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. 2021 Nov 8;6(45):30432–30441. doi: 10.1021/acsomega.1c03735

Facile and Green Synthesis of Silver Quantum Dots Immobilized onto a Polymeric CTS–PEO Blend for the Photocatalytic Degradation of p-Nitrophenol

Fares T Alshorifi †,‡,*, Abdullah A Alswat §, Mohammed A Mannaa ∥, Mohammed T Alotaibi ⊥, Salah M El-Bahy ⊥, Reda S Salama #
PMCID: PMC8600520  PMID: 34805673

Abstract

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Immobilization of inorganic metal quantum dots (especially, noble transition metals) onto organic polymers to synthesize nanometal–polymer composites (NMPCs) has attracted considerable attention because of their advanced optical, electrical, catalytic/photocatalytic, and biological properties. Herein, novel, highly efficient, stable, and visible light-active NMPC photocatalysts consisting of silver quantum dots (Ag QDs) immobilized onto polymeric chitosan–polyethylene oxide (CTS–PEO) blend sheets have been successfully prepared by an in situ self-assembly facile casting method as a facile and green approach. The CTS–PEO blend polymer acts as a reducing and a stabilizing agent for Ag QDs which does not generate any environmental chemical pollutant. The prepared x wt % Ag QDs/CTS–PEO composites were fully characterized through X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy (TEM), thermogravimetric analysis, and UV/visible spectroscopy. The characterization results indicated the successful synthesis of the Ag QDs/CTS–PEO composites by the interactions and complexation between x wt % Ag QDs and CTS–PEO blend sheets. TEM images revealed small granules randomly distributed onto the CTS–PEO blend sheets, indicating the immobilization of Ag QDs onto CTS–PEO composites. The presence of a surface plasmon resonance (SPR) band and the shifting of the absorption edge toward higher wavelengths in the UV/vis spectra indicated the formation of x wt % Ag QDs/CTS–PEO composites. The Ag QDs in the polymeric blend matrix led to remarkable enhancement in the optical, thermal, electrical, and photocatalytic properties of x wt % Ag QDs/CTS–PEO composites. The photocatalytic efficiency of the prepared composites was evaluated by the photodegradation of p-nitrophenol (PNP) under simulated sunlight. The maximum photocatalytic degradation reached 91.1% efficiency within 3 h for the 12.0 wt % Ag QDs/CTS–PEO photocatalyst. Generally, the Ag QDs immobilized onto CTS–PEO blend composites significantly enhance the SPR effect and the synergistic effect and reduce the band gap, leading to a high photocatalytic activity.

1. Introduction

Currently, the rapid advancement of nanoscience has led to the development of new and more active methods for preparing new useful nanomaterials (NMs) with innovative and unique properties. The three-dimensional nanoparticles (3D NPs) with the particles size less than 10 nm are called quantum dots (QDs). Recent studies have revealed that the electrical, catalytic/photocatalytic, and optical properties of nanometal–polymer composites (NMPCs) as a promising material attract greater attention of many researchers as one of the novel significant physicochemical scientific topics. The novel and unique NMPC properties such as higher active surface sites, greater light absorption capacity, surface-plasmon resonance (SPR), larger surface area, and synergistic effects make them ideal NMs for a broad range of new applications in adsorption/photocatalysis; catalysis/separation; solar cells/optical, biomedical, industrial, and health applications; and in optoelectronic/electrochemical devices.1−9 Polymeric composites (PCs) were modified by metals (Ms)/transition metals (TMs) or nonmetals to produce novel MPCs in a nanoscale.10−13 The NMPCs may be used as active polymeric photocatalysts due to their numerous merits, such as non-toxicity, light weight, low cost, high flexibility, good optical/optoelectrical and mechanical properties, environmentally friendly nature, and easy modifiability by easy preparation techniques (sheet or film fabrication process compatibility and band gap tunability).

The novel NMPCs may be used as stable polymeric materials for the photocatalytic degradation of organic pollutants and for many other applications14−17 since environmental water pollution has become the biggest environmental problem in all communities.7,18−24 Multi-fold chemical water wastes produced by many industries such as dyes, textiles, pharmaceuticals, cosmetics, and food industries are identified as a primary water contamination sources causing harm to the environment and human and animal health.25−31 Numerous researchers have investigated numerous methods for the elimination of organic pollutants from wastewater such as adsorption,20,32−34 biological treatment,35,36 microwave catalysis,37,38 chemical oxidation,39,40 and photochemical degradation.41,42 Photocatalytic degradation is considered an effective photocatalytic chemical method for cleaning environmental pollution by transferring the electron covalent band to the conduction band region.5,6,43 From the high energy generated by photoinduced excitation of NMPC molecules, various unique reactions between solid-NMPC photocatalysts and organic pollutants can occur that are not generally available for bulk solid-MPC materials.1,44,45 It is indubitable that all neat organic polymers (OPs) possess some other restrictive defects that prevent their use as photocatalysts such as acid-/alkali-/water-solubility and low thermal stability and has unsatisfactory mechanical/optical and photocatalytic properties. To overcome these limitations, one can attempt to manipulate the OP properties by some fabrication strategies of pure polymer/polymeric blends with/without chemical modification by M-/TM-NPs to create NMPCs.

Chitosan (CTS), a natural biopolymer, is a non-edible, non-toxic, and less expensive carbohydrate; it has an amorphous structure,46−48 which makes it more rigid and highly chemically and thermally stable than other polymers. The poor solubility of CTS in water promotes its effective utilization as a modified polymeric photocatalyst/adsorbent for removing organic/inorganic pollutants from wastewater. The advantages of CTS lie in its amorphous structure with a carbon chain backbone including NH2 groups and OH groups that enable CTS structures to have high affinity toward M/TM ions including Ag, Au, Ti, Bi, Cr, Cu, Fe, Pt, Pd, Cr, V, W, and other TM ions that can be adsorbed by either chelation and/or ion exchange.49 It is interesting to note that CTS acts as both a reducing agent and/or a stabilizer46,50,51 during the formation of TM-NPs and prevents their aggregation. Polyethylene oxide (PEO) is a hydrophilic, non-toxic, simple chain ((CH2CH2O)n) and has a semicrystalline solid structure.52−55 It is an effective ion-conductive polymer with ether (C–O–C) linkages which promote excellent interaction with another organic polymer or/and with inorganic M/TM. The use of M-/TM-NPs for the fabrication of novel NMPCs is triggered by their remarkably good features relative to those of the bulk M/TM structure counterparts.1

Incorporation of TM-NPs into the host polymers can lead to a drastic change in their properties. It is well known that noble TM (Au, Ag, Cu, Pd, and Pt)-NPs strongly absorb visible light due to the SPR effect.1,44,45,56−58 Silver nanoparticles (Ag NPs) show the strongest SPR effect of noble TM-NPs, due to the collective oscillation of conduction electrons which resonate with the incident light’s electromagnetic field on the surface. The photo-excited electrons will return to the thermal equilibrium states of TM-NPs and release heat into the noble TM-lattice and the surrounding medium. This heat may also induce reactions of the molecules adsorbed onto the Ag QD surface. Hence, when illuminated with visible light, the dispersed noble TM-NPs supported by polymers will exhibit a high photocatalytic/catalytic and optical activity. However, M-NPs are not stable as such because of the elevated surface energy. During reactions, M-NPs are destabilized by highly active surface atoms. Hence, immobilization of M-NPs onto an appropriate solid support has come into practice to boost the stability of the material.59−61 In this context, finely dispersed Ag NPs have been immobilized to obtain highly reactive catalysts.

The main purpose of the current work was to fabricate well-stabilized and visible light-driven x wt % Ag QDs/CTS–PEO photocatalysts by a facile and green approach. The polymeric CTS–PEO blend was used as a reducing and a stabilizing agent for Ag QDs because it can easily absorb Ag+ by chelation without further chemical modification and will not result in any environmental toxicity or biological hazards. Ag+ will therefore grow onto the CTS–PEO blend matrix by chelation and/or in situ ion exchange with a large number of O and N donors (OH groups and NH2 groups) in the CTS structure and by attracting the ion pair electrons on O atoms in etheric (C–O–C) linkages in the PEO structure. PNP was used as a model organic pollutant to evaluate the photocatalytic activity of the prepared NMPC photocatalysts under visible light irradiation. The structure, morphology, thermal stability, and optical properties of the prepared x wt % Ag QD/CTS–PEO composites were investigated by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, UV/vis spectroscopy, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA).

2. Experimental Section

2.1. Materials

Silver nitrate (AgNO3), acetic acid (CH3COOH), CTS, and PEO were obtained from Sigma-Aldrich Co. All chemical reagents were of analytical grade and used without further purification.

2.2. Preparation of the CTS–PEO Blend and Ag QDs/CTS–PEO Composites

CTS–PEO blend sheets and Ag QDs/CTS–PEO composites were synthesized by a self-assembly facile casting method. The CTS–PEO composites were prepared as follows: typically, 0.25 g of PEO was dissolved in 40 mL of deionized water under magnetic stirring, and 0.25 g of CTS was dissolved in 40 mL of 1.0% acetic acid. After completely dissolving the two solutions, the two polymer solutions were mixed together:23,26,27 The PEO solution was added drop by drop to the CTS solution under vigorous stirring at 25 °C (approximately for 1 h) until complete homogenization of the CTS–PEO blend solution. Then, 20 mL of freshly prepared aqueous AgNO3 solution [the Ag metal amounts (x) were 1, 3, 6, 9, and 12.0 wt %] was added dropwise to the previous CTS–PEO blend solution under continuous stirring for extra 2 h (the temperature of the reaction was increased to 70 °C using an oil bath). The color of the colloidal x wt % Ag+/CTS–PEO blend suspension changes with the stirring time from colorless to different degrees of yellow and finally to light brown, indicating the formation of colloidal Ag QDs. The formed colloidal x wt % Ag QDs/CTS–PEO composite mixtures were poured onto clean Petri dishes and dried in an oven at 70 °C for 12 h. All x wt % Ag QDs/CTS–PEO composite samples were kept in a desiccator for further drying.

2.3. Characterization Techniques

The chemical structure and the crystal composition of pure and modified photocatalysts were confirmed using a FTIR (Nicolet Magna-IR 550) spectrometer with a resolution of 4 cm–1 and 128 scans in the region of 400–4000 cm–1. XRD patterns for all the x wt % Ag QDs/CTS–PEO samples were recorded using a PW150 (Philips) with a Cu (Kα)-XR-radiation source (λ = 0.1541 nm) at 40 kV and 45 mA. The purity, morphology, and particle size of the as-prepared CTS–PEO blend and x wt % Ag QDs/CTS–PEO samples were measured by TEM (JEOL-JEM-2100). The optical properties and the band gap of the as-prepared composites were obtained using UV/visible diffuse reflectance spectroscopy (ATi Unicam-UV/visible vision software V3.20). The actual contents of Ag NPs supported on CTS–PEO were measured by the inductively coupled plasma-atomic absorption spectroscopy (ICP-AES; Labtam, 8440 Plasmalab) technique.

2.4. Photocatalytic Tests

The photocatalytic activity of the prepared x wt % Ag QDs/CTS–PEO samples was evaluated by the degradation of PNP in simulated sunlight, using a Hg lamp (150 W) with a JB400 filter as a visible light source. Typically, 60 mg of x wt % Ag QDs/CTS–PEO composites was added to 50 mL of (10 μ/mL)-PNP aqueous solution, which was placed in a chamber with a Hg lamp. Before light irradiation, PNP and the prepared composite powder suspensions were magnetically stirred in the dark for 30 min to establish the adsorption–desorption equilibrium of the PNP solution, and then the lamp was turned on. At different time intervals, 2 mL of PNP solution was collected and centrifuged to remove any solid composites in the solution and then analyzed using a UV/vis spectrophotometer (the maximum absorption band at λ = 316 nm).

3. Results and Discussion

3.1. XRD Technique

The XRD patterns of all the prepared composites were obtained to investigate the chemical structure of the prepared composites and the effect of the Ag QD content on the semi-crystal/amorphous structures of the CTS–PEO composite. Figure 1 shows the XRD patterns of pure CTS, pure PEO, and the neat CTS-PEO blend. The spectrum of CTS exhibits three main peaks at 10.1, 15.1, and 20.4° assigned to the set of planes (020), (100), and (101), respectively, indicating the semicrystalline structure of CTS with some order in the intermolecular structure.62 On the other hand, the XRD patterns of pure PEO show two main peaks at 19.1 and 23.2° assigned to the set of planes (120) and (112), respectively, which proved the semicrystalline structure of PEO.49,55 These characteristic peaks overlapped the peaks of the neat CTS–PEO blend, where the three broad peaks of CTS disappeared and the two peaks of PEO became more broad and less intensity, indicating more complexation between PEO and CTS to form CTS–PEO composites. Figure 2 shows the XRD patterns of x wt % Ag QDs/CTS–PEO composites. The figure exhibits new peaks related to Ag QDs at 2θ of 27.6, 32.1, and 46.1° that appeared in all modified composites and assigned to the set of planes (202), (111), and (200), respectively.3,56,57 The two distinct peaks of the neat CTS–PEO blend appeared in all x wt % Ag QDs/CTS–PEO composites, but its intensity was found to decrease with increasing Ag QD contents from 1 to 12.0 wt %. These results revealed the complexation between the Ag QDs and the CTS–PEO blend composite. As shown in Figure 2, the intensity of the Ag QD peaks in 1.0 and 3.0 wt % of Ag QDs/CTS–PEO composites was very small because of the good distortion of Ag QDs within CTS–PEO polymeric matrices or may be as a result of the dilution effect of Ag QDs in the CTS–PEO blend, which led to the complete dissolution/disappearance of Ag QDs in the amorphous regions of the CTS–PEO blend. However, in 6.0, 9.0, and 12.0 wt % Ag QD/CTS–PEO composites, the Ag QD peaks were clearly observed, and the crystallinity of Ag QDs was increased as a result of increased immobilization of Ag QDs on the prepared composites.

Figure 1.

Figure 1

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XRD patterns of pure CTS, pure PEO, and CTS–PEO blend composites.

Figure 2.

Figure 2

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XRD patterns of (a) CTS–PEO blend, (b) 1.0, (c) 3.0, (d) 6.0, (e) 9.0 ,and (f) 12.0 wt % Ag QDs/CTS–PEO.

3.2. FT-IR Technique

One of the most significant technique that is used to identify the functional group of the composites is FTIR spectroscopy. Figure 3 shows the FT-IR spectra of CTS, PEO, and CTS–PEO blend, which exhibit characteristic peaks that matched well with the previous reports.46,54,63 These peak values are presented in Table 1. The difference between PEO and CTS is mainly due to a specific interaction of the hydrogen bonding type between the C–O–C linker in PEO and H atoms from (OH) and (NH2) groups in CTS, as evidenced by FTIR spectroscopy. The complexation occurs between PEO and CTS polymers due to specific intramolecular and intermolecular interactions of polymer chains. From the IR spectrum of the CTS–PEO blend composite, we observed some changes in the position, shape, and intensity of IR absorption bands, indicating that CTS–PEO interacts with each other. The absorption bands of pure CTS at 1556 and 1623 cm–1 appear as very weak peaks, and their intensity is decreased at the CTS–PEO blend sample, which are attributed to the formation of intermolecular hydrogen bonds between CTS and PEO. In addition, the peak of CTS around (3360–3400) cm–1 assigned to the stretching vibration of N–H and O–H is broad, and its intensity is decreased at the CTS–PEO blend sample due to H-bonding of CTS with PEO. The extent of decrease is related to the CTS–PEO blending process, implying that part of the H bonds in CTS molecules was broken by the addition of PEO and new H bonds were formed between the CTS and PEO for the formation of the CTS–PEO blend. On the other hand, Figure 4 shows the FTIR spectra of x wt % Ag QD/CTS–PEO composites, and the immobilization of Ag QDs onto the polymeric CTS–PEO matrix causes some changes in the intensities of some characteristic vibrational bands such as (C–H), (C–O–C), and (O–H) stretching vibrations with increasing Ag QD content. This indicates the interaction of Ag QDs with the CTS–PEO blend in all the prepared composites.

Figure 3.

Figure 3

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FTIR spectra of pure CTS, pure PEO, and CTS–PEO blend composites.

Table 1. FTIR Absorption Bands of Pure CTS and Pure PEO Polymers.

FT-IR peaks (CTS)
 FT-IR peaks (PEO)
λ (cm–1) assignments λ (cm–1) assignments
3443–3360 O–H stretching vibration 3400–3380 O–H stretching vibration
2935, 2890 CH2 stretching vibration 2897 CH2 stretching asymmetric
1623 C=O stretching of amide 1461 CH2 scissoring
1556 bending energy band: in N–H bond 1359 CH2 asym bending
1425 CH2 stretching vibration 1242 CH2 symmetric twisting
1390 bending vibration of CH2 (1146, 1110, 1063) triplet peak of C–O–Cs (semicrystalline-PEO)
1095.1029 C–O–C, C–O stretching 963 out-of-plane rings C–H bending
965, 840 C–H bending and C–C stretching 843 C–C stretching vibrations
655 small wagging mode of (OH)    
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Figure 4.

Figure 4

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FTIR spectra of (a) CTS–PEO blend, (b) 1.0, (c) 3.0, (d) 6.0, (e) 9.0, and (f) 12.0 wt % Ag QDs/CTS–PEO.

3.3. TEM and HRTEM Analyses

Typical TEM micrographs of CTS–PEO blend and x wt % Ag QDs/CTS–PEO composites are obtained to investigate the presence and dispersion of Ag QDs in the prepared composites and to examine the sizes of the immobilized sliver particles. Figure 5 shows that the TEM images of neat CTS–PEO appeared with a uniform morphology as smooth surface sheets. However, the TEM images of x wt % Ag QDs/CTS–PEO composites revealed small granules randomly distributed onto the CTS–PEO blend sheets, indicating the immobilization of Ag QDs onto the CTS–PEO composites. In the HRTEM image, there were some dark spots like spheres after alteration with Ag NPs, which increased with the increase in the amount of Ag NPs. These darks spots ascertained that Ag NPs were homogeneously dispersed on CTS–PEO with the particle size ranging from 3 to 8 nm, as shown in Figure S1. Also, according to the Ag QD size distribution histograms (Figure S2), the average particle sizes of Ag QDs were found to be 5.7, 6.2, 6.8, and 7.6 nm for 1.0, 6.0, 12.0, and 20.0 wt % Ag QDs/CTS–PEO, respectively. A further increase of the Ag content led to increased immobilization of Ag NPs with different small nano-sized and irregular spherical shapes distributed onto the CTS–PEO blend matrix. The 20.0 wt % Ag QDs/CTS–PEO composite showed some aggregation of Ag NPs onto the CTS–PEO surface. These results were confirmed by XRD and FT-IR studies.

Figure 5.

Figure 5

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TEM images of (a) CTS–PEO blend, (b) 1.0, (c) 3.0, (d) 6.0, (e) 9.0 ,and (f) 12.0 wt % Ag QDs/CTS–PEO.

3.4. Thermogravimetric Analysis

Thermal decomposition of the prepared NMPCs was investigated using TGA to gain a better understanding of their thermal stability. Figure 6 shows the TGA curves of the CTS–PEO blend and x wt % Ag QDs/CTS–PEO composites, which displayed that the initial weight loss of the prepared composites occurred at 70–120 °C because of the moisture evaporation. All the prepared samples showed good stability up to 220 °C. The major weight loss for the neat CTS–PEO blend appeared at 235–394 °C, which is attributed to the structural decomposition of the CTS–PEO composite. However, the major weight loss for x wt % Ag QDs/CTS–PEO composites appeared at 302–447 °C, which is attributed to the structural decomposition of x wt % Ag QDs/CTS–PEO composites. We found that the structural decomposition of x wt % Ag QDs/CTS–PEO composites appeared at a higher temperature range compared to neat CTS–PEO composites. This stability could be attributed to the presence of Ag NPs within the structure of the composites, and the thermal stability increased with the increasing Ag QD content. These results have proved that the thermal stability of the prepared composites samples improved after the immobilization of Ag QDs.

Figure 6.

Figure 6

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TGA analysis of the pure CTS–PEO blend and modified Ag QD/CTS–PEO.

3.5. Ultraviolet and Visible Analysis

Figure 7 shows the UV/vis absorption spectra of the CTS–PEO blend before and after immobilization with different weight contents of Ag QDs in the wavelength range of 205 to 900 nm. After immobilization of Ag QDs onto the CTS–PEO blend, the positions of the absorption edge of the prepared composites slightly shifted toward higher wavelengths, which showed the miscibility of Ag QDs and the CTS–PEO blend. We observed a new band that appeared in the visible region (at λ = 400–440 nm) and its intensity continuously increased with increasing Ag QD content onto CTS–PEO composites, which may be related to the characteristic SPR band of the immobilized Ag QDs onto the CTS–PEO blend matrix.46 The observed SPR band and shifting of the absorption edge toward higher wavelengths confirmed the interactions between the Ag QDs and the CTS–PEO blend, that is, the formation of x wt % Ag QDs/CTS–PEO composites by the inter-/intramolecular bonding mainly between Ag QDs and the adjacent OH, NH2, and C–O–C groups of the CTS–PEO blend that are consistent with IR results.

Figure 7.

Figure 7

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UV/vis absorption spectra of (a) CTS–PEO blend, (b) 1.0, (c) 3.0, (d) 6.0, (e) 9.0 ,and (f) 12.0 wt % Ag QDs/CTS–PEO.

The absorption spectrum measurement of the prepared composites is also used to investigate and give information about the energy band structure of materials (band gap energy, Eg). The band gaps of the prepared x wt % Ag QDs/CTS–PEO composites were obtained by Tauc’s equation64

3.5.

where υ, h, and α are the wavenumber, Planck’s constant, and absorption coefficient, respectively. Eg is the energy band gap and A is the coefficient constant, in which n = 1/2 and n = 2 represent indirect transition and direct transition, respectively. Figure S3 shows (αhυ)1/2 as a function of photon energy (hυ), which can be used to calculate its optical energy gap of the prepared NMPCs. These absorption edge values were obtained by extrapolating the linear portions for all curves of the prepared composites. The Eg values decreased with the increase of immobilized Ag QDs onto the CTS–PEO blend. The Eg of the neat CTS–PEO blend was 4.03 eV, while Eg values of x wt % Ag QDs/CTS–PEO composites were equal to 3.76, 3.29, 2.91, 2.56, and 1.99 for 1, 3, 6, 9, and 12.0 wt % Ag QDs/CTS–PEO composites, respectively. The reduction in Eg values with the increase in the content of Ag QDs is attributed to the complexation and the chemical bonding between Ag QDs and the CTS–PEO blend and also may be due to the formation of defects in CTS–PEO polymeric chains, leading to the enhancement of optical properties by producing the localized states in the energy gap. The chemical bonding between Ag QDs and CTS–PEO may lead to the overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital energy bands that makes the lower energy transitions feasible and is responsible for reducing the energy gap when increasing the Ag QD content. Therefore, the appearance of the SPR effect and the small band gap values of the prepared composites make it a good material for absorbing the visible light easily because of the significant improvement in their optical and electrical properties, thus improving the photocatalytic activity.

3.6. Elemental Analysis Using EDS and ICP

The theoretical metal loadings of the prepared catalysts were measured using ICP techniques in which the amount of Ag NPs was measured and then compared with the theoretical weight percentage of Ag loadings, as shown in Table 2. As expected for all the prepared catalysts, practically all the Ag present in solution was deposited on the catalysts. In the case of 1.0 6.0 12.0 ,and 20.0 wt % Ag QDs/CTS–PEO, about 98, 95, 96, and 94% were deposited on the catalysts, respectively, which are very close to the theoretical ratio. Also, EDS analysis was performed to map the existence of different elements. As the EDS image displays, the CTS–PEO sample contains only three metals nitrogen (N), oxygen (O), and carbon (C) in its structure, and no other peaks are observed, which revealed that CTS–PEO has no impurities. After the addition of Ag QDs, a new peak related to Ag NPs was formed, as presented in Figure S4. The data obtained from the figures and Table 2 confirm that the experimental Ag atomic ratio was very close to the theoretical value for the prepared samples.

Table 2. Elemental Analysis of x wt % Ag QDs/CTS–PEO Using ICP-AES and EDS.

  ICP EDS-elemental analysis (wt %)
sample actual Ag (wt %) Ag C O N
1.0 wt % Ag QDs/CTS–PEO 0.986 0.999 49.18 37.78 12.05
6.0 wt % Ag QDs/CTS–PEO 5.726 5.76 46.62 34.56 13.06
12.0 wt % Ag QDs/CTS–PEO 11.589 11.01 42.58 33.85 12.56
20.0 wt % Ag QDs/CTS–PEO 18.802 18.31 39.14 31.57 10.98
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3.7. Photocatalytic Activity

3.7.1. Photocatalytic Degradation of PNP

The photocatalytic PNP degradation was studied at different times by using 60 mg of the prepared x wt % Ag QDs/CTS–PEO composites as a photocatalyst and 10 μg/mL PNP solution. Figure 8 shows the effect of contact time from 0 to 3 h on the photocatalytic PNP degradation efficiency for the prepared composites. The blank experiment without the photocatalyst showed that the PNP concentration was constant after light irradiation for about 4 h. After the addition of x wt % Ag QDs/CTS–PEO photocatalysts, the maximum PNP degradation efficiency was 10.40, 35.30, 53.00 67.80 81.10, 91.10, and 84.90% for the neat CTS–PEO blend and 1.0, 3.0, 6.0, 9.0, 12.0, and 20.0 wt % for Ag QDs/CTS–PEO composite samples, respectively. The photocatalytic PNP degradation was increased with contact time and gradually reached to equilibrium after 2 h.

Figure 8.

Figure 8

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Photocatalytic PNP degradation of (a) blank without the catalyst, (b) CTS–PEO blend, (c) 1.0, (d) 3.0, (e) 6.0, (f) 9.0, (g) 12.0, and (h) 20.0 wt % Ag QDs/CTS–PEO.

The immobilized Ag QDs in the CTS–PEO blend matrix play a significant role in enhancing the photocatalytic activity of x wt % Ag QDs/CTS–PEO composites due to the coverage of noble metals, in which Ag particles are easily leached out and decrease the number of surface-active sites, and the chemical corrosion and photocorrosion can inhibit the activity of the photocatalyst.65,66 By embedding the noble M-NPs in the semiconductors, the noble metals could be protected from aggregation and chemical corrosion and can maximize the intimate surface with semiconductors, which may thus maximize the effects of the SPR, coupling of the energy band, and so forth.67,68 The obtained results display that the photocatalytic activity increases with increasing Ag QD contents until reaching 12.0 wt % Ag QDs/CTS–PEO. However, it was observed that the photodegradation of PNP decreased after increasing the Ag QD content above 12.0 wt % where the Ag NPs played an opposite role as recombination centers and weakened the role of oxygen vacancies as centers of electron capture due to the agglomeration of Ag on the surface.69 These results indicate that the amount of Ag QDs played the crucial factor in enhancing and reducing the photodegradation of PNP.

On other hand, the existence of the CTS–PEO blend sheet makes the excess part of electrons to be very easily transferred and made a separation of electron–hole pairs, so Ag excited state electrons were transferred to CTS–PEO sheets to form negative charges on the CTS–PEO surface. The negative charge on CTS–PEO surfaces can react with oxygen to produce a superoxide anion radical, while the holes (positive charges) can react with water to form a hydroxyl-radical, thus facilitating the degradation of PNP. These results are confirmed by UV/visible spectra, which confirmed the decrease in the band gap with the increase of the Ag QD content. The experimental data confirm that the 12.0 wt % Ag QDs/CTS–PEO composite exhibited the best photocatalytic activity due to increasing Ag QDs, and its band gap was the smallest. To achieve complete degradation (100%) of the PNP pollutant, we repeated the photocatalytic PNP degradation by using 70 mg and 80 mg of the 12.0 wt %Ag QDs/CTS–PEO photocatalyst under the same condition. The results showed that 80 mg of the 12.0 wt % Ag QDs/CTS–PEO photocatalyst was able to achieve almost 100% photocatalytic PNP degradation, as shown in Figure S5.

3.7.2. Mechanism of Photocatalysis

The proposed photocatalytic PNP degradation mechanism1,70−72 over Ag QDs/CTS–PEO photocatalysts under simulated sunlight irradiation has been illustrated in Scheme S1. The PNP pollutant degrades by the photogenerated hydroxyl radical (•OH) or superoxide anion radical (O2•–) and positive holes under simulated sunlight irradiation. In short, under simulated sunlight illumination, the main oxidative species in the photocatalytic PNP degradation process are (•OH), (O2•–), and (h+). When the Ag QDs/CTS–PEO composite absorbs simulated sunlight irradiation, the π orbital electrons of polymeric the CTS–PEO blend are excited and transferred to the π* orbital (π–π* transitions). The π* orbital in the polymeric CTS–PEO blend matches well with the energy level and has a chemical bond interaction with the conduction band (CB) of Ag QDs (d-orbitals of Ag QDs), and a synergic effect was created. These excited electrons to the π* orbital of the CTS–PEO polymer can efficiently migrate to Ag QDs. Then, these photogenerated electrons can be transferred to the surface of the Ag QDs/CTS–PEO photocatalyst to react with O2 and water to yield powerful oxygenous radicals (•OH and O2•–), leading to the oxidation of the adsorbed PNP molecules. Similarly, the valence band (VB) electrons of Ag QDs are excited, leaving positive holes (h+). The photogenerated holes in the Ag QD VB can be transferred directly to the HOMO of CTS–PEO because the HOMO energy levels in CTS–PEO are between the VB–CB of Ag QDs and the Ag QDs VB, which match well with the HOMO of CTS–PEO. These photogenerated holes can easily migrate to the surface of the Ag QDs/CTS–PEO photocatalyst and subsequently oxidize the adsorbed PNP molecules directly.

3.7.3. Reusability of the Catalyst

To confirm the stability of the prepared x wt % Ag QDs/CTS–PEO composite samples, we studied the recycling of the 12.0 wt % Ag QDs/CTS–PEO photocatalyst for four cycles under the same conditions and using 80 mg of the photocatalyst. The results confirmed that the photocatalytic PNP degradation was almost 99.13% in four cycles, which is marginally lower when compared to that in the first cycle (100%), as shown in Figure 9. This slight reduction in the efficiency of the 12.0 wt % Ag QDs/CTS–PEO photocatalyst is attributed to the diminished surface interaction between the reaction intermediates in the photocatalytic PNP degradation process. The results clearly indicate the good stability of the prepared 12.0 wt % Ag QDs/CTS–PEO composites as an excellent heterogeneous photocatalyst. Also, Ag NPs (wt %) of the prepared catalyst (12.0 wt % Ag QDs/CTS–PEO) were measured using the ICP-AES (Labtam, 8440 Plasmalab) technique with an Ar+ ion plasma gas equipped with a charged-couple detector for simultaneous detection, and the weight percentages of Ag NPs after the reusability test were 12.01, 11.94, 11.87, and 11.81 wt % for fresh, first, second, and third runs, respectively. In addition, the results gained from our recent work were compared with the other literature, as shown in Table S1.73−77 The results display that our work shows excellent performance for PNP degradation compared with other catalysts, especially in the range of visible light based on the degradation time and degradation rate (%).

Figure 9.

Figure 9

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Effect of reusing times of the photocatalyst on the photocatalytic degradation of PNP for 12.0 wt % Ag QDs/CTS–PEO.

4. Conclusions

In summary, we have studied the immobilization of Ag QDs onto the CTS–PEO polymeric blend composite by a self-assembly facile casting method as a green approach by using the polymeric CTS–PEO blend as a reducing agent and a stabilizer in the synthesis of Ag QDs. Numerous techniques such as XRD, FT-IR spectroscopy, TEM, TGA, and UV/vis spectroscopy have been carried out for the characterization of the prepared CTS–PEO blend and x wt % Ag QDs/CTS–PEO composites. The XRD results confirmed that the Ag QDs were immobilized onto the CTS–PEO blend matrix, in which the peak intensity of Ag QDs increased with increasing Ag content onto CTS–PEO composites, while that of the two peaks of the CTS–PEO blend decreased and accompanied by an increase in their broadness. From FTIR analysis, an interaction between CTS and PEO and an interaction between Ag QDs and the CTS–PEO blend are evidenced by the changes in the position, shape, and intensity of IR absorption bands, such as the shift of the OH, NH2, and C=O bands to a lower wavenumber. The TEM images of the prepared composites appeared with a uniform morphology as a smooth surface sheet, while the TEM images of x wt % Ag QDs/CTS–PEO composites showed small Ag granules with sizes of 2–8 nm in a polymeric blend matrix, which indicates that the Ag QDs were successfully immobilized onto CTS–PEO composites. From UV/vis data, the position of the absorption edge of the x wt % Ag QDs/CTS–PEO composites was slightly shifted toward higher wavelengths and suggests the miscibility of Ag QDs with the CTS–PEO blend. A new band appeared in the visible region, and its intensity was continuously increased with the increase of the immobilized Ag QDs. This band can be assigned to the SPR band of the Ag QDs. The optical gap decreases with increasing Ag QD contents from 4.86 eV for the neat CTS–PEO blend to 1.99 eV for 12.0 wt % Ag QDs/CTS–PEO composites. The results of photocatalytic PNP degradation indicated that the Ag QDs remarkably improves the photocatalytic activity of the x wt % Ag QDs/CTS–PEO photocatalysts irradiated by simulated sunlight (Hg lamp). This study demonstrates that the as-prepared x wt % Ag QDs/CTS–PEO composites are novel NMPC photocatalysts, which have prospective applications in the field of organic pollutant purification.

Acknowledgments

The authors gratefully acknowledge financial support from Taif University Researchers Supporting Project number (TURSP-2020/135), Taif University, Taif, Saudi Arabia.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c03735.

  • Proposed mechanism of photocatalytic PNP degradation by Ag QDs/CTS–PEO photocatalysts under a simulated sunlight source; HR-TEM images of different weight contents of Ag QDs/CTS–PEO; size distribution histograms with a Gaussian-fitting curve for x wt % Ag QDs/CTS–PEO samples; EDX analysis for x wt % Ag QDs/CTS–PEO samples; effect of catalyst dosage on the photodegradation of PNP over 12 wt % Ag QDs/CTS–PEO; and comparison of degradation efficiency of Ag QDs/CTS–PEO and other photocatalysts (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao1c03735_si_001.pdf (1.4MB, pdf)

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