray scattering(SAXS), where time-resolved analysis revealed oriented
aggregation forming hexagonal mesocrystals (Ungerer etal., 2019; 2020). Oriented attachment was also observed
for the formation of TiO2 nanorods when the reaction system of Joo et al. (2005b) (synthesis of TiO2 from TIP using
oleic acid as a surfactant, Table S1) was reinvestigated byDalmaschio and Leite, where even the detachment ofwell-faceted nanoparticles was observed for longer reaction times and this was interpreted as Rayleigh instabilityof the TiO2 originally formed through oriented attachment(Dalmaschio and Leite, 2012). Although Joo et al. (2005b)
already observed the presence of spherical nanoparticles
along with the synthesized TiO2 nanorods, the reason behind their presence in the product was not further investigated.
Oriented attachment can also be the reason for the formation of ordered aggregates as was observed in the formation of ZnO nanorod bundles (Zhang et al., 2007). Morecomplex geometries could even be obtained due to oriented-attachment mechanisms such as hollow hexagonal ZnO
mesocrystals which form via the oriented attachment oftrapezoidal ZnO nanosheets (Wang et al., 2013), interwoven ZnO nanosheets (Khokhra et al., 2015) and fan- andbouquet-like ZnO nanostructures (Ludi et al., 2012).
Khokhra et al. proposed that the aggregation of ZnOnanosheets to form an interwoven structure was induced by
the interaction of the solvent used with the adsorbed hy-

droxide and acetate ions on the surface of the ZnO nanocrystals via hydrogen bonds (Khokhra et al., 2015). In adetailed study, the temporal evolution of the fan- and bouquet-like ZnO nanostructures was followed from platelet-like ZnO nanoparticles formed in benzyl alcohol(Fig. 16). Overlapping stages of classical nucleation, oriented attachment followed by further growth and surface
smoothening could be observed in this reaction system(Ludi et al., 2012).

7. Benzyl alcohol route for anisotropic
particle growth

The “benzyl alcohol route” has emerged as a unique
non-aqueous synthesis route for multiple metal oxides. The
advantage of this route lies in the fact that it is mainly asolvent-directed route which makes it possible to synthesize metal oxides and control their morphology through thevariation of different experimental parameters without theneed of any surfactants. This is achieved via the coordinating ability of benzyl alcohol to the metal oxide surface,
however, compared to surfactants, a relatively low amountof surface organics results, rendering a “clean surface” thatis accessible for small molecules, which is important for
catalytic or sensing applications (Clavel et al., 2007; Pinna
and Niederberger, 2008).
Benzyl alcohol can act as a solvent, reducing agent andstructure-directing agent for many metal oxides (Pinna etal., 2011; Qamar et al., 2017). The fact that it can also act
as a reducing agent was successfully applied to synthesize
several non-stoichiometric metal oxide nanoparticles,
which is very useful for catalytic applications where these
defects can act as catalytic centers (Qamar et al., 2017).
The structure-directing properties of benzyl alcohol are revealed in the formation of different anisotropic metal oxides. Qamar et al. synthesized a series of non-stoichiometric213

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metal oxides (TiOx, ZnOx, SnOx, CeOx, In2Ox and Ga2Ox) ata high temperature of 245 °C, where for several metal oxides, a square or rectangular plate-like morphology was
obtained (Qamar et al., 2017). ZnO nanorods were successfully synthesized in benzyl alcohol from Zn(acac)2 hydrate
without the need of any surfactant (Djerdj et al., 2008b).
Moreover, Al-doped ZnO formed with a rod-like shape inbenzyl alcohol, in contrast to the synthesis in benzylamine
where no elongated growth was observed (Zellmer et al.,2015). Similarly, TiO2 nanorods were obtained in benzyl
alcohol from TiCl4 at a temperature as low as 70 °C(Abazović et al., 2008; 2006). While shorter synthesistimes of some hours up to one day yielded spherical anatase
TiO2 nanoparticles (Garnweitner and Grote, 2009;
Niederberger et al., 2002a), longer synthesis times of threedays resulted in the formation of nanorods (Abazović et al.,2008; Jia et al., 2009). The use of TIP as a precursor together with minor amounts of silanes in the benzyl alcoholroute yielded platelet-like TiO2 particles (Koziej et al.,2009), while the use of OLA as a surfactant even resulted in
two-dimensional TiO2 nanosheets (Wu et al., 2008). Interestingly, the analogous synthesis in benzylamine also results in nanosheets that are evenly stacked, proving thetendency of amines to guide the reaction towards the formation of 2-D structures (Garnweitner et al., 2008).
Zirconia nanocrystals synthesized in benzyl alcohol using sodium lauryl sulfate as a surfactant showed onedimensional growth when ZrOCl2∙8H2O was used as the
precursor while Zr(SO4)2∙H2O yielded only isotropic
nanoparticles (Siddiqui et al., 2012). The reaction of zirconium n-propoxide in benzyl alcohol was reported to lead tofractal nanostructures, in particular at lower reaction temperatures and extended reaction times (Stolzenburg et al.,2016).
In several cases, the synthesis of metal oxides via the
benzyl alcohol route yielded nanoparticles of narrow size
distribution together with a surprising tendency of forming
well-defined aggregates, sometimes even with twodimensional or complex three-dimensional anisotropic
morphologies. In situ formed organics were often judged toexert a pronounced effect on the nanoparticle and aggregate
morphology formed. An in-depth analysis of the organics
present on the particle surface as well as organic byproducts in the reaction mixture revealed the in-situ formation of benzoate species that adsorb on the nanocrystal
surface and thus act as a capping and structure-directing
agent (Pinna et al., 2005; Pucci et al., 2012). In anotherstudy, the formation of WO3 nanowire bundles was attributed to the partial oxidation of benzyl alcohol to benzaldehyde which contributed to the formation of nanowire
bundles as an inorganic-organic hybrid nanostructure(Polleux et al., 2005; 2006).
The use of additives other than surfactants could also
have a significant effect on the formed nanostructures in the214

Review Paper“benzyl alcohol route”, giving access to novel shapes. For
example, the addition of HCl to Ti(OBu)4 in benzyl alcohol
enabled the formation of walnut-like aggregate structures
composed of rutile-phase nanorods as building blocks. An
increase in the amount of HCl added allowed tuning thesize of the nanorods and thus, of the formed aggregate
structure. It was proposed that the presence of chloride ions
supports the anisotropic growth of rutile TiO2 through selective adsorption to the (110) plane of rutile TiO2 suppressing its growth, thus favoring the growth in the [001]direction (Kim et al., 2013). In the synthesis of tungsten
oxide from WCl6 in benzyl alcohol, the formation of tungstite (WO3∙H2O) nanoplatelets was observed. Upon the
addition of the siderophore deferoxamine mesylate as anadditive, a fundamental change in the morphology of theresulting tungsten oxide was observed. Tungsten oxide
nanowires were obtained, which were arranged in bundles
to form inorganic-organic hybrid superstructures (Polleux
et al., 2005).

8. Summary and outlook

In this article, the different non-aqueous and/or nonhydrolytic reaction systems used for the synthesis of simple
and complex anisotropic metal oxide nanocrystals andnanostructures were summarized. Many examples for themost common metal oxides, in particular titania, zinc andiron oxide as well as for more specialized systems weresummarized, showing that there are many parameters controlling the resultant morphology. Moreover, the common
theories resulting in anisotropic crystal growth in these
non-aqueous systems were elaborated. While some generalmechanisms and concepts have been developed for the
anisotropic crystal growth in non-aqueous media, some reaction systems are still found to possess some uniquefeatures and mechanisms for anisotropic nanostructureformation, such as unique aggregation behavior, and need
to be investigated in detail for each system on its own.
To date, the synthesis of anisotropic metal oxides has
still largely focused on low-dimensional nanoparticles, andmore complex anisotropic structures remain relatively uncommon and need further investigation of the reaction parameters that can yield such structures in a single step or
through multi-step approaches. It should be noted that
anisotropic metal oxide nanoparticles or nanostructures canbe formed directly through either the “truly” anisotropic
growth of a single crystal or via an oriented-attachment orcontrolled-aggregation process, usually leading to polycrystalline anisotropic structures or mesocrystals with certain misalignments and defects. A deeper understanding of
the factors governing the controlled aggregation and oriented attachment of nanoparticles in non-aqueous media
would help to engineer more complex anisotropic metaloxide nanostructures formed from smaller isotropic or
anisotropic building blocks.

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Different parameters have been identified to play an important role in anisotropic growth, where surfactants represent a cornerstone in regulating anisotropic particle growth.
However, the solvent can also fulfill this function and additionally act as a structure-directing agent. Moreover, theprecursor type and its ratio to the solvent, as well as possibly the surfactant, can influence the growth processes andinduce the formation of anisotropic structures. This can be
attributed to specific precursor decomposition mechanisms, as well as the presence of certain counter ions or ligands from the precursor in the system. The precursor may
also contain water from crystallization which can also influence the nanoparticle formation process in various ways.
It has to also be taken into consideration that side products
formed during the non-aqueous synthesis can coordinate to
the nanoparticle surface, resulting in strong modification of
the nanoparticle growth and triggering aggregation, eventhough they might only be formed in very small amounts.
The reaction temperature is another substantial factor that
affects nanocrystal nucleation and the subsequent reaction
rate, which can result in different crystal growth mechanisms. While most of the studies concentrate on factors
such as the surfactant used, surfactant concentration, reaction solvent and metal oxide precursor, attention should
also be paid to several uncommon factors which need morein-depth investigation in non-aqueous systems such as theheating rate, cooling rate, mixing of reaction components,
rate and order of reagent addition, hot-injection of the precursor, etc.
As presented in this contribution, the synthesis of metaloxides in organic media has been intensively studied in theliterature, while the preparation of other metal oxides has
been only rarely investigated in non-aqueous media. It is
important to note that different metal oxide precursors exhibit different reactivity and reaction kinetics, even in similar reaction media, which makes it important to study theshape evolution of each metal oxide in non-aqueous media
separately. There is still a high potential for the development of non-aqueous and non-hydrolytic synthesis routes
capable of synthesizing various anisotropic metal oxide
nanostructures because of the almost infinite possibilities
of combining different organic reaction media, metal oxide
precursors and surfactants, as well as varying the corresponding reaction parameters.

Supplementary information

The online version contains supplementary material
available at https://doi.org/10.14356/kona.2024014.

Acknowledgments

The authors acknowledge funding by the German Research Foundation (Deutsche Forschungsgemeinschaft,
DFG), grants GA 1492/9-2 and NI 414/24-2.

Review Paper

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Review Paper

Authors’ Short Biographies
Sherif Okeil
Sherif Okeil is currently working as a competence team leader for nanomaterial synthesis in Georg
Garnweitner’s group at the Institute for Particle Technology, Technical University of Braunschweig.
He received his bachelor’s degree in Pharmacy at Ain Shams University in Cairo, Egypt in 2009. In
2013, he obtained his masters in Pharmaceutical Analytical Chemistry from the same university. In
2014, he was awarded a scholarship from the German Academic Exchange Service (DAAD) throughwhich he obtained his Ph.D. in Inorganic Chemistry at the Technical University of Darmstadt in2020. His current research interests are inorganic nanomaterial synthesis and their electronic applications.

Julian Ungerer
Julian Ungerer obtained his bachelor’s degree in applied mechanics in 2013 and his master’s degree in the process engineering machines group of Prof. Dr.-Ing Hermann Nirschl at the Karlsruhe
Institute of Technology in 2016, where he also continued his academic research in the field of functional nanomaterials for renewable energy systems before receiving his Ph.D. in 2022.

Hermann Nirschl
Hermann Nirschl received his Ph.D. in Fluid Mechanics from the Technical University of Munich
in 1994. For his habilitation in 1997 he worked on the numerical simulation of the particle-laden
flows. He entered the 3M company in the dental division as the head of process engineering in theyears between 1997 and 2002 where he worked as a project manager for different projects in Munich
and St. Paul/Minnesota. Since 2003 he is Professor for Mechanical Process Engineering at the KIT
in Karlsruhe. The focus of the research is on particle technology with a special emphasis on separation processes, numerical simulations and the development of particle analysis technologies.

Georg GarnweitnerGeorg Garnweitner received a Diploma in Technical Chemistry at Vienna University of Technology, Austria, in 2003 and then moved to the Max Planck Institute of Colloids and Interfaces, Potsdam, Germany, where he obtained his Ph.D. in Colloids Chemistry in 2005. He was appointed as
Professor at Technische Universität Braunschweig, Germany, in 2007. His research is centered on thesynthesis of inorganic nanoparticles and hybrid materials, in particular via nonaqueous approaches.
In addition, the surface modification and functionalization of nanomaterials are studied in his group,targeting diverse application fields such as solid-state and lithium-sulfur batteries, lightweight composites and drug delivery.

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