We report the assembly of nanosized ZnS particles on the 2D platform of a graphene oxide (GO) sheet by a facile two-step wet chemistry process, during which the reduced graphene oxide (RGO, also called GR) and the intimate interfacial contact between ZnS nanoparticles and the GR sheet are achieved simultaneously. The ZnS–GR nanocomposites exhibit visible light photoactivity toward aerobic selective oxidation of alcohols and epoxidation of alkenes under ambient conditions. In terms of structure–photoactivity correlation analysis, we for the first time propose a new photocatalytic mechanism where the role of GR in the ZnS–GR nanocomposites acts as an organic dye-like macromolecular “photosensitizer” for ZnS instead of an electron reservoir. This novel photocatalytic mechanism is distinctly different from all previous research on GR–semiconductor photocatalysts, for which GR is claimed to behave as an electron reservoir to capture/shuttle the electrons photogenerated from the semiconductor. This new concept ...

Graphene Transforms Wide Band Gap ZnS to a Visible Light Photocatalyst. The New Role of Graphene as a Macromolecular Photosensitizer

The separation mechanisms of photogenerated electrons and holes for composite photocatalysts have been a research focus. In this paper, the composite g-C 3 N 4 -WO 3 photocatalysts with different main parts of C 3 N 4 or WO 3 were prepared by ball milling and heat treatment methods. The photocatalytic performance was evaluated by degradation of methylene blue (MB) and fuchsin (BF) under visible light illumination. The photocatalyst was characterized by X-ray powder diffraction (XRD), UV–vis diffuse reflection spectroscopy (DRS), transmission electron microscopy (TEM) and Brunauer–Emmett–Teller (BET) methods. The separation mechanisms of photogenerated electrons and holes of the g-C 3 N 4 -WO 3 photocatalysts were investigated by electron spin resonance technology (ESR), photoluminescence technique (PL), and determination of reactive species in the photocatalytic reactions. When the main part of the g-C 3 N 4 -WO 3 photocatalyst is WO 3 (namely g-C 3 N 4 /WO 3 ), the transport process of the photogenerated electrons and holes adopts the generic band–band transfer. Meanwhile, g-C 3 N 4 is covered by WO 3 powder, and the role of g-C 3 N 4 can not be played fully. The photocatalytic activity of the photocatalyst is not obviously increased. However, when the primary part of the WO 3 -g-C 3 N 4 photocatalyst is g-C 3 N 4 (namely WO 3 /g-C 3 N 4 ), the migration of photogenerated electrons and holes exhibits a typical characteristic of Z-scheme photocatalyst, and the photocatalytic activity of the photocatalyst is increased greatly.

Study on the separation mechanisms of photogenerated electrons and holes for composite photocatalysts g-C3N4-WO3

Porous graphitic carbon nitride (g-C3N4) was prepared by a simple pyrolysis of urea, and then a g-C3N4–Pt-TiO2 nanocomposite was fabricated via a facile chemical adsorption followed by a calcination process. The obtained products were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance absorption spectra, and electron microscopy. It is found that the visible-light-induced photocatalytic hydrogen evolution rate can be remarkably enhanced by coupling TiO2 with the above g-C3N4, and the g-C3N4–Pt-TiO2 composite with a mass ratio of 70 : 30 has the maximum photoactivity and excellent photostability for hydrogen production under visible-light irradiation, and the stable photocurrent of g-C3N4–TiO2 is about 1.5 times higher than that of the bare g-C3N4. The above experimental results show that the photogenerated electrons of g-C3N4 can directionally migrate to Pt-TiO2 due to the close interfacial connections and the synergistic effect existing between Pt-TiO2 and g-C3N4 where photogenerated electrons and holes are efficiently separated in space, which is beneficial for retarding the charge recombination and improving the photoactivity.

Graphitic carbon nitride (g-C3N4)–Pt-TiO2 nanocomposite as an efficient photocatalyst for hydrogen production under visible light irradiation

Novel plasmonic Bi nanoparticles deposited in situ in (BiO)2CO3 microspheres (Bi/BOC) were fabricated via a one-pot hydrothermal treatment of bismuth citrate, sodium carbonate, and thiourea. Different characterization techniques, including XRD, SEM, TEM, XPS, UV–vis DRS, PL, time-resolved fluorescence spectra, and photocurrent generation, were performed to investigate the structural and optical properties of the as-prepared samples. The results indicated that the Bi nanoparticles were generated on the surface of (BiO)2CO3 microspheres via the in situ reduction of Bi3+ by thiourea. The Bi nanoparticle deposited (BiO)2CO3 microspheres were employed for the photocatalytic removal of NO in air under visible light irradiation, and the sample exhibited a drastically enhanced photocatalytic activity and oxidation ability. The highly enhanced activity was attributed to the cooperative contribution of the surface plasmon resonance (SPR) effect, the efficient separation of electron–hole pairs, and the prolonged lif...

Noble metal-like behavior of plasmonic Bi particles as a cocatalyst deposited on (BiO)2CO3 microspheres for efficient visible light photocatalysis

Novel hybrid nanostructures or nanocomposites are receiving increasing attention due to their newly evolved properties. In this work, α-Fe 2 O 3 anchored to graphene oxide (GO) nanosheet (α-Fe 2 O 3 @GO) was synthesized through a facile hydrolysis process and its photo-catalytic performances and durability in heterogeneous Fenton system were fully evaluated. The decolorization rates of methylene blue in α-Fe 2 O 3 @GO + H 2 O 2  + UV system within a wide pH range were approximately 2.9-fold that of classical Degussa P25 TiO 2  + UV and 2.4-fold that of α-Fe 2 O 3  + H 2 O 2  + UV. This enhanced decolorization of methylene blue (MB) in α-Fe 2 O 3 @GO + H 2 O 2  + UV system were attributed to the unique incorporation of GO into the catalyst which not only mediated the morphology of active sites α-Fe 2 O 3 nanoparticles but also offered high electron conductivity and electrostatic attraction between negatively charged GO with positively charged MB. High efficiencies of degradation were achieved on various surface charged organic pollutants (around 96–100%), such as cationic compounds of MB and rhodamine B (RhB), anionic compound Orange II (OII) and Orange G (OG), neutral compounds of phenol, 2-nitrophenol (2-NP) and endocrine disrupting compound 17β-estradiol (E2). The dominant reactive oxygen species (ROS) responsible for decolorization, such as hydroxyl radicals ( OH) and superoxide anion radicals (O 2 − ) generated by activation of H 2 O 2 on the surface of α-Fe 2 O 3 @GO were detected and quantified by free radical quenching methods. The possible degradation mechanism of MB involved the rupture of phenothiazine ring by desulfurization and the rupture of phenyl ring due to the attack of ROS, which was analyzed by LC/MS/MS. The reduction of MB and its intermediates was consistent with the decreasing trend of the acute toxicity towards luminous bacteria with the increasing irradiation time. The results lay a foundation for highly effective and durable photo-Fenton technologies for organic wastewater within wider pH ranges than the conventional photo-Fenton reaction.

Enhanced catalytic degradation of methylene blue by α-Fe2O3/graphene oxide via heterogeneous photo-Fenton reactions

The polyaniline (PANI)/TiO2 nanocomposites have been successfully synthesized via a hydrothermal method and followed by a low-temperature calcination treatment process. We find that such a PANI/TiO2 nanocomposite exhibits higher photocatalytic activity and stability than bare TiO2 and TiO2-xNx toward the liquid-phase degradation of methyl orange (MO) under both UV and visible light (420 nm < λ < 800 nm) irradiation. More noteworthy, the PANI/TiO2 photocatalyst still perform good activity toward MO and 4-chlorophenol (4-CP) under the longer wavelength of light (550 nm < λ < 800 nm). The total organic carbon (TOC) tests show that the mineralization rate of MO and 4-CP over PANI/TiO2 are apparently higher than bare TiO2 under the irradiation of both UV and visible light. The presence of synergic effect between PANI and TiO2 is believed to play an essential role in affecting the photoreactivity. At last, the roles of active species in the photocatalytic process are compared by using different types of active ...

Highly Efficient Photocatalytic Degradation of Organic Pollutants by PANI-Modified TiO2 Composite

Active species such as holes, electrons, hydroxyl radicals (•OH), and superoxide radicals (O2•–) involved in the photodegradation process of methyl orange (MO) over TiO2 photocatalyst were detected by several techniques. Using different types of active species scavengers, the results showed that the MO oxidation was driven mainly by the participation of O2•–, holes and •OH radicals. Characterized by the liquid chromatography/mass spectrometry, the transversion of the degradation products with the light irradiation time was first analyzed. Combined with the measurement of oxidation reduction potential, dissolved oxygen, conductivity, and pH values, the degradation process of MO on TiO2 under the effect of the active species was revealed. This was the first time that electrodes were introduced to track the degradation process in situ, and these parameters would be helpful to explain the degradation processes of other organic pollutants.

Evidence for the Active Species Involved in the Photodegradation Process of Methyl Orange on TiO2

Two facile and green methods without any templates, catalysts, surfactants or organic solvents were applied to synthesize ZnSn(OH)6 nanoparticles, i.e. homogeneous precipitation (HP) and hydrothermal (HT). The photocatalytic activities were evaluated on the degradation of organic pollutants under ultraviolet light illumination. Compared to commercial P25, the photoactivity of ZnSn(OH)6 was remarkably improved. The conversion and mineralization ratio of the photocatalytic degradation of benzene by the ZnSn(OH)6-HP were up to 75% and 68%, which is about 20 and 2 times higher than that of P25. The ZnSn(OH)6 has also exhibited high performance toward other persistent organic compounds as well as methyl orange in suspended solution. The order of photodegradation efficiencies of different catalysts was ZnSn(OH)6-HP > P25 > ZnSn(OH)6-HT. Based on the characterization results and the detection of active species, the possible mechanism of the high photocatalytic activity of ZnSn(OH)6-HP was discussed. The simplified synthesis of zinc hydroxystannate with outstanding activity could be promisingly used in the future photocatalytic application.

High photocatalytic performance of zinc hydroxystannate toward benzene and methyl orange

Modulating the steric-electronic configuration of metal-organic centers is key for tuning the activity and selectivity of heterogeneous reactions, especially multi-electron transfer reactions. Here, three different asymmetric metal-organic complexes with unique steric-electronic structures are immobilized on nanocarbon for an electron-transfer-controlled oxygen reduction reaction. The strong-field ligand-induced low-spin (LS) CoII creates a necessary steric configuration for regulating reaction selectivity through ligand's proton transfer ability, for which acidic diamine ligands facilitate a four-electron transfer (94%), whereas basic ligands drive a highly selective two-electron route (97%). The steric-electronic regulation of the reaction selectivity at catalytic sites is characterized using X-ray absorption spectroscopy, reaction kinetic path analysis, and density functional theory calculation. Our results indicate that an LS state of CoII with asymmetric coordination is necessary to form a unique “flytrap” structure to promote O2 capture for the subsequent proton-coupled electron transfer, which is regulated by the Brønsted acidity of coordinating ligands. 

Regulating electron transfer over asymmetric low-spin Co(II) for highly selective electrocatalysis

Cobalt oxide (assigned as CoOx) is an efficient oxygen evolution reaction (OER) nanocatalyst, which has been extensively studied as a replacement to noble metal-based catalysts. The recent observations and understandings for the interfacial state, adsorbed intermediate products, and rate-determining steps (RDS) on CoOx, however, have remained elusive because of the dynamic transformation of different Co ions and the transient nature of the intermediates formed during the OER process. In this work, we propose that under the chosen experimental conditions, the redox process between Co(III) and Co(IV) species does not follow a proton-coupled electron transfer mechanism that is thought to be common prior to the OER, but it involves a proton-decoupled electron transfer, clarified by isotope labeling experiments and in situ electrostatic modulation. The interfacial state of CoOx is negatively charged prior to the formation of Co(IV)═O species. The theoretical concentration of the resulting Co(IV)═O species is approximately 0.1229 × 1019 cm–2. The Co(IV)═O species are demonstrated to directly regulate the OER performance. Moreover, we experimentally monitor the dynamic evolution behaviors of Co(IV)═O, Co(O)O–, OOH*, and O2–* intermediates during the OER with in situ time-resolved infrared spectroscopy, and the following elementary step OOH* + OH– → OO–* + H2O is likely to be the unexpected RDS in the OER process. 

https://pubs.acs.org/doi/pdf/10.1021/acscatal.1c05598

In Situ Identification and Time-Resolved Observation of the Interfacial State and Reactive Intermediates on a Cobalt Oxide Nanocatalyst for the Oxygen Evolution Reaction

Yangming Lin

Papers

Highly Efficient Photocatalytic Degradation of Organic Pollutants by PANI-Modified TiO2 Composite

Evidence for the Active Species Involved in the Photodegradation Process of Methyl Orange on TiO2

High photocatalytic performance of zinc hydroxystannate toward benzene and methyl orange

Regulating electron transfer over asymmetric low-spin Co(II) for highly selective electrocatalysis

In Situ Identification and Time-Resolved Observation of the Interfacial State and Reactive Intermediates on a Cobalt Oxide Nanocatalyst for the Oxygen Evolution Reaction