The global freshwater scarcity crisis, highlighted by the UN's water report indicating billions lacking access to clean water and sanitation, necessitates innovative solutions. Solar vapor generation devices (SVGDs) and fog harvesting devices (FHDs) are promising strategies. Research has shown that 3D architectures significantly outperform 2D systems in both SVGDs and FHDs. 3D printing offers advantages in design flexibility, material selection, and minimized waste. While many 3D-printed SVGDs and FHDs have been developed, achieving continuous, all-weather water production remains a challenge. Existing bifunctional hydrogels, although capable of continuous harvesting, lack the autonomous adjustment needed for optimal performance in varying conditions. This study introduces an environmentally responsive, autonomous water harvesting system addressing this limitation.

Previous research extensively explored the use of 3D-printed structures for enhanced water harvesting, focusing on improving solar energy absorption, thermal management, and surface properties. 3D architectures inspired by natural structures like cacti, gunnera leaves, and Namib beetles have shown promise in fog collection. However, most existing 3D-printed systems are designed for either solar evaporation or fog harvesting but not both in a continuous, autonomous manner. The integration of self-adjusting mechanisms is crucial for maximizing efficiency throughout the day and night.

This study introduces a novel autonomous water harvesting setup combining a 3D-printed artificial forest (3D AF) and a photothermal actuator. The 3D AF comprises two parts: a bottom foundation coated with Ti₃C₂@PPy for evaporation and top tree-like structures (CNF/PLA) for fog collection. Ti₃C₂@PPy was selected for its wide solar spectrum absorption, light-to-heat conversion efficiency, and 2D water transport pathways. The photothermal actuator (shape memory alloy) enables dynamic orientation adjustment. During the day, the 3D AF tilts downward for maximum solar exposure, while at night it tilts to optimize fog collection. The morphological, structural, and optical absorption characteristics of the 3D AF foundation were studied using SEM, EDS, XRD, and UV-vis-NIR spectroscopy. Solar vapor generation was tested using a lab-made setup under 1 sun irradiation and varying light intensities. Fog collection was evaluated in a controlled humid environment and outdoors, examining the effects of tree branch number and angle of inclination. The continuous water harvesting capability of the combined system was assessed under outdoor conditions over seven days.

The Ti₃C₂@PPy coated 3D AF foundation exhibited superior light absorption compared to the activated 3D AF foundation and PPy coated 3D AF foundation, with a 6.2% increase in the visible region and significant absorption in the NIR region. Under 1 sun irradiation, the 3D AF achieved a water evaporation rate of 2.12 kg m⁻² h⁻¹, comparable to existing 3D-printed SVGDs. The evaporation rate remained high even under lower light intensities (1.02 kg m⁻² h⁻¹ at 0.5 sun and 0.8 kg m⁻² h⁻¹ at 0.3 sun). After 10 cycles of continuous operation, the evaporation rate decreased by only 6%, demonstrating excellent durability. In fog collection tests, three branches per tree yielded the highest efficiency (0.25 g cm⁻² h⁻¹). Optimizing the angle of inclination to 45° further enhanced fog collection to 0.45 g cm⁻² h⁻¹. Continuous fog collection efficiency remained above 93% after 10 cycles. Outdoor testing showed a daily water collection exceeding 5.5 L m⁻² over seven days, showcasing the potential of this combined system.

The results demonstrate the successful integration of advanced materials and autonomous actuation for continuous water harvesting. The 3D AF design, incorporating Ti₃C₂@PPy and a photothermal actuator, overcomes the limitations of previous systems by combining efficient solar vapor generation and fog collection into a single, self-regulating device. The high evaporation rate and fog collection efficiency, coupled with excellent durability and continuous operation, highlight the system's potential for addressing freshwater scarcity in diverse environmental conditions. The superior performance compared to many existing 3D-printed and MXene-based systems points to a significant advancement in the field. The system's adaptability makes it particularly relevant for regions with variable weather patterns.

This study successfully demonstrated a 3D-printed, autonomous artificial forest for continuous freshwater harvesting. The integration of Ti₃C₂@PPy enhanced solar evaporation, while the optimized tree structure and automated alignment maximized fog collection. The system's ability to harvest over 5.5 L m⁻² of freshwater daily outdoors showcases its potential for addressing global water scarcity. Future research could focus on scaling up the system for practical applications and exploring alternative materials for improved efficiency and cost-effectiveness.

While the current study demonstrates excellent performance in controlled and outdoor environments, further investigation is needed to assess long-term durability and reliability under various environmental stressors. The performance of the system may be influenced by factors such as extreme temperatures, strong winds, and the presence of contaminants in the water source or fog. Further optimization of the design and materials could potentially improve the system's overall efficiency and scalability.