Fabrication and Characterization of Metalloporphyrin Nanostructures

Elena Stachew and Ursula Mazur

Characterization of Advanced Materials Research Experience for Undergraduates

Department of Chemistry and Materials Science and Engineering Program, Washington State University, Pullman, WA 99164-4630

AcknowledgementsThis work was supported by the National Science Foundation’s REU program under grantnumber DMR-0755055 and grants CHE-0555696 and 0848511. We also thank Fred Schuetzefor help in programming the furnace, Dave Savage for help in designing the holders for thesource and substrates, Professor K. W. Hipps and Benjamin Friesen for assisting with AFMimaging experiments, and Steve Klase for help with the optical spectra analysis.

IntroductionPorphyrins are a group of organic aromatic macrocycles consisting of four pyrrole

units connected by methine bridges.1 A number of porphyrins and their derivativesare found commonly in nature.2 The most common example is that porphyrins areconstituents in the pigment of red blood cells.

Porphyrins and their derivatives have great potential in the fields of molecularelectronics, catalysis, optoelectronics, and solar energy conversion due to theiroptical and electronic properties.3

To better utilize nanoscale materials for various applications, the relationshipbetween a material’s molecular structure and its properties must be understood.

Porphyrins are an ideal group of molecules to study this relationship due to thefacile substitution of various functional groups, the chelation of a metal ion in thecenter of the porphyrin compound (a.k.a metalloporphyrin), or the adjustment of theaxial ligation of the metalloporphyrins.2 In this study, the metalloporphyrin copper(II) octaethylporphyrin (CuOEtP) nanostructures (Figure 1) were created using a vaportransfer deposition method, and were examined using ultraviolet-visible (UV-Vis)spectroscopy, Fourier transform infrared (FTIR) spectroscopy, attentuated totalreflectance (ATR) infrared spectroscopy, and scanning electron microscopy (SEM).

Figure 1: Molecular structure and CPK model of copper (II) 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine (CuOEtP).

ResultsFigure 3 compares the infrared spectrum of the CuOEtP starting material and that

of the porphyrin deposited for 7 minutes and 1 minute by the VTD method. All threespectra look very similar indicating that no decomposition of the CuOEtP sampleoccurred during the thermal deposition process. The intensities of the bands in the2900 cm-1 region typically associated with CH2 and CH3 vibrations of the ethyl groupson the macrocycle appear weaker in the spectrum of the VTD deposited samples thanthat of the spectrum of CuOEtP in KBr. It is possible that a preferred orientation ofgrowth on the substrates of the CuOEtP nanostructures is responsible for the reducedpeak intensity of the ethyl substituents (Figure 3). The spectra of the CuOEtP startingmaterial and VTD deposited samples were collected using the Perkin-Elmer LimitedSpectrum 1700 FTIR spectrometer and Thermoscientific Nicolet iS10 ATRspectrometer respectively.

UV-Vis spectrum of the CuOEtP of the 7 minute and 1 minute samples depositedon mica indicate that mica’s spectrum is within the same regions as that of thedeposited CuOEtP samples to allow for much distinction in spectral acquisition (Figure4) in transmission mode. The solution spectrum of the porphyrin in chloroform (10-5

M) exhibits a sharp B-band present at 397nm and weaker Q-band progression at530nm and 566nm. These bands are associated with -* electronic transitions. Thespectra were collected using the Perkin-Elmer 300 UV-visible spectrometer .

An FEI Quanta 200F field emission SEM was used to image CuOEtP VTD depositedon the HOPG and mica substrates for 7 min (Figure 5-9). (These samples proved to betoo thick for AFM or STM imaging.) Figures 5 and 6 show similar dense granularfeatures on both the HOPG and mica substrates, indicating a no substratedependence on the CuOEtP nanostructure formation. HOPG proved to be a moresatisfactory substrate for imaging, because it has no surface charging. A highresolution micrograph of CuOEtP deposited on HOPG, Figure 9, shows that thedominant surface structures appear to be spherical in shape and a few secondaryfeatures are nanorod-like. Taking initial approximate size measurements from thisimage, the diameter of the spheres ranges from 2.9µm to 3.7µm. The rods areapproximately 100 nm wide and several microns long. A similar range of features withsimilar dimensions are observed on the mica surface, Figure 7, but the image is lessclear. The average sphere diameters on mica are approximately 3.7µm, the samevalue for the average calculated on CuOEtP spherical nanostructures that are presentthe HOPG surface.

Figure 2: Schematic of the cross-sectional view of the furnace with the vapor transfer deposition setup.

ExperimentalCuOEtP nanostructures were created using a vapor transfer deposition (VTD)

method, using a modified procedure reported earlier.3 A Marshall 1100 Seriesfurnace was used to carry out the fabrication, and two temperature zones werecreated by electrically shorting out a section of the furnace. The porphyrin wasacquired from Frontier Scientific.

CuOEtP powder was placed in an aluminum oxide coated molybdenum boat, andtransferred into a quartz tube which was inserted into a one zone horizontal tubefurnace and placed at the high temperature zone (Figure 2). A KBr disk (2 cmdiameter, 2mm thick), freshly cleaved mica (1 cm x 0.5 cm and 1 cm x 4 cm pieces)and highly ordered pyrolytic graphite, HOPG, (1 cm x 1 cm) were used as thesubstrates. The substrates were placed on a glass plate, inside the quartz tube at thelow temperature zone of the furnace. Tungsten wire was attached to the side of theglass plate to allow for sliding the substrates in and out of the low temperature zone.As the CuOEtP sublimed in the high temperature zone, nitrogen gas was employed tocarry the CuOEtP vapor to the low temperature zone, where it was deposited on thesubstrates. The N2 flow rate was kept constant between 0.35-0.46 scfh.

To determine the placement of the source and the substrates to allow for thegreatest sublimation and deposition, a temperature profile of the furnace was takenalong the length of the quartz tube.

First, a continuous N2 gas flow was established for five minutes prior todeposition experiments. N2 was present during heating and deposition, as well asfor an additional 30 minutes after heating to help cool the substrates to roomtemperature. With just the sample in the quartz tube, the furnace was heated fromroom temperature to 150°C at an average rate of 10 degmin-1, and was kept at thattemperature for one hour. This pre-sublimation was done to eliminate any lowmolecular weight compounds from the CuOEtP sample. After 1 hour, the substrateswere placed in the low-temperature zone, and the furnace was heated to 400°C.Deposition times tried were 15, 7, and 1 minute. In this poster, we present dataprimarily for the 7 minute deposition period, as well as an spectra analysis for the 1min deposition period.

Figure 3: Transmission infraredspectrum of CuOEtP startingmaterial was acquired in a KBrmatrix. ATR accessory was usedto measure the vibrationalspectrum of the thermallydeposited samples of CuOEtP onKBr disk. Spectra were collectedwith 4cm-1 resolution using aTGS detector.

Figure 4: UV-Vis spectrum ofCuOEtP was obtained from a 1X 10-5 M solution of theporphyrin in chloroform.Because mica’s spectrum iswithin similar regions to thatof the thermally depositedsamples of CuOEtP, this doesnot allow for a properspectrum to be obtained.Appropriate references wereused in collecting bothspectra.

References1. Egharevba, G.O; George, R.C.; Maaza, M. Effect of Heat on the Morphology and OpticalProperties of Porphyrin Nanostructures. Synthesis and Reactivity in Inorganic, Metal-Organic and Nano-Metal Chemistry, 2008, 38, 681-687.2. Drain, C.M; Varotto, A.; Radivojevic, I. Self-Organized Porphyrinic Materials. Chem. Rev.2009, 109, 1620-1658.3. Hu, J.S; Ji, H.X.; Wan, L.J. Metal Octaethylporphyrin Nanowire Array and Network towardElectric/Photoelectric Devices. J.Phys.Chem.C 2009, 113, 16259-16265.

Figure 5: HV 2.00 kV SEM image of CuOEtPdeposited on mica for 7 minutes. 500x magnification.

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Figure 7: HV 2.00kV SEM image of CuOEtPdeposited on mica for 7 minutes. 5000xmagnification.

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Figure 8: HV 1.00 kV SEM image of CuOEtPdeposited on HOPG for 7 minutes. 5000xmagnification.

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Figure 9: HV 1.00 kV SEM image of CuOEtPdeposited on HOPG for 7 minutes. 10,000xmagnification.

Conclusions Comparison of the infrared spectra of the CuOEtP starting material and the porphyrin VTD

deposited for 7 minutes and 1 minute indicate that no decomposition of the CuOEtP sampleoccurred during the thermal deposition process. Reduced intensities of the ethyl groupsvibrational bands of the CuOEtP VTD nanostructures indicate the possibility of preferredorientation growth of the porphyrin molecules deposited on the mica and the HOPG .

UV-Vis spectrum of the VTD sample was not observed. It is possible that the substrate(mica) spectrum is located in similar regions to that of the deposited sample, making itdifficult to obtain a proper spectrum of the VTD sample, once the mica spectrum is subtractedas the background. Glass substrates will be used as well as a diffuse reflectance spectrum ofthe samples using an integrated sphere attachment will be taken in the near future.

SEM imaging gave a clear idea of the types of nanostructures formed on the surfaces ofmica and HOPG. While previous studies employing VTD of cobalt(II) octaethylporphyrinreported mostly nanorod growth,3 we have fabricated primarily globular clusters with fewnanorods using a similar deposition method and CuOEtP as the metalloporphyrin. Perhapsthe different metal ions present in the porphyrin affect the shape of nanostructures formedon solid surfaces. Future experiments with varied range of deposition times of CuOEtP as wellas VTD of octaethylporphyrin substituted with different metal ions will demonstrate the rangeof nanostructures formed with metalloporphyrins.

Future WorkMore experiments are under way to better understand and to optimize the growth of VTD

of CuOEtP and other MOEtP compounds by varying the deposition time, temperature, and theN2 flow rate parameters. In particular, we wish to examine how varying these parametersaffects the micro and nanostructure of the MOEtP deposits. The goal is to fabricatereproducible nanowires of metalloporphyrins under controlled VTD conditions and to studytheir optical and electronic properties. Both thicker and thinner MOEtP samples will beprepared. The thinner (several nm) samples will allow for AFM and STM imaging. X-RayDiffraction (XRD) and Transmission Electron Microscopy (TEM) measurements of thenanostructures will be performed to examine the crystallinity of the VTD porphyrin samples.

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Figure 6: HV 1.00 kV SEM image of CuOEtPdeposited on HOPG for 7 minutes. 600x magnification.

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CuOEtP in KBr MatrixCuOEtP VTD deposited for 7 mins on KBrCuOEtP VTD deposited for 1 min on KBr

CuOEtP in Solution

CuOEtP VTD deposited for 7 mins on mica

CuOEtP VTD deposited for 1 min on mica