Synergistic Effect of Redox Dual PdO x /MnO x Cocatalysts on the Enhanced H2 Production Potential of a SnS/α-Fe2O3 Heterojunction via Ethanol Photoreforming.

In the quest for optimal H2 evolution (HE) through ethanol photoreforming, a dual cocatalyst-modified heterocatalyst strategy is utilized. Tin(II) sulfide (SnS) was hybridized with α-Fe2O3 to form the heterocatalyst FeOSnS with a p-n heterojunction structure as confirmed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), UV-vis diffusive reflectance spectroscopy (UV-vis DRS), and Brunauer-Emmett-Teller (BET) techniques. PdO x and PdO x /MnO x cocatalysts were loaded onto the FeOSnS heterocatalyst through the impregnation method, as verified by high-resolution transform electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and elemental mapping. Photocatalytic ethanol photoreforming resulted in the production of H2 as the main product with a selectivity of 99% and some trace amounts of CH4. The FeOSnS2-PdO x 2%/MnO x 1% photocatalyst achieved the highest HE rate of 1654 µmol/g, attributed to the synergistic redox contribution of the PdO x and MnO x species.

Ag-doped BiVO4/BiFeO3 photoanode for highly efficient and stable photocatalytic and photoelectrochemical water splitting.

In a conventional photoelectrochemical (PEC) water splitting system using BiVO4 (BVO), most of the charge carriers have very sluggish photocatalysis reaction kinetics because they are easily recombined from the defects developed from the bulk or the surface of the photoanodes before reaching the fluorine-doped tin dioxide (FTO). Herein, we present a facile design and fabrication technique for a Ag-BVO/BiFeO3 (BFO) heterostructure photoanode by Ag doping and surface passivation with BFO on the as-preparedBVO photoanode. Its photocatalytic properties for PEC water splitting and tetracycline (TC) degradation are compared to those of BVO/BFO, BVO, and Ag-BVO photocatalyst nanoparticle (NP) films. The effect of Ag-doping/BFO surface passivation on the morphological, structural, and optical properties and surface electronic structure of the as-obtainedBVO electrodes was investigated. The photocatalytic degradation of TC in aqueous solution by Ag-BVO/BFO was greatly increased (>1.5-fold) compared to that of BVO. The TC was completely photodegraded in 50 min of visible-light irradiation. The as-preparedAg-BVO/BFO heterojunction photoanode not only exhibited 4-fold higher PEC performance (0.72 mA cm-2 vs. RHE) and stability than those of the pure BVO components, but also the onset potential in the Ag-BVO/BFO photoanode was cathodically shifted by 600 mV compared to that of the bare BVO. The Ag-BVO/BFO photoelectrode with the highest donor density and the lowest charge transfer resistance exhibited a 4.46-fold higher carrier density than that of the pure BVO photoelectrode. More specifically, the Mott-Schottky (MS) and electrochemical impedance spectroscopy (EIS) results demonstrated that the Ag-doping not only effectively increased the carrier charge density of BVO, thus increasing the consumption rate of charge carriers, but also increased the charge transfer and transport efficiencies of the BVO photoanodes.

Effect of pH on photocatalytic and photoelectrochemical (PEC) properties of monoclinic bismuth vanadate.

Monoclinic bismuth vanadate (c-BVO) was prepared via simple calcination of solvothermally processed tetrahedral BiVO4. The physicochemical and morphological properties of c-BVO demonstrated the successful synthesis of the photoactive monoclinic phase from the tetrahedral phase, which has low photoactivity properties. The photoactivities of c-BVO were investigated using the photodegradation of methylene blue (MB) and photoelectrochemical (PEC) measurements in acidic (pH = 2.5), neutral (pH = 6.5) and basic (pH = 9.5) media. The photocatalytic activity of c-BVO was increased with increasing pH, achieved 99% MB degradation in the basic condition, compared with 70 and 45% in the neutral and acidic media, respectively. Although the tetrahedral BiVO4 showed mainly adsorption with negligible photodegradation, c-BVO demonstrated both good adsorption and photodegradation activities. The PEC results indicated that the photocurrent density was affected by both pH and applied voltage. Impedance measurements showed faster charge transfer in the neutral condition than in the acidic and basic electrolytes. The incident photon conversion efficiency (IPCE) showed very low activity for tetrahedral BiVO4, but in comparison it was enhanced by 20- and 10-fold for c-BVO in the visible and simulated solar light, respectively.

Sonochemical-driven ultrafast facile synthesis of WO3 nanoplates with controllable morphology and oxygen vacancies for efficient photoelectrochemical water splitting.

Among the various synthetic techniques, the sonochemical method has emerged as an interesting method for fabricating different photocatalysis materials with unique photoelectrochemical (PEC) properties. In comparison with the classical method without sonication, this study examines the promoting effect of ultrasonic irradiation during the synthesis of tungsten oxide (WO3) nanoplates within short reaction times (15 and 30 min). The shorter ultrasonic reaction time (15 min) was sufficient for the uniform growth of thin and compact layers of WO3 nanoparticles (NPs) on the surface of a tungsten foil. In the classical method, however, partial cracks or patches formed when WO3 samples underwent acid treatment for either 15 min or 30 min at 90° C. The WO3 nanoplates fabricated with 15- or 30-min sonication followed by 15- or 30-min deposition (U-15/30-15/30) showed much higher photocurrent density than the WO3 samples fabricated with the classical method without sonication (C-15/30) at 90 °C. The as-prepared monoclinic WO3 with 30-min ultrasonication and 30-min deposition (U-30/30) showed a maximum photocurrent density of ∼6.51 mA/cm2 under simulated solar light at 1.8 V vs. Ag/AgCl, which was 2.12- to 2.93-fold higher than that of the two classical samples. The ultrasonic samples exhibited extraordinarily high stability for water oxidation by maintaining 98% of their initial photoactivity for 2200 sec, as compared to the low stability (66-61%) of both classical samples. The WO3 nanoplates prepared by sonication method had many advantages, such as facile synthetic route, compact, porous and uniform nanoplate morphology, decreased electron-hole pairs recombination rate and controlled oxygen vacancies for greatly enhanced PEC water splitting performance and stability over extended time.

Photolysis and photocatalysis of tetracycline by sonochemically heterojunctioned BiVO4/reduced graphene oxide under visible-light irradiation.

The widespread use of antibiotics in pharmaceutical therapies and agricultural practice has led to severe environmental pollution. In this study, the simultaneous photolysis and photocatalysis behaviors of tetracycline (TC), one of the most frequently prescribed groups of antibiotics, were investigated using BiVO4 (BVO) supported on reduced graphene oxide (rGO). The resulting BVO/rGO nanocomposite (NC) showed prominent adsorption performance and photocatalytic ability under wide initial pH conditions (from acidic to alkaline: pH 2.5, 6.7, 9.2 and 10.5). This study analyzed the kinetics and proposed a mechanism for the photolytic and photocatalytic degradation of TC under visible light irradiation with BVO and BVO/rGO. The photolysis and photocatalytic degradation efficiency of TC was largely influenced by the solution pH and increased with increasing initial pH. The TC was stable without significant photolysis at pH 2.5, while TC photolysis increased up to 17% at pH 9.2. With further increase in the solution pH from 9.2 to 10.5, the light absorption of TC at 356 nm showed a red shift to 372 nm and new absorption peaks at around 533 nm were formed due to the formation of new colored intermediates. The photocatalytic degradation activities of TC by BVO/rGO under visible light irradiation reached 55, 67, 92 and 99% at initial pH 2.5, 6.7, 9.2 and 10.5, respectively. However, when using BVO only, the photocatalytic degradation of TC was 42, 61, 73 and 85% at pH 2.5, 6.7, 9.2 and 10.5, respectively. The great improvement of photocatalytic activity of BVO/rGO is attributed to the reduced particle size, increased adsorption ability of rGO, extended photo responding range of BVO, and efficient separation of photogenerated charge carriers, which are derived from the ultimate coverage of the BVO by the rGO.

Improved photocatalytic and photoelectrochemical performance of monoclinic bismuth vanadate by surface defect states (Bi1-xVO4).

Due to visible light absorption and photochemical stability, Bismuth vanadate (BiVO4), recognized to be a promising photoanodes for designing high efficiency semiconductor photoelectrochemical (PEC) devices. To improve the photocatalytic and PEC performance of BiVO4, the porous monoclinic BiVO4 with surface bismuth vacancy (Bi1-xVO4 (s-m)) was obtained after the calcination of tetrahedron bismuth vanadate (BiVO4 (s-t)). The photocatalytic experiments showed that despite the relatively lower adsorption capacity of Bi1-xVO4 (s-m) as compared with BiVO4 (s-t), its photocatalytic activity for the photodegradation of tetracyclines (TCs) was 15-fold greater. A four-layer thin films of BiVO4 were deposited by spin coating with intermediate annealing treatment between layers and final calcination at 450 °C in air to shed light on carrier transport in Bi1-x VO4 (s-m). The PEC results indicated that BiVO4 (s-t) showed poor charge carrier mobility, while the Bi1-x VO4 (s-m) showed great improvement by transformation of the tetrahedron BiVO4 (s-t) into monoclinic phase, creation of new surface defect states and formation of a porous structure in Bi1-xVO4 (s-m). Furthermore, Bi1-xVO4 (s-m) showed enhanced and stable photocurrent density of 1.2 mA/cm2 at 1.0 V vs. Ag/AgCl which was achieved under visible light illumination using 0.1 M Na2SO4 aqueous solution. The porous Bi1-xVO4 (s-m) showed the band gap narrowing of 0.08 eV, valence band up-shifting of 0.150 eV and 100 mV cathodic shift in onset potential relative to BiVO4 (s-t). This enhancement is assigned to the longer electron lifetime of Bi1-xVO4 (s-m), 10-fold compared to that of BiVO4 (s-t), resulting in a minimized electron-hole pairs recombination.

A benign ultrasonic route to reduced graphene oxide from pristine graphite.

In this study, we report the synthesis of high purity reduced graphene oxide (rGO) from pristine graphite via a fast and cost-effective one-step ultrasonic reduction method. Ultrasonic treatment was employed to avoid the harsh reaction conditions, including high temperature and use of highly toxic hydrazine, required for the conventional rGO preparation method. The high temperature produced during the ultrasound irradiation at low temperature and short reaction time enabled the reduction of graphene oxide (GO) into rGO without the use of toxic chemicals. The oxygen functional groups on GO were successfully reduced by the sonochemical reduction. The rGO prepared using the ultrasonic method exhibited a curled morphology, a very thin wrinkled paper-like structure, sheet folding, minimal layers (∼4 layers), and a layer spacing of ∼1nm. The sonochemical approach for the synthesis of rGO showed fast, high productivity, much improved safety, less energy, and time consuming characteristics as compared to other methods. More importantly, highly explosive and poisonous hydrazine is not required in this sonochemical technique, opposed to that required in conventional rGO synthesis, making it useful for many industrial applications of rGO.

Low intensity-ultrasonic irradiation for highly efficient, eco-friendly and fast synthesis of graphene oxide.

High quality graphene oxide (GO) with low layer number (less than five layers) and large inter-layer space was produced via a new and efficient method using environmentally friendly, fast and economic ultrasonic radiation. The ultrasonic method neither generated any toxic gas nor required any NaNO3, which have been the main drawbacks of the Hummers methods. The major obstacles of the recently reported improved Hummers method for GO synthesis, such as high reaction temperature (50°C) and long reaction time (12h), were successfully solved using a low intensity-ultrasonic bath for 45min at 30°C, which significantly reduced the reaction time and energy consumption for GO synthesis. Furthermore, ultrasonic GO exhibited higher surface area, higher crystallinity and higher oxidation efficiency with many hydrophilic groups, fewer sheets with higher spaces between them, a higher sp3/sp2 ratio, and more uniform size distribution than classically prepared GO. Therefore, the new ultrasonic method could be applicable for the sustainable and large-scale production of GO. The production yield of the ultrasonic-assisted GO was 1.25-fold greater than the GO synthesized with the improved Hummers method. Furthermore, the required production cost based on total energy consumption for ultrasonic GO was only 6.5% of that for classical GO.

Mechanism of highly efficient adsorption of 2-chlorophenol onto ultrasonic graphene materials: Comparison and equilibrium.

The deficiencies of the recently reported improved Hummers method for the synthesis of graphene oxide (GO), such as high reaction temperature (60°C) and long reaction time (10h), were successfully solved using a low-intensity ultrasonic bath for 30min at 40°C. Furthermore, compared to its conventional synthesis counterpart, a facile and fast, one-step ultrasonic method that excluded hydrazine hydrate was developed to synthesize reduced GO (rGO) from graphite (10min, 50°C) in the presence of hydrazine hydrate (rGO-C, 12h, 90°C). The adsorption characteristics of 2-chlorophenol (2-CP) from an aqueous solution were investigated using rGOs and GOs prepared by ultrasonic (rGO-Us/GO-Us) and conventional (rGO-C/GO-C) methods. Whereas 2-CP was completely removed with rGO-Us after 50min, only 40% of 2-CP was eliminated with rGO-C. The maximum adsorption capacity of 2-CP calculated by the Langmuir model onto rGO-Us (208.67mg/g) was much higher than that onto GO-Us (134.49mg/g). In addition, the ultrasonic graphene adsorption capacities were much higher than the corresponding values of rGO-C (49.9mg/g) and GO-C (32.06mg/g). The enhanced adsorption for rGO-Us and GO-Us is attributed to their greater surface areas, excellent oxygenated groups for GO-Us and superior π-electron-rich matrix for rGO-Us, compared to other adsorbents. The adsorption of 2-CP on the rGO materials increased with increasing solution pH to a maximum around its pKa (pKa=8.85), while the adsorption for the GO materials increased with decreasing solution pH. The adsorption mechanism proceeded via hydrogen bonding in neutral and acidic media, but via π-π electron donor-accepter (EDA) interactions between 2-CP and graphene materials in basic medium. The FTIR spectrum of GO-Us after adsorption indicates that the position and intensity of many peaks of GO-Us were affected due to the adsorption of different 2-CP groups at different pHs.

Novel and facile synthesis of Ba-doped BiFeO3 nanoparticles and enhancement of their magnetic and photocatalytic activities for complete degradation of benzene in aqueous solution.

In this work, Bi1-x Bax FeO3 (x=0.05, 0.1 and 0.2mol%) multiferroic materials as visible-light photocatalysts were successfully synthesized via a simple and rapid sol-gel method, at a low temperature and with rapid calcination. Ba loading brought about a distorted structure of BiFeO3 magnetic nanoparticles (BFO MNPs) consisting of small, randomly oriented and non-uniform grains, leading to increased surface area and improved magnetic and photocatalytic activities. Doping of Ba(2+) into pure BFO (Bi1-x Bax FeO3, x=0.2mol%) greatly increased magnetic saturation to 3.0emu/g and significantly decreased the band-gap energy to 1.79eV, as compared to 2.1emu/g and 2.1eV, respectively, for pure BFO. Bi1-xBa xFeO3 of x=0.2mol% exhibited the greatest photocatalytic degradation effect after 60min of visible light irradiation, and reached 97% benzene removal efficiency, leading to production of a high concentration of carbon dioxide (CO2), with 93% and 82% reductions in chemical oxygen demand (COD) and total organic carbon (TOC), respectively. The identified major intermediate products of photodegradation enabled prediction of the proposed benzene degradation pathway. The enhanced photocatalytic activity of benzene removal is due to both mechanisms, photocatalytic and photo-Fenton catalytic degradation.