Conduction Band Of TiO2 Research Articles

Visible-light photocatalytic degradation of dyes by TiO[formula omitted]–Au inverse opal films synthesized by Atomic Layer Deposition

TiO2–Au composite planar films and inverse opals were synthesized by Atomic Layer Deposition (ALD) using Direct Liquid Injection for TiO2 deposition and for the injection of preformed Au nanoparticles (NPs). A few nanometers thick layer of TiO2 was deposited after the Au NPs. Thermal annealing at 380 °C allowed to remove the polystyrene beads template and to form anatase TiO2.The obtained inverse opals have a layered structure of hollow TiO2 spheres interconnected by channels formed at the template beads contacts. Au NPs are homogeneously distributed on the surface. Their localized surface plasmon resonance (SPR) at 587 nm confirm that they are embedded in TiO2. These photocatalysts were tested for the degradation of methylene blue solutions under visible-light illumination. A significant photocatalytic effect is reported with a 95% degradation after 7 h of exposure only for the composite inverse opal. This results from the transfer of hot electrons from the NPs excited at their SPR to the conduction band of TiO2. They generate reactive oxygen species at the surface of TiO2 which are involved in the initial stage of MB degradation. These results highlight the potential of ALD for the fabrication of complex composite structures for application in photocatalysis.

Carbon-modified TiO2 nanourchin with Ag nanoparticle decoration for environmental remediation

Multifunctional critical nano-composite material assembling Carbon-modified Ag@TiO2 nanourchin was synthesized to investigate visible light photocatalytic activity. The Carbon-modified Ag@TiO2 nanourchin was prepared by a facile two-step chemical synthesis through solvothermal and chemical reduction. Carbon nanospheres as the substrate play a key role for three dimensional nanourchin structure as well as carbon doping. The morphology, crystal structure, and chemical states were characterized carefully by different analytical procedures via transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDAX), X-ray diffraction (XRD), RAMAN spectroscopy, X-ray photoelectron spectroscopy (XPS) and UV–Vis spectroscopy. It shows that the uniform loaded silver (Ag) on TiO2 surface was metallic Ag0 state. The exist of interstitial carbon and Ag NPs on TiO2 nanostructure collectively enhance the visible light photocatalytic activity. The visible light photocatalytic degradation of methyl orange (MO) studies revealed that Ag@C0.1TiO2 exhibited enhanced photocatalytic efficiency compared with the pure TiO2 nanourchin and commercial TiO2 powder. The improved photocatalytic property can be attributed to lower recombination of the photogenerated e− and h+ pairs in catalyst due to the formation of a Schottky barrier between Ag NPs and TiO2 which improved the surface charge separation greatly and presence of carbon serve as a sensitizer to absorb visible light which inject e- into conduction band (CB) of TiO2.

Investigation on the effect of neodymium ion doping in TiO2 on the photovoltaic performance of dye-sensitized solar cells

Titanium dioxide (TiO2), neodymium (Nd3+) doped TiO2 with the doping levels of 0.3% and 0.5% were synthesised by hydrothermal method and were characterized by powder X-ray diffraction (XRD), UV–vis diffused reflectance spectroscopy (DRS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Ultraviolet X-ray photoelectron spectroscopy (UPS) and high resolution transmission electron microscopy (HRTEM). Doping of Nd3+ ions have been created new surface energy states below the conduction band of TiO2 which mediate the photoinjected electrons through TiO2 network to external load without any impediment. The Fermilevel (Ef) of TiO2 was shifted to downward direction upon Nd3+ ion doping confirmed from UPS analysis. An enhanced photoconversion efficiency (PCE) of 10.7% was achieved for the device fabricated using 0.3% Nd3+ ion doped TiO2 which is higher than that of the device with undoped TiO2 (10.1%). The noticeable increment in the PCE was due to the reduced charge recombination, longer electron lifetime and improved charge injection form LUMO of the N719 dye to the CB of TiO2.

MIL-68 (In)-derived In2O3@TiO2 S-scheme heterojunction with hierarchical hollow structure for selective photoconversion of CO2 to hydrocarbon fuels

The hierarchical hollow structured photocatalysts of TiO2 nanosheet arrays-decorated In2O3 hexagonal nanotubes (core–shell structural In2O3@TiO2) were successfully fabricated by the in-situ hydrothermal method. Hierarchical hollow structure of In2O3@TiO2 photocatalysts with large specific surface area can improve the light harvesting efficiency and the adsorption capability for CO2 and H2O reactants. The internal electric field in In2O3@TiO2 photocatalysts provides the strong driving force for S-scheme electron transfer from conduction band of TiO2 to valence band of In2O3. The In2O3@TiO2 catalysts exhibited super performance for selectively photocatalytic CO2 conversion with H2O into hydrocarbon fuel (CH4) without sacrificial agent. Among the catalysts, In2O3@TiO2-10 catalyst has the largest yield (11.1 μmol g−1h−1) and selectivity (88.1 %) of CH4 product. Based on results of the characterizations, the mechanism of selective CO2 photoreduction is proposed: the surface enrichment of photogenerated electron induced by the S-scheme In2O3-TiO2 heterojunction is key-step for selective CO2 photoreduction into CH4. This work provides new ideas for the design and construction of high-efficient S-scheme photocatalysts.

Electron transfer from a new chalcone dye to TiO2 nanoparticles: Synthesis, photophysics, and excited-state dynamics

Efficient electron injection from photosensitizers to semiconductor nanoparticles is essential in many applications such as solar energy harvesting and catalytic reactions. Herein, we present a promising photosensitizer, 4-dimethylamino-2′-hydroxy-4′-carboxychalcone (DHC-COOH), that upon adsorption on TiO2 nanoparticles shows enhanced electron transfer from its excited state to the conduction band (CB) of TiO2 via an intramolecular charge transfer (ICT). In order to fully characterize the current system, three more derivatives were synthesized and characterized, its analogue without the COOH group (DHC), and the molecules without the OH group (DC and DC-COOH). The presence of OH causes a red shift in the absorption and fluorescence spectra, whereas the COOH group induces more red shift due to efficient ICT which is also seen in the density functional theory calculations. Excited-state intramolecular proton transfer is evident in the crystalline/solid form of DHC and DHC-COOH as a large red shift in the fluorescence peak. The red shift was not observed when DHC-COOH was adsorbed on the TiO2 surface due to the involvement of the OH group in binding. This was confirmed by XPS, in addition to a bidentate binding of the COOH group to TiO2. The latter has a major contribution to the electron transfer mechanism which was observed as a much weaker fluorescence intensity and a 50% reduction in the lifetime component that involves the COOH dynamic (from 63 ps to 32 ps). On the other hand, electron-hole recombination dynamics were slowed down on the TiO2 surface (from 311 ps to 440 ps and 1.4 ns to 2.4 ns). From cyclic voltammetry and steady state spectra, it was found that electron injection from the excited state of the dye to the CB of TiO2 is energetically favorable, and regeneration of the oxidized dye by the I−/I3− redox pair (used in dye-sensitized solar cells) is also possible.

Designing of PC31BM‐based acceptors for dye‐sensitized solar cell

AbstractThree push‐pull dyes with triphenylamine as donor, thiophene as spacer and fullerene‐based acceptors have been designed using density functional theory (DFT) for dye‐sensitized solar cells (DSSCs). Here, we have done a systematic case study by varying the acceptor of dye. PCBM type of acceptors are chosen. However, the main focus lies on the fact that fullerene C60, which is used as conventional fullerene in PCBM, is replaced by C30. Further, two more structures are derived, where C30 is changed by two of its doped counterparts, C10B10N10 and C10B10P10. Structural integrity of these dyes is checked and found to be stable. Their electronic properties also provide a good insight into their characteristics. Highest occupied molecular orbital (HOMO) energy level of the dyes is positioned under the redox potential of redox couple, , which means that the dyes can be regenerated. LUMO energy value of dyes with C30 and C10B10N10 lies above the conduction band of semiconductor, TiO2, and hence electrons from the excited state of the dyes can be injected into the conduction band edge of TiO2. For C10B10P10, lowest unoccupied molecular orbital (LUMO) lies slightly below the conduction band of TiO2 (for B3LYP functional). Therefore, few more configurations of C10B10P10 have been checked by replacing B and P atoms with C atoms. Notably, incorporation of solvent rules out this exception and all the dyes become eligible for electron regeneration also. Time‐dependent DFT (TDDFT) studies throw light on their photochemical properties, which is one of the interim criteria for any system getting selected as a dye in the DSSCs.

Influence of the pH of anthocyanins on the efficiency of dye sensitized solar cells

The influence of the pH of anthocyanins on photovoltaic performance in dye-sensitized solar cells has been investigated. Anthocyanins were extracted from crushed leaf stocks of Manihot esculenta Crantz (Cassava plant) using methanol acidified with 0.5% trifluoracetic acid. The filtrate was concentrated using a rotary evaporator and partitioned against ethyl acetate. The anode was prepared by screen printing TiO2 paste on a previously cleaned fluorine-doped tin oxide (FTO) glass substrate. The cathode was made by applying plastisol on a previously cleaned FTO glass substrate using an artistic brush and later annealed at 450 °C for 20 min to activate platinum. The performance of the solar cells was measured using a solar simulator fitted with an AM1.5 air filter. Electron transport was studied using electrochemical impedance spectroscopy (EIS). It was observed that the short circuit current and efficiency dropped from pH 2 to pH 6 and peaked at pH 8, with values of 0.399 mA and 0.390%, respectively. It then drops further as the basicity increases. The open circuit voltage was observed to increase consistently from pH 2 to pH 12. EIS results showed that the electron density in the conduction band of TiO2 increases from pH 2 to pH 10 and drops from pH 10 to pH 12. It was concluded that, while a large number of electrons (∼1016m−3) are injected into the conduction band of TiO2, the majority do not contribute to the current but instead recombine with other electron acceptor species in the solar cell. However, the injected electrons cause an upwards shift in the quasi-Fermi level of electrons in the conduction band of TiO2. This explains the large variation in the open circuit voltage compared to the short circuit current.

New insights into the deterioration of TiO2 based oil paints: the effects of illumination conditions and surface interactions

Titanium dioxide (TiO2) has been used in numerous paintings since its creation in the early 1920s. However, due to this relatively recent adoption by the art world, we have limited knowledge about the nature and risk of degradation in museum environments. This study expands on the existing understanding of TiO2 facilitated degradation of linseed oil, by examining the effect of visible light and crystallographic phase (either anatase or rutile) on the reactivity of TiO2. The present approach is based on a combination of experimental chemical characterization with computational calculation through Density Functional Theory (DFT) modeling of the TiO2-oil system. Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FT-IR) enabled the identification of characteristic degradation products during UV and visible light aging of both rutile and anatase based paints in comparison to BaSO4 and linseed oil controls. In addition, cratering and cracking of the paint surface in TiO2 based paints, aged under visible and UV–vis illumination, were observed through Scanning Electron Microscopy (SEM). Finally, Density Functional Theory (DFT) modeling of interactions between anatase TiO2 and oleic acid, a fatty acid component of linseed oil, to form a charge transfer complex explains one possible mechanism for the visible light activity observed in artificial aging. Visible light excitation of this complex sensitizes TiO2 by injecting an electron into the conduction band of TiO2 to generate reactive oxygen species and subsequent degradation of the oil binder by various mechanisms (e.g., formation of an oleic acid cation radical and other oxidation products).Graphical

Efficient electron extraction by CoS2 loaded onto anatase TiO2 for improved photocatalytic hydrogen evolution

Titanium dioxide (TiO2) as a benchmark photocatalyst has been attracting attention due to its photocatalytic activity combined with photochemical stability. In particular, TiO2 with anatase polymorph holds promise for driving reduction reactions, such as proton reduction to evolve H2 via photocatalysis. In this study, anatase TiO2 is loaded with CoS2 through the hydrothermal route to form a CoS2@TiO2 photocatalyst system. X-ray absorption near edge structure confirms the +2-oxidation state of the Co cation, while extended x-ray absorption fine structure shows that each Co2+ cation is primarily coordinated to six S− anions forming a CoS2-like species. A small fraction of the Co2+ species is also coordinated to O2− anions forming Co x O y species and substitutionally resides at the Ti4+-sites. Further investigations with steady-state IR absorption induced by UV-light and time-resolved microwave conductivity suggest an efficient electron transfer from the conduction band of TiO2 to the surface-loaded CoS2 which acts as a metallic material with no bandgap. The CoS2 shallowly traps electrons at the host surface and facilitates proton reduction. An appreciably enhanced H2 evolution rate (8 times) is recognised upon the CoS2 loading. The CoS2 is here proposed to function as a proton reduction cocatalyst, which can potentially be an alternative to noble metals.

Efficiency and stability improvement of natural dye‐sensitized solar cells using the electrospun composite of TiO 2 nanofibres doped by the bio‐Ca nanoparticles

The dye-sensitized solar cells (DSSCs) can be effectively improved and stabilized by outstanding electrical and morphological characteristics of TiO2 nanofibres combined with bio-calcium doping. The pristine and bio-Ca-doped TiO2 nanofibres were fabricated using a cost-effective electrospinning technique. Biocompatible calcium carbonate nanoparticles (bio-Ca) were synthesized from the cuttlebone of Sepia Pharaonis. Moreover, a facile one-step procedure was employed to fabricate efficient TiO2 nanofibres-based DSSCs using a Pechini-type sol. This approach produced a highly porous dense film of TiO2 upon sintering without the need for the hot-pressing or adhesion layer steps. Based on the results, the DSSCs fabricated by the bio-Ca-doped TiO2 nanofibres showed the highest I sc $$ {I}_{sc} $$ , V oc $$ {V}_{oc} $$ , and η $$ \eta $$ of 2.19 mA, 0.41 V, and 1.48% respectively. This superiority could be due to the higher specific surface area and the relatively smaller average diameter observed for bio-Ca-doped TiO2 nanofibres, which improved dye-loading and guided electron transport respectively. In addition, Ca2+ doping significantly suppressed the photocatalytic activity in the bio-Ca-doped TiO2 nanofibres owing to the formation of the TiO2 rutile-anatase combined phase. Besides, the substitution of Ti4+ with Ca2+ positively affects the conduction band of TiO2 and causes trap sites that retard the charge recombination. Our results also demonstrated that the bio-Ca-doped TiO2 nanofibres-based DSSC maintained about 78.38% of its initial efficiency after two weeks, while DSSCs fabricated by the TiO2 nanofibres and TiO2 nanoparticles retained 63.71% and 27.38% respectively. The superior stability could be due to the combined effect of nanoparticles into nanofibres transformation and bio-Ca doping.

Improving plasmonic photodetector based on ITO/TiO2/Ag configuration and roughened interface

In this work, plasmonic photodetector based on ITO/TiO2/Ag grating configuration specially with nano-scale roughened interface is proposed to achieve high photoresponsivity, in which hot-electron are effectively excited by taking advantages of the narrow distribution of excitation energy in Ag and the high density of states in TiO2 conduction band. Simultaneously, a roughened structure is introduced at Ag/TiO2 interface to alleviate the momentum mismatch of hot-electron. The influences of the grating structures on incident light absorption and hot-electron excitation are simulated by using finite-difference time-domain method. The thicknesses of both metallic Ag and spacer layers modulate the absorption characteristics, and the introduction of the nano-scale roughened interface does not have effect obviously on the absorptivity and response wavelength. The present work provides a valuable guidance in the fields of interface design and structure optimization of high-efficiency plasmonic photodetectors.

Plasmon assisted high ultraviolet to visible broad band photosensitivity in lateral Ag NPs-TiO2 nanocomposite film

Noble metal-TiO2 composite films attract great attention because of the extension of its spectral absorption in the visible range due to surface plasmon resonance. Among them, Ag-TiO2 composite is considered to be an ideal candidate for the visible light photosensitivity because of narrow energy distribution of plasmonic hot electrons in Ag and the high density of states in the TiO2 conduction band. Here, we report high broad band photosensitivity from ultraviolet (UV) to visible wavelength of sol-gel grown TiO2 film decorated with Ag nanoparticles (NPs) in a simple planar photoconductive geometry. In addition to UV light, Ag NPs-TiO2 composite film (for 1 nm Ag) shows very high photosensitivity values of ∼105 and ∼104 and responsivity values of 0.1 A/W and 7×10−3 A/W at 10 V bias for 680 nm and 550 nm illuminations respectively while pristine TiO2 show photosensitivity values of 6×102 and 3.3×102 and responsivity values of 2.4×10−4 A/W and 6.6×10−5 A/W for both the visible illuminations. A highly enhanced broad band photosensitivity due to transfer of plasmonically generated hot electrons from Ag to TiO2 is quite interesting without having any device fabricated. The photosensitivity values for Ag NPs for 3 nm and 6 nm layer thicknesses on TiO2 decrease because of coalescence of metal NPs on surface which acts as scattering center and/or light shielding.

Hydrothermal synthesis of novel heterostructured Ag/TiO2 /CuFe2 O4 nanocomposite: Characterization, enhanced photocatalytic degradation of methylene blue dye, and efficient antibacterial studies.

In this work, we reported the successful synthesis of novel Ag/TiO2 /CuFe2 O4 ternary nanocomposite by hydrothermal technique by using TiO2 /CuFe2 O4 binary nanocomposite precursor that was also prepared by hydrothermal treatment by using TiO2 nanoparticles and CuFe2 O4 nanoparticles synthesized via sol-gel method. The synthesized nanomaterials were accessed for their morphological, structural, and optical properties. X-ray diffraction (XRD) study reveals the formation of pure Ag/TiO2 /CuFe2 O4 ternary nanocomposite in which the Ag, TiO2 , and CuFe2 O4 are in anatase, spinal, and cubic crystal phases, respectively. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) analyses of Ag/TiO2 /CuFe2 O4 ternary nanocomposite indicated granule-shaped morphology with bright spots of silver. The existence of Ti, O, Cu, Fe, and Ag without any other elements in the energy-dispersive X-ray spectroscopy (EDS) spectra of the prepared ternary nanocomposite depict its purity and its polycrystalline nature was confirmed by its selected area electron diffraction (SAED) pattern. The ternary nanocomposite was utilized for the methylene blue dye degradation with an optimum dose of 1.00 g/100 ml under ultraviolet (UV) light; the enhanced photocatalytic activity of the composite is attributed mainly due to the appreciable magnitudinal difference of positive charge of the valence band and negative charge of the conduction band of TiO2 and CuFe2 O4 ; meanwhile, the interfacially placed Ag acts as a sink for the elections. Also, the ternary nanocomposite showed satisfactory antibacterial activities. PRACTITIONER POINTS: The prepared ternary nanocomposite showed effective results in dye degradation and satisfactory antibacterial property. The concentration of methylene dye has decreased considerably in every degradation process which was accessed through UV-vis studies. The highest degradation by using the ternary nanocomposite archived at pH = 6 Appreciable antibacterial activity was achieved against a few Gram-positive strains and Gram-negative strains of bacteria. This research activity can open a broad area of research towards textile dye degradation and antibacterial studies.

Seawater-based electrolytes facilitate charge transfer mechanisms improving the efficiency of dye-sensitized solar cells

In this work, dye-sensitized solar cells containing seawater-based electrolytes were realized and investigated. The influence of the seawater composition on the electrochemical properties of the iodide/triiodide redox mediator was determined. High triiodide diffusion coefficient and ionic conductivity were assessed for seawater electrolytes through cyclic and linear voltammetry and impedance spectroscopy. Moreover, a notable influence of the seawater electrolyte on the charge transfer mechanism at the photoanode/dye/electrolyte interfaces was observed and deeply discussed. The ions, naturally present into seawater, reduce the charge recombination mechanism at the photoanode/electrolyte interface and promote a downward shift of the TiO2 conduction band, thus increasing the final DSSC efficiencies of 23% if compared with traditional devices containing water. The best seawater-based solar cell provides a photo-electrical conversion efficiency equal to 0.37% with 1.09 mA cm−2 as short circuit current density. To the best of our knowledge, this is the first time that seawater is used as a key component for an energy production technology and the obtained results show the great potentiality of this green and recyclable element.

Insight into Interfacial Charge Transfer during Photocatalytic H2 Evolution through Fe, Ni, Cu and Au Embedded in a Mesoporous TiO2@SiO2 Core‐shell

AbstractThe efficiency of charge transfer at the interface of transition metals loaded on TiO2 as well as the active reduction site undoubtedly define the kinetics of H2 production. Here, we report the interfacial charge process of various transition metals (Fe, Ni, Cu, and Au) embedded in mesoporous core‐shell nanostructures, which provides further insight into the photocatalytic H2 evolution. Au was found to be the most active cocatalyst for the H2 evolution. Au nanoparticles (NPs) collect electrons and become a very efficient reduction site, promoting both charge separation and H2 evolution. The scavenging capacity of Au was found to be the highest compared to other NPs. Cu(I), which is as active as Au, but at a higher loading, also shows an ability for electron scavenging and acts as an active site under UV excitation, or rather injects electrons into the TiO2 conduction band under visible excitation. Ni(II) NPs were found to be involved in charge‐carrier separation through hole collection and their contribution to hydrogen evolution is expected to be limited to visible absorption and electron injection. The same behavior occurs with Fe(III), however, it is less efficient for H2 generation due to the weak charge‐carrier lifetime. We show that only Cu(I) could compete with noble metals such as Au, presenting electron‐scavenging behavior. Understanding the charge‐carrier dynamics will help design a very efficient system for photocatalytic H2 generation system.

New D−π–D−π–A Systems Based on Phenothiazine Derivatives with Imidazole Structures for Photovoltaics

Two new compounds with an N-octyl phenothiazine core substituted at C(7) position via a triple bond with imidazole-containing butyl or dodecyl chains bearing on the opposite side at the C(7) cyanoacrylate moiety were synthesized. Photophysical and electrochemical studies were performed to evaluate the possibility of applying them in the dye-sensitized solar cells (DSSCs). The investigations were supported by density functional theory and time-dependent DFT. The synthesized dyes showed a similar absorption profile ranging from 250 to 600 nm in solvents and a high value of light-harvesting efficiency of about 0.8. It was found that its photoluminescence quantum yield was solvent-dependent, and the highest value (about 50%) was reached in DMF solution. Electrochemical measurements revealed that both compounds showed electron injection ability from the LUMO of the molecule to the TiO2 conduction band and the possibility of regeneration by an electrolyte. DSSCs sensitized with the phenothiazine derivatives differing in TiO2 thickness and the type of electrolyte (iodide and cobalt) were prepared. Additionally, transient absorption measurements in solution and solar cells were carried out to study excited-state properties and electron transfer dynamics. The influence of the hydrophobic hydrocarbon chain length of the imidazole unit in these dyes as well as TiO2 thickness and the type of electrolyte on the device performance was observed. DSSCs sensitized with phenothiazine with dodecyl chains showed higher photocurrent, open-circuit voltage, and finally the power conversion efficiency, regardless of TiO2 thickness.

A photoelectrochemical sensor combining CS−GSH−CuNCs and xanthine oxidase for the detection of xanthine

AbstractA photoelectrochemical (PEC) sensor combining chitosan‐coated and glutathione‐protected copper nanocluster (CS−GSH−CuNCs) with peroxidase‐like activity and xanthine oxidase (XAO) was developed for rapid detection of xanthine. Firstly, CS−GSH−CuNCs was prepared by glutathione (GSH) reduction using chitosan (CS) as protective agent, and then was modified on titanium dioxide nanoparticles (TiO2 NPs)/ITO electrode which was prepared by sintering titanium dioxide (TiO2) on the ITO electrode surface by high temperature calcination. After excited by light at 350 nm, the photoinduced electrons from the lowest unoccupied molecular orbital (LUMO) of CS−GSH−CuNCs were transferred to the conduction band of TiO2, which resulted in spatial separation of electron‐hole and enhancement of photocurrent signal. XAO oxidizes xanthine to produce hydrogen peroxide (H2O2). Part of the photoinduced electrons from CS−GSH−CuNCs transferred and catalyzed the reduction of H2O2, resulting in the reduction of the photocurrent. Under optimal conditions, the PEC sensor exhibits good sensitivity and reproducibility with the limit of detection (LOD) of 6.6 nmol ⋅ L−1 and a relative standard deviation of 3.5 % for 10 replicate detections of 1.00 μmol ⋅ L−1 xanthine and the linear range of 0.04–90.0 μmol ⋅ L−1 for xanthine. Furthermore, the PEC sensor presents nice selectivity owing to its enzyme‐like activity and was successfully applied to human urine, indicating its great potential for real sample analysis.

Dysprosium doping in CdTe@CdS type II core/shell and cosensitizing with CdSe for photocurrent and efficiency enhancement in quantum dot sensitized solar cells

Quantum dot sensitized solar cells (QDSSCs) based on the II-VI QDs have become a promising and attractive technology due to their unique properties. However, the applications of the II-VI QDs, especially CdTe QDs, have still been restricted. Herein, a CdTe@Dy-doped CdS core/shell QDs cosensitized with CdSe QDs have been developed as a sensitizer in QDSSCs for enhancing the light-harvesting range and efficiency, accelerating charge separation, and suppressing charge recombination. The fabricated QDSSCs using TiO2/CdTe@Dy-doped-CdS/CdSe/ZnS photoanode exhibited the best short-circuit current (JSC = 20.08 mA/cm2) and power conversion efficiency (ƞ=8.26%) in comparison with that of other photoanodes comprising TiO2/CdTe, TiO2/CdTe@CdS, TiO2/CdTe@CdS/ZnS, TiO2/CdTe@CdS/CdSe/ZnS, and TiO2/CdTe@CdS/CdSe/Dy-Doped ZnS. As the obtained results from Diffuse Reflectance Spectroscopy, Photoluminescence, and Electrochemical Impedance Spectroscopy revealed, the CdTe/CdS type-II core/shell structure QDs leads to a wide absorption range, fast electron injection, and slower recombination rate because of the spatially indirect energy gap formation. Moreover, the presence of Dy cations in CdS crystal lattice boosts the charge transferring rate to the conduction band of TiO2. On the other hand, the cosensitizing of CdTe@Dy-doped CdS QDs with CdSe is advantageous not only to the electron injection and transferring but also to the hole-recovery from the valence band of QDs toward the electrolyte.

An energy level alignment strategy to boost the open-circuit voltage via a Mg:TiO2 compact layer in the planar heterojunction CsPbBr3 solar cells

The suitable energy level alignment between the electron transport layer (ETL) and perovskite absorber for facilitating electron extraction and transfer is essential for planar heterojunction perovskite solar cells (PSCs) to achieve high power conversion efficiency (PCE). The planar heterojunction CsPbBr3 PSCs have attracted increasing attention due to their high stability in ambient in which TiO2 is often used as the ETL. Actually, there is a large energy barrier between the conduction bands of TiO2 and CsPbBr3, which limits the open-circuit voltage (VOC) and PCE of PSCs. Herein, we effectively optimized the energy level alignment between CsPbBr3 and TiO2 by tuning the Mg incorporation level in the latter. The Fermi level of TiO2 ETL was gradually shifted upward using Mg incorporation, which led to a favorable band bending between TiO2 and CsPbBr3 and reduced the interfacial band offset. Consequently, the VOC of the devices was effectively increased from 1.36 to 1.49 V, and the corresponding PCE was also increased from 6.87% to 8.76%. This work highlights the importance of the energy level alignment between ETLs and perovskite absorbers for high-performance devices and offers an effective means to address the large energy barrier between the conduction bands of TiO2 and CsPbBr3.

Density Functional Theory Study and Photocatalytic Activity of ZnO/N-Doped TiO2 Heterojunctions

The beneficial effects on ZnO and TiO2 photocatalytic activity resulting from a suitable combination of doping and heterojunctions were proposed and investigated by a synergistic experimental and theoretical study. In detail, the ZnO/N-doped TiO2 heterojunction was synthesized and tested in the photocatalytic degradation of atrazine under UV and visible light. Wide-angle X-ray diffraction (WAXD) analysis reveals the presence of both ZnO and TiO2 crystalline phases in the heterojunction. UV–vis DRS analysis shows that the simultaneous presence of ZnO and N-doped TiO2 in the heterojunction results in a slight increase in the band gap value in the UV region , while the shoulder in the visible region typically of bare N-doped TiO2 is preserved. On the basis of these experimental observations, density functional theory (DFT) calculations were carried out to model both the N-doped TiO2 structure and to rationalize the experimental decrease in the band gap energy. The DFT modeling of band alignment of the ZnO/N-doped TiO2 heterojunction allowed us to define the “minimal band gap” (MBG), corresponding to the interface gap, which resulted in smaller band gap energy than the two separate semiconductor band gaps. Furthermore, the DFT modeling of the electronic structure of N-doped TiO2 predicted the existence of additional energy levels between the TiO2 valence and conduction bands, which allowed us to justify the activity of the heterojunction under visible light irradiation. Photocatalytic tests showed that the ZnO/N-doped TiO2 heterojunction performance was better than that of both N-doped TiO2 and ZnO alone, allowing us to achieve almost complete atrazine degradation under UV light irradiation, whereas 24% of atrazine degradation was achieved after 180 min of visible light irradiation. Finally, the photocatalytic results achieved by using scavenger molecules for reactive oxygen species showed that ·OH and ·O2– are both reactive species in atrazine photocatalytic degradation under UV irradiation, while ·OH is responsible for the photocatalytic processes under visible irradiation. DFT modeling, validated by these results, finally allowed us to define a model of the band alignment and photogenerated charge-transfer mechanism for the ZnO/N-doped TiO2 heterojunction.