Compared to various organic fuels, hydrogen is designated as a sustainable solar energy source, with a thermal efficiency of 120–142 MJ kg⁻¹. Currently, the global generation of hydrogen exceeds 44.5 million tons [3], and it will be the primary generator of power until 2080 [107]. However, there are numerous methods for producing hydrogen-like substances, and the most common is thermolysis [108], electrolysis [109], biomass [110], photolysis [109], and hydrolysis [111,112,113,114]. Photocatalytic water dissociation is a successful technology that has received a lot of interest due to its broad application for power and sustainability purposes. Artificial photosynthesis is one of the only sustainable, long-term answers to the forthcoming fuel and environmental crises [115]. A group of scientists has already investigated several photocatalyst substances [116]. Nevertheless, the majority of photocatalysts respond to ultraviolet (UV) light, which accounts for only 5% of solar energies [117]. TiO₂ has been intensively investigated as a prospective option for hydrogen production due to its acclaimed physical and chemical qualities, high permanence, earth-abundant, low cost, and non-toxicity. However, its large bandgap (3.0–3.2 eV) limits its absorptivity range [118]. Furthermore, graphitic carbon nitride (g-C₃N₄) has received a great deal of attention due to its physical and chemical characteristics, low cost, earth-abundant, simple preparation, renewability, and, most importantly, visible light responsiveness due to a narrow bandgap of 2.7 eV [119,120,121]. However, the photocatalytic performance is limited by the high recombination rate and low surface area. Although several scientists have examined the heterojunction of TiO₂ and g-C₃N₄ since it has a high surface area, decreases electrons recombination, and absorbs visible light. The photocatalytic performance has not progressed significantly [122].

The hydrothermal bio-template procedure was used to generate a C-doped g-C₃N₄ effectively using the carbonization procedure at 500 °C and nitrogen-coated titanium dioxide as a core-shell heterojunction photocatalyst. During constructing a core-shell heterojunction photocatalyst, kapok fiber was exploited as a bio-template and in-situ carbon coated in CN and titanium dioxide. Furthermore, urea application as a g-C₃N₄ precursor contributes to band-gap narrowing in TiO₂ via in-situ C and N loading. Several characterization approaches were used to investigate the impact of TiO₂ source amount on the formation of core-shell nanocomposite heterojunction photocatalysts, which can impact and enhance catalytic performance. The photoelectrochemical and photoluminescence investigations revealed that the bio-template core-shell nanocomposite heterojunction photocatalysts had a remarkable improvement in photoinduced electron-hole dissociation performance. The improved photogenerated charge carrier dispersion and shorter band gap lead to enhanced photocatalytic activity, with the CCN/T-1.5 material producing the most hydrogen (625.5 μmol h⁻¹ g⁻¹) in methanol medium [123].

Based on Wang et al., 2014, the generated KF with or CKF/AuNPs/CTS nano-composites exhibit excellent catalytic performance and durability for the catalytic reduction decolorization of CR dye, with the color of the CR solution rapidly fading within Three minutes at a modest catalyst dosing of 0.3 g/L. Furthermore, after 20 min of sonication or 1 mol/L acid treatment, the nanocomposite’s catalytic performance can be sustained. The hydrogen was created concurrently with the catalytic decolorization of CR and can be recovered as a clean, renewable energy, with a total output of 430 mL/L. As a result, the nanostructure could be utilized as a catalyst to decolorize dye effluent and create H₂ in a single cycle [128]. The absorbance peak at 533 nm was definitely observable in the UV–vis spectra of KF/CTS/AuNPs and CKF/CTS/AuNPs, confirming that the AuNPs were effectively bonded on the membrane of CTS-coated KF and CKF as predicted . This finding is also clearly displayed in the test procedure, where the purple-red AuNPs solution becomes colorless when added to CTS-coated KF or CKF.

The application of the nano-structure as a catalyst is critical to practical usage. It can be noticed that the color removal rate has not decreased significantly, and 92.6% of the original dye removal rate was attained following nine recoveries. This suggests that the nanocomposite could be used as a reusable catalytic substance for dye removal.

In summary, inserting kapok fiber-based carbon loading in the semiconductor structure using a simple bio-template synthesis technique can improve photocatalytic hydrogen generation in pure semiconductors. Furthermore, a well-constructed mesoporous micro-tubular structure was produced during the fabrication of a C-doped semiconductor employing treated kapok fiber. The generated C-doped semiconductor’s band gap structure and quantity of carbon doping may be easily modified using various impregnation processes. It should be highlighted that the significant increase in photocatalytic performance was due to improved light absorption, an acceptable energy band gap, and quick photogenerated carrier transfer and separation. Developing the C-doped semiconductor-based heterojunction photocatalyst in conjunction with other semiconductor materials can also aid in creating a highly excellent photocatalyst.

Several years ago, the elimination of color was accomplished through adsorbents depending on electron-hole pair contact. However, changes in the shape and texture of photocatalytic components have a favorable impact on the characteristics, such as surface area and photon carrying capacity, which is accompanied by the mobilization of electrons and holes along with the shape [132,133]. Several studies have proven the multiple photocatalytic abilities of several photocatalytic substances for the degradation of various organic molecules available in water, particularly colors, up to the present day [134,135,136,137]. All of these research findings on photocatalytic applications for contaminant degradation show that the bandgap, electron-hole recombination rate, size and shape, crystallinity, phase composition, light infiltration through photocatalytic substances, surface area, and dye adsorption potential on the surface of photocatalysts are important criteria for photocatalytic operation improvement. [138]. Depending on this, scientists have become interested in producing higher surface area photocatalysts, which can offer a larger surface area and more significant dye adsorption on the interface of these compounds.

Metal sulfide-based semiconductors are the most notable photocatalysts among the highly regarded photocatalysts for the removal or dissolution of colors in wastewater applications with minimal expenses, environmentally beneficial, and durable treatment solutions for environmental preservation. In previous decades, environmental contamination has become a severe hazard to the ecosystem and public safety. To combat pollution, loaded and heterojunction-based semiconductor metal sulfide nanostructures (MSNSs) are being explored as photocatalysts for photocatalytic removal or to eliminate massive industrial colors in an environmentally benign and durable approach. In the 1970s, the photocatalytic processing of water splitting to produce hydrogen on semiconductor nanostructures was discovered. The project will then discover the underlying mechanisms that result in photocatalysis and increase the system of photocatalytic removal performance [139]. The bandgap, surface area, quantity of catalyst, and formation of an electron-hole pair are all critical parameters in the photocatalytic removal of hazardous chemicals in an aqueous medium. It has been discovered that, among all parameters, the surface area has a crucial impact on the photocatalytic removal of colors by giving a larger surface area, which results in the increase of dye particle adsorption on the membrane of the photocatalyst and increases photocatalytic effectiveness. Based on the capacity to solve energy and environmental challenges, heterogeneous photocatalysis utilizing semiconductors has gained a lot of interest in recent years as an environmentally friendly and durable solution. Depending on heterogeneous photocatalysis, the analysis of the latest advancements in the synthesizing and usage of semiconductor MSNSs as photocatalysts in the field of heterogeneous photocatalytic removal of multiple colorings by ranging diverse settings like the size of the components, bandgap, light intensity, surface area, and dye solution concentration levels; and their relations with aquatic pollutants [140].

Spontaneous and purposeful oil spills have emerged regularly throughout shipping, manufacturing, and refinement in recent decades, resulting in significant negative consequences for individuals and the natural system [7,143]. Oil-absorbing substances are commonly recognized as the most successful for removing and recovering wasted oil. They are classified into inorganic mineral substances, chemically synthesized polymers, and natural organic components [152,153]. Graphite, organic clay, vermiculite, silica, perlite, fly ash, and zeolites are examples of inorganic mineral minerals [154,155,156,157,158,159,160]. In recent decades, polyacrylate, polyethylene, and polypropylene, a new substance for adsorbing oil, are synthesized organic polymers [160,161,162,163]. Various agricultural items are used in organic natural resources, including cotton fiber, kenaf, straw, milkweed, sawdust, and straw kapok fiber [164,165,166]. Among these organic products, kapok offers several benefits over typical oil-absorbing materials, including inexpensiveness, renewability, innate hydrophobicity, and high sorption ability, making it desirable as an oil-absorbing substance [167]. All recent research regarding the adsorption application of kapok fiber is illustrated in Table 2.

With global warming and resource scarcity becoming increasingly extreme, the electrochemical CO₂ minimization process (ECO₂RR) is an effective and intriguing strategy to convert CO₂ into various valuation compounds as a carbon-neutral way to a sustainable power source [180,181,182]. Among the several compounds created by CO₂ electroreduction, formate or formic acid is a significant fluid biofuel that can be effectively exploited as a super-economic power transporter in fuel cell applications [183,184,185]. Biodegradable resources (animal or botanic) are near-ideal solutions for the synthesis of several high-production catalysts, which have numerous essential advantages, such as inexpensive cost, easy availability, and molecular and geometrical variations [186,187,188]. Kapok fiber is an organic porous fiber generated from the silk-cotton tree with a significant cavity and a thinner covering and is used as an oil and heavy metal ion adsorbent [56,169]. Furthermore, kapok fiber is an ideal carbon source for the fabrication of electrochemical supercapacitors [46,129]. The use of kapok fiber in electrochemical CO₂ removal, on the other hand, has never been recorded. The enormous cylindrical form of kapok-tubes is advantageous for carrying metal nanoparticles; also, kapok-tube has a high capacity for metal ion capturing due to its numerous oxygen active element on the membranes [98,168]. Most of the evidence suggests that it can be used as a novel carbon substrate to manufacture electrocatalysts. The kapok tube is mainly employed as a carbon electrocatalyst for Carbon dioxide electroreduction, and it performs well for electrocatalytic Carbon dioxide to fluid energy conversion without adding active components. Unlike typical carbon nanotubes and graphene, which have weak capacities for formate generation without adding active components, this natural capacity may be attributed to the many mesoporous structures found in MHKTs, which acted as functional spots for formate generation. Additionally, the kapok tube is used for the first time as a novel catalyst substrate for depositing metal nanoparticles in electrocatalytic CO₂ removal. All the metals in-situ bonded on MHKTs are manufactured using a simple one-pot synthetic technique. For the CO₂-to-formate process, the four electro-catalysts, Sn, Bi, Pb, and Cd-MHKTs, exhibited great selection, low overpotential, high existing density, and prolonged stability. These low-cost metal MHKT electrocatalysts offer many potential applications in electrocatalytic CO₂ degradation to formate.

In conclusion, the usage of Kapok fiber for CO₂ conversion under visible light to create solar fuels is discussed, as well as how various properties and structural adjustments may impact the processes and final products. The presence of a flexible structure for adjusting band gaps and imparting lattice distortion allows kapok fiber to control separation, mobility, and the lifetime of photogenerated charges. Demonstrating good selectivity for format generation across a wide range of potentials. The use of kapok fiber offers fresh insight into the production of innovative carbon supports with suitable photocatalytic characteristics.