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REVIEW

Cite this: J. Mater. Chem. A, 2023, 11,10473

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Prussian blue and its analogues as functional
template materials: control of derived structurecompositions and morphologies
Behnoosh Bornamehr, ab Volker Presser, *abc Aldo J. G. Zarbin,
Yusuke Yamauchi *ef and Samantha Husmann *a*dHexacyanometallates, known as Prussian blue (PB) and its analogues (PBAs), are a class of coordination
compounds with a regular and porous open structure. The PBAs are formed by the self-assembly of
metallic species and cyanide groups. A uniform distribution of each element makes the PBAs robust
templates to prepare hollow and highly porous (hetero)nanostructures of metal oxides, sulﬁdes, carbides,
nitrides, phosphides, and (N-doped) carbon, among other compositions. In this review, we examine
methods to derive materials from PBAs focusing on the correlation between synthesis steps andReceived 6th December 2022
Accepted 17th February 2023

derivative morphologies and composition. Insights into catalytic and electrochemical properties resulting
from diﬀerent derivatization strategies are also presented. We discuss challenges in manipulating the

DOI: 10.1039/d2ta09501g

derivatives' properties, give perspectives of synthetic approaches for the target applications and present

rsc.li/materials-aan outlook on less investigated grounds in Prussian blue derivatives.

1. Introduction
Prussian blue (PB) is a well-known classic coordination
compound that has risen from an old painting pigment toa widely studied electrode material.1 PB and its analogues(PBAs) combine the presence of redox-active metallic species
and an open porous structure that can accommodate ions withlittle or no volumetric change. Such characteristics are attractive
for energy storage, catalysis, sensing, and biomedical applications, among others.2–5 Recently, PBAs have gained growinginterest as a template material for preparing porous derivatives.6,7 By providing metal species, carbon, and nitrogen fromcyanide ligands combined with a well-structured, stable, andhighly porous framework, they enable sacricial templating (ortemplate-engaged/self-template) methods. Templating is a very
eﬀective tool to synthesize more complex porous and hollow
structures, and many diﬀerent (hetero)nanostructures derived

INM – Leibniz Institute for New Materials, Campus D2 2, 66123, Saarbrücken,
Germany. E-mail: volker.presser@leibniz-inm.de; husmann.samantha@gmail.com

ab

Department of Materials Science & Engineering, Saarland University, Campus D2 2,66123, Saarbrücken, GermanycSaarene – Saarland Center for Energy Materials and Sustainability, Campus C4 2,66123 Saarbrücken, GermanydDepartment of Chemistry, Federal University of Paraná (UFPR), CP 19032, 81531-980
Curitiba, PR, Brazil. E-mail: aldozarbin@ufpr.bre

School of Chemical Engineering and Australian Institute for Bioengineering andNanotechnology (AIBN), The University of Queensland, Brisbane, QLD 4072,
Australia. E-mail: y.yamauchi@uq.edu.aufInternational Center for Materials Nanoarchitectonics (WPI-MANA), National
Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan

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from PBAs have already been described.8–10 Calcination
methods produce oxides,11–13 while annealing generates
carbides,14,15 alloys,16,17 or metal nanoparticles with diﬀerent
compositions.18–22 Pre-treatments can regulate the morphologyof the derivatives or introduce heteroatoms.22–24
Nevertheless, fast and simultaneous reports within the past
few years have led to many PBA derivative (hereaer referred toas PBDs) compositions and morphologies with little control
over the product, where even diﬀerent derivatives are obtained
under similar processing conditions. This lack of understanding of how PBA characteristics and the derivatization
process aﬀect the composition, structure, and morphology of
PBDs, makes the control and prediction of the nal producta current challenge. Despite that, PBDs perform very well inelectrocatalysis, batteries, and capacitors, far surpassing counterparts obtained by diﬀerent methods. A comprehensive
summary of the PBA derivatization methods and correlation
with PBD composition, morphology, and properties is particularly interesting in this context.
Most reviews on PBDs focus on specic applications such asenergy storage,6,9,10,25 catalysis,10,26 and the environment.9
Nevertheless, very few describe the synthetic methods to obtain
PBDs; noteworthy are the compilation of Fe-based PBDs
prepared by Li et al.,13 and the revisions of Zakaria et al.27 andAzhar et al.28 that largely focus on PBA modication prior toderivatization. The growing chemical compositions and structure complexity described for PBDs require an overview of thesynthesis routes and steps and the approaches utilized for eachcomposition group (oxides, suldes, carbides, etc.).

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In this review, we introduce the concept of using the PB
family of compounds as template materials and present theircharacteristics and advantages, and their potential to be used as
precursors for novel materials. In addition, we will examine thestrategies adopted in the PBA synthesis to create specictemplate structures. The diﬀerent derivatization methods will
be summarized in terms of composition and morphology,
addressing the approaches for introducing additional elements
and creating complex structures. This will provide a guide map
for the derivatization of new compositions and gives a better
insight into planning the synthesis route for the intended
application. As many of the derivatives are used as catalysts andenergy storage electrodes, some examples of diﬀerent processing methods for PBD fabrication and the eﬀect of the steps andthe attained morphology on the performance in these specicapplications will be demonstrated. Finally, we will elaborate on
the key aspects of PBA templating that dictate the composition
and morphology of the derivatives and will discuss the challenges in achieving control over material design.

Review

2. Prussian blue and its analogues as
template materials
Prussian blue is the oldest reported coordination compound,
yet it is still the subject of intense research to the present day.9
PB was initially discovered as a pigment at the beginning of the
18th century. However, aer its structural characterization anddiscovery of its redox properties in the 1970s,29,30 this material
found interest in other areas of application such as (bio)sensors
and absorption, ltration and purication, energy storage andconversion, and drug delivery, among others.31–33
As coordination compounds, the primary structure of the
PBA family is based on two metal centres bridged throughcyanide ligands in an octahedral arrangement, formally denoted as hexacyanometallates. A PBA presents a general formula
AxM[M′(CN)6]y,1−y$nH2O, where M and M′ are transition
metals coordinated to the nitrogen and carbon of CN ligands,
respectively (Fig. 1A); A is normally an alkali cation, , stands

Fig. 1 (A) Theoretical PBA lattice. (B) Defect-free “soluble” PBA with alkali ions in the lattice. (C) “Insoluble” PBA lattice with defects and watermolecules ﬁlling the defect sites. (D) PBA characteristics useful for templating and the parameters aﬀecting the PBD properties. (E) The proposed
approach for PBD derivatization in three steps, and the main feature of each step.

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Reviewfor possible lattice vacancies, and coordinated water can eventually occupy defects or uncoordinated sites. In the case of PB,
the metal centres are Fe(III) and Fe(II) species, resulting in theformula AFeIII[FeII(CN)6]. The defective and alkali-free structureis oen referred to as insoluble, while the highly organized andalkali-containing one is dened as soluble PB (Fig. 1A–C).10
These terms do not correlate with PBA solubility in water orsolvents. The cubic structure formed from the octahedral
coordination of both metals leads to a porous framework, while
the strong bonding energy of the cyanide ligands gives rigidity
to the material.34 During redox reactions of the metallic centres,
ions can be stored in the porous lattice, while the structure
rigidity prevents signicant strain or expansion, enabling stable
ion insertion processes.35 The properties and the size of theinterstitial sites can be tuned by the metal character throughreplacing one or both Fe centres with several other transition
metals such as cobalt, manganese, and nickel, leading to
PBAs.36 These characteristics combined allow a PBA to be
selective towards specic ions, which has signicant implications on its application performance and possibilities.37,38
In 1984, Itaya et al. demonstrated the catalytic eﬀect ofPrussian white (PB reduced form) on H2O2 reduction, which led
to almost 40 years of exponential research on PB in analyticalbio-sensors.39 As a consequence, novel PBAs were described,
bringing new electrochemical properties that naturally diversify
PBAs' application as electrode materials. Much of PBAs
outstanding performances arise from combining (multi)
metallic centres and the described open framework structure,oen compared to metal–organic frameworks (MOFs). Unlike
MOFs, PBAs are water and air-stable under standard ambientconditions,40 are easy to synthesize, and present a highconcentration of redox-active species per cell volume. In addition, the short cyanide ligands create easy access to the metallic
species while holding the structure, which is oen a drawbackof the bulky ligands in MOFs.41,42 Though restricted in terms ofligand nature, PBAs with many diﬀerent combinations oftransition metals have been described.11
An appealing characteristic of PBAs to be used as a templateis that they are biocompatible.43 At the same time, theirsynthesis is normally performed by a fast, easy, and water-based
co-precipitation method using relatively cheap precursors. It iscost-eﬀective and shows a high degree of sustainability. The coprecipitation synthesis of PBAs generally consists of the dropwise addition of an MX salt solution (X = Cl−, NO3−, SO42−, etc.)
to a solution of A6−x[M′x(CN)6]. The M ion coordinates to the[M′x(CN)6] unit, while A ions can be incorporated into thestructure in a generic reaction that can be expressed as:
My+(aq) + [M′x+(CN)6](6−x)−(aq) + (6 − x − y)A+/ (A(6−x−y)M[M′(CN)6]y$nH2O)sParameters like the concentration of precursors, pH,temperature, and mixing rate, among others, inuence thecomposition, crystallinity, defect content, and particle size.44,45
Surfactants or carbonaceous materials are oen added tocontrol the shape or reduce the particle size. In contrast,

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Journal of Materials Chemistry A
additional M salts can be added during co-precipitation to
produce more complex compositions,15,28 resulting in a gradientcomposition between two (or more) PBAs.46,47 An intrinsic
parameter that aﬀects the particle size is the solubility of the
PBA and therefore, its formation speed. This must be considered while changing the chemistry of the PBA and designing its
size and morphology. The solubility diﬀerence can itself be used
to stabilize unstable PBAs. For example, a core–shell PBA on
PBA structure can be synthesized by covering soluble vanadium
hexacyanoferrate (VHCFe) with copper hexacyanoferrate(CuHCFe), which has a lower solubility.48 The synthetic
approaches and the interplay of the controlling variables over
PBA characteristics and morphology are extensively reviewed.
These will not, therefore, be discussed in depth here.28,49,50
Nevertheless, understanding PBA synthesis and particle growthis of great importance, as well as understanding their versatility
in composition and morphology, as they inuence the PBDs
derived from such materials. Therefore, we recommend thatanyone interested in the PBD topic investigate PBA synthesis
literature to be able to successfully tailor PBDs.
PBA characteristics are oen pursued in electrode materials,
such as metal oxides and suldes. Nano and microporosity are
well known to benet the electrochemical properties of materials as they increase the surface area, facilitate ion transport
and improve electron mobility. Complex synthetic methods areoen developed to achieve highly porous/hollow morphologies
of well-known electrode materials and to improve performance.51,52 In this context, PBAs have recently started to be usedas templates to produce porous derivatives (Fig. 1D). Since the
PBA structure benets from the presence of C, N, and metallic
elements, it can also accomplish self-templating.
Derivatization of PB and PBAs can be performed via
multiple approaches, but thermal treatment is mostly used
due to its versatility. By controlling the temperature andatmosphere, for example, using a carrier gas that reacts with
the PBA, the derivative chemistry can be altered in various
ways. Therefore, carbides, metal-containing compounds, or
simply metal alloys can be derived through thermal treatment.
Other methods, such as liquid and hydrothermal syntheses,
have also been used to derive metallic compounds and show
the diversity in the synthesis of PBDs, however, with far lessextent compared to the thermal procedures.27 The next section
will provide an overview of the PBA derivatization strategies
and steps through which diﬀerent morphologies and compositions can be obtained.

3.

PBD synthesis strategies

Synthesis of PBDs can be very versatile and sometimes lengthy;
however, they can be roughly broken down into three main
steps: pre-treatment, derivatization, and post-treatment. The
derivatization is the key step, and it is dened here as the stepthat causes the conversion of a PBA into other materials (oxides,
suldes, carbides, etc.). Oen a pre-treatment is performed
before derivatization, occasionally followed by post-treatment
to complete the entire procedure. Each of these steps ischosen strategically to result in a specic attribute of theJ. Mater. Chem. A, 2023, 11, 10473–10492 | 10475

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Journal of Materials Chemistry A
Table 1

Process steps and methods to achieve PBDs and their key features and limitations

Step

MethodFeatures

Limitations

Pre-treatment;modication of PBA
characteristics

Template

Control of the macrostructure“Cubic-free” morphology possible
Addition of new components/more complex
composition
Hollowness possible
Control of the microstructure (normally byusing salt/surfactants)
Normally achieved in one stepHigh parameter sensibility: ne-tuning
Hollowness and porosity possible (also
combined with etching)
Added porosity and/or hollowness

Multi-step/time demanding
Possible unwanted elements from the template

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Review

Etching

Less morphology variety than templates(cubic-derived morphology only)
High parameter sensibility: diﬃcult to control

Time-dependent morphology: ne-tuning
Combined with directed growth formorphological control
Ion exchange

Preparation of non-stable PBAs

Coating

More complex M/M′ compositions
Additional metals can be incorporated
Combined with etching for control of exchanged
ions and/or added porosity
Control of the particle size
Incorporation of additional elements that canbe used in the derivatization step
Protection against coarsening in derivatization
Added functionality to PBAs and PBDs

Hybrid/composite

Derivatization;
conversion froma PBA to a PBD

ThermalHydrothermal

LiquidPost-treatment; nal
tuning ofcomposition andmorphology

Etching

ThermalComposite

Added components can direct PBA growth but
are not further removed
Accomplishes the widest variety of chemicalcompositions
Time- and temperature-dependent
compositions: tuning possible
Possible porosity due to PBA shrinkage
Control of porosity through reaction time
Larger variety of precursor salts
Less energy intensive
Can be combined with an acid/base for added
porosity and hollowness
Can produce an M′-free PBD with the aid of Scontaining salts
High complexity in morphology and metallic
composition
Can replace an N-coordinated metal
Carbon products through metal leaching
Addition of porosity and/or hollowness
Removal of specic metallic species
Improved crystallinity
Achieved carbon graphitization
Added functionality to PBDs
Versatility in composition and morphology
Can support PB as a substratematerial, such as the morphological, physical, or chemical
properties, and aid the material's performance in its target
application. Each step can be taken via a single or multiple

10476 | J. Mater. Chem. A, 2023, 11, 10473–10492

High sensibility to media and PBA
characteristics
Etching process depends on PBA crystallinity
Particles >100 nm to avoid structure collapse
Normally requires a combined hydrothermal
process
Limited to specic ions (normally oxide/hydroxide salts)
Not all metal species can be exchanged
Generally limited to N-coordinated metal

Normally limited to carbon coatings
Sometimes, multi-step process/time demanding

Not always homogeneous component
distribution
Possible reaction of an added component in thederivation process
Possible loss of morphology, especially at high T(coated samples can help mitigate)
Less control over product than liquid-driven
methods
Sublimation or salt decomposition required for
added elements (F, P, Se, etc.)
Less variety in compositions compared tothermalGenerally longer than thermal
Normally requires a subsequent thermal step to
add crystallinity

So far, limited to hydroxides
So far, limited to pristine cubic PBAs
Slower process
Can cause morphology collapse
Not all metals can be easily or selectively
removedCan cause morphology collapse
Can modify composition during crystallization
Lower interaction between the added
component and PBD compared to pre-treatment
Additional steps/time demanding

processes. For example, two pre-treatment strategies or two
steps of derivatizations can be used. However, adding each step
brings an extra challenge while extending time and energy

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Review
demands. Pre-treatment steps are usually employed to control
the morphology of the PBA, but morphology engineering isrealizable only on particles with a minimum size to tolerate
intricate architecture. Derivatization is the main step indening PBD chemistry. However, this change in chemistryneeds a high degree of control on process parameters topreserve the structure designed by pre-treatment. At last,
post-treatment steps are used to ne-tune the product byetching away unwanted products or adding porosities, but
this comes at the cost of high temperatures or the use of harshacids. Therefore, understanding the full journey from a PBA
to a PBD is essential to design an eﬀective derivatization. A
summary of the derivatization steps and methods covered inthis review and an overview of their key features are describedin Table 1.

3.1

Pre-treatment

Any treatment procedure that retains the PBA character, that is,
being a hexacyanometallate, is considered a pre-treatment.
Here, we summarize the strategies used in the pre-treatment
to alter the PBA morphology and composition and make it
ready for derivatization. These can be divided into templating
and directed growth, etching, ion exchange, coating, andcomposite formation (Fig. 2A). We provide examples of thesynthesis of PBA particles that were further used to prepare
PBDs. The work on composition, morphology, and size control
of PBA particles is extensive. Many pioneering and outstanding
examples were le out simply because there is no work on
producing PBDs from those structures, in particular, theanisotropic growth of PBAs to produce sheets, tubes, rods, orother nanostructures.53,54 Most of the reported studies and theexamples presented here are PBDs with cubic-derived
morphology. This is evidence that many possibilities in PBD
research remain unexplored.
3.1.1 Templating and directed growth. PBAs are sometimes
synthesized through the templating strategy to control theparticle shape and size. The templated PBA is then used asa template to form the PBD. Most PBAs naturally grow as cubic
particles ranging from a few nanometers to a few micrometres,
depending on the synthesis parameters such as solution
concentration and temperature. The PBA templating strategies
can act in two directions: introduce porosity and hollowness inthe particle microstructure, or direct the macrostructure of thebulk material. PBAs can grow around the template material thatis aerward removed to create hollow structures. This can bedone, for example, through so-templating by the use of polymers and surfactants, which are further removed, leaving
a hollow PBA shell. By using diﬀerent surfactants in Co(NO3)2
solution, Liu et al. were able to prepare diﬀerent morphologies
of cobalt hexacyanoferrate (CoHCFe).55 The nature of thesurfactant can direct how the particles will grow. In thisexample, sodium dodecyl benzene sulfonate (SDBS) forms
droplets covered by the cobalt salt, leading to CoHCFe-covered
SDBS spheres (Fig. 2B and C). Sodium dodecyl sulfate (SDS)
covers the particle surface, producing SDS-covered CoHCFe
cubes. Upon heat treatment of the PBA to produce FeCo oxides,

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Journal of Materials Chemistry A
the surfactant is removed, leading to porous cubes, hollow
spheres, or nanoparticles.55
Hard templating has also been used to regulate the microstructure. Hard templating has the advantage of more exibility
in terms of shape and size due to the rigid structure comparedto so templates. Traditional methods like coating siliconparticles and subsequent etching with acid can be employed forthis. Nevertheless, a better way to induce hollowness andporosity is by using sacricial templates, where the particle also
takes part in the PBA reaction and is consumed, leaving an
empty core. For example, Nai et al. prepared CoHCFe nanoframes in the presence of trisodium citrate.56 Initially, typicalcubes of CoHCFe are formed. However, with increased reaction
time, the inner part of the cubes is consumed and reprecipitated at the edges of the original cubes, producing porous
frames aer 36 h of reaction. The citrate plays a key role as it
protects the (100) faces of the cubes, leaving exposed reactive(110) edges. The size of the precursor cubes and the size of theresulting frames could also be tuned by using the concentrationof the reactants. They observed that the soluble K-rich phase is
the preferred structure at the frames compared to the inner
cube, resulting in a less defective material.
Preferential growth engineering is a common approach in
pre-treatment, and tailored morphologies can be synthesizedbased on the coverage of certain edges or faces of the crystal. For
example, Nai et al.57 explored epitaxial growth in CoHCFe toresult in intricate structures such as frames, cages, and boxes.
In this study, the initial PBA is formed rapidly, but over a longer
holding time, a K-rich CoFe PBA starts forming on top of theoriginal CoHCFe cubes. Due to higher activity at the corners andedges, the second phase starts nucleating from these sites. A
higher growth rate was observed in the presence of PVP, and fullcoverage of the PBA template forms cages. Diﬀerent citrateconcentrations at this stage further ripened the shell into a solidor open frame. The authors explain the diﬀerent morphologies
by the diﬀerent planes that PVP ([100]) and citrate ([111])
protect, and the inuence of concentrations on the growth
kinetics (Fig. 2D–F).57
Hard templating is a common approach for tuning themacrostructure of the particles. The template can function asa substrate for the growth of the PBA material, for example,
using layered structures like graphene or MXenes to create PBA
sheets, or wires to create PBA core–shell bres or hollow tubes.
Wang et al. used a nickel foam to grow diﬀerent metallic
structures like Co3O4 nanosheets, Cu(OH)2 nanowires, orNi(OH)2 nanoparticles.58 These materials were used as metal
precursors to form diﬀerent PBAs at the surface of the macrostructure. In this case, the template is not removed and takes
part in the derivatization process, resulting in highly porous 3D
structures.58 This is a common strategy to guarantee that the
template actively acts as a structure support, source of elements,
or results in conductive carbon species.
3.1.2 Etching. Etching of a PBA is normally performed toachieve porosity and hollowness because this characteristic is
desired in many applications for its high specic surface area.
In the etching process, the etchant acts on the most reactive or
weakest part, normally the corners or edges, producing cages.

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Review(A) Strategies to modify PBA particles before derivatization, with the common target results. These treatments are performed in parallel tothe synthesis parameters such as the pH, temperature, or concentration of the salts. (B) Co–Fe PBA precursor and (C) resulting porous spheres
prepared by soft-templating. Reproduced from ref. 55 with permission from the Royal Society of Chemistry. (D–F) Directed growth of the
CoHCFe precursor after resting in the presence of trisodium citrate. Reproduced with permission from ref. 57. Copyright 2018 Elsevier. (G–I)
Etching procedure of the cubic precursor in the presence of HCl59 and (J and K) in the presence of ammonia.23 (G–I) were reproduced from ref. 59
with permission from the Royal Society of Chemistry. (J and K) Were reproduced with permission from ref. 23. Copyright 2017 WILEY-VCH Verlag
GmbH & Co. KGaA, Weinheim. (L and M) TEM images of MnHCFe after the reaction with Co(NO3)2 in the absence (L) and presence (M) of C2H5OH
creating porous CoHCFe cubes. Reproduced with permission from ref. 62. Copyright 2021 Elsevier B.V. (N) Cubic precursors before and (O) after
cation exchange due to the solubility diﬀerence. Reproduced with permission from ref. 22. Copyright 2021 Wiley-VCH GmbH. (P) SEM image of
the PB precursor and (Q) TEM image of core–shell spheres after coating with resorcinol formaldehyde. Reproduced with permission from ref. 67.
Copyright 2019 American Chemical Society. (R) NiCo arrays on carbon cloth as a substrate before and (S) after PBA cube formation. Reproducedwith permission from ref. 75. Copyright 2019 Elsevier B.V.
Fig. 2

However, protective agents like polymers or surfactants candirect the etching process, for example, toward the cube faces,
producing frames, or the interior, leading to hollow cubes. Oneof the rst studies on the selective etching of PBAs was performed by Hu et al., where the faces of PB cubes were protectedby cetyltrimethylammonium bromide (CTAB), creating elongated cuboids in the presence of HCl.59 In the absence of CTAB,
the etching is random, while with its presence, a selectivity isobserved for directional etching. In Fig. 2G–I, the etching of the
cubes from corners, which have higher energy, is shown overtime, leading to the elongated-cube morphology, while faces areprotected by CTAB (Fig. 2I). Similarly, hot etching of PB mesocrystals in the presence of polyvinylpyrrolidone (PVP) can
produce hollow particles.60 In this method, mesocrystals and
PVP were strategically chosen to promote the etching inside theparticles. In comparison, single PB crystals are defect-free anddo not have a high energy grain boundary to let the HCl molecules diﬀuse inside the particles. Therefore, the etching starts atthe surface and creates supercial voids. The role of PVP is tobind with the iron atoms in PB and control the etching rate. A
thicker PVP layer inhibits etching, while too little PVP cannot

10478 | J. Mater. Chem. A, 2023, 11, 10473–10492

guarantee the uniformity of the hollow particles. Depending on
the particle size and temperature, these particles can be transformed upon calcination into diﬀerent phases of iron oxide.61
Most PBAs are unstable in basic media. Thus, using bases like
NaOH or NH4OH became common to etch them. Nevertheless,the presence of OH− can start the simultaneous formation of
metal oxide/hydroxide, directly leading to PBDs.8 Therefore, theconcentration, temperature, and/or etching time must be
carefully controlled. The use of ammonia has consistently been
demonstrated to preferentially etch the corners of the cubic
structures of diﬀerent PBA compositions, indicating a reliable
way to form hollow cages.22–24
Fig. 2J and K showsan example of nickel hexacyanoferrate(NiHCFe) cages produced by ammonia etching.23 As the etchingdepends on defect density, a less crystalline PBA will induce
a faster etching rate, leading to over-etching and collapse of the
cubes at prolonged etching times.23 Due to the increased surface
area, hollow particles are attractive for improving the performance in applications directly aﬀected by the available surface
area and mass diﬀusion. Consequently, etching has beenfurther developed and utilized with other synthesis conditions,This journal is © The Royal Society of Chemistry 2023

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Review
like milder etchants such as urea.20 The control of the etchingrate is a crucial part of tailoring the morphology, producing
uniform hollow/porous particles, and avoiding structure
collapse during the etching or derivatization step. For added
control of the chemical composition of the PBA, etching canalso be carried out along with ion exchange.
3.1.3 Ion exchange. The ion exchange process is a solubility- and equilibrium-driven method to modify the PBA
chemical composition and comprises two sub-categories ofanion and cation exchange. Most of the time, this process alsochanges the morphology, creating core–shell structures, hollow
particles, or other hierarchical structures. Similar to etching,depending on the conditions, ion exchange can also directly
lead to PBDs rather than simply modifying PBA characteristics,
for example, with NaOH treatments, as will be discussed
further.8 The advantage of the PBA co-precipitation method isthat one PBA can be a seed for the growth of a second PBA
composition, creating core–shell structures. The diﬀerence insolubility of the two phases promotes ion-exchange processes,
which can change the morphology by creating yolk–shell orcompletely hollow materials. The diﬀerence in PBA solubility
compared to that of other metal salts can also be used toprepare PBA compositions that are normally not stable by direct
co-precipitation. For example, cobalt in cobalt hexacyanocobaltate (CoHCCo) can be partially exchanged for Mo byadding MoO4 to a CoHCCo suspension22 (Fig. 2N and O). Mo
occupies the M–NC sites due to the weaker character of the Co–
NC bond compared to that of Co–CN. This site occupancy later
plays an essential role in the derivatization process, leading toa hybrid composed of Mo2N and Co3O4. In PBAs, the coordination of the metal to the carbon of cyanide has a muchstronger character than the one to nitrogen. Therefore, in many
ion-exchange strategies, only the M–NC can be replaced in theoriginal structure. Even in the case of complete dissolution of
the PBA to react with the salt additive, the new metal ion willreact with the [M′x+(CN)6](6−x)− unit, retaining the M′–CN character. Zhang et al. combined etching with ion exchange tomodify chemical composition while creating porosity.24 When
cubes of nickel hexacyanocobaltate (NiHCCo) are mixed withammonia, the corners are etched, creating hollow cages. At
prolonged times, the etching continues at the edges until the six
cube faces are separated, leading to plate-like morphology. If
palladium salt is mixed with NiHCCo and ammonia, Pd ionspartially replace Ni (as they are coordinated with nitrogen).
Interestingly, ion exchange accelerates the morphology change
caused by ammonia, directly leading to PdNiHCCo plates rather
than hollow cages. Even if the NiHCCo cubes are rst etchedand then mixed with Pd, hollow cages cannot be accomplished,indicating that ion exchange preferentially takes place at theedges, causing dissolution and collapse of the cubes. A similar
cation exchange based on this principle was conducted bysynthesizing a more soluble manganese hexacyanoferrate(MnHCFe) and then adding cobalt nitrate that replaced Mn
atoms to form CoHCFe in the presence of C2H2OH (Fig. 2L andM).62 Ion-exchange in this way can be regarded as similar to thetemplating method, and the initial PBA before ion-exchange isregarded as the sacricial template.

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Journal of Materials Chemistry A
3.1.4 Coating and surface modication. The coating of PBA
particles is performed to improve the electrical conductivity of
PBDs as most of the derivatives, like metal oxides, or suldes,have intrinsic low conductivity. As a secondary eﬀect, thecoating can also prevent particle aggregation and coarsening
during derivatization.63 Some studies report the successful
capitalization of carbon atoms of cyanide ligands as the source
to produce conductive carbon within the PBDs.24 Nevertheless,this is achieved under specic and mild conditions like lowtemperature (<400 °C) and a N2 atmosphere, which is unsuitable for most derivatization processes. Therefore, the coating
step is usually performed with a carbon precursor that can laterbe converted into a conductive carbon shell. The coating can be
performed by using polymers that bind to the material. Thisbinding ability can be an inherent property of the polymer or
has to be facilitated by treating the surface and adding elementsthat ease the attachment of the coating precursor to thesubstrate. Two main examples are resorcinol formaldehyde (RF)and polydopamine (PDA).63–67 RF can be used as a shell on PBA
particles to produce core–shell morphology (Fig. 2P and Q).
These particles can then be converted to a yolk–shell by heat
treatment. A RF resin shell can be implemented on particles byusing a surfactant, such as CTAB, that is stabilized on the PB
particle surface and promotes the polymerization of resorcinol
and formaldehyde.67 Coating with PDA can be performed ina single step and simultaneously with PBA co-precipitation.63,65
This is due to the self-polymerization of dopamine hydrochloride through its oxidation in the presence of Fe3+ or some otheroxidized metals. The polymerization takes place by noncovalent bonding of Fe3+ and catechol. The pH of the solution
or the presence of oxidative agents further promotes polymerization kinetically and thermodynamically.65
A recent study observed that the incorporation of a PDA
aﬀected the particle size and material composition. In thesynthesis of CuHCFe, the presence of a PDA coating stabilized
potassium in the PBA crystal, which subsequently aﬀected thenal derivative composition.63 Electrospinning is also an
adopted strategy to cover the PBAs with polymeric carbon
precursors. By dispersing the synthesized PBA particles in thespinning solution, bres of PBAs embedded in a carbon matrix
are produced. Upon heat treatment, the particles are converted
into the targeted PBD, while the bres can carbonize and forma network of conductive carbon.68–70 Besides acting upon
conductivity, the PBA surface can be modied or coated withother materials that provide extra elements in the derivatization
process. For example, adding thiols to the material can provide
a source of sulfur to produce PBD metal suldes.71 Anchoring
metal precursors can alter the metallic derivative's composition
and morphology. For example, CoHCCo cubes are modiedwith Ni(OH)2, creating a core–shell composition. Upon suldation, both materials are converted, creating a CoS2@NiS2
hierarchical structure rather than a homogeneous mixture of
CoS2 and NiS2.72
3.1.5 Composites and hybrids. Creating composites or
hybrids with other materials that do not necessarily change PBA
particles but rather act as a support or active material in PBD

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application is one of them. For example, carbon nanotubes(CNT),73,74 carbon cloth,75 graphene,76,77 and nickel foam,58,78–80
among others, can act as a substrate for particle growth or
simply anchoring. In cases where direct anchoring of PB on thesubstrate is impossible or ineﬃcient, a second substrate can beused. Fig. 2R and S show an example where NiCo arrays wererst formed by a hydrothermal procedure on carbon cloth, andthen NiHCCo was formed on the arrays.75 These substrates arerather stable up to high temperatures and do not actively
change PBA particles nor modify them upon derivatization.
However, they act as a support for a PBD material while
improving mechanical and electrical properties. Materials withtarget properties can also be added that do not necessarily
decompose or react during the derivatization process. Wanget al. modied the surface of PB cubes with CeO2 nanoparticles.81 The modication occurs through a redox reaction
between Ce(OH)2+, formed due to hydrolysis of hexamethylenetetramine, and Fe(III) species of PB. This mechanism leads
to an intimate interaction between the as-produced CeO2
particles and the PB surface, preventing the aggregation orsegregation of the particles during heat treatment.
3.1.6 Other approaches. Simply modifying PBA synthetic
conditions can also signicantly change the morphology orcomposition. For example, by varying the ratio between Co2+and [Fe(CN)6]3− in solution, the morphology of CoHCFe canprogress from rounded cubes to nanoframes.82 At the sametime, increased ageing time can lead to hollow particles, whichis possibly a greener alternative to the etching methods. Most of
the PBA studies rely on co-precipitation. There are alternativesynthetic methods for PBA preparation, for example, electrodeposition which can better control particle size, or the hydrothermal method, which can tune porosity. Wang et al.15
controlled
manganese
hexacyanocobaltate(MnHCCo)
morphology and porosity by varying reaction times duringhydrothermal processing. The particles evolve from rounded
cubes to cross-shaped etched faces.15 The morphology is thenretained upon pyrolysis. In this case, no pre-treatment per se isapplied, but the PBA synthesis is rationally designed. As previously mentioned, the synthetic routes for controlling PBA
composition, growth, and morphology are plenty, and many (ifnot most) of them remain unexplored for PBD synthesis.28 The
combination of the synthesis routes and parameters with pretreatment methods gives space for improving the control over
PBD morphology and composition and better tuning of theirproperties.

3.2

Derivatization

We dene derivatization as the step in which a PBA is used asa template/precursor and converted into another material. The
two widely adopted methods are hydrothermal and thermal
syntheses, through which many chemical compositions can bederived. They are usually relatively simple and fast methods
with great scalability potential. Liquid methods are far less reported. However, they enable complex compositions andmorphologies. The following metallic compounds have beenreported as derivatives from PBAs: carbides, oxides, suldes,10480 | J. Mater. Chem. A, 2023, 11, 10473–10492

Reviewuorides, hydroxides, nitrides, phosphides, phosphosuldes,selenides, and selenophosphides. In addition, metal alloys orpure metals and carbon can be derived from PBAs by extra
steps, besides composites or hybrids within these compositions. A primary factor separating each group of compounds isthe temperature of the treatment. Because they have diﬀerent
thermal stabilities, one can control the dominant phase bycontrolling the temperature. Both thermal and hydrothermal
treatments can be used for lower-temperature treatments, but
only thermal procedures can be conducted for higher temperatures. The nal chemistry of the derived compound depends
on the environment during derivatization, besides the (pretreated) PBA composition itself. In thermal treatments, theatmosphere can carry the reactive element, for example, oxygen(in synthetic air or O2) or nitrogen (in NH3). It can also be
a carrier of the reactive material typically placed upstream in the
furnace or mixed with the PBA. The gas can also be an inert
component, enabling the synthesis of carbides, for example.
Similarly, additional salts can be placed in the media topromote specic compositions in hydrothermal or liquid
methods. For example, by hydrothermal treatment of a Co–Ni
PBA with ammonium thiomolybdate, mixed Ni–Co–Mo suldecan be derived.83 Here, an overview of the derivatization strategies is provided for each compound group. These procedures
are summarized and shown schematically in Fig. 3 and Table 2.
3.2.1 Oxides. These compounds were the rst and mostwidely reported PBDs, as they can be easily prepared througha simple standalone thermal step, or subsequent to hydrothermal, and liquid methods.8,11,12,22,55–59,61,67,69,81,84–87 During
thermal treatment, oxidation can be achieved in open air,synthetic air, or oxygen. To successfully derive metal oxide or
mixed metal oxides from PBAs, the temperature and duration of
the treatment have to be controlled with precision. A broad
range of temperatures from 250 °C to 900 °C can be implemented for oxide derivatization, depending on the desired oxide
phase. PB has been extensively investigated to produce ironoxides, given the two iron centres present in its structure. A
great compilation focused on PB-derived FexOy can be foundelsewhere.13 In general, the thermal decomposition of PB
enables the stabilization of other phases of Fe2O3 rather than aFe2O3. The crystallinity and resulting phase depend not only on
the temperature but also on the PB particle size andmorphology. In a multi-step approach, Liu et al. reported yolk–
shell spheres of Fe2O3@C that were further treated to produce
suldes (Fig. 3C).67 Hu et al. prepared hollow PB particles ofdiﬀerent sizes based on the PVP method.60 When calcined at
250 °C, the smaller particles (110 nm) led to a mixture ofamorphous and g-Fe2O3, while the larger ones (190 nm) resulted in the g-phase only.61 Similarly, when calcined at 400 °C, thelarge particles oxidized into single a-Fe2O3 while the smaller
ones were composed of a mixture of a- and g-Fe2O3. In addition,
if standard non-hollow small PB cubes (110 nm) are heattreated, no crystallinity is achieved even at 400 °C. This indicates that the smaller the surface area, the slower the crystallization and conversion. When diﬀerent metals are present in the
structure, more complexity is added to the oxidation of PBAs.
Since most metals have a high oxygen aﬃnity, homogeneous

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Review

Fig. 3 (A and B) Overview of typical compounds derived from PBAs and the common route for derivatization. (C) Consecutive treatments of PB
to obtain FeS2@C yolk–shell structures. Coating with resorcinol followed by 2-step annealing leads to a Fe2O3@C yolk–shell (ii). Furtherhydrothermal treatment with thioacetamide leads to the ﬁnal derivative. Reproduced with permission of ref. 67. Copyright 2019 American
Chemical Society. (D) Carbon-coated mixed metal sulﬁde nanoparticles obtained by thermal treatment in H2S/Ar gas. Reproduced withpermission of ref. 63. Copyright 2022 American Chemical Society. (E) Cages of mixed selenides obtained through etching with ammonia solution(ii) followed by thermal treatment with Se powder in N2. Reproduced with permission of ref. 23. Copyright 2017 WILEY-VCH Verlag GmbH & Co.
KGaA, Weinheim. (F) Several derivatives obtained from PB by diﬀerent methods: (ii and iii) treatment with NaOH produces multi-shelled (ii) or
hollow (iii) metal hydroxide cubes with hydrothermal treatment (ii) or in room temperature solution (iii). (iv) New metals can be incorporated by
the dissociation of metal oxide salts and form partially hollow cubes of hydroxide/oxide mixtures that can be heat treated in air to form oxides (v)and further etched with acid to remove one phase (vi) creating single metal oxide hollow cubes. Reproduced with permission of ref. 8. Copyright2013 American Chemical Society (G) Hierarchical metal nitrides coated on Ni foam/Co(OH)2 obtained by thermal treatment with NH3. The higher
the temperature, the more coarsening occurs (ii and iii). Reproduced with permission of ref. 58. Copyright 2018 WILEY-VCH Verlag GmbH & Co.
KGaA, Weinheim. (H) Porous cubes of metal ﬂuoride mixtures directly obtained from thermal treatment with NH4F in argon. Reproduced withpermission of ref. 21. Copyright 2019 Elsevier B.V. (I) Ternary mixed metal phosphide core–shell particles are obtained by annealing with NaH2PO2
in N2. Reproduced with permission of ref. 102. Copyright 2021 American Chemical Society. (J) Porous hollow cross-shaped frames of mixed metal
carbide@carbon nanotubes obtained by direct annealing of the PBA in Ar. Reproduced with permission of ref. 15. Copyright 2020 Wiley-VCH
GmbH. CT: coating, T + P: thermal + powder, T + G: thermal with gas, HT: hydrothermal, L: liquid.

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J. Mater. Chem. A, 2023, 11, 10473–10492 | 10481

10482 | J. Mater. Chem. A, 2023, 11, 10473–10492(ET + IE), (NH4)2S, hierarchicalmicrospheres(CT), PDA, core–shell

NiHCCo, cubes(T + P), sulfur, N2, 350 °C, 2 h,hierarchical microspheres(T + G) Ar, 600 °C, 2 h, hollow
nanocages(T + P) NaH2PO2, Ar, 400 °C, 2 h,hollow nanocages(T + G) air, 450 °C, 2 h, cubes(T + G) NH3, 450 °C, 2 h, cubes(T + G, HT) Ar, 150 °C, 2 h, 600 C,
2 h, yolk–shell Fe2O3@C(T + G), air, 300 °C, 6 h, yolk–shell
porous box(T + G), air, 500 °C, 6 h, a-Fe2O3/
SnO2 hollow box(T + G), air, 200 °C, 2 h, nanobers(T + P), NaH2PO2, Ar, 300 °C, 2 h,porous nanoplates(L) 2 M NaOH, RT, 10 min,amorphous porous nanoplates(T + G), air, 300 °C, 1 h, porous
nanoplates(T + P) NaH2PO2, N2, 300 °C, 2 h,porous nanoframes(T + P), thiourea, N2, 350 °C, 2 h,core–shell(T + G), air, 300 °C, 6 h, hollow box

Derivatization (method)b,
conditions, morphology

Hydrothermal, TAA,
160 °C, 20 h

Thermal annealing
800 °C, Ar/H2

HCl etching

Post-treatment

OER

NiO

Co3O4 + Fe3O4
Co5.47N + Fe3N
FeS2@CNiCoP + C

NiCo + C

NiS2

SnO2, porous hollow
box
NiCo alloy@C

Li-ion battery

OER and HER

Li–S battery

Solar cells

OER

Li-ion battery

Li-ion battery

a-Fe2O3
a-Fe2O3

Capacitor

CoS2@NiS2

HER and OER

OER

Ni(OH)2

Co0.6Fe0.4P + C + N

OER

Application

Ni5P4 + Ni2P + C

PBD

67

58

65

94

16

8

72

20

105

Ref

a
ET: etching, IE: ion exchange, CT: coating, and TP: template. b T + P: thermal + powder, T + G: thermal with gas, HT: hydrothermal, and L: liquid. c CoHCFe: cobalt hexacyanoferrate. FeHCFe: ironhexacyanoferrate (PB). HER: hydrogen evolution reaction. NiHCCo: nickel hexacyanocobaltate. NiTCNi: Ni(H2O)2[Ni(CN)4]$xH2O. OER: oxygen evolution reaction. RT: room temperature. and(TAA): thioacetamide.

CoHCFe, cubes on
Co(OH)2 sheets
FeHCFe, cubes

NiHCCo, cubes

NiHCCo, cubes(CT + ET), resorcinol, urea,
core–shell(CT + L), Ni(NO3)2 + urea, core–shell(ET + HT), 0.2 M NaOH, hollow
box(ET), RT, 0.2 M NaOH, yolk–shell
porous box(ET + IE), K2SnO3$3H2O, Fe(OH)3/
SnO2 hollow box(TP), electrospinning nanobers

CoHCCo, cubes

FeHCFe, cubes(ET + HT), urea, frames

CoHCFe, cubes

NiTCNi, nanoplates

Pre-treatment (method)a,b,
conditions. morphology

Examples of the PBA derivatization process and application of the resulting PBDc

PBA, morphology

Table 2

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Review

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Reviewderivatization might not occur at low temperatures anda shortderivatization time. This might result in mixtures of singlemetal oxides over mixed-metal oxides due to a higher oxygenaﬃnity of one of the metallic species. For example, uponoxidation of CoHCFe nanoframes at 350 °C for 2 h, the resulting
oxide is composed of a mixture of Co3O4, CoFe2O4, andK2CoO3.56 Meanwhile, oxidation of hollow nanocubes andnanospheres of CoHCFe at 500 °C for 3 h led to a single phase of
FeyCoxO4.55 Phase diagrams can be used as an estimation guide
while reducing the strength of oxidation by decreasing thesynthetic air-to-argon ratio or using carbon dioxide gas can help
to achieve better control over oxidation. Li et al. reported
a copolymer-co-morphology synthetic strategy to prepare spineloxides, where metal ratios initially tuned the composition of theoxide in the PBA.11 In this approach, Mn–Fe hexacyanocobaltate(MnyFe1−yHCCo) with diﬀerent Mn/Fe ratios is prepared bysimultaneous addition of FeCl2 and Mn(NO3)2 to K3[Co(CN)6].
When heat-treated at 500 °C for 1 h, spinel MnxFe1.8−xCo1.2O4 (0< x < 1.8) hollow nano-dice are obtained. Besides tuning thecomposition, the decrease in the Mn content leads toa morphological progression from cubes to spheres, demonstrating that the chemical composition also plays a role inmorphology control. In terms of morphology, shrinkage of PBA
particle is normally reported when being oxidized due to theremoval of the CN units. This is followed by an overall increasein surface roughness and surface area, which is desired in many
applications. Though many studies report a direct templating of
PBA into PBDs, that is, changing the composition but retaining
the original PBA morphology, that is not always the case. Zhang
et al. demonstrated the morphological evolution of PB cubes at
diﬀerent treatment temperatures.85 When crystalline PB cubesof 1 mm are heat treated at 350 °C, the outer part of the cubes isoxidized, while the centre atoms diﬀuse outwards, creatinga Fe2O3 shell-microbox. When the temperature is increased to
550 °C, there is particle coarsening, resulting in porous hollow
microboxes, which further progress to a hierarchical structureof hollow boxes composed of Fe2O3 nanoplates at 650 °C. Thisshows that even in the absence of pre-treatments, hollowness
and morphology can be tuned with heat treatment parameters.
Hollowness in PBA particles through heat treatment has been
explored for other compositions as well. This is achieved
through the Kirkendall eﬀect, where the diﬀerence in thediﬀusion of the metals, or the metals and the gas, leads tointernal voids in the PBD particles. If well controlled,morphologies like yolk–shell can be achieved.88
Though less explored, liquid methods are also used forderiving PBA into oxides. However, this is achieved by the initialderivatization of PBA into hydroxides and further thermal
treatment of PBD–OH. Therefore, the liquid approaches will be
discussed in the hydroxide section.
3.2.2 Suldes and selenides. Derivatization of PBA to thesechalcogenides can be performed by both hydrothermal andthermal procedures, while suldes have also been achieved byliquid methods.23,67,71,72,75,76,83,89,90 In the thermal method, H2S
can be directly used, or sulfur powder can be heated to produce
sulfur vapour and combined with H2 gas to diﬀuse as H2S gas in
the PBA structure. Sometimes sulfur or sulfur-containing

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Journal of Materials Chemistry A
additives are incorporated in the PBA particles in pretreatment steps, acting as a source of sulfur during the heat
treatment.71 Similar procedures can be performed by putting
selenium powder in the upstream side of the furnace or simply
mixing the ground powder with other reactants. Temperatures
between 300 and 500 °C usually provide the necessary energy forthe formation of homogeneous chalcogenides.72,75 Nevertheless,partial suldation/selenization through thermal methods ismore diﬃcult to control, normally resulting in complete
conversion of the treated materials.63,71
Bornamehr et al. were able to completely convert CuHCFe
into chalcopyrite within only 10 minutes of treatment at 300 °C
with H2S gas.63 In this work, the authors demonstrated the eﬀectof the coating on the derivative composition. The polydopamineused to coat and control the particle size also led to a potassium-rich and less defective CuHCFe phase. As a result, thesulde derived from the coated material was KCuFeS2, while forthe uncoated, it was CuFeS2 (Fig. 3D). This work also presents
signicantly smaller derivative particles (<100 nm) than theusual 0.5–2 mm range. Higher surface area can be achieved withsmaller particles; however, it is more diﬃcult to incorporate
morphology complexity in this size range.
In the hydrothermal procedure, a sulfur-containing
compound such as the Na2S salt or thioacetamide can releaseS2− anions and promote anion exchange in the PB lattice toform metal sulde.67,91 For selenides, the selenium powder canbe dissolved in water to use as the reactive environment inhydrothermal methods.92 Reaction temperatures for forming
metal chalcogenides in hydrothermal synthesis are reported
between 120 and 200 °C. However, the product of hydrothermal
synthesis is oen subjected to a subsequent thermal annealing
process to achieve complete conversion and crystallinity. Liuet al. reported an interesting multi-step procedure to prepareyolk–shell spheres of FeS2@C derived from PB (Fig. 3C).67
Initially, PB particles are coated with a resorcinol–formaldehyde
resin (RF). Upon two-step annealing in Ar at 150 °C and 600 °C,
the PB particles shrink and oxidize to Fe2O3 while the RF
coating graphitizes into a carbon shell, forming a yolk–shellstructure. The spheres were modied with thioacetamide, andthrough hydrothermal synthesis at 160 °C, FeS2@C yolk–shellparticles were achieved. The coating prevents agglomerationand coarsening of particles, while annealing at higher temperatures is essential to achieve conductivity in the formed carbon
shell. Another way to achieve suldes with yolk–shellmorphology is through the Kirkendall mechanism and ion
exchange. This was performed, for example, by reacting
cadmium hexacyanoferrate (CdHCFe) with Na2S at 150 °C under
10 atm.93 The S2− ions are exchanged with [Fe(CN)6]3−, forming
CdS. As the Cd2+ ions diﬀuse outwards faster than S2− candiﬀuse inwards, hollowness starts to appear between thesurface and the centre of the cubes, resulting in a CdS yolk–shellstructure. When NiHCCo is reacted with Na2S at 100 °C, thecube faces are etched rst, and the morphology evolves to NiSx
nanocages rather than yolk–shell structures.91 Using (NH4)S
instead of Na2S drastically changes the morphology. Huang
et al. prepared hierarchical NiS2 particles by treating NiHCCo
cubes with (NH4)S by the hydrothermal method at 100 °C.94
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Through hydrolysis, OH− and Sx− are formed, and OH− slowly
etches the Ni+ ions, leading to g-NiOOH. At prolonged times,
NiHCCo cubes are completely dissolved, and g-NiOOH ispartially converted to NiSx while the morphology evolves tohierarchical microspheres of g-NiOOH/NiSx. These particles are
crystallized and converted to NiS2 by thermal treatment at 350 °C with sublimed sulfur powder. These last studies are the few
examples where the metal from the [M′(CN)6] unit is completely
removed from the nal derivative, as no iron93 or cobalt91,94
remains in the material. Nai et al. also utilized ammonia toprepare cages of NiHCFe,23 by heat treatment with Se powder at
350 °C, obtaining a mixture of NiSe2 and FeSe2 (Fig. 3E). Like
oxides, obtaining mixed metal chalcogenides rather thana mixture of single metal components also requires careful
tuning of derivatization parameters. Selenide powder was also
used as a post-treatment to dope (CoFe)S2.95 Hollow cubes of
CoHCFe were prepared in the presence of trisodium citrate andthen mixed with sulfur powder. Aer heat treatment at 350 °C
for 2 h, (CoFe)S2 is formed. Mixing the sulde with selenium
powder and treating it at 250 °C for 8 h leads to the Se-doped(CoFe)S2 hollow boxes. The inverse process, rst selenization
followed by suldation was also performed, resulting in S(CoFe)Se2.
3.2.3 Hydroxides. The conversion of PBAs into hydroxides
can be realized through liquid and hydrothermal procedures.8,58,94 Perhaps the most extensive example is the work by
Zhang et al.8 In this work, PB cubes were transformed into
diﬀerent hollow and hierarchical cubes with diﬀerent compositions driven by basic etching and ion exchange. In the presence of NaOH solutions, PB is converted to Fe(OH)3. The
morphology of the resulting cubes depends on the NaOH
concentration and temperature, generating yolk–shell, hollow
cubes, or multi-shelled microboxes (Fig. 3F(ii) and (iii)). In thepresence of MOx salts (K2SnO3$3H2O, Na2SiO3, Na2GeO3,
NaAlO2, or NaBO2$4H2O), basic etching from salt hydrolysis
and ion exchange takes place, resulting in homogeneous
Fe(OH)3/MOx hollow cubes (Fig. 3F(iv)). Upon annealing, they
can be converted into mixtures of Fe2O3 and MOx (SnO2, SiO2,
GeO2, etc.) and further single MOx hollow cubes by etching out
the Fe2O3 phase (Fig. 3F(v) and (vi)). This work demonstrates theversatility of changing morphology and composition by nely
tuning reaction parameters.
As discussed in the sulde section, (NH4)S2 can also go
through hydrolysis, producing NH3 and H2S which are decomposed into OH− and Sx2−.94 This leads to a simultaneoustransformation of the PBA into oxyhydroxide and partial suldation with a complex morphology evolution. In general, theuse of a metal salt rather than a strong base or ammonia leads
to slower and more controlled derivatization into hydroxide.
This is also a pathway to introduce new elements to the PBA/PBD structure, especially if they do not typically form stable
PBA complexes, like Ge, Si, Al, and B in the work of Zhang et al.8
This strategy was also used by Hu et al. to form oxide mixtures ofFe and W.96 By stirring PB cubes with Na2WO4 at 50 °C, Fe(OH)3/WO3−x hollow cages are formed. Through calcination at 550 °C
for 6 h, Fe2O3/W18O49 composites are then obtained. The liquid
methods allow the synthesis of more complex or hierarchical10484 | J. Mater. Chem. A, 2023, 11, 10473–10492

Review
structures, which is usually not the case for thermal treatments.
Therefore, PBD–OH is typically used as a precursor for other
PBD compositions like PBD-oxides, instead of direct oxidationof PBAs, and PBD-suldes, by heat treatment with sulfur
powder.94
3.2.4 Fluorides and nitrides. These compounds remain
little explored and are mainly synthesized by thermal procedures. For nitrides, derivatization can be achieved with the aidof NH3 gas.58,97 Some studies capitalized on the nitrogen fromCN ligands and prepared metal nitrides as well.22 Nevertheless,most commonly, nitrogen from CN is either removed or resultsin N-doped carbon rather than metal nitrides. Of the few reports
on nitrides, the majority covers the conversion of PB into Fe2N,
and most metals/compositions are still to be studied.97,98 Wanget al. demonstrated that under an NH3 ow, CoHCFe templated
over Co(OH)2/nickel foam is converted into a mixture of FeN
and CoN.58 At a lower temperature (325 °C), interconnectednanosheets are obtained. When the temperature increases to
350 °C, 450 °C, and 500 °C, coarsening and coalescence occur,
resulting in complete loss of the initial nanostructure (Fig. 3G).
The higher the temperature, the less nitrogen is incorporated
into the material, resulting in mixtures of nitrides with diﬀerent
stoichiometric ratios. Wang et al. described the formation of
FeCoNiN on nickel foam (NF) by treatment of CoHCFe/Ni(OH)2/
NF with ammonia. However, the large amount of Ni suppresses
more structural and composition insights into Co- and Federived components, with evidence of Ni3N.99 Fluoride-based
PBDs are also quite limited, though more restricted in terms ofsynthetic approaches, given the toxicity and hazards of uorinated compounds, especially at high temperatures. One report
uses the templating approach to grow NiHCFe onto
Ni(OH)2@Ni foam nanosheets.79 This material is then uorinated by a thermal process through the decomposition of NH4F
at 350 °C for 2 h and the eﬀect of the NH4F ratio and temperature were evaluated. They show that uorine suppresses therelease and decomposition of CN units, producing a morestable hierarchical structure compared to that of the same
sample annealed in argon. Fluorination results in F-doped
NiFeOOH rather than a uoride compound. Nevertheless, Fdoping plays an important role in the catalytic activity of thematerial. Diﬀerently, when bare CoHCFe cubes are heat treatedat 400 °C with NH4F, etched porous cubes of CoF2 and FeF2
mixtures are formed (Fig. 3H),21 indicating that Co/Fe mixtures
might be more prone to uorination than Ni/Fe.
3.2.5 Phosphides and phosphor-containing compounds.
Due to their high hydrogen evolution activity as catalyst andtheir low cost as opposed to the noble metals, diﬀerent
phosphor-containing compounds have been investigated as
PBDs, being the emerging PBD class of compounds aer thepioneering oxides.20,64,65,74,75,77,78,80,82,100,101 Metal phosphides arederived by a thermal procedure using NaH2PO2 powder under
an inert gas ow such as Ar or N2. Similar to selenization andsuldation, the powder can be mixed with the reactants orplaced on the upstream side of the tube.20,65 Zhou et al.102
prepared mixtures of 3 diﬀerent phosphides by treating
FeHCCo with NiCl2 to form NiFeHCCo, and further heat treatment with NaH2PO2 in N2. The heating leads to a core–shellThis journal is © The Royal Society of Chemistry 2023

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Review
structure that could be related to the higher concentration of Ni
at the surface (Fig. 3I). As observed for the derived-oxides, thePBA composition also aﬀects the morphology of the derived
phosphide. Ding et al. prepared CoHCFe, NiHCFe, and
MnHCFe by co-precipitation and phosphorized them under thesame conditions (400 °C, Ar, and NaH2PO2). Aer the heat
treatment, all materials present a rough surface. However,NiFe–P shows higher particle coarsening while MnFe–P presents small particles adhered to the cubes.101 The addition of Se
or S powder can result in the formation of selenophosphide orphosphosuldes.75,89 Similar to other thermal treatments,
a temperature range of 300–500 °C has shown the best resulting
products in terms of homogeneity. For example, Liu et al. performed suldation, phosphorization, and phosphosuldationof NiHCFe, to evaluate the eﬀect of time and temperature on thecomposition and morphology.89 At 350 °C and under a N2 owin the presence of both sulfur and NaH2PO2, the cubic
morphology evolves to smaller nanosheets aer 1 h of annealing, leading to a complete loss of the cubic shape and morphing
into nanoowers aer 2.5 h. The ower morphology is not obtained in the absence of either sulfur or NaH2PO2, and only
deformed and shrunk cubes are produced. Analysis of composition indicates that the owers are mixed metal phosphosulde (NiFe)PS3 rather than individual phosphosuldes. In
another phosphide derivatization, similar to the RF strategy,
NiHCCo 450 nm cubes were coated by dopamine through theself-polymerization process prior to derivatization.65 The material was rst annealed at 600 °C to ensure carbonization of thecoating while forming a Ni–Co alloy. This results in carboncoated NiCo hollow cubes. The PBD was then treated with
NaH2PO2 at 400 °C to convert the alloy into a mixed NiCo
phosphide. Jiang et al. also utilized dopamine for PBD preparation. They demonstrated a tubular PB morphology rather thanthe usual cubes.64 Aer coating tubular PB particles withdopamine, the material was converted to carbon-coated FeP.
They used the original PB as a cathode and the PBD FeP as an
anode in a sodium-ion battery full cell. In many cases, the PBAs
are combined with carbon structures prior to the phosphorization process.65,74,77 The carbon structures, like carbon nanotubes and graphene, enhance the overall electronic conductivityof the derivative and anchor the PBD particles, demonstrating
improved catalytic performances compared to the bare PBD.77
3.2.6 Alloys, carbides, and carbon. By heating the PBAs in
an inert atmosphere such as Ar, without providing otherelements,
alloysandmetalcarbidescanbeobtained.13–17,24,62,65,68,70,84,103,104 For example, the treatment
parameters used for oxidation in air can be replicated under Ar
to produce a metallic alloy/carbon composite.84 While mild
conditions will form alloys and carbon mixtures, the formationof carbides requires higher temperatures (>500 °C). This canmake it challenging to achieve carbides with diﬀerent
morphological features like high porosity and hollowness. As
demonstrated by Wang et al., highly porous cages of MnHCCo
can be prepared through prolonged reaction times (Fig. 3J).15
Nevertheless, when annealed at 700 °C for 2 h, the structureswith higher hollowness degrees collapsed. The presence of
metal and carbon at high temperatures and under inert

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Journal of Materials Chemistry A
conditions can also favour the growth of by-products likecarbon nanotubes, aﬀecting the nal morphology of the PBD
product.15 Aer the derivatization of the metal carbide or alloy/C
mixture, porous carbon can be produced through a subsequentetching of the metallic species, for example, by using HF or HCl.
This carbon material with high porosity can be very attractive
for absorption applications.103,104
3.3

Post-processing

Some PBD synthesis methods involve post-processing steps toreach the desired product quality and characteristics. This canbe liquid etching to remove impurities or additives, such as
removing metals in the derivative to produce porous carbon.103
Simultaneous with etching, more intricate morphologies can beachieved due to ion exchange.8 Etching can also be utilized asa post-processing step that removes unwanted phases. For
instance, if two oxide phases are formed aer derivatization,and one is undesired or detrimental to the performance, it canbe selectively etched. It also causes in situ porosity formationthat can be benecial to the target product.86 Post-processing
can also be in the form of mono- or multi-stage annealing,
resulting in higher hybridization and graphitization degrees orhigher crystallinity of the PBD phases. The PBD particles can befurther incorporated into other materials, for example, within
MXene layers, to form MXene/PBD composites.87 Though they
do not fundamentally relate to the PBA derivatization process,these steps aﬀect the resulting PBD composite properties fortheir target application.

4. PBD properties: structure design
and applications
Understanding the structure/property correlation is a key step
to enabling PBD use for diﬀerent applications. Therefore, thederivatization procedure's steps are oen engineered to achievea specic structure. The structure design can be reviewed fromtwo perspectives: rst, the elaboration on the PBD synthesis
route in each step of pre-treatment, derivatization, and posttreatment; and second, the formation of hybrids and composites by assembling PBAs and other components together. The
result of these design strategies is either a nal sophisticatedmorphology such as hierarchical and multi-walled structuresthat aid the material with improved surface area, mechanicalproperties, and other characteristics that are benecial to thenal application, or in a chemical composition that improves
conductivity, magnetic properties, etc. Since each chemicalgroup (carbides, phosphides, oxides, etc.) have a diverse eﬀect
on performance, the eﬀect of composition can be studied fora particular application, as described in previous revisions.6,9,10,25,26 Here, attention will be focused on the eﬀect ofmorphology on the properties and applications.
4.1

Structure design and PBD characteristics

Pre-treatment usually has a crucial eﬀect on the structure
design, mainly when etching and ion exchange are employed.
For example, by using PVP or citrate, crystal growth can be

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Journal of Materials Chemistry A
controlled to result in a frame or box morphology.57 The derivatization step can heavily inuence the PBD structure.84,105
Sometimes, due to diﬀerent precursors (sulfur, selenium, or
NaH2PO2 powder) and diﬀerent growth mechanisms of thederivative crystals, the same structure evolves into diﬀerent
morphologies.19,89 Heat-treatment parameters can also aﬀect
the design. It has been shown that treating PBA particles under
the same atmosphere but with diﬀerent temperatures can resultin diﬀerent oxide morphologies.85
Besides the synthesis parameters, even morphological attributes such as the cavity size in the PBA can aﬀect the subsequent derivative morphology. This has been shown for diﬀerent
iron oxide phases derived from PB cages with diﬀerent cavity
sizes under the same derivatization parameters.61 Particle sizes
start from around 200 nm when designing more complex andhierarchical structures. The morphology of smaller particles isusually limited to dense cubic shapes that are not well developed and grown. Smaller particles are more prone to collapse
when inducing hollowness and porosity and do not provide thenecessary volume for morphology design. Furthermore,methods to modify particle morphology rely on protecting
specic planes/surfaces and/or attacks on reactive sites. Thisrequires well-structured particles with high crystallinity. The
high surface energy of smaller particles makes it more diﬃcult
to control and guide morphology changes.
While the elemental composition of PBDs can be relatively
easy to control, the resulting phases and stoichiometry rely on
many kinetic and thermodynamic aspects of the derivatization
process. A key aspect of PBA composition is the relatively
limited amount of stable metallocyanide salts, typically varying
between [FeII/III(CN)6]4−/3− and [Co(CN)6]3−, sometimes[Mn(CN)6]3− or [Ru(CN)6]4−, though the latter is quite costprohibitive. Therefore, most PBA compositions consist of M
[M′(CN)6] with M′ = Fe, Co, Mn, and sometimes Ru. Even
though the M element could vary through a large portion oftransition metals, nearly all studies report on PBAs with M = Fe,
Co, Ni, and Mn. A few examples of alternative metals alreadyreported – though not all for PBD synthesis – are Cu, V, Mg, Zn,
Cd, Ag, Ti, Pd, and Mo (Fig. 5). Mixing diﬀerent M units has
become one strategy to increase composition complexity or
stabilize diﬀerent metals in the lattice that would not forma standalone PBA, where even a high entropy PBA with 5 different M is described.11,106 In principle, mixing diﬀerent[M′(CN)6]6−x units are also possible and has been successfully
reported.107 However, the versatility remains limited. Besides,
the M unit can be more easily replaced in the structure in pretreatment procedures like ion exchange due to the lower M–
NC bonding strength, providing more exibility in composition
design. The composition of the PBDs will be modied in thederivatization process by incorporating, for example, O, S, or P,
and eliminating N and C from the ligands while retaining themetal species (in the absence of post-treatment). Therefore, tosome extent, it is possible to predict probable chemicalcomponents of PBDs. Nevertheless, the derivatization parameters and morphological features of the PBA precursor will aﬀect
the resulting phases. Higher porosity and hollowness provide
a larger surface area that can accelerate the conversion or

10486 | J. Mater. Chem. A, 2023, 11, 10473–10492

Review
facilitate the diﬀusion of the metallic species, resulting in
diﬀerent phases or segregation of components, for example,
metal oxides versus mixed metal oxides.55,56,61
Though not covered in this review, the PBA synthetic strategy
presents many possibilities for altering morphology andcomposition. Most of the examples presented here involve thesynthesis of PBA by co-precipitation and further modifying thesurface, morphology, or composition through pre-treatment
and derivatization. The synthesis parameters in coprecipitation can also be tuned to change morphology, likethe solvent (H2O, ethanol), temperature, pH or M/M′ concentration ratio. A few template strategies utilize diﬀerent materials
as the support for PBA nucleation, directing the 3D macrostructure,58,79,108 or as a support and metallic source, where thetemplate itself reacts with the metallocyanide salt to form
a PBA, creating better interaction between the template andPBA.109 Nevertheless, other synthesis methods and processings
exist. One approach involves the single-precursor synthesis
through the decomposition of [M′(CN)6] salt, either by acidinduced or hydrothermal methods.110,111 This enables the slow
release of M′ ions, creating highly crystalline nanoparticles
below the 100 nm range, though M = M′ limits the composition.
Electrodeposition has the same controlled nucleation andgrowth eﬀect but allows for a more varied composition (MsM′), especially if the M source is immobilized in theelectrode.112–114 Electrodeposition can also orient themorphology115 and create PBA composites with the electrodematerial, for example, carbon nanotubes112 or graphene.116
Layer-by-layer assembly is another method for controlledorientation of PBA particles while modifying the surface withpotential active materials for derivatization, like polymers (forcarbonization and conductivity), thiols or amides (for suldesor N-doped materials), or nanoparticles (for an additional
metallic source and compositional complexity).80,117,118 Thoughcarbon coating through surface modication is the standard
strategy to induce conductivity while avoiding particle aggregation, there are other ways to achieve that, such as electrospinning. Electrospun carbon bers provide high porosity,
conductivity, and specic surface area. PBA particles can beassembled inside the electrospun bers by electrospinning the
PBA particles16,68,69 or anchoring the particles on the electrospun
carbon bers.100 Aer derivatization, the carbonized bers act
as a conductive matrix for the PBD while providing exibility
and mechanical stability.

4.2

PBD morphology and applications

The applications of PBDs are varied since they can present many
compositions. The most attention is given to electrochemical
applications such as catalysis, energy storage, and conversion.
The eﬀect of properties on the application can be discussed
from two viewpoints, namely, the eﬀect of chemical composition and the eﬀect of structure and morphology. The chemicalcomposition results from the used precursor and the media
chosen for derivatization. This is usually better controlled andeasily tailored, though unpredicted or secondary phases can beformed during the derivatization. The unwanted phases can be

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Review

Fig. 4

Journal of Materials Chemistry A

Main strategies to tune the structure of PBDs, their resulting properties, and examples in applications that are aﬀected by such strategies.

controlled by changing processing parameters or etching theextra phases as a post-treatment step. The chemical composition will have diﬀerent eﬀects depending on the application, as
the target properties will vary. Therefore, here the properties
and their eﬀect on the application are focused on themorphology since there are similar morphological features thatare benecial to diﬀerent applications, for example, tailoring
the structure for smaller particle size to reduce the diﬀusion
paths and increase the reaction rate. Typical morphologicalfeatures are depicted in Fig. 4, and some examples are
addressed in Table 2. These standard features can be divided
into the following.
4.2.1 Small particle size. Decreasing the particle size is themost common approach to enhance properties, especially inelectrochemical applications. This is a rather simple step thatenhances electron transfer and element diﬀusion by reducing
the diﬀusion paths. This method is extensively explored in the
PBA application. It uses the synthesis parameters, such as thesalt concentration during co-precipitation or the pH to control
the growth of the nuclei. A careful treatment keeps the small
size attribute of the template. Therefore, studies on the derivatization of the PBA usually do not focus on the eﬀect of thederivative particle size on the properties but rather use strategies in the pre-treatment step to control the PBA size. Li et al.13
provided an overview of PB derivatives and the eﬀect oftemperature and environment on the particle size and phasechange, and their application in energy storage and conversion.
By cross-comparing diﬀerent reported studies, they showed thata larger particle size usually results in higher crystallinity due tosmaller surface area and defects. The eﬀect of particle shape canalso be observed in parallel to the size eﬀect. For example,
a nanosphere morphology can provide better mechanicalstability than a nano-cubic structure and enhance the stability
during the cycling of a battery electrode. However, with thesame diameter, a cubic structure provides a higher surface area.
Liu et al.55 prepared porous nanospheres and nanocubes ofiron–cobalt oxide. With the similar size of both morphologies,
the spheres provided higher capacity and stability when tested
as a lithium-ion battery anode material compared to the cubes

This journal is © The Royal Society of Chemistry 2023

due to the higher cobalt loading and better porosity achieved
during the processing.
4.2.2 Hollow and porous structures. This method benets
applications that rely on high surface area and fast diﬀusion.
Nevertheless, such properties can also be achieved by reducing
the particle size or through sheet morphologies. Their ability tobuﬀer volume expansions in applications where redox andconversion reactions occur makes hollow structures characteristic and attractive. The most prominent example is metal-ion
batteries, which rely on the diﬀusion of ions (like lithium or
sodium) into and out of the particles in each charge anddischarge step. Depending on the material, alloying reactions
can also be part of the charge storage mechanism, which
presents a much more signicant volumetric expansion (100–400%). Zhang et al.8 provided extensive work on hollow metaloxide boxes. They compared the eﬀect of box layers on thestability of the material as an electrode for Li-ion batteries. A
higher number of box layers showed better cycling stability,
indicating better structural stability during the charge anddischarge. For the synthesis of PBDs, they employed all threesteps with ion exchange as the pre-treatment by using NaOH,
which simultaneously derived iron hydroxide from the PB
template, followed by an annealing step to produce crystalline
iron oxide. In another report, Liu et al.67 prepared yolk–shell
spheres by coating PB particles with resorcinol formaldehyde.
They performed initial annealing and hydrothermal treatment
via thioacetamide to prepare FeS2 yolks and carbon shells. The
present free space between the yolk and the shell provides roomfor volumetric expansion, which occurs during the conversion
reactions for lithiation and de-lithiation. The shell is not an
active material and does not aggregate during the reactions;
therefore, a stable cycling performance is achieved.
4.2.3 Hierarchical structure. Hierarchical materials enable
macrostructure arrangements with complex morphologies thatprovide a higher accessible surface area which is critical for
applications reliant on surface activity such as catalysis or solar
cells. Huang et al. prepared nickel sulde mesospheres made ofan assembly of 2D nanoakes and compared the performanceof this material to a similar nickel sulde phase in nanoparticle

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Journal of Materials Chemistry A
morphology as a catalyst for solar cells.94 They started with
NiHCCo and performed ion exchange and etching to remove
cobalt by using (NH4)2S salt and further derivatization via
a thermal treatment by using sublimed sulfur powder. The
hierarchical nanosphere material presented better long-term
stability and faster electrolyte regeneration, assigned to lower
resistance and diﬀusion impedance compared to thenanoparticles.
4.2.4 Carbon composite. Using carbonaceous materials
with a high ratio of sp2 hybridized carbon atoms increases the
conductivity, whether when used as a carbon coating,a substrate, or a matrix, and is a standard approach in applications that rely on facile electron transfer. Forming a coating
has the added benet of the entrapment of unwanted species.
Wu et al.65 explored bimetallic Ni–Co phosphide as a separator
and polysulde mediator in Li–S batteries. To prevent polysulde shuttling during battery cycling, they designed carboncoated hollow cages of NiCo phosphide. Although the results
are not compared to a coating-less counterpart, successful
polysulde mediation is demonstrated. For this, two strategies
are combined: rst, a hollow structure to mitigate the volume
expansion in polysulde adsorption, and second, the carbon
coating by using polydopamine to entrap these species andincrease the capacity stability of the cell. A rather less explored
strategy is employing electrospinning to either decorate electrospun bres by the PBD or to directly electrospin the PBA/D. A
1D structure can be produced by controlling the bre size andReview
providing a high surface area and active site availability.
Moreover, the polymer-based bre can be heat treated and used
as a carbon substrate to increase the overall conductivity of thecomposite.70 For example, Wei et al.100 prepared FeCo phosphide on nitrogen-doped carbon bres through electrospinning. They further annealed and calcined the material andthen performed a second derivatization by using NaH2PO2
powder as the phosphor source. Electrospinning provides
a high surface area and access sites to evenly distributed
particles while protecting them from agglomeration due to theopen bre morphology, which aids in the catalytic performance.

5.

Summary and outlook

The Prussian blue family of compounds emerged as a new classof precursors/templates for synthesizing advanced materials.
They provide a source of diﬀerent metallic species, carbon, andnitrogen arranged in open framework cubic structures. The
cubic morphology can be tuned into various complex, hierarchical, and porous/hollow structures, while composition can bechanged through surface modication and ion exchange
processes. In addition, the template synthesis is water-based
and usually low-cost, making it a sustainable and scalable
derivatization template. Such structures can be converted intoa wide array of metal-based compound compositions whosemorphology is a direct product of the PBA template. Conversion
is achieved by diﬀerent derivatization methodologies, mainly

Fig. 5 Outlook on PBDs. Many compositions remain unexplored. Further investigating new synthetic methods, composites and material processing can improve PBD performance in current and new applications.

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Reviewconsisting of thermal and hydrothermal methods. Most derivatives are characterized by increased surface area and homogeneous morphology and composition. The diﬀerent PBDs andtheir composites have enhanced performance for several
applications, such as catalysis and diverse energy storage
technologies.
Though PBD reports are quickly growing, several aspects ofusing PBAs as templates are overlooked. First, regardless of theincreased interest in PBDs, most reports are restricted toa limited number of synthetic approaches and compositions. In
addition to the classic PBA synthesis route via a liquid method,cyanogels can aid in expanding the PBA chemistry past thetransition metal combinations.119,120 The extensive work on PBA
synthesis and particle control should be taken advantage of to
explore and achieve a well-tailored PBA template for derivatization. In terms of composition, much can be done bytemperature control, while the ratio of metals will playa dening role in the phases that can be obtained. Combining
multiple M species could also contribute to a more diverse
range of compositions. For example, the gradient synthesis of
PBAs is an unexplored method for PBDs.46,47 The gradientenables complex compositions, a mixture of diﬀerent metal
species, and could easily create PBDs with M1@M2 core–shellstructures. Even though PBAs with diﬀerent M species havebeen reported, most PBD studies consist of M = Fe, Co, Ni, andMn (Fig. 5). Meanwhile, a challenge is posed for compositions
without Fe and Co, given the availability of [M′(CN)6] salts.
Currently, ion-exchange synthesis seems to be the only
described way to remove M′ species, besides specic acidwashing for Fe removal.
PBA can show diﬀerent structures with lower or higher
degrees of defects (as in the soluble and insoluble forms) andcation content. However, there is still a lack of understanding ofwhether and how such defects and cations inuence thederivative. For example, a few studies observed the presence ofalkali-containing phases in the resulting derivative arising fromthe PBA alkali-rich structure.56,63 The counter-cation largely
inuences PBA structure and properties,112 therefore, more
attention should be given to counter-cations present in the PBA
synthesis, as it is retained in derivatization through heat treatment. For example, adding Li+ in the PBA synthesis could leadto a lithiated PBD phase for energy storage applications. The
lack of counter-cations in PBA is associated with a defective
structure. Defect engineering is a promising method to introduce inherent conductivity rather than adding other components like carbon, for example, in metal oxides. More insight
into the eﬀect of defects in the original PBA structure would
help better to design PBA synthesis and pre-treatment for
defect-oriented PBDs.
In terms of derivatization methods, liquid methods remain
limited, even though they have the advantage of mild/room
temperature conditions which can prevent the collapse ofhighly porous/hollow structures. More elaborate compositions
and complex morphologies could arise from diving into liquid
ion exchange and etching methods during the pre-treatment
and derivatization steps. In this case, post-processing could
be limited to low-temperature treatments to induce

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Journal of Materials Chemistry A
crystallization, while temperatures for graphitization might beprohibitive. Besides, new experimental approaches could beconsidered for transforming PBAs into PBDs, like plasmaassisted and electrochemical methods.
When using multiple metallic components, a challenge is toderive single-phase compounds. The weaker bond between M–
NC compared to that between M′–CN will likely promote better
diﬀusion of M species upon the heat treatment, which can leadto phase separation.121 Using a gas with lower reactivity (e.g.,
CO2) instead of synthetic air can lower the oxidation rate andretain carbon species. Using H2S gas instead of sublimed sulfur
powder also provides a stable sulfur supply that can result inbetter homogeneity and control in size and phase.
Much was discussed in terms of morphology change anddesign during PBD synthesis steps. However, that is mainly
done starting from cubic particles. Elaborating on the anisotropic growth of PBA particles could unlock a new range ofmorphologies and properties of PBDs.53,54 Expanding on thetemplating of PBA during their synthesis could also increase themorphological complexity of the resulting PBD. So far, nickel
foam is the main adopted substrate for PB growth. Nevertheless,template-oriented growth can be widely varied to produce lms,
rods, wires, and arrays, among others.7,122–125
Most PBA derivatization results in PBD powders for electrochemical applications. This means processing into electrodematerials with the aid of binder, carbon, solvents, and coatingprocesses. Exploring new combinations of PBA synthesis andderivatization like electrospinning, aerogels, 3D printed structures, or self-supporting lms can enable the preparation of
free-standing and exible PBD materials. Similarly, the PBDs
can also be further processed in post-treatment steps to create
new composites or assemblies that are not possible before thederivatization method, for example, the addition of conductive
polymers (that would otherwise decompose in the treatments),
MXenes (that could themselves convert into oxides, suldes,etc.), nanoparticles (that would coarsen), etc.
In terms of application, PBAs are primarily used as templates
due to the porous structure with ion-compatible voids and highsurface area desired in catalysis and energy storage. Nevertheless, each compound class (oxides, suldes, carbides, etc.) has
numerous applications. Both selenides and suldes haveessential applications in solar cells, and only a few reports of
PBDs address such use.90,94 Waste treatment is also an overlooked potential application of PBDs. The high porosity andsurface area make them ideal for absorption, while the tunable
voids and metallic species can induce selectivity towards
specic compounds. The HER and OER are primarily studied in
the widely explored catalysis eld. However, (photo)catalytic
degradation could greatly benet from such structured materials. Iron species are known to provide high photocatalyticeﬃciency. The straightforward implementation of additional
metals like Ti, Sn, or Zn in the PBA synthesis can further tune
the catalytic activity. At the same time, target pre-treatment andderivatization can result in a highly active material. Similarly,
other energy storage systems besides Li-ion, Li–S, and Na-ion
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Journal of Materials Chemistry A
composition and structure of PBDs. For example, the catalytic
properties could be used in the ORR in Li–air and Zn–air
cathodes.
Using PBA as precursors and templates is an exciting strategy
to get unique, specic, and sophisticated materials aiming attarget properties and applications. The PBD properties strongly
correlate to the PBA structure/composition and the experimental steps employed to derivate the PBA. Exciting scienticand technological challenges emerge from this topic. We expect
this review to be useful for chemists, physicists, materials
scientists, and engineers interested in exploring this fascinating
subject.

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

Acknowledgements
S. H. acknowledges the funding of the PBDS project (HU 2959/21) by the German Research Foundation (DFG, Deutsche Forschungsgemeinscha). The INM authors thank Eduard Arzt
(INM) for his continuing support. A. J. G. Z. thanks INCT
Nanocarbono, INCT NanoVida, CAPES, and CNPq for nancial
support.

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