Role In Photocatalysis Research Articles

In situ decoration of g-C3N4 quantum dots on 1D branched TiO2 loaded with plasmonic Au nanoparticles and improved the photocatalytic hydrogen evolution activity

The hybrid 1D branched TiO2 loaded with surface plasmonic Au and decorated with g-C3N4 QDs were successfully fabricated which played a significant role in photocatalysis. The 1D branched TiO2 prepared by electrospinning followed by an alkali-hydrothermal process. The Au particles decorated by photo deposition and after that g-C3N4 QDs were grafted over it by a chemical vapor deposition method. The composite shows great improvement in photocatalytic hydrogen evolution and evolved about 2.22 mmol g−1 h−1 hydrogen. The composite sample has a quantum efficiency of 19.5% for hydrogen production at 420 nm light that depicts the performance behavior of the composite. The 1D branched TiO2 fiber facilitates the fast transfer of electrons and Au reduces the charge recombination due to its surface plasmonic effect while g-C3N4 QDs facilitate the visible light absorption by improving the optical properties and reduces the bandgap. The formation of the type II heterostructure system remarkably promotes the separation and transfer of electron holes and facilitates the photo-reduction reaction.

High photocatalytic activity of 2D sheet structure ZnO/Bi2WO6 Z-scheme heterojunction under simulated sunlight

The ZnO/Bi2WO6 sheet structure was synthesized by a simple two-step hydrothermal method, and the structure in which the sheet was in close contact with the sheet improved the photocatalytic performance of Bi2WO6. Under simulated sunlight, ZnO/Bi2WO6 showed excellent photocatalytic activity for the degradation of phenolic contaminants. The degradation rate of (1:1) ZnO/Bi2WO6 to phenol, p-nitrophenol and p-chlorophenol was about 4 times, 7.69 times and 5 times than pure Bi2WO6, respectively. The increase in photocatalytic activity is attributed to the Z scheme heterostructure formed by the two semiconductors, which can effectively separate photogenerated carriers. Compared with a single catalyst, ZnO/Bi2WO6 exhibits excellent charge transport properties in photoelectric conversion. Trap experiments of active species have shown that h + and play a major role in photocatalysis. This study proposes a feasible solution for the treatment of a class of pollutant wastewater.

Construction of 2D‐ZnS@ZnO Z‐Scheme Heterostructured Nanosheets with a Highly Ordered ZnO Core and Disordered ZnS Shell for Enhancing Photocatalytic Hydrogen Evolution

AbstractHeterostructure and defects in photocatalyst play highly important role in photocatalysis. Formation of 2D‐ZnS@ZnO Z‐scheme heterostructure and introduction of zinc interstitial defects into ZnO were successfully achieved through a facile in‐situ ion‐exchange hydrothermal approach, and the photocatalysts exhibited high photocatalytic activity for hydrogen evolution. The heterointerface between the highly ordered ZnO core and disordered ZnS shell, and the zinc interstitial, facilitate the transfer and separation of charge carriers, resulting in high photocatalytic activity. The 2D‐ZnS@ZnO photocatalyst with a disordered ZnS layer of about 8 nm in thickness exhibited a photocurrent density about 7.2 times than that of ZnO, which is mainly attributed to the facilitative effect on interface transfer and the increased charge density. In addition, the photocatalytic activity can be adjusted via changing the thickness of the disordered ZnS overlayer. This work provides a strategy for the design of the heterointerface photocatalysts combined with defect engineering, through which the photocatalytic activity can be remarkably improved.

Bcl Morphology Formation Strategy on Nanostructured Titania via Alkaline Hydrothermal Treatment

Titanium dioxide (TiO2) is a semiconductor material that plays an important role in photocatalysis. Bicontinuous concentric lamellar (bcl) is an interesting morphology with an open channel pore structure that has been successfully synthesized on silica-based materials. If bcl morphology can be applied in TiO2 system, then many surface properties of TiO2 can be enhanced, i.e. photocatalytic activity. A simple and effective strategy has been demonstrated to transform aggregated and spherical TiO2 particles to bcl morphology via alkaline hydrothermal route. Alkaline hydrothermal treatment successfully transforms TiO2 particle surface to have bcl morphology through swelling with ammonia then followed by phase segregation process. We proposed this strategy as a general pathway to transform the particle surface with any shape to have bcl morphology.

Effects of ball milling on the photochemistry of biochar: Enrofloxacin degradation and possible mechanisms

Ball-milled biochar with enhanced physicochemical and sorption properties has been intensively investigated, but its photochemistry is far less studied. In this study, biochars produced at various pyrolysis temperatures were ball-milled and tested to determine enrofloxacin (EFA) photocatalytic degradation and its mechanisms. Ball-milled biochar could generate more O2− under visible light irradiation, which favored the photocatalytic degradation of EFA. The ball-milled biochar pyrolyzed at 300 °C (BM300) exhibited the highest EFA degradation rate (80.2% compared with 13.9% for unmilled biochar) and mineralization ability (66.4% compared with 0% for unmilled biochar). The characterization results suggested that more oxygen-containing functional groups played a significant role in the enhancement of photocatalytic performance. Based on the characterization and experiment results, it was supposed that a semiconductor-like structure played a main role in photocatalysis of BM300. A new mechanism suggested that the electrons in carbon defects (as valence band) could be excited by visible light and move to oxygen-containing functional groups (as conduction band), then the electrons in conduction band was further transferred to dissolved oxygen to produce O2−, and h+ could be formed in valence band at the same time. This work applied ball-milled biochar to the field of photocatalysis for the first time, and provided a new opportunity for the removal of EFA in aqueous solutions.

Ultrafast Hot Carrier Injection in Au/GaN: The Role of Band Bending and the Interface Band Structure

Plasmon photochemistry can potentially play a significant role in photocatalysis. To realize this potential, it is critical to enhance the plasmon excited hot carrier transfer and collection. However, the lack of atomistic understanding of the carrier transfer across the interface, especially when the carrier is still "hot", makes it challenging to design a more efficient system. In this work, we apply the nonadiabatic molecular dynamics simulation to study hot carrier dynamics in the system of a Au nanocluster on top of a GaN surface. By setting up the initial excited hole in Au, the carrier transfer from Au to GaN is found to be on a subpicosecond time scale. The hot hole first cools to the band edge of Au d-states while it transfers to GaN. After the hole has cooled down to the band edge of GaN, we find that some of the charges can return back to Au. By applying different external potentials to mimic the Schottky barrier band bending, the returning charge can be reduced, demonstrating the importance of the internal electric field. Finally, with the understanding of the carrier transfer's pathway, we suggest that a ZnO layer between GaN and Au can effectively block the "cold" carrier from returning back to Au but still allow the hot carrier to transfer from Au to GaN.

Polypyrrole/cadmium sulfide hollow fiber with high performance contaminant removal and photocatalytic activity fabricated by layer-by-layer deposition and fiber-sacrifice template approach

A recyclable polypyrrole (PPy)/cadmium sulfide (CdS) hollow fiber photocatalyst was innovatively fabricated for solving the loss issue of the current powder-form photocatalyst in slurry system. Core-sheath structure CdS/polyacrylonitrile (PAN) fiber was prepared via successive ionic layer adsorption and reaction (SILAR) method on the surface of PAN fiber. PPy was further deposited on the CdS/PAN fiber by vapor deposition polymerization. After the removal of interior PAN template, PPy/CdS hollow fiber was yielded. The hollow structure of PPy/CdS hollow fiber was confirmed by morphology observation. The resulting PPy/CdS hollow fiber presents low energy band gaps of 1.9 eV, which accounts for enhanced visible light photocatalytic activity after PPy deposition. PPy/CdS hollow fiber shows good dye removal efficiency of 73.06 wt% (dosage of the product as low as 5 mg·10 mL−1), and praiseworthy H2 production rate up to 269.7 μmol·g−1·h−1. PPy/CdS hollow fiber maintained high and sustainable photocatalytic activity compared to CdS/PAN fiber after 8 cycles, indicating that PPy effectively improved the stability of CdS. Here, PPy plays key synergistic role in photocatalysis of PPy/CdS hollow fiber for the promotive and protective effects based on the actual photocatalytic performance and inductively coupled plasma optical emission spectrometer (ICP-OES) results. Compared with nano-sized photocatalysts, the fiber-formed PPy/CdS hollow fiber is highly bulky and easy to recycle. PPy/CdS hollow fiber has great potential for scale-up in industrial application because of its excellent grabbing ability and degradation to contaminants, and ease of disposal.

Energy-absorption-based explanation of the photocatalytic activity enhancement mechanism of TiO2 nanofibers

As is reported, the photocatalytic activity will increase significantly when TiO2 nanoparticles are agglomerated into TiO2 nanofibers (NFs), but the photocatalytic activity enhancement mechanisms are still not fully understood. As is widely accepted, the optical absorption process plays a key role in photocatalysis, and it can even be said that the optical absorption capability of the photocatalyst directly determines its photocatalytic activity, while the influence of the structure on the optical absorption characteristics of TiO2 has largely been ignored in the existing explanations. In this paper, optical simulations are introduced into analyzing optical characteristics of TiO2 Nanofibers with which, the photocatalytic activity enhancement mechanism is further discussed, and a photocatalytic activity enhancement mechanism of TiO2 Nanofibers is proposed.

The preparation and characterization of TiO2/r-GO/Ag nanocomposites and its photocatalytic activity in formaldehyde degradation

ABSTRACT A series of TiO2-rGO-Ag nanocomposites were prepared in this work via a facile one-pot hydrothermal method utilized for formaldehyde (HCHO) photodegradation; using TiO2, graphene oxide(GO) as well as AgNO3 as the raw materials, and sodium citrate as a reducing agent. Characterization by X-ray diffraction (XRD), Raman spectra, Transmission electron microscopy (TEM) and Field emission scanning electron microscopy (FESEM) demonstrated that GO and Ag+ were reduced during the formation of TiO2-rGO-Ag nanocomposites. X-ray photoelectron spectroscopy(XPS), UV–vis diffuse reflectance spectroscopy (DRS), photoluminescence spectra(PL) and Photocurrent spectrum measurement were applied to quantitatively characterize the bonding between TiO2 and rGO, the band gap energy of catalysts as well as electron–hole pairs recombination rate. The results revealed that the introduction of rGO sheets and Ag nanoparticles reduced the band gap energy of catalysts; it also suppressed the recombination of electron–hole pairs. However, C–O–Ti bond, which played a key role in photocatalysis, was reduced to some extent by the existence of Ag. Photodegradation results showed that, when the Ag loading was 9 mol% of TiO2, the reaction rate constant of formaldehyde (HCHO) removal improved distinctly, by about 22.3 times that of TiO2. The radical scavenger tests and electron paramagnetic resonance(EPR) analysis revealed that superoxide radical (·O2 −), hole (h+), and hydroxylradical (·OH) were reactive species of formaldehyde photodegradation.

Nitrogen-doped black TiO2 spheres with enhanced visible light photocatalytic performance

N-doped black TiO2 spheres (N2–b-TiO2) were prepared by solvothermal reaction and calcination under a nitrogen atmosphere. N-doping introduces new impurity levels above the TiO2 valence band, enhancing the effective absorption of visible light. The presence of OV and Ti3+ in the disordered outer layer inhibits electron–hole pair recombination, and the spherical structure provides many active sites. Those were confirmed by X-ray diffraction, Fourier transform IR, X-ray photoelectron spectroscopy and High-resolution transmission electron microscopy. UV–visible absorption indicates that the nitrogen-doped black TiO2 samples has a reduced band gap and a strong visible light absorption, which is attributed to the doping of OV, Ti3+ and N. The photocatalytic efficiency of the best sample (N2–b-TiO2) for potassium dichromate and rhodamine B was 96.2% and 99.5%, respectively. The superoxide radical (•O2−) played a dominant role in photocatalysis by scavenging experiments. Thus, a photocatalytic mechanism with reduced band gap has been proposed. This study provides a simple and practical method for non-metallic doping to control the photocatalytic performance of semiconductors.

Revealing the Relative Electronic Landscape of Colloidal ZnO and TiO2 Nanoparticles via Equilibration Studies

Interest in metal oxide semiconductors for energy processes has increased due to their prominent roles in photocatalysis, electrical energy storage, and conversion. However, an understanding of the thermochemistry of electron transfer (ET) reactions of these systems has lagged behind photophysical studies. This report investigates ET equilibria between reduced forms of well-characterized, ligated ZnO and TiO2 nanoparticles (NPs) suspended in toluene. Multiple electrons were added to each type of NP, either photochemically or with a chemical reductant. Equilibration experiments monitoring these added electrons are used to construct a qualitative band diagram. Surprisingly, the difference between the “reducible” oxide TiO2 and the formally “nonreducible” ZnO is reflected not in the relative band energies but rather in the relative width of the bands (the density of trap and/or band states). Moreover, the position of the electron equilibrium shifts upon addition of excess dodecylamine or oleic acid capping ligands. The directions of the equilibrium shifts suggest that they are due to the acid/base or hydrogen bond donor/acceptor properties of capping ligands. This suggests a coupling of protons with the electron transfers in these systems. These findings provide a more nuanced and detailed picture of ET thermodynamic landscapes at nanoparticles than what is provided in a typical nanoparticle band energy scheme. Aspects of this understanding could be valuable for the use of nanoscale oxides in energy technologies.

Highly efficient visible light-driven Ag/FeOOH/MMT composite photo-catalyst for degrading phenol

A deposition and photo-activated method was used to prepare the Ag/FeOOH photo-catalyst on montmorillonite (MMT). Different characterization techniques are performed on the Ag/FeOOH/MMT composite to investigate its structure, morphology, valence state and optical property. During the photo-catalytic experiments, a certain amount of Ag can improve the photo-catalytic efficiency of FeOOH/MMT for phenol degradation, and h+ and ∙O2 play the key roles in photo-catalysis according to the radical scavengers experiment. The mechanism of phenol degradation is also explored: The target organic pollutant phenol is degraded to some intermediates with weak acid, eventually to CO2 and H2O. With excellent stability and recyclability, the prepared Ag/FeOOH/MMT is a promising photo-catalyst for wastewater treatment under sunlight.

Noble metal-free integrated UiO-66-PANI-Co3O4 catalyst for visible-light-induced H2 production.

The discriminate etching chemistry (crystal engineering) of metal-organic frameworks (MOFs) offers promising opportunities for tailoring electron-hole separation, and charge-carrier utilization plays a central role in photocatalysis. Herein, the noble metal-free examples of polyaniline (PANI)-sensitized cobalt oxide functionalized perfect crystal UiO-66 and defective crystal UiO-66D photocatalysts were synthesized and evaluated for photocatalytic hydrogen generation from water under visible light irradiation. A viable route for the simple yet effective modification of UiO-66 by implanting PANI and cobalt oxide was demonstrated for the development of a series of UiO-66-based catalysts. The UiO-66-PANI-Co3O4 catalyst showed staggeringly enhanced photocatalytic activity, which was 24 times that of the pristine UiO-66(Zr).

Ag/AgCl/Ag2MoO4 composites for visible-light-driven photocatalysis

Ag2MoO4 shows low photocatalytic activity under visible light. It would be interesting to develop Ag2MoO4-based composite/heterojunction photocatalysts working efficiently under visible light. Herein, novel Ag/AgCl/Ag2MoO4 composites were obtained via photoreduction of AgCl/Ag2MoO4 composites prepared by sequential precipitation. The composition, morphologies, and optical properties of the samples were studied via various characterization techniques. Photocatalytic degradation of rhodamine B (RhB), methylene blue (MB), methyl orange (MO), norfloxacin (NOF), and tetracycline hydrochloride (TC) solutions was conducted under visible-light irradiation. An optimal Ag/AgCl/Ag2MoO4 catalyst showed much higher photocatalytic activity than Ag2MoO4, and it also showed reasonably fine recyclability. Through radical-capturing experiments, photogenerated holes (h+) were determined to be the main active species whereas hydroxyl radicals (OH) were found to play a secondary role in photocatalysis. Possible photocatalytic mechanisms were proposed.

RNA-seq analysis reveals the significant effects of different light conditions on oil degradation by marine Chlorella vulgaris

Marine Chlorella vulgaris, an efficient hydrocarbon-degrading organism, is easily affected by light. In this study, we investigated the direct effects of different light conditions on crude oil degradation by C. vulgaris and its crude enzyme. Under 12 h illumination, the crude enzyme improved hydrocarbon removal by 39.36%, whereas the addition of the enzyme and C. vulgaris increased the degradation rate by 121.73%. Conversely, the addition of enzyme under heterotrophic condition was negatively related to oil degradation by C. vulgaris, and the degradation rate decreased from 74.32% to 48.65% and further reduced by 34.54%. The results of RNA sequencing analysis suggested that hydrocarbons removal was attributed to C. vulgaris metabolism in heterotrophic physiological state. While enhanced removal efficiency of hydrocarbons was achieved in mixotrophic physiological state due to the coupling of C. vulgaris metabolism with photocatalytic oxidation. Functional enzymes played key roles in photocatalysis and biodegradation processes.

Mixed-Phase 2D-MoS2 as an Effective Photocatalyst for Selective Aerobic Oxidative Coupling of Amines under Visible-Light Irradiation.

Metal-semiconductor heterojunctions play a vital role in photocatalysis, yet their preparation can be exceedingly difficult. The heterojunction helps not only to separate the charges and inhibit recombination processes but also to transfer the photogenerated electron/hole to the reacting molecule. Here, we demonstrate 1T (metallic)-2H (semiconducting) phase boundaries intrinsic to individual sheets of chemically exfoliated 2D-MoS2 can serve as heterojunctions for enhanced photocatalysis in comparison to only semiconducting phase. Due to the abundance of heterojunctions in these multiphasic materials, chemically exfoliated 2D-MoS2 provides improved stability and transfer of photogenerated charges to the reactants, giving better yield. We demonstrate that this easy to synthesize material is an effective photocatalyst for the aerobic oxidative coupling of amines to imines under visible light irradiation. Given its broad applications, we believe mixed phase 2D-MoS2 can be of interest to several industrial synthetic applications related to semiconductor-based photocatalysis. As an added advantage, this heterogeneous photocatalyst can be recycled for several times up to five cycles without any significant loss in the activity.

Ultrathin CdS nanosheets with tunable thickness and efficient photocatalytic hydrogen generation

Two-dimensional (2D) materials play a crucial role in photocatalysis due to their excellent electrical and optical properties. Herein, thickness-controlled CdS nanosheets were synthesized through a simple and low-cost oil-bath method. Sodium citrate, as the structure-directing agent, is paramount in systematically tailoring the thickness and an optimized growth condition in crucial. In particular, the thinnest CdS nanosheets with 1.5-nm-thickness can be obtained when the molar ratio of cadmium source to sodium citrate is 3:5. Moreover, there is a strong correlation between the thickness and photocatalytic hydrogen generation activity of CdS nanosheets. Thinner nanosheets exhibited higher photocatalytic activity, which can be ascribed to the shorter charge transport distance, more active sites and sufficiently negative conduction band edge. A few nanometer difference in thickness has a critical effect on the photocatalytic performance. Meanwhile, the thinnest sample exhibits the optimal performance of photocatalytic hydrogen production (2155 μmol g−1 h−1), which is about 3.7 times than that of CdS nanoparticles (582 μmol g−1 h−1). This work highlights the influence of CdS nanosheet thickness on photocatalytic hydrogen generation activity and provides new inspiration for the design of high-efficiency 2D photocatalysts.

Tuning Solvated Electrons by Polar-Nonpolar Oxide Heterostructure.

Solvated electron states at the oxide/aqueous interface represent the lowest energy charge-transfer pathways, thereby playing an important role in photocatalysis and electronic device applications. However, their energies are usually higher than the conduction band minimum (CBM), which makes the solvated electrons difficult to utilize in charge-transfer processes. Thus it is essential to stabilize the energy of the solvated electron states. Taking LaAlO3/SrTiO3 (LAO/STO) oxide heterostructure with H2O-adsorbed monolayer as a prototypical system, we show using DFT and ab initio time-dependent nonadiabatic molecular dynamics simulation that the energy and dynamics of solvated electrons can be tuned by the electric field in the polar-nonpolar oxide heterostructure. In particular, for LAO/STO with p-type interface, the CBM is contributed by the solvated electron state when LAO is thicker than four unit cells. Furthermore, the solvated electron band minimum can be partially occupied when LAO is thicker than eight unit cells. We propose that the tunability of solvated electron states can be achieved on polar-nonpolar oxide heterostructure surfaces as well as on ferroelectric oxides, which is important for charge and proton transfer at oxide/aqueous interfaces.

Novel Strategy of Defect-Induced Graphite Nitride Carbon Preparation and Photocatalytic Performance

Based on the fact that nitrogen defective can greatly affect the electronic structure and the properties of photo-excited charge transmission and plays a substantial role in photocatalysis, defect-induced graphite nitride carbon (g–C3N4−x(X)NiS2) was synthesized by a facile one step with sodium hydroxide-assisted route in this paper. The photocatalytic activity of nitrogen defective g–C3N4−x(0.1)NiS2 composite photocatalysts exhibited much more high activities, namely, the maximum amount of H2 production was reached up to 143.5 µmol under continuous visible light irradiation for 5 h, which is about 72 times of pristine g–C3N4 photocatalyst and two times of g–C3N4(0)NiS2 composite photocatalysts. Such high enhanced abilities were greatly attributed to the nitrogen defective on the photocatalysts. A series of characterizations such as SEM, TEM, XPS, XRD, BET, FTIR, UV–vis diffuse reflectance spectroscopy (DRS), photocurrent and transient fluorescence etc. were conducted and the results of which were in fully congruent with each other. Furthermore, the possible reaction mechanism over nitrogen defective g–C3N4−x(X)NiS2 composite photocatalyst under visible light irradiation was proposed.

Elucidating Charge Separation Dynamics in a Hybrid Metal–Organic Framework Photocatalyst for Light-Driven H2 Evolution

Metal–organic frameworks (MOFs) have emerged as novel scaffolds for artificial photosynthesis due to their unique capability in incorporating homogeneous photosensitizer and catalyst to their robust heterogeneous matrix. In this work, we report the charge separation dynamics between molecular Ru-photosensitizer and Pt-catalyst, both of which were successfully incorporated into a Zr-MOF that demonstrates excellent activity and stability for light-driven H2 generation from water. Using optical transient absorption (OTA) spectroscopy, we show that charge separation in this hybrid MOF occurs via electron transfer (ET) from Ru-photosensitizer to Pt-catalyst. Using Pt L3-edge X-ray transient absorption (XTA) spectroscopy, we observed the intermediate reduced Pt site, directly confirming the formation of charge separated state due to ET from Ru-photosensitizer and unraveling their key roles in photocatalysis.