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Capturing phase evolution during solvothermal synthesis of metastable CuO Zhelong Jiang, Shiliang Tian, Shuqi Lai, Rebecca D. McAuliffe, Steven P Rogers, Moonsub Shim, and Daniel P. Shoemaker Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b00421 • Publication Date (Web): 19 Apr 2016 Downloaded from http://pubs.acs.org on April 20, 2016

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Chemistry of Materials

Capturing phase evolution during solvothermal synthesis of metastable Cu4O3 Zhelong Jiang,† Shiliang Tiang,‡ Shuqi Lai,† Rebecca D. McAuliffe,† Steven P. Rogers,† Moonsub Shim,† and Daniel P. Shoemaker∗,† †Department of Materials Science and Engineering, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA ‡Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA E-mail: [email protected]

Abstract

Introduction

The metastability of Cu4 O3 has long hindered the synthetic preparation of bulk samples with substantial crystallinity. The lack of suitable samples has thwarted the detailed understanding of the magnetic properties of Cu4 O3 and the ability to tune its properties. While Cu4 O3 was recently shown to form in solvothermal reactions, the results are unpredictable and the crystals are small. We developed a new, more uniform synthesis technique using sealed fused silica tubes. Interrogation of the solid and liquid phases resulting from this reaction has shed more light on the kinetic evolution of copper-containing phases and the microstructural correlation between different precipitation products. We find that direct conversion of the intermediate phase Cu2 (NO3 )(OH)3 to Cu4 O3 is a likely consequence of dimethylformamide (DMF) triggered in situ reduction. The optimal reduction environment should be more straightforward to attain given the improved reliability of our method, and it remains under investigation. We verify the formation of Cu4 O3 via X-ray diffraction, Raman microscopy, and SQUID magnetometry.

The binary copper oxide family of cuprous oxide Cu2 O, cupric oxide CuO, and paramelaconite Cu4 O3 has numerous functional applications due to their unique structures and electronic configurations. Cu2 O rectifiers were commonplace before the introduction of Si diodes, and were reviewed in 1951 by Brattain. 1 Cu2 O is a natural p-type semiconductor, and has been extensively applied in solar cells and optoelectronic devices. 2–7 Recently, attention has turned to excitonic BoseEinstein condensation 8–10 and spin-phonon interaction 11 in Cu2 O, as well as electronphonon coupling 12 and spin-phonon interactions 13 in CuO. Both Cu2 O and CuO are also prevalent in catalysis, 14–16 gas sensing, 17 electrochromism, 18 spintronics, 19,20 and lithium ion battery research. 21–23 Less is known about paramelaconite Cu4 O3 due to its scarcity and the complexities of its synthesis. After its discovery in the Copper Queen mine (Bisbee, Arizona, USA), 24 its structure was determined to be tetragonal I41 /amd (Figure 1(A)). 25,26 It is a mixedvalence compound with equal numbers of two-fold linearly-coordinated Cu+ and fourfold square-planar Cu2+ ions. The spin-active Cu2+ sublattice in Cu4 O3 is an analog of the pyrochlore lattice (Figure 1(B)). Oxides of the

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an understanding of the reaction mechanisms. We show that the ethanol-induced mineralization of Cu(NO3 )2 ·3H2 O into Cu2 (NO3 )(OH)3 is followed by DMF-induced reductive conversion of Cu2 (NO3 )(OH)3 into Cu4 O3 . Our transparent, uniform reaction vessel should enable further in situ characterization 48 that can lead to optimization of this reaction for phase purity and crystallinity.

U500 (500 MHz) with a mixture of DMF-d7 and ethanol-d5 (1:2 v/v) as an external standard and one peak of DMF-d7 at 2.74 ppm was used for referencing spectra. Inductively coupled plasma–optical emission spectroscopy (ICP-OES) measurements of the liquid samples were performed with a PerkinElmer 2000DV spectrometer. The liquid samples were dried in ICP tubes for 2 days at room temperature prior to ICP-OES measurement. X-band electron paramagnetic resonance (EPR) spectra of the liquid samples were collected at 77 K on a Varian-122 spectrometer equipped with an Air Products Helitran cryostat and temperature controller, with a collection frequency of 9.29 GHz, power of 0.2 mW and field modulation of 4.0 G. EPR spectrum was simulated with SIMPOW6. 49 Ultraviolet-visible (UV-Vis) spectroscopy measurements for the liquid samples were performed on a Varian Cary5G spectrometer with transmission attachment, with source and detector changeover both at 600 nm. The undiluted samples were used to record the UV-Vis spectra from 350 to 1000 nm, while samples diluted to 4 vol% with ethanol were used to record the spectra from 290 to 450 nm. All light path lengths were 1 cm in this study. Powder X-ray diffraction (XRD) patterns of the solid samples were collected on a Bruker D8 ADVANCE diffractometer equipped with a Mo-Kα source and LYNXEYE XE detector in transmission geometry. Rietveld refinements were performed using TOPAS 4.2. Scanning electron microscopy (SEM) images of Au/Pd-coated samples were taken with a JEOL JSM-6060LV microscope with 15 kV acceleration voltage. Micro-Raman spectra of the solid samples were acquired on a Jobin Yvon Labram HR800 micro-Raman spectrometer with 532 nm laser excitation (laser spot size ∼ 1 µm) with laser intensity around 3 µW/µm2 . The samples were spread over standard glass microscope slides for micro-Raman measurements. Visible light microscopy images associated with the micro-Raman studies were taken with Olympus BX41 microscope. Crystal structures were visualized using VESTA. 50 Magnetization measurements were performed

Experimental Synthesis Ethanol and DMF were mixed in a 1:2 volume ratio to form the reaction solvent. Cu(NO3 )2 ·3H2 O was dissolved in this solvent to specified concentrations (either 70 or 50 mM) to make precursor solutions. In autoclave attempts, 18 mL of the precursor solution was placed in a 23 mL capacity PTFElined autoclave (Parr Instruments). In sealed tube attempts, 15 mL of the precursor solution was poured into a fused silica tube with 16 mm outer diameter and 14 mm inner diameter. The end of the tube was submerged into liquid nitrogen to freeze the solution. The tube was then evacuated and sealed. In both autoclave and tube reactions, the precursor solution was placed in an oven pre-heated to 130◦ C, held for a specific amount of time, then removed from the oven and air cooled to room temperature. The tubes were tilted about 30◦ from vertical during reactions. The products were collected and centrifuged to separate the liquid and solid phases. All silica tube reactions presented here were prepared from the same two batches (50 mM and 70 mM) of precursor.

Characterization Fourier transform infrared (FTIR) spectra of the liquid samples were collected on a Thermo Nicolet Nexus 670 spectrometer with an attenuated total reflection attachment. 1 H nuclear magnetic resonance (NMR) spectra of the liquid samples were recorded on a Varian


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Figure 2: EPR spectra for the liquid phase after reactions of (A) 70 mM and (B) 50 mM Cu(NO3 )2 ·3H2 O in 1:2 v/v ethanol:DMF at 130◦ C. The insets in (C) and (D) show the variation in Cu+ concentration, calculated as the difference between total Cu (ICP-OES) and Cu2+ (EPR) concentrations. Cu+ concentrations for both sets of reactions peaked at 2 hours before dropping to a small value after 5 hours. Cu2 O and CuO were obtained. The above reactions in PTFE-lined autoclaves led to inconsistent proportions of crystalline phases. Furthermore, different solid products were found adhering to different surfaces (walls versus bottom) of the liners. The conspicuous difference in roughness of these two surfaces and the difficulty of fully cleaning the liners between reactions precluded a systematic investigation of Cu4 O3 synthesis using this technique. To reduce reaction variability we performed syntheses in fused silica tubes as described in the Experimental section. Variations in products were minimized and improved reproducibility was seen over PTFE-contained reactions because a new fused-silica tube was used

using a Quantum Design MPMS3 operating in DC mode with a sample mass of 12.2 mg in a standard snap-shut powder capsule in a brass trough holder.

Results and Discussion Variability of reactions in PTFE versus fused silica We initially attempted the synthesis described by Zhao et al. 44 and reacted 50 mM and 70 mM Cu(NO3 )2 ·3H2 O in 1:2 volume ratio DMF:ethanol at 130 ◦ C in PTFE-lined autoclaves. The liquid phase changed color and various amounts of Cu4 O3 , Cu2 (NO3 )(OH)3 ,


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for every reaction.

creased quickly with time, commensurate with precipitation of >95% of the total solvated copper after 5 hours. ICP-OES and EPR show that Cu+ did not exist at the start of the reaction, peaked in concentration at 2 hours, then decreased to a small, steady value after 5 hours. Maximum Cu+ concentrations for both precursor conditions were similar: 12.6% for 70 mM and 12.5% for 50 mM. In the final steady state (>7 h), the system reached ca. 1.0% Cu+ concentration and 24 h, spherical particles varied in size, coarsening, and location.


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side of silica wall, opposite to the concave up silica wall where the plates and other spheres were found. These films persisted after long reaction times (not explicitly shown in Figure 6 beyond 7 h). The 50 mM precursor series, despite having different phase composition, experienced similar morphology development as seen in SEM. Cu2 (NO3 )(OH)3 plate aggregates appeared first, followed by spheres attached to the Cu2 (NO3 )(OH)3 surface (Figure 6(G,H)). Self-assembled films of spheres also appeared (Figure 6(I)), and after the full consumption of Cu2 (NO3 )(OH)3 , some spheres’ surfaces became corrugated (Figure 6(J)). Micro-Raman spectroscopy permits phase assignment of individual particles in these samples because the Raman peaks for the 4 solid phases are well-separated. The four Raman spectra in Figure 7(A-D) are selected single-phase micro-Raman scans from this study: Figure 7(A) from 70 mM precursor 1 h, Figure 7(B,C) from 70 mM precursor 9 h, and Figure 7(D) from 50 mM precursor - 2 h. Even though our micro-Raman spectra were collected from micron-sized single particles, the results are consistent with Debbichi et al.’s report. 54 Micro-Raman spectroscopy also verified that the particle color viewed by VLM can provide phase information (Figure S7(A,B) Supporting Information): emerald blocks are Cu2 (NO3 )(OH)3 , red particles are Cu2 O, and shiny black spheres are either Cu4 O3 or Cu2 O. Micro-Raman spectroscopy confirmed the nucleation of Cu2 O spheres on Cu2 (NO3 )(OH)3 plates as early as 3 h in a 70 mM precursor reaction (Figure 7(E)). As in SEM, spherical particles (ca. 1 µm) adhering to large aggregated plates were seen in VLM. Micro-Raman spectra showed that the emerald body is indeed Cu2 (NO3 )(OH)3 , and when the laser probe is focused around the red spheres, the 219 cm-1 peak (unique to Cu2 O) appeared (Figure 7(E)right). Such phase identification is consistent with our XRD analysis (Figure 4(C)). The spheres attached to Cu2 (NO3 )(OH)3 plates were identified as a mixture of Cu2 O and Cu4 O3 when the reaction was extended

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to 4 h (with 70 mM precursor) (Figure 7(F)). VLM observations showed that the spheres at this stage had two colors: red and black (Figure 7(F)-left). As in Figure 7(E), the red spheres were confirmed to be Cu2 O. MicroRaman spectra of the black spheres revealed Cu4 O3 (Figure 7(F)-right). The phenomenon of black Cu4 O3 spheres being much larger than red Cu2 O spheres in Figure 7(F) is universal across our samples. Before the full consumption of Cu2 (NO3 )(OH)3 , the microstructure of Cu2 O and Cu4 O3 attached to Cu2 (NO3 )(OH)3 plates did not change much as the 70 mM precursor reactions progressed. Over time, Cu2 (NO3 )(OH)3 plates shrunk and the oxide spheres grew larger. Cu4 O3 spheres remained larger in size than Cu2 O. The self-assembled films composed of spheres (seen after 7 h reaction) were red in VLM and were found to be Cu2 O from micro-Raman spectroscopy (Figure S7(C), Supporting Information). After the depletion of Cu2 (NO3 )(OH)3 , days of reaction resulted in the surfaces of Cu4 O3 spheres being covered by a layer of Cu2 O (Figure S7(D) Supporting Information), giving rise to their corrugated appearance seen by SEM (Figure 6(F)). Reactions with 50 mM precursor displayed similar morphology development, but with all three binary copper oxides (Cu4 O3 , CuO, and Cu2 O) attaching to the Cu2 (NO3 )(OH)3 surface (Figure 7(G,H)). At 2 h reaction time, black spherical particles were spotted on the emerald Cu2 (NO3 )(OH)3 plate aggregates (Figure 7(G)-left). Their micro-Raman spectrum showed CuO (Figure 7(G)-right). At 3 h of reaction, the spheres attached to the surface were either black or red. Micro-Raman spectra showed that the black spheres are CuO and Cu4 O3 . The Raman spectrum of one Cu4 O3 sphere is displayed in Figure 7(H)-right. The red spheres were identified to be universally Cu2 O (Figure 7(H)-right). By using microRaman spectroscopy to distinguish CuO and Cu4 O3 , we found that CuO and Cu2 O had comparable sphere sizes, while Cu4 O3 spheres were mostly larger in diameter. Further reactions of 50 mM precursor series showed similar events as in the 70 mM case: formation of


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Figure 7: Micro-Raman spectra of (A) Cu2 (NO3 )(OH)3 , (B) Cu2 O, (C)Cu4 O3 , and (D) CuO. Visible light microscopy images and the corresponding micro-Raman spectra of (E) 70 mM precursor 3 h, (F) 70 mM precursor - 4 h, (G) 50 mM precursor - 2 h, and (H) 50 mM precursor - 3 h samples. Micro-Raman analysis shows that particles of all three binary copper oxides can be found on Cu2 (NO3 )(OH)3 plates, and that Cu4 O3 spheres tend to be larger than Cu2 O or CuO.


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a self-assembled CuO film and the corrugation of Cu4 O3 spheres by Cu2 O coverage. The ability of all three binary copper oxides to nucleate on Cu2 (NO3 )(OH)3 plate surfaces by adjusting reaction conditions is proven by our micro-Raman analysis. It was also observed that the size of Cu4 O3 spheres was significantly larger than those of Cu2 O and CuO.

Antiferromagnetic Cu4 O3

signature

of Figure 8: Magnetic susceptibility of Cu4 O3 and diamagnetic Cu2 O obtained from a reaction with 70 mM precursor concentration and 48 h reaction time. Measured in field H = 104 Oe. The arrow corresponds to the reported N´eel temperature TN = 42 K reported for natural crystals, which coincides with our observed antiferromagnetic ordering for Cu4 O3 .

Controlled synthesis of bulk Cu4 O3 is a mandatory goal before the magnetic properties of this system can be studied. We present the first verification that bulk synthetic paramelaconite displays the same N´eel temperature as natural crystals, despite the limitation to micronsized agglomerates currently imposed by the solvothermal synthesis route. Figure 8 shows the magnetic susceptibility of a sample generated from a 70 mM Cu(NO3 )2 ·3H2 O precursor solution reacted for 48 h, which consists of Cu4 O3 and diamagnetic Cu2 O. The drop in susceptibility at 42 K agrees with the value reported for natural crystals. 32 We expect this transition to sharpen as crystallinity and yield improve.

Progression of Cu valence Tracking the total copper valence in these reactions provides insight on how the reaction can be further optimized. We have seen that Cu(NO3 )2 ·3H2 O, when dissolved in ethanol alone, precipitates as Cu2 (NO3 )(OH)3 at elevated temperatures (equation (1)). The copper nitrate precursor concentration had little impact on this behavior (50 or 70 mM).

8 Cu(NO3 )2 · 3 H2 O −−→ 4Cu2 (NO3 )(OH)3 +12 NO2 (g)+3 O2 (g)+18 H2 O (1) Including DMF in the solvent has two effects. First, it encourages precipitation of dissolved copper ions into binary copper ox-

ides. Second, the presence of DMF leads to reduction of Cu2+ . Chang et al. demonstrated the first effect by showing that reaction with DMF can convert Cu(NO3 )2 ·3H2 O directly into CuO, without the involvement of Cu2 (NO3 )(OH)3 . 55 The second effect is most apparent from our results. With the inclusion of 33 vol% DMF in the solvent, Cu+ became the dominant ion after 5 h (Figure 2(C,D)) and Cu+ -containing Cu4 O3 and Cu2 O formed (Figure 4(C,D)). Therefore, a reducing agent must be present in the solution. This can either be DMF itself, or a chemical species generated by the reaction of DMF and ethanol. Chang et al. proposed that the hydration of DMF at similar reaction temperatures produced dimethylamine and formic acid (equation (2)). 55 Dimethylamine was proposed to be responsible for precipitating solvated Cu2+ as CuO (equation (3)), whereas formic acid is credited to be the reducing agent (equation (4)). 44,55,56 Identifying the reacting species in situ will require further concerted effort. Gas chromatography mass spectroscopy has been used by Kaye et al. to examine the postreaction solution of transition metal nitrate in N-alkylformamide. 57 Large molecules (e.g.


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1-alkylpyrrolidine-2,5-dione and alkyl urea) were detected but the mass range of smaller molecues (such as formic acid) has not been examined. Within the sensitivity limit our NMR and FTIR, we were unable to detect the presence of any liquid species other than the starting solvent in this study (Figure S1 and S2, Supporting Information). Nevertheless, the importance of DMF in binary oxide formation and Cu valence reduction in this system is apparent. HCON(CH3 )2 + H2 O −−→ HCOOH + NH(CH3 )2 (2) Cu2+ +H2 O+2 NH(CH3 )2 −−→ CuO(s)+2 NH2 (CH3 )2+ (3) 2+ + + 2 Cu +HCOOH −−→ 2 Cu +CO2 (g)+2 H (4) If the reducing power of the system determines the final balance of copper oxides, then Cu4 O3 would require an intermediate power between that of CuO and Cu2 O. The average Cu valance in all four solid phases during reactions are displayed in Figure 9(A). It is evident the that copper valence was continuously reduced with reaction time. The reducing agents in solution must remain active for at least 24 h. Figure 9(A) also demonstrates that the reducing power was weaker with 50 mM Cu(NO3 )2 ·3H2 O precursor than with 70 mM. Further decrease of the Cu(NO3 )2 ·3H2 O presursor concentration to 30 mM resulted in inefficient reducing power, as evidenced by greater CuO production (Figure S8, Supporting Information). Although the difference in average Cu valence may seem small in Figure 9(A), the effect is magnified in the distribution of oxide phases (Figure 9(B,C)). For 50 mM precursor reactions, the oxide Cu valence is centered around 1.5+, which seems ideal given our target of Cu4 O3 , but the distribution is wide, limiting the purity of Cu4 O3 . The 70 mM products are skewed towards Cu+ in Cu2 O (Figure 9(B)). Controlling more degrees of freedom (e.g. solvent composition, reaction temperature) will be necessary to obtain pure Cu4 O3 .

Figure 9: Panel (A) shows the change in overall Cu valence in the solid precipitates with time for 70 mM and 50 mM precursor reactions. The distribution of copper atoms between phases with average valence of 1 (Cu2 O), 1.5 (Cu4 O3 ) and 2 (CuO) are shown for (B) 70 mM and (C) 50 mM precursor conditions. The overall Cu valence in the solid decreases even after the liquid has stabilized. The 50 mM reactions generated products with a more symmetrical distribution of Cu about Cu4 O3 .

Proposed reaction mechanism Based on our extensive characterization and earlier published studies of related systems, we propose a reaction scheme leading to the production of variACS Paragon Plus Environment

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Laufhutte and Dr. Ratna Dutta in the Microanalysis Laboratory at the University of Illinois at UrbanaChampaign for their assistance in ICP-OES measurement, and the Department of Materials Science and Engineering for financial support. Characterization was carried out in part in the Frederick Seitz Materials Research Laboratory Central Facilities, University of Illinois.

the precipitation of Cu2 (NO3 )(OH)3 from copper nitrate in the presence of ethanol, the in situ reductive transformation from Cu2 (NO3 )(OH)3 to Cu4 O3 triggered by DMF, and a suitable reduction potential in the reaction environment. Competing processes include the direct oxide formation from solvated Cu ions, and the re-dissolution of Cu2 (NO3 )(OH)3 . It would be desirable to suppress these competitive oxide formation pathways to improve Cu4 O3 yield. With continued exploration of the reactions mechanisms described here, this solvothermal system has the potential for highpurity Cu4 O3 production.

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Conclusion In this study, extensive characterization was performed to understand the reaction mechanism of Cu4 O3 formation from a Cu(NO3 )2 ·3H2 O solvothermal reaction in ethanol and DMF. By adapting solvothermal reactions in sealed fused silica tubes, experimental variation was significantly reduced and an overview on the evolution of copper-containing species was obtained. Combining quantitative data with microscopic phase identification, the progression of the reactions was examined in detail. Copper oxide formation resulted from the mineralization effects of both ethanol and DMF. We found that CuO and Cu2 O precipitated from dissolved copper ions, while Cu4 O3 was likely formed by in situ transformation of the Cu2 (NO3 )(OH)3 predecessor. Key elements of stabilizing metastable Cu4 O3 crystals were identified to be the generation of Cu2 (NO3 )(OH)3 and a suitable reduction potential for transformation into Cu4 O3 .

(3) Yoon, K. H.; Choi, W. J.; Kang, D. H. Photoelectrochemical Properties of Copper Oxide Thin Films Coated on an N-Si Substrate. Thin Solid Films 2000, 372, 250–256. (4) Akimoto, K.; Ishizuka, S.; Yanagita, M.; Nawa, Y.; Paul, G. K.; Sakurai, T. Thin Film Deposition of Cu2 O and Application for Solar Cells. Solar Energy 2006, 80, 715–722. (5) Liu, Y.; Turley, H. K.; Tumbleston, J. R.; Samulski, E. T.; Lopez, R. Minority Carrier Transport Length of Electrodeposited Cu2 O in ZnO/Cu2 O Heterojunction Solar Cells. Appl. Phys. Lett. 2011, 98, 162105. (6) Wang, M.; Xie, F.; Xie, W.; Zheng, S.; Ke, N.; Chen, J.; Zhao, N.; Xu, J. B. Device Lifetime Improvement of Polymer-Based Bulk Heterojunction Solar Cells by Incorporating Copper Oxide Layer at Al Cathode. Appl. Phys. Lett. 2011, 98, 183304.

Supporting Information

(7) Meyer, B. K. et al. Binary Copper Oxide Semiconductors: From Materials Towards Devices. Phys. Status Solidi B 2012, 249, 1487– 1509.

1 H NMR peak intensity ver-

FTIR and NMR spectra, sus time, 50 mM UV/Vis spectra, CuCl UV/Vis spectrum, 50 mM XRD time series, additional 70 mM micro-Raman/VLM, detailed 48 h Rietveld refinements.

(8) Snoke, D. Spontaneous Bose Coherence of Excitons and Polaritons. Science 2002, 298, 1368–1372.

Acknowledgment

(9) Lin, J. L.; Wolfe, J. P. Bose-Einstein Condensation of Paraexcitons in Stressed Cu2 O. Phys. Rev. Lett. 1993, 71, 1222–1225.

We thank Prof. Yi Lu, Prof. Paul Braun, and Dr. Lingyang Zhu for helpful discussions, Dr. Rudiger


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