The rapid development of industrialization in the world today has triggered two most urgent issues: environmental pollution and energy crisis [1,2]. The excessive use of traditional energy sources such as coal and oil not only exacerbates the energy crisis but also causes serious environmental pollution. In contrast, clean energy sources like solar and wind power have lower carbon emissions and are renewable. Hydrogen peroxide (H2O2) is an environmentally friendly oxidant that is now considered as the next-generation energy carrier because it is water-soluble and releases H2O as its sole by-product [3]. Due to its applications in areas such as food and paper, medical purification, wastewater treatment, and chemical synthesis, it has attracted significant attention from researchers in recent years [4]. In addition to the energy crisis, environmental pollution is also a pressing issue facing humanity, with wastewater pollution having a direct impact on human health. Widespread applications of Brilliant black BN bis-azo dye in food dyeing, auto-mobile painting and textile dyeing industry implies that BN dye has got a higher likelihood of polluting water sources. The toxicity of Azo dye additives in food has attracted more and more attention because the metabolites of Azo dye may have potential carcinogenicity [5,6]. In the current scenario of continuously increasing global energy demand and environmental pollution, there is an urgent need for a multifunctional material to address two existing issues. So far, semiconductor photocatalysis has been recognized as a green approach to effectively harness sunlight and convert it into chemical energy [7].

Numerous semiconductor-based photocatalysts have been optimized for the production of H2O2 and the degradation of pollutants, such as TiO2, graphitic carbon nitride, CeO2, etc. have widely been explored since the advent of photocatalytic technology [8]. Graphitic carbon nitride (GCN) is a metal-free organic semiconductor that has received increasing attention because of its high thermal, and chemical stability and its attractive electronic structure [9,10].The combination of adequate band alignment, desirable band gap energy, environmentally benevolent nature, and low cost makes carbon nitride a reliable and effective visible light catalyst [11]. But its low light absorption, small specific surface area (SSA), and sluggish charge dynamics, which significantly restricts its photocatalytic ability [12,13]. The mainstream approach is to overcome the weaknesses of GCN through element doping, chemical functionalization, and constructing heterojunctions. For instance, the P, N, co-doped nanocarbon-embedded GCN hollow sphere nanoreactor was capable of generating 239.5 μmol h−1 g−1 in the absence of the sacrificial agent [14]. Ma et al. emphasized on the H2O2 production (115 μmol h−1 g−1) using carbon nitride quantum dots/GCN homojunction through type-I charge transfer pathway [15]. Lignin is the most abundant methoxylated polyphenolic compound present in nature and is a macromolecule with great development potential due to its excellent nontoxic, low cost, high yield, and sustainable source [16,17]. Lignin contains delocalized electrons in its circular π bonds can be photoexcited to energetically higher levels, causing them to participate in photoinduced electron transfer for solar-driven redox chemistry [18]. At present, researchers have studied the photophysical properties of lignin through photoelectrochemical analysis, and used lignin sulfonate as a photocatalyst to form H2O2 through the synergistic formation of O2 reduction and H2O oxidation [19].

In this work, uniformly dispersed lignin microspheres with a diameter of 300 nm were prepared through self-assembly and innovatively loaded onto nitrogen-doped carbon nanotubes. The lignin spheres prepared by self-assembly have a larger surface area, exposing more reactive sites. Additionally, the rich benzene ring structure in lignin can conjugate with the triazine ring in nitrogen-doped carbon, promoting pollutant adsorption and degradation through π-π interaction. The structure of the synthesized material CNL was observed through X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), and nuclear magnetic resonance (2D-NMR). Furthermore, the photodegradation pathway and structure-function relationship of CNL were revealed through experiments on hydrogen peroxide production and free radical quenching. This work not only elucidates the photocatalytic degradation mechanism of the unique materials formed by combining lignin and nitrogen-doped carbon, but also provides new ideas for the design and fabrication of biomass photocatalysts with high catalytic performance.