Effective Fluorescence Detection of Hydrazine and the Photocatalytic Degradation of Rhodamine B Dye Using CdO-ZnO Nanocomposites

Abstract

CdO-ZnO nanocomposites were synthesized using a simple solution approach, and several characterization approaches were used to examine the morphological, structural, phase, vibrational, optical, and compositional properties of these CdO-ZnO nanocomposites. The FESEM study revealed the development of aggregates ranging in size from 250 nm to 500 nm. These aggregates were composed of various CdO-ZnO nanoparticle shapes and sizes. XRD investigation revealed hexagonal wurtzite and cubic phases in ZnO and CdO, respectively. The crystal size was 28.06 nm. The band-gap energy of the produced nanocomposites was calculated using UV-Vis analysis and was determined to be 2.55 eV. The CdO-ZnO nanocomposites were employed as a promising material for the effective fluorescence detection of hydrazine and for the quicker photocatalytic degradation of Rhodamine B (RhB) dye. Within 120 min of UV light exposure, the RhB dye was 87.0% degraded in the presence of the CdO-ZnO nanocomposites and the degradation process followed zero-order and pseudo-first-order kinetics. Based on 3σ IUPAC criteria, the limit of detection for fluorescent hydrazine sensing was 28.01 µM. According to the results presented here, CdO-ZnO nanocomposites may function as both a photocatalyst for the breakdown of organic pollutants as well as an effective luminous sensor for the detection of harmful analytes.

1. Introduction

Recent years there have seen a rise in the number of organic pollutants in the environment due to the development of agriculture and the continued growth of industry. Because of commercial operations, like tanneries, petrochemicals, and pharmaceuticals, numerous organic contaminants are released into water bodies [1,2,3]. These organic effluents are generally non-biodegradable and have a tremendous influence on the environment and people’s health [4,5,6].

Among the various pollutants, hydrazine (N₂H₄), an oily, colorless chemical with a strong odor, is a frequently used raw material in the emulsifier, metal corrosion inhibitor, pharmaceutical, and textile industries [7,8]. It is also a common propellant used in satellite launch systems, space shuttles and rockets. However, due to its instability and toxicity, hydrazine has seriously and adversely affected people’s health [9]. In addition to causing cancer, hydrazine poisoning can damage the liver, kidneys, and lungs. The US Environmental Protection Agency (EPA) has set the hydrazine threshold value in drinking water at 10 parts per billion [10]. Therefore, it is crucial to establish appropriate methods for hydrazine monitoring to protect the environment and avoid diseases.

The cosmetics, dyeing, food processing, leather, paint manufacturing, paper and textile sectors are just a few of the industries that use dyes widely [11,12]. Due to their chemical composition and large molecular size, many industrial dyes are not biodegradable, which has led to a significant environmental problem worldwide [13,14]. Rhodamine B (RhB), with the chemical formula C₂₈H₃₁ClN₂O₃ and a molecular weight 478.5 g·mol⁻¹, is one such dye that is used in the production of ballpoint pens, stamp pad inks, paints, dye-based lasers, dye-sensitized solar cells, carbon sheets and crackers in addition to being utilized as a textile coloring agent [15,16]. It is a neurotoxin and carcinogenic dye that irritates the skin, gastrointestinal tract, eyes and respiratory systems in both humans and animals. Long-term exposure to RhB is dangerous and can damage the thyroid and liver [17]. Many nanostructured materials, like α-Fe₂O₃ hollow spheres [18,19], silicon nanowires [20], ZnO nanorods [21], TiO₂ nanoparticle [22], and α-MnO₂/Palygorskite composites [23], have been reported as efficient photocatalysts.

The ZnO-CdO nanocomposites have been designed to improve on the benefits and minimize the drawbacks of individual oxides, i.e., ZnO and CdO. When compared to pure ZnO and CdO, the composite of CdO and ZnO has a much better charge-isolating performance [24]. Furthermore, because of the existence of structural defects, including oxygen and Zn vacancies, CdO-ZnO composites have shown excellent optical characteristics in diverse parts of the optical spectrum [25,26]. Because of their higher transmittance and lower resistance as compared to individual oxides, such composites have been used in optoelectronic devices.

CdO-ZnO nanocomposites of various morphologies and shapes, such as nanorods [27], nanofiber arrays [28], three-dimensional graded nanosphere [29], necklace-like nanofibers [30], coral-shaped [31], nanoblocks [32], hexagonal nanocones [33], cauliflower-like [34], quantum wells [35], flower-like hollow microspheres [36], and many others, have been prepared using different synthetic methods and reaction conditions. These composites have been investigated for a wide range of applications. SILAR-deposited ZnO-CdO thin-film nanocomposites have been investigated as liquefied petroleum gas sensors [37]. Well-crystalline composite CdO-ZnO hexagonal nanocones synthesized using a simple low-temperature hydrothermal process were used in the fabrication of photo-anodes and working electrodes for the designing of dye-sensitized solar cells, as well as a 4-nitroaniline chemical sensor [33]. CdO-ZnO hollow microspheres composites with a 2.6:100 (Cd:Zn) molar ratio demonstrated a roughly 16-fold higher gas sensitivity to ethanol than pure ZnO at 250 °C [36]. The improved performance was ascribed to electron transport from the ZnO conduction band to CdO, which results in the creation of the hole depletion and electron accumulation layers on the surface of ZnO and CdO respectively. This electron transfer from the ZnO conduction band to CdO is also responsible for the slower rate of photo-induced electron-hole recombination of charge carriers, which is a desirable property in photocatalytic applications [38,39]. Since both CdO and ZnO are n-type semiconductors, the formation of an n-n heterojunction between CdO and ZnO also leads to enhanced photocatalytic activity. After 180 min of irradiation, CdO/ZnO nano-fibrous materials generated through electrospinning demonstrated 98% degradation of methylene blue and 93% degradation of methyl orange dyes [40]. Under UV light irradiation, ZnO-CdO integrated with reduced graphene oxide as a photocatalyst mineralized bisphenol A, thymol blue, and ciprofloxacin by approximately 98.5, 98.38 and 99.28% within 180, 120, and 75 min, respectively [41].

The potential of ZnO nanoparticles as fluorescence sensors has also been investigated because of their photo-oxidation and photoluminescent quenching properties, whether in their pure form or modified forms [42,43]. In the current study, we have created a CdO-ZnO nanocomposite utilizing a facile solution technique, keeping in mind the aforementioned characteristics of CdO-ZnO nanocomposites. When using several characterization techniques, the as-synthesized sample was evaluated for its morphological, crystal, phase, vibrational, optical and compositional characteristics. Under exposure to UV light, the photocatalytic behavior of the CdO-ZnO nanocomposites was assessed against the degradation of RhB dye. The composites were also investigated as sensor materials for hydrazine fluorescence sensing.

2. Materials and Experimental Details

2.1. Materials

For the synthesis of the CdO-ZnO nanocomposites, zinc nitrate hexahydrate [Zn(NO₃)₂·6H₂O] and cadmium nitrate tetrahydrate [Cd(NO₃)₂·4H₂O] were purchased from Sigma–Aldrich, Bengaluru, India. Hydrazine of AR grade for fluorescence sensing was also procured from Sigma–Aldrich, Bengaluru, India. Rhodamine dye B dye (RhB), as the model dye, was supplied by M.P. Biomedicals, Mumbai, India. Without additional purification, all of the compounds were utilized as obtained. For all the experimental studies, the solutions were prepared with deionized (DI) water.

2.2. Synthesis of CdO-ZnO Nanocomposites

CdO-ZnO nanocomposites were synthesized utilizing a simple solution method using Zn(NO₃)₂·6H₂O and Cd(NO₃)₂·4H₂O salts. Equi-molar solutions of Zn(NO₃)₂·6H₂O (100 mL) and Cd(NO₃)₂·4H₂O (100 mL) were mixed in a beaker. A 1:1 ammonia solution was added to the mixture solution dropwise to keep the pH of the solution at 10.0. For a half-hour, the mixture was magnetically stirred. The mixed solution was refluxed at 70 °C for 5 h after stirring. The temperature of the solution was reduced to room temperature, and the resulting product was washed thoroughly with DI water and ethanol multiple times before being allowed to dry at room temperature. Finally, the obtained product was annealed for 1 h in the air at 500 °C.

2.3. Characterization Techniques for CdO-ZnO Nanocomposites

Field emission scanning electron microscopy (FESEM: JEOL-JSM-7600F, Hitachi, Tokyo, Japan) was used to characterize the morphology and surface features of the as-synthesized CdO-ZnO nanocomposites. The elemental details were analyzed using energy dispersive spectroscopy in conjunction with FESEM. X-ray diffractometer (XRD; PANalytical X’Pert PRO, Cambridge, UK) measurements with source Cu-Kα radiations, having wavelength 0.1542 nm with a scan speed of 8°/min, was used to analyze the structural, phase, and crystallinity of the nanocomposites. At room temperature, the UV-Vis spectrum of the produced CdO-ZnO nanocomposites was recorded using a Carry 100 Bio UV-Vis spectrophotometer (Agilent, Santa Clara, CA, USA) to examine their optical and band-gap characteristics. Using a Perkin Elmer LS55 fluorescence spectrophotometer (Waltham, MA, USA), the PL spectrum of the nanocomposites was acquired at room temperature.

2.4. Photocatalytic Dye Degradation

The degrading performance of the synthesized photocatalysts was investigated using the RhB dye. CdO-ZnO nanocomposite (0.05 g) was dispersed in 100 mL of 20 ppm aqueous dye solution taken in a 250 mL beaker for this process. To ensure the proper dispersion of the photocatalyst in the dye solution, ultra-sonication was carried out for 15 min. To maintain adsorption ⇌ desorption equilibrium, the solution was placed in the dark and agitated for 30 min. Before light exposure, the dye solution (3 mL) was extracted and labeled as A_o (initial dye absorbance). A 125 W mercury lamp was employed as a source of UV radiation. The degradation rate of RhB was calculated by measuring the change in absorbance at λ_max = 553 nm. The degradation of the RhB dye was investigated by recording the absorbance of aliquots extracted after a regular time interval of 20 min using a UV-visible spectrophotometer. Using Equation (1), the photocatalytic degradation percentages of the CdO-ZnO nanocomposite as photocatalyst for the degradation of the RhB dye were estimated.

$$\mathrm{Percent} \mathrm{photodegradation}=\frac{{\mathrm{A}}_{\mathrm{o}}-\mathrm{A}}{{\mathrm{A}}_{\mathrm{o}}}\times 100$$

(1)

where A_o and A are the absorbance of the aqueous RhB dye solution before and after UV light exposure to the dye solution in the presence of CdO-ZnO nanocomposite as photocatalyst.

2.5. Fluorescence-Based Hydrazine Chemical Sensor

The luminous sensor characteristics of the as-synthesized CdO-ZnO nanocomposites towards hydrazine were investigated using photoluminescent studies. Initially, a 0.01M hydrazine stock solution in a phosphate buffer solution with a pH of 7.0 was prepared. The CdO-ZnO nanocomposites were dispersed in DI water, and the photoluminescence spectrum was measured between excitations, ranging from 360–440 nm. The maxima were observed at 404 nm. Then, in phosphate buffer solution, different concentrations of hydrazine, ranging from 10–220 µM, were prepared and added to the dispersion of the nanocomposites. Before the measurements, the resulting solutions were equilibrated for 20 min. At room temperature, fluorescence was measured using a Perkin Elmer LS55 fluorescence spectrophotometer.

3. Results and Discussion

3.1. Characterization of CdO-ZnO Nanocomposites

FESEM was used to examine the microstructural and morphological features of the CdO-ZnO nanocomposites. Figure 1a–c shows the FESEM images of the CdO-ZnO nanocomposites at three different magnifications. The FESEM images (Figure 1a,b) indicate the formation of aggregates of varied dimensions (diameters ranging from 250 nm to 500 nm) as a result of annealing at 500 °C. CdO-ZnO nanoparticles of various shapes and sizes agglomerate together, as seen in the high-resolution FESEM image (Figure 1c). The EDS analysis was explored to identify the elemental composition of the as-synthesized nanocomposites. The existence of Zn, Cd, and O composites is confirmed by the EDS image (Figure 1d).

Powder XRD measurements were carried out to identify the crystallographic phase structure of the as-produced CdO-ZnO nanocomposites. Figure 2a displays these XRD patterns. Well-defined diffraction reflections at 31.75°, 34.40°, 36.25°, 47.50°, 56.55°, 62.80°, 67.90°, 69.00°, 72.55°, and 76.95°, respectively, correspond to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (1 1 2), (2 0 1), (0 0 4) and (2 0 2) diffraction planes of ZnO respectively. When compared to the information from the JPCDS card number 36–1451, all of the XRD peaks perfectly match the hexagonal wurtzite phase. The typical peaks for the cubic phase of CdO can be found in the XRD data at 2θ = 23.45°, 30.35°, 43.85°, 49.85°, and 66.30°, which correspond to the (1 1 0), (1 1 1), (2 0 0), (2 1 1), and (2 2 0) diffraction planes, respectively. These peaks match the JCPDS (Joint Committee for Powder Diffraction Studies (JCPDS) File No. 05-0640 and the literature very closely [44,45]. The formation of extremely crystalline CdO-ZnO nanocomposites is suggested by the strong and compacted diffraction peaks in the XRD spectrum of these materials.

Using the common Debye–Scherer equation, the crystal size (d) of the CdO-ZnO nanocomposites was also estimated (Equation (2)) [46,47,48,49].

$$\mathrm{d}=\frac{0.90 \mathsf{\lambda }}{\mathsf{\beta } \cos \mathsf{\theta }}$$

(2)

where λ = 0.1542 nm, θ = diffraction angle, and β = the full width half maximum (FWHM). In order to investigate crystal size, the FWHM values of the seven most strong diffraction peaks were calculated. For the CdO-ZnO nanocomposites, the crystal size was 28.06 nm (

Table 1. Crystalline parameters for the CdO-ZnO nanocomposites.

Diffraction Planes (h k l)	Diffraction Angles (°)	FWHM (β)	Crystal Size (nm)
(1 0 0)	31.75	0.25494	32.06
(0 0 2)	34.40	0.26447	31.12
(1 0 1)	36.25	0.27482	30.10
(1 0 2)	47.50	0.30312	28.34
(1 1 0)	56.55	0.33747	26.45
(1 0 3)	62.80	0.37188	24.77
(1 1 2)	67.90	0.40221	23.57

).

The FTIR spectrum provides information regarding the development of metal-oxygen bonds in CdO-ZnO nanocomposites. The FTIR spectrum was investigated in the 400–4000 cm⁻¹ region, revealing distinctive vibrational peaks at 511, 854, 1392, 1620, and 3442 cm⁻¹ (Figure 2b). The stretching vibration of the Zn-O and Cd-O bonds is shown by the prominent wide peak at 511 cm⁻¹ [50,51]. Another absorption peak at 1392 cm⁻¹ could be connected to the $C{O}_3^{2-}$ ions, which often show up in the spectrum when the FTIR samples are produced and examined in the air. The out-of-plane bending modes of these $C{O}_3^{2-}$ ions are responsible for yet another peak at 830 cm⁻¹ [52]. A broad band at 3442 cm⁻¹ may be attributed to the stretching vibration of the O-H bond of physisorbed water molecules. The existence of a transient band at 1620 cm⁻¹ can be attributed to the first overtone of the primary stretching mode of the O-H bond of physisorbed water [52].

The optical characteristics of the synthesized CdO-ZnO nanocomposites were investigated using UV-Vis absorption spectroscopy. At room temperature, the primary absorption peak for nanocomposites was seen at about 403 nm (Figure 2c). This is due to the direct transition of electrons between the valance band and conduction bands. The optical band gap (E_g) of these CdO-ZnO nanocomposites was also estimated using the absorption coefficient ‘α’ and optical band gap relationship [53,54] (Equation (3)).

$${(\mathsf{\alpha }\mathrm{h}\mathsf{\nu })}^2=\mathrm{A} \left(\mathrm{h}\mathsf{\nu }-{ \mathrm{E}}_{\mathrm{g}}\right)$$

(3)

where A = the parameter related to the effective masses of the valence and conduction bands, h = Plank’s constant, and ν = optical frequency.

The band-gap energy of the nanocomposites was computed by extrapolating the linear section of the graph between the function (αhv)² and photon energy (hν) and was found to be 2.55 eV (Figure 2d). This band gap is extremely small when compared to pure ZnO nanostructures (3.25–3.37 eV) [55,56,57] but higher than the CdO nanomaterials (2.2–2.5 eV) [58,59] previously reported in the literature.

3.2. Photocatalytic Degradation Applications of CdO-ZnO Nanocomposites

Based on the photodegradation of RhB dye as a pollutant substance under UV light irradiation at fixed operational parameters, i.e., a dye concentration of 20 ppm, a catalyst dose of 0.05 g, a 100 mL volume of solution, and an irradiation time of 120 min, the photocatalytic performance of the CdO-ZnO nanocomposites was investigated. In order to compare the photocatalytic effectiveness of the CdO-ZnO nanocomposites, the degradation processes were also examined under UV irradiationonly without any catalyst and also in the presence of a catalyst when the dye solution was not exposed to UV radiation.

The photocatalytic activity of the CdO-ZnO nanocomposites was investigated by measuring the primary absorption peak of RhB at 553 nm. Figure 3a depicts the UV-vis spectra of the RhB dye over the nanocomposite, and it can be seen that the strongest absorbance peak of RhB diminishes with increasing UV irradiation time. The variations inphotodegradation extent (A/A_o) and percentage photodegradation of the RhB dye as a function of exposure duration are depicted in Figure 3b,c respectively. When the dye solution underwent UV irradiation solely without any catalyst or when no UV irradiations were applied to the dye solution dispersed with a photocatalyst, no discernible degradation of the RhB dye was observed. However, the percentage degradation of the RhB dye increases significantly in the presence of CdO-ZnO nanocomposites, with about 87% degradation observed within 120 min of UV light exposure.

Three kinetics models: zero, pseudo-first, and pseudo-second order, were studied by using the following equations (Equations (4)–(6)) [60,61].

$${\mathrm{C}}_0-\mathrm{C}={ \mathrm{k}}_0\mathrm{t}+0$$

(4)

$$\ln \frac{{\mathrm{C}}_0}{\mathrm{C}}={ \mathrm{k}}_1\mathrm{t}+0$$

(5)

$$\frac{1}{\mathrm{C}}-\frac{1}{{\mathrm{C}}_0}={ \mathrm{k}}_2\mathrm{t}+0$$

(6)

where C₀ and C are the initial dye concentration and final dye concentrations after irradiation time (t), and k₀, k_1, and k₂are the rate constants for zero-order, pseudo-first-order and pseudo-second-order rate expressions, respectively. The equilibrium RhB concentrations were determined from the calibrated curves.

The kinetics parameters were determined by graphing the data and fitting them to a linear plot (Figure 4a–c).

Table 2. Kinetic parameters for the degradation of RhB dye using CdO-ZnO nanocomposites.

Kinetic Model	Rate Constant (×10−2)	Half-Life Time (t1/2)	R2 (COD)
Zero-order	k0 = 2.406	68.25 min	0.99431
Pseudo-first-order	k1 = 1.196	57.94 min	0.95846
Pseudo-second-order	k2 = 1.086	28.04 min	0.80346

lists the different kinetic parameters. In the presence of the CdO-ZnO nanocomposites, the photocatalytic degradation of the RhB dye followed zero-order and pseudo-first-order kinetics, as is evident from the coefficient of determinant (COD; R²) values.

The energy gap between the valence and conduction band, the oxidation potential of photogenerated, positively charged holes, and the effectiveness in separating photogenerated electrons and holes are all factors that affect the photocatalytic efficacy of a semiconductor photocatalyst, according to Reddy et al. [62]. When exposed to UV light, photogenerated electrons move from the ZnO valence band to the conduction band, leaving holes in the valence band. In the CdO-ZnO nanocomposite, CdO acts as a sink for the conduction band electrons of ZnO and induces a shift in the Fermi levels toward lower potentials [17,63]. This transfer further lowers the recombination of electron-hole pairs, thereby increasing photocatalytic activity. While the holes at the conduction band oxidize the hydroxyl ions, the electrons at the conduction band reduce the dissolved oxygen [64]. Additionally, the n-n heterojunction that results from the combination of n-type CdO and ZnO semiconductors speeds up the electronic transport processes. The hydroxyl radicals (OH^•) formed, owing to the oxidation of the adsorbed H₂O or adsorbed OH⁻ ions, are the prominent strong oxidants that degrade the complex dye molecules to simpler ones (Figure 5) (Equations (7)–(17)) [11,65,66].

Table 3. Comparison of photocatalytic degradation of RhB dye for CdO-ZnO nanocomposites, with previously reported photocatalysts.

Photocatalyst	[RhB] (ppm)	Catalyst Dose (g/L)	Time (min)	Degradation (%)	Light Source	k1 (×10−2 min−1)	Ref.
CdO-ZnO nanocomposites	20	0.5	120	87.0	UV	1.196	This work
ZnO-Cu0.5O heterostructure	10	0.05	120	73.5	UV	21.70	[67]
CuO nano-whiskers	1	-	260	84.0	Visible	0.71	[68]
In-doped ZnO nanoparticles	20	0.5	120	76.0	UV	-	[69]
ZnO nanoparticles	10 μM	0.2	200	98.0	Solar	1.7	[70]
Au-ZnO nanoparticles	10	0.3	180	95.0	UV	2.47	[71]
Ce-doped spinel CuFe2O4	~4.25	2.0	120	88.0	Visible	-	[72]
TiO2/ZrO2 composites	10	0.5	270	90.5	UV	-	[73]
Cauliflower shaped ZnO	10	0.5	120	75.0	Solar	-	[74]

compares the photocatalytic parameters for the breakdown of the RhB dye, as measured here, with the other previously reported parameters.

$$\mathrm{ZnO} \stackrel{ \mathrm{hv} }{\to } \mathrm{ZnO}\left({\mathrm{e}}_{\mathrm{CB}}^{-}\right)+ \mathrm{ZnO}\left({\mathrm{h}}_{\mathrm{VB}}^{+}\right)$$

(7)

$$\mathrm{CdO} \stackrel{ \mathrm{hv} }{\to } \mathrm{CdO}\left({\mathrm{e}}_{\mathrm{CB}}^{-}\right)+ \mathrm{CdO}\left({\mathrm{h}}_{\mathrm{VB}}^{+}\right)$$

(8)

$$\mathrm{ZnO}\left({\mathrm{e}}_{\mathrm{CB}}^{-}\right)+ \mathrm{CdO}\stackrel{}{\to } \mathrm{CdO}\left({\mathrm{e}}_{\mathrm{CB}}^{-}\right)+\mathrm{ZnO}$$

(9)

$${\mathrm{O}}_{2 \left(\mathrm{ads}\right)}\stackrel{{\mathrm{e}}_{\mathrm{CB}}^{-}}{\to }{\mathrm{O}}_2^{-•}$$

(10)

$${\mathrm{O}}_2^{-•}+{ \mathrm{H}}_2\mathrm{O}\stackrel{}{\to }{\mathrm{OH}}^{-}+{ \mathrm{HO}}_2^{•}$$

(11)

$${\mathrm{O}}_2^{-•}+{ \mathrm{H}}_2\mathrm{O}\stackrel{}{\to }{\mathrm{OH}}^{-}+{ \mathrm{HO}}_2^{•}$$

(12)

$$2{ \mathrm{HO}}_2^{•}\stackrel{}{\to }{\mathrm{H}}_2{\mathrm{O}}_{2 }+{ \mathrm{O}}_{2 }$$

(13)

$${\mathrm{H}}_2{\mathrm{O}}_2\stackrel{{\mathrm{O}}_2^{-•}}{\to }{\mathrm{OH}}^{-}+{\mathrm{OH}}^{•}+{ \mathrm{O}}_{2 }$$

(14)

$$\mathrm{ZnO}\left({\mathrm{h}}_{\mathrm{VB}}^{+}\right)+{ \mathrm{H}}_2\mathrm{O} \stackrel{}{\to }{\mathrm{OH}}^{•}+{\mathrm{H}}^{+}+ \mathrm{ZnO}$$

(15)

$$\mathrm{CdO}\left({\mathrm{h}}_{\mathrm{VB}}^{+}\right)+{ \mathrm{H}}_2\mathrm{O} \stackrel{}{\to }{\mathrm{OH}}^{•}+{\mathrm{H}}^{+}+ \mathrm{CdO}$$

(16)

$${\mathrm{OH}}^{•}+ \mathrm{Dye} \stackrel{}{\to } \mathrm{Dye} \mathrm{degradation} \mathrm{products}$$

(17)

3.3. Sensing Applications of CdO-ZnO Nanocomposites

Investigations were conducted on the efficacy of the CdO-ZnO nanocomposites for hydrazine fluorescence sensing. Photoluminescence emission is greatly impacted by the presence of hydrazine in composite suspensions. With each subsequent administration of hydrazine, there is a definite indication of the quenching of fluorescence emission. As demonstrated in Figure 6, the fluorescence intensity of the CdO-ZnO nanocomposites decreased as the hydrazine concentration increased from 10 to 220 μM.

The fluorescence data were evaluated using the basic Stern–Volmer equation to better understand the quenching process of the CdO-ZnO nanocomposites by hydrazine (Equations (18) and (19)) [75,76,77].

$$\frac{{\mathrm{I}}_{\mathrm{o}}}{\mathrm{I}}={ \mathrm{K}}_{\mathrm{sv}}\left[\mathrm{Q}\right]+1$$

(18)

$$\frac{{\mathrm{I}}_{\mathrm{o}}}{\mathrm{I}}-1={ \mathrm{K}}_{\mathrm{sv}}\left[\mathrm{Q}\right]$$

(19)

where K_sv = Stern–Volmer quenching constant, [Q] = hydrazine (quencher) concentration, I_o = fluorescence intensity in the absence of quencher, and I = fluorescence intensity after the addition of hydrazine.

Due to the nonlinear nature of the graph, however, no significant information can be drawn from the plot between (I_o/I)-1 and [Q] (Figure 7a). Still, the graph exhibited linear behavior for the quenching of the nanocomposites for the hydrazine concentration, ranging between 10–130 µM. These findings show that static quenching outperforms dynamic quenching in the linear zone; however, when the concentration of the analyte hydrazine increases, the divergence from linearity corresponds to the equality of static and dynamic quenching by hydrazine as a quencher [78,79].

A modified Stern–Volmer equation was applied to better understand the quenching mechanism (Equations (20) and (21)) [80,81].

$$\frac{{\mathrm{I}}_{\mathrm{o}}}{\mathrm{I}}={ \mathrm{K}}_{\mathrm{sv}}\left[\mathrm{Q}\right]{\mathrm{e}}^{\mathrm{V}\left[\mathrm{Q}\right]}+1$$

(20)

Equation (20) can be re-written as Equation (21)

$$\frac{{\mathrm{I}}_{\mathrm{o}}-\mathrm{I}}{{\mathrm{I} \mathrm{e}}^{\mathrm{V}\left[\mathrm{Q}\right]}}={ \mathrm{K}}_{\mathrm{sv}}\left[\mathrm{Q}\right]$$

(21)

where V = the static quenching constant.

A linear fit of the plot $\frac{{\mathrm{I}}_{\mathrm{o}}-\mathrm{I}}{{\mathrm{Ie}}^{\mathrm{V}\left[\mathrm{Q}\right]}}$ vs. [Q] exhibits a COD (R²) = 0.98565 for the CdO-ZnO nanocomposites (Figure 7b). The value of V fits 0.015 with a K_SV of 1030 M⁻¹ for the CdO-ZnO nanocomposites. It was observed that the fluorescent hydrazine sensing detecting limit was 28.01 µM. This indicates that CdO-ZnO nanocomposites can be used as an efficient luminescent sensor for the detection of hydrazine.

4. Conclusions

By using a simple solution growth technique, CdO-ZnO nanocomposites were created. A comprehensive examination verified the development of highly aggregated composites produced at high densities with outstanding crystallinity and purity. When exposed to UV rays in the presence of the as-produced CdO-ZnO nanocomposites, the RhB dyes degraded. Both zero-order and pseudo-first-order kinetics were followed in the degrading process. The inclusion of hydrazine showed a significant influence on the photoluminescence emission of the CdO-ZnO nanocomposites and its concentration further enhanced it. The detection limit for hydrazine using fluorescence sensing was found to be 28.01 µM. The findings of the current study support the notion that synthesized CdO-ZnO nanocomposites represent a promising material for photocatalytic and sensing applications.