Organic Framework Thin Films with Enhanced Photocurrent

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Cite This: ACS Appl. Energy Mater. 2018, 1, 249−253

Photon Up-Conversion via Epitaxial Surface-Supported Metal− Organic Framework Thin Films with Enhanced Photocurrent Shargeel Ahmad,† Jinxuan Liu,*,† Chenghuan Gong,† Jianzhang Zhao,† and Licheng Sun*,†,‡ †

State Key Laboratory of Fine Chemicals, Institute of Artificial Photosynthesis, Dalian University of Technology, 116024 Dalian, China ‡ Department of Chemistry, School of Chemical Science and Engineering, KTH Royal Institute of Technology, 10044 Stockholm, Sweden S Supporting Information *

ABSTRACT: We report a new triplet−triplet annihilation photon up-conversion (TTA-UC) system using an epitaxial Zn-perylene surface-supported metal−organic framework (SURMOF) grown on metal oxide surface as “emitter”, and a platinum octaethylporphyrin (PtOEP) as “sensitizer” in [Co(bpy)3]2+/3+ acetonitrile solution. It has been demonstrated that the photocurrent can be significantly enhanced relative to epitaxial Zn-perylene SURMOF due to the TTAUC mechanism. This initial result holds promising applications toward SURMOF-based solar energy conversion devices. KEYWORDS: triplet−triplet annihilation, up-conversion, metal−organic framework thin film, photocurrent

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emission from polymer, the dyes are embedded in the rubbery polymer matrixes to permit a certain degree of dye mobility in order to overcome the quenching by oxygen and promote TTA-UC efficiencies.36 The liquid up-conversion systems comprise a triplet sensitizer and an acceptor. In the TTA-UC process, the triplet energy transfer from sensitizer to acceptor generates the two excited-state acceptor triplets, which interact with each other to produce a singlet excited state of the acceptor. In electronically coupled cells,37−40 the TTA-UC materials (organic dyes) are incorporated into the cell with one dye (A) capable of absorbing lower-energy photons as “sensitizer” and the other dye (B) as “emitter”. Upon photoexcitations, the excited triplet states of dye A interact with a ground-state dye B, leading to a triplet state of dye B, where the TTA-UC occurs and generates one higher-energy injecting state. With this strategy, the low-energy excited states can be converted into higher excited states, and further increase energy conversion efficiency. Herein, we reported a new electronically coupled TTA-UC system using epitaxial SURMOF anchored on TiO2 substrate as emitter, and platinum(II) octaethylporphyrin (PtOEP) as sensitizer in a [Co(bpy)3]2+/3+ acetonitrile solution26 as schematically shown in Figure 1. Further, we demonstrated the enhancement of photocurrent via utilization of TTA-UC in

etal−organic frameworks (MOFs) or porous coordination polymers (PCPs) are a class of porous, crystalline materials constructed by metal-oxo connectors and organic linkers.1,2 Because of their tunable chemical and physical properties, MOFs have attracted enormous research efforts in gas storage,3 purification and separation,4 as well catalysis5 and sensing6 applications. In recent years, the use of MOFs as energy materials for applications in photovoltaics,7,8 electronics,9,10 and energy storage devices11 has received enormous attention, which requires the deposition of MOFs on a solid surface. The surface-supported metal−organic framework thin films (SURMOFs) with well-defined orientation and thickness become an attractive platform for fabrication of devices.12,13 The regular and precise arrangements of organic linkers within SURMOFs, in particular, the well-defined organic/organic interface within multilayer SURMOFs, are suited for efficient energy transfer via triplet−triplet annihilation up-conversion (TTA-UC).14 This process allows for the conversion of a high wavelength into a low wavelength via Dexter energy transfer15 at the heterojunction between donor and acceptor followed by TTA-UC within the MOF between two acceptors by using lowintensity noncoherent light.16−19 In the past decade, a wide variety of TTA photon upconversion systems have been developed.16,20−30 With regard to the TTA-UC systems used in solar energy conversion, these can be categorized into two strategies, i.e., optically and electronically coupled cells. The optical devices rely on UC emission from a polymer-based31−33 or solution-based16,27,34,35 filter back to a conventional solar cell. For the devices with UC © 2018 American Chemical Society

Received: October 11, 2017 Accepted: January 26, 2018 Published: January 26, 2018 249

DOI: 10.1021/acsaem.7b00023 ACS Appl. Energy Mater. 2018, 1, 249−253

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ACS Applied Energy Materials a photoelectrochemical device containing [Co(bpy)3]2+/3+ acetonitrile solution.

orientation. Further analysis of XRD data reveals that the (001) peak at 2θ = 5.8° corresponds to a d value of 1.5 nm, which has the exact same length as 3,9-perylenedicarboxylic acid and the Zn paddle-wheel structure. In combination with simulated XRD patterns shown in Figure 2a, the Zn-perylene SURMOF exhibits a structure similar to our previously reported SURMOF 2 constructed with nonlinear linker: 2,6- naphthalene dicarboxylic acid (2,6-NDC) exhibiting a P2 symmetry as a result of the nonlinearity of the carboxyl functional groups on the ligand43 with layers perpendicular to the TiO2 surface containing one-dimensional channels with a diameter of ∼1.5 nm, and a layer distance of ∼0.58 nm as shown in Figure 2b. The morphology of the Zn-perylene SURMOF films prepared with the LPE method on TiO2 substrate was characterized with scanning electron microscope (SEM) as displayed in Figure S1, which exhibits a homogeneous and compact surface with a thickness of ∼200 nm (20 LPE cycles).44 The infrared characterization can be found in Figure S2. The electronic properties of Zn-perylene SURMOF (emitter), PtOEP (sensitizer), and Zn-perylene SURMOF + PtOEP were further characterized by ultraviolet−visible (UV− vis) absorption spectroscopy. The recorded UV−vis spectra of these samples in acetonitrile solution are shown in Figure 3.

Figure 1. Schematic illustration of the electronically coupled TTA-UC system using an epitaxial Zn-perylene SURMOF anchored on mesoporous TiO2 substrate as emitter, and PtOEP as sensitizer in a [Co(bpy)3]2+/3+ acetonitrile solution.

The SURMOF was fabricated by using a well-established liquid phase epitaxy (LPE) approach via alternative deposition of 3,9-perpylenedicarboxylic acids24,41 and zinc acetates onto a mesoporous TiO2 substrate, which leads to the formation of homogeneous and epitaxial Zn-perylene SURMOF.42 The detailed preparation procedures regarding substrate and Znperylene SURMOFs can be found in the Supporting Information. The as-prepared Zn-perylene SURMOF thin film was characterized with X-ray diffraction (XRD) as shown in Figure 2. The pronounced (001) and weak (002) peaks observed in an out-of-plane XRD pattern (Figure 2a) suggest that the fabricated Zn-perylene SURMOF has grown exclusively along the [001] direction on the TiO2 surface, which is in accordance with the simulated XRD diffractogram with preferred [001] Figure 3. UV−vis spectra of Zn-perylene SURMOFs (in red), PtOEP (in black), and Zn-perylene SURMOFs + PtOEP (in blue). All of the spectra were recorded in acetonitrile solution.

The observed broad band centered at 408 nm for Znperylene SURMOF is blue-shifted compared with those of free perylenedicarboxylic acids45 (438 and 460 nm in acetonitrile solution, Figure S3), which are associated with the S1(B1u) ← S0(Ag) transition of perylene units.46 In the case of PtOEP in acetonitrile solution, a typical absorption characteristic of metalated porphyrin compounds was observed with an intense Soret band at 377 nm, and Q bands at 532 and 497 nm due to singlet−singlet absorption.47 For the combined system of Znpeylene SURMOF + PtOEP (Figure 3 (blue)), the absorption bands at 377, 408, 497, and 532 nm are identical to those bands observed for Zn-peylene SURMOF and PtOEP, respectively. In order to confirm whether up-conversion can be realized in the heterogeneous Zn-perylene SURMOF + PtOEP system (Figure 4a), we carried out fluorescence experiments. The Znperylene SURMOF was placed into a sealed cuvette filled with acetonitrile solution containing PtOEP sensitizers, which was purged with N2 for 30 min to get rid of the influence of O2. The recorded emission spectrum of Zn-perylene SURMOF + PtOEP is shown in Figure 4b (in black). As reference, the

Figure 2. (a) Out-of-plane XRD patterns of Zn-perylene SURMOF grown on TiO2 substrate (in red) and simulated XRD pattern of Znperylene SURMOF with preferred (001) orientation. (b) Proposed ideal structure of Zn-perylene SURMOF with lattice parameters of a = b = 1.5 nm, c = 0.58 nm. The proposed structure was generated by using Materials Studio based on previously reported SURMOF 2 in ref 43. 250

DOI: 10.1021/acsaem.7b00023 ACS Appl. Energy Mater. 2018, 1, 249−253

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Figure 4. (a) Schematic representation of the molecular structure of Zn-perylene SURMOF + PtOEP on the TiO2 surface. (b) Emission spectra of PtOEP (dotted green, λex = 530 nm), FTO/TiO2-Zn-perylene SURMOF (dotted blue, λex = 430 nm), and FTO/TiO2-Zn-perylene SURMOF + PtOEP (solid black line, λex= 530 nm). All the spectra were recorded in acetonitrile solution. The 530 nm excitation peak is attributed to the wavelength of laser source. Excitation power density of 4.6 mW/cm2 was used to for 530 and 430 nm laser sources.

emission spectra of Zn-perylene SURMOF (dotted blue), PtOEP (dotted green), and 3,9-perylenedicarboxylic acid in acetonitrile were displayed in Figure 4b and Figure S5, respectively. Excitation wavelength of 530 nm was used for the Znperylene SURMOF + PtOEP system generating the emission signal centered at ∼460 and 643 nm (Figure 4b, solid black), which is in accordance with the emission signal from Znperylene SURMOF (Figure 4b, dotted blue, λex = 430 nm) and PtOEP (Figure 4b), dotted green, λex = 530 nm). The use of 430 nm excitation wavelength aims to make comparative analysis of the upconversion signal generated from the Znperylene SURMOFs when the sample is excited with green light. A cell with similar architecture was prepared in the absence of PtOEP at λex = 530 nm, which gave no emission at 460 nm as shown in Figure S4. We attribute the emission signal at 460 nm observed in the PtOEP + Zn-perylene SURMOFs spectrum to direct triplet energy transfer (TET) from PtOEP to Zn-perylene SURMOF at the interface followed by TTA-UC between the neighboring perylene molecules within Znperylene SURMOF (Figure 1). The result demonstrates that TiO2-perylene SURMOF + PtOEP is an effective architecture to facilitate λex = 530 nm to λem = 460 nm up-conversion via triplet−triplet annihilation on surface-anchored metal−organic framework thin film (TTA-UC-SURMOF). In order to utilize the TTA-UC-SURMOF system, we assembled TiO2-Zn-perylene + PtOEP into a standard electrochemical cell by using TiO2-Zn-perylene + PtOEP or TiO2-Zn-perylene, or TiO2 + PtOEP as working electrodes, Ag/AgNO3 as reference electrode, and platinum wire as counter electrode in 0.01 μM [Co(bpy)3]2+/3+ acetonitrile solution (with applied external potential 0 V vs Ag/AgNO3) as illustrated in Figure 5a. The electrochemical cell was irradiated by using simulated solar light (AM1.5 solar) passing through a 530 nm long-pass filter coupled with an automatic shutter control the light irradiation, i.e., light on and light off. In Figure 5b (left), upon irradiation with the 530 nm light, the transient photocurrents of ∼2 μA/cm2 for TiO2-Znperylene + PtOEP, ∼0.1 μA/cm2 for TiO2-Zn-perylene, and ∼0.2 μA/cm2 for TiO2 + PtOEP were generated, respectively. By analysis of the transient photocurrents, we found that the photocurrent was substantially enhanced for TiO2-Zn-perylene + PtOEP with a factor of 10 compared with photocurrents for

Figure 5. (a) Schematic illustration of photoelectrochemical cell using TiO2-Zn-perylene + PtOEP as working electrode, Ag/AgNO3 as reference electrode, and platinum wire as counter electrode in 0.01 μM [Co(bpy)3]2+/3+ acetonitrile solution. (b, left) The i−t curves for photoelectrochemical cell containing TiO2 + PtEOP, TiO2-Znperylene SURMOF, and TiO2-Zn-perylene SURMOF + PtEOP photoanodes under AM1.5 solar irradiation passing through a 530 nm long-pass filter (power density = 35 mW/cm2) at external applied potential 0 V vs Ag/AgNO3 and (right) photocurrent density from photoelectrochemical cell containing TiO2-Zn-perylene SURMOF + PtEOP photoanode with respect to 530 nm excitation intensity with external applied potential 0 V vs Ag/AgNO3.

TiO2-Zn-perylene SURMOF and TiO2 + PtOEP. Under 530 nm irradiation, the singlet excited state of the PtOEP sensitizer is converted into a triplet state via intersystem crossing (ISC). 251

DOI: 10.1021/acsaem.7b00023 ACS Appl. Energy Mater. 2018, 1, 249−253

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ACS Applied Energy Materials *E-mail: [email protected].

The generated excited triplet states transferred from the PtOEP sensitizer to Zn-perylene SURMOF (the perylene at the top layer) resulting in the triplet excited states of perylene, followed by the TTA-UC between perylene molecules within Znperylene SURMOF, which diffused through the Zn-perylene SURMOF to the TiO2 surface followed by charge separation leading to the higher photocurrent relative to Zn-perylene SURMOF (acceptor) and PtOEP (sensitizer) (Figure 5a). In order to confirm that the photocurrent enhancement is due to a TTA-UC mechanism, the experiment of photocurrent density as a function of 530 nm excitation power density for TiO2-Zn-perylene SURMOF + PtOEP was carried out as shown in Figure 5b, right. It can be seen that the TiO2-Znperylene SURMOF + PtOEP exhibited a quadratic (slope ≈2) to linear (slope ≈1) intensity-dependent behavior, which indicates a TTA-UC mechanism;40 i.e., the sensitizers were excited with low-energy light, followed by energy transfer, TTA-UC, and electron injection from the up-converted state. As a control experiment, photocurrent density with respect to 430 nm excitation power density was performed for TiO2-Znperylene SURMOF + PtOEP as displayed in Figure S6. The observed linear dependence (slope ≈1) behavior suggests that with high-energy excitation the generation of photocurrent results from the electron injection to TiO2 from singlet excited states of perylene (acceptor). The observed up-converted signal shown in Figure 4b, together with the quadratic to linear behavior of photocurrent versus power density, strongly supports that the enhanced photocurrent is due to the TTA up-conversion in the low-power region. In conclusion, we have demonstrated the realization of upconversion using epitaxial Zn-perylene SURMOF as acceptor and PtOEP as sensitizer in a photoelectrochemical device. The photocurrent was enhanced by a factor of 10 for the hybrid TiO2-Zn-perylene SURMOF + PtOEP system relative to Znperylene SURMOF (acceptor) and PtOEP (sensitizer) under the green light irradiation (530 nm) due to TTA-UC in the sensitized SURMOF. As a matter of fact, the present system has the ability to transfer the triplet energy from the PtOEP to the Zn-perylene SURMOFs interface to achieve enhanced photoelectrochemical current as compared to those of Zn-perylene SURMOF (acceptor) and PtOEP (sensitizer). However, more research efforts are needed to have a deeper understanding of this phenomenon such as an investigation of the TTA-UC mechanism; a search for suitable redox mediator, energy transfer yields, and quantum yields; optimization of the oxygen removal technique; and improvement of TTA-UC efficiency, etc. With the first example of the generation of photocurrent via TTA-UC from the surface-anchored metal−organic framework thin film, this may open avenues for further development of MOF-based solar energy conversion devices.

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ORCID

Jinxuan Liu: 0000-0002-6306-1359 Jianzhang Zhao: 0000-0002-5405-6398 Licheng Sun: 0000-0002-4521-2870 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Basic Research Program of China (973 program) (2014CB239402), the Natural Science Foundation of China (NSFC 21673032, 51372028), the Fundamental Research Funds for the Central Universities (DUT17LK21), and the State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University (201507), is gratefully acknowledged.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.7b00023. Details of preparation and characterization of the surfacesupported metal−organic framework thin films and photoelectrochemical measurements (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. 252

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DOI: 10.1021/acsaem.7b00023 ACS Appl. Energy Mater. 2018, 1, 249−253