Letter pubs.acs.org/NanoLett
Inkjet Printing Assisted Synthesis of Multicomponent Mesoporous Metal Oxides for Ultrafast Catalyst Exploration Xiaonao Liu,† Yi Shen,‡,⊥ Ruoting Yang,§ Shihui Zou,† Xiulei Ji,∥ Lei Shi,†,# Yichi Zhang,∥ Deyu Liu,∥ Liping Xiao,† Xiaoming Zheng,† Song Li,‡ Jie Fan,*,† and Galen D. Stucky*,∥ †
Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang Province 310027, China Department of Mathematics, Zhejiang University, Hangzhou, Zhejiang Province 310027, China § Institute for Collaborative Biotechnologies, University of California, Santa Barbara, California 93106, United States ∥ Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States ‡
S Supporting Information *
ABSTRACT: We describe an inkjet printing assisted cooperative-assembly method for high-throughput generation of catalyst libraries (multicomponent mesoporous metal oxides) at a rate of 1 000 000-formulations/hour with up to eight-component compositions. The compositions and mesostructures of the libraries can be well-controlled and continuously varied. Fast identification of an inexpensive and efficient quaternary catalyst for photocatalytic hydrogen evolution is achieved via a multidimensional group testing strategy to reduce the number of performance validation experiments (25 000-fold reduction over an exhaustive one-byone search). KEYWORDS: Mesoporous metal oxides, high-throughput, inkjet printing, photocatalyst, heterogeneous catalysis, group testing
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for producing acrylonitrile17). Split-and-pool synthesis using nanoparticles as catalyst building blocks is an efficient method for making up highly diverse inorganic material libraries comprising thousands to hundreds of thousands of compounds on an inert support material such as Al2O3 or SiO2.18−20 However, the practical limitation of this method is the encoding of the libraries. Among the synthesis techniques, inkjet printing has been widely and actively used for the high-throughput nanofabrication of materials.21−23 However, only a few inorganic materials can be inkjet printed, due to the dissimilar condensation kinetics and chemistry of the precursors of metal species leading to difficulty in preparing inkjet-printable precursors. In addition, redox reactions and phase transformations, including bulk crystallization, can limit the stability of complexes at elevated temperatures. Several groups have reported the successful printing of mesoporous SiO2,24−27 SnO2,28 or TiO2,29 and the special nonmesoporous metal oxides (e.g., Fe−Cs−Nd−Cu−O21) at low resolution. However, a simple, general, and inexpensive formulation of the necessary precursor inks for printing synthesis MMMOs has not been proposed to date. Moreover, current printing techniques mainly depend on overprinting, which prints one
eterogeneous catalysis is of vital importance in our society and constitutes a cornerstone of life from biological processes to the large-scale production of bulk chemicals. A promising candidate family for heterogeneous catalysis is that of the mesoporous metal oxides (MMOs).1−7 The MMOs possess high internal surface areas, variable pore sizes, and designable pore networking that can be used for the support of a wide variety of active sites and supported catalyst species, as well as the reactant/product residence time control that is required for the creation of highly selective and highyield catalysts. Because of the sensitivity of catalytic performance to catalyst structural, electronic, and compositional variables, catalyst design and optimization is a formidable challenge.8−10 We believe that developing an improved highyield synthetic route to produce multi-component mesoporous metal oxides (MMMOs) integrated with an efficient screening strategy would greatly enable the design and discovery of highperformance solid-state catalysts. State-of-the-art, high-throughput inorganic-catalyst syntheses, including MMMOs, have been highly successful and reported to provide some thousands of samples per day up to four components by thin film deposition and solution-based synthetic methods.11−14 However these capabilities are still far from constructing a standard library for multidigit precision complex metal oxide catalysts (e.g., Mo1.0V0.3Te0.23P0.15On for the aerobic partial oxidation of propane,15 Al0.04-K0.004-Ca0.006Fe0.95On for ammonia synthesis,16 and Mo1.0V0.33Nb0.11Te0.22On © 2012 American Chemical Society
Received: August 10, 2012 Revised: October 2, 2012 Published: October 10, 2012 5733
dx.doi.org/10.1021/nl302992q | Nano Lett. 2012, 12, 5733−5739
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Figure 1. Scheme of MMMO syntheses using inkjet printing assisted cooperative-assembly method. (Step 1) A stable colloidal nanoparticle “ink” system consists of nonaqueous solvents, structure-directing agents (SDA), acids, and suitable metal precursors. (Step 2) The color-management system includes an “Absolute Colorimetric” profile to create a one-to-one output between the “percent value” of each color in an image and the “printing volume” of each ink and a “Multichannel Combiner” to extend the printable space. (Step 3) An evaporation-induced cooperative-assembly process is used to generate the desired mesoporous structures of printed libraries (up to gram-scale).
color on top of another with a fixed ink dosage. To print a continuous landscape made up of different compositions, overprinting makes it necessary to print many layers, which is time-consuming and substantially affects the mixing and dispersion of ink solutions during synthesis. Importantly, even if given a one-million-catalyst library by an appropriate high-throughput synthesis method, screening this library oneby-one would easily cost multiple months of hard work and require substantial associated material consumption. Here we propose an inkjet printing assisted cooperativeassembly (IJP-A) method for ultrafast exploration of MMMO catalysts. The IJP-A synthetic platform is set up to reproducibly generate a continuous landscape of candidate catalysts at a rate of 1 000 000-formulations/hour containing up to eight different metal species. More importantly, the composition and mesostructure of the catalyst libraries can be well-controlled and continuously varied over a wide range to better explore the possible catalytic features that might be present and transitioned to via bulk synthesis procedures. In parallel with IJP-A syntheses, a multidimensional group testing (m-GT) methodology is applied to rapidly explore the entire candidate library space that greatly reduces the number of performance validation experiments required and enables one to sequentially analyze catalytic performance using generic analytical tools (25 000-fold reduction over what is needed for exhaustive one-byone searching using a two-digit printing precision configuration). We demonstrate the use of this approach by identifying an inexpensive, yet efficient, quaternary catalyst for photocatalytic hydrogen evolution in the liquid-phase reforming of a formaldehyde-aqueous solution. Inkjet Printing Assisted Cooperative-Assembly Synthesis of MMMOs. MMMO materials were fabricated using inkjet printing assisted cooperative-assembly synthesis as outlined in Figure 1. First, we use a nanoparticle sol−gel cooperative-assembly system to formulate the inorganic precursor inks for IJP-A synthesis (Figure 1, step 1).30,31 The ink system is composed of an amphoteric nonaqueous solvent, metal species, and blockcopolymers to optimize the performance of the ink. Using traditional nonaqueous solvents and the judicious addition of complexing ligands such as acids to control growth and condensation kinetics,24 our ink solutions can form stable colloidal nanoparticles that are monodispersed in size for most common metal precursors. These colloidal nanoparticles are used as building blocks and can be cooperatively assembled
with each other, with elemental inorganic species, and with appropriate cosolvents, block copolymers, or surfactants to form the desired mesostructured library. Moreover, amphoteric solvents such as alcohols or acetic acid ensure appropriate solubility, viscosity, and surface tension for all species. When a selection of metal species is stabilized in the ink system, a large number of compounds can be easily and reproducibly obtained by inkjet printing. Block-copolymers (e.g., F127, P123, Brij 76) coassembled with metal ions and nanoparticles are used as structure-directing agents (SDA), as binders to template a variety of high surface area mesoporous structures of metal oxides (2−50 nm)5 and also to formulate a high quality “ink” solution. Over 25 different single-element formulations of colloidal nanoparticles, and/or a large number of composition combinations of these elements, can be made into colloidal ink. This ink formulation is a major breakthrough that makes it possible to meet the stringent requirements for the IJP-A technique. For example, as illustrated in Figure S1 of the Supporting Information, the printing drops monitored by a high-speed camera system32 have excellent roundness, which guarantees the uniform topography and quality of metal precursor ink solutions during the entire printing procedure. Table S1 summarizes the performance parameters for a total of 25 metal precursor ink solutions that are compatible with our IJP-A system. Because the “ink” typically is made with multicolloidal nanoparticles, it is possible to create particles with different mesoporous structures and variable inorganic wall structure compositions (Figure 2) that can be arbitrarily selected, which when printed give homogeneous atomic compositions within the mesostructure wall on the nanostructure length scale, which is typically a few nanometers. Second, a series of CMYK (cyan, magenta, yellow, black) color images are input into the color-management software for printing the compositional variation of the libraries (Figure 1, step 2). This printing process is programmed and reproducible. To create a continuous controlled catalyst landscape with uniform topography by the printer, researchers have employed gradient color patterns (e.g., CMYK) designed by Adobe Photoshop or Microsoft Powerpoint for inkjet printing syntheses of combinatorial materials.21 However, for our multicolor inkjet printer (more than four colors), the original printer driver uses multilevel color technology (e.g., two black ink cartridges) to increase the quality of professional color prints. For any desired color, this technology mixes the light5734
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separate eight ink cartridges into eight independent channels (C1, M1, Y1, K1, C2, M2, Y2, and K2) that can create a one-toone correspondence map between the “percent value” of each color in an image and the “printing volume” of each channel.33,34 Most importantly, we developed a “Multichannel Combiner” function to merge two carefully designed CMYK images (C1M1Y1K1 + C2M2Y2K2) into a new image (C′M′Y′K′) that does not have any overlaps with its precursor inks. In this manner, we are able to double the capability of our IJP-A system to eight metal components. In principle, the number of components that can be synthesized is equal to the number of channels of the printer head. Third, an evaporation-induced cooperative-assembly process is realized in IJP-A to generate the desired mesoporous structures of printed libraries (Figure 1, step 3). After evaporating and aging, as-synthesized films are calcined at an appropriate temperature to remove the block−copolymer surfactants. The oxides with different compositions and mesostructures are fabricated using various temperatures and durations of calcination conditions. In addition to overcoming the limitation of inkjet printing methods that only create thin films,23,35 our IJP-A method can be easily applied to make thick films, free-standing membranes, powders, and monoliths in bulk amounts (Figure S2 of the Supporting Information). This
Figure 2. Transmission electron microscopy (TEM) images and dispersive X-ray spectroscopy (EDX) element mapping of mesostructured (a) Mo0.05V0.01Te0.01Si1.0Ow and (b) Cu0.004TiOw, revealing a homogeneous distribution of each component in the printed mesoporous materials.
color channel with the standard color channel CMYK. As a result, to obtain a 100% cyan color, the printer mixes two inkscyan and light cyan. This technology removes the independence of the channels and cannot be directly implanted into our desired synthesis protocol. To solve this challenge, we developed a color management system that defines an “Absolute Colorimetric” profile to
Figure 3. Characterizations of MMMOs (calcined at 350 °C) created by a modified Epson Stylus Pro 4880 inkjet printer. (a) TEM image of IJP-A synthesized Mg0.1Ni0.1Mn0.06Cr0.1Cu0.07Fe0.1Co0.1Ti1.0Ow at a resolution of 360 dpi × 360 dpi and element mapping of a single Mg0.1Ni0.1Mn0.06Cr0.1Cu0.07Fe0.1Co0.1Ti1.0Ow particle. (b) Optical image of CoxFeyNizTi1.0Ow arrays printed on a glass slide at a resolution of 2880 dpi × 1440 dpi (0 < x, y, z < 1.0; size of each sample is 1 mm × 1 mm; space between samples is 1 mm). (c) M/Ti molar ratio (M = Cu, Mg, and Ni) of a MgxNiyCuzTi1.0Ow composite (0 < x, y, z < 1.0, 360 dpi × 360 dpi printing resolution) obtained from EDX element analyses as a function of the programmed color management deposition of components, r2 > 0.99. Inset: high correlation between molar ratio and desired molar ratio is still conserved in a smaller scale (0.01 printing precision). (d) TEM images of the CuxTi1.0Ow (0 < x < 1.0, 360 dpi × 360 dpi printing resolution) samples with a different x value (left, 0.005; middle, 0.05; right, 0.2). 5735
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Figure 4. Identification of the optimal active catalyst for hydrogen production by m-GT methodology. (a) 5000 MgxNiyCuzTi1.0Ow (0 < x, y, z < 1.0) samples were designed to pool into eight groups (G1−G8) by the bisection method. G4 (Mg0.5−1Ni0−0.35Cu0.5−0.9Ti1.0Ow) was selected for the next iteration. (b) Because G4-2 (Mg0.7−1Ni0−0.17Cu0.5−0.65Ti1.0Ow) and G4-6 (Mg0.7−1Ni0.17−0.35Cu0.5−0.65Ti1.0Ow) were the regions above the threshold, they were reserved. (c) Subgroup G4-2-2 (Mg0.8−0.9Ni0−0.09Cu0.5−0.55Ti1.0Ow) was chosen to implement the fourth iteration. (d) The optimal catalyst was discovered in G4-2-2-3, Mg0.8−0.85Ni0−0.05Cu0.5−0.55Ti1.0Ow.
3c). The high correlation between printed molar ratio and desired molar ratio of different elements is still conserved on a smaller scale (0.01 printing precision, Figure 3c inset). Varied structural characteristics of calcined printed samples were observed corresponding to different metal compositions. TEM analyses (Figure 3d) show that the structures of the printed CuxTi1.0Ow (0 < x < 1.0) samples were consistent with those obtained by conventional benchtop syntheses, ranging from ordered (left) to wormlike (middle), to disordered mesostructure configurations (right).30 Multidimensional Group Testing Strategy for Exploring Catalysts. Exploration of the full-formulation space of MMMO catalysts (e.g., quaternary materials) with high formulation precision (0.01) is very much like looking for a needle in a haystack. For this reason, many researchers have heuristically picked a few combinations of interest and locally identified possible variations in the neighborhood.37 Although this reduces the number of screenings, the searching process is vulnerable to local-optimum traps.38 A few stochastic global optimization strategies can be used to escape the local minimum traps,11,39 but stochastic methods use many random seeds in each step to approach the global optimal solution and thus significantly increase the number of tests. In light of the search challenge, we propose a new ultrafast multidimensional group testing (m-GT) methodology to explore large numbers of active and inactive compounds. The major concept of group testing is to reduce the number of tests by pooling samples. This idea was initiated by Robert Dorfman, who was struck by the wastefulness of subjecting blood samples from millions of draftees to identical analyses to detect a few thousand cases of syphillis in World War II.40 The essence of
is important for exploring heterogeneous catalyst performance with different morphologies and length scales. Using this IJP-A technique, we have demonstrated the preparation of multicomponent complexes, for example, binary, quaternary, and for the first time, octonary metal oxides. A typical printing process is shown in Video S1. Figure 3a shows the morphology of an octonary metal oxide Mg0.1Ni0.1Mn0.06Cr0.1Cu0.07Fe0.1Co0.1Ti1.0Ow calcined at 350 °C. The material has a wormhole36 mesostructure as determined by transmission electron microscopy (TEM). An energy dispersive X-ray spectroscopy (EDX analysis) element mapping measurement made at a resolution of ∼2 nm exhibits a uniform X-ray intensity throughout the mesoporous particles, indicating the homogeneous distribution at this resolution of eight components in the octonary sample (Figure 3a). Figure 3b shows an optical image of a CoxFeyNizTi1.0Ow (0 < x, y, z < 1.0) library formed on a glass slide by IJP-A. Each sample is 1 mm × 1 mm. The space between samples is 1 mm. Since it only takes about seven minutes for an Epson Stylus Pro 4880c inkjet printer to print an A2-paper (420 mm × 594 mm) library at 2880 dpi × 1440 dpi resolution, more than one million materials can be prepared in an hour. Figure 3c further demonstrates that the printing compositions can be controlled precisely by the color management system. Scanning electron microscopy (SEM) coupled with EDX analyses was conducted to verify the compositions of mIJP synthesized quaternary MgxNiyCuzTi1.0Ow metal oxides (0 < x, y, z < 1.0). The result shows that the M/Ti (M = Cu, Mg, and Ni) molar ratio of a composite is linear with the programmed color management deposition of components in a wide range of 0.0−1.0 molar ratio of M/Ti (r2 > 0.99, Figure 5736
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were designed to pool into new eight groups (G4-1 to G4-8) by the bisection method. For example, for G4-1, there were 625 n ew variable form ulat io ns within the g ro up o f Mg0.5−0.7Ni0−0.17Cu0.5−0.65Ti1.0Ow. The calculated threshold in this iteration was 355 μmol·g−1·h−1. G4-2 and G4-6 with a yield of 414 μmol·g−1·h−1 and 357 μmol·g−1·h−1 was reserved for the next iteration, as shown in Figure 4b. Similarly, precise formulation and large-number syntheses and test procedures based on our IJP-A system and m-GT strategy were carried out until the optimal catalyst was identified. In the third iteration (Figure 4c-1 and 4c-2), the calculated threshold was 360 μmol·g−1·h−1. G4-2-2 exhibits the highest amount of hydrogen generation, 461 μmol·g−1·h−1. The catalytic activities of four groups (G4-2-2-1 to G4-2-2-4) give similar results (standard deviation
