3D Printing of Artificial Leaf with Tunable Hierarchical Porosity for CO2

Jan 24, 2018 - The development of new pathways for 3D artificial photosynthetic systems (APS) with controllable architectures and tunable hierarchical...
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Article Cite This: Chem. Mater. 2018, 30, 799−806

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3D Printing of Artificial Leaf with Tunable Hierarchical Porosity for CO2 Photoreduction Liao Chen,‡ Xingwei Tang,‡ Peiwen Xie,‡ Jun Xu, Zhihan Chen, Zuocheng Cai, Peisheng He, Han Zhou,* Di Zhang, and Tongxiang Fan* State Key Lab of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai 200240, P. R. China S Supporting Information *

ABSTRACT: The development of new pathways for 3D artificial photosynthetic systems (APS) with controllable architectures and tunable hierarchical porosity on a large scale is significant. Herein, we demonstrate a 3D printing approach for fabricating artificial microleaves with 3D architectures spanning orders of magnitude from nanometers to centimeters in a rapid, programmable, and scalable manner. TiO2-based inks served as a preliminary prototype, with surfactants and silica nanospheres incorporated for porosity modification. Thus, a TiO2-based ink is developed to allow for the fabrication of porosity-tunable hierarchical 3D architectures with high surface area (up to ∼259 m2g−1) and structural integrity with well-designed patterns. The artificial microleaves have macropore architectures comparable to those of natural leaves, indicating their efficient mass transfer ability. Artificial photosynthesis via CO2 reduction enhances CO and CH4 evolution on the 3D printed APS by up to 2-fold and 6-fold, respectively, compared with the levels observed for the corresponding powder counterparts. Furthermore, gas diffusion behaviors, closely related to the gas-phase reaction, are investigated by theoretical simulation to reveal the hierarchical structural effects on catalytic efficiency. The strategy is proven to be critical and demonstrates obvious advantages in the potential scale-up of 3D APS device manufacturing.

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fabrication of structures with multiple length scales.12,13 Among the 3D printing techniques used today, direct ink writing is one of the most versatile because of its simple printing principles and low cost. 3D direct writing has been applied for the fabrication of numerous materials such as metals, ceramics, carbon-based materials, polymers, and composites,14−16 covering various structures including microcoils, interdigitated architectures, woodpile structures, and cellular structures.17−19 The applications cover a wide range of areas, including microfluidics, energy storage and conversion, microelectronics, and tissue engineering.20−22 However, to our knowledge, there are few reports on the controlled production of artificial photosynthetic systems via this technique. For CO2 photoreduction, mass transfer is one of the determining factors that affects final performance. Interestingly, leaves’ hierarchical architecture at different scales has been demonstrated as a perfect structural model for efficient mass transfer (e.g., gas diffusion).7 Once CO2 molecules diffuse within the leaf, they suffer minimal intercellular diffusion due to the presence of numerous macropores. Through direct experiments, Pieruschka et al.23 demonstrated that CO2 diffusion inside leaves enhanced photosynthesis. We previously

INTRODUCTION Artificial photosynthesis inspired by nature is an attractive strategy for converting solar energy to sustainable fuels.1,2 CO2 photoreduction is more urgent than water splitting.3,4 Artificial photosynthesis has been the focus of both fundamental studiesto understand the principles of natural photosynthesis by using models and biomimetic systemsand applied studies, toward potential applications.5,6 The design of novel artificial photosynthetic systems (APS) with optimized architectures is important.7,8 Materials with three-dimensional (3D) micro/ nanoarchitectures exhibit many beneficial mechanical and optical properties and superior mass transfer ability and thus are emerging as an important trend in this research area.9,10 In particular, the construction of 3D APS by mimicking natural leaves’ key architectural elements with the integration of functional units into integrated materials/devices is appealing but also challenging. 3D APS have been produced via biotemplating, electrodeposition, reactive-ion etching, and hydrothermal synthesis, among other techniques.7,11 However, many 3D micro/nanoarchitectures are generally limited by structural design and scalability for integrated systems. Therefore, the development of new pathways for the fabrication of 3D APS with controllable architectures and tunable hierarchical porosity on a large scale is of high significance. 3D printing is a versatile and effective additive manufacturing technique that allows for fast, designable and high-accuracy © 2018 American Chemical Society

Received: October 13, 2017 Revised: January 24, 2018 Published: January 24, 2018 799

DOI: 10.1021/acs.chemmater.7b04313 Chem. Mater. 2018, 30, 799−806

Article

Chemistry of Materials

Figure 1. (a) Illustration of a leaf’s cross-section, indicating CO2 diffusion within the leaf’s porous architecture. (b) Cross-section of lotus leaf observed under confocal laser scanning microscope (CLSM). (c) SEM image of the cross-section of a cherry blossom leaf. (d) SEM image of the cross-section of a cherry blossom leaf’s veins after carbonization. (e) SEM images of 3D-printed DBSA-TIA scaffolds. (f) Optical images of a 3Dprinted artificial leaf. (g) (h) SEM and optical images of SiO2-DBSA-TIA scaffolds after calcination and etching with hot KOH. Here, SiO2-DBSATIA refers to 3.2 wt % SiO2/TIA mass ratio and 14.4 wt % DBSA/TIA mass ratio.

sized interconnecting air spaces (Figure 1b, c) that are much larger than the mean free path of gas molecules. Therefore, there is negligible resistance of intercellular diffusion because of molecular diffusion.27 Another unique structural feature is the venation architecture with large pores in the range of tens of micrometers (Figure 1d). Thus, on the basis of the understanding of the important structural elements of natural leaves, the key step to mimicking these structures is to simplify them and reproduce their function. To demonstrate the direct-write assembly of APS devices (Figure S1 of the Supporting Information, SI), we create 3D woodpile architecture spanning orders of magnitude, from nanometers to centimeters (Figure 2). A suitable ink formula is highly important for programmable printing, which should meet two basic requirements: (1) desired rheological and printing behaviors and (2) high porosity and surface areas for the final products. Although 3D printing of TiO2 has been reported elsewhere,28,29 the features of high surface area and tunable porosity have not yet been achieved. Here, our inks were designed by creating a sol−gel precursor solution, titanium diisopropoxide bis(acetylacetonate) (TIA),28 with two acetylacetone (acac) and two isopropoxide radicals surrounding one Ti atom (Figure 3a) with dodecyl benzenesulfonic acid (DBSA) added to increase the surface area (Figure 3b). The contents of DBSA were adjusted to obtain an optimum surface area (64.17 m2g−1) (Figure S2a) with typical mesoporous structures, and the pore volume was optimized to reach ∼0.1 cm3/g. Printable TIA-based ink development is challenging because dilute TIA suspensions do not possess the required rheological behavior for 3D printing, as they are low-viscosity Newtonian fluids. By a gradual solvent evaporation method, in this work, the apparent viscosity of a DBSA-TIA ink with a 14.4 wt % DBSA/TIA mass ratio reached 104−105 Pa s (Figure 3c), making the concentrated DBSA-TIA ink a promising nonNewtonian fluid with strong shear-thinning behavior, which is necessary for 3D printing.30−32 To manufacture 3D architectures on a large scale, the DBSA-TIA ink should have the ability to maintain its cylindrical shape immediately after being

reproduced a leaf’s hierarchical structure via biotemplating and preliminarily demonstrated macro-/mesoporous architectures for promoting gas diffusion efficiency.7 Therefore, the reproduction of a leaf’s key structural elements for fast mass transfer involves constructing 3D hierarchical macro-/mesoporous architectures with high surface areas. Moreover, theoretical simulation is a powerful tool for deeply understanding and investigating gas diffusion behaviors in different porous media. Hua et al.24 used computational fluid dynamicsbased numerical methods to study hydrogen combustion and investigated the microcombustion mechanism. Seyed et al.25 proposed a mathematical model for studying CO2 capture behavior in a membrane container via the CFD numerical method. A transient finite-element model was developed by Kromp et al.26 to investigate the combination of reforming chemistry and gas transport. Nevertheless, for artificial photosynthesis, theoretical simulation of gas diffusion effects on performance enhancements has rarely been explored. Here, we put forward a 3D direct writing strategy for the manufacture of artificial leaves with 3D architectures spanning orders of magnitude, from nanometers to centimeters. TiO2based sol−gel inks serve as a preliminary prototype in this study. Surfactants and silica nanospheres are then added to the inks for porosity modification. Thus, we develop a TiO2-based ink for the production of porosity-tunable 3D hierarchical architectures with high surface area and structural integrity with well-patterned features. Artificial photosynthesis via CO2 reduction in the gas phase is further investigated. Moreover, computational simulation is conducted to reveal the 3D hierarchical structural effects on gas diffusion efficiency and the promotion of artificial photosynthesis activity. Our strategy is proven to be critical and demonstrates advantages in the potential scale-up of 3D APS device manufacturing.

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RESULTS AND DISCUSSION Characterization of 3D-Printed Artificial Leaf. As illustrated in Figure 1a, during photosynthesis, CO2 can diffuse via the air spaces inside a leaf. The main gas exchange surfaces are the spongy mesophyll cells,27 with numerous micrometer800

DOI: 10.1021/acs.chemmater.7b04313 Chem. Mater. 2018, 30, 799−806

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Chemistry of Materials

The DBSA-TIA-based patterns shown in Figure 1e are composed of several woodpile layers, in which each layer is orthogonal to its adjacent layers and filaments are bonded to one another through spanning gaps between underlying filaments. Beyond the example of a woodpile, 3D artificial microleaves could also be printed scalably (Figure 1f). Notably, the obtained artificial microleaf patterns (Figure 1e) are comparable to a natural leaf’s macroporous architecture (Figure 1d), indicating the potential ability for efficient mass transfer. SiO2 nanospheres were mixed into the precursor to obtain higher porosity and larger mesopores after SiO2 removal33 (Figure 2). Two sizes of SiO2 nanospheres (15 and 30 nm) were added in different amounts (Figure S3). The rheological properties of the SiO2-DBSA-TIA ink were similar to those of the DBSA-TIA ink (Figure 3c, d), demonstrating its suitability for printing. After being mixed with 15 nm SiO2 nanospheres (SiO2/TIA = 3.2 wt %, equals 16.2 wt % SiO2 in the calcined sample), the ink showed shear-thinning and solidification behavior. We patterned the SiO2-DBSA-TIA ink using 30 μm nozzles (Figure S4); thus, the solidification slope of the SiO2DBSA-TIA ink was much slower than that of the DBSA-TIA ink because of the former’s higher solid content. The specific surface areas and pore size distributions can be tuned by varying the SiO2/TIA mass ratio and SiO2 particle size (Figure 4a, b). The as-obtained samples showed typical mesoporous structures (Figure S5a). Similarly, excessive SiO2 had a negative effect on ink homogeneity, preventing the ink from flowing smoothly at SiO2/TIA mass ratios greater than 9.6 wt %. On the basis of the optimized ink formula, considering both the surface area and the rheological properties, well-patterned hierarchical macro-/mesoporous APS could be fabricated via

Figure 2. Illustration of 3D direct writing process for macro-/ mesoporous periodic artificial microleaf. SiO2 nanospheres and DBSA were added to the as-prepared TIA sols. After concentration, soluble linear chains formed, and a homogeneous sol−gel ink with proper rheological properties was obtained, which was able to flow through a micronozzle. A hierarchical porous microlattice structure was obtained after solidification, calcination, and finally etching with hot KOH.

Figure 3. (a) Molecular structure of TIA used to formulate DBSA-TIA and SiO2-DBSA-TIA inks, RCH(CH3)2. (b) Photograph of DBSATIA ink in a barrel connected to a borosilicate glass micronozzle (d = 10 μm). Log−log plots of (c) viscosity as a function of shear rate. (d) G′ and G″ as a function of shear stress of DBSA-TIA ink and SiO2DBSA-TIA ink.

extruded through a micronozzle and span gaps over the underlying layers. Elasticity is also an important characteristic for 3D printing. As revealed in Figure 3d, when the shear stress exceeded the yield stress (τy) of ∼500 Pa, the loss modulus (G″) exceeded the storage modulus (G′), allowing the ink to readily flow through the fine nozzle. In the low-shear-stress region, the storage modulus (G′) reached a plateau value of 104 Pa, fulfilling the required elastic response when the shear stress disappeared.

Figure 4. (a) Specific surface areas of samples with different SiO2/TIA mass ratios; the DBSA/TIA mass ratio was constant at 14.4 wt %. (b) Pore size distribution of SiO2-DBSA-TIA inks with different SiO2/TIA mass ratios; the SiO2 particle size was 15 nm. For the 12.8 wt % SiO2/ TIA mass ratio ink, we squeezed the ink out of the needle to maintain the same procedure for obtaining the data. 801

DOI: 10.1021/acs.chemmater.7b04313 Chem. Mater. 2018, 30, 799−806

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Chemistry of Materials Table 1. A List of Parameters of the Products with SiO2-DBSA-TIA Precursors samplesa

SiO2/TIA (wt %)

surface area (m2/g)

SiO2 contents before KOH etching (wt %)b

SiO2 contents after KOH etching (wt %)c

pore volume (cm3/g)

mean pore size (nm)

1 2 3 4

3.2 6.4 9.6 12.8

237.3 251.3 259.3 267.0

16.25 27.95 36.79 43.79

1.35 1.38 1.51 1.73

0.42 0.57 0.69 0.80

5.3 9.1 12.6 14.8

a

DBSA/TIA ratio is 14.4 wt % in all samples. Silica nanosphere size is 15 nm. bData obtained by theoretical calculation based on the original precursors. cData obtained according to the XPS data.

3D printing, calcination, and finally etching (Figure 2). After calcination at 550 °C, the inks were transformed into anatase phase (Figure S6). Complete organic decomposition occurred at 500 °C (Figure S7). Furthermore, KOH etching at 180 °C was performed for the removal of SiO2. Surprisingly, the treated TiO2 scaffold retained its original morphology and integrity (Figure 1 g, h). SEM-EDS data (Figure S8) show that the remaining SiO2 content was only ∼0.3 wt %, far below the original content. XPS data further demonstrate that SiO2 could be mostly removed by this procedure (Table 1, Figure S5b). Artificial Photosynthesis on 3D-Printed Leaf. Artificial photosynthesis was carried out in a gas-phase reaction using H2O and CO2 as the reactants without any sacrificial agents under UV−visible light irradiation. Bare systems evolved CO and CH4 as the products; however, the yields were negligible. Therefore, Au and RuO2 were loaded for activity promotion. Au is a widely used cocatalyst for accelerating CO2 reduction.34 Ruthenium and its complexes have been demonstrated to promote CO2 photo- or electroreduction with high quantum yields.35 Other possible hydrocarbon fuels such as formic acid or methanol failed to be detected, likely because of the strong oxidizing ability of photoexcited holes, which could react with the intermediates and products.36,37 Control experiments (without CO2, catalyst or light) were conducted; almost no CO or CH4 could be detected (Figure 5a, b). Notably, the CO and CH4 evolution rates reached 0.21 and 0.29 μmol−1 g−1 h−1, respectively, for the 3D APS with the highest surface area (DBSA/SiO2 inks), exhibiting approximately 2-fold and 6-fold improvements in CO (0.11 μmol−1 g−1 h−1) and CH4 (0.05 μmol−1 g−1 h−1) evolution, respectively, relative to the rates measured for the corresponding powder counterparts (Figure 5a,b). The powder counterparts were ground based on the 3D APS (DBSA/SiO2 inks) such that they had the same surface area and mesoporosity but no 3D macroporous feature. Moreover, the CO and CH4 evolution rates of the 3D APS containing only DBSA for ink modification were only 0.15 μmol−1 g−1 h−1 and 0.13 μmol−1 g−1 h−1, respectively, appreciably lower than the activity of the 3D APS (DBSA/ SiO2 inks). Our results are comparable to those reported for other TiO2-based CO2 photoreduction systems (Table S1). However, as the experimental conditions (light intensity, wavelength, catalysts amount, etc.) are different, the photocatalytic performances vary considerably. Simulations on the Gas Diffusion Behaviors. In this work, CO2 photoreduction occurred in a gas-phase system; thus, the reaction rates were significantly influenced by gastransport-related behaviors. The catalytic activity improvements could be attributed to the combination of high gas flow velocity and high gas diffusion efficiency because of the 3D macro-/ mesoporous APS structure, as illustrated in Figure 6. Here, we focus on gas transport performance, including gas flow velocity and gas diffusion, with the finite element software COMSOL

Figure 5. CO2 photoreduction activity in gas-phase reaction without sacrificial agents. (a) Photocatalytic CH4 evolution activity. (b) Photocatalytic CO evolution activity. Au/RuO2 3D TiO2 (DBSA/ SiO2) refers to 3.2 wt % SiO2/TIA mass ratio and 14.4 wt % DBSA/ TIA mass ratio, while Au/RuO2 3D TiO2 (DBSA) refers to 14.4 wt % DBSA/TIA mass ratio.

simulation approach to investigate the effects of microstructure on device properties theoretically.38 Figure 6 compares the 3D APS structure and the corresponding powder (contrast model); both are composed of a large number of mesopores, as discussed above. We set the physical field as the reacting flow at the porous media interface (the same mesoporosity and the same mesopore distribution are set, Table S2); thus, the difference between the two models is only the 3D macropore arrays. This physics interface supports both free-flow domains and porous-media domains, which provides a convenient method for establishing the whole 3D macro-mesoporous structure.26 Basic simulation parameters are presented in Table S2. The gas diffusion is modeled by the Navier−Stokes equations under isothermal conditions in the porous medium in both the steady state and dynamic state.39 (The main function is described by eqs S1−S8) Figure 7a shows the static state for the CO2 velocities in the cross-sectional area, which illustrates that the CO2 flow rate 802

DOI: 10.1021/acs.chemmater.7b04313 Chem. Mater. 2018, 30, 799−806

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Chemistry of Materials

the product CO concentration in both structures over time. The comparison indicates that the CO concentration in the contrast structure changes slowly as the reaction progresses, which means that it is difficult for the concentrated CO inside to diffuse out of the bulk. The dynamic processes vividly demonstrate the higher gas diffusion efficiency of the APS structure, which could increase the catalytic reaction rate correspondingly. Furthermore, Figure 7d shows a comparison of the CO2 concentration in the two structures in the static state. The CO2 concentration in the APS structure decreases obviously near the outlet when the system is balanced, which indicates that more CO2 could participate in the reaction. The simulation results are consistent with our expectation and the experimental data; the gas velocity and gas diffusion efficiency can be appreciably improved for the APS structure compared with those for the contrast model.

Figure 6. Illustration of APS structure model and the contrast structure model. The outside boxes are the reaction container (a1 and a2). CO2 flows from the left side, and the products CO and CH4 flow out from the right side. The porous catalytic beds (c1 and c2) are placed in the container. Both models have mesopores distributed in the structures (b1 and b2).

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CONCLUSIONS We demonstrate a new pathway for the construction of 3D microleaves by 3D direct writing. TiO2 based inks were modified to possess an enhanced surface area (up to 259 m2g−1) and suitable rheological properties. Complex patterns are available because of high modulus values and the inks’ ability to flow readily and solidify quickly in air. The asobtained 3D microleaves have macropore architectures comparable to those of natural leaves. Artificial photosynthesis via CO2 reduction enhances CO and CH4 evolution on the 3Dprinted APS by up to 2-fold and 6-fold, respectively, compared with the rates measured for the corresponding powder counterparts. Gas diffusion behaviors are further studied by theoretical simulation to reveal the 3D hierarchical structural effects on catalytic performance. Moreover, as a proof of concept, the 3D direct writing approach is used for the scalable fabrication of 3D APS. A similar approach could be extended to other 3D-patterned ceramics (e.g., SrTiO3, CaTiO3, etc.) and may provide new possibilities for biomimetic pattern fabrication with special underlying architectures and models.43 Our approach opens a new pathway to constructing APS architectures with exceptional versatility and potential scalability. This strategy can be readily extended to incorporate multiple functional components into a fully integrated system (e.g., inorganic-biological hybrid system44) as a powerful platform for applications including but not limited to solar energy conversion, environmental purification, and electromagnetic shielding.

Figure 7. Simulation results for (a) the static state CO2 velocity distribution. (b) Comparison of dynamic state CO2 velocity distribution after 0.001 s (b1 and b4), 0.01 s (b2 and b5), and 0.1 s (b3 and b6). (c) Dynamic state CO concentration distribution after 0.1 s (c1 and c4), 0.2 s (c2 and c5), and 0.3 s (c3 and c6). (d) Static state CO2 concentration distribution.

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EXPERIMENTAL SECTION

Ink Preparation. Titanium diisopropoxide bis(acetylacetonate) (TIA) was used as the initial precursor. First, 0.31 g polyvinylpyrrolidone (PVP) was dissolved into 4 mL ethanol, and 1.0 g dodecylbenzenesulfonic acid (DBSA) was added. Polyvinylpyrrolidone (PVP) was added to the precursor solution to prevent the sol−gel ink from excessive stress during drying and calcination. After being stirred for 5 min, 6.25 g TIA was added to the solution, with the color of the solution changing to yellow. Finally, 4.0 mL ethanol, 0.93 mL H2O, and 0.67 mL concentrated NH4OH (25%∼28%) were successively added to the precursor solution and heated to approximately 60−80 °C under constant stirring for approximately 6−8 h to form concentrated sol−gel inks for printing. In addition to DBSA, four other types of surfactants were added: sodium dodecyl sulfate (SDS), hexadecyl trimethylammonium bromide (CTAB), poly(ethylene oxide)−poly(propylene oxide)−poly(ethylene oxide) (P123), and Pluronic F127. However, after concentration at 80 °C, only the DBSA-containing ink remained a sol, while the others precipitated.

distribution in the contrast model is 2−3-fold lower on average. The results of dynamic studies (Figure 7b) show that the contrast model reaches dynamic equilibrium after approximately 0.1 s according to the initial flow rate (Table S2), while the 3D APS structure reaches equilibrium after only 0.01 s. Thus, the 3D APS structure exhibits lower resistance to gas diffusion. Gas diffusion in the 3D APS structure is determined by multiple effects of molecular diffusion and Knudsen diffusion, which is similar to the leaf structure.27,40 Knudsen diffusion is important in mesopores, and molecular diffusion plays a dominating role in macropores.41 For the contrast model, which is mainly composed of mesopores, molecular diffusion can be ignored.42 Figure 7c depicts the distribution of 803

DOI: 10.1021/acs.chemmater.7b04313 Chem. Mater. 2018, 30, 799−806

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Chemistry of Materials

Photocatalytic CO2 Reduction. CO2 photocatalytic reduction was performed in a commercial evaluation system with a 300 W Xe lamp and a sealed quartz reactor (Figure S9). The evolved products (CO and CH4) were detected with a gas chromatograph (GC-7920) equipped with a flame ionization detector (FID) and argon as the carrier gas. Typically, a photocatalyst sample (50 mg) was spread on a quartz Petri dish, and the Petri dish was placed in the quartz reactor. Then, 2 mL water was added to the quartz reactor. For comparison, powder counterparts were also measured, which were ground based on the 3D APS. Thus, the 3D macroporous feature was destroyed while the same surface area was retained. The same amount of sample was used for the measurements. Gas Diffusion Simulation. Gas diffusion was studied by the finite element method (FEM) with the COMSOL Multiphysics software (COMSOL 5.0). The Reacting Flow in Porous Media Interface model was set in the software. The APS structure was built according to the SEM images, and the contrast structural model was established based on the APS structure model without macropore patterns. Gas diffusion was modeled by the Navier−Stokes equations under isothermal conditions in the porous medium in both the steady state and dynamic state.

Therefore, the inks containing the other four surfactants were not suitable for further printing. For the ink containing silica nanospheres, typically, 0.5 g silica powder (15 or 30 nm) was added into 15 mL ethanol, and the mixture was treated under sonication for 2 h. PVP, DBSA, TIA, H2O, and NH4OH were then added as described for the inks prepared as indicated above. 3D Direct Writing. 3D printing was carried out on a benchtop robotic system using a programmed patterning procedure. The inks were stored in a 50 mL cylindrical syringe barrel attached to a borosilicate glass micronozzle (diameter = 10, 30 μm). An air-powered transport system provided the appropriate pressure for extruding the inks through the nozzle and controlling their flow rate. The ink delivery system was mounted on a three-axis micropositioning platform, controlled with CAD software. Printing parameters, including pressure, printing speed, platform temperature, and barrel temperature, can be adjusted directly by the accompanying software. The 3D structures were printed onto a petroleum jelly plane substrate, which was fabricated by melting and solidifying the petroleum jelly gel on quartz glass. 3D periodic patterns were directly written by placing several layers of parallel filaments in a woodpile pattern. With different ink viscosities, the printing pressure varied from 0.1 mPa to 0.3 mPa. The printing speed should also match the ink viscosity and the pressure, which was typically in the range of 0.5−1 mm s−1. The entire printing process was conducted in air at room temperature (20−25 °C). Treatment after Printing. The as-printed materials were calcined at 550 °C for 6 h under constant air flow. For samples containing silica nanospheres, etching was applied. Typically, the as-obtained calcined samples were immersed in 25 mL of a 6 M KOH solution and then transferred into a stainless steel autoclave (interior volume 50 mL) and reacted at 180 °C for 3 days to etch the silica nanospheres. Afterward, the structures were collected carefully, washed, and dried at 80 °C overnight. Co-Catalysts Loading. One wt % Au was deposited on the TiO2 structures via a precipitation method using HAuCl4 as a Au source. Specifically, 78 mg TiO2 structures were immersed in 10 mL of a 0.4 mM HAuCl4 solution (pH = 9). Then, the mixture was shaken in a water bath thermostatic shaker for 12 h at 70 °C. Afterward, the TiO2 structures were recovered, washed, and dried at 80 °C for 12 h before heat treatment at 200 °C for 4 h in air. RuO2 loading: First, 65.8 g TiO2 structures (after Au cocatalyst loading) were immersed with triruthenium dodecacarbonyl (Ru3(CO)12, 1.41 mg) in tetrahydrofuran. Second, the mixture was shaken for 6 h, and the as-obtained materials were annealed at 350 °C in air for 2 h. Powder samples used for comparison were prepared by the same procedures and further ground in a mortar for use. Characterization. Optical images were captured by digital microscopy (Keyence VHX-1000). Fluorescence microscopy was performed with a laser scanning microscope (LSM 510, Carl Zeiss). The blue fluorescence from cell walls was excited by a 790 nm titanium sapphire laser. SEM images were obtained by a scanning electron microscopy (Hitachi S-4800). TEM images of the ground structure were obtained using a transmission electron microscopy (JEM-2100F). Rheological properties were investigated with a rheometer (Bohlin Instruments Gemini 200 HR) using a 20 mm flat plate. Specifically, the ink was extruded onto a rheometer platform from our syringe barrel; then, the 20 mm flat plate was lowered to contact the ink completely. A strain sweep from 0.00159 to 15.9155 s−1 was conducted to record the viscosity as a function of shear rate, and a stress sweep from 0.1 to 500 Pa was performed to measure the variations in the storage and loss moduli as functions of stress. XRD (Rigaku D-max/2550) was used for phase characterization. Porosity was characterized by a chemical adsorption analyzer (Micrometrics ASAP 2010+). The samples were degassed at 180 °C for 4 h prior to the adsorption measurement. The thermal decomposition of the TiO2-based inks was monitored using a thermal gravimetric analyzer (Pyris 1 TGA, PerkinElmer, U.S.). The samples were heated in air to 650 °C. The chemical composition was investigated by an X-ray photoelectron spectrometer (AXIS Ultra DLD, Kratos Analytical, Japan).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04313. Image of the 3D printing apparatus, surface areas of samples with different DBSA/TIA mass ratios, photographs of inks with SiO 2 nanosphere filler, N 2 adsorption−desorption isotherms, XPS spectrum of the as-obtained TiO2, XRD spectra, thermogravimetric analysis, and comparison with other TiO 2 -based CO2photoreduction systems under gas phase and details about COMSOL simulation (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.Z.). *E-mail: [email protected] (T.X.F.). ORCID

Tongxiang Fan: 0000-0003-4255-5138 Author Contributions ‡

L.C., X.W.T., and P.W.X. contributed equally. L.C. and H.Z. designed the research. L.C., X.W.T. conducted the experiments and data analysis. P.W.X. conducted the simulation. J.X., Z.H.C., Z.C.C., and P.S.H. carried out part of materials synthesis. H.Z., L.C., and P.W.X. cowrote the manuscript. H.Z. and T.X.F. supervised the project. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support of the Foundation for National Natural Science Foundation of China (51425203, 51772191), Foundation for the Author of National Excellent Doctoral Dissertation of PR China (201434), Innovation Program of Shanghai Municipal Education Commission (15ZZ008), Program of Shanghai Subject Chief Scientist (15XD1501900), Shanghai Rising-Star Program (15QA1402700), Natural Science Foundation of Shanghai (17ZR1441100), and International Science & Technology 804

DOI: 10.1021/acs.chemmater.7b04313 Chem. Mater. 2018, 30, 799−806

Article

Chemistry of Materials

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Cooperation Program of China (2015DFE52870). We thank the reviewers for the helpful suggestions and comments.

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DOI: 10.1021/acs.chemmater.7b04313 Chem. Mater. 2018, 30, 799−806

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Chemistry of Materials Effect Designs for Applications in Solar Energy Manipulation. Adv. Funct. Mater. 2017, 1705309. (44) Nichols, E. M.; Gallagher, J. J.; Liu, C.; Su, Y.; Resasco, J.; Yu, Y.; Sun, Y.; Yang, P.; Chang, M. C.; Chang, C. J. Hybrid bioinorganic approach to solar-to-chemical conversion. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 11461−11466.

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DOI: 10.1021/acs.chemmater.7b04313 Chem. Mater. 2018, 30, 799−806