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
Introduction
Porphyrins are a group of organic aromatic macrocycles consisting of four pyrrole
units connected by methine bridges.1 A number of porphyrins and their derivatives
are found commonly in nature.2 The most common example is that porphyrins are
constituents in the pigment of red blood cells.
Porphyrins and their derivatives have great potential in the fields of molecular
electronics, catalysis, optoelectronics, and solar energy conversion due to their
optical and electronic properties.3
To better utilize nanoscale materials for various applications, the relationship
between a material’s molecular structure and its properties must be understood.
Porphyrins are an ideal group of molecules to study this relationship due to the
facile substitution of various functional groups, the chelation of a metal ion in the
center of the porphyrin compound (a.k.a metalloporphyrin), or the adjustment of the
axial ligation of the metalloporphyrins.2 In this study, the metalloporphyrin copper
(II) octaethylporphyrin (CuOEtP) nanostructures (Figure 1) were created using a vapor
transfer deposition method, and were examined using ultraviolet-visible (UV-Vis)
spectroscopy, Fourier transform infrared (FTIR) spectroscopy, attentuated total
reflectance (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-octaethyl21H,23H-porphine (CuOEtP).
Experimental
CuOEtP nanostructures were created using a vapor transfer deposition (VTD)
method, using a modified procedure reported earlier.3 A Marshall 1100 Series
furnace was used to carry out the fabrication, and two temperature zones were
created by electrically shorting out a section of the furnace. The porphyrin was
acquired from Frontier Scientific.
CuOEtP powder was placed in an aluminum oxide coated molybdenum boat, and
transferred into a quartz tube which was inserted into a one zone horizontal tube
furnace and placed at the high temperature zone (Figure 2). A KBr disk (2 cm
diameter, 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 the
substrates. The substrates were placed on a glass plate, inside the quartz tube at the
low temperature zone of the furnace. Tungsten wire was attached to the side of the
glass 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 to
carry the CuOEtP vapor to the low temperature zone, where it was deposited on the
substrates. 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 the
greatest sublimation and deposition, a temperature profile of the furnace was taken
along the length of the quartz tube.
Results
Figure 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 three
spectra look very similar indicating that no decomposition of the CuOEtP sample
occurred during the thermal deposition process. The intensities of the bands in the
2900 cm-1 region typically associated with CH2 and CH3 vibrations of the ethyl groups
on the macrocycle appear weaker in the spectrum of the VTD deposited samples than
that of the spectrum of CuOEtP in KBr. It is possible that a preferred orientation of
growth on the substrates of the CuOEtP nanostructures is responsible for the reduced
peak intensity of the ethyl substituents (Figure 3). The spectra of the CuOEtP starting
material and VTD deposited samples were collected using the Perkin-Elmer Limited
Spectrum 1700 FTIR spectrometer and Thermoscientific Nicolet iS10 ATR
spectrometer respectively.
UV-Vis spectrum of the CuOEtP of the 7 minute and 1 minute samples deposited
on mica indicate that mica’s spectrum is within the same regions as that of the
deposited CuOEtP samples to allow for much distinction in spectral acquisition (Figure
4) 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 at
530nm and 566nm. These bands are associated with -* electronic transitions. The
spectra were collected using the Perkin-Elmer 300 UV-visible spectrometer .
An FEI Quanta 200F field emission SEM was used to image CuOEtP VTD deposited
on the HOPG and mica substrates for 7 min (Figure 5-9). (These samples proved to be
too thick for AFM or STM imaging.) Figures 5 and 6 show similar dense granular
features on both the HOPG and mica substrates, indicating a no substrate
dependence on the CuOEtP nanostructure formation. HOPG proved to be a more
satisfactory substrate for imaging, because it has no surface charging. A high
resolution micrograph of CuOEtP deposited on HOPG, Figure 9, shows that the
dominant surface structures appear to be spherical in shape and a few secondary
features are nanorod-like. Taking initial approximate size measurements from this
image, the diameter of the spheres ranges from 2.9µm to 3.7µm. The rods are
approximately 100 nm wide and several microns long. A similar range of features with
similar dimensions are observed on the mica surface, Figure 7, but the image is less
clear. The average sphere diameters on mica are approximately 3.7µm, the same
value for the average calculated on CuOEtP spherical nanostructures that are present
the HOPG surface.
CuOEtP in KBr Matrix
CuOEtP VTD deposited for 7 mins on KBr
CuOEtP VTD deposited for 1 min on KBr
Figure 3: Transmission infrared
spectrum of CuOEtP starting
material was acquired in a KBr
matrix. ATR accessory was used
to measure the vibrational
spectrum of the thermally
deposited samples of CuOEtP on
KBr disk. Spectra were collected
with 4cm-1 resolution using a
TGS detector.
CuOEtP in Solution
CuOEtP VTD deposited for 7 mins on mica
CuOEtP VTD deposited for 1 min on mica
Figure 4: UV-Vis spectrum of
CuOEtP was obtained from a 1
X 10-5 M solution of the
porphyrin in chloroform.
Because mica’s spectrum is
within similar regions to that
of the thermally deposited
samples of CuOEtP, this does
not allow for a proper
spectrum to be obtained.
Appropriate references were
used in collecting both
spectra.
Figure 2: Schematic of the cross-sectional view of the furnace with the vapor transfer
deposition setup.
First, a continuous N2 gas flow was established for five minutes prior to
deposition experiments. N2 was present during heating and deposition, as well as
for an additional 30 minutes after heating to help cool the substrates to room
temperature. With just the sample in the quartz tube, the furnace was heated from
room temperature to 150°C at an average rate of 10 degmin-1, and was kept at that
temperature for one hour. This pre-sublimation was done to eliminate any low
molecular weight compounds from the CuOEtP sample. After 1 hour, the substrates
were 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 data
primarily for the 7 minute deposition period, as well as an spectra analysis for the 1
min deposition period.
200 µm
Figure 5: HV 2.00 kV SEM image of CuOEtP
deposited on mica for 7 minutes. 500x
magnification.
200 µm
Figure 6: HV 1.00 kV SEM image of CuOEtP
deposited on HOPG for 7 minutes. 600x
magnification.
20 µm
20 µm
Figure 7: HV 2.00kV SEM image of CuOEtP
deposited on mica for 7 minutes. 5000x
magnification.
20 µm
Figure 8: HV 1.00 kV SEM image of CuOEtP
deposited on HOPG for 7 minutes. 5000x
magnification.
Figure 9: HV 1.00 kV SEM image of CuOEtP
deposited on HOPG for 7 minutes. 10,000x
magnification.
200 µm
10 µm
Conclusions
200 µm
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 sample
occurred during the thermal deposition process. Reduced intensities of the ethyl groups
vibrational bands of the CuOEtP VTD nanostructures indicate the possibility of preferred
orientation 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 it
difficult to obtain a proper spectrum of the VTD sample, once the mica spectrum is subtracted
as the background. Glass substrates will be used as well as a diffuse reflectance spectrum of
the 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 of
10 µm
mica and HOPG. While previous studies employing VTD of cobalt(II) octaethylporphyrin
reported mostly nanorod growth,3 we have fabricated primarily globular clusters with few
nanorods using a similar deposition method and CuOEtP as the metalloporphyrin. Perhaps
the different metal ions present in the porphyrin affect the shape of nanostructures formed
on solid surfaces. Future experiments with varied range of deposition times of CuOEtP as well
as VTD of octaethylporphyrin substituted with different metal ions will demonstrate the range
of nanostructures formed with metalloporphyrins.
Future Work
More 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 the
N2 flow rate parameters. In particular, we wish to examine how varying these parameters
affects the micro and nanostructure of the MOEtP deposits. The goal is to fabricate
reproducible nanowires of metalloporphyrins under controlled VTD conditions and to study
their optical and electronic properties. Both thicker and thinner MOEtP samples will be
prepared. The thinner (several nm) samples will allow for AFM and STM imaging. X-Ray
Diffraction (XRD) and Transmission Electron Microscopy (TEM) measurements of the
nanostructures will be performed to examine the crystallinity of the VTD porphyrin samples.
References
1. Egharevba, G.O; George, R.C.; Maaza, M. Effect of Heat on the Morphology and Optical
Properties of Porphyrin Nanostructures. Synthesis and Reactivity in Inorganic, MetalOrganic 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 toward
Electric/Photoelectric Devices. J.Phys.Chem.C 2009, 113, 16259-16265.
Acknowledgements
This work was supported by the National Science Foundation’s REU program under grant
number DMR-0755055 and grants CHE-0555696 and 0848511. We also thank Fred Schuetze
for help in programming the furnace, Dave Savage for help in designing the holders for the
source and substrates, Professor K. W. Hipps and Benjamin Friesen for assisting with AFM
imaging experiments, and Steve Klase for help with the optical spectra analysis.