Synergistically Enhanced Photocatalytic Degradation by Coupling Slow-Photon Effect with Z-Scheme Charge Transfer in CdS QDs/IO-TiO2 Heterojunction

Lower light absorption and faster carrier recombination are significant challenges in photocatalysis. This study introduces a novel approach to address these challenges by anchoring cadmium sulfide quantum dots (CdS QDs) on inverse opal (IO)-TiO2, which increases light absorption and promotes carriers’ separation by coupling slow-photon effect with Z-scheme charge transfer. Specifically, the IO-TiO2 was created by etching a polystyrene opal template, which resulted in a periodic structure that enhances light absorption by reflecting light in the stop band. The size of CdS quantum dots (QDs) was regulated to achieve appropriate alignment of energy bands between CdS QDs and IO-TiO2, promoting carrier transfer through alterations in charge transfer modes and resulting in synergistic-amplified photocatalysis. Theoretical simulations and electrochemical investigations demonstrated the coexistence of slow-photon effects and Z-scheme transfer. The system’s photodegradation performance was tested using rhodamine B as a model. This novel hierarchical structure of the Z-scheme heterojunction exhibits degradability 7.82 and 4.34 times greater than pristine CdS QDs and IO-TiO2, respectively. This study serves as a source of inspiration for enhancing the photocatalytic capabilities of IO-TiO2 and broadening its scope of potential applications.


Introduction
The rapid industrial development observed in recent decades has been accompanied by widespread environmental contamination, urgently requiring green and efficient solutions. Photocatalytic degradation, an emerging technology, offers a facile, controllable, and environmentally friendly approach when compared to traditional techniques such as adsorption, precipitation, and reverse osmosis [1,2]. This solar to chemical energy conversion is a process that harnesses sunlight and converts it into chemical energy, which can be stored and utilized for various applications. This conversion process involves the capture of solar energy through photovoltaic and its subsequent utilization to drive chemical reactions that store energy in the form of chemical bonds. Solar to chemical energy conversion plays a crucial role in achieving sustainable and renewable energy systems, as it allows for the efficient utilization of abundant and clean solar energy resources to produce storable and transportable chemical fuels. The above advantages are also highly attractive because it does not require additional energy [3][4][5], making it an increasingly popular choice in recent years [6][7][8].
The cornerstone of photocatalysis is photocatalytic material. In 1972, Fujishima et al. first reported TiO 2 as electrodes for photocatalytic water splitting [6]. Since then, TiO 2 has been intensively explored as a promising photocatalytic material due to its excellent chemical stability, durability, strong tenability, environmental compatibility, and cost effectiveness [9][10][11]. However, TiO 2 has two notable limitations. Firstly, its large bandgap (3.2
Polystyrene (PS) opal colloidal crystals were used as sacrificial templates to construct inverse opal TiO 2 (IO-TiO 2 ). However, the photocatalytic ability of the IO structure alone was insufficient, which led us to introduce a self-built-in electric field in IO-TiO 2 to maximize its photocatalytic efficiency. In the context of photocatalysis, a self-built-in electric field refers to the generation of an electric field within a photocatalytic material without the need for external bias or applied voltage. This electric field arises as a result of the spatial distribution of charge carriers and the energy band alignment within the material. The self-built-in electric field plays a crucial role in enhancing the separation and migration of photo-generated charge carriers, namely electrons and holes. It creates a driving force that promotes the movement of these charge carriers toward their respective reaction sites, facilitating efficient redox reactions and minimizing recombination losses. The generation of a self-built-in electric field is often achieved through the design of heterojunctions or interfaces between different semiconducting materials. Generally, Type II heterojunction is a common and facile way to construct a self-built-in electric field [40][41][42]. It enhances spatially separating photogenerated electron-hole carriers through band alignment between two semiconductors [43]. Additionally, inside the type II heterojunction, the photogenerated electrons (or holes) could migrate from the higher potential to the lower one without any extra bias [44,45]. This distinguished performance makes it very suitable for biomaterials analysis. However, type II heterojunction also exhibits several problems when used as photocatalysts. The inherent electrostatic repulsion between the same charges makes it difficult for electrons or holes to migrate to the electron-rich conduction band (CB) or the hole-rich valence band (VB) [46,47]. Moreover, the lower reduction (oxidation) potential of type II heterojunctions seriously limits their redox ability in practical applications.
Inspired by plant photosynthesis, a new electron migration route exists in the chloroplast through photosystems I and II (PS I and PS II), which has perfectly solved defects of type II heterojunction. This electron migration route looks like the letter "Z", thus denoted as the "Z-scheme" [48][49][50]. In the Z-scheme, unlike the electron migration in type-II heterojunction, photo-generated electrons can transfer from semiconductor A's CB to semiconductor B's VB (not from the CB to CB) due to the electrostatic attraction between the electron and hole [51,52]. This Z-scheme heterojunction not only preserves the efficient photo-generated carriers' spatial separation but also retains the heterojunction with a higher redox potential, greatly enhancing its photocatalytic performance [53][54][55]. Semiconductor quantum dots (QDs) have recently gained attention in photocatalysis due to their unique properties, low cost, and effectiveness in light harvesting and charge separation [56][57][58]. In this undertaking, cadmium sulfide quantum dots (CdS QDs), a direct-band semi-conductor with a narrow bandgap, were selected and used to enhance the photocatalytic ability of IO-TiO 2 . The size of quantum dots is a critical factor that determines their properties. Generally, as the size of CdS QDs decreases, the bandgap energy increases, leading to a redshift in the absorption spectrum towards the visible light region. This allows CdS QDs to effectively utilize a broader range of solar irradiation for photocatalytic reactions. Veerathangam [59][60][61]. Overall, the size-dependent properties of CdS QDs make them promising candidates for photocatalytic applications, and further research is being conducted to optimize their size and explore their full potential in various photocatalytic processes. By adjusting the reaction time, CdS QDs with various sizes (ranging from 2.7-4.7 nm) were prepared to obtain the optimal CdS QDs/IO-TiO 2 heterojunction. To validate the photocatalytic performance of this heterojunction, we selected rhodamine B (RhB), a highly toxic heterocyclic dye with wide industrial application, as a model pollutant. The resulting photocatalysts successfully combined the slow-photon effect of the IO structure with the Z-scheme charge migration mechanism, resulting in improved photocatalytic performance. Compared with pristine CdS QDs and IO-TiO 2 , 4.7 nm of CdS QDs/IO-TiO 2 photocatalyst exhibits promising photocatalysis degradation, and its photodegradable performance was 7.82 and 4.34 times higher, respectively. This study provides a constructive approach to optimize Z-scheme heterojunctions for efficient photocatalytic reactions.

Characterizations of As-Prepared IO-TiO 2
The preparation process of hierarchical IO-TiO 2 is illustrated in Figure 1a, and the CdS QDs loaded IO-TiO 2 are shown in Figure 1b. The detailed characterizations of IO-TiO 2 are illustrated in Figure 2. The SEM image shows that the synthesized PS microspheres diameter is ca. 260 nm. In addition, the prepared PS colloidal crystal template is closepacked with a highly ordered, face-centered cubic arrangement ( Figure 2a). Then, the TiO 2 precursor penetrates the PS template, and the PS template is removed through calcination. Finally, a hierarchical IO-TiO 2 structure was obtained, as presented in Figure 2b. their properties. Generally, as the size of CdS QDs decreases, the bandgap energy creases, leading to a redshift in the absorption spectrum towards the visible light reg This allows CdS QDs to effectively utilize a broader range of solar irradiation for ph catalytic reactions. Veerathangam et al. significantly improved photoelectrochemical version efficiency by utilizing different-sized CdS QDs loaded onto the surface of titan dioxide. Zhu et al. investigated the fluorescence effects of CdTe/CdS core-shell QDs w different sizes. Li et al. utilized CdS QDs with different sizes to enhance the photoelec chemical conversion efficiency, which was applied in photoelectrochemical sensing 61]. Overall, the size-dependent properties of CdS QDs make them promising candid for photocatalytic applications, and further research is being conducted to optimize t size and explore their full potential in various photocatalytic processes. By adjusting reaction time, CdS QDs with various sizes (ranging from 2.7-4.7 nm) were prepare obtain the optimal CdS QDs/IO-TiO2 heterojunction. To validate the photocatalytic formance of this heterojunction, we selected rhodamine B (RhB), a highly toxic heter clic dye with wide industrial application, as a model pollutant. The resulting photoc lysts successfully combined the slow-photon effect of the IO structure with the Z-sch charge migration mechanism, resulting in improved photocatalytic performance. C pared with pristine CdS QDs and IO-TiO2, 4.7 nm of CdS QDs/IO-TiO2 photocatalyst hibits promising photocatalysis degradation, and its photodegradable performance 7.82 and 4.34 times higher, respectively. This study provides a constructive approac optimize Z-scheme heterojunctions for efficient photocatalytic reactions.

Characterizations of as-Prepared IO-TiO2
The preparation process of hierarchical IO-TiO2 is illustrated in Figure 1a, and CdS QDs loaded IO-TiO2 are shown in Figure 1b. The detailed characterizations of TiO2 are illustrated in Figure 2. The SEM image shows that the synthesized PS mi spheres diameter is ca. 260 nm. In addition, the prepared PS colloidal crystal templa close-packed with a highly ordered, face-centered cubic arrangement (Figure 2a). T the TiO2 precursor penetrates the PS template, and the PS template is removed thro calcination. Finally, a hierarchical IO-TiO2 structure was obtained, as presented in  (XPS) and X-ray diffraction (XRD) characterizations were carried out. XPS spectra the existence of Ti2p and O1s (Figure 2c). Ti 2p3/2 and 2p1/2 of Ti 4+ had corresponding ing energies of 457.9 eV and 463.7 eV, respectively ( Figure 2d). The XPS spectrum o is three peaks (Figure 2e); this indicates three oxygen chemical states in the as-pre sample. These results validated pure composition without chemical changes. The pattern of prepared IO-TiO2 is highly analogous compared to the anatase TiO2 sta spectrum (JCPDS No: 65-0190) (Figure 2f).

Proof of the Slow-Photon Effect
The PBG wavelength (λ) of the inverse opal (IO) structure can be calculated foll Bragg's law Equation (1): where D is the IO-TiO2 aperture, n represents the refractive index, and f is the v percentage of TiO2, commonly taken as 0.26. θ is the light incident angle. When th incident angle is 0 o , the wavelength (λ) is only related to the pore size (D). UV-Vis DRS shows various Bragg reflection peak positions of as-prepared IO centered at 477 and 322 nm ( Figure 3a). To further prove the existence of PBG, w Optiwave OptiFDTD 7 to carry out the theoretical simulation ( Figure 3b). The fini ference time-domain (FDTD) method was employed to calculate the electromagneti distribution by solving Maxwell's equations. Figure 3b presents the simulated electr netic field intensity distribution. It is revealed that the enhanced electromagnetic fie localized in the proximity of the IO-TiO2 interfaces [64]. As shown, the as-obtained IO-TiO 2 aperture is ca. 205 nm with a skeleton thickness of 25 nm. This interconnected macropore hierarchical structure could promote the fast transport of photo-generated carriers [62,63]. Compared with the corresponding PS templates, the pore sizes exhibit~21% shrinkage, which is ineluctable in the calcination process. Nevertheless, it still formed highly ordered, crack-free micron scale inverse opal films ( Figure S1). To confirm this material's component, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) characterizations were carried out. XPS spectra show the existence of Ti2p and O1s (Figure 2c

Proof of the Slow-Photon Effect
The PBG wavelength (λ) of the inverse opal (IO) structure can be calculated following Bragg's law Equation (1): where D is the IO-TiO 2 aperture, n represents the refractive index, and f is the volume percentage of TiO 2 , commonly taken as 0.26. θ is the light incident angle. When the light incident angle is 0 • , the wavelength (λ) is only related to the pore size (D). UV-Vis DRS shows various Bragg reflection peak positions of as-prepared IO-TiO 2 centered at 477 and 322 nm (Figure 3a). To further prove the existence of PBG, we used Optiwave OptiFDTD 7 to carry out the theoretical simulation (Figure 3b). The finite-difference time-domain (FDTD) method was employed to calculate the electromagnetic field distribution by solving Maxwell's equations. Figure 3b presents the simulated electromagnetic field intensity distribution. It is revealed that the enhanced electromagnetic fields are localized in the proximity of the IO-TiO 2 interfaces [64]. Molecules 2023, 28, x FOR PEER REVIEW  ) also shows wide-ranging fluorescence peaks of CdS QDs, which are attributed to the semiconductor surface recombination of trapped charge carriers. Additionally, the quantum size effect causes fluorescence spectrum red shift. Furthermore, the increased CdS QDs diameter also causes the UV−Vis spectrum of CdS QDs (Figure 4f) to show well-defined absorption peaks at 408, 423, and 452 nm for 1 h, 2 h, and 3 h heating, respectively. As we know, the performance of QDs immensely depends on their size. The precise bandgaps of various size CdS QDs were calculated by Equation (2): Molecules 2023, 28, x FOR PEER REVIEW  After calculation, the precious bandgaps of various size CdS QDs are listed in Table 1. The corresponding bandgaps are 3.18 eV (408 nm), 3.05 eV (423 nm), and 2.82 eV (452 nm), respectively. After coupling IO-TiO 2 with CdS QDs, the smooth surface of IO-TiO 2 became rough (Figure 5a,b). This indicates that the skeleton of the inverse opal structure provides abundant spatial sites for fixing CdS QDs. As illustrated in Figure 5c

Characterizations of CdS QDs/IO-TiO2
After coupling IO-TiO2 with CdS QDs, the smooth surface of IO-TiO2 became r (Figure 5a,b). This indicates that the skeleton of the inverse opal structure provides a dant spatial sites for fixing CdS QDs. As illustrated in Figure 5c     traits. As is well known, the n-type semiconductor conduction band potential (E CB ) is approximately equal to E fb ; thus, the E CB of IO-TiO 2 and various CdS QDs can be calculated as −0.89, −2.24, −1.91, and −1.53 V (vs. Ag/AgCl), respectively. Based on these calculated results, the energy band schematic illustrations of IO-TiO 2 and various CdS QDs are elaborated in Figure 6e. The primary premise of constructing a Z-scheme heterojunction is matching band alignment; the CB of semiconductor A should be as near to semiconductor B's VB as possible [65]. The appropriate VB (~0.63 eV) of 4.7 nm CdS QDs is the nearest to the IO-TiO 2 CB, making it an optimal semiconductor to construct a Z-scheme heterojunction with IO-TiO 2 .

Band Structure of an Internal Electric Field
Molecules 2023, 28, x FOR PEER REVIEW equal to Efb; thus, the ECB of IO-TiO2 and various CdS QDs can be calculated as −0.89 −1.91, and −1.53 V (vs. Ag/AgCl), respectively. Based on these calculated results, t ergy band schematic illustrations of IO-TiO2 and various CdS QDs are elaborated ure 6e. The primary premise of constructing a Z-scheme heterojunction is matching alignment; the CB of semiconductor A should be as near to semiconductor B's VB a sible [65]. The appropriate VB (~0.63 eV) of 4.7 nm CdS QDs is the nearest to the IO CB, making it an optimal semiconductor to construct a Z-scheme heterojunction w TiO2.

Photocatalytic Mechanism and Verification
EPR spectroscopy was carried out to ascertain the generation of reactive oxyge cies (ROS). In this experiment, DMPO was selected as a spin-trapping reagent, and a DMPO-•OH signal can be observed in the dark. Upon photoexcitation, the charac peaks of DMPO-•OH (1:2:2:1) can be monitored for CdS QDs/IO-TiO2 methanol disp liquid (Figure 7a). This result indicates that the charge migration route followed a ard Z-scheme transfer mode. Photoluminescence (PL) spectra are commonly emplo investigate the efficiency of charge carrier trapping, migration, and transfer in sem ductor particles. PL spectra provide valuable insights into the fate of electron-hol as the PL emission originates from the recombination of free carriers. By analyz spectra, researchers can gain a better understanding of the behavior and dynam charge carriers in semiconductors, which is crucial for optimizing the performance ious optoelectronic and photonic devices. The intensity of the PL emission spectra termined by the recombination of excited electrons and holes. A lower PL emission sity indicates a lower recombination rate in the samples, suggesting more favora combination properties. This can be attributed to efficient charge carrier trapping, tion, or transfer processes, which minimize the non-radiative recombination pat and enhance overall photoluminescence efficiency. Thus, monitoring the PL emiss tensity provides valuable information about the recombination behavior. Figure S4 the PL spectra of IO-TiO2, CdS QDs, and CdS QDs/IO-TiO2. The excitation wave was set at 300 nm to initiate the absorption of light by the sample. As shown, the IO exhibits strong emission peaks in the wavelength ranges of 380 to 430 nm. After the duction of CdS QDs, a significant decrease in PL emission intensities was observed

Photocatalytic Mechanism and Verification
EPR spectroscopy was carried out to ascertain the generation of reactive oxygen species (ROS). In this experiment, DMPO was selected as a spin-trapping reagent, and a weak DMPO-·OH signal can be observed in the dark. Upon photoexcitation, the characteristic peaks of DMPO-·OH (1:2:2:1) can be monitored for CdS QDs/IO-TiO 2 methanol dispersion liquid (Figure 7a). This result indicates that the charge migration route followed a standard Z-scheme transfer mode. Photoluminescence (PL) spectra are commonly employed to investigate the efficiency of charge carrier trapping, migration, and transfer in semiconductor particles. PL spectra provide valuable insights into the fate of electron-hole pairs as the PL emission originates from the recombination of free carriers. By analyzing PL spectra, researchers can gain a better understanding of the behavior and dynamics of charge carriers in semiconductors, which is crucial for optimizing the performance of various optoelectronic and photonic devices. The intensity of the PL emission spectra is determined by the recombination of excited electrons and holes. A lower PL emission intensity indicates a lower recombination rate in the samples, suggesting more favorable recombination properties. This can be attributed to efficient charge carrier trapping, migration, or transfer processes, which minimize the non-radiative recombination pathways and enhance overall photoluminescence efficiency. Thus, monitoring the PL emission intensity provides valuable information about the recombination behavior. Figure S4 shows the PL spectra of IO-TiO 2 , CdS QDs, and CdS QDs/IO-TiO 2 . The excitation wavelength was set at 300 nm to initiate the absorption of light by the sample. As shown, the IO-TiO 2 exhibits strong emission peaks in the wavelength ranges of 380 to 430 nm. After the introduction of CdS QDs, a significant decrease in PL emission intensities was observed. In the heterostructures, the introduction of Au NPs and CdS nanolayers effectively suppressed the recombination of photogenerated electron-hole pairs. This can be observed by the decrease in photoluminescence (PL) emission intensities, indicating enhanced charge carrier trapping and reduced recombination processes. The presence of CdS QDs creates a favorable energy landscape for efficient charge transfer and separation, leading to improved photocatalytic performance. A schematic illustration of the detailed Z-scheme charge migration pathway in the heterojunction system is shown in Figure 7b. Under visible light irradiating, many photo-induced electrons (e − ) with enough energy could jump into the CB of IO-TiO 2 and CdS QDs, respectively. Then, the internal electric field could drive the electrons on the CB of IO-TiO 2 to combine with the holes on the VB of CdS QDs, which could form the Z-scheme electron migration route. Finally, the photo-generated electrons aggregated on the CdS QDs' CB and photo-generated holes accumulate on its VB. This electron transmission mechanism imparted superior redox ability to the Z-scheme system. Photocurrent measurement was also carried out to confirm the excellent photoelectric performance of CdS QDs/IO-TiO 2 . Figure 7c shows the electrochemical impedance spectroscopy (EIS) of pristine CdS QDS, IO-TiO 2 , and their composites. In comparison, CdS QDs/IO-TiO 2 exhibits the smallest semicircle, while CdS QDs exhibit the largest. The semicircle of CdS QDs/IO-TiO 2 is significantly decreased compared to that of pristine IO-TiO 2 and CdS QDs, indicating the Z-scheme mode can reduce the interfacial charge transfer resistance. The photocurrent response provides the same conclusion. All the tested materials show a light response feature when exposed to a 300 W Xe lamp. (Figure 7d). The highest photocurrent observed for CdS QDs/IO-TiO 2 is indicative of the most efficient separation and migration.
Molecules 2023, 28, x FOR PEER REVIEW heterostructures, the introduction of Au NPs and CdS nanolayers effectively supp the recombination of photogenerated electron-hole pairs. This can be observed by t crease in photoluminescence (PL) emission intensities, indicating enhanced charge trapping and reduced recombination processes. The presence of CdS QDs creates a able energy landscape for efficient charge transfer and separation, leading to imp photocatalytic performance. A schematic illustration of the detailed Z-scheme char gration pathway in the heterojunction system is shown in Figure 7b. Under visibl irradiating, many photo-induced electrons (e − ) with enough energy could jump in CB of IO-TiO2 and CdS QDs, respectively. Then, the internal electric field could dr electrons on the CB of IO-TiO2 to combine with the holes on the VB of CdS QDs, could form the Z-scheme electron migration route. Finally, the photo-generated ele aggregated on the CdS QDs' CB and photo-generated holes accumulate on its VB electron transmission mechanism imparted superior redox ability to the Z-scheme s Photocurrent measurement was also carried out to confirm the excellent photoelectr formance of CdS QDs/IO-TiO2. Figure 7c shows the electrochemical impedance sp copy (EIS) of pristine CdS QDS, IO-TiO2, and their composites. In comparison QDs/IO-TiO2 exhibits the smallest semicircle, while CdS QDs exhibit the largest. Th icircle of CdS QDs/IO-TiO2 is significantly decreased compared to that of pristine IO and CdS QDs, indicating the Z-scheme mode can reduce the interfacial charge tr resistance. The photocurrent response provides the same conclusion. All the tested rials show a light response feature when exposed to a 300 W Xe lamp. (Figure 7d highest photocurrent observed for CdS QDs/IO-TiO2 is indicative of the most efficie aration and migration.

Photocatalytic Activity
The investigation of CdS QDs/IO-TiO 2 as a heterogeneous photocatalyst for RhB degradation was conducted. The corresponding results are presented in Figure 8a. As depicted, RhB exhibits minimal degradation when exposed to single-component CdS QDs and IO-TiO 2 due to their poor charge separation. However, the degradation rate of CdS QDs/IO-TiO 2 composites is notably improved, reaching up to 85% (4.7 nm CdS QDs/IO-TiO 2 , 50 min). The UV-Vis absorption spectra and digital photographs are shown in Figures  S4 and S6. Additionally, the UV-Vis absorbance spectra of composites under dark adsorption are shown in Figure S5. These findings strongly support the Z-scheme migration mechanism contribution to the efficiency of the CdS QDs/IO-TiO 2 heterojunction system. To better understand detailed photocatalytic degradation efficiency, the corresponding reaction rate constant "k" is calculated by kinetic fitting reaction degradation curve (Figure 8b). The calculation details follow Equation (3): The investigation of CdS QDs/IO-TiO2 as a heterogeneous photocatalyst for RhB de radation was conducted. The corresponding results are presented in Figure 8a. As d picted, RhB exhibits minimal degradation when exposed to single-component CdS QD and IO-TiO2 due to their poor charge separation. However, the degradation rate of Cd QDs/IO-TiO2 composites is notably improved, reaching up to 85% (4.7 nm CdS QDs/IO TiO2, 50 min). The UV-Vis absorption spectra and digital photographs are shown in Fi ures S4 and S6. Additionally, the UV-Vis absorbance spectra of composites under da adsorption are shown in Figure S5. These findings strongly support the Z-scheme migr tion mechanism contribution to the efficiency of the CdS QDs/IO-TiO2 heterojunction sy tem. To better understand detailed photocatalytic degradation efficiency, the correspon ing reaction rate constant "k" is calculated by kinetic fitting reaction degradation curv (Figure 8b). The calculation details follow Equation (3): According to fitting results, the composite composed of 4.7 nm of CdS QDs demo strates the highest degradation rate, being 13 and 7.8 times higher than that of pristin CdS QDs (0.003 min −1 ) and IO-TiO2 (0.005 min −1 ), respectively (Figure 8c). Moreover, catalysts, it is essential to achieve high, stable catalytic activity. After the fourth recyclin test, the prepared photocatalysts still maintained a high level of photocatalytic activit indicating excellent reusability (Figure 8d). Additionally, the XRD pattern of the phot catalysts is shown in Figure S7, which reveals slight photo corrosion. It is worth notin that long-term exposure to light is inevitable.  According to fitting results, the composite composed of 4.7 nm of CdS QDs demonstrates the highest degradation rate, being 13 and 7.8 times higher than that of pristine CdS QDs (0.003 min −1 ) and IO-TiO 2 (0.005 min −1 ), respectively (Figure 8c). Moreover, as catalysts, it is essential to achieve high, stable catalytic activity. After the fourth recycling test, the prepared photocatalysts still maintained a high level of photocatalytic activity, indicating excellent reusability (Figure 8d). Additionally, the XRD pattern of the photocatalysts is shown in Figure S7, which reveals slight photo corrosion. It is worth noting that long-term exposure to light is inevitable.

Reagents and Apparatus
All reagents were purchased from commercial suppliers without further purification. Detailed information on the reagents and apparatus is available in Supplementary Materials.

Synthesis of PS Microspheres
PS spheres were synthesized according to the method in [54]. Firstly, 0.6 g potassium persulfate and 0.45 g sodium dodecyl sulfate were dissolved in 20 mL aqueous ethanol solution (50% v/v); then, the mixture was heated to 75 • C under a nitrogen atmosphere and 5 mL styrene was added. After continuous stirring for 19 h, the white emulsion (PS microspheres suspension) was obtained.

Assembly of Ordered PS Array Film Templates
The PS array film templates were assembled using the "evaporative deposition" method; a total of 500 µL of prepared PS sphere suspension was dispersed into 25 mL deionized water and sonicated for 30 min to ensure the dispersion was uniform. The cleaned ITO-coated glass slides (alternately, ultrasonically cleaned with ethanol, acetone, and deionized water for 5 min) were vertically immersed into the beaker and kept in a 75 • C oven. The hexagonal close-packed PS array film formed while the liquid was dried out.

Preparation of IO-TiO 2 Structure
In this work, IO-TiO 2 was prepared using an infiltration method. The prepared PS arrays film as sacrificial templates and are fabricated by filling the interstitial space between the spheres with the TiO 2 precursor. The details are as follows: The TiO 2 precursor, consisting of TiBALDH, 0.1M HCl, and ethanol (1:1:1.5 volume ratio), was stirred for 1 h at room temperature. Then, 10 µL precursor was deposited onto the as-prepared PS template and kept at 35 • C for 4 h. The electrodes were heated from room temperature to 450 • C at 1.0 • C·min −1 ramp rate and held for 2 h. Finally, the IO-TiO 2 structure was formed.

Synthesis of CdS QDs
Synthesizing CdS QDs was based on a previous study with a slight, rutile modification [55]. Firstly, add 104 µL TGA (98%) to CdCl 2 ·2.5H 2 O aqueous solution (34 mL, 0.175 mM) and thiourea (22 mg, 0.289 mmol). The molar ratio of Cd 2+ : thiourea: TGA was 1:1.7:2.3. Deaerate the mixture with nitrogen-bubbling for 30 min. Meanwhile, adjust the mixture to pH 10 with 1.0 M NaOH. Then, transfer into a Teflon-lined stainless steel autoclave, keeping the autoclave at 100 • C for a designed time and cooling to room temperature. Finally, purify the different sizes of CdS QDs three times using ethanol-water (v:v 1:4) and centrifugation at 10,000× g rpm for 5 min. The purified QDs were redispersed in water and stored at 4 • C for further use.

Preparation of CdS QDs/IO-TiO 2 Heterojunction
The as-prepared IO-TiO 2 ITO electrodes were immersed in the CdS QDs solution until they reached saturated adsorption. The whole process was carried out in a dry place, protected from light.

Measurement of Photocatalytic Activity
The CdS QDs/IO-TiO 2 heterojunction photocatalytic effect was examined using the degrading organic contaminant RhB. In this experiment, CdS QDs/IO-TiO 2 heterojunction photocatalyst (ca. 3 mg) was fixed on the ITO electrode and immersed in 30 mL RhB aqueous solution (ca. 50 mg·L −1 ). Then, this solution mixture was continuously agitated in the dark for 5 min to achieve saturation adsorption. A 300 W Xenon lamp (a cut-off wavelength of~400-800 nm with 0.15 mW·cm −2 light intensity) recorded the photocatalytic responses. The mixture was sampled at certain intervals and then filtered through a 0.22 µm filter before UV spectrophotometer analysis. The digital photograph of the experimental setup is shown in Figure S8.

Conclusions
In this study, we have demonstrated the adaptability of the IO structure for further modifications such as the incorporation of quantum dots. By adjusting the size of CdS QDs loaded onto the IO-TiO 2 surface, we achieved excellent synergistic photochemical amplification of the slow-photon effect and the Z-scheme charge transfer in CdS QDs/IO-TiO 2 . Herein, an extensive study has been conducted on the photoelectric properties, morphology, and structural characteristics of this material to better understand the enhanced mechanism. Thanks to this synergy, the novel hierarchical structure of the Z-scheme heterojunction exhibits degradability 7.82 and 4.34 times greater than pristine CdS QDs and IO-TiO 2 , respectively. This proof of concept allows for integrating various physical or chemical enhancements, making it valuable in photochemistry and related fields.