Optimum Band Gap Research Articles

Visible light photocatalytic degradation of dyes by Ag3PO4/g-C3N4/CQDs composite

To facilitate the realize high-value utilization of coal tar pitch, carbon quantum dots (CQDs) were synthesized using ball-milling-assisted H₂O₂ oxidation method with tar pitch as carbon source. Subsequently, Ag₃PO₄/g-C₃N₄/CQDs composites were prepared through the electrostatic self-attraction of CQDs, Ag₃PO₄ and g-C₃N₄. These composites were then employed for the photocatalytic degradation of dyes, including methyl orange (MO), methylene blue (MB) and rhodamine B (RhB). A case study of MO, the degradation rate constant of Ag₃PO₄/g-C₃N₄/CQDs reached 0.02469 min−1, which was 11 times and 1.6 times higher than that of g-C₃N₄ and Ag₃PO₄. It found that Ag₃PO₄:g-C₃N₄ mass ratio of 1:1 and 5 mg/L CQDs solution loading were optimal. Characterization analysis revealed the specific surface area of the catalyst was increased and optimized band gap structure after the combination of CQDs as the co-catalyst. The possible reaction mechanism was also expounded, Ag₃PO₄/g-C₃N₄/CQDs constructed Z-scheme heterostructure, with CQDs as electron bridge. In addition, the impact of catalyst mass, initial concentration, and pH on the degradation process was studied, as well as the cyclic stability of the catalyst. Based on capture and ESR tests, the main active substances involved in the reaction were in the following order: h+ > ·OH > ·O2−. These experiments provided a new idea on the application of coal-based CQDs in the field of visible light catalytic degradation.

Design and optimization of the dual-functional lattice-origami metamaterials

This study proposes a multi-scale composite lattice-origami metamaterial (MCLOM) to achieve excellent bandgap characteristics and energy absorption capacities. The MCLOMs are constructed by considering the high impedance mismatch of lattice structures, the spatial deformability of origami structures, and the tunability of the components in multi-scale composite materials. Firstly, elastic wave propagation characteristics are analyzed in the Bloch wave framework, revealing the realization of complete bandgaps and their generation mechanism by mode shape analysis and transmission spectrum. Subsequently, an optimization framework integrating the particle swarm optimization (PSO) algorithm is developed to maximize the first bandgap’s bandwidth by adjusting various component parameters. Under optimal distribution, the proposed metamaterials achieve remarkable improvements of 289% and 271% in the design objectives of two lattice-origami metamaterials with 90∘ dihedral angle compared to the initial distribution. It can be demonstrated that non-uniform distributions of multi-scale composite materials are dramatically effective for broadband wave attenuation. Additionally, while striving to widen the bandgap, the energy absorption capacities of structures are also crucial. The effect of the distribution of multi-scale composite materials with the optimal bandgap on the energy absorption performance is investigated. The results reveal that the optimal distribution of the lattice-origami metamaterials yields notable improvements of 48.26% and 34.86% under low-velocity impact, and 37.41% and 25.19% under medium-velocity impact. This work presents innovative concepts and approaches for devising and implementing novel dual-functional metamaterials, undoubtedly propelling the continual progress of material science and engineering technology in the times ahead.

Ultra-Stable ITO-Free Organic Solar Cells and Modules Processed from Non-Halogenated Solvents under Indoor Illumination.

Organic Photovoltaics (OPV) is a very promising technology to harvest artificial illumination and power smart devices of the Internet of Things (IoT). Efficiencies as high as 30.2% have been reported for OPVs under warm white light-emitting diode (LED) light. This is due to the narrow spectrum of indoor light, which leads to an optimal bandgap of ≈1.9eV. Under full sunlight, OPV devices often suffer from poor stability compared to the established inorganic PV technologies such as crystalline silicon. This study focuses on a potentially very cost-effective Indium Tin Oxide (ITO) free cell stack with absorber materials processed from non-halogenated solvents. These organic solar cells and modules with efficiencies up to 21% can already achieve remarkable stabilities under typical indoor illumination. Aging under 50,000lux LED lighting leads to very little degradation after more than 11000h. This light dose corresponds to more than 110years under 500lux. For modules encapsulated with a flexible barrier, extrapolated lifetimes of more than 41years are achieved. This shows that OPV is mature for the specific application under indoor illumination. Due to the large number of potential organic semiconducting materials, further efficiency increase can be expected.

Impact of alkaline-earth doping on the mechanical, electronic, optical and photocatalytic properties of CsPb0.875AE0.125Br3 (AE = Be, Mg, Ca, Sr and Ba): Insights from DFT perspective

Density functional theory was used to examine the stability, mechanical properties, photoelectric properties, and photocatalytic performance of the inorganic perovskite CsPbBr3 (AE = Be, Mg, Ca, Sr, and Ba) in order to enhance its properties and lessen the presence of lead. The predicted band gap values (1.31–2.06 eV) were quite close to the optimal band gap for photoelectric conversion. Additionally, CsPb0.875Sr0.125Br3 has effective electron and hole masses that are significantly lower than those of CsPbBr3 in its original form, making it a better candidate for carrier transport. All CsPb0.875AE0.125Br3 materials are mechanically stable and exhibit favorable all-around characteristics. And it has a lower energy loss function and reflectivity in the visible light spectrum, which helps the conversion efficiency. According to the sideband potential and variances in the hole and electron effective masses, CsPb0.875Ca0.125Br3, CsPb0.875Sr0.125Br3, and CsPb0.875Ba0.125Br3 all have a tremendous potential for improving photocatalytic performance and converting solar energy into chemical energy.

Optoelectronics characteristics of chemically synthesized Cu(In1-xGax)Se2 based Schottky and heterojunctions

CIGS is an important photovoltaic material in recent years. In this work, we have performed a comparative analysis of various conduction mechanism in CIGSe2 based Schottky and hetero junction nanostructures. Cu(In1-xGax)Se2 nanostructure dispersed in PVP matrix has been synthesized through chemical bath deposition method at a compositional variation x = 0-1. The structural and optical characterization are done and correlated with the In/Ga fraction ratio. The conduction mechanism for both Schottky and hetero structures has been analyses and systematically co-related with compositional variation of In/Ga. By using the thermionic theory, the diode parameters ideality factor, saturation current, barrier height is determined in hetero junction device and co related with composition of Cu(In1-xGax)Se2. Richardson-Schottky(RS) mechanism is found to be dominant in Schottky type junction whereas both Pool-Frenkel(PF) and Richardson-Schottky(RS) mechanism is involved in hetero junction at different range of bias. Our findings points to Cu(In1-xGax)Se2 nanostructures synthesized at x = 0, 0.3,0.5 as promising candidate for solar cell because of their optimum diode parameters and band gap for absorption of maximum solar radiation.

Optoelectronic optimization of graded-bandgap thin-film AlGaAs solar cells. Part II: optimal antireflection front-surface texturing.

In Part I [Appl. Opt.59, 1018 (2020)APOPAI0003-693510.1364/AO.381246], we used a coupled optoelectronic model to optimize a thin-film AlGaAs solar cell with a graded-bandgap photon-absorbing layer and a periodically corrugated Ag backreflector combined with localized ohmic Pd-Ge-Au backcontacts, because both strategies help to improve the performance of AlGaAs solar cells. However, the results in Part I were affected by a normalization error, which came to light when we replaced the hybridizable discontinuous Galerkin scheme for electrical computation by the faster finite-difference scheme. Therefore, we re-optimized the solar cells containing an n-AlGaAs photon-absorbing layer with either a (i) homogeneous, (ii) linearly graded, or (iii) nonlinearly graded bandgap. Another way to improve the power conversion efficiency is by using a surface antireflection texturing on the wavelength scale, so we also optimized four different types of 1D periodic surface texturing: (i) rectangular, (ii) convex hemi-elliptical, (iii) triangular, and (iv) concave hemi-elliptical. Our new results show that the optimal nonlinear bandgap grading enhances the efficiency by as much as 3.31% when the n-AlGaAs layer is 400nm thick and 1.14% when that layer is 2000nm thick. A hundredfold concentration of sunlight can enhance the efficiency by a factor of 11.6%. Periodic texturing of the front surface on the scale of 0.5-2 free-space wavelengths provides a small relative enhancement in efficiency over the AlGaAs solar cells with a planar front surface; however, the enhancement is lower when the n-AlGaAs layer is thicker.

Triple junction solar cell with Cu In1-x Gex Se as absorbents layers

The design of multi-junction solar cells is guided by both the theoretical optimum bandgap combinations as well as the realistic limitations to materials with these bandgaps. Nowadays, triple-junction III-V multi-junction solar cells are commonly used as GaAs, InGaAs; InGaP ... In this work, we are interested in studying triple junctions based on thin-film solar cells Cu(In1-xGax) Se2, CuInSe2, and CuGaSe2 quaternaries using Silvaco ATLAS software. Incorporating Cu(In0.34Ga0.66) Se2 as an absorber in the middle sub-cell increased the open-circuit voltage by 0.72 V. The highest cell efficiency is 20.89 % (Voc = 2.33 V, Jsc = 9.97 mA/cm2, FF = 89.62%). This triple-junction solar cell demonstrates the potential and limitations of future improvements when voltage and current are considered.

Interstellar photovoltaics

The term 'Solar Cell’ is commonly used for Photovoltaics that convert light into electrical energy. However, light can be harvested from various sources not limited to the Sun. This work considers the possibility of harvesting photons from different star types, including our closest neighbor star Proxima Centauri. The theoretical efficiency limits of single junction photovoltaic devices are calculated for different star types at a normalized light intensity corresponding to the AM0 spectrum intensity with AM0 = 1361 W/m2. An optimal bandgap of > 12 eV for the hottest O5V star type leads to 47% Shockley-Queisser photoconversion efficiency (SQ PCE), whereas a narrower optimal bandgap of 0.7 eV leads to 23% SQ PCE for the coldest red dwarf M0, M5.5Ve, and M8V type stars. Organic Photovoltaics (OPVs) are the most lightweight solar technology and have the potential to be employed in weight-restricted space applications, including foreseeable interstellar missions. With that in mind, the Sun’s G2V spectrum and Proxima Centauri’s M5.5Ve spectrum are considered in further detail in combination with two extreme bandgap OPV systems: one narrow bandgap system (PM2:COTIC-4F, Eg = 1.14 eV) and one wide bandgap system (PM6:o-IDTBR, Eg = 1.62 eV). Semi-empirically modeled JV-curves reveal that the absorption characteristics of the PM2:COTIC-4F blend match well with both the G2V and the M5.5Ve spectrum, yielding theoretical PCEs of 22.6% and 12.6%, respectively. In contrast, the PM6:o-IDTBR device shows a theoretical PCE of 18.2% under G2V illumination that drops sharply to 0.9% under M5.5Ve illumination.

Rational Approach for High‐Efficiency Dopant‐Free Solar Cells Characterized with Key Parameters

Crystalline silicon (c‐Si) solar cells have dominated the photovoltaic market due to their superior mechanical and thermal robustness, nonhazardous nature, optimal energy bandgap, and the availability of established manufacturing techniques. However, conventional c‐Si solar cells fabricated through thermal doping processes present challenges such as high recombination rates and increased production costs. Recently, dopant‐free solar cells have emerged as a promising next‐generation approach; they offer numerous advantages, such as reduced recombination, cost‐effectiveness, environmental friendliness, and applicability to nano‐ and submicrometer structures. This review evaluates the strengths and weaknesses of dopant‐free passivating contact materials for emitters, tracing the development of dopant‐free solar cells from the earliest reports to the current state‐of‐the‐art. A systematic evaluation of these materials based on their electrical and optical properties, coating conformality, stability, and overall photovoltaic performance is presented. Moreover, the limitations of dopant‐free solar cells are identified and strategies to further enhance their efficiency are proposed.

Direct band-gap iodide double perovskite solar cell materials by doping strategy: First-principles predictions

As alternatives to lead-based halide perovskite materials, the lead-free halide double-perovskite materials have many advantages but often show wide indirect band gaps and exceed the optimum band-gap range of 0.9–1.6 eV. Based on first-principles calculations, we scan a serial of halide double-perovskites Cs2B′B''X6 (B′ = Li, Na, K; B'' = In, Bi; X = Cl, Br, I) and find that most of Cs2B′B''X6 double-perovskite materials have direct band gaps but larger than 1.6 eV, while the direct band gaps of Cs2LiInI6 and Cs2NaInI6 are smaller than 0.9 eV. We modulate the electronic structure of Cs2B′InI6 (B′ = Li, Na, K) by an ionic doping strategy through substituting partial In ions with Bi ions and discover more halide double-perovskite solar cell materials with suitable band gap. By exploring the stability, electronic structures and optical properties of the doped double-perovskite Cs2B′In1-xBixI6 (B′ = Li, Na, K, x = 0.25 and 0.75), we discover the Cs2B′In0.75Bi0.25I6 (B′ = Li, Na and K) DP materials show direct band gaps within the optimal band-gap range of 0.90 − 1.60 eV for promising solar-cell materials. Meanwhile, the doped Cs2B′In1-xBixI6 DP materials show good carrier mobility as high as 103 cm2S−1V−1. Moreover, the predicted new materials exhibit excellent light absorption coefficients of 106 cm−1 in the visible light range. These new lead-free inorganic DP materials may offer great promise as candidates for highly efficient solar absorber materials for photovoltaic applications.

First-principles study of metals, metalloids and halogens doped monolayer MoSe2 to tune its electronic properties

Owing to their small band-gap characteristics, two-dimensional transition metal dichalcogenides (TMDCs) have garnered a lot of attention. Herein, we studied the electronic as well as structural properties of doped monolayered MoSe2 with different dopants at the selenium (Se) site employing the density functional theory (DFT) using generalized gradient approximation (GGA). To ascertain bonding nature electron localization function (ELF) was used and Phonon spectra computations were done for dynamical stability tests. We systematically explored the electronic properties of monolayer MoSe2 doped with X-atoms (X comprises of metals Li, Be, Al; metalloids B, Si; non-metals (NMs) C, N, P, O and the NM atoms belonging to halogen group (F, Cl). We compared our outcomes with available experimental and other calculated results in the literature. The appearance of impurity lines in the energy gap causes to diminish the band gaps of doped MoSe2. The change in doping site and dopant element results in a significant reduction in the bandgap of doped MoSe2. To gain more insight into the electronic properties of X-doped MoSe2 monolayer, the partial density of states (PDOS) of the dopants and neighbour Mo atoms are attached to the band structures. Our results give pleasing data regarding the optimization of bandgap which shows that halogen dopant provided n-type doping while the rest of the elements provided p-type doping in MoSe2, which helps to understand the electrical conductivity in addition to their utilization in semiconductor and other optoelectronic devices. We are optimistic that the findings in this work will help experimentalists to carry out their research on a real-world application, which will also widen the investigation of transition-metal dichalcogenides (TMDs) in other sectors.

Effect of Temperature Variation on Band Gap Value in Thin Layers of Nano Photocatalyst Fe2O3/CuO/MnO2

Water is one of the primary needs of all living things in daily life. However, the availability of clean water is currently starting to decrease along with the decline in the quality and quantity of water in the environment. Industrialization and rapid economic development have caused concern due to the habit of disposing of waste without proper management. Attempts to get clean water free from pollution are to utilize technology with photocatalytic properties that can reduce liquid pollutants with Infra-Red light. The purpose of this study was to determine the optimum conditions of the band gap to photocatalyst by temperature variation. Fe2O3, CuO, and MnO2 are doped semiconductor materials for the application of photocatalyst properties. The materials were milled using a High Energy Milling (HEM-3ED) to obtain nanoparticle sizes. The synthesis was carried out by sol-gel and spin-coating methods to make a thin film in enhancing photocatalytic activity in industrial applications. Characterization was conducted using ultraviolet-visible (Uv-Vis) spectrophotometry analysis. The results showed that temperature affects photocatalyst properties and the band gap value obtained at a temperature variation of 400°C is 1.36 eV which is the most optimum semiconductor band gap energy.

Electrodeposited CZTS loaded titania nanotubes for photocatalytic water splitting

Copper Zinc Tin Sulphide (Cu2ZnSnS4 (CZTS)) is a promising semiconductor material with optimum direct band gap of 1.4-1.6 eV, large light absorption coefficient (> 104 cm-1) and p-type electrical conductivity. In addition, its earth abundant and non-toxic elemental constituents make it a good candidate to replace the CdTe and CIGS absorber layers used in thin film solar cells. Also, it has properties that make it useful as a buffer layer that helps to reduce the effective band gap of TiO2 when used as an anode for photocatalytic splitting of water. We prepared titania nano tube (TNT) arrays by anodization of titanium foil. Raman spectrum of TNT confirmed the presence of pure anatase phase of TiO2. Field emission scanning electron microscopy images were obtained to analyse the surface morphology of TNT. CZTS sensitized titania nano tube arrays were then prepared by the electrodeposition of CZTS thin films over TNT arrays. FESEM images of the CZTS sensitized titania nano tube arrays showed that the electrodeposited CZTS layer had a fibrous structure. A photo electro chemical (PEC) cell was set up with CZTS sensitized titania nanotubes as anode. An anodic current density of 0.19 mA/cm2 was shown by the PEC cell when the anode was irradiated with blue light of wavelength 430 nm at power 50 mW/cm2. The incident photon to current conversion efficiency was 1.1 %.

Synergistic modulation of oxygen vacancies and carbon atoms in TiO2 via a simple pot‐in‐pot reaction for improved visible‐light‐driven photocatalytic performances

AbstractNovel synthesis methods are urgently expected for visible‐light‐driven photocatalysts. Hereby we report a series of TiO2‐based photocatalysts (T‐x) by using a simple pot‐in‐pot reaction approach. In this approach, two reaction media are isolated from each other by the crucible reactor. The obtained T‐x sample shows nanosheet‐aggregate structure and wide visible‐light absorption property with an optimal bandgap of 2.35 eV. Factors that influence the bandgap modulation, including nitrogen, fluorine, carbon and oxygen vacancy etc., are discussed in detail. Characterization results demonstrate a substantial increase of oxygen vacancies and carbon atoms in T‐x sample, which were proven to the main reason for the extended visible‐light absorption and enhanced photocatalytic performance. Under visible‐light irradiation, T‐x sample shows excellent photocatalytic degradation activity for RhB with a k value of 0.027 min−1 and good photoelectrochemical performance. Trapping experiments reveal that ⋅OH are the main active species to oxidize the RhB dye. This new pot‐in‐pot reaction approach enables synthesis of novel photocatalytic materials with wide visible‐light‐response.

Multi-objective optimized design of two-dimensional multi-phase phononic crystals with materials self-selection

In this paper, the multi-objective optimization problem (MOOP) of selecting suitable materials from four materials for phononic crystals layouts is investigated based on an optimization method combined nondominated sorting-based genetic algorithm II (NSGA-II) and finite element method (FEM). Driven by different optimization objectives, the changes in the optimized structure compared to the seed structure are the change of the bandgap mechanism from a local resonance mechanism to a Bragg scattering mechanism, the change of the included materials from four to three, and the significant change in the location and extent of the pores and other materials, respectively. The obtained nondominated Pareto solution of MOOP can balance the bandgap and the structural mass, which provides the decision-maker trade-off to select the appropriate optimized solution. Compared with the single-objective optimization problem (SOOP), the MOOP not only obtains a nondominated solution close to the result of SOOP, but also obtains other nondominated solutions of MOOP, which proves the effectiveness of the optimization algorithm in this paper. The design method in this paper can be easily extended to select suitable materials from a wider variety of materials for bandgap optimization design of PnCs, which has very promising applications.

Thermodynamic stability and electronic properties of 2D all-inorganic halogen perovskites

Two-dimensional (2D) all-inorganic metal halide perovskites capture more attentions due to its superior stability compared with three-dimensional (3D) perovskites. However, the power conversion efficiency of 2D all-inorganic perovskite is needed to be largely improved and few theoretical research focuses on its structural stability and optoelectronic properties in depth. Herein, we exploit the electronic structure, thermodynamically stable phase, effective masses, and optical properties of Ruddlesden-Popper (RP) 2D inorganic perovskites Csn+1MnIn+1Cl2n (M = Pb or Sn, n = 1–4 and ∞) by employing density functional theory calculations. It is found that the most stable configuration is closely related to the specific position of the halogen atoms. Moreover, the thermodynamic stability of Pb-based 2D perovskites is further calculated based on the phase diagram, which is notably more stable than that of Cs2SnI2Cl2. In addition, our calculated results show that the band gaps decrease as the number of layers increase in both Sn- and Pb-based perovskites. Moreover, Sn-based 2D perovskites can be adjusted to superior electronic properties with the optimal bandgap ranging from 0.9 to 1.6 eV and small effective mass of carriers considering its future application. This systematic theoretical study provides a significant methodology for fabricating stable, new-type 2D inorganic photoelectric materials according to a reasonable stoichiometric ratio.

From non-stoichiometric CTSe to single phase and stoichiometric CZTSe films by annealing under Sn+Se atmosphere

One of the new materials for next-generation thin film solar cells is Cu2ZnSnSe4 (CZTSe). However, achieving a single-phase CZTSe compound remains a challenge. This study describes the development of Cu2ZnSnSe4 thin films through the sequential deposition of stacked films of non-stoichiometric Cu2SnSe3 (CTSe) and ZnSe by magnetron sputtering. The structural, morphological, and electrical properties as well as the surface chemistry of the films were investigated and compared depending on the growth sequence of the thin films. By using Raman spectroscopy and grazing incidence X-ray diffraction, the tetragonal CZTSe structure was confirmed. Scanning electron microscopy and energy-dispersive spectroscopy measurements of the morphological and compositional properties indicated large grains and dense surfaces with an elemental composition close to the desired stoichiometry in SLG\\SnSe2\\Cu\\ZnSe and SLG\\SnSe2\\Cu2Se\\ZnSe stacks. To ascertain the surface chemistry and unique characteristics of the produced films, additional X-ray photoemission spectroscopy experiments were carried out. The optimal band gap values for the absorber layers were found using conventional spectroscopy, and they ranged from 0.88 to 1.47 eV. According to the electrical measurements, all the films were p-type and have high carrier concentrations between 1016 and 1020 cm−3. Our findings demonstrate that employing a sequential deposition approach and annealing in different atmospheres can yield CZTSe absorber layers with desirable properties, overcoming the challenge of non-stoichiometric CTSe precursors.

A Multi-Channel Frequency Router Based on an Optimization Algorithm and Dispersion Engineering.

Integrated frequency routers, which can guide light with different frequencies to different output ports, are an important kind of nanophotonic device. However, frequency routers with both a compact size and multiple channels are difficult to realize, which limits the application of these frequency routers in nanophotonics. Here, a kind of bandgap optimization algorithm, which consists of the finite element method and topology optimization, is proposed to design a multi-channel frequency router. Channels supporting photonic edge states with different frequencies are built through the synthetic dimension of translational deformation. Due to the help of the developed optimization algorithms, the number of channels and output ports can be increased up to nine while maintaining ultracompact device size. The device operates within a working band of 0.585-0.665 c/a, corresponding to 1.504-1.709 μm when the lattice constant is set as 1 μm, covering the telecom wavelength of 1.55 μm. The average crosstalk is about -11.49 dB. The average extinction ratio is around 16.18 dB. Because the bus of the device can be regarded as a part of a topological rainbow, the results show that the structure is robust to fabrication errors. This method is general, which can be used for different materials and different frequency ranges. The all-dielectric planar configuration of our router is compact, robust, and easy to integrate, providing a new method for on-chip multi-channel broadband information processing.

Eco-friendly perovskite/CZTSSe tandem cell exceeding 28% efficiency through current matching and bandgap optimization: a numerical investigation

Eco-friendly perovskite/CZTSSe tandem cell exceeding 28% efficiency through current matching and bandgap optimization: a numerical investigation

Crystallization Engineering of CuNi2 S4 Ultra-Fine Nanocrystals with Optimized Band Structures for Efficient Photocatalytic Pollutant Degradation and Hydrogen Production.

The mono-dispersed cubic siegenite CuNi2 S4 ultra-fine (≈5nm) nanocrystals are fabricated through crystallization engineering under hot injection. The strong hydroxylation on mostly exposed CuNi2 S4 (220) surface leads to the formation of multi-valence (Cu+ , Cu2+ , Ni2+ , Ni3+ ) species with unsaturated hybridization and coordination micro-environments, which can induce rich redox reactions to optimize interfacial kinetics for the adsorbed reaction intermediates. The as-synthesized CuNi2 S4 nanocrystals with ultra-small particle size and the characteristics of being highly dispersed can increase specific surface area and hydroxylated active sites, which considerably contribute to the improvement of photocatalytic activities. Experimental and theoretical studies indicate that the CuNi2 S4 with unique surface condition can properly modulate the charge density distribution and the electronic band structure, thus achieving an optimal band gap for enhancing visible light absorption. Additionally, the strong hydroxylation on CuNi2 S4 (220) surface can not only make the photocatalytic process stable in alkaline environment but also bring about an impurity level between conduction and valence band, which facilitates the separation of photo-induced charge carriers by suppressing the rapid re-combination of exited electrons and holes. The optimization of band structure should be the intrinsic reason for the efficient photocatalytic pollutant degradation and hydrogen production under visible light illumination.