Duality in the Mechanism of Hexagonal ZnO/Cu_xO Nanowires Inducing Sulfamethazine Degradation under Solar or Visible Light

Abstract

This study presents the first evidence for the photocatalytic performance of ZnO/Cu_xO hexagonal nanowires leading to sulfamethazine (SMT) degradation. The chemical composition of the nanowires was determined by X-ray fluorescence (XRF). The sample with the composition ZnO/Cu_x=1.25O led to faster SMT-degradation kinetics. The SMT-degradation kinetics were monitored by high performance liquid chromatography (HPLC). The morphology of the hexagonal nanowires was determined by scanning electron microscopy (SEM) and mapped by EDX. The redox reactions during SMT degradation were followed by X-ray photoelectron spectroscopy (XPS). The interfacial potential between the catalyst surface and SMT was followed in situ under solar and indoor visible light irradiation. SMT-degradation was mediated by reactive oxidative species (ROS). The interfacial charge transfer (IFCT) between ZnO and Cu_xO is shown to depend on the type of light used (solar or visible light). This later process was found to be iso-energetic due to the potential energy positions of ZnO and Cu_xO conduction bands (cb). The intervention of surface plasmon resonance (LSPR) species in the SMT degradation is discussed.

1. Introduction

Sulfamethazine (SMT) is one of the most important members in the family of sulfonamide antimicrobials and has been widely used in pharmaceuticals to protect animals against bacterial infections [1,2,3,4]. SMT is a persistent non-biodegradable environmental pollutant [5,6]. Residual SMT is excreted by animals to the surroundings and found in soils, food crops and drinking water [7,8,9,10], posing a threat to human health and the ecological system [11].

Several methods have been developed to remove SMT such as adsorption [12,13,14], biodegradation [15,16,17], advanced oxidation processes (AOPs) [18,19,20,21,22], and photocatalytic degradation [23,24]. It has also been reported that SMT is not completely eliminated by adsorption. Biodegradation by active sludge [15], Microbacterium sp. [16] and Achromobacter denitrificans PR1 [17] is possible, but requires long time periods and is restricted to low concentrations of SMT. Advanced oxidation processes (AOPs) such as Fenton and Fenton-like [18], photo-Fenton [19], electro-Fenton [20], ozonation [21], and gamma irradiation [22] have been reported to lead to SMT degradation. However, they need a high reagent concentration. Photocatalytic degradation of SMT mediated by Bi₂MoO₆ was recently reported by C. Guo et al., [23] with AgI/Bi₁₂O₁₇Cl₂ s in aqueous solution [24]. The present study reports novel a semiconductor-oxide nanowire leading to SMT degradation. This study describes the catalyst preparation, the SMT degradation kinetics and the nanowire surface properties. A reaction mechanism is suggested based on the experimental data found during the course of this work.

ZnO is a widely studied photocatalyst for environmental remediation. Some recent reviews reflect the interest of ZnO in the field of applied photochemistry [25]. The solar light utilization of ZnO is limited due to the low absorption in the visible region < 4–5%. By coupling ZnO with oxide semiconductors with Cu_xO, the light absorption of the double-oxide reaches 800 nm [25]. Catalysts presenting optical absorption into the visible region for environmental applications is a subject of topical interest [26,27]. Depending on nanoparticle size and the preparation, ZnO presents a conduction band (cb) at 0.1–0.2 V (SHE) and a valence band (vb) positioned at ∼3.4 (SHE) [28]. Cu–oxide has been chosen to decorate the ZnO to extend its spectral absorption into the visible range. For this objective, low amounts of Cu–oxide are needed compared to Mn and Fe [25] doping of ZnO. The use of Cu_xO is also an alternative to the addition of Ag and Pt to ZnO. This avoids the use of higher cost noble metals. ZnO is an inexpensive semiconductor widely used in the electronic industry due to its high electrical conductivity. At pH 1, the ZnO band gap (bg) is 3.2–3.4 eV depending on the preparation route used.

The notation Cu_xO is used for Cu–oxide since two oxidation states are present: CuO and Cu₂O. CuO presents typically a band gap of 1.7 eV and a conduction band at −0.4 V, and Cu₂O a band gap of 2.2 eV and a conduction band of −0.20 or −0.25 V [29]. Both Cu–oxides present visible light absorption [30,31].

This study presents the preparation of ZnO/Cu_xO hexagonal nanowires enhancing the SMT degradation with respect to bare ZnO. Evidence for two different interfacial charge transfer (IFCT) mechanisms between Cu_xO and ZnO under solar or visible light are reported. The use of TiO₂/Cu_xO leading to SMT degradation has been recently reported by our laboratory [32]. A dual mechanism was found to lead to SMT degradation under visible or sunlight irradiation. The surface properties of the ZnO/Cu_xO nanowires are reported in this study.

2. Results and Discussion

2.1. Description of the ZnO/CuxO (5%) Surface by Physical Methods

Figure 1 shows the images and morphology of the optimized ZnO/Cu_x=1.25O nanowire samples. Figure 1a shows the hexagonal nanowires with a media diameter of 1 to 2 microns and a length of 6–8 μm. The image was referenced in a 2 micron scale. Figure 1b shows the SEM imaging of ZnO/Cu_x=1.25O nanowires in a 2 micron scale. The arrows in Figure 1b indicate the presence of Cu_xO NPs that tend to aggregate. Using imageJ software, the particles size distribution of Cu_xO–NPs is in the range 18 to 28 nm as presented in Figure 1c. The values presented are the average of two distinct measurements (two big aggregates were left aside). Figure 1d–f show the EDX mapping of Zn, O and Cu, respectively.

Cu nanoparticles contain plasmons with free moving electrons presenting a collective oscillation and generating a local electromagnetic field. This affects the electronic states of the surrounding molecules. The plasmons transfer their energy into the nanowire-activating regions away from the excitation spot. The optical activity of NPs is determined by the wavelength of the plasmon resonance. This is a function of the particle size, shape and surrounding environment [33]. Plasmon studies have been widely reported for silver, with a peak around 400 nm [34]. Cu is an abundant and low-cost material compared to noble metals presenting inter-band transitions between 350 nm and 600 nm [35] with a plasmonic peak/doublet around 500 nm [36]. Figure 1 shows Cu–NPs of 25–45 nm decorating the ZnO nanowires. Light absorbers like STM interact with the plasmon nanowires. This probably involves electron tunneling between both oxides.

The XRD diffractogram of ZnO/Cu_xO (5%) is presented below in Figure 2. The XRD diffractogram shows a structure typical of wurtzite hexagonal high crystallinity ZnO (JCPDS card No.36-1451) [37] with lattice parameters of a = 0.325 nm and c = 0.521 nm. The peaks centered at 35.4°, 38.7° and 48.7° were assigned to CuO (JCPDS card No. 45-0937). The crystallite sizes for ZnO and CuO were estimated by means of the Scherrer equation from the strongest peak signal:

$$D=\frac{K\lambda }{\beta \cos \theta }$$

where D stands for the crystal size in nm, K is the shape factor which is usually fixed as 0.89, λ is the wavelength which is taken as 0.154 nm, β is the full width at half maximum for the prominent peak, and θ is the Bragg’s diffraction angle in radians. The values found were 40.24 nm for ZnO and 22.18 nm nm for CuxO. In Figure 2, the characteristic peaks of ZnO and CuO overlap as the ZnO and CuO appear at the same positions in the diffractogram. The system could be estimated to consist of CuxO decorating the ZnO crystallites since ZnO is the major component and CuO is present at a very low concentration.

2.2. Redox Catalysis and Interfacial Potential during SMT Degradation

The surface atomic concentration percentages and the change in Cu valence states in the ZnO/Cu_xO were determined by XPS. The results are shown in Figure 3a–d. No other elements besides Cu, O and Zn were detected except C. The latter element appears due to the sample exposure to air contamination. Figure 3b shows a shift for the Zn2p peaks from 1022.2 eV to 1021.6 eV during SMT photodegradation. This is evidence for the generation of Zn⁺ species during SMT degradation. ZnO corrosion is due to the pH decrease produced by acids formation as intermediate species during SMT degradation (please see Figure 4). The observed reduction of Zn²⁺ to Zn⁺ is similar to the reduction of TiO₂ to Ti³⁺ reported during the degradation of several pollutants [38]. This shows redox reactions occurring at the interface of ZnO nanowires during SMT degradation. The corrosion of ZnO is further confirmed by the data presented in Figure 4. The pH of the solution shifts from 7.2 to ~6.4 during the photocatalytic degradation of SMT under solar-simulated light. This is due to ZnO corrosion at pH < 6.5. This last pH value depends on the make-up and chemical species present in the suspension.

The BE peak shifts observed in the deconvolution of Cu2p illustrated in Figure 3c presents the evidence for redox reactions occurring during the SMT photocatalytic degradation. The peaks at 935.4 eV and 956.2 eV correspond to Cu²⁺, with typical CuO. The satellite peaks appear at 941.2 eV and 943.4 [39,40,41]. XPS shows that Cu₂O oxide was the predominant Cu species on the ZnO/Cu_x=1.25O. After SMT degradation as shown in Figure 3c, the Cu₂O peak (net Cu⁺) decreases. This occurs on localized surface plasmon resonance (LSPR) present on the nanowire surface. The LSPR contribution to the SMT degradation will be further discussed in the section below. The surface atomic percentage of Cu in the ZnO/Cu_x=1.25O sample is seen to be 7.32% in

Table 1. Surface atomic percentage concentration of the elements determined by X-ray photoelectron spectroscopy (XPS) in the ZnO/Cu_x=1.25O sample.

	C1s	N1s	O1s	Na1s	Cu2p	Zn2p
Before SMT degradation	13.78	0.00	42.58	0.00	7.32	36.32
After SMT degradation	14.02	0.00	41.99	0.00	6.92	37.06

.

The O1s deconvolution in Figure 3d shows the presence of three peaks located at 532.0 eV, 530.3 eV and 528.5 eV and assigned to the O^2- deficient regions of Zn–O and Cu–O [39,40]. Table 1 presents the atomic surface percentage concentration of the elements in the ZnO/Cu_x=1.25O sample before and after SMT degradation. The C1s percentage on the initial sample is due to C residues from the air and from the residual C in the solutions used to prepare the samples. The C content slightly increases after the SMT degradation due to the non-mineralized SMT species (see equation 1 below). The O1s and Cu2p percentage decrease due to the C1s being deposited on the sample during SMT degradation. The small increase in the Zn2p percentage observed after SMT degradation is due to the destruction of impurities present on the initial pristine ZnO by oxidative radical species (see below Section 3.4).

Figure 4a,b show the surface potential and local pH of the ZnO/Cu_x=1.25O nanowires under: (a) simulated solar light and (b) visible light irradiation. Figure 4a shows that the initial potential increases up to 170 min and increases again due to the presence of the SMT intermediates generated during the SMT degradation. Figure 5 shows that the SMT degradation reaches ~75% after 170 min (trace g). A similar pattern was seen for the surface potential variation in Figure 4b for the SMT degradation under visible light. In both cases, the initial potentials (Eigenvalues) were seen to be different. The changes in the interfacial potential during SMT photodegradation in Figure 4b present a trend similar to the one shown in Figure 4a. This suggests in both cases a similar mechanism.

The variation of the initial pH 7 during SMT degradation is shown in Figure 4a,b (right-hand axes). This variation was due to the organic acids (branched or not) generated during SMT degradation. Additional cationic species contribute to the pH drop as the degradation time progresses. The production of organic acid intermediates was reported by Kraeutler and Bard [42] due to the photo-Kolbe reaction as shown in Equation (1) below

$$SMT+ZnO/C{u}_{x}O\stackrel{solar/visblelight}{\to }RCOO-+intermediateacids/cations+C{O}_2$$

(1)

A pH decrease from 7.2 down to 6.4 was observed for the SMT degradation under solar light in Figure 4a,b. This is equivalent to an eight-fold increase in the concentrations of H⁺ in the solution. Short-chain carboxylic acids (C₁–C₄) with pKa values around ∼3.0 [38] are generated in the solution and account for the pH drop. The SMT follows the sequence: sulfanilic acid → benzoquinone → maleic/fumaric acid → acetic/oxalic/formic acid → CO₂ as reported in references [20,21,22,23,24,25]. The intermediate carboxylic acids are subsequently eliminated in the last step of the SMT degradation as CO₂. Supplementary Tables S1 and S2 provide the values for the pH decreases.

2.3. SMT-Degradation on ZnO/CuxO: Effect of Light Intensity

Figure 5 and Figure 6 present the degradation of SMT on ZnO/CuO under solar-simulated and visible light irradiation in a solution with initial pH 7, respectively. The SMT degradation kinetics are slower for the sample ZnO/CuxO (10%) in Figure 6 (trace f) compared to the ZnO/CuO(5%) sample in (trace g). CuO doping has been reported to decrease the ZnO band gap.

The effects of SMT and of ZnO/Cu_xO concentrations on the degradation kinetics are shown in the Supplementary Figures S1 and S2. The single HPLC peaks during the SMT degradation on ZnO/Cu_x=1.25O samples is shown in Figure 7.

SMT presents two pKa values at 2.8 and 7.4 giving rise to SMT⁺, SMT and SMT^- species [43]. Sulfamethazine is a weak acid due to the strong electron attraction of the -SO₂ substituent and the resonance stabilization of the sulfamethazine anion [44]. SMT degradation runs at different pH values are reported in Supplementary Figure S4. Figure S4 shows that the fastest degradation takes place at pH values of 3. At pH 3, ZnO corrosion lead to the release of Zn²⁺-ions in the solution since ZnO is stable at pH > 6.5. These Zn²⁺-ions catalyze the SMT degradation via Fenton-like reactions [45,46].

The ZnO presents an isoelectric point (IEP) of ~9.3, and the value for CuO is ~9.5 [47]. The two oxides are positively charged at pH values ~7. The negatively-charged groups of SMT and the positive ZnO/CuO interface lead to an electrostatic interaction between both components. The SMT equilibria as a function of the solution pH is shown in Scheme 1. At pH 7, an SMT-zwitterion is present since this solution pH is positioned between pKa1 and pKa2.

Figure 8 presents the effect of light intensity on the SMT degradation kinetics on ZnO/Cu_x=1.25O %) under solar light and visible light. The SMT degradation efficiency increased with the applied light intensity. This is evidence that the photocatalyst presents semiconductor behavior during SMT degradation under band-gap irradiation. A larger amount of photogenerated charges is generated by a semiconductor at higher light doses leading to a faster SMT degradation. This is shown in Figure 8a,b.

The optical absorption of Cu_xO occurs between 300 nm and 800 nm [42] and the SPR absorption peaks were reported at 390, 570, and 590 nm depending on the Cu_xO size [48,49]. When Cu_–oxide is exposed to light, the conduction electrons in the nanoparticle oscillate coherently due to the electromagnetic field associated with the incoming light. The electric field associated with the incident light excites the Cu_xO upper electron shells. This increases the electron oscillations, inducing a Cu_xO electric field on the surface of the Cu_xO nanocrystals [50,51].

2.4. Determination of the ROS-Species during SMT-Degradation

To understand the photocatalytic mechanism of SMT degradation, the determination of the ROS species was carried using the appropriate radical scavengers: methanol (^⦁OH scavenger); p-benzoquinone (^⦁O₂⁻ scavenger); NaN₃ (¹O₂ scavenger); and disodium ethylene-diamine-tetra-acetate dehydrate (EDTA-2Na), a hole scavenger. A scavenger concentration of 0.5 mM reagent was used in all cases. Figure 9 shows that the SMT degradation under visible light was decreased by 60% and 58% when EDTA-2Na and methanol were added to the solution. In this way, vbh⁺ and ^⦁OH-radicals were found as the two main radicals leading to SMT degradation. The SMT degradation was reduced by 20% when benzoquinone as an HO₂/^⦁O₂⁻ scavenger was added to the solution. The unambiguous determination by NaN₃ of ¹O₂ and CH₃OH of ^⦁OH-radicals was not possible. NaN₃ also scavenges the ^⦁OH radicals. Possibly, ¹O₂ and ^⦁OH-radicals intervene jointly in SMT degradation. The ^⦁OH radical intervention (^⦁OH/HO⁻ −1.90–2.20 V vs NHE) [52] predominates, leading to SMT-oxidation. The ¹O₂/O₂⁻ potential of +0.08V NHE [53] is much lower compared to the ^⦁OH/HO⁻ reaction. The identification of ROS radicals followed the approach used in TiO₂ photocatalysis when investigating the ROS intervention, leading to pollutant degradation. However, ZnO/CuxO-mediated photocatalysis may not follow the pattern valid for TiO₂ photocatalysis [54]. Figure 10 shows the data for scavenging of ROS carried under visible light.

2.5. IFCT Mechanisms Suggested under Solar and Visible Light: Involvement of the LSPR

Figure 11a presents the IFCT for reactions photocatalyzed by ZnO/Cu_x=1.25O under low-intensity solar-simulated light. The ZnO in Figure 11a generates the holes leading to the SMT degradation [55]. During the IFCT, the ZnOvb transfers holes (h+) to the Cu_xOvb, increasing the carrier lifetime. This partially hinders the charge-pair recombination in Cu_xOvb.

Figure 11b presents the visible light irradiation of the ZnO/CuO samples where Cu_xO absorbs the visible light > 410 nm. The absorber reacts by the build-up of surface plasmon resonance species (LSPR). In this case, charge recombination occurs on the Cu_xO surface due to: (a) the close proximity of the photo-generated charges on the Cu_xO surface facilitating the charge recombination and precluding charge diffusion to the non-absorbing ZnO component. The higher recombination rate limits the amount of free charges leading to SMT degradation. The SMT degradation under visible light irradiation does not involve reactions on the ZnO valence band (see Figure 11a)

Scheme 2 shows the SMT functional group coordination with ZnO and ZnO/CuO. Low potential (or sub-potential) photoinduces holes/mid-gap states in the ZnO/Cu_x=1.25O samples. In the presence of SMT, these mid-gap holes would photo-induce ¹O₂ through the oxidation of O₂^−⦁ in a thermodynamically allowed reaction [29,42]. Photo-induced holes that have a lower oxidation potential would oxidize O₂^−⦁ (+0.34 V vs NHE). The O₂^−⦁ occurs due to the reduction of molecular oxygen by cb electrons as shown above in Figure 11. The oxidation of superoxide radicals (see Section 3.4) by photoinduced vb holes may also lead to the singlet oxygen/hydroxyl radicals (^⦁OH) [52,53].

A mechanism for SMT degradation is suggested in Equations (2)–(7) as outlined in Figure 11a for SMT degradation under solar-simulated light:

ZnO/Cu_xO + solar light → ZnOcbe⁻ + ZnOvbh⁺ + CuOcbe⁻ + CuxOvbh⁺

(2)

ZnOcbe⁻ + O₂ → O₂⁻ + H+ → HO₂^⦁

(3)

HO₂^⦁ + SMT → CO₂ + H₂O,N,S

(4)

The crucial step under solar irradiation in Figure 11a is the electron injection from ZnOcbe^- into the minority component Cu_xO in the ZnO/Cu_x=1.25O. The heterojunction between ZnO and Cu_xO has been reported [56,57] for the detection of H₂S [58] and for the monitoring of H₂ production [59]. Another mechanism to consider besides the one suggested in Equations (2)–(7) involves photosensitization of ZnO/Cu_x=1.25O during SMT degradation. This would be more important during the initial stages of the STM degradation.

A mechanism for SMT degradation under visible light is suggested in Figure 11b as:

Cu_xO + visible light → Cu_xOcbe⁻ + Cu_xOvbh⁺

(5)

CuxOcbe⁻ + O₂ → O₂⁻ + H+ → HO₂^⦁ → HO₂^⦁−

(6)

HO₂^⦁ + SMT → CO₂ + H₂O,N,S

(7)

Different SMT degradation mechanisms are suggested for pollutants’ degradation under solar or visible light as noted in Figure 11a,b.

The reduction of the Cu₂O shown in Figure 3 may proceed by two mechanisms: (i) radiative energy transfer from Cu-plasmon state to Cu_xO enhancing the electrons’ transition from the bonding orbitals in the valence band to the anti-bonding orbitals in the conduction band. This would weaken the Cu–O bonding leading to Cu₂O reduction, and (ii) electronic states of ZnO or CuO excited by the type of light used interacting with Cu_xO LSPR. In the latter case, the electron injection can be direct or indirect depending on the Cu oxidation state during SMT degradation. The plasmons interact with the anti-bonding Cu_xO states. However, in the indirect process, Landau damping takes place involving plasmon decay and leading to hot electrons. These electrons are subsequently transferred to Cu₂O. Scheme 3 below suggests two LSPR mechanisms for ZnO/Cu_xO surface under light. Scheme 3 suggests the direct electron injection into Cu_xO during SMT degradation. This is favored by the increase in the dielectric function on the photocatalyst surface. Under solar light, the latter process may proceed faster compared to visible light illumination. Both mechanisms suggested above for the photocatalytic degradation of SMT are not completely understood at the present time and more work is needed in this direction.

3. Materials and Methods

3.1. Preparation of ZnO/Cu_xO Hexagonal Wurtzite Nanowires

The synthesis of ZnO hexagonal nanowires decorated with Cu_xO was carried out aiming at fast STM degradation [60]. A solution of 200 mL of 0.1 M zinc nitrate containing Cu(NO₃)₂ and 200 mL of 0.1 M hexa-methylene-tetramine (HMT) was mixed under stirring. This solution was transferred to a 500-mL flask and refluxed at 90 °C for 3 days. The final suspension was filtered, washed with de-ionized water and ethanol, and later oven-dried in air atmosphere. The resulting powders were calcined at 500 °C for 1 h with a heating rate of 5 °C/min. Calcination of ZnO/Cu_xO nanowires: In separate experiments, Cu_xO were prepared by heating a Cu(NO₃)₂ aqueous solution to dryness at 110 °C followed by calcination at 500 °C for 1h. A long series of experiments with various ZnO:CuO ratios were run to optimize the ratio of the ZnO to CuO leading to the fastest SMT photodegradation. The sample leading to the fastest SMT degradation was prepared by mixing solutions of ZnO and CuO in the ratio of 20:1. This ratio can also be formulated as ZnO/Cu_xO (5%).

3.2. Nanowires Properties Determined by Surface Physical Methods

The composition of the optimized photocatalyst ZnO/CuxO(5%) was determined by X-ray fluorescence (XRF) and found the present composition of ZnO/Cu_x=1.25O. The XRD unit used was an Oribs PC Micro EDXRF analyzer (Shanghai, China) with a Rh source (20 kV, 1 mA) and an Apollo XRF-ML50 Silicon Drift Detector (Shanghai, China) with an energy resolution lower than 135 eV at the MnKa. The elementary crystal sizes for ZnO and Cu_xO were estimated by the Scherrer equation from the strongest peak signal.

SEM imaging was carried out in a field-emission scanning electron microscopy (FESEM, Hitachi S4800, Shanghai, China) operated at 20 kV. The images were mapped by EDXS. Particle size distribution assessment of the Cu_xO NPs were carried out by means of Image J software 1.50i (Wayne Rasband, National Institutes of Health, Maryland, USA).

3.3. X-ray Photoelectron Spectroscopy and Interfacial “In Situ” Potentials Determination

The atomic surface concentration percentage and chemical state of Zn and Cu of the ZnO/Cu_x=1.25O were determined by X-ray photoelectron spectroscopy (XPS) in an AXIS NOVA X-ray photoelectron spectrometer from Kratos Analytical, Manchester, UK provided for with a monochromatic AlK (hν = 1486.6 eV) anode. The binding energies (BE) were calibrated against the standard C1s binding energy at 284.8 eV. The spectra were deconvoluted by means of a Multipak (version 9 software) using 50:50 Gaussian-Laurentzian. Background subtraction correction was Shirley [61]. At this level, it is worth reminding the surface percentage calculation/estimation method. To estimate the surface atomic percentage of the elements of the peaks of interest, first, one should divide the intensity by the X-ray flux, then by the photoionization cross-section for the core level. This allows estimating the probability that a photoelectron will be emitted. This emission is different for each atomic core level at each photon energy. After normalizing these results, the ratio of the elemental percentages can be found.

The in situ interfacial potential during the SMT-degradation was monitored as a function of time in a pH/mV/Temp meter (Jenco 6230N, San Diego, USA) equipped with a microprocessor and a RS-232-C IBM interface.

3.4. Irradiation Procedures and ROS Analysis.

The photodegradation of SMT was carried out in the cavity of a Suntest solar simulator CPS (Atlas GmbH, Hanau, Germany). In a typical run, 25 mg of the selected photocatalyst was placed in a 50-mL photochemical reactor with 2.5 mg/L SMT. The pH of the pollutant solution was adjusted by 0.1 M NaOH or 0.1 M HCl solutions. Prior to irradiation, the samples were equilibrated in the dark for 30 min. At preselected times, 2 mL aliquots were withdrawn to evaluate the SMT degradation kinetics. For runs in the visible light range, a cut-off filter (λ > 410 nm) was inserted during the irradiation runs in the solar simulator.

The amount of SMT was determined by high performance liquid chromatography in an HPLC Agilent 1100 instrument (Waldbronn, Germany) equipped with a C18 column (Supercosil LC-18, 5 µm particle size, length 15 cm, id. 3mm) and a UV detector (λ = 270 nm). SMT aliquots of 20 µL were injected into the HPLC acetonitrile with 0.1% formic acid solution mobile phase (40:60) and a flow rate of 0.85 mL/min. The effect of the light intensity was monitored in the Suntest simulator by varying the applied power to the Xe-light source. Scavenging experiments were carried out to determine the reactive oxygen species (ROS) using: p-benzoquninone as the HO₂^⦁− radical scavenger, NaN₃ as the singlet oxygen ¹O₂ scavenger, methanol as the ^⦁OH-radical scavenger and EDTA-2Na as the hole (vbh⁺) scavenger [30].

The sulfamethazine structural formula is shown next in Scheme 4.

The results deviation was estimated using the ANOVA statistical method. All the kinetic experiments reported in this study were carried out in triplicates.

4. Conclusions

Two mechanisms are suggested for ZnO/Cu_xO leading to SMT degradation under solar or visible light. The ZnO/Cu_x=1.25O sample led to the fastest SMT degradation. CuO was the predominant form of the Cu–oxide species as detected by XPS. When Cu_xO is irradiated under visible light, the SMT degradation kinetics are slower compared to the kinetics attained under solar irradiation. The SEM/EDXS images for ZnO/Cu_x=1.25O provide evidence for nanowires with a hexagonal structure. The solution parameters intervening in the SMT degradation kinetics were investigated in detail as well as the properties of the photocatalyst surface. This study provides the first evidence for ZnO/Cu_xO iso-energetic electron transfer between Cu_xO to ZnO, leading to the degradation of SMT. There are some initial reports about ZnO/Cu_xO nanowires photoelectrochemical-mediated H₂ production activated by visible light due to the extended visible light absorption spectrum of Cu_xO. This is an advance with respect to ZnO-mediated generation of H₂ that proceeds with a low efficiency using a costly UV-light. Some preliminary studies reported on ZnO/Cu_xO nanowires as humidity and pH sensors, but no commercial devices have been developed and commercialized until now. Further work to exploit the practical potential of the ZnO/CuO nanowires is warranted at this time. Several studies in scientific journals have reported on ZnO/Cu_xO nanowires addressing the degradation of cationic and anionic dyes. However, no flow reactors leading to photoelectrochemical dye degradation-mediated ZnO/Cu_xO nanowires have been reported so far.