Synthesis of Highly Efficient (0D/1D) Z-Scheme CdS-NPs@ZnO-NRs Visible-Light-Driven Photo(electro)catalyst for PEC Oxygen Evolution Reaction and Removal of Tetracycline

Abstract

Herein, we synthesized the cadmium sulfide nanoparticles (CdS-NPs) coated zinc oxide nanorods (ZnO-NRs) core-shell like CdS-NPs@ZnO-NRs heterojunction for photo(electro)chemical applications. The CdS-NPs and ZnO-NRs were synthesized through a simple hydrothermal path. The physicochemical and optoelectronic properties of the as-prepared catalysts are characterized by various spectroscopy techniques, such as FTIR, XRD, SEM, TEM, EDX, VB-XPS, DRS, and PL. The photocatalytic performances of the CdS-NPs@ZnO-NRs catalyst were evaluated by photodegradation of tetracycline (TC) in aqueous media under visible-light irradiation, which demonstrated 94.07 % of removal (k’ = 0.0307 min⁻¹) within 90 min. On the other hand, the photoelectrochemical (PEC) water-oxidation/oxygen-evolution reaction (OER) was performed, which resulted in the photocurrent density of 3.002 mA/cm² and overpotential (at 2 mA/cm²) of 171 mV (vs RHE) in 1.0 M KOH under AM 1.5G illumination. The reactive species scavenging experiment demonstrates the significant contributions of photogenerated holes towards TC removal. Furthermore, the Z-scheme CdS-NPs@ZnO-NRs core-shell heterojunction exhibits high efficiency, recyclability, and photostability, demonstrating that the CdS-NPs@ZnO-NRs is a robust photo(electro)catalyst for visible-light PEC applications.

1. Introduction

Semiconductor materials have a widespread application in various fields owing to their exceptional physical and chemical properties that can be explored to attain the maximum possible outputs. They have a largescale application in photocatalysis, chemical sensors, electronics, batteries, and so on [1,2,3,4]. Both n-type and p-type semiconductors have several advantages towards photoelectrochemical applications with their major active reaction species (e⁻, h⁺). They can be modified and tuned by some physical and chemical techniques to enhance the desired yield or efficiency towards the application of interest. Converging on the n-type semiconductors (e.g., ZnO, TiO₂, ZnS, CdS, and CdSe) with moderate bandgap energies 2~3.2 eV, they possess electrons as major charge carriers that determine the conductivity [5,6,7]. In contrast, the p-type semiconductor materials, such as PbS, NiO, Cu₂O, and Co₃O₄, dominate with holes as majority carriers [8,9]. Among the several modification methodologies, doping and composite preparation are simplistic yet efficient ways to synthesize a catalytic material with suitable band structures for the respected applications [10,11,12]. On the other hand, the defective surface sites of the semiconductor materials are beneficiaries of the enlarged light absorption and act as an active centre that traps the photogenerated charge carriers. Yet, an additional benefit of composite material or heterojunction formation is that it facilitates the high photocatalytic activity by means of bandgap narrowing, increases the charge separation, and diminishes the fast charge recombination [13,14].

With the increasing demand in food industries owing to the intensified population demands worldwide over the past few decades, shortcut methods to meet the needs have also been triggered by means of the excess usage of antibiotics and other supplements that have shallow influence yet are lethal when subject to prolonged consumption [15,16]. In the early 20th century, the discovery of antibiotics was celebrated and considered a boon to human discoveries, yet comparing it with the current situation, the view is juxta-posed due to the adverse effects of the same by over usage [17]. The persistent presence of antibiotics in the food and water systems makes them an emerging environmental concern owing to their hostile effects [18,19,20]. Characteristically, antibiotics are chemotherapeutic agents that inhibit the growth of microorganisms, e.g., bacteria, protozoans, viruses, microalgae, and fungi. They are of different types based on their mode of administration and mechanism of action, source, and so on [21]. Hence, all kinds of natural, semi-synthetic, and synthetic antibiotics have been widely used in poultry, livestock, cattle, and even in some aquatic organisms to prevent diseases. After the administration, only a part of those antibiotics is metabolized in their bodies, and the remaining antibiotics will be released via urine and faeces into the ecosystem, and small traces of the antibiotics will remain indefinitely as residue in their bodies. Moreover, the consumption of traces of those antibiotics from food or drinking water over a long period can develop antibiotic resistance in our bodies [22,23]. This could lead to severe concerns in the case of health issues related to microbial infections. Among various types and groups of antibiotics that have been extensively used, tetracycline (TC) is a primary group of antibiotics broadly used for veterinary purposes, agricultural needs, and human therapy [24].

In the past several decades, the photocatalytic/photoelectrochemical conversion of solar energy to chemicals has attracted much attention. Among these, the photo(electro)chemical water splitting has fascinated significant interest towards the production of clean fuels (i.e., oxygen (O₂) and hydrogen (H₂)). To achieve better efficiency or quantum yield, the PEC water-splitting photocatalyst should fulfil the following requirements: appropriate electronic bandgap energies (~1.23 eV), ultrafast photogenerated charge separation, reduced recombination rate, cost-effectiveness, and excellent physicochemical stabilities. Likewise, the band edge potentials of the semiconductor materials have been modified through several typical methods, such as metal/non-metal doping, the construction of rational heterojunctions and nanostructures, and binary/ternary composite synthesis. Besides, the surface morphology of the photo(electro)catalyst plays a vital role in the PEC water-splitting efficiencies. The photoelectrochemical oxygen evolution (PEC-OER) reaction is also known as an anodic-half-cell four-electron transfer process. Thus, it is essential to develop well-organized OER catalysts to accomplish high visible-light harvesting and O₂ generation.

Several photocatalytic systems, including single/combined heterojunctions, have been utilized for photocatalytic degradation and photoelectrochemical applications. Among them, the coupled semiconductor materials exhibited a significant enhancement in their photoactivity in terms of oxide/oxide combinations and sulphide/oxide combinations [25,26,27]. According to Tak et al., two fundamental reasons for this improved catalytic activity are based on the fact that, in the coupled semiconductor systems, the wide bandgap semiconductor can utilize more visible light since it is coupled with the narrow bandgap semiconductor. While the system has different energy levels, charge transfer from one system to another enhances the longer and more efficient charge separations, which overcome the rapid electron-hole pair recombination [28]. With the path of water splitting, CdS is the most studied photo(electro)catalyst under visible light conditions [29,30,31]. Besides the sulphide/oxide coupled combinations, Zong et al. have reported sulfide/sulfide systems for enhanced photoelectrochemical applications [32].

Herein, we have synthesized a CdS-NPs@ZnO-NRs core-shell-like Z-scheme heterojunction through a simple hydrothermal method for the photodegradation and photoelectrochemical water splitting reactions. The physicochemical properties of as-synthesized materials were characterized by several spectroscopic and microscopic analyses. The as-prepared CdS-NPs@ZnO-NRs heterojunction composite shows extended visible-light absorbance and excellent charge transfer abilities. Moreover, the photocatalytic activities of the CdS-NPs@ZnO-NRs catalyst were evaluated by the photocatalytic removal of tetracycline (TC) under visible-light irradiation. The role of reactive species and the effect of pH on photodegradation reactions were also analyzed. The plausible photo(electro)catalytic mechanism has been derived from the support of UV-DRS, PL, VB-XPS, and electrochemical Mott-Schottky analyses.

2. Results and Discussion

The different crystalline phases of the ZnO-NRs, CdS-NPs and CdS-NPs@ZnO-NRs catalysts are investigated by X-ray diffraction (XRD) studies. As shown in Figure 1A, the characteristic Bragg’s patterns located at 31.79°, 34.46°, 36.28°, 47.55°, 56.62°, 62.89°, 66.41°, 67.91°, 69.25°, 72.57°, and 31.79° are attributed to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202) planes, respectively, which confirms the hexagonal wurtzite phase (JCPDS Card No.: 05-0640) of ZnO [33,34]. Similarly, the XRD patterns of CdS-NPs positioned at 21.53°, 23.77°, 28.28°, 30.52°, 43.88°, and 50.03° are ascribed to the following (100), (002), (100), (200), (110), and (112) planes of both hexagonal greenockite/wurtzite and cubic hawleyite phases (JCPDS No.: 10-0454; 02-0549) [35,36,37,38]. The above results confirm the pure crystallinity phases of CdS and ZnO.

Consequently, the CdS-NPs@ZnO-NRs exhibit the Bragg’s patterns of both CdS-NPs (marked as “▼”) and ZnO-NRs (marked as “*”), which demonstrates the successful formation of binary core-shell-like CdS-NPs@ZnO-NRs heterostructures. The various chemical environment and functional groups present in the samples were characterized by Fourier-transform infrared spectroscopy (FTIR) analysis employed at room temperature in the range of 4000–400 cm⁻¹ (Figure 1B). The CdS-NPs show the following characteristic bands at 650 cm⁻¹, 1021 cm⁻¹, 1160 cm⁻¹, and 1440 cm⁻¹ are due to the stretching vibration modes of Cd–S and stretching and bending vibration modes of C–S/C–H groups, respectively. Likewise, FTIR spectra of ZnO illustrate that the typical peaks located below 550 cm⁻¹, 1435 cm⁻¹ are related to the metal-oxide (Zn–O) bond; bending and stretching vibration modes of C–H and C–O bonds, respectively. Furthermore, the resulting vibration bands at 1550 cm⁻¹, 1620 cm⁻¹, 3450 cm⁻¹, and 3200–3600 cm⁻¹ were ascribed to the O–H bending vibrations and surface O–H groups of adsorbed water molecules [39], respectively. Hence, the resulting FTIR spectra of CdS-NPs@ZnO-NRs confirm the existence of significant vibration bands of CdS and ZnO and evidence the successful formation of the core-shell-like binary heterostructure [36,40].

The structural morphologies and the different elemental percentages of the ZnO-NRs, CdS-NPs, CdS-NPs@ZnO-NRs catalysts were studied by field-emission scanning electron microscopy (FESEM), and energy dispersive X-ray spectroscopy (EDX) analyses, respectively. Figure 2A,B represents the ZnO nanorods with an average diameter of ~100–200 nm. Moreover, we can see some hexagonal nanorods with sharp edged (pencil-like) morphologies. Figure 2D,E shows the agglomerated small CdS nanocrystals with an average particle size of ~5–15 nm. Furthermore, the CdS-NPs@ZnO-NRs heterojunctions are displayed in Figure 2G,H. This clearly shows the core-shell-like heterojunction formation owing to the deposition of CdS-NPs on the surface of ZnO-NRs. In general, the 1D nanorod-like structures (ZnO-NRs) have a high conductive ability, and 0D nanoparticles (CdS-NPs) have a higher reactive surface area, which is more beneficial for photoelectrocatalytic activity enhancement. Consequently, the EDX spectra of ZnO-NRs (Figure 2C), CdS-NPs (Figure 2F), and CdS-NPs@ZnO-NRs (Figure 2I) catalysts show the respective weight percentages of different elements, such as Zn, O, Cd, and S. This clearly states that the as-synthesized samples are formed without the presence of any impurities. The uniform distributions of elements present in different ZnO-NRs, CdS-NPs, and CdS-NPs@ZnO-NRs catalysts were also confirmed via elemental mapping images (Figures S1–S3), respectively. The morphological analysis is carried out using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) techniques. The TEM images of ZnO-NRs are shown in Figure 3A–C, which exhibits the hexagonal-pencil-nip-like nanorod morphology. Moreover, the TEM images of CdS-NPs are displayed in Figure 3D–F. We can see the agglomerated CdS nanoparticles with an average size of 25~45 nm. Figure 3G–I shows the core-shell-like (0D/1D) CdS-NPs@ZnO-NRs heterostructures. A yellow arrow indicates the surface-bounded CdS-NPs on ZnO-NRs. The obtained TEM results are well consistent with those of FE-SEM results.

The optical absorbance properties of the ZnO-NRs, CdS-NPs, and CdS-NPs@ZnO-NRs materials were scrutinized by UV-Vis diffuse reflectance spectroscopy (UV-Vis-DRS) analysis (Figure 4A). The resultant DRS spectrum of CdS-NPs, ZnO-NRs, and CdS-NPs@ZnO-NRs show the absorption edges at around ~560 nm, ~410 nm, and ~585 nm, which indicates that the increase in the absorption of CdS-NPs@ZnO-NRs core-shell-heterostructure is due to the synergistic effect of both CdS-NPs and ZnO-NRs. The increased light absorption is also beneficial for the enriched photo(electro)catalytic performance. The Tauc plots were derived from the Kubelka–Munk function (αhυ = A(hυ-E_g)ⁿ) (Figure 4B) [41,42], where α is the absorption coefficient, ν is frequency, h is Planck’s constant, E_g is the bandgap energy of the catalysts, n = ½ for direct allowed transition, and A is the proportionality constant.

The determined optical bandgap (E_g) energies of the ZnO-NRs (3.06 eV), CdS-NPs (2.23 eV), CdS-NPs@ZnO-NRs (2.12 eV) suggest that the deposition of CdS nanoparticles on ZnO nanorod surface significantly increases the visible-light absorbance and reduces the optical E_g energies which is more appropriate for the photooxidation reactions. The optical E_g of all CdS-NPs, ZnO-NRs and CdS-NPs@ZnO-NRs lie within the visible light region, and the E_g of CdS-NPs@ZnO-NRs heterojunction is smaller than those of CdS-NPs, and ZnO-NRs, owing to the synergistic effect of CdS and ZnO. Accordingly, the visible light absorbance of the nanocomposite can be credited to the interactions of CdS and ZnO. The expanded and substantial visible-light absorption range of the photocatalyst allows more photons to be absorbed, resulting in more photoinduced charge carriers and enhanced photo(electro)catalytic efficiency.

In addition, the photoinduced charge carrier separation and recombination processes can be seen from the photoluminescence (PL) spectroscopy studies. Herein, the high PL intensity refers to the fast recombination of photogenerated electrons and holes, whereas the reduced or abridged PL intensity is related to the quenched recombination process. The ZnO-NRs, CdS-NPs, and CdS-NPs@ZnO-NRs samples exhibited PL peaks at 544 nm, 556 nm, and 612 nm (Figure 4C), respectively, which are quite matched with their absorbance spectrum and E_g values. The significant quenching in the charge recombination is also owing to the charge transfer kinetics or electron-hole migration via the Z-scheme heterojunction mode of transportation. The electrochemical Mott-Schottky (M–S) measurement was performed to investigate the donor density and the flat-band potentials at electrode-electrolyte interfaces. The M–S plots of as-prepared CdS-NPs, ZnO-NRs, and CdS-NPs@ZnO-NRs are measured in 0.1 M Na₂SO₄ electrolyte solution in the dark and shown in Figure 4D. The M–S equation mentioned below (Equations (1) and (2)) gives the relationship of capacitance (C) vs. applied voltage (V) at the electrode/electrolyte interface.

$$\frac{1}{{\mathrm{C}}^2}=\left(2/{\mathrm{N}}_{\mathrm{d}}{\mathrm{e}}_0\mathsf{\epsilon }{\mathsf{\epsilon }}_0{\mathrm{A}}^2\right)\times \Delta \left(\mathrm{x}-\mathrm{intercept}\right)$$

(1)

$$\Delta \left(x-intercept\right)=V-V_{fb}-k_{B}T/e_0$$

(2)

where C = capacitance; N_d = carrier/donor density; ε = dielectric constant; ε₀ = permittivity of free space (8.854 × 10⁻¹² F/m); A = area of working electrode (1 × 1 cm²); V = potential (applied); V_fb = flat-band potential; k_B = Boltzmann constant (1.38 × 10⁻²³ m² kg s⁻² K⁻¹); T = absolute temperature (298 K); and e₀ = electron charge (1.6 × 10⁻¹⁹ C) [43]. Whereas the extrapolation of the linear line towards the X-axis intercept is directly proportional to the flat-band (E_fb) potential of semiconductors. The E_fb is equivalent to the fermi-levels (i.e., E_CB and E_VB) of n-type and p-type semiconductors, respectively. The E_fb (E_CB) obtained for CdS-NPs, ZnO-NRs, and CdS-NPs@ZnO-NRs from the M–S plots are –1.09 V, –0.69 V, and –1.04 V, respectively. The results also suggest the n-type nature of CdS-NPs, ZnO-NRs, and CdS-NPs@ZnO-NRs semiconductor materials.

The surface elemental analysis and chemical states were carried out using X-ray photoelectron spectroscopy measurements (XPS). Figure 5A shows XPS-survey scan spectra of ZnO-NRs, CdS-NPs, and CdS-NPs@ZnO-NRs samples measured from 0 to 1200 eV binding energy range, which evidences the incidence of Cd, Zn, S, and O elements in the pristine and core-shell composite material. The core-level XPS spectra of Cd 3d and S 2p obtained from pristine CdS-NPs, and CdS-NPs@ZnO-NRs are shown in Figure 5B–E. In Figure 5B, the two distinctive peaks located at 406.67 eV and 413.57 eV are attributed to the Cd 3d_5/2 and Cd 3d_3/2, respectively, present in the pristine CdS-NPs sample. Compared to the above results, Cd 3d in the CdS-NPs@ZnO-NRs sample shows the Cd 3d_5/2 and Cd 3d_3/2 peaks at slightly lower binding energies of 405.41 eV and 412.23 eV (Figure 5D), respectively, indicating the decreased electron cloud on CdS-NPs surface owing to the charge transfer. Similarly, the high-resolution spectra of S 2p peaks of CdS-NPs and CdS-NPs@ZnO-NRs can be deconvoluted into two peaks, ascribed to the S 2p_3/2 and S 2p_1/2, respectively. When compared to the S 2p spectrum (2p_5/2: 162.14 eV; 2p_3/2: 163.30 eV) of CdS-NPs, the S 2p spectrum (2p_5/2: 161.09 eV; 2p_3/2: 162.27 eV) of CdS-NPs@ZnO-NRs shifted towards lower binding energy shows the quenched electron cloud on CdS-NPs surface.

On the other hand, the core-level XPS O 1s spectra can be deconvoluted into three peaks (Figure 5F,H). The O 1s spectrum of ZnO-NRs and CdS-NPs@ZnO-NRs shows two distinctive peaks at 529.65 eV, 531.18 eV, 532.27 eV and 529.58 eV, 530.93 eV, 532.01 eV respectively, in which the peak at lower binding energy is ascribed to the (Zn–O) O^2– lattice oxygen, the peak at middle binding energy is attributed to the chemisorbed oxygen molecules or oxygen vacancy at the vicinity of Zn, and the peak at higher binding energy refers to the hydroxyl groups or surface adsorbed oxygen from water molecules [44,45,46]. Thus, the above results confirm that the O 1s binding energy of the composite is shifted to the higher binding energy (red-shift) due to the increased surface electron cloud caused by the photoexcited electrons transfer from the CB of CdS-NPs to the CB of ZnO-NRs. Moreover, the XPS high-resolution Zn 2p spectra of ZnO-NRs (Figure 5G) and CdS-NPs@ZnO-NRs (Figure 5I) samples exhibit one Zn 2p_3/2 peak located at 1022.48 eV and 1022.49 eV binding energies, respectively, referring to the Zn present in the Zn-O [47]. Therefore, the XPS analysis confirms the surface chemical states of the elements present in the ZnO-NRs, CdS-NPs and CdS-NPs@ZnO-NRs materials, and the shift in the binding energies evidences the Z-scheme mode of charge transfer mechanism over CdS-NPs@ZnO-NRs heterostructure. In addition, the valance-band X-ray photoelectron spectroscopy (VB-XPS) analysis was also conducted to observe the change in the electronic band structures and offsets of the as-synthesized CdS-NPs and ZnO-NRs (Figure 5J,K). From the VB-XPS spectra, the valance band maximum values of CdS-NPs and ZnO-NRs are estimated to be 1.09 eV and 2.38 eV, respectively, evaluated by the extrapolation of the curve to the baseline. Compared to the pristine CdS-NPs and ZnO-NRs, the VB maximum of the CdS-NPs@ZnO-NRs slightly upshifted towards a more positive potential of 1.93 eV (Figure S4).

2.1. Photo(electro)catalytic Performances of ZnO-NRs, CdS-NPs, and CdS-NPs@ZnO-NRs Catalysts

The photoelectrochemical oxygen evolution reaction (PEC-OER) reaction was demonstrated by the polarization curves obtained by linear sweep voltammetry (LSV) analysis with the operating potential window of 1.0–2.2 V (vs. RHE) in 1.0 M KOH under simulated solar light irradiation (AM 1.5G/100 mWcm²). Figure 6A displays the obtained LSV curves for the different ZnO-NRs/ITO, CdS-NPs/ITO, and CdS-NPs@ZnO-NRs/ITO photoanodes (illumination area 1 cm²). The photocurrent density (J) of CdS-NPs@ZnO-NRs/ITO photoanode is around 3.002 mA/cm^2, which is about 1.44 and 1.19 folds higher than that of pristine ZnO-NRs/ITO (2.079 mA/cm²) and CdS-NPs/ITO (2.512 mA/cm²), respectively (Figure 6B). Furthermore, the overpotential for the PEC-OER (η_OER) reaction was determined at 2 mA/cm², which depicts that the lowest η_OER of 171 mV (vs RHE) was accomplished by CdS-NPs@ZnO-NRs/ITO photoanode. Moreover, the pristine CdS-NPs/ITO and ZnO-NRs/ITO exhibit η_OER of 212 mV and 216 mV (vs RHE), respectively, which is comparatively 1.24 and 1.26 times less than the CdS-NPs@ZnO-NRs/ITO composite material. The significant improvement in the PEC-water oxidation efficiency of CdS-NPs@ZnO-NRs is owing to the effective photogenerated charge separation, the introduction of more active sites for the oxidation reactions, and quenched electron-hole recombination rate by the Z-scheme heterojunction between CdS-NPs and ZnO-NRs. The schematic representation of the possible charge transfer mechanism and PEC-oxygen evolution reactor system is shown in Scheme 1.

The photocatalytic performances of the ZnO-NRs, CdS-NPs and CdS-NPs@ZnO-NRs catalysts are further evaluated by the removal of tetracycline (TC) under visible-light illumination. In brief, 0.04 g of as-synthesized catalysts were mixed with 100 mL of 10 mgL⁻¹ of TC and magnetically stirred for 30 min under dark conditions for the catalyst-TC equilibrium. After the light illumination, the progress of TC photodegradation was evaluated by the absorbance (λ_max) peak reduction at every 15 min interval using a UV-Vis spectrophotometer, and the photodegradation percentage was calculated by the below equation (Equation (3)),

$$\mathrm{photodegradation}(\%)\mathrm{of}\mathrm{TC}=\left(\frac{{\mathrm{C}}_{\mathrm{TC}}-{\mathrm{C}}_{0\mathrm{TC}}}{{\mathrm{C}}_{\mathrm{TC}}}\right)\times 100\%$$

(3)

where C is the initial concentration of TC; C₀ is TC concentration at time (t). A plot of change in TC concentration (C/C₀) against light irradiation time (Figure 6C) shows the photocatalytic degradation efficiency of different ZnO-NRs, CdS-NPs, and CdS-NPs@ZnO-NRs catalysts. Among them, CdS-NPs@ZnO-NRs heterojunction attains 94.07% of TC removal, and that is 1.37, 1.15, and 11.05 folds higher than that of pristine CdS-NPs (68.44%), ZnO-NRs (81.9%), and blank (8.51%), respectively. The enhanced photocatalytic activity of the binary CdS-NPs@ZnO-NRs heterojunction exhibits the combined synergistic optoelectronic properties of surface-bound 0D CdS nanoparticles and 1D ZnO nanorods. Moreover, the following equations can express the overall photo(electro)catalytic process over the CdS-NPs@ZnO-NRs heterojunction (Equations (4)–(7)).

$$CdS-NPs@ZnO-NRs+hv \to \left(CdS\right)e_{CB}^{-}+\left(ZnO\right)h_{VB}^{+}$$

(4)

$$\left(ZnO\right)h_{VB}^{+}+TC \to I.M \to C{O}_2+H_{2}O$$

(5)

$$\left(ZnO\right)4h_{VB}^{+} \left(working electrode\right)+2H_{2}O \to 4H^{+}+O_2 \left(gas\right)$$

(6)

$$4e_{CB}^{-}\to Pt electrode+4H^{+} \to 2H_2 \left(gas\right)$$

(7)

Additionally, the kinetics and rate of TC photodegradation reaction over CdS-NPs@ZnO-NRs heterojunction were derived from the plot of the logarithmic value of (C/C₀) against reaction time (Equation (8)). As shown in Figure 6D, the rate constants (k’) of different catalysts can be obtained from the slope of the linear fitting. The k’ value of CdS-NPs@ZnO-NRs (0.0307 min⁻¹) is 1.59, 2.51, and 38.4 folds greater than that of ZnO-NRs (0.0193 min⁻¹), CdS-NPs (0.0122 min⁻¹), and blank (0.0008 min⁻¹), respectively.

$$-\ln \left(\frac{{\mathrm{C}}_{\mathrm{TC}}}{{\mathrm{C}}_{0\mathrm{TC}}}\right)={\mathrm{K}}_{\mathrm{app}}\times \mathrm{t}$$

(8)

where C = TC concentration, C₀ = initial concentration of TC, t = light illumination time, TC photodegradation and K_app = apparent rate constant. The above results suggest that the photocatalytic removal of TC over CdS-NPs@ZnO-NRs is the pseudo-first order reaction. Consequently, the correlation coefficient (R²) values for this model also calculated for ZnO-NRs, CdS-NPs, and CdS-NPs@ZnO-NRs catalysts are 0.9971, 0.9616, and 0.9903, respectively.

The photocatalytic activity enhancement was further supported by the transient photocurrent (TPC) and electrochemical impedance spectroscopy analyses (Figure 6E,F). From Figure 6E, the CdS-NPs@ZnO-NRs show a maximum photocurrent density of 0.5311 μA than that of pristine CdS-NPs (0.4259 μA) and ZnO-NRs (0.2488 μA). This confirms the excellent photogenerated charge carrier excitation and separation. The enlargement in the transient photocurrent response of the CdS-NPs@ZnO-NRs catalyst also evidences the suppressed recombination of photoinduced electron-hole pairs. Figure 6F displays the Nyquist plots of ZnO-NRs, CdS-NPs, and CdS-NPs@ZnO-NRs samples. The charge transfer resistance (R_ct) between the heterojunction and electrode-electrolyte interface can be calculated by the radius of the semicircle obtained from the EIS-Nyquist plots. The 0D CdS-NPs show a high resistance of 43.4 Ω, and 1D ZnO-NRs exhibit a relatively lower resistance of 24.84 Ω. After the formation of CdS-NPs@ZnO-NRs core-shell-type heterojunction, it shows a significantly lower R_ct value of 27.39 Ω. Therefore, the EIS results confirm the significant increment in the conductivity and charge density available for the photo(electro)catalytic reactions. Moreover, the cyclic voltammetry technique was used to measure the electrochemically active surface area (ECSA) of the different photoelectrodes. The electrically double layer capacitance (C_dl) can be calculated by the slope of ∆J vs. scan rate plot obtained in the non-Faradaic region in 1.0 M KOH (Figure S5). The obtained results show that the CdS-NPs@ZnO-NRs have a high ECSA of 32.71 × 10⁻² μF cm^−2, which is about 1.09 and 1.13 folds more than that of pristine CdS-NPs (29.88 × 10⁻² μF cm⁻²) and ZnO-NRs (28.79 ×10⁻² μF cm⁻²), respectively. These results also confirm and evidence the excellent photo(electro)chemical activities of CdS-NPs@ZnO-NRs heterojunction.

The predominant active species during the photocatalytic degradation of tetracycline (TC) using CdS-NPs@ZnO-NRs was analyzed through reactive radical trapping tests under identical conditions as the degradation process (Figure 7A). For the scavenger test, 100 mmol of acrylamide (AA), isopropyl alcohol (IPA), silver nitrate (SN), and triethanolamine (TEOA) were used as trapping for superoxide radicals (●O₂⁻), hydroxyl radicals (●OH), holes (h⁺), and electrons (e⁻), respectively. Assessing the obtained samples after 90 min of light irradiation, samples containing the IPA and SN species have degradation nearly unaffected by their presence, indicating that ●OH radical and e⁻ have negligible contribution in the photodegradation of TC. Following that, the addition of AA (i.e., ●O₂⁻) has little effect on the photodegradation. Finally, the addition of TEOA greatly influences the degradation by reducing the tetracycline removal by 49.18% compared to the blank, which shows that h⁺ is the primary active species involved in TC removal. Since the solution pH influences the surface charge of the catalytic material, the photodegradation of TC over CdS-NPs@ZnO-NRs heterojunction was performed with different pHs ranging from (pH 3~11) to determine the optimal pH suitable for excellent catalyst-TC adsorption (Figure 7B). The results suggest that the TC removal is significantly reduced under acidic conditions (pH 3~5), demonstrating the repulsive electrostatic forces between CdS-NPs@ZnO-NRs and TC. On the other hand, we could see 88.66% TC removal at neutral (pH 7) condition, and the highest TC removal percentages (i.e., >90%) were achieved at alkaline conditions (pH > 8). This reflects that the high adsorption of TC on the catalyst surface is owing to electrostatic attraction forces and thus enhances the photocatalytic degradation efficiency. The above pH studies further confirm the three different acid dissociation (pK_a = 3.30—acidic; 7.68—neutral; 9.69—alkaline) constants of the TC molecule.

2.2. The Possible Mechanism for the PEC-OER and Photocatalytic TC Degradation Reactions

The optoelectronic band structures of the ZnO-NRs, CdS-NPs, and CdS-NPs@ZnO-NRs photo(electro)catalysts were derived from DRS-Tauc plot analysis (Figure 4B). In addition to that, the valance band (VB) potential was directly obtained by VB-XPS analysis. Consequently, the conduction band (CB) potential was calculated from the following expression (Equation (9)),

$${\mathrm{E}}_{\mathrm{CB}}={\mathrm{E}}_{\mathrm{VB}}-{\mathrm{E}}_{\mathrm{g}}$$

(9)

where E_CB is the conduction band potential, E_VB is the valance band potential, and E_g is the optical bandgap energy. From Figure 4B, the bandgap energies of CdS-NPs, ZnO-NRs and CdS-NPs@ZnO-NRs materials are calculated to be 2.23 eV, 3.06 eV, and 2.12 eV, respectively. Moreover, Figure 5J,K show the VB potentials of CdS-NPs (1.09 eV) and ZnO-NRs (2.38 eV). In addition, the CB potentials were calculated to be –1.14 eV and –0.68 eV, respectively, by the equation mentioned as Equation (9).

Concerning the calculated VB and CB potentials of CdS-NPs and ZnO-NRs, the overall band structure is shown in Scheme 1. When both CdS and ZnO photocatalytic systems are exposed to light irradiation, the electrons are excited from the top of the VB bands to the bottom of the CB bands. Since the VB/CB (1.09/–1.14 eV) potentials of CdS-NPs is less positive and more negative than ZnO-NRs (2.38/–0.68 eV), making a staggered band alignment. The formation of a built-in-electric field in the CdS-NPs@ZnO-NRs heterojunction could reduce the thermal diffusion of electrons from CdS-NPs to ZnO-NRs, thus initiating a thermal equilibrium state in the heterojunction. When the CdS-NPs@ZnO-NRs system is introduced into the light irradiation, the photogenerated charge carriers (i.e., electrons in CB and holes in VB) reach a non-equilibrium state. This helps to overcome the thermal equilibrium state, and the built-in-electric field at CdS-NPs@ZnO-NRs heterojunction forced the electron flow from CB of ZnO-NRs to the VB of CdS-NPs. The photogenerated electrons recombine with photoexcited holes, and the remaining electrons in the CB of CdS-NPs and holes in the VB of ZnO-NRs actively participate in the photocatalytic reactions [48]. As a result of the above-generated built-in electric field, the charge transfer process over the CdS-NPs@ZnO-NRs heterojunction is elucidated as direct Z-scheme mode. This process will be helpful for the tremendous charge separation from the recombination of electrons in the CBs of CdS and ZnO. Further, the Z-scheme mode of carrier transportation is confirmed by XPS analysis via a downshift in the binding energy of Zn–O. Moreover, the CB potential of CdS-NPs and VB potential of ZnO-NRs is higher than that of water oxidation (H₂O/●OH; H₂O/O₂ = 2.38/1.23 V) and reduction (O₂/●O₂⁻; H₂O/H₂ = −0.33/0 V) potentials, respectively [49]. Henceforth, the residual photogenerated electrons exist in the CB of CdS, and holes in the VB of ZnO can effectively produce superoxide (•O₂⁻) and hydroxyl (•OH) radicals, respectively, for the photocatalytic redox reactions. Thus, the direct Z-scheme charge transportation over the binary CdS-NPs@ZnO-NRs heterojunction enables higher photo(electro)catalytic oxidation performances.

2.3. Comparative Analysis of the Photocatalytic Degradation Performances of CdS-NPs@ZnO-NRs Heterojunction

The results of photocatalytic removal of TC over CdS-NPs@ZnO-NRs Z-scheme heterojunction photocatalyst were compared with those previously published results (

Table 1. Comparative analysis on photocatalytic ability of various catalytic materials over different contaminant concentration levels.

S.No	Catalyst	Conc. (ppm)	Cat. Load (mg)	Time (min)	Deg. (%)	References
1.	SDS/ZnO	40	20	150	49	[50]
2.	α-MnO2	100	50	70	51.55	[51]
3.	C3N4ZnxOy	10	50	120	92.7	[52]
4.	Bi2WO6/CMS	20	20	150	78.4	[53]
5.	N, Fe-CDs/G-WO3	10	50	180	54.5	[54]
6.	TiO2	10	10	120	35.7	[55]
7.	Bi24O31Br10	20	100	60	91	[56]
8.	BiOI microspheres	40	50	120	94	[57]
9.	Au/Pd/g-C3N4	20	20	210	92.8	[58]
10.	CdS-NPs@ZnO-NRs	10	20	90	94.07	This Work

Conc. = Concentration; ppm = parts per million; Cat. load. = Catalyst loading amount; Deg. (%) = Degradation percentage.

.)

2.4. Stability and Recyclability of CdS-NPs@ZnO-NRs Z-Scheme Heterojunction Photoelectrocatalyst

The photocurrent stability of the CdS-NPs@ZnO-NRs heterojunction was studied by i-t measurement at 2 mA/cm² (171 mV vs. RHE) in 1.0 M KOH (Figure 8A). After continuous irradiation of 30 min, the current density slightly gets reduced to 1.89 mA/cm². This demonstrates the excellent photostability of as-synthesized CdS-NPs@ZnO-NRs. The cyclic stability of the CdS-NPs@ZnO-NRs photocatalyst was evaluated for the three successful degradation reaction cycles of TC (Figure 8B). The used catalysts were recovered by centrifugation and tried at 60 °C for the following reaction cycle. Thus, the obtained results demonstrate the initial degradation of TC over CdS-NPs@ZnO-NRs (94.07%) heterojunction catalyst is slightly reduced to 91.87% and 90.75% for the second and third cycles, respectively, under 90 min of visible-light irradiation. The second and third cycles of degradation reaction result in 2.2% and 3.32% loss of efficiency due to the minimal product loss during the catalyst recovery processes. Figure 8C shows the FT-IR spectrum of the CdS-NPs@ZnO-NRs catalyst after three successive reaction cycles, which revealed the robust nature and excellent chemical stability of the CdS-NPs@ZnO-NRs Z-scheme heterojunction catalyst. Besides, the FE-SEM characterization was performed for the recycled CdS-NPs@ZnO-NRs catalyst to confirm the structural/morphological stabilities after the photo(electro)chemical reactions, as shown in Figure 8D. Therefore, the recyclability tests, FT-IR, and FE-SEM analysis further confirm the excellent stability and sustainability of CdS-NPs@ZnO-NRs photo(electro)catalyst towards energy and environmental applications.

3. Materials and Methods

3.1. Materials

Zinc acetate dihydrate (Zn (CH₃COO)₂.2H₂O), sodium sulfide (Na₂S), sodium hydroxide (NaOH), triethanolamine (C₆H₁₅NO₃), potassium hydroxide (KOH), acrylic acid (C₃H₄O₂), potassium chloride (KCl), methyl alcohol (CH₃OH), potassium ferricyanide (K₃[Fe(CN)₆]), silver nitrate (AgNO₃), cadmium acetate (Cd (CH₃COO)₂), isopropyl alcohol (C₃H₈O), potassium ferrocyanide (K₄[Fe(CN)₆]), hydrochloric acid (HCl), and tetracycline hydrochloride (C₂₂H₂₄N₂O₈.HCl) chemicals were obtained from Sigma-Aldrich and used without any further purification. Deionized water (DI) was used as a solvent throughout the experiment unless otherwise stated.

3.2. Characterizations

The optoelectronic properties of the photocatalytic materials were measured by UV-Visible diffuse reflectance spectroscopy (UV-DRS, Jasco-4700 with 15 mm integrated sphere) and room temperature photoluminescence (PL, UniNanoTech, Gyeonggi-Do, Republic of Korea). The crystallography properties were studied by X-ray diffraction (XRD, PANalytical X’pert Pro, Almelo, The Netherlands) analysis. Physicochemical properties of samples were analyzed using Fourier-transform infrared spectroscopy (FT-IR-ATR, PerkinElmer-Frontier, OH, USA), energy dispersive X-ray spectroscopy (EDX, Oxford Instruments), and X-ray photoelectron spectroscopy (XPS, JOEL, JPS-7010, Tokyo, Japan). Furthermore, the morphological and structural characterizations were analyzed using field-emission scanning electron microscopy (FESEM, JOEL, JSM-5010, Tokyo, Japan) and transmission electron microscopy (TEM, JOEL, JEM-2100Plus, Tokyo, Japan).

3.3. Synthesis of CdS-NPs@ZnO-NRs Core-Shell Heterojunction

Firstly, for the synthesis of ZnO-NRs, 0.1 M zinc acetate in 10 mL of ethanol solution is mixed with 20 mL of 0.5 M sodium hydroxide solution under stirring for 30 min. Then, the mixture is transferred to the autoclave, kept in a hydrothermal jacket, and placed in the box furnace at 150 °C for 24 h. Then the precipitate obtained is centrifuged, washed with ethanol, and kept overnight drying at 60 °C. Secondly, for the synthesis of CdS-NPs, 0.1M cadmium nitrate and 0.1 M sodium sulfide are dissolved in deionized water under stirring for 30 min. Then, the obtained solution is transferred to an autoclave and kept under hydrothermal for 5 h at 110 °C, and the acquired precipitate is centrifuged and dried overnight in a hot air oven to get CdS-NPs powder. Finally, equal parts of CdS-NPs and ZnO-NRs (1:1) are taken in a beaker containing 30 mL of methanol and placed under magnetic stirring at 50 C for total solvent evaporation (TSE) (Scheme 2). Then, the obtained composite is dried in a hot air oven and subjected to systemic characterizations and applications.

3.4. Photo(electro)chemical Measurements

The photo(electro)catalytic performances were evaluated by PEC oxygen evolution reaction and photocatalytic degradation of tetracycline (TC). For the PEC-OER reactions, the ZnO-NRs, CdS-NPs, and CdS-NPs@ZnO-NRs samples coated ITO photoanodes were analyzed by linear sweep voltammetry (LSV) technique at 10 mV/s in 1.0 M Na₂SO₃ in 0.1 M potassium phosphate buffer (pH~7.5) under AM 1.5G irradiation (300 W Xenon Lamp, 100 mW cm⁻²). Furthermore, the photoanodes were fabricated by dispersing 10 mg of as-synthesized catalyst in 2 mL of DI water and 2-propanol mixture (3:1) and sonicated for 30 min. Then, the 50 μL of this homogeneous mixture was drop cast on 1 × 1 cm² ITO plates, place in an oven, and dried at 60 °C. On the other hand, the photodegradation reactions were carried out in the presence of 10 mg L⁻¹ aqueous solution of tetracycline hydrochloride. In brief, about 0.04 g of ZnO-NRs, CdS-NPs, and CdS-NPs@ZnO-NRs samples were dispersed and sonicated for 5 min. Then, the catalyst-TC mixture was stirred under the dark for 30 min to attain an equilibrium state, followed by simulated solar light irradiation. To evaluate the progress of TC, approximately 3 mL of aliquots were collected every 10 min from the reaction mixture and measured by UV-Vis spectrophotometer. The photoelectrochemical OER transient photocurrent experiments were carried out using the CHI1121C instrument, electrochemical impedance (EIS), and Mott-Schottky measurements were measured by Autolab (PGSTAT-FRA32M module integrated with NOVA) instrument with the conventional three-electrode system.

4. Conclusions

We have synthesized a robust CdS-NPs@ZnO-NRs heterojunction photo(electro)catalyst through hydrothermal and total solvent evaporation/impregnation techniques. UV-Vis-DRS and Tauc measurements confirm the increased absorption band edges in the visible-light region and reduced optical bandgap energies. The XPS, VB-XPS, and Mott-Schottky analyses demonstrate the various chemical states of the surface elements, VB, and CB potentials of the CdS-NPs and ZnO-NRs samples. Besides, the excellent charge separation and transfer resistance of CdS-NPs@ZnO-NRs were derived by TPC and EIS studies. The PEC-OER resulted in a high photocurrent density of 3.002 mA/cm² and overpotential (at 2 mA/cm²) of 171 mV (vs. RHE), which is 1.24 and 1.26 folds higher than that of CdS-NPs and ZnO-NRs, respectively. Moreover, the photocatalytic removal of TC under visible-light irradiation resulted in 94.07% of degradation (k’ = 0.0307 min⁻¹) within 90 min compared to CdS-NPs (68.44%) and ZnO-NRs (81.90%), respectively. Active species trapping experiments confirmed the role and significant contribution of photogenerated holes. According to the obtained VB/CB potentials, the plausible Z-scheme charge transfer mechanism was adopted for the photocatalytic/photoelectrochemical reactions over CdS-NPs@ZnO-NRs heterojunction under visible-light irradiation. Therefore, the outstanding optoelectronic properties of the Z-scheme CdS-NPs@ZnO-NRs photo(electro)catalyst confirmed that it could be used for energy/environmental and other related applications.