Achieving ultra-trace analysis and multi-light driven photodegradation toward phenolic derivatives via a bifunctional catalyst derived from a Cu(I)-complex-modified polyoxometalate

Abstract

In the field of water decontamination, the design of a new catalyst for sensitive detection and effective removal of toxic phenolic compounds are important and challenging. Herein, a new bifunctional Cu(I)-naphthalene-amide-complex-modified catalyst, [Cu^I₄(L)₄(SiW₁₂O₄₀)(H₂O)₄] (1, L = N,N′-bis(3-methylpyridin-yl)naphthalene-2,6-dicarboxamide), was obtained by incorporating [SiW₁₂O₄₀]⁴⁻ clusters into the Cu-L 3D supramolecular framework. 1 can be used as a peroxidase-like enzyme to detect seven kinds of phenolic compounds (phenol, 4-chlorophenol, o-cresol, p-cresol, 4-nitrophenol, resorcinol and phloroglucinol), which has an “nM” level LOD, satisfactory selectivity, stability, and applicability in various water environments. Using 1 as the photocatalyst, the effects of different irradiation light sources (UV light, visible light, NIR light, full spectrum light, and sunlight) on the photocatalytic degradation of phenol were studied. Taking visible light as representative irradiation, the photocatalytic performance toward the above seven phenolic compounds was also investigated. The removal efficiencies by photocatalytic degradation of the above phenolic derivatives within a relatively short time are satisfactory. By photocurrent response experiments and electrochemical impedance spectroscopy, the corresponding relationships between the types of irradiation light sources and the degradation effect of catalysts on phenol under different excitation light sources were studied. The photocatalytic mechanisms were also investigated using radical trapping experiments, VB-XPS and Mott–Schottky plots in detail.

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1. Introduction

With the rapid development of the global economy and industry, environmental pollution, especially water pollution, has attracted widespread attention.^1–4 Among the various water contaminants, phenol is a representative water contaminant and widely discharged from textiles, packing materials, and agrochemicals, which has seriously harmful effects on human health and ecosystem.^5,6 Moreover, phenol has been listed as a priority pollutant by relevant environmental agencies in the United States and other countries. Therefore, it is significant to achieve the sensitive detection and effective removal of phenol in wastewater.^7,8 So far, various methods (chromatographic, adsorptive, electrochemical, etc.) have been reported to detect the concentration of phenol. Although these methods can provide precise detection results toward phenol, the detection process is complicated, high-cost, and time-consuming.^9,10 Therefore, the colorimetric detection method, which can effectively overcome the above problems and detect phenol with the naked eye based on colour changes, has offered unique advantages including high selectivity, fast response and low cost.^11,12

On the other hand, among many developed technologies for the removal of phenol, photocatalytic degradation of phenol is a useful and environmentally friendly technology, which can degrade phenol into nontoxic products (CO₂ and H₂O) without secondary pollution. Recently, some excellent photocatalysts including Au@Ni/rGO,¹³ MgAC-Fe₃O₄/TiO₂,¹⁴ Fe₃O₄/rGO/MOF,¹⁵ FeTCPP-PW₁₂, and FeTCPP-PMo₁₂¹⁶ have been reported, which displayed outstanding catalytic properties for the degradation of phenol compounds in the ultraviolet or visible light region. As we know, the near-infrared light (NIR) range commanded more than 50% of the full-spectrum region for solar energy, which can provide an opportunity to broaden the solar-light absorption region of photocatalysts. Thus, it is meaningful to synthesize new photocatalysts, which display full-spectrum solar energy absorption, to degrade phenol effectively.

Polyoxometalates (POMs) have been intensively studied owing to their potential applications in various fields by the multielectron-transfer process.^17–22 However, most of the POMs that can act as UV-light-active photocatalysts with high solubility and low stability are restricted in applications.^23,24 POM-based metal–organic complexes (POMOCs) provide a useful strategy for combining the outstanding properties of POMs and the characteristics of MOCs.^25–30 These materials can not only avoid the problems of POM dissolution, leaching, and aggregation, but also broaden the light absorption range of POMOCs from UV to Vis or NIR.^31,32 Some POMOCs applied to the colorimetric detection and degradation of phenol have been exploited, for example, Su et al. have reported a POM-based Cu(II)-complex used for the degradation of phenolic compounds, and the degradation rates of phenol, 4-chlorophenol and 2,4-dichlorophenol are 90.5, 97.0 and 96.9%, respectively.³³ Ma et al. have synthesized a new POM-based Co-complex used as a multifunctional catalyst for the colorimetric detection of phenol and visible photocatalytic degradation to 4-chlorophenol.³⁴ Recently, our group obtained a novel 1D ladder-like POM-based Cu-complex that acted as a bifunctional catalyst, which showed excellent detection and visible photocatalytic degradation abilities towards phenolic compounds.³⁵ These reported POMOCs show the potential applications of colorimetric detection and photocatalytic degradation, and most of them were researched in the process of colorimetric detection and photocatalytic degradation individually or in a single light region (ultraviolet or visible light region). It is very rare to use POMOCs for the colorimetric detection of phenol with a low LOD and the degradation of phenol under different light regimes (especially the NIR or full spectrum region). Therefore, it is still a challenge to synthesize new POMOCs used as bifunctional catalysts, which can be applied for both the ultra-trace detection and multi-light driven photodegradation of phenol.

In this work, the photosensitive ligand L (N,N′-bis(3-methylpyridin-yl) naphthalene-2,6-dicarboxamide, Scheme S1†) was combined with [SiW₁₂O₄₀]⁴⁻ and Cu²⁺ by hydrothermal reactions, resulting in a novel 1D POM-based Cu(I)-complex [Cu^I₄(L)₄(SiW₁₂O₄₀)(H₂O)₄] (1). 1 can act as a colorimetric detector to detect phenolic compounds, the limits of detection (LODs) for phenol, 4-chlorophenol, o-cresol, p-cresol, 4-nitrophenol, resorcinol, and phloroglucinol are 56 nM, 24 nM, 12 nM, 7.8 nM, 730 nM, 150 nM and 48 nM, respectively. The above LODs are obviously lower than those of most reported colorimetric sensors. The colorimetric sensor exhibits satisfactory selectivity, stability and applicability in tap water, industrial discharge water, lake water and river water. Under UV light, visible light, NIR light, full spectrum light, and sunlight irradiation, 1 also has good photocatalytic degradation performance for phenol with a high removal efficiency of 97.69%–99.06% within 60 min–100 min. The removal efficiencies under visible light toward other phenolic derivatives are more than 80.00%. By photocurrent response and electrochemical impedance spectroscopy (EIS) measurements, the corresponding relationships between the degradation effect of catalysts on phenol under different excitation light sources and the types of excitation light sources were studied. The photocatalytic mechanisms were also investigated using the radical trapping experiment, VB-XPS and Mott–Schottky in detail.

2. Experimental

2.1 Synthesis of [Cu^I₄(L)₄(SiW₁₂O₄₀)(H₂O)₄] (1)

A mixture of L (0.13 g, 0.33 mmol), H₄[SiW₁₂O₄₀]·xH₂O (0.51 g, 0.17 mmol) and Cu(NO₃)₂·3H₂O (0.48 g, 1.98 mmol) was dissolved in deionized water (4 mL) and acetonitrile (2 mL) and stirred for 2 h, then pH was adjusted to 3.68 by NaOH (1 M). After this the mixture solution was moved to a 25 mL Teflon-lined stainless steel autoclave and heated at 120 °C for 4 days. After cooling to room temperature (pH_end = 4.6), yellow crystals were obtained. Yield: 46.5% based on H₄[SiW₁₂O₄₀]·xH₂O. Anal. calcd for C₉₆H₈₈Cu₄N₁₆O₅₂SiW₁₂: C, 24.11; H, 1.86; N, 4.69. Found: C, 24.09; H, 1.85; N, 4.68. IR (KBr): 3485 (w), 3374 (w), 3335 (w), 3049 (w), 2927 (w), 1630 (s), 1525 (s), 1433 (m), 1317 (s), 1255 (w), 1194 (m), 1139 (w), 1010 (m), 968 (s), 906 (s), 888 (w), 790 (s), 692 (w), 532 (w). The crystallographic data structure refinement information and selected bond distances (Å) of 1 are summarized in Tables S1 and S2,† respectively.

2.2 Colorimetric detection measurements of phenolic compounds

Various concentrations of phenolic compounds (phenol, 4-chlorophenol, o-cresol, m-cresol, p-cresol, resorcinol, and phloroglucinol) were added to centrifuge tubes containing 1 (0.2 mg mL⁻¹), 4-AAP (2.0 mg mL⁻¹), and then H₂O₂ (4 mM) was added at room temperature. The final total volume of solution is 3 mL. After 30 min, the absorbance intensity was measured with an ultraviolet-visible spectrometer.

2.3 Photocatalytic degradation of phenolic compounds

5.0 mg of complex 1 was dispersed in 10 mL solution with 400 mg L⁻¹ phenolic compounds (pH = 1–12) and reached the adsorption and desorption equilibrium after stirring for 30 min in the dark. Then a certain amount of fresh 30% H₂O₂ (10 μL–400 μL) was added to the mixture. After that, the suspension was irradiated under UV light, visible light, NIR light, and full spectrum light. The temperature of the reactor was maintained at 25 °C using a circulating water control system.

3. Results and discussion

3.1 Structural description

Bond-valence-sum (BVS) calculations show that the valence of Cu, Si, and W atoms are +1, +4 and +6 (Table S3†), verifying the charge balance of 1.³⁶ XPS (Fig. S1a and b†) was also carried out to further confirm the valence of Cu⁺ in 1.³⁷ Structural analysis displayed that 1 is in monoclinic space group C2/c. The structural unit of 1 contains one [SiW₁₂O₄₀]⁴⁻ anion, four Cu^I ions, four ligands, and four coordination water molecules. In 1, Cu1 exhibits four-coordinated pattern, provided by two N atoms (N1, N2) from two L ligands and two coordination water molecules [Cu1–N1: 2.028(10) Å; Cu1–N2^#3: 2.141(10) Å; Cu1–O1W: 2.064(16) Å; and Cu1–O2W: 2.198(14) Å] (Chart S1a†). Cu2 is three-coordinated by two N atoms (N3 and N4) and one amide O atom (O1) from three L ligands [Cu2–N3: 1.896(10) Å; Cu2–N4: 1.896(11) Å; and Cu2–O1^#4: 2.478(10) Å] (Chart S1a†). There is a μ₂-L and a μ₃-L in 1 (Fig. 1a). One μ₃-L acting as a 3-connected node coordinates with one Cu1 and two Cu2 (Chart S1b†). Two μ₃-L connect two Cu1 and two Cu2 to form a Cu₄ metal–organic subunit (Chart S1c†). One μ₂-L links one Cu1 and one Cu2 (Chart S1d†). Then, a pair of μ₂-L bridges the adjacent Cu₄ subunits forming a wave-like nanoribbon (Fig. 1b and Chart S1e†). There are two types of hydrogen bonds (Chart S1f and h†). One type of hydrogen bond between coordinated water molecules (O1 W) combined with Cu1 and amide O atoms (O2) from μ₂-L link parallel nanoribbons forming a 3D supramolecular framework (Chart S1f and g†). The other hydrogen bonds between amide N atoms (N7) from μ₂-L and O atoms (O11) from SiW₁₂ anions modifies SiW₁₂ on either side of the nanoribbon, finally making SiW₁₂ occupy the non-open channel of the above 3D supramolecular framework (Fig. 1c, d and Chart S1h–j†).

Fig. 1 (a) μ₂-L and μ₃-L. (b) Schematic view of the 1D metal–organic nanoribbon. (c) Basket-like structural unit. (d) 3D stacking structure based on SiW₁₂ anions and metal–organic nanoribbons.

3.2 Characterization

The experimental powder X-ray diffraction (PXRD) patterns of complex 1 match well with the simulated pattern, indicating that the phase purity of 1 is good (Fig. S2†). The characteristic bands observed at 1010 cm⁻¹–888 cm⁻¹ in IR spectra are attributed to v(W=O), v(Si–O), v(W–O–W), and v(W–O–W), respectively. The peaks at 2927 cm⁻¹–3049 cm⁻¹, 692 cm⁻¹–790 cm⁻¹ and 1433 cm⁻¹–1630 cm⁻¹ are attributed to v(C–H) and v(C–N). The characteristic absorption peaks of the naphthalene ring are in the range of 1433 cm⁻¹–1630cm⁻¹ (Fig. S3†).³⁸

The solid-state ultraviolet-visible spectroscopy (UV-vis) of 1 was carried out for the evaluation of the photochemical properties (Fig. 2a). The band gap energy (E_g) of 1 was 2.72 eV (Fig. 2b), which displayed that 1 has potential as a semiconductor photocatalyst in the broad spectrum region.³⁹ In order to determine the flat-band potential (generally regarded as the Fermi level, E_f), the Mott–Schottky studies of 1 was performed at frequencies of 1000 Hz. The positive slope of the curve shows that 1 exhibits typical n-type semiconductor characteristics with the Fermi level E_f of −0.34 eV vs. Ag/AgCl, corresponding to −0.14 eV vs. NHE (Fig. 2c).⁴⁰ The gap between the E_f and VB potentials of 1 was obtained by VB-XPS, which is 0.81 eV (0.57 eV vs. NHE) (Fig. 2d).⁴¹ Therefore, the VB of 1 was determined to be 0.43 eV (vs. NHE); combining the band gap value obtained above, the CB of 1 was calculated to be −2.29 eV (vs. NHE).

Fig. 2 (a) UV-vis absorption spectrum of 1. (b) The optical band gap of 1. (c) Mott–Schottky plot of 1. (d) The VB-XPS of 1.

3.3 Ultra-trace colorimetric detection of phenolic derivatives

Phenolic compounds are commonly found in industrial wastewater and are regarded as a threat to human health. Therefore, a rapid and accurate method such as colorimetric method to detect phenolic compounds is more desirable.^42,43 Phenol was chosen as a representative to introduce the detailed experimental process. H₂O₂ was used as an oxidant in the oxidative coupling reaction between phenol and 4-AAP to verify the peroxidase-like characteristics of 1. The results are shown in Fig. 3a. The mixture of 4-AAP and phenol exhibited little absorption intensity at room temperature. When H₂O₂ or 1 was added to the 4-AAP + phenol system, the corresponding absorption intensity was also very low. The difference is that when H₂O₂ and 1 were both added to the 4-AAP + phenol system, an obvious absorption peak (about 501 nm) appeared and the color of the reaction system changed from colorless to pink. When the same molar mass of H₂O₂, 1, SiW₁₂, L or Cu²⁺ were separately added to the 4-AAP + phenol system, the intensity of the absorption peak at about 501 nm did not increase obviously, indicating that the oxidative coupling reaction did not occur (Fig. 3b). These results suggest that 1 can be used as a mimic of peroxidase to catalyze the oxidation of phenol and 4-AAP to form quinoneimine by hydrogen peroxide. The reaction time of the 4-AAP + phenol + H₂O₂ + 1 system was further explored. The absorbance increased gradually within 0–30 min, and there was no obvious increase in absorbance within 30–40 min (Fig. 3c). Therefore, the 4-AAP + H₂O₂ + 1 system could act as an effective colorimetric platform for phenol detection.

Fig. 3 (a) and (b) The absorbance spectra of diverse systems. (c) The absorbance spectra of the 4-AAP + phenol + H₂O₂ + 1 system vs. time. (d) ˙OH-trapping photoluminescence spectra (0.5 mg mL⁻¹ of 1, 0.5 mM of TA, 5 mM of H₂O₂).

Photoluminescence experiments were carried out to investigate the mechanism of 1 as a peroxidase and the corresponding experimental details are given in the ESI.† ^44,45 As shown in Fig. 3d, TA + H₂O₂ solution (III) and TA + H₂O₂ + 1 solution (IV) have similar emission peaks at 435 nm. The fluorescence intensity of (IV) solution was higher than that of (III) solution. As we know, ˙OH can extract electrons from the hydroxyl of phenols because of its strong oxidation ability. Then quinone free radicals were subsequently generated. Finally, pink quinoneimine was obtained because of the oxidative coupling of 4-AAP with the quinone radical led by the superfluous ˙OH (Fig. 4a).⁴⁶ The above investigations proved that the peroxidase-like activity of 1 was mainly derived from ˙OH, which is from the decomposition of H₂O₂.

Fig. 4 (a) The proposed mechanism of phenol catalyzed by 1. (b) The absorbance spectra with the increase of the phenol concentration. (c and d) the absorbance intensities vs. concentrations of phenol, Inset: linear calibration plot for phenol.

In order to make full use of the colorimetric performance of the 4-AAP + H₂O₂ + 1 system, the experimental conditions such as concentration of 4-AAP (0.5–2.5 mg mL⁻¹), pH (2–5), concentration of the catalyst (0.1–0.5 mg mL⁻¹), and concentration of H₂O₂ (2–7 mM) were studied using phenol as a model (Fig. S4a–d†). Thus the optimized conditions (4-AAP: 2 mg mL⁻¹; pH = 3; catalyst: 0.2 mg mL⁻¹; H₂O₂: 4 mM) were obtained and the quantitative phenolic compound detection was carried out under these conditions. In a classical experiment, phenolic compounds (phenol, 4-chlorophenol, o-cresol, p-cresol, 4-nitrophenol, resorcinol, and phloroglucinol) with different concentrations (0.001–0.1 mM) were added to the solution containing the 1 + 4-AAP system, H₂O₂ is subsequently added (Fig. 4b, c and Fig. S5, S6†). With the increase of the concentration of phenolic compounds (0.001 mM–0.1 mM), the corresponding absorbance intensities increased gradually. In the range of 0.001 mM–0.01 mM, the absorbance intensity and the concentration of phenolic compounds showed a good linear relationship (Fig. 4c and Fig. S6†). The detection limits (S/N = 3) for phenol, 4-chlorophenol, o-cresol, p-cresol, 4-nitrophenol, resorcinol, and phloroglucinol were satisfactory (56 nM, 24 nM, 12 nM, 7.8 nM, 730 nM, 150 nM and 48 nM). According to the literature research, oxide, oxide@complex, metal element and complex have been used as colorimetric detectors for the detection of phenol (Table S4†). The LODs of the reported materials are in the range of 0.86–3.79 μM. The LOD of 1 toward phenol is 56 nm, which is apparently lower than the above reported colorimetric detectors.

In order to demonstrate the selectivity of phenolic compounds, the possible interference of coexisting species such as NaCl, NaNO₂, CH₃CH₂OH, CH₃COCH₃, 2-ethylimidazole, acetophenone, nitrobenzene, and isobutyraldehyde on phenolic compounds was studied (Fig. S7†). When the above substances were added to the 1 + H₂O₂ + 4-AAP systems, the solution remained colorless, namely, the addition of the above substances had little effect on the detection of phenolic compounds.

3.4 The application of colorimetric detection of phenol

To estimate the practicability test in the real water sample, tap water (Jinzhou), industrial discharge water (a petrochemical plant) and water from JinLing Reservoir (Jinzhou) and Daughter River (Jinzhou) were selected for practical application toward the detection of phenol. The results are listed in Table 1. The recoveries of the three samples ranged from 93.33 to 110.00%, the RSDs were achieved in the range of 1.00–8.67% (n = 3), indicating that 1 had great potential for the trace detection of phenol in actual water samples.

Table 1 Results of the determination of phenol in three water samples

Sample	Original amount (mM)	Added (mM)	Found (mM)	Recovery (%)			RSD (%)
			1	2	3	1	2	3
a N.D., not detected.
Tap water	N.D.^a	0.002	0.0021	0.0022	0.0019	105.00	110.00	95.00	7.39
		0.006	0.0058	0.0056	0.0061	96.67	93.33	101.67	4.32
		0.01	0.0101	0.0102	0.0099	101.00	102.00	99.00	1.52
Industrial discharge water	N.D.^a	0.002	0.0019	0.0021	0.0020	95.00	105.00	100.00	5.00
		0.006	0.0062	0.0058	0.0060	103.33	96.67	100.00	3.33
		0.01	0.0099	0.0101	0.010	99.00	101.00	100.00	1.00
Lake water	N.D.^a	0.002	0.0019	0.0021	0.0021	95.00	105.00	105.00	5.68
		0.006	0.0062	0.0058	0.0058	103.33	96.67	96.67	3.89
		0.01	0.0099	0.0101	0.0101	99.00	101.00	101.00	1.15
River water	N.D.^a	0.002	0.0019	0.0019	0.0022	95.00	95.00	110.00	8.67
		0.006	0.0060	0.0062	0.0057	100.00	103.33	95.00	4.22
		0.01	0.0100	0.0103	0.0102	100.00	103.00	102.00	1.50

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3.5 Multi-light driven photodegradation of phenolic derivatives

Based on the above experimental results, 1 exhibited peroxidase-like activity and could act as an efficient colorimetric sensor toward phenolic compounds. Besides detection, the degradation of phenolic compounds was also important in water treatment. Thus, four factors (pH, and the concentration of H₂O₂, catalyst and phenolic compounds) were considered to investigate the photocatalytic degradation activity of complex 1 for phenolic compounds. As we know, the pH range of wastewater is wide, we chose the pH of phosphate buffered saline (PBS) solution which is 1–12 to explore phenol degradation. The results showed that the removal efficiency of phenol was the highest when pH = 8 (Fig. 5a). The effect of the initial H₂O₂ concentration on phenol degradation is shown in Fig. 5b, 99.06% of phenol can be removed within 70 min when 200 μL of H₂O₂ was added to the PBS solution. As shown in Fig. 5c, the degradation efficiency of phenol varies with the catalyst dosage, and reaches the maximum when the dosage of the catalyst is 5 mg. In addition, the degradation ability of 1 with different concentrations of phenol was also explored. During the experiment, catalyst 1 showed satisfactory removal efficiency for phenol at concentrations of 200 (99.16%) and 400 (99.06%) mg L⁻¹, but when phenol concentration increased to 600 mg L⁻¹ or even higher, the degradation efficiency for phenol was limited (Fig. 5d). Thus, the degradation of phenol (400 mg L⁻¹) was carried out under optimal conditions (pH = 8, reaction time: 70 min, H₂O₂: 200 μL and catalyst 1: 5 mg).

Fig. 5 Degradation efficiency of phenol under different reaction conditions within 70 min. (a) pH; (b) concentration of H₂O₂; (c) effect of catalyst dosage; and (d) effect of phenol concentration.

The photocatalytic activities of complex 1 for the degradation of phenol under different irradiation conditions were also explored. When full spectrum light irradiation was used, the degradation efficiency of phenol was 98.42% within 60 min. The corresponding degradation efficiency using visible light, UV light and NIR light irradiation are 99.06% (within 70 min), 97.69% (within 90 min) and 97.72% (within 100 min) (Fig. 6a, b and Fig. S8a–c†). The results show that complex 1 has a good degradation ability toward phenol in the whole spectrum range. Besides, to build a comprehensive study, the analysis of reaction kinetics using the Langmuir–Hinshelwood (L–H) model was also carried out. Experimental results show that the reaction kinetics of the photocatalytic process was consistent with the pseudo-first-order mode.^47–49 The ln (c₀/c) and reaction time of phenol exhibits a good linear relationship, in which the linear correlation coefficient (R²) is greater than 0.9.^50,51 The rate constants of 1 for phenol are 0.0581 min⁻¹, 0.0572 min⁻¹, 0.0397 min⁻¹, and 0.0306 min⁻¹, with the corresponding half-lives of 11.93 min, 12.12 min, 17.46 min and 122.65 min (Fig. 6c and Fig. S8d–f†). The raw materials (L, SiW₁₂ and Cu(NO₃)₂·3H₂O) for the synthesis of 1 were also used as catalysts for the degradation of phenol under visible light irradiation. As a result, the corresponding degradation efficiencies are only 41.78%, 42.65%, and 42.62%, which are lower than those without irradiation, catalyst or H₂O₂ (Fig. 6d).

Fig. 6 (a) The photocatalytic degradation of phenol with 1 under different light regimes with the increase of time. (b) The absorbance spectra vs. reaction time under visible light irradiation. (c) The linear relationship between ln (c₀/c) and reaction time (t) of phenol. (d) The photocatalytic degradation of phenol under different reaction conditions.

It is not difficult to see from Fig. 6a that different irradiation conditions have different effects on the degradation efficiency of phenol. Therefore, the transient photocurrent response and electrochemical impedance spectroscopy (EIS) of 1 under different light regimes were investigated to explore the corresponding relationships between the degradation effect of catalysts under different excitation light sources and the types of excitation light sources. As shown in Fig. S9,† the stable photocurrent response of 1 shows almost a two-fold enhancement compared to blank in multiple on–off cycles under different light regimes (full spectrum light, visible light, UV light, and NIR light), which illustrates the generation of electron–hole pairs by photoexcitation and effective separation. The photocurrent responses of 1 under full spectrum light, visible light, UV light, and NIR light are from strong to weak (Fig. 7a). This result displays that the activation effect of different light regimes on the catalyst is different. On the other hand, the electrochemical impedance spectroscopy (EIS) results show that the resistance of 1 is the smallest under full spectrum irradiation, and is the biggest under NIR (Fig. 7b). These electrochemical results suggest that the fastest charge transports of 1 are achieved under full spectrum irradiation. The corresponding values are the lowest under NIR. The EIS and photocurrent response results correspond to those of phenol removal efficiency.^52–54

Fig. 7 (a) Transient photocurrent response of 1 under different light regimes. (b) Nyquist plots of 1 under different light regimes.

In addition, the catalytic activity of complex 1 toward phenolic compounds (4-chlorophenol, o-cresol, p-cresol, 4-nitrophenol, resorcinol and phloroglucinol) under visible light irradiation was also investigated under the optimum conditions. The corresponding removal efficiencies are 96.27%, 92.43%, 82.96%, 87.67%, 80.00% and 83.38% within 70 min (Fig. S10a–f†). The linear relationship between ln (c₀/c) and reaction times was good. The linear correlation coefficient (R²) is greater than 0.9. The appearance rate constants of 1 for the above seven phenolic compounds are 0.0363 min⁻¹, 0.0276 min⁻¹, 0.0223 min⁻¹, 0.0270 min⁻¹, 0.0217 min⁻¹, and 0.0200 min⁻¹, with the corresponding half-life of 19.09 min, 25.11 min, 31.08 min, 25.67 min, 31.94 min, and 34.66 min (Fig. S11a–f†). The above results prove that 1 is an efficient catalyst for the removal of phenolic compounds.

Complex 1 as the heterogeneous catalyst has also been investigated in the recycling experiments. The removal efficiency of phenol under visible light irradiation showed no evident changes after 5 cycles using the recovered catalyst (Fig. 8a). The XPS (Fig. 8b), PXRD patterns (Fig. S12†) and IR spectra (Fig. S13†) before and after the detection and degradation under visible light irradiation remained broadly consistent, which show good stability of 1.

Fig. 8 (a) The phenol removal efficiency after five cycles. (b) The XPS spectral patterns of 1 before and after detection/degradation under visible light irradiation.

Based on the above experimental results and analyses, the reactive species trapping experiments were conducted to determine the mechanism of the photocatalytic process.^55,56 To address the role of the active species involved in the photocatalytic degradation of phenolic compounds in the presence of 1, we have used different well-known scavengers to trap different active species. Silver nitrate (0.01 M), ammonium oxalate (AO, 0.01 M), tert-butanol (TBA, 0.01 M), and benzoquinone (BQ, 0.01 M) were used as scavengers of the photogenerated electrons (e⁻), holes (h⁺), hydroxyl radicals (˙OH), and superoxide anion radicals (˙O₂⁻). As a result, the corresponding removal efficiencies of phenol decrease to 79.31%, 46.61%, 72.58% and 49.69%, respectively (Fig. 9a). The results show that e⁻, h⁺, ˙OH, and ˙O₂⁻ influence the process of phenol degradation. Among them, h⁺ and ˙O₂⁻ play the major roles. On the other hand, the Raman spectra of 1 were measured to verify the formation of O–O. After the treatment of 1 with H₂O₂, a new peak at 879 cm⁻¹ was clearly observed (Fig. 9b), which is assigned to O–O stretching.⁵⁷

Fig. 9 (a) The phenol removal efficiency in the presence of different sacrificial agents under visible light irradiation. (b) Raman spectra of 1 before and after treatment with H₂O₂.

A possible mechanism of phenol degradation by 1 is shown in Fig. 10. Under light irradiation, the photogenerated electrons and holes on 1 are separated. In the removal process of phenol, the excited holes oxidize phenol directly to generate CO₂ and H₂O. The [Cu^I(L)(H₂O)]₄⁴⁺ chain coordination units and [SiW₁₂O₄₀]⁴⁻ in 1 facilitate the electron transfer. e⁻ could capture O₂ to produce ˙O₂⁻, and ˙O₂⁻ reacts with H⁺ to produce additional ˙OH. Simultaneously, the aggregation of h⁺ in the VB of 1 could trap H₂O to create ˙OH. Eventually, phenolic compounds could be degraded to CO₂ and H₂O.^13,34,58

Fig. 10 Schematic view of the possible mechanism of degradation of phenolic compounds by 1.

3.6 The application of the degradation of phenol under sunlight

Sunlight is a clean and renewable energy source and the utilization of sunlight is of great significance to environmental protection.⁵⁹ Therefore, sunlight was used for irradiation to investigate the catalytic activity of complex 1 toward phenol under optimal conditions. All the reactions were carried out on bright sunny days from 12 am to 2 pm in summer and autumn at Jinzhou, China. As a result, around 95.31%–98.68% degradation of phenol was achieved within 70 min (Fig. S14†). These results can provide experimental reference for the degradation of phenolic pollutants under environmentally friendly conditions.

4. Conclusions

In summary, a new Cu(I)-naphthalene-amide-complex polyoxometalate-based complex has been successfully synthesized by combining 3D supramolecular framework and [SiW₁₂O₄₀]⁴⁻ clusters. As a bifunctional catalyst, it can be used for the ultra-trace detection of phenolic derivatives (phenol, 4-chlorophenol, o-cresol, p-cresol, 4-nitrophenol, resorcinol, and phloroglucinol) with an “nM” level LOD (7.8 nM–730 nM) and hydroenvironmental adaptability. Under UV light, visible light, NIR light, full spectrum light, and sunlight irradiation, the title complex exhibits good removal efficiency (97.69%–99.06%) by photocatalytic degradation of phenol within a relatively short time. Under visible light irradiation, the other 6 above phenolic derivatives can be degraded by 1 within 70 min with a high removal efficiency of 80.00%–96.27%. The results of the photocurrent response test and electrochemical impedance spectroscopy display that different lights have a significant influence on the photocatalytic effect toward the degradation of phenolic compounds. This work provides important guidance for the development of POMOC as bifunctional catalysts for the detection and multi-light driven photodegradation of phenolic compounds, showing good industrial application potential.

Author contributions

Shuang Li: synthesis of the N-ligand and the complex, design and execution of experiments, and writing – original draft. Bingqian Wang: characterization of the complex. Guo-Cheng Liu: formulation of the overarching research goal, revision of the draft and the finalization of the manuscript. Xiaohui Li: performance analysis of the complex. Chang Sun: structural analysis of the complex. Zhong Zhang: design and adjustment of the experimental scheme, manuscript reviewing and funding support. Xiuli Wang: manuscript reviewing and funding support.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the NSFC (no. 21901018, 22271021 and 22201021), the Natural Science Foundation of Liaoning Province (2022-MS-373 and 2021-MS-312).

References

1. W. X. Shi, Z. Y. Liu and Z. M. Zhang, CdS-POM nanosheets for highly efficient visible-light-driven H2 evolution, Inorg. Chem. Front., 2023, 10, 4534.
2. Y. Ding, C. H. Wang, L. Pei, S. Maitra, Q. N. Mao, R. T. Zheng, M. J. Liu, Y. H. Ng, J. S. Zhong, L. H. Chen and B. L. Su, Emerging heterostructured C₃N₄ photocatalysts for photocatalytic environmental pollutant elimination and sterilization, Inorg. Chem. Front., 2023, 10, 3756.
3. J. J. Liu, S. N. Sun, J. Liu, Y. Kuang, J. W. Shi, L. Z. Dong, N. Li, J. N. Lu, J. M. Lin, S. L. Li and Y. Q. Lan, Achieving high-efficient photoelectrocatalytic degradation of 4-chlorophenol via functional reformation of titanium-oxo clusters, J. Am. Chem. Soc., 2023, 145, 6112–6122.
4. S. S. Wang and G. Y. Yang, Recent advances in polyoxometalate-catalyzed reactions, Chem. Rev., 2015, 115, 4893–4962.
5. J. Yan, Q. H. Meng, X. J. Shen, B. F. Chen, Y. Sun, J. F. Xiang, H. Z. Liu and B. X. Han, Selective valorization of lignin to phenol by direct transformation of C_sp2−C_sp3 and C–O bonds, Sci. Adv., 2020, 6, eabd1951.
6. X. Z. Du, H. J. Fan, S. L. Liu and Z. C. Zhang, Selective nucleophilic α-C alkylation of phenols with alcohols via Ti=Cα intermediate on anatase TiO₂ surface, Nat. Commun., 2023, 14, 4479.
7. W. Xu, Y. F. Zhang, X. Zhang, X. X. Xu and Q. Wang, One stone, two birds: A Cu-S cluster as a laccase-mimicking nanozyme and sulfite activator for phenol remediation in marine environments, J. Hazard. Mater., 2023, 457, 131776.
8. Z. N. Gu, Z. Y. Zhang, N. Ni, C. Z. Hu and J. H. Qu, Simultaneous phenol removal and resource recovery from phenolic wastewater by electrocatalytic hydrogenation, Environ. Sci. Technol., 2022, 56, 4356–4366.
9. R. Song, H. B. Chi, Q. L. Ma, D. F. Li, X. M. Wang, W. S. Gao, H. Wang, X. L. Wang, Z. L. Li and C. Li, Highly efficient degradation of persistent pollutants with 3D nanocone TiO₂-based photoelectrocatalysis, J. Am. Chem. Soc., 2021, 143, 13664–13674.
10. Y. M. Lin, F. Wang, J. Yu, X. Zhang and G. P. Lu, Iron single-atom anchored N-doped carbon as a ‘laccase-like’ nanozyme for the degradation and detection of phenolic pollutants and adrenaline, J. Hazard. Mater., 2022, 425, 127763.
11. T. D. Tran, P. T. Nguyen, T. N. Le and M. Kim, DNA-copper hybrid nanoflowers as efficient laccase mimics for colorimetric detection of phenolic compounds in paper microfluidic devices, Biosens. Bioelectron., 2021, 182, 113187.
12. J. W. Kim, D. Monllor-Satoca and W. Y. Choi, Simultaneous production of hydrogen with the degradation of organic pollutants using TiO₂ photocatalyst modified with dual surface components, Energy Environ. Sci., 2012, 5, 7647.
13. G. Darabdhara and M. R. Das, Dual responsive magnetic Au@Ni nanostructures loaded reduced graphene oxide sheets for colorimetric detection and photocatalytic degradation of toxic phenolic compounds, J. Hazard. Mater., 2019, 368, 365–377.
14. Y. Jang, V. K. H. Bui, P. T. Nguyen, Y. C. Lee and M. Kim, UV-light-driven enhancement of peroxidase-like activity of Mg-aminoclay-based Fe₃O₄/TiO₂ hybrids for colorimetric detection of phenolic compounds, Chemosensors, 2021, 9, 219–228.
15. Y. Wang, M. Z. Zhao, C. Hou, X. N. Yang, Z. J. Li, Q. J. Meng and C. Liang, Graphene-based magnetic metal organic framework nanocomposite for sensitive colorimetric detection and facile degradation of phenol, J. Taiwan Inst. Chem. Eng., 2019, 102, 312–320.
16. Z. N. Xia, L. B. Wang, Q. Zhang, F. Y. Li and L. Xu, Fast degradation of phenol over porphyrin-polyoxometalate composite photocatalysts under visible light, Polyoxometalates, 2022, 1, 9140001.
17. Z. Zhang, Y. L. Wang, Y. Liu, S. L. Huang and G. Y. Yang, Three ring-shaped Zr(IV)-substituted silicotungstates: syntheses, structures and their properties, Nanoscale, 2020, 12, 18333–18341.
18. D. H. Li, X. Y. Zhang, J. Q. Lv, P. W. Cai, Y. Q. Sun, C. Sun and S. T. Zheng, Photo-activating biomimetic polyoxomolybdate for boosting oxygen evolution in neutral electrolytes, Angew. Chem., Int. Ed., 2023, 62, e202312706.
19. X. Chang, X. F. Yan, Y. Qiao, S. Wang, M. H. Zhang, J. Xu, D. H. Wang and X. H. Bu, Confined heteropoly blues in defected Zr-MOF (bottle around ship) for high-efficiency oxidative desulfurization, Small, 2020, 16, 1906432.
20. J. N. Lu, J. J. Liu, L. Z. Dong, J. M. Lin, F. Yu, J. Liu and Y. Q. Lan, Synergistic metal-nonmetal active sites in a metal-organic cage for efficient photocatalytic synthesis of hydrogen peroxide in pure water, Angew. Chem., Int. Ed., 2023, 62, e202308505.
21. L. F. Lian, H. Y. Zhang, S. An, W. Chen and Y. F. Song, Polyoxometalates-based heterogeneous catalysts in acid catalysis, Sci. China: Chem., 2021, 64, 1117–1130.
22. X. X. Li, C. H. Li, M. J. Hou, B. Zhu, W. C. Chen, C. Y. Sun, Y. Yuan, W. Guan, C. Qin, K. Z. Shao, X. L. Wang and Z. M. Su, Ce-mediated molecular tailoring on gigantic polyoxometalate {Mo₁₃₂} into half-closed {Ce₁₁Mo₉₆} for high proton conduction, Nat. Commun., 2023, 14, 5025.
23. Y. N. Liu, J. Wang, L. N. Li, S. Meng, K. H. Ji, P. T. Ma, J. P. Wang and J. Y. Niu, Enhanced photoinduced Baeyer−Villiger oxidation of ketones by introducing trinuclear ruthenium clusters into polyoxometalate based metal−organic frameworks, Chem. Mater., 2023, 35, 3941–3950.
24. C. L. Xiao, L. Zhang, K. F. Wang, H. P. Wang, Y. Y. Zhou and W. Z. Wang, A new approach to enhance photocatalytic nitrogen fixation performance via phosphate-bridge: a case study of SiW₁₂/K-C₃N₄, Appl. Catal., B, 2018, 239, 260–267.
25. J. X. Liu, X. B. Zhang, Y. L. Li, S. L. Huang and G. Y. Yang, Polyoxometalate functionalized architectures, Coord. Chem. Rev., 2020, 414, 213260.
26. H. P. Xiao, Y. S. Hao, X. X. Li, P. Xu, M. D. Huang and S. T. Zheng, A water-soluble antimony-rich polyoxometalate with broad spectrum antitumor activities, Angew. Chem., Int. Ed., 2022, 61, e202210019.
27. M. L. Wang, Y. J. Guo, G. Y. Zhao, B. K. Chen and Y. F. Bi, Ni₄-thiacalix[4]arene sandwiched Mo8 polyoxometalate bimetallic nanoclusters for electrocatalytic glucose oxidation, Chin. Chem. Lett., 2023, 34, 107365.
28. J. C. Liu, J. F. Wang, Q. Han, P. Shangguan, L. L. Liu, L. J. Chen, J. W. Zhao, C. Streb and Y. F. Song, Multicomponent self-assembly of a giant heterometallic polyoxotungstate supercluster with antitumor activity, Angew. Chem., Int. Ed., 2021, 60, 11153–11157.
29. Y. R. Gong, Y. T. Zhang, C. Qin, C. Y. Sun, X. L. Wang and Z. M. Su, Bottom-up construction and reversible structural transformation of supramolecular isomers based on large truncated tetrahedra, Angew. Chem., 2019, 131, 790–794.
30. Z. W. Guo, L. H. Lin, J. P. Ye, Y. Chen, X. X. Li, S. Lin, J. D. Huang and S. T. Zheng, Core-shell-type all-inorganic heterometallic nanoclusters: record high-nuclearity cobalt polyoxoniobates for visible-light-driven photocatalytic CO₂ reduction, Angew. Chem., Int. Ed., 2023, 62, e202305260.
31. L. Yang, Z. Zhang, C. N. Zhang, S. Li, G. C. Liu and X. L. Wang, An excellent multifunctional photocatalyst with a polyoxometalate–viologen framework for CEES oxidation, Cr(VI) reduction and dye decolorization under different light regimes, Inorg. Chem. Front., 2022, 9, 4824–4833.
32. X. J. Sun, J. Zhang and Z. Y. Fu, Polyoxometalate cluster sensitized with copper-viologen framework for efficient degradation of organic dye in ultraviolet, visible, and near-infrared light, ACS Appl. Mater. Interfaces, 2018, 10, 35671–35675.
33. Y. Lu, T. Zhang, Y. X. Zhang, X. J. Sang, F. Su, Z. M. Zhu and L. C. Zhang, A POM-based copper-coordination polymer crystal material for phenolic compound degradation by immobilizing horseradish peroxidase, Dalton Trans., 2021, 50, 15198–15209.
34. J. J. Xin, H. J. Pang, Z. X. Jin, Q. Wu, X. J. Yu, H. Y. Ma, X. M. Wang, L. C. Tan and G. X. Yang, Two polyoxometalate-encapsulated two-fold interpenetrating dia metal−organic Frameworks for the detection, discrimination, and degradation of phenolic pollutants, Inorg. Chem., 2022, 61, 16055–16063.
35. S. Li, J. Y. Sun, G. C. Liu, S. Zhang, Z. Zhang and X. L. Wang, A new Keggin-type polyoxometallate-based bifunctional catalyst for trace detection and pH-universal photodegradation of phenol, Chin. Chem. Lett., 2023 DOI:10.1016/j.cclet.2023.109148.
36. I. D. Brown and D. Altermatt, Bond-valence parameters obtained from a systematic analysis of the inorganic crystal structure database, Acta Crystallogr., Sect. B: Struct. Sci., 1985, 41, 244–247.
37. S. Zhang and L. Zhao, A merged copper(I/II) cluster isolated from Glaser coupling, Nat. Commun., 2019, 10, 4848.
38. X. M. Luo, N. F. Li, Z. B. Hu, J. P. Cao, C. H. Cui, Q. F. Lin and Y. Xu, Polyoxometalate-based well-defined rodlike structural multifunctional materials: synthesis, structure, and properties, Inorg. Chem., 2019, 58, 2463–2470.
39. P. M. Stanley, A. Y. Su, V. Ramm, P. Fink, C. Kimna, O. Lieleg, M. Elsner, J. A. Lercher, B. Rieger, J. Warnan and R. A. Fischer, Photocatalytic CO₂-to-syngas evolution with molecular catalyst metal-organic framework nanozymes, Adv. Mater., 2023, 35, 2207380.
40. S. P. Tandon and J. I. Gupta, Measurement of forbidden energy gap of semiconductors by diffuse reflectance technique, Phys. Status Solidi, 1970, 38, 363–367.
41. J. Li, X. Y. Wu, W. F. Pan, G. K. Zhang and H. Chen, Vacancy-rich monolayer BiO_2−-x as a highly efficient UV, visible, and near-infrared responsive photocatalyst, Angew. Chem., Int. Ed., 2018, 57, 491–495.
42. P. Watthaisong, P. Pongpamorn, P. Pimviriyakul, S. Maenpuen, Y. Ohmiya and P. Chaiyen, A chemo-enzymatic cascade for the smart detection of nitro- and halogenated phenols, Angew. Chem., 2019, 131, 13388–13392.
43. Y. W. Guan, Y. X. Lu, J. Y. Zhao, W. Huang and Y. Y. Liu, Cobalt-based zeolitic imidazole framework incorporated with well-dispersed bimetallic nanoparticles/ions as a multifunctional nanozyme for the degradation of environmental pollutants and discrimination of various phenolic substances, Chem. Eng. J., 2023, 465, 142703.
44. L. Zhang, Q. G. Tan, S. J. Xiao, G. P. Yang, Q. Q. Zheng, C. Sun, X. L. Mao, J. Q. Fan, R. P. Liang and J. D. Qiu, Reversed regulation effects of ssDNA on the mimetic oxidase and peroxidase activities of covalent organic frameworks, Small, 2023, 19, 2207798.
45. D. K. Li, Y. S. Fang and X. M. Zhang, Bacterial detection and elimination using a dual-functional porphyrin-based porous organic polymer with peroxidase-like and high near-infrared-light-enhanced antibacterial activity, ACS Appl. Mater. Interfaces, 2020, 12, 8989–8999.
46. N. N. Xing, Y. S. Lyu, J. Yang, X. L. Zhang, Y. Han, W. L. Zhao, D. H. L. Ng and J. Li, Motion-based phenol detection and degradation using 3D hierarchical AA-NiMn-CLDHs@HNTs-Ag nanomotors, Environ. Sci.: Nano., 2022, 9, 2815–2826.
47. Y. R. Wang, J. M. Ma, X. L. Wang, Z. S. Zhang, J. H. Zhao, J. Yan, Y. P. Du, H. B. Zhang and D. Ma, Complete CO oxidation by O₂ and H₂O over Pt−CeO₂−δ/MgO following Langmuir−Hinshelwood and mars−van krevelen mechanisms, respectively, ACS Catal., 2021, 11, 11820–11830.
48. S. Bernadet, E. Tavernier, D. M. Ta, R. A. L. Vallée, S. Ravaine, A. Fécant and R. Backov, Bulk photodriven CO₂ conversion through TiO₂@Si(HIPE) monolithic macrocellular foams, Adv. Funct. Mater., 2019, 29, 1807767.
49. W. A. Thompson, E. S. Fernandez and M. M. Maroto-Valer, Probability Langmuir-Hinshelwood based CO₂ photoreduction kinetic models, Chem. – Eur. J., 2020, 384, 123356.
50. R. Farouq, Investigation of the kinetics and optimization of photocatalytic degradation of methylene blue, J. Chin. Chem. Soc., 2018, 65, 1333–1339.
51. Z. H. Wang, C. Z. Wang, C. W. Zhang and W. J. Li, Ultrasound-assisted enzyme catalyzed hydrolysis of olive waste and recovery of antioxidant phenolic compounds, Innovative Food Sci. Emerging Technol., 2017, 44, 224–234.
52. Y. J. Guo, J. H. Li, G. Y. Zhao, B. K. Chen, X. X. Hang and Y. F. Bi, Post-synthetic atomically precise single-Cu^II sites in a thiacalix[4]arene-supported octahedral Na@Co₂₄ cluster: the Cu–Co synergistic effect for selective photocatalytic conversion of CO₂ to CO, Chem. Commun., 2023, 59, 5591–5594.
53. J. J. Zhang, T. Y. Bai, H. Huang, M. H. Yu, X. B. Fan, Z. Chang and X. H. Bu, Metal–organic-framework-based photocatalysts optimized by spatially separated cocatalysts for overall water splitting, Adv. Mater., 2020, 32, 2004747.
54. D. Yang, Z. G. Li, X. H. Zhang, Z. H. Ren, W. H. Lu, H. N. Liu, X. M. Guo, J. J. Zhang and X. H. Bu, Rational design of ZnCdS/TpPa-1-COF heterostructure photocatalyst by strengthening the interface connection in solar hydrogen production reactions, Nano. Res., 2023 DOI:10.1007/s12274-023-5991-5.
55. X. Shi, P. Q. Wang, L. Wang, Y. Bai, H. Q. Xie, Y. Zhou and L. Q. Ye, Change in Photocatalytic NO Removal Mechanisms of Ultrathin BiOBr/BiOI via NO₃⁻ Adsorption, Appl. Catal., B, 2019, 243, 322–329.
56. S. Shajahan and M. A. Haija, Effective removal of azithromycin by novel g-C₃N₄/CdS/CuFe₂O₄ nanocomposite under visible light irradiation, Chemosphere, 2023, 337, 139372.
57. J. H. Son, D. H. Park, D. A. Keszler and W. H. Casey, Acid-stable peroxoniobophosphate clusters to make patterned films, Chem. – Eur. J., 2015, 21, 6727–6731.
58. L. Yang, Z. Zhang, C. N. Zhang and X. L. Wang, A bifunctional POM-based Cu-viologen complex with mixed octamolybdate clusters for rapid oxidation desulfurization and effective photogeneration of hydrogen, Rare Met., 2024, 43, 236–246.
59. S. Sorcar, Y. Hwang, J. Lee, H. Kim, K. Grimes, C. Grimes, J. Jung, C. Cho, T. Majima, M. Hoffmannd and S. In, CO₂, water, and sunlight to hydrocarbon fuels: a sustained sunlight to fuel (Joule-to-Joule) photoconversion efficiency of 1%, Energy Environ. Sci., 2019, 12, 2685.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2263177. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3qi02513f

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