Visible Light Absorption Efficiency Research Articles

CuO/PbTiO3: A new-fangled p–n junction designed for the efficient absorption of visible light with augmented interfacial charge transfer, photoelectrochemical and photocatalytic activities

The asymmetric photocurrent in opposite directions and the rectifying behaviour of all the CuO/PbTiO3 samples reveal the formation of a p–n junction between them, this helps to augment the charge anti-recombination process at the interface and enhance the photocatalytic performance.

Optical properties of a new Bi38Mo7O78 semiconductor with fluorite-type δ-Bi2O3 structure

Bi3+-containing inorganic materials usually show rich optical and electronic properties due to the hybridization between 6s and 6p electronic components together with the lone pair in Bi3+ ions. In this work, a new semiconductor of bismuth molybdate Bi38Mo7O78 (19Bi2O3·7MoO3) was synthesized by the sol-gel film coating and the following heat process. The samples developed into nanoparticles with average size of 40nm. The phase formation was verified via the XRD Rietveld structural refinement. Orthorhombic Bi38Mo7O78 can be regarded to be derived from the cubic δ-phase Bi2O3 structure. The microstructure was investigated by SEM, EDX, TEM, BET and XPS measurements. The UV-vis absorption spectra showed that the band gap of Bi38Mo7O78 (2.38eV) was greatly narrowed in comparison with Bi2O3 (2.6eV). This enhances the efficient absorption of visible light. Meanwhile, the conduction band of is wider and shows more dispersion, which greatly benefits the mobility of the light-induced charges taking part in the photocatalytic reactions. Bi38Mo7O78 nanoparticles possess efficient activities on the photodegradation of methylene blue (MB) solutions under the excitation of visible-light. The photocatalysis activities and mechanisms were discussed on the crystal structure characteristics and the measurements such as photoluminescence, exciton lifetime and XPS results.

Boron-doped TiO2 Nanotube Array: In Situ Electrochemical Preparation and Study on Its Photocatalytic Activity

Through anodization in a NH4BF4-containing glycerol solution, TiO2 nanotube arrays were obtained and showed characteristics of both boron-doping and double-layer walled structure. Boron doping inhibited the growth of anatase crystals during the annealing process and enhanced the UV–visible light absorption efficiency of the TiO2 nanotube array. For boron-doped inside, the TiO2 nanotube arrays demonstrated improved photocatalytic activities under both UV and visible light irradiation. This work demonstrates the feasibility of electrochemical preparation of a boron-doped TiO2 nanotube array with double-layer walled structure and its potential application for photocatalysis.

Rational Band Structure Engineering of TiO2 for Photoelectrochemical Water Splitting

The search for new semiconducting materials or the engineering of existing semiconductors for commercially viable photoelectrochemical (PEC) water splitting has been extremely challenging. Meeting that challenge requires the discovery of a semiconductor with several tightly coupled material property criteria such as appropriate band gap (1.6 – 2.2 eV), efficient visible light absorption, high carrier mobility, and correct band edge positions that straddle the water redox potentials. However, previous searches/modifications of semiconducting materials for PEC water splitting application have often focused on a particular individual criterion such as band gap, neglecting the possible detrimental consequence to other important criteria. In this talk, general strategies for the rational design of semiconductors such as TiO2 to simultaneously meet all of the requirements for a high efficiency solar-driven PEC water splitting device are discussed. Density-functional theory calculations reveal that with appropriate donor-acceptor co-incorporation, heavily doped anatase TiO2 hold great potential to satisfy all of the criteria for a viable PEC device. Other approaches to modify the band structure of TiO2, such as the application of strain, will also be discussed.

(Invited) Si/NiCoSex Nanorods Photocathode for Enhanced Photoelectrochemical Hydrogen Production

Hydrogen is considered as the ideally sustainable alternative to fossil fuels. Photoelectrochemical (PEC) water splitting is attractive as it can store solar energy in the form of hydrogen. Silicon as an earth-abundant and efficient visible light absorption material has been widely used in PEC for hydrogen generation. However, achieving a high photocurrent density still faces a challenge when utilizing silicon as a photocathode owing to its sluggish hydrogen evolution reaction (HER) kinetics and low light harvesting capability. Here, we design a novel one-dimensional photocathodes - Si/NiCoSex nanorods using a facile photo-assistant electrodeposition method and demonstrate their enhanced PEC hydrogen production performance with high photocurrent density. Si nanorods were covered with NiCoSex electrocatalysts, where electrons efficiently diffuse across the electrode/electrolyte interface. The synergy effects of the high light harvesting of Si nanorod morphology and NiCoSex electrocatalyst enhanced the production and utilization of photogenerated electrons. The high quality junctions between one-dimensional semiconductor and electrocatalyst for charge collection and utilization is significant to enhance the efficiency of water splitting for hydrogen generation. The improved charge separation and hydrogen evolution reaction kinetics on the electrode-electrolyte interface were confirmed. Furthermore, we expect that inducting the buried junction of Si or optimizing the electrode/electrolyte interface might further boost the PEC-HER performance of the Si/NiCoSex nanorods, making it a more competitive candidate for PEC-HER photocathode. The reported new Si/NiCoSex nanorods photocathode offers a new strategy for preparing efficient solar energy conversion devices.

Enhanced visible light photocatalytic activity of CdMoO4 microspheres modified with AgI nanoparticles

Visible-light-driven AgI/CdMoO4 composite was successfully fabricated via a hydrothermal process combined with a facile deposition–precipitation method at room temperature. The obtained samples were characterized using various analytical techniques. The experimental results showed that the AgI/CdMoO4 composite displayed enhanced visible light photocatalytic activity for degrading organic dyes. The 32.9%-loaded AgI/CdMoO4 photocatalyst showed the highest photocatalytic activity and maintained good stability. The enhanced photocatalytic activity could be ascribed to the efficient visible light absorption and separation of electrons and holes by using CdMoO4 as an electron trap.

Photocatalytic degradation of bisphenol A by oxygen-rich and highly visible-light responsive Bi12O17Cl2 nanobelts

Visible light responsive photocatalysts can directly harvest energy from solar light, offering a desirable way to resolve environmental pollution problems through utilizing solar energy. Bismuth oxychloride (BiOCl) with a band gap of about 3.4eV is widely recognized as an effective photocatalyst for the degradation of organic dye molecules under visible light irradiation, but such a photocatalytic degradation has to be assisted by dye-sensitization. Thus, preparation of BiOCl photocatalyst to achieve visible light response without dye-sensitization is greatly desired, as this would greatly expand their practical applications for the degradation of non-dye pollutants. In this work, oxygen-rich Bi12O17Cl2 nanobelts with a band gap of 2.07eV were synthesized by using a solvothermal route, and their photocatalytic performance was evaluated through photodegrading a colorless contaminant bisphenol A (BPA) in an aqueous solution. In comparison with BiOCl, which is not sensitive to visible light, the oxygen-rich Bi12O17Cl2 nanobelts exhibited a drastically enhanced visible-light photoreactivity and were also superior to the well-known photocatalyst TiO2 (P25). The greatly enhanced photocatalytic performance of the Bi12O17Cl2 nanobelts was attributed to their efficient visible light absorption. Our findings might be helpful to explore visible light bismuth-based photocatalysts for pollutant degradation and water treatment.

Multilayer tungsten-alumina-based broadband light absorbers for high-temperature applications

Efficient broadband absorption of visible and near-infrared light by low quality-factor metal-insulator-metal (MIM) resonators using refractory materials is reported. Omnidirectional absorption of incident light for broad angles of incidence and polarization insensitivity are observed for the fabricated MIM resonator. Excellent thermal stability of the absorber is demonstrated at high operating temperatures (800 °C). The experimental broadband absorption spectra show good agreement with simulations. The resonator with 12 nm top tungsten and 100 nm alumina spacer film shows absorbance above 95% in the range of 650 to 1750 nm. The absorption window is tunable in terms of the center wavelength, bandwidth, and the value of maximum absorbance (~98%) by simple variation of appropriate layer thicknesses. Owing to their flexibility, ease of fabrication and low cost, the presented absorbers have the potential for a wide range of applications, including the use in commonly used infrared bands or absorbers for (solar) thermo-photovoltaic energy conversion, where high absorbance and simultaneously low (thermal) re-radiation is of paramount importance.

Large enhancement of the photovoltaic effect in ferroelectric complex oxides through bandgap reduction.

Tuning the bandgap in ferroelectric complex oxides is a possible route for improving the photovoltaic activity of materials. Here, we report the realization of this effect in epitaxial thin films of the ferroelectric complex oxide Bi3.25La0.75Ti3O12 (BLT) suitably doped by Fe and Co. Our study shows that Co (BLCT) doping and combined Fe, Co (BLFCT) doping lead to a reduction of the bandgap by more than 1 eV compared to undoped BLT, accompanied by a surprisingly more efficient visible light absorption. Both BLCT and BLFCT films can absorb visible light with a wavelength of up to 500 nm while still exhibiting ferroelectricity, whereas undoped BLT only absorbs UV light with a wavelength of less than 350 nm. Correlated with its bandgap reduction, the BLFCT film shows a photocurrent density enhanced by 25 times compared to that of BLT films. Density functional theory calculations indicate that the bandgap contraction is caused by the formation of new energy states below the conduction bands due to intermixed transition metal dopants (Fe, Co) in BLT. This mechanism of tuning the bandgap by simple doping can be applied to other wide-bandgap complex oxides, thereby enabling their use in solar energy conversion or optoelectronic applications.

Multilayered MoS2 coated TiO2 hollow spheres for efficient photodegradation of phenol under visible light irradiation

Multilayered MoS2 coated hollow TiO2 heterostructures were fabricated by a two-step hydrothermal process using TiO2 hollow spheres as precursors. Compared with pure TiO2, the heterostructures exhibited greatly enhanced visible-light-responsive activity on photodecomposition of colorless phenol, due to the enlargement of visible light absorption and improvement separation efficiency of photogenerated electron-hole pairs. Photocatalytic activity was influenced by the MoS2/TiO2 ratio and the optimized composition was obtained with high stability during recycling.

Electrochemical Deposition of Cu2O Thin Film As Efficient Photocatalyst for Water Splitting Reaction

Solar water splitting can form the basis of a sustainable hydrogen based energy economy, however the system requires optimisation in terms of cost and performance on a scale commensurate with the global energy demand. Photocatalysts consist of solar spectrum absorber semiconductor materials and co-catalysts as the redox mediator for hydrogen and oxygen evolution reactions. The key factors for efficient solar water splitting process to occur with such photocatalysts are a semiconductor with a sufficiently narrow band gap to absorb visible lights, a suitable thermodynamic potential for the redox reactions, and stability against photocorrosion. However, current photocatalysts also suffer from undesirable photoelectron-hole (charge carrier) recombination within the semiconductor due to their short diffusion path lengths and lifetimes, which results in fewer than 10% of the incident photons being used for water splitting. Cuprous Oxide (Cu2O), one of the few naturally occurring p-type metal oxides, is the most promising candidate for an efficient photocathode because of its earth abundance, a band gap of 2.0 eV, which enables efficient visible light absorption, a scalable CMOS compatible deposition technique (e.g., electrodeposition), and favourable energy band position i.e. conduction band lies 0.7 eV negative of the hydrogen evolution potential that drives off half of the water reduction reaction without of any external bias. However, the application of Cu2O as photocathode is limited by two drawbacks; fast photoelectron–hole recombination owing to short carrier diffusion length and self-photo corrosion in aqueous solution under illumination since the potentials of Cu2O reduction and oxidation to metallic Cu and CuO, respectively, lie within the bandgap. Hence, the solar-to-fuel conversion efficiency on Cu2O depends on how fast the photoelectron can be extracted to the reduction reaction sites and inhibition of electrolyte access to Cu2O surface. A thin protective coating and a heterostructure in the seed layer of the electrochemically deposited Cu2O film enhance the overall photocatalytic performance versus the Cu2O film alone. The thickness of the Cu2O film was optimised to avoid the charge carrier recombination towards the efficient and stable photocatalytic performance. The photocatalytic performance of Cu2O thin film is shown in Fig 1, in which the film performance is unexpectedly high and only dropped to 22% after the first cycle at 0.0 VRHE, after which it stabilises. This presentation will discuss our current research to develop cheap and efficient photocatalysts and their enhancement mechanism towards photocatalytic water splitting reactions. Figure 1

Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties

Black TiO2 nanotube arrays were synthesized via a two-step anodization procedure by controlling the duration of anodization and annealing in an argon atmosphere. Oxygen vacancies were introduced in black TiO2 nanotube arrays during the anodization and annealing process due to an insufficient oxygen atomosphere, as confirmed by EPR, XRD, SEM, TEM, XPS, and Raman analysis. Black TiO2 nanotube arrays prepared in 6 h (denoted as Ar TNT-6h) exhibited excellent photoelectrocatalytic performance and stable photoelectrochemical performance due to a considerable amount of oxygen vacancies, which ensured charge separation efficiency and strong visible light absorption. The photoelectrocatalytic activities of the samples were monitored by the decomposition of Rhodamine B (RhB). Notably, the black TiO2 nanotube arrays exhibited lower charge-transfer resistance in electrochemical impendence spectroscopy (EIS). The photocurrent density of Ar TNT-6h (1.2 mA/cm2) was nearly twelve times higher than that of air-TNTs (pristine TiO2 nanotube arrays annealed in air, 0.1 mA/cm2). The formation of oxygen vacancies in black TiO2 nanotube arrays was influenced by the duration of anodization and the annealing atmosphere. A considerable amount of oxygen vacancies significantly improved the separation efficiency of photogenerated electrons and holes, thus enhancing the photoelectrocatalytic efficiency.

Synthesis and photocatalytic application of visible-light active β-Fe2O3/g-C3N4 hybrid nanocomposites

Hybrid organic/inorganic nanocomposites comprised of nanocrystalline iron oxide at the metastable β-phase and graphitic carbon nitride (g-C3N4) were prepared via a facile in-situ growth strategy embedded in a solid state process. The hybridized β-Fe2O3/g-C3N4 nanomaterials were thoroughly characterized by a variety of techniques, including UV–vis absorption, nitrogen physisorption, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and transmission electron microscopy (TEM). Their photocatalytic activity was evaluated under both simulated solar light and pure visible light irradiation against the photodegradation of methyl orange (MO), rhodamine B (RhB) and phenol. The prepared β-Fe2O3/g-C3N4 nanocomposites were proven durable and significantly more efficient than the single components. The β-Fe2O3 content in the final material was tuned to optimize the photocatalytic performance, with particular attention to the activity under visible light. The enhanced photoactivity was attributed to a) the improved optical properties of the prepared nanocomposites, presenting narrower band-gap energies and increased visible light absorption efficiency, and b) to the efficient separation of the photoinduced charge carriers driven by the matched band edges in the heterostructure. The predominant active species responsible for the photodegradation activity were determined and a possible mechanism is proposed.

Facile synthesis of surface N-doped Bi2O2CO3: Origin of visible light photocatalytic activity and in situ DRIFTS studies

Bi2O2CO3 nanosheets with exposed {001} facets were prepared by a facile room temperature chemical method. Due to the high oxygen atom density in {001} facets of Bi2O2CO3, the addition of cetyltrimethylammonium bromide (CTAB) does not only influence the growth of crystalline Bi2O2CO3, but also modifies the surface properties of Bi2O2CO3 through the interaction between CTAB and Bi2O2CO3. Nitrogen from CTAB as dopant interstitially incorporates in the Bi2O2CO3 surface evidenced by both experimental and theoretical investigations. Hence, the formation of localized states from NO bond improves the visible light absorption and charge separation efficiency, which leads to an enhancement of visible light photocatalytic activity toward to the degradation of Rhodamine B (RhB) and oxidation of NO. In addition, the photocatalytic NO oxidation over Bi2O2CO3 nanosheets was successfully monitored for the first time using in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). Both bidentate and monodentate nitrates were identified on the surface of catalysts during the photocatalytic reaction process. The application of this strategy to another relevant bismuth based photocatalyst, BiOCl, demonstrated that surface interstitial N doping could also be achieved in this case. Therefore, our current route seems to be a general option to modify the surface properties of bismuth based photocatalysts.

Visible light-driven water oxidation with a subporphyrin sensitizer and a water oxidation catalyst.

A new subporphyrin was synthesized for use as a molecular sensitizer in electrochemical and dye-sensitized photoelectrochemical water oxidation. A photoelectrochemical cell with a TiO2 electrode modified with the sensitizer and a molecular water oxidation catalyst generated higher photocurrent than reference cells that have electrodes modified with either the photosensitizer or the catalyst under visible light (λ > 500 nm) illumination. Oxygen evolution was confirmed after photolysis by GC and GC-MS analyses using isotope-labeling experiments. The large molar extinction coefficients of the ring-contracted porphyrin in the visible region enabled kinetic analysis by time-resolved transient absorption spectroscopy, which also supported the photocatalytic activity.

Synthesis of carbon-doped BiVO4@multi-walled carbon nanotubes with high visible-light absorption behavior, and evaluation of their photocatalytic properties

In order to realize high efficiency visible-light absorption and electron–hole separation of bismuth vanadate (BVO), we synthesized carbon-doped BVO (C-BVO) with high visible-light absorption behavior. We used polyvinylpyrrolidone K-30 as a template and L-cysteine as the carbon source in a one-step hydrothermal synthesis method, and then obtained the carbon-doped BVO@multi-walled carbon nanotubes (C-BVO@MWCNT) by a two-step method. The carbon nanotubes were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, specific surface area, electron spin resonance, and transient photocurrent responses. The XRD analysis confirmed that all photocatalysts were in the same crystal form with a single monoclinic scheelite structure. Combining this with the other characterization results, we showed that the carbon element was successfully doped in BVO and the resulting C-BVO was successfully coupled with multi-walled carbon nanotubes. The removal ratio of rhodamine B by C-BVO@MWCNT was much higher than those by BVO and C-BVO under visible-light irradiation. Recycling experiments verified the stability of C-BVO@MWCNT, which was proved to offer excellent adsorption, strong visible-light absorption behavior, and high electron–hole separation efficiency. Such properties are expected to be useful in practical applications.

Enhanced visible light photocatalytic activity by Cu2O-coupled flower-like Bi2WO6 structures

Cu2O nanodots decorated flower-like Bi2WO6 were prepared via an interfacial self-assembly method. The Cu2O nanodots, with an average diameters of 20nm, were dispersed on the surface of Bi2WO6 uniformly, as evidenced by characterization of the structure and composition. The as-prepared Cu2O/Bi2WO6 hybrid photocatalysts revealed the lower charge-transfer resistance, higher photocurrent intensity and the outstanding photocatalytic activity. The 3wt% Cu2O/Bi2WO6 composites showed the highest degrade rate for methylene blue (MB), which was 2.14 and 12.25 times that of the pure Bi2WO6 and Cu2O, respectively. Significantly, the superior stability was also observed in the five cyclic runs. The enhanced photocatalytic performance was attributed to the enhancement of visible light absorption efficiency as well as the efficient photo-generated charge separation originated from a strong interaction in the intimately contact interface, which was confirmed by the results of photocurrent and EIS measurements. Based on the experimental results, a mechanisms on enhancement of photocatalytic activity have been emphasized.

Novel ternary component Ag-SrTa2O6/g-C3N4 photocatalyst: Synthesis, optical properties and visible light photocatalytic activity

In this work, we report on the synthesis of a novel ternary component Ag-SrTa2O6/g-C3N4 photocatalytic system with efficient visible light photocatalytic activity toward Cr(VI) photoreduction and methyl orange degradation. The samples were carefully characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, UV–vis diffuse reflectance spectroscopy, Fourier transformed infrared spectroscopy and photocatalytic test. It is found that SrTa2O6 was deposited on the surface of g-C3N4 and that obtained SrTa2O6/g-C3N4 heterojunction photocatalyst showed strong absorption in the visible light region. Photocatalytic test indicated that the as-prepared SrTa2O6/g-C3N4 heterojunction showed increased photocatalytic activity toward Cr(VI) photoreduction and methyl orange degradation in comparison with the bare SrTa2O6 and g-C3N4 under visible light irradiation. The matching of the band structure between SrTa2O6 and g-C3N4 induced an efficient photogenerated electron transfer from the conduction band of g-C3N4 to the conduction band of SrTa2O6, resulting in efficient separation of the photogenerated electron–hole pairs and the subsequent promotion of photocatalytic activity. Moreover, the decoration of Ag on SrTa2O6/g-C3N4 led to the formation of ternary component Ag-SrTa2O6/g-C3N4 photocatalyst, which can highly enhance the visible light absorption efficiency and robustlypromote the photocatalytic activity by a factor of 2.8 for Cr(VI) photoreduction and 4.6 for methyl orange degradation, respectively. Ag decoration on SrTa2O6/g-C3N4 can not only extend the visible light absorption region due to surface plasmon resonance effects, but also act as an electron mediator for efficient migration of photogenerated electrons and simultaneously prevent the recombination of photogenerated electron–hole pairs as much as possible. Finally, a possible photocatalytic mechanism of the charge transfer in Ag-SrTa2O6/g-C3N4 photocatalyst was proposed.

Construction of novel ternary component photocatalyst Sr0.25H1.5Ta2O6·H2O coupled with g-C3N4 and Ag toward efficient visible light photocatalytic activity for environmental remediation

This work reports on the fabrication of a novel ternary component Ag–Sr0.25H1.5Ta2O6·H2O/g-C3N4 photocatalytic system with highly enhanced visible light photocatalytic activity toward Cr(VI) photoreduction and methyl orange degradation. The result indicated that Sr0.25H1.5Ta2O6·H2O nanoparticles were deposited on the surface of g-C3N4 with high dispersion and that obtained Sr0.25H1.5Ta2O6·H2O/g-C3N4 heterojunction photocatalyst showed strong absorption in the visible light region. The Sr0.25H1.5Ta2O6·H2O/50wt%–g-C3N4 composite displayed increased photocatalytic activity for Cr(VI) photoreduction and methyl orange degradation in comparison with the pristine Sr0.25H1.5Ta2O6·H2O and g-C3N4 under visible light irradiation. The matching of the band structure between Sr0.25H1.5Ta2O6·H2O and g-C3N4 induced an efficient photogenerated electron transfer from the conduction band of g-C3N4 to the conduction band of Sr0.25H1.5Ta2O6·H2O, resulting in efficient separation of the photogenerated electron–hole pairs and the subsequent promotion of photocatalytic activity. Moreover, Ag decorated on Sr0.25H1.5Ta2O6·H2O/g-C3N4 can enhance the visible light absorption efficiency and robustly improve the photocatalytic activity by a factor of 4.7 for Cr(VI) photoreduction and 14.2 for methyl orange degradation, respectively. Ag nanoparticles on Sr0.25H1.5Ta2O6·H2O/g-C3N4 not only extended the visible light absorption region due to surface plasmon resonance effects, but also acted as an electron mediator as well as an electron acceptor for efficient charge separation.

Rational Design of Semiconductor for Photoelectrochemical Water Splitting

The search for semiconducting materials for commercially viable photoelectrochemical (PEC) water splitting has been extremely challenging. Meeting that challenge requires the engineering of a semiconductor with the following tightly coupled material property criteria: appropriate band gap (1.6 – 2.2 eV), efficient visible light absorption, high carrier mobility, correct band edge positions that straddle the water redox potentials, and durability in solution environment. In this talk, I will discuss some general strategies for the rational design of semiconductors to simultaneously meet the requirements for a high efficiency solar-driven PEC water splitting devices. Density functional theory (DFT) calculations reveal that with appropriate donor-acceptor pairs or acceptor-acceptor co-incorporation, anatase TiO2 holds great potential for a viable PEC device. Other approaches to modify the band structure of TiO2, such as the application of strain and band structure engineering of other materials such as CoX2O4 and BiVO4, will also be discussed.