Crystallite Size Dependent and the Effect of Nano-Ceria Support

Here we study the stability of nano-cuprite against reduction as a function of its crystallite size and upon interaction with a nano-ceria support. A ...
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Reduction of Nano-Cu2O: Crystallite Size Dependent and the Effect of Nano-Ceria Support Junhua Song,† Philip P. Rodenbough,‡,† Wenqian Xu,§ Sanjaya D. Senanayake,§ and Siu-Wai Chan*,† †

Department of Applied Physics and Applied Mathematics/Materials Science & Engineering Program, Columbia University, New York, New York 10027, United States ‡ Chemistry Department, Columbia University, New York, New York 10027, United States § Chemistry Department, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *

ABSTRACT: Copper(I) oxide (Cu2O) is an effective catalyst in the CO oxidation reaction. While high surface to volume ratio in nanoparticles will increase their catalytic efficiency, it posts a stability problem. Here we study the stability of nano-cuprite against reduction as a function of its crystallite size and upon interaction with a nano-ceria support. A systematic analysis of isothermal reduction of a series size of monodispersed Cu2O nanocrystals (±7%) with time-resolved X-ray diffraction (TR-XRD) provides the time-resolved phase fraction of Cu2O and the time when reduction product of Cu (fcc) first appears. The initial phase fraction of nano-Cu2O is less than one with the balance attributed to an amorphous CuO shell. Since no peaks of crystalline CuO (monoclinic) were observed, a core−shell structure with an amorphous CuO shell is proposed. From the analysis, Cu2+ content in corresponding to shell increases from 0 to 33% as Cu2O decreases to 8 nm from the bulk. Based on the reduction profiles, a time size reduction (TSR) diagram is constructed for the observed Cu2O phase behavior during reduction. The incorporation onto a nano-CeO2 support (7 nm) significantly stabilizes our nano-Cu2O in a reducing atmosphere. The oxygen supply propensity in terms of oxygen nonstoichiometry of CeO2−y is shown to be lower when a larger crystallite size CeO2 (20 nm) support is used. The larger oxygen capacity in smaller nano-CeO2 support is analyzed and explained by the “Madelung model” with sizedependent bulk modulus of nano-ceria.

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INTRODUCTION Copper(I) oxide, cuprite, with its cation Cu having three possible valence states (Cu0, Cu+, and Cu2+), makes it a desirable amphoteric catalyst in redox-based thermochemical reactions.1−3 Cu2O has a direct band gap of 2.2 eV and is used in electronic devices and as photocatalysts.4 Although reduction studies have been conducted on Cu2O,5−10 few have investigated the reduction dynamics as affected by their crystal sizes. For nanomaterials, the size effect, such as larger surface to volume ratio and more active surface sites, has a profound influence on their chemical activity and electrical performance compared to their bulk counterparts.11 Here, we present a systematic reduction study of a range of sizes of Cu2O nanoparticles under a carbon monoxide reducing atmosphere. The measurement of size-dependent reduction rate in nanocopper(I) oxide is helpful in understanding and refining nanoparticle application. Traditionally, one can analyze the durability and stability of catalytic materials based on the reaction temperature and time.12 Here, we provide a new perspective from measuring the durability of nano-cuprite from the reduction rate at a particular crystal size. This method can be used in estimating stability range for different sized nanocuprite during isothermal reduction. It is well-known that the resistance to reduction of oxide materials can be greatly enhanced with a proper supporting material, often a synergistic redox interplay between the © 2015 American Chemical Society

support and the active catalyst. Cerium oxide (ceria), in this case, is a desirable support due to its extraordinary tolerance of nonstoichiometry and extended ability to supply and absorb oxygen.13 There is little evidence, however, showing how the crystal size of the support (ceria) affects the reduction of Cu2O. By combining different sizes of ceria nanoparticles with Cu2O, we probe the oxygen transfer between nano-Cu2O and nanoCeO2. Our study of nano-ceria-supported nano-Cu2O brings to light a more rational design for a binary nano-oxide system for catalysis of the active catalyst and the support.

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EXPERIMENTAL SECTION Chemicals. Copper nitrate trihydrate (Cu(NO)2·3H2O, 99−104%), L-ascorbic acid, sodium hydroxide pellets (NaOH, 97%), and LS colloidal silica (SiO2, 30 wt %) are purchased from Sigma-Aldrich and used without further purification. Hexamethylenetetramine (HMT, 99+%), copper(I) oxide (Cu2O, 99%), and cerium(III) nitrate hexahydrate (CeN3O9· 6H2O, 99.5%) are purchased from Alfa Aesar and used without further purification. Purified water was prepared to 18 MΩ·cm with a Barnstead Nanopure Infinity system. Received: April 29, 2015 Revised: June 8, 2015 Published: June 9, 2015 17667

DOI: 10.1021/acs.jpcc.5b04121 J. Phys. Chem. C 2015, 119, 17667−17672

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The Journal of Physical Chemistry C Synthesis of Cu2O Nanoparticles. Direct Precipitation Method.14 400 mL of aqueous 0.005 M Cu(NO)2·3H2O was prepared and stirred. Separately but simultaneously, an aqueous solution of 400 mL of NaOH and 0.009 M ascorbic acid were mixed and stirred. After 10 min of independent stirring, the two solutions were combined and mixed for 30 min. The final solution was then centrifuged at 10 °C at 12K rpm for 2 h. The supernatant was discarded, and the solid mass was left to dry in air overnight. The solid was then mechanically ground with a mortar and pestle. Size control was achieved by adjusting the molar concentration of NaOH from 0.038 to 0.9 M, which resulted in uniform cube-shape particles ranging from 14 nm to 1 μm, respectively. Particle sizes are obtained by measuring over 100 particles using TEM (Figure S1). Precipitation-Reduction Method.7 500 mL of aqueous 0.005 M Cu(NO)2·3H2O was prepared and stirred. Simultaneously, 500 mL of 0.5 M HMT solution was prepared and stirred. After 30 min independent stirring, two solutions were combined and stirred at 40 °C in a heating circulator (Thermo Scientific NESLAB Ex10). The resulting CuO nanoparticles are reduced by annealing at 250 °C to Cu2O in 5% CO balanced with a 95% He atmosphere. Size control was achieved by adjusting the annealing time from 10 to 25 min. This method resulted in uniform aggregation of nano-Cu2O crystals ranging from 8 to 20 nm. Particle sizes are obtained by measuring over 100 particles using TEM (Figure S2). Synthesis of CeO2 Nanoparticles.15 250 mL of aqueous 0.0375 M Ce(NO3)3·6H2O was prepared and stirred for 30 min. Separately but simultaneously, 250 mL of aqueous 0.5 M HMT was prepared and stirred for 30 min. For synthesizing 7 nm CeO2, the solutions were combined and stirred for 6 h. The solution was then centrifuged at 10 °C at 12K rpm for 2 h. The supernatant was discarded, and the solid mass was left to dry in air overnight. The solid was then mechanically ground with a mortar and pestle. The 20 nm CeO2 was obtained by annealing the 7 nm one for 2 h at 550 °C. Fabrication of Cu2O/CeO2 Nanostructure.14 A binary nanostructure was achieved by mixing 20 mg of Cu2O nanoparticles and 40 mg of CeO2 nanoparticles with 1.5 g of colloidal silica. After 15 min of ultrasonication and stirring, the gel solution was dried in an oven at 80 °C for an hour. The resulting solid was then ground with a mortar and pestle. Characterization. TR-XRD. The TR-XRD data were collected isothermally (250 °C) at X7B beamline of National Synchrotron Light Source at Brookhaven National Laboratory (λ = 0.3196 Å). Diffraction patterns were collected using a PerkinElmer amorphous silicon detector at 32 s intervals. The powders were loaded into a quartz capillary tube with 0.5 mm inner diameter. Position of samples was secured by quartz wool, inserted on each side of the capillary. Gas flow rate (≈10 cm3/ min) of 5% CO/95% He was held at constant and read on both inlet and outlet of the cell by a flow meter. A mass spectroscope was attached at the end of exhaust tube to monitor gas composition after reaction. The capillary was heated from the bottom by a kanthal wire. In order to maintain accuracy, a thermal couple was inserted into samples to monitor reaction temperature. Prior to collecting data, instrumental profile was obtained by a measurement of LaB6 powder. From the TRXRD data, phase fraction was analyzed by GSAS (general structure analysis system) program, equipped with Rietveld analysis.16 In GSAS, the change of XRD peak intensity of Cu2O is tracked. The change of characteristic peak intensity

represents the change of the concentration of its corresponding phase.

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RESULTS Reduction of Cu2O. Isothermal reduction for a series Cu2O crystallite sizes was conducted using TR-XRD. The normalized Cu2O phase fraction, deducted from XRD peaks intensity, as a function of reduction time is shown (Figure 1). Only 8 nm, 50

Figure 1. Normalized Cu2O phase fraction as a function of time in reducing 8 nm, 50 nm, and bulk Cu2O.

nm, and bulk samples were presented here for clarity. The reduction of our synthesized Cu2O nanoparticles appears as a two-step reaction unlike the bulk. In the 8 nm Cu2O, for example, the first step involves reducing Cu2+ to Cu+, indicated by an increase of Cu2O phase fraction from 0.67 to unity; the second step begins with losing surface oxygens and their migration in cuprite which gradually creates enough oxygen vacancies and then reduces to copper metal.17 Before the heating begins, neither observable CuO nor Cu XRD peaks were observed in the XRD pattern (not shown here) at room temperature. In addition, we observed that the starting phase fraction of Cu2O increases with increasing crystal size as well as longer incubation period before the Cu (fcc) peaks appear. The size-dependent phase fraction of Cu+ at t = 0 coincides with our previous X-ray absoption near-edge spectroscopy (XANES) study on the copper cation’s valence state with different crystallite sizes (see Figure 2).18 Time Size Reduction (TSR) Diagram. Since the crystal size plays a vital role in dictating the oxide reductive behavior, an observation of the reduction in terms of crystal size is desirable for directing sample preparation and catalytic performance. Unlike the time−transformation−temperature (TTT) diagram,11 the TSR diagram is useful in presenting the size influence of oxides when reduced isothermally. To construct the TSR diagram, the normalized Cu2O phase fraction for 8, 5, 23, 40, and 50 nm as well as bulk (designated as 100 nm) in diameter are plotted against reduction time (see Figure 3a) during the second reductive step. The reduction curves shift to shorter reduction time as particle size decreases. For the 8 nm Cu2O, the slightly longer redcution time than that of 14 nm Cu2O attributes to the aggregated particles, which slow the crystals inside the aggregation from reducing. The extracted start time, halfway, and end time of reduction were 17668

DOI: 10.1021/acs.jpcc.5b04121 J. Phys. Chem. C 2015, 119, 17667−17672

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Figure 2. Comparison of initial Cu+ concentration versus crystallite size from XRD and XANES. The 8 nm sample is synthesized by the HMT precipitation-reduction method.

ploted versus their corresponding crystallite sizes (see Figure 3b). The middle dashed line represents the reduction halfway from Cu2O to Cu metal (i.e., 50/50 phase fraction). The subsequent TSR digram for Cu2O reduction consists of three regions with the corresponding phase fractions: (1) Cu2O > 99%, (2) Cu2O/Cu region, and (3) Cu2O ≤ 0.5%. The shape of TSR diagram adopts a narrower transition region, which gradually opens up with increasing Cu2O crystallite size. When supported by nano-CeO2, we observe that the 50/50 line shifts to longer time in the TSR diagram. Reduction Rate of Cu2O and CeO2 Supported Cu2O. The reduction rate (RR) determines the stability and thus the performance of redox-based catalytic materials. RR is conventionally denoted as the reciprocal of the time needed for reduction to reach halfway to completion.10 The time-resolved in situ XRD pattern of CeO2 (7 nm) supported Cu2O (14 nm) shows that the reduction of Cu2O to Cu starts at 20 min and becomes having more Cu as time proceeded (see Figure 4). We also found that the size of CeO2 support influences the RR in Cu2O reduction. When supported by nano-CeO2, the RR of Cu2O (14 nm) decreases as the crystal size of the ceria support decreases. For comparison, a plot of RR versus crystallite size of both Cu2O and supported Cu2O is shown in Figure 5. As we can see, the RR of nano-Cu2O decreases as the crystal size increases from 8 to 50 nm to bulk. In the study of sizedependent reduction rate of nano-Cu2O, we are interested in the stability of nano-Cu2O as a catalyst in a reductive environment. The CeO2 support proves to extend this stability by significantly decreasing the reduction rate. Lattice Parameter of CeO2 Support. In this study, the Cu+ to Cu metal reduction of Cu2O is accomplished by a oxygen depletion process, which is greatly impeded when supported by CeO2. The structual responses of different sized nano-CeO2, associated with its oxygen activity, is important for unraveling its role as a support in Cu2O reduction. During the isothermal reduction of nano-Cu2O, we observed an increasing lattice paramter of nano-CeO2 before and during Cu+−Cu reduction (Figure 6). The oscillations of lattice parameter in the plots are due to the noise in structural refinement. In addition, the CeO2 support of a smaller crytal size tends to have a greater capacity of donating oxygen and lattice expansion. The lattice paramter of 7 nm CeO2 support expands 0.37% and

Figure 3. (a) Nomalized phase fraction of a crystal size series of nanoCu2O versus reducing time. (b) Time size reduction diagram constructed from the reduction profile. The polydispersity of each size is as follows: 8 ± 0.52 nm; 14 ± 0.98 nm; 23 ± 1.70 nm; 40 ± 2.64 nm; 50 ± 3.56 nm; 1 ± 0.1 μm.

0.31% when combined with 50 and 14 nm Cu2O, respectively. Both are larger than the 0.27% in 20 nm CeO2. The lowest RR of 7 nm Cu2O coincides with the largest lattice expansion of nano-CeO2 when it is supporting 14 nm Cu2O.

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DISCUSSION AND ANALYSIS Reduction Mechanism of CeO2 Supported Cu2O. Since cerium dioxide is notably capable of losing lattice oxygens upon heating (CeO2−x),11 the extended stability of supported nanoCu2O indicates a possible diffusion of oxygen from nano-ceria lattice.19 As a result, nano-Cu2O does not become too oxygen deficient to become Cu as the removed oxygens in Cu2O matrix are supplemented from the CeO2 support, i.e., a distinct transfer of oxygen from one oxide to the other. The existence of this facile transfer stabilized nano-Cu2O with a lower reduction rate comparable to the unsupported Cu2O samples. Thermally activated diffusion of oxygen is considered to be associated with the formation of oxygen deficient CeO2 with considerable concentrations of Ce3+ cations and oxygen 17669

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Figure 4. Time-resolved in situ X-ray powder diffraction patterns (λ = 0.3196 Å) during reduction of Cu2O/CeO2 in a CO atomsphere.

Figure 6. Lattice parameter of nano-CeO2 versus reduction time. The dashed lines represent the onset of the appearing of Cu (fcc).

Figure 5. Different particle sizes in controlling phase reduction rate for pure Cu2O and CeO2 supported Cu2O during reduction in 5% CO/ 95% He gas (10 cm3/min flow). Figure 7. Interactive reaction takes places when CeO2 is partially reduced by lending oxygen to prevent Cu+ from reducing.

vacancies. The oxygen deficient ceria formation can be represented as CeO2 → (y/2)O2 + CeO2−y. Upon this reaction, we have one oxygen vacancy generated for every two Ce3+ at Ce4+ sites. In Kröger−Vink notion, the doping of Ce2O3 into

considered to be the only contributor in lattice expansion here. We have previously calculated the Ce3+ content and oxygen nonstoichiometry in nano-ceria by assuming the lattice expansion appears from the increasing concentrations of larger Ce3+ ions and oxygen vacancies with diminishing ceria particle size.14 The calculation is based on the relationship between the increase of CeO2 lattice parameter per trivalent cation concentration and increasing rare-earth cation radius.21 Here, we use the Ce3+ radius of 1.143 Å and estimate the corresponding concentrations of Ce3+ions and oxygen vacancies from lattice parameters as shown in Table 1. It is also worth mentioning that the nanosize effect, Madelung negative pressure, explains 60% of the lattice expansion of 6 nm ceria using the compressibility of bulk ceria.22 With recent experimental results on a crystal size dependent compressibility of nano-ceria, the bulk modulus (K) and the reciprocal of

CeO2

CeO2 can be written as Ce2O3 ⎯⎯⎯⎯→ 2Ce′Ce + VÖ . Based on previous work,20 the concentration of Ce3+ (here simply designated as [Ce3+]) in nano-ceria increases with decreasing crystal sizes. The oxygen exchanging process with nano-Cu2O is facilitated by the fast diffusion of oxygen vacancies in nanoCeO2 support. Thus, it is reasonable to consider an interaction between nano-ceria support and nano-Cu2O during the reduction process (see Figure 7). When exposed to CO, Cu2O is first reduced to form an oxygen-deficient Cu2O1−x phase. Meanwhile, the fast oxygen transportation from CeO2−y continuously prevents the oxygen-deficient Cu2O1−x from further reduction. The oxygen transportation can be indirectly measured by the lattice expansion of the CeO2 support before and during the Cu+ to Cu reduction. Since there is no thermal expansion during isothermal reduction, oxygen loss is 17670

DOI: 10.1021/acs.jpcc.5b04121 J. Phys. Chem. C 2015, 119, 17667−17672

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Table 1. Ce3+ Concentrations and Oxygen Vacancy Concentrations with Different CeO2 Support Sizes Calculated from the Lattice Expansion CeO2 size d (nm)

Cu2O size d (nm)

CeO2 change of lattice parameter Δa (Å)

y in CeO2−y

[Ce3+]

[Ce3+] corrected with Madelung model

corrected y in CeO2−y

7 ± 1.05 7 ± 1.05 20 ± 3.12

14 ± 0.98 50 ± 2.81 14 ± 0.98

0.016 0.018 0.014

0.29 0.32 0.25

0.58 0.65 0.51

0.18 0.21 0.28

0.09 0.11 0.14

Database (COD ID: 78-2076) and used as a constant throughout the calculation. The comparison between both calculated and measured CuO cencentration are plotted against their corresponding crystal sizes Cu2O in Figure 9. The

compressibility of 20 and 7 nm ceria are 290 GPa and193 GPa, respectively.23 These K values of different sized ceria nanoparticles indicate that the lattice expansion caused by the Madelung model is overestimated at 1.3 times of 20 nm nanoceria and underestimated at 0.9 times of 7 nm nano-ceria using “bulk” modulus. By considering a possibly reduced surface adhesive ionic interaction, the overall calculation of oxygen deficiency may be overestimated using the Madelung model.24 The Madelung corrected [Ce3+] and oxygen concentrations are listed in Table 1 for comparison. It is now obvious to see how size of nano-ceria affects the oxygen capacity as support material against Cu2O from reducing. Compared to 20 nm nano-ceria support, the oxygen supplying ability is diminished by 66% using 7 nm nano-ceria due to its lower K value. Thus, we demonstrate that nano-ceria functions as oxygen buffer for Cu2O against reduction by supplying its lattice oxygen. Surface/Volume Effect of Amorphous CuO Surface in Nano-Cu2O. An important charateristic that distinguishes nanomaterials from their bulk counterpartes is their surface-tovolume ratio. According to the step reactions we observed in reduction of Cu2O (Figure 8), the absence of CuO XRD peak

Figure 9. Blue line represents the ratio of Cu ions from the calculated Cu(I) concentration with increasing crystallite size (i.e., R = Cu+/ Cu2+); The red dots shows the ratio from the measured Cu(I) concentration.

reference CuO shell thickness is held as a constant in estimating Cu(I)/Cu(II) ratio (R) for different sizes Cu2O samples. (The derivation can be seen in the Supporting Information as eq 1.) It is clear that the calculated Cu+/Cu2+ ratio differs from that observed as the nano-Cu2O decreases. This difference indicates that the formation of more Cu2+ in the shell layer at smaller Cu2O crystallites than a constant surface layer thickness of amorphous CuO (3.5 Å). Instead, it is natural for nano-Cu2O adopting this core−shell structure with a thicker amorphous CuO layer at a smaller crystal size.

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Figure 8. Schematic drawing to show a proposed model of two-step reduction of Cu2O nanoparticles.

CONCLUSIONS A range of crystal sizes of monodispersed nano-Cu2O without and with nano-CeO2 support was isothermally reduced. Timeresolved X-ray powder diffraction measurements have shown that smaller Cu2O crystallites, under a carbon monoxide atmosphere, reduce to Cu metal faster than larger Cu2O crystallites. When supported by the smallest nano-CeO2 (7 nm), the reduction rate of Cu2O (14 nm) decreases by a factor of 2−3, comparable to the unsupported micron-sized Cu2O. Before Cu+ to Cu0 reduction begins, the amorphous CuO shell layer first reduces to Cu2O. The initial phase fraction of this amorphous CuO shell is 33% in 8 nm Cu2O. The time size reduction (TSR) diagram predicts a narrower reduction region with smaller Cu2O crystallite sizes. Present results clearly show that both the amorphous CuO shell layer and the nano-CeO2 support retard the reduction of nano-Cu2O in CO.

suggests the presence of an amorphous surface layer of CuO. Both TR-XRD and XANES results show the increasing thickness of this amorphous layer is formed on smaller nanoCu2O chemically, a result of a higher surface-to-volume ratio (S/V) in smaller nano-Cu2O. Here, we consider two possible factors in determing CuO content at different crystallite sizes: (1) change of S/V with constant thickness of the amorphous layer; (2) tendency of forming more stable Cu(II)−O bonds than Cu(I)−O bonds. Since the amorphous CuO shell has a negligible influence on the crystal strucuture of the Cu2O core, we examine the value of S/V in determining the Cu2+ content by assuming a spherical core−shell Cu2O nanoparticles with a fixed thickness of the amorphous CuO layer. The value of the unit cell volume was obtatined from Crystallography Open 17671

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(5) Kim, J.; Rodriguez, J.; Hanson, J.; Frenkel, A.; Lee, P. Reduction of CuO and Cu2O with H2: H Embedding and Kinetic Effects in the Formation of Suboxides. J. Am. Chem. Soc. 2003, 125, 10684−10692. (6) Yang, F.; Choi, Y.; Liu, P.; Hrbek, J.; Rodriguez, J. A. Autocatalytic Reduction of a Cu2O/Cu (111) Surface by CO: STM, XPS, and DFT Studies. J. Phys. Chem. C 2010, 114, 17042−17050. (7) Hua, Q.; Chen, K.; Chang, S.; Bao, H.; Ma, Y.; Jiang, Z.; Huang, W. Reduction of Cu2O Nanocrystals: Reactant-dependent Influence of Capping Ligands and Coupling between Adjacent Crystal Planes. RSC Adv. 2011, 1, 1200−1203. (8) Pike, J.; Chan, S.; Zhang, F.; Wang, X.; Hanson, J. Formation of Stable Cu2O from Reduction of CuO Nanoparticles. Appl. Catal., A 2006, 303, 273−277. (9) Goldstein, E.; Mitchell, R. Chemical Kinetics of Copper Oxide Reduction with Carbon Monoxide. Proc. Combust. Inst. 2011, 33, 2803−2810. (10) Yao, S.; Mudiyanselage, K.; Xu, W.; Johnston-Peck, A. C.; Hanson, J. C.; Stacchiola, T.; Wu, D.; Rodriguez, J. A.; Zhao, H.; Beyer, K. A.; Chapman, K. W.; Chupas, P. J.; Martínez-Arias, A.; Si, R.; Bolin, T. B.; Liu, W.; Senanayake, S. D. Unraveling the Dynamic Nature of a CuO/CeO2 Catalyst for CO Oxidation in Operando: A Combined Study of XANES (Fluorescence) and DRIFTS. ACS Catal. 2014, 4, 1650−1661. (11) Roduner, E. Size Matters: Why Nanomaterials are Different. Chem. Soc. Rev. 2006, 35, 583−583. (12) Callister, W. Materials Science and Engineering: An Introduction, 7th ed; John Wiley & Sons: New York, 2007. (13) Tschope, A. Redox Activity of Nonstoichiometric Cerium Oxide-Based Nanocrystalline Catalysts. J. Catal. 1995, 957, 42−50. (14) Cao, Y.; Xu, Y.; Hao, H.; Zhang, G. Room Temperature Additive-free Synthesis of Uniform Cu2O Nanocubes with Tunable Size from 20 nm to 500 nm and Photocatalytic Property. Mater. Lett. 2014, 114, 88−91. (15) Zhang, F.; Chan, S.; Spanier, J.; Apak, E.; Jin, Q.; Robinson, R.; Herman, I. Cerium Oxide Nanoparticles: Size-Selective Formation and Structure Analysis. Appl. Phys. Lett. 2002, 80, 127−127. (16) Rietveld, H. A Profile Refinement Method for Nuclear and Magnetic Structures. J. Appl. Crystallogr. 1969, 2, 65−71. (17) White, E. B. Chemistry and Catalysis at the Surface of Nanomaterials. Ph.D. Dissertation, Columbia University, 2007. (18) Song, J.; Rodenbough, P. P.; Zhang, L.; Chan, S. SizeDependent Crystal Properties of Nanocuprite. Int. J. Appl. Ceram. Technol. 2015, submitted. (19) Skårman, B.; Grandjean, D.; Benfield, R. E.; Hinz, A.; Andersson, A.; Wallenberg, L. Carbon Monoxide Oxidation on Nanostructured CuOx/CeO2 Composite Particles Characterized by HREM, XPS, XAS, and High-Energy Diffraction. J. Catal. 2002, 211, 119−133. (20) Zhang, F.; Wang, P.; Koberstein, J.; Khalid, S.; Chan, S. Cerium Oxidation State in Ceria Nanoparticles Studied with X-ray Photoelectron Spectroscopy and Absorption Near Edge Spectroscopy. Surf. Sci. 2004, 563, 74−82. (21) Mcbride, J.; Hass, K.; Poindexter, B.; Weber, W. Raman and Xray Studies of Ce1−xRExO2−y, where RE = La, Pr, Nd, Eu, Gd, and Tb. J. Appl. Phys. 1994, 76, 2435−2435. (22) Perebeinos, V.; Chan, S.; Zhang, F. “Madelung Model” Prediction for Dependence of Lattice Parameter on Nanocrystal Size. Solid State Commun. 2002, 123, 295−297. (23) Rodenbough, P. P.; Song, J.; Walker, D.; Clark, M.; Simon; Kalkan, B.; Chan, S. Size Dependent Compressibility of Nano-ceria: Minimum near 33 nm. Appl. Phys. Lett. 2015, 106, 163101. (24) Diehm, P.; Á goston, P.; Albe, K. Size-Dependent Lattice Expansion in Nanoparticles: Reality or Anomaly? ChemPhysChem 2012, 13, 2443−2454.

The size effect in the reduction of Cu2O is a two-step reaction process. The supported nano-Cu2O shows better resistance against reduction in a CO atmosphere than the unsupported ones. In addition, the thermal activated oxygen is more available from smaller nano-CeO2, which is responsible in stabilizing nano-Cu2O against reduction. The Madelung negative pressure model explains the diminished oxygen supplying ability in 20 nm ceria with a minimum compressibility of ceria nanocrystals. The higher surface-to-volume ratio in nano-Cu2O is not the dominating factor for a higher concentration of the CuO in the smaller Cu2O crystals. The amorphous Cu (II)−O surface layer is thicker in smaller Cu2O nanocrystals. From the reduction profile, the rate of the reduction is not limited by the nucleation of Cu metal. It is likely that Cu grows epitaxially on Cu2O as oxygens diffuse out to react with CO.

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ASSOCIATED CONTENT

* Supporting Information S

TEM image of Cu2O nanoparticles synthesized by direct precipitation and precipitation-reduction methods; derivation of surface to volume effect in Cu+/Cu2+ for a series size of Cu2O. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcc.5b04121.

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AUTHOR INFORMATION

Corresponding Author

*(S.-W.C.) E-mail [email protected]; Tel 212-854-8519. Present Address

W.X.: X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was primarily supported by National Science Foundation-DMR 1206764. Research at Brookhaven National Laboratory, in the Chemistry Department and at the National Synchrotron Light Source, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-SC0012704. The authors thank Professor James Im for his constructive discussion. We also acknowledge SWC’s former student Jenna Pike for her investigation in cuprite nanoparticle synthesis and Yousun Hardware Manufactory, Shenzhen, China for machining parts for the heating stage, which makes in situ XRD possible.

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b04121 J. Phys. Chem. C 2015, 119, 17667−17672