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).

This journal is �c The Royal Society of Chemistry 2009 Chem. Soc. Rev., 2009, 38, 1418–1429 | 1419

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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.

1420 | Chem. Soc. Rev., 2009, 38, 1418–1429 This journal is �c The Royal Society of Chemistry 2009

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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.

This journal is �c The Royal Society of Chemistry 2009 Chem. Soc. Rev., 2009, 38, 1418–1429 | 1421

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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.

1422 | Chem. Soc. Rev., 2009, 38, 1418–1429 This journal is �c The Royal Society of Chemistry 2009

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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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