The Ferroelectric Effects of Rhombohedral and Tetragonal BiFeO3 in Photoelectrochemical Water Splitting.

The phase of BiFeO3 (BFO) as well as its domain configuration can be tuned by strain engineering. Phase change may greatly influence the properties of the polarization field and hence charge separation. However, the photoelectrochemical properties of different BFO phases have rarely been addressed. Here, the photoelectrochemical study of tetragonal (T-) and rhombohedral (R-) phase BFO films was conducted under visible light illumination. The photocurrent density of R-BFO is 5 times that of T-BFO. A ferroelectric domain study shows that T-BFO features single domain structure in contrast to the polydomain structure of R-BFO. Higher charge separation efficiency is achieved in R-BFO, dominated by the domain walls as conducting pathways for efficient charge separation and transfer. This work provides a fundamental understanding of the photoelectrochemical properties of T- and R-BFO, offering valuable insights for the development of BFO-based materials for solar energy conversion.

Distribution, occurrence, and leachability of typical heavy metals in coal gasification slag.

Coal gasification slag (CGS) contains variable amounts of heavy metals, which can negatively impact the environment. The mineral composition, element distribution, occurrence, and leaching characteristics of heavy metals in coal gasification coarse slag (CGCS) and coal gasification fine slag (CGFS) are studied to explain the leaching behavior of heavy metals in CGS. The movable components of heavy metals in CGFS (0.06 %-63.03 %) are significantly higher than those in CGCS (0 %-18.72 %). Leaching Environmental Assessment Framework 1313 data shows that heavy metals Zn, Cr, Cd, As, Pb, Ni, and Cu exhibit high leaching rates at low pH conditions, with Zn leaching concentrations as high as 2.11 mg/L at pH 2. Zn, Cr, and As exhibit obvious amphoteric leaching characteristics, and the leaching concentration of As at high pH (1.34 mg/L) even exceeds that at low pH (1.31 mg/L). Except for Cu, all heavy metals in CGS exceed the class III groundwater standard in some cases. Therefore, evaluation is needed before resource utilization of CGS due to potential leaching of some heavy metals.

Recent advances in in-situ transmission electron microscopy techniques for heterogeneous catalysis.

The process of heterogeneous catalytic reaction under working conditions has long been considered a "black box", which is mainly because of the difficulties in directly characterizing the structural changes of catalysts at the atomic level during catalytic reactions. The development of in situ transmission electron microscopy (TEM) techniques offers opportunities for introducing a realistic chemical reaction environment in TEM, making it possible to uncover the mystery of catalytic reactions. In this article, we present a comprehensive overview of the application of in situ TEM techniques in heterogeneous catalysis, highlighting its utility for observing gas-solid and liquid-solid reactions during thermal catalysis, electrocatalysis, and photocatalysis. in situ TEM has a unique advantage in revealing the complex structural changes of catalysts during chemical reactions. Revealing the real-time dynamic structure during reaction processes is crucial for understanding the intricate relationship between catalyst structure and its catalytic performance. Finally, we present a perspective on the future challenges and opportunities of in situ TEM in heterogeneous catalysis.

Boosting Photocatalytic Water Oxidation on Photocatalysts with Ferroelectric Single Domains.

Ferroelectric materials are considered as promising photocatalysts due to their efficient charge separation via a polarization-induced built-in electric field. However, the polydomain structures hinder spatial charge separation and transfer due to the cancellation of polarization vectors in the domains. In this work, taking BiFeO3 (BFO) as a prototype, single-domain BFO nanosheets with visible-light absorption are prepared, as evident by piezoresponse force microscopy (PFM), spatially resolved surface photovoltage spectroscopy (SRSPS), and photodeposition experiments. The single-domain BFO nanosheets show nine times activity in photocatalytic water oxidation reaction under visible-light irradiation, compared with that of the polydomain BFO particles. With the asymmetric driving force for charge separation in a single domain, selective deposition of cocatalysts further enhances the photocatalytic activity of single-domain ferroelectric BFO nanosheets. These results demonstrate the role of the single-domain structure in constructing the driving force of charge separation in ferroelectric photocatalysts. The fabrication of single-domain structures in ferroelectric photocatalysts to achieve enhanced photocatalytic activity offers a path to efficiently utilize the photogenerated charges in solar energy conversion.

Determination of Crystallographic Orientation and Exposed Facets of Titanium Oxide Nanocrystals.

Titanium dioxide (TiO2 ) nanocrystals have attracted great attention in heterogeneous photocatalysis and photoelectricity fields for decades. However, contradicting conclusions on the crystallographic orientation and exposed facets of TiO2 nanocrystals frequently appear in the literature. Herein, using anatase TiO2 nanocrystals with highly exposed {001} facets as a model, the misleading conclusions that exist on anatase nanocrystals are clarified. Although TiO2 -001 nanocrystals are recognized to be dominated by {001} facets, in fact, anatase nanocrystals with both dominant {001} and {111} facets always co-exist due to the similarities in the lattice fringes and intersection angles between the two types of facets (0.38 nm and 90° in the [001] direction, 0.35 nm and 82° in the [111] direction). A paradigm for determining the crystallographic orientation and exposed facets based on transmission electron microscopy (TEM) analysis, which provides a universal methodology to nanomaterials for determining the orientation and exposed facets, is also given.

Tuning the Anisotropic Facet of Lead Chromate Photocatalysts to Promote Spatial Charge Separation.

A crucial issue in artificial photosynthesis is how to modulate the behaviors of photogenerated charges of semiconductor photocatalysts. Here, using lead chromate (PbCrO4 ) as an example, we conducted the morphology tailoring from parallelepiped (p-PbCrO4 ) to truncated decahedron (t-PbCrO4 ) and elongated rhombic (r-PbCrO4 ), resulting in exposed anisotropic facets. The spatial separation of photogenerated charges closely correlates to the anisotropic facets of crystals, which can only be realized for t-PbCrO4 and r-PbCrO4 . The charge-separation efficiencies exhibit a quasilinear relation with the surface photovoltage difference between anisotropic facets. The r-PbCrO4 gives an apparent quantum efficiency of 6.5 % at 500 nm for photocatalytic water oxidation using Fe3+ ions as electron acceptors. Moreover, the oxidation reverse reaction from Fe2+ to Fe3+ ions was completely blocked with ∼100 % of Fe3+ conversion achieved on the anisotropic PbCrO4 crystals.

Enhancement of Plasmon-Induced Photoelectrocatalytic Water Oxidation over Au/TiO2 with Lithium Intercalation.

Plasmon-induced chemical reaction is an emerging field but its development faces huge challenges because of low quantum efficiency. Herein, we report that the solar energy conversion efficiency of Au/TiO2 in plasmon-induced water oxidation is greatly enhanced by intercalating Li+ into TiO2 . An incident photon-to-current efficiency as high as 2.0 %@520 nm is achieved by Au/Li0.2 TiO2 in photoelectrocatalytic water oxidation, realizing a 33-fold enhancement in photocurrent density compared with Au/TiO2 . The superior photoelectrocatalytic performance is mainly ascribed to the enhanced electric conductivity and higher catalytic activity of Li0.2 TiO2 . Furthermore, the ultrafast transient absorption spectroscopy suggests that lithium intercalation into TiO2 could change the dynamics of hot electron relaxation in Au nanoparticles. This work demonstrates that intercalation of alkaline ions into semiconductors can promote the charge separation efficiency of the plasmonic effect of Au/TiO2 .

Crystal facet modulation of Bi2WO6 microplates for spatial charge separation and inhibiting reverse reaction.

We experimentally demonstrated that spatial charge separation can take place between the {010} and {001} facets of Bi2WO6 microplates. Further assembly of the reduction and oxidation cocatalysts leads to a remarkable enhancement of photocatalytic water oxidation activity in the presence of Fe3+ ions while the reverse oxidation of Fe2+ to Fe3+ ions is totally inhibited. The origin of the driving force is theoretically proven to be the difference in surface work function between the co-exposed facets, which shows a feasible strategy for developing efficient photocatalysts for solar energy conversion.

A high value utilization process for coal gasification slag: Preparation of high modulus sodium silicate by mechano-chemical synergistic activation.

Coal gasification coarse slag (CGCS) is solid waste generated during coal gasification. The mainly treatment method of CGCS is storage and landfill, which causes severe environmental pollution and waste of land resources. Sodium silicate can be synthesized using CGCS after impurities are removed for the high content of amorphous silica. In this work, a novel method of acid activation depolymerization-dilute alkali dissociation is proposed to synthesize high-modulus, low-impurity sodium silicate using CGCS under mild conditions. In the acid activation depolymerization process, the content of impurities such as CaO and Fe2O3 can be reduced from over 30% to below 3%. SiO2 composition can be enriched from 35.75% to 60.60%. The SiOAl bond is broken, the coordination structures of Q4(2Al) and Q4(3Al) are depolymerized, and the reactive Q4(0Al) and Q3(0Al) coordination structures of amorphous silica are formed. Numerous defects appear in the aluminosilicate structure, exposing a large number of active SiOH bonds. Efficient desilicated ratio is increased from 7.59% to 73.45%. During the process of dilute alkali dissociation, a large number of reactive SiOSi bonds with network structure defects are broken with the destruction of hydroxyl groups, while SiO and SiOH bonds are formed. Amorphous silica is leached into the liquid phase in the form of oligomers, and high-modulus sodium silicate can be obtained. Under optimal conditions, the removal ratio of amorphous silica and modulus of sodium silicate can reach 80% and 3.53, respectively. The synthesized sodium silicate can be used to produce hydrated silica, adhesives and surface coatings. This process not only reduces pollution, but also alleviates the shortage of high-purity quartz sand resources and promotes the clean development of coal chemical enterprises.