Morphology Controlled Solution-Based Synthesis of Cu2O Crystals for

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Morphology Controlled Solution-based Synthesis of CuO Crystals for the Facets-Dependent Catalytic Reduction of Highly Toxic Aqueous Cr(VI) ABHAYA KUMAR MISHRA, and Debabrata Pradhan Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00186 • Publication Date (Web): 17 Jun 2016 Downloaded from http://pubs.acs.org on June 18, 2016

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Crystal Growth & Design

Morphology Controlled Solution-based Synthesis of Cu2O Crystals for the Facets-Dependent Catalytic Reduction of Highly Toxic Aqueous Cr(VI) Abhaya Kumar Mishra and Debabrata Pradhan* Materials Science Centre, Indian Institute of Technology, Kharagpur, W. B. 721 302, India

ABSTRACT In this study, we demonstrate the systematic shape evolution of Cu2O crystals from the octahedron, through truncated octahedron, cube, and finally to truncated cube by varying the reaction temperature with an optimum precursor concentration of 25 mM Cu(NO3)2⋅3H2O and 1 g polyvinylpyrrolidone (PVP) as the shape controlling reagent. The average size of these crystals increased with temperature from ~70 nm (at 40°C) to ~1 µm (at 100°C). With a much lower (6 mM) and higher (250 mM) precursor concentration, nanoparticles and polyhedron-shaped crystals are respectively formed in the studied temperature region (40−120°C). The role of precursor concentration, PVP quantity, reaction medium, and reaction temperature in the formation of diverse Cu2O crystals morphologies are demonstrated and discussed. Furthermore, the catalytic activity of the as-synthesized Cu2O crystals is tested for the reduction of Cr(VI) at room temperature. The toxic Cr(VI) is found to be rapidly reduced to nontoxic Cr(III) in a short span of 4 minutes in presence of Cu2O cubes in the acidic medium. The repeat catalytic measurements of Cr(VI) reduction for 20 cycles confirm higher stability of cube-shaped Cu2O crystals with {100} exposed facets as compared to octahedrons with {111} exposed facets, a classic example of facets-dependent catalytic properties of crystals. Key words: Cr(VI) reduction, catalysis, cubes, octahedrons, shape control *

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1. INTRODUCTION Controlled growth of inorganic polyhedral nano/microcrystals has been one of the forefront research topics in the field of nanoscience.1,2,3 The shape and size-dependent physical and chemical properties of inorganic materials are well established, prompting an intense research to synthesize novel nano- and micro-structured materials with controlled morphology.4,5,6 The exposed facets of ionic solids such as metal oxides with a cubic crystal structure can be controlled by varying its shape.7 The shape of the nano/microstructure is normally tuned by using organic or inorganic additives that can preferentially adsorb on the specific crystal planes.8,9 The systematic shape evolution with polyhedral morphologies has been achieved with several semiconductors including Cu2O, TiO2, PbS, and PbSe.10,11,12,13 In particular, Cu2O is an ideal material for the exposed facet-dependent property investigation because of its non-toxicity, abundance, and wide applicability. Cu2O shows a variety of interesting properties that has been exploited in several fields such as facets-dependent electrochemical property,14 photocatalytic water splitting,15 oxygen reduction reaction,16 photocatalyst,17 and as gas sensor.18 However, in the aqueous solution, Cu2O is unstable and liable towards phase change and formation of CuO.19,20 Thus controlled fabrication of Cu2O without any impurities such as Cu and/or CuO is an challenging issue.21 In the present work, we report the synthesis of phase pure Cu2O crystals in aqueous medium using a simple solution chemistry route. Various approaches such as hydrothermal,22 microemulsion,23 polyol,24 microwave,25 electrodeposition,26,27 and metallic copper oxidation28 techniques have been employed to synthesize Cu2O crystals. Zhao et al. hydrothermally synthesized Cu2O microcrystals with horn/cube-shape using formic acid as reducing agent in an ethanol-water mixed solvent at 150°C for 2h.22 Pang et al. reported glycine-assisted, mixed-solvothermal approach for the synthesis of Cu2O crystals with different morphologies.29 The morphology and thereby exposed facets of crystals play an important role in the catalysis. The Cu2O octahedra with {111} exposed facets is reported as the best catalyst for the synthesis of 1,2,3-triazoles among different shaped Cu2O crystals.30 On the other hand, rhombic dodecahedra bound by {110} facets reported to show superior catalytic performance for the photodegradation of methyl orange.17 However, Cu2O cubes with {100} exposed facets are reported to be inactive toward photodegradation of methyl orange.31,32 In the present study, we demonstrate not only the higher catalytic activity of cubes with {100} exposed facets for the reduction of Cr(VI) but also their high cyclic stability. There 2 ACS Paragon Plus Environment

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are only a few reports on the Cr(VI) reduction using Cu2O that involves visible light irradiation.33,34 On the contrary, here we demonstrate the catalytic Cr(IV) reduction at a much faster rate using Cu2O crystals in the absence of light (under dark). Chromium exists almost exclusively in two oxidation states such as hexavalent (VI) and trivalent (III). Cr(VI) is one of the highly toxic, carcinogenic, and mutagenic component, which can exists in a wide range of pH and normally found in the industrial (metal finishing, electroplating, leather tanning, dyeing, and textiles) contaminated water.35 Depending on the pH, Cr(VI) can be found in water as chromate (CrO42−), dichromate (Cr2O72−), hydrogen chromate (HCrO4−), and dihydrogen chromate (H2CrO4).36 Thus the removal or degradation/reduction of Cr(VI) from the contaminated water is highly essential.37 On the other hand, Cr(III) is a nontoxic, less mobile, relatively inert, and considered as human nutrient.38 Various methods have been employed to remove toxic Cr(VI) ions from aqueous solution. These include biological processes, chemical precipitation, ion exchange, reverse osmosis, membrane processes, evaporation, electrochemical precipitation, reduction, solvent extraction, and adsorption.39,40,41 Among these, catalytic reduction has been recognized as an effective, green, and costcompetitive technique for the elimination of reducible pollutants in water.42 Recently Sarkar et al. reported the use of copper(I) metallogel for the reductive removal of Cr(VI) from aqueous solution.43 In the present study, we report a facile solution-phase chemistry route for the synthesis of phase-pure Cu2O crystals with diverse morphologies by varying the reaction parameters such as reaction temperature, precursor concentration, polyvinylpyrrolidone (PVP) quantity, and reaction medium. The reaction temperature is found to play prime role in controlling the growth of crystals with different shapes and thus the exposed facets. This prompted us to examine the facets-dependent catalytic property of as-synthesized Cu2O crystals through the reduction of Cr(VI) into Cr(III). The important contributions of the present work are quick (4 min) reduction of Cr(VI) to Cr(III) and high cyclic stability of Cu2O cubes for the Cr(VI) reduction. 2. EXPERIMENTAL 2.1 Chemicals. Copper nitrate trihydrate [Cu(NO3)2⋅3H2O], dimethylformamide (DMF), sodium borohydrate

(NaBH4),

potassium

dichromate

(K2Cr2O7),

sulphuric

acid

(H2SO4),

ethylenediaminetetraacetic acid (EDTA), acetic acid (AA), citric acid (CA) from Merck, India, 3 ACS Paragon Plus Environment


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and PVP from Loba Chemical, India. All the above reagents were analytical grade and used without further purification. 2.2 Synthesis of Cu2O crystals. Typically, 241 mg of copper nitrate trihydrate (25 mM) was dissolved in 40 mL mixed solvent of DMF and H2O with a volume ratio of 1:1. Then 1 g of PVP was added to the above solution and stirred vigorously for 10 min, followed by addition of 40 mg sodium borohydride (25 mM) which act as reducing agent. The resultant black solution with further stirring at room temperature for 1 h produced a yellow color precipitate. The precipitate was collected by centrifuging and washing with distilled water and ethanol several times to remove unreacted chemicals. The powder product was finally dried at 60°C for 4 h in an air oven. To find the role of different experimental parameters on the formation of Cu2O crystals, precursor concentration, reaction temperature, reaction medium, and PVP quantity were varied. Experiments were first performed at 40 and 100°C on a hot plate by varying the concentration of precursor (i.e. copper nitrate trihydrate) in the range of 6 mM to 250 mM with PVP quantity (1.0 g) and reaction medium (DMF to H2O ratio 1:1) fixed. With an optimized precursor concentration of 25 mM copper nitrate and 1.0 g PVP, reaction was performed at 40, 80, 100, 120, and 140°C. Finally PVP quantity was varied in the range of 0.1 to 2.0 g to find its role in the formation of Cu2O crystals. Experiments were also performed with either water or DMF or mixed solvent. While no product was obtained with only DMF, the product obtained with only water was non-uniform in shape [Supporting Information (SI), Figure S1]. Thus, the solvent ratio i.e. DMF to water ratio was kept constant at 1:1. 2.3 Characterization. The surface morphology of the as-prepared Cu2O nano/micro crystals was examined using a Zeiss SUPRA 40 field-emission scanning electron microscope (FESEM). The structural properties of the as-synthesized samples were studied with a PANalytical high resolution x-ray diffractometer (HRXRD) (PW 3040/60) operated at 40 kV and 30 mA, using Co Kα x-rays of wavelength 1.79 Å. The microstructure of the as-synthesized crystals was studied with a FEI TECNAI G2 Transmission electron microscope (TEM) operated at 200 kV. The x-ray photoelectron spectroscopy (XPS) measurement was performed by a PHI 5000 VersaProbe II Scanning XPS Microprobe with a monochromatic Al Kα source (1486.6 eV). The N2 adsorptiondesorption isotherm was recorded at 77 K using Autosorb-iQ (Quantachrome Instruments). 2.4 Catalytic Study. The catalytic activity of as-prepared Cu2O crystals was studied by the reduction of Cr(VI) in the presence of an acid (H2SO4, EDTA, acetic acid, or citric acid) at pH 4. 4 ACS Paragon Plus Environment

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The pH value of 4 was optimized from the results obtained by performing catalytic Cr(VI) reduction at different pH. Typically 20 mg of Cu2O powder was suspended in 100 mL aqueous solution of potassium dichromate (5 × 10−5 mol/L) and sonicated for different durations. The reduction of Cr(VI) as a function of time was measured using UV-vis absorption spectroscopy (Perkin Elmer, Lambda 750). 3. RESULTS AND DISCUSSION 3.1 Morphology of Cu2O nanostructures. Effect of precursor concentration. To study the effect of precursor concentration on the morphology of Cu2O crystals formation, the concentrations of Cu(NO3)2⋅3H2O was first varied in the range of 6 to 250 mM. All the reactions were performed at 40 and 100°C with 1 g PVP. Figure 1 shows the FESEM images of Cu2O product obtained at 40 and 100°C with different molar concentrations of the precursor. At a very low concentration (6 mM) of copper nitrate, nanoparticles are obtained as shown in the Figure 1a and 1b. The average size of these nanoparticles is found to be increased with temperature from 70 nm at 40°C (Figure 1a) to 187 nm at 100°C (Figure 1b) for 60 min. With increasing the precursor concentration to 25 mM, Cu2O octahedrons (average edge length ~318 nm, Figure 1c) and cubes (~660 nm, Figure 1d) are obtained at 40°C (for 60 min) and 100°C (for 30 min), respectively. These octahedrons and cubes, obtained at 25 mM Cu(NO3)2⋅3H2O, are found to be quite uniform in shape and size with {111} and {100} exposed facets, respectively. With 50 mM Cu(NO3)2⋅3H2O at 40°C, octahedrons are obtained (Figure 1e) albeit of larger size (>1 µm) than that obtained at 25 mM. However, polyhedrons along with a few octahedrons are obtained at 100°C as shown in Figure 1f. With further increasing precursor concentration to 75 mM, micron sized truncated octahedrons and polyhedrons are obtained both at 40°C (Figure 1g) and 100°C (Figure 1h). At a much larger concentration of 250 mM, polyhedron shaped Cu2O crystals are also obtained (Figure S2, SI). It is important to be noted that the sizes of Cu2O crystals are increased as a function of precursor concentration, reaction temperature and/or duration of reaction for all the cases. The detail reaction parameters and the product morphology are presented in Table 1. It is found that 25 mM Cu(NO3)2⋅3H2O produced distinctly different but uniform morphology i.e. octahedrons and cubes at 40°C and 100°C, respectively. Thus 25 mM Cu(NO3)2⋅3H2O is considered optimized precursor concentration for further study. 5 ACS Paragon Plus Environment


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Figure 1. FESEM images of Cu2O crystals obtained by varying Cu(NO3)2⋅3H2O molar concentration and reaction temperature (a,b) 6 mM, (c,d) 25 mM, (e,f) 50 mM, (g,h) 75 mM, and reaction temperature of (a,c,e,g) 40 °C and (b,d,f,h) 100 °C. The PVP content was kept constant at 1 g in all these experiments.


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Table 1. Effect of various synthesis parameters on the shape and size of Cu2O crystals Cu(NO3)2⋅3H2O conc. (mM)

Temp. (°°C) and Duration (min)

PVP (g)

25

40, 60 100, 60 40, 60 100, 30 40, 60 100, 60 40, 60 100, 60 40, 60 100, 60 40, 60

25

60, 60

1

25 25 25 25

80, 30 100, 30 120, 30 140, 30 40, 60

1 1 1 1

6 25 50 75 250

25 25 25 25 25

100, 30 40, 60 100, 30 40, 60 100, 30 40, 60 100, 30 40, 60 100, 30

1 1 1 1 1 1

0 0.1 0.5 1.0 2.0

Shape

Average size (nm)

Spherical Spherical Octahedron Cube Octahedron Polyhedron Truncated octahedron Polyhedron Polyhedron Polyhedron Octahedron Edge and corner truncated octahedron Cube Cube Cube Truncated Cube Spherical Spherical particle and distorted cube Truncated Octahedron Cube Octahedron Cube Octahedron Cube Polyhedron with distorted surface Cube

70 187 318 663 1361 2306 1545 2214 1856 3468 318 730 230, 696 663 757 1264 214 322 231 530 500 861 318 663 396 590

Effect of synthesis temperature. Keeping the Cu(NO3)2⋅3H2O concentration fixed at 25 mM, the reaction temperature was varied at 20°C difference i.e. at 40, 60, 80, 100, 120, and 140°C. Figure 1c shows the octahedron morphology obtained at 40°C. With increasing the synthesis temperature to 60°C, truncated octahedrons (of average size ~730 nm) are obtained as shown in Figure 2a. This shape of crystal indicates the formation of {100} facets at six vertices 7 ACS Paragon Plus Environment


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of the octahedron which is due to a higher growth rate along [111]. These octahedrons are evolved to cubes at 80°C with slight truncation at the corners (shown later in TEM) with two different average sizes (230 nm and 696 nm) as shown in the Figure 2b. This suggests the completion of growth along [111] with the formation cubes with majorly {100} exposed facets. With further increasing the reaction temperature to 100°C, highly uniform perfect cubes of average size 663 nm across diagonal are obtained as shown in Figure 1d. The cube shape remains with increasing the reaction temperature to 120°C though average size slightly increased to ~757 nm across the diagonal (Figure 2c). However, at 140°C, corner truncated cubes with concave {100} facets are obtained as shown in Figure 2d. This suggests the change in the growth rate along different planes at different temperatures. In particular, initial growth along {111} facets is faster up to 120°C forming cubes and then growth on {100} facets predominates forming corner truncated cubes at 140°C. Previous efforts on the shape evolution of Cu2O crystal involve varying the volume of reducing agent. In particular, Huang et al. controlled the morphology of Cu2O nanocrystals from cubic to rhombic dodecahedral by simply adjusting the amounts of NH2OH⋅HCl.17 Recently, Tsai et al. controlled the shape of Cu2O nanocrystals by varying the hydrazine volume.30 The change in shape and size with increasing the precursor concentration and growth temperature is also presented schematically in Figure 3a and 3b, respectively. As shown in Figure 3, the size of the Cu2O crystals is found to be increased with precursor concentration and reaction temperature.


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Figure 2. FESEM images of Cu2O crystals obtained at (a) 60 °C, (b) 80 °C, (c) 120 °C, and (d) 140 °C with fixed 25 mM concentration of Cu(NO3)2⋅3H2O and 1 g PVP.

Figure 3. Change in the size and shape of Cu2O crystals by varying the (a) precursor concentration at 40 and 100°C and (b) growth temperature.

The microstructural investigation of different shaped Cu2O crystals obtained by varying the reaction temperature was carried out by the TEM. Figure 4 displays the TEM images, selected-area electron diffraction (SAED) patterns, and high-resolution TEM images of 9 ACS Paragon Plus Environment


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octahedrons, truncated octahedrons, and cube-shaped Cu2O crystals. Figure 4a and 4b show the TEM images at different magnifications of octahedrons obtained at 40°C using 25 mM Cu(NO3)2⋅3H2O and 1 g PVP. The size of octahedrons (300−350 nm) in the TEM analysis is found to match the FESEM image (Figure 1c). Figure 4c shows the SAED pattern taken in the region marked as circle in Figure 4b. The regular spot SAED pattern ascertains the perfectly single crystalline nature of octahedrons.30 A high-resolution TEM image (Figure 4d) further confirms the crystalline nature of octahedron with a lattice spacing of 0.25 nm corresponding to (111) plane of cubic Cu2O. Figure 4e and 4f show the TEM images of truncated octahedrons and cubes with slight corner truncated obtained at 60°C and 80°C, respectively, with rest of the synthesis parameters fixed. The TEM images of truncated octahedrons and corner truncated cubes as obtained at 60°C and 80°C are closely matched to the FESEM images shown in Figure 2a and 2b, respectively. At 100°C, uniform cubes (Figure 1d) are obtained as shown in Figure 4g with sharp edges and corners. Figure 4h shows a square SAED pattern of cube along [100] suggesting their single crystalline nature and growth orientation.31

Figure 4. (a,b) TEM images and corresponding (c) SAED and (d) high-resolution TEM image of Cu2O octahedrons synthesized at 40°C. TEM images of (e) truncated octahedrons and (f) cubes with corner truncated obtained at 60°C and 80°C, respectively. (g) TEM image and corresponding (h) SAED pattern of Cu2O cubes obtained at 100°C.


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Effect of PVP quantity. To find the exact role of PVP in the formation of Cu2O crystals, synthesis was performed in the absence and presence of varying PVP quantity while keeping the rest of the reaction parameters fixed. Figure 5a shows an FESEM image of agglomerated nanoparticles obtained in the absence of PVP at 40°C. The size of these individual particles is measured to be ~214 nm. At 100°C, similar particles and distorted cuboids are obtained as shown in Figure 5b. The size of the particles and cuboids obtained at 100°C is found to be larger than that obtained at 40°C. With 0.1 g of PVP, edge- and corner-truncated octahedrons (size ~232 nm) with eight {111} planes, twelve {110} planes, and six {100} vertices are obtained at 40°C as illustrated in Figure 5c. On the other hand, cubes (Figure 5d) are obtained at 100°C with 0.1 g PVP. An increased PVP quantity of 0.5 g leads to formation of uniform truncated octahedrons of average size ~500 nm at 40°C. The area of {100} vertices of these truncated octahedrons are found to be decreased to a large extent with increase in PVP in the reaction medium. At 100 °C, with 0.5 g PVP, cubes are obtained (Figure 5f) and several of these cubes are appeared penetrated to each other. The formation of uniform Cu2O octahedrons and cubes with 1.0 g PVP has been discussed in the previous section referring to Figure 1c and 1d, respectively. However on further increasing the PVP quantity to 2.0 g, polyhedrons with a rough surface are obtained at 40°C (Figure 5g). Moreover, at 100°C with 2.0 g PVP, cubes are also obtained with some of the cubes having concave faces on the top at shown in Figure 5h. Zhang et al. systematically studied the morphology evolution of Cu2O crystal with the variation of [PVP]/[Cu2+] ratio.44 They reported formation of thermodynamically more stable cubic shaped crystal of size 900 nm in the absence of PVP. With increasing [PVP]/[Cu2+] ratio in basic medium, crystals with different ratios of {111}/{100} exposed facets were reported. This was attributed to selective surface stabilization by PVP. Although the surface stabilization and shape controlling role of PVP is beyond doubt, here we demonstrate the role of synthesis temperature in controlling the shape of Cu2O crystals with fixed quantity of PVP at 1.0 g as discussed earlier. This suggests that the reaction medium and other reaction parameters are also important in controlling the shape of Cu2O crystals.



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Figure 5. SEM images of Cu2O crystals obtained with different PVP quantities (a,b) no PVP, (c,d) 0.1 g (e,f) 0.5 g, and (g,h) 2.0 g PVP at (a,c,e,g) 40°C and (b,d,f,h) 100°C.

Effect of reaction medium. The reaction medium is known to play an important role in the crystal growth. The results of the experiments presented above were performed with 1:1 volume ratio of DMF and water. This ratio was considered because of non-uniform shaped product obtained with only water (Figure S1, SI) and no product with only DMF as solvent. Thus, 12 ACS Paragon Plus Environment

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experiments were also carried out in the mixed solvent with DMF:water ratio of 1:3 and 3:1 keeping the rest of parameters fixed (25 mM precursor, 40 or 100°C, 1 g PVP, and 1 h). In addition to the solvent role, DMF also acts as ligand and structure-directing agent in the synthesis of inorganic nanomaterials. However, no product in only DMF could be due to its poor anion solvating ability.45 Interestingly, upon adding DMF to water (i.e. DMF:water = 1:3) the crystals were found to take shape and produced truncated octahedrons and truncated cubes at 40°C (Figure S3a, SI) and 100°C (Figure S3b, SI), respectively. The uniform octahedrons and cubes obtained with DMF:water = 1:1 is discussed earlier (Figure 1c,d). However, with higher DFM content (i.e. DMF:water = 3:1) the crystals are again found to be truncated octahedrons at 40°C (Figure S3c, SI) and cubes at 100°C (Figure S3d, SI). This suggests the effect of reaction medium in the growth of Cu2O crystals. Growth mechanism. For the synthesis, copper nitrate trihydrate was used as copper precursor, which was reduced by sodium borohydride forming Cu2O precipitate in a mixed solvent of DMF and water (1:1 v/v). The complete aqueous medium promotes uncontrolled growth and produced Cu2O crystals with non-uniform morphology and uneven surface (Figure S1, SI). On the other hand, reaction in DMF only produces no product. In the mixed solvent, DMF controls the growth either by suitable capping and/or forming complex with copper. In addition, PVP was used as shape-controlling agent to produce crystals of specific shapes and as stabilizer to prevent the aggregation of particles. After Cu2O nucleates as particles, crystal growth occurs at different rates along different planes. The crystal growth rate is primarily controlled by the precursor and additive concentration, reaction temperature, and surface free-energy of the crystals.46 In particular, growth rate increases with increasing the precursor concentration and reaction temperature as observed in the present work (Figure 3a and 3b). However, growth can be terminated on a selected plane by using suitable capping agent. In the present case, a lower precursor concentration (6 mM) produced particles of near-spherical shape. Similarly, reaction in the absence of surfactant (PVP) produced near-spherical particles (Figure 5a,b). The faceted Cu2O crystals are found to be formed with increasing the concentration of copper precursor to 25 mM in presence of PVP, suggesting the role of both. The formation of faceted crystals is mainly due to anisotropic growth along different directions, which is due to the surface free-energy of different facets. The surface energy of cubic phase materials varies as γ{111} < γ{100} < γ{110} as per



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Wulff’s construction.47 With an optimum copper precursor concentration of 25 mM, octahedral and cube-shaped Cu2O crystals are obtained at 40 and 100°C, respectively. With increasing the concentration to 50 mM or higher (reaching the supersaturation), polyhedrons are produced in accord with previous reports.46,48,49 This suggests that the growth along [001] is dominated at relatively lower concentration (lower supersaturation) and thus producing octahedrons at 40°C with low-energy {111} exposed facets.46 In addition, PVP is known to adsorbs on {111} facets more strongly than that on {100} and thus produced truncated octahedrons with 0.1 g (Figure 5c) and 0.5 g (Figure 5e) PVP at 40°C.44,50 With increasing the PVP quantity to 1.0 g, capping effect increases on {111} facets which facilitates growth on the {100} planes producing perfect octahedrons (Figure 1c). Further increasing the PVP quantity to 2.0 g, near spherical particles are obtained as shown in Figure 5g, which could be due to coverage of PVP on all the exposed planes. This trend of growth and role of PVP corroborate the previous report by Sui et al.46 Interestingly, growth at 100°C does not follow the same mechanism as described above for 40°C. The growth along {111} facets is found to be significantly enhanced producing cube shaped crystals at 100°C with different PVP concentration. Even in the absence of PVP, agglomerated particles with a few cube shaped particles are obtained at 100°C (Figure 5b). This clearly suggests that the reaction temperature disturbed the thermodynamic growth condition and facilitate the growth to occur on {111} facets at an optimum PVP quantity of 0.1−0.5 g and thus produced near-perfect cubes. Zhang et al. also reported similar shift of growth direction i.e. {100} at 25°C to {111} at 75°C for Cu2O.44 At a higher PVP concentration (2.0 g), several cubes are found with excess growth on {111} planes and thus showing concave top surface (Figure 5h). The change in crystal growth direction from {100} for octahedrons to {111} for cubes is further demonstrated by performing the growth at 60, 80, and 120°C (Figure 2). It is obvious that at 60°C, truncated octahedrons are obtained due to slower growth along {111} plane and thereby with partial {111} exposed facets. With increasing the temperature to 80−120°C, growth predominates on {111} planes producing near-perfect cubes. With further increasing the temperature to 140°C, slightly corner-truncated cubes are formed. This could be due to the shift of growth direction again to {100}. In particular, surface free-energy of crystal facet not only changed with the additives used in the reaction system but also temperature.50,51 This suggests


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that the initial copper precursor concentration, PVP quantity, and temperature play crucial role in determining the growth direction and thus producing Cu2O crystals with diverse shapes.

3.2 Crystal structure and chemical composition. The structural properties of the assynthesized Cu2O crystals were analyzed by powder XRD. Figure S4 (SI) shows the powder XRD patterns of Cu2O nanoparticles (Figure 1a, 1b and 5b) synthesized at low precursor concentration (6 mM) and in the absence of PVP. Figure 6 shows the XRD patterns of Cu2O crystals with different primary geometries such as octahedron, truncated octahedron, truncated cube, and cube. All the diffraction peaks in the XRD patterns in Figure S4 and Figure 6 are indexed to the cubic phase of Cu2O (a = 7.619 Å, c = 32.329 Å, JCPDS: 00-004-2787). No characteristic diffraction peaks from possible impurity such as metallic copper and/or cupric oxide (CuO) was observed. The diffraction peaks of Cu2O nanoparticles (Figure S4) are found to broader than the faceted crystals (Figure 6) as expected due to smaller crystallite size of former. The change in XRD intensity of faceted crystals (Figure 6) can be attributed to different growth rate along different crystal plane of Cu2O crystal. The facets with a slower growth rate are exposed to a greater extent and thus show stronger diffraction intensity in the XRD pattern.52 The octahedrons have predominately {111} exposed facets due to slower growth on the same plane and thus have much higher diffraction intensity than that of {200} planes (Figure 6a). The intensity ratio [{111}/{200}] of the as-synthesized crystals is found to decrease as shape of crystal evolved from octahedrons to cubes: octahedrons (3.04) > truncated octahedrons (2.66) > truncated cubes (1.36) > cubes (0.91) (Figure S5, SI). This is due to the development of {200} planes along the six vertices of octahedron until the formation of cube completes. The {111}/{200} intensity ratio for octahedrons obtained here is close to 3.05 reported by Sui et al.46 The change in XRD intensities of different planes with the evolution of different shape of Cu2O crystals in the present work match the previous reports with similar growth orientation.53,54,55


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(311) (222)

Cubes (220)

(111)

(110)

(d)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(200)


(c)

Truncated Cubes

(b)

Truncated Octahedrons

(a)

Octahedrons

20

40

60

80

2θ (degree) Figure 6. Powder XRD patterns of (a) octahedrons, (b) truncated octahedrons, (c) truncated cubes, and (d) cubes, obtained at 40, 60, 80, and 100°C, respectively.

Figure 7a(i) and 7a(ii) show the survey XPS spectra of Cu2O octahedrons and cubes obtained at 40°C and 100°C, respectively, with 25 mM Cu(NO3)2⋅3H2O. In addition to the major photoelectron peaks of copper and oxygen, peak at binding energy of ~285.0 eV and a minor peak at ~400 eV are assigned to C 1s and N 1s, respectively, contributed from the surface impurity and PVP used in the synthesis. Figure 7b and 7c show the Cu 2p and O 1s region XPS spectra of the as-synthesized samples (octahedrons and cubes), respectively. The Cu 2p XPS features for both the octahedrons and cubes are found to be similar with the peak at 932.6 eV assigned to Cu 2p3/2 of Cu2O matching the previous reports.56,57 The +1 oxidation states of copper oxide is further confirmed from the absence of satellite peak normally found at 942.0 eV for CuO and a smaller full width at half maximum of Cu 2p3/2 (1.28 eV for octahedrons and 1.3 eV for cubes) of Cu2O.58 The O 1s region XPS spectra show two peaks centered at 530.3 eV and 532.2 eV, assigned to oxide and surface hydroxide, respectively (Figure 7c). 16 ACS Paragon Plus Environment

Cu 3s Cu 3p

C 1s

N 1s

O 1s

Cu LMM

Cu 2p3/2

Cu 2p1/2

(a)

(i) Octahedrons (ii) Cubes

1000

800

600

400

200

0

Binding Energy (eV) (c) O 1s

Cu 2p3/2

(b) Cu 2p Cu 2p1/2

Counts per second

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Counts per second

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(i) Octahedrons

(i) Octahedrons

(ii) Cubes

970

960

(ii) Cubes

950

940

930 540

536

532

528

Binding Energy (eV)

Binding Energy (eV)

Figure 7. (a) Survey XPS spectra, (b) Cu 2p, and (c) O 1s region XPS spectra of Cu2O octahedrons and cubes

3.3 Optical property Figure 8a and 8b show the UV-vis absorption spectra and corresponding band gap plots of Cu2O octahedrons and cubes, respectively. The absorbance profiles appear similar with significant absorption in the visible region of the electromagnetic radiation ( truncated octahedrons (0.16 min−1) > octahedrons (0.13 min−1) > particles obtained at 40°C (0.07 min−1). This clearly suggests the role of {100} facets for higher Cr(VI) reduction activity. The reduction of Cr(VI) to Cr(III) is further analyzed by COD (chemical oxygen demand) analysis in separate experiments. For the COD analysis, Cu2O octahedrons and cubes were added to the conventional COD analysis solution containing potassium dichromate. It is found that COD value increases from 1151 mg/L (without Cu2O) to 2129 mg/L (with Cu2O octahedrons) and to 2186 mg/L (with Cu2O cubes). This confirms higher Cr(VI) reducing ability of Cu2O and it was higher for cube morphology than that of octahedrons. The previous reports on Cr(VI) reduction using Cu2O involves visible light which take much longer duration than that demonstrated here.33,34 Li et al. reported complete reduction of Cr(VI) in 2.5 h using visible light irradiation that has been ascribed to easy formation of electrons and holes on the (111) planes of Cu2O.33 However, Qin et al. found EDTA > CA > AA as displayed in Figure 9d. This concludes that the milder acids can also be used for the Cr(VI) reduction.



(a) Octahedrons

2.0 1.5

(b) Cubes

2.5

Duration 0 min 2 min 4 min 6 min 8 min

Absorbance (%)

Absorbance (%)

2.5

1.0 0.5 0.0

Duration 0 min 2 min 4 min

2.0 1.5

0 min 2 min 4 min

1.0 + NaOH

0.5 0.0

300

350

400

450

500

550

600

300

350

400

450

500

550

600

Wavelength (nm)

Wavelength (nm) 120

(c)

Cubes Octahedrons

100

(d)

1.0 0.8

80

C/C0

Efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.6 EDTA AA CA AA + Cu2O

0.4

60

0.2

40

CA + Cu2O EDTA + Cu2O

0.0

H2SO4 + Cu2O

20 0

5

10

15

-2

20

0

2

4

6

8

Time (min)

Cycle Number

Figure 9. UV-vis absorption spectra of aqueous potassium dichromate solution upon stirring with Cu2O (a) octahedrons and (b) cubes for different duration suggesting the reduction of Cr(VI) to Cr(III) with decrease in absorbance. Inset of (b) shows photographs of potassium dichromate solution with change in color in presence of Cu2O cubes for 0, 2, and 4 min. The transparent solution obtained in 4 min is due to complete reduction of Cr(VI) to Cr(III). This was confirmed by adding NaOH and obtaining cyan color solution due to formation of Cr(OH)3. (c) Cr(VI) reduction efficiency as a function of cycle number with same catalyst i.e. octahedrons and cubes. It is to be noted that octahedrons were eroded and dissolved in the potassium dichromate solution in 5 cycles. (d) Effect of different acids on Cr(VI) reduction efficiency in presence of respective acids and Cu2O cubes. 22 ACS Paragon Plus Environment

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Further insights on the stability of Cu2O octahedrons and cubes are obtained by studying the morphology, phase, and composition of crystals after catalytic reduction of Cr(VI) for the selected cycles in presence of H2SO4. Figure 10a and 10b show the FESEM images of Cu2O octahedrons at different magnifications after two cycles of Cr(VI) reduction. After two cycles of Cr(VI)

reduction,

the

surface

of

several

octahedrons

appears

rough

indicating

erosion/degradation. After four cycles, the shape of Cu2O octahedrons becomes almost spherical as shown in Figure 10c and 10d. The erosion and degradation is thereby ascertain the poor Cr(VI) reduction ability of Cu2O octahedrons (Figure 9c). Interestingly, the shape of Cu2O cubes remains intact even though surface appear rough after 20 cycles of Cr(VI) reduction as shown in Figure 10e and 10f. This clearly demonstrates the high stability of Cu2O cubes for the reduction of Cr(VI). The high catalytic activity and stability of Cu2O cubes can be attributed to its {100} exposed facets. Hua et al. reported that in an weak acid solution, the stability of different faces of Cu2O crystals follow the order of {100}>>{111},>{110}.65 This is due to the shorter bond length of Cu−O on the {100} facets (1.76 Å) than that of {111} facets (1.83 Å) of Cu2O crystal as optimized by the density functional theory calculation.65 The present experimental catalytic investigation in the acidic medium with Cu2O crystals with either {100} or {111} exposed facets and the high stability of cubes thus further corroborates the earlier theoretical study by Hua et al.65 Figure 11a shows the XRD pattern of Cu2O cubes after being used in the catalytic reduction for 80 min (20 cycles). The diffraction peaks are found to be remaining sharp and match the cubic phase with no additional peaks from other phases of Cu. This suggests phasepure Cu2O after the catalytic Cr(VI) reduction cycles. Furthermore, XPS survey spectrum (Figure 11b) indicates the presence of Cu, O, and C elements only. The absence of Cr(III) or Cr(VI) features in the XPS survey spectrum ascertains that Cr species are released from the surface of Cu2O cubes after the reduction. The insets in the top left, top right, and bottom left of Figure 11b show the O 1s, Cu 2p, and Cr 2p high resolution region XPS spectra. The O 1s and Cu 2p XPS peaks of Cu2O cubes are found to be similar before and after 20 catalytic Cr(VI) reduction cycles. These results confirm that the Cu2O phase remains unchanged and Cr-free after the catalytic reduction of Cr(VI) to Cr(III) with Cu2O cubes.



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Figure 10. FESEM images of Cu2O octahedrons collected after (a,b) two and (c,d) four cycles of Cr(VI) reduction in presence of H2SO4. Each reduction cycle with Cu2O octahedrons was 8 min. (e,f) FESEM images of Cu2O cubes after twenty cycles of Cr(VI) reduction. Each reduction cycle with cubes was 4 min.


40

50

60

Cu 2p

70

1000

80

2θ (degree)

580

Cu LMM

590

800

600

950 940 930

Cubes After stability test

C 1s

Cu 3s Cu 3p

530

Cu 2p3/2

Cu 2p1/2

535

O 1s

(311)

(220)

(111)

30

Cr 2p

After stability test

(110)

20

(b) O 1s

Cubes

Counts (a.u.)

(a)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(200)

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400

200

0

Binding energy (eV)

Figure 11. (a) Powder XRD pattern (b) XPS survey spectrum of Cu2O cubes after 20 Cr(VI) reduction cycles. The insets of (b) show the O 1s, Cr 2p, and Cu 2p region XPS spectra.

4. CONCLUSIONS A facile and an effective route for the shape-controlled synthesis of Cu2O crystals using simple solution-based technique is demonstrated here. The reaction temperature is found to play an important role in controlling the shape of Cu2O crystals and thus their exposed facets. At 40°C, uniform Cu2O octahedrons are obtained. With increasing the reaction temperature, {100} facets evolved at the six vertices of octahedrons and becomes fully grown cubes at 100°C with optimum 25 mM concentration of Cu(NO3)2⋅3H2O and 1.0 g PVP. The role of Cu(NO3)2⋅3H2O concentration, reaction medium, and PVP quantity are also studied in the present work. In particular, growth rate along {111} of Cu2O is found to be increased with increase in precursor concentration and reaction temperature. More importantly, an exposed facets-dependent catalytic property is demonstrated for the reduction of Cr(VI) in presence of several acids at pH 4 at room temperature. The cube-shaped Cu2O crystals show much higher catalytic performance for the Cr(VI) reduction suggesting the higher activity of {100} facets than that of {111} facets. Furthermore, the repeat cyclic reduction of Cr(VI) confirms higher stability of cube-shaped Cu2O crystals as tested for 20 cycles. This concludes that the cube-shaped Cu2O crystals has potential for the reduction of highly toxic Cr(VI) in the industrial wastewater to milder and nontoxic Cr(III). 25 ACS Paragon Plus Environment


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ASSOCIATED CONTENTS Supporting Information. SEM images of Cu2O product obtained by varying the reaction medium and at higher precursor concentration, powder XRD of Cu2O nanoparticles, N2 sorption isotherm, catalytic Cr(VI) reduction with Cu2O crystals of diverse shapes, effect of pH on Cr(VI) reduction. This material is available free of charge via the Internet http://pubs.acs.org.

AUTHOR INFORMATION Corresponding author *[email protected] Notes Authors declare no competing financial interest.

ACKNOWLEDGEMENTS The present work is supported by Science and Engineering Research Board (SERB), New Delhi, India through the grant SB/S1/IC-15/2013. AKM is thankful to UGC, New Delhi, India for the fellowship.

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Morphology Controlled Solution-based Synthesis of Cu2O Crystals for the FacetsDependent Catalytic Reduction of Highly Toxic Aqueous Cr(VI) Abhaya Kumar Mishra and Debabrata Pradhan

A solution chemistry route is employed to synthesize Cu2O crystals of diverse shapes by varying the reaction parameters. The as-synthesized Cu2O crystals is employed for the reduction of toxic Cr(VI) to nontoxic Cr(III). The cube-shaped Cu2O crystals with {100} exposed facets demonstates superior Cr(VI) reduction activity along with high cyclic stability than that of octahedrons with {111} exposed facets.


Morphology Controlled Solution-Based Synthesis of Cu2O Crystals for

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