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Article

TiO2 Decorated onto Three-Dimensional Carbonized Osmanthus Fragrans Leaves for Solar-Driven Clean Water Generation

by
Yali Ao
1,†,
Li Wang
1,†,
Lin Yang
1,2,
Chengjie Duan
1,
Qizhe Gui
1,
Songyun Cui
2,
Shutang Yuan
2 and
Jiaqiang Wang
1,*
1
School of Materials and Energy, Institute of International Rivers and Eco-Security, Yunnan Province Innovation Center for New Materials and Equipment Technology in Water Pollution Control, Yunnan Frontier Water Environment Industry Research Institute, Yunnan University, Kunming 650091, China
2
Kunming Branch of Yunnan Hydrology and Water Resources Bureau, Dianchi Lake Ecosystem Observation and Research Station of Yunnan Province, Kunming 650032, China
*
Author to whom correspondence should be addressed.
†
These authors contributed equally to this work.
Nanomaterials 2025, 15(7), 504; https://doi.org/10.3390/nano15070504
Submission received: 24 February 2025 / Revised: 12 March 2025 / Accepted: 17 March 2025 / Published: 27 March 2025
(This article belongs to the Section Energy and Catalysis)
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Versions Notes

Abstract

:
Solar steam generation (SSG) has garnered significant attention for its potential in water purification applications. While composites with physically combined structures based on semiconductors or biomass have been developed for SSG, there remains a critical need for low-cost, high-efficiency devices. In this study, TiO2 composites exhibiting excellent stability, high solar absorption, porous microstructure, and hydrophilic surfaces were identified as effective materials for SSG and water purification for the first time. A novel SSG device was designed by decorating TiO2 onto three-dimensional carbonized Osmanthus fragrans leaves (TiO2/carbonized OFL). Compared to directly carbonized OFL (without TiO2) and Osmanthus fragrans leaves with templated TiO2 (OFL-templated TiO2), the TiO2/carbonized OFL carbon composites demonstrated enhanced solar absorption, achieving over 99% in the visible region and more than 80% in the near-infrared region. Under solar illumination of 1 kW·m−2, the TiO2/carbonized OFL device achieved a high water evaporation rate of 2.31 kg·m−2·h−1, which is 1.6 times higher than that of carbonized OFL and 3.45 times higher than OFL-templated TiO2. Additionally, the TiO2/carbonized OFL system exhibited remarkable efficiency in treating pharmaceutical wastewater, with a chemical oxygen demand (COD) removal efficiency of 98.9% and an ammonia nitrogen removal efficiency of 90.8% under solar radiation.

1. Introduction

Solar-driven interfacial steam generation has emerged as a promising technology for diverse applications, including sterilization, desalination, and water purification [1]. The development of highly efficient photothermal materials is crucial to maximize the utilization of natural sunlight. To date, significant progress has been made in exploring various photothermal materials, including natural materials [2,3,4,5], carbon-based materials [6,7,8,9], metallic plasmonic materials [10,11,12], semiconductor materials [13,14,15,16], porous polymers [17,18,19,20,21], and composite materials [22,23,24]. Despite these advancements, the development of materials that simultaneously exhibit high photothermal conversion efficiency, cost effectiveness, and environmental friendliness remains a significant challenge [25,26].
Carbon materials derived from biomass carbonization have gained considerable attention due to their natural micro- and macro-structures, high hydrophilicity, and excellent light-harvesting capabilities. Various plant-derived materials, such as mushrooms [27], lotus seedpods [28], rice leaves [29], sunflower heads [30], Enteromorpha prolifera [31], daikon [32], loofah [33], sweet lime peels [34], kelp [35], tofu [26], carrot [36], and rice straw [37], have been successfully employed as photothermal materials after carbonization [38,39]. These carbonized plant materials not only exhibit broadband light absorption and efficient photothermal conversion but also retain the unique structural features of plants that facilitate water transport. Particularly, leaves have attracted significant interest for solar steam generation due to their exceptional water transport and light-harvesting capabilities [40]. For instance, carbonized lotus leaves have demonstrated remarkable SSG performance with an evaporation rate of 3.1 kg·m−2·h−1 [41].
Among various leaf structures, Osmanthus fragrans leaves (OFLs) are particularly noteworthy. As the primary structure for transpiration in evergreen broad-leaved vascular tree species [42], OFLs account for more than 90% of water evaporation [43]. The unique straight-hole structure within OFLs facilitates multiple light scattering and absorption while providing efficient pathways for moisture and vapor transmission, both of which are crucial for SSG processes [44]. The non-toxicity, stability, natural transpiration properties, excellent structural features, and hydrophilicity of OFLs make them an ideal source for photothermal materials. Consequently, carbonized OFL presents significant potential as a continuous and porous 3D network carbon sponge for solar-driven water transpiration applications.
The integration of different materials to form composites can yield synergistic effects, potentially enhancing photothermal efficiency beyond that of the individual components. For example, carbonized radish coated with titanium nitride and titanium oxide demonstrated significantly higher evaporation rates compared to pristine biomass carbon materials [29,45]. The combination of carbon materials with suitable semiconductors can create synergistic effects that not only improve photothermal conversion efficiency but also enable pollutant degradation through photocatalytic processes. Semiconductor materials like TiO2 are particularly promising due to their light absorption and hydrophilic properties under UV irradiation [46]. Recent studies have reported composite materials combining TiO2-nanoparticle-based photocatalytic functions with Au-nanoparticle-based plasmonic evaporation [47], as well as compound films composed of Au@TiO2 core-shell nanoparticles that enhance solar water evaporation [48]. The TiO2 in the composite can significantly increase the energy transfer efficiency under sunlight due to the light absorption and hydrophilic properties with UV irradiation [49]. A novel composite coagulation technique with built-in electric potential was proposed to fabricate high-performance uniaxial moisture-driven generators [50]. This study rationally designed a manganese oxide/poly-L-lysine co-modified carbon fiber cloth (CFC) composite that achieves high-efficiency antifouling solar-driven desalination through enthalpy reduction of water evaporation coupled with synergistic enhancement of broadband light absorption and antibacterial efficacy [51].
In this study, we developed a novel TiO2-decorated three-dimensional carbon material derived from carbonized Osmanthus fragrans leaves (TiO2/carbonized OFL) using biotemplating and hydrothermal methods. The TiO2/carbonized OFL composite was designed to synergistically combine the advantages of carbon and TiO2, while preserving the inherent porous structure of plant leaves. We systematically compared the solar evaporation efficiency of TiO2/carbonized OFL with carbonized OFL and OFL-templated TiO2 to demonstrate the enhancement in solar evaporation performance. Furthermore, we applied TiO2/carbonized OFL in the treatment of actual pharmaceutical wastewater under natural solar radiation, demonstrating its potential for practical water purification applications. This work not only establishes TiO2/carbonized OFL as a promising solar photothermal material but also provides new insights for future research in solar-driven wastewater treatment technologies, particularly through the synergistic combination of TiO2 and carbonized biomass.

2. Experimental

2.1. Materials

All chemicals were analytical grade and could be used without further purification. The osmanthus leaves (OFLs) used in this study were collected from pruning waste on the campus of Yunnan University during autumn. These selected mature leaves exhibited a healthy green coloration without parasitic infestation, with an average dimension of 4 cm × 11 cm. Anhydrous glucose (C6H12O6, AR) was purchased from Tianjin Fengchuan Chemical Reagent Technology Co., Ltd. (Tianjin, China); isopropyl alcohol titanium (Ti4(OCH3)16, 98%) was purchased from Beijing Inocai Technology Co., Ltd. (Beijing, China); glutaraldehyde (C5H8O2, 50%) was purchased from Tianjin Damao Chemical Reagent Factory; and acetylacetone (C5H8O2, 99%) was purchased from Guangdong Guanghua Technology Co., Ltd. (Guangzhou, China). Pharmaceutical wastewater was collected from a production facility in Songming, Yunnan Province.

2.2. Synthesis of Carbonized OFL

The Osmanthus leaves were cut into small 1 cm × 1 cm pieces and dried. Then, a certain mass of dried osmanthus leaves (OFLs) was mixed with 60 mL of deionized water, and 3% mass fraction of glucose was added. The glucose was stirred until it was completely dissolved, and the mixture was heated in a water bath autoclave reactor at 200 °C for 24 h. After that, the prepared material was dialyzed in pure water for 2–3 days, followed by freeze-drying for 48 h to obtain carbonized OFL material.

2.3. Synthesis of TiO2/OFL Composites

The preparation method of TiO2/OFL composite materials combines biological templates with hydrothermal synthesis, using OFL as a biological template carrier and TiO2 as the precursor solution. The osmanthus leaves (OFLs) were cut into appropriate sizes and then soaked in a 4% glutaraldehyde solution for 24 h to fix the plant template. Then, it was rinsed with pure water 3 to 4 times and placed at room temperature to remove surface moisture. Then, gradient dehydration of the material was performed using 30%, 50%, 70%, 90%, and 100% ethanol solutions. The dehydrated material was soaked in 10% HCl for 24 h, then washed with pure water until neutral, and finally soaked in ethanol for 2 h. After that, the materials used to remove the surface ethanol solution were placed in a fume hood to air dry naturally at room temperature for about one week, resulting in the OFL biological template carrier.
To obtain the Ti precursor solution, 10 mL of 98 wt% TTIP solution, 190 mL of 99 wt% acetylacetone, and 1 mL of anhydrous ethanol were mixed. The ratio of TTIP to acetylacetone was chosen to ensure optimal TiO2 nanoparticle formation during the hydrothermal process. An appropriate amount of air-dried OFL biotemplate carrier was weighed and immersed in the precursor solution for 24 h. The TiO2/OFL composite material was freeze-dried to form it. After that, the material soaked in the precursor solution was placed in a quartz boat and heated in a tube furnace under an N2 atmosphere, with a programmed heating rate of 5 °C/min until reaching 280 °C, where it was held for 2 h. Then, it continued to warm to 460 °C for 4 h to obtain the TiO2/carbonized OFL composite materials.

2.4. Solar Steam Generation

For the solar evaporation experiment, a solar simulator was used to test three different types of materials, namely OFL, carbonized OFL, and TiO2/OFL composites. These materials were cut into disc shapes and immersed in a container filled with water for a solar evaporation experiment. To prevent heat loss, a layer of cotton fiber was wrapped around the outer wall of the container (glass bottle). The solar evaporation rate was measured under different solar radiation intensities (1 kW/m2, 3 kW/m2, 5 kW/m2, 10 kW/m2). An analytical electronic balance was used to record the change in water mass (Δm) during the evaporation process in sunlight at 15 min intervals, while monitoring the temperature (T) of the material with an infrared imaging thermometer. The evaporation rate (η) is calculated as follows:
η = Δm/(T × S)
In this, η represents the evaporation rate, measured in kg·m−2·h−1; Δm is the change in water weight, measured in kg; T is time, measured in hours; and S is the cross-sectional area of the corresponding material, measured in m2. Please note that the diameter of each material is 0.01 m, so S remains constant in this study.

2.5. Characterization

The phase composition of the samples was characterized using a TTR-III X-ray diffractometer (XRD) (Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation (λ = 0.15406 nm), performing continuous scanning in the 2θ range of 10–90°. Elemental composition and chemical states were investigated by X-ray photoelectron spectroscopy (XPS) on a K-Alpha+ system (Thermo Fisher Scientific Inc., Waltham, MA, USA), where the binding energy scale was calibrated using the adventitious carbon C 1s peak at 284.8 eV, followed by peak deconvolution analysis with Avantage 5.9 software. Morphological features were examined using an FEI Nova NanoSEM450 field-emission (FEI Company, Hillsboro, OR, USA) scanning electron microscope (SEM) operated at 15 kV. Optical absorption properties were evaluated by UV-vis-NIR spectroscopy (Shimadzu Corporation UV-2401PC, Kyoto, Japan) over the spectral range of 200–2500 nm. Surface functional groups were identified through Fourier-transform infrared spectroscopy (FTIR) measurements conducted on a Thermo Nicolet 8700 spectrometer (Thermo Fisher Scientific Inc., Waltham, MA, USA) equipped with a diamond ATR accessory, collecting 32 scans at a 4 cm−1 resolution.

3. Results and Discussion

3.1. Synthesis and Characterization

Pre-treatment of biomass before pyrolysis is considered an effective way to improve the properties of biochar [52]. Vacuum freeze-drying has been proven to be an effective pre-treatment method [53]. During the freeze-drying process, the transformation through water-ice-sublimation improved the crystalline structure and hydrophobicity of the biomass, resulting in biochar with a uniform pore surface structure after pyrolysis [54]. Hydrothermal pretreatment is also a type of pretreatment method. Hydrothermal pretreatment can enhance the specific surface area, pore volume, content of oxygen-containing functional groups, and crystallinity of biochar. In our work, a facile hydrothermal method was developed to prepare carbonized OFL. The primitive pore structure of the leaves was preserved by controlling the temperature and pyrolysis time while carbonizing. At the same time, glucose solution as a hydrothermal solvent can establish chemical engineering of the material surface (such as improving the hydrophilicity of the OFL). The combination of biotemplating and hydrothermal methods enables the uniform growth of TiO2 on both the inner and outer surfaces of the material. On the basis, we introduced the biotemplating method in the preparation process of carbonized OFL to obtain a TiO2/OFL composite that could fully replicate the surface and internal structure of OFL. The intuitive images of carbonized OFL, TiO2/OFL, OFL temp-TiO2, and their precursors can be seen in Figure 1. The carbonized OFL and TiO2/OFL were black in color, and the OFL-templated TiO2 were white; they all inherited the flake shape of the natural leaf.
The XRD pattern of the TiO2/OFL composite is presented in Figure 2a, showing distinct diffraction peaks at 2θ ≈ 25°. Distinct diffraction peaks are observed, with the most prominent peak appearing at 2θ ≈ 25°, corresponding to the (101) crystal plane of anatase TiO2 (JCPDS No. 21-1272). The well-defined position and intensity of this characteristic peak confirm the crystalline nature of TiO2 in the composite material. Furthermore, additional diffraction peaks corresponding to other crystal planes of anatase TiO2, including (004), (200), (105), and (211), are clearly identified, providing strong evidence for the successful incorporation of crystalline TiO2 onto the OFL biochar matrix. In the higher 2θ range (2θ > 60°), the diffraction pattern shows a gradual decrease in peak intensity with the emergence of a broad background, suggesting the coexistence of amorphous carbon phases within the composite structure.
For comparison, Figure 2b displays the XRD pattern of pristine OFL biochar, which exhibits a broad diffraction hump centered around 2θ ≈ 24°, characteristic of amorphous carbon materials. This pattern indicates that the high-temperature carbonization of osmanthus leaves primarily yields biochar with a disordered carbon structure, lacking long-range crystalline order. The distinct contrast between the XRD patterns of TiO2/OFL composite and pure OFL biochar clearly demonstrates the successful integration of crystalline TiO2 nanoparticles with the amorphous carbon matrix through the composite preparation process.
The XPS analysis of the TiO2/OFL composite provides detailed insights into its chemical composition and bonding states. Figure 2c presents the high-resolution Ti 2p spectrum, which exhibits two well-defined peaks at binding energies of 458.5 eV and 464.3 eV, corresponding to Ti 2p3/2 and Ti 2p1/2, respectively. These peaks are characteristic of the Ti4+ oxidation state, confirming the presence of TiO2 in the anatase phase. The spin-orbit splitting energy of 5.8 eV between the Ti 2p3/2 and Ti 2p1/2 peaks is consistent with the reported values for crystalline TiO2, further validating the successful incorporation of TiO2 into the composite material.
Figure 2d displays the deconvoluted O 1s spectrum, which reveals the complex oxygen environment within the composite. The spectrum can be resolved into three distinct components: (1) a dominant peak at 530.0 eV, attributed to lattice oxygen in the Ti-O bonds of TiO2; (2) a secondary peak at 531.5 eV, where the oxygen associated with C=O and -OH functional groups may originate from the biochar matrix; and (3) a minor peak at 533.0 eV, corresponding to weakly bonded oxygen species such as C-O or adsorbed water molecules. The presence of these oxygen species suggests a synergistic interaction between the TiO2 nanoparticles and the oxygen-rich functional groups on the biochar surface. This multifunctional oxygen environment not only enhances the material’s surface reactivity but also potentially improves its performance in applications such as photocatalytic water treatment or solar-driven evaporation, where surface chemistry plays a critical role. The coexistence of Ti-O bonds and oxygen-containing functional groups further confirms the successful integration of TiO2 with the biochar matrix, creating a composite material with tailored surface properties.
The morphological evolution of the materials at different stages of processing is presented in Figure 3. Figure 3a reveals the surface structure of natural osmanthus fragrans leaves (OFL), exhibiting a characteristic rough fibrous surface with naturally occurring pores, which are intrinsic to the plant’s native morphology. A higher magnification view in Figure 3b provides further insight into the microstructure, revealing the presence of surface particulate matter. The transformation upon carbonization is evident in Figure 3c,d, where the carbonized OFL displays a well-defined, hierarchical porous structure with uniform pore distribution and enhanced surface openness, compared to its natural counterpart. The TiO2-modified OFL, as shown in Figure 3e,f, demonstrates successful surface functionalization, with TiO2 nanoparticles uniformly dispersed across the substrate. The high-resolution image (100,000×) in Figure 3f reveals a dense and homogeneous distribution of TiO2 particles, suggesting effective surface modification. This nanostructured surface is particularly advantageous for photocatalytic applications and may significantly influence the material’s wettability properties.
Due to the agglomerated nanoparticles of pure TiO2 [55], the introduction of OFL in the TiO2/carbonized OFL composite significantly suppresses particle aggregation, resulting in more dispersed and smaller particles. This reduction in particle size and improved dispersion can enhance the solar-driven water evaporation rate, expose more active sites, and promote pollutant adsorption and degradation.
Notably, the modification process has preserved and enhanced the natural stomatal structures of the leaf, as evidenced by the high-magnification images. The removal of the natural waxy coating during processing has exposed these stomatal features, which are known to enhance light-trapping capabilities [56,57,58,59]. The replication of these biological structures in the TiO2/carbonized OFL composite is particularly advantageous for solar-driven applications, as it combines the light-harvesting efficiency of natural plant structures with the photocatalytic properties of TiO2.
Cross-sectional analysis (Figure 4) further confirms that both carbonized OFL and TiO2/carbonized OFL maintain the intrinsic straight-pore architecture of natural OFL, while demonstrating improved structural regularity and channel alignment. This preservation of the natural vascular structure, combined with the introduced modifications, creates an optimized system for water transport and light absorption. The biomimetic approach employed in this study successfully translates the evolutionary advantages of natural leaf structures into engineered materials, providing an effective structural foundation for enhanced solar steam generation (SSG) performance. The hierarchical porosity, from macro-scale channels to nano-scale features, facilitates efficient water transport while maximizing light absorption and vapor generation capabilities.

3.2. Evaporation Performance of the Materials

To evaluate the photothermal performance, the materials were subjected to simulated solar irradiation at varying power densities (1 sun, 3 suns, 5 suns, and 10 suns) in a controlled experimental setup. The mass changes of water were monitored at 10 min intervals over a 1 h period. For comparative analysis, pristine OFL, carbonized OFL, TiO2/carbonized OFL, and pure water were tested under identical environmental conditions, maintained at an ambient temperature of 24 °C and relative humidity of 45%.
The experimental results, as summarized in the accompanying table and illustrated in Figure 5, demonstrate the water evaporation rates for pure water, carbonized OFL, and TiO2/carbonized OFL composite under different solar intensities (1 sun, 5 suns, and 10 suns). The data reveal that both TiO2/carbonized OFL and carbonized OFL exhibit significantly enhanced evaporation rates compared to natural OFL and pure water across all tested light intensities.
As shown in Figure 6, the variation of transpiration efficiency with evaporation time for TiO2/TiO2 OFL and TiO2/TiO2 OFL under different light intensities is presented. Under 1-sun irradiation (1 kW/m2), the TiO2/carbonized OFL composite demonstrated remarkable performance, with a cumulative mass loss of 0.12 g after 50 min of illumination, significantly outperforming previously reported carbonized biomass materials (e.g., carbonized lotus leaves with an evaporation rate of 3.1 kg·m−2·h−1). This represents a 3-fold and 1.3-fold increase in evaporation rate compared to pure water and carbonized OFL, respectively. The superior performance of TiO2/OFL can be attributed to the synergistic combination of the biological template, TiO2 photocatalyst, and carbon matrix, which collectively enable efficient water transport and enhanced light absorption. The black surface of the composite further contributes to its excellent photothermal conversion efficiency. These results clearly demonstrate that the TiO2/carbonized OFL composite, with its optimized structural and compositional design, significantly outperforms simply carbonized materials in solar steam generation applications.
Temperature changes were monitored using infrared (IR) thermal imaging. Two sets of infrared images illustrate the temperature variations of the carbonized OFL and TiO2/carbonized OFL under 1-sun illumination during the experiment (Figure 7a,b). The images were taken from a top view. As shown in Figure 7b, the surface temperature of TiO2/carbonized OFL reached 41.5 °C from room temperature within approximately 20 min, and the temperature steadily increased and reached 43.7 °C after 50 min. However, the surface temperature of the carbonized OFL only increased to 39.7 °C after being exposed to sunlight for 50 min. The surface temperature of TiO2/carbonized OFL changed more rapidly than that of carbonized OFL and reached the highest value under the same irradiation time. Additionally, as TiO2 particles grow on both the surface and interior of the composite, thin water films form on these areas. This setup offers two key advantages: (i) The TiO2 particles and carbon on the top layer absorb solar energy multiple times, heating the water film on the surface and causing a sharp rise in temperature. (ii) Due to the linear microchannels and pinholes in the OFL, convection occurs between the hot water in the top layer and the colder water at the backside. This facilitates heat transfer through the channels until thermal equilibrium is reached. This indicates that TiO2/carbonized OFL can effectively absorb and convert solar energy into heat with less heat loss to the bulk water.
To evaluate the performance of different solar-driven water evaporation materials, we compared three materials, analyzing their synthesis methods, water evaporation rates, advantages, and limitations (Table 1). The results indicate that all these materials exhibit high photothermal conversion efficiency but differ in stability, fabrication complexity, and practical applicability.
Among them, PVA@PCLS, synthesized using the sol-gel method, achieved the highest water evaporation rate (2.33 kg·m−2·h−1) [60]. This excellent performance can be attributed to its efficient solar-to-vapor conversion capability and well-structured porous architecture. Additionally, the utilization of biomass-derived precursors enhances resource recycling. However, the preparation of biochar via the sol-gel method requires high energy input, which may limit its large-scale application. Furthermore, its long-term stability and durability in practical environments remain to be evaluated.
The CFC/MnO2/PLL composite, prepared via electrostatic adsorption, exhibited a slightly lower evaporation rate (2.20 kg·m−2·h−1) [51]. The primary advantages of this material include its reduced water evaporation enthalpy, which facilitates efficient energy utilization, and its enhanced light absorption properties, which contribute to a higher evaporation rate. Additionally, the incorporation of MnO2 endows the material with antibacterial properties, making it more promising for water treatment applications. However, its long-term durability remains uncertain, and the synthesis process involves multiple steps, making it more complex and costly compared to other methods.
The TiO2/carbonized OFL composite, synthesized through the hydrothermal method, exhibited an evaporation rate of 2.31 kg·m−2·h−1, which is comparable to that of PVA@PCLS. This material demonstrates high stability and excellent water purification capability, making it a promising candidate for integrated water treatment applications. The hydrothermal synthesis method allows for well-dispersed TiO2 nanoparticles on the carbonized substrate, enhancing both photothermal and photocatalytic properties. However, its applicability to diverse pollutants is still limited, which may constrain its effectiveness in treating complex wastewater compositions.
From the above comparison, it is evident that each material has distinct strengths and weaknesses. While PVA@PCLS shows the highest evaporation rate, its high energy consumption during fabrication may hinder large-scale deployment. CFC/MnO2/PLL introduces antibacterial properties and energy-efficient evaporation, yet its high synthesis cost and potential durability issues require further optimization. TiO2/carbonized OFL offers a balance between high stability and water purification ability but needs further modifications to broaden its pollutant removal capacity. Future research should focus on optimizing synthesis methods to reduce fabrication energy consumption and cost while maintaining high evaporation efficiency. Additionally, improving the long-term stability and broadening the water purification capabilities of these materials are essential for practical solar-driven water treatment applications.
To quantitatively study the solar-driven evaporation performance of the TiO2/carbonized OFL composite with different content of the TiO2, we changed the addition ratio of the titanium source in the precursor solution to prepare six groups of composite materials with different TiO2 content. The water evaporation performance test and XPS characterization of the material were carried out. The results are shown in Figure 8. The composite material with TiO2 content of 1.31% exhibited the highest water evaporation efficiency, and the maximum water evaporation rate can reach 2.31 kg·m−2·h−1. The composite materials with TiO2 content of 1.65% and 2.03% also have good water evaporation performance, and the water evaporation rate can reach 2.02 kg·m−2·h−1 and 1.89 kg·m−2·h−1, respectively. With the increase of the proportion of titanium source in the precursor solution, when the TiO2 content of the composite material is 2.19%, the water evaporation rate decreases significantly, making it even lower than that of carbon materials. As a comparison, the evaporation efficiency of the OFL-templated pure TiO2 without carbon source was also tested in the same condition. The evaporation efficiency of SSG with OFL-templated pure TiO2 is very low and close to the efficiency of pure water. The photothermal conversion property of the composite material, consisting of biochar and titanium dioxide, initially increases with the loading but then decreases as the load continues to rise. This is because titanium dioxide particles with excessively large diameters hinder the interaction between water molecules and the heated components. Additionally, these larger particles may obstruct the movement of water molecules. Therefore, we can conclude that the water evaporation performance of TiO2/carbonized OFL depends on two key factors: the structure of the material and the relative content of TiO2 in the composite.

3.3. UV-Vis-NIR Diffuse Reflectance Spectra

Light absorption properties of the materials were measured in the solar spectrum range (wavelengths from 300 to 2200 nm). A comparison of the absorption spectra of carbonized OFL and TiO2/carbonized OFL is presented in Figure 9. The carbonized OFL exhibits higher absorbance in the ultraviolet region, with relatively stable absorbance around 400–600 nm, close to 1. As the wavelength increases, the absorbance gradually decreases, particularly beyond 800 nm, where it significantly declines and remains low after 1200 nm. In contrast, TiO2/carbonized OFL shows minimal changes in absorbance across the entire wavelength range, with a gentle downward trend. Its absorbance remains relatively high in the 400–1200 nm range, demonstrating excellent broadband absorption characteristics. While carbonized OFL has strong absorbance in the ultraviolet region, its absorbance rapidly diminishes in the near-infrared region. On the other hand, TiO2/carbonized OFL maintains relatively high absorbance over a broader wavelength range, exhibiting superior broadband absorption in both the ultraviolet and visible light regions, which is more favorable for applications such as photocatalysis. In summary, compared to carbonized OFL, TiO2/carbonized OFL demonstrates better absorption over a wider spectral range, while carbonized OFL excels in the ultraviolet region but performs poorly in the near-infrared region. First of all, the structures of micron/submicron holes as well as the rough surface of materials enable strong and broad optical resonances, resulting in efficient light trapping and absorption enhancement of carbon-based materials [44]. At the same time, due to the continuous scattering and refraction of light incident on the composite film containing TiO2 crystals in the Au@TiO2 core-shell nanoparticle structure, the light absorption performance of carbon is enhanced [48]. Based on the abovementioned effects, the solar evaporation improved with the use of the TiO2/carbonized OFL composites.

3.4. FT-IR Spectra

The Fourier-transform infrared (FTIR) spectra in Figure 10 illustrate the transmittance variations of carbonized OFL and TiO2/carbonized OFL in the wavenumber range of 500–4000 cm−1. Figure 10a represents the FTIR spectrum of carbonized OFL. The band at 3300.52 cm−1 corresponds to O-H stretching vibrations, attributed to hydroxyl groups or residual water on the material surface. The peak at 2940.9 cm−1 is characteristic of C-H stretching vibrations, indicating the presence of hydrocarbon groups in the material. The band at 1686.0 cm−1 is typically associated with C=O stretching vibrations, suggesting the presence of carbonyl groups. The bands at 1272.2 cm−1 and 800.4 cm−1 are possibly linked to vibrations of C-O or other C-X bonds, which might originate from oxygen-containing functional groups formed during the carbonization process. Figure 10b shows the FTIR spectrum of TiO2/carbonized OFL. The peak at 3385.9 cm−1 falls within the typical range of O-H stretching vibrations, originating from hydroxyl groups or water on the TiO2 surface. The band at 1690.5 cm−1 corresponds to the bending vibrations of C=O or O-H bonds. The band at 1201.0 cm−1 is likely associated with the stretching vibrations of C-O or Si-O bonds, potentially arising from functional groups in organic compounds or inorganic oxides.
A comparative analysis reveals that both samples exhibit O-H stretching vibration peaks in the range of 3300–3385 cm−1, with TiO2/carbonized OFL showing a higher peak, potentially indicating a higher surface hydration or hydroxyl content. For C=O and C-H absorptions, carbonized OFL exhibits more pronounced peaks for C=O (1686.0 cm−1) and C-H (2940.9 cm−1), which are characteristic of hydrocarbons and carbonyl groups formed during carbonization. These peaks are weaker or absent in TiO2/carbonized OFL. In the low-wavenumber region (1200–800 cm−1), both carbonized OFL and TiO2/carbonized OFL display distinct characteristic peaks, likely originating from stretching vibrations of C-O, Si-O, or Ti-O bonds. These peaks reflect the compositional and structural differences between the two materials. In summary, carbonized OFL exhibits more prominent carbon-based functional group characteristics, while TiO2/carbonized OFL demonstrates more surface features related to TiO2.

3.5. Purification of Pharmaceutical Wastewater

The carbonized OFL and TiO2/carbonized OFL composite were applied for evaporative purification of pharmaceutical wastewater under 8 h solar irradiation. As shown in Figure 11, we fabricated a device designed to directly generate clean water using solar energy. This device consists of two compartments: one for contaminated water and another for condensed purified water. The side walls were constructed with steel plates equipped with thermal insulation layers to minimize solar absorption losses. The top of the device was covered with glass. The carbonized OFL and TiO2/carbonized OFL composite floated at the air–water interface of the contaminated solution to absorb solar energy.
During the experiment, solar steam generation was influenced by climatic parameters including solar irradiance, ambient temperature, and wind speed. Solar irradiance was recorded throughout the process using a pyranometer. The temperature of the top glass surface exceeded the ambient temperature. The ambient temperature affected the temperature difference (ΔT) between the bulk water in the basin and the inner surface of the glass cover. Wind speed enhanced the natural condensation process in the solar steam generation system by reducing the ambient temperature around the system and slightly cooling the top glass surface for condensation. The average solar irradiance and ambient temperature were 573 W/m2 and 26.6 °C, respectively.
Figure 12a shows the surface water temperature in the solar steam generation (SSG) system under real solar irradiance, recorded by thermocouples. During the 8 h pharmaceutical wastewater evaporation treatment, the water temperature in the TiO2/carbonized OFL device remained consistently higher than that of the carbonized OFL and blank control groups. The peak surface water temperature reached 66 °C at 14:00 (with an irradiance of 0.62 kW/m2). The mass of collected purified water was measured using an electronic balance, as illustrated in Figure 12b. After 8 h of irradiation, approximately 30 mL of purified water was collected from the TiO2/carbonized OFL device, compared to 19 mL from the carbonized OFL device and only 9 mL from the blank group. The solar still equipped with TiO2/carbonized OFL achieved a productivity of 6.9 L/m2.
In solar-driven water evaporation research, selecting an appropriate experimental duration is crucial for evaluating system performance. Generally, the experimental duration should align with the daylight hours in actual applications to ensure the reliability and practical relevance of the results. The effective daylight hours typically range from 6 to 10 h, making an 8 h experimental duration a widely accepted choice. For example, in one study, researchers developed an environmentally friendly photothermal hydrogel evaporator for efficient solar-driven water purification and conducted a continuous 12 h test [61]. The results showed that the evaporation performance remained stable over the long duration, demonstrating the high reliability of the system. Additionally, previous studies have shown that TiO2 possesses excellent thermal stability. For instance, a study investigated the stability of electron-beam-deposited TiO2 monolayers and TiO2/SiO2 high-reflective layers during annealing at temperatures ranging from 300 to 1100 °C and found that the main optical properties of the material did not significantly degrade below 900 °C [62]. Furthermore, TiO2, as a widely used photocatalytic material, has been thoroughly validated for its heat resistance and chemical stability. Research has shown that TiO2 nanotube array films retain their tubular structure after being calcined at 650 °C for 2 h, further proving their excellent stability under high-temperature conditions [63]. In conclusion, an 8 h experimental duration effectively simulates system performance under actual sunlight conditions. Combining performance trend analysis, the inherent stability of the materials, and support from the relevant literature, it can be inferred that the TiO2/carbonized OFL system demonstrates good stability in long-term use.
As shown in Figure 13a, after purification by SSG, the color of the collected water is close to clear, indicating SSG based on TiO2/carbonized OFL and carbonized OFL can be effectively applied for removal of pollutants. Figure 14 shows the COD and ammonia nitrogen removal performance by carbonized OFL and TiO2/carbonized OFL composite. The SSG with TiO2/carbonized OFL was able to reduce COD concentrate and ammonia nitrogen concentrate from 54471 mg/L and 57.26 mg/L to 579 mg/L and 5.27 mg/L, respectively. The COD concentrate and ammonia nitrogen concentrate were also reduced to 786 mg/L and 7.24 mg/L for the carbonized OFL, while the collected purified water was 6070 mg/L (COD) and 17.18 mg/L (ammonia nitrogen) for the blank group. The SSG with carbonized OFL removed 98.5% of COD and 87.4% of ammonia nitrogen in the pharmaceutical wastewater after evaporation, indicating its good water purification capacity. By comparison, more than 98.9% of the COD and 90.8% of the ammonia nitrogen was removed by the SSG with TiO2/carbonized OFL. The enhanced performance should be attributed to the combined synergetic water purification effects. It is proved that the TiO2/carbonized OFL composite combining the advantage of TiO2 and carbon can not only improve the water evaporation performance of the SSG but also play a role in the further purification of water during the evaporation process.

4. Conclusions

In summary, a composite material incorporating TiO2 and porous carbon derived from OFL was designed for efficient clean water generation powered by abundant solar energy. The uniformly distributed TiO2 particles within the composite improve solar light absorption and facilitate the conversion of solar energy into localized heat at the interface. This heat directly drives the evaporation-condensation process, effectively distilling clean water from the contaminated solution. Compared with the carbonized OFL, the TiO2/carbonized OFL composite played a better role in promoting solar evaporation of water. The TiO2/carbonized OFL solar steam generator was able to generate steam at a rate of 2.31 kg·m−2·h−1 under 1-sun illumination, which is 3.45 times the bare water system. Moreover, the productivity of clean water of the solar still with the TiO2/carbonized OFL was 6.9 L/m2.day even in outdoor conditions with low solar intensities. At the same time, the TiO2/carbonized OFL composite achieved 98.8% COD removal efficiency and 90.8% ammonia nitrogen removal efficiency after evaporation. The excellent performance of the TiO2/carbonized OFL composite, particularly its high evaporation rate and pollutant removal efficiency, should motivate further research in low-cost, facile, and scalable water purification technologies. Future studies could explore the scalability of this material for large-scale water treatment applications.

Author Contributions

Methodology, Y.A.; Validation, S.C. and S.Y.; Investigation, C.D. and Q.G.; Data curation, L.Y.; Writing—original draft, L.W.; Project administration, J.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Major Project (FWCY-ZD2024001) from Yunnan Provincial Department of Education for Service Key Industries, National Natural Science Foundation of China (22062026); Key Project (202401BF070001-028) for Double First-Class Joint Special Project between Yunnan Provincial Department of Science and Technology; and Special Project for The Construction of Field Scientific Observation and Research Stations (202305AM340008). The authors also thank the Yunling Scholar (YNWR-YLXZ-2019-002) and Institute of Frontier Technologies in Water Treatment, R & D Project (2022 No4), from the Water Resources Department of Yunnan Province.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Intuitive images of natural OFL, carbonized OFL, TiO2/OFL, and OFL-templated TiO2.
Figure 1. Intuitive images of natural OFL, carbonized OFL, TiO2/OFL, and OFL-templated TiO2.
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Figure 2. (a) XRD pattern of the TiO2/OFL; (b) XRD pattern of the carbonized OFL; (c,d) XPS spectra of TiO2/OFL.
Figure 2. (a) XRD pattern of the TiO2/OFL; (b) XRD pattern of the carbonized OFL; (c,d) XPS spectra of TiO2/OFL.
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Figure 3. (a,b) The SEM surface morphology of natural OFL; (b) the morphology of natural OFL at 100,000 times magnification; (c,d) SEM surface of carbonized OFL morphology diagram; and (e,f) SEM surface morphology of TiO2/carbonized OFL, where (f) is the SEM surface morphology of TiO2/carbonized OFL at 100,000 times magnification.
Figure 3. (a,b) The SEM surface morphology of natural OFL; (b) the morphology of natural OFL at 100,000 times magnification; (c,d) SEM surface of carbonized OFL morphology diagram; and (e,f) SEM surface morphology of TiO2/carbonized OFL, where (f) is the SEM surface morphology of TiO2/carbonized OFL at 100,000 times magnification.
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Figure 4. (a) The internal micro SEM morphology of natural OFL; (b) the internal micro SEM morphology of carbonized OFL; and (c) the internal micro SEM morphology of TiO2/carbonized OFL.
Figure 4. (a) The internal micro SEM morphology of natural OFL; (b) the internal micro SEM morphology of carbonized OFL; and (c) the internal micro SEM morphology of TiO2/carbonized OFL.
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Figure 5. (a) Changes of the evaporation rate of the carbonized OFL under different light intensities with time; and (b) changes of the evaporation rate of the TiO2/carbonized OFL under different light intensities with time.
Figure 5. (a) Changes of the evaporation rate of the carbonized OFL under different light intensities with time; and (b) changes of the evaporation rate of the TiO2/carbonized OFL under different light intensities with time.
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Figure 6. Transpiration efficiency of carbonized OFL and TiO2/carbonized OFL under different light intensities as a function of evaporation time.
Figure 6. Transpiration efficiency of carbonized OFL and TiO2/carbonized OFL under different light intensities as a function of evaporation time.
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Figure 7. (a) Infrared images of carbonized OFL under an irradiation intensity of 1 sun. (b) Infrared images of TiO2/carbonized OFL under an irradiation intensity of 1 sun.
Figure 7. (a) Infrared images of carbonized OFL under an irradiation intensity of 1 sun. (b) Infrared images of TiO2/carbonized OFL under an irradiation intensity of 1 sun.
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Figure 8. The evaporation performance of the TiO2/carbonized OFL composite with different amounts of TiO2.
Figure 8. The evaporation performance of the TiO2/carbonized OFL composite with different amounts of TiO2.
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Figure 9. The UV-Vis diffuse reflectance spectra of carbonized OFL and TiO2/carbonized OFL.
Figure 9. The UV-Vis diffuse reflectance spectra of carbonized OFL and TiO2/carbonized OFL.
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Figure 10. Fourier infrared spectrum of the material in the range of 500–4000 nm; (a) shows the Fourier infrared absorption spectrum of the carbonized OFL; (b) shows the Fourier infrared absorption spectrum of the TiO2/carbonized OFL.
Figure 10. Fourier infrared spectrum of the material in the range of 500–4000 nm; (a) shows the Fourier infrared absorption spectrum of the carbonized OFL; (b) shows the Fourier infrared absorption spectrum of the TiO2/carbonized OFL.
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Figure 11. Variations in solar irradiance and ambient temperature over an 8 h period in a practical solar-driven evaporation wastewater treatment system.
Figure 11. Variations in solar irradiance and ambient temperature over an 8 h period in a practical solar-driven evaporation wastewater treatment system.
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Figure 12. (a) The change of wastewater surface temperature in evaporation experiment; (b) the amount of purified water collected in the 8 h evaporation experiment.
Figure 12. (a) The change of wastewater surface temperature in evaporation experiment; (b) the amount of purified water collected in the 8 h evaporation experiment.
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Figure 13. (a) Pharmaceutical wastewater; (b) pharmaceutical wastewater treated by carbonized OFL; and (c) pharmaceutical wastewater treated by TiO2/carbonized OF.
Figure 13. (a) Pharmaceutical wastewater; (b) pharmaceutical wastewater treated by carbonized OFL; and (c) pharmaceutical wastewater treated by TiO2/carbonized OF.
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Figure 14. Changes in ammonia nitrogen and COD content before and after treatment of pharmaceutical wastewater with the materials prepared in this work.
Figure 14. Changes in ammonia nitrogen and COD content before and after treatment of pharmaceutical wastewater with the materials prepared in this work.
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Table 1. Comparative analysis of solar-driven water evaporation performance among engineered materials.
Table 1. Comparative analysis of solar-driven water evaporation performance among engineered materials.
Material NameSynthesis MethodWater Evaporation Rate (kg·m−2·h−1)AdvantagesDisadvantagesReference
PVA@PCLSSol-gel method2.33Resource recycling; high solar-to-vapor conversion efficiencyHigh energy consumption in the preparation process of biochar; long-term stability and durability not yet evaluated[60]
CFC/MnO2/PLLElectrostatic adsorption2.20Low evaporation enthalpy; enhanced light absorption and antibacterial propertiesInsufficient long-term stability and durability; complex preparation methods and high costs[51]
TiO2/car-bonized OFLHydrothermal method2.31High stability; efficient water purification capabilityLimited applicability to pollutantsThis work
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Ao, Y.; Wang, L.; Yang, L.; Duan, C.; Gui, Q.; Cui, S.; Yuan, S.; Wang, J. TiO2 Decorated onto Three-Dimensional Carbonized Osmanthus Fragrans Leaves for Solar-Driven Clean Water Generation. Nanomaterials 2025, 15, 504. https://doi.org/10.3390/nano15070504

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Ao Y, Wang L, Yang L, Duan C, Gui Q, Cui S, Yuan S, Wang J. TiO2 Decorated onto Three-Dimensional Carbonized Osmanthus Fragrans Leaves for Solar-Driven Clean Water Generation. Nanomaterials. 2025; 15(7):504. https://doi.org/10.3390/nano15070504

Chicago/Turabian Style

Ao, Yali, Li Wang, Lin Yang, Chengjie Duan, Qizhe Gui, Songyun Cui, Shutang Yuan, and Jiaqiang Wang. 2025. "TiO2 Decorated onto Three-Dimensional Carbonized Osmanthus Fragrans Leaves for Solar-Driven Clean Water Generation" Nanomaterials 15, no. 7: 504. https://doi.org/10.3390/nano15070504

APA Style

Ao, Y., Wang, L., Yang, L., Duan, C., Gui, Q., Cui, S., Yuan, S., & Wang, J. (2025). TiO2 Decorated onto Three-Dimensional Carbonized Osmanthus Fragrans Leaves for Solar-Driven Clean Water Generation. Nanomaterials, 15(7), 504. https://doi.org/10.3390/nano15070504

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