Microreactor for High Yield Solution Deposition of ZnO Nanowires Kevin M. McPeak and Jason B. Baxter Department of Chemical and Biological Engineering, Drexel University, Philadelphia PA Background & Motivation Solution deposition is a simple deposition method requiring only that a substrate be placed in a vessel containing a heated aqueous solution of common precursors. Advantages of solution deposition include low cost, operation at low temperature and atmospheric pressure, and scalability to large area substrates. One of the major drawbacks of solution deposition is inefficient utilization of reactants and significant waste solvent generation. Deposition on the substrate is limited by competing precipitation in solution and deposition on the reactor walls. Careful design of the reactor geometry is critical to efficient utilization of reactants and minimization of waste solvent. Microreactors offer the following advantages over traditional reactors: High surface-to-volume ratio suppresses homogeneous reactions Low thermal mass → precise temperature control Scale up without re-engineering the reactor We report on the use of a microreactor to deposit ZnO nanowires and films from a sub-millimeter reaction channel with the following results: Order of magnitude increase in yield 60% increase in deposition rate Improved bulk crystal quality Fundamentals of Solution Deposition Aqueous Precursors (Metal Salts/Complexing Agents) Speciation Diffusion to surface Adsorption onto surface Surface Reaction Deposition Precipitation Bulk Reaction Traditional reactors have low surface-to-volume ratios Bulk reaction dominates → massive precipitation → low yield Microreactor for Solution Deposition Plexiglass Aluminum Deposition channel Substrate Contact heater 1mm Gasket ZnO deposition Clamp Sub-millimeter reaction channel Contact-heated substrate forms one wall of reaction channel Batch Reactor Case Study We compared ZnO nanowires deposited using a traditional vial reactor and microreactor by investigating: Morphology of deposition ZnO deposition mass Bath [Zn 2+ aq ] Crystal quality Solution deposition can be used to deposit oxides & chalcogenides (CdS, CdSe, etc.), we chose ZnO because: ZnO Properties II-VI semiconductor Wide band gap (3.37 eV) Large exciton binding energy (60 meV) ZnO Applications Transparent conducting oxides Nanostructured photovoltaics Gas sensors UV Lasers Experimental conditions for the batch reactors Vial Reactor Microreactor Substrate Soda lime glass Seeding Method Dip coat in Zinc Acetate, Ethanol, MEA then anneal Aqueous Precursors 0.025 M Zn(NO 3 ) 2 & 0.025 M Methenamine (HMT) Bath Volume 27 mL 0.8 mL Temperature 90 ◦ C 90 ◦ C at interface Results from Batch Reactor Case Study Deposition has similar morphology for both reactors Microreactor Vial Reactor 15 min 2 h 4 h 30 min 2 h 4 h Microreactor Vial Reactor 30 31 32 33 34 35 36 37 Intensity [a.u.] 2θ [deg.] (100) (002) (101) Nanowires Seeds (x10) Hexagonal Dense, well-aligned from seeding 80-100 nm diameter Measuring deposition volume from SEM images results in errors due to non-uniform morphology Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analysis of acid digested ZnO deposition gives precise ZnO deposition mass 0 1 2 Mass of ZnO Deposition [μg/mm 2 ] Microreactor Vial Reactor 0 0.25 0.5 0.75 1 0 1 2 3 4 5 6 ZnO Deposition Rate [(μg/mm 2 )/h] Time [h] Microreactor Vial Reactor 0 0.25 0.5 0.75 0 0.5 [μg/mm 2 ] Time [h] Microreactor’s fast heating increases growth rate Microreactor reduces induction time Microreactor results in 60% higher deposition rate Vial reactor’s large volume deposits more Microreactor provides high yield deposition %Yield = ZnO [moles ] deposited Zn [moles ] initial × 100 Microreactor gives order of magnitude better yield than vial reactor Slower heating rates result in higher yield 0 10 20 30 40 50 0 1 2 3 4 5 6 % Yield Time [h] Microreactor (8 °C/min) Microreactor (34 °C/min) Vial Reactor What species control the ZnO deposition rate? C 6 H 12 N 4 + 6H 2 O - * ) - 6 HCHO + 4 NH 3 NH 3 + H 2 O - * ) - NH + 4 + OH – 2 OH – + Zn 2+ - * ) - ZnO (s) + H 2 O 0.0001 0.001 0.01 0.1 1 10 100 2 3 4 5 6 7 8 9 10 11 12 13 % Zn pH Zn 2+ ZnOH + Zn(OH) 2(aq) Zn(OH) 3 - Zn(OH) 4 2- ZnO (s) pH range during deposition Zn 2+ is the predominant zinc species at deposition conditions. We examined the deposition rate as a function of [Zn 2+ ] in the bath. [OH - ] initially limiting Initial excess Zn 2+ Slow thermohydrolysis of HMT - → OH – Becomes [Zn 2+ ] limited pH gradually rises throughout reaction Zn 2+ is depleted 0 0.25 0.5 0.75 1 1.25 1.5 0 0.005 0.01 0.015 0.02 0.025 ZnO Deposition Rate [(μg/mm 2 )/h] Zn 2+ (aq) [mol/L] [OH - ] limited [Zn 2+ ] limited [OH - ] limited [Zn 2+ ] limited Microreactor Vial Reactor -4 -3 -2 -1 0 0 0.005 0.01 0.015 0.02 ln(ZnO Deposition Rate) Zn 2+ (aq) [mol/L] Results from Batch Reactor Case Study (cont.) Raman shows equal quality nanowires from microreactor Microreactor’s faster growth rate did not alter E 2 peak position Indicates no excess stress in the deposition Differences in E 2 peak height due to void fraction differences 200 300 400 500 600 700 Intensity [a.u.] Raman Shift [cm -1 ] E 2 (high): 438 Microreactor Vial Reactor Glass Substrate Continuous Flow Microreactor The batch microreactor’s small bath volume limits the deposition thickness. To overcome this limitation we have developed a continuous flow microreactor for solution deposition. Removes deposition thickness limitation Bath concentration constant at fixed position i.e. plug flow reactor Allows for further investigation of growth mechanisms Preliminary Results from Continuous Microreactor Interference fringes highlight changes in deposition thickness as reactants are depleted ZnO nanowire length vs. position downstream Flow rate affects deposition uniformity Slow flow → graded deposition Fast flow → more uniform deposition 0 0.25 0.5 0.75 1 0 4 8 12 16 20 24 28 Nanowire Length (normalized) Distance from Inlet [mm] 4 h growth at various flow rates 0.72 mL/h 1.44 mL/h 2.88 mL/h Conclusions Nanowire morphology equivalent in both reactors Microreactor increased deposition yield by order of magnitude Deposition rate increased by 60% in microreactor Raman shows equal crystal quality in microreactor Batch microreactor’s small bath volume limits deposition thickness, continuous microreactor lifts this limitation Flow rate in continuous microreactor affects deposition uniformity References 1. McPeak, K.M.; Baxter, J.B., Microreactor for High-Yield Solution Deposition of Semiconductor Nanowires and Films. I&EC Research, Accepted. 2. McPeak, K.M.; Baxter, J.B., Drexel University (2008) Microreactor for Solution Deposition and Method of Use, Prov. U.S. Pat. 61,054,911.