CARBON DOTS-BASED PHOTOCATALYTIC ELECTRODE FOR SIMULTANEOUS ORGANIC MATTER DEGRADATION AND HEAVY METAL REDUCTION AND USE THEREOF

Abstract

The present invention discloses a carbon dots-based photocatalytic electrode for simultaneous organic matter degradation and heavy metal reduction and preparation method and use thereof, which belong to the field of multifunctional environmental materials and water treatment. With respect to the insufficient ability of simultaneous organic matter degradation and heavy metal reduction of existing photocatalytic electrodes, the present application provides a photocatalytic electrode with a Z-type heterojunction structure constructed by using carbon dots (CDs) as an electronic assistant. The directional transfer ability of photo-generated electrons is improved, while the recombination efficiency of photo-generated electrons and holes is reduced. The performance of a photocatalytic electrode in simultaneous organic matter degradation and heavy metal reduction is thereby improved. The invention provides a scientific basis and technical support for developing highly-efficient photocatalytic electrode materials and ensuring water quality safety.

Claims

1. A method for preparing a carbon dots (CDs)-based photocatalytic electrode for simultaneous organic matter degradation and heavy metal reduction, comprising: forming a carbon dots electron transport layer on a semiconductor I; forming a semiconductor II on the carbon dots electron transport layer.

2. The method according to claim 1, wherein the step of forming a carbon dots electron transport layer on the semiconductor I comprises: immersing the semiconductor I in a mixed solution comprising 10 vol %-30 vol% (e.g., 15 vol %, 20 vol % or 25 vol %) mercaptopropionic acid (MPA) and 1-10 g/L (e.g., 2 g/L, 5 g/L or 8 g/L) CDs (preferably, the immersion time is 24-48 h), and then taking out the semiconductor I-CDs electrode.

3. The method according to claim 1, wherein the semiconductor I is a TiO.sub.2 nanotube or a Fe.sub.2O.sub.3 nanotube.

4. The method according to claim 3, wherein the TiO.sub.2 nanotube is a TiO.sub.2 nanotube prepared by anodization, and the Fe.sub.2O.sub.3 nanotube is a Fe.sub.2O.sub.3 nanotube prepared by anodization.

5. The method according to claim 1, wherein the semiconductor II is an organic semiconductor or an inorganic semiconductor, wherein the organic semiconductor is polyaniline, reduced graphene oxide or carbon nitride; and the inorganic semiconductor is WO.sub.3 or MoS.sub.2.

6. The method according to claim 1, wherein the method of preparing carbon dots comprises: dissolving glucose in concentrated H.sub.2SO.sub.4, heating at 180-220 C. (e.g., 190 C., 200 C. or 210 C.) for 3-5 h (e.g., 3.5 h, 4 h or 4.5 h), cooling to room temperature, adjusting the pH of the mixed solution to 6.9-7.1, extracting the supernatant after centrifugation of the mixed solution by a solid phase extraction column, purging the extract with nitrogen and freeze-drying it (for example, freeze-drying time is 24-48 h) to obtain solid particles of carbon dots.

7. The method according to claim 6, wherein the solid phase extraction column is an HLB solid phase extraction column; preferably, the extraction step comprises rinsing the solid phase extraction column with methanol and then removing the residual methanol in the solid phase extraction column by ultrapure water; extracting the CDs solution by the solid phase extraction column; washing the solid phase extraction column by ultrapure water; and rinsing the solid phase extraction column by methanol to desorb the CDs to obtain a high-purity CDs-methanol solution.

8. A photocatalytic electrode prepared by the method of claim 1.

9. A method for simultaneously degrading organic matter and reducing heavy metals using the photocatalytic electrode according to claim 8, comprising: immersing the photocatalytic electrode in a solution containing organic pollutants and heavy metals, and performing the degradation of organic pollutants and reduction of heavy metals under light exposure.

10. The method according to claim 9, wherein the reaction conditions are as follows: light intensity: more than 50 mW.Math.cm.sup.2; wavelength: more than 200 nm; the ratio of electrode working area to solution volume: 1-10 cm.sup.2.Math.L.sup.1; concentration of the organic pollutant(s): less than 1 M; concentration of the heavy metal(s): less than 10 M; and reaction time: 30-120 min (such as 60 min or 90 min).

Description

BRIEF DESCRIPTION OF DRAWINGS

[0029] The following drawings are only intended to schematically illustrate and explain the present invention, and are not intended to limit the scope of the present invention.

[0030] FIG. 1 is an XPS diagram of CDs obtained in an example of the present invention.

[0031] FIG. 2 is a transmission electron microscope diagram of CDs obtained in an example of the present invention.

[0032] FIG. 3 is a scanning electron microscope image obtained in an example of the present invention, (a) TiO.sub.2 nanotubes, (b) TiO.sub.2 nanotubes-CDs, (c) TiO.sub.2 nanotubes-CDs-WO.sub.3, (d) TiO.sub.2 nanotubes-CDs-PANI.

[0033] FIG. 4 is a structural diagram of a TiO.sub.2 nanotube-CDs-PANI electrode obtained in an example of the present invention.

[0034] FIG. 5 is a graph showing the light absorption properties of (a) TiO.sub.2 nanotube-CDs-WO.sub.3 and (b) TiO.sub.2 nanotube-CDs-PANI obtained in the examples of the present invention.

[0035] FIG. 6 is a graph showing the photoelectric conversion performance of (a) TiO.sub.2 nanotube-CDs-WO3 and (b) TiO.sub.2 nanotube-CDs-PANI obtained in an example of the present invention.

[0036] FIG. 7 is a scanning electron microscope image of Fe.sub.2O.sub.3 nanotube-CDs-carbon nitride obtained in an example of the present invention.

[0037] FIG. 8 is a graph showing the photoelectric conversion performance of Fe.sub.2O.sub.3 nanotube-CDs-carbon nitride photocatalytic electrode obtained in an example of the present invention.

[0038] FIG. 9 is a graph showing the organic matter degradation efficiency of an example of the present invention.

[0039] FIG. 10 is a graph showing the heavy metal reduction efficiency of an example of the present invention.

[0040] FIG. 11 is a graph showing the organic matter degradation efficiency in Comparative Example 1 of the present invention.

[0041] FIG. 12 is a graph showing the heavy metal reduction efficiency in Comparative Example 1 of the present invention.

[0042] FIG. 13 is a graph showing the organic matter degradation efficiency in Comparative Example 2 of the present invention.

[0043] FIG. 14 is a graph showing the heavy metal reduction efficiency in Comparative Example 2 of the present invention.

DETAILED DESCRIPTION

[0044] In order to facilitate understanding of the above features and advantages of the present invention, the present invention is illustrated in detail by the embodiments in combination with the accompanying drawings, but the present invention is not limited thereto.

[0045] In some embodiments of the present invention, a TiO.sub.2 nanotube-CDs-PANI photocatalytic electrode is prepared using TiO.sub.2 nanotubes as semiconductor I and polyaniline (PANI) as organic semiconductor II, and a TiO.sub.2 nanotube-CDs-WO.sub.3 photocatalytic electrode is prepared using TiO.sub.2 nanotubes as semiconductor I and WO.sub.3 as inorganic semiconductor II.

[0046] As shown in FIG. 1, there are three distinct characteristic peaks in the C 1 s high-resolution spectrum of CDs, which are located at 284.5 eV, 286.2 eV and 288.1 eV, respectively. It is indicated that there are CC (CC), CO and CO on the surface of CDs, consistent with the characteristics of CDs.

[0047] As shown in FIG. 2, the diameter of CDs is mainly distributed in the range of 2-4 nm. According to the lattice fringes, the spacing between the crystal planes is 0.21 nm, indicating that the exposed surface of CDs is the (100) crystal plane. The successful synthesis of CDs is proven by FIG. 1 in combination with FIG. 2.

[0048] As shown in FIG. 3, an obvious TiO.sub.2 nanotube array structure can be observed from FIG. 3a, wherein the outer diameter of the TiO.sub.2 nanotube is about 90 nm, and the inner diameter is about 80 nm. From FIG. 3b, it can be observed that large amounts of aggregated CDs are adsorbed on the surface of the nanotube tube. It can be observed from FIG. 3c that a large amount of WO.sub.3 nanoparticles are successfully loaded on the surface of the TiO.sub.2 nanotube-CDs electrode. From FIG. 3d, it can be observed that a large number of PANI nanowires are successfully loaded on the surface of the TiO.sub.2 nanotube-CDs electrode.

[0049] As shown in FIG. 4, mercaptopropionic acid (MPA) has both COOH and SH functional groups, respectively connected to TiO.sub.2 nanotubes and CDs, to ensure the stability of TiO.sub.2 nanotube-CDs photocatalytic electrode. CDs and PANI are connected in the forms of PANI-H...O-CQDs...OTiO.sub.2, PANI-N...O-CQDs, PANI-N...H bond...O-CQDs, PANI-H...O-CQDs and the like to ensure the stability of TiO.sub.2 nanotube-CDs-PANI.

[0050] As shown in FIG. 5, in contrast to TiO.sub.2 nanotubes, the band gap width of TiO.sub.2 nanotube-CDs-PANI and TiO.sub.2 nanotube-CDs-WO.sub.3 photocatalytic electrodes is significantly reduced, while the light absorption capacity in the range of 200-800 nm is significant enhanced, especially the absorption of visible and near-infrared light with a wavelength greater than 420 nm. Therefore, the photocatalytic electrodes show good potential photocatalytic performance.

[0051] As shown in FIG. 6, in contrast to TiO.sub.2 nanotubes, the photoelectric conversion performance of TiO.sub.2 nanotubes-CDs-PANI and TiO.sub.2 nanotubes-CDs-WO.sub.3 photocatalytic electrodes is significantly improved by 8.1 times and 18.8 times, respectively. Therefore, the photocatalytic electrodes show good potential photocatalytic performance.

[0052] In other embodiments of the present invention, Fe.sub.2O.sub.3 nanotube-CDs-carbon nitride photocatalytic electrode is prepared by using Fe.sub.2O.sub.3 nanotubes as semiconductor I and carbon nitride as semiconductor II.

[0053] From FIG. 7, the obvious Fe.sub.2O.sub.3 nanotube-CDs-carbon nitride structure can be observed, wherein the outer diameter of the Fe.sub.2O.sub.3 nanotube is about 80 nm, and the inner diameter is about 70 nm. It can be observed that large amounts of aggregated CDs are adsorbed on the surface of the nanotubes. In addition, carbon nitride nanoparticles with a diameter of about 100 nm can also be observed. It is noted that due to the magnetic properties of Fe.sub.2O.sub.3, astigmatism is inevitable when observed through scanning electron microscopy.

[0054] As shown in FIG. 8, in contrast to Fe.sub.2O.sub.3 nanotubes, the photoelectric conversion performance of Fe.sub.2O.sub.3 nanotubes-CDs-carbon nitride photocatalytic electrode is significantly improved by 4.1 times. Therefore, the photocatalytic electrode shows good potential photocatalytic performance.

[0055] It should be understood that the semiconductor I and the semiconductor II in the present invention are not limited to the semiconductor materials in the Examples.

EXAMPLE 1

[0056] CDs were prepared by the following steps. 2.5 g glucose was dissolved in 150 mL 98 wt % concentrated H.sub.2SO.sub.4. After introduced into a reactor with an inner tank of polytetrafluoroethylene, the mixed solution of glucose and H.sub.2SO.sub.4 was heated at 180 C. for 3 h and then naturally cooled to room temperature. Next, the pH of the mixed solution was adjusted to 7 with Na.sub.2CO.sub.3. The mixed solution was centrifuged for 15 min at 12000 r/min and the resultant supernatant was extracted with a solid phase extraction column (Oasis HLB, 3 cc/60 mg, Waters). The extraction step comprises four steps: an activation step, an enrichment step, a separation step and an elution step. Specifically, in the activation step, the solid phase extraction column was rinsed with 20 mL methanol, and the residual methanol in the solid phase extraction column was then washed away with 40 mL ultrapure water. In the enrichment step, 20 mL CDs solution was extracted with a solid phase extraction column. In the separation step, the solid phase extraction column was rinsed with 40 mL ultrapure water to dissolve impurities such as inorganic salts. In the elution step, the solid phase extraction column was rinsed with 20 mL methanol to desorb the CDs to obtain high-purity methanol solution of CDs as the extract. Finally, the extract was purged with nitrogen and freeze-dried for 48 h to obtain CDs solid particles.

[0057] TiO.sub.2 nanotubes were prepared by anodization. The TiO.sub.2 nanotubes were immersed in a mixed solution of 10% by volume mercaptopropionic acid (MPA) and 7 g/L CDs, and were taken out after immersion for 24 h to obtain TiO.sub.2 nanotube-CDs electrode.

[0058] The TiO.sub.2 nanotube-CDs-PANI photocatalytic electrode was prepared by in-situ electropolymerization. The TiO.sub.2 nanotube-CDs electrode, a platinum sheet and Ag/AgCl were used as a working electrode, a counter electrode and a reference electrode, respectively. The electrolyte was an acetone solution comprising 0.2 M aniline and 0.05 M citric acid. The electrolyte was deoxygenated with nitrogen for 30 minutes before in-situ electropolymerization. In-situ electropolymerization of aniline was carried out using cyclic voltammetry (potential: 0-0.8 V; number of polymerization cycles: 10 cycles). The working electrode after polymerization was dried at 40 C. for 48 h to obtain a TiO.sub.2 nanotube-CDs-PANI photocatalytic electrode.

[0059] TiO.sub.2 nanotube-CDs-WO.sub.3 photocatalytic electrode was prepared by electrodeposition method. The TiO.sub.2 nanotube-CDs electrode, a platinum sheet and Ag/AgCl were used as a working electrode, a counter electrode and a reference electrode, respectively. The electrolyte was 25 mM Na.sub.2WO.sub.4 and 30 mM H.sub.2O.sub.2 in water, and the pH of the solution was adjusted to 1.40.1 with 0.01 M HNO.sub.3. The deposition voltage was 0.437 V.sub.Ag/AgCl, and the deposition time was 150 s. The resulting electrode was cured at 60 C. for 24 h to obtain the TiO.sub.2 nanotube-CDs-WO.sub.3 photocatalytic electrode.

[0060] An experiment of simultaneous organic matter degradation and heavy metal reduction was performed using the prepared photocatalytic electrode. In particular, the photocatalytic electrode was immersed in a solution containing a certain concentration of organic pollutants and heavy metals, and then the working surface of the photocatalytic electrode was exposed to simulated sunlight for organic pollutant degradation and heavy metal reduction. The specific conditions were as follows: light intensity: 100 mW.Math.cm.sup.2, wavelength: more than 420 nm; the ratio of electrode working area to solution volume: 7.5 cm.sup.2.Math.L.sup.1; organic pollutant (carbamazepine) concentration: 1 M; heavy metal (hexavalent chromium) concentration: 0.68 mM; reaction time: 60 min; no requirement of adjusting the pH value of the reaction system; and no requirement of aerating the reaction system.

[0061] As shown in FIG. 9, after loading of CDs and PANI, or CDs and WO.sub.3, the degradation efficiency of the photocatalytic electrode on organic matter was increased from 21% (TiO.sub.2 nanotubes) to 100% (TiO.sub.2 nanotubes-CDs-PANI) and 84% (TiO.sub.2 nanotubes-CDs-WO.sub.3), showing an excellent degradation efficiency of organic matter.

[0062] As shown in FIG. 10, after loading of CDs and PANI, or CDs and WO.sub.3, the reduction efficiency of the photocatalytic electrode on heavy metals was increased from 10% (TiO.sub.2 nanotubes) to 76% (TiO.sub.2 nanotubes-CDs-PANI) and 57% (TiO.sub.2 Nanotube-CDs-WO.sub.3), showing an excellent reduction efficiency of heavy metal.

EXAMPLE 2

[0063] CDs were prepared by the following steps. 1 g glucose was dissolved in 200 mL 98 wt % concentrated H.sub.2SO.sub.4. After introduced into a reactor with an inner tank of polytetrafluoroethylene, the mixed solution of glucose and H.sub.2SO.sub.4 was heated at 200 C. for 5 h and then naturally cooled to room temperature. Next, the pH of the mixed solution was adjusted to 7 with Na.sub.2CO.sub.3. The mixed solution was centrifuged for 15 min at 12000 r/min and the resultant supernatant was extracted with a solid phase extraction column (Oasis HLB, 3 cc/60 mg, Waters). The extraction step comprises four steps: an activation step, an enrichment step, a separation step and an elution step. Specifically, in the activation step, the solid phase extraction column was rinsed with 20 mL methanol, and the residual methanol in the solid phase extraction column was then washed away with 40 mL ultrapure water. In the enrichment step, 20 mL CDs solution was extracted with a solid phase extraction column. In the separation step, the solid phase extraction column was rinsed with 40 mL ultrapure water to dissolve impurities such as inorganic salts. In the elution step, the solid phase extraction column was rinsed with 20 mL methanol to desorb the CDs to obtain high-purity methanol solution of CDs as the extract. Finally, the extract was purged with nitrogen and freeze-dried for 48 h to obtain CDs solid particles.

[0064] TiO.sub.2 nanotubes were prepared by anodization. The TiO.sub.2 nanotubes were immersed in a mixed solution of 30% by volume mercaptopropionic acid (MPA) and 10 g/L CDs, and were taken out after immersion for 48 h to obtain TiO.sub.2 nanotube-CDs electrode.

[0065] The TiO.sub.2 nanotube-CDs-PANI photocatalytic electrode was prepared by in-situ electropolymerization. The TiO.sub.2 nanotube-CDs electrode, a platinum sheet and Ag/AgCl were used as a working electrode, a counter electrode and a reference electrode, respectively. The electrolyte was an acetone solution comprising 0.2 M aniline and 0.05 M citric acid. The electrolyte was deoxygenated with nitrogen for 30 minutes before in-situ electropolymerization. In-situ electropolymerization of aniline was carried out using cyclic voltammetry (potential: 0-0.8 V; number of polymerization cycles: 20 cycles). The working electrode after polymerization was dried at 60 C. for 36 h to obtain a TiO.sub.2 nanotube-CDs-PANI photocatalytic electrode.

[0066] TiO.sub.2 nanotube-CDs-WO.sub.3 photocatalytic electrode was prepared by electrodeposition method. The TiO.sub.2 nanotube-CDs electrode, a platinum sheet and Ag/AgCl were used as a working electrode, a counter electrode and a reference electrode, respectively. The electrolyte was 25 mM Na.sub.2WO.sub.4 and 30 mM H.sub.2O.sub.2 in water, and the pH of the solution was adjusted to 1.40.1 with 0.01 M HNO.sub.3. The deposition voltage was 0.437 V.sub.Ag/AgCl, and the deposition time was 150 s. The resulting electrode was cured at 40 C. for 24 h to obtain the TiO.sub.2 nanotube-CDs-WO.sub.3 photocatalytic electrode.

[0067] An experiment of simultaneous organic matter degradation and heavy metal reduction was performed using the prepared photocatalytic electrode. The photocatalytic electrode was immersed in a solution containing a certain concentration of organic pollutants and heavy metals, and then the working surface of the photocatalytic electrode was exposed to simulated sunlight for organic pollutant degradation and heavy metal reduction. The specific conditions were as follows: light intensity: 100 mW.Math.cm.sup.2, wavelength: more than 420 nm; the ratio of electrode working area to solution volume: 7.5 cm.sup.2.Math.L.sup.1; organic pollutant (carbamazepine) concentration: 100 M; heavy metal (hexavalent chromium) concentration: 68 mM; reaction time: 60 min; no requirement of adjusting the pH value of the reaction system; and no requirement of aerating the reaction system.

[0068] As shown in FIG. 9, after loading of CDs and PANI, or CDs and WO.sub.3, the degradation efficiency of the photocatalytic electrode on organic matter was increased from 21% (TiO.sub.2 nanotubes) to 100% (TiO.sub.2 nanotubes-CDs-PANI) and 79% (TiO.sub.2 nanotubes-CDs-WO.sub.3), showing an excellent degradation effect of organic matter.

[0069] As shown in FIG. 10, after loading of CDs and PANI, or CDs and WO.sub.3, the reduction efficiency of the photocatalytic electrode on heavy metals was increased from 10% (TiO.sub.2 nanotubes) to 73% (TiO.sub.2 nanotubes-CDs-PANI) and 56% (TiO.sub.2 Nanotube-CDs-WO.sub.3), showing an excellent reduction efficiency of heavy metal.

EXAMPLE 3

[0070] CDs were prepared by the following steps. 5 g glucose was dissolved in 150 mL 98 wt % concentrated H.sub.2SO.sub.4. After introduced into a reactor with an inner tank of polytetrafluoroethylene, the mixed solution of glucose and H.sub.2SO.sub.4 was heated at 220 C. for 5 h and then naturally cooled to room temperature. Next, the pH of the mixed solution was adjusted to 7 with Na.sub.2CO.sub.3. The mixed solution was centrifuged for 15 min at 12000 r/min and the resultant supernatant was extracted with a solid phase extraction column (Oasis HLB, 3 cc/60 mg, Waters). The extraction step comprises four steps: an activation step, an enrichment step, a separation step and an elution step. Specifically, in the activation step, the solid phase extraction column was rinsed with 20 mL methanol, and the residual methanol in the solid phase extraction column was then washed away with 40 mL ultrapure water. In the enrichment step, 20 mL CDs solution was extracted with a solid phase extraction column. In the separation step, the solid phase extraction column was rinsed with 40 mL ultrapure water to dissolve impurities such as inorganic salts. In the elution step, the solid phase extraction column was rinsed with 20 mL methanol to desorb the CDs to obtain high-purity methanol solution of CDs as the extract. Finally, the extract was purged with nitrogen and freeze-dried for 48 h to obtain CDs solid particles.

[0071] TiO.sub.2 nanotubes were prepared by anodization. The TiO.sub.2 nanotubes were immersed in a mixed solution of 25% by volume mercaptopropionic acid (MPA) and 6 g/L CDs, and were taken out after immersion for 24 h to obtain TiO.sub.2 nanotube-CDs electrode.

[0072] The TiO.sub.2 nanotube-CDs-PANI photocatalytic electrode was prepared by in-situ electropolymerization. The TiO.sub.2 nanotube-CDs electrode, a platinum sheet and Ag/AgCl were used as a working electrode, a counter electrode and a reference electrode, respectively. The electrolyte was an acetone solution comprising 0.2 M aniline and 0.05 M citric acid. The electrolyte was deoxygenated with nitrogen for 30 minutes before in-situ electropolymerization. In-situ electropolymerization of aniline was carried out using cyclic voltammetry (potential: 0-0.8 V; number of polymerization cycles: 30 cycles). The working electrode after polymerization was dried at 80 C. for 24 h to obtain a TiO.sub.2 nanotube-CDs-PANI photocatalytic electrode.

[0073] TiO.sub.2 nanotube-CDs-WO.sub.3 photocatalytic electrode was prepared by electrodeposition method. The TiO.sub.2 nanotube-CDs electrode, a platinum sheet and Ag/AgCl were used as a working electrode, a counter electrode and a reference electrode, respectively. The electrolyte was 25 mM Na.sub.2WO.sub.4 and 30 mM H.sub.2O.sub.2 in water, and the pH of the solution was adjusted to 1.40.1 with 0.01 M HNO.sub.3. The deposition voltage was 0.437 V.sub.Ag/AgCl, and the deposition time was 150 s. The resulting electrode was cured at 80 C. for 24 h to obtain the TiO.sub.2 nanotube-CDs-WO.sub.3 photocatalytic electrode.

[0074] An experiment of simultaneous organic matter degradation and heavy metal reduction was performed using the prepared photocatalytic electrode. The photocatalytic electrode was immersed in a solution containing a certain concentration of organic pollutants and heavy metals, and then the working surface of the photocatalytic electrode was exposed to simulated sunlight for organic pollutant degradation and heavy metal reduction. The specific conditions were as follows: light intensity: 100 mW.Math.cm.sup.2, wavelength: more than 420 nm; the ratio of electrode working area to solution volume: 7.5 cm.sup.2.Math.L.sup.1; organic pollutant (carbamazepine) concentration: 1M; heavy metal (hexavalent chromium) concentration: 0.68 M; reaction time: 60 min; no requirement of adjusting the pH value of the reaction system; and no requirement of aerating the reaction system.

[0075] As shown in FIG. 9, after loading of CDs and PANI, or CDs and WO.sub.3, the degradation efficiency of the photocatalytic electrode on organic matter was increased from 21% (TiO.sub.2 nanotubes) to 100% (TiO.sub.2 nanotubes-CDs-PANI) and 81% (TiO.sub.2 nanotubes-CDs-WO.sub.3), showing an excellent degradation efficiency of organic matter.

[0076] As shown in FIG. 10, after loading of CDs and PANI, or CDs and WO.sub.3, the reduction efficiency of the photocatalytic electrode on heavy metals was increased from 10% (TiO.sub.2 nanotubes) to 71% (TiO.sub.2 nanotubes-CDs-PANI) and 53% (TiO.sub.2 nanotube-CDs-WO.sub.3), showing an excellent reduction efficiency of heavy metal.

EXAMPLE 4

[0077] CDs were prepared by the following steps. 3 g glucose was dissolved in 180 mL 98 wt % concentrated H.sub.2SO.sub.4. After introduced into a reactor with an inner tank of polytetrafluoroethylene, the mixed solution of glucose and H.sub.2SO.sub.4 was heated at 190 C. for 4 h and then naturally cooled to room temperature. Next, the pH of the mixed solution was adjusted to 7 with Na.sub.2CO.sub.3. The mixed solution was centrifuged for 15 min at 12000 r/min and the resultant supernatant was extracted with a solid phase extraction column (Oasis HLB, 3 cc/60 mg, Waters). The extraction step comprises four steps: an activation step, an enrichment step, a separation step and an elution step. Specifically, in the activation step, the solid phase extraction column was rinsed with 20 mL methanol, and the residual methanol in the solid phase extraction column was then washed away with 40 mL ultrapure water. In the enrichment step, 20 mL CDs solution was extracted with a solid phase extraction column. In the separation step, the solid phase extraction column was rinsed with 40 mL ultrapure water to dissolve impurities such as inorganic salts. In the elution step, the solid phase extraction column was rinsed with 20 mL methanol to desorb the CDs to obtain high-purity methanol solution of CDs as the extract. Finally, the extract was purged with nitrogen and freeze-dried for 48 h to obtain CDs solid particles.

[0078] TiO.sub.2 nanotubes were prepared by anodization. The TiO.sub.2 nanotubes were immersed in a mixed solution of 18% by volume mercaptopropionic acid (MPA) and 7 g/L CDs, and were taken out after immersion for 36 h to obtain TiO.sub.2 nanotube-CDs electrode.

[0079] The TiO.sub.2 nanotube-CDs-PANI photocatalytic electrode was prepared by in-situ electropolymerization. The TiO.sub.2 nanotube-CDs electrode, a platinum sheet and Ag/AgCl were used as a working electrode, a counter electrode and a reference electrode, respectively. The electrolyte was an acetone solution comprising 0.2 M aniline and 0.05 M citric acid. The electrolyte was deoxygenated with nitrogen for 30 minutes before in-situ electropolymerization. In-situ electropolymerization of aniline was carried out using cyclic voltammetry (potential: 0-0.8 V; number of polymerization cycles: 15 cycles). The working electrode after polymerization was dried at 60 C. for 24 h to obtain a TiO.sub.2 nanotube-CDs-PANI photocatalytic electrode.

[0080] TiO.sub.2 nanotube-CDs-WO.sub.3 photocatalytic electrode was prepared by electrodeposition method. The TiO.sub.2 nanotube-CDs electrode, a platinum sheet and Ag/AgCl were used as a working electrode, a counter electrode and a reference electrode, respectively. The electrolyte was 25 mM Na.sub.2WO.sub.4 and 30 mM H.sub.2O.sub.2 in water, and the pH of the solution was adjusted to 1.40.1 with 0.01 M HNO.sub.3. The deposition voltage was 0.437 V.sub.Ag/AgCl, and the deposition time was 150 s. The resulting electrode was cured at 60 C. for 18 h to obtain the TiO.sub.2 nanotube-CDs-WO.sub.3 photocatalytic electrode.

[0081] An experiment of simultaneous organic matter degradation and heavy metal reduction was performed using the prepared photocatalytic electrode. The photocatalytic electrode was immersed in a solution containing a certain concentration of organic pollutants and heavy metals, and then the working surface of the photocatalytic electrode was exposed to simulated sunlight for organic pollutant degradation and heavy metal reduction. The specific conditions were as follows: light intensity: 100 mW.Math.cm.sup.2, wavelength: more than 420 nm; the ratio of electrode working area to solution volume: 7.5 cm.sup.2.Math.L.sup.1; organic pollutant (carbamazepine) concentration: 1M; heavy metal (hexavalent chromium) concentration: 6.8 M; reaction time: 60 min; no requirement of adjusting the pH value of the reaction system; and no requirement of aerating the reaction system.

[0082] As shown in FIG. 9, after loading of CDs and PANI, or CDs and WO.sub.3, the degradation efficiency of the photocatalytic electrode on organic matter was increased from 21% (TiO.sub.2 nanotubes) to 100% (TiO.sub.2 nanotubes-CDs-PANI) and 85% (TiO.sub.2 nanotubes-CDs-WO.sub.3), showing an excellent degradation efficiency of organic matter.

[0083] As shown in FIG. 10, after loading of CDs and PANI, or CDs and WO.sub.3, the reduction efficiency of the photocatalytic electrode on heavy metals was increased from 10% (TiO.sub.2 nanotubes) to 79% (TiO.sub.2 nanotubes-CDs-PANI) and 58% (TiO.sub.2 nanotube-CDs-WO.sub.3), showing an excellent reduction efficiency of heavy metal.

EXAMPLE 5

[0084] CDs were prepared by the following steps. 2 g glucose was dissolved in 150 mL 98 wt % concentrated H.sub.2SO.sub.4. After introduced into a reactor with an inner tank of polytetrafluoroethylene, the mixed solution of glucose and H.sub.2SO.sub.4 was heated at 200 C. for 3 h and then naturally cooled to room temperature. Next, the pH of the mixed solution was adjusted to 7 with Na.sub.2CO.sub.3. The mixed solution was centrifuged for 15 min at 12000 r/min and the resultant supernatant was extracted with a solid phase extraction column (Oasis HLB, 3 cc/60 mg, Waters). The extraction step comprises four steps: an activation step, an enrichment step, a separation step and an elution step. Specifically, in the activation step, the solid phase extraction column was rinsed with 20 mL methanol, and the residual methanol in the solid phase extraction column was then washed away with 40 mL ultrapure water. In the enrichment step, 20 mL CDs solution was extracted with a solid phase extraction column. In the separation step, the solid phase extraction column was rinsed with 40 mL ultrapure water to dissolve impurities such as inorganic salts. In the elution step, the solid phase extraction column was rinsed with 20 mL methanol to desorb the CDs to obtain high-purity methanol solution of CDs as the extract. Finally, the extract was purged with nitrogen and freeze-dried for 48 h to obtain CDs solid particles.

[0085] TiO.sub.2 nanotubes were prepared by anodization. The TiO.sub.2 nanotubes were immersed in a mixed solution of 10% by volume mercaptopropionic acid (MPA) and 10 g/L CDs, and were taken out after immersion for 48 h to obtain TiO.sub.2 nanotube-CDs electrode.

[0086] The TiO.sub.2 nanotube-CDs-PANI photocatalytic electrode was prepared by in-situ electropolymerization. The TiO.sub.2 nanotube-CDs electrode, a platinum sheet and Ag/AgCl were used as a working electrode, a counter electrode and a reference electrode, respectively. The electrolyte was an acetone solution comprising 0.2 M aniline and 0.05 M citric acid. The electrolyte was deoxygenated with nitrogen for 30 minutes before in-situ electropolymerization. In-situ electropolymerization of aniline was carried out using cyclic voltammetry (potential: 0-0.8 V; number of polymerization cycles: 10 cycles). The working electrode after polymerization was dried at 60 C. for 24 h to obtain a TiO.sub.2 nanotube-CDs-PANI photocatalytic electrode.

[0087] TiO.sub.2 nanotube-CDs-WO.sub.3 photocatalytic electrode was prepared by electrodeposition method. The TiO.sub.2 nanotube-CDs electrode, a platinum sheet and Ag/AgCl were used as a working electrode, a counter electrode and a reference electrode, respectively. The electrolyte was 25 mM Na.sub.2WO.sub.4 and 30 mM H.sub.2O.sub.2 in water, and the pH of the solution was adjusted to 1.40.1 with 0.01 M HNO.sub.3. The deposition voltage was 0.437 V.sub.Ag/AgCl, and the deposition time was 150 s. The resulting electrode was cured at 60 C. for 12 h to obtain the TiO.sub.2 nanotube-CDs-WO.sub.3 photocatalytic electrode.

[0088] An experiment of simultaneous organic matter degradation and heavy metal reduction was performed using the prepared photocatalytic electrode. The photocatalytic electrode was immersed in a solution containing a certain concentration of organic pollutants and heavy metals, and then the working surface of the photocatalytic electrode was exposed to simulated sunlight for organic pollutant degradation and heavy metal reduction. The specific conditions were as follows: light intensity: 100 mW.Math.cm.sup.2, wavelength: more than 420 nm; the ratio of electrode working area to solution volume: 7.5 cm.sup.2.Math.L.sup.1; organic pollutant (carbamazepine) concentration: 10 M; heavy metal (hexavalent chromium) concentration: 0.68 mM; reaction time: 60 min; no requirement of adjusting the pH value of the reaction system; and no requirement of aerating the reaction system.

[0089] As shown in FIG. 9, after loading of CDs and PANI, or CDs and WO.sub.3, the degradation efficiency of the photocatalytic electrode on organic matter was increased from 21% (TiO.sub.2 nanotubes) to 100% (TiO.sub.2 nanotubes-CDs-PANI) and 84% (TiO.sub.2 nanotubes-CDs-WO.sub.3), showing an excellent degradation efficiency of organic matter.

[0090] As shown in FIG. 10, after loading of CDs and PANI, or CDs and WO.sub.3, the reduction efficiency of the photocatalytic electrode on heavy metals was increased from 10% (TiO.sub.2 nanotubes) to 73% (TiO.sub.2 nanotubes-CDs-PANI) and 57% (TiO.sub.2 nanotube-CDs-WO.sub.3), showing an excellent reduction efficiency of heavy metal.

EXAMPLE 6

[0091] CDs were prepared by the following steps. 4 g glucose was dissolved in 180 mL 98 wt % concentrated H.sub.2SO.sub.4. After introduced into a reactor with an inner tank of polytetrafluoroethylene, the mixed solution of glucose and H.sub.2SO.sub.4 was heated at 180-220 C. for 3 h and then naturally cooled to room temperature. Next, the pH of the mixed solution was adjusted to 7 with Na.sub.2CO.sub.3. The mixed solution was centrifuged for 15 min at 12000 r/min and the resultant supernatant was extracted with a solid phase extraction column (Oasis HLB, 3 cc/60 mg, Waters). The extraction step comprises four steps: an activation step, an enrichment step, a separation step and an elution step. Specifically, in the activation step, the solid phase extraction column was rinsed with 20 mL methanol, and the residual methanol in the solid phase extraction column was then washed away with 40 mL ultrapure water. In the enrichment step, 20 mL CDs solution was extracted with a solid phase extraction column. In the separation step, the solid phase extraction column was rinsed with 40 mL ultrapure water to dissolve impurities such as inorganic salts. In the elution step, the solid phase extraction column was rinsed with 20 mL methanol to desorb the CDs to obtain high-purity methanol solution of CDs as the extract. Finally, the extract was purged with nitrogen and freeze-dried for 48 h to obtain CDs solid particles.

[0092] TiO.sub.2 nanotubes were prepared by anodization. The TiO.sub.2 nanotubes were immersed in a mixed solution of 25% by volume mercaptopropionic acid (MPA) and 15 g/L CDs, and were taken out after immersion for 48 h to obtain TiO.sub.2 nanotube-CDs electrode.

[0093] The TiO.sub.2 nanotube-CDs-PANI photocatalytic electrode was prepared by in-situ electropolymerization. The TiO.sub.2 nanotube-CDs electrode, a platinum sheet and Ag/AgCl were used as a working electrode, a counter electrode and a reference electrode, respectively. The electrolyte was an acetone solution comprising 0.2 M aniline and 0.05 M citric acid. The electrolyte was deoxygenated with nitrogen for 30 minutes before in-situ electropolymerization. In-situ electropolymerization of aniline was carried out using cyclic voltammetry (potential: 0-0.8 V; number of polymerization cycles: 20 cycles). The working electrode after polymerization was dried at 60 C. for 24 h to obtain a TiO.sub.2 nanotube-CDs-PANI photocatalytic electrode.

[0094] TiO.sub.2 nanotube-CDs-WO.sub.3 photocatalytic electrode was prepared by electrodeposition method. The TiO.sub.2 nanotube-CDs electrode, a platinum sheet and Ag/AgCl were used as a working electrode, a counter electrode and a reference electrode, respectively. The electrolyte was 25 mM Na.sub.2WO.sub.4 and 30 mM H.sub.2O.sub.2 in water, and the pH of the solution was adjusted to 1.40.1 with 0.01 M HNO.sub.3. The deposition voltage was 0.437 V.sub.Ag/AgCl, and the deposition time was 150 s. The resulting electrode was cured at 60 C. for 12 h to obtain the TiO.sub.2 nanotube-CDs-WO.sub.3 photocatalytic electrode.

[0095] An experiment of simultaneous organic matter degradation and heavy metal reduction was performed using the prepared photocatalytic electrode. The photocatalytic electrode was immersed in a solution containing a certain concentration of organic pollutants and heavy metals, and then the working surface of the photocatalytic electrode was exposed to simulated sunlight for organic pollutant degradation and heavy metal reduction. The specific conditions were as follows: light intensity: 100 mW.Math.cm.sup.2, wavelength: more than 420 nm; the ratio of electrode working area to solution volume: 7.5 cm.sup.2.Math.L.sup.1; organic pollutant (carbamazepine) concentration: 10 M; heavy metal (hexavalent chromium) concentration: 0.68 mM; reaction time: 60 min; no requirement of adjusting the pH value of the reaction system; and no requirement of aerating the reaction system.

[0096] As shown in FIG. 9, after loading of CDs and PANI, or CDs and WO.sub.3, the degradation efficiency of the photocatalytic electrode on organic matter was increased from 21% (TiO.sub.2 nanotubes) to 100% (TiO.sub.2 nanotubes-CDs-PANI) and 85% (TiO.sub.2 nanotubes-CDs-WO.sub.3), showing an excellent degradation efficiency of organic matter.

[0097] As shown in FIG. 10, after loading of CDs and PANI, or CDs and WO.sub.3, the reduction efficiency of the photocatalytic electrode on heavy metals was increased from 10% (TiO.sub.2 nanotubes) to 76% (TiO.sub.2 nanotubes-CDs-PANI) and 56% (TiO.sub.2 nanotube-CDs-WO.sub.3), showing an excellent reduction efficiency of heavy metal.

EXAMPLE 7

[0098] CDs were prepared by the following steps. 4 g glucose was dissolved in 180 mL 98 wt % concentrated H.sub.2SO.sub.4. After introduced into a reactor with an inner tank of polytetrafluoroethylene, the mixed solution of glucose and H.sub.2SO.sub.4 was heated at 180 C. for 3 h and then naturally cooled to room temperature. Next, the pH of the mixed solution was adjusted to 7 with Na.sub.2CO.sub.3. The mixed solution was centrifuged for 15 min at 12000 r/min and the resultant supernatant was extracted with a solid phase extraction column (Oasis HLB, 3 cc/60 mg, Waters). The extraction step comprises four steps: an activation step, an enrichment step, a separation step and an elution step. Specifically, in the activation step, the solid phase extraction column was rinsed with 20 mL methanol, and the residual methanol in the solid phase extraction column was then washed away with 40 mL ultrapure water. In the enrichment step, 20 mL CDs solution was extracted with a solid phase extraction column. In the separation step, the solid phase extraction column was rinsed with 40 mL ultrapure water to dissolve impurities such as inorganic salts. In the elution step, the solid phase extraction column was rinsed with 20 mL methanol to desorb the CDs to obtain high-purity methanol solution of CDs as the extract. Finally, the extract was purged with nitrogen and freeze-dried for 48 h to obtain CDs solid particles.

[0099] Fe.sub.2O.sub.3 nanotubes were prepared by anodization. The Fe.sub.2O.sub.3 nanotubes were immersed in a mixed solution of 25% by volume mercaptopropionic acid (MPA) and 15 g/L CDs, and were taken out after immersion for 48 h to obtain Fe.sub.2O.sub.3 nanotube-CDs electrode.

[0100] Carbon nitride was formed on the CDs electron transport layer by a hydrothermal method. The prepared Fe.sub.2O.sub.3 nanotube-CDs electrode was immersed in an aqueous solution containing melamine 10 wt % and incubated at 80 C. for 24 h to obtain the Fe.sub.2O.sub.3 nanotube-CDs-carbon nitride photocatalytic electrode.

[0101] An experiment of simultaneous organic matter degradation and heavy metal reduction was performed using the prepared photocatalytic electrode. The photocatalytic electrode was immersed in a solution containing a certain concentration of organic pollutants and heavy metals, and then the working surface of the photocatalytic electrode was exposed to simulated sunlight for organic pollutant degradation and heavy metal reduction. The specific conditions were as follows: light intensity: 100 mW.Math.cm.sup.2, wavelength: more than 420 nm; the ratio of electrode working area to solution volume: 7.5 cm.sup.2.Math.L.sup.1; organic pollutant (carbamazepine) concentration: 10 M; heavy metal (hexavalent chromium) concentration: 0.68 mM; reaction time: 60 min; no requirement of adjusting the pH value of the reaction system; and no requirement of aerating the reaction system.

[0102] As shown in FIG. 9, after loading of CDs and carbon nitride, the degradation efficiency of the photocatalytic electrode on organic matter was increased from 6% (Fe.sub.2O.sub.3 nanotubes) to 70% (Fe.sub.2O.sub.3 nanotubes-CDs-carbon nitride), showing an excellent degradation efficiency of organic matter.

[0103] As shown in FIG. 10, after loading of CDs and carbon nitride, the reduction efficiency of the photocatalytic electrode on heavy metals was increased from 4% (Fe.sub.2O.sub.3 nanotubes) to 79% (Fe.sub.2O.sub.3 nanotubes-CDs-carbon nitride), showing an excellent reduction efficiency of heavy metal.

EXAMPLE 8

[0104] CDs were prepared by the following steps. 4 g glucose was dissolved in 180 mL 98 wt % concentrated H.sub.2SO.sub.4. After introduced into a reactor with an inner tank of polytetrafluoroethylene, the mixed solution of glucose and H.sub.2SO.sub.4 was heated at 220 C. for 3 h and then naturally cooled to room temperature. Next, the pH of the mixed solution was adjusted to 7 with Na.sub.2CO.sub.3. The mixed solution was centrifuged for 15 min at 12000 r/min and the resultant supernatant was extracted with a solid phase extraction column (Oasis HLB, 3 cc/60 mg, Waters). The extraction step comprises four steps: an activation step, an enrichment step, a separation step and an elution step. Specifically, in the activation step, the solid phase extraction column was rinsed with 20 mL methanol, and the residual methanol in the solid phase extraction column was then washed away with 40 mL ultrapure water. In the enrichment step, 20 mL CDs solution was extracted with a solid phase extraction column. In the separation step, the solid phase extraction column was rinsed with 40 mL ultrapure water to dissolve impurities such as inorganic salts. In the elution step, the solid phase extraction column was rinsed with 20 mL methanol to desorb the CDs to obtain high-purity methanol solution of CDs as the extract. Finally, the extract was purged with nitrogen and freeze-dried for 48 h to obtain CDs solid particles.

[0105] Fe.sub.2O.sub.3 nanotubes were prepared by anodization. The Fe.sub.2O.sub.3 nanotubes were immersed in a mixed solution of 25% by volume mercaptopropionic acid (MPA) and 15 g/L CDs, and were taken out after immersion for 48 h to obtain Fe.sub.2O.sub.3 nanotube-CDs electrode.

[0106] Carbon nitride was formed on the CDs electron transport layer by a hydrothermal method. The prepared Fe.sub.2O.sub.3 nanotube-CDs electrode was immersed in an aqueous solution containing melamine 30 wt % and incubated at 80 C. for 72 h to obtain the Fe.sub.2O.sub.3 nanotube-CDs-carbon nitride photocatalytic electrode.

[0107] An experiment of simultaneous organic matter degradation and heavy metal reduction was performed using the prepared photocatalytic electrode. The photocatalytic electrode was immersed in a solution containing a certain concentration of organic pollutants and heavy metals, and then the working surface of the photocatalytic electrode was exposed to simulated sunlight for organic pollutant degradation and heavy metal reduction. The specific conditions were as follows: light intensity: 100 mW.Math.cm.sup.2, wavelength: more than 420 nm; the ratio of electrode working area to solution volume: 7.5 cm.sup.2.Math.L.sup.1; organic pollutant (carbamazepine) concentration: 10 M; heavy metal (hexavalent chromium) concentration: 0.68 mM; reaction time: 60 min; no requirement of adjusting the pH value of the reaction system; and no requirement of aerating the reaction system.

[0108] As shown in FIG. 9, after loading of CDs and carbon nitride, the degradation efficiency of the photocatalytic electrode on organic matter was increased from 6% (Fe.sub.2O.sub.3 nanotubes) to 65% (Fe.sub.2O.sub.3 nanotubes-CDs-carbon nitride), showing an excellent degradation efficiency of organic matter.

[0109] As shown in FIG. 10, after loading of CDs and carbon nitride, the reduction efficiency of the photocatalytic electrode on heavy metals was increased from 4% (Fe.sub.2O.sub.3 nanotubes) to 66% (Fe.sub.2O.sub.3 nanotubes-CDs-carbon nitride), showing an excellent reduction efficiency of heavy metal.

COMPARATIVE EXAMPLE 1

[0110] TiO.sub.2 nanotubes were prepared by anodization. The TiO.sub.2 nanotube-PANI photocatalytic electrode was prepared by in-situ electropolymerization. The TiO.sub.2 nanotube electrode, a platinum sheet and Ag/AgCl were used as a working electrode, a counter electrode and a reference electrode, respectively. The electrolyte was an acetone solution comprising 0.2 M aniline and 0.05 M citric acid. The electrolyte was deoxygenated with nitrogen for 30 minutes before in-situ electropolymerization. In-situ electropolymerization of aniline was carried out using cyclic voltammetry (potential: 0-0.8 V; number of polymerization cycles: 15 cycles). The working electrode after polymerization was dried at 60 C. for 24 h to obtain a TiO.sub.2 nanotube- PANI photocatalytic electrode.

[0111] TiO.sub.2 nanotube-WO.sub.3 photocatalytic electrode was prepared by electrodeposition method. The TiO.sub.2 nanotube electrode, a platinum sheet and Ag/AgCl were used as a working electrode, a counter electrode and a reference electrode, respectively. The electrolyte was 25 mM Na.sub.2WO.sub.4 and 30 mM H.sub.2O.sub.2 in water, and the pH of the solution was adjusted to 1.40.1 with 0.01 M HNO.sub.3. The deposition voltage was 0.437 V.sub.Ag/AgCl, and the deposition time was 150 s. The resulting electrode was cured at 60 C. for 18 h to obtain the TiO.sub.2 nanotube-WO.sub.3 photocatalytic electrode.

[0112] An experiment of simultaneous organic matter degradation and heavy metal reduction was performed using the prepared photocatalytic electrode. The photocatalytic electrode was immersed in a solution containing a certain concentration of organic pollutants and heavy metals, and then the working surface of the photocatalytic electrode was exposed to simulated sunlight for organic pollutant degradation and heavy metal reduction. The specific conditions were as follows: light intensity: 100 mW.Math.cm.sup.2, wavelength: more than 420 nm; the ratio of electrode working area to solution volume: 7.5 cm.sup.2.Math.L.sup.1; organic pollutant (carbamazepine) concentration: 1 M; heavy metal (hexavalent chromium) concentration: 6.8 M; reaction time: 60 min; no requirement of adjusting the pH value of the reaction system; and no requirement of aerating the reaction system.

[0113] As shown in FIG. 11, after loading of PANI and WO.sub.3, respectively, the degradation efficiency of the photocatalytic electrode on organic matter was increased from 21% (TiO.sub.2 nanotubes) to 29% (TiO.sub.2 nanotubes-CDs-PANI) and 23% (TiO.sub.2 nanotubes-CDs-WO.sub.3), showing a slight increase in degradation efficiency of organic matter.

[0114] As shown in FIG. 12, after loading of PANI and WO.sub.3, respectively, the reduction efficiency of the photocatalytic electrode on heavy metals was increased from 10% (TiO.sub.2 nanotubes) to 12% (TiO.sub.2 nanotubes-PANI) and 11% (TiO.sub.2 Nanotube-WO.sub.3), showing a slight increase in reduction efficiency of heavy metal.

[0115] It was found that the efficiency of simultaneous organic matter degradation and heavy metal reduction could be significantly increased by introducing CDs as an electronic assistant into the photocatalytic electrode.

COMPARATIVE EXAMPLE 2

[0116] CDs were prepared by the following steps. 4 g glucose was dissolved in 180 mL 98 wt % concentrated H.sub.2SO.sub.4. After introduced into a reactor with an inner tank of polytetrafluoroethylene, the mixed solution of glucose and H.sub.2SO.sub.4 was heated at 220 C. for 3 h and then naturally cooled to room temperature. Next, the pH of the mixed solution was adjusted to 7 with Na.sub.2CO.sub.3. The mixed solution was centrifuged for 15 min at 12000 r/min and the resultant supernatant was extracted with a solid phase extraction column (Oasis HLB, 3 cc/60 mg, Waters). The extraction step comprises four steps: an activation step, an enrichment step, a separation step and an elution step. Specifically, in the activation step, the solid phase extraction column was rinsed with 20 mL methanol, and the residual methanol in the solid phase extraction column was then washed away with 40 mL ultrapure water. In the enrichment step, 20 mL CDs solution was extracted with a solid phase extraction column. In the separation step, the solid phase extraction column was rinsed with 40 mL ultrapure water to dissolve impurities such as inorganic salts. In the elution step, the solid phase extraction column was rinsed with 20 mL methanol to desorb the CDs to obtain high-purity methanol solution of CDs as the extract. Finally, the extract was purged with nitrogen and freeze-dried for 48 h to obtain CDs solid particles.

[0117] Fe.sub.2O.sub.3 nanotubes were prepared by anodization. Two Fe.sub.2O.sub.3 nanotubes were immersed in a mixed solution of 25% by volume mercaptopropionic acid (MPA) and 15 g/L CDs and a solution of 15 g/L CDs without mercaptopropionic acid, and were taken out after immersion for 48 h to obtain TiO.sub.2 nanotube-CDs electrode.

[0118] Carbon nitride was formed on the CDs electron transport layer by a hydrothermal method. The prepared Fe.sub.2O.sub.3 nanotube-CDs electrode was immersed in an aqueous solution containing melamine 30 wt % and incubated at 80 C. for 72 h to obtain the Fe.sub.2O.sub.3 nanotube-CDs-carbon nitride photocatalytic electrode.

[0119] An experiment of simultaneous organic matter degradation and heavy metal reduction was performed using the prepared photocatalytic electrode. The photocatalytic electrode was immersed in a solution containing a certain concentration of organic pollutants and heavy metals, and then the working surface of the photocatalytic electrode was exposed to simulated sunlight for organic pollutant degradation and heavy metal reduction. The specific conditions were as follows: light intensity: 100 mW.Math.cm.sup.2, wavelength: more than 420 nm; the ratio of electrode working area to solution volume: 7.5 cm.sup.2.Math.L.sup.1; organic pollutant (carbamazepine) concentration: 10 M; heavy metal (hexavalent chromium) concentration: 0.68 mM; reaction time: 60 min; no requirement of adjusting the pH value of the reaction system; and no requirement of aerating the reaction system.

[0120] As shown in FIG. 13, in the case of no addition of MPA, the degradation efficiency of the Fe.sub.2O.sub.3 nanotube-CDs-carbon nitride photocatalytic electrode on organic matter was increased from 6% (Fe.sub.2O.sub.3 nanotubes) to 25%, and further increased to 65% after addition of MPA. It was confirmed that the addition of MPA facilitated immobilization of CDs and could further increase the degradation efficiency of organic matter.

[0121] As shown in FIG. 14, in the case of no addition of MPA, the reduction efficiency of the Fe.sub.2O.sub.3 nanotube-CDs-carbon nitride photocatalytic electrode on heavy metals was increased from 4% (Fe.sub.2O.sub.3 nanotubes) to 12%, and further increased to 66% after addition of MPA. It was confirmed that the addition of MPA facilitated immobilization of CDs and could further increase the reduction efficiency of heavy metals.

[0122] The specific embodiments further describe the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that these are only specific embodiments of the present invention and are not intended to limit the present invention. Within the spirit and principle of the present invention, any modifications, equivalent replacements, modifications, etc., shall be included in the scope of the present invention.