Formation and Structure of Copper (II) Oxalate Layers on Carboxy

Sep 16, 2014 - Copper(II) oxalate was grown on carboxy-terminated self-assembled monolayers using a step-by-step approach by dipping the surfaces ...
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Formation and Structure of Copper(II) Oxalate Layers on CarboxyTerminated Self-Assembled Monolayers I. Schrader,† L. Wittig,‡ K. Richter,‡ H. Vieker,§ A. Beyer,§ A. Gölzhaü ser,§ A. Hartwig,‡ and P. Swiderek*,† †

Institute of Applied and Physical Chemistry, University of Bremen, Fachbereich 2 (Chemie/Biologie), Leobener Straße/NW 2, Postfach 330440, D-28334 Bremen, Germany ‡ Fraunhofer Institute for Manufacturing Technology and Advanced Materials, Wiener Straße 12, D-28359 Bremen, Germany § Physics of Supramolecular Systems, University of Bielefeld, D-33615 Bielefeld, Germany ABSTRACT: Copper(II) oxalate was grown on carboxy-terminated self-assembled monolayers using a step-by-step approach by dipping the surfaces alternately in ethanolic solutions of copper(II) acetate and oxalic acid with intermediate thorough rinsing steps. The deposition was monitored by reflection absorption infrared spectroscopy (RAIRS), a quartz microbalance with dissipation measurement (QCM-D), scanning electron microscopy (SEM), and helium ion microscopy (HIM). Amounts of material corresponding to a coverage of 75% of a monolayer are deposited in each dipping step in copper(II) acetate solution while deposition of oxalic acid produces a viscoelastic layer that is partially removed by rinsing. This points toward initial aggregation but acid not bound to Cu2+ ions as oxalate ions is removed by the rinsing steps. RAIRS further indicates that the material grows as copper(II) oxalate ribbons similar to the crystal structure but with ribbons oriented roughly parallel to the surface. SEM and HIM give evidence of the formation of needle-shaped structures which are a possible explanation for the viscoelastic behavior of the layer.

1. INTRODUCTION Surface films with well-defined composition, thickness, and structure are of utmost importance for the study of interfacial processes. To name only a few examples, self-assembled molecular monolayer films (SAMs)1 can control etching processes,2 give insight into electrical transport through interfaces,3,4 serve as binding sites for biomolecules,5 and can act as templates for surface crystallization.6 The latter process can also be exploited as initial step in a layer-by-layer deposition of materials on surfaces such as thin films of metal−organic frameworks (MOFs), the so-called surface MOFs (SurMOFs).5,7,8 MOFs consist of a three-dimensional network of metal ions bridged by multifunctional rigid organic chelating agents. They attract considerable interest because they offer the perspective to taylor their porosity and functionality by varying the size and side groups of the organic component. SurMOFs are versatile materials for sensor applications and provide detailed insight into absorption and diffusion processes.8 SurMOFs are prepared by exposing a suitable surface in turn to a solution containing metal ions and another solution containing organic linker molecules, each dipping step being followed by thorough rinsing with the solvent. Prior to this process, a SAM carrying suitable functional end groups is deposited on the surface. The SAM provides binding sites for the first layer of metal ions and can even control the structure and orientation of the SurMOF layer. This has been shown for the example of HKUST-1, a MOF material prepared from © 2014 American Chemical Society

solutions of copper(II) acetate and benzene-1,3,5-tricarboxylic acid (btc).7,8 In fact, HKUST-1 grows with different orientation on surfaces of carboxy- or hydroxyl-terminated SAMs.9,10 The surface growth of MOFs such as HKUST-1 results from the binding geometry of the metal ions to the surface of the SAM. Copper(II) acetate prevails in solution as complex of stochiometry Cu2Ac4 (Ac = H3CCO2−) in which a Cu2+ dimer is bridged by four carboxylate groups in a paddlewheel-like arrangement in which further ligands such as H2O may bind to the apical positions (Figure 1a).10 This Cu2+ dimer complex forms the so-called secondary building unit (SBU) of the MOF. The observed crystal growth on the carboxy-terminated SAM suggested that the SBU reacts with either one or two carboxylic acid groups of the SAM which replace one acetate ligand each so that the Cu2+ ion pair is oriented parallel to the SAM surface if the latter is well-oriented.10 The remaining acetate ligands are then replaced by carboxyl groups of the linker in the subsequent dipping step. Because of its rigid nature, the linker btc can only form bridges between adjacent Cu2+ dimer SBUs with each of the three carboxyl groups chelating a different SBU (Figure 1b). Besides btc, a variety of different linkers have been suggested including benzene-1,4-dicarboxylic acid (bdc) or oxalic acid, Received: July 4, 2014 Revised: September 7, 2014 Published: September 16, 2014 11945

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two carboxylate groups of the oxalate ions (Figure 1f) and surrounded by two such oxalate ions leading to a zigzag chain structure.12−15 This structural motif is also encountered in other copper coordination polymers14 and oxalato complexes.16 Adjacent ribbons are perpendicular to each other in a fishbone arrangement allowing for additional coordinative interactions between oxalate ions of a specific ribbon and Cu2+ ions of the next ribbon, thus stabilizing the structure.13 The variable coordination geometry may be the reason, besides the small size of the oxalate linker, why this compound has to the best of our knowledge not been applied to the synthesis of MOFs. On the other hand, this discrepancy provides an interesting perspective to investigate the structure directing behavior of the Cu2+ dimer SBU. We have therefore investigated the layer-by-layer growth of copper(II) oxalate on carboxy-terminated SAMs using the established deposition procedure for HKUST-1. To this end, btc was simply replaced by oxalic acid in the protocol for formation of HKUST-1 SurMOFs.7,8,10 In fact, a continuous growth of a copper(II) oxalate layer upon increasing number of dipping cycles was observed by use of both reflection−absorption infrared spectroscopy (RAIRS) and mass deposition measurements using a quartz crystal microbalance with dissipation monitoring (QCM-D). For comparison, analogous growth experiments were performed by replacing copper(II) acetate with copper(II) nitrate and oxalic acid by potassium oxalate. The combined results reveal details of the growth processes and of copper(II) oxalate surface layers. Based on a comprehensive review of the infrared spectra of copper(II) oxalate and additional images obtained by scanning electron microscopy (SEM) and helium ion microscopy (HIM), the most likely orientation of oxalate ions within the layer is discussed.

2. EXPERIMENTAL SECTION 2.1. Preparation of Copper(II) Oxalate Surface Layers. Copper(II) oxalate surface layers were grown using a layer-by-layer approach on carboxylic acid-terminated SAMs deposited on Au surfaces. First, commercial Si substrates coated with a 5 nm layer of Ti and a 200 nm layer of Au (Georg Albert PVD) were cleaned in a 1:3 mixture of H2O2 (30%, VWR) and concentrated H2SO4 (95%, VWR) for 15 min (caution: piranha solution is an oxidizing mixture that can be explosive when in contact with organic materials), rinsed with deionized water (18.2 MΩ, Millipore purification system) and ethanol (99.9%, VWR), and dried in a stream of N2. SAMs of 11-mercaptoundecanoic acid (MUA: HSC10H20CO2H, 95%, Sigma-Aldrich) were prepared by immersing the cleaned Au substrates into a 1 mM solution of MUA in ethanol (99.8%, SigmaAldrich) under N2. After an incubation time of 24 h, the substrates were rinsed with ethanol, dried in a stream of N2, and used immediately for the deposition experiments or as a background for RAIRS. Copper(II) oxalate surface layers were prepared following the protocol of ref 10, but replacing btc by oxalic acid. Au substrates functionalized with MUA-SAMs were thus immersed first in a 1 mM ethanolic solution of copper(II) acetate monohydrate (≥99.0%, Sigma-Aldrich) for 30 min and then in a 0.1 mM ethanolic solution of oxalic acid dihydrate (≥99.5%, Merck) for 60 min. This cycle was repeated several times to deposit additional layers of copper(II) oxalate. After each immersion the substrate was rinsed with ethanol and dried in a stream of N2. Additional layer growth experiments were performed by immersing MUA SAMs subsequently in either 1 mM ethanolic solution of copper(II) nitrate trihydrate (p.a., Riedel-de Haën) for 30 min and in a 0.1 mM ethanolic solution of oxalic acid dihydrate for 60 min or in a 1 mM ethanolic solution of copper(II) acetate monohydrate for 30 min

Figure 1. Structural motifs of metal−organic frameworks and copper(II) oxalate layers: (a) copper(II) acetate complex known to serve as structure determining secondary building unit in growth of MOFs with apical positions that can bind solvent molecules marked by arrows,5,7−11 (b) possible binding situations of copper(II) acetate SBU on the surface of a carboxy-terminated self-assembled monolayer (SAM), (c) exemplary organic linkers for growth of MOFs (btc, bdc) and the smallest possible linker oxalic acid, (d) coordination geometry of bdc to copper(II) acetate SBUs, (e) analogous hypothetic coordination geometry of oxalate ion to copper(II) acetate SBUs, and (f) coordination geometry of oxalate ions in the copper(II) oxalate crystal structure.12−15

both providing two chelating groups that point in opposite direction.11 However, while the binding geometry for bdc is again unique in the sense that each of the two carboxyl groups binds to a different Cu2+ dimer (Figure 1c), two distinct arrangements are possible in the case of oxalato complexes. In analogy to the structure of HKUST-1 (Figure 1d) and other MOFs, one would expect a geometry in which the CC bond is oriented along to line connecting two Cu2+ dimers, and consequently each carboxyl group binds to one of the two connected metal centers (Figure 1e). In contrast, the crystal structure of copper(II) oxalate has been proposed to consist of a coordination polymer, i.e., ribbons of alternating oxalate ions and metal ions, the latter being located “sideways” between the 11946

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and then in a 0.1 mM aqueous solution of potassium oxalate (p.a., Merck) for 60 min. 2.2. Reflection Absorption Infrared Spectroscopy of Surface Layers. RAIR spectra between 4000 and 550 cm−1 were recorded using an evacuated FT-IR-spectrometer (IFS 66v/S, Bruker Optics GmbH) by accumulating 400 scans. The spectrometer was equipped with a grazing incidence reflection unit and a liquid nitrogen-cooled MCT detector with sensitivity range extending down to 750 cm−1. The resolution was set to 4 cm−1 and the aperture to 1.5 mm. The chamber pressure was 4 mbar. The system was purged with nitrogen to eliminate water vapor and carbon dioxide. Background spectra were recorded on a MUA SAM prepared as described above. 2.3. Preparation of Bulk Material by Precipitation and IR Characterization. Bulk copper(II) oxalate was prepared by precipitation upon combining equal amounts of the same solutions of copper(II) acetate monohydrate (1 mM in ethanol) and oxalic acid (0.1 mM in ethanol) as also used in surface layer growth. The product was dried in air at 120 °C for 5 days, at 200 °C for 3 days, and at 220 °C for 3 days to ensure that all water has been removed. Infrared spectra of the precipitate as well as of a commercial copper(II) oxalate hemihydrate (98%, Alfa Aesar) were obtained from KBr pellets using a N2-flushed FT-IR-spectrometer (AVATAR 370, Thermo Nicolet). The resolution was set to 4 cm−1. The dryness of the KBr used was carefully checked by recording IR spectra of pellets prepared from the pure salt. 2.4. Measurements of Copper(II) Oxalate Surface Layer Deposition by QCM-D. The surface deposition of copper(II) oxalate was monitored in situ by using a quartz crystal microbalance with dissipation measurement equipped with four sensor flow modules (QCM-D E4 instrument and software, Q-Sense AB, Gothenborg, Sweden). AT-cut and gold-coated polished crystals with a fundamental resonance frequency of 5 MHz were used (LOT-QuantumDesign GmbH, Germany). The crystals were cleaned with piranha solution and rinsed with ethanol prior to deposition of MUA SAMs by 24 h immersion in 1 mM ethanolic solution of MUA under N2. Copper(II) oxalate thin films were grown on the surface of the MUA-coated QCM-D crystal by alternately flowing ethanolic solution of copper(II) acetate monohydrate (1 mM) and oxalic acid dihydrate (0.1 mM) over the surface with ethanol washing steps in between. For comparison, growth was also investigated on the Au-coated crystal without SAM. The flow rate for the three solutions was maintained constant at 0.1 mL/min by a peristaltic pump IPC-N4 from Ismatec (Wertheim, Germany). After the coating process the crystals were washed with ethanol and dried in a nitrogen stream. 2.5. SEM Measurements. Representative QCM-D crystals were investigated by SEM after the deposition of copper(II) oxalate layers without additional coating as the conductivity was high enough to prevent surface charges. A FE-SEM Leo 1530 Gemini from Carl Zeiss (Oberkochem, Germany) was used. 2.6. HIM Measurements. Helium ion microscopy (HIM) employs a finely focused beam of He+ ions with a diameter down to 0.35 nm that is scanned over the sample. The secondary electrons (SE) generated by the ion impact are detected. HIM was performed with a Carl Zeiss Orion Plus. The helium ion beam was operated at 34.7 kV acceleration voltage at a current of 0.3 pA. The working distance was about 10.8 mm at a sample tilt of 30°. A dwell time per pixel of 1 μs at 64 lines averaging was used. The HIM micrograph was recorded with a pixel size of 0.98 nm.

compared to bulk spectra, and their assignment is established as far as possible through a comprehensive comparison with previous results. The infrared spectra of both bulk samples and of surface layers of copper(II) oxalate in the range between 750 and 2000 cm−1 are shown in Figure 2, and the band positions are listed in

Figure 2. Comparison of RAIR spectra of surface grown copper(II) oxalate and transmission infrared spectra of copper(II) oxalate bulk material in KBr disks: (a) RAIRS of a copper(II) oxalate layer grown on a carboxy-terminated SAM by 10 subsequent dipping cycles in ethanolic solutions of copper(II) acetate monohydrate and oxalic acid dihydrate. Bands denoted with an asterisk are assumed to arise from interference effects. (b) Transmission spectrum of copper(II) oxalate prepared by precipitation from ethanolic solutions of copper(II) acetate monohydrate and oxalic acid and subsequent annealing to remove H2O. (c) Transmission spectrum of commercial copper(II) oxalate hemihydrate.

Table 1 together with previous data. As reference, the infraredactive vibrations of oxalate ions in the ionic solid potassium oxalate are included and shown in Figure 3.19 The infrared bands of copper(II) oxalate observed in the present experiments agree well with previous results,20,21 but the relative intensities are characteristically different in the RAIR spectra of surface layers with the band above 1600 cm−1 being significantly less intense while the intensity of the sharp signal around 1365 cm−1 increases relative to the bands at lower wavenumbers of about 1320 cm−1 (Figure 2). Most of the signals correlate closely with the spectrum of potassium oxalate that has been investigated in depth using isotope-labeled derivatives,22 suggesting that the orientation of the TDMs for copper(II) oxalate may be derived from the results on potassium oxalate. However, copper(II) oxalate reveals two strong bands between 1300 and 1400 cm−1 similar to different oxalato complexes,16,23 while only one band occurs in potassium oxalate (Table 1). Also, the previous assignments for copper(II) oxalate differ from each other (Table 1), thus hampering a straightforward assignment of the present spectra. Furthermore, the band above 1600 cm−1 that has been assigned to an asymmetric CO2 stretching vibration12 may be overlapped by the bending mode of crystal water.21 These issues need to be resolved to arrive at a reliable basis for the interpretation of orientation effects on the RAIR spectra of copper(II) oxalate surface layers. In contrast to the relatively sharp signals at lower wavenumbers, the band above 1600 cm−1 is very broad with

3. RESULTS AND DISCUSSION 3.1. Infrared Spectra of Bulk Material and Surface Layers of Copper(II) Oxalate. The orientation of adsorbed molecules with respect to the underlying metal surface can, in principle, be derived from the RAIR spectra by use of the surface selection rule.17,18 This, however, requires that the orientation of the transition dipole moment (TDM) must be known for at least some of the observed infrared vibrations. To arrive at a detailed assignment of the vibrational bands, the spectra of surface grown layers of copper(II) oxalate are first 11947

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Table 1. Dominant Infrared Bands of Copper(II) Oxalate Surface Layers and Solid and of Potassium Oxalate Solid (cm−1)a surface layerb precipitate from ethanol,b annealed to remove H2O commercialb Cu(II)C2O4·0.5H2O (nom) precipitate from H2Oc Cu(II)C2O4·0.2H2O (TGA)

1612 1626 (br) 1633 (vbr) 1614 (vs) (br)

assignmentsc

ν(C−O)

commerciald Cu(II)C2O4·2H2O (nom) assignmentsd

1670 (s) ν(C−O)

K2C2O4·H2Oe assignmentse (Herzbergf)

copper(II) oxalate 1367 1323 1363 1317 1363 1319 1363 (vs) 1319 (vs) ν(C−O)

ν(C−O)

1365 (s) 1320 (m) ν(C−O) CO2 wag potassium oxalate 1621 1315 ν8 b2u (ν9 b2u) ν5 b1u νas(CO2) (ν11 b3u) νas(CO2)

831 822 822 823 (vs) δ(CO2) + ν(Cu−O)

501, 490 505 509 (vs), 490 (vs) ν(Cu−O) + ring def

824 (s) δ(CO2) 776 ν6 b1u (ν12 b3u) δas(CO2)

526 ν12 b3u (ν7 b1u) ρwag(CO2)

353 ν9 b2u (ν10 b2u) ρrock(CO2)

a

nom = nominal water content as stated by supplier, TGA = water content determined by thermogravimetric analysis. Note that the assignment of ref 22 deviates from the Herzberg notation as used throughout the text. bPresent work. cReference 20. dReference 21. eReference 22. fReference 19.

width and position of maximum shifting between the different samples (Figure 2 and Table 1). The bending vibration of crystal water is very likely responsible for the very broad band in the commercial sample that is nominally a copper(II) oxalate hemihydrate (Figure 2c). Such broad bands were also reported previously,12,20,24 but unfortunately the most comprehensive study of the vibrational spectra of copper(II) oxalate, presumably performed on a dihydrate,21 only presented a list of wavenumbers with approximate intensity and not the infared spectrum itself so that a close comparison is difficult. The width of the band above 1600 cm−1 is reduced in the case of the precipitate obtained from ethanolic solution (Figure 2b). However, it still dominates the spectrum similar to the result obtained from a sample with stochiometry Cu(II)C2O4·0.2H2O as derived from thermogravimetric analysis.20 It must be noted that annealing in air for several days at temperatures up to 220 °C did not significantly alter the spectrum of the ethanolic precipitate. In line with a previous report that water can be removed from copper(II) oxalate by annealing at 120 °C in air for several days,25 we thus conclude that the samples shown in Figure 2 were essentially free of water. The remaining width of the band is somewhat surprising but is supported by data on potassium oxalate showing also an intense and relatively broad band that undergoes a characteristic shift upon 13C isotope substitution.22 In conclusion, this band can be safely assigned to an asymmetric CO2 stretching vibration as no other types of vibrations of the oxalate ions fall into this range of wavenumbers.21,22 Before proceeding with the assignments of the infrared spectrum of copper(II) oxalate, the geometry of the oxalate ions must be considered in detail. This is particularly important regarding the assignment of the band around 1365 cm−1 that does not correlate with the spectrum of potassium oxalate (Table 1). The strictly mutually exclusive infrared and Raman spectra of solid potassium oxalate give evidence of a planar geometry (point group D2h) of the oxalate ions.22 On the other hand, oxalate ions are known to be twisted by 28° (D2) in ammonium oxalate and even by 90° (D2d) in aqueous solution, thus leading to a modified infrared activity.21,22 As only the four CO2 stretching vibrations appear above 1000 cm−1 in potassium oxalate,21,22 only their possible change in infrared activity upon twisting must be considered here. A close inspection reveals, however, that a CO2 stretching mode of a twisted isolated oxalate ion cannot explain the 1365 cm−1 band.

Figure 3. Infrared-active normal vibrations b1u, b2u, and b3u and directions of the transition dipole moments (TDM) as well as infraredinactive YX stretching normal vibrations of a X2Y4 molecule (point group D2h) after Herzberg and approximate mode descriptions for free oxalate.19 This notation will be used throughout the text.

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First, while the ν5 (b1g) mode (here and in the following text the Herzberg notation of Figure 2 is used) located at 1604 cm−1 in potassium oxalate22 in fact will attain a TDM upon twisting because the dipoles of the two vibrating CO2 subunits will not cancel anymore as in the planar geometry, this mode becomes degenerate with ν9 (b2u) at 90° twist. It thus cannot shift to the 1300−1400 cm−1 range. Regarding the orientation of the local transition dipole moments of the two carboxyl groups, we can also deduce that the TDM of the band above 1600 cm−1 must in any case be oriented perpendicular to the C−C bond of the oxalate ion. Second, the totally symmetric CO2 stretching mode ν1 (ag) located at 1445 cm−1 in potassium oxalate remains infrared-inactive upon twisting both in point groups D2d and D2. This appears to contradict the previous assignment of the 1365 cm−1 band as a CO2 stretching mode20 unless ν11 (b3u) of potassium oxalate has in fact shifted upward in copper(II) oxalate as also suggested previously.21 With the 1365 cm−1 band assigned to a CO2 stretching mode, an earlier study has interpreted the then remaining 1320 cm−1 band to a CO2 wagging mode.21 This assignment, on the other hand, appears unlikely given that the wagging mode is located much lower at 526 cm−1 in potassium oxalate.22 An alternative explanation for the appearance of two bands between 1300 and 1400 cm−1 is suggested by considering the infrared spectra of oxalate complexes which also show the two bands.16,23 When two or more oxalate ions are coordinated to a common central atom, the coupling between the subunits can lead to splitting of the initially degenerate vibrations of the equivalent ligands. A similar splitting is very likely effective in the case of the solid coordination polymer copper(II) oxalate. However, the directions of the TDMs for the resulting coupled normal modes are difficult to predict without detailed knowledge of the solid structure and strength of coupling. In consequence, while it is tempting to assign the 1320 cm−1 band to ν11 (b3u) due to the close correspondence of its position with the band observed in potassium oxalate, its origin and consequent TDM in copper(II) oxalate require a more detailed theroretical study. In contrast, due to the lack of band dublication in the range of wavenumbers below 1000 cm−1, it is reasonable to identify the bands near 820 and 500 cm−1 with the nearby modes of potassium oxalate. The former is thus assigned to the asymmetric CO2 deformation ν12 (b3u) with TDM oriented along the C−C axis irrespective of the twist. The second would then relate to the wagging mode ν7 (b1u) with TDM perpendicular to the molecular plane for D2h geometry and perpendicular to the C−C axis for a twisted geometry. Based on the preceding analysis of the infrared spectra of copper(II) oxalate, the assignments are unique for two bands appearing in the surface spectrum, namely, the asymmetric CO2 stretching vibration ν 9 above 1600 cm −1 with TDM perpendicular to the CC bond and in plane if the oxalate ion assumes a planar geometry and the asymmetric CO2 bending vibration ν12 near 820 cm−1 with TDM along the CC bond. These two bands are thus particularly suited as indicators of the average orientation of the oxalate ions with respect to the underlying surface in the layer-by-layer deposited films. While the intensity ratio I(ν9)/I(ν12) as determined using the band maxima is roughly 2 in the transmission spectrum obtained from the ethanolic copper(II) oxalate precipitate, it is reversed in the case of the RAIR spectrum of the surface layer. This suggest that on average the oxalate ions are arranged with the carboxylate groups close to a parallel orientation with respect to

the underlying surface. In contrast, the CC bond must, again on average, have a more upright orientation. As it is not possible to derive a complete structure model from this information, we consider, in the following, several specific situations that can be anticipated from the crystal structure of copper(II) oxalate12−15 and from the structures of MOFs.5,7,8 Assuming that the first layer of Cu2+ dimer SBUs is adsorbed as described before10 and summarized in section 1, the Cu2+ dimers should be oriented approximately parallel to the surface. As a first structure model we assume that oxalate binds to single Cu2+ ions in a “sideways” arrangement as depicted in Figure 1f, leading to formation and chain growth of the copper(II) oxalate ribbon structure during repeated deposition cycles. If these ribbons grew upright on the surface, the CC bonds and thus the TDM of the asymmetric CO2 bending vibration ν12 would be oriented parallel to the surface while a major fraction of the TDM resulting from the asymmetric CO2 stretching vibration ν9 should be oriented upright. This, however, does not reflect the observed intensity ratio in the RAIR spectrum of Figure 2a. This growth model can thus be ruled out. Considering that the copper(II) oxalate ribbons are not densely packed during the first few deposition cycles, it appears likely that adjacent chains have a tendency to interact by tilting toward the surface. A second growth model thus assumes that the copper(II) oxalate ribbons grow parallel to the surface. If all ribbons are oriented parallel to the surface, the TDM of both the asymmetric CO2 stretching vibration ν9 and the asymmetric CO2 deformation ν12 (b3u) would be oriented close to the plane of the surface and thus carry only little intensity. However, adjacent copper(II) oxalate ribbons in the crystalline state have been described as being oriented perpendicular to each other in a fishbone-type arrangement.13 Assuming that the same is true for ribbons that grow along the surface, the CC bond would, on average, be tilted away from the surface by 45° so that ν12 is active in RAIRS. This model is thus in accord with the experimental result (Figure 2). Assuming as a third model that instead of forming a copper(II) oxalate crystal structure, the surface layers grow in a MOF-type arrangement (Figure 1e), the selection rules for RAIRS would be similar to model 2. Growth models 2 and 3 are thus difficult to distinguish. However, the discrimination between these two models will be further considered in section 3.2. Finally, it is worth noting that the two bands between 1300 and 1400 cm−1 behave differently in the transmission spectra (Figure 2b,c) and in RAIRS (Figure 2a). The relative intensity of the band near 1320 cm−1 as compared to the asymmetric CO2 deformation ν12 (b3u) near 820 cm−1 is the same in both, suggesting in fact an assignment to ν11 (b3u). On the other hand, the 1365 cm−1 band has a similar intensity as ν12 (b3u) in the transmission spectra but is about twice as intense in RAIRS. The reason for this enhancement, however, remains unclear due to the lack of a convincing assignment. In addition to the experiments described so far, we also attempted to deposit copper(II) oxalate layers using different starting materials. However, replacing oxalic acid by a solution of potassium oxalate completely suppressed layer growth. This may be traced back to the formation of soluble complexes of stochiometry K2Cu(C2O4)2 when insoluble copper(II) oxalate is washed with an excess of potassium oxalate.21 As an alternative explanation, oxalate itself may by incapable of removing acetate ions from the SBU because the latter cannot be protonated and thus withdrawn from the equilibrium. As another variation, copper(II) acetate was replaced by copper11949

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(II) nitrate. Similar to previous results concerning the surface growth of HKUST-1,7−10 also the deposition of copper(II) oxalate was much less pronounced in the case of the copper(II) nitrate precursor. This suggests that the pre-formed SBU is also important for the formation of copper(II) oxalate surface layers. 3.2. Growth of Copper(II) Oxalate Surface Layers Monitored by RAIRS. The growth of copper(II) oxalate surface layers upon alternating dipping steps in ethanolic solutions of copper(II) acetate and oxalic acid was monitored by recording RAIR spectra of the samples after each deposition step during the first five deposition cycles and again during several later cycles. Representative spectra are shown in Figure 4, and the peak intensities of all characteristic bands as a

Figure 5. Peak intensities in the RAIR spectra of surface grown copper(II) oxalate as a function of dipping cycles in ethanolic solutions of copper(II) acetate monohydrate (integral cycle numbers) and oxalic acid (intermediate values of cycle numbers).

selection rule. Oxalate ions with CC bond close to parallel to the surface and molecular plain oriented at least partly tilted away from the surface, however, can explain the observed intensities for the first layer. Later, the relative intensity of the asymmetric CO2 deformation ν12 as compared to ν9 increases, indicating that the CC bond assumes a more upright average orientation. This is in line with the assumption that the copper(II) oxalate layers grow as ribbons similar to the crystal structure12−15 but tilt toward the surface during later dipping steps and consequent chain growth. The copper(II) oxalate ribbons would thus grow parallel to the surface and in line with the crystal structure with ribbons oriented in a fishbone arrangement with respect to their neighboring chains. The spectra obtained after immersion of the samples in copper(II) acetate solution show an additional small band at 1443 cm−1 that can be ascribed to the acetate ions of the copper(II) acetate SBU.26 As antipicated for a layer-by-layer growth, the acetate band indeed shows a simple on/off behavior throughout the full range of deposition cycles (Figure 5, bottom). This suggests that all of the acetate ligands remaining attached when the Cu2+ SBU binds to the growing layer are removed upon deposition of oxalic acid. In this step and as already shown to be essential for layer growth of copper(II) oxalate (section 3.1), proton transfer must occur between oxalic acid and the acetate ligands to yield oxalate which binds to the Cu2+ SBU and acetic acid which is released. As another detail that is clearly more pronounced after the immersion steps in copper(II) acetate solution, the spectra show a negative signal at 1740 cm−1 that can be assigned to the CO stretching vibration of the carboxylic acid groups.27 This signal can be explained by considering that the spectra of the copper(II) oxalate layers were recorded against a MUA SAM as background. Upon deposition of copper(II) acetate, at least a part of the carboxylic acid groups of the SAM become involved as ligands to the Cu2+ SBU. In consequence, the CO stretching signal at 1740 cm−1 is smaller in the sample than in the background leading to a negative peak. However, immersion in

Figure 4. RAIR spectra of surface grown copper(II) oxalate after the indicated selected numbers of dipping steps in ethanolic solutions of copper(II) acetate monohydrate (Cu) and oxalic acid dihydrate (Ox). The inset shows an enlargement of the same spectra in the range of the symmetric acetate CO2 stretching vibration.26

function of dipping step are plotted in Figure 5. In fact, the intensities of the four characteristic oxalate bands increase with increasing number of dipping steps. A constant linear increase is observed for the three bands at lower wavenumbers after each dipping step in oxalic acid solution while, as expected, these intensities remain constant upon immersion in copper(II) acetate solution. In contrast to the very regular behavior of the other oxalate signals, the intensity of the asymmetric CO2 stretching vibration ν9 above 1600 cm−1 first goes through a maximum during the first dipping cycle, then decreases, and increases again starting with dipping cycle 3. Also, the relative intensity of this band as compared to the other oxalate signals is much higher during the first deposition cycle. This finding suggests that the MOF-type arrangement of the oxalate ions (Figure 1e) does not well represent the first layer of oxalate ions. In this arrangement the TDM of ν9 would be oriented roughly parallel to the surface and thus not be visible according to the surface 11950

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Figure 6. QCM-D results showing changes for various harmonics of the frequencies and their dissipations during several copper(II) acetate and oxalic acid deposition cycles with intermediate rising steps with ethanol. The experimental procedure is summarized by the labels during the fifth deposition cycle but was the same during all cycles. The Au-coated crystals were modified by deposition of a MUA SAM prior to the experiment. The inset shows an enlargement of the data during the time of the first copper(II) acetate deposition step.

oscillating frequency in the present experiment coincide during the initital copper(II) acetate deposition, indicating that the change in dissipation is negligible, and the Sauerbrey equation can be applied during this deposition step.29 The surface mass coverage is thus directly proportional to the change in oscillating frequency.29,30 Assuming that the copper(II) acetate SBU binds to the hydroxyl groups of the SiO2 surface via its apical sites (Figure 1a) so that the Cu dimer is oriented perpendicular to the surface, the HKUST-1 growth study estimated that a nearly complete monolayer coverage had been reached.28 As the present results were obtained with an equivalent setup, we can estimate from the smaller frequency that a surface coverage of 75% as compared to the deposition on SiO2 was reached in the present experiments. The difference can be attributed to both a different binding geometry on the SAM (Figure 1b) and possible defects in the underlying SAM. Following the initial deposition of the copper(II) acetate SBU and an intermediate rinse with pure ethanol, a solution of oxalic acid in ethanol was flushed over the crystal. While deposition of btc led to a further frequency decrease of about 15 Hz and again no significant increase in dissipation pointing to formation of a stiff and strongly interlinked network in HKUST-1,28 a drastically different result was obtained upon exposure of the sample to oxalic acid (Figure 6). Here, the frequency change was much larger (e.g., about 180 Hz for the third overtone) and differed strongly between the different overtones. At the same time, the dissipation increase was more than one order of magnitude larger compared with the change of dissipation measured for deposition of btc.28 Both values reached a plateau within 10 s in the present experiment. The strong increase in dissipation indicates that oxalic acid forms a viscoelastic layer at the surface so that a Voigt analysis would be required to determine the thickness of the deposit.29,30 This is beyond the scope of the present study. While it has been reported previously that QCM-D frequency changes can be larger during deposition of the organic linker than during adsorption of the metal-containing SBU,28,31 the present results appear unusual due to the large

solution of oxalic acid removes most of this signal, suggesting that a large fraction of the initial carboxylic acid groups of the SAM are recovered. This points toward a replacement reaction of anchoring groups from the SAM by oxalate ions during which the oxalic acid now protonates the SAM similar to the replacement of acetate ligands by oxalate. In consequence, copper(II) oxalate layers as grown here may not be firmly attached to the surface of the SAM after several dipping cycles. 3.3. Growth of Copper(II) Oxalate Surface Layers Monitored by QCM-D. The deposition processes upon subsequent exposure to solutions of copper(II) acetate and oxalic acid were also monitored under continuous flow conditions using a quartz crystal microbalance with dissipation measurement (QCM-D). The same equipment and technique has previously been applied to investigate the growth kinetics of HKUST-1 on a SiO2 surface.28 It is thus highly instructive to compare the present results to this study which is so far the only work that has investigated the dissipation changes upon formation of a SurMOF. In analogy to this previous work, the experiment started by flowing pure ethanol over the QCM-D crystal, which was in the present experiments coated with Au and a MUA SAM. For comparison, a few deposition experiments were also performed on bare Au-coated crystals. As the results were qualitatively similar to those obtained with the SAM-coated crystals, they are only briefly shown in section 3.4. To initiate deposition, a 1 mM ethanolic solution of copper(II) acetate was flushed over the sample. This led to a sudden and rapid decrease of the crystal oscillating frequency reaching a constant value within 15 s (inset of Figure 6). The overall frequency change of 3 Hz is similar to the drop by 4 Hz observed previously on the SiO2 surface, the latter having been completed after 2 min.28 The different deposition rates can be traced back to a 5 times higher copper(II) acetate concentration in the present study as compared to the HKUST-1 deposition experiment.28 In accord with the results on HKUST-1 growth,28 the QCMD results for the various overtones of the fundamental crystal 11951

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Figure 7. SEM images of copper(II) oxalate layers formed after six cycles of deposition of copper(II) acetate and oxalic acid. Layers were grown on (a) a Au-coated QCM-D crystal modified by MUA SAM and (b) on a bare Au-coated QCM-D crystal.

crystals after copper(II) oxalate deposition experiments. The grain structure of the gold is clearly visible in the images shown in Figure 7. This figure shows the results for deposition on both a Au-coated crystal covered by a MUA SAM and a bare Aucoated crystal. In both cases, but more strongly on the bare Au surface, needle-shaped structures are seen. The visibility of surface structures is enhanced in HIM by imaging the secondary electrons (SE) generated by the He+ impact.34 In SE imaging, the topology of the sample produces contrast as more electrons are ejected when the ion beam hits the sample at glancing incidence. The energy of the SE is very low, which results in a high surface sensitivity. In fact, a HIM image reveals again that the copper(II) oxalate grows as needletype structures that are oriented parallel to the surface (Figure 8).

dissipation changes. They are in fact more reminiscent of different biomolecular adsorbates.29,32 For example, the changes in frequency and dissipation observed upon deposition of oxalic acid are also qualitatively similar to but still larger than those described previously for deposition of a mussel-adhesive protein29,33 or for adsorption of polymeric charged nanoparticles on a neutral lipid bilayer,32 both forming layers which are presumably loosely coupled to the surface. This suggests that also oxalic acid forms, within 10 s, an extended but selfterminating soft layer that dampens the oscillation efficiently. However, a significant part of oxalic acid is washed off during rinsing, leading to frequency increase. This points toward aggregation above the monolayer coverage. However, as RAIRS detects no significant amount of free acid after rinsing (section 3.2), we can assume that aggregated oxalic acid was efficiently washed off in the RAIRS experiments so that only oxalate ions remained after each layer growth step by immersion into oxalic acid solution and subsequent rinsing. The viscoelastic nature of the layer may thus result from flexible interchain interactions. Another possible explanation is a facile and relatively free movement of growing copper(II) oxalate ribbons that are not densly packed on the surface as suggested by the submonolayer SBU coverage reached during the initial deposition step. Unlike typical SurMOFs, copper(II) oxalate layers prepared by the same deposition method can thus not be regarded as a stiff framework. During the subsequent deposition cycles, the described changes in crystal oscillating frequency and dissipation are qualitatively the same. However, while the changes upon exposure to the copper(II) acetate solution are roughly constant, the steps in oscillating frequency and dissipation upon deposition of oxalic acid decrease with the number of deposition steps. In particular, the changes are much more pronounced during the first three deposition cycles, i.e., within the regime of transition in the orientation of the oxalate ions as deduced from RAIRS (section 3.2). This behavior may be explained by assuming that oxalic acid strongly aggregates by formation of hydrogen-bonded networks at carboxylic acid groups of the SAM that have not been consumed by binding to copper(II) acetate SBUs in the previous deposition step. With increasing number of deposition cycles, these sites of the SAM become increasingly involved in binding of the SBU so that less carboxylic acid groups remain as initiators of oxalic acid aggregation. 3.4. SEM and HIM of Copper(II) Oxalate Layers. SEM images were obtained from the surfaces of several QCM

Figure 8. HIM image of a copper(II) oxalate layer formed after 16 cycles of deposition of copper(II) acetate and oxalic acid on a carboxyterminated SAM grown on a Au surface.

Needle-type structure are not unexpected for copper(II) oxalate considering the ribbon-type arrangement of the metal and the ligands (Figure 1e).12−15 However, an unequivocal assignment cannot be given based on the present results. A closer inspection of the SEM and HIM images suggests that the needles are preferentially aligned with domain boundaries of the underlying polycrystalline Au layer. Again, this is not 11952

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(6) Plain, J.; Pallandre, A.; Nysten, B.; Jonas, A. M. Nanotemplated crystallization of organic molecules. Small 2006, 2, 892−897. (7) Zacher, D.; Shekhah, O.; Wöll, C.; Fischer, R. A. Thin films of metal−organic frameworks. Chem. Soc. Rev. 2009, 38, 1418−1429. (8) Shekhah, O.; Liu, J.; Fischer, R. A.; Wöll, C. MOF thin films: existing and future applications. Chem. Soc. Rev. 2011, 40, 1081−1106. (9) Biemmi, E.; Scherb, C.; Bein, T. Oriented growth of the metal organic framework Cu3(BTC)2(H2O)3·xH2O tunable with functionalized self-assembled monolayers. J. Am. Chem. Soc. 2007, 129, 8054− 8055. (10) Shekhah, O.; Wang, H.; Zacher, D.; Fischer, R. A.; Wöll, C. Growth mechanism of metal−organic frameworks: Insights into the nucleation by employing a step-by-step route. Angew. Chem., Int. Ed. 2009, 48, 5038−5041. (11) Janiak, C.; Vieth, J. K. MOFs, MILs and more: concepts, properties and applications for porous coordination networks (PCNs). New J. Chem. 2010, 34, 2366−2388. (12) Birzescu, M.; Niculescu, M.; Dimitru, R.; Budrugeac, P.; Segal, E. Copper(II) oxalate obtained through the reaction of 1,2-ethanediol with Cu(NO3)2·3H2O - Structural investigations and thermal analysis. J. Therm. Anal. Calorim. 2008, 94, 297−303. (13) Jongen, N.; Bowen, P.; Lemaître, J.; Valmalette, J.-C.; Hofmann, H. Precipitation of self-organized copper oxalate polycrystalline particles in the presence of hydroxypropylmethylcellulose (HPMC): Control of morphology. J. Colloid Interface Sci. 2000, 226, 189−198. (14) Kitagawa, S.; Okubo, T.; Kawata, S.; Kondo, M.; Katada, M.; Kobayashi, H. An oxalate-linked copper(II) coordination polymer, [Cu2(oxalate)2(pyrazine)3]n constructed with two different copper units: X-ray crystallographic and electronic structures. Inorg. Chem. 1995, 34, 4790−4796. (15) Wu, W.-Y.; Zhai, L. X. Poly[diaqua-l-oxalato-copper(II) monohydrate]. Acta Crystallogr., Sect. E 2007, 63, m429−m430. (16) Scott, K. L.; Wieghardt, K.; Sykes, A. G. μ-Oxalato-cobalt(III) complexes. Inorg. Chem. 1973, 12, 655−663. (17) Tolstoy, V. P.; Chernyshova, I. V.; Skryshevsky, V. A. Handbook of Infrared Spectroscopy of Ultrathin Films; Wiley: Hoboken, NJ, 2003. (18) Bradshaw, A. M.; Schweizer, E. In Spectroscopy of Surfaces;Clark, R. J. H., Hester, R. E., Eds.; Wiley: New York, 1988; p 413. (19) Paul, J.; Williams, G. P.; Hoffmann, F. M. Carbon dioxide activation and alkali compound formation. I. Vibrational characterization of oxalate intermediates. Surf. Sci. 2003, 531, 244−264. (20) D’Antonio, M. C.; Palacios, D.; Coggiola, L.; Baran, E. J. Vibrational and electronic spectra of synthetic moolooite. Spectrochim. Acta, Part A 2007, 68, 424−426. (21) Edwards, H. G. M.; Farwell, D. W.; Rose, S. J.; Smith, D. N. Vibrational spectra of copper(II)oxalate dihydrate, CuC2O4·2H2O, and dipotassium bis-oxalato copper(II) tetrahydrate, K2Cu(C2O4)2·2H2O. J. Mol. Struct. 1991, 249, 233−243. (22) Clark, R. J. S.; Firth, S. Raman, infrared and force field studies of K212C2O4·H2O and K213C2O4·H2O in the solid state and in aqueous solution, and of (NH4)212C2O4·H2O and (NH4)213C2O4·H2O in the solid state. Spectrochim. Acta, Part A 2002, 58, 1731−1746. (23) Fujita, J.; Martell, A.; Nakamoto, K. Infrared spectra of metal chelate compounds. VI. A normal coordinate treatment of Oxalato Metal Complexes. J. Chem. Phys. 1962, 36, 324−331. (24) Rahimi-Nasrabadi, M.; Pourmortazavi, S. M.; DavoudiDehaghani, A. A.; Hajimirsadeghi, S. S.; Zahedi, M. M. Synthesis and characterization of copper oxalate and copper oxide nanoparticles by statistically optimized controlled precipitation and calcination of precursor. CrystEngComm 2013, 15, 4077−4086. (25) Lamprecht, E.; Watkins, G. M.; Brown, M. E. The thermal decomposition of copper(II) oxalate revisited. Thermochim. Acta 2006, 446, 91−100. (26) Heyns, A. M. The low-temperature infrared spectra of the copper(II)acetates. J. Mol. Struct. 1972, 11, 93−103. (27) Arnold, R.; Azzam, W.; Terfort, A.; Wöll, C. Preparation, modification, and crystallinity of aliphatic and aromatic carboxylic acid terminated self-assembled monolayers. Langmuir 2002, 18, 3980− 3992.

unexpected as domain boundaries are defects that commonly act as nucleation sites. Most importantly, however, the size of the needle-type structures is comparable to those of the polymeric nanoparticles investigated previously by the QCM-D technique.32 If the nanoscale structures obtained in the present experiment are again coupled loosely to the underlying surface as also suggested by the disappearance and reappearance of free carboxylic acid groups in RAIRS (section 3.2), they can in fact explain the large dissipation observed in the present QCM-D measurements. This again supports that the copper(II) oxalate layers as grown here may not be firmly attached to the surface of the SAM following several dipping cycles.

4. CONCLUSIONS This study provides evidence that copper(II) oxalate can be deposited on surface using a step-by-step approach in which a surface terminated by carboxylic acid groups is alternately dipped into solutions of copper(II) acetate and oxalic acid. QCM-D results show that well-defined amounts of Cu2+ ions are adsorbed to the growing surface in each copper(II) acetate deposition step although oxalic acid appears to aggregate during deposition. RAIRS indicates that copper(II) oxalate layers most likely do not grow in a MOF-type structure with the two carboxylate groups binding to different copper(II) acetate SBUs but in the ribbon-type structure of the copper(II) oxalate crystals. However, after a few initial steps the ribbons tend to grow parallel to the surface as also supported by needle-like structures observed in SEM and HIM. A large dissipation observed in QCM-D measurements further supports this picture by giving evidence that a viscoelastic layer is formed that is consequently rather loosely attached to the surface. In consequence, a good control over the amount of deposited copper(II) oxalate material has been achieved. This approach can be exploited for the deposition of well-defined amounts of metal as a basis for studies on radiation-induced nanoparticle formation and is pursued as such in ongoing experiments.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (P.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.S. thanks Mathias Wickleder (Universität Oldenburg) and Jens Beckmann (Universität Bremen) for valuable discussions. REFERENCES

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