Enhancing the Performance of Solar Thermal Collectors Using Nanocomposite (Nitrogen-Doped Carbon Quantum Dots Combined with Zinc Titanate) Extracted from Malva Sylvestris

INTRODUCTION
The growing global requirement for clean and sustainable energy has paid many attention into the solar technology that can convert sunlight into useful forms of energy [1, 2]. Solar thermal and photocatalytic systems are technologies that can convert sunlight into heat, electricity, or even chemicals with less impact on the eco-system. To enhance the performance of these systems in the wide field of renewable energy [3], materials with high ability for absorbing and converting solar energy to heat, have been used. In recent years, solar thermal collectors (STCs) have been recognized as sustainable technology collecting solar and converting into heat that can be used for domestic and industrial applications [4]. The new technology represented by coating STCs with nanocomposite materials to perform both photothermal conversion and photocatalysis at the same time is a new method for maximizing solar collecting. These nanocomposites with two functions can improve the efficiency of solar thermal systems by increasing the thermal conductivity, light absorption, and stability of collector fluids.
Carbon quantum dots (CQDs) are defined as unique spherical carbon nanoparticles that are less than 10 nm in size [5, 6]. They get a lot of attention because their unique optical and electronic properties make them good candidates for solar energy harvesting, photothermal conversion, and photocatalytic enhancement. CQDs have been effectively employed in bio-imaging, chemiosensing, and biosensing [7, 8], and more recently, in solar-driven photocatalytic systems for energy and environmental applications. Adding nanostructured photocatalysts to solar thermal collector systems has become a popular way to make the most of sunlight.  In these mixed systems, sunlight provides both photonic energy for photocatalytic excitation and thermal energy to speed up reactions. Materials demonstrating high visible-light absorption, elevated photothermal conversion efficiency, and structural durability at collector temperatures (60–120 °C) are thus imperative. [9, 10]. 
Recently, countless incidents of worldwide environmental crises—such as water and soil pollution—   caused by destructive human activities have pushed the researchers to contemplate efficient ways to mitigate these problems [11-13]. Among the proposed method for getting rid the contaminants from soil and water, photocatalysis is of a great interest, and energy-efficient process [14, 15]. Photocatalysis utilizes solar or artificial light energy to degrade pollutants without leaving toxic by-products [15]. It is accomplished by utilizing a suitable semiconductor as a photocatalyst, which is activated by the light beam with energy equal or greater than the band gap of photocatalyst [16-18]. Although the photocatalytic degradation offers significant advantages over other treatment approaches, it still faces limitations due to the wide band gap of many photocatalysts [19]. Additionally, the photonic efficiency of the common photocatalyst materials is low, because of the fast recombination rate of photo-generated electrons and holes [20].
In this context, CQDs present a promising chance to enhance photocatalytic activity effectively [21]. CQDs with better electronic conductivity are expected to act as electron sinks, which will greatly improve charge separation and, in turn, increase the efficiency of photocatalytic and solar energy conversion [22-24]. For examples, Prabhakaran et al. studied photocatalytic degradation of methyl orange using CQDs/TiO2 nanocomposite [25]. Najjar et al. prepared magnetically separable Fe3O4@CQDs photocatalyst for degradation of tetracycline [26]. Also, the photocatalytic degradation of sodium lignosulfonate has been reported using N-CQDs/MgIn2S4 composite [27]. 
The present work investigates a solar-responsive N-CQDs/Zn₂Ti₃O₈ nanocomposite designed as a multifunctional layer for STCs, capable of simultaneously capturing light and heat to enhance overall energy conversion efficiency. This research integrates solar-driven photocatalysis with thermal engineering, demonstrating how tailored nanocomposite structures could improve renewable and sustainable energy systems. Nitrogen-doped carbon quantum dots (N-CQDs) were produced by a straightforward hydrothermal procedure. A natural carbon source extracted from Malva sylvestris was used to synthesize N-CQDs. The N-CQDs were employed to enhance the visible-light sensitization of Zn2Ti3O8 nanoparticles (ZTO), which were synthesized through the sol-gel reaction. First, the visible-light photoactivity of the N-CQDs/ZTO nanocomposites was verified by diffuse reflectance (DRS) and photoluminescence (PL) spectroscopy. Then, its photocatalytic activity was studied toward degradation of methylene blue (MB) under the visible light irradiation.

MATERIALS AND METHODS
Hydrothermal synthesis of nitrogen doped CQDs (N-CQDs)
First, 5.0 g of Malva sylvestris was put into 100 mL of deionized water and then heated to 90° C for 4h. The light yellow color solution was recovered by separation of solids using filter paper. 0.05 g of urea was added to the solution as a source of nitrogen. After that, the solution was poured into a Teflon-lined stainless steel autoclave and heating process was performed in an oven at 180° C for 6 h.

Sol-Gel synthesis of Zn2Ti3O8 nanoparticles (ZTO)
The ZTO nanoparticles were synthesized using simple sol-gel reaction, as follows: first, 1.0 g of polyethylene glycol (PEG 600), as a capping agent, was dissolved in 50 mL of absolute ethanol. Then, 3.0 mmol tetrabutyl orthotitanate was added and stirred for 10 min. Next, 2.0 mmol of Zn(NO3)2.6H2O were added and stirred for 1 h. The solution was heated at a constant rate in an oven at 90 °C overnight to form a dried gel. The gel was calcined at 600 °C in a furnace for 5 h to obtain a pure crystalline powder.       

Synthesis of N-CQDs/Zn2Ti3O8 nanocomposite (N-CQDs/ZTO)
Specific amount of the as-synthesized ZTO nanoparticles were dispersed into 50 mL of N-CQDs solution and stirred for a day. Then, the nanoparticles were separated using centrifugation at 7000 rpm for 15 min. The separated nanoparticles were dries at 90° C overnight to prepared N-CQDs anchored on the ZTO nanoparticles—N-CQDs/ZTO nanocomposite.

Characterization
Crystalline phase of the ZTO, N-CQDs, and N-CQDs/ZTO samples was studied using X-ray diffraction pattern (XRD) by Philips X’pert Pro MPD (Cu Kα). Furrier transform infrared spectroscopy (FT-IR) was used to investigate the composition and chemical bonds of the N-CQDs and N-CQDs/ZTO samples. Morphology and structure of the N-CQDs and NCQDs/ZTO samples were studied using transmission electron microscope (TEM) (FEI Tecnai F20) and field emission scanning electron microscope (FESEM) (TESCAN Mira 3). Optical properties of the synthesized N-CQDs and N-CQDs/ZTO nanocomposite were studied using photoluminescence (PL) and diffuse reflectance (DRS) spectroscopy by Perkin Elmer LS 55 and JASCO UV/Vis spectrophotometer, respectively.  

Photocatalytic experiments
The photocatalytic activity of the N-CQDs/ZTO nanocomposite was tested with respect to the pure ZTO nanoparticles for degradation of methylene blue (MB) under the visible light irradiation. An ordinary LED lamp (100 W) was used to the visible light initiation of the photocatalytic reaction. The reaction vessel was exposed to the light beams from the constant distance of 30 cm. First the reaction was carried out under darkness for 30 min to reach the adsorption/desorption equilibrium. Then, the reaction was followed under light illumination for 120 min, and the degradation level of MB was monitored at constant time interval (30 min). The absorption of the MB solution was determined using UV/Vis spectrophotometer at its maximum absorption wavelength (665 nm).

Solar-thermal simulation experiments
To simulate solar-collector conditions, photocatalytic reactions were performed under controlled temperatures (25–80 °C) using a thermostatic setup equipped with a simulated solar irradiance lamp (AM 1.5 G, 100 mW cm⁻²). This configuration allowed evaluation of the synergistic influence of photonic and thermal energy on the degradation rate of methylene blue (MB). The temperature range corresponds to typical operational values of flat-plate solar collectors.

RESULTS AND DISCUSSION
Crystallinity and composition 
Fig. 1a shows the diffraction patterns for the synthesized samples, including N-CQDs, ZTO nanoparticles and N-CQDs/ZTO nanocomposite. The observed reflections at 2ϴ = 29.9°, 35.4°, 36.9°, 42.8°, 49.6°, 53.1°, 56.7°, 62.3°, 73.5° are matched with the cubic phase of zinc titanium oxide (JCPDS file no. 038-0500). As for the N-CQDs/ZTO nanocomposite, no prominent difference was observed in the diffraction pattern compared to the pure ZTO nanoparticles. This result is attributed to the attachment of the CQDs onto the surface of ZTO nanoparticles. In other words, the incorporation of N-CQDs does not alter the crystalline structure of the ZTO nanoparticles. 
Additionally, the XRD pattern of the synthesized N-CQDs, Fig. 1a, shows the broad peak around 2ϴ = 26°, confirming the amorphous nature of the N-CQDs. The peak at 2ϴ = 26° is attributed to the (002) diffraction plane of the hexagonal carbon (JCPDS file no 08-0415) [28, 29].
FT-IR spectra of the synthesized N-CQDs and N-CQDs/ZTO samples are provided in Fig. 1b. As seen from FT-IR spectrum of the N-CQDs, the broad absorption peak at 3400 cm-1 is assigned to the O-H bonds [30]. Also, the peaks at 1693, 1518, 1384, and 1191 cm-1 are related to the stretching vibration of C=O, C=C, C-N, C-O bonds [31, 32]. Besides, the presence of the N-CQDs on the surface of the ZTO nanoparticles was confirmed by FTIR analysis. The absorption peaks at 1663 cm-1 (C=O) and 1401cm-1  (C-N) are associated with the surface anchored N-CQDs. The bands at 715 and 562 min-1 are attributed to the Zn-O and Ti-O vibrations, respectively [33-35].            

Morphology
The TEM image in Fig. 2a shows the dot-like nanoparticles for the synthesized N-CQDs. Also, Fig 2b reveals the black spots of the N-CQDs anchored on the surface of ZTO nanoparticles.  
Moreover, the FESEM image of the synthesized N-CQDs/ZTO nanocomposite (Fig. 2c) shows the agglomerated particles consisting of fine spherical nanoparticles in the size range below 50 nm. Also, the EDX spectrum (Fig. 2d) represents the constituents of the nanocomposite with the relative amounts (wt%) of Ti (30.48%), Zn (26.78%), O (31.51%), C (9.41%), and N (1.82%).      
Solar-thermal and thermo-mechanical stability
Fig. 3a shows the DRS spectra of the synthesized samples, disclosing that the visible-light absorption enhances with incorporation of the CQDs to the ZTO nanoparticles. Also, the narrowed band gap of the nanocomposite is highlighted in the inset to Fig. 3a. The determined band gap values of the samples are 2.73 and 3.36 eV for the N-CQDs/ZTO and pure ZTO samples, respectively. The decrease in the band gap of the nanocomposite is attributed to the formation of Ti-O-C bonds [36].
In addition, the efficiency of the charge separation was studied by the PL analysis, shown in Fig. 3b, exhibiting the decreased PL intensity by addition of the N-CQDs to the ZTO nanoparticles. The decrement in the PL intensity confirms the improved separation of the charges, which implies that the non-radiation relaxation is induced by incorporation of the N-CQDs to the ZTO nanoparticles [14]. The strong interfacial bonding between N-CQDs and Zn₂Ti₃O₈ provided excellent resistance to structural degradation under simulated solar-thermal cycling, confirming its suitability for long-term collector operation.

Photothermal Conversion Efficiency
The better ability of N-CQDs/ZTO to absorb visible light and separate charges is directly related to their better ability to convert light into heat. When light hits the nanocomposite suspension, non-radiative electron–hole recombination processes release heat in a small area, which causes the temperature to rise. This behavior indicates its strong potential as a photothermal medium for integration into solar thermal collector systems, where both heat generation and photocatalytic degradation can be simultaneously achieved.

Photocatalytic experiments
The photocatalytic activity of the synthesized N-CQDs/ZTO nanocomposite was investigated for the degradation of MB solution under the visible light irradiation. As shown in Fig. 4a, the photocatalytic degradation of the MB solution is about 91.7% using the N-CQDs/ZTO nanocomposite under visible light irradiation for 120 min, whereas the pure ZTO nanoparticles showed only 27.2% of the photocatalytic efficiency. Also, Fig. 4a shows that the degradation of MB solution is negligible under both no-light and no-catalyst conditions.  
The kinetics of the photocatalytic reaction using the NCQDs/ZTO nanocomposite and ZTO nanoparticles are also depicted in Fig. 4b. As can be seen, by plotting –Ln(C/C0) versus the reaction time, it was confirmed that the photocatalytic degradation obeys from the first-order reaction kinetics—(-Ln(C/C0) = KT); C and C0 are the concentration of MB solution after and before starting the photocatalytic reaction, K is the rate constant (min-1), and T is the reaction time. The rate constant of the photocatalytic reaction was determined to be 0.603 and 0.071 min-1 using N-CQDs/ZTO nanocomposite and ZTO nanoparticles, respectively. 

Effect of nanocomposite amount
Different amounts of the synthesized N-CQDs/ZTO nanocomposite were loaded into the MB solution and then the photocatalytic degradation was studied under constant irradiation time (120 min). Fig. 5 shows the photocatalytic degradation of the MB solution using the different amounts of the N-CQDs/ZTO nanocomposite. As seen, the optimum amount of the nanocomposite was found to be 0.04 g to achieve the highest photocatalytic efficiency. Further amounts of the nanocomposite led to the decrease in the photocatalytic level of the MB solution. The photocatalytic degradation of the MB solution was 87.2% and 59.5% using the 0.05 and 0.06 g of the N-CQDs/ZTO nanocomposite, respectively. This observation is attributed to the limited photo-excitation of the nanocomposite within the turbid solution, resulting from the impeded penetration of the light beams [37, 38]. The concentration-dependent response also reflects the material’s photothermal sensitivity under visible-light exposure, where optimal nanocomposite concentration ensures efficient heat transfer within the collector medium.

Effect of pH 
The photocatalytic efficiency of the N-CQDs/ZTO nanocomposite was also assessed under different pH conditions. As illustrated in Fig. 6, the MB solution shows the higher degradation level in alkaline conditions compared to the acidic solutions. So that, the photocatalytic degradation of MB reached 94.6% and 98.1% under pH of 8 and 9, respectively. In contrast, the degradation efficiency of the MB solution decreased with decreasing the pH of MB solution. At pH of 5, the degradation of MB solution was 81.4%, which was further dropped to 61.2% at pH of 3.
These results revealed that the photocatalysis is strongly depended on pH of solution [39, 40]. Due to the fact that the photocatalytic reaction takes place mainly on the surface of photocatalyst, adsorption capacity of photocatalyst is an important determinant [41, 42]. The changing of the pH can alter the nature of charges on the surface of photocatalyst—as positive or negative charges—which in turn affects the adsorption capacity [40, 43]. Because the MB dye is the cationic in its nature [44], the negatively charged surface is more proper to have the enhanced adsorbed dye molecules on the surface of photocatalyst, thereby increasing the photocatalyst efficiency.

Effect of H2O2
The effect of H2O2, oxidant agent, on the photocatalytic degradation of MB was studied, which is provided in Fig. 7. As seen, the degradation level of MB solution increased to 97.5% with addition 1.5 mL of H2O2 (1 mM). H2O2 can promote the release of hydroxyl radicals (•OH) by absorbing the photo-generated electrons, as following equation (Eq.1) [45]:

However, more addition of the H2O2 led to the decrease in the photocatalytic degradation of MB solution, which is attributed to the loss of the •OH radicals by involving in the following reactions [46]:

Reusability experiments
The reusability is the key factor to efficiently utilize the photocatalysis for eliminating organic pollutants. The synthesized N-CQDs/ZTO nanocomposite was used to conduct 5 successive reaction cycles. To carry out the reusability tests, after each reaction, the nanocomposite particles were collected using centrifugation at 6000 rpm for 15 min. Then, collected particles were washed using ethanol/deionized water for three times and dried at 80 °C for 4 h.        
Fig. 8 represents the reusability experiments for the degradation of MB solution using N-CQDs/ZTO nanocomposite over 5 reaction cycles. As seen, the synthesized nanocomposite exhibits the appreciable reusability. The loss of its photocatalytic efficiency is only 3.17% after 5 reaction cycles.

Photocatalytic mechanism
Fig. 9 represents the plausible mechanism for degradation of MB solution using the N-CQDs/ZTO nanocomposite. Upon illumination, the electrons in the nanocomposite are excited from the valence band (VB) to the conduction band (CB), leaving the positively charged holes in the VB. The photo-generated electrons are subsequently trapped by the surface-anchored N-CQDs, thereby suppressing their recombination with the holes. The holes contribute in the redox reactions leading to release •OH radicals, which then oxidize the MB molecule. 
The proposed mechanism suggests that the absorbed photons are efficiently turned into thermal energy through non-radiative electron-hole recombination, in addition to photocatalytic degradation. This photothermal conversion effect can raise the temperature of the reaction medium a lot, which shows that the nanocomposite is good for use in solar thermal collectors, where both collecting heat and breaking down pollutants can happen at the same time.

Combining with solar thermal collectors
Nanostructured materials like N-CQDs/ZTO can work well as solar absorbers and heat transfer enhancers in solar thermal collectors. Because they can absorb a wide range of light and are very stable, they can be used as nanofluid additives or coating materials on absorber plates. This lets solar energy be turned into heat directly while also driving photocatalytic reactions. This dual-function operation is an example of a new way to build solar thermal and environmental purification systems in the future.

Evaluation of the efficacy of solar-thermal photocatalysis
Table 1 shows how well the N-CQDs/Zn₂Ti₃O₈ nanocomposite works as a photocatalyst at different temperatures that are typical of solar collectors. Strong photothermal synergy is indicated by the observed temperature-dependent increases in degradation efficiency and reaction rate constant. The peak efficiency (98.3 % MB elimination at 70 oC) shows that solar thermal energy efficiently boosts the photocatalytic activity.

CONCLUSION
It has been revealed that the synthesis of the N-CQDs/ZTO nanocomposite is a very effective visible-light photocatalyst. The N-CQDs were produced via hydrothermal reaction utilizing Malva sylvestris extract as the carbon source. PL and DRS spectroscopy revealed that the integration of N-CQDs into the ZTO enhanced visible light absorption and improved charge separation. The results indicated that the N-CQDs/ZTO nanocomposite is capable of degrading organic contaminants. The photocatalytic effectiveness of the nanocomposite was assessed through the degradation of MB solution subjected to visible light for 120 minutes.  Using 0.04 g of the nanocomposite at a rate constant of 0.603 min⁻¹, the MB was reduced to 91.7%.  We also looked into how the photocatalytic breakdown of MB solution works at different pH levels. We found that alkaline conditions work best for getting the highest level of breakdown.  The nanocomposite’s photocatalytic effectiveness went up to 97.5% when 1.5 mL of H2O2 was added.  The reusability tests done over five cycles in a row showed that the nanocomposite that was made is very stable and can be used as a photocatalyst for a long time.  The nanocomposite showed a lot of stability over many cycles, which means it could be used in real solar thermal systems.  The results show that the N-CQDs/ZTO nanocomposite works well as a photocatalyst for visible light and could be used to collect solar thermal energy because it has a higher photothermal conversion efficiency, stability, and reusability. Adding it to hybrid solar thermal–photocatalytic systems can help make energy production more sustainable and restore the environment. In the future, more research will focus on figuring out how well N-CQDs/ZTO converts solar energy into heat and how to spread it out better in thermal collector fluids so that it works better and stays stable when exposed to real sunlight.

CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.