Visible Light Absorption Efficiency Research Articles

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.

Surface decoration of BiPO4 with BiOBr nanoflakes to build heterostructure photocatalysts with enhanced photocatalytic activity

BiOBr nanoflakes-decorated BiPO4 (BiOBr/BiPO4) composite photocatalysts were prepared by a simple hydrothermal method. The BiOBr nanoflakes were tightly dispersed on the surface of the BiPO4 and formed p–n heterojunctions between BiOBr and BiPO4, as evidenced by characterization of the structure and composition. The p–n heterojunctions BiOBr/BiPO4 composites could greatly improve photocatalytic activity, and the 60wt% BiOBr/BiPO4 composite showed the highest degrade rate for methylene blue (MB), which was eight times and two times than that of pure BiPO4 and BiOBr, meanwhile, the BiOBr/BiPO4 composite also exhibited superior activities for phenol degradation under visible light. The enhanced photocatalytic performance was attributed to the enhancement of visible-light absorption efficiency as well as the formation of p–n heterojunctions in BiOBr/BiPO4, which could greatly accelerate the separation efficiency of photogenerated charge carriers. In addition, the roles of the radical species were investigated, and the O2− and h+ were thought to dominate the photocatalytic process. Based on the experimental and theoretical results, the possible photocatalytic mechanism was proposed.

Hollow Microporous Organic Networks Bearing Triphenylamines and Anthraquinones: Diffusion Pathway Effect in Visible Light-Driven Oxidative Coupling of Benzylamines.

Hollow microporous organic networks were prepared by using silica spheres as the template and tris(4-ethynylphenyl)amine and 2,6-diiodo-9,10-anthraquinone as the building blocks for the Sonogashira coupling. The resultant materials bearing triphenylamine and anthraquinone moieties showed efficient visible light absorption and catalytic activities in the photochemical oxidative coupling of benzylamines. Through the comparison studies of hollow and nonhollow catalytic materials, the diffusion pathway effect of the substrates was clearly observed in the photochemical conversion of benzylamines.

Optical properties and visible-light-driven photocatalytic activity of Bi8V2O17 nanoparticles

Bi8V2O17 (4Bi2O3·V2O5) nanoparticles with the uniform size of about 50 nm were fabricated through the Pechini method. The crystal structure was investigated by X-ray powder diffraction and the structural refinement. The surface of the as-synthesized samples was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy, and X-ray photoelectron spectroscopy. The optical properties, band structure, and the degradation mechanisms were discussed. The experimental results demonstrate that Bi8V2O17 nanoparticles have an efficient visible-light absorption with band-gap energy of 1.85 eV and a direct allowed electronic transition. The photocatalytic activity was evaluated by the photodegradation of the methylene blue (MB) under visible-light irradiation (λ > 420 nm) as a function of time. These results indicate that Bi8V2O17 could be a potential photocatalyst driven by visible light. The effective photocatalytic activity was discussed on the base of the crystal structure characteristic.

The effects of precursors and loading of carbon on the photocatalytic activity of C–BiVO4 for the degradation of high concentrations of phenol under visible light irradiation

A series of carbon modified BiVO4 (C–BiVO4) photocatalysts were synthesized by impregnation calcination method of BiVO4 with glucose, sucrose and cellulose as precursors, respectively. The prepared photocatalysts were characterized by XRD, HR-TEM, SEM, Raman, BET, UV–vis DRS and PL. Their photocatalytic activities were investigated by degradation of phenol under the visible light. The precursor and loading of carbon had remarkable effects on physical–chemical properties of samples and degradation activities of phenol. The increase of the number of carbon ring in precursors led to a better crystallinity, smaller grain size, larger surface area and higher separation efficiency of electron–hole pairs. The increase of loading led to a smaller catalyst grain size, larger surface area, higher visible light absorption and higher separation efficiency of electron–hole pairs. The results showed that the 0.5wt.% of C–BiVO4–cellulose has the best photocatalytic activity and degraded 88.7% of 5mg/L phenol. Besides, addition of 25mmol KBrO3 as an electron capture agent can degrade 27.4% of 1000mg/L phenol. The degradation efficiency of the same system above mentioned under sunlight irradiation was 22.6%. In addition, the relationship between the photocatalytic activity of C–BiVO4 and its physical–chemical properties was also discussed.

Optical properties and visible light-driven photocatalytic activity of Bi11VO19 nanoparticles with δ-Bi2O3-structure

The δ-Bi2O3-structural Bi11VO19 nanoparticles with the average size of ∼70nm were fabricated through the Pechini method for a possible photocatalytic application. The crystal structure was investigated by X-ray powder diffraction (XRD) and the structural refinement. The as-synthesized sample was also characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), UV–vis spectroscopy and photoluminescence measurements. The experimental results demonstrate that Bi11VO19 nanoparticles have an efficient visible light absorption with band-gap energy of 2.23eV. The photocatalytic activity was evaluated by the photodegradation of the methylene blue (MB) under visible light irradiation (λ>420nm) as a function of time. All the results indicate that Bi11VO19 could be a potential photocatalyst driven by visible-light. The effective photocatalytic activity was discussed on the base of the crystal structure characteristic.

Photocatalytic activity of anatase–nickel ferrite heterostructures

The simple co‐precipitation route was used to couple commercial TiO2 anatase nanopowder with nickel ferrite (NiFe2O4). The morphology and the crystalline structure of composite nanoparticles were characterised by TEM, N2 adsorption‐desorption, XRD and Rietveld refinement, XPS and XAS. The optical and magnetic properties were investigated. After co‐precipitation NiFe2O4 nanoparticles, composed of spinel ferrite crystal phase, were formed on the surface of TiO2 anatase nanopowder. The TiO2/NiFe2O4 composite oxide demonstrated large specific surface area, high visible light absorption efficiency and efficient charge carrier separation, compared to pristine anatase TiO2 or pristine NiFe2O4, representatively. The obtained TiO2/NiFe2O4 composite oxides, with different nickel ferrite contents (5, 10, 25, 50 and 75 wt%) showed decent visible light photocatalytic efficiency, up to three times higher than pure anatase or pure NiFe2O4. However, TiO2/NiFe2O4 composite oxides did not demonstrate high magnetic properties.

Synthesis of a visible-light active V2O5–g-C3N4 heterojunction as an efficient photocatalytic and photoelectrochemical material

Synergistic enhancement of the photocatalytic degradation of DR81 using V2O5–g-C3N4 is due to an increase in visible-light absorption efficiency and rapid photoinduced charge separation.

Simultaneous doping and heterojunction of silver on Na2Ta2O6nanoparticles for visible light driven photocatalysis: the relationship between tunable optical absorption, defect chemistry and photocatalytic activity

In this work, we report the synthesis of Na2Ta2O6/Ag heterojunctions with the aim of tailoring their structural, electronic and photocatalytic properties. The as-prepared samples were characterized by X-ray diffraction, transmission electron microscopy, UV-vis optical diffuse reflectance spectroscopy, and X-ray photoelectron spectroscopy. The effects of silver doping and heterojunctions on the electronic structure and photodegradation of a rhodamine B dye solution were investigated experimentally and theoretically. It was found that silver played critical roles in modulating the electronic structure, defect chemistry and visible light photocatalytic activity. Partial incorporation of Ag ions into the Na2Ta2O6 host lattice led to a monotonous lattice expansion and introduced an interband for band gap narrowing. Meanwhile, some of the Ag ions were reduced, and crystallized into Ag nanoparticles on Na2Ta2O6 surfaces that can promote the visible light absorption efficiency of Na2Ta2O6/Ag heterojunctions. Moreover, the doping and heterojunction of silver can also result in the modulation of defective oxygen species that have a great impact on photocatalytic performance. As a consequence of the well-controlled electronic structure, surface defect chemistry, Ag particle size, BET surface area, and Ag+ doping effects, the photocatalytic performance of the Na2Ta2O6/Ag heterojunctions was optimized at an as-measured Ag/Ta molar ratio of 1.17.

Synthesis of nitrogen-doped graphene–Ag2CO3 composites with enhanced photocatalytic efficiency

A series of nitrogen-doped graphene–Ag2CO3 (NG–Ag2CO3) composites with different weight addition ratios of NG as a visible-light-induced photocatalyst were fabricated through a facile precipitation method. It was found that NG–Ag2CO3 composite with a proper addition amount of NG exhibited higher photocatalytic activity and improved anti-photocorrosion than Ag2CO3 toward degradation of rhodamine B (RhB) in water under visible light irradiation. The rodlike Ag2CO3 sub-microcrystals with 4.5wt% NG content exhibited the highest photodegradation efficiency of RhB (92% in 60min), which was about 1.2 times of pure Ag2CO3 sub-microcrystals (76% in 60min). The improved photocatalytic performance could be ascribed to the enhanced visible light absorption efficiency and more effective separation of photo-generated electron–hole pairs.

Study on improving visible light photocatalytic activity of Ag3PO4 through morphology control

Improving the visible light activity of Ag3PO4 photocatalyst through morphology control was studied using a precipitation synthetic method. It was found that the morphology depended on the reactants' constitution. With its alteration, polyhedral microcrystals, spherical and irregular nanocrystals were obtained, respectively. Among them, the polyhedrons exhibited the highest efficiency in degrading methylene blue. This was attributed to the higher visible-light absorption efficiency and more effective electron–hole separation ability of the polyhedrons, according to UV–visible diffuse reflectance spectroscopy (UV–vis DRS) and photoluminescence (PL) investigation.

One-step fabrication and high photocatalytic activity of porous graphitic carbon nitride/graphene oxide hybrid by direct polymerization of cyanamide without templates

Graphene oxide modified porous g-C3N4 (porous g-C3N4/GO) had been synthesized by means of one-step calcination of cyanamide for efficient photocatalysis under visible light irradiation (λ > 400 nm). We expect that the photocatalytic activity of this hybrid photocatalyst could be enhanced by the efficient visible light absorption due to the porous structure and efficient photo generated charge separation at the heterojunction formed between porous g-C3N4 and GO. Scanning electron microscopy (SEM) images demonstrated that the as prepared photocatalyst is composed of GO and porous g-C3N4. The UV-vis diffuse reflectance spectrum shows that optical absorption of porous g-C3N4/GO is more intensive than for pristine g-C3N4. The enhanced generation of photocurrent under visible light irradiation (λ > 400 nm) is observed for the porous g-C3N4/GO. The results of photocatalytic experiments reveal that the pseudofirst-order kinetic constant of photocatalytic degradation of methylene blue (MB) using the porous g-C3N4/GO is 6 times higher than that of pristine g-C3N4.