Preparation And Dye Degradation Mechanism Of Carbon Dots/Titanium Dioxide Composite Photocatalytic Materials

The annual water consumption of printing and dyeing industry in our country is 10billion tons,and a large number of printing and dyeing wastewater containing organic matter such as dye has caused great damage to ecological environment.In recent years,the use of photocatalytic materials to treat textile printing and dyeing wastewater has become a research hotspot,among which titanium dioxide（Ti O₂）has attracted widespread attention due to its excellent dye-degradation performance.However,Ti O₂ has an excessively band gap（about 3.2e V）,and its maximum absorption wavelength is only 395 nm,unable to absorb visible light accounting for 40%of the solar spectrum.As a result,the utilization rate of sunlight in the practical application of photocatalysis is very low（<5%）.The above shortcomings limit the practical application of Ti O₂ in photocatalysis.Doping modification can narrow the band gap of Ti O₂,so that it can absorb part of visible light.However,most doping materials are precious metals with high prices.Therefore,the use of non-metallic carbon materials to improve photocatalytic performance has become an important topic in the current research field.In this paper,Ti O₂ was modified by carbon dot（C-dot）hybrid to form a direct Z-scheme C-dot/Ti O₂ composite photocatalyst,which widened the absorption range of Ti O₂ to visible light and promoted the separation and transmission of photogenerated electrons and holes,so as to improve the performance of photocatalytic degradation of printing and dyeing wastewater.The main contents of this paper are as follows:（1）Using urea and citric acid as raw materials,C-dot with high fluorescence performance and wide spectral response was prepared by space-limited vacuum heating method,and the fluorescence color,particle size,structure,optical properties and dispersion stability of C-dot were studied.The experimental results show that C-dot solution exhibits green fluorescence at the excitation wavelength of 395 nm.TEM（Transmission electron microscopy）and FT-IR（Fourier infrared spectroscopy）showed that the average particle size of C-dot was 4.1 nm,and its surface groups were composed of hydroxyl,carboxyl and amino groups.According to the results of ultraviolet-visible absorption spectra（UV-Vis）and fluorescence spectra（PL）,it was found that C-dot exhibited two significant absorption peaks at 270 nm and 405 nm,and a narrow emission peak at 525 nm.By analyzing the TRF（time-resolved fluorescence spectrum）and Zeta potential,the results show that the average fluorescence lifetime of C-dot is 4.13 ns,and its potential in solution is-9.23 m V,showing excellent dispersion stability.（2）Using ultrasonic assisted method,C-dot was modified on the surface of Ti O₂,and a composite photocatalyst C-dot/Ti O₂ responsive to visible light was successfully prepared.The obtained composite photocatalyst was characterized by TEM,FT-IR and XPS（X-ray diffraction）.The experimental results showed that C-dot and Ti O₂ were connected by Ti-O-C bond,and C-dot could be stably modified on the surface of Ti O₂.UV-Vis experiment results show that C-dot doping makes the absorption wavelength of Ti O₂ redshift and enhances its response to visible light.PL results show that the fluorescence intensity of C-dot is significantly higher than that of C-dot/Ti O₂,because the electron-hole recombination rate of C-dot is faster,and C-dot can effectively inhibit the photoexcited electron-hole recombination in Ti O₂,thus reducing the fluorescence intensity of C-dot/Ti O₂ composites.TRF research results show that the photogenerated electron lifetime of C-dot/Ti O₂ is significantly longer than that of C-dot,which indicates that C-dot can improve the separation and transport efficiency of photogenerated charge and prolong the life of photogenerated electron by modifying Ti O₂.The Zeta potential study results show that the interaction between C-dot and Ti O₂ can be strengthened under the action of ultrasound,so that they are more closely combined.The average potential of the ultrasonic C-dot/Ti O₂dispersion solution is 30.1m V,indicating that the dispersion solution has excellent stability,which is due to the large number of hydrophilic groups contained in the composite system.The C-dot/Ti O₂ composite material forms a stable dispersion liquid in water to prevent the agglomeration and precipitation of the composite material.（3）Using natural dyes（turmeric）,chemical dyes（methylene blue,Rhodamine B）and industrial printing and dyeing wastewater as pollutants,the photocatalytic performance of C-dot/Ti O₂ was tested under visible light.The experimental results of degradation of methylene blue by Ti O₂ with different C-dot doping amount under visible light show that the photocatalytic efficiency of composite photocatalyst is the best when the doping weight percentage of C-dot is 9.The degradation rate of methylene blue can reach 90%within 30minutes,which is 9 times of Ti O₂.The methylene blue degradation experiment also showed that the photocatalytic efficiency of 9wt%C-dot/Ti O₂ was stable and efficient.Rhodamine B（Rh B）can be completely degraded by 9wt%C-dot/Ti O₂ under indoor fluorescent lamp（4μW/cm²）within 12 hours.After 8 minutes of simulated sunlight（100 m W/cm²）irradiation,the 9wt%C-dot/Ti O₂ composite can efficiently degrade the natural dye turmeric and completely decompose it.In order to fit the practical industrial application,the dyeing wastewater in textile factory was tested by visible light degradation.The results showed that 95%of the dye in dyeing wastewater could be degraded within 100 minutes.The possible mechanism of the degradation of printing and dyeing wastewater was verified by EPR test.The EPR results showed that the direct Z-scheme（DZS）structure was formed by the combination of C-dot and Ti O₂,which produced a large number of strong oxidizing·OH（hydroxyl radical）and·O₂^-（superoxide radical）under visible light irradiation,so that it had efficient photocatalytic performance.The free radical trapping experiment showed that·O₂^-was the main active species.The research of this paper provides theoretical basis and feasible method for the efficient and stable catalytic system to realize the treatment of photodynamic dye industry wastewater.