Photocatalysis is one of the most potential techniques to solve energy crisis and environmental pollution problems, and how to obtain photocatalysts with a greater catalytic efficiency is always a research hotspot in the photocatalysis. The heterogeneous photocatalyst is proved as a promising efficient strategy to address this issue. The heterojunction photocatalysts can be utilized to expedite the separation of photo-induced electron-hole pairs. In particular, Z-scheme heterojunctions exhibit the dual advantages of suppression of recombination of electron-hole pairs, reserving a high redox ability for both semiconductors. The visible-light-driven photocatalysts Bi₂WO₆ and g-C₃N₄ exhibit a poor photocatalytic activity because of their fast recombination of photo-generated carriers. It is advisable to choose the g-C₃N₄ and Bi₂WO₆ to form an all-solid-state Z-scheme photocatalytic system to obtain the higher photocatalytic performance. It is reported that carbon materials often put up a low work function when they modify the electronic and optical properties of hybrids. In this paper, a novel amorphous carbon/g-C₃N₄/Bi₂WO₆ (C/C₃N₄/Bi₂WO₆) Z-scheme heterojunction with a visible light response was synthesized. The performance of C/C₃N₄/Bi₂WO₆ was measured by photo-degrading TC under the visible light (i.e., ≥420 nm). The as-synthesized composites played an enhanced visible-light photoactivity than the pure g-C₃N₄ and Bi₂WO₆. Furthermore, the possible mechanism of the enhanced performance of the C/C₃N₄/Bi₂WO₆ composite was discussed.

All reagents with analytical grade were used without further purification. First, the C/C₃N₄ sample was prepared by a milling/roasting method. When a mass ratio of glucose to urea was 0.1%, the as-fabricated C/C₃N₄ composite exhibited an optimum photocatalytic performance. The C/C₃N₄/Bi₂WO₆ nanocomposites were prepared through a facile hydrothermal method. The obtained products were denoted as C/C₃N₄/Bi₂WO₆-1, C/C₃N₄/Bi₂WO₆-2, C/C₃N₄/Bi₂WO₆-3 and C/C₃N₄/Bi₂WO₆-4, when the mass ratios of C/C₃N₄∶Bi₂WO₆ were 1.0∶0.1, 1.0∶0.4, 1.0∶0.7 and 1∶1, respectively. For the comparison, a pure Bi₂WO₆ sample was synthesized through the same procedur without introducing C/C₃N₄.

The crystal structure was detected by X-ray diffraction with Cu K_α radiation (λ=0.154 06 nm). The morphology and microstructures of the synthesized samples were investigated on high-resolution transmission electron microscopy (HRTEM). The ultraviolate-visible (UV-Vis) diffuse reflectance spectra (DRS) were taken on UV-Vis-near infrared (NIR) spectrophotometry with BaSO₄ as a reference. The chemical composition and chemical state of C/C₃N₄/Bi₂WO₆ were analyzed by X-ray photoelectron spectroscopy (XPS) with Al K_α excitation source. The photocurrent response under simulated solar light irradiation was recorded with an electrochemical workstation in a sandwich-type configuration, a Pt slice as a counter electrode, a saturated calomel electrode (SCE) as ae reference, and 0.1 mol/L Na₂SO₄ solution as electrolyte. A 300 W xenon arc lamp equipped with a simulated solar light filter calibrated to 100 mW/cm², which was measured with a radiometer, employed as a light source. Electrochemical impedance spectroscopy (EIS) Nyquist plots were obtained at 0.6 V with a small alternating current amplitude of 5 mV in a frequency range of 0.1 Hz-10⁵ Hz.

The photocatalytic activity of the C/C₃N₄/Bi₂WO₆ sample was determined by degrading the simulated pollutant tetracycline (TC). In the experimental setup, a 300W Xe lamp was employed as a light source and a 420 nm cut-off filter was used to provide only visible-light irradiation. The photocatalyst of 100 mg was added to 100 mL of TC solution (1×10^-4 mol/L). Before being irradiated, the suspensions were magnetically stirred in the dark for 3 h to reach the adsorption-desorption equilibrium between photocatalysts and TC. The solution was exposed to visible light irradiation under magnetic stirring. The aliquot of 3 mL was collected from the suspension and centrifuged immediately for every 10 min. The degradation of TC was monitored by checking the absorbance at 357 nm using an UV-Vis spectrometer.

The degradation ratios of TC are 25% and 50% after irradiation for 240 min by visible-light in the presence of g-C₃N₄ and Bi₂WO₆, respectively. Furthermore, it is clear that the C/C₃N₄ composite exhibits a higher photocatalytic activity in the degradation of TC than the g-C₃N₄ sample. All the C/C₃N₄/Bi₂WO₆ photocatalysts exhibit better photocatalytic performance than the C/C₃N₄ composite and Bi₂WO₆ under the same experimental condition. Specifically, the photocatalytic activities of C/C₃N₄/Bi₂WO₆ nanocomposites gradually improve as Bi₂WO₆ ratio increases, and the photocatalytic performance is optimal when the mass ratio of C/C₃N₄∶Bi₂WO₆ is 1.0∶0.7. The photocatalytic activities decrease when Bi₂WO₆ ratio continues to increase. Hence, the as-prepared C/C₃N₄/Bi₂WO₆-3 nanocomposite has the optimum photocatalytic performance, and the maximum degradation efficiency of 97.1% is attained after the irradiation for 240 min.

The improved photocatalytic performance of C/C₃N₄/Bi₂WO₆ composites is ascribed to the formed amorphous carbon-mediated Z-scheme heterojunction. When the heterojunction C/C₃N₄/Bi₂WO₆ is exposed to visible light (λ>420 nm), the electrons generated from CB of Bi₂WO₆ can transfer to the VB of g-C₃N₄ via the amorphous carbon mediator due to the good electrical conductivity of amorphous carbon, and then annihilate with the holes induces from the VB of g-C₃N₄, the photo-induced carriers can be effectively separated.

We constructed Z-scheme C/C₃N₄/Bi₂WO₆ heterojunction photocatalyst with enhanced photocatalytic activities in degradation of TC. When the mass ratio of C/C₃N₄∶Bi₂WO₆ was 1.0∶0.7, the as-fabricated C/C₃N₄/Bi₂WO₆ heterojunction exhibited an optimum photocatalytic performance. The mechanism illuminated that the efficient separation of charge carriers generated by the amorphous carbon-mediated Z-scheme mechanism mainly contributed to the improved photocatalytic performance of C/C₃N₄/Bi₂WO₆ heterojunction. We anticipated that the results of this work could favor the application of solar energy in water pollution.