A Convenient Route to High Area, Nanoparticulate TiO₂ Photoelectrodes Suitable for High-Efficiency Energy Conversion in Dye-Sensitized Solar Cells

Ethanol-soluble amphiphilic TiO2 nanoparticles (NPs) of average diameter ∼9 nm were synthesized, and an α-terpineol-based TiO2 paste was readily prepared from them in comparatively few steps. When used for fabrication of photoelectrodes for dye-sensitized solar cells (DSSCs), the paste yielded highly transparent films and possessing greater-than-typical, thickness-normalized surface areas. These film properties enabled the corresponding DSSCs to produce high photocurrent densities (17.7 mA cm−2) and a comparatively high overall light-to-electrical energy conversion efficiency (9.6%) when deployed with the well-known ruthenium-based molecular dye, N719. These efficiencies are about ∼1.4 times greater than those obtained from DSSCs containing photoelectrodes derived from a standard commercial source of TiO2 paste

Photovoltaic Effects of CdS and PbS Quantum Dots Encapsulated in Zeolite Y

Zeolite Y films (0.35–2.5 μm), into which CdS and PbS quantum dots (QDs) were loaded, were grown on ITO glass. The CdS QD-loaded zeolite Y films showed a photovoltaic effect in the electrolyte solution consisting of Na2S (1 M) and NaOH (0.1 M) with Pt-coated F-doped tin oxide glass as the counter electrode. In contrast, the PbS QD-loaded zeolite Y films exhibited a negligible PV effect. This contrasting behavior was proposed to arise from the large difference in driving force for the electron transfer from S2– in the solution to the hole in the valence band of QDs, with the former being much larger (∼2 eV) than the latter (∼1 eV). In the case of CdS QD-loaded zeolite Y with a loaded amount of CdS of 6.3 per unit cell, the short circuit current, open circuit voltage, fill factor, and efficiency were 0.3 mA cm–2, 423 V, 28, and 0.1%, respectively, under the AM 1.5, 100 mW cm–2 condition. This cell was stable for more than 18 days of continuous measurements. A large (3-fold) increase in overall efficiency was observed when PbS QD-loaded zeolite Y on ITO glass was used as the counter electrode. This phenomenon suggests that the uphill electron transfer from ITO glass to S in the solution is facilitated by the photoassisted pumping of the potential energy of the electron in ITO glass to the level that is higher than the reduction potential of S by PbS QDs. Under this condition, the incident-photon-to-current conversion efficiency (IPCE) value at 398 nm was 42% and the absorbed-photon-to-current conversion efficiency (APCE) value at 405 nm was 82%. The electrolyte-mediated interdot charge transport within zeolite films is concluded to be responsible for the overall current flow

New Insights into CdS Quantum Dots in Zeolite−Y

When dry Cd2+-exchanged zeolites Y are exposed to dry H2S under a rigorously anhydrous condition, CdS quantum dots (QDs) are formed in the supercages of zeolite−Y regardless of the loading levels of CdS from 0.01% to 32% and regardless of the Si/Al ratio of zeolite−Y between 1.8 and 2.5. The absorptions with the maximums (λmax) ≤ 290 nm are assigned as those arising from isolated CdS QDs with the sizes smaller than or equal to the size of a supercage (1.3 nm); the absorptions with λmax between 290 and 380 nm are assigned as those arising from interconnected CdS QDs that were formed by the interconnection of isolated CdS QDs through the supercage windows; and the absorptions with λmax > 400 nm are assigned as those arising from mesosized (3−10 nm) CdS QDs residing in or on the surfaces of amorphous aluminosilicate. The H+ ions alone, which are generated during the formation of CdS, do not destruct the zeolite−Y framework causing the formation of amorphous aluminosilicate. Instead, the water-induced agglomeration of isolated and interconnected CdS QDs to mesosized CdS QDs in the presence of H+ ions leads to the destruction of the zeolite−Y framework. The size of the interconnected CdS QD which is formed by moisture adsorption increases as the loaded amount of CdS increases for a given zeolite and as the size of the zeolite host increases. The presence of a tetraethylammonium ion in each supercage not only gives rise to the formation of very small QDs within zeolites Y but also prevents the zeolite framework from destruction

Photocurrent Enhancement by Surface Plasmon Resonance of Silver Nanoparticles in Highly Porous Dye-Sensitized Solar Cells

Localized surface plasmon resonance (LSPR) by silver nanoparticles that are photochemically incorporated into an electrode-supported TiO2 nanoparticulate framework enhances the extinction of a subsequently adsorbed dye (the ruthenium-containing molecule, N719). The enhancement arises from both an increase in the dye’s effective absorption cross section and a modest increase in the framework surface area. Deployment of the silver-modified assembly as a photoanode in dye-sensitized solar cells leads to light-to-electrical energy conversion with an overall efficiency of 8.9%. This represents a 25% improvement over the performance of otherwise identical solar cells lacking corrosion-protected silver nanoparticles. As one would expect based on increased dye loading and electromagnetic field enhanced (LSPR-enhanced) absorption, the improvement is manifested chiefly as an increase in photocurrent density ascribable to improved light harvesting

Effect of Water on the Behavior of Semiconductor Quantum Dots in Zeolite Y: Aggregation with Framework Destruction with H−Y and Disaggregation with Framework Preservation for NH₄−Y

Treatment of dry M2+-exchanged zeolite Y (M2+ = Cd2+, Mn2+, and Zn2+) with dry H2S leads to the formation of isolated, ligand-free, subnanometer MS quantum dots (QDs) in zeolite Y with no framework destruction and with H+ as the countercation. Treatment of the dry H+/CdS QD-incorporating zeolites Y with dry NH3 leads to the neutralization of H+ to NH4+. During this process, the framework structure remains intact. However, small amounts of interconnected CdS QDs were formed within the zeolite Y by coalition of isolated CdS QDs at the windows. With H+ as the countercation, isolated CdS QDs rapidly aggregate into interconnected and mesosized QDs with accompanying destruction of ∼50% of sodalite cages leading to the framework rupture. With NH4+ as the countercation, however, the isolated QDs and zeolite framework remain intact even after exposure to the moist air for 4 weeks. Interestingly, the interconnected QDs that were formed during neutralization of H+ with NH3 disintegrate into isolated QDs in the air. Similar results were obtained from ZnS and MnS QDs generated in zeolite Y. Thus, ligand-free, naked, subnanometer QDs can now be safely preserved within zeolite pores under the ambient conditions for long periods of time. This finding will expedite the generation and dispersion of various QDs in zeolite pores, their physicochemical studies, and applications

Direct in Situ Conversion of Metals into Metal–Organic Frameworks: A Strategy for the Rapid Growth of MOF Films on Metal Substrates

The fabrication of metal–organic framework (MOF) films on conducting substrates has demonstrated great potential in applications such as electronic conduction and sensing. For these applications, direct contact of the film to the conducting substrate without a self-assembled monolayer (SAM) is a desired step that must be achieved prior to the use of MOF films. In this report, we propose an in situ strategy for the rapid one-step conversion of Cu metal into HKUST-1 films on conducting Cu substrates. The Cu substrate acts both as a conducting substrate and a source of Cu2+ ions during the synthesis of HKUST-1. This synthesis is possible because of the simultaneous reaction of an oxidizing agent and a deprotonating agent, in which the former agent dissolves the metal substrate to form Cu2+ ions while the latter agent deprotonates the ligand. Using this strategy, the HKUST-1 film could not only be rapidly synthesized within 5 min but also be directly attached to the Cu substrate. Based on microscopic studies, we propose a plausible mechanism for the growth reaction. Furthermore, we show the versatility of this in situ conversion methodology, applying it to ZIF-8, which comprises Zn2+ ions and imidazole-based ligands. Using an I2-filled HKUST-1 film, we further demonstrate that the direct contact of the MOF film to the conducting substrate makes the material more suitable for use as a sensor or electronic conductor

Exploiting Microwave Chemistry for Activation of Metal–Organic Frameworks

Microwave is thought of as a useful electromagnetic radiation tool because it is often used in real life as well as in a variety of chemical processes. Meanwhile, activation of metal–organic frameworks (MOFs), which must be essentially done to remove coordinating and pore-filling solvents before the use of MOFs for various applications, has been performed commonly with the methods of heat supply or solvent exchange. Here, we show a new methodological microwave activation (MA), realizing it with various MOFs such as HKUST-1, UiO-66, and MOF-74s. For instance, microwave irradiation to the MOF samples for 4–35 min leads to the complete activation of the MOFs without structural damage. As described below, we further demonstrate that the solvent-assisted MA, which is the MA process performed after the solvent exchange, can substantially reduce the time for the activation by 4 min

Diffusion Control in the in Situ Synthesis of Iconic Metal–Organic Frameworks within an Ionic Polymer Matrix

Ionic polymers that possess ion-exchangeable sites have been shown to be a greatly useful platform to fabricate mixed matrices (MMs) where metal–organic frameworks (MOFs) can be in situ synthesized, although the in situ synthesis of MOF has been rarely studied. In this study, alginate (ALG), an anionic green polymer that possesses metal-ion-exchangeable sites, is employed as a platform of MMs for the in situ synthesis of iconic MOFs, HKUST-1, and MOF-74­(Zn). We demonstrate for the first time that the sequential order of supplying MOF ingredients (metal ion and deprotonated ligand) into the alginate matrix leads to substantially different results because of a difference in the diffusion of the MOF components. For the examples examined, whereas the infusion of BTC^3– ligand into Cu²⁺-exchanged ALG engendered the eggshell-shaped HKUST-1 layers on the surface of MM spheres, the infusion of Cu²⁺ ions into BTC^3–-included alginate engendered the high dispersivity and junction contact of HKUST-1 crystals in the alginate matrix. This fundamental property has been exploited to fabricate a flexible MOF-containing mixed matrix membrane by coincorporating poly­(vinyl alcohol). Using two molecular dyes, namely, methylene blue and rhodamine 6G, further, we show that this in situ strategy is suitable for fabricating an MOF-MM that exhibits size-selective molecular uptake

A Chemical Role for Trichloromethane: Room-Temperature Removal of Coordinated Solvents from Open Metal Sites in the Copper-Based Metal–Organic Frameworks

Open coordination sites (OCSs) in metal–organic frameworks (MOFs) have shown potential in applications such as molecular separation, sorption, catalysis, and sensing. Thus, the removal of coordinated solvent has been viewed as an essential step that needs to be performed prior to the use of the MOFs in the above applications. To date, a thermal method that is normally performed by applying heat and vacuum has been the most commonly employed activation method despite its negative influence on the structural integrity of the MOFs. In this report, we demonstrate that commonly inert trichloromethane (TCM) can activate OCSs; the TCM treatment process serves as an alternative chemical route to activation that does not require the external thermal energy. On the basis of the Raman study, we suggest a possible mechanism for the chemical activation process where TCM may weakly coordinate to the OCSs and then spontaneously dissociate. In addition, we prove that the chemical activation behavior is substantially boosted when a small amount of external heat energy (55 °C, 2.6 meV) is supplied during the TCM treatment. Using an HKUST-1–polyvinylidene fluoride (PVDF) mixed matrix (MM), we also demonstrate that this chemical activation strategy is a safe way to activate thermally deformable MOF–polymer mixed matrices

Coordination-Chemistry Control of Proton Conductivity in the Iconic Metal–Organic Framework Material HKUST-1

HKUST-1, a metal–organic framework (MOF) material containing Cu^II-paddlewheel-type nodes and 1,3,5-benzenetricarboxylate struts, features accessible Cu^II sites to which solvent or other desired molecules can be intentionally coordinated. As part of a broader investigation of ionic conductivity in MOFs, we unexpectedly observed substantial proton conductivity with the “as synthesized” version of this material following sorption of methanol. Although HKUST-1 is neutral, coordinated water molecules are rendered sufficiently acidic by Cu^II to contribute protons to pore-filling methanol molecules and thereby enhance the alternating-current conductivity. At ambient temperature, the chemical identities of the node-coordinated and pore-filling molecules can be independently varied, thus enabling the proton conductivity to be reversibly modulated. The proton conductivity of HKUST-1 was observed to increase by ∼75-fold, for example, when node-coordinated acetonitrile molecules were replaced by water molecules. In contrast, the conductivity became almost immeasurably small when methanol was replaced by hexane as the pore-filling solvent