Ag3PO4-Deposited TiO2@Ti3C2 Petals for Highly Efficient Photodecomposition of Various Organic Dyes under Solar Light

Two-dimensional Ti3C2 MXenes can be used to fabricate hierarchical TiO2 nanostructures that are potential photocatalysts. In this study, the photodecomposition of organic dyes under solar light was investigated using flower-like TiO2@Ti3C2, deposited using narrow bandgap Ag3PO4. The surface morphology, crystalline structure, surface states, and optical bandgap properties were determined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption analysis, and UV-Vis diffuse reflectance spectroscopy (UV-DRS). Overall, Ag3PO4-deposited TiO2@Ti3C2, referred to as Ag3PO4/TiO2@Ti3C2, demonstrated the best photocatalytic performance among the as-prepared samples, including TiO2@Ti3C2, pristine Ag3PO4, and Ag3PO4/TiO2 P25. Organic dyes, such as rhodamine B (RhB), methylene blue (MB), crystal violet (CV), and methylene orange (MO), were efficiently degraded by Ag3PO4/TiO2@Ti3C2. The significant enhancement of photocatalysis by solar light irradiation was attributed to the efficient deposition of Ag3PO4 nanoparticles on flower-like TiO2@Ti3C2 with the efficient separation of photogenerated e-/h+ pairs, high surface area, and extended visible-light absorption. Additionally, the small size of Ag3PO4 deposition (ca. 4–10 nm diameter) reduces the distance between the core and the surface of the composite, which inhibits the recombination of photogenerated charge carriers. Free radical trapping tests were performed, and a photocatalytic mechanism was proposed to explain the synergistic photocatalysis of Ag3PO4/TiO2@Ti3C2 under solar light.


Introduction
Globally, as industrialization and urbanization accelerate, the demand for food and consumer goods (clothing, electronics, furniture, vehicles, etc.) increases. Unsustainable resource use and inefficient waste management, however, contribute to environmental problems, which have become a source of concern [1][2][3]. Numerous harmful pollutants endanger the environment and human health, such as heavy metal ions, toxic gases, and organic dye [4,5]. The majority of organic dyes found in wastewater originate from various industrial waste streams, including paint, dyeing clothing, bleaching paper, synthesizing rubber, and processing plastic, thereby, resulting in water pollution.
Organic dyes at high concentrations (5-1500 mg/L) cause significant environmental and organismal toxicity, including carcinogenic, mutagenic, and teratogenic effects [6][7][8]. Owing to their low removal capacity, conventional biological processes used to remove dyes, such as flocculation, filtration, precipitation, and coagulation, have gradually become obsolete [9,10]. In comparison, heterogeneous photocatalysts used in advanced oxidation processes are considered promising alternatives for the removal of a wide variety of organic pollutants with a highly efficient degradation rate [11].
Titania (TiO 2 ) is the most widely used and recognized photocatalytic material in environmental remediation owing to its superior photocatalytic performance, non-toxicity, and stability [12][13][14]. However, pure TiO 2 has a low quantum efficiency and a large systems at 140 • C for 15 h. The dispersion was then allowed to cool naturally to room temperature, and the samples were washed with deionized (DI) water and ethanol several times before being dried in an oven at 60 • C for 12 h. The dried sample was soaked in a 0.05 M HCl solution (500 mL) for 12 h to ensure ion exchange. After heating the sample in a furnace at 500 • C for 5 h, the TiO 2 @Ti 3 C 2 composite was obtained. The formation of TiO 2 is presented by the following equations:

Preparation of Ag 3 PO 4 Particles
To prepare Ag 3 PO 4 particles, a simple method was followed. To generate Ag 3 PO 4 , 0.1 M Na 3 PO 4 (8 mL) was added dropwise to 0.1 M AgNO 3 solution (24 mL) and stirred for 5 h in the dark. To remove redundant ions, the precipitate was centrifuged and washed with deionized water. The purified sample was dried overnight at 60 • C to obtain the Ag 3 PO 4 powder.

Preparation of Ag 3 PO 4 /TiO 2 @Ti 3 C 2
Ag 3 PO 4 /TiO 2 @Ti 3 C 2 composites were prepared by adding various amounts of AgNO 3 (x) and Na 3 PO 4 (y) solutions with a ratio of x:y = 3:1. First, 0.05 g TiO 2 @Ti 3 C 2 was added to 0.1 M AgNO 3 solutions. The mixed solutions were vigorously stirred and sonicated for 10 min. Thereafter, 0.1 M Na 3 PO 4 was gradually dropped into the solution and stirred for 5 h to form Ag 3 PO 4 . To avoid photocorrosion, the reaction was conducted in the dark. The resulting powder products were washed, centrifuged with DI water, and dried at 60 • C to obtain Ag 3 PO 4 /TiO 2 @Ti 3 C 2 . The powder obtained from mixing quantities x = 1, 2, 4, 6, and 8 mL AgNO 3 with y = 0.33, 0.67, 1.33, 2, and 2.67 mL Na 3 PO 4 corresponds to samples marked A1 to A5, respectively). The samples were kept in the dark throughout the preparation process to avoid photocorrosion. In comparison with sample A4, Ag 3 PO 4 /TiO 2 P25 was also prepared under the same conditions. The fabrication procedure for the composites is depicted in Scheme 1. First, Ti 3 C 2 MXene was subjected to hydrothermal oxidation, ion exchange, and heat treatment steps, transforming its accordion-like structure into a flower shape. Thereafter, Ag 3 PO 4 NPs containing varying amounts of Ag 3 PO 4 were deposited by in-situ precipitation on the flower-like structure to form the Ag 3 PO 4 /TiO 2 @Ti 3 C 2 composites. Photodegradation tests were conducted under solar-driven light.

Photocatalysis of Dye Degradation
A Xe lamp (1000 W) was used as an artificial solar light source with a light intensity of 1000 mW/cm 2 . Under simulated solar light, the photocatalytic activities of the as-prepared samples were evaluated for the photodecomposition of RhB dye. Typically, 10 mg of each sample was added to 20 mL of an aqueous solution containing RhB (9 mg/L). Prior to light irradiation, the adsorption experiments of Ag 3 PO 4 , TiO 2 , and Ag 3 PO 4 /TiO 2 @Ti 3 C 2 were performed by stirring the solutions in the dark for 15 min to improve dispersion and adsorption-desorption equilibrium. Thereafter, the solution was exposed to solar light. Following that, an aliquot of each sample (1 mL) was removed and centrifuged at the indicated intervals to obtain the supernatant for UV-vis spectrophotometer evaluation.
The degradation efficiency of the dye materials can be described by the following equation: degradation percentage (%) = C o − C t /C o × 100. To determine the rate constant of RhB degradation, the degradation kinetics were assumed to follow the pseudo-first order model [21,24]: where C o and C t denote the initial concentration of RhB and a specific concentration of RhB after exposure to light for t minutes, respectively. Here, "k" is the pseudo-first-order rate constant calculated from the linear slope of ln(C 0 /C) versus t (time). 10 mg/L concentration of MB, MO, and CV were used for photodegradation with the same procedure. The absorbance changes for each dye were determined using a UV-Vis spectrometer at different wavelengths, RhB (554 nm), MB (664-665 nm), MO (464 nm), and CV (590 nm).

Photocatalysis of Dye Degradation
A Xe lamp (1000 W) was used as an artificial solar light source with a light intensity of 1000 mW/cm 2 . Under simulated solar light, the photocatalytic activities of the as-prepared samples were evaluated for the photodecomposition of RhB dye. Typically, 10 mg of each sample was added to 20 mL of an aqueous solution containing RhB (9 mg/L). Prior to light irradiation, the adsorption experiments of Ag3PO4, TiO2, and Ag3PO4/TiO2@Ti3C2 were performed by stirring the solutions in the dark for 15 min to improve dispersion and adsorption-desorption equilibrium. Thereafter, the solution was exposed to solar light. Following that, an aliquot of each sample (1 mL) was removed and centrifuged at the indicated intervals to obtain the supernatant for UV-vis spectrophotometer evaluation.
The degradation efficiency of the dye materials can be described by the following equation: degradation percentage (%) = Co − Ct/Co × 100. To determine the rate constant of RhB degradation, the degradation kinetics were assumed to follow the pseudo-first order model [21,24]: where Co and Ct denote the initial concentration of RhB and a specific concentration of RhB after exposure to light for t minutes, respectively. Here, "k" is the pseudo-first-order rate constant calculated from the linear slope of ln(C0/C) versus t (time). 10 mg/L concentration of MB, MO, and CV were used for photodegradation with the same procedure. The absorbance changes for each dye were determined using a UV-Vis spectrometer at different wavelengths, RhB (554 nm), MB (664-665 nm), MO (464 nm), and CV (590 nm).

Characterization
Scheme 1. Schematic indicating the fabrication processes, beginning with multilayer Ti 3 C 2 MXene and ending with the formation of Ag 3 PO 4 -deposited flower-shaped TiO 2 @Ti 3 C 2 .

Characterization
The X-ray diffraction (XRD) patterns of the samples were determined using a Rigaku Smartlab X-ray diffractometer with Cu-Kα radiation (λ = 1.544 Å) (Rigaku Corporation, Tokyo, Japan). The morphologies and microstructures of the samples were investigated using a Hitachi S-4700 field emission scanning electron microscope (FE-SEM) (Hitachi Ltd., Tokyo, Japan) and FEI Tecnai transmission electron microscopy (TEM) (FEI, Hillsboro, OR, United States). XPS measurements were conducted using an X-ray photoelectron spectrometer (XPS, Multilab 2000, Thermo Scientific, Waltham, MA, United States). The surface area, pore size, and pore volume were measured using an N 2 adsorption-desorption apparatus (ASAP 2020, Micromeritics Instrument Corp, Norcross, GA, USA). Optical properties of the samples were determined using a Jasco V770 UV-Vis diffuse reflectance spectrophotometer (Jasco Inc., Easton, MD, USA)
In the case of the composites, Figure 1b shows the diffraction patterns of Ag3PO4/TiO2@Ti3C2 (samples A3, A4, and A5), which clearly display both Ag3PO4 crystal and TiO2 anatase phase peaks, confirming that the composites were composed of Ag3PO4 and TiO2. The diffraction peaks of TiO2 in the heterostructure composite are weaker than those of Ag3PO4 owing to the lower crystallinity of TiO2. However, samples A1 and A2 containing a lower concentration of Ag3PO4 mostly exhibit peaks of TiO2 anatase at = 25.3° (JCPDS card No. 21-1272), instead of Ag3PO4 peaks due to self-corrosion [22].

Morpholoy Analysis by SEM, TEM
The accordion-like shape of Ti3C2 is shown in the SEM image in Figure 2a. After oxidization, ion exchange, and heat treatment, accordion-shape Ti3C2 transforms into TiO2@Ti3C2 with a flower shape (Figure 2b). Figure 2c depicts pure Ag3PO4 NPs with an irregular shape and a diameter of ca. 300-500 nm [19]. The SEM image in Figure 2d shows aggregated Ag3PO4-deposited TiO2@Ti3C2 that retains its flower-like shape. To overcome certain limitations of SEM analysis, which focuses on the surface morphology of samples, TEM and HRTEM images were used to clarify the transmission morphology and crystalline structure of Ag3PO4/TiO2@Ti3C2 (sample A4). According to the EDX spectrum ( Figure  S1), the main elements present in the sample are carbon, oxygen, titanium, silver, and In the case of the composites, Figure 1b shows the diffraction patterns of Ag 3 PO 4 / TiO 2 @Ti 3 C 2 (samples A3, A4, and A5), which clearly display both Ag 3 PO 4 crystal and TiO 2 anatase phase peaks, confirming that the composites were composed of Ag 3 PO 4 and TiO 2 . The diffraction peaks of TiO 2 in the heterostructure composite are weaker than those of Ag 3 PO 4 owing to the lower crystallinity of TiO 2 . However, samples A1 and A2 containing a lower concentration of Ag 3 PO 4 mostly exhibit peaks of TiO 2 anatase at = 25.3 • (JCPDS card No. 21-1272), instead of Ag 3 PO 4 peaks due to self-corrosion [22].

Morpholoy Analysis by SEM, TEM
The accordion-like shape of Ti 3 C 2 is shown in the SEM image in Figure 2a. After oxidization, ion exchange, and heat treatment, accordion-shape Ti 3 C 2 transforms into TiO 2 @Ti 3 C 2 with a flower shape ( Figure 2b). Figure 2c depicts pure Ag 3 PO 4 NPs with an irregular shape and a diameter of ca. 300-500 nm [19]. The SEM image in Figure 2d shows aggregated Ag 3 PO 4 -deposited TiO 2 @Ti 3 C 2 that retains its flower-like shape. To overcome certain limitations of SEM analysis, which focuses on the surface morphology of samples, TEM and HRTEM images were used to clarify the transmission morphology and crystalline structure of Ag 3 PO 4 /TiO 2 @Ti 3 C 2 (sample A4). According to the EDX spectrum ( Figure  S1), the main elements present in the sample are carbon, oxygen, titanium, silver, and phosphorous. C, O, and Ti were obtained from TiO 2 @Ti 3 C 2 , with a portion of the O derived from Ag 3 PO 4 . It can be demonstrated that Ag 3 PO 4 is coated on the surface of TiO 2 @Ti 3 C 2 . Figure 3a shows a typical TEM image of the flower-shaped TiO 2 @Ti 3 C 2 with some petal fragments and large agglomerated Ag 3 PO 4 on it. However, the higher magnification images in Figure 3b,c demonstrate that the dominant Ag 3 PO 4 NPs were formed and deposited on each TiO 2 @Ti 3 C 2 nanorod. Positively charged silver ions were attracted to the surface of negatively charged TiO 2 , and the reaction between Ag + and PO 4 3− occurred immediately upon the addition of Na 3 PO 4 solution, resulting in the deposition of Ag 3 PO 4 NPs on TiO 2 @Ti 3 C 2 rods [26]. The TEM images show that precipitating Ag 3 PO 4 using TiO 2 @Ti 3 C 2 nanorods as a template can reduce particle aggregation owing to the high specific surface area of the nanorods, which affects Ag 3 PO 4 nucleation and reduces the diameter to around 4-10 nm.   Figure 3a shows a typical TEM image of the flower-shaped TiO2@Ti3C2 with some petal fragments and large agglomerated Ag3PO4 on it. However, the higher magnification images in Figure 3b,c demonstrate that the dominant Ag3PO4 NPs were formed and deposited on each TiO2@Ti3C2 nanorod. Positively charged silver ions were attracted to the surface of negatively charged TiO2, and the reaction between Ag + and PO4 3− occurred immediately upon the addition of Na3PO4 solution, resulting in the deposition of Ag3PO4 NPs on TiO2@Ti3C2 rods [26]. The TEM images show that precipitating Ag3PO4 using TiO2@Ti3C2 nanorods as a template can reduce particle aggregation owing to the high specific surface area of the nanorods, which affects Ag3PO4 nucleation and reduces the diameter to around 4-10 nm.
Additionally, the Ag3PO4 nanoparticles were relatively stable and did not detach during sonication as part of the TEM preparation process, whereas the agglomerated Ag3PO4 fragments moved and changed position during real-time TEM measurements. Figure 3c,d show the high-resolution TEM (HR-TEM) images of the Ag3PO4/TiO2@Ti3C2 composite. TiO2@Ti3C2 (101) has lattice fringes with an interplanar spacing of 0.35 nm, while cubic Ag3PO4 (210) has an interplanar spacing of 0.26 nm (Figure 3d). A SAED image further demonstrates the clear formation and polycrystallinity of these materials, which is shown in Figure S2. These TEM images can assist in resolving the initial ambiguity regarding the morphology of Ag3PO4/TiO2@Ti3C2 shown in the SEM images.   Additionally, the Ag 3 PO 4 nanoparticles were relatively stable and did not detach during sonication as part of the TEM preparation process, whereas the agglomerated Ag 3 PO 4 fragments moved and changed position during real-time TEM measurements. Figure 3c,d show the high-resolution TEM (HR-TEM) images of the Ag 3 PO 4 /TiO 2 @Ti 3 C 2 composite. TiO 2 @Ti 3 C 2 (101) has lattice fringes with an interplanar spacing of 0.35 nm, while cubic Ag 3 PO 4 (210) has an interplanar spacing of 0.26 nm (Figure 3d). A SAED image further demonstrates the clear formation and polycrystallinity of these materials, which is shown in Figure S2. These TEM images can assist in resolving the initial ambiguity regarding the morphology of Ag 3 PO 4 /TiO 2 @Ti 3 C 2 shown in the SEM images. Figure 4 depicts the scanning TEM (STEM) image of Ag 3 PO 4 /TiO 2 @Ti 3 C 2 with elemental mapping. The high-angle annular dark-field (HAADF) STEM image of Ag 3 PO 4 /TiO 2 @Ti 3 C 2 shows areas with different contrasts, with the brighter regions representing Ag 3 PO 4 particles and the darker regions representing TiO 2 @Ti 3 C 2 rods. The elemental mapping analysis displays C, O, P, K, and Ag elements from an arbitrary area. In detail, the Ti signal is primarily associated with the backbone of TiO 2 @Ti 3 C 2 , whereas the P and Ag signals are associated with the large agglomerated Ag 3 PO 4 fragments on top of the flower and Ag 3 PO 4 NPs decorated alongside the petals. Additionally, the O signals are distributed uniformly throughout the composite. The elemental mapping results provide more comprehensive and clear observations of the elemental distribution across the whole composite, corresponding to the SEM and HR-TEM above.

XPS Results
To characterize the chemical compositions and elemental states of the as-prepared samples, XPS measurements were conducted. The survey spectra of Ti3C2 MXene, TiO2@Ti3C2 flowers, and Ag3PO4/TiO2@Ti3C2 illustrate the characteristic elemental peaks at approximately 532. 35, 455.16, and 284.96 eV allocated to O 1s, Ti 2p, and C 1s, respectively ( Figure S3). The weak peak of F 1s at 685.1 eV in Ti3C2, indicates that some fluoride was left over from the synthesis process [17].
The TiO2@Ti3C2 heterostructure exhibits a much stronger O 1s peak intensity when compared to Ti3C2, indicating the existence of an appreciable amount of oxide in the heterostructure due to the oxidation process and the formation of TiO2 [4]. In the spectra results of composite Ag3PO4/TiO2@Ti3C2, the peaks of P 2p and Ag 3d represent Ag3PO4 in the composite. Generally, Ag 3d peaks have a higher intensity than Ti 2p, suggesting that many silver elements or their compounds are present on the surface of the solid sample, decorating the TiO2@Ti3C2 flower shape. This result is consistent with the SEM and TEM images discussed previously.

XPS Results
To characterize the chemical compositions and elemental states of the as-prepared samples, XPS measurements were conducted. The survey spectra of Ti 3 C 2 MXene, TiO 2 @Ti 3 C 2 flowers, and Ag 3 PO 4 /TiO 2 @Ti 3 C 2 illustrate the characteristic elemental peaks at approximately 532.35, 455.16, and 284.96 eV allocated to O 1s, Ti 2p, and C 1s, respectively ( Figure S3). The weak peak of F 1s at 685.1 eV in Ti 3 C 2 , indicates that some fluoride was left over from the synthesis process [17].
The TiO 2 @Ti 3 C 2 heterostructure exhibits a much stronger O 1s peak intensity when compared to Ti 3 C 2 , indicating the existence of an appreciable amount of oxide in the heterostructure due to the oxidation process and the formation of TiO 2 [4]. In the spectra results of composite Ag 3 PO 4 /TiO 2 @Ti 3 C 2 , the peaks of P 2p and Ag 3d represent Ag 3 PO 4 in the composite. Generally, Ag 3d peaks have a higher intensity than Ti 2p, suggesting that many silver elements or their compounds are present on the surface of the solid sample, decorating the TiO 2 @Ti 3 C 2 flower shape. This result is consistent with the SEM and TEM images discussed previously. Figure 5 demonstrates the Ti 2p, C 1s, and O 1s high-resolution spectra of Ti 3 C 2 , TiO 2 @Ti 3 C 2 , and Ag 3 PO 4 /TiO 2 @Ti 3 C 2 (sample A4). The Ti-C peak at 454.26 eV in Ti 2p spectra indicates the presence of Ti 3 C 2 ; however, there is no Ti-C peak in TiO 2 @Ti 3 C 2 [27,28]. Additionally, the intensification of the Ti-O peak (Ti 4+ ) at 458.83 eV indicates the formation of TiO 2 in the flower-shaped TiO 2 @Ti 3 C 2 . The Ti-C bond is absent in TiO 2 @Ti 3 C 2 in the high-resolution C 1s; however, C-C/C-H, C-O, and O-C = O bonds exist at approximately 284.88, 286.40, and 288.72 eV, respectively [28][29][30]. Owing to the use of TiO 2 @Ti 3 C 2 as a template for precipitate Ag 3 PO 4 , the Ti 2p and C 1s spectra of Ag 3 PO 4 /TiO 2 @Ti 3 C 2 (sample A4) are similar to those of TiO 2 @Ti 3 C 2 .
Nanomaterials 2022, 12, x 9 of 18 The binding energies of 366.83 and 372.91 eV assigned to Ag 3d5/2 and Ag 3d3/2, respectively, indicate that Ag + is dominated in the composites. The deconvoluted Ag 3d peaks are slightly shifted toward lower binding energy, and the FHMW is also wider. This is attributed to self-corrosion after exposure to the environment for a period of time [22]. The peak of the P 2p spectra located at 131.82 eV corresponds to P 5+ in the PO4 3+ [31].

Surface Area and Optical Analysis
The N2 adsorption-desorption isotherms and the corresponding BET surface areas, pore volumes, and pore sizes of TiO2@Ti3C2, Ag3PO4/P25, and Ag3PO4/TiO2@Ti3C2 are shown in Figure 6a and Table 1. Based on the isotherm curves, all samples belong to type IV, and the pore size indicates that they are mesopores [4]. As can be seen, the TiO2@Ti3C2 flowers have the highest BET surface area (53 m 2 g −1 ). After the deposition of Ag3PO4, the surface area and pore volume of the Ag3PO4/TiO2@Ti3C2 sample (40 m 2 g −1 ) decreased slightly but remained higher than that of Ag3PO4/TiO2 P25 (27 m 2 g −1 ). This high surface area enables photocatalytic reactions to occur. Nevertheless, the differences in O 1s XPS spectra among the three samples were unavoidable owing to the changing environment in which oxygen was present. Ti 3 C 2 exhibited a distinct peak attributed to C-Ti-OH linkage at 530.72 and 532.59 eV, whereas the others showed the strongest Ti-O peak at 529.60 eV. Furthermore, Ti 3 C 2 MXene also exhibited F 1s spectra ( Figure S4a), which could be ascribed to residual fluorine after the etching steps. The Ag 3d and P 1s spectra of Ag 3 PO 4 /TiO 2 @Ti 3 C 2 (sample A4) are presented in Figure S4b,c. The binding energies of 366.83 and 372.91 eV assigned to Ag 3d 5/2 and Ag 3d 3/2 , respectively, indicate that Ag + is dominated in the composites. The deconvoluted Ag 3d peaks are slightly shifted toward lower binding energy, and the FHMW is also wider. This is attributed to self-corrosion after exposure to the environment for a period of time [22]. The peak of the P 2p spectra located at 131.82 eV corresponds to P 5+ in the PO 4 3+ [31].

Surface Area and Optical Analysis
The N 2 adsorption-desorption isotherms and the corresponding BET surface areas, pore volumes, and pore sizes of TiO 2 @Ti 3 C 2 , Ag 3 PO 4 /P25, and Ag 3 PO 4 /TiO 2 @Ti 3 C 2 are shown in Figure 6a and Table 1. Based on the isotherm curves, all samples belong to type IV, and the pore size indicates that they are mesopores [4]. As can be seen, the TiO 2 @Ti 3 C 2 flowers have the highest BET surface area (53 m 2 g −1 ). After the deposition of Ag 3 PO 4 , the surface area and pore volume of the Ag 3 PO 4 /TiO 2 @Ti 3 C 2 sample (40 m 2 g −1 ) decreased slightly but remained higher than that of Ag 3 PO 4 /TiO 2 P25 (27 m 2 g −1 ). This high surface area enables photocatalytic reactions to occur.  The light-harvesting capability is important for evaluating photocatalytic activity as demonstrated by UV-Vis absorption spectra in Figure 6b. Owing to its metallic properties, Ti3C2 MXene exhibits a nearly horizontal spectrum over the entire wavelength range. The sequential transformations of MXene (metallic material) to the TiO2@Ti3C2 heterostructure (semiconductor material) throughout the processes resulted in the gradual development of the TiO2@Ti3C2 absorption peak [4,16,17]. Its light absorption is improved compared with that of the commercial TiO2 P25.
It can be seen that the decoration of Ag3PO4 on the flower-shaped TiO2@Ti3C2 extends the optical absorption by the combination of Ag3PO4 and TiO2@Ti3C2. Due to the high surface area of the 3D nanoflower structure, the harvested and scattered light is omnidirectional. In this case, the forward and backward light results in constructive interference, which can extend the photon lifetime and improve the absorbance [16]. These findings further confirm that the Ag3PO4-deposited TiO2@Ti3C2 composite can enhance the lightharvesting ability, making it a promising photocatalyst under visible illumination. Figure 6. (a) BET surface area plot of TiO 2 @Ti 3 C 2 , Ag 3 PO 4 /P25, and Ag 3 PO 4 /TiO 2 @Ti 3 C 2 ; (b) UV-Vis DRS spectra of the as-prepared samples, including pristine TiO 2 P25, TiO 2 @Ti 3 C 2 , pure Ag 3 PO 4 , and Ag 3 PO 4 /TiO 2 @Ti 3 C 2 (sample A4). Table 1. Specific surface area, pore volume, and pore size distribution of TiO 2 @Ti 3 C 2 , Ag 3 PO 4 /TiO 2 P25, and Ag 3 PO 4 /TiO 2 @Ti 3 C 2 (sample A4).

Sample
BET Surface Area (m 2 g −1 ) Pore Volume (cm 3 g −1 ) Pore Size (nm) The light-harvesting capability is important for evaluating photocatalytic activity as demonstrated by UV-Vis absorption spectra in Figure 6b. Owing to its metallic properties, Ti 3 C 2 MXene exhibits a nearly horizontal spectrum over the entire wavelength range. The sequential transformations of MXene (metallic material) to the TiO 2 @Ti 3 C 2 heterostructure (semiconductor material) throughout the processes resulted in the gradual development of the TiO 2 @Ti 3 C 2 absorption peak [4,16,17]. Its light absorption is improved compared with that of the commercial TiO 2 P25.
It can be seen that the decoration of Ag 3 PO 4 on the flower-shaped TiO 2 @Ti 3 C 2 extends the optical absorption by the combination of Ag 3 PO 4 and TiO 2 @Ti 3 C 2 . Due to the high surface area of the 3D nanoflower structure, the harvested and scattered light is omnidirectional. In this case, the forward and backward light results in constructive interference, which can extend the photon lifetime and improve the absorbance [16]. These findings further confirm that the Ag 3 PO 4 -deposited TiO 2 @Ti 3 C 2 composite can enhance the light-harvesting ability, making it a promising photocatalyst under visible illumination.
3.5. Photocatalytic Performance 3.5.1. The Effects of Ag 3 PO 4 Content on the Photodegradation of Rhodamine B Figure 7a illustrates the photocatalytic degradation of RhB (9 mgL −1 ) using Ag 3 PO 4 , TiO 2 and Ag 3 PO 4 /TiO 2 @Ti 3 C 2 composites (samples A1, A2, A3, A4, and A5) at a concentration of 0.5 gL −1 under solar light irradiation. Overall, the composite exhibited effective photodegradation reaction under solar light irradiation in a short period of time, with sample A4 displaying significant enhanced photocatalytic performance by demonstrating 97% RhB degradation within 12 min. Nanomaterials 2022, 12, x 11 of 18 partially degrade 28.9% and 59.9% RhB, respectively. Although sample A5 contains more Ag3PO4 than sample A4, there was no significant difference in the photocatalytic activities between samples A5 and A4. When comparing degradation performances within 20 min, pure Ag3PO4 had a degradation efficiency of 95% and Ag3PO4/TiO2 P25 86%. The lower photocatalytic performance of Ag3PO4/TiO2 P25 is most likely due to self-corrosion in the composite after prolonged storage [22]. Figure 7b shows the calculated rate constant (k) values of 0.017, 0.046, 0.161, 0.289, and 0.278; 0.223; 0.095 min −1 ; and 0.0093 min −1 for samples A1-A5, Ag3PO4, Ag3PO4/TiO2 P25, and flower-like TiO2@Ti3C2, respectively. These results indicate that, among the asprepared samples, sample A4 exhibited the greatest photocatalysis performance. These results indicate that combining TiO2 derived from MXene with Ag3PO4 can significantly improve its photocatalytic performance when exposed to solar illumination.

Photodegradation of Other Organic Dyes
The three dyes, MB, CV, and MO (10 mg L −1 , 0.5 gL −1 ) were also photodegraded under solar light irradiation. The adsorption and photocatalytic performance of sample A4 over time for the three organic dyes are shown in Figures 8 and S5. The results indicate that the composite is effective in adsorbing cationic dyes (MB and CV) but has no effect on anionic dye concentration (MO) after 15 min in the dark. It can be attributed to the composite's surface, which is anionic and negatively charged in the aqueous solution, which enables it to readily attract and adsorb cationic dyes in comparison to its anionic dye counterpart [32]. These results aid in the cationic dyes' molecules more easily reaching the surface of materials, leading to higher degradation performance in MB and CV compared to MO. After 6 min, the photocatalyst degraded 94.4% of MB, nearly 99.2% CV in 14 min, and 92.4% MO after 40 min. The pseudo-first-order rate constants for MB, CV, and MO were calculated to be 0.489, 0.279, and 0.073 min −1 , respectively. As samples A1 and A2 contain less Ag 3 PO 4 and may be susceptible to self-corrosion and photocorrosion, as discussed in the XRD diffractogram, these samples took 20 min to partially degrade 28.9% and 59.9% RhB, respectively. Although sample A5 contains more Ag 3 PO 4 than sample A4, there was no significant difference in the photocatalytic activities between samples A5 and A4. When comparing degradation performances within 20 min, pure Ag 3 PO 4 had a degradation efficiency of 95% and Ag 3 PO 4 /TiO 2 P25 86%. The lower photocatalytic performance of Ag 3 PO 4 /TiO 2 P25 is most likely due to self-corrosion in the composite after prolonged storage [22]. Figure 7b shows the calculated rate constant (k) values of 0.017, 0.046, 0.161, 0.289, and 0.278; 0.223; 0.095 min −1 ; and 0.0093 min −1 for samples A1-A5, Ag 3 PO 4 , Ag 3 PO 4 /TiO 2 P25, and flower-like TiO 2 @Ti 3 C 2 , respectively. These results indicate that, among the asprepared samples, sample A4 exhibited the greatest photocatalysis performance. These results indicate that combining TiO 2 derived from MXene with Ag 3 PO 4 can significantly improve its photocatalytic performance when exposed to solar illumination.

Photodegradation of Other Organic Dyes
The three dyes, MB, CV, and MO (10 mg L −1 , 0.5 gL −1 ) were also photodegraded under solar light irradiation. The adsorption and photocatalytic performance of sample A4 over time for the three organic dyes are shown in Figures 8b and S5. The results indicate that the composite is effective in adsorbing cationic dyes (MB and CV) but has no effect on anionic dye concentration (MO) after 15 min in the dark. It can be attributed to the composite's surface, which is anionic and negatively charged in the aqueous solution, which enables it to readily attract and adsorb cationic dyes in comparison to its anionic dye counterpart [32]. These results aid in the cationic dyes' molecules more easily reaching the surface of materials, leading to higher degradation performance in MB and CV compared to MO. After 6 min, the photocatalyst degraded 94.4% of MB, nearly 99.2% CV in 14 min, and 92.4% MO after 40 min. The pseudo-first-order rate constants for MB, CV, and MO were calculated to be 0.489, 0.279, and 0.073 min −1 , respectively. Compared with TiO2@Ti3C2, pristine Ag3PO4, and Ag3PO4/TiO2 P25, the Ag3PO4/TiO2@Ti3C2 composite exhibited superior photocatalytic activity. Recent publications on photocatalytic materials indicate the possibility of a highly promising photocatalyst for flower shaped TiO2@Ti3C2 with a high surface area. Combining TiO2@Ti3C2 with Ag3PO4 results in synergetic photodegradation of various dyes. The flower-like structure of TiO2 derived from Ti3C2 can improve its light-harvesting ability, and its high specific surface area allows the solvent to approach the reactive sites more easily.
Additionally, Ag3PO4 in the composites can shorten the diffusion paths for photoexcited carriers, thereby, inhibiting the recombination of electron-hole pairs [4,25]. The photocatalytic activity of the optimized sample was compared to that of other reported similar photocatalysts. As demonstrated in Table 2, the Ag3PO4/TiO2@Ti3C2 composite (sample A4) can degrade a variety of pollutants, including both cationic and anionic dyes, in a relatively short period of time, as compared to the state-of-the-art works listed in Table 2.  Compared with TiO 2 @Ti 3 C 2 , pristine Ag 3 PO 4 , and Ag 3 PO 4 /TiO 2 P25, the Ag 3 PO 4 /TiO 2 @Ti 3 C 2 composite exhibited superior photocatalytic activity. Recent publications on photocatalytic materials indicate the possibility of a highly promising photocatalyst for flower shaped TiO 2 @Ti 3 C 2 with a high surface area. Combining TiO 2 @Ti 3 C 2 with Ag 3 PO 4 results in synergetic photodegradation of various dyes. The flower-like structure of TiO 2 derived from Ti 3 C 2 can improve its light-harvesting ability, and its high specific surface area allows the solvent to approach the reactive sites more easily.
Additionally, Ag 3 PO 4 in the composites can shorten the diffusion paths for photoexcited carriers, thereby, inhibiting the recombination of electron-hole pairs [4,25]. The photocatalytic activity of the optimized sample was compared to that of other reported similar photocatalysts. As demonstrated in Table 2, the Ag 3 PO 4 /TiO 2 @Ti 3 C 2 composite (sample A4) can degrade a variety of pollutants, including both cationic and anionic dyes, in a relatively short period of time, as compared to the state-of-the-art works listed in Table 2.

Scavenger Trapping and Recycling Tests of RhB Degradation
To ascertain the main active species in the photocatalytic reactions, scavenger trapping tests were conducted on RhB photodegradation over the Ag 3 PO 4 /TiO 2 @Ti 3 C 2 composite (sample A4). The three types of scavengers were tert-butanol (a quencher of •OH), Na 2 -EDTA (a quencher of h + ), and p-benzoquinone (p-BQ, a quencher of •O 2− ). The dye solution containing a specified amount of catalyst was added to 2 mL of trapping agent (0.01 M), stirred for 15 min in the dark, and exposed to solar light for 20 min. As shown in Figure 9a, the addition of tert-butanol had no effect on the photocatalytic performance, thereby, indicating that free •OH radicals were not the dominant oxidizing species. The presence of Na 2 -EDTA and p-BQ, however, decreased the degradation efficiency to 4.76% and 21.7%, respectively. From these results, it can be concluded that h + and •O 2 − radicals play a critical role in the photodecomposition of RhB.

Scavenger Trapping and Recycling Tests of RhB Degradation
To ascertain the main active species in the photocatalytic reactions, scavenger trapping tests were conducted on RhB photodegradation over the Ag3PO4/TiO2@Ti3C2 composite (sample A4). The three types of scavengers were tert-butanol (a quencher of •OH), Na2-EDTA (a quencher of h + ), and p-benzoquinone (p-BQ, a quencher of •O 2− ). The dye solution containing a specified amount of catalyst was added to 2 mL of trapping agent (0.01 M), stirred for 15 min in the dark, and exposed to solar light for 20 min. As shown in Figure 9a, the addition of tert-butanol had no effect on the photocatalytic performance, thereby, indicating that free •OH radicals were not the dominant oxidizing species. The presence of Na2-EDTA and p-BQ, however, decreased the degradation efficiency to 4.76% and 21.7%, respectively. From these results, it can be concluded that h + and •O2 − radicals play a critical role in the photodecomposition of RhB. Along with the photocatalytic efficiency, stability evaluations of photocatalysts are necessary for practical use. Figure 9b shows the RhB degradation efficiency of sample A4 Along with the photocatalytic efficiency, stability evaluations of photocatalysts are necessary for practical use. Figure 9b shows the RhB degradation efficiency of sample A4 over three consecutive recycling runs. The degradation efficiency was significantly reduced after repeated exposure to solar light, indicating that the photocatalyst was not stable and rapidly deteriorated due to the photocorrosion of Ag 3 PO 4 NPs [22]. In the first run, a degradation efficiency of 97% was obtained in 12 min, whereas the degradation efficiencies of RhB were approximately 74% and 39% after the second and third runs, respectively. After a few recycling tests under solar light, the photocorrosion of the catalyst resulted in a low photodecomposition rate by forming an uncontrolled amount of Ag 0 , which can agglomerate and hinder the photocatalytic ability [31].

Proposed Photocatalytic Mechanism
Based on the obtained scavenger trapping and recycling results, the photocatalytic mechanism of Ag 3 PO 4 /TiO 2 @Ti 3 C 2 is schematically shown in Figure 10. Under solar illumination, electrons from the valence band (VB) of Ag 3 PO 4 could be excited to its conduction band (CB), resulting in the formation of holes in the valence band (VB). Since the E VB of TiO 2 is more negative than that of Ag 3 PO 4 , holes from Ag 3 PO 4 also transferred to TiO 2 and directly degraded RhB, which was absorbed on the surface of TiO 2 during the oxidization process [25,31]. According to literature, the conduction potential of Ag 3 PO 4 is higher than the activation energy of single-electron oxygen, preventing photogenerated electrons from being captured by dissolved oxygen.
Based on this proposed mechanism, the stability of the composite did not significantly improve. This might be due to a longer electron transfer distance between TiO2 and Ti3C2 than between Ag3PO4 and Ti3C2. Second, an unknown amount of Ti3C2 on the surface and/or insufficient interfacial contact between Ag3PO4 and Ti3C2 might lower the composite's stability. Both reasons could not prevent the rapid formation of Ag, reducing the stability and photocatalytic performance of the composite.

Conclusions
This study successfully synthesized Ag3PO4/TiO2@Ti3C2 composites by precipitating Ag3PO4 NPs on the surface of TiO2@Ti3C2 flowers. Some characterization methods were conducted to investigate the surface morphology, structural composition, surface area, and optical properties of Ag3PO4/TiO2@Ti3C2. The characteristics of Ag3PO4-deposited TiO2@Ti3C2 endow superior photocatalytic activities in the comparison with TiO2@Ti3C2, pristine Ag3PO4, and Ag3PO4/TiO2 P25, particularly sample A4. This composite exhibited excellent photocatalytic performance for various organic dyes (degraded 97% RhB, 94% MB, 99% CV, and 92% MO) within a short period of time when exposed to solar light irradiation. Figure 10. A schematic illustration of the proposed photocatalytic mechanism of Ag 3 PO 4 /TiO 2@ Ti 3 C 2 (A4) exposed to solar light.
Instead, a small amount of Ag is formed as a result of photocorrosion [31]. Electrons and holes in Ag could also be excited by light energy, with electrons migrating to the CB of TiO 2 , while the remaining holes could be recombined with photoexcited electrons in Ag 3 PO 4 CB, thereby, partially preventing further corrosion [31]. Furthermore, some electrons were transferred from TiO 2 and Ag 3 PO 4 to Ti 3 C 2 MXene, resulting in band bending and the formation of a Schottky junction as well as a uniform Fermi level [4,17,39]. The photo-reduction reaction occurring on the surface of Ti 3 C 2 and the TiO 2 conduction band generated •O 2 − radicals, which are required for effective RhB decomposition. The production of •O 2 − and h + could easily degrade a variety of dyes, including RhB, MB, CV, and MO, under solar light. The proposed mechanism is consistent with scavenger trapping experiments and is formulated as follows:

1.
Photo-oxidation reaction: Based on this proposed mechanism, the stability of the composite did not significantly improve. This might be due to a longer electron transfer distance between TiO 2 and Ti 3 C 2 than between Ag 3 PO 4 and Ti 3 C 2 . Second, an unknown amount of Ti 3 C 2 on the surface and/or insufficient interfacial contact between Ag 3 PO 4 and Ti 3 C 2 might lower the composite's stability. Both reasons could not prevent the rapid formation of Ag, reducing the stability and photocatalytic performance of the composite.

Conclusions
This study successfully synthesized Ag 3 PO 4 /TiO 2 @Ti 3 C 2 composites by precipitating Ag 3 PO 4 NPs on the surface of TiO 2 @Ti 3 C 2 flowers. Some characterization methods were conducted to investigate the surface morphology, structural composition, surface area, and optical properties of Ag 3 PO 4 /TiO 2 @Ti 3 C 2 . The characteristics of Ag 3 PO 4 -deposited TiO 2 @Ti 3 C 2 endow superior photocatalytic activities in the comparison with TiO 2 @Ti 3 C 2 , pristine Ag 3 PO 4 , and Ag 3 PO 4 /TiO 2 P25, particularly sample A4. This composite exhibited excellent photocatalytic performance for various organic dyes (degraded 97% RhB, 94% MB, 99% CV, and 92% MO) within a short period of time when exposed to solar light irradiation.
The high photocatalytic activity of the composite was due to the high surface area of TiO 2 @Ti 3 C 2 with small Ag 3 PO 4 particles (4-10 nm) that can easily reach dye molecules. The e − /h + transfer throughout the composite system also contributes to increased charging transfer, expanded light absorption wavelength, and decreased electron-hole pair recombination, which gives synergistic effects for photocatalytic activity. Based on the scavenger trapping and recycling test results, the mechanism was proposed.