Photoinduced Electron Injection Research Articles

Electron Transfer Dynamics at Dye-Sensitized SnO2/TiO2 Core/Shell Electrodes in Aqueous/Nonaqueous Electrolyte Mixtures.

The dynamics of photoinduced electron transfer were measured at dye-sensitized photoanodes in aqueous (acetate buffer), nonaqueous (acetonitrile), and mixed solvent electrolytes by nanosecond transient absorption spectroscopy (TAS) and ultrafast optical-pump terahertz-probe spectroscopy (OPTP). Higher injection efficiencies were found in mixed solvent electrolytes for dye-sensitized SnO2/TiO2 core/shell electrodes, whereas the injection efficiency of dye-sensitized TiO2 electrodes decreased with the increasing acetonitrile concentration. The trend in injection efficiency for the TiO2 electrodes was consistent with the solvent-dependent trend in the semiconductor flat band potential. Photoinduced electron injection in core/shell electrodes has been understood as a two-step process involving ultrafast electron trapping in the TiO2 shell followed by slower electron transfer to the SnO2 core. The driving force for shell-to-core electron transfer increases as the flat band potential of TiO2 shifts negatively with increasing concentrations of acetonitrile. In acetonitrile-rich electrolytes, electron injection is suppressed due to the very negative flat band potential of the TiO2 shell. Interestingly, a net negative photoconductivity in the SnO2 core is observed in mixed solvent electrolytes by OPTP. We hypothesize that an electric field is formed across the TiO2 shell from the oxidized dye molecules after injection. Conduction band electrons in SnO2 are trapped at the core/shell interface by the electric field, resulting in a negative photoconductivity transient. The overall electron injection efficiency of the dye-sensitized SnO2/TiO2 core/shell photoanodes is optimized in mixed solvents. The ultrafast transient conductivity data illustrate the crucial role of the electrolyte in regulating the driving forces for electron injection and charge separation at dye-sensitized semiconductor interfaces.

Self-powered photoelectrochemical sensing for sensitive detection of chloramphenicol based on sulfur-vacancy engineered MoS2 nanoribbons/plasmonic Ti3C2 MXene with continual injection of photoinduced electrons

Accurate detection of antibiotic concentration in a matrix is vital for determining the contamination status. In this study, we have developed an ultrasensitive self-powered photoelectrochemical (PEC) sensor based on MoS2/Ti3C2 heterojunction, where sulfur vacancy engineered MoS2 nanoribbons were first grown in situ onto the plasmonic Ti3C2 MXene via a facile thermal treatment method. With the introduction of nonmetallic plasmonic Ti3C2, the constructed MoS2/Ti3C2 heterostructure facilitated the continuous photoelectron injection from MoS2 to Ti3C2 MXene, resulting in stable surface plasmon resonance (SPR) effect and enhanced photoelectric conversion. Compared to MoS2 alone, the PEC response of as-prepared MoS2/Ti3C2 heterojunction was increased by five times. Due to the excellent PEC properties of the MoS2/Ti3C2 heterostructure, a new ultrasensitive PEC aptasensor was fabricated with the assistance of chloramphenicol (CAP) aptamer. The developed aptasensor displayed satisfactory stability over a period of 11 days and high selectivity, a wide linear response range (0.1 nM to 0.01 mM) and a low detection limit (0.034 nM). Therefore, it can be successfully applied to CAP detection in environmental samples. This work provided a signal amplification strategy to fabricate efficient self-powered PEC sensing platforms.

Photoinduced Electron Transfer Modulated Photoelectric Signal: Toward an Organic Small Molecule-Based Photoelectrochemical Platform for Formaldehyde Detection.

Photoelectrochemical (PEC) sensing has been rapidly evolving in recent years, while the introduction of small molecules with specific recognition functions into the sensing interface remains a nascent area of study. In this work, we reported a PEC biosensor for formaldehyde (FA) detection based on photoinduced electron transfer (PET)-gated electron injection between organic small molecules and inorganic semiconducting substrates. Specifically, an FA-responsive probe (NA-FA-COOH) and TiO2 nanoarrays were integrated to construct a PEC platform (NFC/TiO2) via a coordination bond. NFC served simultaneously as a target-specific recognition element and a modulator of photoinduced electron injection. Treatment of NFC/TiO2 by FA would suppress the intramolecular PET process, with the quenched photocurrent signal due to the changed carrier transfer pathway, thus establishing the PEC platform for FA based on effective PET modulation. The proposed PEC system exhibited high selectivity and sensitivity, with a low detection limit of 0.071 μM. This study presents a novel perspective on the use of organic small molecules with a PET effect for advanced PEC bioanalysis.

Unraveling an Ultrafast Electron Transport Mechanism in a Photocatalytic "Micromachine" for Their Potential Light Harvesting Applications.

Following the seminal discovery of Richard Feynman, several micromachines have been made that are capable of several applications, such as solar energy harvesting, remediation of environmental pollution, etc. Here we have synthesized a nanohybrid combining TiO2 nanoparticle and light harvesting robust organic molecule RK1 (2-cyano-3-(4-(7-(5-(4-(diphenylamino)phenyl)-4-octylthiophen-2-yl)benzo[c][1,2,5] thiadiazol-4-yl)phenyl) acrylic acid) as a model micromachine having solar light harvesting ability potential for application in photocatalysis, preparation of solar active devices, etc. Detailed structural characterization, including High Resolution Transmission Electronic Microscopy (HRTEM) and Fourier-transform infrared spectroscopy (FTIR), has been performed on the nanohybrid. We have studied the excited-state ultrafast dynamics of the efficient push-pull dye RK1 in solution, on mesoporous semiconductor nanoparticles, and in insulator nanoparticles by streak camera (resolution of the order of 500 fs). The dynamics of such photosensitizers in polar solvents have been reported, and it has been observed that completely different dynamics occur when they are attached to the surface of the semiconductor/insulator nanosurface. A femtosecond-resolved fast electron transfer has been reported when photosensitizer RK1 has been attached to the surface of the semiconductor nanoparticle, which in turn plays a crucial role in the development of an efficient light harvesting material. The generation of reactive oxygen species as a result of femtosecond-resolved photoinduced electron injection in the aqueous medium is also investigated in order to explore the possibility of redox-active micromachines, which are found to be crucial for efficient and enhanced photocatalysis.

Optically Tunable Transient Plasmons in InSb Nanowires.

Optical carrier incubation can effectively alter the electron transport properties of semiconductors; thus, optical switching of the plasmonic response of the semiconductor enables the ultrafast manipulation of the light at the nanoscale. Semiconductor nanostructures are promising platforms in on-chip high-speed plasmonic devices, owing to their high photoinduced electron injection efficiency at sub-picosecond and compatibility with contemporary semiconductor technologies. The pure single crystalline InSb nanowires are promising plasmonic materials in the mid-infrared region due to their high electron mobility and small electron effective mass. Here, the pump-probe nanoscopy is utilized to investigate the pump fluence dependency and the dynamics of the non-equilibrium plasmons in the InSb nanowires. The InSb plasmon is successfully switched by injecting the photoinduced electrons and the practical tuning of the plasmon frequency to one octave is shown by increasing the pump fluence from 0 to 90µJcm-2 . The density of the photoinduced electrons in InSb nanowires is 18.8×1018 cm-3 with pump fluence as low as 90µJcm-2 . The high electron mobility of the InSb supports the low-loss plasmon with a damping rate of ≈200cm-1 . The InSb nanowires' excellent plasmonic properties ensure that they are a promising platform for upcoming high-speed mid-infrared plasmonic materials for informatic devices.

Photoelectrochemical C-H Activation Through a Quinacridone Dye Enabling Proton-Coupled Electron Transfer.

Dye-sensitized photoanodes for C-H activation in organic substrates are assembled by vacuum sublimation of a commercially available quinacridone (QNC) dye in the form of nanosized rods onto fluorine-doped tin oxide (FTO), TiO2 , and SnO2 slides. The photoanodes display extended absorption in the visible range (450-600 nm) and ultrafast photoinduced electron injection (<1 ps, as revealed by transient absorption spectroscopy) of the QNC dye into the semiconductor. The proton-coupled electron-transfer reactivity of QNC is exploited for generating a nitrogen-based radical as its oxidized form, which is competent in C-H bond activation. The key reactivity parameter is the bond-dissociation free energy (BDFE) associated with the N⋅/N-H couple in QNC of 80.5±2.3 kcal mol-1 , which enables hydrogen atom abstraction from allylic or benzylic C-H moieties. A photoelectrochemical response is indeed observed for organic substrates characterized by C-H bonds with BDFE below the 80.5 kcal mol-1 threshold, such as γ-terpinene, xanthene, or dihydroanthracene. This work provides a rational, mechanistically oriented route to the design of dye-sensitized photoelectrodes for selective organic transformations.

(Invited) Discovering Bottlenecks and Opportunities in Hybrid Solar Light Harvesting Systems By Ultrafast X-Ray Spectroscopy

A prerequisite for advancing hybrid solar light harvesting systems is a comprehensive understanding of photoinduced electronic dynamics across a wide range of spatial and temporal scales. Time-resolved spectroscopy techniques in the optical domain have proven instrumental in identifying critical timescales of fundamental processes, such as interfacial charge transfer, charge migration, and carrier lifetimes, which are intimately linked to a system's performance. New light source and instrument developments open opportunities to extend such studies into the X-ray regime. Due to their inherent elemental specificity and sensitivity to local electronic environments, time-domain X-ray spectroscopy techniques are uniquely suited to gain a microscopic picture of dynamics from well-defined reporter sites within often complex nanoscale light harvesting architectures. We will present new insights into fundamental photoinduced dynamics in several archetypical heterostructures based on femtosecond and picosecond time-resolved X-ray photoemission spectroscopy (TRXPS).The photoinduced transient charge redistribution in the model hybrid system of nanoporous zinc oxide (ZnO) sensitized by ruthenium bipyridyl chromophores is probed independently from the viewpoints of the molecular electron donor and the semiconductor acceptor.[1] Charge injection from the chromophore into the semiconductor is found to proceed via a transiently populated interfacial charge transfer (ICT) complex on an overall timescale of ~300 ps. Our results show that, even after release from the ICT states into the ZnO conduction band, the injected electrons remain localized within less than 6 nm from the interface, due to enhanced downward band-bending by the photo-injected charge carriers. This spatial confinement suggests that light-induced charge generation and transport in nanoscale ZnO photocatalytic devices proceeds predominantly within the defect-rich surface region, which may lead to enhanced surface recombination and explain their lower performance compared to titanium dioxide (TiO2)-based systems. The quantitative nature and surface sensitivity of TRXPS also provides direct access to the spatial separation of the holes remaining on the bipyridyl chromophores after charge injection from the semiconductor surface with sub-nm precision.Complementary femtosecond and picosecond TRXPS studies of photoinduced dynamics in copper-phthalocyanine(CuPc)-C60 heterojunctions provide a deeper understanding of both exciton migration pathways toward the interface as well as the absolute efficiency by which ICT states dissociate into separate charges.[2] The measurements reveal surprisingly efficient charge generation from triplet excitons and from the lowest lying ICT states, both of which were previously believed to be inactive during light-to-charge conversion.Photoinduced charge transfer dynamics between spherical gold nanoparticles (AuNPs) and a nanoporous film of TiO2 is monitored by picosecond TRXPS, providing direct access to the absolute photon-to-charge conversion efficiency and subsequent electron-hole recombination dynamics.[3] Preliminary results will be presented for the extension of the measurements into the ambient pressure regime, taking a first step toward the study of photoinduced interfacial chemistry by TRXPS in this model system for nanoplasmonic light harvesting applications.[1] S. Neppl et al., “Nanoscale Confinement of Photo-Injected Electrons at Hybrid Interfaces”, J. Phys. Chem. Lett. 12, 11951 (2021).[2] F. Roth et al., “Direct observation of charge separation in an organic light harvesting system by femtosecond time-resolved XPS”, Nat. Commun. 12, 1196 (2021).[3] M. Borgwardt et al., “Photoinduced Charge Carrier Dynamics and Electron Injection Efficiencies in Au Nanoparticle-Sensitized TiO2 Determined with Picosecond Time-Resolved X-ray Photoelectron Spectroscopy”, J. Phys. Chem. Lett. 11, 5476 (2020). Figure 1

Immersion solvent dependent photophysical and photovoltaic properties of Porphyrin/TiO2 interface

In this work, the role of immersion solvents on light harvesting efficiency, photoinduced electron injection and photovoltaic properties of porphyrin/TiO2 have been investigated. Time resolved fluorescence technique was used to characterize the kinetics of electron injection at the porphyrin/TiO2 interface. The results show that electron injection from photoexcited porphyrin to the conduction band of TiO2 is highly sensitive to the nature of the solvent used during self-assembly of the TiO2-porphyrin interface. Based on the findings, the t-BuOH:ACN solvent is suitable for achieving long lived electrons in a H2MP/TiO2 device.

Photoinduced Electron Injection in a Fully Solvated Dye-Sensitized Photoanode: A Dynamical Semiempirical Study

Dye-sensitized solar cells and dye-sensitized photoelectrochemical cells have attracted much interest in recent years for solar energy conversion. More effort is still required to increase the efficiency of these devices, which is closely linked to the crucial process of photoinduced charge separation. Computational studies can provide insights into this fundamental process and suggest molecular components and interfaces that feature optimal energy-level alignment before time-consuming trial-and-error experimental realization. Here, we use a combination of density functional based tight binding and an extended Huckel approach to perform quantum classical simulations of photoinduced electron injection in a TiO2 dye-sensitized photoanode with explicit solvation at a reasonable computational cost. In particular, we evaluate injection capabilities of core-extended naphthalene diimide (NDI) dyes with three different anchoring groups. Our results stress the importance of nuclear motion as well as conformational and trajectory sampling for a realistic description of the injection process. Furthermore, explicit solvation highly influences the conformational space explored by the dye and anchoring molecules, especially concerning the adsorption mode. Taking these effects into account, the core-extended NDI with a catechol-based anchoring moiety is shown to be the most promising ultrafast electron injector. Our strategy allows for a more systematic computational search for appropriate molecular chromophores in dye-sensitized devices for solar energy conversion.

Linear and nonlinear photophysical properties of ZnSe/CdS/ZnS core/shell/shell type II nanocrystals

In this work, one kind of type II ZnSe/CdS/ZnS core/shell/shell nanocrystals (NCs) is synthesized, and their linear and nonlinear photophysical properties are investigated. Through measurements of the temperature-dependent photoluminescence spectra of NCs, their excitonic properties, including the coefficient of the bandgap change, coupling strength of the exciton acoustic phonons, exciton longitudinal optical (LO) phonons, and LO–phonon energy are revealed. Femtosecond transient absorption spectroscopy was employed to obtain insight into ultrafast processes occurring at the interface of ZnSe and CdS, such as those involving the injection of photo-induced electrons into the CdS shell, interfacial state bleaching, and charge separation time. At the end, their multiphoton absorption spectra were determined by using the z-scan technique, which yielded a maximum two-photon absorption cross section of 3717 GM at 820 nm and three-photon absorption cross section up to 3.9 × 10 − 77 cm 6 · s 2 · photon − 2 at 1220 nm, respectively. The photophysical properties presented here may be important for exploiting their relevant applications in optoelectronic devices and deep-tissue bioimaging.

One-step solid state synthesis of facet-dependent contact TiO2 hollow nanocubes and reduced graphene oxide hybrids with 3D/2D heterojunctions for enhanced visible photocatalytic activity

One-step solid state calcination strategy via a template-engaged topotactic transformation process has been developed to construct serials of 3D TiO2 hollow nanocubes (TiO2-HNCs) with 2D reduced graphene oxide (rGO) heterojunction hybrids with facet-dependent contact for the first time. The effect of calcination temperature and TiOF2/GO mass ratio on the morphology and photocatalytic activity of TiO2-HNCs/rGO hybrids was systematically discussed. The study indicated that 3D TiO2 hollow nanocubes were assembled by six regular nanosheets and GO can effectively reduce the solid-state calcination temperature and facilitate the topological transformation from cubic TiOF2 to TiO2-HNCs through 3D/2D heterojunctions hybridized to TiO2-HNCs, which also provided a transferring photo-induced electrons bridge to sensitize TiO2-HNCs for enhancing visible light response. The infrared spectra (IR), X-ray photoelectron spectroscopy (XPS) and Raman spectroscopic studies revealed that rGO principally hybridized on the surface as sp2 graphitic carbon in extended p-π conjugated environments and a small percentage of rGO created carbonate structure by substituted surface Ti atoms in the TiO2-HNCs lattice. Due to the superior sensitization and charge carrier mobility of rGO, the photodegradation activity of Rhodamine B (RhB) of as-prepared T300/5rGO hybrid improved 4.6 times compared to P25 with a degradation yield of 99% within 90 min under simulated sunlight irradiation (>420 nm) and still remained over 95% after 5 recycles. The enhanced photoreactivity of TiO2-HNCs/rGO was mainly attributed to the strong hybridized interaction through chemical bonds between TiO2-HNCs and rGO, which formed 3D/2D facet-dependent contact heterojunctions and further verified by its good dispersibility and stability. The unique facet-dependent contacting rGO with {1 0 1} instead of {0 0 1} facets of TiO2-HNCs facilitated the injection and transfers of photo-induced electrons from {1 0 1} facet of TiO2-HNCs to rGO, thus retarding the recombination of electron-hole pairs, which was not only increasing the numbers of active sites, but also facilitating the adsorption of dye molecules, and improving light-harvesting ability, thereby enhancing visible photoreactivity of TiO2-HNCs/rGO hybrids.

Photocatalytic Water Splitting Cycle in a Dye-Catalyst Supramolecular Complex: Ab Initio Molecular Dynamics Simulations

A dye-sensitized photoelectrochemical cell (DS-PEC) is a promising device for direct conversion of solar energy into fuel. The basic idea, inspired by natural photosynthesis, is to couple the photoinduced charge separation process to catalytic water splitting. The photo-oxidized dye coupled to a water oxidation catalyst (WOC) should exert a thermodynamic driving force for the catalytic cycle, while water provides the electrons for regenerating the oxidized dye. These conditions impose specific energetic constraints on the molecular components of the photoanode in the DS-PEC. Here, we consider a supramolecular complex integrating a mononuclear Ru-based WOC with a fully organic naphthalene-diimide (NDI) dye that is able to perform fast photoinduced electron injection into the conduction band of the titanium-dioxide semiconductor anode. By means of constrained ab initio molecular dynamics simulations in explicit water solvent, it is shown that the oxidized NDI provides enough driving force for the whole photocatalytic water splitting cycle. The results provide strong evidence for the significant role of spin alignment and solvent rearrangement in facilitating the proton-coupled electron transfer processes. The predicted activation free energy barriers confirm that the O–O bond formation is the rate-limiting step. Our results expand the current understanding of the photocatalytic water oxidation mechanism and provide guidelines for the optimization of high-performance DS-PEC devices.

Electron injection in anthocyanidin and betalain dyes for dye-sensitized solar cells: a DFT approach

A theoretical investigation was carried out using density functional theory at the MN12SX/6-311+G(d,p) level of theory to calculate and compare the photoinduced electron injection for selected members of the anthocyanidin and betalain families. The rate constant of the injection for both families was calculated, and it was verified that the regeneration in the dye during the DSSC operation is caused by an energetic electron injection of the matched electrons in the dye. The present research shows that the rate constant of electron injection of the selected anthocyanidin family was very rapid: Aurantinidin has the overall best injection rate for both families. Aromaticity, the open-circuit voltage \({V}_{{\mathrm{oc}}}\), the \(\Delta {G}_{\mathrm{inject}}\) and \(\Delta {G}_{\mathrm{reg}}\) values for the selected molecule play a vital role in the comparison of both selected families. A prediction of the best dye based on the above properties was made for the maximum efficiency of the DSSC to be achieved.

A two-photon excited O2-evolving nanocomposite for efficient photodynamic therapy against hypoxic tumor

This paper reported on a two-photon excited nanocomposite FCRH to overcome tumor hypoxia for enhanced photodynamic therapy (PDT). Through modified by ruthenium (Ⅱ) complex (Ru(bpy)32+) and hyperbranched conjugated copolymer with poly (ethylene glycol) arms (HOP), the water-splitting mediated O2 generation can be triggered via two-photon irradiation from iron-doped carbon nitride (Fe-C3N4) for the first time. While exposured to two-photon laser, Ru(bpy)32+ was activated to generate singlet oxygen (1O2) and Fe-C3N4 was triggered to split water for oxygen supply in the mean time. Owing to the injection of photoinduced electrons from excited Ru(bpy)32+ to Fe-C3N4, O2 generation by Fe-C3N4 was significantly accelerated. After accumulation of the nanocomposite by enhanced permeability and retention (EPR) effect, FCRH was demonstrated to alleviate the tumorous hypoxia and consequently enhance the antitumor efficacy of PDT. Furthermore, tumor metabolism evaluations explained the capability of the nanocomposite in reducing intratumoral hypoxia. Our results provide a new diagram for ameliorating the hypoxic tumor microenvironment and accelerating 1O2 generation under two-photon excitation, which will find great potential for spatiotemporally controlled tumor treatment in vivo.

Electronic and photocatalytic properties of modified MoS2/graphene quantum dots heterostructures: A computational study

Solar energy absorption and photo-induced charge separation and transfer are crucial to enhance photocatalytic properties. Here, the structural, electronic and photocatalytic properties of graphene quantum dots (GQDs), pure and O, N, Sv-modified MoS2 monolayers, GQDs-based heterostructures have been studied by theoretical calculations based on density functional theory. Compared with the GQDs, the obvious red shifts of the absorption peaks can be observed, the absorption intensities increases evidently, and the typical type-II band alignments can be formed after the construction of the pure, O, N, Sv-MoS2/GQDs heterostructures, which is beneficial to promote photo-induced charge transfer and more visible light harvesting. Subsequently, it is worth mentioning that N-MoS2/GQDs heterostructure also has the lower binding energy and higher absorption in the infrared region and impactful photo-induced electron injection from GQDs to N-MoS2 surface. Therefore, this work provides an instrumental and promising approach in designing new GQDs-based heterostructures to enhance the solar energy absorption and conversion.

Graphene oxide enwrapped polyimide composites with efficient photocatalytic activity for 2,4-dichlorophenol degradation under visible light irradiation

In this study, graphene oxide (GO) enwrapped polyimide (PI) composite photocatalyst was prepared via a facile ultrasonic chemical method. The as-synthesized GO/PI hybrid composite can serve as a stable metal-free photocatalyst for 2,4-dichlorophenol (2,4-DCP) degradation under visible-light irradiation. After loading of GO, the GO/PI composites displayed enhanced photocatalytic performance. The apparent rate constant for 2,4-DCP degradation of the composites was approximately 4.5 times as high as that of bare PI. The results of radical trapping experiments and electron spin resonance indicated that the photogenerated hole (h+) and O2− were the main responsible for the 2,4-DCP degradation. The energy band diagram derived from Mott-Schottky and UV–vis diffuse absorption spectra revealed that the formed heterojunction between PI and GO is type-II. The formed heterojunction with well matched energy levels leads to an efficient injection of photoinduced electrons from PI to GO, and facilitates the separation of photoinduced charge carrier. The combined transient photocurrent and electrochemical impedance spectroscopy results further verified the enhancement was mainly ascribed to the improved charge carrier separation performance of composite.

Effect of the second chromophore energy gap on photo-induced electron injection in di-chromophoric porphyrin-sensitized solar cells.

This work investigates the effect of the second chromophore energy gap on charge generation in porphyrin-based di-chromophoric dye-sensitized solar cells (DSSCs). Three di-chromophoric porphyrin dyes (PorY, PorO and PorR) containing three organic chromophores with decreasing frontier orbital energy offsets, including a carbazole-triphenylamine chromophore (yellow, Y), a carbazole fused-thiophene chromophore (orange, O) or a carbazole-thiophene benzothiadiazole thiophene chromophore (red, R), were investigated using optical and electrochemical methods, steady-state photoluminescence and photovoltaic device characterization. Energy transfer from the organic chromophore to the porphyrin was suggested in PorY and PorO as the main charge generation mechanism in DSSCs using these di-chromophoric dyes. On the other hand, electron transfer from the photo-excited porphyrin to the organic chromophore as a competing pathway leading to the loss of photocurrent is suggested for PorR-sensitized solar cells. The latter pathway leading to a loss of photocurrent is due to the lower lying lowest unoccupied molecular orbital of the additional organic chromophore (R) and suggests the limitation of the current di-chromophoric approach to increase the overall efficiency of DSSCs.

In Situ Shape Change of Au Nanoparticles on TiO2 by CdS Photodeposition: Its Near-Field Enhancement Effect on Photoinduced Electron Injection from CdS to TiO2.

Hemisphere-like gold nanoparticles (NPs) were loaded on TiO2 (Au/TiO2) by the deposition–precipitation method. Subsequent photodeposition of CdS on the Au surface of Au/TiO2 at 50 °C yields Au(core)–CdS(shell) hybrid quantum dots with a heteroepitaxial (HEPI) junction on TiO2 (Au@#CdS/TiO2), whereas nonHEPI Au@CdS/TiO2 was formed by CdS photodeposition at 25 °C. In the HEPI system, the shape of the Au core changes to an angular shape, whereas it remains in a hemisphere-like shape in the nonHEPI system. The hot photodeposition technique was applied to the Au NP-loaded mesoporous TiO2 nanocrystalline film (Au/mp-TiO2). Using Au@CdS/mp-TiO2 and Au@#CdS/mp-TiO2 as the photoanodes, two-electrode quantum dot-sensitized photoelectrochemical cells were fabricated for hydrogen (H2) generation from water, and the performances of the cells were evaluated under illumination of simulated sunlight. In the photocurrent and the rate of H2 evolution, the Au@#CdS/mp-TiO2 photoanode cell surpasses the CdS/mp-TiO2 and Au@CdS/mp-TiO2 ones. Three-dimensional finite-difference time-domain calculations for the model systems indicated that the angular shape Au core generates an intense electric field at the corners and edges, extending the electric field distribution over the Au core–CdS shell interface. The striking shape effect on the cell performances can originate from the promotion of the CdS excitation and charge separation due to the near-field enhancement by the deformed Au core.

Tuning the BODIPY core for its potential use in DSSC: a quantum chemical approach

Boron dipyrromethene (BODIPY) is a highly promising candidate for use in dye-sensitized solar cell (DSSC), because of its attractive absorption characteristics such as strong extinction coefficients in the visible and near-IR ranges (70000–80000 $$\hbox {M}^{-1}$$ $$\hbox {cm}^{-1})$$ , large quantum yields, longer excited-state lifetime and also high solubility in many organic solvents. Moreover, the absorption peaks can be shifted towards longer wavelengths when functionalized at suitable positions of the BODIPY core. Herein, on the basis of density functional theory (DFT) and time-dependent DFT, we provide the opto-electronic properties of BODIPY core-functionalized dyes to see their applicability in organic DSSC. Our systematic analyses reveal that the 2,6 substituted dyes show better photovoltaic properties compared to the 3,5 substituted ones. On the basis of empirical relationship, we have also calculated the photo-induced electron injection times of some dye- $$\hbox {TiO}_{2}$$ composites, which seem to be in the ultrafast time scale, thus rendering them a promising candidate for DSSC applications. Our theoretical studies provide that judiciously designed BODIPY core-derived dyes show certain unique spectroscopic and electronic features that make them highly advantageous in DSSC applications as compared to other organic dyes.

Rational tailoring of C275 towards promising organic dyes: Enhancing light absorption and charge separation

Designed with electron-donating and electron-withdrawing moieties as donors and acceptors, respectively, a series of novel organic dyes for dye-sensitized solar cells (DSCs) are simulated using density functional theory (DFT) and time-dependent DFT (TD-DFT) methods The interacting dye/TiO2 systems are also studied to further explore the interfacial injection process of photo-excited electron. The model dye C275 is featured with donor/acceptor (D/A) molecular architecture, which is primary to achieve the easy charge-transfer excitation and suitable energy levels alignment. To date, considerable efforts are devoted to the design and synthesis of D/A type organic dyes, and the initial configurations are typically comprised of N-annulated perylene donor, porphyrin linker, and carboxylic acid acceptor. Our results show that, the electronic levels and optical absorption properties can be tuned gradually by introducing auxiliary donor units or electron-deficient spacer in acceptor groups. Moreover, the designed dyes also exhibit good performance in terms of photoinduced charge transfer, electron injection, and dye regeneration, etc. Through structural modification of donor moieties, the new tailored dyes provide appropriate energy levels, strong light-harvesting and easy charge separation. Meanwhile, the distinct red-shift of optical absorption and large amount of transferred charges are also obtained by structural tailoring of acceptor groups, whereas the large values of injection time may be harmful for the interfacial photoelectron injection. Therefore, comparing with donor modification, it should be more carefully to choose appropriate acceptor group with the balance of various energetic and kinetic factors for the rational design of high-efficient organic dyes.