SUPPLEMENTARY INFORMATION

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1 3D self-assembly of aluminium nanoparticles for plasmon-enhanced solar desalination Lin Zhou 1,3, Yingling Tan 1, Jingyang Wang 1, Weichao Xu 1, Ye Yuan 1, Wenshan Cai 2, Shining Zhu 1 and Jia Zhu 1* *Correspondence to: jiazhu@nju.edu.cn contributed equally Supplementary Information Contents S. I Fabrication Process S. I. 1 Self-assembly of Al nanoparticles S. I. 2 Feasibility for scalable and high throughput fabrication S. II Characterizations of Al NPs S. II. 1 TEM images of Al NPs with surface oxide layers S. II. 2 Particle size distributions of Al NPs S. II. 3 Demonstrations on role of Al NPs for solar absorption S. III Comparison between our structure with conventional solar absorbing materials S. IV Dependence of solar steam efficiency with optical concentration S. V Durability of plasmon enhanced solar desalination S. V. 1 Salinity after cycles S. V. 2 Long time durability test for solar desalination S. VI Comparison between the plamon enhanced solar desalination with conventional desalination strategies S. VII References NATURE PHOTONICS 1

2 S. I Fabrication Procedure S. I. 1 Self-assembly of Al nanoparticles inside AAM The nanoporous anodized alumina membranes (AAM) were fabricated and transferred to an electron-beam evaporator (FU-20PEB-RH) for the deposition of Al nanoparticles. As shown in Figure S1, at low gas pressures (< ~ Pa), the aluminum ions fly ballistically from the target in straight lines, travel deep (~ µm) into the pores, and energetically impact on the sidewall of each pore in the template. Therefore, by finely tuning the pore size of the nanoporous AAM and the gas pressure of the evaporator system, random sized and positioned aluminum nanoparticles (Al NPs) can be deposited on both the top surface and sidewalls of the nanopores in the templates. During the process of deposition, the deposited Al on the surface (rather than sidewall) forms a metallic film. Meanwhile, Al NPs down in the naopores would adhere to the walls and aggregate, forming a randomly distributed profile (the experimental SEM images with increasing deposition time and schematic diagrams for the self-assembly mechanism is shown in Fig. S1). 2 NATURE PHOTONICS

3 SUPPLEMENTARY INFORMATION Figure S1 From left to right, time evolution of the Al NPs inside nanopores of AAM, which illustrates the self-assembly process: (a-c) SEM images for 4 min, 7 min and 9 min, respectively; (d-f) schematic diagrams. S. I. 2 Feasibility for scalable and high throughput fabrication We will show that our fabrication process is highly scalable, especially compared to traditional top down approaches. There are mainly two steps in the process: 1) nanoporous templates fabrications, 2) Al NPs with physical vapor deposition (PVD). a) Fabrications of nanoporous templates Actually the alumina-based nanoporous template (or called anodic aluminium oxide) we used in this study has been extensively investigated and widely used for several decades in material sciences and several major industries. For example, currently alumina porous templates have been widely applied as a scalable process in fabrications, optics as well as solar harvesting systems, etc 1-3. The nanoporous templates have been massively produced and commercially available (with $50 for a 4-inch piece currently, even cheaper for large scale manufactures). See links: b) Al NPs with PVD The physical vapor deposition (PVD) process has been widely used for large scale manufacture in several major industries, such as semiconductors, displays ( For large NATURE PHOTONICS 3

4 scale systems in the semiconductors and large area displays (~ 20 m 2 per hour), the cost of PVD operation (both material and process cost) is estimated to be ~ $ 0.3/m 2. The material cost of the traditional RO membrane (~ $ 36/m 2 ) 4. S. II Characterizations of Al NPs S. II. 1 TEM images of Al NPs with surface oxide layers Surface oxide layer is critical for plasmonic effect as well as stability. To carefully evaluate the thickness of the surface oxide layer, aluminum nanoparticles are carefully examined under transmission electron microscopy, as shown in Figure S2. Figure S2 TEM images of aluminum nanoparticles with surface oxide layers. It is confirmed that the Al NPs are coated with a thin oxide coating layer (~ 2 nm). S. II. 2 Particle size distributions of Al NPs Figure 1f reveals that the Al NPs have a broad size distribution. Taking Al NPs of one of the nanopores in Fig. 1j for example (ticked by red square in inset of Fig. R4), the statistical size distribution of Al NPs is shown in Figure S3. It can be found that Al NPs with diameter around 12 nm account for more than 70% of the total quantity. 4 NATURE PHOTONICS

5 SUPPLEMENTARY INFORMATION Figure S3 Size distributions of Al NPs inside one of the nanopores in Fig. 1j of the manuscript. S. II. 3 The role of Al NPs for solar absorption The superior performance of the broadband solar absorption of our structure is a result of judicious design that combines a few important characteristics of plasmonic responses to realize unique broadband perfect absorption. The most critical component responsible for solar absorption and desalination is Al NPs along the sidewalls of pores, while Al film and AAM provide assistant role. We will provide detailed simulation and experimental results as following to justify this point. Firstly, we demonstrate the role of Al NPs for solar absorption through comparison of absorption (both simulation and experiment) of various structures. Figure S4 shows the comparison between bare AAM, Al film/aam, Al NPs/AAM and our structure (ticked by Al NPs/Al film/aam for clear demonstration). It reveals that the nanoporous template (bare AAM) is almost transparent over the entire solar spectrum. When there is only Al film (with periodic pore array) on AAM, the overall absorbance slightly increases (red line in Fig. S4). The broadband and highly efficient absorbance can be achieved only when the random Al NPs are introduced. (Al NPs/Al film/aam, blue and black lines in Fig. S4). Thus, it is clear that Al NPs instead of Al film that plays the crucial role for solar absorption (the slight absorption increase from Al NPs/AAM to Al NPs/Al film/aam infers the assistant role of Al film). NATURE PHOTONICS 5

6 Fig. S4 (a) Simulated absorbance of bare AAM, Al film/aam, Al NPs/AAM and Al NPs/Al film/aam (the proposed structure in the manuscript), respectively. (b) Experimental absorption spectra for three of the samples. S. III Comparison between our structure with conventional solar absorbing materials Figure S5 compares the energy conversion efficiency of our structure with these of other structures, such as carbon nanotube forests (CNTs film with ~ 900 m in thickness, absorbance ~ 100%), CNT powder filtrated on a paper, Al film and water. It is clear that the overall energy transfer efficiency of our structure is the highest. 6 NATURE PHOTONICS

7 SUPPLEMENTARY INFORMATION Figure S5 Energy transfer efficiency by several control absorbers under 4 sun illuminations. This part has been added to the Supplementary information section III. S. IV Dependence of solar steam efficiency with optical concentration According to the definition of the solar steam efficiency mh LV / Pin, the efficiency is proportional to the evaporation rate m and inversely proportional to optical concentration C opt or illumination light intensity P in (Phase change entropy h LV only slightly varied with steam temperature and will not discuss in details below). The evaporation rate m does not increase linearly with P in (smaller slope for low C opt, larger slope for higher C opt see Figs. 3C-D), as m is positively related to the thermal motion or steam temperature T steam, which is not linearly increased with C opt or P in (see Figure S6). For the extreme case, the temperature of steam will reach a steady state for the open system, in which case ultra-violent steam generation (or called as boiling phenomena) can be observed. Therefore, solar evaporation under 4 sun illuminations would be much intensive than that of 1 sun illumination. is NATURE PHOTONICS 7

8 Figure S6 Measured temperature of steam as a function of illumination intensity. S. V Durability of solar desalination S. V. 1 Salinity residual after cycles We have taken the SEM images of our plasmonic structure before and after durability test, as shown in Figure S7. We can find that most of the Al NPs remained inside the nanopores, as expected. It is confirmed that the durability would not cause notable change to the penetration depth as well as the distribution profile of Al NPs. In addition, there are no obvious salt crystals observed in these pores. 8 NATURE PHOTONICS

9 SUPPLEMENTARY INFORMATION Figure S7 SEM images of our plasmonic structure before (a) and after (b) durability test. To further carefully examine the salt residue within these pores, both EDS and ICP-OES are performed, as shown in Figure S8 and S9. Figure S8 reveals that Na and Cl (potential elements from sea water) are close to the detection limit (0.1 wt%) of energy dispersive spectroscopy (EDS) of SEM. Figure S8 EDS of different possible elements inside our structure after long time solar desalination. In addition, a sample after stability test is put into a 10 ml deionized water for ultrasonic cleaning treatments twice (30 min for each time). The washed water after the first and second treatments were gathered and then carefully measured by the inductively coupled plasma spectroscopy (ICP - OES), as shown in Figure S9. It is confirmed that there is the existence of salt residue (confirmed by the increase of salinity after first wash), and these residues could be effectively removed by washing (confirmed by the decrease of salinity after the second wash). NATURE PHOTONICS 9

10 Figure S9 ICP measured salinity of washed water. The salinity of deionized water is used for reference. S. V. 2 Long time durability test for solar desalination We have performed the desalination experiment for 7 cycles with each cycle lasted for 4 hours, as shown in Figure S10. The durability test reveals that our material is quite stable for a long period of time. Figure S10 Stability test under 1 sun illumination with each cycle lasts for 4 hours. S. VI Comparison between plasmon enhanced solar desalination with conventional desalination strategies In our opinion, perhaps the most interesting feature of our approach is that it potentially provides an efficient and effective portable or personalized solution for 10 NATURE PHOTONICS

11 SUPPLEMENTARY INFORMATION solar desalination, which is very different from most (if not all) of current centralized desalination technologies such as reverse osmosis (RO) and distillation. In addition, the extensive comparison with the two most popular desalination technologies, reverse osmosis (RO) and distillation, in terms of energy consumption, efficiency and cost, is provided as following. M-RO is famous for the relatively low investment cost and widely employed currently. However, there are several inherent limitations. 1) M-RO should consume high grade electrical energy (44% of its cost) 5. 2) The membranes require frequent replacement due to the severe corrosion of saline water 6. 3) The requirements for pretreatment and operations are high. 4) In addition, the total dissolved solids (TDS) after M-RO treatment is still high (on the order of 100 ppm 7 ). Based on the evaporation processes, conventional distillation technology such as multi-stage flash or multiple effect distillation are advantageous on water quality (TDS < 10 ppm) and the operation process is simple. However, they still consume thermal energy (~50% of the cost) and many stages are needed to maintain the efficiency. Therefore, the conventional distillation-based desalinations are confined to applications for large scale water supply with rather high investment costs. Solar desalination can be much less expensive for high quality fresh water for the solar energy consumption as well as distill based process. However, the current solar desalination designs are suffering from the low efficiency and production rate. Two most important issues are the inefficient solar energy harvesting and large amount of thermal loss to the bulk saline water. To the best of our knowledge, the commercial solar desalination efficiency for the simple basin passive setup is in the range of 30 45% 8. Our plasmoic enhanced solar desalination approach combines the advantages of low cost, high efficiency and minimized carbon footprint. It can greatly increase the NATURE PHOTONICS 11

12 output efficiency and productivity as compared to the conventional solar distills. 1) Our Al NPs/AAM structure is broadband and efficient for solar absorption (AM 1.5 weighted absorbance ~ 96.5% for nm), the effective thickness of aluminum is ~ 85 nm, much less than absorbing materials for traditional solar desalinations. 2) Our Al NPs/AAM structure is highly porous and can float on the water surface. Our structure can locally heat the saline water in the proximity of water interface and the energy transfer of evaporation can be much more efficient than the conventional bulk heating geometries (traditional absorbing materials sink at the bottom of the saline water). 3) The Al NPs AAM structure can support the Al-based plasmonic near field effect, which can greatly enhance the solar energy density around the Al NPs and finally contribute to the energy transfer efficiency. Based on the above advantages, our Al NPs plasmonic structure based solar desalination reaches average energy transfer efficiency ~ 88% at only 4 sun illumination, overpassing most if not all of the reported passive solar distill desalination so far. S. VII References Lee W et al. (2008) Structural engineering of nanoporous anodic aluminium oxide by pulse anodization of aluminium. Nature Nanotech 3: Fan ZY et al. (2009) Three-dimensional nanopillar-array photovoltaics on low-cost and flexible substrates. Nature Mater 8: Martin J, Martin-Gonzalez, FJF, Caballero-Calero, O (2014) Ordered three-dimensional interconnected nanoarchitectures in anodic porous alumina. Nature Commun 5: Banat F, Jwaied N (2008) Economic evaluation of desalination by small-scale autonomous solar-powered membrane distillation units. Desalination 220: National Research Council, Review of the desalination and water purification technology roadmap, 2004, National Academies Press, Washington, DC. 6. Shannon, M. A. et al. Science and technology for water purification in the coming decades. Nature 452, (2008). 12 NATURE PHOTONICS

13 SUPPLEMENTARY INFORMATION 7. Busch, M. and Mickols, W. E. Reducing energy consumption in seawater desalination. Desalination 165, 299 (2004). 8. P. Durkaieswaran and K. Kalidasa Murugavel, Renew. and Sust. Energy Rev. 49, (2015) NATURE PHOTONICS 13