Fabrication And Photocatalytic Activities Of Mixed Metal Oxides/Carbon Nanotubes From Hydrotalcite Precursor

As one kind of the advanced oxidation technologies,heterogeneous photocatalysis based on semiconductors by utilizing solar light is a promising technology in the field of environment and energy,and exploiting novel and efficient photocatalysts with visible light activities is one of the reseach focuses.In the present work,a series of ZnCr-LDH and LDH-CNTs precursors were fabricated by the co-precipitation method,taking advantage of the special crystalline structure of layerd double hydroxides（LDHs）and the excellent electronic transmission of carbon nanotubes（CNTs）.Mixed metal oxides（MMO）and MMO-CNTs nanohybrids were derived from the composite precursors of ZnCr-LDH and ZnCr-LDH-CNTs upon vacuum calcination,which were then utilized in the photodegradation of bisphenol A（BPA）under solar light irradiation.The physicochemical features of the samples were analyzed by spectroscopy,micro-beam and micro-zone techniques.It provides more comprehensive theories for the crystal transformation of LDHs-based materials,and new preparation methods for LDHs and CNTs in the field of photocatalysis.The main contents are as follows:（1）The carbonate intercalated ZnCr-LDH with a desired molar ratio of Zn/Cr at 2 was synthesized as a precursor to prepare ZnCr-MMO nanocomposites.A range of calcination temperatures from 400-800 °C was performed in different calcination atmospheres（i.e.,vacuum,air,nitrogen）,respectively.According to XRD,BET,FT-IR,SEM,TEM,UV-vis and XPS analysis,we found that the physicochemical features of the samples were thoroughly affected by different thermal treatment.The XRD patterns showed that the well-crystallized ZnO and ZnCr₂O₄ spinel structure were formed after calcination at 500 °C and above.It was found that increasing calcination temperature led to a reduction of SBET and an increase of crystallinity of the samples.The photocatalytic activity of ZnCr-MMO was highly improved compared with ZnCr-LDH precursor.Additionlly,a superior efficiency for ZnCr-MMO calcined at 700℃ in the photodegradation of BPA,in comparison with the counterparts calcined at different temperatures was observed.（2）Further investigation indicated that the sample calcined in vaccum（MMO-V-700）showed a better crystallinity,smaller particle size and larger SBET than the samples calcined in air（MMO-A-700）or nitrogen（MMO-N-700）.Furthermore,it could be clearly seen that sample MMO-V-700 was pale green,while MMO-A-700 and MMO-N-700 was brown gray.FTIR and XPS results showed that the chemical composition of samples was incorrelation with calcination atmosphere,and there were a little of Cr⁶⁺ existed in MMO-A-700 and MMO-N-700 because of partial oxidation of Cr³⁺.Raman and ERS showed revealed the existence of a handful of oxygen vacancies in sample MMO-V-700,which could inhibit the recombination of photoinduced electrons and holes.According to the PL and transient photocurrent tests,MMO-V-700 exhibited the most effective surface charge transfer,which contributed to the enhanced photocatalytic activity for degradation of pollutants.In addition,it was found that MMO-V-700 presented favorable recycling and stability for the treatment of BPA wastewater in near-neutral conditions.（3）A series of LDH-CNTs precursors were successfully fabricated via in-situ growth of ZnCr-LDH on CNTs surface.The MMO-CNTs nanohybrids were then derived from the LDH-CNTs precursors upon vacuum calcination.The photocatalytic behaviors of MMO-CNTs nanohybrids were investigated for the degradation of bisphenol A（BPA）under solar light irradiation.Particular attention was paid to the physicochemical and optical properties of the obtained nanohybrids.It was found that CNTs were well incorporated into the MMO nanoparticles to form a nanohybrid structure dependent upon intimate interfacial contact.The incorporation of CNTs led to reduced crystal size,improved particle dispersibility and enhanced visible light absorptivity of the nanohybrids.The experimental results revealed that the obtained MMO-CNTs nanohybrids exhibited significantly enhanced photocatalytic performance for bisphenol A（BPA）degradation under simulative solar light irradiation,in comparison to pristine MMO.MMO-CNTs-0.10 possessed the highest photocatalytic activity with the BPA degradation efficiency of ¹00%.The key role of A-CNTs played in enhancing the photocatalytic activity of the nanohybrids was probably ascribed to the larger surface area,higher visible light absorption and the more efficient restriction of charge carriers recombination.