Hybrid CuxOâTiO2 Nanopowders Prepared by Ball Milling for Solar

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Hybrid CuxO-TiO2 Nanopowders Prepared by Ball Milling for Solar Energy Conversion and Visible-light-induced Wastewater Treatment Pradip Basnet, Erik C. Anderson, and Yiping Zhao ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00325 • Publication Date (Web): 13 Mar 2019 Downloaded from http://pubs.acs.org on March 19, 2019

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ACS Applied Nano Materials

Hybrid CuxO-TiO2 Nanopowders Prepared by Ball Milling for Solar Energy Conversion and Visible-light-induced Wastewater Treatment Pradip Basnet*†,a, Erik Andersonb, and Yiping Zhaoa aDepartment

of Physics and Astronomy, and Nanoscale Science and Engineering Center, University of Georgia, Athens, GA, 30602, USA. bGeorge W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, 30313, USA.

Abstract Hybrid nanocomposites of Cu2O-(R)TiO2, CuO-(R)TiO2, and Cu3TiO4-(R)TiO2 (where R represents the Rutile phase of TiO2) nanopowders (NPs) were produced via solid state reaction followed by 20 hrs of ball milling, their photocatalytic activities were evaluated for methylene blue (MB) degradation under visible light intensity (~65 mW/cm2) and compared to Degussa P25 under both ultraviolet (UV) and visible light irradiations. The highest MB degradation rate under the visible light irradiation was observed to be 0.271 ± 0.010 hr-1 for Cu2O-(R)TiO2 NPs, which was 2.5 times higher than that of P25, while under UV illumination both the Cu2O-(R)TiO2 and Cu3TiO4-(R)TiO2 NPs were slightly less active than that of the P25, and CuO-(R)TiO2 was the least active amongst all. The solar energy conversion performance of the CuxO-(R)TiO2 NPs were observed to be controlled by the applied potentials. The highest stable cathodic photocurrent density (6.3 μA/cm2) was observed for Cu2O-(R)TiO2 NPs at a low negative bias voltage (-0.3 V vs. Ag/AgCl, 3M KCl), and under the solar simulator (AM 1.5G). This method to design multifunctional visible-light-active metal oxides is simple, scalable, and has the potential to prepare other efficient photocatalysts for solar energy conversion. Keywords: Cu2O-TiO2 solid state reaction, Ball milling, Visible-light-active photocatalysts, Photoelectrochemical, Solar energy conversion, Charge separation, Redox reaction.

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Introduction Titanium dioxide (TiO2) micro-/nano-structures have been studied for more than four decades as an efficient photocatalyst for various applications, including photoelectrochemical (PEC) and pollutant degradation.1-5 TiO2 has three polymorphs: anatase, rutile, and brookite, where the mixed phase of different polymorphs are known to have enhanced effects on photocatalytic activity compared to the respective phases.6 The most common commercial photocatalyst is known as Degussa P25, which consists of both rutile and anatase crystallites. Despite the unique physical and chemical properties, such as exceptional stability, low-toxicity, and good photoresponsivity, TiO2 use is limited to ultraviolet (UV) light due to large bandgaps (~3.2 eV for anatase, (A)TiO2; ~3.0 eV for rutile, (R)TiO2; ~3.4 eV for brookite, (B)TiO2).5-7 Recently, many efforts have been made to develop efficient visible-light-active photocatalysts (VLAPs) that can harness most of the solar spectrum (λ ≥ 400 nm) either by modifying well-known large bandgap semiconductors or by finding a suitable low bandgap material.8-13 Numerous pristine low bandgap semiconductors, such as Fe2O3, Cu2O, CuO, CoO, Bi2O3, CuBi2O4 have been studied for various applications in wastewater decontamination, antimicrobial, and solar-energy conversion.3, 10, 12, 14 Unfortunately, most single phase materials do not meet all the requirements to be an ideal VLAP, including high surface area for adsorption of targeted species, high conductivity, long-charge lifetimes, direct pathways to transport photogenerated charge, and good chemical stability in water. Mixed phase or doped materials, however, exhibit relatively higher charge separation at the interfaces resulting in higher photocatalytic activities and have begun to attract greater attentions as a viable VLAP.1516

Traditional photocatalysts modified by doping with metals or non-metals, or coupling with other

low bandgap materials such as CuxO (x = 1, 2), Fe2O3, CdSe, CoO, Bi2O3, and Bi2WO6 have also been reported.9, 11, 13, 17-18


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For environmental remediation and solar-energy conversion applications, TiO2 and CuxO mixtures possess great potential, in part due to their low toxicity, but also in lieu of their suitable energy band positions as illustrated in Figure 1.12, 19-20 Cu2O and CuO are both p-type visible-lightactive materials, while TiO2 is an n-type UV active photocatalyst.19, 21 Thus, the resultant CuxOTiO2 composite has the potential to utilize the full UV-to-visible spectrum of sunlight, and also enable further charge separation relative to the individual materials.10,

22-24

For TiO2, if no

recombination occurs the photogenerated electrons (e-) and holes (h+) are expected to migrate toward the surface of the catalyst and initiate redox reactions with the adsorbed molecules (Figure 1). For instance, photoexcited e- in the conduction band (CB) can react with O2 to form superoxide radical (O2•-) and may induce degradation of organic pollutants. Similarly, photogenerated h+ in the valance band (VB) can oxidize the adsorbed molecules with the generated hydroxyl radical (OH•).9, 12 Comparing the VBs of CuxO and TiO2, only the oxidation potential of holes in TiO2 is large enough to oxidize H2O to OH• (+2.38 V vs. standard hydrogen electrode, SHE), while the reduction potential of electrons in both Cu2O and TiO2 are suitable to reduce O2 to O2•- (-0.33 V vs. SHE). In addition, the redox potentials in all CuxO and TiO2 are enough to split water with solar light absorption since their bandgaps are above 1.23 eV. Concisely, both the redox reaction with organics and water splitting is expected with TiO2. On the other hand, Cu2O experiences water splitting and reduction (but no oxidation), and CuO experiences only water splitting (without redox reaction). To date, different forms of CuxO-TiO2 composite have been studied for various photocatalytic applications, including pollutant degradation,25-26 antimicrobial,27 hydrogen production and CO2 reduction.20, 28-29 Barreca et. al. demonstrated the promising applications of the CuxO-TiO2 nanocomposites, with or without the Au nanoparticles coatings.26, 30



Various chemical and physical methods, such as sol-gel,25 chemical vapor deposition (CVD),26, 30

and impregnation,27-28 have been used to fabricate the CuxO-TiO2 nanocomposites. Gombac et.

al. synthesized CuO nanoparticles embedded in TiO2 (CuOx@TiO2) using water-in-oil microemulsion (ME), and Cu-impregnated TiO2 via standard impregnation technique followed by an oxidation of Cu to make the CuxO-TiO2 structure.31 They reported higher photocatalytic performance in this embedded system and the improved photocatalytic activity could be controlled by either the structure or CuxO-TiO2 interface properties. There are advantages and disadvantages of these different preparation methods. Making embedded photocatalysts through microemulsion requires large volumes of solvents and sol-gel requires costly starting materials. The use of hydrazine monohydrate during ME is also potentially carcinogenic. Comparatively, ball milling is a low cost, eco-friendly, and facile approach to implement on the industrial level. However, there is potential for contamination when the same milling balls are used for different materials. This is typically not an issue at the industrial level where only a single material is synthesized; though at the research level the tendency to reuse equipment for different materials is advised against. In this work, we report a simple and scalable fabrication of hybrid CuxO-TiO2 nanopowders (NPs) prepared by a high temperature solid state reaction followed by facile ball milling method. Herein, we provide additional experimental evidence to support recent findings by Nie et.al. (2017) on eutectic temperature of CuO and TiO2 reaction,32 which has been a controversial issue for a long time, with variation that could be based on heavily disparate experimental conditions reported through the literature.33 The sample preparation strategy entails first mixing an appropriate amount of Cu2O and TiO2 homogeneously, heating the mixture under an optimized condition, then ball milling the product to obtain the desired particle size and crystallinity (Figure


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2). Ball milling is promising because it is a low-cost, simple, and a very reliable technique which can be scaled for mass-production.17-18, 34-35 Further, it does not involve the as many parameters that might adversely affect the final product of typical wet chemical or vapor deposition methods, e.g. chamber pressure, temperature, precursor concentration, etc. Hence, the morphology and composition of the ball mill-synthesized micro-/nanostructures can be well-predicted. We determined the optimized calcination temperatures and the environments to fabricate three different hybrid CuxO-TiO2 NPs, (Cu2O-(R)TiO2, CuO-(R)TiO2, and Cu3TiO4-(R)TiO2). All hybrid CuxO-TiO2 NPs are characterized for visible-light-induced activity. The photocatalytic performance is evaluated and compared by using methylene blue (MB) as the probe molecule for degradation. The PEC performance is evaluated in a homemade cell to compare the sample conversion efficiency from solar energy to electrical current.

Results and discussion Morphology and composition of hybrid CuxO-TiO2 NPs An analysis of particle composition was performed to compare three different hybrid nanoparticle samples: Cu2O-(R)TiO2 (sample S1), CuO-(R)TiO2 (sample S2), and Cu3TiO4(R)TiO2 (sample S3). After 20 hrs of ball milling, SEM images reveal the particle sizes ranged from 30 to 300 nm in diameter (Figure 3(a-c)). The average size of each of three samples were similar: 70 ± 30 nm, 70 ± 50 nm, and 60 ± 30 nm for S1-S3 (see Figure S1 for statistical data, in the Supporting Information, SI). We also find the average particle size decreases exponentially with milling time (Figure 3(d)). Optical photos of the samples after ball milling reveal a variation in color: S1 is light brown, S2 is greyish black, and S3 is dark brown in appearance (Figure 2(a-c)). Since all the samples have roughly the same size particles, we interpret the color difference as coming from the material’s



composition. In particular, the different phases of CuxO affect the samples’ intrinsic optical properties. The qualitative compositions of all three samples were determined by EDX measurements (Figure S3). The detailed morphology of individual NPs, especially the CuxO/TiO2 interfaces, were examined by HRTEM. In Figure 4 we see that the interfaces of CuxO and TiO2 are randomly formed and are heterogeneously distributed in all three samples. No core-shell like structures were formed. In sample S1, two domains with lattice spacing of 0.32 nm and 0.25 nm are seen (Figure 4(a″)), corresponding to the (110) and (111) crystallographic planes of (R)TiO2 and Cu2O. Similarly in S2 and S3, the lattice fringes of 0.23 nm and 0.26 nm agree well with the lattice plane of (111)CuO and (101)Cu3TiO4 (Figure 4(b″, c″)). The inhomogeneous mixture of CuxO and TiO2 is reasonable since the formation of either the Cu2O or CuO is controlled by high temperature solid state reaction, and the ball milling does not have a significant effect on alloying but can mix the CuxO and TiO2 phases homogeneously with high mechanical force. It is also expected that such a mixture creates many p-n junctions at the CuxO-TiO2 interfaces. XRD Characterization The XRD measurements were also performed to determine the global crystalline phases of the three samples and to corroborate the TEM results. All peaks in the XRD pattern can be perfectly indexed to the mixture of Cu2O-(R)TiO2, CuO-(R)TiO2, and Cu3TiO4-(R)TiO2 (see Figure S4 in SI). All three types NP samples are observed to exhibit the standard diffraction patterns of tetragonal (R)TiO2, namely, the diffraction angles at 2θ = 27.45°, 36.01°, 39.20°, 41.23°, 44.05°, 54.33°, 56.65°, 62.74°, 64.00°, 69.01°, and 69.80° represent the crystal planes (110), (101), (200), (111), (210), (211), (220), (002), (310), (301), and (112) (JPCDS #00-021-1276). Similarly, all the Cu2O, CuO, and Cu3TiO4 are observed to be crystalline phase, no trace amount of amorphous


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materials or impurity are observed within the detection limit of XRD. The diffraction peaks of the Cu2O and CuO are consistent with our recently published results.12 In particular, the diffraction patterns of both the Cu2O and CuO match well with the standard JCPDS reference #078-2076 and #048-1548, respectively. Similarly, the copper titanium oxide, Cu3TiO4 (JCPDS #048-1548), is also confirmed by the diffraction peaks at 2θ = 31.23°, 35.0°, 37.65°, 41.70°, 46.9°, 53.05°, 59.90°, 60.95°, 70.12°, and 73.95°, which correspond to its (004), (101), (102), (103), (104), (105), (106), (110), (114), and (202) crystal planes. All these results agree with the HRTEM results. Both XRD and HRTEM results indicate that the Cu2O in the (CuxO-TiO2) mixture does not undergo oxidation under the N2 annealing atmosphere at 1000°C. Also, no Cu2TiO3 or Cu3TiO4 could be formed. Hexagonal Cu3TiO4 was first reported by Hayashi et.al. in 1974, and was prepared by heating Cu2O, CuO, and TiO2 with a mole ratio of 1:1:1 in Pt crucible at 1050°C in air and cooling rapidly.36 Interestingly, no one has reported the catalytic/photocatalytic properties of Cu3TiO4 or any composites since. Hence, one of our goals here is also to explore the photocatalytic activity of Cu3TiO4/TiO2 mixture. In addition, the average grain sizes of CuxO-TiO2 crystallites are estimated using the Scherrer equation, 𝑑 = 𝐾𝜆/𝛽cos 𝜃. Here d is the diameter of the crystalline grain, the shape factor is K = 0.9, λ (Cu-Kα1) = 1.5406 Å, and β is the FWHM of the selected diffraction peak.12 Table S1 summarizes crystallite sizes of the different components in each sample, estimated from the most prominent XRD peaks (see Figure S4 in SI). The average crystallite grain sizes of the (R)TiO2, Cu2O, CuO, and Cu3TiO4 are about the same as average particle sizes determined from SEM. Hence, each particle typically corresponds to a single crystal, while the fluctuations in crystalline size for the different samples could be related to different local environment during calcination. Please note that the XRD cannot detect the amorphous phase in the composite, therefore, we



further characterized the samples with XPS and EDX measurements, and the results are discussed below. XPS Characterization XPS was performed to investigate the chemical states and compositions of CuxO-TiO2 nanoparticles as well as to confirm if there were any contamination coming from the vial/balls during the ball milling and/or any processing step. For the Ti 2p spectrum (Figure 5(a)), two distinct peaks of were observed around 457.0 ± 0.1 eV and 462.7 eV. These peaks correspond to Ti 2p3/2 and Ti 2p1/2, respectively, and indicate the Ti4+ species within the TiO2. For sample S3, a significant satellite shoulder peak appears at 459.1 eV, which we attribute to the Ti-O-Cu structure in the Cu3TiO4-TiO2. The Cu 2p spectrum similarly exhibits Cu 2p3/2 and Cu 2p1/2 peaks (Figure 5(b)), with satellite peaks in the range of 939-945 eV. Deconvolution suggests the presence of two phases: Cu+ and Cu2+. Sample S1 reveals primarily Cu+ species dominating at 931.6 and 950.8 eV, corresponding to the Cu2O. Sample S2 instead exhibits almost exclusively Cu2+ at 934.0 and 953.9 eV, corresponding to CuO. The increased Cu2+ content in sample S2 is attributed to the open-air heating during sample preparation. Cu is prone to oxidation under an oxygen-abundant environment, resulting in CuO formation. The O 1s spectrum (Figure 5(c)) depicts two main peaks for S1 and S2 at 529.2 and 530.7 eV, indicating O=O bonding linked to the Ti-O and Cu-O structures. Deconvolution of S3 reveals four peaks at 529.1, 530.5, 531.9, and 533.7 eV, which suggests additional Ti-O-Cu species expected for Cu3TiO4-TiO2. Using the XPS data, we estimate the composition of CuxO-TiO2 to be 63%, 66%, and 84% of CuOx for samples S1-S3, respectively. XPS did not reveal any significant peaks for Nitrogen (N) nor any potential metal impurities (e.g. Fe). We think that the absence of metal contamination is due to higher mass of source material used for ball milling, namely, Cu2O (0.642 g) and TiO2 (0.358 g). This may also be due to the use of high-quality hardened stainless steel (HSS) vial and balls. This result is consistent with the


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literature for the ball milling of TiO2 using the HSS.34 It is worth noting that the contamination can be dependent on different parameters such as pre-processing steps, milling time, and grinding materials, as reported in the literature.34-35

Optical properties Samples’ optical properties were determined by measuring transmission and diffuse reflectance. The extinction coefficients of NP solutions were used to estimate the absorbance of visible light starting with different concentrations (Figure S5). The concentration of NPs was chosen such that at least 90% of visible light is absorbed for all photocatalytic activity tests. By converting the diffuse reflectance spectra of the powder samples into the Kubelka-Munk function (Figure S6) we determine the bandgaps.18 From Figure S6, the estimated bandgaps are 2.65, 2.58, and 2.76 eV for Cu2O-(R)TiO2 (S1), CuO-(R)TiO2 (S2), and Cu3TiO4-(R)TiO2 (S3). The values for Cu2O-(R)TiO2 and CuO-(R)TiO2 NPs are consistent with literature, whereas the bandgap of Cu3TiO4-(R)TiO2 is relatively higher than either CuxO-TiO2 NPs.37-38 More important, the different bandgaps may explain the color variation we observed between samples (Figure 2(a-c)). Photocatalytic Activity To ensure an accurate comparison of catalytic and photocatalytic activities, an optimized concentration of nanoparticles is used for all three samples (see Figure 6 and Figure S7 in SI for details). The catalytic activity of the NP samples is tested under completely dark conditions, while the photocatalytic activity is tested under UV or visible illumination. The absorbance peak of the MB solution at 664 nm is then monitored for catalytic and photocatalytic activities. The change of the MB absorbance peak  (t ) at 664 nm versus illumination time t for all samples exhibits exponentially decaying behavior. Therefore, as reported in our previous papers,12-13 we use a firstorder exponential model, to quantify the results (Figure 6(a)). The dark catalytic activity is



significantly slower than the photocatalytic activity for each sample, indicating that all CuxO-TiO2 samples are photocatalytically active. The estimated photocatalytic and catalytic decay rates of the samples are summarized in Table 1. The catalytic activity, including the dark adsorption, of Cu2O NPs samples, prepared by grinding the Cu2O powder for 20 hrs, and the visible-light-induced photocatalytic activity tests of both the Cu2O NPs and P25 samples were performed as controls. Note that the catalytic and the photocatalytic performances of single phase CuO and Cu3TiO4 NP samples were not included here as the CuO phase is proven to be less photocatalytically active12, while the single phase of Cu3TiO4 NPs could not be prepared. We point out that both the catalytic and photocatalytic performances follow the same order: Cu2O-(R)TiO2 > CuO-(R)TiO2 > Cu3TiO4-(R)TiO2. It is well accepted that the equilibrium dark adsorption of dye molecules, or the equivalent loss of dye concentration, roughly represent the total surface area of the photocatalyst available for dye adsorption.39 However, the detailed adsorption kinetics can be complicated sometimes depending on whether just the surface area of a photocatalyst is playing a role of more complex reaction is causing that concentration change. The dark or catalytic activity of the S1 and pure Cu2O NP samples indicate that the Cu2O NP sample induces the redox reaction slightly faster than that of the S1 sample as the presence of TiO2 slows down the reduction of Cu2O. In contrast, the visible-light-induced MB degradation rate of the S1 sample is about 3 times higher than that of the pure Cu2O NP sample. The improved visible-light-induced photocatalytic performance of Cu2O-(R)TiO2 NP samples could be attributed to better charge separation at the Cu2O/TiO2 interface due to well-matched CB and VB positions of these two materials (Figure 1). Also, the CB and VB positions of both Cu2O and TiO2 are suitable for redox reactions of MB, as explained in our recently published papers.11-12 In brief, not only do the CB and VB positions of the metal oxide or oxide composites determine photocatalytic activity, but the redox potential of the gaseous


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or liquids molecules (i.e. adsorbed onto the oxides’ surface) also plays a key role. For example, the higher MB degradation rate of sample S1 can be attributed to improved transfer of electrons (from Cu2O to TiO2) and generation of O2•- and OH• radicals, resulting in mineralization of the MB via reduction and oxidation, respectively. While the charge (e-) transfer in sample S2 (from TiO2 to CuO) can be detrimental to the reduction of the MB, that is, regardless of whether the generated O2•- and OH• radicals can still oxidize the MB solution depending on the reduction and oxidation potential of MB as reported by Khan et.al.40 The relatively low catalytic and photocatalytic activity of the CuO-(R)TiO2 sample could be due to more stable CuO in the composite and the unfavorable CB and VB positions of CuO lying in a disadvantageous position for charge separation and redox reaction. Thus, the photogenerated e- and h+ could be accumulated in the CuO of the CuO-(R)TiO2 sample, resulting in the lower photocatalytic activity. The inferior photocatalytic performance of the Cu3TiO4-(R)TiO2 could be from the formation of Cu-Ti-O complex, inducing an unfavorable bandgap structure and causing less charge transfer at the interfaces. To assess the long-term performance of the samples, we measured the percentage of MB removal after 10 hrs with and without UV and visible illumination (Figure 6(b)). To ensure reliability, experiments were repeated without significant variation. Commercial P25 was also used as a control under the same experimental conditions. We see that the removal of MB with the CuxO-TiO2 NPs samples under visible light illumination are remarkable. About 95% MB is removed for sample S1 within 10 hrs, whereas only 38% was removed for the P25 and 60% for the Cu2O control samples. However, S2 and S3 are relatively less active under both visible and UV illumination (Table 1). It is interesting to see such high activity from S1, even though the (R)TiO2 cannot absorb the visible light appreciably; we think this is due to good charge transfer across the Cu2O/TiO2 interface. As mentioned above, the exact charge transfer at the interface of



heterojunction metal oxides or composites can be a complex process when involving a redox reaction at the CB and VB positions. However, the overall photocatalytic performance, e.g. dye mineralization in our case, can be controlled by both the relative CB and VB positions of the composite (via charge separation) and the redox potentials of the dye used (via charge transfer). To shed light on how the charge separation/transfer occurs at the different heterojunction metal oxides, causing different MB degradation activity, we direct the reader to our prior published papers.11-13 We also include an overview of our previous works (Table S2) to provide context for comparing the relative photocatalytic efficiencies of the film. Photoelectrochemical (PEC) properties The PEC performance of the samples are characterized using CV, LSV, and chronoamperometry. Figure 7 shows the representative CV curves recorded in dark for the three samples. We point out that the potential V (vs. Ag/AgCl) is not the same as the potential for reversible hydrogen electrode, V(RHE), and potential for saturated calomel electrode, V(SCE), reported in the literature. However, they can be converted according to V(RHE) = [V(Ag/AgCl) + 0.197 + 0.059  pH],41 where the pH of our electrolyte is 7.0. For S1, all the cycles have distinctive anodic and cathodic peaks at +0.23 and -0.25 V (Figure 7). After each CV cycle the current density, j, at the cathodic and anodic peaks decrease slowly (Figure S8(a-c)). The lower j could be attributed to a loss of material from the electrode since the NPs may just weakly bond onto the electrode substrate. A similar trend has been obtained for samples S2 and S3 apart from slight changes in their anodic and cathodic potentials (Figure S8(a-c)), likely due to material-dependent properties. When the applied potential is negative, the decreasing magnitude of the cathodic current could be from electrons accumulating in the CB of each sample. We will discuss this result more with EIS spectra. Since the onset potential of a photocatalyst is an important parameter, the LSV data were further used to estimate and compare their onset potentials (see S9 in SI for details).42-


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43

In figure S9 we see both positive and negative onset potentials, and they are respectively

termed as potential for OER (Oxygen evolution reaction) and HER (Hydrogen evolution reaction). OER is the primary anodic process, while HER is the cathodic process. Result showed that the onset potentials of all three samples are considerably smaller compared to the P25. The improved photoresponse of the samples, especially samples S1 and S3, at these lower potentials can be attributed to both lower bandgap and favorable CB and VB locations of all three samples as explained above (Figure 1). It is obvious from Figure S8 and Figure S9 that all three samples generate some dark current, and both the cathodic and anodic potentials are changing. These indicate that the samples undergo continuous redox reactions within the potential range. Despite the appreciable photocurrent and the low onset potentials, none of the samples appear stable enough for practical use. We believe this instability is caused by the presence of CuxO NPs in the mixture while the mechanism of selfredox reaction of the CuxO nanostructures is well-described in the literature.12, 44 For example, the CuO is reported to be the most stable phase, while Cu2O is least stable. However, our study could not confirm the phase change of any samples from the XRD patterns (Figure S10). For more comprehensive details about redox reaction of CuxO/TiO2 in such experimental conditions, the rate influencing factors, and potential experiments to confirm the Cu leaching (in the solution) we refer the reader to papers by and Montini, et.al. and Lennox et.al.45-46 Figure 8 shows the dynamic photoresponse curves of three samples recorded at -0.3 V, under the illumination of AM1.5G and chopped at a frequency of 0.1 Hz. The cathodic current densities j of the sample S1, S2, and S3 were estimated by subtracting the dark j values (light OFF) from j values with the solar illumination (light ON) and reported only their magnitudes. The net estimated j values of the samples S1 to S3 are 0.0063, 0.0020, and 0.0037 mA/cm2 (Figure 8 inset). This result



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is consistent with the photocatalytic activity performance under visible light illumination, as discussed above. The cathodic photocurrents with all three samples at the negative bias potential can be quickly stabilized within a minute (sample S2 stabilizes almost immediately), and no significant changes are observed for longer times. All three samples do not generate noticeable photocurrent spikes with turning the light “ON” as reported in the literature.47 This can be attributed to a better electron transportation on all the electrode surfaces. Because of the different experimental parameters used in the literature, including the nature of the light source, the properties of electrolyte, and the material thickness, it is hard to compare only the current density (j) values with the other reports. However, the observed j of the sample S1 under the AM1.5G illumination is not as high as that of the single phases TiO2, Cu2O, and CuO nanorods (NRs) fabricated by an e-beam assisted physical vapor deposition method.12, 47 Specifically, in Ref. 12, j = 0.06 and 0.18 mA/cm-2 are reported for Cu2O and CuO NRs respectively, while j = 0.015 mA/cm-2 is reported in Ref. 55 for TiO2. Note that in the reports of TiO2, Cu2O, and CuO NRs, the nanorods have different lengths and the tests conducted in 0.5 M NaClO4 or in 0.5 M Na2SO4. Finally, the interface between electrolyte and all three samples are investigated using the MottSchottky (M-S) analysis, with an aim to relate the effect of flat band potential (Vfb).18,

47

In

particular, the capacitance profiles of all three samples are recorded at a constant frequency (50 Hz) (see Figure S11 in SI). Even though different protocols are reported in the literature, the true and accurate determination of Vfb is difficult due to its complex nature.48 The capacitance measurement in aqueous solution might involve several parameters, including electrolyte pH, absence or presence of interfacial layer (also known as Helmholtz layer), and applied frequency. These variables can account for the slope change in M-S plot, as reported by Gryse et. al.49 A paper


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by D. Watson and Gerald J. Meyer discussed the Vfb calculation with band bending across an oxide space-charge layer, among other techniques.48 However, our main purpose is to compare the Vfb values for all three samples and explore how the relative CB and VB positions, i.e., with respect to the negative and positive Vfb, play the role in the photocatalytic and PEC performances. As seen in the Figure S11, the existence of both the positive and negative slopes for all three samples verify the n- and p- type characteristics of the TiO2 and CuxO (x = 1, 2) (and also Cu3TiO4) as reported in the literature.50 The M-S plots indicated that all three samples are the composite structure, forming p-n junctions. The positive and negative potentials for samples S1–S3 are estimated as 0.044, 0.004, 0.024 V and -0.186, -0.206, and -0.286 V, respectively (Figure S11(a-b)). Thus, the change from n-type to p-type is very abrupt for all three samples: in the region of -0.186 to +0.044V (S1), -0.206 to +0.004V (S2), and -0.286 to +0.024 V (S3). The estimated negative and positive potentials can further be utilized to relate to the observed PEC performances of all three samples. The negative value of the flat band potential directly reflects the position of the CB edge of the n-type (R-TiO2) semiconductors (Figure 1), which plays a vital role in the PEC performance. The difference between CB edge and the negative flat band potential is very small. Similarly, the positive values of the flat band potential are related to the position of the VB edge of the p-type (CuxO or Cu3TiO4) semiconductors, which also determines the PEC and photocatalytic performances. Again, the difference between VB edge and the positive flat band potential is assumed to be very small. Therefore, the PEC performances of the samples (S1 > S3 > S2) at negative bias voltage is observed to be controlled by the relative positions of their VBs (+0.044 V > +0.024 V > +0.004 V).



Conclusions Solid state reactions followed by the ball-milling method have been successfully used to fabricate hybrid nanocomposites of Cu2O-(R)TiO2, CuO-(R)TiO2, and Cu3TiO4-(R)TiO2 NPs by simply varying the calcination temperatures and environments. This is a remarkably simple and effective method for large scale multifunction photocatalyst production. Among all three samples, Cu2O-(R)TiO2 NPs showed the highest MB degradation rate under the visible light illumination, and also exhibited the highest photocurrent at a low negative bias and under solar simulator (AM 1.5G) at a neutral pH of 7.0. It is observed that the photocatalytic performance of Cu2O-(R)TiO2 NPs was considerably better than that of commercial P25 under the visible light illumination. The improved performance is attributed to the better charge transfer at the Cu2O/(R)TiO2 interfaces due to favorable CB and VB positions. In addition, a new photocatalyst, Cu3TiO4 -(R)TiO2 NPs, is also produced, though its photocatalytic performance is not as good as those of the other two CuxO-(R)TiO2 NP samples. We anticipate that, with further optimization, the photocatalytic performance of this material could be improved, or other phases of CuTixOy compounds could be formed with tuneable bandgaps and photocatalytic properties. By further doping the materials with proper metal or non-metal, or coupling them with other low bandgap material, other new photocatalytic materials can be created. Thus, the proposed method can be extended to explore other photocatalytic compounds and the stable and non-water soluble Cu2O-(R)TiO2 nanocomposite can be a promising candidate for the environmental remediation and sustainability.

Experimental Section Materials and Methods Cu2O (97%, Fisher Scientific), TiO2 pieces (99.9%, Kurt J. Lesker), and P25 (80% anatase and 20% rutile; Evonik Degussa Corp.) were used. A 1:1 molar ratio of Cu2O (0.642 g) and TiO2


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(0.358 g) was hand-grinded for 2 hrs with a mortar and pestle. The powder mixture was compressed to 2 mm thick pellets and 12 mm diameter, as shown in Figure 2. To obtain different phases of CuxO (x = 1, 2) and combinations with TiO2, the pellets were calcined in a quartz tube furnace (Lindberg/Blue M Company) around 1000°C under varying conditions, such as in open air or while flowing N2. The as-prepared CuxO-TiO2 pellets were then milled for up to 20 hrs in a mixer mill (SPEX) to obtain the desired nanostructure. Substrates used for characterizing the nanopowder coatings included glass slides (Gold Seal® #3010), ITO-coated glass (Rs = 10 Ω/sq, Delta Tech. Ltd.), and Si (100) wafers. High-purity methylene blue (MB, C16H18CN3S; CAS:122965-43-9) was purchased from Alfa-Aesar. Deionized water was used as a solvent for all experiments.

Hybrid CuxO-TiO2 NPs and Electrode Three different hybrid CuxO-TiO2 samples (Cu2O-TiO2, CuO-TiO2, and Cu3TiO4-TiO2, denoted by S1 to S3 respectively) were obtained by heating the pellets at three different conditions: (S1) 5°C/min up to 500°C and then 1000°C for 2 hrs under continuous N2 flow at 20 sccm, (S2) 1000°C for 10 hrs in open air, and (S3) 5°C/min up to 1000°C and then 1050°C for 2 mins under 20 sccm N2. S1 and S2 were allowed to cool down to room temperature inside the furnace whereas S3 was taken out rapidly to cool. The as-prepared hybrid CuxO-TiO2 pellets were individually ball milled in a hardened steel vial with the 6.35 mm steel grinding balls (SPEX™ SamplePrep 8001). The ratio of the mass of the balls to the mass of the pellet was fixed to be about 10:1 for all experiments. Prior to ball milling, all the balls were immersed in ethanol in the vial for an hour. The balls and vials were then rinsed with DI water, and dried in a 200 ºC oven for 2 hours. At various times throughout ball-milling, ~1 mL of slurry was removed to characterize size as a function of milling time.



Finally, after 20 hrs of grinding the suspension was dried in air. The as-prepared NPs were used for photocatalytic activity testing, while the coated ITO substrates were used as electrodes for PEC characterizations. The CuxO-TiO2 electrodes were prepared following the same procedures reported in our recent paper:14 ~1 μm thick electrodes were prepared by spraying the 0.5 gm/mL CuxO-TiO2 sols in DI water onto the pre-heated ITO substrates. Details are described in the Section S2 of Supporting Information (SI) (see Figure S2).

Characterization The morphology of the three hybrid CuxO-TiO2 NP samples was examined by a fieldemission scanning electron microscope (SEM) capable of energy dispersive X-ray spectroscopy (EDX). To investigate the detailed morphology of the individual NPs, especially the CuxO/TiO2 interfaces, transmission electron microscopy (TEM) and highresolution TEM (HRTEM) images were acquired on a Hitachi HD-2000 scanning TEM system using a carbon-coated Cu grid. Particle surfaces were characterized using a Hitachi 9500 TEM (300 kV, 0.10 nm lattice resolution). For statistical analysis, images of randomly selected particle surfaces were recorded. Structural properties were characterized by a PANalytical X’Pert PRO MRD X-ray diffractometer (XRD) with a fixed incidence angle of 1.5°. The XRD patterns were recorded with a Cu Kα radiation (λ = 1.5406 Å) in the 2θ range from 20-80°. The optical extinction coefficients of NP suspensions were extracted by a double beam UV-visible light (UV-vis) spectrophotometer (JASCO V-570), while diffuse reflectance spectra of the NPs were obtained by a Shimadzu 2450 UV-Vis spectrometer. Further, the extinction spectra were used to estimate the optical absorption, while the diffuse reflectance spectra were used to determine the bandgaps of the materials.


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The photocatalytic activities of the CuxO-TiO2 NP samples were evaluated by the photocatalytic degradation of a 10 ppm (~ 31 μM) MB aqueous solution (pH = 6.2) under both the UV (BLAK-RAY, Model B 100AP) and visible (250 W quartz halogen lamp: UtiliTech) light irradiations. The halogen source covered a wavelength range from 400 to 800 nm, while the UV source generated a sharp spectral peak at 365 nm. Different concentration of CuxO-TiO2 NPs in the range of 0.061 to 1 mg/mL (MB solution) was used for the photocatalytic activity evaluation. The optimized concentration to achieve the maximum MB degradation was used to compare the photocatalytic properties of all three samples S1, S2, and S3. Specifically, the concentration of all three samples in the MB solution was fixed at 0.5 mg/mL with 3.0 mL of MB test solution. A detailed optimization of NPs concentration is presented in Sections 4 and 5 of Supporting Information. Prior to illumination, each suspension was stirred in the dark for 30 min in order to acquire the dye adsorption-desorption equilibrium, and to observe any dark decay activity. Then, each cuvette was illuminated for up to 7 hrs with either visible (65 mW/cm2) or UV light (10 mW/cm2). A water circulation filter was placed in front of the cuvette to absorb the IR light and to avoid heating the reaction container. The test suspensions were shaken every 30 mins to avoid possible NP precipitation between photodegradation measurements. The behavior of free standing of NPs or dispersion stability was confirmed by recording the insitu absorbance spectra; and no obvious change in the maximum absorption peak was observed at the given wavelength in 30 mins. The photodegradation of the MB solution was measured by examining the UV-vis transmission spectra of the MB solution using an Ocean Optics spectrophotometer (USB 2000). The absorbance peak at 664 nm was monitored every 30 min to evaluate the photodegradation rate of MB.



Page 20 of 29

For PEC characterization, cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry

(i-t),

and

electrochemical

impedance

spectroscopy

(EIS)

measurements were performed. A home-made single compartment PEC cell was used for all PEC characterization. The PEC cell had a quartz window (>90% visible transmission), and a conventional 3-electrode arrangement connected to a potentiostat (CH Instruments Inc., CHI1040C). The reference and counter electrodes were 3 M Ag/AgCl and platinum wire respectively, while the ITO-coated sample was the working electrode. 0.5 M borate solution (with pH adjusted to 7.0 using KOH) was used as an electrolyte for the aforementioned characterizations.51 LSV was carried out at a scan rate of 0.005 V/s, while the CVs were carried out at a rate of 0.02 V/s in a potential range from -0.6 V to +0.6 V. The chronoamperometric measurements were carried out at a bias voltage of ±0.25 V (vs. Ag/AgCl). The electrode was illuminated from the back using a solar simulator (100 mW/cm2). EIS was performed at an amplitude of 20 mV, frequency 50 Hz, and potential range of ±1.0 V.


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ASSOCIATED CONTENT Supporting Information. Size distributions of 20-hrs ball milled Cu2O-(R)TiO2, CuO-(R)TiO2, and Cu3TiO2-(R)TiO2 samples, Photoelectrode sample preparation, EDX and XRD patterns of Cu2O-(R)TiO2, CuO-(R)TiO2, and Cu3TiO2-(R)TiO2 samples, Absorbance of CuxO-(R)TiO2 NP suspensions, Optimization of MB degradation test with CuxO-(R)TiO2 NP samples, Stability test of CuxO-(R)TiO2 NP samples: XRD results, and Mott-Schottky analysis for the flat band potentials of the Cu2O-(R)TiO2, CuO-(R)TiO2, and Cu3TiO2-(R)TiO2 samples. This material is available free of charge via online version of this article. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Ph.: +1(706)-542-6230. Fax: +1(706) 542 2492 ORCID Pradip Basnet: 0000-0002-5619-7581

Present Addresses †School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors would like to thank Mr. Jarryd N. Ashby (for helping in PEC measurements), Dr. Taku Cowger (for taking the TEM images), Mr. Connor A. Frost (for his assistance in running the Ball Mill and literature search), Mr. Steven Larson (for taking the SEM images of spray coatings on ITO substrates), and Prof. Ramaraja P. Ramasamy for the use of his PEC characterization tools. Prof. Yiping Zhao is partially supported by National Science Foundation under contract number ECCS-1303134.



REFERENCES 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.

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Page 24 of 29

Table 1. Summary of catalytic and photocatalytic activities of S1 (Cu2O-(R)TiO2), S2 (CuO(R)TiO2), and S3 (Cu3TiO2-(R)TiO2) samples. P25 and Cu2O (prepared by grinding for 20 hrs) samples were used as controls.

Sample S1 S2 S3 P25 Cu2O

MB Decay Rates (h-1)

Amount of MB % removal in 10 hr

Dark 0.036 ± 0.001

Visible 0.270 ± 0.020

Dark 18 ± 2

Visible 95 ± 5

UV 80 ± 7

0.019 ± 0.001

0.130 ± 0.010

8±1

75 ± 5

40 ± 4

0.006 ± 0.001

0.100 ± 0.010

5±1

65 ± 3

75 ± 7

--

--

0.5 ± 1.0

38 ± 5

92 ± 5

0.040 ± 0.005

0.087 ± 0.020

33 ± 2

60 ±10

--


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Figure 1. Representative schematic of energy band positions of Cu2O, TiO2, and CuO to show the possible redox reaction with organics and water splitting under UV or visible light irradiation. Possible electron (e-) and hole (h+) transfer in the CuxO-TiO2 composite is indicated by arrows. Potential band bending is not shown.

Figure 2. A scheme describing the preparation of CuxO-TiO2 NPs via solid state reaction followed by a ball milling technique. Fabrication steps are numbered from (1) to (4): (1) Grind the mixture of source materials for about 2hrs then compress to pellet/s, (2) sinter the pellet/s at different pre-determined temperatures (1000 – 1050 °C), under different environments, (3) ball mill for 20 hrs and (4) dry the as prepared CuxO-TiO2 NPs. Note that the ball milling grinding vial is shown in sectional view.



Figure 3. (a) to (c) SEM images of 20 hrs milled Cu2O-TiO2, CuO-TiO2, and Cu3TiO4-TiO2 NPs samples. (d) Effect of ball milling time on particle size for representative sample S1.

Figure 4. (a-c) TEM images of samples S1-S3, respectively. Images on the right with a single (‘) and double (“) prime represent their corresponding HRTEM images for their crystallographic orientation of each exposed surface.


Page 26 of 29

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(a)

Ti 2p

2p3/2 457.0 eV

2p1/2 462.7 eV

Intensity (a.u.)

S3

S2

S1 450

455

460

465

470

Binding Energy (eV)

(b)

Cu 2p

2p3/2

2p1/2

Cu2+ +

Cu

Intensity (a.u.)

Satellite

Cu2+

S3

+

Cu

S2

S1

925

930

935

940

945

950

955

960

Binding Energy (eV)

(c)

O 1s

529.2 eV 530.7 eV

S3

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


S2

S1 525

530

535

Binding Energy (eV)

Figure 5. XPS spectra of S1 (Cu2O-(R)TiO2), S2 (CuO-(R)TiO2), and S3 (Cu3TiO2-(R)TiO2) samples. (a) Ti 2p, (b) Cu 2p, and (c) O 1s spectrum.



(a)

100

S2

S1

0.8 0.6 0.4 S1

0.2

(b) S1 S2 S3 P25

80

S3

[(1-C)/C0]100

1.0

-ln[(t)/(0)]

60 40 20

S2 S3

0.0 0

1

2

3

4

5

6

7

8

0

Time t (h)

Dark

Visible

UV

Figure 6. Dark adsorption and photocatalytic activity comparison of the samples S1 (Cu2O(R)TiO2), S2 (CuO-(R)TiO2), and S3 (Cu3TiO2-(R)TiO2). (a) First order exponential approximation,  (t ) /  (0)  e  K t , of MB degradation kinetics in the dark (dotted lines) and under visible illumination (solid lines). (b) Relative percent of MB removed after 10 hrs in dark, and under visible and UV illuminations. c

Current density j (mA/cm2)

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

Page 28 of 29

0.23 V 0.1 0.0 -0.1 -0.25 V

Sample S1 Sample S2 Sample S3

-0.2 -0.3

-0.6 -0.4 -0.2

0.0

0.2

0.4

0.6

Potential (V vs Ag/AgCl)

Figure 7. Dark CV curves of samples S1 (Cu2O-(R)TiO2), S2 (CuO-(R)TiO2), and S3 (Cu3TiO2(R)TiO2) for comparison. The anodic and cathodic peaks are +0.23 and -0.25 V for S1, +0.22 and -0.22 V for S2, and +0.20 and -0.22 V for S3.


Page 29 of 29 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


Figure 8. The photocurrent response of the samples S1 (Cu2O-(R)TiO2), S2 (CuO-(R)TiO2), and S3 (Cu3TiO2-(R)TiO2) in 0.5 M borate (pH adjusted to 7.0 using KOH) solution under solar simulator (AM 1.5G) at –0.3 V vs. Ag/AgCl, 3 M KCl.


Hybrid CuxOâTiO2 Nanopowders Prepared by Ball Milling for Solar

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