Comparison of modification strategies towards enhanced charge carrier separation and photocatalytic degradation activity of metal oxide semiconductors (TiO2, WO3 and ZnO)

Most previous reported photocatalysts designs show that, either photocatalytic oxidation (PCO) or reduction (PCR) is inhibited due to the insufficient govern of the photo-excited h+ and e−. To overcome this problem, we consider that both PCO and PCR could be improved simultaneously by employing suitable photocatalyst. Herein, the novel 3D marigold flower-like hierarchical architecture CoO@MnCo2O4 was successfully synthesized by anchoring CoO nanoparticles (NPs) on MnCo2O4 flower using a facile solvothermal method followed thermal annealing treatment under N2 atmosphere for the first time. This strategy relies on the formation of a p-n heterostructure with matching energy band gaps, in which CoO NPs tightly adhere to the surface of hierarchical MnCo2O4 microflowers, and the flower-like MnCo2O4 act as scaffold to disperse CoO NPs. The as-fabricated CoO@MnCo2O4 hybrid not only presented remarkable performance for PCO of tetracycline (TC) but also exhibited excellent PCR of hexavalent chromium (Cr (VI)) under visible light irradiation. Meanwhile, the photocatalyst also can be used for treatment a mixture of them and exhibited promoted photocatalytic efficiency. That's the primary reason that h+ and e- can play their own roles to accomplish PCO and PCR, respectively. Moreover, the unique 3D hierarchical microflowers structure not only provide a higher specific surface area, sufficient active sites and enhanced light harvesting, but also significantly decrease the aggregation of CoO particles. The appearance photocatalytic mechanism of the p-n heterostructure system was also discussed in detail. The facile synthesis strategy and outstanding photocatalytic performance make the 3D hierarchical architecture CoO@MnCo2O4 a promising candidate as a visible light photocatalyst.

Incorporation of CoO nanoparticles in 3D marigold flower-like hierarchical architecture MnCo2O4 for highly boosting solar light photo-oxidation and reduction ability

A Z-scheme g-C3N4/Ag/MoS2 ternary plasmonic photocatalyst in a flowerlike architecture of diameter about 0.4–0.6 μm is successfully synthesized by a reliable and effective method. The as-synthesized g-C3N4/Ag/MoS2 photocatalyst showed excellent improvement for visible-light absorption and separation efficiency of photoinduced electron–hole pairs. The g-C3N4/Ag/MoS2 system exhibits optimum visible-light-induced photocatalytic activity in degrading Rhodamin B (RhB), which is 9.43-fold and 3.56-fold of Ag/MoS2 and g-C3N4/MoS2 systems, and 8.78-fold and 2.08-fold in the production of hydrogen (H2) out of water, respectively. The excellent photocatalytic activities are attributed to the synergetic effects of Ag, g-C3N4, and MoS2 nanophase structures in the g-C3N4/Ag/MoS2 composites, which result in a Z-scheme-mechanism-assisted fast separation and slow recombination of photoinduced electron–hole pairs and thereby higher photocatalytic activity.

Highly Efficient Visible-Light-Induced Photoactivity of Z-Scheme g-C3N4/Ag/MoS2 Ternary Photocatalysts for Organic Pollutant Degradation and Production of Hydrogen

Effective enhancement of the separation and transfer of photogenerated electrons and holes of semiconductor photocatalysts is key to increase their photocatalytic efficiency. This study aims at improving the photocatalytic performance of visible-light-responsive, CdI2-type layered SnS2 by employing good conductivity N-doped reduced graphene oxide (NRG) and polyaniline (PANI) as the receptors and transporters of photogenerated electrons and holes, respectively. PANI/SnS2/NRG ternary composites were prepared by a three-step method, and the optimum conditions for preparing the most efficient product were explored. The photocatalytic experiments demonstrated that the PANI/SnS2/NRG ternary composite prepared under the optimum conditions (PANI/SnS2/NRG-2%) had markedly higher photocatalytic activity than SnS2 nanoplates, SnS2/PANI and SnS2/NRG binary composites in the reduction of aqueous Cr(VI) under the irradiation of visible-light (λ > 420 nm). Based on comparison of the photoabsorption, photoluminescence and electrochemical impedance properties of PANI/SnS2/NRG, SnS2/PANI and SnS2, it was thought that the coupling of SnS2 with both PANI and NRG had synergistic effects in increasing the absorption of visible-light as well as the separation and transfer of photogenerated electrons and holes, which contributed to the exceptionally high photocatalytic activity of PANI/SnS2/NRG ternary composite. Moreover, it was discovered that the photocatalytic reduction of Cr(VI) over PANI/SnS2/NRG-2% was dominantly through the reduction by photogenerated electrons, whereas both photogenerated electron and superoxide anion radical (O2 –) reduction played important roles in the photocatalytic reduction of Cr(VI) over SnS2. Besides, the influences of photocatalytic testing conditions (including starting pH and concentration of Cr(VI) solution, and dosage of photocatalyst) on the efficiency of PANI/SnS2/NRG-2% in treatment of aqueous Cr(VI) were also examined.

Exceptional synergistic enhancement of the photocatalytic activity of SnS2 by coupling with polyaniline and N-doped reduced graphene oxide

Thermoelectric energy conversion is an all solid-state technology that relies on exceptional semiconductor materials that are generally optimized through sophisticated strategies involving the engineering of defects in their structure. In this review, we summarize the recent advances of defect engineering to improve the thermoelectric (TE) performance and mechanical properties of inorganic materials. First, we introduce the various types of defects categorized by dimensionality, i.e. point defects (vacancies, interstitials, and antisites), dislocations, planar defects (twin boundaries, stacking faults and grain boundaries), and volume defects (precipitation and voids). Next, we discuss the advanced methods for characterizing defects in TE materials. Subsequently, we elaborate on the influences of defect engineering on the electrical and thermal transport properties as well as mechanical performance of TE materials. In the end, we discuss the outlook for the future development of defect engineering to further advance the TE field.

Defect engineering in thermoelectric materials: what have we learned?

Visible-light-driven (VLD) CoOx loaded TiO2-based nanosheets (Co-TNSs) with surface-enrich CoOx nanoparticles were successfully synthesized by using a facile one-pot hydrothermal method. The photocatalysts were characterized by SEM, AFM, HRTEM, XRD, Raman, XPS, FL and the surface photocurrent (SPC) technique, etc. The as-synthesized samples exhibit sheet-like structure with large specific surface area (260–350 cm2/g) and small thickness (3–4 nm). The CoOx nanoparticles (1–2 nm) are highly dispersed on the surface of TNSs. Loaded CoOx nanoparticles not only have influenced the crystal structure, crystallinity and surface area of the TNSs, but also resulted in considerable enhancement of visible-light (VL) absorption and a red-shift of the band gap of the TNSs. XPS results reveal that cobalt mainly exists in the form of Co2+ and Co3+, and the binding energies of Ti O bonds are changing with the loaded amount of CoOx nanoparticles. FL and SPC results indicate appropriate amount of CoOx (2.5 at.%) loading can effectively inhibit the recombination of photo-generated electron-hole pairs, thus improve the separation efficiency of charge carriers. The photocatalytic activity of the samples was evaluated by the dichromate (Cr(VI)) solution under VL irradiation. It can be found that the Co-TNSs photocatalysts showed excellent VL photocatalytic activity for the removal of Cr(VI). Initially the photocatalytic activity increased with the content of CoOx, and then decreased after attaining a maximum value at an optimal content (2.5 at.%) for degradation of aqueous Cr(VI) solution. The addition of Ca2+ promotes photocatalysis owing to its ionic bridging function in the form of TiOH+ Cr(VI)–Ca2+ Cr(VI) linkages, while SO42− slightly inhibits the photo-reduction of Cr(VI), indicating good synergy of photocatalysis and adsorption even at higher ionic strength of electrolyte. The photocatalytic reduction of Cr(VI) is also significantly promoted by phenol and synergism between Cr(VI) reduction and degradation of phenol which is demonstrated by measuring the effect of multiple usage of Co-TNSs on its photocatalytic efficiency. Desorbed Co-TNSs (Co-TNSs-Des) are easily regenerated and reused for Cr(VI) removal with excellent performance. A possible alternate mechanism for the enhancement of photocatalytic activity under visible light irradiation is also proposed.

Visible light induced photocatalytic removal of Cr(VI) over TiO2-based nanosheets loaded with surface-enriched CoOx nanoparticles and its synergism with phenol oxidation

Facile one-pot fabrication and high photocatalytic performance of vanadium doped TiO 2 -based nanosheets for visible-light-driven degradation of RhB or Cr(VI)

Order–disorder transition in clathrate Ba6Ge25 studied by positron annihilation

High purity $$\hbox {In}_2\hbox {O}_3$$
 nanopowders were subjected to different thermal treatments to investigate the role of defects on the lattice thermal conductivity. The powders were first treated by spark plasma sintering (SPS) at $$700\,^{\circ}\hbox {C}$$
 and annealed in air between 700 and $$1300\,^{\circ}\hbox {C}$$
 . X-ray diffraction measurements show that the samples are single phase, and the diffraction peaks become sharper with increasing of annealing temperature, indicating improvement in crystallinity and increase in grain size. The $$\hbox {In}_2\hbox {O}_3$$
 nanopowders were also treated by SPS sintering at different temperatures without subsequent annealing. On the contrary, the average grain size of $$\hbox {In}_2\hbox {O}_3$$
 treated by SPS has no obvious change with the increase in sintering temperatures. Positron annihilation measurements reveal large amounts of monovacancies and vacancy clusters in the $$\hbox {In}_2\hbox {O}_3$$
 nanocrystals. The monovacancies gradually recover and the vacancy clusters transform into smaller vacancies with increasing annealing or sintering temperatures. The lattice thermal conductivity increases with the increase in annealing or sintering temperature, which shows close correlation with the recovery of vacancy defects after heat treatment. This gives us strong evidence that vacancy defects play an important role on the suppression of lattice thermal conductivity in nanostructured thermoelectric materials.

Role of vacancy defects on the lattice thermal conductivity in In2O3 thermoelectric nanocrystals: a positron annihilation study

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Visible light induced photocatalytic removal of Cr(VI) over TiO2-based nanosheets loaded with surface-enriched CoOx nanoparticles and its synergism with phenol oxidation

Facile one-pot fabrication and high photocatalytic performance of vanadium doped TiO 2 -based nanosheets for visible-light-driven degradation of RhB or Cr(VI)

Order–disorder transition in clathrate Ba6Ge25 studied by positron annihilation

Role of vacancy defects on the lattice thermal conductivity in In2O3 thermoelectric nanocrystals: a positron annihilation study