In Situ Observations of UV-Induced Restructuring of Self-Assembled

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In Situ Observations of UV-Induced Restructuring of Self-Assembled Porphyrin Monolayer on Liquid/Au(111) Interface at Molecular Level Yongman Kim, Won Hui Doh, Jeongjin Kim, and Jeong Young Park Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00418 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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In Situ Observations of UV-Induced Restructuring of Self-Assembled Porphyrin Monolayer on Liquid/Au(111) Interface at Molecular Level Yongman Kim,†,‡ Won Hui Doh,‡ Jeongjin Kim†,‡ and Jeong Young Park*,†,‡ † Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea ‡ Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon 34141, Republic of Korea KEYWORDS: Scanning Tunneling Microscopy, Metalloporphyrin, Au(111), UV-Induced restructuring

ABSTRACT

Porphyrin-derived molecules have received much attention for use in solar energy conversion devices, such as artificial leaves and dye-sensitized solar cells. Because of their technological importance, a molecular-level understanding of the mechanism for supramolecular structure formation in a liquid, as well as their stability under ultraviolet (UV) irradiation are important. Here, we observed the self-assembled structure of free-base, copper (II), and nickel (II) octacethylporphyrin formed on Au(111) in a dodecane solution using scanning tunneling microscopy (STM). As evident in the STM images, the self-assembled monolayers (SAM) of these three porphyrins on the Au(111) surface showed hexagonal close-packed structures when in dodecane solution. Under UV irradiation (λ = 365 nm), the porphyrin molecules in the SAM

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or the dodecane solution move extensively and form new porphyrin clusters on the Au sites that have a high degree of freedom. Consequently, the Au(111) surface was covered with disordered porphyrin clusters. However, we found that the porphyrin molecules decomposed under UV irradiation at 254 nm. Molecular-scale observation of the morphological evolution of the porphyrin SAM under UV irradiation can provide a fundamental understanding of the degradation processes of porphyrin-based energy conversion devices.

INTRODUCTION

In the photosynthetic processes in plants and bacteria, solar energy is first absorbed in porphyrin-based chromophores and the absorbed light energy is then converted to chemical energy. Much research on porphyrin-based photoelectronics (e.g., artificial leaves and dyesensitized solar cells) has been carried out in recent decades, motivated by this high-efficiency energy conversion process.1-6 Porphyrin is a macrocyclic compound consisting of a 16-atom ring that is composed of four tetrapyrrolic compounds and four methine bridges. The highly conjugated π orbitals of the porphyrin allow it to form very stable porphyrin films on a substrate and to effectively absorb light energy. Furthermore, the optical, physical, chemical, and photovoltaic properties are easily modified, depending on the central transition metal and the functional groups located in the β position.7-12 Thus, porphyrin is capable of having various derivatives and being used in many photoelectronic device applications. Self-assembly is one of the simplest ways to form defect-free and ideal porphyrin-based films in the liquid phase. The driving force of self-assembly are the differences in enthalpy and


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entropy between the initial and final states that result in free energy.13 In this process, molecules spontaneously form a non-covalent thin film by balancing four interactions: (i) intermolecular interactions, (ii) molecule–substrate interactions, (iii) molecule–solvent interactions, and (iv) solvent–substrate interactions under the sophisticated adsorption thermodynamics and kinetics of the SAM formation process.13-15 In the case of Au-thiol self-assembled monolayers (SAM), vacancy islands are formed by chemisorbed thiols on the Au substrate, which is a main defect in SAM.16-19 However, the porphyrin molecules are adsorbed on the substrate by van der Waals interactions (physisorption) to form a well-ordered monolayer with few defect sites. In previous research, scanning tunneling microscopy (STM) was used to characterize the surface adsorption structure of porphyrin SAM on a substrate at the molecular level under ultrahigh vacuum (UHV) conditions.12, 20-24 However, to understand the behavior of porphyrin in liquid media, which are used in many applications, it is desirable to analyze the porphyrin in the liquid phase. Information about the liquid/solid interface becomes gradually more and more important in modern technology, from conventional processes (e.g., spin casting, lubrication, and crystallization) to new fields that utilize liquid media (e.g., inkjet printing of organic electronics).25,26 Therefore, much research on porphyrin-based films on Au(111) and highly ordered pyrolytic graphite (HOPG) has been carried out in the liquid phase.27-32

Image

simulations are often coupled with high-resolution STM images to improve the accuracy of the adsorption geometry of the porphyrin monolayers33 or the atomic structure of the alkyl-chain connections of the porphyrin.34-36 At room temperature, the thermal energy is similar to the ordering and desorption energy of the porphyrin molecules, and STM measurements in the liquid phase are very difficult at the molecular level. Thus, many scientists have investigated temperature-dependent experiments of


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porphyrins in liquid.30, 37-39 For example, kinetics, thermodynamics, and reaction mechanisms have been considered at temperatures of 10–70 °C, which are common operating temperatures for solar cells.14, 40 For photoelectronic devices, however, durability under sunlight has not been fully considered, especially under ultraviolet (UV) irradiation. In this paper, we used STM to investigate the structures of self-assembled free-based, Cu(II), and Ni(II) octaethylporphyrin (H2OEP, CuOEP, and NiOEP, respectively) monolayers on Au(111) in liquid. Changes to the adsorption structure of the porphyrin SAM were also monitored with respect to UV radiation (UV-A: 365 nm) exposure time. When exposed to UV light, the porphyrin molecules quickly aggregate along the Au reconstruction lines. Since UV rays have enough energy (365 nm ≈ 3.4 eV) for the electrons to hop from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) (3.0 ~ 3.4 eV),41 this electron transfer process might have an effect on the SAM in liquid.

EXPERIMENTAL DETAILS

2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphine (H2OEP); 2,3,7,8,12,13,17,18-octaethyl-21, 23H-porphine copper (II) (CuOEP); 2,3,7,8,12,13,17,18-octaethyl-21H, 23H-porphine nickel (II) (NiOEP); and anhydrous dodecane (≥ 98%) were purchased from Aldrich. A fresh mica (Ted Pella Inc.) surface was obtained by cleaving the mica sheet using a platinum-coated razor blade (DORCO). Au was deposited on the freshly cleaved mica sheets on the same day to minimize contamination, such as moisture adsorption or surface oxidation. Au deposition on the mica surface was carried out using an e-beam evaporator that kept the pressure of the chamber at 7 x 10−7 Torr during the gold deposition step. The Au on mica samples were then tailored using


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titanium nitrile coated scissors into 1 × 1 cm2 squares to fit into the STM sample holder. Prior to use as a substrate for the SAM, the Au on mica samples were flame annealed using Clavilier's method to make the Au(111) surface.42 The annealed Au surface has a typical Au(111) reconstruction structure with a monatomic step height, as confirmed by STM (Figure S1). All the images were obtained using an RHK-STM (ATM300) on a pneumatic vibration isolation table with control electronics (R9) in the constant-current mode at 25 °C and 1 atmosphere.43 To calibrate the STM measurements, x, y, and z directions were respectively corrected with reference to HOPG. All the images were obtained with different scan rates to confirm the effects of thermal drift. All the lattice parameters and internal angles of the materials are indicated in the image captions. The error bars in all captions were calculated using the standard deviation of the parameters and internal angles, which are extracted from multiple images and various fitting points in the image using R9s analysis software supplied by RHK Technology. Owing to the different measurement environments, we tested HOPG in air and in solution to improve the accuracy of the calibrations (see Supporting Information Figure S2). Since the measured lattice constants in solution were slightly smaller than in air, all images taken in solution were calibrated following the above factors. Although the decrease in the unit parameter remains as an unknown issue in this paper, thermal contraction of the piezo or a difference in the tip length may have been a factor. The STM tips were prepared by mechanically cutting an annealed Pt0.8Ir0.2 wire purchased from Goodfellow. Depending on the sample conditions, a bias voltage of 0.5–1.0 V was applied. For the STM measurements of the SAM structure at the gas/solid interface, H2OEP, CuOEP, and NiOEP solutions were separately drop casted on different Au(111)/mica substrates. The three porphyrin solutions were prepared by dissolving each solution separately in chloroform to


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make a 1.0 × 10−5 M solution.30 25 µL of each solution was then drop casted on the Au(111)/mica inside a petri dish. After enclosing with parafilm, the sample was placed in a fume hood to dry for several hours to minimize contamination from air. To image the SAM structure on the liquid/solid interface, 3.3 × 10−5 M solutions of the H2OEP, CuOEP, and NiOEP were prepared by dissolving each separately in dodecane.37 About 0.3 mL of a given solution was then dropped onto the Au substrate mounted in the liquid sample holder. A UV flashlight (3W/365 nm, Rayman) was used for continuous irradiation on the porphyric SAM on Au(111)/mica samples during the time series of STM measurements. In addition, UV-C (4W/254 nm) and UV-A (4W/352 nm) lamps (Sankyo Denki), which have greater energy than the gap between the HOMO and LUMO of the porphyrins, were used to examine the effect of UV irradiation on the oxidation state of the central metal ion and the molecular structures of the porphyrin. These properties were measured using ex situ experiments (e.g., Fourier transform infrared (FTIR), ultraviolet-visible (UV-Vis) spectroscopy).

RESULTS AND DISCUSSION

Porphyrin molecules have a coplanar macrocyclic structure that consists of four alternately connected pairs of pyrrole rings and methine bridges. In the case of octaethylporphyrin (OEP), eight hydrogens in β-pyrrolic positions are substituted with eight ethyl groups. The molecular structure of the free-base OEP and metalloporphryin are depicted in Figure 1. The smallest separation from the substrate appears when all the ethyl groups are oriented upward in a crown shape. The theoretical average molecular size was reported to be 0.5 nm high and 1.5 nm wide.24, 41, 44


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In the liquid phase, however, it is extremely difficult to probe the ethyl groups using STM measurements, even for UHV-based STM, owing to scan instability. In addition, since STM observes the geometric structures as well as the electronic density of states of the materials, the apparent height of the porphyrin molecules is generally reported as 0.3 nm in liquid media.24 The measured molecular sizes of the porphyrin in previous reports are very different and depend on which solvent was used.32, 45 To examine the effects of the solvent on the self-assembled H2OEP molecules on the Au(111)/mica substrate, H2OEP SAM were prepared using two different methods: First, prior to STM measurements, 25 µL of the 1.0 × 10−5 M solution of H2OEP in chloroform was drop casted on the Au(111)/mica substrate, followed by drying in air. After the solvent (chloroform) was fully evaporated, STM topography of the H2OEP was observed in air. Similar to that on HOPG,32 quasi-orthogonal SAM of H2OEP molecules formed on the Au(111)/mica substrate are shown in Figure 2a,c,e. The possible unit cell parameters are depicted in Figure 2e. The lattice constants are a = 1.32 ± 0.08 nm and b = 1.49 ± 0.12 nm, and the internal angle is θ = 99.2 ± 3.9°. In the absence of a transition metal at the center of the H2OEP, a relatively low tunneling current is measured by the STM, which appears as the dark color in the center of the H2OEP molecules (Figure 2c). Secondly, STM measurements of the H2OEP SAM on the Au(111)/mica substrate in the 3.3 × 10−5 M solution of H2OEP were performed. Dodecane was used as the solvent because of its physical properties; dodecane is a non-polar molecule, thus it is relatively stable in an electric field. Thus, STM tip insulation is not required to avoid leakage current that flows through unwanted conductive paths.46 In addition, owing to its low vapor pressure at room temperature, there are fewer solvent evaporation issues and the accompanying changes in concentration. Even


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after the H2OEP SAM was formed on the Au(111)/mica surface, the distance between the reconstructed Au line pairs was maintained at about 6 nm in the [11ത0] direction (refer to Figure S1). These results indicate that the H2OEP SAM is well-formed along the Au reconstruction lines. STM topography of the H2OEP on Au(111)/mica substrate in the liquid phase shows a wellordered SAM structure (Figure 2b,d,f). Instead of showing orthogonal self-assembly in the liquid, the quasi-hexagonal lattice arrangements are observed with a = 1.42 ± 0.04 nm, b = 1.32 ± 0.01 nm, and the angle between a and b was 66.1 ± 5.2°, which is similar to that on HOPG in the dodecane solution.32 The self-assembled molecular structures in air and in dodecane solution are very different. This result might be attributed to not only the absence of a center metal, but also to the linear alkane structure of dodecane. Previous research indicated that the different self-assembly geometries resulted from the lack of a center metal and from the effect of the solvent on the sample– substrate interaction.32, 45 In spite of the invisible dodecane arrangement when using STM, which is based on many studies on self-assembly of an n-alkane monolayer on Au(111), it is easily inferred that dodecane can coabsorb with porphyrin molecules on the Au surface.47-49 Thus, dodecane might influence the total interaction between the porphyrin molecules and the Au(111) surface and produce different molecular structures, like HOPG.32 The similar line profiles in Figure 2g,h (from Figure 2c,d, respectively) were obtained that represent the molecular corrugation of the H2OEP monolayer as 0.05 nm. Since the Au substrate was entirely covered with H2OEP molecules within the maximum scanning area (6 µm x 6 µm), it was hard to acquire STM images that simultaneously have both the H2OEP layer and the Au terrace. Therefore, it is not possible to assign the molecular height of the H2OEP from the bare


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Au(111)/mica surface. It should be noticed that the lattice constants of the self-assembled H2OEP monolayer on Au(111)/mica surface are very similar in both the gas and liquid phases. The self-assembled NiOEP and CuOEP monolayers on the Au(111)/mica substrate in the 3.3 × 10−5 M porphyrin solutions were probed using STM (Figure 3a,b). In both images, the porphyrin SAM show well-ordered hexagonal adsorption structures on a wide range of Au(111)/mica surfaces. For the NiOEP SAM, the intermolecular distances are a = 1.38 ± 0.08 nm, b = 1.39 ± 011 nm, and the internal angle θ is 65.3 ± 9.4° (Figure 3c,e), which are similar to previous results.24 In the case of the CuOEP, the lattice constants are a = 1.31 ± 0.07 nm and b = 1.38 ± 0.08 nm with θ = 66.2 ± 3.6° (Figure3d,f), which are also similar to other studies.32 Even after the formation of SAM, the Au(111) still had wide terraces and herringbone reconstruction, as is clearly shown in Figure 3a,b. Furthermore, the line profile also confirms the Au monatomic step height (≈ 0.25 nm). According to the four-orbital model theory, porphyrin has four orbitals consisting of the highest occupied π orbitals and the lowest unoccupied π* orbitals.7, 50 Transitions involving the HOMO, SHOMO, LUMO, and SLUMO orbitals contribute to the adsorption bands of the porphyrin. This orbital mixing generates two different energy states: Q-bands and Soret bands. The Soret bands are correlated with transitions from the ground state to the second excited state and these bands usually absorb light in the range of 380–500 nm. The Q-bands appear in the 500–750 nm range and are related to weak transitions to the first excited state.8 Thus, the porphyrin chromophore is capable of strongly absorbing light energy in the Soret and Q-bands. If the light energy is high enough, electrons in the HOMOs could excite to the LUMOs or even higher states, which leads to decomposition of the porphyrin molecules or other actions. UV irradiation could thus affect the durability of porphyrin-based solar cells.


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STM images were obtained after UV radiation exposure at 365 nm for 5 min on the CuOEP and H2OEP SAM (Figure 4). The hexagonal molecular arrangements of both SAM (i.e., CuOEP and H2OEP) remained in a large area, however, randomly dispersed agglomeration or cluster formation were also observed. As shown in the line profiles in Figure 4, the extrusion sizes are about 3 ~ 5 nm wide and the corrugation height was about 0.07 ~ 0.17 nm. Aggregation can occur via the migration of porphyrin molecules in the SAM or in the dodecane solution or by the precipitation of porphyrin fragments decomposed by UV irradiation. To confirm the origin of the porphyrin aggregation, UV light at 254 and 352 nm was used (see Supporting Information Figure S3). Although this analysis was not quantitative and is different from the STM measurements, we successfully monitored the UV-induced degradation in the solution phase depending on UV wavelength. Since the porphyrin has intense absorption bands, it generally appears as a specific color that is dependent on the number of pyrrole units.7,8, 51 To verify whether the UV-induced phenomena are general or not, we tested cobalt (II) octacethylporphyrin (CoOEP), which has a relatively higher 3d orbital energy than NiOEP, CuOEP and H2OEP.52 The color changed from purple to colorless and transparent following several minutes of UV irradiation (at 254 nm). Additionally, the characteristic UV-Vis bands (e.g., Q-band and Soret-band) disappeared entirely from the UV-Vis spectrum (Figure S3a). However, UV irradiation at 352 nm did not have an effect on the color nor on the overall UV-Vis spectrum (Figure S3b), except for a slightly decreased peak intensity. The FTIR spectra after UV irradiation at 254 nm showed that the UV light decomposed the conjugated system of porphyrin molecules (Figure S4). Therefore, the migration of porphyrin molecules in the SAM or dodecane solution at wavelengths above 352 nm could be a possible aggregation mechanism based on this analysis. It is also more likely that the aggregation seen in Figure 4 is attributed to the migration


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of porphyrin molecules rather than the precipitation of decomposed fragments created by UV irradiation. Successive STM measurements were carried out to confirm the changes in the physical properties of the porphyrin molecules, such as molecular expansion, following UV exposure. As shown in Figure S5, however, the structure and unit parameters of the H2OEP SAM were similar in all cases. The effect of the bias voltage during the STM measurements was also examined as a function of time while the bias voltage was kept at 1.0 V. However, no movement of porphyrin molecules was observed even after several STM measurements (Figure S6), which indicates that the bias voltage and the STM tip do not affect porphyrin molecule migration in the SAM or dodecane solution. Another factor for aggregation is a difference in concentration that originates from the precipitation of porphyrin molecules near the surface. However, Hipps et al.53 reported that a small change in the solution concentration might not be sufficient to change the surface concentration. They explained that once an organic monolayer is formed on the surface, the desorption rate from the cluster is very slow because of its low potential energy. Therefore, the aggregation of porphyrin molecules could be the result of an energy redistribution process of porphyrin molecules excited by UV irradiation. A series of STM images was obtained for CuOEP and H2OEP SAM on the Au(111)/mica substrate in a 3.3 × 10−5 M solution of each type of porphyrin to examine the effect of UV irradiation time. For this, the samples were continuously exposed to UV radiation at 365 nm. Changes in the features on the SAM surface are depicted in Figure 5 as a function of UV radiation time. In both cases, hexagonal wellordered molecular arrangements were observed when UV exposure began (at t = t0). After that, the porphyrin molecules migrated to aggregate along the gold reconstruction lines. The black arrows indicate newly formed agglomerations or clusters of porphyrin molecules. Increasingly


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disordered aggregations of the porphyrin molecules were observed over time. The height of the porphyrin molecule aggregates is 0.8-1.2 Å (see Fig. S7), which that is lower than the height of a monolayer of porphyrin molecules (~2.5 Å) and indicates that vertical molecule stacking is not the case here. According to the potential energy graph,53 the reconstructed lines consisting of perturbed Au atoms can act like defect sites. Owing to their high degree of freedom, these line pairs can result in having a relatively low activation barrier to make clusters of porphyrin molecules. Thus, these could have a stable potential energy.53,54 This corresponds well with our results that the molecular aggregation first occurs along the gold reconstruction lines.

CONCLUSION

In conclusion, we have demonstrated UV-induced restructuring of self-assembled monolayers (SAM) of porphyrin compounds that are related to the durability of porphyrin-based solar cells. SAM of free-base octaethlyporphyrin and metalloporphyrin were fabricated on Au(111)/mica substrates using a dodecane-mediated porphyrin solution. STM measurements in the liquid media measured the adsorbed structure of the porphyrins and structural changes when under UV irradiation. All the porphyrin compounds adsorbed on the Au substrate to form a hexagonal wellordered adsorption structure, except for H2OEP in air. Under UV irradiation at 254 nm, the porphyrin molecules decomposed, as is evident in the UV-Vis and FTIR results. However, UV irradiation at 365 nm does not actively promote the decomposition of molecules, but rather promotes porphyrin molecule migration in the SAM or dodecane solution. The porphyrin molecules started to migrate and aggregate along the gold reconstruction lines. Eventually, random aggregate molecular structures are observed on the surface. This and other current studies of the molecular-scale degradation processes on porphyrin SAM can lead to a


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fundamental understanding of the durability of porphyrin-based energy conversion devices, including for solar cell applications.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.#######. Preparation and confirmation of Au(111), UV-ozone treatment results, infrared spectra and UV-Vis spectra after treatment, and STM images in dodecane solution (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Institute for Basic Science (IBS) [IBS-R004]. REFERENCES 1. Campbell, W. M.; Burrell, A. K.; Officer, D. L.; Jolley, K. W., Porphyrins as light harvesters in the dye-sensitised TiO2 solar cell. Coord. Chem. Rev. 2004, 248 (13), 1363-1379. 2. Campbell, W. M.; Jolley, K. W.; Wagner, P.; Wagner, K.; Walsh, P. J.; Gordon, K. C.; Schmidt-Mende, L.; Nazeeruddin, M. K.; Wang, Q.; Grätzel, M., Highly efficient porphyrin sensitizers for dye-sensitized solar cells. J. Phys. Chem. C 2007, 111 (32), 11760-11762. 3. Walter, M. G.; Rudine, A. B.; Wamser, C. C., Porphyrins and phthalocyanines in solar photovoltaic cells. J. Porphyrins Phthalocyanines 2010, 14 (09), 759-792.


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Figure 1. Molecular structure of free-base octaehtylporphryin (H2OEP) and metalloporphryin (M(II)OEP). Blue, nitrogen; gray, carbon; white, hydrogen; and red, metal (M).


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Figure 2. STM images of self-assembled H2OEP molecules on the Au(111)/mica substrate in (a), (c), (e) air and (b), (d), (f) liquid dodecane solution. (Vs = + 0.7 V, It = 300 pA). The line profiles in (g) and (h) are obtained from the STM images in (c) and (d), respectively. The molecular architectures are different in air and dodecane.


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Figure 3. STM images of self-assembled (a), (c), (e) NiOEP and (b), (d), (f) CuOEP on the Au(111)/mica substrate in liquid dodecane solution. (Vs = + 0.7 V, It = 300 pA). The line profiles in (g) and (h) are obtained from the dotted lines in the STM images in (a) and (b), respectively.


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Figure 4. STM images of self-assembled CuOEP (left) and H2OEP (right) on the Au(111)/mica substrate after UV irradiation at 365 nm for 5 min in liquid dodecane solution. (Vs = + 0.7 V, It = 300 pA). The line profiles are obtained from the corresponding STM images. In the early stages of UV radiation exposure, agglomeration of the porphyrin molecules was observed along the Au reconstruction lines.


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Figure 5. Series of STM images of self-assembled CuOEP (left) and H2OEP (right) on Au(111)/mica substrate as a function of UV irradiation time (minutes) at 365 nm in liquid dodecane solution. (Vs = + 0.6 V, It = 250 pA).

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Graphical abstract

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In Situ Observations of UV-Induced Restructuring of Self-Assembled

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