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This article was published as part of the
2009 Metal–organic frameworks issueReviewing the latest developments across the interdisciplinary area of
metal–organic frameworks from an academic and industrial perspective Guest Editors Jeffrey Long and Omar Yaghi
Please take a look at the issue 5 table of contents to access the other reviews.
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Thin films of metal–organic frameworksw
Denise Zacher,aOsama Shekhah,
bChristof Woll
band Roland A. Fischer*
a
Received 6th October 2008
First published as an Advance Article on the web 9th March 2009
DOI: 10.1039/b805038b
The fabrication of thin film coatings of metal–organic frameworks (MOFs) on various substrates is
discussed in this critical review. Interestingly, the relatively few studies on MOF films that have
appeared in the literature are limited to the following cases: [Zn4O(bdc)3] (MOF-5;
bdc = 1,4-benzenedicarboxylate), [Cu3(btc)2] (HKUST-1; btc = 1,3,5-benzenetricarboxylate),
[Zn2(bdc)2(dabco)] (dabco = 1,4-diazabicyclo[2.2.2]octane), [Mn(HCOO)], [Cu2(pzdc)2(pyz)] (CPL-1;
pzdc = pyrazine-2,3-dicarboxylate, pyz = pyrazine), [Fe(OH)(bdc)] (MIL-53(Fe)) and
[Fe3O(bdc)3(Ac)] (MIL-88B; Ac = CH3COO�). Various substrates and support materials have been
used, including silica, porous alumina, graphite and organic surfaces, i.e. self-assembled monolayers
(SAMs) on gold, as well as silica surfaces. Most of the MOF films were grown by immersion of the selected
substrates into specifically pre-treated solvothermal mother liquors of the particular MOF material. This
results in more or less densely packed films of intergrown primary crystallites of sizes ranging up to several
mm, leading to corresponding film thicknesses. Alternatively, almost atomically flat and very homogenous
films, with thicknesses of up to ca. 100 nm, were grown in a novel stepwise layer-by-layer method. The
individual growth steps are separated by removing unreacted components via rinsing the substrate with the
solvent. The layer-by-layer method offers the possibility to study the kinetics of film formation in more
detail using surface plasmon resonance. In some cases, particularly on SAM-modified substrates, a highly
oriented growth was observed, and in the case of the MIL-53/MIL-88B system, a phase selective deposition
of MIL-88B, rather than MIL-53(Fe), was reported. The growth of MOF thin films is important for smart
membranes, catalytic coatings, chemical sensors and related nanodevices (63 references).
1. Introduction
Smart membranes, catalytic coatings, chemical sensors,
and many other related nanotechnological devices and
applications depend on the fabrication of thin films and
coatings of defined porosity, combined with tuneable chemical
functionality. Zeolites, organic polymers, metal oxides
and activated carbon are the typical materials used for this
purpose. Zeolites and related siliceous materials have interest-
ing properties in comparison with the others, namely very well
defined, highly regular pore structures, and often exceptionally
high chemical and thermal stabilities. However, the range of
control of functionality on a molecular level is nevertheless
limited with zeolites and its inorganic congeners. Coordination
a Anorganische Chemie II-Organometallics and Materials,Ruhr-Universitat Bochum, D-44780 Bochum, Germany.E-mail: roland.fischer@rub.de; Fax: +49 234 321 4174;Tel: +49 234 322 4174
b Physikalische Chemie 1, Ruhr-Universitat Bochum,D-44780 Bochum, Germany
w Part of the metal–organic frameworks themed issue.
Denise Zacher
Denise Zacher, born 1981,obtained her BSc at theUniversiat Duisburg-Essenand received her MSc inchemistry at the Ruhr-Universitat Bochum in 2006.She is currently doing herdoctoral thesis under thesupervision of Roland A.Fischer on the growth ofMOF thin films and nano-crystals, within the frame ofthe Priority Programm 1362‘‘Metal Organic Frameworks’’of the German Research Foun-dation (DFG). Her work is
also associated with the specific targeted research project‘‘SURMOF’’ of the European Union (6th FP).
Osama Shekhah
Osama Shekhah, born 1973,studied chemistry in Jordan.He received his PhD in thefield of physical and surfacechemistry in 2001 from theFreie Universitat Berlin underthe supervision of Prof. R.Schlogl. He then worked forone year as a postdoc at theFritz-Haber institute in Berlin.In 2005 he joined the physicalchemistry group of Prof. C.Woll at the Ruhr-UniversitatBochum as a staff scientist.His research interests includesurface chemistry, self-
assembled monolayers, and metal–organic frameworks and theirgrowth on organic surfaces.
1418 | Chem. Soc. Rev., 2009, 38, 1418–1429 This journal is �c The Royal Society of Chemistry 2009
CRITICAL REVIEW www.rsc.org/csr | Chemical Society Reviews
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polymers with permanent porosity (PCPs), or so-called metal–
organic frameworks (MOFs), are hybrid inorganic–organic
solid state compounds with zeolite-like structures and
properties, but transgressing the limitations of zeolites in terms
of materials chemistry, at least in principle.1 A large body of
research on MOFs is directed to unravel the rules of reticular
synthesis and to develop the tool box needed for a ‘‘design’’ of
MOFs with desired properties.2–5 Consequently, the work in
this area concentrates on the synthesis of novel MOFs and
structure/property/function correlations, in particular looking
at responsive, flexible frameworks and introducing interesting
chemical functionality.6–8 This is also reflected by the
collection of articles in this themed issue of Chem. Soc. Rev.
on metal–organic frameworks. Recently, Kitagawa and
Matsuda pointed out: ‘‘Researchers control the size, shape,
and distribution of pores and will establish this engineering in
the near future. However, even when they have nanosized
channels or cavities, compounds are at least mm-sized micro-
crystals, insoluble in any solvent and therefore hard to prepare
in a thin layer form. . . The ultimate goal is the ability to
control the arrangements of channels of porous modules for
various nanodevices’’.8 Thus, there are two challenges: the
deposition or growth of MOF thin films on substrates, ideally
in a dense, homogeneous and oriented fashion, and also the
preparation of size-, shape- and surface-functionalized MOF
nanocrystals, which can act as wells, wires, rods and dots.
Both areas are, however, connected and the general principles
of how to approach these problems are similar to the related
research on zeolite thin films9–12 and zeolite nanocrystals.13,14
This review will discuss the most recent progress in the
fabrication of MOF thin films. It should be noted that the
specific topic of MOF thin films is closely related to the more
general field of bottom-up synthesis of functional coatings on
surfaces. In particular, the controlled growth of non-porous
coordination polymers or related inorganic–organic hybrid
structures on surfaces aimed at specific functions is an
important area, but cannot be covered in this review here.
Some reference is, however, given in sections 4 and 5 below.
There are three different concepts of MOF thin film fabrica-
tion: (A) the direct growth/deposition from solvothermal
mother solutions, (B) the assembly of preformed, ideally size
and shape selected, nanocrystals and (C) the stepwise layer-
by-layer growth onto the substrate. To the best of our
knowledge, MOF thin films have only been manufactured by
employing methods of type A and type C so far. Some very
limited research has been carried out on MOF nanocrystals
and/or MOF colloids, of which the results are reviewed by
Spokoyny et al. in this themed issue of Chem. Soc. Rev..15 The
influence of the surface chemistry (functionality) of the chosen
substrate and the use of MOF seeds on the nucleation,
orientation, as well as on the adhesion of the MOF films,
are important factors to be addressed. In particular,
self-assembled organic monolayers (SAMs)16 have been used
to direct the nucleation, orientation and structure of the
deposited MOFs. The control of crystallisation of inorganic
solid state materials by the influence of organic macro-
molecules is the underlying principle of biomineralisation,17
which can be transferred to MOFs as inorganic–organic
hybrid polymers. SAMs may be used as a well-defined
artificial organic interface that mimics the structure-directing
power of the complex biointerfaces, and the oriented
growth of zeolites on substrates has been demonstrated by
using SAMs.18
Interestingly, the relatively few studies on MOF films
are limited to the following: [Zn4O(bdc)3] (MOF-5; bdc =
1,4-benzenedicarboxylate),19–21 [Cu3(btc)2] (HKUST-1;
btc = 1,3,5-benzenetricarboxylate),22–25 [Zn2(bdc)2(dabco)]
(dabco = 1,4-diazabicyclo[2.2.2]octane),22 [Mn(HCOO)],26
[Fe(OH)(bdc)] (MIL-53(Fe)) and [Fe3O(bdc)3(Ac)] (MIL-88B;
Ac = CH3COO�).27 Some of the most recent new reports,
Christof Woll
Christof Woll studied Physicsin Gottingen and received hisPhD in 1987 under theguidance of Peter Toennies inthe field of surface science.After a postdoctoral stayat the IBM Research Lab,Almaden, USA (1988–1989),he joined Heidelberg Univer-sity, where he obtainedhis Habilitation in 1992and was then appointedHochschulassistent (AssistantProf.). In 1994 he wasawarded a Heisenberg Fellow-ship from the German DFG.
Since 1997 he has held a chair in Physical Chemistry at RuhrUniversitat, Bochum. His current research focuses on thechemistry of oxide surfaces, heterogeneous catalysis, organicmolecular beam deposition, organic surfaces exposed by self-assembled monolayers, the nucleation of metal–organic frame-works, as well as photoelectron spectroscopy and scanningtunnelling microscopy.
Roland A. Fischer
Roland A. Fischer studiedChemistry at the TechnischeUniversitat Munchen (TUM)and received his Dr rer. nat. in1989 under the guidance ofWolfgang A. Herrmann. Aftera postdoc with Herb Kaesz atUCLA, he returned to TUMin 1990, obtained his Habilita-tion in 1995 and was appointedAssociate Prof. at theRuprecht-Karls Universitat,Heidelberg in 1996. He movedto Ruhr Universitat, Bochumin 1998 for a chair in InorganicChemistry. Currently he is
Dean of the Ruhr Universitat Research School. His researchinterests focus on group 13/transition metal bonds and clusters,precursor chemistry for inorganic materials, chemical vapourdeposition (CVD), thin films, nanoparticles, colloids and hostguest chemistry of porous coordination polymers (MOFs).
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which the authors became aware of during editing the
manuscript, are cited in section 5 (Conclusions and perspec-
tives). The deposition of non-porous coordination polymers in
the form of coatings or free-standing thin films are not
included in this review.28 We will start our discussion with a
summary of the growth of MOFs on bare oxidic substrates,
such as alumina, silica and carbon.
2. Deposition of MOF coatings on alumina, silica
and carbon substrates
Hermes et al. described the deposition of MOF-5 micro-
crystals (5–10 mm) on c-plane sapphire at 25 1C over 24 h
using a cooled, aged solvothermal mother solution (in N,N-
diethylformamide (DEF)).19 The density of nucleation and/or
anchoring sites for the seeds and growing MOF crystals is low
on very smooth (ideally atomically flat) and defect-free
surfaces. The deposition of an amorphous Al2O3-type buffer
layer onto the c-plane sapphire substrate by means of ALD
(atomic layer deposition)20 leads to a denser coating with
intergrown MOF-5 crystals (Fig. 1). Interestingly, it was not
possible to deposit MOF-5 on silica substrates (e.g. the native
SiO2 coating on a silicon wafer) by using the same
conditions.20,21 A related study by Zacher et al. on the
deposition of HKUST-1 at 120 1C directly from the solvo-
thermal mother solution (water–ethanol) gave quite similar
results (Fig. 1).22 The striking difference between SiO2 and
Al2O3 surfaces was attributed to the isoelectric points of these
materials, and the obvious requirement of a basic, i.e. electro-
statically compatible, surface for anchoring MOF-5
and HKUST-1 under these conditions. Interestingly, the
deposition experiments of [Zn2bdc2(dabco)], again under
solvothermal conditions at 120 1C in dimethylformamide,
showed no substrate selectivity.
Dense coatings (see Fig. 1) were obtained on SiO2 and
amorphous Al2O3 as well. The bifunctional nature of this
pillared, layer-based MOF, with acidic bdc linkers and basic
dabco pillars, is favourable for binding to acidic SiO2 as well
as basic Al2O3 substrates. The chemical and structural identity
of the deposited materials were confirmed by X-ray diffraction
studies and comparison with authentic bulk materials. The
adsorption properties of the MOF coatings (after activation)
were qualitatively tested by exposure to a coloured volatile
organometallic compound, as shown in Fig. 2. Up to now,
only three groups have followed these initial studies. Yoo and
Jeong demonstrated the rapid deposition of MOF-5 on
carbon-coated anodic aluminium oxide (AAO) by micro-
wave-induced thermal deposition, 21 Gascon et al. developed
a seeding approach for the deposition of HKUST-1 and
Fig. 1 The deposition of MOF-5, HKUST-1 and [Zn2(bdc)2(dabco)] on SiO2, alumina and COOH/CF3-modified surfaces (SAMs). Figures
adapted from ref. 20 with permission, Copyright 2007, American Chemical Society and ref. 22 with permission, Copyright 2007, RSC Publishing.
Fig. 2 A qualitative demonstration of the adsorption properties of
MOF-5 coatings on alumina: optical images (digital photographs) of
an empty 5 mm thick MOF-5 thin film on sapphire substrates before
(left) and after (middle) exposure to the vapour of the deep red
MOCVD precursor [(Z5-C5H5)Pd(Z3-C3H5)]. Subsequent treatment
of the loaded film with UV light converts [(Z5-C5H5)Pd(Z3-C3H5)]@
MOF-5 into Pd@MOF-5, visible by the colour change to deep black
(right). Reproduced from ref. 20 with permission, Copyright 2007,
American Chemical Society.
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obtained very dense coatings,25 and Arnold et al. studied the
oriented crystallisation of [Mn(HCOO)2] on porous alumina,
as well as on graphite.26 The microwave method required a
conductive surface; there was little MOF-5 deposition on bare
AAO, but a coating of well-developed cubic crystallites of
14 mm on amorphous carbon/AAO and a dense, smooth
coating of smaller crystallites (7 mm) on graphite/AAO. The
higher nucleation density on graphite/AAO was attributed to
the better absorption of the microwaves (i.e. local heating).
The authors speculated about the origin of the observed
strong adhesion and the preferred orientation with the [10-2]
direction perpendicular to the substrate. The intense micro-
wave Joule heating may cause the formation of
carboxylic acid groups on the graphite surface under the
conditions of the experiment, which is beneficial for both
adhesion and orientation. However, the authors provided no
analytical evidence for that idea. At least MOF-5 was shown
to preferentially bind to COOH-functionalized SAMs.19 The
adhesion of MOF-5 to graphite/AAO was tested by using a
sonication method. About 80% of the coating remained after
sonication for 1 h. It should be noted here that rapid synthesis
of MOFs using rather highly concentrated solutions, and in
particular microwave assisted heating, may be complicated by
the adsorption of unreacted metal species, organic linkers and
precursors of the secondary building units (SBUs) in the
cavities of the growing MOF. A detailed study on microwave
assisted synthesis procedures of bulk samples of MOF-5 was
published by Hafizovic et al.,29 also showing that rapid
synthesis may cause the formation of an interpenetrated phase
of MOF-5. However, Yoo and Jeong did not provide any
porosity or adsorption experiments on their MOF-5 films.
Gascon et al. reported dense coatings of HKUST-1 on
a-alumina supports by a combination of suitable seeding with
low concentration mother liquors. The best results in terms of
thin film morphology (studied by scanning electron micro-
scopy (SEM), see Fig. 3) were obtained by seeding
(spin coating) with a slurry of the 1D coordination polymer,
catena-triaqua-m-(1,3,5-benzenetricarboxylate)copper(II)[Cu(Hbtc)(H2O)3], obtained by modification of the original
HKUST-1 recipe. Using water as the solvent instead of a 1 : 1
ethanol–water mixture, only two of the three carboxylic
groups of the 1,3,5-benzenetricarboxylic acid are deproto-
nated. Dense coatings of small intergrown octahedral micro-
crystals (B2 mm) are formed by immersion of the pre-treated
substrates into a diluted solvothermal mother solution at
110–120 1C over a period of 12–18 h. Alternatively, slurries
of amorphous so-called ‘‘proto-HKUST-1’’ can be used for
seeding purposes to obtain coatings with slightly larger
crystals (B5 mm). This proto form of HKUST-1 precipitates
when concentrated water–ethanol solutions of copper(II)-
nitrate and btc are combined at room temperature. Based on
PXRD studies, the authors claimed the coatings consisted of
phase-pure HKUST-1, without preferential orientation. The
authors point out that no traces of the characteristic peaks of
the 1D seeding MOF were detected in the XRD pattern of the
coating, and suggest a rather complete conversion of the
seeding material into the HKUST-1 phase under the
conditions of the experiment. However, the quality of the
published PXRD pattern (broad lines and high background
noise) was surprisingly low compared to the microcrystalline
HKUST-1 reference samples. Also, neither qualitative nor
quantitative data on the adsorption and porosity properties
of the obtained coatings were given in the publication.
The general problem of inhibited nucleation, and thus low
crystal density, on bare alumina and graphite was also
observed in the case of deposition of [Mn(HCOO)2] under
solvothermal conditions at 115 1C (1,4-dioxane–DEF).
Arnold et al. reported densities of 10 crystals mm�2 for
alumina and 80 crystals mm�2 for graphite.26 Electrostatic
repulsion between the substrate (alumina) and the MOF nuclei
and growth species was suggested as the key factor when
[Mn(HCOO)2] is synthesized according to the original recipe
by Dybtsev et al.30 By developing the so-called ‘‘formate’’
route (i.e. using sodium formate rather than formic acid) and
oxidized graphite substrates (i.e. creating different oxo-
functionalities at the surface), nucleation was enhanced and
rather dense coatings of [Mn(HCOO)2] with crystallite sizes
4100 mm were obtained. The important factor for membrane
applications, however, is the orientation of the 1D channels of
the [Mn(HCOO)2] structure in a perpendicular, or at least
tilted, fashion with respect to the underlying (meso/macro)
porous substrate. Only in case of the ‘‘formate’’ route with
functionalized graphite as a substrate were the authors
successful in obtaining a coating with a preferred orientation
and a tilt angle of about 341 of the 1D channel system
(see Fig. 4). The same authors studied the methanol uptake
and anisotropic mass transport on individual single crystals of
Fig. 3 SEM micrographs of HKUST-1 layers obtained under
different synthesis conditions according to Gascon et al. Adapted
from ref. 25 with permission, Copyright 2008, Elsevier.
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[Mn(HCOO)2] by interference microscopy,31 but again, data
on the porosity and transport properties of the deposited
MOF coatings were not given.
3. Deposition of MOFs on substrates modified
by self-assembled organic monolayers
The above cited observation of inhibited nucleation of MOFs
on bare metal oxide and graphite supports, together with the
known property of SAMs to direct the nucleation and growth
of zeolites,18 motivated the first study of MOF growth at
SAM-modified substrates. Again, MOF-5 was chosen as the
first test case. By immersion of SAM-modified Au substrates
into a preconditioned, supersaturated mother solution in DEF
at 25 1C, Hermes et al. obtained the selective deposition
of well-shaped MOF-5 cubes of 0.5–1 mm at the COOH-
terminated sites of the patterned, mixed COOH/CF3-
terminated SAM surface.19 The thiol-type SAMs used on
Au are not thermally robust; thus, the growth studies were
undertaken at room temperature. Note, that MOF-5 can be
synthesized at room temperature in DMF in the presence of
triethylamine as base.32 In case of the standard solvothermal
recipe in DEF at 105 1C, however, the situation is more
complex, and the thermal decomposition of DEF is an
important process.33 The obtained MOF-5/SAM coating was
activated by the known process of washing with DEF and
CHCl3, and gentle drying in vacuo at elevated temperatures.
The adsorption of an organometallic Pd-precursor from
the gas phase was used as a qualitative test for the
adsorption properties, similarly to the above cited case of
MOF-5/alumina coatings.20
It is well known, that MOF-5 deposits as perfect cubes of
several mm in length from the standard solvothermal mother
solution (Fig. 5). This perfect cubic shape persists from the
early stages of homogeneous crystal growth after the so far
still unknown nucleation step, as shown by in situ time
resolved light scattering.34 Also, the Zn2+ sites in the bulk
structure of MOF-5 are coordinatively saturated, which
possibly limits the influence of structure- and orientation-
directing additives being present during the crystal growth
and deposition on the chosen substrates. This is different for
HKUST-1, which consists of btc-bridged Cu2 units. These
paddle-wheel type copper dimers exhibit weakly bound water
molecules in the apical position of the Cu2+ centres, which can
be fully desorbed in a drying step, as well as being reversibly
exchangeable against other (weak) ligands, such as pyridine.22
Therefore, Biemmi et al. chose HKUST-1 as candidate for a
study aiming at the oriented growth of HKUST-1 on differ-
ently terminated SAMs.24 They used preconditioned, aged
(8 d, 75 1C) and filtered standard HKUST-1 mother solutions
cooled down to room temperature (presumably containing
only low concentration of the growth species). The growth of
HKUST-1 microcrystals on Au substrates modified with thiol-
based SAMs using HS(CH2)10COOH, HS(CH2)10CH2OH and
HS(CH2)10CH3 was monitored by SEM over a period of 16 to
100 h. The key finding was a highly oriented growth of
individual crystals over time. Under the conditions of the
Fig. 4 SEM micrographs of [Mn(HCOO)2] layers on an oxidized graphite support according to Arnold et al. Adapted from ref. 26 with
permission, Copyright 2007, Wiley-VCH.
Fig. 5 MOF-5 deposition of SAM-modified silica/silicon substrates.
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experiment, the COOH-terminated SAM favoured orientation
along the [100] direction, which resulted in the formation
of pyramids. The OH-terminated SAM favoured the [111]
direction, which led to the formation of octahedral crystals
resting on one triangular face (see Fig. 6).
Interestingly, the authors observed a growth on CH3-
terminated SAMs as well. An even faster growth process
was reported than on the other two polar and coordinatively
more strongly interacting surfaces. However, a strongly
preferred orientation was absent. The growth on CH3-
termination is a somewhat surprising result; however,
dispersive forces between the supposedly organic-terminated
crystal faces and the alkyl-terminated SAM may be dominant
in that case. One should note that in the case of CF3-
terminated thiol-based SAMs on Au, as well as silane-based
SAMs on SiO2/Si substrates (vide infra), neither MOF-5 nor
HKUST-1 could be grown.19,20,22 Nevertheless, the protocol
to achieve the highly selective growth developed by Bemmi
et al. appears a bit complicated. The authors argue that the
thermal pre-treatment of the synthesis solution (8 d at 75 1C)
induces the crystallization process. After filtration and
removal of the deposited solid product (see also Gascon
et al.), they suggest the existence of colloidal nanocrystals or
small molecular building blocks of HKUST-1 in the possibly
rather diluted solution.
Prolonged exposure of 4100 h of all the SAM-modified
substrates to the growth solution resulted in the formation of
about 600 nm thick films of intergrown crystals. In parallel,
and independently, Zacher et al. studied the deposition of
HKUST-1 on COOH- and CF3-terminated (patterned) silane-
based SAMs on SiO2/Si-substrates, as well as on c-plane
sapphire under the usual solvothermal growth conditions of
110–120 1C.22 Silane SAMs are known to be much more
thermally robust than thiol-based SAMs. Again, a highly
selective growth of HKUST-1 on the COOH-terminated sites
of the SAMs was observed. However, the deposited crystals
were orientated along the [111] direction, just the orientation
that Bemmi et al. observed for OH-terminated SAMs, but not
for COOH-termination. Note, that the carboxylic acid termi-
nating groups of the SAM can interact in various ways
with surface-exposed Cu2-dimers in order to complete the
paddle-wheel structural motif. In fact, a SAM with COOH-
termination is compatible with both orientations, either along
[100], or along [111] (see Fig. 7). This discrepancy of the two
results on COOH-terminated SAMs may be related to the very
different growth conditions; on the one hand the low tempera-
ture growth and possibly rather low concentration of growth
species, and on the other hand, high temperature and higher
concentrations. More detailed studies on the temperature and
concentration dependence of HKUST-1 growth on top of
SAMs are clearly warranted.
In addition to the oriented growth of MOFs on SAMs, the
structure-directing influence of SAMs on heterogeneous
nucleation was recently demonstrated. Scherb et al. reported
the growth of MIL-88B(Fe) on SAMs of mercaptohexa-
decanoic acid on Au;27 this is quite similar to the above
outlined study on deposition of HKUST-1/SAM, but now
using a preconditioned mother solution for the MIL(Fe)
synthesis. In the Fe3+/bdc system, several different MOF
Fig. 6 (a) X-Ray diffraction patterns (background corrected) of thin
films of HKUST-1 on functionalized gold surfaces, compared with a
randomly oriented powder reference sample measurement. Each
pattern is normalized to the most intense reflection. Bottom: schematic
illustrations of the oriented growth of HKUST-1 nanocrystals
controlled via surface functionalization; (b) on an 11-mercapto-
undecanoic acid SAM, and (c) on 11-mercaptoundecanol-modified
gold surfaces. The alkanethiol self-assembled monolayers are repre-
sented with a tilt of ca. 301 from the surface normal as reported in the
literature. Figures adapted from ref. 24 with permission, Copyright
2007, American Chemical Society.
Fig. 7 The matching of different lattice planes of HKUST-1 with
COOH-terminated SAMs.22
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structures are known, including the so-called MIL-53(Fe) and
MIL-88B types.
These frameworks are quite flexible and the exact cell
parameters strongly depend on the interaction with guest
molecules, which makes these materials particularly interest-
ing candidates for sensors and smart membranes.35,36 In the
monoclinic structure of [Fe(OH)(bdc)(py)0.85], the Fe3+
version of MIL-53, chains of FeO6 octahedra are connected
by bdc. Thus, rhombic 1D channels are formed that run along
the a axis of the structure. The hexagonal 3D structure of
MIL-88B is built up from trimers of FeO6 octahedra linked to
bdc. Thus, the 3D pore system of MIL-88B consists of tunnels
along the c axis connected by bipyramidal cages
(see Fig. 8).27,37 The SAM-functionalized gold substrates were
placed face-down in a crystallization solution, obtained by the
solvothermal treatment of a synthesis mixture for MIL-53 at
150 1C for 2 d, filtration, and further treatment of the clear
solution at 150 1C for 5 d. The X-ray diffraction analysis of the
crystals attached to the SAM revealed a phase-selective
growth of the MIL-88B material, with an orientation along
the [001] direction. During the experiment, however, a
precipitate formed again. This precipitate was identified as
the other phase, pure MIL-53(Fe). The authors discussed this
observation as follows. As indicated in Fig. 8, the structure of
MIL-53(Fe), the product of homogeneous nucleation from the
crystallization solution, differs dramatically from that of
MIL-88B(Fe), the product of heterogeneous nucleation. This
effect is attributed to symmetry transfer, that is, the different
symmetry relationships between the COOH-terminated SAM
and the two crystal systems. No crystal growth was observed
with hydroxy- and alkyl-terminated SAMs, or with untreated
gold slides. Presented with a surface exposing (approximately)
hexagonal symmetry, the growth species, e.g. the SBUs and
bdc linkers, clearly prefer to assemble in the form of a
hexagonal MIL-88B structure type instead of monoclinic
MIL-53. However, we suggest using terms homogeneous and
heterogeneous nucleation with care in these cases. Virtually
nothing is known about the composition of the growth
solutions in terms of the molecular species of relevance for
nucleation and further growth. It is possible that nuclei, and
even larger nanocrystals of both MIL species, are formed in
the solution in a homogeneous manner, but only the MIL-88B
types matches to the ‘‘sticky’’ SAM surface, which then leads
to the observed selectivity. There is no direct evidence for a
‘‘heterogeneous’’ mechanism of the nucleation step of
MIL-88B growth on the particular SAM. Also, one should
keep in mind that evidence on the persistence of SBUs during
the formation of MOFs under the typical solvothermal, and
in fact heterogeneous, conditions is quite shallow.38 The
adsorption properties of the deposited crystals of
MIL-88B(Fe) were tested by taking advantage of the strong
breathing effect of the structure, depending on the accommo-
dation of guest molecules. The samples were exposed to a
vapour phase saturated with DMF for 24 h and the expected
breathing effect upon exposure/drying cycles was observed by
X-ray diffraction. Obviously the attachment to the SAM, i.e.
some elasticity of the organic interface to the substrate, can
accommodate the shrinkage and expansion cycles.
The obvious and common drawback of all of the above
mentioned pioneering reports on the deposition of MOFs on
SAMs from somehow pre-treated mother solutions, is that in
fact smooth and dense MOF thin films were not obtained at
all. Rather scattered, more or less isolated crystals, or island of
crystals, or rough coatings with many cracks were deposited,
even after reaction times of several days.
4. Stepwise layer-by-layer liquid epitaxy of MOFs
In contrast to the established synthesis protocols of MOFs,
where the educts (primary building blocks, typically two) are
mixed and reacted under solvothermal conditions, the stepwise
layer-by-layer growth mode of MOFs as introduced by
Shekhah et al.23,39 is based on the combination of the reaction
partners in a sequential, stepwise fashion. The individual steps
are separated by removing unreacted components via rinsing
with a solvent, see Fig. 9. The principles of such a layer-
by-layer growth mode of supramolecular architectures on
surfaces is well known,40 but has only recently been
transferred to the fabrication of MOF thin films. As an
intriguing example for the bottom-up synthesis of functional
inorganic–organic hybrid thin film materials in a more general
sense, we would like to cite the more recent work of Altman
et al. and refer to further references given therein.41
Obviously, the layer-by-layer and bottom-up concepts of
the assembly of supramolecular structures rely on surface
chemistry. From a more general point of view, it is also related
to the well-established solid-phase synthesis of complex
(bio)organic polymers, such as peptides, DNA, etc. The ideal
substrates to start with in such a layer-by-layer deposition of
organic ligands or metal-oxo coupling units, are organic
surfaces as exposed by SAMs.16 Since a fairly large number
of MOF structures reported so far are based on carboxylate
coupling units, the choice of COOH-terminated SAMs
exposing a COOH-terminated surface is quite obvious. It
has to be noted, however, that the fabrication of such
COOH-terminated surface from the corresponding organo-
thiols is not trivial.42,43 Indeed, by using the COOH-
functionalized organic surface of a mercaptohexadecanoic
acid SAM (MHDA) as a 2D anchoring and nucleation site,
Fig. 8 Left: in the Fe3+/bdc system, MIL-53(Fe) forms as the
product of ‘‘homogeneous nucleation’’, while MIL-88B(Fe) deposits
in an oriented fashion on COOH-terminated SAMs as the product of
‘‘heterogeneous nucleation.’’ Right: a schematic representation of the
observed oriented growth of the MIL-88B(Fe) phase on mercapto-
hexadecanoic acid SAMs on gold substrates at 25 1C from aged
mother solutions. Figures adapted from ref. 27 with permission,
Copyright 2008, Wiley-VCH.
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the growth of homogenous, structurally well-defined MOF
structures in a step-by-step fashion was demonstrated by
Shekhah et al. for the case of HKUST-1 (Fig. 9).23
The two components, copper(II)acetate (CuAc2) and 1,3,5-
benzenetricarboxylic acid (H3btc), were separately dissolved in
ethanol and the substrate was immersed into each solution in a
cyclic way, while each step was followed by rinsing with pure
ethanol. By starting with copper(II)acetate, a linear increase of
thickness of the deposited HKUST-1 layer with the number of
(alternating) immersion cycles in CuAc2 and btc could be
demonstrated in situ by surface plasmon resonance (SPR)
spectroscopy (see Fig. 10). Most importantly, XRD data
recorded for these deposited MOF layers revealed that
only one orientation is grown, with the [100] direction
perpendicular to the surface. Interestingly, when replacing
copper(II)acetate with zinc(II)acetate, instead of the growth
of oriented MOFs, only the deposition of a non-porous,
amorphous Zn2+/btc polymer was observed, in contrast to
the known solvothermal chemistry, which leads to MOFs.44,45
When the MHDA SAM (COOH-terminated surface)
was replaced by a mercaptoundecanol (MUD) SAM
(OH-terminated surface) again an oriented growth of
HKUST-1 takes place.46 This time, however, with the [111]
direction of HKUST-1 orientated normal to the substrate
surface.
The observation that on a COOH-terminated surface,
HKUST-1 grows along the [100] direction and on an
OH-terminated surface along the [111] direction, is in full
accord with the observation reported by Biemmi et al.
(see discussion above), who observed that the deposition of
MOFs from a ‘‘mother liquor’’ at elevated temperature also
leads to polycrystalline, but well-ordered, MOF films with the
same preferential orientations. A particular advantage of
the layer-by-layer method is the possibility to directly monitor
the deposition of both ligands and coupling units using SPR
spectroscopy. The data in Fig. 10 show that subsequently
adding CuAc2 and btc at room temperature leads to step-
by-step deposition of layers on both MHDA and MUD
SAMs. A closer inspection of the data reveals that the height
of the deposition steps is different for the first layers. In future
work it will be important to investigate the importance of
deposition parameters (concentrations and temperature) and
the film deposition kinetics. The XRD data shown in Fig. 11
(recorded after 40 deposition cycles) provides unambiguous
evidence for the formation of highly ordered HKUST-1 on the
MUD-modified Au substrate. This works therefore goes
significantly beyond earlier sequential deposition of organic
layers, e.g. in the case of polyelectrolytes or dithiol-Cu
multilayers.47,48 In the work by Shekhah et al.,23 the
Fig. 9 A schematic diagram for the step-by-step growth of MOFs on SAMs by repeated growth cycles separated by washing: first immersion in a
solution of metal precursor and subsequently in a solution of the organic ligand. Here, for simplicity, the scheme simplifies the assumed structural
complexity of the carboxylic acid coordination modes. Reproduced from ref. 23 with permission, Copyright 2007, American Chemical Society.
Fig. 10 The SPR signal as a function of time, recorded in situ during
sequential injections of (A) CuAc2, (B) ethanol, and (C) btc in the SPR
cell containing MHDA SAM (above)23 and MUD SAM (below).46
Fig. 10 above adapted from ref. 23 with permission, Copyright 2007,
American Chemical Society.
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gas-loading properties of the deposited HKUST-1 layers were
also studied via NH3/water exchange experiments. IR and
near-edge X-ray absorption fine structure spectroscopy
(NEXAFS) data allowed them to conclude that the loading
with NH3 was similar to that observed for bulk HKUST-1,49
including a substantial irreversibility.
Another particular advantage of using the organic surfaces
exposed by Au/organothiol SAMs is the availability of
methods to laterally pattern the organic surface, e.g. via micro
contact printing (mCP),50 and thus to achieve a selective
growth of MOF films on pre-defined areas of the surface.
In the work by Munuera et al., such patterned SAMs have
been used in connection with quantitative SFM to follow the
growth of oriented MOFs on the COOH-terminated surface
and to study their morphological characteristics (Fig. 12).51
The results verify the selective growth of the HKUST-1 on the
COOH-terminated surface (Fig. 12A) and the homogeneity of
the deposited layers. It also demonstrates the strength of
the step-by-step preparation procedure employed and its
capability to fabricate high quality MOFs films on surfaces.
The observed monotonous thickness increase indicates that,
within experimental error, the number of layers grown is
proportional to the number of immersion cycles, implying
that once the first [Cu3btc2(H2O)n] layer nucleates on the
COOH-terminated regions, all of the subsequent material
is deposited on top of the previously nucleated layers.
Interestingly, in that work it was observed that the increase
in thickness per immersion cycle amounted to half a unit cell in
the [100] direction, see Fig. 12B.
With regard to applications, it is particularly relevant that
the topmost film surface roughness is fairly low and does not
increase substantially with SURMOF thickness. In contrast to
the immersion of substrates into preconditioned solvothermal
mother liquors, the step-by-step synthesis yields extraordinary
homogeneous films of 4100 nm thickness and a roughness in
the order of only one elementary cell in regions in the range of
several mm2. As a result, the layer-by-layer approach may also
be suited for the fabrication of thicker layers with a very
homogenous, flat surface, e.g. in sensor applications and for
fabricating membranes.
Using the step-by-step approach, it was also possible
to graft a monocarboxylic-substituted polychlorotriphenyl-
methyl radical (PTMCOOH) onto a COOH-functionalized
MHDA SAM using Cu2+ ions as linkers between the carboxyl
groups of the SAM and the PTMCOOH ligand (Fig. 13).52 In
this case, the organic ligand only has one coupling unit, so that
only the deposition of a monolayer can be achieved. In
this study, the rather well-defined metal radical adlayer
was characterized thoroughly using different surface analysis
Fig. 11 Out-of-plane XRD data for HKUST-1 films: (a) bulk,
(b) growth on a MHDA SAM (simulation), (c) growth on MHDA
SAM (experimental), (d) growth on MUD SAM (simulation),
(e) growth on MUD SAM (experimental).46 Adapted from ref. 23
with permission, Copyright 2007, American Chemical Society.
Fig. 12 (A) Two different ways of measuring thickness (averaged
profiles and histograms). Left: (a) a topographic image (6.5 � 6.5 mm)
and (b) a selected area for accurate thickness estimation; right: the
corresponding height histogram (top) and averaged profile (bottom)
calculated over the whole area in (b). The red lines in the histogram
represent the corresponding Gaussian fits. (B) (a) A series of topo-
graphic SFM images for different samples corresponding to n = 10,
20, 23, 30 and 45 immersion cycles from left to right, respectively. The
total colour scale (total height range) is 110 nm for all the images.
Because of the low topography of the 10 cycles sample, the inset shows
the same image with the scale magnified by a factor of two. (b) Film
thickness as a function of the number of immersion cycles. The red
dashed line corresponds to the proposed ‘‘half-layer’’ growth, whereas
the grey one corresponds to a single unit cell or complete layer growth
(see text). (c) The root mean square (rms) surface roughness as a
function of the number of immersion cycles calculated for different
scan sizes (see inset). The black horizontal line corresponds to the rms
of the starting substrate while the blue dashed line has been drawn as a
visual aid. Error bars represent the standard deviation values. Repro-
duced from ref. 51 with permission, Copyright 2008, RSC Publishing.
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techniques, such as contact angle, IRRAS, XPS, SPR,
ToF-SIMS, SFM and NEXAFS. The magnetic character of
the grafted radical ions was confirmed by EPR. The density of
unoccupied electronic states was investigated using X-ray
absorption spectroscopy and a low energy peak in the
NEXAFS spectrum directly revealed the presence of partially
occupied electronic levels, thus proving the open-shell
character of the grafted ligands. SEM measurements on a
laterally patterned sample prepared by mCP of MHDA in a
matrix of hexadecane thiolate (a CH3-terminated SAM) was
performed to demonstrate that the metal-assisted anchoring of
the open-shell ligand occurs selectively on the COOH-
terminated SAM. These results represent an easy and new
approach to anchor organic radicals on surfaces, and consti-
tute the first step towards the growth of magnetic metal–
organic radical-based frameworks on solid substrates.
5. Conclusions and perspectives
Although comparably few groups have studied the deposition
of porous coordination polymers (PCPs) and/or MOFs at
surfaces, some significant advances have been made, as
discussed in this article above. The first examples of phase
pure MOF thin films have been described, showing promising
morphology. However, more detailed characterisation of the
adsorption properties of the films or coatings in comparison to
the known bulk materials needs to be done. Liu et al. most
recently reported on a rather convincing, and in fact first
example, of a true continuous and well-intergrown MOF-5
membrane, which was successfully prepared on porous
a-alumina substrate by in situ solvothermal synthesis.53 The
BET measurements on crystals taken from the same mother
liquor that was used for membrane synthesis yielded a
Langmuir surface area of 2259 m2 g�1, and a narrow pore
size distribution centered at 1.56 nm. The permeation data for
H2, CH4, N2, CO2 and SF6 of the grown MOF-5 membrane
show that the diffusion of simple gases follows the Knudsen
diffusion behaviour.
Despite the fact that self-assembled organic monolayers
(SAMs) have been shown to be very useful to direct the
growth and even allow the control of the orientation, as well
as the deposited MOF phase, the morphologies of the
obtained MOF films from the solvothermal mother solutions
are poor. The demonstration of the oriented layer-by-layer
growth of smooth, very homogeneous and quasi epitaxial
MOFs on SAMs/Au points in a novel direction, which holds
much promise and is quite similar to related oriented layer-by-
layer growth and bottom-up assemblies of other hybrid
inorganic–organic materials at surfaces.40,41,54–57 However,
the particular combination of porosity with chemical and
physical functionalities of the coordination framework8 will
be most interesting, and will possibly transgress the limitation
of less ordered and non-porous thin films of coordination
polymers also grown at surfaces, which, for example, are
interesting as redox or photo-functional molecular systems
on electrodes.56 In this context, we would like to quote another
recent and very nice example of unique perspectives on MOF
thin films as ‘‘pars pro toto’’ in order to stimulate further
research. Allendorf et al. communicated the concept of
stress-induced chemical detection using MOFs by integrating
a thin film of the HKUST-1 with a microcantilever surface.58
Their results showed that the energy of molecular adsorption,
which causes slight distortions in the MOF crystal structure, is
converted to mechanical energy to create a highly responsive,
reversible and selective sensor. This sensor responds to water,
methanol and ethanol vapours, but yields no response to either
N2 or O2. The magnitude of the signal, which is measured by a
built-in piezoresistor, is correlated with the concentration and
can be fitted to a Langmuir isotherm.
Aside from the applications for MOF thin film device
fabrication indicated above, the layer-by-layer preparation
method offers new prospects to study the kinetics andmechanism
of MOF formation itself in more detail from a new
perspective.37,59 The systematic in situ SPR monitoring of
MOF film growth is expected to provide a new insight on
the assembly process of the frameworks being not only
dependent on solvents, temperature, pH etc., but also depen-
dent on the offered building blocks, in particular on the
precursors for the metal-containing SBUs. Recently, we found
evidence for the preferred layer-by-layer growth of HKUST-1
using Cu(Ac)2 as a Cu2+ source (see Figs. 10–12) in contrast to
Cu(NO3)2�3H2O.60 Since solutions of Cu(Ac)2 in fact contain
the SBU-like dimeric species, [Cu2(CH3COO)4], this finding
directly supports the mechanistic implications of the
Fig. 13 A schematic representation of (a) mononuclear and (b) dinuclear copper(II) complexes obtained by reacting PTMCOOH or its
carboxylate PTMCOO� with copper(II)acetate, and (c) idealized representation for grafting the PTMCOO� ligand on top of a COOH-terminated
SAM. Reproduced from ref. 52 with permission, Copyright 2008, American Chemical Society.
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controlled SBU concept of MOF synthesis.1,38 The overall
kinetics of layer-by-layer MOF growth were found to be
strictly linear.23,60 However, non-liner growth modes for the
related formation of nanoarchitectures at surfaces were
observed in the case of self replicating amphiphilic mono-
layers, for example in case of polyelectrolytes.61 A similar
non-liner mechanism was recently found for inorganic–
organic hybrid systems, similar to MOFs or surface coordina-
tion polymers (SCPs).62 It will be interesting to compare these
different chemical systems, including MOFs, in terms of the
underlying growth mechanisms, possibly even aimed at an
accelerated self-propagating growth of oriented MOFs at
surfaces. Furthermore, the step-by-step method obviously
offers the unique opportunity to grow MOF-like ordered
structures, which cannot be obtained by established solvo-
thermal routes. For example, the deposition of MOFs with
alternating layers (heterostructures), possibly with non-
periodic combinations of different metal ions and/or different
linkers should be feasible. As an example for this direction of
MOF thin film research, we finally like to highlight the recent
work of Kanaizuka et al. on the construction of highly
oriented crystalline, but non-porous, SCPs, which are
composed of copper dithiooxamide complexes. The authors
suggest that such homo- and also heterostructured SCPs,
quite similar to MOF thin films, may be useful for many
applications, including Josephson junctions of super-
conductors, magnetic spin valves, capacitance, screen displays,
fuel cells and catalytic devices.63
Acknowledgements
The authors acknowledge financial support by the EU STREP
SURMOF (NMP4-CT-2006-032109) and the Priority
Program 1362 ‘‘Metalorganic Frameworks’’ of the German
Research Foundation. D. Z. is grateful for additional support
by the Ruhr University Research School (http://www.
research-school.rub.de).
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