Band Gap CdS Research Articles

Tailoring Morphology in Hydrothermally Synthesized CdS/ZnS Nanocomposites for Extraordinary Photocatalytic H2 Generation via Type-II Heterojunction

CdS@ZnS core shell nanocomposites were prepared by a one-pot hydrothermal route. The morphology of the composite was tuned by simply changing the Zn2+ precursor concentration. To characterize the samples prepared, various techniques were employed, including XRD, FESEM, TEM, XPS and UV-vis DRS. The band gaps of CdS and ZnS were measured to be 2.26 and 3.32 eV, respectively. Compared with pure CdS, the CdS@ZnS samples exhibited a slight blue shift, which indicated an increased band gap of 2.29 eV. The CdS@ZnS core shell composites exhibited efficient photocatalytic performance for H2 generation under simulated sunlight illumination in contrast to pure CdS and ZnS. Additionally, an optimized H2 generation rate (14.44 mmol·h−1·g−1cat) was acquired at CdS@ZnS-2, which was approximately 4.6 times greater than that of pure CdS (3.12 mmol·h−1·g−1cat). Moreover, CdS@ZnS heterojunction also showed good photocatalytic stability. The process of charge separation over the photocatalysts was investigated using photoelectrochemical analysis. The findings indicate that the CdS@ZnS nanocomposite has efficient charge separation efficiency. The higher H2 generation activity and stability for CdS@ZnS photocatalysts can be attributed to the intimate interface in the CdS@ZnS core–shell structure, which promoted the light absorption intensity and photoinduced charge separation efficiency. It is expected that this study will offer valuable insights into the development of efficient core shell composite photocatalysts.

Two-dimensional buckled tetragonal cadmium chalcogenides including CdS, CdSe, and CdTe monolayers as photo-catalysts for water splitting.

Pure hydrogen production via water splitting is an ideal strategy for producing clean and sustainable energy. Two-dimensional (2D) cadmium chalcogenide single-layers with a tetragonal crystal structure, namely Tetra-CdX (X = S, Se, and Te) monolayers, are theoretically predicted by means of density functional theory (DFT). Their structural stability and electronic and optical properties are investigated. We find that Tetra-CdX single-layers are thermodynamically stable. Their stability decreases as we go down the 6A group in the periodic table, i.e., from X = S to Se, and Te which also means that the electronegativity decreases. All considered novel monolayers are indirect band gap semiconductors. Using the HSE06 functional the electronic band gaps of CdS, CdSe, and CdTe monolayers are predicted to be 3.10 eV, 2.97 eV, and 2.90 eV, respectively. The impact of mechanical strain on the physical properties was studied, which indicates that compressive strain increases the band gap and tensile strain decreases the band gap. The optical properties of the Tetra-CdX monolayers show the ability of these monolayers to absorb visible light. Due to the suitable band gaps and band edge positions of Tetra-CdX, these newly discovered 2D materials are promising for photocatalytic water splitting.

Ternary Co3O4/CdS/SrTiO3 core-shell pn junctions toward enhanced photocatalytic hydrogen production activity

A ternary Co3O4/CdS/SrTiO3 core-shell pn junction is fabricated via a simple hydrothermal-chemical-photodeposition method. Herein, a CdS shell is deposited on the surface of a SrTiO3 nanosphere, and then p-type Co3O4 lamellas are grown on the surface of the CdS/SrTiO3 core-shell heterojunction. Evaluated by its hydrogen evolution ability, the as-prepared ternary Co3O4/CdS/SrTiO3 core-shell pn junction exhibits an obvious photocatalytic enhancement of ~15-fold compared to single SrTiO3 nanospheres. This improvement can be mainly ascribed to the narrow band gaps of CdS and Co3O4 increasing the visible light response, and the ternary core-shell pn junction quickly transferring/diffusing photogenerated charge carriers into water. Additionally, the core-shell structure can provide sufficient active sites, and the ultrathin Co3O4 lamellas can shorten the photoelectron transport path to improve the photocatalytic stability.

Energy level alignment and nanoscale investigation of a-TiO2/Cu-Zn-Sn-S interface for alternative electron transport layer in earth abundant Cu-Zn-Sn-S solar cells

Efficiency of earth abundant and pure sulfide kesterite Cu-Zn-Sn-S (CZTS) solar cell has been stagnant around 9.4% for years, while its counterpart Cu-In-Ga-Se (CIGS) reports an efficiency of more than 22%. Low open circuit voltage (VOC) is the major challenging factor for low efficiency due to severe nonradiative interface recombinations. The existence of higher defect states at the conventional CZTS-CdS interface due to undesirable energy level alignment and lattice misfit promotes trap-assisted recombinations and results in low VOC. In this work, amorphous TiO2 (Eg=3.8eV) is proposed as a promising substitute to the conventional and low bandgap CdS (Eg=2.4eV) layer. The surface and interface of the CZTS-TiO2 layer were investigated using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The result reveals favorable “spike”-like conformations at the CZTS-TiO2 interface with a conduction band offset value of 0.17 eV. The nanoscale probing of the interface by Kelvin probe force microscopy across CZTS-TiO2 layers shows a higher potential barrier for interface recombination at CZTS-TiO2 in contrast to the conventional CZTS-CdS interface. Finally, the fast decay response and lower persistent photoconductivity of photogenerated carriers for CZTS-TiO2 heterojunction based photodetectors further validate our results. The energy level alignment and nanoscale interface studies signify TiO2 as a promising alternate buffer layer for earth abundant CZTS solar cells.

The promising photoanode of Pt coupled TiO2 NFs/CdS QDs with enhanced photoelectrochemical performance

CdS quantum dots (QDs) were deposited on TiO2 nanoflowers (NFs) by a facile successive ionic layer adsorption and reaction method to construct TiO2 NFs/CdS QDs. The structural, morphological and optical properties were measured for the prepared TiO2 NFs/CdS QDs. TiO2 NFs/CdS-6 showed the absorption edge up to about 500 nm with broaden visible light response. By taking full advantages of narrow band gap CdS QDs and 3D TiO2 NF, TiO2 NFs/CdS-6 showed the highest photocurrent density of 1.25 mA/cm2 and 1.18 mA/cm2 at 1.0 V vs. SCE under AM 1.5G and visible light illumination, respectively, which was about 48 and 118 times higher than that of pure TiO2 NFs. After employing Pt layer to form Pt/TiO2 NFs/CdS QDs structure, the PEC performance is further elevated by 39.8% enhancement under AM 1.5G illumination, which was further raised to 164.5% in the visible light regime. This facile and effective coupling Pt layer approach makes it possible to design metal/semiconductor/QDs structures for other general photoanode improvement.

Nanoheterojunction through PbS nanoparticles anchored CdS nanowires towards solar cell application

Through simple chemical route, nanoheterojunction has been developed between CdS-nanowires (NWs) as core with PbS nanoparticles as a shell for photoelectrochemical solar cell application. The use of simple, cost effective and industry scalable chemical bath deposition (CBD) and successive ionic layer adsorption and reaction (SILAR) approaches for the deposition of CdS NWs and PbS nanoparticles, respectively has the added advantage. Well-tuned nano particle size of PbS through number of SILAR cycles over wide band gap CdS resulted into optical red shift which maximize the absorption region of solar spectrum responsible for yielding enhancement in light conversion efficiency than bare CdS. Device configured solar cell yields 15 fold enhancement in the power conversion efficiency in liquid configuration due to the well aligned nanoheterojunction between CdS and PbS analysed through photoelectron X-ray spectroscopy. This enhancement in efficiency has been correlated through flat band potential, electron life time, rate of recombination and external quantum efficiency.

Bandgap tunable CdS:O as efficient electron buffer layer for high-performance Sb2Se3 thin film solar cells

Antimony selenide (Sb2Se3) is a promising low-cost and low-toxicity photovoltaic material. The electron buffer layer is of great significance for superstrate Sb2Se3 thin film solar cells. Herein, high-performance Sb2Se3 solar cells were fabricated by using sputtered, bandgap-tunable oxygenated cadmium sulfide (CdS:O) as a novel class of electron buffer layers. The optical transparency and energy band levels of CdS:O buffers were precisely adjusted by controlling the oxygen content in CdS:O layers. With the incorporation of an optimized wide band-gap CdS:O electron buffer layer, the device parameters JSC, VOC and FF were all improved. An efficiency of as high as 6.29% was achieved. It's found that, besides the reduced light absorption in the CdS:O layer, the high-quality CdS:O/Sb2Se3 junction played a vital role in the efficiency enhancement. The optimization of interfacial band alignment and defects passivation by atomic oxygen both contribute to the improved quality of CdS:O/Sb2Se3 heterojunction. This study demonstrates that the sputtered CdS:O is a promising electron buffer layer for preparation of high-efficiency and large-area Sb2Se3 thin film solar cells.

Single Crystal, High Band Gap CdS Thin Films Grown by RF Magnetron Sputtering in Argon Atmosphere for Solar Cell Applications

Single Crystal, High Band Gap CdS Thin Films Grown by RF Magnetron Sputtering in Argon Atmosphere for Solar Cell Applications

Solar hybrid photo-thermochemical sulfur-ammonia water-splitting cycle: Photocatalytic hydrogen production stage

One of the main limitations of existing solar thermochemical water-splitting cycles (WSC) are that they utilize only thermal component of the solar irradiation neglecting its photonic component. A new hybrid photo-thermochemical sulfur–ammonia (HySA) WSC developed at the Florida Solar Energy Center allows circumventing this shortcoming. In the HySA cycle, water splitting occurs by means of solar beam splitting which enables utilization of the quantum (UV–Vis) portion of the solar spectrum in the hydrogen production stage and the thermal (IR) portion in the oxygen production stage. Present work investigates the photocatalytic hydrogen production step using narrow band gap CdS and CdSZnS composite photocatalysts, and ammonium sulfite as an electron donor. The choice of the electron donor was determined by the considerations of its regenerability in the thermal stages of the HySA cycle. This article examines the impact of photocatalyst and cocatalyst loading, temperature, and light intensity on hydrogen production rates. Photocatalysts, cocatalysts and photoreaction products were analyzed by a number of materials characterization (XRD, SEM, TEM, EDS) and analytical (GC and IC) methods. The experimental data obtained provide guidance for the improved solar photoreactor design.

Efficient photovoltaic devices based on p-ZnSe/n-CdS core–shell heterojunctions with high open-circuit voltage

Large band-gap ZnSe nanowires and CdS films are used to construct core–shell heterojunction solar cells with high open circuit voltage and efficiency.

Towards efficient visible-light active photocatalysts: CdS/Au sensitized TiO2 nanotube arrays

A visible-light active photocatalyst, CdS/Au/TiO2 nanotube array (NTA) photoelectrode, was prepared by electrodeposition of Au nanoparticles onto TiO2 NTA with subsequent deposition of visible-light absorbable 2.4eV band-gap CdS quantum dots using successive ion layer adsorption and reaction (SILAR). The Au nanoparticles here act as electron sinks facilitating charge carrier separation. Under AM1.5G illumination a photoconversion efficiency of 4.06% was achieved for the CdS/Au/TiO2 NTA photoelectrode, suggesting the promise of the material architecture for achieving high-performance cost-effective materials.

High-sensitivity and self-driven photodetectors based on Ge–CdS core–shell heterojunction nanowires via atomic layer deposition

Core–shell p–n heterojunction nanowires (NWs) hold great promise for optoelectronic applications due to the large effective junction area and minimized carrier recombination. However, the rational design and synthesis of core–shell heterojunction NWs are hindered by the complex fabrication process and the potential damage to core NWs during shell deposition. Herein, we demonstrated the fabrication of Ge–CdS core–shell heterojunction NWs via a facile atomic layer deposition (ALD) technique. The ALD technique allowed the conformal deposition of polycrystalline CdS shells on single-crystalline Ge NWs with high uniformity and controllable thickness. The heterojunction NWs exhibited excellent diode characteristics, with a pronounced photoresponse under light illumination. Significantly, owing to the existence of a built-in electric field, the heterojunctions could serve as self-driven photodetectors, with a high photosensitivity of 18000%, which is remarkably much better than previous reports on Ge NW photodetectors. The complementary bandgaps of Ge and CdS also ensured that the device had a capability for broadband detection from visible to infrared light. This study represents an important advance in fabricating core–shell heterojunction NWs for high-performance optoelectronic applications.

Efficient utilization of photogenerated electrons and holes for photocatalytic selective organic syntheses in one reaction system using a narrow band gap CdS photocatalyst

Selective syntheses of aromatic aldehydes and anilines were realized in one coupled system under mild reaction conditions and their yields can be successfully achieved at the theoretical ratio.

Influence of wide band gap oxide substrates on the photoelectrochemical properties and structural disorder of CdS nanoparticles grown by the successive ionic layer adsorption and reaction (SILAR) method.

The photoelectrochemical properties of nanoheterostructures based on the wide band gap oxide substrates (ZnO, TiO2, In2O3) and CdS nanoparticles deposited by the successive ionic layer adsorption and reaction (SILAR) method have been studied as a function of the CdS deposition cycle number (N). The incident photon-to-current conversion efficiency (IPCE) passes through a maximum with the increase of N, which is ascribed to the competition between the increase in optical absorption and photocarrier recombination. The maximal IPCE values for the In2O3/CdS and ZnO/CdS heterostructures are attained at N ≈ 20, whereas for TiO2/CdS, the appropriate N value is an order of magnitude higher. The photocurrent and Raman spectroscopy studies of CdS nanoparticles revealed the occurrence of the quantum confinement effect, demonstrating the most rapid weakening with the increase of N in ZnO/CdS heterostructures. The structural disorder of CdS nanoparticles was characterized by the Urbach energy (EU), spectral width of the CdS longitudinal optical (LO) phonon band and the relative intensity of the surface optical (SO) phonon band in the Raman spectra. Maximal values of EU (100–120 meV) correspond to СdS nanoparticles on a In2O3 surface, correlating with the fact that the CdS LO band spectral width and intensity ratio for the CdS SO and LO bands are maximal for In2O3/CdS films. A notable variation in the degree of disorder of CdS nanoparticles is observed only in the initial stages of CdS growth (several tens of deposition cycles), indicating the preservation of the nanocrystalline state of CdS over a wide range of SILAR cycles.

Ultraviolet stimulated emission from high-temperature-annealed MgO microcrystals at room temperature

Research on semiconductor nanowires underlies the development of the miniaturization of laser devices with low cost and low energy consumption. In general, nanowire lasers are made of direct band gap semiconductors, e.g., GaN, ZnO and CdS, and their band-edge emissions are used to achieve optically pumped laser emission. In addition to the existing class of nanowire lasers, we here show that air-annealed micrometer-sized MgO cubic crystals with well-defined facets exhibit room-temperature stimulated emission at 394 nm under pulsed laser pumping at ∼350 nm. Surface midgap states are assumed to be responsible for the excitation and emission processes. The present findings will not only provide opportunities for the development of miniaturized lasers composed of insulating oxides, but will also open up functionality in various families of cubic crystalline materials.

The effects of high temperature processing on the structural and optical properties of oxygenated CdS window layers in CdTe solar cells

High efficiency CdTe solar cells typically use oxygenated CdS (CdS:O) window layers. We synthesize CdS:O window layers at room temperature (RT) and 270 °C using reactive sputtering. The band gaps of CdS:O layers deposited at RT increase when O2/(O2 + Ar) ratios in the deposition chamber increase. On the other hand, the band gaps of CdS:O layers deposited at 270 °C decrease as the O2/(O2 + Ar) ratios increase. Interestingly, however, our high temperature closed-space sublimation (CSS) processed CdTe solar cells using CdS:O window layers deposited at RT and 270 °C exhibit very similar cell performance, including similar short-circuit current densities. To understand the underlying reasons, CdS:O thin films deposited at RT and 270 °C are annealed at temperatures that simulate the CSS process of CdTe deposition. X-ray diffraction, atomic force microscopy, and UV-visible light absorption spectroscopy characterization of the annealed films reveals that the CdS:O films deposited at RT undergo grain regrowth and/or crystallization and exhibit reduced band gaps after the annealing. Our results suggest that CdS:O thin films deposited at RT and 270 °C should exhibit similar optical properties after the deposition of CdTe layers, explaining the similar cell performance.

Strain-assisted structural transformation and band gap tuning in BeO, MgTe, CdS and 2H-SiC: A hybrid density functional study

Structural transformations and electronic structures of (0001) BeO, MgTe, CdS and 2H-SiC films under equibiaxial in-plane strains are studied using the HSE06 range-separated hybrid functionals. The main results are summarized as follows: I) The structural transition from the polar wurtzite to the nonpolar graphite-like phase is predicted for BeO, MgTe and CdS but not for 2H-SiC, which is more covalent in nature. II) Either a direct or an indirect band structure is displayed in wurtzite BeO and 2H-SiC based on the values of strain, while only a direct band gap is displayed in wurtzite MgTe and CdS. At large tensile strains, the band gaps of graphite-like BeO and CdS are always indirect, whereas the graphite-like MgTe undergoes a direct-to-indirect band gap transition. Furthermore, the decrease in band gap is observed for both types of strain, thereby enabling a number of important technological applications.

Extended photo-response of ZnO/CdS core/shell nanorods fabricated by hydrothermal reaction and pulsed laser deposition

Heterogenous nanostructures shaped with CdS covered ZnO (ZnO/CdS) core/shell nanorods (NRs) are fabricated on indium-tin-oxide by pulsed laser deposition of CdS on hydrothermally grown ZnO NRs and characterized through morphology examination, structure characterization, photoluminescence and optical absorption measurements. Both the ZnO cores and the CdS shells are hexagonal wurtzite in structure. Compared with bare ZnO NRs, the fabricated ZnO/CdS core/shell NRs present an extended photo-response and have optical properties corresponding to the two excitonic band-gaps of ZnO and CdS as well as the effective band-gap formed between the conduction band minimum of ZnO and the valence band maximum of CdS.

Hierarchical growth of SnO2 nanostructured films on FTO substrates: structural defects induced by Sn(II) self-doping and their effects on optical and photoelectrochemical properties.

Direct hydrothermal growth of Sn(II)-doped SnO2 films on fluorine-doped tin oxide (FTO) substrates results in the formation of upstanding SnO2 nanosheet arrays covered by hierarchical SnO2 nanoflowers. The n-type semiconductor films show extended photoresponse in the visible spectrum arising from the coexistence of Sn(II) dopant ions and oxygen vacancies in these hierarchical SnO2 nanostructures, which leads to a narrowed bandgap. Photoluminescence spectroscopy revealed that the emission in the UV, blue and red spectral ranges is related to the evolution of Sn(II) dopants and oxygen vacancies with annealing temperature, whereas oxygen vacancies are mostly responsible for visible emission. The Sn(II)-doped SnO2 films show higher photocurrent when sensitized with narrow bandgap CdS nanoparticles, serving as efficient electron acceptors.

Rational and scalable fabrication of high-quality WO3/CdS core/shell nanowire arrays for photoanodes toward enhanced charge separation and transport under visible light

High-quality one-dimensional WO3/CdS core/shell nanowire arrays used as photoanodes in photoelectrochemical (PEC) cells were for the first time prepared via a rational, two-step chemical vapor deposition process. The narrow band-gap CdS shell was homogeneously coated on the entire surface of as-grown WO3 core nanowire arrays, forming coaxial heterostructures. The one-dimensional core/shell heterostructure facilitates the photogenerated electron-hole pair separation and the electron transfer from CdS to WO3 nanowires under visible light illumination. Moreover, the core nanowire arrays provide a direct pathway for the electron transport. The present results imply that the WO3/CdS core/shell heterostructure nanowire arrays may be useful in the design of nanostructure photoanodes toward highly efficient PEC cells.