CdS Particles Research Articles

Ta2O5 coatings modified with CdS, ZnO, and Dy2O3 particles prepared by plasma electrolytic oxidation of tantalum for the photocatalytic degradation of methyl orange

Ta2O5/CdS, Ta2O5/ZnO and Ta2O5/Dy2O3 coatings for application in photocatalytic degradation of methyl orange were prepared by plasma electrolytic oxidation of tantalum in an alkaline electrolyte with different concentrations of CdS, ZnO, and Dy2O3 particles up to 2.0 g/L. The coatings were characterized by SEM/EDS, XRD, Raman spectroscopy, DRS and photoluminescence spectroscopy. The morphology, thickness, phase structure and absorption properties of all coatings are almost identical and independent of the content of CdS, ZnO, and Dy2O3 embedded in Ta2O5 coatings. The orthorhombic and monoclinic phases of Ta2O5 are the main constituents of the coatings. Due to the low content of embedded CdS, ZnO, and Dy2O3 in Ta2O5 coatings, they exhibit high absorption in the middle ultraviolet region, which is characteristic of Ta2O5. The photocatalytic activity (PA) of Ta2O5/CdS, Ta2O5/ZnO and Ta2O5/Dy2O3 is higher than that of pure Ta2O5 and depends on the amount of CdS, ZnO, and Dy2O3 in the electrolyte, respectively. The highest PA was observed for coatings formed in an electrolyte containing 0.5 g/L of CdS, ZnO, and Dy2O3. The improved PA of Ta2O5/CdS, Ta2O5/ZnO and Ta2O5/Dy2O3 compared to Ta2O5 is related to the reduction of photogenerated electron/hole recombination. The mechanism of the photogenerated electron/hole transfer process is presented and discussed.

Surface sulphur vacancies of CdS/MIL-68(In)–NH2 for Boosting Visible-Light photocatalytic hydrogen evolution

The incorporation of sulfur vacancies (SVs) during the synthesis of CdS represents an essential approach for enhancing its photocatalytic hydrogen production performance. The CdS/MIL-68(In)–NH2 photocatalytic material with SVs was successfully synthesized via a facile two-step method and employed for visible light photocatalytic hydrogen evolution. Among various loading ratios, the CdS(30)/MIL-68(In)–NH2 composite exhibited remarkable photocatalytic performance, demonstrating hydrogen evolution rates 3.7 times and 7.3 times higher than those of pure CdS and mechanically mixed CdS with MIL-68(In)–NH2, respectively, while maintaining significant photocatalytic stability. The incorporation of MIL-68(In)–NH2 effectively enhanced the dispersion of CdS particles, thereby increasing the availability of active sites for efficient photocatalytic reactions. Moreover, the presence of SVs on the surface of CdS served as electron trapping levels, significantly improving charge separation efficiency. This study not only provides novel insights into fabricating composite photocatalysts but also explores a defect engineering regulation strategy.

Surface structure-based model to calculate thermal and optical properties in nanoscale and quantum dot semiconductors

Surface combination structure was used to design the construction of nano and quantum films, particles, and dots. The created model was applied to tetrahedral semiconductor nanoparticles and quantum dots with a successful performance. It works better than reported experimental data across the whole size range, from the bulk state to the critical size of quantum dots. The model helps to figure out the melting point that changes with the nanosize scale as applied on Si and Sn, which are simple tetrahedral semiconductors, and for CdS and CdSe, which are among II-VI group compounds. It was also used to calculate melting-related parameters such as cohesive energy, Debye temperature, vibrational entropy, and melting enthalpy. The nanosize dependence of the lattice volume and the atom's ionization energy describes the solid surface structure. From the model, an equation was obtained to calculate the energy gaps in Si, CdS, and CdSe particles that are nano- and quantum-dot-sized, and the results are compared to the experimental data that goes with these particles.

One-Step Photochemical Preparation of CdS/Poly(MMA-co-MAA) Composite with Enhanced Photocatalytic Activity.

This paper presents a one-step photochemical method for the preparation of CdS/Poly(MMA-co-MAA) composite photocatalyst, based on the concept of simultaneous photocatalytic polymerization of organic monomers during UV-light induced formation of CdS. The preparation is carried out in an aqueous solution of Na2S2O3, CdSO4, methyl methacrylate (MMA) and methacrylic acid (MAA), under a UV lamp. The continuously formed CdS particles with photocatalytic activity act the role of initiator to directly initiate the copolymerization of MMA and MAA, resulting in the in situ formation of the composite and full contact of the CdS particles with the oxygen-containing groups in the polymer. Taking the photocatalytic degradation of methylene blue as a case study, the composite exhibited significantly higher activity under simulated solar light compared to the pure CdS. By analysis on various data, the enhanced photocatalytic activity is attributed to the enhanced visible light absorption, and especially the high electron-hole separation efficiency caused by the electrostatic interaction between photogenerated holes and carbonyl oxygen atoms with negatively charged features. Furthermore, the composite displays excellent sunlight activity and recyclability, suggesting its potential for practical applications. Such a one-step construction strategy relying only on photo-energy is green, low-cost and promising in obtaining high-performance semiconductor/polymer composite photocatalysts.

Effect of Additions of Mixtures of Sodium Sulfite and Formic and Acetic Acid on the Photocatalytic Reduction of Water with Cadmium Sulfide Suspensions

The photochemical reduction of water with cadmium sulfide suspensions containing mixtures of sodium sulfite and formic and acetic acid solutions has been studied by electromotive force measurements and gasometry. The sulfite ions are oxidized on the anode in the sulfite–acetate suspensions; the sulfite ions and formic acid, in the sulfite–formate solutions. Hydrogen peroxide was found among the products of the photochemical reaction; it is involved in the oxidation of the radicals of sulfite ions and molecular sulfur of CdS particles and prevents the reduction of water.

Improving the Performance of Polydimethylsiloxane-Based Triboelectric Nanogenerators by Introducing CdS Particles into the Polydimethylsiloxane Layer.

Energy harvesting and power generation technologies hold significant potential for meeting future energy demands and improving environmental sustainability. A triboelectric nanogenerator (TENG), which harnesses energy from the surrounding environment, has garnered significant attention as a promising and sustainable power source applicable in various fields. In this study, we present a technique to improve the triboelectric performance of a PDMS-based TENG by incorporating nanostructured cadmium sulfide (N-CdS). This study investigates the utilization of CdS nanomaterials in TENG production, where mechanical energy is converted into electrical energy. We conducted a comparative analysis of TENGs utilizing N-CdS/PDMS, commercial CdS/PDMS (C-CdS/PDMS), and pure PDMS substrates. The N-CdS/PDMS substrates demonstrated superior triboelectric performance compared to TENG devices based on pure PDMS and C-CdS/PDMS. The triboelectric open-circuit voltage (Voc) and short-circuit current (Isc) of the N-CdS/PDMS-based TENG device were approximately 236 V and 17.4 µA, respectively, when operated at a 2 Hz frequency. These values were approximately 3 times and 2.5 times higher, respectively, compared to the pure PDMS-based TENGs. They were further studied in detail to understand the effect of different parameters such as contact-separation frequency and contact force on the TENGs' operation. The stability of the TENG devices was studied, and their potential to be integrated into self-powered smart textiles as power sources was demonstrated.

Effects of incorporating manganese in CdS thin films elaborated by CBD and the performance of Schottky diodes TCO/CdS:Mn/C

This study focuses on manganese doped CdS thin films by chemical bath deposition (CBD). Thin films and Schottky diodes of CdS and CdS:Mn are synthesized by a simple chemical route. The influence of the incorporation of Mn2+ cations in the crystalline structure of CdS has been characterized by techniques such as XRD, SEM, XPS, UV–Vis spectroscopy, and I–V curves. Doped thin films have a uniform hexagonal structure and preferentially grow along the (100) and (002) planes. The higher the amount of Mn2+ cations, the preferential orientation of the (100) plane gradually disappears, while the undoped sample shows a preferential orientation in the (002) plane. In addition, the morphology, when incorporating manganese, exhibits a drastic change, generating scale-like and needle-like structures, and the average size of the particles varies from 67 nm to 107 nm, in contrast to virgin CdS particles that show a mean size of 23 nm and spherical structures. The variation of the bandgap is 0.067 eV due to the presence of the preferential orientation in the (100) crystal plane in the Mn-doped samples and the doping increase in crystallite size. Doped Schottky diodes exhibit a saturation current of 2 nA and a potential barrier of 0.66 eV, compared to undoped diodes that exhibit a saturation current of 0.93 nA and a potential barrier of 0.823 eV. In addition, it was possible to modify the rectification speed of the diodes by almost 70 % by incorporating manganese. Overall, our study reveals that there is great potential in the design of affordable high-speed optoelectronic devices, although we are aware that we are still at an early stage in achieving these goals.

Synthesis and Characterization Conducting Polymer Supported CDS-ZNS Nano Composites Used for Various Applications

Keys include undertaking polymer synthesis and physical property research. Conductive is polyaniline. The presence of pristine nano CdS particles in the matrix was demonstrated by X-ray diffraction (XRD).
 In this study, four sol-gel techniques are used to prepare nanostructured polyaniline (PANI). PANI molecular structure was characterized using X-ray diffraction to assess the morphology of PANI samples.
 The results of the prepared PANI's characterization reveal that the unconventional synthesis methods utilized to create it drastically changed its morphology, chemical composition, crystallinity, conductivity, and surface area. The outcomes of the characterization supported this. The manufacturing and properties of polyaniline conducting polymers are examined in this research. The classic and grafted techniques of polyaniline synthesis have been examined. The relationship between structure and properties is established by this high-accuracy development procedure. In the future, polyaniline with altered structure may be employed.
 PANI/CdS nanocomposites have higher DC conductivity than PANI, according to observations.
 In situ production of conducting polyaniline nanocomposites with CdS nanoparticles was achieved by oxidizing aniline with cadmium sulphate at 3.5 pH. Electrical conductivity of polyaniline is influenced by CdS-nanoparticles. In contrast to pure polyaniline, the electrical conductivity of polyaniline nano-composite is increased by the homogenous intercalation of CdS nanoparticles, which results in a cooperative phenomenon between polyaniline and the nanoparticles.

Carbon nanotubes as electron mediators for CNTs/CdS/MoS2 high efficient photodegradation of tetracycline under visible light

The ternary nanocomposites of CNTs/CdS/MoS2 was synthesized by a simple two-step hydrothermal method and characterized accordingly. In situ growth of CdS on the surface of CNTs decreased the size of CdS particles and increased its dispersibility. Moreover, CdS took advantage of the good conductivity of CNTs to accelerate the conduction of photogenerated holes and reduced the recombination efficiency of charge carriers. MoS2 grew in situ on the surface of CdS, and the interface between them formed a close contact heterostructure. The built-in electric field formed at the interface between the two forced the photogenerated electrons of CdS and the holes of MoS2 to recombine here. This property not only changed the path of photogenerated electrons and holes, but also made the surface of MoS2 becoming the active center of reduction reaction. The hybrid structure of CNTs/CdS/MoS2 in-situ composite significantly improved the photocatalytic performance. When CNTs/CdS/MoS2 was used as photocatalyst, the degradation rate of tetracycline was 96.7% under 100 min of visible irradiation, and the data was still above 95% after 5 cycles indicate. This indicated that the ternary complex has good photocatalytic performance and stability. The successful preparation of CNTs/CdS/MoS2 provides an economically feasible approach for the degradation of tetracycline.

Significantly enhanced photocatalytic hydrogen production performance of MoS2/CNTs/CdS with carbon nanotubes as the charge mediators

Photocatalysis technology can efficiently produce hydrogen, which is essential for the sustainable development of clean energy. Herein, molybdenum sulfide/carbon nanotubes/cadmium sulfide (MoS2/CNTs/CdS) ternary composites are designed as catalysts to obtain an efficient photocatalytic hydrogen production system. In particular, the in-situ growth of CdS on the surface of MoS2/CNTs improves the dispersion of CdS nanoparticles, reduces the size of CdS particles, and increases the active site of photocatalytic reaction. A built-in electric field is formed between the interface between CdS and MoS2, which causes photogenerated electrons in conduction band (CB) of CdS and photogenerated holes in valence band (VB) of MoS2 to move in Z-type conduction mechanism. The excellent charge-conducting ability of CNTs reduces the recombination of photogenerated electrons and holes in CdS, further improving the hydrogen production efficiency and the stability of photocatalytic performance. Experiments showed that MoS2/CNTs/CdS (15%) have the highest hydrogen production rate (101.18 mmol h−1 g−1), which was 253 times higher than that of pure CdS. This study is expected to provide new ideas for the design and preparation of ternary hybrid photocatalysts for hydrogen production by photolysis of water.

Rational Design of the Electronic Structure of CdS Nanopowders.

In this study, various techniques, such as energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Raman spectroscopy, and spectrophotometry, were used to analyze the properties of nanometric CdS particles synthesized with varying precursor concentrations. EDX analysis revealed the nonstoichiometric composition manifested by an increase in the Cd/S ratio from 1.02 up to 1.43 with increasing precursor concentration. The growth of lattice parameters and unit cell volume accompanying preferential crystallization of the hexagonal phase along with an increasing Cd/S ratio was confirmed by XRD analysis. This indicated the presence of interstitial cadmium in nonstoichiometric Cd1+xS. The formation of shallow Cdi donor levels below the bottom edge of the conduction band impacts the bang-gap energy; a decrease from 2.56 to 2.21 eV along with increasing nonstoichiometry was observed. This is accompanied by a widening of the range of absorption of light, which creates conditions that can lead to an increase in the efficiency of redox reactions in photochemical processes.

Structural, Optical and Antimicrobial Properties of Carbohydrate-Capped Cadmium Sulfide Nanoparticles

Cadmium sulfidenanoparticles (CdSNPs) were prepared through the chemical precipitation process where sweet potato starch and glucose-functioned as capping agents. Ultraviolet-Visible (UV-Vis) spectroscopy, Fourier Transform Infrared (FTIR) spectroscopy and X-ray diffraction (XRD) techniques were applied for the analysis of prepared CdSNPs. The absorption peaks in the UV-Vis spectroscopy towards the lower wavelength region compared to the bulk CdS (512 nm) indicated the formation of CdS particles in the nanometer scale range. FTIR spectroscopy showed the characteristic absorption peaks of the CdSNPs along with the functional groups present in starch and glucose, suggesting the possible interaction between CdSNPs and capping agents. The XRD revealed crystallographic information such as the average crystallite size and shape of CdSNPs. Using the Scherrer equation, the average crystallite size of CdSNPs was determined to be 11.48 nm and 16.03 nm for starch-capped and glucose-capped CdSNPs (CdS-St andCdS-Gl)respectively. Antibacterial and antifungal activities of the prepared CdSNPs were found to be highly efficient for various pathogenic bacteria and fungi. Hence in the present study, the new synthetic route of CdSNPs was applied, and antimicrobial activities were performed

Three-dimensional maple leaf CdS/g-C3N4 nanosheet composite for photodegradation of benzene in water

In this study, composites of graphitic carbon nitride (g-C3N4) and maple leaf CdS (CdS/g-C3N4) were synthesized by hydrothermal method. To investigate the charge-carrier interface for photocatalytic improvement, CdS/g-C3N4 composites in different molar ratios were prepared. X-ray diffraction (XRD), scanning electron microscopy, X-ray photoelectron spectroscopy, photoluminescence (PL) spectroscopy, ultraviolet–visible diffuse reflectance spectroscopy (UV-DRS), and electrochemical impedance spectroscopy were used to examine the coupling of the CdS particles to g-C3N4 nanosheets (EIS). With a well-defined three-dimensional hierarchical dendrimer-like structure and high purity, as demonstrated by elemental mapping and XRD patterns, optical characteristics were analyzed using UV-DRS, PL spectroscopy, and EIS. When the optical properties of CdS were examined using UV-DRS, PL spectroscopy, and EIS, it was found that the material's high purity and well-defined hexagonal and cubic crystal phase nanostructure were clearly visible. Furthermore, the material's electron-hole recombination rate was significantly reduced. The samples were used to assess the visible light photodegradation of benzene. The maximum benzene degradation and highest stability for up to four cycles were observed in the 40 % CdS/g-C3N4 sample. For the combined photocatalyst and degradation process, a Z-scheme was established.

A simple and low-cost new procedure for synthesis of nickel(II) and cadmium(II) sulfides in situ thiourea metal–chelation precursor

ABSTRACT. Nickel(II) and cadmium(II) sulfides are a promising chemical material in various advanced research areas such as solar cells, supercapacitors, catalysts, and of significant interest for their practical implementations in up to photonics and electronics. Cadmium and nickel sulfides were synthesized with stoichiometries in aqueous media at elevated temperature from Ni(NO3)2.6H2O, NiCl2.6H2O, NiSO4.6H2O, CdCl2, Cd(NO3)2.4H2O, CdSO4.8H2O and thiourea as a sulfur precursor using a direct chemical reaction. At room temperature an octahedral and tetrahedral geometries result from the reactions between different nickel(II), cadmium(II) and thiourea with the molecular formulas can be presented as [Ni(NH2CSNH2)6]X2 and [Cd(NH2CSNH2)4]X2 where X = Cl-, NO3-, SO42. The novelty of our study is precipitated yellow cadmium sulfide and black nickel sulfide at elevated temperature ~80 oC through the molar ratio 1:10 (M2+: thiourea) in aqueous media. Infrared spectra, XRD, TEM including EDX, and elemental analysis were used for characterization the metal sulfides products. The average crystalline size of obtained CdS and NiS particles in range 20-25 nm. Different reaction conditions effects were evaluated on the size, morphology and particle size.
 
 KEY WORDS: Nickel and cadmium sulfide, Chelation, Thiourea, XRD, TEM
 Bull. Chem. Soc. Ethiop. 2023, 37(3), 663-674. 
 DOI: https://dx.doi.org/10.4314/bcse.v37i3.10 

Continuous Flow Synthesis of Cd1-x Znx S and CdS/ZnS Core/Shell Semiconductor Nanoparticles by MicroJet Reactor Technology.

From aqueous precursor solutions of metal salts and sodium sulfide using MicroJet Reactor (MJR) technology Cd1-x Znx S and CdS/ZnS core/shell semiconductor nanoparticles were synthesized. The MJR approach represents an automated, continuous, flexible and scalable route for nanoparticle synthesis, providing a tight control over process parameters and thus simple size, shape and composition control. Since particle sizes below the excitonic Bohr radius were obtained by MJR, the nanoparticulate materials exhibit quantum confinement effects. By varying the precursor ratio the band gap of Cd1-x Znx S Quantum Dots (QDs) could be targeted from 3.1 to 3.6 eV. CdS/ZnS core/shell QDs were prepared by enclosing CdS particles from MJR with ZnS produced by thermal decomposition of a Zn-MPA complex. Adjustment of the shell thickness increased the photoluminescence intensity by 43 %. Synthesis of ternary sulfides in the form of core/shell particles broadens the spectrum of materials accessible by MJR and demonstrates the extraordinary flexibility of the technology.

Sequestration and efflux largely account for cadmium and copper resistance in the deep-sea Nitratiruptor sp. SB155-2 (phylum Campylobacterota).

In deep-sea hydrothermal vent environments, metal-enriched fluids and sediments abound, making these habitats ideal to study metal resistance in prokaryotes. In this investigation, we employed transcriptomics and shotgun proteomics with scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy (STEM-EDX) to better understand mechanisms of tolerance for cadmium (Cd) and copper (Cu) at stress-inducing concentrations in Nitratiruptor sp. SB155-2 (phylum Campylobacterota). Transcriptomic profiles were remarkably different in the presence of these two metals, displaying 385 (19%) and 629 (31%) differentially transcribed genes (DTG) in the presence of Cd(II) and Cu(II), respectively, while only 7% of differentially transcribed (DT) genes were shared, with genes for non-specific metal transporters and genes involved in oxidative stress-response predominating. Transcriptomic and proteomic analyses confirmed that metal-specific DT pathways under Cu(II) stress, including those involving sulfur, cysteine, and methionine, are likely required for high-affinity efflux systems, while flagella formation and chemotaxis were over-represented under Cd(II) stress. Consistent with these differences, STEM-EDX analysis revealed that polyphosphate-like granules (pPLG), the formation of CdS particles, and the periplasmic space are crucial for Cd(II) sequestration. Overall, this study provides new insights regarding metal-specific adaptations of Campylobacterota to deep-sea hydrothermal vent environments.

Ligand-Decomposition assistant formation of CdS/TiO2 hybrid nanostructure with enhanced photocatalytic activity

CdS decorated anatase TiO 2 without capping ligands between them was constructed by the reaction of amorphous TiO 2 sphere modified by thioglycolic acid with Cd 2+ during the heating process (95 °C) directly. We have confirmed that the crystallization of amorphous TiO 2 and the formation of CdS particles are driven by the decomposition of thioglycolic acid. Moreover, the dissociation of thioglycolic acid also avoids the influence of ligand on the charge transfer between TiO 2 and CdS which is beneficial to improving the photocatalytic activity. Benefiting from this fabrication strategy, the photocatalytic activity of the obtained photocatalysts presents the enhanced activity in the wastewater treatment under visible light irradiation compared to conventionally prepared photocatalysts with the ligand. • CdS/TiO 2 composites without capping ligands was fabricated. • The absence of ligand is beneficial to the transient photocurrent response of CdS/TiO 2 . • The crystallization of amorphous TiO 2 and the formation of CdS particles are driven by the decomposition of thioglycolic acid. • The different interactions between original TiO 2 and metal ions can influence the morphology of sulfide/TiO 2 composites.

Structural and electrical properties of CdS nanocomposite solid films for electrolyte applications

ABSTRACT Solid Polymer electrolyte-based energy devices are playing a vital role for future energy sources. Solid state Poly (ethylene Oxide) polymer electrolyte was prepared by incorporating CdS nanoparticles. The precursor Cd(NO3)2 was incorporated in the host polymer to form a film and H2S gas was exposed on the film to convert it into CdS. The host polymer itself acted as a capping agent to reduce the size of CdS nanoparticles. The size distribution of CdS particles inside the matrix and the surface property were studied using Transmission Electron Microscope (TEM) and Atomic force microscope (AFM). The functional group present in the composites had been analyzed using Fourier Transform spectroscopy (FTIR). The electrical characteristics of the polymer nano composite electrolyte were characterized using impedance analyzer. The increase in ionic conductivity by varying the ratio of CdS:PEO was observed from the electrical studies.

Electronic Structure of CdS Nanoparticles and CdSe/CdS Nanosystems

The electronic states of “wurtzite” CdS nanoparticles and CdSe/CdS nanosystems with up to 80 pairs of Cd-Se or CdS atoms were calculated.The results for CdS particles were compared with the results obtained earlier for CdSe particles of the same size and with published calculations of other authors. The calculated gap values in the range of 2.84 eV ~ 3.78 eV are typical for CdS particles of studied sizes in accordance with results of published data. The CdSe/CdS nanosystems were considered as layered ones and as quantum dots. The layered CdSe/CdS systems with twolayer CdS coverings can be interpreted in terms of combinations of two semiconductors with different energy band gaps (2.6 eV and 3.3 eV), while analogous systems with single-layer CdS coverings do not demonstrate a two-gap electron structure. Simulation of a CdSe/CdS quantum dot shows that the single-layer CdS shell demonstrates a tendency for the formation of the electronic structure with two energy gaps: approximately of 2.5 eV and 3.0 eV.

Sustainable and stable hydrogen production over petal-shaped CdS/FeS2 S-scheme heterojunction by photocatalytic water splitting

One-dimensional CdS nanorods have garnered interest because of their highly visible light response, narrow bandgap, and negative potential at the conduction band edge, which are suitable for proton reduction. However, their poor charge separation and surface photocorrosion remain unresolved. In this study, CdS was synthesized with a 3D dendrite-like morphology to reduce its surface instability and junctioned with inexpensive FeS2 particles to extend the absorption region toward visible light and improve its photoactivity. The photocurrent density was increased 8.3 times, and the photoluminescence reduced by half in petal-shaped CdS/15% FeS2 compared with pure CdS. The petal-shaped CdS/15% FeS2 heterojunction catalyst exhibited significantly enhanced photostability and photocatalytic activity; when 10% lactic acid was used as a hole scavenger, the hydrogen generation rate was 22.91 mL g−1 for 10 h in pure CdS particles and 107.56 mL g−1 in the petal-shaped CdS/15% FeS2 particles. Moreover, the amount of hydrogen generated was maintained until 8th recycling experiments. The Cd and S ions eluted via photocorrosion were not detected after the reaction was complete. This was attributed to the petal-shaped CdS/FeS2 heterojunction system which protected the unstable CdS surface owing to its controlled morphology. The FeS2 junction improved visible light absorption facilitating the separation of photogenerated charges.