Facile synthesis of quantum dots metal oxide for photocatalytic degradation of organic hazardous materials and factory effluents

2.1 Materials

Packages of zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) and sodium hydroxide (NaOH) bought from Merck Company and a commercial pack of nano zinc oxide from Aldrich-Sigma. The chemicals used in this work are also analytical grade and used without further purification, and for all formulated solutions deionized water was used. Indigo Carmine dye (IC) (C₁₆H₈N₂Na₂O₈S₂, M. Wt. = 466.36 g/mol) was also obtained from Fluka and picked by one of the largest Spinning & Weaving Egyptian companies as one of the industrial red color dyes used in the dying process.

2.2 Preparation of quantum dots oxides

Using zinc acetate as a starting material and sodium hydroxide solution to add oxygen (O) to the production of quantum dots, ZQs were synthesized via the modified precipitation process. Sodium hydroxide (0·1 M) was applied slowly dropwise (0·5 ml/min) under intense stirring at 9000 rpm to the zinc acetate aqueous solution (0·1 M). The aging reaction has done for 48 h to validate the formation of a dense white precipitate by addition and stability. ZQs isolated by centrifugation and washed until neutral pH (pH = 7) were obtained with deionized water, followed by methanol washing.

For the complete elimination of solvent and any reactive impurities, ZQs samples were dried in an oven at 60 ⁰C for 24 h. The ZQs (ZQ1 and ZQ2) samples were calcinated after drying in the muffle furnace at 450 and 550 ⁰C for 20 min, respectively. Eventually, the ZQs samples produced were stored under airtight conditions.

2.3 Characterization

X-ray powder diffraction (XRPD-Philips Holland) was used for phases and crystal structure analysis of the prepared quantum dot catalyst. The Xpert MPD model uses 0.154 nm, 50 kV, and 40 mA copper -K al radiation, and the data recorded is 100 s/step at 0.0170 step size. A high-resolution TEM (A JEOL HRTEM- Philips CM-120) was determined the surface properties of the samples. The bandgap energies for all ZQs prepared samples were determined by UV–vis diffuse reflectance spectra (DRS, Varian Cary 300). Also, Shimadzu 240 spectrometer-JAPAN was used for determining the absorption spectrum shifts of the chosen dyes.

A spectrophotometer has calculated photoluminescence output (Shimadzu-JAPAN) and from the slope of the relationship between the time curves for fluorescence intensity and irradiation, the apparent rate constant (k_f) was determined. The Brunauer Emmett-Teller (EBT) study measured the surface areas (SA) of the prepared materials. After drying the samples at 150 ⁰C for 60 min, the SA values (eq. (1)) determined using the Tristar 3000 Micrometrics (Micrometrics apparatus-Norcross-GA) method depended on the adsorption isotherm of N₂ gas as a liquid nitrogen temperature at 77 K.

where s, $\rho$ and d is the specific surface area, density of ZnO (5.63 g/Cm³) and the particles average diameter, respectively (Mohamed et al., 2021; Singh et al., 2019).

2.4 Photocatalytic performance of quantum dots oxide samples

The first method used to evaluate the photocatalytic activity performance of the ZQs prepared samples is the photodegradation process of the Indigo Carmine dye as a selective industrial textile dye. 0.5 g/L of the prepared ZQs samples were dispersed in 100 ml of Indigo Carmine dye solution (5 × 10–5 M) at pH = 6.6 during the experimental procedures (Wahba et al., 2020). The dye solution was stirred in the darkroom for 15 min in the presence of the prepared samples to detected the adsorption–desorption equilibrium. A photoreactor used in the photodegradation process has a xenon arc lamp as a light source (from the Engineering Firm, Egypt). Also, a water-cooling cycle system is used in the reactor to remove the lamp temperature effect. The Xenon lamp has suitable power strength for this proposal which is 100 W/Cm² and its wavelength limits between 100 and 1100 nm. The photodegradation test was investigated under the Xenon light bulb as a light source which the ZQ's particles are extracted in the final step of photodegradation after the illumination process, centrifuged for a further 15 min at 8000 rpm.

Total organic carbon (TOC) of the investigated dye and factory effluents samples after the photodegradation processes is measured by the TOC analyzer (Model TOC-VCPH from Shimadzu Firm) to determine the maximum photodegradation percentage for each sample.

Also, the fluorescence rate of 7-Hydroxychromen-2-one (coumarin) photooxidation is the second way of estimating the photocatalytic performance of the ZQs prepared samples. The experiment starts with the preparation of both coumarin solution (0.146 g/L), referred to as (solution A), and the catalysts prepared solution (ZQs) (0.5 g/L), referred to as (solution B). Then mixed solution B which is dissolved in solution A and vigorously stirred at the UV source. Fluorescence spectrum emissions have been measured over a specific period after revealed the light at solution excitation (λ_ex = 332 nm). The results indicate that under these parameters, coumarin is not active.

Finally, the solar photocatalytic process for dyes with a high environmental hazard is considered an interesting application to evaluate the photocatalytic activity performance of the ZQs prepared samples. The procedure was carried out at pH 7 (neutral medium) under sunlight as the third approach for photocatalytic activity efficiency for samples prepared by ZQs.

During this analysis, the sunlight doze has 2.9 mW/Cm² and 1091 mW/Cm² of UV radiation and visible light, respectively. Also, the mineralization efficiency determined by using of COD 'C-99′ model - HANNA Company and as the following formula

where COD₀ and COD_t are the chemical oxygen demand at zero and time t, respectively.

3.1 XRPD-Characterization

Analysis of X-ray powder diffraction (XRPD) is a distinguishable way to evaluate the crystalline phases and structure of ZQs prepared samples. The resulting evidence gives direct proof of the single wurtzite crystal phase structure of the prepared ZQS compared to the crystallographic data for index card No. 36–1451 in the JCPDS-ICDD. With the two theta values of 31.67, 34.4, 36.2, 47.58, 56.47, 62.61, and 67.92⁰, the major characteristic XRPD peaks of the ZQs prepared samples observed to belong to the following planes of (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2), respectively (Fig. 1 (a)).

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Fig. 1

XRPD (a), relative refinements (b) and XRPD demonstrator parameters (c) for ZQs prepared samples.

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The lack of any additional diffraction peaks in the pattern indicated the superior purity of the samples. The prepared quantum dot samples are similar depending on the relative refinement technique and have the same packing structure as represented in Fig. 1 (b). The Debye-Scherer equation was applied with the following formula to calculate the crystallite size (D) of the prepared ZQs (Mohamed et al., 2020).

Where the symbols λ, β and θ represent the wavelength radiation of the X-ray radiation, the full width at the half-peak maximum (FWHM), and the maximum peak position of the diffraction angle, respectively, the Scherrer constant is 0.9 (Fan et al., 2013; Mohamed et al., 2020; Yang et al., 2016). The calculated results obtained indicate a decrease in the mean crystallite size value for the first ZQ1 sample (4.4 nm) compared to the second ZQ2 sample (5.3 nm) shown in Fig. 1 (c), which has XRPD demonstrator parameters for the ZQ1 and ZQ2.

3.2 TEM

High-resolution transmission electron microscopy was a potent and effective way to investigate surface morphology and the particle size of ZQ2 prepared samples. The superfine nanoparticles, extreme crystallinity composition, and ellipsoidal elongated shape are in Fig. 2. This observation forms one of the most outstanding surface qualities for the prepared samples.

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Fig. 2

TEM image of ZQs prepared samples (a and b) along with their particle size distribution map (c and d) and QDs presence percentages (e).

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The observed average particle size is 4.4 nm and 5.3 nm for the samples ZQ1 and ZQ2, respectively, was determined by using the Debye-Scherer equation. These results indicate increases in the hydroxyl volume during the preparation process, leading to increases in particle size of the prepared samples.

The approximate crystallite size values calculated by HRTEM images give significantly higher values by 10% from the actual size than that measured by XRPD, it may be due to the limited agglomeration found for the prepared quantum dot samples as a result of aging the samples until they measured.

3.3 Bandgap energies

Using Ultraviolet–visible absorption spectroscopy of the optical properties of the prepared catalysts was carried out. The absorption spectra of the prepared ZQ2 sample are greater than those of the ZQ1, as seen in Fig. 2, which shows the significant impact of the additional hydroxide concentrations in the prepared samples. One of the interesting features derived from the optical analysis is the determination of energy bandgap values due to its significant effect on the photocatalytic behavior of ZQs samples.

Tauc’s gap equation was employed to measure the optical bandgap energies by any of the following formulas:

where E_g, A, n, hυ, β and α is optical bandgap energy, constant, power factor of the transition mode, photon energy, band tailing parameter constant and the absorption coefficient, respectively.

The measuring process of the second relationship generates a clear line in a lower region of the curve when photon energy (hυ) plotted against (αhυ)², where the bandgap (E_g) values obtained by crossing the extrapolation of the corresponding straight line with the axis of hυ. ZQs samples prepared in Fig. 3, 3.53 eV, and 3.50 eV were showed for ZQ1 and ZQ2 samples, respectively. By comparison to (3.37 eV) for the bulk ZnO. These results appear when a distinctive quantum effect (quantum confinement effect) and the change in the blue area of the absorption band are detected.

From absorption spectra, as shown in Fig. 3 (b), A small blue shift from 370 to 374 nm observed indicate increases in frequency within decreasing in wavelength, the observed tunning due to decreases ZQs particle size. On the other side, from the emission spectra (Fig. 3 (c)), a distinguishable blue shift was observed nearly about 11 nm from 550 to 561 nm, which is excitation wavelength decreased from 374 to 370 nm as shown in Fig. 3 (b). This observation is expected as the same blue shift observed in the absorption spectra due to the decrease in the ZQs size from ZQ2 (5.3 nm) to ZQ1(4.4 nm). Triggering as an additional factor of ZQs surface-functionalized may be led to a decrease and/or increase of the defects of the ZQs surface (van Dijken et al., 2000). A broad emission band occurred by a defined type of electronic relaxation observed when trapping inside the surface O2⁻/O⁻ states occurred by the photo-generated holes. Trapped holes formed during tunneling processes and subsequent recombination with the oxygen vacancies can return to the bulk phase or larger ZQs size (from ZQ1 to ZQ2). This observation led to the emission effect is more intense for the smaller ZQs size (ZQ1) according to the observed recombination in the visible range (Ambade et al., 2014; van Dijken et al., 2000).

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Fig. 3

Band gap energies (a), absorption spectra (b), emission spectra (c) and UV-DRS spectra (d) of ZQs samples.

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Also, UV–vis diffuse reflectance spectra presented in (Fig. 3 (d)) suggest very low reflection rate in the visible region; thus, verifying the amount of light absorption by ZQ1 is higher as compared to ZQ2. This observation due to the photocatalytic activity of ZQ1 is higher than ZQ2 because of ZQ1 is the smaller in size and larger in surface area than ZQ2.

3.4 Surface area (BET)

Large surface areas observed are considered one of the most distinctive surface characteristics due to identify the efficiency and performance of photocatalysts prepared for various applications. The BET theory is an outstanding instrument to measure the surface (S) of ZQs prepared samples. For the ZQ 1 and ZQ 2 tests, the approximate surface area values are 233.25 and 197.81 m²/g. The relation between BET, E_g, and quantum particle size values seen for each of the ZQ samples (Fig. 4). These results demonstrate that the reduction in the optical bandgap and the particle size led to increasing the surface area affected by hydroxyl ion lack.

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Fig. 4

Correlation between quantum particle size, E_g and BET of ZQs samples.

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3.5 Photocatalytic activity of ZQs

The photocatalytic assessment of ZQs prepared samples using three applications of manufacturing, economic and environmental significance.

Indigo Carmine (IC) is one of the commercially organic dyes consumed in the food and garment industry in Egypt. The safe handling is an important environmental issue to assess the catalytic efficiency of ZQs prepared samples via the IC dye photodegradation process, multiple experiments, and relationships are determined.

The first evaluation way is the changes in the absorbance spectra of the dye within the different degradation periods under xenon lamp irradiation as a light source, as seen in Fig. 5, demonstrating the linear relationships between ln (C_o/C_t) with irradiation time for the photodegradation process. Also, the photodegradation process reaction rate constant (K_app) was determined using formulas (6).

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Fig. 5

Kinetics photodegradation rate plot of Indigo Carmine dye in presence of ZQs.

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Both ZQ1 and ZQ2 have equal overall photodegradation percentages of 99.93 and 99.91, respectively. But ZQ1 has a higher photodegradation rate than ZQ2, 17.18 × 10^-3, and 15.25 × 10^-3, respectively. It refers to the improvement of the photodegradation rates accompanied by decreases in the photodegradation times and quantum particle size seen in Fig. 5. Concurrently, the results demonstrate that the excellent photodecontamination rate is directly proportional to the minimum time needed for photodegradation, active surface area, and energy gap. Also, the photodecontamination rate is inversely proportional to the average size of the ZQs particles (Fig. 6).

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Fig. 6

Correlation between irradiation times, max. photodegradation percentages, Photodegradation rates and quantum particle size of ZQs prepared samples.

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The starting point of the suggested mechanism for the IC dye photodegradation process in presence of the prepared samples is in the valence band (VB) region, where the electrons are transferred to the conduction band (CB) region when the catalyst exposure to the light source. This process will result in the relocation of the electron to the upper level of exciting negative electrons (e^-), while at the lower level a positive hole (h⁺) formed at the previous level. So, the reactive species formed due to superoxide and hydroxyl radicals created by the reaction of e- and h + with the oxygen (O2) and water (H2O) molecules, respectively. By targeting the subsequent active components of the target IC dye molecule, the degradation mechanism completed and the full photodegradation mechanism is published in our previous work (Mohamed et al., 2020; Ambade et al., 2014; Foo et al., 2014; Hirai et al., 2005; Wahba et al., 2020).

Under the previous condition, ZQs prepared samples are revealed to a substantial improvement for the photodegradation process by shortening the recombination step time required led to increases in the photodegradation rate of the investigated dye. TOC analysis determines the highest percentage of organic carbon disappearance (dissociation) during the photodegradation processes seen in Fig. 7.

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Fig. 7

TOC analysis for photodegradation process of Indigo Carmine in presence of ZQs.

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A phenolic organic compound belonging to the Coumarin family is 7-Hydroxychromen-2-one, which has numerous biological applications such as anti-microbial, anti-oxidant, and anti-carcinogenic cells. The photo-oxidation process of Coumarin (7-hydroxychromen-2-one) is the second way of testing the photocatalytic behavior of ZQs by the fluorescent probe instrument. Fig. 8 shows the effect of the photocatalysts on the coumarin emission spectra and fluorescence rate constant (K_f) under illumination with UV light for one hour. A pseudo-zero-order reaction followed because the photo-oxidation rate improved in the first sample than the second sample as in the IC dye photodegradation process and the values listed in Table 1.

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Fig. 8

Emission spectra of coumarin in presence of different ZQs samples.

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Coumarin dye oxidation boosted in the presence of the ZQ1 (4.4 nm) sample, which is the smallest particle size prepared sample. ZQ1 has a high energy bandgap (3.53 eV), the area of the surface (233.25 m²/g), the rate of fluorescence (51.46 × 10^-3), apparent OH radical formation rate (35.91 × 10^-3), and have a greater e-/h + pair separation efficiency than that of ZQ2 (5.3 nm) sample. All above results have a distinguishable performance compared to literature and our previous work (Mohamed et al., 2021; Chen et al., 2019; Mohamed et al., 2020;).

3.6 Solar photocatalytic process for hazardous factory effluents

Industrial waste and especially wastewater contain many organic dyes, which are known for their high degradation resistance. They are a source of many environmental and health issues under normal conditions. Organic effects on waste are measured using an accurate study of the need for chemical oxygen to detect the amount of oxygen needed to turn organic waste into environmentally safe end products (CO₂ and H₂O). Organic waste is measured by chemical oxygen demand (COD).

The particle size was the main factor effect in COD values for the wastewater sample estimated during the wastewater treatment process (Fig. 9). The reported results aligned to the safe limits of the environmental law of Egypt (COD <1000 ppm) (2009).

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Fig. 9

COD evaluation for industrial wastewater in presence of ZQs prepared samples.

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January-February an experimental investigation period with one of the largest Egyptian Spinning & Weaving companies that showed high COD values ranging between 4215 and 6138 (ppm) shown in Table 2.

Table 2 COD values for solar photocatalytic process of industrial wastewater samples in 2021 in presence of ZQs prepared samples.

It can clarify from the results obtained that the spectrum of COD values for 16 COD investigated samples for hazardous factory effluents as industrial wastewater samples allowed a cap for the use of ZQ1 (4.4 nm) as a photocatalyst except for NO. 13 and NO. 4, 11, and 13 for ZQ2 (5.3). But there are seven samples in the risky limit range (901 to 978) for ZQ2 and only two samples for ZQ1. From the above findings, better results were obtained for the smallest particle size ZQ1 than ZQ2, which leads to an improvement in industrial wastewater mineralization. (photodegradation) over an assessment period owing to an increase in photo-oxidation processes with a decrease in quantum particle size. From the above results, there are 13 samples from 16 investigated factory effluents samples subject to the allowed COD limit (less than 899 ppm) for ZQ1 but only six samples in case of ZQ2 in the allowed COD limit.

The future work plan is to use various forms of ZnO depending on their particle sizes as quantum dots or nanoparticles to be economical. Prospective future research would also provide a local economic strategy for ZQ preparation since there are more exciting uses for ZQs as sophisticated new optics and solar cell applications.

3.7 Recycling of ZQs during industrial wastewater treatment of factory effluents

The recycling method is one of the more effective studies in the economic and environmental sector for preserving and repeatedly using prepared catalysts for two photocatalytic activity evaluation processes for both Indigo Carmine and industrial wastewater of factory effluents.

Indigo Carmine destruction occurred was replicated ten times using a variety of (ZQs) prepared samples with a concentration (0.5 g/l). The photodegradation rates were decreased by 36.5 % from (17.18 × 10^-3 to 10.92 × 10^-3) and 38.5 % from (15.25 × 10^-3 to 9.41 × 10^-3) in the presence of ZQ1 and ZQ2 samples, respectively (Fig. 10 (a and b). The correlation curve between repetitions using the number of various prepared ZQ samples and the photodegradation rates of Indigo Carmine dye shown in (Fig. 10 (c)). Also, nanofiltration membranes used to avoid any weight loss, so the repetition (recycling) reached to 10 times with high photocatalytic efficiency for the photocatalysts prepared. The regular accumulation of catalysts is the explanation key for the depression rate, which eventually results in the reduction of photocatalytic activity that can be seen in the ZQ2 than ZQ1 sample, indicating the impact of the particle size on the effectiveness of the prepared materials.

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Fig. 10

(a and b) photodegradation rates, (c) Correlation between recycling process number and photodegradation rate of Indigo Carmine dye and (d) COD limits for recycling process.

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The recycling process for the solar photocatalytic process of industrial wastewater samples depended on using ZQs as a photocatalyst. Also, the recycling process efficiency has been investigated by determining COD values at ten replicate times for industrial wastewater recycling in the Egyptian Spinning and Weaving Company as shown in Table 3 and 10 (d).

Table 3 COD values for recycling process of industrial wastewater in presence of ZQs samples.

Aggregation of the prepared ZQs samples during the recycling process led to decreased recycling activity because increasing its particle size due to decreases in its active surface area. These findings were observed because ZQ samples prepared behaved like a very interesting active photocatalyst.