pH-Control as a way to fi ne-tune the Cu / Cu 2 O ratio in radiation induced synthesis of Cu 2 O particles †

In this work we have optimized the γ-radiation induced synthesis of Cu–Cu2O particles from aqueous CuSO4 solution by investigating the effect of pH. The obtained precipitate was analyzed by XRD and SEM techniques. The results indicated that at solution pH lower than 3.75, quasi-spherical Cu agglomerates can be formed while at pH higher than 4.40 only octahedron-shaped Cu2O particles are produced. At solution pH in the range from 3.75 to 4.40, a Cu–Cu2O mixture is produced. It was found that the relative amount of Cu2O in the Cu–Cu2O precipitate increases with pH in the studied range. The influence of solution pH on the Cu/Cu2O ratios in the product can be explained on the basis of pH-dependent competition kinetics between the reactions leading to either Cu or Cu2O formation. As a consequence, the composition and morphology of the Cu–Cu2O precipitate can be tuned by controlling pH of the aqueous CuSO4 solution during the γ-radiation induced synthesis.


Introduction
Metal and metal oxide particles have received increasing attention during the last decades due to their catalytic, 1 magnetic, 2 conductive 3 and optical 4 properties.6][7][8][9] Cu 2 O particles have potential applications in solar energy conversion, 10 photocatalysis, 5 biosensing 11 and organic synthesis. 12The expanding application of Cu 2 O requires the materials to be produced with well controlled size, shape, morphology and composition.One of the possible routes to synthesis of Cu 2 O is to precipitate it from solution by changing the redox potential.Kumar et al. 13 prepared Cu 2 O nanocubes with a hierarchical structure by a one-pot synthesis method employing polyethylene glycol and glucose as a structure-directing agent and reductant, respectively.Xu et al. 14 synthesized octahedral Cu 2 O crystal by reducing copper hydroxide with hydrazine.Yu et al. 15 prepared CuO/Cu 2 O composite hollow microspheres with controlled diameter and composition by hydrothermal synthesis using Cu(CH 3 COO) 2 •H 2 O as a precursor.
In the current study, a γ-radiation induced method is used to produce Cu 2 O particles.The use of ionizing radiation in synthesizing particles has several advantages compared to other methods.7][18][19][20] Gold nanoparticles, 21 Palladium catlysts 22 and Pt/CNTs catalysts 23 have been produced by γ-radiation induced synthesis methods.2][33][34][35][36] Also, solution pH influences the yield of the obtained Cu precipitate. 37he aim of the current study is to investigate the influence of pH of the Cu precursor solution on the composition, conversion and morphology of the Cu-Cu 2 O precipitate obtained by γ-radiation induced synthesis, as well as to work out the optimal conditions for synthesis of crystalline Cu 2 O without Cu admixtures.

Radiation induced synthesis method
Radiation-induced formation of particles in solution is in general based on reactions between solvent radiolysis products and solute precursors.To produce metal or metal oxide particles, metal ions are usually used as solute precursors.In this work, water is used as solvent for the reaction system.Upon absorption of ionizing radiation water is decomposed into oxidizing and reducing species: 24 The chemical change induced by γ-irradiation can be quantified via the radiation chemical yield or G-value which is the number of moles of a given species produced or consumed per absorbed Joule of radiation energy [mol J −1 ]. 24 Among the water radiolysis products, e − aq and OH • are produced with the highest yield at pH 7 in γ-irradiated solutions. 24The G-values of e − aq and OH • are both 0.28 μmol J −1 at times >10 −6 s (ref.24) after the initial absorption of the radiation energy.The G-values are pH independent in pH range from 3 to 13. 38 The hydrated electron, e − aq , is a very strong reductant, E 0 (H 2 O/e − aq ) = −2.87V vs. SHE, capable of reducing most metal ions to lower valence states. 39It acts as the most effective reducing agent in the system, the rate constant of the reaction between e − aq and Cu 2+ ions is 8.2 × 10 9 dm 3 mol −1 S −1 . 40OH • is a very strong oxidant capable of oxidizing metal ions to a higher oxidation states. 24For γ-radiation induced synthesis, the redox conditions of solution can be tuned to either oxidizing or reducing condition by adding radical scavengers prior to irradiation.The scavengers can react with undesired radicals and sometimes also convert them into other radicals with the desired properties.In the present work, the reducing route was selected to produce Cu 2 O particles in Cu 2+ solution.This was achieved by using isopropanol which can react with OH • and produces the reducing radical Furthermore, via the reaction (2) the isopropanol scavenges the oxidizing radical OH • and produce same amount of reducing radical (CH 3 ) 2 CO • H. Hence, the total G-value for reducing radicals in this system will be doubled (G(red) = G(e − aq ) + G(OH • )) and equal to 5.6 × 10 −7 mol J −1 .

Synthesis condition
In a typical preparation, the aqueous solution (Millipore Milli-Q) contained 0.01 M CuSO 4 •5H 2 O (Merck, 99.8%), 2.0 M isopropanol (Sigma-Aldrich, 99.9%) and 0.50 M CH 3 COOH/ CH 3 COO − buffer.0.008 M sodium dodecyl sulfate (SDS, Fluka, 99.0%) was used as surfactant in every experiments.The CH 3 COOH/CH 3 COO − ratio was varied to adjust pH between 3.75 and 4.40.The solutions were purged with high purity nitrogen (Standmollen, 99.999%) for 30 min to remove oxygen from the solution prior to irradiation.Thereafter, the solutions were irradiated using Cs-137 γ source at a dose rate of 0.124 Gy s −1 for 20 hours.The total absorbed dose was 8.9 kGy for each sample.After irradiation, the obtained red precipitate was washed with deionized water several times and filtered.

Characterization
Optical absorption spectra were acquired using a JASCO V-730 spectrophotometer to measure the Cu(II)-concentration in the copper sulfate solutions before and after irradiation.The intensities of the absorption peaks at a wavelength of 777 nm typical for Cu 2+ aq were used. 42Powder X-ray diffraction (XRD) patterns of synthesized particles were recorded by a PANalytical X'Pert PRO diffraction system using Cu Kα radiation (λ = 1.54 Å) in Bragg-Brentano geometry.XRD scans were recorded in 2θ range from 5°to 95°with a step of 0.01°.Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were performed using a VP-SEM S3700N setup.

Results and discussion
The powder X-ray diffraction patterns of the precipitated particles produced at the different pH-values of the copper sulfate solutions are shown in Fig. 1.It should be noted that the solution pH before and after irradiation was found to be similar within the uncertainty of 0.05 pH-units.The pH values of the solutions are denoted in the figure.4][45] While for the sample synthesized at pH 3.85, the characteristic peaks of Cu 2 O are observed 13 in addition to those which belong to Cu.The two  S1 † and Fig. 3(a).Thus, by varying the copper sulfate solution pH without changing other synthesis parameters, one can tune the composition of the precipitate obtained by γ-radiation induced synthesis.
The effect of pH can be understood from the E-pH diagram 46 of the Cu-H 2 O system with 0.01 M Cu 2+ shown in Fig. 2. As seen in the diagram at solution pH < 3.5, only metallic copper can be formed when lowering the potential, Cu 2+ → Cu 0 .At 3.5 < pH < 5 there is a potential interval where Cu 2 O(s) is stable.However, upon further reduction of the potential Cu 0 is formed.
To elucidate the mechanism of Cu 2 O formation, we plotted the ratios of the integral intensities of the XRD peaks I Cu 2 O /(I Cu + I Cu 2 O ) as a function of solution pH, see Fig. 3(a).Also the conversion of Cu 2+ (ΔCu 2+ ), i.e., the change in Cu(II)-concentration upon irradiation, is plotted as a function of solution pH as well, see Fig. 3(b).As seen in Fig. 3 Cu 2+ will be reduced to form Cu 2 O through the following reactions ( 3)-( 5). 42,47Metallic Cu particles could either be produce by two consecutive one-electron transfer steps from Cu 2+ via Cu + to Cu 0 , reactions ( 3) and ( 6), 40 or through disproportionation of Cu + , reaction (7): The change in I Cu 2 O /(I Cu + I Cu 2 O ) with pH implies that there is a pH-dependent competition between the reaction yielding Cu 2 O (4) and the reactions yielding Cu ( 6) and (7), where reaction ( 4) is favored at higher pH.The apparent pH-effect on the  Cu 2+ conversion at the given absorbed dose directly follows from this since two electrons are required to convert Cu 2+ to Cu while only one electron per Cu 2+ is required to produce Cu 2 O according to proposed scheme.This is confirmed by the results shown in Fig. 3.The influence of scavenger (isopropanol) concentration on Cu 2 O formation was also investigated and the results are shown in Fig. 4. As the results indicate, the ratio of I Cu 2 O /(I Cu + I Cu 2 O ) increases with increasing isopropanol concentration in the range 0.5 M to 1.0 M.This could partly be attributed to hydroxyl radicals reacting with the buffer in competition with the desired reaction with isopropanol, reaction (2).However, based on the relative rate constants of these reactions (k i = 2.3 × 10 −9 mol −1 dm 3 s −1 for hydroxyl radicals react with isopropanol, k a = 9.2 × 10 −6 mol −1 dm 3 s −1 for hydroxyl radicals react with acetic acid), this effect is expected to be quite small. 48,49iven the magnitude of the isopropanol concentration effect, it is more likely that it can be attributed to solvent effects on the relative rates of the competing reactions.Since the isopropanol concentration effect is only observed below 1.00 M, 2.00 M of isopropanol was selected for the current study.
It was also found that solution pH influences the morphologies of produced Cu 2 O particles.SEM images of the precipitate were collected and are shown in Fig. 5  ticles is gradually decreased, whereas the Cu 2 O octahedrons becomes the main phase of the produced particles.Note that the average size of the Cu 2 O particles is decreased from 13.5 μm to around 7 μm (Fig. 5(f )).This could be due to the fact that at high solution pH the nucleation of Cu 2 O is favorable and therefore, likely to more nucleation centers of Cu 2 O are produced.This could lead to smaller particle size and likely to more narrow size distribution. 29,30,53Solely Cu 2 O particles with octahedron shape are synthesized at pH 4.40 and is shown in Fig. 5(e).

Conclusions
Cuprous oxide particles were synthesized by the reduction of dissolved Cu 2+ ions from CuSO 4 aqueous solution using a γ-radiation induced synthesis route.
Fe 2 O 3 ,25 UO 2 ,26 Co 3 O 420 and Cr 2 O 3 .27,28There are several studies about γ-radiation induced synthesis of Cu 2 O particles.He et al.29 investigated the sizecontrolled preparation of Cu 2 O octahedron nanocrystals by γ-irradiation and found that the average edge of the octahedron Cu 2 O nanocrystals varied from 45 to 95 nm as a function of the γ-radiation dose rate.Furthermore, Liu et al.30 reported a shape controlled synthesis of Cu 2 O particles, such as eight-pod cubic, six-armed star-like, octahedral and spindlelike structure, by γ-irradiation with cetyltrimethylammonium bromide (CTAB) as a nucleation and growth controller.

Fig. 1
Fig. 1 XRD patterns of the Cu-Cu 2 O powders synthesized at different pH values, rhombus and hearts denote XRD peaks for Cu and Cu 2 O, respectively.
(a) the ratio of I Cu 2 O /(I Cu + I Cu 2 O ) changes gradually from 0 to 1, indicating co-existence of pure crystalline Cu and Cu 2 O phases in the solution in the pH range used in the experiments.In the pH range from 3.75 to 4.0, the Cu-Cu 2 O precipitate can be obtained.It is also clear that the Cu 2+ conversion increases gradually with solution pH and its value doubles when solely Cu 2 O forms.

Fig. 2 E
Fig. 2 E-pH diagram showing predominant phases in the Cu-H 2 O system (0.01 M Cu 2+ aqueous solution).

Fig. 3
Fig. 3 The ratios of relative XRD intensities I Cu 2 O /(I Cu + I Cu 2 O ), I Cu 2 O /(I Cu + I Cu 2 O ) was the ratios of integral intensities of XRD peaks, which I Cu 2 O and I Cu represented the sum of integral intensities of peaks for Cu 2 O ((110), (111), (200) and (220)) and for Cu ((111) and (200)), respectively.(a) and Cu 2+ conversion yield ΔCu 2+ (b) as a function of solution pH, dash lines are guide for eyes.

Fig. 4
Fig. 4 The isopropanol concentration as a function of the ratios of I Cu 2 O /(I Cu + I Cu 2 O ) prepared at the solution pH 4.40.I Cu 2 O /(I Cu + I Cu 2 O ) was the ratios of integral intensities of XRD peaks, which I Cu 2 O and I Cu represented the sum of integral intensities of peaks for Cu 2 O ((110), (111), (200) and (220)) and for Cu ((111) and (200)), respectively.
It was found that by changing the solution pH the composition and morphology of the obtained precipitate can be tuned.Thus, at solution pH < 3.75 metallic copper quasi-spherical agglomerates are formed while at pH > 4.40 just Cu 2 O octahedron-shaped particles are precipitated.In the pH range from 3.75 to 4.40, a mixture of both metallic Cu particles and Cu 2 O particles are produced and the relative amount of crystalline Cu 2 O in the precipitate increases with increasing pH.As a direct consequence, the overall conversion of Cu 2+ is increased with increasing solution pH.The observed pH effect can be attributed to pH-dependent competition between the reactions yielding Cu and the reactions yielding Cu 2 O. Thus, by controlling pH of the CuSO 4 solution during γ-radiation induced synthesis one can tune the composition and morphology of the obtained precipitate.