Single source heterobimetallic precursors for the deposition of Cu–Ti mixedmetal oxide thin films†‡§Asif Ali Tahir,a Mazhar Hamid,a Muhammad Mazhar,*a Matthias Zeller,b Allen D. Hunter,b
Muhammad Nadeemc and Muhammad Javed Akhtarc
Received 6th August 2007, Accepted 23rd November 2007First published as an Advance Article on the web 2nd January 2008DOI: 10.1039/b712031a
Heterobimetallic molecular precursors [Ti4(dmae)6(l-OH)(l-O)6Cu6(benzoate)9] (1) and[Ti4(dmae)6(l-OH)(l-O)6Cu6(2-methylbenzoate)9] (2) were prepared by the interaction of Ti(dmae)4
[dmae = N,N-dimethylaminoethanolate] with Cu(benzoate)2·2H2O for (1) andCu(2-methylbenzoate)2·2H2O for (2), respectively, in dry toluene, for selective deposition of Cu/Tioxide thin films for possible technological applications. Both the complexes were characterized bymelting point, elemental analysis, FT-IR, thermal analysis and single crystal X-ray analysis. Complex(1) crystallizes in the triclinic space group P-1 and complex (2) in the rhombohedral space group R-3.The TGA analysis proves that complexes (1) and (2) undergo facile thermal decomposition at 550 ◦C toform copper titanium mixed metal oxides. The SEM/EDX and XRD analyses suggest the formation ofcarbonaceous impurity free good quality thin films of crystalline mixtures of b-Cu3TiO4 and TiO2 forboth (1) and (2), with average grain sizes of 0.29 and 0.74 lm, respectively. Formation of two differenthomogenously dispersed oxide phases is also supported by electrical impedance measurements.
Introduction
Perovskite-like titanium based oxides of the composition MTiO3
(M: Cu, Ni, Pb, Fe, Mg, Co and Zn) are well known as functionalinorganic materials with wide applications in electrodes of solidoxide fuel cells (SOFC),1 metal–air barriers,2 gas sensors3 andhigh performance catalysts.4 Recent techniques for the formationof such materials, including chemical vapor deposition (CVD),sol–gel processes, metal–organic decomposition and molecularbeam epitaxy, to name but a few, require metal–organic moleculesas precursors with specific physical and chemical properties. Thepreparation of inorganic materials from metal–organic precursorsgenerally has the advantages over ‘traditional routes’ of lowtemperatures of formation and/or crystallization, better compo-sitional uniformity and conformal coverage in the case of films.5
The synthetic approaches to these ceramic materials developedso far include solid state reactions between fine powders6,7 or evennanoparticles8,9 of homometallic oxides with titanium oxide,10
co-precipitation reactions in aqueous media11,12 and the alkoxidebased sol–gel approach (only for MgTiO3), based on hydrolysis ofsolutions of n-butoxides in nBuOH.13 It was observed that neitherthe metal–organic precursor based approach has been proposed
aDepartment of Chemistry, Quaid-I-Azam University, Islamabad, 45320,Pakistan. E-mail: [email protected] Consortium @ YSU & Department ofChemistry, Youngstown State University, 1 University Plaza, Youngstown,OH, 44555-3663, USAcPhysics Division, PINSTECH, P.O. Nilore, Islamabad, Pakistan† The HTML version of this article has been enhanced with colour images.‡ CCDC reference numbers 655806 & 655807. For crystallographic datain CIF or other electronic format see DOI: 10.1039/b712031a§ Electronic supplementary information (ESI) available: Numerical crystaldata files and EDX scan files. See DOI: 10.1039/b712031a
nor the soluble/volatile heterometallic single source precursor forthe synthesis of copper titanate have been reported so far.
Therefore, we were attracted to MTiO3-type perovskite titanates(where M = Cu, Ca, Sr, Ba, Pb) as they are key materialsfor use as critical components in electronic devices because oftheir outstanding electrical properties.14 The discovery of a giantdielectric constant in CaCu3Ti4O12 has further increased interestin this type of perovskite oxide.15–17
Following our previous work directed towards the designof heterobimetallic complexes for mixed metal oxide systemsand taking advantage of aminoalcohol ligands such as N,N-dimethylaminoethanol (dmaeH), which can coordinate to themetal atoms in several ways with the possibility of gaining highnuclearity species18,19 we were thus able to investigate the possi-bility of developing soluble and volatile metallo-organic singlesource precursors. The reaction of Ti(dmae)4 with Cu(O2C7H4–R)2·2H2O [R = H, 2-CH3] gave crystalline Ti4(dmae)6(l-OH)(l-O)6Cu6(benzoate)9 (1) and Ti4(dmae)6(l-OH)(l-O)6Cu6(2-methylbenzoate)9 (2) under very mild conditions. Both the cagecomplexes (1) and (2) are volatile under the experimental condi-tions of Aerosol Assisted Chemical Vapour Deposition (AACVD)and found suitable for the deposition of carbonaceous impurityfree homogeneous thin films of b-Cu3TiO4 for industrial and othertechnological applications.
Experimental
All manipulations were carried out under an inert atmo-sphere of dry argon using Schlenk tube and glovebox tech-niques. Solvents were rigorously dried and distilled over sodiummetal/benzophenone. Benzoic acid, 2-methylbenzoic acid andTi(OC2H5)4 were purchased from Aldrich Chemicals and were
stored in a glovebox under argon. Ti(dmae)4 and Cu(O2C7H4–R)2·2H2O [R = H, mp: 300 ◦C dec., 2-CH3, mp: 280 ◦C dec.] wereprepared by literature procedures.20,21
All other reagents were purchased from Fluka Chemicals.Melting points were recorded on a Mitamura Riken Kogyo(MT-D) apparatus and are uncorrected. Elemental analyses wereperformed using a CHN Analyzer LECO model CHNS-932.FT-IR spectra were recorded with a Bio-Red Excalibur FT-IRmodel FTs 300 MX spectrometer using KBr pellets. Magneticmeasurements were conducted using a commercial vibratingsample magnetometer (VSM) model BHV-50, Perkin Denshi Co.(Ltd.), Japan.
Controlled thermal analyses (TGA) of the complexes wereinvestigated using a Perkin Elmer Thermogravimetric AnalyzerTGA-7 with computer interface. The measurements were carriedout in an alumina crucible under an atmosphere of flowingnitrogen gas (25 ml min−1) at a heating rate of 10 ◦C min−1.
Scanning electron microscopy (SEM) of thin films was carriedout using a JEOL JSM-5910 scanning electron microscope.Metallic elemental ratios were recorded on an EDX analyzerInca-200 of Oxford Instruments, UK. XRD peak patterns ofthe thin films were collected using a PANanalytical, X′Pert PROdiffractometer with Cu-Ka radiation.
Single crystal diffraction data of complexes (1) and (2) werecollected on a Bruker AXS SMART APEX CCD diffractometerat 100(2) K using monochromatic Mo-Ka radiation with x scantechnique. The unit cells were determined using SMART22 andSAINT+23 and the data were corrected for absorption usingSADABS in SAINT+. The structures were solved by directmethods and refined by full matrix least squares against F 2 withall reflections using SHELXTL.24 All non-hydrogen atoms wererefined anisotropically.
For compound (1), 1441 A3 (or 24%) of the unit cell volumeconsists of toluene filled voids. Attempts to define and refinethe severely disordered solvent molecules did reveal the presenceof toluene as the solvate molecules, but no satisfactory modelcould be developed to define the disordered molecules and thusthe solvent molecules have been “squeezed” out using the Platonprogram.25
One benzoate ligand and one dmae ligand neighboring eachother are disordered over two positions with an occupancy ratio of0.606(5) to 0.394(5). Similarity restraints were applied to achieve ameaningful and stable refinement of the disordered atoms, detailsare given in the cif file.†
For compound (2), the structure has solvent accessible voidsof 305.0 A3 in the asymmetric unit (1638 A3 or 9.7% for thewhole unit cell). The voids are located along the three fold axisand seem to be filled with solvent disordered around this axis. Asno obvious model for the disorder is discernable, the remainingelectron density within the voids (not exceeding 1.372 e A−3) wasignored.
Two of the 2-methylbenzoic acid ligands are flip disordered withone of the orientations dominating. The disorder of one of thesegroups is associated with a disorder of the coordinated copperatoms. The refined occupancy ratios are 0.897(1) to 0.103(1) forthis site and 0.947(4) to 0.053(4) for the site with only the liganddisordered. Again similarity restraints were applied to achieve ameaningful and stable refinement of the disordered atoms, detailsare given in the cif file.†
All carbon bonded hydrogen atoms of (1) and (2) were placedin calculated positions and all hydrogen atoms were isotropicallyrefined with a displacement parameter 1.5 (methyl, hydroxyl) or1.2 times (all others) that of the adjacent carbon or oxygen atom.The O–H distances of the hydroxyl groups in (1) and (2) were setto be 0.85 A within a standard deviation of 0.02.
Impedance studies of the thin films have been conducted at roomtemperature using an Alpha-N analyzer (Novocontrol, Germany).Windeta software has been used for data acquisition, which hadbeen fully automated by interfacing the analyzer with a PC. Beforeimpedance experiments, the dispersive behaviors of the leads havebeen carefully checked to ensure the absence of any extraneousinductive or capacitative coupling in the experimental frequencyrange. The amplitude of the ac signal has been set to 1.0 V andelectrical contacts have been made by silver paint on one side ofthe thin film sample. The results of the complex impedance arepresented as Z = Z′ + jZ′′ and M = M ′ + jM ′′ (where Z′, Z′′, M ′
and M ′′ are the real and imaginary parts of the impedance andmodulus, respectively).
Synthesis of Ti4(dmae)6(l-OH)(l-O)6Cu6(benzoate)9 (1)
2.0 g (5.86 mmol) of Cu(benzoate)2·2H2O was added to asolution of 1.67 g (4.18 mmol) of Ti(dmae)4 in 25 ml oftoluene at room temperature. After stirring for 2 h, the excessof Cu(benzoate)2·2H2O was eliminated by filtration through acannula and the reaction mixture was evaporated to dryness underreduced pressure. The solid was washed with hexane to remove thereaction by-products and redissolved in 5 ml of toluene to give 95%of the crystalline product after five days. mp: 205 ◦C; Analysis:Calc. for C87H106N6O31Ti4Cu6: C, 45.30; H, 4.59; N, 3.64; FoundC, 45.80; H, 4.52; N, 3.43%. IR/cm−1 3434 br, 3063 s, 3022 w, 2970w, 2889 w, 2868 w, 2798 w, 2239 w, 1707 w, 1609 s, 1568 s, 1549 w,1447 w, 1376 s, 1304 w, 1278 s, 1173 s, 1092 s, 1068 s, 1052 s, 951 s,897 s, 836 s, 789 s, 717 s, 678 s, 616 s, 583 w, 544 w, 449 s. TGA:148–213 ◦C (4.68% wt. loss); 213–550 ◦C (residue of 32.5%).
Synthesis of Ti4(dmae)6(l-OH)(l-O)6Cu6(2-methylbenzoate)9 (2)
2.5 g (6.72 mmol) of Cu(2-methylbenzoate)2·2H2O was added toa solution of 1.91 g (4.80 mmol) of Ti(dmae)4 in 25 ml of tolueneat room temperature. After stirring for 2 h, the excess of Cu(2-methylbenzoate)2·2H2O was eliminated by filtration through acannula and the reaction mixture was evaporated to dryness underreduced pressure. The solid was washed with hexane to remove thereaction by-products and redissolved in 5 ml of toluene to give 95%of the crystalline product after three days. mp: 195 ◦C; Analysis:Calc. for C96H124N6O31Ti4Cu6(C6H5–CH3): C, 48.99; H, 5.23; N,3.32; Found C, 48.84; H, 5.06; N, 3.20%. IR/cm−1 3436 br, 3061 s,3015 w, 2960 w, 2867 w, 2797 w, 1708 w, 1610 s, 1570 s, 1551 w, 1438w, 1376 s, 1279 s, 1154 s, 1090 s, 1050 s, 1015 w, 949 s, 887 w, 840 s,778 s, 743 s, 660 s, 615 s, 587 w, 541 s, 450 s. TGA: 156–236 ◦C(12.4% wt. loss); 233–550 ◦C (residue of 30.2%).
Thin film deposition
Mixed metal oxide thin films were prepared on a glass substrateusing a self-designed assembly having an ultrasonic nebulizer togenerate an aerosol as described elsewhere.26 The substrates, 1.5 ×4 cm glass slides, were horizontally placed in a glass tube inserted in
a tube furnace fitted with an aerosol generating assembly. Carriergas (N2) and sample solution flow rate was manually controlled.Parameters for the growth of thin films are listed in Table 1.
Results and discussion
Taking advantage of ligands with bifunctional donor groupsheterobimetallic precursors can be readily prepared by couplingtwo different monometallic complexes.27–29 Ligands with oxy-gen donors such as carboxylates, bifunctional alcohols and b-diketones have been extensively explored, and also aminoalcoholswere found to be well suited for this purpose due to the presenceof the amine and alkoxy groups as donor sites towards differentmetal atoms.30,31
Metal carboxylates and metal alkoxides/aminoalkoxides arethe most commonly used reactants for the synthesis of suchheterobimetallic complexes because of their bridging or bridging–chelating coordination properties. Metal alkoxides and carboxy-lates often react with one another by the elimination of an esteras a volatile byproduct32 but in the reactions observed here, it ismore likely that oxo and hydroxo complexes were formed throughhydrolysis by water from the hydrated starting materials accom-panied by the loss of acetate and dmae ligands. However, suchreactions can occur under very mild conditions (room temperatureand nonpolar solvents) and thus the complexes can be synthesizedby simple mixing of the starting materials in an appropriatesolvent. The reactions between divalent copper(II) carboxylatesand Ti(dmae)4 illustrate these features. These reactions proceedsmoothly and quantitatively in hydrocarbons over 1 or 2 h stirring,with progressive dissolution of the metal carboxylate according toeqn (1) and (2). The excess of metal carboxylate is easily removedby filtration and reaction by-products are eliminated by washingwith hexane while the compounds are crystallized out almostquantitatively from the filtrate.
4Ti (dmae)4 + 6 Cu (O2C7H5)2 · 2H2Otoulene−−−−−→
r.t.
Ti4 (dmae)6 (l-OH) (l-O)6 Cu6 (O2C7H5)9
+ 3HO2C7H5 + 5 H2O + 10 (dmaeH) (1)
4Ti (dmae)4 +6 Cu (O2C8H7)2 · 2H2Otoulene−−−−−→
r.t.
Ti4 (dmae)6 (l-OH) (l-O)6 Cu6 (O2C8H7)9
+ 3 HO2C8H7 + 5 H2O + 10 (dmaeH) (2)
Both complexes (1) and (2) were characterized by melting point,elemental analysis, FT-IR and single crystal X-ray crystallography.In the IR spectrum, the broad bands at 3422 and 3436 cm−1 in
complexes (1) and (2), respectively, can be assigned to bridgingOH-groups. For the carboxylate ligands the difference D(masCO2 −msCO2) = 234 cm−1 suggests a chelating or bridging–chelatingbehavior for these ligands.33 Magnetic measurements show thatthe complexes are paramagnetic.
The single crystal structures of both (1) and (2) have beendetermined by single crystal diffraction. Both crystal structuresexhibit solvent filled voids (see Experimental) and in both casessome of the ligands show orientational disorder. In the followingdiscussion only the major orientations will be considered.
Both molecules have slight variations of the same structuralmotif: six partially connected copper units arranged around acentral admantane-like cage of Ti4(l-O)6. Due to crystallizationand packing effects, however, slight differences between thestructures of the two compounds are observed in the solid state.Complex (2) retains its three fold symmetry and crystallizes in arhombohedral setting with the point group R-3 and the moleculeis located on the crystallographic three fold axis. Complex (1), onthe other hand, forms triclinic crystals and the complex molecule islocated on a general position, thus losing its non-crystallographicthree fold symmetry in the solid state. For (1) several of the metal–ligand distances are becoming unequal. Simplified depictions ofthe structures of (1) and (2) are shown in Fig. 1–4, crystal andstructure refinement data are given in Table 2, bond distances inTable 3 and angles can be found in the ESI.§
Fig. 1 ORTEP drawing at the 30% probability level for Ti4(dmae)6-(l-OH)(l-O)6Cu6(benzoate)9 (1). For the disordered ligands only the majororientation is shown.
The entire complex comprises six copper and four titaniumcenters, linked together by oxygen atoms which are doubly/triplybridging between the metal atoms. Three of the copper units areconnected to the Ti4(l-O)6 unit solely via this titanium atom, the
Fig. 2 ORTEP drawing at the 30% probability level for Ti4(dmae)6-(l-OH)(l-O)6Cu6(2-methylbenzoate)9 (2). For the disordered ligands onlythe major orientation is shown.
other three are arranged as a rim around the Ti3(l-O)3 base ofthe adamantane cage. This effectively splits the molecules intotwo units only connected via one octahedral TiO6 unit: a top unitof Ti1Cu3(l-OH)(O2C7H4–R)3(dmae)3 and a bottom unit of Ti4(l-O)6Cu3(O2C7H4–R)6(dmae)3 (see Fig. 3 and 4).
The core of the bottom Ti4(l-O)6Cu3(O2C7H4–R)6(dmae)3 sub-unit is built by the adamantane like Ti4(l-O)6 cage (Fig. 3). The Ti–Ti distances from the central Ti atom to each of the other titaniumatoms are significantly shorter than the Ti–Ti distances within the“cyclohexane-like” Ti3(l–O)3 base. The former values (3.3776(13),3.3812(13), and 3.3855(14) A for (1) and 3.3835(8) A for (2)) areconsistent with literature values for similar compounds,34,35 thelatter are all larger than 3.468 A and above the threshold for a Ti–Ti bonding interaction. The three titanium atoms of the base areeach coordinated to three cage l-oxide anions and oxygen atomsof two carboxylate ligands and dmae molecules are completingthe close to octahedral coordination mode. The dmae ligand andone of the l-oxide anions are bridging each titanium atom to acopper atom which is also in direct metal–metal interaction withthe titanium center (2.949(11), 2.9567(12) and 2.9568(11) A for(1), 2.9499(6) A for (2)). To the other side each two carboxylateligands are bridging the base titanium atoms to another coppermoiety. The copper centers have a close to ideal square pyramidalCuO4N environment. The ligating atoms are made up by eachone bridging dmae oxygen atom, one dmae nitrogen atom, twocarboxylate ligands oxygen atoms, and one l-oxide anion of theTi3(l-O)3 base.
The top Ti1Cu3(l-OH)(O2C7H4–R)3(dmae)3 moieties of (1) and(2) each have a centre which can be seen as a TiCu3O4 cube (seeFig. 4). The connectivity and the general motif are the same forboth complexes but the individual bond distances, especially theCu–O ones, differ significantly. In the case of complex (2), theTiCu3O4 moiety is located on the three fold crystallographic axisand all the three copper atoms have a square planar environmentsurrounded by three oxygen and one nitrogen donor atoms,augmented by two additional but only loosely coordinated oxygendonor atoms. Each oxygen atom of the dmae ligands bridgesthe titanium with the copper atoms, which in turn are bondedto a terminal l-hydroxyl anion. For complex (1), this symmetric
Fig. 3 ORTEP drawing of the bottom parts of molecules in (a) complex (1) and (b) complex (2) at the 30% probability level. For disordered parts of themolecules only the major orientations are shown.
Fig. 4 ORTEP drawing of the top parts of molecules in (a) complex (1) and (b) complex (2) at the 30% probability level. For disordered parts of themolecules only the major orientations are shown. Dotted bonds indicate substantially elongated Cu–O interactions.
Table 2 Crystal and structure refinement data for compounds (1) and (2)‡
Empirical formula C87H106Cu6N6O31Ti4 C96H124Cu6N6O31Ti4
Formula weight 2304.62 2430.79Solvent Toluene TolueneCryst. shape, colour Plate, blue Block, blueTemperature/K 100(2) 100(2)Crystal system Triclinic RhombohedralSpace group P-1 R-3Unit cell dimensions a = 16.427 (6) A, b = 17.267 (6) A, c =
21.893 (3) A, a = 102.256 (7)◦, b = 99.706(7)◦, c = 92.352 (7)◦
a = 19.6795 (6) A, b = 19.6795 (6) A, c =50.516 (3) A, a = 90◦ b = 90◦ c = 120◦
Volume/A3 5963(3) 16942.9(12)Z 2 2Density (calculated) 1.284 1.429Absorption coefficient / mm−1 1.366 1.446F(000) 2360 7512Crystal size/mm 0.30 × 0.20 × 0.06 0.40 × 0.38 × 0.37h range for data collection / ◦ 0.97 to 28.28 1.44 to 28.28Index ranges −21 ≤ h ≤ 21, −23 ≤ k ≤ 23, −29 ≤ l ≤ 29 −26 ≤ h ≤ 26, −26 ≤ k ≤ 26, −65 ≤ l ≤ 67Reflections collected 61919 57859Independent reflections 28462 R(int) = 0.0806) 9357 R(int) = 0.0274Absorption correction Multi-scan Multi-scanMax. and min. transmission 0.921 and 0.602 0.586 and 0.428Data/restraints/parameters 29462/27/1245 9357/98/447Goodness-of-fit on F2 0.871 1.127Final R indices [I>2r (I)] R1 = 0.0601, wR2 = 0.1285 R1 = 0.0424, wR2 = 0.1237R indices (all data) R1 = 0.1151, wR2 = 0.1425 R1 = 0.0567, wR2 = 0.1383Largest diff. peak and hole/e A−3 0.576 and −0.527 1.372 and −0.414
arrangement around the three fold axis is broken and especiallythe coordination of the benzoate ligands is quite asymmetric. Inthe case of (2), each methylbenzoate carboxylate group is tightlybonded to one copper atom and only loosely connected to another(2.455(4) A) (see Fig. 4). For (1), on the other hand, two of thebenzoate ligands have slightly stronger interactions with the sec-ond copper atom (2.338(3) and 2.380(3) A), but the third one hasno interaction via its second O atom with any metal at all (Cu–Odistance larger than 2.85 A). This change of the coordination modefor the copper atoms then results in a significant rearrangementof the bond distances within the TiCu3O4 cube as shown in Fig. 4
(substantially elongated Cu–O interactions are indicated by dottedlines). While each Cu atom still has a square planar environmentsurrounded by three oxygen and one nitrogen donor atoms, aug-mented by two other weakly coordinated oxygen atoms, the rolesof the strong and loosely bonded oxygen donor atoms has beenpartially reversed in (1) when compared to (2). This significantdifference between the two complexes in the solid state, however,does not translate into a different chemical nature for both com-plexes as the bonding environment around Cu(II) is very flexibleand it can be assumed that in solution (i.e. upon release of packingeffects) both complexes will attain the same connectivity pattern.
Thermal decomposition studies and characterization ofthin films
The thermal behaviour of complexes (1) and (2) was studied bythermogravimetric analysis over a range of 50–700 ◦C, performedunder an inert atmosphere of flowing nitrogen gas (25 ml min−1)and a heating rate of 10 ◦C min−1. The TGA plot (Fig. 5) shows thatboth the complexes smoothly undergo thermal decomposition.The decomposition of the molecular complexes begins at 148 ◦Cand is completed at 550 ◦C. It proceeds in two steps and themaximum weight loss (approximately 67%) takes place between213 and 500 ◦C. The residual weight of 32.5% for (1) and 30.2%for (2) is slightly less, but close to the expected composition forCu6Ti4O12, [2(Cu3TiO4 + TiO2)], as compared to the calculatedvalue of 33.1 and 31.4% for (1) and (2) respectively, indicating thatthe complexes decompose quantitatively to oxides. The residual
weight also indicates the removal of carbonaceous matter fromthe organic groups leaving no carbon impurity in the final metaloxides. In these complexes each metal centre is co-coordinativelysaturated by the oxygen atoms of the chelating carboxylate anddmae ligands thus eliminating the need of additional oxygento form oxides. Thus the new robust precursors are suitablefor deposition of clean copper titanium oxide thin films at therelatively low temperature of 550 ◦C.
Fig. 5 Thermogravimetric plot showing loss in weight with increase intemperature for complexes (1) and (2).
Thin films of mixed metal oxides were prepared as describedin the experimental section and were studied and characterizedfor their thickness, surface morphology, metallic composition andelemental distribution by SEM/EDX. The nature of the crystallinephases was investigated by XRD. Physically, both the thin films arereflecting light in multi-shaded fringes, are stable towards air andmoisture and qualify the ‘scotch tap test’ indicating good adhesionto the substrate.
The thickness of the film deposited from precursor (1) variesfrom 0.133 to 0.4 lm. The micrograph of the film (Fig. 6) indicates
Fig. 6 SEM micrograph of oxide thin film deposited from complex (1) byAACVD at 550 ◦C.
an average particle size of 0.29 lm homogeneously dispersed in adense microstructure without any distinguishable phase. The SEMimage of the film grown out of complex (2) is more compact andsmooth (see Fig. 7) with an average particle size of 0.74 lm andwith a thickness ranging from 0.23 to 1.5 lm. EDX analyses ofboth films indicate a largely homogeneous distribution of Cu, Tiand O throughout the surface and absence of any impurities eitherfrom the ligand or from the carrier gas.
Fig. 7 SEM micrograph of oxide thin film deposited from complex (2) byAACVD at 550 ◦C.
The X-ray diffractograms of the oxides obtained from com-plexes (1) and (2) (Fig. 8) indicate the formation of each twodifferent types of oxides according to eqn (3) and (4).
Fig. 8 X-Ray diffractograms of oxides obtained from complexes (1) and(2). x = TiO2 [01-071-1167]36 (Anatase) and y = Cu3TiO4 [01-083-1284].37
Both the oxide mixtures exist as highly crystalline phaseswith ratios of 77 to 23% for Cu3TiO4 to TiO2, respectively. TheTiO2
36 formed in both the reactions is in the Anatase formhaving a tetragonal crystal system with space group I41/amd
with cell parameters a = b = 3.7892 A and c = 9.5370 A,while the copper titanium oxide Cu3TiO4
37 is of the b-type withhexagonal crystal system and space group P63/mmc (a = b =3.0400 A, c = 11.4590 A) for complex (1) and (2). It has beenreported that in titanium-based ceramics with titanium in the4+ oxidation state prolonged heating at high temperatures (ca1350 ◦C) leads to its reduction to Ti3+ and other low valent speciesthus deteriorating the dielectric properties and quality of the oxidefilms.38–40 Since complexes (1) and (2) decompose at a relativelylow temperature (ca 550 ◦C), oxides containing Ti4+ with gooddielectric properties can be prepared without any deteriorationof the titanium component. It has been reported that the copperdoped TiO2 phases (like Cu3TiO4) are very effective photocatalystsfor the decomposition of organic material such as methylene blue41
and may also be used as sorbents for hot gas cleanup especiallyfor the removal of H2S from exhaust gases.42
Impedance spectroscopy
Electrical impedance measurements were carried out to obtainadditional information about the make up and electrical propertiesof the thin films. Fig. 9 shows a typical plane plot of the real (Z′)against the imaginary (Z′′) parts of the measured impedance. Theintersection of the semicircle with the Z′ axis at the right hand sidegives the value of the resistance corresponding to the dc values(Rdc as frequency → 0). On the left hand side of the semicircleone can see the appearance of a second arc, shown in the inset ofFig. 9, which is not fully developed within the available frequencyrange.
Fig. 9 Impedance plane plot of thin film sample. The black squares aremeasured data, the continuous line is based on the (R1Q1)(R2Q2) equivalentcircuit model fit. The inset shows the second semicircle arc at higherfrequencies.
In order to correlate the electrical properties of the thin filmswith the microstructure of the sample, we employed a standardequivalent electrical circuit model (R1Q1)(R2Q2), where R1, Q1
and R2, Q2 are the resistance and the effective capacitance(or constant phase elements, CPE) of the two materials. Theexperimental data and the fitting results using the (R1Q1)(R2Q2)equivalent circuit model are shown in Fig. 9. The derived fittingparameters are given in Table 4. Normally the resistance R of thesemiconducting TiO2 phase can be considered to be higher than
Table 4 Fitting parameter derived using the equivalent circuit model(R1Q1)(R2Q2). R = resistance, Q = effective capacities or constant phaseelements (CPE)
Cu3TiO4 phase TiO2 phase
R1 = 2.03 × 106 X R2 = 3.50 × 108 XQ1 = 4.82 × 10−12 F Q2 = 7.39 × 10−13 Fn = 0.81 n = 0.94
that of the high conductivity copper titanate phase. By comparingthe resistance R1 of Cu3TiO4 with the reported value for coppertitanate perovskite,43 R1 is found to be 2.03 × 106 X while that of theTiO2 network (R2) is in the order of 3.5 × 108 X. The resistance ofthe TiO2 phase, as compared with reported values,44 is considerablyhigher than expected, which may be due to poor connectivityof the TiO2 particles in the matrix. These results indicate thepresence of a mixed phase matrix with different resistances R1
(the resistance of the Cu3TiO4 phase) and R2 (resistance of theTiO2 phase), which further supports the results from the X-raydiffraction experiments. The impedance results can be interpretedon the basis of the Frick model45 for two-phase dispersions. Asdiscussed in the XRD section TiO2 makes up only 23% of thecrystalline phases in the present system, which is less than thecritical value for the percolation limit (∼30%) where particles arestill expected to connect to form a continuous path. Thus one caninfer that there is a continuous medium of Cu3TiO4 in which asecond phase of poorly connected TiO2 is embedded.
The time constants of the two phases are not very differentfrom each other, thus causing the relatively low resolution ofthe impedance plane plots in Fig. 9. Fig. 10 shows the moduluspresentation of the data, M ′ versus M ′′, where the low frequency arccorresponds to the high conductivity continuous Cu3TiO4 phaseand is apparently a perfect semicircle with its centre on the real axis.The incomplete high frequency arc corresponds to the dispersed23% TiO2 phase with its centre below the real axis, and this non-ideal behaviour might imply heterogeneity in the sample.
Fig. 10 Modulus spectrum resolving two arcs for a two phasemicrostructure.
Conclusion
The heterobimetallic complexes Ti4(dmae)6(l-OH)(l-O)6Cu6-(benzoate)9 (1) and Ti4(dmae)6(l-OH)(l-O)6Cu6(2-methyl-benzoate)9 (2) were synthesized by simple and routine chemicalreactions of Ti(dmae)4 with Cu(benzoate)2·2H2O for (1), andCu(2-methylbenzoate)2·2H2O for (2) under very mild conditions.The high solubility in organic solvents makes (1) and (2)attractive precursors for preparation of Anatase doped goodquality Cu3TiO4 thin films by chemical vapour deposition,which were characterized by SEM/EDX and XRD. Impedancemeasurements co-relate the electrical properties with themicrostructural properties of the thin films based upon the twophase dispersion model, which further supports the XRD data.This system thus opens new approaches to the synthesis ofnanoparticles with controlled geometries, which in turn mightfind use in advanced technological and industrial applications.
Acknowledgements
AAT, MH and MM acknowledge the Pakistan Science Founda-tion and the Higher Education Commission Islamabad, Pakistanfor financial support through the Project No. PSF/R & D/C-QU/CHEM.(218) and the “Merit Scholarship Scheme for PhDStudies in Science & Technology (200 Scholarships)”. The SmartApex diffractometer was funded by NSF grant 0087210, by OhioBoard of Regents grant CAP-491 and by YSU.
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