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J. Phys. Chem. C 2007, 111, 80-85
Applications for Metal-Organic Frameworks (MOFs) as Quantum Dot Semiconductors Francesc X. Llabre´ s i Xamena, Avelino Corma,* and Hermenegildo Garcia* Instituto de Tecnologı´a Quı´mica CSIC-UPV, UniVersidad Polite´ cnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain ReceiVed: June 9, 2006; In Final Form: September 22, 2006
The structure of MOF-5 is considered as being constructed of discrete semiconductor Zn4O13 quantum dots stabilized and interconnected by terephthalate linkers. Terephthalate can absorb light and sensitize the semiconductor dots. In this way, MOFs exhibit the photocatalytic activity of MOF-5 for the degradation of phenols. As a photocatalyst, the most remarkable feature of MOF-5 is the observation of reverse shapeselectivity in which large phenolic molecules that cannot access the interior of the micropores are degraded significantly faster than those others that can enter into the particle. MOF-5 also exhibits activity as an active component in photovoltaic and electroluminescence cells.
1. Introduction Metal-organic frameworks (MOFs) are a class of crystalline hybrid materials whose crystal structure is made up of extended 3D networks of metal ions or small discrete clusters connected through multidentate organic spacers.1-4 To become a MOF, these building units organize spatially in such a way that crystalline porous structures defining channels and cavities of regular size and shape on the nanometer scale are formed. The crystal framework of MOFs is analogous to that of zeolites and other related purely inorganic porous materials, but there are clear differences between them concerning porosity, range of stability, and feasibility of building a crystal structure by design. In particular, MOFs exhibit values of specific surface areas of up to 3000 m2 g-1 and specific pore volumes of up to 1 g cm-3, which are among the highest values ever reported for any material. MOFs have an extremely wide-open structure in which the free space available for host molecules can reach up to 90% of the crystal volume.5 This results in the lowest densities attained so far for any crystalline material (a value as small as 0.21 g cm-3 has been reported for IRMOF-16).5 Also in the case of MOFs it is possible to achieve a fine control over the chemical environment and the topology of the internal voids by selecting appropriate building blocks (dimensions and functionalities of the organic linkers and size of the metal clusters) and the way in which they are connected. However, due to the presence of organic building blocks, MOFs are obviously much less stable than zeolites to thermal treatments, to moisture, and to chemical agents. Despite their novelty and in contrast to the ample use of zeolites,6-10 MOFs have found up to now only limited application, mainly in gas separation and storage.11,12 It is evident that, considering the potential of MOFs, there is a large interest in developing other applications that fully exploit the presence of organic components in a highly crystalline and porous environment. Considering the use in heterogeneous catalysis and in spite of their high metal content, the potential of MOFs is largely hampered by the fact that the coordination sphere of the metal ion is usually completely blocked by the organic linkers and * Corresponding authors. Fax: (+34) 96 387 7809. E-mail: hgarcia@ qim.upv.es.
by the poor thermal and chemical stability of MOFs. On the other hand, MOFs may show their superior behavior when used in room-temperature applications that require a response from the material. It appears to us that they can be particularly appropriate for nanotechnology applications that take advantage of the presence (in some MOFs) of inorganic semiconductor quantum entities (such as dots or wires) in close contact with organic molecules. The latter can serve to activate these semiconductor quantum dots upon external chemical, photochemical, or electrochemical stimuli. In the search of these applications for MOFs, we have found very promising activities in fields that have been so far unexplored and where zeolites are inactive due to their inaccessible band gap. Specifically, we have observed the following responses for MOFs: (i) as reverse shape-selective photocatalysts, (ii) as photoactive materials for photovoltaic cells, and (iii) as components for electroluminescence devices. It is the aim of this contribution to show the promising potential of MOFs in these fields in which these novel materials have not yet been used. Also, we want to set the bases for future developments and improvements. 2. Experimental Section 2.1. Materials Preparation. MOF-5 was synthesized according to Huang et al.,13 and its structure was confirmed by X-ray diffraction. The synthesis of the hybrid tin-containing metal-organic compound quoted in the text was also adapted from this recipe, using SnCl2‚2H2O as the precursor for Sn2+, and keeping all other synthesis conditions unchanged. XRD showed that the material thus prepared was amorphous. 2.2. Photocatalytic Tests. Photocatalytic tests of a single phenol were carried out, with stirring in open air and at room temperature, on an air-saturated aqueous suspension (20 mL) of phenol or 2,6-di-tert-butylphenol (40 ppm) in the presence of MOF-5 (80 mg). The suspension was stirred for at least 10 min in the dark before irradiation. The samples were placed in a series of independent Pyrex test tubes (25-mL capacity), each of them provided with a magnetic stirring bar. The test tubes were placed in a thermostated water bath around a waterrefrigerated Pyrex well containing a 125-W medium-pressure Hg lamp. The course of the irradiation was followed by removing each test tube at the required reaction time and
10.1021/jp063600e CCC: $37.00 © 2007 American Chemical Society Published on Web 12/06/2006
Applications for MOFs in Nanotechnology analyzing not only the concentration of the phenols in the supernatant aqueous solution but also the amount adsorbed on the solid. The amount of phenol adsorbed on the solids was recovered by sonicating the powder for 30 min after redispersing the solid in 3 mL of fresh water. The combined extracts (supernatant plus products extracted from the solid) were analyzed by phase reverse HPLC (Suprasil column, diode array detector monitoring at 254 nm). For competitive photodegradation assays, the experimental setup adopted was analogous; however, aqueous solutions containing mixtures of the two analytes (20 ppm each) were used instead. Blank controls revealed that no degradation of phenol occurs in the absence of the photocatalyst and that over 85% of the adsorbed phenol (40 ppm) can be recovered by our solid-liquid extraction procedure based on sonication. XRD showed that the crystal structure of the photocatalyst used is maintained. 2.3. Preparation of the Photovoltaic Solar Cells. For the preparation of photovoltaic solar cells, a thin layer of MOF-5/ DMF paste was deposited onto a transparent indium tin oxide (ITO) electrode, using double-sided adhesive tape to control the layer area and thickness (1 × 1 cm2 and 50 µm, respectively). Then, a platinum electrode was placed on top of the cell and sealed. The solar cells were then exposed to a solar simulator (525 W) through an AM1.5 filter. The characteristic open-circuit voltage (VOC), short-circuit current (ISC), and fill factor (FF%) were evaluated from the corresponding I-V curves of the cell upon irradiation. The photocurrent spectrum was obtained using a 75-W Xe lamp provided with a monochromator. 2.4. Preparation of the EL Cells. EL (electroluminescence) devices were prepared by depositing a uniform thin layer of a MOF-5/DMF paste onto a transparent ITO electrode. The thickness (50 µm) and the area (1 × 1 cm2) of the film were controlled by double-sided adhesive tape placed on the ITO. Then, an aluminum counter electrode was placed on top of the device and sealed. When a square alternating current (ac) was applied between the two electrodes, EL emission from the cell was observed. The light emitted by the device was recorded as a function of the wavelength. The influence of the voltage and of the frequency of the ac current on the emitted light intensity and λEL(max) was checked. The best results concerning light intensity were obtained for a voltage of 60-80 V and a frequency of 180 Hz. No significant changes in the value of λEL(max) were observed in the range of frequencies examined. Emission decreased significantly or even disappeared for dry MOF-5 cells. 3. Results and Discussion 3.1. Reverse Shape-Selective Photocatalysis by MOFs. Looking for potential applications of MOFs and considering as the starting point that the structure of MOF-5 can be considered as Zn4O13 quantum dots interacting with terephthalate linkers, we initially anticipated that MOF-5 can exhibit activity as a photocatalyst. An interesting observation related to our present study on the photocatalytic activity of MOF-5 was recently reported by Zecchina and co-workers, who observed emission from the zinc oxide clusters of MOF-5 upon excitation at terephthalate λmax.14 Indeed, the Zn4O13 clusters found in MOF-5 are structurally stable quantum dots isolated by the terephthalate linkers. In general, the realization that MOF-5 is a semiconducting material in the broad sense of the term opens up a series of novel applications in those fields in which semiconducting properties of the material are required, such as those reported here
J. Phys. Chem. C, Vol. 111, No. 1, 2007 81 TABLE 1: Photodegradation Rates (k, ppm × min-1) for Phenol (P) and 2,6-Di-tert-butylphenol (DTBP), Irradiated Separately or in a Mixture, and Using MOF-5 as a Photocatalyst separate irradiation k(P)pure k(DTBP)pure k(DTBP)pure/k(P)pure
mixture 0.352 0.387 1.10
k(P)mix k(DTBP)mix k(DTBP)mix/k(P)mix
0.067 0.296 4.42
(photocatalysis, photovoltaic cells, and electroluminescence). The same ideas could also apply in principle to many other metal-organic materials besides MOF-5. Taken the reported uses of zeolites to orient the quest for novel applications of MOFs, there are numerous precedents describing heterogeneous photocatalysts based on the encapsulation inside the micropores of zeolites of photoactive components.15,16 However, in the reported examples of zeolite-based photocatalysts and due to their large band gap, zeolites behave as passive components of the photochemical events and they were not directly responsible for the generation of electrons and holes. Exceptions to the lack of photocatalytic activity are some crystalline titanosilicates, such as ETS-4 and ETS-10. These zeotypes have structural Ti-O semiconducting chains that can be excited by UV light in a manner similar to that of TiO2, and then they behave as photocatalysts.17-20 In contrast to zeolites, metal-organic frameworks contain in their structure light-absorbing organic chromophores and photoexcitable semiconductor metal clusters. Moreover, a charge-transfer interaction between the organic moiety as donor and the Zn4O13 clusters as acceptor is responsible for the extended absorption at longer wavelengths than those of the organic and inorganic components independently. Therefore, these materials combine properties typical of semiconductor quantum dots with those conferred by the organic spacer. The organic molecule can act as a photon antenna for lightharvesting, photosensitizing by energy transfer or electron injection the metal quantum dot that ultimately will be responsible for the photocatalytic activity. At the same time, MOFs may exhibit some effect arising from the dimensions, geometries, and the chemical environment of their channels and cavities. As a first test for demonstrating the photocatalytic activity of MOF-5, we have considered the photodegradation of phenol (P) in aqueous solution using Pyrex-filtered light (polychromatic radiation, λ < 300 nm) as it is a standardized test proposed by the IUPAC to compare the photocatalytic activity of different solids.21,22 To check for any possible shape-selective effects, we have also studied the competitive photodegradation of 2,6di-tert-butylphenol (DTBP), a molecule considerably bulkier than phenol. The results are reported in Table 1 and Figure 1. Importantly, XRD of the MOF-5 after its use as a photocatalyst has revealed that its crystal structure has remained intact during the process. The first important conclusion that can be drawn from the results obtained is that MOF-5 shows photocatalytic activity, since phenol is largely degraded, although with a lower degradation rate than that of the conventional P-25 TiO2 standard (roughly 50% degradation of phenol is attained after 180 min of irradiation in the presence of MOF-5, while 50% phenol degradation is achieved at 25 min with P-25). Better results are obtained for the photocatalytic degradation of DTBP, as in this case 180 min of irradiation causes the almost complete degradation of the molecule. The initial rate constants for the two separate systems (calculated as the slope of the time conversion plots at short irradiation times) result in a k(DTB-
82 J. Phys. Chem. C, Vol. 111, No. 1, 2007
Llabre´s i Xamena et al.
Figure 1. Photodegradation curves of phenol (P) and 2,6-di-tert-butylphenol (DTBP) obtained using MOF-5 as a photocatalyst. (a) Curves correspond to photodegradation of 40 ppm of the pure species; (b) curves correspond to competitive photodegradation (irradiation of a mixture of 20 ppm of both molecules). Solid lines are the best fit to the experimental data obtained with a first-order exponential decay. Dotted straight lines show the initial degradation rates.
P)pure/k(P)pure ratio of 1.10. In other words, when these phenols are irradiated in the presence of MOF-5 independently, DTBP is degraded at an initial rate comparable to that of P. A second and more relevant conclusion is that MOF-5 exhibits reverse shape-selectivity. Indeed, when a solution containing both P and DTBP is irradiated in the presence of MOF-5, DTBP is degraded with a rate constant ratio of 4.42fold higher with respect to phenol (i.e., k(DTBP)mix/k(P)mix value in Table 1). This value corresponds to a selective photodegradation of about 82% toward bulkier DTBP with respect to P. The results obtained for MOF-5 as a photocatalyst indicate that large molecules (i.e., molecules with a high steric hindrance with respect to the pore openings of MOF-5) are degraded faster than small molecules. This result is in line with the conclusions pointed out in the literature for the titanosilicate ETS-10,18,20 and therefore the explanation and conclusions given in these precedents of reverse shape-selectivity photocatalysis can also be applied here. The authors explained the different behavior of the material toward small and large molecules by adducing that small molecules can freely diffuse to the internal space of the material, where they are less prone to undergo photodegradation. In contrast, bulky molecules remain at the external surface of the photocatalyst where they are rapidly degraded. In other words, degradation of the organic molecules takes place at a different rate in the internal and on the external surface of the photocatalyst (being significantly lower at the internal surface), and this is the key factor governing the shape-selective properties of the photocatalyst. It is noteworthy to remark here that, with respect to zeolites, MOFs present a clear advantage in tailoring of the pore size, shape, and chemical environment, due to the recent progress in understanding and exploiting the concept of “modular chemistry” found behind their synthesis.4 In principle, it is possible to design and construct an ad hoc photocatalyst that selectively degrades molecules on the basis of their size by simply selecting the organic linker that better matches the dimensions of the target species. On the other hand, it is obvious that organic chromophores other than terephthalate adsorbing at longer wavelengths could render the photocatalytic activity of MOFs more efficient and of higher practicality.
3.2. MOFs in Photovoltaic Cells. The basic structure of a photovoltaic cell consists of a transparent conductive electrode [usually a thin indium tin oxide (ITO) layer deposited onto glass] that contains a thin (submillimetric) film of a semiconductor in contact with an electrolyte solution. The counter electrode of the cell can be an inert metal, and the short-circuit between the electrodes is avoided by an insulating separator that seals the cell. The activity of MOF-5 as a semiconductor photocatalyst prompted us to check whether or not MOFs can be used as photoactive materials for photovoltaic cells. Thus,we prepared a device using MOF-5 as the active material and tested its performance in a photovoltaic solar cell. Note that such a rudimentary photovoltaic cell does not contain either sensitizing dyes (that should enlarge the response of the cell to visible or near-IR wavelengths) or electrolytes (that should increase the rate of charge transport minimizing competitive electron-hole recombination of the charge-separated excited state). Therefore, our photovoltaic cell is far from being optimized since our purpose here is only to prove the principle that MOF-5 can be used as photoactive materials for photovoltaic cells. We were pleased to observe a photocurrent upon illumination of the MOF-5 photovoltaic cell with the output light of a solar simulator. The I-V curve obtained for the MOF-5 solar cell upon irradiation with an AM1.5-filtered solar light simulator is shown in Figure 2a. The shape of the curve obtained is typical of a power-generating photovoltaic device, since the electric power (i.e., the product P ) I × V) is strictly negative for all voltage values. From this curve, the following characteristic parameters of the MOF-5/DMF cell without any electrolyte can be deduced: VOC ) 0.33 V, ISC ) 0.7 µA, and FF ) 44%. For comparison, a photovoltaic cell prepared in our laboratory with standard P25 TiO2 (with no dye) under the same conditions as the one with MOF-5 described here gives the following photovoltaic parameters: VOC ) 0.52 V, ISC ) 209 µA, and FF ) 58%. Another characterization of the photovoltaic cell consists in recording its photocurrent spectrum (Figure 2b). For this measurement, the cell is irradiated with monochromatic light of a given wavelength and the electric current generated is
Applications for MOFs in Nanotechnology
Figure 2. (a) I-V curve obtained for the photovoltaic solar cell prepared with MOF-5 upon irradiation with an AM1.5-filtered lamp (525 W) using an ITO-glass electrode. (b) Photocurrent spectrum of MOF-5 obtained with a transparent electrode consisting of ITO on glass. Note that this electrode is not completely transparent for wavelengths
