Layer-by-Layer Growth and Photocurrent Generation in MetalâOrganic

Article pubs.acs.org/JPCC

Layer-by-Layer Growth and Photocurrent Generation in Metal− Organic Coordination Films Jennifer T. Joyce,†,‡ Fathima R. Laffir,‡ and Christophe Silien*,†,‡ †

Department of Physics and Energy, University of Limerick, Limerick, Ireland Materials and Surface Science Institute, University of Limerick, Limerick, Ireland

‡

S Supporting Information *

ABSTRACT: A series of metal−organic coordination films with tetra(4-carboxyphenyl)porphyrin (TCPP) and trimesic acid (TMA) ligands and Cu2+ ion linkers were grown layer-bylayer on TiO2-modified indium tin oxide electrodes and the cathodic short-circuit photocurrent that they generate was studied. The layer-by-layer growth was verified by using absorption spectroscopy in the visible and X-ray photoelectron spectroscopy, highlighting the consistent coverage of TCPP in each layer. The photocurrent increases nonlinearly with the number of TCPP layers and with the number of TMA layers introduced as a buffer between TCPP and substrate. These observations are rationalized by considering substrate screening of the TCPP photoexcited state and charge transport across the metal−organic multilayers.

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INTRODUCTION Metal−organic frameworks (MOFs) are robust hybrid materials made of organic ligands and metal ion linkers that can exhibit a porosity similar to that of zeolites.1 With the limitless combinations of ligands and metal ions affording an unprecedented tunability in pore size and shape, crystalline MOFs have found applications in the storage of gaseous molecules,2 in the adsorption and separation of gases and vapors,3−5 in catalysis,6,7 as molecular sieves,8 in chemical sensing,9 in biomedical imaging and drug delivery,10,11 in nonlinear optics,12−16 and as host for metal nanoparticles.17,18 Porphyrins and in particular tetra(4-carboxyphenyl)porphyrin (TCPP), a prototypical chromophore in photovoltaic applications, have also been used as a building block in MOFs19−25 and highly ordered MOFs of TCPP and Cu2+ were grown layer by layer by using a Langmuir−Blodgett scheme.26 Frameworks of trimesic acid (TMA) with Cu2+ have been well documented from three-dimensional MOF growth by solvothermal synthesis,27 MOFs deposited on self-assembled monolayers (SAM), 28−30 and MOFs formed layer by layer.31−34 In the last case, the MOF components were deposited onto a SAM alternately, from separate solutions, adopting thus a surface functionalization approach,35 which affords in principle smooth and homogeneous films.36 The generation of cathodic and anodic photocurrent by electrodes modified with porphyrin derivatives and other chromophores37−54 and the influence of the film structure (thickness, molecular orientation, etc.)39,55,56 have been widely studied. The quenching of the photoexcited porphyrin by the electrode has been repeatedly highlighted40,42,43,47,57 and the © XXXX American Chemical Society

effect of introducing spacers has been documented for different spacer chain length58−63 and thickness,64,65 or for multilayered spacers.66 In many cases, the spacers were attached to the photoactive moiety in solution before adsorption onto the substrate.43,63,67,68 In this paper, we study the liquid phase layer-by-layer growth of TCPP metal−organic coordination multilayers, with and without TMA buffer layers, on indium tin oxide (ITO) electrodes modified with a thin film of titanium dioxide (TiO2), and a systematic study of the suitability of the layered films for photocurrent generation in an electrolyte is presented. Absorption spectroscopy in the visible and X-ray photoelectron spectroscopy (XPS) were used to demonstrate the consistent layer-by-layer growth of the TCPP films on TiO2/ITO with copper acetate as linker. A homemade two-electrode cell was used to probe the photocurrent generated by TCPP films of various thicknesses, and separated from the substrate by a thin TMA buffer (see Figure 1), and the measurements are used to evaluate the relative contributions between the various relaxation pathways of the TCPP photoexcited state for different layered film composition, so that the generation of a cathodic current can be maximized.

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EXPERIMENTAL SECTION Tetrakis(4-carboxyphenyl)porphyrin (TCPP, >97.0% TCI EUROPE), 1,3,5-benzenetricarboxylic acid (trimesic acid, Received: December 24, 2012 Revised: May 26, 2013

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dx.doi.org/10.1021/jp312708e | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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Figure 1. Illustrations of the metal−organic coordination multilayers grown for this study: (a) (TCPP/Cu2+)n/TCPP/TiO2/ITO with n = 0, 1, 2, and 3, corresponding to films of 1, 2, 3, and 4 TCPP layers. (b) TCPP/TiO2/ITO and (TCPP/Cu2+)/(TMA/Cu2+)n/TMA/TiO2/ITO with n = 0, 1, and 2, corresponding to buffers of 0, 1, 2, and 3 TMA layers.

(1 mM TMA in DI water for 12 min), ethanol (DI water) rinsing, and N2 drying. XPS analysis was performed at fixed pass energy of 20 eV with an AXIS 165 (KRATOS) with use of a monochromatized Al X-ray source (1486 eV) and a surface-sensitive takeoff angle of 30° with respect to the sample surface. Binding energies were set by adjusting the C 1s peak at 284.8 eV or at 284.5 eV when TCPP was used.70 Mixed Gaussian−Lorenzian functions and Shirley-type backgrounds were used for reconstruction and fitting of the high-resolution spectra. Absorption spectra in the visible were measured by using a CARY 300 in transmission through the modified TiO2/ITO electrodes. The data are presented after subtraction of a background spectrum measured for a freshly prepared but otherwise unmodified TiO2/ITO substrate. Photocurrents were measured by using a homemade twoelectrode cell (see the Supporting Information, Figure S1) consisting of a Viton O-ring (estimated sample area: 0.2 cm2) clamped between a modified TiO2/ITO electrode and a clean microscope glass slide (1.0−1.2 mm thick). The O-ring was pierced with a 0.8 mm (outer diameter) needle to insert a freshly annealed Pt wire (diameter 0.25 mm, >99.99%) and two Teflon catheters (22g × 32 mm, VENISYSTEMS ABBOCATH-T) used as tubing (bottom and top of cell) to fill/empty the cell (50 mM MV2+ in 0.1 M NaClO4 aqueous) with use of a syringe (1 mL, cell volume ∼0.03 mL). The cell is typically overfilled and it is ensured that no air bubble is trapped. The sample was irradiated through the ITO with the light emitted by an off-the-shelf halogen incandescent bulb filtered by using band-pass filters (±40 nm) centered at 400, 450, 500, 550, and 600 nm (THORLABS). The spectral power density of the light source, at the sample position, with and without band-pass filter, is presented in the Supporting Information (Figure S2). The current between the modified TiO2/ITO electrode and the Pt wire was measured with an operational amplifier acting as

TMA, 95% SIGMA-Aldrich), methyl viologen dichloride hydrate (MV2+, 96% SIGMA-Aldrich), copper(II) acetate (Cu2+, 99.999% ALFA AESAR), ethanol reagent (99.5%, SIGMA-Aldrich), titanium(IV) butoxide (TiBuO, 97% SIGMA-Aldrich), ethanolamine (>99%, SIGMA-Aldrich), propanol-2 (PROLABO, VWR), and acetone (EMPLURA, MERCK MILLIPORE) were used as received. The ITO-coated glass substrates (indium-tin-oxide, ITO, 12−15 Ω·cm) were purchased from DATASIGHTS Ltd. Prior to growing the TiO2 films, the ITO substrates were cleaned by 10 min ultrasonication at 80 °C in 20 wt % ethanolamine in deionized water (DI, 18 MΩ·cm) and by 5 min ultrasonication in acetone and 5 min ultrasonication in isopropanol at room temperature. The ITO substrates were finally rinsed with ethanol and DI water, and dried in N2 gas. Functionalization of the electrode with hydroxyl moieties was realized by deposition of a TiO2 (see the section below for a justification of the composition stoichiometry) thin film following the sol−gel procedure described in ref 69. Briefly, the ITO electrodes were immersed in 0.1 M Ti(OBu)4 in toluene:ethanol (1:1 v/v) for 3 min and subsequently rinsed with ethanol and dried with N2 gas. Hydrolysis was achieved by immersion in DI water for 2 min. The TiO2/ITO samples were finally dried with N2 gas. The first molecular layer of TCPP (TMA) was attached to freshly prepared TiO2/ITO electrodes by immersion in 1 mM TCPP in ethanol for 30 min (1 mM TMA in DI water for 12 min) followed by rinsing with ethanol (DI water) and drying with N2 gas. The subsequent deposition cycles of TCPP/Cu2+ (TMA/ Cu2+)33 were achieved by first functionalizing the surface with Cu2+ by immersion in 1 mM Cu(OAc)2 in ethanol for 30 min, rinsing with ethanol, and N2 gas drying, and by adding the TCPP (TMA) by immersion in 1 mM TCPP in ethanol for 3 s B



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Figure 2. XPS analysis of the core levels In 3d (a), Ti 2p (b), Cu 2p3/2 (c), and N 1s (d). The samples are ITO (i), TiO2/ITO (ii), TCPP/TiO2/ ITO (iii), Cu2+/TCPP/TiO2/ITO (iv), and TCPP/Cu2+/TCPP/TiO2/ITO (v).

current−voltage converter.71 The Pt wire and electrode were otherwise not biased, and to ensure proper signal-to-noise ratio the light source was chopped at ca. 675 Hz and the corresponding ac component of the current was detected with a lockin amplifier (E&G PRINCETON APPLIED RESEARCH 5207). It has been verified that the chopping frequency did not alter the signal magnitude and the frequency was thus chosen for maximizing the signal-to-noise ratio. Time traces of the ac photocurrent were recorded by using an analogdigital converter (NATIONAL INSTRUMENT, NI USB6212) controlled with LABVIEW and a shutter was introduced between the light source and sample to control the irradiation of the sample. All the photocurrent values reported below were normalized to the bandpass-filtered incident power on the sample (see the Supporting Information, Figure S2). The reported values were typically averaged over three samples and error bars are used to reveal the statistical distribution of photocurrent for the individual samples.

substrate before and after the sol−gel procedure. According to the literature, the Ti2p3/2 core level peaks at 453.8, 455.3, 457.1, and 458.6 eV for Ti(0), Ti(II), Ti(III), and Ti(IV), respectively.73 The Ti 2p3/2 level is measured here at 458.4 eV (Ti 2p1/2 at 464.66 eV), which matches with the binding energy for the oxidation state Ti4+, suggesting that the composition stoichiometry is dominantly TiO2.74 The Cu 2p3/2 levels are shown in Figure 2c. Two components are seen at 932.7 and 934.6 eV for both Cu2+/ TCPP and TCPP/Cu2+/TCPP on TiO2/ITO. The component at 934.6 eV and the satellite peaks seen between ca. 938 and 946 eV are readily associated with the Cu2+ at the interface, bonded to the COOH moieties and possibly (see below) into the TCPP cores.75−77 Earlier works have shown the sensitivity of Cu2+ to X-ray irradiation within typical XPS experimental conditions and the low binding energy (BE) component at 932.7 eV which marks Cu(I) species is thus understood accordingly as an X-ray induced artifact.78 Noteworthy, the relatively larger magnitude of the low BE peak for TCPP/Cu2+/ TCPP can be identified with a doubled irradiation time for that sample. Consequently, although the observation of Cu 2p core levels demonstrates clearly the addition of the Cu ions at the interface, the levels cannot be used for a thorough discussion of the chemistry involved. XPS spectra of the N 1s core levels are shown in Figure 2d for TCPP, Cu2+/TCPP, and TCPP/Cu2+/TCPP on TiO2/ ITO. After immersion of the TiO2/ITO substrate in the TCPP solution, two N 1s components are observed at 399.6 and 397.5 eV. These are characteristic of free-base TCPP,79 and mark respectively the presence of protonated and nonprotonated pyrrole moieties in the porphine molecular ring. The intensity of the component at larger BE is ca. 1.7 times larger and, in line with earlier observations, the asymmetry in intensity between the two components is believed to result from an electron withdrawing from the meso-substituents (i.e., here, the carboxylic acid moieties).79,80 After immersion in the Cu2+ bath, a new component at BE 397.9 eV is detected. The peak marks the partial incorporation of Cu2+ in the TCPP core.79

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RESULTS Layer-by-Layer Metal−Organic Film Growth on ITO. XPS spectra of In 3d, Ti 2p, Cu 2p, and N 1s at various stages of substrate preparation and TCPP layer-by-layer growth are presented in Figure 2. The In 3d5/2 (444.05 eV) and In 3d3/2 (451.5 eV) core levels72 are shown in Figure 2a for a freshly cleaned ITO substrate before and after sol−gel deposition of TiO2, as well as after successive deposition of TCPP, Cu2+, and TCPP (respectively marked as step i, ii, iii, iv, and v). The intensity of the In 3d peaks decreases after each step of modification demonstrating the addition of materials above the ITO substrate. Although the effective mean free path of the photoelectrons in each layer is difficult to assess, and quantification of their respective thickness and density thus not feasible, the persistent observation of the In 3d levels in Figure 2a suggests that, after each deposition step, only very thin or even monolayer-like layers of materials are added. The formation of a thin titanium oxide layer is demonstrated in Figure 2b, where the Ti 2p core levels are shown for the ITO C



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For TCPP/Cu2+/TCPP, the N 1s integrated intensity is increased and the three components at 399, 398, and 397 eV are all visible, suggesting that a bilayer of TCPP is formed as expected, with a mixture of free-base and Cu-base TCPP. Absorption spectra of TCPP multilayers are presented in Figure 3. The spectra were recorded for a single layer of TCPP

expected to be the same. The evolution of the Soret band integrated area with three subsequent TCPP/Cu2+ deposition cycles is plotted in the inset. The data fit with a line passing through the origin of the plot, confirming that each cycle induces the deposition of a monolayer of density equivalent to that of the first TCPP layer. Scanning electron microscopy was used to assess the evolution of the sample roughness and presented in the Supporting Information (Figure S3). SEM revealed no change in the overall sample corrugation for ITO, TiO2/ITO, (TCPP/ Cu2+)3/TCPP/TiO2/ITO, and TCPP/(Cu2+/TMA)2/TiO2/ ITO samples, with the surface exhibiting grains of roughly 20−60 nm in all cases. Moreover, diffraction experiments did not reveal any features that could be assigned to ordered TCPP MOF, and it is thus deduced that the films do not exhibit longrange order. However, the XPS and visible absorption data demonstrate and validate the layer-by-layer growth scheme for TCPP/Cu2+ metal−organic film on TiO2/ITO, and the shortcircuit photocurrent generation is now addressed. Short-Circuit ac Photocurrent. Measurements of the short-circuit photocurrent were carried out in a 0.1 M NaClO4 aqueous solution containing 50 mM of the electron acceptor methyl viologen (MV2+). For all samples containing TCPP, a photocurrent was readily detected and measured after removing the shutter placed between the light source and the sample. Figure 4a shows the photocurrent with a 400 nm band-pass filter for a sample prepared with 3 TCPP layers. The current is obviously correlated with the removal of the shutter at 120 s and its reintroduction at 360 s. Also shown is the photocurrent measured for an unmodified TiO2−ITO electrode, which shows no variation when the shutter is removed. Although it is currently unclear why, a neat difference in noise is observed in the data in the absence of TCPP. The variation of photocurrent for the TCPP modified electrode with incident wavelength is

Figure 3. Absorption spectra of TCPP/TiO2/ITO before and after 1, 2, and 3 TCPP/Cu2+ deposition cycles (i.e., for 1, 2, 3, and 4 TCPP layers). The integrated area of the Soret band (ca. 420 nm) as a function of the number of TCPP layer is shown in the inset.

self-assembled on the TiO2/ITO substrate and for the next three modification cycles with TCPP/Cu2+. The absorption spectrum of the bare TiO2/ITO was subtracted from all the data. All the spectra exhibit a strong absorption band at ca. 420 nm and weaker ones between 500 and 700 nm. These are readily assigned to the Soret and Q bands of porphyrin derivatives.81 The absorbance peaks at 0.04 in the case of a single layer of TCPP, which matches perfectly with data reported in the literature for porphyrin-derivative monolayers.38,55,56 The molecular arrangement and density are thus

Figure 4. (a) Photocurrent measured for (TCPP/Cu2+)2/TCPP/TiO2/ITO with the light source at 400 nm and with the shutter closed until 120 s, open from 120 to 360 s, and closed from 360 s. (b) Photocurrent for TiO2/ITO with 4 layers of TCPP for different visible bands of wavelength. (c) ac photocurrent at 400 nm for TiO2/ITO after depositing a layer of TCPP and 1, 2, and 3 additional TCPP/Cu2+ deposition cycles. The differential increase in current consecutive to each TCPP/Cu2+ deposition cycle is shown in the inset. (d) Photocurrent at 400 nm for TiO2/ITO after deposition of 0, 1, 2, and 3 TMA layers with one TCPP layer on top. For all data, the electrolyte solution is 0.1 M NaClO4 aqueous with 50 mM MV2+. D



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illustrated in Figure 4b for band-pass filters centered at 400, 450, 500, 550, and 600 nm. The increase in photocurrent seen at shorter wavelengths fairly follows the visible absorption spectra of TCPP peaking at ca. 420 nm (see Figure 3), even though the band-pass filter width of 80 nm and the spectrally uneven power distribution do not allow for a high spectral resolution (see Figure S2). These lead for example to the generation of a normalized photocurrent larger at 400 nm than at 450 nm since for each filter the spectral fraction of irradiation power at the sample dominates at longer wavelength, meaning a poorer match of the ca. 420 nm TCPP absorption peak with the 450 nm filter (see the Supporting Information, Figure S4). The spectral evolution agrees with earlier investigations39,40,58,81−83 and strongly suggests the involvement of TCPP in the photocurrent generation. The measurements were found to be highly repeatable for samples subject to the same number of TCPP/Cu2+ deposition cycles, as seen from the error bars magnitude. The photocurrents measured at 400 nm for films made of 1, 2, 3, and 4 TCPP layers are plotted in Figure 4c. The increase in photocurrent is clearly not linear with the number of TCPP layers. The nonlinearity is better assessed by observing that the differential increase in current following each TCPP deposition cycle is not constant (see the inset in Figure 4c); the current increase is significantly larger after deposition of the third TCPP layer and, although still large, is seen to be less for the fourth layer. As demonstrated by the visible absorption spectra in Figure 3 the TCPP layers are, however, similar in molecular density. Thus a discussion of the role of the screening of the photoexcited state of TCPP by the TiO2/ITO substrate and the probability of charge transfer to the solution must be made. Prior to that it is interesting to observe the effect of introducing molecular layers of the nonphotoactive TMA between the TiO2/ITO and a TCPP layer. Because of the carboxylic acid moieties, a monolayer of TMA readily bonds to the TiO2/ITO substrate and additional layers could be deposited layer-by-layer by cycles of Cu2+ and TMA immersion, following the procedure reported in ref 32. Absorption spectra did not reveal significant absorption by TMA (see the Supporting Information, Figure S5). The photocurrent was recorded at 400 nm for samples terminated by a single TCPP/ Cu2+ layer deposited directly on TiO2/ITO and above 1, 2, and 3 layers of TMA and the measurements are plotted in Figure 4d. The current increases with the number of TMA layers and peaks at 0.33 ± 0.042 nA/mW for two layers of TMA before slightly decreasing. The current with a buffer made of two TMA layers is thus more than 5 times the current that is generated by the sample where the TCPP is directly adsorbed on the TiO2/ ITO, and between the values seen for films made of two and three TCPP layers (see Figure 4c). Noteworthy, by recording the absorption spectra, we have verified that the integrated area of the Soret band remained constant at 2.3 ± 0.3 nm for all those samples, which agrees with the result shown in Figure 3 for a single layer of TCPP.

of MV2+ or O2 at the interface and by transfer of electron from the TiO2/ITO to the porphyrin cation P•+ and subsequent recovery of the ground-state P. This cathodic electron flow is, however, in competition with the quenching of 1P* by the substrate, creating an alternative deactivation pathway for the photoexcited state. Although it is less effective than for other metals such as Au,40 the quenching in ITO must a priori also be taken into account to analyze our data. Noteworthy, the introduction of spacers (usually chemically attached to the porphyrin) between TCPP and substrates is known to reduce the magnitude of the quenching.58−68 Finally, because our samples are multilayers, the charge transport across the film must also be considered, since TCPP molecules not in direct contact with the electrolyte will have a lower probability of ceding an electron to the electrolyte when photoexcited, leading to a deexcitation of 1P* without contribution to the photocurrent. It was indeed shown recently by others that 40 layer thick MOFs of TMA/Cu2+ are insulating.84 The experimental results presented in Figure 4 can be rationalized from these earlier observations. From Figure 4d, it is clear that the introduction of a TMA buffer reduces the quenching of the excited 1P* by ITO. However, when the TMA film is more than 2 molecular layers thick, the photocurrent is reduced; a fact that can be understood from a reduced electron transfer between TCPP and ITO and thus recovering of the electron from the solution. From that analysis the nonlinear increase in photocurrent upon increase in the number of TCPP molecular layers in Figure 4c can then be understood as a lesser contribution from the first two TCPP layers as a result of screening by ITO and by a reduction of the inner layers’ contribution upon addition of the fourth layer because the buried TCPP layers are then further from the solution and direct recombination becomes a more preferred deexcitation pathway.

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CONCLUSIONS

We have studied the generation of a photocurrent in a metal− organic coordination multilayer of TCPP with Cu2+ grown on TiO2 thin film on ITO. XPS and absorption in the visible clearly show that the layer-by-layer growth is achieved by the sequential immersion of the sample in TCPP and Cu2+ solutions, with a high homogeneity in the molecular layer density. There is also an indication that although most TCPP remain free-base, Cu2+ are incorporated in some molecules. When the samples are placed in an aqueous electrolyte and irradiated with visible light a cathodic photocurrent flows between the interface and a freshly annealed Pt wire. The current is imputed to the reduction of MV2+ and/or O2 at the interface as proposed earlier by others for monolayers of various TCPP derivatives and for TCPP multilayers made by sol−gel approaches.43,82 The introduction of a TMA buffer is seen to drastically reduce the quenching from the substrate without sacrificing the TCPP molecular density and ability to generate photocurrent. A thickness of 2 TMA layers is found to be optimum for our TiO2/ITO substrates. MOFs have today shown their potential for a wide range of applications and our study further highlights the validity of the metal−organic coordination scheme for the generation of photocurrent in an electrolyte.

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DISCUSSION The mechanisms of photoexcitation and charge transfer in TCPP and similar porphyrin derivatives in aqueous media with MV2+ and O2 have been proposed by others40,43,58 and are summarized here to facilitate the discussion of our data (see also the Supporting Information, Figure S6). After absorption of a photon the porphyrin molecules are excited into the singlet state 1P* and cathodic photocurrent is established by reduction E



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ASSOCIATED CONTENT

S Supporting Information *

Sketch of the homemade two-electrode cell (Figure S1); spectral power density of the halogen light source (Figure S2); scanning electron microscopy images of samples of unmodified ITO (a), TiO/ITO (b), (TCPP/Cu2+)3/TCPP/TiO2/ITO (c), and TCPP/(Cu2+/TMA)2/TiO2/ITO (d) (Figure S3); relative power absorption by TCPP for the filters at 400 and 450 nm (Figure S4); visible absorption spectra TCPP and TMA films (Figure S5); and photocurrent mechanism illustrating the pathways for electron flow (Figure S6). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.S. acknowledges funding from the Integrated Nanoscience Platform for Ireland (INSPIRE), initiated by the Higher Education Authority in Ireland within the PRTLI4 framework. The authors acknowledge David Richardson for his contribution to the design of the photoelectrochemical cell used in this study.

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REFERENCES

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