Control of particle growth and enhancement of photoluminescence, adsorption efficiency, and photocatalytic activity for zinc sulfide and cadmium sulfide using CoAl-layered double hydroxide system

The fabrication of zinc sulfide (ZnS) and cadmium sulfide (CdS) hybrids was carried out by the sulfidization of Zn(II) or Cd(II) adsorbed in dodecylsulfate modified CoAl-layered double hydroxide through solid-liquid reaction. The TEM images showed the nanocrystals of ZnS (2.61 nm) or CdS (3.29 nm) orderly distributed on the nanosheets. The BET surface area of ZnS (1.13 m2/g) and CdS (0.78 m2/g) was largely improved by intercalating in the interlayer space of CoAl-layered double hydroxide system (15–20 m2/g). The spectroscopic observations further confirmed the formation of ZnS or CdS nanoparticles in the hybrid as the evidence of the blue-shifted absorption onset (39–44 nm), and the increase of the photoluminescence intensity (3–4 times) relative to those of bare ZnS and CdS. The nanohybrids could be applicable as the adsorbent and photocatalyst on purifying wastewater contaminated with Congo red dye. By the adsorptive removal, the hybrids exhibited the maximum adsorption capacity of 216–234 mg/g, resulting from the effect of CoAl-layered double hydroxide. In addition, the photocatalytic degradation was completely conducted by using CdS hybrid with the rate constant of 0.0115 min-1, because the host-guest and/or guest-guest interactions promoted the greater optical performance, and adsorption and photocatalytic efficiencies.


Introduction
Optical materials have been received much interest because their optical characteristics are tunable as controlling its size and shape, namely size-dependent optical properties, which are extensively applied on fabricating advanced technology and innovation . Zinc sulfide (ZnS) and cadmium sulfide (CdS) are a dominant metal sulfide classified as UV light and visible light-responsive semiconductor for many optical activities, especially photocatalyst (Chandrasekaran et al. 2019). Considering metal sulfides Chandrasekaran et al. 2019), ZnS and CdS nanoparticles are mostly chosen to hybridize with another inorganic material with tailoring photoluminescence intensity and performance for optical applications such as therapy, nano-optical device, and environmental activities (Khaorapapong et al. 2010;Kabilaphat et al. 2015;Intachai et al. 2017b;Zhou et al. 2018;Amani-Ghadim et al. 2019;José-Gil et al. 2020). In addition, the valence electrons of Zn(II) and Cd(II) are fulfilled in d orbital (d 10 ), which promotes high stability of its oxidation state and compound. However, the formation of bulk particles depreciates the optical material in terms of the decrease in fluorescence intensity and stability as well as the optical applications. Besides, the effects of high pressure and heat and strong acid-base solution can destroy their structure and properties (Chenglong et al. 2008;Trenczek-Zajac et al. 2019). As a result, the fabrication and development of ZnS and CdS nanoparticles based on the durable and high-fluorescent material must be investigated continuously. To date, light-responsive nanohybrids comprised of semiconductor and layered inorganic host have been intensively developed with the enhanced photoluminescence and high optical performance for long-term storage, and thermal stabilities (Intachai et al. 2017b;Poosimma et al. 2018). The optical properties, stabilities, adsorption, and photocatalytic activity of ZnS and CdS were improved by accommodating in the interlayer space of smectite and layered double hydroxide due to host-guest and/or guestguest interactions, (Khaorapapong et al. 2010;Kabilaphat et al. 2015;Zhou et al. 2018;Amani-Ghadim et al. 2019;José-Gil et al. 2020).
Layered double hydroxide (LDH) is a class of two-dimensional inorganic hosts, where the layered structure is generated by bearing of positive-charged brucite-type nanosheets and anions with an electrostatic interaction (Liu et al. 2006;Ogawa et al. 2014;Qiu et al. 2015). Many interesting attractions of LDH as a host material are large surface area, anion exchange capacity, swelling property, structural flexibility, and so on Karim et al. 2022). Various structures of LDH, [M II 1-x M III x (OH) 2 ] y+ ⋅y/n(A n-)⋅zH 2 O] can be considerably synthesized with varying divalent (M II ), trivalent (M III ), and interlayer anion (A n-), which show different intrinsic properties such as magnetic and optical features used as a potential catalyst, absorbent, supercapacitor, drug carrier, and so on (Qiu et al. 2015;Li et al. 2018;Zhou et al. 2018;José-Gil et al. 2020;Karim et al. 2022). Relative to montmorillonite and saponite (Kabilaphat et al. 2015;Intachai et al. 2017b) as well as other materials (Zhang et al. 2001;Yogamalar et al. 2015;Jia et al. 2020;Radoor et al. 2020), the ion exchange capacity of LDH is greater, which can accommodate ZnS and CdS nanoparticles in more amount, and possibly higher photoluminescence intensity. CoAl-LDH is mostly used as a host material because it can be prepared by many available routes with yielding large amount, and applied as efficient adsorbent and photocatalyst, in addition to Co(II) central atom which is environmental friendly (Liu et al. 2006;Ogawa et al. 2014;Qiu et al. 2015). However, the presence of the interlayer carbonate anion (CO 3 2-) is hard to substitute by any guest due to the strong affinity to positive-charged LDH nanosheet. To eliminate the interlayer CO 3 2-, many researches have been manipulated by the intercalation of dodecylsulfate (Liu et al. 2006;Ogawa et al. 2014), which has still been a supporting method for the intercalation of large-size and unstable guest species (Ruan et al. 2013;Demel et al. 2014).
Up to now, metal sulfide and LDH have been hybridized so far and mostly used as adsorbent and photocatalyst Zhou et al. 2018;Amani-Ghadim et al. 2019;José-Gil et al. 2020), where the photocatalytic role has received much interest and challenge due to the degradation of toxic organic structures Yaghoot-Nezhad et al. 2023). However, the CdS-and/or ZnS-based samples have not seriously reported on the photoluminescence, and were performed through complex method. To control the size and size distribution, and improve the optical performance and application of ZnS and CdS, the intercalation of ZnS or CdS in CoAl-LDH was conducted by the intercalation of Zn(II) or Cd(II) in the interlayer space of dodecylsulfate modified CoAl-LDH and reacted to sodium sulfide (Na 2 S), because it was found that the direct mixing of sulfide (an aqueous solution of Na 2 S) to an aqueous dispersion of CoAl-LDH produced the formation of Co x S y species with the obtained black precipitate (Jing et al. 2019). Additionally, their adsorptive and photocatalytic capacities were investigated by removing anionic Congo red dye in water due to its serious toxicity on living things (Hassani et al. 2020;Madihi-Bidgoli et al. 2021). The obtained hybrid materials may provide high removal efficiency of Congo red (anion) through simultaneous adsorption and photocatalytic processes by using CoAl-LDH with positively charged surface as the adsorbent and co-photocatalyst in the presence of visible light-driven ZnS or CdS nanocatalyst (Qiu et al. 2015;Chandrasekaran et al. 2019). This study may be an alternative approach to generate a smart nanohybrid material in the viewpoints of high efficiencies and stabilities on photoluminescence, and adsorption and photocatalytic activity.

Preparation of host material
The preparation of CoAl-LDH was conducted through a urea hydrolysis at about 97 °C, using the aqueous solution of CoCl 2 (11.8965 g), AlCl 3 (6.0358 g), and CO(NH 2 ) 2 (10.5096 g) with the molar ratio of 2:1:7, respectively. After the ultrasonication for 30 min, the aqueous solution of the precursors was mixed in which two-necked round bottom was capped with reflux system under magnetic stirring for 2 days through N 2 flowing. The obtained solid was separated by centrifugation, washed with deionized (DI) water until the formation of AgCl precipitate happened by dropping of AgNO 3 solution, then ethanol several times, and dried at 60 °C. The exchange of the interlayer CO 3 2by dodecylsulfate (DS) anion was conducted by mixing of an aqueous solution of sodium dodecylsulfate and an aqueous suspension of 1 3 CoAl-LDH under magnetic stirring for 10 min. The loading amount of DS was equal to 30 times of the theoretical AEC (300 meq/100 g) of the LDH. Subsequently, the resulting mixture was further reacted in the hydrothermal condition at 120 °C for 24 h. The host solid was collected by centrifugation, washing with DI water then ethanol several times, and dried at 60 °C, where the sample was labeled as DS/ CoAl-LDH.
By the experiment, the increase of DS amounts over 30 times of the AEC of LDH could not completely eliminate the interlayer CO 3 2and yielded less DS/CoAl-LDH. It was due to the difference in the chemical affinity, structure, and size between CO 3 2and DS to LDH. As a result, the loading amount of DS with 30 times of the AEC of LDH was optimized for replacing the interlayer CO 3 2in CoAl-LDH together with broadening the nanospace.

Formation of ZnS or CdS in DS/CoAl-LDH
The intercalation of ZnS or CdS in DS/CoAl-LDH was carried out by solid-liquid reaction at room temperature. An aqueous solution of ZnCl 2 or CdCl 2 and a DI water-ethanol (1:1 v/v) suspension of DS/CoAl-LDH were mixed under magnetic stirring for 10 min. The loading amount of Zn(II) or Cd(II) was equal to 100% of the AEC of the LDH (DS/ CoAl-LDH). Then an aqueous solution of Na 2 S was mixed for the sulfidization to Zn(II) or Cd(II) in DS/CoAl-LDH under magnetic stirring further 24 h. The molar ratio of Zn(II) or Cd(II) to sulfide (Na 2 S) was 1:1. The resulting hybrid solid was separated by centrifugation, washing with DI water several times to get rid of the excess species, then dried at 60 °C. The products were named as ZnS/CoAl-LDH@DS and CdS/CoAl-LDH@DS.

Adsorption and photocatalytic activities
Firstly, 0.01 g of the as-prepared samples was used to adsorb Congo red in water (50 mL 30 ppm) in the dark for 60 min under magnetic stirring. In addition, the amount of the adsorbents was varied from 0.01 to 0.05 g for achieving the dye removal, and the optimized adsorbent dosage was used to study the adsorption kinetic and isotherm by the corresponding equations in Table S1. Secondly, after the adsorption equilibrium (0.01 g adsorbent), the visible-light bulb (200-W tungsten halogen lamp) was turned on to explore the photodegradation of Congo red. At the reaction time interval, the supernatant was separated by centrifugation to measure the absorbance. The photocatalytic effectiveness was assessed as the equations in Table S1. On the reusability test, CdS/ CoAl-LDH@DS, representing as the adsorbent and photocatalyst, was rerun on removing Congo red in water

Congo red removal
Scheme 1 Schematic diagram for the synthesis of the hybrids and the utilization as adsorbent and photocatalyst until the third cycle. On each reusing, the spent sample was cleaned by mixing in 0.5M NaOH solution under magnetic stirring for 3 h, washed with DI water and ethanol several times, where the schematic diagram for the synthesis of the hybrids and the utilization as adsorbent and photocatalyst is represented in Scheme 1.

Intercalation of ZnS or CdS in the hybrid
The XRD patterns of ZnS/CoAl-LDH@DS, CdS/CoAl-LDH@DS, DS/CoAl-LDH, and CoAl-LDH are shown in Fig. 1, and the XRD data are listed in Table 1. In Fig. 1(a), the XRD pattern of the pristine CoAl-LDH showed the diffraction peaks at 2theta (2θ) = 11.74° (d = 0.75 nm) and 23.83° (d = 0.39 nm), corresponding to the (003) and (006) reflections of the LDH in the brucite structure, respectively, which confirmed the formation of CoAl-LDH intercalated with CO 3 2- (Liu et al. 2006). The intercalation of any guest in the nanospace of LDH can be verified by the expansion of the interlayer space, which is calculated by subtracting the thickness of LDH nanosheet (0.48 nm) from the resulting d 003 value. In Table 1, the expansion of the interlayer spaces of DS/CoAl-LDH was 0.27 and 2.05 nm, relating to the size of CO 3 2-, and hydrated DS, indicating the segregation of CO 3 2and DS in the nanospaces of CoAl-LDH with monolayer and paraffin arrangement, respectively (Demel et al. 2014;Ogawa et al. 2014). Considering Table 1, the calculated d 003 and interlayer expansion of the hybrids were reduced to 0.90 nm and 0.42 nm for ZnS/CoAl-LDH@ DS, and of 0.91 nm and 0.43 nm for CdS/CoAl-LDH@DS relative to those of DS/CoAl-LDH (2.53 nm and 2.05 nm). To prove, the products were heated at 200 °C for 2 h to evaporate the hydrated molecules, then analyzed by XRD technique. It was found that the XRD patterns of both heattreated hybrids (Table 1) were almost no change in comparison with the unheated one, where the d 003 was 0.90 nm for ZnS/CoAl-LDH@DS ( Fig. 1(c)) and 0.91 nm for CdS/ CoAl-LDH@DS ( Fig. 1(d)). This could be indicative of the interlayer expansion in the hybrids without water molecule. Besides, the rearrangement of the intercalated DS from paraffin (the interlayer expansion = 2.05 nm) to monolayer was also impossible because the nanospace of the hybrids (just 0.40 nm) was smaller than that of the head size of DS (0.58 nm) (Ruan et al. 2013). By the previous reports, the nanospace of montmorillonite was expanded by 0.26 nm and 0.29 nm for the intercalation of NiS and MnS, respectively (Khaorapapong et al. 2009) and other semiconductor nanoparticles (Khaorapapong et al. 2010;Intachai et al. 2017a). Besides, there was no appearance of any reflection peak of the as-prepared ZnS and CdS crystals (Fig. S1) in the XRD patterns of all products. Consequently, it was highly possible that ZnS or CdS nanoplates were intercalated in the interlayer space of CoAl-LDH hybrid.
After the sulfidization, the powder colors of ZnS/ CoAl-LDH@DS and CdS/CoAl-LDH@DS were into pale magenta and yellowish orange, respectively (Fig. 1). Relative to those of bare ZnS (white) and CdS (orange) (Fig. S1), and the samples without metal sulfide (Fig. 1), the change in the color of product powder might indicate the presence of ZnS or CdS in the hybrid with the difference in its particle size and amount. By the elemental analysis, the amounts of Zn(II) or Cd(II) in the product were found to be about 290 and 287 meq/100 g of the LDH that might confirm the presence of ZnS or CdS in the hybrid.
To interpret the guests based on the functional group in the hybrids, the FT-IR spectra of sodium dodecylsulfate, CoAl-LDH, DS/CoAl-LDH, ZnS/CoAl-LDH@DS, and CdS/CoAl-LDH@DS were investigated (Fig. S2). In  Fig. S2c, the FT-IR spectrum of DS/CoAl-LDH showed the additional absorption bands due to DS (Fig S2a) at 1470, 2859, and 2932, as well as 2950 cm −1 that were assigned to the CH 2 bending, and CH 2 asymmetric and symmetric stretching, as well as CH 3 stretching vibration modes, respectively (Liu et al. 2006;Ruan et al. 2013;Demel et al. 2014). Meanwhile, the FT-IR absorbance due to the interlayer CO 3 2at 1359 cm −1 was mostly decreased, indicating the substitution of most CO 3 2by DS (30 times of AEC). After the intercalation of ZnS or CdS in the hybrid, the spectra were almost no change in the functional groups based on the wavenumber (Figs. S2d and S2e) in comparison with DS/CoAl-LDH ( Fig. S2c) and sodium dodecylsulfate (Fig. S2a); however, the FT-IR absorbance (at 2859-2950 cm -1 ) due to the intercalated DS decreased. These observations were attributed to the presence of DS and very small amounts of CO 3 2in the products. As a result, the FT-IR and XRD results, as well as the resulting color of the product, could describe the intercalation of ZnS or CdS in the nanospaces of CoAl-LDH together with the interlayer CO 3 2-, where small DS amounts were captured on the external surface.
It has been extensively reported that the average size of the semiconductors intercalated in the restricted nanospaces was less than 10 nm, relating to the host-guest and guest-guest interactions (Khaorapapong et al. 2009;Intachai et al. 2017a;Poosimma et al. 2018). The size and size distribution of ZnS or CdS in the hybrid were studied by TEM images (Fig. 2). It was evident that the round-shape darker points due to ZnS crystal (Fig. 2a) or CdS crystal (Fig. 2b) were well distributed on a lighter region due to the nanosheets of the LDH. A few hundred of the darker points was measured, where the average diameter size was 2.61 nm for ZnS/CoAl-LDH@DS (Fig. 2c) and 3.29 nm for CdS/CoAl-LDH@DS (Fig. 2d); therefore, the obtained nanoparticles of ZnS or CdS with orderly particle size distribution could confirm the intercalation in the interlayer space. The distinct sizes of the intercalated ZnS and CdS nanoparticles might be because of the difference in their bonding lengths of ZnS (~ 258 pm) and CdS (~ 279 pm) that affected the particle growth (Kabilaphat et al. 2015). Considering the expansion of the interlayer space in Table 1, the intercalated nanoparticles were plate-or disk-like shape based on the diameter size of 2.61 nm for ZnS/CoAl-LDH@DS and of 3.29 nm for CdS/CoAl-LDH@DS, and their thickness about 0.4 nm, which arranged parallel to LDH nanosheet together with few interlayer CO 3 2ions. Besides, it can be seen in the SEM images (Fig. S3) that the larger sized ZnS (1-4 μm, Fig. S3e) or CdS (2-6 μm, Fig. S3f) particles were not observed on the external surface of ZnS/ CoAl-LDH@DS (Fig. S3c) and CdS/CoAl-LDH@DS (Fig. S3d). As a result, CoAl-LDH could control the nanoparticle growth of ZnS or CdS with the pre-expansion  Fig. S4. Relative to CoAl-LDH with the interlayer CO 3 2- (Fig. S4a) and DS (Fig. S4b) (Liu et al. 2006), relating to that observed at 212 °C on the DTA curve of CoAl-LDH (Fig. S4g), whereas the exothermic peaks in the temperature range of 366-540 °C were defined the oxidative decomposition of DS (Demel et al. 2014) that corresponded with those observed at 174-517 °C on the DTA curve of DS/CoAl-LDH (Fig. S4h). Besides, their mass losses were extended to 800 °C, relating the decomposition of the sulfate residue from the decomposition of DS (Demel et al. 2014). Meanwhile, the dehydroxylation of brucite-like LDH structure overlappingly occurred at the temperature range of 265-600 °C, corresponding to the exothermic peak at 300 °C (Fig. S4g). The variation on the mass losses and the associating DTA curves of DS for DS/CoAl-LDH and both products might be because of the difference in the environments and arrangements (Liu et al. 2006). In the TG-DTA curves of the products, no any exothermic peak was observed below 80 °C on the DTA curves of the products, indicating the absence of water molecule in the interlayer space (Ni et al. 2009), corresponding to the XRD result. It was found that the mass losses of the intercalated ZnS or CdS clearly did not appear, and no observation of any DTA peak relative to the oxidation of ZnS at 682 °C (Fig. S4e) with the exothermic peak at 671 °C (Fig. S4l) and the decomposition of CdS at 811 °C (Fig. S4f) with the endothermic peak at 806 °C (Fig. S4k). It might be because the decomposition of ZnS or CdS nanocrystal in the hybrid was stable at higher temperature, which was suggested to the thermal protection by

Optical properties
The optical properties of ZnS or CdS in the hybrid were investigated by UV-visible and photoluminescence (PL) spectroscopies; all of the optical data are listed in Table 1. The band gap energy (E g ) of the optical sample was determined by Kubelka-Munk equation (Fig. S5) (Kabilaphat et al. 2017) that corresponded to its absorption onset. The absorption spectrum of DS/CoAl-LDH (Fig. 3(e)) was based on the optical characteristic of CoAl-LDH (Fig. 3(f)), where the charge transfer from Co 2+ to O 2occurred at 238 nm, and three electronic transitions of Co 2+ (d 7 ) due to 4 T 1g (F) → 4 T 1g (P), 4 T 1g (F) → 4 A 2g , and 4 T 1g (F) → 4 T 2g were splitted at 450, 526, and 615 nm, respectively (Khodam et al. 2018).
After the intercalation of ZnS or CdS, the new absorption shoulder band and absorption onset (λ onset ), as well as higher absorbance, were observed ( Fig. 3(a, b)), implying to the effect of ZnS or CdS (Kabilaphat et al. 2015). In UV-visible spectrum of ZnS/CoAl-LDH@DS ( Fig. 3(a)), the absorption shoulder band, λ onset , and E g were observed at 312 nm, 343 nm, and 3.66 eV, respectively. Meanwhile, those of CdS/ CoAl-LDH@DS ( Fig. 3(b)) were observed at 367 nm, 500 nm, and 2.50 eV, respectively, where its λ onset was determined as shown in Fig. S6. The blue-shifted λ onset and higher E g were obtained 39 nm and 0.34 eV for ZnS/CoAl-LDH@ DS, and 44 nm and 0.22 eV for CdS/CoAl-LDH@DS when compared to those in ZnS (382 nm and 3.32 eV) (Fig. 3(c)) and CdS (554 nm and 2.28 eV) (Fig. 3(d)). The result could confirm the formation of ZnS or CdS nanoparticles in the hybrid that corresponded to the TEM result, and the synergistic effect of the strong quantum size effect (Khaorapapong et al. 2010;Wang et al. 2018), whereas the λ onset (414 nm) and E g (2.99 eV) of ZnS/CoAl-LDH@DS were also located in the visible region, which indicated as both UV and visible light-responsive material. Considering Table 2, the assynthesized hybrids showed the blue-shifted λ onset due to ZnS or CdS in the hybrid than those in other systems, which could be indicative of the formation of the smaller nanosized ZnS or CdS (Intachai et al. 2017b;Poosimma et al. 2018;Chandrasekaran et al. 2019). It was due to the difference in the microstructure and environment of host material, the amounts of the metal sulfide in and on the host material, and the preparation method. In Fig. 4(c) of the PL spectrum of bulk ZnS, the board spectrum was covered in the wavelength range of 400-625 nm with the maxima wavelength at 470 nm that attributed to non-radiative emission of the various defects such as zinc vacancy (at 469-470 nm), sulfide vacancy (at 420-430 nm), and sulfide interstitial (at 490 nm) (Manzoor et al. 2003;Devarajan et al. 2012), whereas ZnS/CoAl-LDH@ DS showed the deep-blue PL band centered at 422 nm and the blue PL shoulder at 469 nm, which were arisen from the sulfide vacancy and zinc vacancy, respectively. Besides, its PL intensity (the band centered at 469 nm) was 2.6 times higher than that of ZnS (470 nm). The resulting PL characteristics of ZnS/CoAl-LDH@DS were thought due to the host-guest and/or guest-guest interactions (Khaorapapong et al. 2010;Kabilaphat et al. 2015;Intachai et al. 2017b), which might be a new part or support of smart optical devices and applications. By the hybridization of CoAl-LDH@DS to CdS, the PL observations exhibited the blue shift of the emission wavelength about 13 nm from 549 (bulk CdS, Fig. 4(d)) to 536 nm (CdS/CoAl-LDH@DS, Fig. 4(b)) and the enhanced PL intensity up to 3.5 times, which could be indicative of the quantum size effect (Chandrasekaran et al. 2019). As a result, the intercalation of ZnS or CdS in CoAl-LDH system was an efficient method to tailor PL intensity, reflecting to the increase of the electron-hole carriers (Kabilaphat et al. 2015). In Table 2, the PL of ZnS in host matrices had not been seriously studied (Zhang et al. 2001;Chen et al. 2010;Li et al. 2017), whereas the PL band and intensity of CdS nanoparticles in montmorillonite were altered by the preparation methods (Khaorapapong et al. 2010;Intachai et al. 2017b), and the low PL intensity of CdS nanoparticles was reported due to the larger sized particle growth on the surface of LDH and graphene (Yogamalar et al. 2015;Li et al. 2018;Zhou et al. 2018;Trenczek-Zajac et al. 2019). This work was more excellent, where the formation of CdS in CoAl-LDH system gave higher PL intensity of 1.5 times relative to CdS-montmorillonite prepared by solid-liquid system and 33.7 times prepared by solid-state system, and the quantum confinement (blueshifted PL band) relative to CdS nanosol deposited on CoAl-LDH. It was suggested that the effective route by the intercalation of ZnS or CdS in CoAl-LDH system on the enhanced photoluminescence may be an alternative pathway for applying in the various optical applications.

Optical performance
The optical performance of ZnS/CoAl-LDH@DS and CdS/ CoAl-LDH@DS after the storage for 12 months at ambient conditions was analyzed by spectroscopic observation, which was comparative to the unhosted bulk particles. In Fig. 5, the λ onset of ZnS or CdS in the hybrid was slightly shifted to longer wavelength (red-shift) about 4-14 nm in comparison with the fresh one; meanwhile, the red-shifted λ onset about 18-20 nm was observed for the bulk ZnS and CdS particles. The results could be indicative of the larger sized particles Chandrasekaran et al. 2019), whereas the nanoparticles of ZnS/CoAl-LDH@DS and CdS/CoAl-LDH@DS after the storage for 12 months were slightly aggregated due to the protection by the host material (Intachai et al. 2017b;Poosimma et al. 2018), which corresponded to slightly larger average sizes 3.69 nm and 4.66 nm, respectively (Fig. 6). Corresponding to the UVvisible observation, it can be seen the red-shifted PL band of CdS/CoAl-LDH@DS, ZnS, and CdS after the storage for 12 months in comparison with the as-synthesized one in Fig. 7. Meanwhile, PL band of ZnS/CoAl-LDH@DS centered at 470 nm due to zinc vacancy was higher intensive relative to the PL band centered at 422 (sulfide vacancy) of the as-prepared product, suggesting the variety in the PL characteristics. It was found that the particle size was larger, which decreased the PL (Fig. 7). The PL intensity for ZnS/ CoAl-LDH@DS, CdS/CoAl-LDH@DS, ZnS, and CdS after the storage for 12 months (Table 2) was reduced to 18, 19, 44, and 40, respectively. Interestingly, the PL intensity of the hybrids after the storage for a year was quite stable relative to the product after the preparation. As a result, the usage of CoAl-LDH system was achieved to preserve the optical performance of ZnS and CdS nanoparticles.

Adsorption and photocatalytic abilities
The surface characteristic and optical functionality of the samples were assessed by the adsorption and/or photodegradation of Congo red in water. In Fig. 8, by increasing the adsorbent dosage from 10 to 50 mg ( Fig. 8(a-c)), the Congo red amounts were more captured due to the larger active sites (Intachai et al. 2022), where CoAl-LDH was the best adsorbent compared to other adsorbents (Fig. 8(d)). It was found that difference in the adsorption capacity of each adsorbent was based on the specific BET surface area, where the surface area (Table 3) of CoAl-LDH was the largest (30.1 m 2 /g) among those of ZnS/CoAl-LDH@DS (19.7 m 2 /g), CdS/ CoAl-LDH@DS (14.7 m 2 /g), DS/CoAl-LDH (5.4 m 2 /g), ZnS (1.1 m 2 /g), and CdS (0.8 m 2 /g).
To investigate the adsorption reaction, the adsorption data ( Fig. 8(c)) were linearly fitted as the pseudo-first-order and pseudo-second-order models (Figs. S7a and S7b); the data  Table S2. The R 2 of all adsorbents obtained by fitting as the pseudo-second-order reaction was more nearly equal to 1 than that of the pseudo-first-order reaction, which could be indicative of the adsorption-dependent on the amounts of adsorbent and adsorbate (Suppaso et al. 2021). In Figs. S7c and S7d, the adsorption isotherms were studied by plotting as Langmuir and Freundlich models. Considering the R 2 in Table S2, the adsorption mechanism of Congo red in water by all the as-prepared samples was best followed as Langmuir model based on monolayer coverage (Intachai et al. 2022). The maximum adsorption capacity (q m ) of CoAl-LDH, DS/CoAl-LDH, ZnS/CoAl-LDH@DS, and CdS/CoAl-LDH@DS was about 251, 126, 234, and 216 mg/g, respectively, that was quite high compared to other adsorbents (Table 3), attributing to an efficient adsorbent for removing anionic dye in water. It was thought that the LDH-based adsorbents showed the efficient adsorption of Congo red in water due to the chemical affinity between the positively charged surface of the LDH and Congo red anion, together with H bonding and physical interactions in accordance with the previous studies (Bharali et al. 2017;Huang et al. 2017;Sriram et al. 2020). It was further supported by the pH at zero point charge (pH zpc ) carried out by the drift method (Intachai et al. 2022), where the data are listed in Table 3. Consideringly, the obtained solution pH (6.1-6.6) and pHzpc (6.7-8.7) could confirm the ionization of Congo red as anion (two anionic sites) due to pKa 1 = 3.70 and pKa 2 = 5.50 (Sriram et al. 2020), and the positive-charged surface of CoAl-LDH due to pH < pH zpc (Lafi et al. 2016). As a result, the adsorption of Congo red by using CoAl-LDHbased sample was selectively proceeded by the electrostatic interaction with tailoring the adsorption and photocatalytic activity.
After the adsorption equilibrium, the photocatalytic removal of Congo red was further proceeded by breaking the chromophore down. In Fig. 9(a), after the visible-light irradiation, the removal efficiency of Congo red was further increased by the different photocatalyst potential as follows CdS/CoAl-LDH@DS > ZnS/CoAl-LDH@DS > CoAl-LDH > DS/CoAl-LDH > CdS > ZnS. The results were due to the effect of visible-light responsive species, where the CdS nanoparticle in the hybrid could be easily and highly generated; the electron and hole carriers then promoted the increase of photocatalytic activity (Zhu et al. 2009;Kaushik et al. 2022). The lower E g (2.99 eV) of ZnS/CoAl-LDH@ DS (another higher E g = 3.66 eV) could describe the photocatalytic activity under visible-light irradiation, as a result of the host-guest and/or guest-guest interaction relative to pure ZnS. Meanwhile, the photocatalytic activity of CoAl-LDH and DS/CoAl-LDH occurred through d-d transitions (Suppaso et al. 2021), which could increase the photodegradation of Congo red by using the hybrids. It was thought that DS did not present the photocatalytic role, and it might obstruct contacting Congo red (large sized dye with two anionic sites, Fig. 9(b) (inset)) to the active sites in accordance with lower adsorption and lower photocatalytic activity. It 1 3 can be seen in Fig. 9(b), by increasing the reaction time, the absorption intensity of three bands (at 499 nm due to the azo group, at 344 due to naphthalene moieties, and at 236 nm due to benzene rings) was dramatically decreased, whereas the maximum absorption bands were slightly shifted to lower wavelength (for 6 h irradiation), indicating smallersized derivatives due to the photodegradation in which the mechanisms were possibly proposed followed by the previous researches by two pathways. By pathway 1 due to CdS nanoparticle (Bhoi et al. 2016;Kaushik et al. 2022), the CdS nanoparticles in the hybrid could generate the active carriers based on the photoexcited electron (e -) and hole (h + ) by shining the visible light energy (hν), as shown in (1). Then the carriers further reacted with the oxidizing agent (O 2 ) and reducing agent ( OH − ) to produce O 2 ⋅− (2) and OH • (3), respectively, in order to decompose the adsorbed Congo red dye on the surface to be the smaller-sized derivatives, H 2 O and CO 2 (4).
By pathway 2 (Suppaso et al. 2021), the electronic transition due to CoAl-LDH co-photocatalyst happened through the d-d transition of Co 2+ as described in the (1) CdS hybrid + hν (≥ 2.50 eV) → e − + h + O 2 ⋅− + OH • + h + + adsorbed Congo red dye → degraded dye derivatives + CO 2 + H 2 O Fig. 9 Photocatalytic activity (a) UV-visible spectra of Congo red solution at any phodegradation time (b), pseudo-first order plot (c), and its reusability (d) on removing Congo red in water using CdS/CoAl-LDH@DS as catalyst UV-visible spectra (Fig. 3) that could react to O 2 with the produced O 2 ⋅− (5), whereas the Al (CoAl-LDH sheet) could be the oxidant to stabilize Co 3+ through metal to metal charge transfer (6), and the resulting Al could be the oxidant for OH − to produce OH • (7). As a result, all active species ( O 2 ⋅− and OH • ) could attack Congo red dye to be the degraded dye derivatives and other low toxic species (8).
The photocatalytic rate constant (k app ) was determined by the pseudo-first-order plot as shown in Fig. 9(c), and the data are listed in Table S3, where the k app of CdS/ CoAl-LDH@DS was quite high relative to the other photocatalysts (Table 4), attributing to a smart material on removing Congo red in water. To evaluate the photocatalytic performance, the first spent CdS/CoAl-LDH@ DS was rerun on the second and the third photocatalytic activity, respectively ( Fig. 9(d)). Surprisingly, the photocatalytic efficiency for all reuses was almost the same; it was thought that the DS amount coated on the photocatalyst surface was slightly cleaned and/or photodegraded that promoted the slight increase of the adsorption efficiency and photocatalytic activity. To verify the stability, the quantities of Co, Al, and Zn or Cd elements (5) O 2 ⋅− + OH • + adsorbedCongo red dye → degraded dye derivatives + CO 2 + H 2 O in the supernatant (3rd rerun) for the spent hybrid photocatalysts investigated by ICP-OES were almost the same relative to the fresh one just less 1% of leaching amount. This result was indicative of high performance on the adsorbent and photocatalyst.

Conclusions
It was a delight to propose a new potential process for generating two nanohybrids of ZnS and CdS nanoparticles in CoAl-layered double hydroxide@dodecylsulfate (abbreviated as CoAl-LDH@DS) by in situ intercalation of Zn(II) or Cd(II) and sulfidization by Na 2 S in the interlayer space. By using CoAl-LDH system, the control of particle growth of ZnS and CdS was achieved to be 2.61 nm and 3.29 nm that corresponded to the blue-shifted absorption onset of 39 nm and 44 nm, respectively, as well as the increase of their photoluminescence intensity and performance. The appearances of the deep-blue emission of ZnS/CoAl-LDH@ DS at 422 nm and the blue-shifted photoluminescence band of CdS/CoAl-LDH@DS were indicative of the host-guest and/or guest-guest interactions. Besides, C d S / C o A l -L D H @ D S a n d Z n S / C o A l -L D H @ D S showed the high potential on removing Congo red in water through the adsorption and photocatalysis. This study exhibited an effective template for the preparation of smart hybrid solids on controlling small nanoparticle and tailoring photoluminescence, and adsorption and photocatalytic efficiencies.