Ti3C2-MXene/NiO Nanocomposites-Decorated CsPbI3 Perovskite Active Materials under UV-Light Irradiation for the Enhancement of Crystal-Violet Dye Photodegradation

Ti3C2-MXene material, known for its strong electronic conductivity and optical properties, has emerged as a promising alternative to noble metals as a cocatalyst for the development of efficient photocatalysts used in environmental cleanup. In this study, we investigated the photodegradation of crystal-violet (CV) dye when exposed to UV light using a newly developed photocatalyst known as Ti3C2-MXene/NiO nanocomposite-decorated CsPbI3 perovskite, which was synthesized through a hydrothermal method. Our research investigation into the structural, morphological, and optical characteristics of the Ti3C2-MXene/NiO/CsPbI3 composite using techniques such as FTIR, XRD, TEM, SEM–EDS mapping, XPS, UV–Vis, and PL spectroscopy. The photocatalytic efficacy of the Ti3C2-MXene/NiO/CsPbI3 composite was assessed by evaluating its ability to degrade CV dye in an aqueous solution under UV-light irradiation. Remarkably, the Ti3C2-MXene/NiO/CsPbI3 composite displayed a significant improvement in both the degradation rate and stability of CV dye when compared to the Ti3C2-MXene/NiO nanocomposite and CsPbI3 perovskite materials. Furthermore, the UV–visible absorption spectrum of the Ti3C2-MXene/NiO/CsPbI3 composite demonstrated a reduced band gap of 2.41 eV, which is lower than that of Ti3C2-MXene/NiO (3.10 eV) and Ti3C2-MXene (1.60 eV). In practical terms, the Ti3C2-MXene/NiO/CsPbI3 composite achieved an impressive 92.8% degradation of CV dye within 90 min of UV light exposure. We also confirmed the significant role of photogenerated holes and radicals in the CV dye removal process through radical scavenger trapping experiments. Based on our findings, we proposed a plausible photocatalytic mechanism for the Ti3C2-MXene/NiO/CsPbI3 composite. This research may open up new avenues for the development of cost-effective and high-performance MXene-based perovskite photocatalysts, utilizing abundant and sustainable materials for environmental remediation.


Introduction
Many environmental concerns are currently occurring as a result of the impact of many natural and man-made elements on the Earth's crust [1].Environmental pollution, in general, refers to any undesirable and unacceptable alterations in the environment resulting from various human activities.These alterations can manifest as direct or indirect changes in the biological, chemical, and physical characteristics of natural water bodies, leading to detrimental effects on both human populations and aquatic ecosystems [2].Various factors contribute to the pollution of natural water bodies, including rapid population growth, urbanization, and extensive industrialization.One significant source of water contamination is azo dyes, which pose a considerable threat to the environment due to their non-biodegradable and hazardous nature [3].Azo dyes, a type of dye with diverse applications, account for more than 50% of global dye production.Synthetic azo dyes find extensive use in textiles, food, cosmetics, lithography, and certain medical products [4].Additionally, they play a crucial role in various technological applications, such as photonics devices, laser dyes, photovoltaics, and antidiabetic drugs.Azo dyes consist of diazotized amines attached to amines or phenols, often containing one or more azo linkages.The precursor compounds of azo dyes are aromatic amines.Some azo compounds exhibit remarkable stability and persist in the environment for extended periods, making them resistant to removal from wastewater through conventional methods.There are several ways to break down dyes in wastewater, including chemical, biological, and physical techniques.Nevertheless, a lot of these methods have significant deterioration and upkeep expenses.Moreover, several of these techniques produce secondary waste products that need to be treated further, making them inappropriate and expensive for treating wastewater [5].
Nanotechnology, on the other hand, has gained a tremendous impetus in this quickly rising technological period by producing an abundance of scientific concepts to compete with the daily problems of growing technology.Nanomaterials have garnered immense interest due to their myriad applications and unique properties, which arise from their distinct size, shape, and surface-area characteristics [6].One such nanomaterial, MXene, represents a novel class of 2D materials derived from the etching of Ti 3 C 2 , a transition metal carbide, nitride, or carbonitride.MXene has drawn considerable attention as a promising cocatalyst material in the development of heterostructure systems, particularly for photocatalysis [7].This heightened interest can be attributed to MXene's remarkable attributes, including its structural stability, an abundance of hydrophilic functional groups (e.g., -O and -OH) on its surface, excellent metal conductivity, and enhanced redox reactivity emanating from its terminal Ti sites.Utilized as a carrier substrate, Ti 3 C 2 -MXene serves a dual purpose.It prevents the agglomeration of nanoscale photocatalysts and effectively captures photoexcited electrons, thus promoting the separation of electron-hole pairs during the photocatalytic process.Nonetheless, the self-stacking tendency of Ti 3 C 2 -MXene sheets can lead to undesirable outcomes, such as reduced surface area and diminished active accessible sites [8].This self-stacking phenomenon results in a transition of Ti 3 C 2 -MXene properties from metallic to semiconducting.To address this challenge, Ti 3 C 2 -MXene materials are viewed as valuable auxiliary components that can modify the conductivity of active materials when integrated into devices alongside other substances like metal oxides.Consequently, 2D Ti 3 C 2 -MXene materials hold promise as cocatalysts, enhancing the photocatalytic performance of photocatalysts by effectively promoting the separation of photogenerated carriers [9].
Metal oxide semiconductors perform better when two different semiconductors with different photogenerated electron-hole pair energy levels are connected because of their interfacial activity [10].Nonetheless, one significant challenge associated with these nanomaterials is their propensity to aggregate into secondary particles, which significantly limits their catalytic performance in various applications.Notably, a p-n junction can be formed at the interface between p-type and n-type binary semiconductor oxides, which effectively helps to separate electron-hole pairs [11].Among the several p-type oxides, nickel oxide (NiO) is a particularly active compound with a broad band gap between 3.6 and 4.0 eV.For a variety of uses, such as chemical sensors, photovoltaic devices, gas sensing, catalysis, magnetic materials, electrochromic films, and battery cathodes, it has been thoroughly investigated [12].Research indicates that nanoscale materials can exhibit novel and unique properties, and, among them, semiconductor oxides belonging to the group of photocatalyst nanomaterials, such as Fe 3 O 4 , NiO, TiO 2 , and ZnO, hold significant potential for advanced oxidation processes in the context of environmental pollution remediation [13].While some researchers have investigated the synthesis methods and characteristics of NiO nanoparticles, there is a noticeable lack of reports regarding their functionality as photocatalysts for dye degradation and an examination of the factors influencing photocatalytic degradation in the available scientific literature.
Perovskite materials had previously only been used in semiconducting applications, but their photocatalytic activity and capacity to actively break down constituent particles were investigated for water-refining applications [14].In recent times, perovskite-based catalysts have piqued the interest of researchers due to their versatile bandgap adjustability, high stability, rapid mobility of photoinduced electrons and holes (e − /h + ), and exceptional photocatalytic activity [15].In particular, lead trihalide perovskites have become a fascinating family of materials with great potential for applications in the next generation.Superior optical qualities, a high-attenuation coefficient, a configurable bandgap, adaptable surface chemistry, long-range electron-hole diffusion, and high carrier mobility are just a few of their impressive features [16].These materials typically adhere to the general formula ABX 3 , with A representing a cation (organic or inorganic), B as a divalent metal (Pb 2+ , Sn 2+ , Ge 2+ ), and X as an anion (Cl − , Br − , I − , or a combination thereof).Semiconductor materials are widely used as photocatalysts in the energy and environmental domains because of their low cost and special physiochemical properties [17].The use of inorganic lead trihalide perovskites, their derivatives, and composites as photocatalysts has been the subject of multiple reports in recent times.It has been noted that, among the lead halides based on cesium, pure iodide-based compounds with a broad bandgap provide difficulties for photocatalysis [18][19][20].However, various treatments, such as creating heterostructures or modifying typical ligands, can render them optimal choices.To the best of our knowledge, there has been no prior research conducted on the utilization of Ti 3 C 2 -MXene/NiO nanocomposite-decorated CsPbI 3 perovskite and its photocatalytic activity in degrading crystal-violet (CV) dye.Ti 3 C 2 -MXene/NiO/CsPbI 3 is an effective passivator in three main ways, according to systematic experimental results: (i) it can modify the energy levels of perovskite materials and create a hole-transfer pathway that is efficient; (ii) it can passivate defects and lessen nonradiative recombination at the interface; and (iii) it forms a barrier layer that keeps water out and improves the stability of CsPbI 3 materials.
The amalgamation of halide perovskites and MXene materials in various configurations represents a cutting-edge frontier in photocatalysis for environmental remediation.The novel insights into the construction of halide perovskite-based photocatalysts, exploring enhanced properties through composites, mechanochemical synthesis mechanisms, and innovative heterostructure designs [21][22][23].Additionally, the integration of MXene materials into composite electrodes for supercapacitors showcases a breakthrough in energy-storage technologies.The interplay between perovskite structures, such as LaNiO 3 and MnTiO 3 , and their synergistic interactions with other materials reveal promising advancements in the photocatalytic degradation of pollutants, providing a foundation for sustainable and efficient environmental solutions.This literature presents an innovative approach that combines different nanomaterials, such as Ti 3 C 2 -MXene, NiO, and CsPbI 3 perovskite, to create a composite material.The focus of the study lies in exploring the photocatalytic capabilities of these materials, particularly in the degradation of crystal-violet (CV) dye in an aqueous solution under UV-light exposure.The Ti 3 C 2 -MXene/NiO/CsPbI 3 composite exhibits exceptional photocatalytic performance, leading to significant CV dye degradation.The paper delves into the degradation pathway and elucidates the mechanisms involved in the photocatalytic process, emphasizing the composite's efficiency.Factors contributing to this efficiency include trapping sites for electrons, hindrance of electron-hole pair recombination, a larger surface area, and a lower recombination rate.Therefore, the study provides valuable insights into the design and application of composite nanomaterials for advanced photocatalysis, suggesting potential implications for environmental pollution remediation and diverse future applications.
Herein, for the first time, Ti 3 C 2 -MXene/NiO nanocomposites with CsPbI 3 perovskite materials were successfully synthesized through the hydrothermal method and characterized by FTIR, XRD, TEM, SEM-EDS mapping, XPS, UV-Vis, and PL techniques.The photocatalytic capabilities of three different materials, namely Ti 3 C 2 -MXene, Ti 3 C 2 -MXene/NiO, and Ti 3 C 2 -MXene/NiO/CsPbI 3 composites, were assessed by their ability to degrade crystal-violet (CV) dye in an aqueous solution when exposed to UV light.
In comparison to the other catalysts, the Ti 3 C 2 -MXene/NiO/CsPbI 3 composite exhibited a highly advantageous photocatalytic performance, achieving an impressive 92.8% degradation of the CV dye within just 90 min of UV-light exposure.The effectiveness of the Ti 3 C 2 -MXene/NiO/CsPbI 3 composite can be attributed to its ability to provide ample trapping sites for electrons, which, in turn, hinders the recombination of electronhole pairs and contributes to the enhanced removal of CV dye through photocatalysis.The paper also delves into the degradation pathway of CV dye and the mechanisms involved in the photocatalytic process.In this context, the rapid and efficient degradation of dye molecules is achieved through a combination of factors, including the larger surface area and a lower electron-hole recombination rate.These characteristics make the Ti 3 C 2 -MXene/NiO/CsPbI 3 composites highly efficient photocatalysts and promising candidates for a wide range of future applications.

Materials
Ti 3 AlC 2 powder (approximately 400 mesh), hydrofluoric acid (HF, 40% weight), CsI, PbI 2 , n-butyl acetate, and dimethyl sulfoxide (DMSO) were procured from Sigma-Aldrich (St. Louis, MO, USA).NiSO 4 •6H 2 O and NaOH were obtained from TCI Chemical Reagents.All chemical reagents employed in this study were of analytical grade and were used without any additional purification.Deionized water served as the solvent in all synthesis procedures.

Preparation of Ti 3 C 2 -MXene Nanosheets
In accordance with the methodology outlined in a prior study [24], Ti 3 C 2 -MXene nanosheets were prepared by selectively etching the aluminum (Al) layer from Ti 3 AlC 2 .The Ti 3 AlC 2 powder was immersed in a concentrated hydrofluoric acid (HF) solution with a concentration of 50% by weight.This immersion was conducted at room temperature and lasted for 24 h, facilitating the removal of the aluminum (Al) atoms from the Ti 3 AlC 2 structure.The resulting suspension was transferred into a 45 mL centrifuge tube, followed by centrifugation at 3500 rpm for 5 min.This step was performed to separate the etched Ti 3 C 2 -MXene nanosheets from the solution.The centrifuged material was then washed with deionized (DI) water five times to remove any residual HF acid.Subsequently, 0.2 g of the Ti 3 C 2 -MXene powder were combined with 15 mL of dimethyl sulfoxide (DMSO).This mixture was subjected to magnetic stirring for 24 h at room temperature.The final product obtained after the etching process was centrifuged again, this time at 10,000 rpm for 30 min, and then washed with DI water.The washed material was further subjected to vacuum drying to obtain the delaminated Ti 3 C 2 -MXene powder for subsequent use.

Preparation of Ti 3 C 2 -MXene/NiO Composite
The synthesis of the Ti 3 C 2 -MXene/NiO composite was achieved through a hydrothermal method [25].Initially, 200 mg of the previously prepared Ti 3 C 2 -MXene and 150 mg of NiSO 4 •6H 2 O were dispersed in a solution of 50 mL of NaOH.Ultrasonication was applied for 30 min to ensure effective mixing, followed by continued magnetic stirring.The resulting mixture was then sealed within an 80 mL Teflon-lined autoclave and maintained at a temperature of 150 • C for a duration of 12 h.The product obtained after the hydrothermal treatment was a black slurry.It was subjected to filtration and washed with deionized water five times to remove any impurities.Finally, the resulting material was dried in a vacuum oven at 60 • C for 24 h, leading to the formation of the Ti 3 C 2 -MXene/NiO composite.For comparative purposes, pure NiO was also synthesized using the following abbreviated procedure.A dropwise addition of 50 mL of NaOH solution was made to 15 mL of NiSO 4 •6H 2 O.The mixture was stirred for 2 h, and, as a result, a precipitate of Ni(OH) 2 was obtained.The Ni(OH) 2 precipitate was subsequently lyophilized and then subjected to calcination at 350 • C for 2 h under a nitrogen (N 2 ) atmosphere to yield NiO.

Preparation of CsPbI 3
The synthesis of the CsPbI 3 powders was accomplished through a chemical precipitation method using CsI and PbI 2 as precursor materials, with methanol as the solvent [26].First, 10 mL of methanol were used to dissolve 1.0 mM of CsI.After that, the solution was heated in a water bath to about 60 • C; 4 mL of n-butyl acetate were used to dissolve 1.0 mM of PbI 2 powders in a different container.After carefully adding the PbI 2 solution to the heated CsI solution, the mixture was thoroughly mixed.Fine yellow CsPbI 3 powders precipitated as a result of this.To make sure the reaction was finished, the mixture was stirred for a further half hour at a speed of 300 revolutions per minute (rpm) in a water bath set at 60 • C. Centrifugation was used to separate and twice wash the solid CsPbI 3 precipitates with n-butyl acetate.The cleaned CsPbI 3 precipitates were then dried in an oven at 80 • C overnight to obtain the desired CsPbI 3 powders.

Preparation of Ti 3 C 2 -MXene/NiO/CsPbI 3 Composites
The Ti 3 C 2 -MXene/NiO/CsPbI 3 composites were prepared via a hydrothermal treatment using an aqueous solution containing Ti 3 C 2 -MXene nanosheets, NiO, and CsPbI 3 powders.Initially, 50 mg of Ti 3 C 2 -MXene nanosheets were dissolved in 50 mL of deionized (DI) water.To achieve delamination, the mixture was subjected to stirring and probe sonication for 5-10 min, respectively, at room temperature.This resulted in the formation of a homogeneous colloidal solution of Ti 3 C 2 -MXene nanosheets.Subsequently, 15 mg of NiO and 15 mg of CsPbI 3 were added to the prepared colloidal solution.Again, the mixture was stirred and probe-sonicated for a duration of 5-10 min.The hydrothermal reaction was then started after the resultant solution was put into a 100 mL stainless steel container lined with Teflon.For twelve hours, the reaction was run at 160 • C with a 2 • C per minute heat ramp-up rate.Following the completion of the hydrothermal reaction, room temperature was allowed to settle in the Teflon container.Centrifugation was used to gather the product, and the centrifuge ran for 5 min at 2000 RCF.After collecting the material, it was cleaned and then freeze-dried for 48 h to produce the final composite powder.

Photocatalytic Degradation of Dye
The assessment of the newly synthesized photocatalyst performance in crystal-violet (CV) degradation under UV-light irradiation.It involved dispersing a set quantity of the photocatalysts (25 mg) in 100 mL of CV-dye solutions with a concentration of 15 mg/L.A UV protection cabinet equipped with a UV medium-pressure immersion lamp was used during the experiments.The photocatalyst, composed of Ti 3 C 2 -MXene nanosheets, Ti 3 C 2 -MXene/NiO, and Ti 3 C 2 -MXene/NiO/CsPbI 3 composites was mixed with CV and homogenized using a magnetic stirrer.To validate the equilibrium adsorption-desorption characteristics, a UV-Vis measurement was conducted for 30 min in a dark space before light illumination.A 400 W lamp producing a line spectrum in the ultraviolet and visible range (200-800 nm) and a high-power output density of about 100 W/cm 2 in the UVC range (200-300 nm) served as the UV-light source.Subsequently, the light source was activated to initiate the photocatalytic dye degradation process.At specific time intervals (every 15 min), 3 mL aliquots were extracted, and these aliquots were then centrifuged to separate the nanosized photocatalyst during the degradation process.Photocatalytic degradation tests were conducted three times, and the average values were reported.The CV concentrations were measured using a UV-Vis spectrometer, and the removal efficiency (%) was determined using the following formula.

Removal efficiency
where C 0 and C t (mg/L) are the initial and final concentrations of CV at time t, respectively.
To assess the stability and recyclability of the Ti 3 C 2 -MXene/NiO/CsPbI 3 composites, a series of recycling photocatalytic experiments were carried out.These experiments consisted of three successive cycles for the degradation of CV.After each photocatalytic cycle, the Ti 3 C 2 -MXene/NiO/CsPbI 3 composites were collected, subjected to centrifugation, and washed multiple times with double-distilled water.Subsequently, they were dried at a temperature of 60 • C.After that, the recovered photocatalyst was used again in the same conditions as the first experiment to break down CV dye.In the FTIR spectra, the Ti3C2-MXene nanosheets exhibited distinct bands at 3428, 1629, 1388, 1096, and 655 cm −1 .These bands corresponded to the stretching vibrations of -OH, C=O, O-H, C-F, and Ti-O bonds, respectively.These observations were consistent with a previously reported study [27].The FTIR spectrum of NiO revealed broad absorp- In the FTIR spectra, the Ti 3 C 2 -MXene nanosheets exhibited distinct bands at 3428, 1629, 1388, 1096, and 655 cm −1 .These bands corresponded to the stretching vibrations of -OH, C=O, O-H, C-F, and Ti-O bonds, respectively.These observations were consistent with a previously reported study [27].The FTIR spectrum of NiO revealed broad absorption bands at 3395 and 1381 cm −1 associated with O-H stretching vibrations.Additional stretching vibrations were observed at 1629 and 1021 cm −1 , which were attributed to the surface-adsorbed moisture and physical absorption of CO 2 during sample preparation.A distinctive absorption band at 459 cm −1 supported the formation of NiO nanoparticles.The FTIR spectrum of the Ti 3 C 2 -MXene/NiO displayed broadbands at 3451 and 1598 cm −1 , supporting the presence of a hydroxyl functional group (O-H) on the surface of Ti 3 C 2 -MXene/NiO.The characteristic absorption band of NiO was evident at 462 cm −1 , corresponding to the tensile vibration of Ni-O [28].The antisymmetric C-H stretching modes were identified in the FTIR spectrum of CsPbI 3 by the signals at 2921 and 2849 cm −1 , and the COO-stretching modes were identified in the signals at 1531 and 1411 cm −1 [29].Furthermore, the FTIR spectra displayed peaks at 569, 1028, 1112, 1181, 1381, and 1628 cm −1 , corresponding to -C-I, -CH, -NH, -C=O, -C=C, and -OH bonds, respectively.These peaks aligned with the characteristic bonds of both CsPbI 3 and Ti 3 C 2 -MXene/NiO, indicating the presence of an interfacial interaction between CsPbI 3 and Ti 3 C 2 -MXene/NiO.In the Ti 3 C 2 -MXene/NiO/CsPbI 3 composites, most of these peaks were present with slightly reduced intensities compared to Ti 3 C 2 -MXene/NiO and CsPbI 3 alone.However, the characteristic peaks of CsPbI 3 and Ti 3 C 2 -MXene/NiO in the Ti 3 C 2 -MXene/NiO/CsPbI 3 composites exhibited significantly higher intensities than in Ti 3 C 2 -MXene/NiO alone.This observation indicated a stronger interaction effect between CsPbI 3 and the few-layered Ti 3 C 2 -MXene/NiO, which is consistent with the XRD results.

Morphological Properties of Ti 3 C 2 -MXene/NiO/CsPbI 3 Composites
The morphology of the as-prepared Ti 3 C 2 -MXene nanosheets, Ti 3 C 2 -MXene/NiO, and Ti 3 C 2 -MXene/NiO/CsPbI 3 composites were analyzed by transmission electron microscopy (TEM).Figure 2a,b provide TEM images of the Ti 3 C 2 -MXene nanosheets, confirming their thin and electron-transparent nature, with a thickness comparable to graphene.Some local regions exhibit folding, which is attributed to their high flexibility and elasticity [33].Figure 2c presents a high-resolution TEM image of the Ti 3 C 2 -MXene, reaffirming their graphene-like morphology.The lattice fringes observed in this image, with a spacing of 0.42 nm, correspond to the typical (110) plane of the layered structure of Ti 3 C 2 -MXene nanosheets.Moving on to the Ti 3 C 2 -MXene/NiO composite, the TEM images in Figure 2d clearly illustrate the well-dispersed NiO particles on the thin and transparent Ti 3 C 2 -MXene nanosheets without altering the initial structure of the Ti 3 C 2 -MXene nanosheets [34].Differentiable lattice fringes and grain boundaries between the NiO particles and Ti 3 C 2 -MXene nanosheets can be seen in the high-resolution TEM (HRTEM) image displayed in Figure 2e-g.The image-derived lattice spacing of 0.24 nm is consistent with the cubic NiO (111) crystal-plane spacing.A TEM analysis also confirms that the lattice spacing of 0.42 nm corresponds to the (110) crystal plane of Ti 3 C 2 -MXene [25].An additional TEM analysis (Figure 2h) demonstrates the presence of CsPbI 3 particles loaded onto the Ti 3 C 2 -MXene/NiO composite.Figure 2h reveals that NiO and CsPbI 3 particles, each of a narrow size, entirely cover the surface of Ti 3 C 2 -MXene.Furthermore, the HRTEM image of the Ti 3 C 2 -MXene/NiO/CsPbI 3 composites reveals distinctly different phases [35].The HRTEM lattice fringe pattern displays interplanar spacings of 0.62, 0.24, and 0.42 nm (Figure 2i-k Figure 3 represents the FESEM image of the Ti3C2-MXene nanosheets, NiO, Ti3C2-MXene/NiO, CsPbI3, and Ti3C2-MXene/NiO/CsPbI3 composites phase.In Figure 3a,b, the SEM micrographs illustrate the distinctive structure of Ti3C2-MXene nanosheets, characterized by a typical 2D and sheet-like arrangement with only a few layers, resembling the graphene-like structure [25].The surface of these nanosheets appears smooth, and the   3a,b, the SEM micrographs illustrate the distinctive structure of Ti 3 C 2 -MXene nanosheets, characterized by a typical 2D and sheet-like arrangement with only a few layers, resembling the graphene-like structure [25].The surface of these nanosheets appears smooth, and the presence of a well-defined layered structure confirms the existence of Ti 3 C 2 -MXene nanosheets (Figure 3b). Figure 3c presents SEM micrographs of the NiO nanoparticles, displaying a generally spherical shape with some degree of aggregation.In contrast, Figure 3d,e show SEM images of the Ti 3 C 2 -MXene/NiO composite.After combining with NiO, the surface of Ti 3 C 2 -MXene/NiO becomes notably rough, with some NiO particles covering the surface of Ti 3 C 2 -MXene, while others are randomly embedded within the layers of Ti 3 C 2 -MXene [25,28].For the SEM images of CsPbI 3 shown in Figure 3f, the majority of the particles exhibit a spherical shape, with a few particles exhibiting faceted (cubic) shapes and a brighter contrast [36].In Figure 3g-i  To further corroborate the formation of the Ti3C2-MXene/NiO/CsPbI3 composites, elemental mapping was performed using energy-dispersive spectroscopy (EDS) (Figure 4).As depicted in Figure 4, NiO particles envelop CsPbI3, and this combined structure is adhered to the Ti3C2-MXene nanosheets.Elemental mapping reveals the distribution of various elements, such as Ti, C, Ni, O, Cs, Pb, and I, within the nanocomposite structure, providing further evidence of composite formation [25,37].To further corroborate the formation of the Ti 3 C 2 -MXene/NiO/CsPbI 3 composites, elemental mapping was performed using energy-dispersive spectroscopy (EDS) (Figure 4).As depicted in Figure 4, NiO particles envelop CsPbI 3 , and this combined structure is adhered to the Ti 3 C 2 -MXene nanosheets.Elemental mapping reveals the distribution of various elements, such as Ti, C, Ni, O, Cs, Pb, and I, within the nanocomposite structure, providing further evidence of composite formation [25,37].

Optical Properties of Ti 3 C 2 -MXene/NiO/CsPbI 3 Composites
The UV-vis absorption properties and band gap of the Ti 3 C 2 -MXene nanosheets, Ti 3 C 2 -MXene/NiO, and Ti 3 C 2 -MXene/NiO/CsPbI 3 composites were investigated by UVvis spectroscopy, as shown in Figure 6. Figure 6a presents the UV-Vis absorption spectrum of the Ti 3 C 2 -MXene nanosheets in a dilute aqueous medium, displaying distinct peaks at 275 nm [39].When UV-Vis spectroscopy was performed on a mixed solution of Ti 3 C 2 -MXene/NiO, it is evident that the typical peak position of NiO was at 380 nm, and that of Ti 3 C 2 -MXene/NiO was also located at 380 nm [40].Notably, a new absorption band appears at the position of a peak around 465 nm after the incorporation of CsPbI 3 .With successive additions of CsPbI 3 , the absorption spectra of the Ti 3 C 2 -MXene/NiO/CsPbI 3 composites retained the characteristics of the native Ti 3 C 2 -MXene/NiO behavior.The peak position of NiO at 380 nm remained virtually unchanged, indicating that the structure of Ti 3 C 2 -MXene/NiO was unaffected by the addition of CsPbI 3 .However, the peaks of Ti 3 C 2 -MXene nanosheets shifted from 275 nm to 270 nm, confirming the successful addition of CsPbI 3 to the Ti 3 C 2 -MXene/NiO composites.Additionally, the presented Figure 6b-d illustrates the determination of the energy band gap using the Tauc plots of Ti 3 C 2 -MXene nanosheets, Ti 3 C 2 -MXene/NiO, and Ti 3 C 2 -MXene/NiO/CsPbI 3 composites.αhυ = A(hυ − E g ) n ; in this relation, α represents the absorption coefficient, hυ corresponds to the photon energy, E g is the band gap, and n is set to 1/2 for direct transitions.Figure 6b-d features a plot of (αhυ) 2 against the hυ axis for the determination of the band gap.The determined band gap values are 1.60 eV for the Ti 3 C 2 -MXene nanosheets and 3.10 eV for Ti 3 C 2 -MXene/NiO.Notably, the band gap of the Ti 3 C 2 -MXene/NiO/CsPbI 3 composites experiences a slight reduction to 2.41 eV.This decrease in the band gap implies electronic interaction and enhanced coupling among the Ti 3 C 2 -MXene nanosheets, NiO, and CsPbI 3 particles, resulting in modified optical properties [41].

Charge-Transfer Behavior of Ti 3 C 2 -MXene/NiO/CsPbI 3 Composites
Photoluminescence (PL) spectroscopy was employed to examine the rate of recombination of photoinduced charge carriers in the prepared materials, as depicted in Figure 7.
The PL intensity serves as a measure of the semiconductor electronic behavior, particularly with regard to the charge recombination rate, which has a direct impact on the photocatalytic performance of the materials.The PL spectra of Ti 3 C 2 -MXene nanosheets, NiO, Ti 3 C 2 -MXene/NiO, and Ti 3 C 2 -MXene/NiO/CsPbI 3 composites reveal emission peaks at 530, 517, 515, and 532 nm, respectively.The charge recombination rate is indicated by the peak's intensity in these PL spectra.Consequently, in comparison to the Ti 3 C 2 -MXene/NiO/CsPbI 3 composites, which show a weaker intensity peak, the PL spectra of Ti 3 C 2 -MXene/NiO, with a high peak intensity, indicate a higher electron-hole recombination rate [41].Because of this, the process of creating Ti 3 C 2 -MXene/NiO/CsPbI 3 composites decreases the amount of photogenerated charge carriers that recombine, which increases the quantity of charge carriers that are available for photocatalytic degradation [42].

Optical Properties of Ti3C2-MXene/NiO/CsPbI3 Composites
The UV-vis absorption properties and band gap of the Ti3C2-MXene nanosheets, Ti3C2-MXene/NiO, and Ti3C2-MXene/NiO/CsPbI3 composites were investigated by UVvis spectroscopy, as shown in Figure 6. Figure 6a presents the UV-Vis absorption spectrum of the Ti3C2-MXene nanosheets in a dilute aqueous medium, displaying distinct peaks at 275 nm [39].When UV-Vis spectroscopy was performed on a mixed solution of Ti3C2-MXene/NiO, it is evident that the typical peak position of NiO was at 380 nm, and that of Ti3C2-MXene/NiO was also located at 380 nm [40].Notably, a new absorption band energy, Eg is the band gap, and n is set to 1/2 for direct transitions.Figure 6b-d features a plot of (αhυ) 2 against the hυ axis for the determination of the band gap.The determined band gap values are 1.60 eV for the Ti3C2-MXene nanosheets and 3.10 eV for Ti3C2-MXene/NiO.Notably, the band gap of the Ti3C2-MXene/NiO/CsPbI3 composites experiences a slight reduction to 2.41 eV.This decrease in the band gap implies electronic interaction and enhanced coupling among the Ti3C2-MXene nanosheets, NiO, and CsPbI3 particles, resulting in modified optical properties [41].

Charge-Transfer Behavior of Ti3C2-MXene/NiO/CsPbI3 Composites
Photoluminescence (PL) spectroscopy was employed to examine the rate of recombination of photoinduced charge carriers in the prepared materials, as depicted in Figure 7.The PL intensity serves as a measure of the semiconductor electronic behavior, particularly with regard to the charge recombination rate, which has a direct impact on the photocatalytic performance of the materials.The PL spectra of Ti3C2-MXene nanosheets, NiO, Ti3C2-MXene/NiO, and Ti3C2-MXene/NiO/CsPbI3 composites reveal emission peaks at 530, 517, 515, and 532 nm, respectively.The charge recombination rate is indicated by the peak's intensity in these PL spectra.Consequently, in comparison to the Ti3C2-MXene/NiO/CsPbI3 composites, which show a weaker intensity peak, the PL spectra of Ti3C2-MXene/NiO, with a high peak intensity, indicate a higher electron-hole recombination rate [41].Because of this, the process of creating Ti3C2-MXene/NiO/CsPbI3 composites decreases the amount of photogenerated charge carriers that recombine, which increases the quantity of charge carriers that are available for photocatalytic degradation [42].
In the study, to gain insights into the relationship between the electrochemical performance and the resistance behavior of various materials, including Ti3C2-MXene nanosheets, Ti3C2-MXene/NiO, and Ti3C2-MXene/NiO/CsPbI3 composites, they used In the study, to gain insights into the relationship between the electrochemical performance and the resistance behavior of various materials, including Ti 3 C 2 -MXene nanosheets, Ti 3 C 2 -MXene/NiO, and Ti 3 C 2 -MXene/NiO/CsPbI 3 composites, they used Nyquist plots derived from electrochemical impedance spectroscopy (EIS) tests, as depicted in Figure 8a.Nyquist plots are powerful tools for analyzing electrochemical systems.In these plots, the impedance spectra are represented by characteristic shapes.In the high-frequency regions, one can observe sharp semicircles that intersect the real axis, whereas, in the low-frequency regions, nearly vertical lines become apparent.These shapes provide valuable information about the material electrical properties.The Ti 3 C 2 -MXene nanosheets exhibited a distinctive behavior in the Nyquist plot, with their curve and the horizontal axis intersecting at the smallest point in Figure 8a.This intersection indicates that this particular sample has the lowest equivalent internal resistance among all the tested materials.Notably, the Ti 3 C 2 -MXene nanosheets lack a significant arc in the high-frequency region, implying low charge-transfer resistance.This is a promising sign for their electrochemical performances.Conversely, the Ti 3 C 2 -MXene/NiO composite displayed a larger semicircular diameter in the high-frequency region, suggesting a higher internal resistance to electron conduction.This behavior can be attributed to the presence of NiO sheets on the surface of Ti 3 C 2 -MXene, which contribute to this increased resistance.The Ti 3 C 2 -MXene/NiO/CsPbI 3 composites stood out in the Nyquist plot by displaying the smallest diameter among the samples [43].This observation confirmed the lowest resistance and highest conductivity compared to the Ti 3 C 2 -MXene nanosheets and Ti 3 C 2 -MXene/NiO samples [44,45].This enhanced interfacial charge transfer within the composites is a positive indication of their superior electrochemical performance, ultimately contributing to improved photocatalytic activity.Therefore, the Nyquist plots derived from EIS tests offer valuable insights into the electrical properties and performance of the tested materials, shedding light on their suitability for various applications, particularly in the context of photocatalysis.To further explore the efficacy of separating photoexcited electrons and holes, we conducted a series of photocurrent measurements on Ti

Photocatalytic Degradation of Dye
The photocatalytic performance of the Ti3C2-MXene nanosheets, Ti3C2-MXene/NiO, and Ti3C2-MXene/NiO/CsPbI3 composites was assessed by the degradation of crystal-violet (CV) aqueous solution under UV-light irradiation.During the initial hour of the investigation, the degradation process commenced in dark conditions and continued for an additional 1.5 h with the presence of UV-light irradiation.In this study, the photocatalytic degradation of crystal violet (CV) was monitored at 590 nm, since there was no significant shift in the main peak of CV during the photocatalytic experiment [46].The recorded UVvisible absorption spectra of the CV dye are illustrated in Figure 9a-c.

Photocatalytic Degradation of Dye
The photocatalytic performance of the Ti 3 C 2 -MXene nanosheets, Ti 3 C 2 -MXene/NiO, and Ti 3 C 2 -MXene/NiO/CsPbI 3 composites was assessed by the degradation of crystalviolet (CV) aqueous solution under UV-light irradiation.During the initial hour of the investigation, the degradation process commenced in dark conditions and continued for an additional 1.5 h with the presence of UV-light irradiation.In this study, the photocatalytic degradation of crystal violet (CV) was monitored at 590 nm, since there was no significant shift in the main peak of CV during the photocatalytic experiment [46].The recorded UV-visible absorption spectra of the CV dye are illustrated in Figure 9a-c.Photocatalytic degradation of CV by the Ti3C2-MXene-based composite of NiO and CsPbI3 demonstrated excellent performance, attributed to the remarkable properties of Ti3C2-MXene, including its large surface area and surface functional groups.As the UVlight exposure duration increased, the color of the solutions containing Ti3C2-MXene nanosheets, Ti3C2-MXene/NiO, and Ti3C2-MXene/NiO/CsPbI3 composites also changed.The results for photodegradation are presented in the inset of Figure 9a-c.The degradation efficiency of the Ti3C2-MXene nanosheets, Ti3C2-MXene/NiO, and Ti3C2-MXene/NiO/CsPbI3 composites can be explained in terms of Ct/C0, where C0 represents the initial concentration of the CV dye and Ct represents the concentration at a specific time interval (Figure 10a) [47].The photocatalytic performance of these materials was evaluated based on the kinetics of CV-dye photodegradation, and it was found to follow first-order kinetics [48,49].A straight line is obtained in Figure 10b when plotting −ln(Ct/C0) against time (t), and the rate constant 'k' in min⁻¹ can be calculated from the slope of this straight line [50].The rate constant (k) values, as shown in Figure 10c, for the degradation of CV dye by Ti3C2-MXene nanosheets, Ti3C2-MXene/NiO, and Ti3C2-MXene/NiO/CsPbI3 composites are 0.0042, 0.0132, and 0.0199 min⁻¹, respectively.The faster degradation of CV dye with Ti3C2-MXene/NiO/CsPbI3 composites is primarily attributed to the effective charge separation within these composites, possibly resulting from the formation of a heterojunction between Ti3C2-MXen, NiO, and CsPbI3 [51].In addition, the Ti3C2-MXene nanocomposite contains NiO and CsPbI3, which not only provide a large surface area to improve CV-dye adsorption but also suppress electron and hole recombination, enhancing photocatalytic activity [48][49][50][51][52].
Radical scavenger experiments were carried out for the photocatalytic decomposition of CV in the presence of different scavengers in order to investigate the photocatalytic reaction mechanism of Ti3C2-MXeneNiO/CsPbI3 composites towards CV [54].Using methanol (MeOH), isopropanol (IPA), and ammonium oxalate (AO) as scavengers, Figure 11a shows the photocatalytic breakdown of CV and validates the effects of superoxide radicals (•O2 − ), hydroxyl radicals (•OH), and photogenerated holes (h + ), respectively.The substantial contribution of photogenerated holes to CV breakdown is confirmed by the noticeably lower photodegradation of CV (31.2%) in the presence of AO.Nonetheless, the moderate decrease in photodegradation (43.4% and 69.2%) when IPA and MeOH are present validates the partial contribution of hydroxyl radicals (•OH) [55].Therefore, the photodegradation of CV is primarily mediated by hydroxyl radicals (•OH) and photogenerated holes (h + ) [56].Additionally, the reusability of Ti3C2-MXeneNiO/CsPbI3 composites was explored through eight successive cycles of CV degradation, as illustrated in Figure 11b.The enhanced catalytic efficiency of Ti 3 C 2 -MXene/NiO (62.9%) compared to Ti 3 C 2 -MXene nanosheets (31.3%) is attributed to the defects generated by doping.Furthermore, the dopants act as trapping agents for holes and electrons, reducing their recombination phenomenon [53].In the case of Ti 3 C 2 -MXene/NiO/CsPbI 3 composites, the percentage of CV-dye degradation reaches 92.8%.This remarkable performance can be attributed to the Ti 3 C 2 -MXene nanosheets, which prevent the agglomeration of NiO and CsPbI 3 , resulting in an increased surface-to-volume ratio.Consequently, Ti 3 C 2 -MXene/NiO/CsPbI 3 composites, with their larger surface area, adsorb a higher amount of CV dye, leading to a promising catalytic efficiency of 92.8%.
Radical scavenger experiments were carried out for the photocatalytic decomposition of CV in the presence of different scavengers in order to investigate the photocatalytic reaction mechanism of Ti 3 C 2 -MXeneNiO/CsPbI 3 composites towards CV [54].Using methanol (MeOH), isopropanol (IPA), and ammonium oxalate (AO) as scavengers, Figure 11a shows the photocatalytic breakdown of CV and validates the effects of superoxide radicals (•O 2 − ), hydroxyl radicals (•OH), and photogenerated holes (h + ), respectively.The substantial contribution of photogenerated holes to CV breakdown is confirmed by the noticeably lower photodegradation of CV (31.2%) in the presence of AO.Nonetheless, the moderate decrease in photodegradation (43.4% and 69.2%) when IPA and MeOH are present validates the partial contribution of hydroxyl radicals (•OH) [55].Therefore, the photodegradation of CV is primarily mediated by hydroxyl radicals (•OH) and photogenerated holes (h + ) [56].Additionally, the reusability of Ti 3 C 2 -MXeneNiO/CsPbI 3 composites was explored through eight successive cycles of CV degradation, as illustrated in Figure 11b.It is noteworthy that a minimal decline of 2.9% (from 92.3% to 89.4%) in the photocatalytic degradation activity of CV dye was observed.This slight reduction might be attributed to the potential loss of catalyst material occurring during the centrifugation, drying, and washing steps involved in the recovery process.This underscores the importance of considering the various stages of the experimental procedure and their potential impact on the composite performance over multiple cycles.

Proposed Photocatalytic Mechanism
The efficiency of photoinduced carrier migration, transfer, and separation during the photocatalytic degradation process was examined using photoluminescence (PL) spectra [57].PL emission usually results from photoexcited electron-hole pairs recombining [58].A semiconductor with a lower PL intensity has a higher photocatalytic activity.Figure 12 shows a schematic representation of a suggested photocatalytic mechanism for the improved performance of Ti3C2-MXene/NiO/CsPbI3 composites based on the earlier findings.It is noteworthy that a minimal decline of 2.9% (from 92.3% to 89.4%) in the photocatalytic degradation activity of CV dye was observed.This slight reduction might be attributed to the potential loss of catalyst material occurring during the centrifugation, drying, and washing steps involved in the recovery process.This underscores the importance of considering the various stages of the experimental procedure and their potential impact on the composite performance over multiple cycles.

Proposed Photocatalytic Mechanism
The efficiency of photoinduced carrier migration, transfer, and separation during the photocatalytic degradation process was examined using photoluminescence (PL) spectra [57].PL emission usually results from photoexcited electron-hole pairs recombining [58].A semiconductor with a lower PL intensity has a higher photocatalytic activity.Figure 12 shows a schematic representation of a suggested photocatalytic mechanism for the improved performance of Ti 3 C 2 -MXene/NiO/CsPbI 3 composites based on the earlier findings.It is noteworthy that a minimal decline of 2.9% (from 92.3% to 89.4%) in the photocatalytic degradation activity of CV dye was observed.This slight reduction might be attributed to the potential loss of catalyst material occurring during the centrifugation, drying, and washing steps involved in the recovery process.This underscores the importance of considering the various stages of the experimental procedure and their potential impact on the composite performance over multiple cycles.

Proposed Photocatalytic Mechanism
The efficiency of photoinduced carrier migration, transfer, and separation during the photocatalytic degradation process was examined using photoluminescence (PL) spectra [57].PL emission usually results from photoexcited electron-hole pairs recombining [58].A semiconductor with a lower PL intensity has a higher photocatalytic activity.Figure 12 shows a schematic representation of a suggested photocatalytic mechanism for the improved performance of Ti3C2-MXene/NiO/CsPbI3 composites based on the earlier findings.In the PL spectra, Ti 3 C 2 -MXene nanosheets exhibit weak intensity, while NiO and Ti 3 C 2 -MXene/NiO exhibit the strongest PL intensity (Figure 7), indicating rapid recombination of photoexcited charge carriers [59].The PL intensity of Ti 3 C 2 -MXene/NiO/CsPbI 3 composites decreases following modification with CsPbI 3 and NiO, indicating that CsPbI 3 and NiO, functioning as an electron mediator, efficiently encourages the separation of photoexcited charge carriers.As a result, the composites Ti 3 C 2 -MXene/NiO/CsPbI 3 show the lowest PL-emission intensity, suggesting a higher carrier-separation rate and greater production of reactive species for the degradation of pollutants.These PL spectroscopy findings are consistent with the evaluation of photocatalytic activity, confirming that photogenerated carrier-separation efficiency in semiconductors does, in fact, influence photocatalytic activity.In the previous literature, when exposed to light, MXenes generate electron-hole pairs, with electrons in the conduction band participating in reduction reactions and holes in the valence band engaging in oxidation reactions [60].In the context of Cr(VI) reduction, for instance, MXenes facilitate the conversion of toxic Cr(VI) species to less harmful forms [61].As a catalyst, MXenes remain unaltered during the reaction, providing surfaces for reactant adsorption and bringing them into proximity to enhance reaction possibility [62].The final products of photocatalysis depend on the specific reactants, such as the reduction of Cr(VI) leading to Cr(III) species [63].In addition, the surface properties, including defects and functional groups, are crucial for determining active sites, emphasizing the importance of understanding MXene surface chemistry to optimize the photocatalytic activity [64].The photocatalytic systems achieving high solar-to-hydrogen efficiency in photocatalytic water splitting are crucial for renewable hydrogen production.Various materials and strategies are employed to enhance the efficiency of this process, including the use of semiconductor photocatalysts and optimizing reaction conditions [65].ZnIn 2 S 4 is a semiconductor material used in photocatalysis for hydrogen evolution.Designing heterostructured photocatalysts involves combining different materials to create interfaces that enhance charge separation and improve the overall efficiency in hydrogenevolution reactions [66].Also, the metal-organic frameworks (MOFs) are porous materials with unique properties.Constructing double Z-scheme heterojunctions involves creating specific arrangements of MOFs to facilitate efficient charge transfer and enhance photocatalytic activity, particularly in the degradation of pollutants [67].This topic likely involves the development of a strategy to spatially separate redox reactions in order to achieve efficient and highly selective photoconversion of amines to imines [68].Furthermore, this kind of research can have implications for the design of more precise and selective chemical processes.This proposed mechanism states that photogenerated electrons from the CsPbI 3 conduction band (CB) migrate to Ti 3 C 2 -MXene and NiO CB migrate to the Ti 3 C 2 -MXene valence band (VB), resulting in Fermi levels that coincide [69].These electrons then mix with the h + produced by photocatalysis in the VB of Ti 3 C 2 -MXene/NiO VB, which may lead to a low migration rate or the accumulation of charge carriers.In order to mitigate these problems, CsPbI 3 is incorporated at the interface between NiO and Ti 3 C 2 -MXene, acting as a bridge to accelerate the rate of electron migration from NiO to Ti 3 C 2 -MXene [70]

Conclusions
A novel ternary composite composed of Ti 3 C 2 -MXene/NiO/CsPbI 3 is successfully synthesized and studied as the best photocatalyst for the degradation of crystal-violet dye.The as-prepared Ti 3 C 2 -MXene/NiO/CsPbI 3 composite was investigated in comparison with Ti 3 C 2 -MXene nanosheets, NiO, CsPbI 3 , and a binary composite of Ti 3 C 2 -MXene/NiO.Various properties of the composite were analyzed, including optical, structural, and morphological properties, using techniques such as FTIR, XRD, TEM, SEM-EDS mapping, XPS, UV-Vis, and PL spectroscopy.The study found that the Ti 3 C 2 -MXene/NiO/CsPbI 3 composite exhibited superior photocatalytic efficiency, degrading 92.8% of the target molecules after 90 min of UV-light irradiation, compared to pristine Ti 3 C 2 -MXene nanosheets (31.3%) and the binary composite Ti 3 C 2 -MXene/NiO (62.9%).The Ti 3 C 2 -MXene/NiO/CsPbI 3 composite also showed the highest rate constant (0.0199 min −1 ) and improved photocatalytic activity due to the reduction in band gap and strong synergistic effects at the interface between Ti 3 C 2 -MXene/NiO and CsPbI 3 .The addition of CsPbI 3 enhanced the transport and separation of photogenerated electron-hole pairs at the NiO and Ti 3 C 2 -MXene interface.Therefore, the formation of Ti 3 C 2 -MXene/NiO/CsPbI 3 composites involving perovskite materials and carbon-based materials proved to be an effective approach for removing organic pollutants from water under UV light.The improved performance, attributed to the larger surface area and lower electron-hole recombination rate, makes Ti 3 C 2 -MXene/NiO/CsPbI 3 composites a highly efficient photocatalyst with promising applications in various fields.

Figure 3
Figure 3 represents the FESEM image of the Ti 3 C 2 -MXene nanosheets, NiO, Ti 3 C 2 -MXene/NiO, CsPbI 3 , and Ti 3 C 2 -MXene/NiO/CsPbI 3 composites phase.In Figure3a,b, the SEM micrographs illustrate the distinctive structure of Ti 3 C 2 -MXene nanosheets, characterized by a typical 2D and sheet-like arrangement with only a few layers, resembling the graphene-like structure[25].The surface of these nanosheets appears smooth, and the presence of a well-defined layered structure confirms the existence of Ti 3 C 2 -MXene nanosheets (Figure3b).Figure3cpresents SEM micrographs of the NiO nanoparticles, displaying a generally spherical shape with some degree of aggregation.In contrast, Figure3d,e show SEM images of the Ti 3 C 2 -MXene/NiO composite.After combining with NiO, the surface of Ti 3 C 2 -MXene/NiO becomes notably rough, with some NiO particles covering the surface of Ti 3 C 2 -MXene, while others are randomly embedded within the layers of Ti 3 C 2 -MXene[25,28].For the SEM images of CsPbI 3 shown in Figure3f, the majority of the particles exhibit a spherical shape, with a few particles exhibiting faceted (cubic) shapes and a brighter contrast[36].In Figure3g-i, typical SEM images of the Ti 3 C 2 -MXene/NiO/CsPbI 3 composites reveal that Ti 3 C 2 -MXene nanosheets are evenly covered with spherical NiO and CsPbI 3 particles, while the ordered layer structure of Ti 3 C 2 -MXene is still retained.These morphologies confirm the successful seeding and growth of NiO and CsPbI 3 particles on the surface of Ti 3 C 2 -MXene, resulting in an enhanced surface roughness[25].
Figure 3 represents the FESEM image of the Ti 3 C 2 -MXene nanosheets, NiO, Ti 3 C 2 -MXene/NiO, CsPbI 3 , and Ti 3 C 2 -MXene/NiO/CsPbI 3 composites phase.In Figure3a,b, the SEM micrographs illustrate the distinctive structure of Ti 3 C 2 -MXene nanosheets, characterized by a typical 2D and sheet-like arrangement with only a few layers, resembling the graphene-like structure[25].The surface of these nanosheets appears smooth, and the presence of a well-defined layered structure confirms the existence of Ti 3 C 2 -MXene nanosheets (Figure3b).Figure3cpresents SEM micrographs of the NiO nanoparticles, displaying a generally spherical shape with some degree of aggregation.In contrast, Figure3d,e show SEM images of the Ti 3 C 2 -MXene/NiO composite.After combining with NiO, the surface of Ti 3 C 2 -MXene/NiO becomes notably rough, with some NiO particles covering the surface of Ti 3 C 2 -MXene, while others are randomly embedded within the layers of Ti 3 C 2 -MXene[25,28].For the SEM images of CsPbI 3 shown in Figure3f, the majority of the particles exhibit a spherical shape, with a few particles exhibiting faceted (cubic) shapes and a brighter contrast[36].In Figure3g-i, typical SEM images of the Ti 3 C 2 -MXene/NiO/CsPbI 3 composites reveal that Ti 3 C 2 -MXene nanosheets are evenly covered with spherical NiO and CsPbI 3 particles, while the ordered layer structure of Ti 3 C 2 -MXene is still retained.These morphologies confirm the successful seeding and growth of NiO and CsPbI 3 particles on the surface of Ti 3 C 2 -MXene, resulting in an enhanced surface roughness [25].Nanomaterials 2023, 13, x FOR PEER REVIEW 10 of 22
3 C 2 -MXene nanosheets, Ti 3 C 2 -MXene/NiO, and Ti 3 C 2 -MXene/NiO/CsPbI 3 composite materials.The photocurrent profiles, presented in Figure 8b, illustrate the periodic on-off responses to UV light illumination for these materials.Notably, the Ti 3 C 2 -MXene/NiO/CsPbI 3 composite exhibits a superior photocurrent response compared to the Ti 3 C 2 -MXene nanosheets and the Ti 3 C 2 -MXene/NiO, which is consistent with the observed photocatalytic activity.Measurements in a 3 mol L −1 KOH solution reveal a photocurrent density of 0.18 µA cm −2 for the Ti 3 C 2 -MXene/NiO/CsPbI 3 composite superior to 0.15 µA cm −2 for Ti 3 C 2 -MXene nanosheets and 0.16 µA cm −2 for Ti 3 C 2 -MXene/NiO.This outcome indicates that the integration of Ti 3 C 2 -MXene/NiO with CsPbI 3 enhances photocurrent density, suggesting improved separation efficiency of photoexcited carriers.Therefore, these findings highlight the robust capacity of the Ti 3 C 2 -MXene/NiO/CsPbI 3 composite to transfer and generate photoexcited charge carriers under UV light, an essential factor in supplementing its photocatalytic performance.Nanomaterials 2023, 13, x FOR PEER REVIEW 14 of 22 context of photocatalysis.To further explore the efficacy of separating photoexcited electrons and holes, we conducted a series of photocurrent measurements on Ti3C2-MXene nanosheets, Ti3C2-MXene/NiO, and Ti3C2-MXene/NiO/CsPbI3 composite materials.The photocurrent profiles, presented in Figure 8b, illustrate the periodic on-off responses to UV light illumination for these materials.Notably, the Ti3C2-MXene/NiO/CsPbI3 composite exhibits a superior photocurrent response compared to the Ti3C2-MXene nanosheets and the Ti3C2-MXene/NiO, which is consistent with the observed photocatalytic activity.Measurements in a 3 mol L −1 KOH solution reveal a photocurrent density of 0.18 µA cm −2 for the Ti3C2-MXene/NiO/CsPbI3 composite superior to 0.15 µA cm −2 for Ti3C2-MXene nanosheets and 0.16 µA cm −2 for Ti3C2-MXene/NiO.This outcome indicates that the integration of Ti3C2-MXene/NiO with CsPbI3 enhances photocurrent density, suggesting improved separation efficiency of photoexcited carriers.Therefore, these findings highlight the robust capacity of the Ti3C2-MXene/NiO/CsPbI3 composite to transfer and generate photoexcited charge carriers under UV light, an essential factor in supplementing its photocatalytic performance.

Figure 9 .
Figure 9. Absorption spectra of CV-dye photodegradation over, (a) Ti 3 C 2 -MXene nanosheets, (b) Ti 3 C 2 -MXene/NiO, and (c) Ti 3 C 2 -MXene/NiO/CsPbI 3 composites.Photocatalytic degradation of CV by the Ti 3 C 2 -MXene-based composite of NiO and CsPbI 3 demonstrated excellent performance, attributed to the remarkable properties of Ti 3 C 2 -MXene, including its large surface area and surface functional groups.As the UV-light exposure duration increased, the color of the solutions containing Ti 3 C 2 -MXene nanosheets, Ti 3 C 2 -MXene/NiO, and Ti 3 C 2 -MXene/NiO/CsPbI 3 composites also changed.The results for photodegradation are presented in the inset of Figure 9a-c.The degradation efficiency of the Ti 3 C 2 -MXene nanosheets, Ti 3 C 2 -MXene/NiO, and Ti 3 C 2 -MXene/NiO/CsPbI 3 composites can be explained in terms of C t /C 0 , where C 0 represents the initial concentration of the CV dye and C t represents the concentration at a specific time interval (Figure10a)[47].The photocatalytic performance of these materials was evaluated based on the kinetics of CV-dye photodegradation, and it was found to follow first-order kinetics[48,49].A straight line is obtained in Figure10bwhen plotting −ln(C t /C 0 ) against time (t), and the rate constant 'k' in min −1 can be calculated from the slope of this straight line[50].The rate constant (k) values, as shown in Figure10c, for the degradation of CV dye by Ti 3 C 2 -MXene nanosheets, Ti 3 C 2 -MXene/NiO, and Ti 3 C 2 -MXene/NiO/CsPbI 3 composites are 0.0042, 0.0132, and 0.0199 min −1 , respectively.The faster degradation of CV dye with Ti 3 C 2 -MXene/NiO/CsPbI 3 composites is primarily attributed to the effective charge separation within these composites, possibly resulting from the formation of a heterojunction between Ti 3 C 2 -MXen, NiO, and CsPbI 3[51].In addition, the Ti 3 C 2 -MXene nanocomposite contains NiO and CsPbI 3 , which not only provide a large surface area to improve CV-dye adsorption but also suppress electron and hole recombination, enhancing photocatalytic activity[48][49][50][51][52].

Figure 11 .
Figure 11.(a) Effect of different scavengers and (b) cyclic stability performance of Ti3C2-MXene/NiO/CsPbI3 composites in the photocatalytic removal of CV.