Changxia Li, Sijia Cao, Jana Lutzki, Jin Yang, Thomas Konegger,
Freddy Kleitz, Arne Thomas
A Covalent Organic Framework/Graphene
Dual-Region Hydrogel for Enhanced Solar-Driven
Water Generation
Open Access via institutional repository of Technische Universität Berlin
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Citation details
Li, C., Cao, S., Lutzki, J., Yang, J., Konegger, T., Kleitz, F., & Thomas, A. (2022). A Covalent Organic
Framework/Graphene Dual-Region Hydrogel for Enhanced Solar-Driven Water Generation. In Journal of the
American Chemical Society (Vol. 144, Issue 7, pp. 3083–3090). American Chemical Society (ACS).
https://doi.org/10.1021/jacs.1c11689.
This document is the Accepted Manuscript version of a Published Work that appeared in final form in the
Journal of the American Chemical Society, copyright © American Chemical Society after peer review and
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1A Covalent Organic Framework/Graphene Dual-Region Hydrogel for
2Enhanced Solar-Driven Water Generation
3Changxia Li,
#
Sijia Cao,
#
Jana Lutzki, Jin Yang, Thomas Konegger, Freddy Kleitz, and Arne Thomas*
4ABSTRACT: Solar-driven water generation is a sustainable water
5treatment technology, helping to relieve global water scarcity issues.
6However, this technology faces great challenges due to the high energy
7consumption of water evaporation yielding low evaporation rates. Here, a
8covalent organic framework (COF)/graphene dual-region hydrogel,
9including hydrophilic regions and hydrophobic regions in one material,
10 is developed through a facile in situ growth strategy. The hydrophilic COF
11 is covering parts of the hydrophobic graphene regions. Through accurate
12 control of both wetting regions, the hybrid hydrogel shows effective light-
13 harvesting, tunable wettability, optimized water content, and lowered
14 energy demand for water vaporization. Acting as solar absorber, the dual-
15 region hydrogel exhibits a steam generation rate as high as 3.69 kg m−2h−1
16 under 1 sun irradiation (1 kW m−2), which competes well with other state-
17 of-the-art materials. Furthermore, this hydrogel evaporator can be used to produce drinkable water from seawater and sewage,
18 demonstrating the potential for water treatment.
19 ■INTRODUCTION
20 The shortage of freshwater is a growing challenge for social
21 development.
1,2
It is reported that 29% of the global
22 population lacks access to safe drinking water and the demand
23 for clean water is still increasing due to the population growth
24 and environmental issues.
3
Especially in the context of the
25 current outbreak of coronavirus disease (COVID-19)
26 pandemic, clean water concerning sanitation is particularly
27 important.
4
Sunlight-driven water evaporation is regarded as
28 one of the most promising technologies to produce clean water
29 from seawater and wastewater using solar light as the only
30 power input.
5−8
This technology strongly depends on the solar
31 absorbers, which act as evaporators to convert solar energy to
32 heat for vapor generation. To achieve a high water evaporation
33 rate and solar-thermal conversion efficiency, an ideal material
34 should possess a broad absorbance for effectively using most
35 parts of the solar spectrum, low thermal conductivity for
36 suppressing the heat loss, and three-dimensional (3D) porous
37 channels for effective water transportation.
9−12
To date,
38 various photothermal materials have been developed, such as
39 carbon-based materials (e.g., graphene or carbon nano-
40 tubes),
5,6,9−13
polymers,
7,14−16
metal and metal oxides,
8,17
41 MXenes,
18,19
and hybrid materials therefrom.
20−23
Among
42 these, carbon-based aerogels or foams acting as evaporators
43 have been explored extensively due to their excellent
44 stability,
24
high surface area,
25−27
good light absorbance
45 ability, and tunable water transportation pathways.
5,6,12,13,28
46 However, the evaporation rates of most of the reported
47 carbon-based evaporators are less than 2 kg m−2h−1under 1
48
sun (1 kW m−2), which is much lower than that of hydrophilic
49
polymer hydrogels (2.5−4.0 kg m−2h−1),
7,14−16
while the
50
polymer hydrogels usually have reduced water vaporization
51
enthalpy for facilitating vapor generation. Therefore, to reduce
52
the energy requirement of water vaporization in carbon
53
materials would be an effective approach to break the current
54upper limit of vapor generation rate.
55
Covalent organic frameworks (COFs) are emerging porous
56
crystalline materials with low density,
29−34
high surface
57
area,
35−38
tunable bandgaps,
39,40
and low thermal conductiv-
58
ity.
41
More importantly, the monomers of COFs are designable
59
at the molecular level. Therefore, COFs should be perfectly
60
suitable as building blocks for fabricating photothermal
61
materials. Nevertheless, there are currently just very few
62
reports about COFs-based solar-thermal evaporators. Most
63
recently, a COF/polyvinyl alcohol hybrid hydrogel with a
64
water evaporation rate of 2.5 kg m−2h−1was reported.
42
65
However, this COF was synthesized using the conventional
66
solvothermal method, which requires suitable organic solvents
67
and elevated temperatures during the reaction process. In a
68recent effort, we have shown that COF can grow in situ on
1
69 graphene employing a green hydrothermal method and the
70 composites displayed hierarchical porosity with ultralow
71 density.
34
72 In this contribution, through the rational design and
73 accurate control of sulfonic acid-functionalized COF (COF-
74 SO3H) onto reduced graphene oxide (rGO) using a hydro-
75 thermal approach, we have successfully fabricated a COF/
76 graphene hydrogel (CGH)-based solar vapor generator, which
77 is composed of both hydrophilic COF-loaded rGO (COF@
f1 78 rGO) regions and hydrophobic pure rGO regions (Figure 1).
79 Through systematic regulation of the ratio of these two
80 regions, an enhanced light absorbance, suitable water content,
81 more weakly bonded water within the hydrogel, and a lowered
82 evaporation enthalpy were achieved. Benefiting from these
83 merits, a high solar-vapor conversion performance has been
84 obtained with a vapor generation rate up to 3.69 kg m−2h−1
85 and 92% solar-to-vapor efficiency under 1 sun irradiation.
86 Building upon this high performance, the CGH also
87 demonstrates the ability of effective solar-driven seawater
88 desalination, contaminated water purification, and heavy metal
89 ions removal.
90 ■RESULTS AND DISCUSSION
91 COF-SO3H was synthesized applying a hydrothermal method
92 using 1,3,5-triformylphloroglucinol (Tp) and 2,5-
93 diaminobenzenesulfonic acid (DASA) as monomers, p-
94 toluenesulfonic acid (PTSA) as catalyst, and water as the
95 only solvent. After shaking the mixture for 20 min using a
96 vortex shaker and heating for 2 days at 120 °C in an oven, a
f2 97 dark-red COF powder was obtained (Figure 2a and Figure S1).
98 Scanning electron microscopy (SEM) measurements show that
99 this COF has rod-like morphology with a diameter of close to
100 1μm and a length of several micrometers (Figure S2). The
101 successful synthesis of COF-SO3H was proven by powder X-
102 ray diffraction (XRD) and Fourier transform infrared (FT-IR)
103 spectroscopy (Figure S3). Within the XRD pattern, the peaks
104 at 2θ= 4.6°, 8.1°, and 26.7°correspond to the (100), (110),
105 and (001) planes, respectively, which are in good agreement
106 with the simulated AA stacking model. The FT-IR spectrum
107 shows strong characteristic peaks at 1655 cm−1(CO), 1568
108 cm−1(CC), and 1220 cm−1(C−N), which can be attributed
109
to the formation of the β-ketoenamine linked framework
110
structure. In addition, the distinctive absorption bands at 1429
111
cm−1, 1077 cm−1, 1024 cm−1, and 984 cm−1confirm the
112
presence of −SO3H groups (Figure S3b). The permanent
113
porosity and surface area of COF-SO3H were explored by N2
114adsorption measurement at 77 K (Figure S3c). The Brunauer−
115
Emmett−Teller (BET) surface area and pore size of COF-
116
SO3H are calculated to be 242 m2g−1and 1.4 nm, respectively.
117
The value of BET surface area is a little higher than that of
118
COF-SO3H prepared via a solvothermal method (158.6−215
119
m2g−1),
43,44
showing that hydrothermal synthesis is also an
120
effective method for COF-SO3H preparation. Furthermore, the
121
water contact angle of ∼10°indicates the superhydrophilic
122
property of the COF due to the large amount of −SO3H
123groups (Figure S3d).
124
The presence of a 3D network of water transport channels is
125
a fundamental condition for water evaporation, which the COF
126
powder itself cannot form. In order to construct 3D capillary
127
channels and adjust the wettability simultaneously, COF
128monomers were added to graphene oxide (GO) dispersion
129to form oligomers on the GO surface through shaking, then
130
another portion of a pure GO dispersion was added and the
131
combined dispersions were well mixed through continuous
132
stirring (Figure S4). During the following hydrothermal
133
process at 120 °C, the oligomers further grow to build the
134
COF on the GO surface while the GO was reduced (see details
135
in Figure S5). After cooling down to room temperature, black
136
hydrogels were obtained with both hydrophilic COF@rGO
137
regions and hydrophobic pure graphene regions. According to
138the volume percentage of the COF-monomers/GO dispersion
139
to the entire GO dispersion, the CGH samples are denoted as
140
CGH-0 (i.e., pure rGO), CGH-25, CGH-50, CGH-75, and
141
CGH-100, respectively. Overall, the content of rGO remains
142
the same in all the CGHs, while the COF coverage should
143
increase from CGH-0 to CGH-100 (see more details in Table
144
S1). With the increase of COF modification, the volume of the
145
hydrogel expands, evidencing that the introduction of COF
146
can inhibit the π−πstacking of graphene nanosheets to a
147
certain extent (Figure S4).
34
Although this expansion leads to a
148slight decrease in mechanical strength (Figure S6) of the dry
149
samples, CGH-50 could still maintain its structure stability
150
during the long-term measurement (see detailed discussion
151
below). Moreover, the hydrogel can be prepared in different
152scales by just using larger autoclaves (Figure 2b).
153SEM was measured after freeze-drying the as-prepared
154
hydrogels. The CGH samples possess macroporous capillary
155
channels for internal water transport, in which the average pore
156
sizes are 2.3, 3.8, 8.7, 10.3, and 12.3 μm from CGH-0 to CGH-
157
100, respectively (Figure 2c and Figures S7−S9). Furthermore,
158
mercury porosimetry was performed. The results show the
159
same trend regarding the pore size but with a widening of the
160
apparent pore size distribution and a larger average pore
161
diameter (Figure S10), which is attributed to a partial material
162deformation and disintegration during application of intrusion
163pressure. As the amount of COF precursors is the only
164
parameter that is varied in this protocol, the change of pore
165
size can be ascribed to the amount of COF present in the
166
CGHs. The representative transmission electron microscopy
167
(TEM) and high-magnification SEM images exhibit a rough
168
surface with a lot of wrinkles (Figure 2d and Figure S11). It
169
should be noted that no isolated COF rods or particles are
170
observed in all CGH samples, evidencing that the 2D COF
171grows in situ along the surface of 2D rGO nanosheets. Taking
Figure 1. Scheme of the COF/graphene dual-region hydrogel for
accelerating solar-driven water evaporation. The 3D porous channels
are capable of transporting water upward rapidly. The dual-region
functions synergistically to regulate channel size from ∼2.3 μmto
∼12.3 μm, enhance solar absorption, optimize water content, and
lower energy requirement for evaporation.
2
172 CGH-100 as an example, the corresponding energy-dispersive
173 spectroscopy (EDS) mapping images show the distribution of
174 C, O, N, and S, indicating that the individual graphene sheets
175 are fully covered by COF-SO3H in the absence of extra pure
176 rGO (Figure S12). For CGH-50, rGO and COF@rGO are
177 mixed uniformly inside the whole network from the low-
178 magnification SEM-mapping (Figure 2e), and the resolution of
179 the EDS mapping is not enough to distinguish their respective
180 regions. FT-IR measurements of CGHs show similar character-
181 istic peaks as for pure COF-SO3H, providing direct proof for
182 the effective growth of COF on rGO (Figure 2f). However, the
183 XRD patterns show just the low intensity (110) plane peak of
184 COF probably as the COF forms rather thin layers on the rGO
185 sheets (Figure S13a). To further confirm that in situ growth of
186 the COF has happened and not just a physical rGO-COF
187 mixture is formed, we mixed the COF-SO3H and rGO in the
188 same proportion as for CGH-50. Notably, the XRD patterns
189 exhibit a distinct difference for the (002) interplane peak
190 (Figure S13b). For CGH-50, the maximum of the pronounced
191 peak at 25.6°(2θ) is between the one for pure rGO and pure
192 COF, while a broader peak that covers both (002) planes is
193 observed in the physical mixture sample, which provides strong
194 evidence that graphene layers are overlaid with the COF in
195 CGHs.
196 The X-ray photoelectron spectroscopy (XPS) spectra show a
197 complete view of the surface chemical composition of CGHs
198 (Figure 2g, Figure S14, and Figure S15). The binding energy at
199 ∼400 eV and ∼167 eV can be assigned to N 1s and S 2p,
200
respectively. Furthermore, an increase in the intensity of N and
201
S manifests an increasing coverage of COF on the surface of
202
rGO. The growth of COF on graphene was further validated
203by high-resolution XPS spectra. COF-SO3HandCGHs
204
samples have similar high-resolution N 1s core-level spectra
205
(Figures S14b and S15a), which can be divided into three
206
peaks, i.e., free secondary amine (−NH−, 399.9 eV),
207
protonated secondary amine (−NH2+−, 401.9 eV), and
208
charging effects (404.4 eV).
44,45
The amine signals come
209
from the condensation reaction between −CHO and −NH2
210groups of respective monomers (Tp and DASA). Similarly, the
211
S 2p binding energy of COF-SO3H and CGHs shows two
212
types of sulfurs: one at 167.4 eV for the sulfonate ions
213
(−SO3
−) and the other at 168.5 eV for the sulfonic acid groups
214
(−SO3H).
44
Apparently, the sulfonic acid group can protonate
215
a part of the amine and become a sulfonate ion (Figure S14d).
216It is reported that the sulfonate groups are able to generate
217activated water for fast steam generation.
15
The atomic
218
percentages of elements from XPS analysis are shown in
219
Table S2. In addition, the amount of COF loading in CGHs is
220
calculated from elemental analysis (Table S3). On the basis of
221
the amount of sulfur, the COF loadings from CGH-0 to CGH-
222100 are 0, 3.42%, 22.05%, 34.11%, and 42.05%, respectively.
223
To gain further insight into the coverage of COF, atomic
224
force microscopy (AFM) measurements of CGH-0, CGH-50,
225
and CGH-100 were performed. For CGH-100, the COF-
226
loaded graphene has an average thickness of 2.5−3.5 nm
227(Figure S17), showing an increased thickness compared to that
Figure 2. (a) Chemical structure of COF-SO3H. (b) Photograph of CGH-50 synthesized using 20 mL (left) and 120 mL (right) autoclaves,
respectively. (c, d) SEM images of CGH-50. (e) SEM image and the corresponding EDS mapping of CGH-50. (f) FT-IR spectra of COF-SO3H
and CGHs. (g) XPS survey spectra of CGHs. (h, i) AFM image and the corresponding height profiles of CGH-50.
3
228 of rGO (1.2−1.7 nm) (Figure S18). This increase in thickness
229 results from the uniform growth of a few layers of COF on
230 rGO surface. For CGH-50, we detected the coexistence of pure
231 rGO and COF@rGO from AFM (Figure 2h,i and Figure S19).
232 This strong proof points toward the formation of dual-region
233 hydrogel. Notably, the growth of COF on the surface of
234 graphene increases the roughness of graphene (Figure S19),
235 which will be beneficial for decreasing the light reflection and
236 further enhancing light absorption. Accordingly, all the above
237 results suggest the successful formation of COF-decorated
238 dual-region hydrogels.
239 The water contact angles from CGH-0 to CGH-100 were
240 measured to be 130.6 ±4.6°, 112.2 ±2.1°, 88.3 ±0.3°, 82.1 ±
f3 241 0.7°, and 69.7 ±1.7°, respectively (Figure 3a−e). The pure
242 COF is superhydrophilic with a contact angle of 10°(Figure
243 S3d). With increasing the coverage of rGO with COF-SO3H,
244 an increasing hydrophilicity is observed, which confirms that
245 COF-SO3H can effectively tune the surface wetting states of
246 graphene. The absorbance characterization was performed
247 using ultraviolet−visible near-infrared (UV−vis−NIR) spectra
248 of ∼1 mm thin slices of the samples to compare the
249 absorbance of CGHs (Figure 3f). The introduction of COF
250 demonstrates an apparent improvement of broadband light
251 absorbance compared with CGH-0. Especially, CGH-50 and
252 CGH-75 show ∼2-fold enhancement over pure rGO. The
253 CGH-50 exhibits ignorable transmittance and low reflectance
254 of average ∼6.1% in the wavelength range from 200 to 2000
255 nm (Figure S20). Thus, the calculated absorptivity of CGH-50
256 is ∼93.9%. A control experiment using a −SO3H group
257 modified rGO (rGO-SO3H) was carried out by just applying
258 the sulfuric acid functionalized monomer (DASA) but not the
259 second, cross-linking monomer (Tp) in the hydrogel synthesis
260 to illustrate the unique function of COF layers. The resulting
261 rGO-SO3H showed an increased hydrophilicity (water contact
262angle = 85.8 ±0.9°) compared to pure rGO, but the light
263absorption of rGO-SO3H was almost the same as rGO (Figure
264S21). The enhanced absorption of CGHs indicates that the
265introduction of COF-SO3H can harvest the incident solar light
266efficiently.
267In COF/rGO hydrogels, the hydrophilic region provides
268plenty of sulfonic acid groups that have a significant influence
269on the water content and water state, both of which will affect
270the water evaporation behavior. Too low water content causes
271a shortage of water supply, while too high water content would
272block the pore channels and hinder water from moving
273freely.
46
The water content (Q) is calculated by the equation
=
Q
MM/
wd 274
(1)
275where Mwand Mdare the weights of water in the hydrogel and
276of the corresponding dried aerogel, respectively. The Qvalues
277in the hydrogels rise with increasing hydrophilic region from
278CGH-0 to CGH-100, which are 61.8, 91.8, 107.9, 137.7, and
279147.6 g g−1, respectively (Figure 3g). It is worth noting that
280rGO-SO3H has a higher sulfur content of 1.30 atom % (i.e.,
281more sulfonic acid groups on the surface) than CGH-50 (0.90
282at%), but the Qin rGO-SO3H (68.1 g g−1) is not much above
283the one for CGH-0. The rise of Qin CGHs thus can be
284attributed to three aspects. First, the introduction of the COF
285enlarges the pores of the hydrogel network, enabling the
286accommodation of more water, while merely introducing
287sulfonic acid groups on the surface of graphene is unable to
288adjust the size of the pores (Figure S22). Second, the
289hydrophilic −SO3H/−SO3
−groups in the COF form strong
290electrostatic interaction and hydrogen bonds with water
291molecules so that more water can bind inside the network.
292Third, the COF itself, as a porous material, can also adsorb
293plenty of water molecules. Furthermore, the water transport in
294the hydrogels is evaluated by the dynamic analysis of water
Figure 3. (a−e) Water contact angles of the freeze-dried samples and the corresponding schematic drawing of the dual-region in the CGHs where
the blue area represents pure rGO, while the orange area represents COF@rGO. (f) UV−vis−NIR absorbance of CGHs. (g) Water content and
the ratio of intermediate water (IW) and free water (FW) in CGHs. (h) Dark evaporation rate and evaporation enthalpy of pure water and water in
CGHs. The error bars in (g) and (h) represent standard deviation obtained from five measurements using different samples.
4
295 absorption of the hydrogel network (see Figure S23 for more
296 details). The CGHs show high water transport rates, which are
297 24.7, 36.7, 43.2, 55.1, and 59.0 g min−1from CGH-0 to CGH-
298 100, respectively. The fast water transport could ensure
299 continuous water replenishment and stable vapor generation
300 during the evaporation process. To demonstrate the water state
301 in the CGHs, Raman spectra were carried out to analyze the
302 intermediate water (IW) and free water (FW) content (see
303 Figure S24 for detailed information). The evaporation of IW
304 could be much faster than that of FW due to the weaker
305 interaction of IW with surrounding water molecules.
7,15
As
306 shown in Figure 3g, the IW/FW ratio first increases from
307 CGH-0 to CGH-50, indicating that hydrophilic −SO3H/−
308 SO3
−groups raise the IW amount. However, with further
309 increasing of the COF coverage, the content of IW reaches its
310 threshold, while the free water content continues going up;
311 thus the ratio of IW/FW decreases from CGH-50 to CGH-
312 100. This phenomenon suggests that there is an optimum ratio
313 of surface functionality and pore size of the large pores in the
314 hydrogels, both of which would influence the water content
315 and state. While a larger COF fraction increases the
316 hydrophilicity of the pore walls, which should yield more
317 IW, it also increases the size of the macropores, thus higher
318 water content, which yields more FW. Therefore, the CGH-50
319 achieves the highest IW/FW ratio.
320 The dark evaporation process at ambient condition was
321 tracked to study the different water vaporization behavior of
322 bulk water and CGHs. As demonstrated in Figure 3h, the dark
323 evaporation rates of the CGHs are much higher than that of
324 pure water, suggesting that the CGHs are capable of activating
325 water and improving the vaporization of water effectively. The
326 tendency under dark conditions agrees with that of the IW/
327
FW ratio, indicating that the moderate ratio of hydrophobic
328
region and hydrophilic region is a primary factor for water
329
activation. It is worth mentioning that the CGH-50 presents an
330
evaporation rate of 0.50 kg m−2h−1under dark conditions,
331
which is higher than that of reported materials without special
332
surface processing (Figure S25).
5,14,20−22,47,48
We postulate
333
that the fast evaporation behavior arises from the CGH-50
334
having a lower evaporation enthalpy (i.e., energy consumption
335
of water vaporization). To prove this origin, the evaporation
336
enthalpy under dark conditions was measured by spontaneous
337
evaporation and differential scanning calorimetry (DSC),
338
respectively (see detailed discussion in Figure S26 and Table
339
S4). As shown in Figure 3h and Table S4, the water confined
340
in CGHs exhibits lower evaporation enthalpy (1043 J g−1for
341
CGH-50) than bulk water (2450 J g−1) and the trend is similar
342
to the ratio of IW/FW, suggesting that water evaporation can
343
be tuned in these hydrogels. Such a low evaporation enthalpy
344
for CGH-50 is ascribed to the adjusted ratio of hydrophilic/
345
hydrophobic regions, which seems to be optimal within CGH-
346
50 to create the highest amount of weakly bonded
347
intermediate water. Overall, all of the above results show
348
that a dual hydrophilic/hydrophobic region hydrogel with
349
interpenetrating pore channels could be a good candidate as
350
solar evaporator with its advantages in 3D transport channels,
351
tunable hydrophilicity, enhanced solar absorbance, optimized
352water content and state, and lowered evaporation enthalpy.
353
Solar vapor generation performance over time was examined
354
by placing the CGH (∼2cm
2area, 0.2 cm thickness) on a
355
polystyrene (PS) foam as a heat insulating layer to isolate bulk
356 f4
water for reducing heat loss (Figure 4a). Upon exposure to 1
357
sun irradiation, the temperature of CGH-50 ascends sharply
358and reaches a plateau of around 32 °C in an open environment
Figure 4. (a) Schematic of the setup used in the water evaporation test. (b, c) Time-dependent temperature and mass change of pure water and
CGHs under 1 sun irradiation (1 kW m−2). (d) Water evaporation rate and energy efficiency under 1 sun. The error bars represent standard
deviation obtained from five measurements using different samples. (e) Comparison of evaporation rate showing the high performance of CGH-50
among carbon-based evaporators. (f) Stability test upon simulated seawater of CGH-50 over seven cycles.
5
359 while the pure water is around 27 °C(Figure 4b). The mass
360 reduction in water with and without CGHs is recorded after
361 the temperature reaches a steady state. It is obvious that the
362 vapor generation rate with CGHs is much faster than pure
363 water under identical illumination conditions (Figure 4c). The
364 water evaporation rates are calculated from the slope of mass
365 change curves. Under 1 sun illumination, CGH-50 achieves the
366 highest rate up to 3.69 kg m−2h−1among all CGHs samples
367 (Figure 4d). This value surpasses the current reported
368 graphene/carbon-based systems (Figure
369 4e)
5,6,9,10,12−14,20−23,49
and other hybrid materials, even
370 comparable to that of polymer-based photothermal evapo-
371 rators (Table S5). The vapor generation rate increases first and
372 then decreases, which follows the trend observed for solar
373 absorption (Figure 3f) and IW/FW ratio (Figure 3g) and the
374 opposite trend of the evaporation enthalpy (Figure 3h). In
375 addition, the excessive water content in CGH-75 and CGH-
376 100 will decrease the energy utilization efficiency because more
377 energy is used for heating the water.
14
These results reveal that
378 the excellent performance of CGH-50 can be attributed to its
379 high IW/FW ratio and light absorption, low evaporation
380 enthalpy, and optimized water content. In comparison, rGO-
381 SO3H delivers a water yield of just 2.72 kg m−2h−1(Figure
382 S27), illustrating the important role the COF plays in the
383 evaporator. It should be noted that even under 0.5 sun
384 illumination, the steam generation rate of CGH-50 can reach
385 1.46 kg m−2h−1, which is important for practical application
386 considering inadequate light illumination in some cases. The
387 solar energy conversion efficiency (η) is calculated by the
388 equation
η
=mE P/
equ 0
389 (2)
390 where mrepresents the net evaporation rate calculated by
391 subtracting evaporation rate under dark condition from
392 evaporation rate under 1 sun irradiation, Eequ refers to the
393 equivalent evaporation enthalpy of water in CGHs. P0is the
394 solar irradiation power of one sun (1 kW m−2). The energy
395 efficiency of CGH-50 is calculated to be ∼92% (see Energy
396 Conversion Efficiency and Energy Loss Analysis in Supporting
397 Information for details), suggesting its potential as an ideal
398 photothermal material. Furthermore, the CGH-50 sample was
399 used to measure the steam generation performance upon
400 simulated seawater under 1 sun illumination. The CGH-50
401 reaches an evaporation rate of 3.45 kg m−2h−1and maintains
402 the high seawater evaporation performance for seven cycles
403 (Figure 4f). During this process, salt precipitates are formed on
404 the surface of evaporator, which can be easily redissolved in
405 seawater (Figure S28). Moreover, the CGH-50 can retain its
406 structural stability and hydrophilicity during the evaporation
407 process, as shown from SEM images, FT-IR, AFM, and contact
408 angle measurements (Figures S29 and S30). The outstanding
409 performance of CGH can be attributed to the rational design
410 of the dual-region hydrogel with its advantages in appropriate
411 water content for rapid water supply, and high IW/FW ratio
412 for reduced water vaporization energy consumption.
413 Given the outstanding steam generation performance, the
414 practical applications of CGH-50 for solar-light driven
415 desalination and water treatment were further explored. As a
416 proof-of-concept, a custom-designed water collection setup
417 was employed to collect the condensed water from seawater
f5 418 and contaminated water (Figure 5a). We first used the
419 simulated seawater as water source, and the quality of treated
420 water was estimated by inductively coupled plasma mass
421spectrometry (ICP-MS). After desalination by our solar water
422evaporation system, the ion concentrations (Na+,K
+,Ca
2+, and
423Mg2+) were significantly decreased by 4 orders of magnitude
424(Figure 5b). In addition to seawater, the CGH-based solar
425evaporation system can be utilized to purify wastewater
426containing heavy metal. After purification, the concentration
427of heavy metal ions, such as Cr3+,Cu
2+,Ni
2+,Zn
2+,Ga
3+, drops
428to ∼0.1 ppm (Figure 5c). The desalination efficiency and
429removal efficiency for heavy metal ions of CGH-50 are
430estimated to be above 99.95% and 99.98%, respectively,
431meeting the standard of drinking water from the World Health
432Organization.
50
The CGH-50 can also be ideal for purifying
433industrial dye wastewater. By use of methyl orange as a
434representative contaminant, the sewage is transformed into
435clear and colorless water after purification using our solar
436evaporation system (Figure 5d). According to UV−vis
437absorption characterization, the purification efficiency in the
438collected water can reach 100%, which further demonstrates
439that the versatile CGH-based photothermal generator
440possesses promising potential for wastewater treatment.
441
■CONCLUSIONS
442In summary, a dual-region CGH with hydrophilic COF-loaded
443rGO regions and hydrophobic pure rGO regions has been
444developed. Both regions function synergistically to regulate
445channel size and wettability, which further adjust the water
446content and state, as well as the energy consumption of steam
447generation. By use of the right ratio of hydrophilic and
448hydrophobic regions, a CGH can be produced from which
449water is evaporated with a high rate of 3.69 kg m−2h−1with
45092% energy efficiency under 1 sun irradiation. More
451importantly, this hydrogel enables still a high rate of
452evaporation even under weak sunlight. The ability to use the
453CGH for desalination and wastewater treatment further shows
454the promising potential of this material in environmental
455applications. This work demonstrates a novel and effective
456approach not only for the formation of dual-region hybrid
457materials but also for the regulation of the water state in
458hydrogels. Such an approach to create highly porous materials
Figure 5. (a) Photograph of the setup for solar-driven clear water
collection. (b) Ion concentration of simulated seawater before and
after desalination. (c) Heavy ion concentration before and after
purification. (d) UV−vis spectra and photograph of dye contaminated
water and purified water.
6
459 with tunable hydrophilic and hydrophobic regions should also
460 be interesting for other applications, for example, in
461 membranes and other separation and purification devices.
468 ■AUTHOR INFORMATION
469 Corresponding Author
470 Arne Thomas −Department of Chemistry, Division of
471 Functional Materials, Technische Universität Berlin, 10623
472 Berlin, Germany; orcid.org/0000-0002-2130-4930;
473 Email: [email protected]
474 Authors
475 Changxia Li −Department of Chemistry, Division of
476 Functional Materials, Technische Universität Berlin, 10623
477 Berlin, Germany; Department of Inorganic Chemistry,
478 Functional Materials, Faculty of Chemistry, University of
479 Vienna, 1090 Vienna, Austria
480 Sijia Cao −Department of Chemistry, Division of Functional
481 Materials, Technische Universität Berlin, 10623 Berlin,
482 Germany; Department of Chemistry, Humboldt Universität
483 zu Berlin, 12489 Berlin, Germany
484 Jana Lutzki −Stranski-Laboratorium fur Physikalische und
485 Theoretische Chemie, Institut fur Chemie, Technische
486 Universität Berlin, 10623 Berlin, Germany
487 Jin Yang −Department of Chemistry, Division of Functional
488 Materials, Technische Universität Berlin, 10623 Berlin,
489 Germany
490 Thomas Konegger −Institute of Chemical Technologies and
491 Analytics, Technische Universität Wien, 1060 Vienna, Austria
492 Freddy Kleitz −Department of Inorganic Chemistry,
493 Functional Materials, Faculty of Chemistry, University of
494 Vienna, 1090 Vienna, Austria; orcid.org/0000-0001-
495 6769-4180
498 Author Contributions
#
499 C.L. and S.C. contributed equally.
500 Notes
501 The authors declare no competing financial interest.
502 ■ACKNOWLEDGMENTS
503 This work was financially supported by the China Scholarship
504 Council (CSC) and the Deutsche Forschungsgemeinschaft
505 (DFG, German Research Foundation) under Germany’s
506 Excellence Strategy, EXC 2008/1 (UniSysCat), Grant
507 390540038. C.L. and F.K. also acknowledge the funding
508 support of the University of Vienna, Austria. We acknowledge
509 Prof. M. Gradzielski in the Stranski lab/work group Physical
510 Chemistry/Molecular Material Science for the support in the
511 measurements of AFM and contact angle. We thank C.
512 Eichenauer and M. Unterweger for their assistance with N2
513 sorption, TGA, and XRD measurements. We also thank Prof.
514 Dr. R. Schomäcker from Technische Universität Berlin for the
515 assistance in experimental equipment. S.C. acknowledges for
516the support of Dr. M. Schwalbe from Humboldt Universität zu
517Berlin.
518
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