Semiconductor Energy Band Research Articles

Dynamical preparation and control of stable population inversion among semiconductor energy bands

Dynamical preparation and control of stable population inversion among semiconductor energy bands

Z-Scheme BiVO4/g-C3N4 Photocatalyst-With or Without an Electron Mediator?

The hybrid system BiVO4/g-C3N4 is a prospective photocatalyst because of the favorable mutual alignment of the energy bands of both semiconductors. However, the path of the photocatalytic process is still unclear because of contradictory information in the literature on whether the mechanism of charge carrier separation at the BiVO4/g-C3N4 interface is band-to-band or Z-scheme. In this work, we clarified this issue by comparative photocatalytic studies with the use of systems without a mediator and with different kinds of mediators including Au nanoparticles, fullerene derivatives, and the Fe3+/Fe2+ redox couple. Additionally, the charge transfer dynamics at the BiVO4/g-C3N4 and BiVO4/mediator/g-C3N4 interfaces were investigated by time-resolved photoluminescence (TRPL) measurements, while the influence of the mediator on the surface recombination of the charge carriers was verified by intensity-modulated photocurrent spectroscopy (IMPS). We proved that the charge carrier separation at the BiVO4/g-C3N4 interface occurs according to the mechanism typical for a heterojunction of type II, while the incorporation of the mediator between BiVO4 and g-C3N4 leads to the Z-scheme mechanism. Moreover, a very strong synergetic effect on caffeine (CAF) degradation rate was found for the system BiVO4/Au/g-C3N4 in the presence of Fe3+ ions in the CAF solution.

Effects of self-consistent extended Hubbard interactions and spin-orbit couplings on energy bands of semiconductors and topological insulators

Effects of self-consistent extended Hubbard interactions and spin-orbit couplings on energy bands of semiconductors and topological insulators

Fabrication of CoTiO3/MgIn2S4 S–scheme heterojunctions with efficient charge separation to enhance the photocatalytic activities of hydrogen generation and formaldehyde removal

The efficient separation of photoinduced charge carriers is a primary factor in the catalytic performance of photocatalysts. Constructing S-scheme heterojunctions is a prospective strategy to efficiently and spatially separate photoinduced charges while retaining strong oxidation and reduction capacities of the catalyst system. Herein, CoTiO3/MgIn2S4 (abbreviated x% CTO/MIS, where x% represents the mass ratio of CTO-to-MIS; x = 5, 10, 15, 20, and 25) heterojunctions were successfully synthesized using a hydrothermal method. These heterojunctions exhibited highly efficient and reusable visible-light photocatalytic properties for hydrogen (H2) generation and formaldehyde (HCHO) degradation. Notably, the 15 % CTO/MIS demonstrated an admirable H2 evolution activity of 631.77 μmol·g−1·h−1, with AQE = 2.25 % at λ = 420 nm, and an HCHO degradation rate of 67.18 %. The intermediate products generated during HCHO removal were identified applying in situ diffuse reflectance infrared Fourier transform spectroscopy. The exceptional catalytic performance of the synthesized materials was attributed to the boosted light absorption and utilization, rapidly separation, and transfer of photoproduced charge carriers, which resulted from the internal electric field of the S-scheme mechanism. Moreover, the S-scheme electron transfer pathway for the CTO/MIS catalyst system during the photocatalytic process was confirmed using electron spin resonance spectroscopy, free-radical capturing tests, density functional theory calculations, in situ X-ray photoelectron spectroscopy, and semiconductor energy band theory. The study offers an innovative perspective for establishing S-scheme heterojunctions with highly efficient and stable photocatalytic activity for H2 generation and HCHO removal.

Enhanced catalytic activity of modified g-C3N4 heterojunction for photocatalytic degradation of methylene blue from wastewater

Rapid recombination of photoinduced charge carriers and poor photocatalytic degradation performance greatly hinder the large-scale application of g-C3N4 photocatalysis. Therefore, the g-C3N4 is modified by BiPO4 to overcome its poor photocatalytic performance, which is investigated by methylene blue (MB) removal. The S scheme heterojunction is constructed by modifying g-C3N4 with BiPO4, which can effectively preserve the redox activities of holes and electrons on VB and CB, exhibiting good photocatalytic activity. Therefore, the modified g-C3N4 exhibits excellent photocatalytic activity with efficient separation of the photoelectron-hole, compared to pure g-C3N4 and BiPO4. The modified g-C3N4 exhibits large MB degradation removal of 96.7 %, which is significantly larger than the pure g-C3N4 (46.3 %) and BiPO4 (3.3 %) owe to synergy, respectively. The photodegradation rate constant of the modified g-C3N4 is 4.8 and 43 times larger than that of the pure g-C3N4 and BiPO4, respectively. The modified g-C3N4 still has large MB removal of 92.9 % after 5 cycles, indicating excellent stability and recyclability. The fractal density calculation of the modified g-C3N4 is investigated combined with LUMO and HOMO analysis. The MB photocatalytic degradation process follows the S scheme charge transfer mechanism, based on degradation mechanism analysis combined with semiconductor energy band and density functional theory calculation analysis. The MB photocatalytic degradation path is systemically analyzed based on MS-UPLC result analysis, which indicates that MB is finally decomposed into CO2 and H2O, achieving the degradation of MB.

Piezoelectric-mediated two-dimensional copper-based metal–organic framework for synergistic sonodynamic and cuproptosis-driven tumor therapy

Sonodynamic therapy (SDT) is a minimally invasive therapeutic approach that utilizes sonosensitizers to catalyze substrates and generate reactive oxygen species (ROS) under ultrasound stimulation, ultimately inducing tumor cell death. Enhancing the piezoelectric properties of nanomaterials and modulating the semiconductor energy band are effective strategies to improve the catalytic efficiency of sonosensitizers. In this study, we developed a two-dimensional (2D) copper-based piezoelectric metal–organic framework (MOF) sonosensitizer, denoted as CM, through the coordination of copper and dimethylimidazole. The unique 2D MOF structure imparts CM with piezoelectric characteristics, enabling it to enhance SDT efficacy by modulating the semiconductor bandgap and carrier mobility. Upon ultrasound irradiation, CM catalyzes oxygen to undergo a cascade reaction, producing highly toxic singlet oxygen. Additionally, cupric ions in CM can be reduced by glutathione, facilitating the spontaneous catalysis of hydrogen peroxide in tumors to generate hydroxyl radicals and deplete glutathione, thereby inducing oxidative damage. Moreover, cupric ions in CM can trigger tumor cell cuproptosis, which, in combination with the generated ROS, accelerates cell death. Thus, this study establishes a MOF-based system for controllably inducing multi-pathway cancer cell death and provides a foundation for enhancing ultrasound-catalyzed tumor therapy through the optimization of piezoelectric properties.

Boosted photocatalytic performance of hydrogen evolution and formaldehyde degradation with a S-scheme PMo12/MgIn2S4 heterojunction

Fabrication of a S-scheme heterojunction with staggered energy band structures can accelerate the charge separation and retain high redox potentials. Herein, a series of x wt% H3PMo12O40/MgIn2S4 (abbr. x% PMo12/MIS; x = 7.5, 10.0, 12.5, 15.0 and 17.5) S-scheme heterojunctions were successfully constructed by a hydrothermal technology and exhibited efficient and durable visible-light-driven photocatalytic properties of H2-evolution and formaldehyde (HCHO) degradation. Particularly, the 12.5 % PMo12/MIS sample represented the optimal H2 evolution rate of 232.53 μmol·g−1·h−1 with the apparent quantum efficiency (AQE) value of 1.52 % (λ = 420 nm), and HCHO removal rate of 64.20 % in 60 min. Moreover, the intermediate products formed during the removal process of HCHO were revealed through in situ DRIFTS method. The superior photocatalytic activities mainly stemmed from the enhanced absorption of visible light, and effective separation of photoinduced charge carriers. Furthermore, the S-scheme photocatalytic mechanism for PMo12/MIS system was confirmed via in situ irradiated X-ray photoelectron spectroscopy, radical capturing tests and electron spin resonance data, and semiconductor energy band theory. The current study affords a valuable perspective for fabricating highly efficient S-scheme heterojunction photocatalysts for hydrogen evolution and HCHO elimination.

A visible-light-driven dual Z-scheme CeVO4/g-C3N4/LaNiO3 hierarchical nanostructure photocatalyst for effective removal of organic pollutants

The major obstacle to restrict the practical application of photocatalysts is the low separation efficiency of photo-induced charge carriers. To overcome the above problem, a novel ternary CeVO4/g-C3N4/LaNiO3 hierarchical nanostructure photocatalyst with dual Z-scheme heterojunction is designed and fabricated by feat of semiconductor energy levels matching engineering. First, one-dimensional (1D) LaNiO3 nanobelts are fabricated by an electrospinning technique. Second, zero-dimensional (0D) g-C3N4 nanoparticles are fixed on 1D LaNiO3 nanobelts through a direct gas-solid reaction. Finally, 1D CeVO4 needle-like nanorods are fixed on g-C3N4/LaNiO3 hierarchical nanostructures through a hydrothermal method. Following exposure to visible-light irradiation for 180 min, the optimized CeVO4/g-C3N4/LaNiO3 hierarchical nanostructure photocatalyst exhibits the highest photocatalytic degradation rate (87.57%) for tetracycline hydrochloride (TCH). Additionally, under the same conditions, the photocatalyst demonstrates its broad applicability by effectively degrading various organic pollutants, including methylene blue (MB) (60.56%), bisphenol A (BPA) (68.00%), ibuprofen (IBU) (57.56%), cefuroxime sodium (CRO) (77.34%), carbamazepine (CBZ) (59.65%) and ciprofloxacin (CIP) (75.16%). The exceptional photocatalytic performance of the CeVO4/g-C3N4/LaNiO3 hierarchical nanostructure photocatalyst can be attributed to the formation of a dual Z-scheme heterojunction among the 1D CeVO4 needle-like nanorods, 0D g-C3N4 nanoparticles, and 1D LaNiO3 nanobelts. The photocatalyst has a large specific surface area and suitable semiconductor energy band positions, providing the hierarchical nanostructures with numerous surface-active sites, rapid charge transfer ability and small charge recombination rate. The design ideas and preparation method proposed in this work have guiding significance for developing other dual Z-scheme heterojunction photocatalysts.

Constructing a link between multivariate titanium-based semiconductor band gaps and chemical formulae based on machine learning

Titanium-based semiconductors are wildly recognized as one of the most commonly used photocatalysts for photocatalysis. Energy band modulation is a key aspect of the catalytic activity of photocatalytic semiconductors, but the acquisition of semiconductor energy bands is still a complex and important task. In recent years, machine learning has played an important role in materials prediction, where the crystal structure of a material is usually used as input in energy band prediction. However, existing machine learning algorithms cannot accurately predict the energy bands of materials from structural components. Here, we convert the chemical formula into component descriptors after comparing first principles and ultraviolet-visible spectrophotometry (UV–vis) errors on bandgap values, and the component and bandgap values form a set of labeled data pairs. The chemical formula components are used as input to accurately predict the energy bands of the material. In our evaluation, the model outperforms existing machine learning methods in predicting energy bands, yielding mean absolute value errors of about 0.277 eV, and possesses a significant advantage in component prediction. In particular, this method of predicting the photocatalytic semiconductor energy band gap from chemical formula components provides a new way of thinking about photocatalyst selectivity.

Solar Energy Dependent Supercapacitor System with ANFIS Controller for Auxiliary Load of Electric Vehicles

Innovations are required for electric vehicles (EVs) to be lighter and more energy efficient due to the range anxiety issue. This article introduces an intelligent control of an organic structure solar supercapacitor (OSSC) for EVs to meet electrical load demands with solar renewable energy. A carbon fibre-reinforced polymer, nano zinc oxide (ZnO), and copper oxide (CuO) fillers have been used in the development of OSSC prototypes. The organic solar cell, electrical circuits, converter, controller, circuit breaker switch, and batteries were all integrated for the modelling of OSSCs. A carbon fibre (CF)-reinforced CuO-doped polymer was utilised to improve the concentration of electrons. The negative electrodes of the CF were strengthened with nano ZnO epoxy to increase the mobility of electrons as an n-type semiconductor (energy band gap 3.2–3.4 eV) and subsequently increased to 3.5 eV by adding 6% π-carbon. The electrodes of the CF were strengthened with epoxy-filled nano-CuO as a p-type semiconductor to facilitate bore/positive charging. They improve the conductivity of the OSSC. The OSSC power storage was controlled by an adaptive neuro-fuzzy intelligent system controller to meet the load demand of EVs and auxiliary battery charging. Moreover, a fully charged OSSC (solar irradiance = 1000 W/m2) produced 561 W·h/m2 to meet the vehicle load demand with 45 A of auxiliary battery charging current. Therefore, the OSSC can save 15% in energy efficiency and contribute to emission control. The integration of an OSSC with an EV battery can minimise the weight and capacity of the battery by 7.5% and 10%, respectively.

Enlightening the bimetallic effect of Au@Pd nanoparticles on Ni oxide nanostructures with enhanced catalytic activity

Bimetallic decoration of semiconductor electrodes typically improves catalytic and sensing performances because of a well-claimed synergistic effect. A microscopic and quantitative investigation of such an effect on energy bands of semiconductor can be really useful for further exploitation. Au, Pd and Au@Pd (core@shell) nanoparticles (10–20 nm in size) were synthesized through chemical reduction method and characterized with scanning and transmission microscopy, Rutherford backscattering spectrometry, cyclic voltammetry electrochemical impedance spectroscopy and Mott–Schottky analysis. The nanoparticles have been used to decorate Ni-based nanostructured electrodes with the aim to quantitatively investigate the effect of decoration with mono or bimetallic nanoparticles. Decorated electrodes show higher redox currents than bare ones and a shift in redox peaks (up to 0.3 V), which can be ascribed to a more efficient electron transport and improved catalytic properties. These effects were satisfactorily modeled (COMSOL) employing a nano Schottky junction at the nanoparticle–semiconductor interface, pointing out large energy band bending (up to 0.4 eV), space charge region and local electric field (up to {10}^{8}mathrm{ V }{mathrm{m}}^{-1}) in bimetallic decoration. Sensing test of glucose and H2O2 by decorated Ni oxide electrodes were performed to consolidate our model. The presence of bimetallic nanoparticles enhances enormously the electrochemical performances of the material in terms of sensitivity, catalytic activity, and electrical transport. The modification of energy band diagram in semiconductor is analyzed and discussed also in terms of electron transfer during redox reactions.

Effects of Charge Traps on Hysteresis in Organic Field-Effect Transistors and Their Charge Trap Cause Analysis through Causal Inference Techniques.

Hysteresis in organic field-effect transistors is attributed to the well-known bias stress effects. This is a phenomenon in which the measured drain-source current varies when sweeping the gate voltage from on to off or from off to on. Hysteresis is caused by various factors, and one of the most common is charge trapping. A charge trap is a defect that occurs in an interface state or part of a semiconductor, and it refers to an electronic state that appears distributed in the semiconductor's energy band gap. Extensive research has been conducted recently on obtaining a better understanding of charge traps for hysteresis. However, it is still difficult to accurately measure or characterize them, and their effects on the hysteresis of organic transistors remain largely unknown. In this study, we conduct a literature survey on the hysteresis caused by charge traps from various perspectives. We first analyze the driving principle of organic transistors and introduce various types of hysteresis. Subsequently, we analyze charge traps and determine their influence on hysteresis. In particular, we analyze various estimation models for the traps and the dynamics of the hysteresis generated through these traps. Lastly, we conclude this study by explaining the causal inference approach, which is a machine learning technique typically used for current data analysis, and its implementation for the quantitative analysis of the causal relationship between the hysteresis and the traps.

High-performance broadband photodetector based on plasmonic semiconductor-semiconductor p-n Cu1.8S/AZO interface

Plasmonic semiconductor nanocrystals (NCs) provide intriguing optical properties which offer opportunities to extend the photo-response corresponding to photons with energies less than the semiconductor energy band gap. However, the performance of photodetectors based on individual plasmonic semiconductors is restricted due to very short-living photo-carriers, followed by carrier recombination (∼ 100 ps). Herein, a plasmonic semiconductor-semiconductor p-n interface (Cu1.8S/AZO NCs) was constructed representing 2-fold optical characteristics containing an inter-band electronic transition from the valence to conduction band in response to ultraviolet-visible (UV–vis) excitation and plasmon-induced interfacial charge-transfer transition (PICTT) in response to near infrared (NIR) excitation. We demonstrated that AZO NCs, are appropriate candidates to generate chemical interface damping (CID) of plasmons in Cu1.8S NCs as an acceptor, which is required for PICTT pathway. The device displayed outstanding broadband UV–vis–NIR photoresponsivity (R) and response-time (Trise/Tfall) as high as R = 51.48 A/W, Trise/Tfall= 130/100 ms in 390 nm, R = 37.2 A/W, Trise/Tfall= 50/60 ms in 535 nm, R = 45.6 A/W, Trise/Tfall= 400/10 ms in 750 nm and R = 38.7 A/W and Trise/Tfall= 60/50 ms in 950 nm. This study demonstrates the promising potential of plasmonic semiconductor-semiconductor p-n hybrid systems as high-performance broadband photodetectors (PDs).

Nanomaterials as Photocatalysts—Synthesis and Their Potential Applications

Increasing demand for energy and environmental degradation are the most serious problems facing the man. An interesting issue that can contribute to solving these problems is the use of photocatalysis. According to literature, solar energy in the presence of a photocatalyst can effectively (i) be converted into electricity/fuel, (ii) break down chemical and microbial pollutants, and (iii) help water purification. Therefore, the search for new, efficient, and stable photocatalysts with high application potential is a point of great interest. The photocatalysts must be characterized by the ability to absorb radiation from a wide spectral range of light, the appropriate position of the semiconductor energy bands in relation to the redox reaction potentials, and the long diffusion path of charge carriers, besides the thermodynamic, electrochemical, and photoelectrochemical stabilities. Meeting these requirements by semiconductors is very difficult. Therefore, efforts are being made to increase the efficiency of photo processes by changing the electron structure, surface morphology, and crystal structure of semiconductors. This paper reviews the recent literature covering the synthesis and application of nanomaterials in photocatalysis.

Energy band modulation of Mg-doped ZnO electrolyte for low-temperature advanced fuel cells

Mg-doped ZnO was developed as an ion conductor electrolyte that achieves an ionic conductivity of 0.24 S cm−1 at 550 °C. An open-circuit voltage of 1.12 V and a maximum power density of 859 mW cm−2 was achieved by using 5% Mg-doped ZnO having the bandgap of 3.241 eV and Fermi level value of −3.43 eV. X-ray photoelectron spectroscopy studies showed that the improved performance was due to the high concentration of oxygen vacancies. UV diffuse reflectance and UV photoelectron spectroscopy analysis confirmed that Mg doping can modulate the energy band structure of ZnO to improve its electrochemical properties. In addition, 5% Mg-doped ZnO can form an optimal Schottky junction with the metal Ni generated by the reduction of the anode to prevent the cell from short-circuiting and accelerate the ion transport rate between the anode and electrolyte interface. This study provides a new idea for the application of wide bandgap semiconductor energy band modulation to low temperature advanced fuel cell electrolyte materials.

Direct light-driven electrodeposition of Bi and Cu without external voltage: Light-induced metal recovery and energy conversion

Seeking novel solar energy utilization and conversion methods has always been a frontier topic in energy research. Here, we propose a new way to directly photoelectrodeposit metals using solar energy based on semiconductor energy band theory. Three kinds of semiconductor photoanodes (TiO2, BiVO4 and Fe2O3) are employed to deposit metallic Cu or Bi at the cathode under illumination without external voltage. It is found that when TiO2 and BiVO4 are used as photoanodes, Cu, Cu2O and Bi can be continuously deposited on the cathode, but when Fe2O3 photoanode is irradiated, no material comes out. The results prove that the potential of electrons in the conduction band determines whether photoelectric deposition occurs or not. The related mechanism involves the oxygen reduction reaction (OER) of the valence band holes of the photoanode, the collection of conduction band electrons by back electrode, as well as the reduction of metal ions on the counter electrode. In addition, we also put forward the concept of using the deposited Cu or Bi as the metal electrode to form a new metal-air battery to release chemical energy of the deposited metal.

Mixed valence Ce-doped TiO2 with multiple energy levels and efficient charge transfer for boosted SERS performance

Considering the variable valence characteristics of rare earth elements, they can be in a variety of valence forms coexistence. Doping of rare earth element with different valence states may produce different energy levels to tune the semiconductor energy band structure. We utilize rare earth element Ce doping TiO2 for the development of high-performance semiconductor surface-enhanced Raman scattering (SERS) substrates based on an energy-level tuning strategy. Ce doping not only forms multiple energy levels including Ce3+ and Ce4+ metal doping energy levels in the bandgap of TiO2, but also enriches the surface state level of TiO2 itself, which together promote the separation of photogenerated carriers and improve charge transfer efficiency between substrates and absorbed molecules. This endows TiO2 semiconductor substrate with a higher SERS enhancement factor, which can reach 2.2 × 106. The detectable concentration of methylene blue can be as low as 10−10 mol/L. Moreover, the semiconductor substrate exhibits excellent uniformity and stability. This study not only provides a new strategy to develop excellent semiconductor SERS substrate with multiple energy levels, but also lays the foundation for promising practical application of semiconductor substrate.

Oxygen vacancy-rich 2D GO/BiOCl composite materials for enhanced photocatalytic performance and semiconductor energy band theory research

Bismuth-based materials are extensively studied for photocatalytic applications because of their unique crystal structure. Herein, we reported a binary graphene oxide (GO)/BiOCl composite material prepared by a hydrothermal method and analyzed its photodegradation mechanism through the semiconductor energy band theory. The degradation rate of the GO/BiOCl composite towards Rhodamine B could reach 93.6% within 8 min, and its performance exceeded that of most photocatalysts. The influencing factors for improving the photocatalytic activity are as follows: (1) abundant oxygen vacancies generated on the tight recombination interface; (2) a 2D-2D electron transfer channel between GO and BiOCl; and (3) GO acting as a load to provide more reaction sites for BiOCl nanosheets. This work provides a simple solution and theoretical explanation for the rapid degradation of pollutants, and has broad application prospects.

Photo-assisted Al-air batteries based on gel-state electrolyte

The use of new concept electrochemical devices to store and release solar energy has always been a research hotspot in the society of resource shortage. In this work, we firstly introduce sunlight into the Al-air battery by CuO and Cu2O semiconductor electrode materials to improve the discharge voltage under illumination and the calcium alginate hydrogel-based electrolyte is also applied to the Al-air battery. The calcium alginate hydrogel-based electrolyte can greatly inhibit the self-corrosion of aluminum, resulting in the specific capacity of Al-air battery being significantly increased. At the current density of 1 mA cm−2, the CuO air cathode shows better performance, the maximum discharge voltage is 1.37 V and 1.23 V under illumination in aqueous sodium hydroxide solution and hydrogel, respectively, while the maximum discharge is 0.9 V and 0.78 V in the darkness. And the specific capacity in the hydrogel is about 20 times that of the aqueous sodium hydroxide solution. In addition, the semiconductor energy band structure and the discharge mechanism of the photo-assisted Al-air battery under illumination are also studied. This study provides a prospect for the conversion and application of solar energy in the photo-assisted Al-air battery.

Synthesis of Co3O4 nanospheres for enhanced photo-assisted supercapacitor

The design of the novel photo-assisted supercapacitor electrodes is considered an effective way to store abundant solar energy and improve energy storage performance. However, the mechanism of photo-induced charge transfer undergoing semiconductor energy band levels is still unclear. Herein, we have synthesized Co3O4 nanospheres (p-type semiconductor) for photo-assisted supercapacitors, and proposed its mechanism of enhanced photo-assisted energy storage by supported experiments and density functional theory (DFT) investigations. Results show the photo-generated charge produced by Co3O4 with semiconductor characteristics will promote the charging reaction under continuous light. After the charging process, the semiconductor characteristics gradually disappear, and the generated Co2+−OH is not sensitive to light response. On the basis of this result, we further propose a new type of light-assisted strategy in tandem with a n-type semiconductor (TiO2) additional electrode, which can provide photo-generated electrons and holes in the charge/discharge processes. The specific capacitance (523 F g−1) under light with TiO2 assisting is increased by 16.7% when compared with that without TiO2-assisted under dark (450 F g−1) at 1 A g−1. This work may provide a theoretical basis and new strategy for improving the energy storage capacity of supercapacitors.