- Nano Express
- Open Access
Enhanced Photocatalytic Performance of Luminescent g-C3N4 Photocatalyst in Darkroom
© Li et al. 2016
- Received: 8 September 2015
- Accepted: 6 February 2016
- Published: 16 February 2016
Graphitic-C3N4(g-C3N4), a low-cost visible-light-driven photocatalyst, was used for the photocatalytic oxidation of aqueous methylene blue (MB) in the dark with Sr4Al14O25:(Eu,Dy) assistance. The Sr4Al14O25:(Eu,Dy)/g-C3N4 photocatalysts were fabricated through the ultrasonic dispersion method. The commercial Sr4Al14O25:(Eu,Dy) phosphor was used as a long afterglow supplier for exciting g-C3N4 in the dark. The results demonstrated that the metal-free g-C3N4 photocatalyst could use the eye-visible long afterglow to photocatalytically decompose MB dyes in the dark. This work may expand the appealing application of g-C3N4 for the environmental cleanup.
- Dye degradation
- Water clear
- Long afterglow assistance
- In darkroom
The organic dye is one of the most significant identified pollutants in waste waters because of its high toxicity and possible accumulation in the environment . At the same time, the dye, generally as a water-soluble organic colorant, is gradually increased owing to the tremendous increase of industrialization and requirements of human beings for color . The presence of dyes in textile wastewater is an environmental problem due to their high visibility, resistance, and toxic impact . In addition, dyes have direct and indirect toxic effects on humans as they are associated with cancer, jaundice, tumors, skin irritation, allergies, heart defects, and mutations , and . Among various treatments, photocatalytic oxidation is found to be one of the most effective ways to degrade dyes in a wastewater and gets rid of their deep color .
The photocatalytic performance of semiconductor-based (such as TiO2, ZnO, and SrTiO3) photocatalysis has received considerable attention for the attractive strategy of conversing solar energy into the formation of hydrogen by the treatment of organics in waste waters [7–9]. However, the lack of visible-light utilization ability and/or the low quantum yield greatly limited their performance and large-scale application. Therefore, designing and optimizing the efficient visible-light responsive photocatalysts have attracted worldwide attention [10, 11]. Ag3PO4, a breakthrough on visible-light-driven photocatalyst , had created the wave of research interest and participation widely . Nevertheless, it was also found that Ag3PO4 exhibited the low photochemical stability [14, 15], photo-corrosion, [16, 17] and “self-corrosion” characteristic , which prevent its use in environment and energy regions. Besides Ag3PO4, several other visible-light-driven photocatalysts, such as Bi2Fe4O9 , Bi3NbO7 , and Bi2WO6 , have also been investigated. To date, however, most of the reported photocatalysts with high photocatalytic ability for water treatment under visible-light irradiation are metal compound-based semiconductors. Considering that metals are relatively expensive materials because of the limited resource, alternative photocatalysts based on precious metal-free materials have been actively pursued .
As a metal-free photocatalyst, polymeric graphite-like carbon nitride (g-C3N4) with a band gap of 2.70 eV has attracted much attention in H2 production and contaminants degradation [23–25]. Nevertheless, as other single-phase semiconductors with the narrow band gaps, the high recombination rate of photo-generated electrons and holes of g-C3N4 restricts its further application in the field of photocatalysis . Up to date, numerous of g-C3N4-based composites, such as TaON/g-C3N4 , Ag3VO4/g-C3N4 , and S-TiO2/g-C3N4 , have been reported. The hybrid photocatalysts presented much higher activity than the pure one, mainly due to the coupling effect between the g-C3N4 and semiconductors .
On the other hand, the long-lasting phosphorescence, which persists the luminescence for a long time after the removal of the excitation light source, is an interesting phenomenon . Our previous works have revealed that the long afterglow phosphor-assisted Ag3PO4 photocatalyst can efficiently decompose aqueous organic pollutants even after turning off the irradiation light . However, under lamp irradiation, Ag3PO4 shows the low chemical stability and photo-corrosion, while this phenomenon does not occur in the g-C3N4 photocatalytic system. Herein, the present work reports for the first time that the Sr4Al14O25:(Eu,Dy)/g-C3N4 exhibits good performance on wastewater cleaning both under lamp irradiation and turning off light. This work not only provides a new promising strategy for the full time wastewater purification but also broadens the application of long afterglow phosphors in photocatalysis and deepens the understanding on the combination mechanism of g-C3N4-based hybrids.
All chemicals were purchased from Aladdin (Shanghai, China) except for Sr4Al14O25:(Eu,Dy) powders from Lumin (Dalian, China) and used as received without further purification. The Sr4Al14O25:(Eu,Dy) long afterglow phosphor was used as a phosphorescence assistance because of its stability in aqueous solution for a long time as shown in our previous work . G-C3N4 was prepared by the thermal polycondensation of melamine . Typically, 5.0 g of melamine in a covered alumina crucible was put into a muffle furnace and heated to 550 °C for 2 h with a heating rate of 10 °C/min. The resulted yellow product was collected and ground into powders. In order to combine Sr4Al14O25:(Eu,Dy) with g-C3N4, 0.1 g of g-C3N4 powders were, firstly, dispersed into 10 mL of ammonia solution (25 wt%) via stirring at room temperature for 5 h . The final product was collected by centrifugation and dried at 60 °C under vacuum overnight. Before combination, Sr4Al14O25:(Eu,Dy) and g-C3N4 were firstly modified by ammonia solution treatment, respectively . Then, Sr4Al14O25:(Eu,Dy)/g-C3N4 composites were achieved by a simple method [26, 32]. In a typical procedure, 0.12 g of g-C3N4 powders and 0.08 g of Sr4Al14O25:(Eu,Dy) powders were separately added into 50 mL of methanol and sonicated for 30 min. Then, these two solutions were mixed and continuously stirred in a covered beaker at room temperature for 24 h. After volatilizing the methanol, Sr4Al14O25:(Eu,Dy)/g-C3N4 powders were obtained after volatilizing methanol and drying. According to this method, the different mass ratios of hybrid photocatalysts were prepared. The obtained composites are named as x % composite sheet (CS) (x = 20, 40, 60, 80), where x refers to the g-C3N4 weight percent in the composite. For comparison, pure g-C3N4 and Sr4Al14O25:(Eu,Dy) were similarly treated by methanol and dried, respectively.
The X-ray diffraction (XRD) patterns of the catalysts were measured from 10° to 80° of 2θ using a Bruker AXS D2 Phaser X-ray diffractometer and graphite-monochromic CuKα radiation. The catalyst morphology was observed by using an FEI Tecnai G2 F30 transmission electron microscope (TEM) with a Gatan imaging filter (GIF) system. The diffuse reflectance spectra (DRS) were determined using powder samples (PE Lambda 950), and BaSO4 was used as a reference. The vibration spectra were characterized by Fourier transform infrared spectroscopy (FT-IR) (NEXUS 670, Nicolet). X-ray photoelectron spectroscopy (XPS) measurement was done using a Kratos AXIS Ultra DLD XPS system with a monochromatic AlKα source and a charge neutralizer; all the binding energies were referenced to the C1s peak at 284.6 eV of the surface adventitious carbon. The photoluminescence (PL) spectra were obtained on a FLS-920T fluorescence spectrophotometer (excitation wavelength 325 nm). The decay curve was then measured with a PR 305 afterglow phosphorescence instrument at 15 °C. The surface charge in aqueous solution was measured using a zeta-potential analyzer (Malvern Zetasizer Nano-ZS 90).
Methylene blue (MB) was taken as a target pollutant to evaluate the photocatalytic activities of the Sr4Al14O25:(Eu,Dy)/g-C3N4 composites. Sr4Al14O25:(Eu,Dy), a kind of long afterglow phosphor, emits an eye-visible blue and green luminescence with the peak wavelength at λ = 490 nm at room temperature. To compare with the long afterglow-assisted photocatalytic activity, a weak-intensity lamp with visible light was used as a light source. Therefore, the visible lamp light-induced photocatalytic reaction experiments were conducted using a 350-W Xe lamp (Au-Light, CEL-LAX 350) equipped with a 420-nm cutoff filter as the visible-light source (ca. 0.11 mW/cm2). Typically, a 100 mL of MB aqueous solution at a concentration of 5 mg/L was mixed with 0.08 g of sample in a 500-mL beaker for reaction. Prior to light irradiation, the suspensions were continuously stirred in the dark for 60 min to reach the adsorption/desorption equilibrium. Then, the light was turned on, and 5 mL of the suspension was withdrawn every 20 min, centrifuged, and filtered to remove the solid particles. The filtrates were analyzed by recording variations of the maximum absorption peak (664 nm for MB). The decoloration efficiency was recorded as C/C 0, where C is the MB concentration after adsorption or photocatalysis and C 0 is the initial concentration.
To investigate the effect of long afterglow assistance on photocatalytic activity, the catalytic reaction was proceeded in the dark for 10 h after the photoexcitation of the sample to generate the long afterglow. Typically, 0.01 g of the sample was first irradiated by an 8-W black lamp for 30 min and then well dispersed in 25 mL MB dye solution (2.5 mg/L) for ca. 10 h in the dark. Then, the sample powders were removed from the solution to determine the concentration of MB dye remained in the solution. After that, the sample was irradiated by the black lamp for 30 min again and put into the MB dye solution in the dark. These procedures were repeated 15 times, i.e., the total reaction time was 150 h. In order to avoid the influence of other light, the photocatalytic reaction proceeded in the darkness with an opaque material hood. The MB dye solution with dispersed photocatalyst sample was put into a 50-mL centrifuge tube with black tapes. The experimental procedure and evaluation of persistent photocatalytic degradation reaction are similar to those in the previous work .
The 60 % CS hybrid material also displays two C1s peaks at 284.8 and 287.8 eV, indicating a little difference in the binding energies compared with pure g-C3N4. It suggests that Sr4Al14O25:(Eu,Dy) hybridization with g-C3N4 resulted in an inner shift of the C1s orbit. The N1s peaks of g-C3N4 and 60 % CS are observed at 401.1, 400.1, and 398.4 eV in Fig. 4c. The main signal shows the occurrence of C–N–C bond (398.4 eV) and tertiary nitrogen N-(C)3 groups (400.1 eV) in g-C3N4 and 60 % CS. It also reveals an additional signal at 401.1 eV, indicative of the amino functions carrying hydrogen (C–N–H). In addition, the N1s peak maintained the same binding energy which suggests that a suitable combination of g-C3N4 and Sr4Al14O25:(Eu,Dy) inducing the N1s orbit offset can be ruled out. The O1s peak at 537.5 and 531.3 eV shown in Fig. 4d can be assigned to the O element in the Sr4Al14O25:(Eu,Dy).
At the same time, the O1s peak at 532.6 eV of g-C3N4 is derived from the hydroxyl group. After the combination with Sr4Al14O25:(Eu,Dy), the peak blue shifts to 531.9 eV, indicating there is the chemical bond between them. To further observe the chemical interaction, the Sr 3d XPS spectra are shown in Fig. 4e. It can be seen that the Sr 3d peak at 138.5 eV can be only found in pure Sr4Al14O25:(Eu,Dy), as well as the Sr 3d peaks at 133.5 and 135.1 eV for pristine Sr4Al14O25:(Eu,Dy) and 60 % CS hybrid shift to 134.1 and 135.6 eV, respectively, firmly confirming the interaction between the two components.
The red shift of the Sr 3d value indicates that the interaction can increase the effective negative charge of the Sr species. It is also supported by the result that the g-C3N4 possesses the surface hydroxyl groups. A similar phenomenon is also found in the XPS spectra of the Al 2p (Fig. 4f). The binding energies of the Al 2p (74.1 and 78 eV) of pure Sr4Al14O25:(Eu,Dy) are higher than that (74.4 eV) of 60 % CS hybrid. Such results can be similarly attributed to the interaction of g-C3N4 with Sr4Al14O25:(Eu,Dy), resulting in an inner shift of the Al 2p orbit. The analyses distinctly indicate the presence of chemical bonds between g-C3N4 and Sr4Al14O25:(Eu, Dy), rather than a simple physical mixing.
In summary, a facile and efficient process for the MB dye degradation in the dark has been achieved over the Sr4Al14O25:(Eu,Dy)-assisted g-C3N4 composite photocatalysts. Compared with pure g-C3N4, the hybrids with long afterglow phosphor can dramatically realize the photocatalytic aqueous MB oxidation in the dark. The enhanced photocatalytic efficiency of composites is attributed to the long afterglow assistance from Sr4Al14O25:(Eu,Dy) and the visible-light absorptivity of g-C3N4. Moreover, the studies of the photocatalytic performance of the hybrid photocatalysts revealed that the Sr4Al14O25:(Eu,Dy)/g-C3N4 composite consisting of 60 wt% g-C3N4 exhibited the highest catalytic activity in the dark. Considering the high price and low photochemical stability of Ag3PO4, the metal free g-C3N4 photocatalyst displays the desirable potential in the field of long afterglow-assisted photocatalysis in the dark. This work opens a new avenue for the development of g-C3N4 photocatalyst for the environmental cleanup.
This research was supported by the National Natural Science Foundation of China (No. 51402139), Technology Foundation for Selected Overseas Chinese Scholar (Department of Human Resources and Social Security of Gansu Province), the basic scientific research business expenses of the central university, and Open Project of Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University (No. LZUMMM2014008 and No. LZUMMM2015001).
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