Effect of Moisture on Dopant Segregation in Solid Hosts - The Journal

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Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

Effect of Moisture on Dopant Segregation in Solid Hosts Pragathi Darapaneni,† Natalia S. Moura,† Darrell Harry,†,‡ David A. Cullen,§ Kerry M. Dooley,† and James A. Dorman*,† †

Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, United States Department of Chemistry, Southern University and A&M College, Baton Rouge, Louisiana 70807, United States § Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡

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S Supporting Information *

ABSTRACT: Transition metal-doped semiconductor materials are extensively employed for light harvesting and photocatalytic applications owing to their increased light absorption and charge mobility. In this work, spatial tailoring of the Ni dopant in TiO2 nanostructures is performed by varying the secondary processing parameters to engineer the resulting optoelectronic properties for select applications. Specifically, the aging of the dried Ti sol and the resulting Ni segregation are observed to be moisturedriven phenomena based on the infrared and time-resolved UV−vis spectroscopy measurements. While X-ray diffraction and scanning transmission electron microscopy coupled with electron energy-loss spectroscopy characterizations show a clear difference in the crystal structures between pristine TiO2 powders and phase-segregated NiO− TiO2, the thermogravimetric measurements reveal substitution of the ethoxy group by ambient moisture, resulting in the ejection of hydroxylated Ni clusters. Furthermore, the doped system could be locked into a metastable state by rapidly annealing the amorphous powders. Finally, the photocatalytic activity of these different TiO2:Ni2+ (15 mol %) nanoparticles under AM 1.5G solar light highlights the relationship between the photocatalytic activity and the dopant position. This ability to spatially control dopants within highly doped materials allows for direct control of specific optoelectronic properties, paramount for photoelectrochemical devices.

1. INTRODUCTION TiO2 is one of the most extensively studied compounds for a myriad of applications such as photocatalysis,1,2 solar cells,3 and electrochemistry,4,5 owing to its nontoxicity,6 oxidation power,7 chemical stability,8 and resistance to corrosion.6,9,10 Unfortunately, the relatively high band gap of TiO2 (above 3.0 eV)11 and the fast carrier recombination rate12 reduce the efficiency of TiO2 in the photochemical processes. Therefore, in order to extend the optical absorption of TiO2 from the UV to the visible region,13−15 while simultaneously improving the photochemical performance, various methods such as chemical doping,5,16 dye sensitization,17 and narrow gap semiconductor coupling18,19 have been employed to modify the electronic structure. Among all of these, doping with transition metals (TMs) has received great attention because of the effective narrowing of the band gap and increased carrier concentration.1 Although high-doping concentrations result in significant red shifts of the band gap,20 they are seldom employed because of the limitations in solubility and the formation of recombination centers.16,21 Although there have been many studies that have focused on optimizing the doping concentration to yield a select photophysical response,16,22 the positioning of the dopant (substitution vs interstitial, surface vs bulk) in the host lattice still remains a synthetic challenge. The dopant spatial distribution is often ignored in the literature1,23,24 despite the direct effect on the physical, chemical, optoelectronic, and magnetic properties © XXXX American Chemical Society

of the nanoparticles (NPs). Moreover, the homogeneous distribution of dopants in host TiO2 is frequently validated based on bulk characterization methods [i.e., X-ray diffraction (XRD), UV−vis, etc.] that are insensitive to the local dopant environment in the host lattice.25−27 At doping concentrations beyond the solubility limit, the formation of metallic/metal oxide clusters is reported without discussion of their impact on derived properties.28 Therefore, the ability to control the dopant position within the nanostructure is critical to the photochemical performance and must be fully understood. In the present work, Ni2+-doped TiO2 NPs, with doping concentrations up to 15 mol %, were synthesized via sol−gel chemistry. Secondary processing conditions of the sol, such as drying and annealing, were observed to influence the segregation of NiO clusters in TiO2, altering the resulting optoelectronic properties. The bulk crystal structures of TiO2:Ni2+ (15 mol %) NPs were identified from XRD, and the spatial distribution of dopants was probed via scanning transmission electron microscopy (STEM) coupled with electron energy-loss spectroscopy (EELS). Fourier transform infrared (FTIR) and timeresolved UV−vis spectroscopic results show that the aging of the dried TiO2:Ni2+ sol is a moisture-sensitive phenomenon. Received: February 1, 2019 Revised: April 15, 2019

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DOI: 10.1021/acs.jpcc.9b01067 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. TiO2:Ni2+ (15 mol %) NPs that are formed from pristine and aged powders are quantified and imaged using (a) XRD, (b) UV−vis absorption, (c,d) STEM imaging with the insets showing the Ni chemical maps, and (e,f) EEL spectra at the specified beam point in Figures (c,d). These STEM−EEL point spectra demonstrate that NiO NPs are segregated for air-exposed and annealed TiO2:Ni2+ (15 mol %) NPs.

2. EXPERIMENTAL SECTION 2.1. Materials. Titanium(IV) isopropoxide (TTIP, Acros Organics, >98%), nickel (II) chloride hexahydrate (NiCl2· 6H2O, BTC, > 99%), hydrochloric acid (HCl, 36−38.5% purity, ACS grade), and reagent alcohol (ethanol, 5 mol %).33−35 Furthermore, UV−vis absorption measurements were performed on these NPs to corroborate the XRD observations and identify the Ni coordination in the TiO2 matrix. The peaks in the absorption spectra (Figure 1b) are labeled according to the crystal field theory for Oh coordination of Ni2+.36−39 In addition to the spin-allowed transitions, the spin-forbidden transitions, 3A2g−1T1g (G) and 3A2g−1E (D), are observed in the reference NiO and NiO-segregated TiO2 NPs and are attributed to the spin−orbit coupling and antiferromagnetic ordering in NiO.40,41 To better understand the local structural differences in the TiO2:Ni2+ (15 mol %) NPs formed from pristine and aged powders, HAADF−STEM imaging was performed. The HAADF image of the TiO2:Ni2+ (15 mol %) NPs that are formed by annealing the pristine powders does not show high contrast ratio regions, suggesting homogeneous Ni distribution (Figure 1c). This was further confirmed from the EELS chemical map of Ni (inset of Figure 1c) of the highlighted region. Lattice fringe spacing of 3.47 Å was identified for these NPs (Figure S4) for the (101) plane of anatase TiO2.42 The corresponding EEL spectrum (Figure 1e) at the specified beam spot in Figure 1c exhibits peaks corresponding to the Ti L (458 eV), O K (532 eV), and Ni L (852 eV) edges. For the aged and annealed TiO2:Ni2+ (15 mol %) NPs, separate cubic NiO NPs can be observed (Figure 1d). Furthermore, the EELS chemical maps of the rectangular region in Figure 1d indicate the segregation of Ni into separate cubic NPs (inset of Figure 1d). Accordingly, the EEL spectrum at the specified spot in this region is composed of only Ni L and O K edge peaks (Figure 1f). Moreover, the darker regions exhibit peaks corresponding to Ti L and O K edges (Figure S5), showing no traces of Ni. From the high-resolution TEM (HRTEM)−EELS characterization, it is evident that the NiO NPs are segregated from the host TiO2 for the aged and annealed TiO2:Ni2+ (15 mol %) NPs.

by the same method without the addition of Ni precursor. The amorphous powders were annealed in air at 450 °C for 2 h to form crystalline powders. 2.3. Photocatalytic Activity Evaluation. The photocatalytic activity of as-synthesized TiO2:Ni2+ (15 mol %) NPs was determined from the degradation of the MB dye under solar light AM 1.5G, simulated using a Sinus-220 solar simulator (intensity 785 W/m2). In a typical experiment, 240 mg of TiO2:Ni2+ (15 mol %) NPs was added to 200 mL of 0.015 mM MB under vigorous stirring and air bubbling.31 The stirring was continued in the dark for an hour to reach adsorption equilibrium, and the initial concentration was measured as Co. Aliquots were taken every 30 min from the continuously stirred solution, and the absorption spectra were recorded to determine the concentration of MB dye (Ct) after exposure time, t. The log(Co/Ct) is plotted against exposure time (t) to determine the reaction rate constant. The absorption peak of MB at 664 nm was taken as a reference to calculate the MB concentration in the solution. 2.4. Structural and Optical Characterization. The shape and size of TiO2:Ni2+ NPs were determined by TEM using a JEOL JEM-1400 operating at an accelerating voltage of 120 kV. TEM images were collected using a Gatan Orius 2k × 2k CCD camera. The powder sample was dispersed in toluene and dropcasted on a lacey carbon type-A, 300 mesh copper grid prior to imaging. Furthermore, to determine the dopant spatial distribution, aberration-corrected high-angle annular dark-field (HAADF)−STEM imaging was performed using the NionUltraSTEM 100 (U100) electron microscope operated at 100 kV and equipped with third-generation C3/C5 aberration corrector, Gatan Enfina electron energy-loss spectrometer, and a cold FEG source. The crystal structure was identified by performing powder XRD using a PANalytical X-ray diffractometer operating at 45 kV and 40 mA. The θ−2θ radial scan was performed over the range 5−70° with a step size of 0.03° with dwell times up to 60 s, using Cu Kα1 (λ = 1.54 Å) as a radiation source. TGA was performed with a TA SDT Q600 DSC−TGA under air flow to monitor the crystallization process of dried TiO2:Ni2+ powders. The temperature was programmed from 25 to 450 °C at 4 °C/min. The optical absorption spectra were recorded using a PerkinElmer Lambda 900 UV−vis−NIR spectrometer equipped with an integrating sphere and a center-mounted sample holder. The absorption scans ranging from 1300 to 300 nm with a scan rate of 1 nm/s were obtained on the NPs dried on the glass substrates. The change in monochromators was set to occur at 900 nm. FTIR spectroscopy was performed in the diffuse reflectance (DRIFTS) mode using a Thermo Scientific Nicolet 380 FTIR with a DTGS detector. The data are reported in Kubelka−Munk (f(R)) units, with background taken after 30 min of N2 purge, and a resolution of 4 cm−1 from 4000 to 1000 cm−1. 2.5. Electronic Characterization. The oxidation states of Ti, Ni, and local chemical environment around Ni dopant were identified using X-ray photoelectron spectroscopy (XPS) measurements. A Scienta-Omicron ESCA 2SR XPS system equipped with Al Kα monochromatic source and hemispherical analyzer with a 128 channel detector was used to perform these measurements on TiO2:Ni2+ (15 mol %) NPs. The collected spectra were calibrated to adventitious C 1s peak at 284.6 eV. C


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The Journal of Physical Chemistry C Before turning to understand the effect of these structural differences on the optoelectronic properties of TiO2:Ni2+ (15 mol %) NPs, it is necessary to shed light on the processing parameters of the NPs that impacted the formation of these different crystal structures. As shown above, the aging of the dried powders is the key parameter in controlling the segregation of NiO clusters in the host TiO2. This aging response in air is likely due to hydroxylation of the dopant followed by cluster segregation. To verify this, FTIR studies were performed on the dried TiO2:Ni2+ (15 mol %) powders to identify the surface functional groups (Figure 2). Distinct peaks

Figure 3. UV−vis absorption spectra of dried TiO2:Ni2+ (15 mol %) powders exposed to air for different aging times. The absorption spectrum is blue shifted with the increase in the aging time, consistent with the inset pictures that show color change from yellow to green.

bright yellow color is a result of distorted octahedral environment, whereas the green color corresponds to octahedral symmetry around the Ni2+.40 This change in the value of 10 Dq and the corresponding in the local symmetry suggest that the interaction of atmospheric moisture with the dopant is causing a change in the local bonding environment around Ni2+ in the host TiO2. The calculated time-dependent 10 Dq values for the aged TiO2:Ni2+ (15 mol %) sol are tabulated in Table 1, indicating the bonding of the cation (Ti/Ni) to more electronegative hydroxyl groups.48,49

Figure 2. FTIR spectra of dried TiO2:Ni2+ (15 mol %) powders showing the dampening of organic peaks and increase in the surface hydroxyl concentration upon aging.

Table 1. Increase in the Value of 10 Dq of Dried TiO2:Ni2+ (15 mol %) Powders with Aging Time

corresponding to the C−H alkane stretches (2930 and 2970 cm−1) and CH2 bending modes (1380 cm−1) are identified for the pristine powders.6,43−45 Furthermore, a broad O−H stretching mode is observed in the 3300−3700 cm−1 region. These O−H stretches, corresponding to terminally bound and bridged hydroxyls, increase in intensity for the aged powders,46 specifically for the terminally bound −OH species (3540 cm−1), and are attributed to the physisorbed water molecules.47 Simultaneously, a corresponding decrease in the organic peaks (C−H stretches/CH2 bending modes) is observed for the aged powders. Although ethoxide and ethanol groups are adsorbed on the surface of the dried TiO2:Ni2+ (15 mol %) powders, the dampening of the C−H peaks is attributed to the substitution with water.44 A similar increase in the hydroxyl concentration and decrease in the organics are observed for the aged TiO2 powders (Figure S6). These FTIR results demonstrate that the dried TiO2:Ni2+ (15 mol %) powders undergo hydroxylation upon atmospheric exposure, and as a result, the surface hydroxyl concentration is increased for the aged powders. To correlate the observed increase in the hydroxyl concentration with the changes induced in the ligand field strength of the dried powders upon aging, time-resolved UV−vis absorption spectra were collected on the powders prior to annealing. The powder was exposed to the atmosphere, and samples were collected at regular time intervals up to 48 h (Figure 3). From visual inspection, a color change from yellow (pristine powders) to green (aged powders) is observed upon aging in air (inset of Figure 3), consistent with the blue shift of the absorption spectrum. The yellow to green color change is indicative of a change in the crystal field splitting energy, 10 Dq. Moreover, previous studies on complex Ni oxides report that the

transition

time (h)

A2−3T2 (F)

0

6

48

expt. 10 Dq calc. 10 Dq

0.92 eV 0.94 eV

0.97 eV 1.04 eV

1.00 eV 1.13 eV

3

Although UV−vis and FTIR spectroscopic results suggest the hydroxylation of the dried powders upon aging, they cannot predict the subsequent structural changes during the annealing process. Therefore, to understand the bond formation during this process, TGA−DSC studies (Figure 4) were performed. The TGA curve in Figure 4a shows that the weight loss for pure TiO2 (1.5%), pristine (3.4%), and aged (29.9%) powders of dried TiO2:Ni2+ (15 mol %) sol in the lower temperature region (25−125 °C) is attributed to the physisorbed solvents. This increase in the weight loss with aging time was confirmed by performing similar heat treatment studies on the 3 h aged powders of TiO2:Ni2+ (15 mol %) (Figure S7). The higher weight loss associated with the 48 h aged powders indicates the volatilization of ethanol from the hydrated organic matrix, which is consistent with the UV−vis and FTIR results, suggesting hydroxyl substitution upon aging. Accordingly, in the DSC plot (Figure 4b), a broad endothermic peak is observed for pure TiO2, whereas the sharp peaks observed at 70 and 100 °C for pristine and aged powders of TiO2:Ni2+ (15 mol %) are due to the evaporation of physisorbed ethanol50,51 and water,46 respectively. With further increase in the temperature for pure TiO2, an exothermic peak corresponding to the combustion of organic species is observed at 330 °C, followed by anatase D


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Figure 4. (a) TGA and (b) DSC plots of dried TiO2:Ni2+ (15 mol %) powders at different aging times. The crystallization of TiO2 and NiO is combined in a broad exothermic peak in the aged TiO2:Ni2+ (15 mol %) powders.

Figure 5. Schematic showing the hydration of amorphous organometallic matrix of Ti, Ni upon aging in air. After annealing, the crystalline phases of NiO and TiO2 co-exist because of the stabilization of Ti(OH)4 clusters and the lower calcination temperature of NiO.

crystallization at 400 °C.6 Similar peaks are observed at 230 and 400 °C for the pristine powders of TiO2:Ni2+ (15 mol %).52 For the aged powders, the first region (220−330 °C) is attributed to the removal of the unhydrolyzed isopropoxide groups6 and the second region from 330 to 400 °C corresponds to the formation of the nickel oxide phase.53,54 This co-existence of NiO and anatase TiO2 phases in aged then annealed TiO2:Ni2+ (15 mol %) NPs was observed earlier in the XRD patterns and HAADF− STEM images (Figure 1). On the basis of above observations, the dopant incorporation and segregation mechanism in solid hosts are illustrated in Figure 5. The doping of Ni in TiO2 follows a nucleation-doping mechanism,55 as the Ti−O−Ni bonds are formed during the polycondensation step.56,57 After drying the sol, an amorphous organic matrix consisting of Ti−OR−Ni bonds is formed. During the aging process, the ethoxy groups are displaced by hydroxides from ambient water.44 The hydroxylated clusters of Ti and Ni coexist in the aged powders with the Ti(OH)4 being more stable compound because of its low hydroxylation energy.58,59 Upon annealing, the nanocrystals undergo a selfpurification effect via diffusion of the Ni(OH)2,60,61 followed by lattice ejection due to the lower NiO crystallization temperature.62 Moreover, the segregation of NiO NPs is clearly a function of the concentration of the parent hydroxylated Ni clusters, resulting in a strong NiO diffraction peak for the aged NPs in the XRD pattern (Figure 1a), and is attributed to the higher probability of Ni−OH formation. This key finding for the

highly doped TiO2 nanocrystalsthat the dopant position in the host lattice can be tailored by varying the secondary processing parameters of the doped TiO2 solwill provide an important means to selectively tune the optoelectronic properties of these materials. In all of the previously mentioned TiO2:Ni2+ (15 mol %) NPs, longer annealing times (2 h) were employed. However, recent literature reports show that the transformation of amorphous titania into a crystalline anatase phase can be achieved within 30 min of thermal treatment at temperatures as low as 350 °C.63,64 In order to take advantage of this rapid heat treatment for doped TiO2, pristine powders of the TiO2:Ni2+ (15 mol %) were rapidly annealed at 450 °C for 30 min. The FTIR spectra (Figure S8a) show that lowering the annealing time from 2 h to 30 min has no effect on the combustion of organic species. However, rapidly annealed dried powders exhibit a poorly crystallized anatase structure (Figure S8b). Furthermore, the dopant distribution in these NPs, probed using HAADF−STEM coupled with EELS (Figure S8c), suggests uniform distribution of Ni dopants in TiO2. The corresponding EEL spectrum at the beam spot in Figure S8c shows the peaks for Ti L edge, O K edge, and Ni L edges (Figure S8d). These results indicate that rapid annealing can be performed to lock the dopant in the host lattice. The doped system is therefore quenched in the metastable state (Ti0.85Ni0.15O2) during the lattice incorporation phase. E


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photophysical responses can be modified by tailoring the dopant position, reinforcing the structure−property relationship in these TM-doped materials. Moreover, higher photocatalytic reaction rates can be obtained for optimum doping concentrations of TiO2:Ni2+ NPs, which is however, not the focus of this paper.

The literature available for doped TiO2 reports a wide range of optimum doping concentrations, between 0.5 and 15 mol %, for photochemical investigations with little reported on the dopant position.31,65,66 To probe the role of dopant spatial distribution in photocatalysis, the degradation of MB was performed over these TiO2:Ni2+ (15 mol %) NPs (Figure 6). The degradation

4. CONCLUSIONS In summary, TiO2:Ni2+ (15 mol %) NPs were synthesized using a sol−gel chemistry. The difference in the crystal structure and the optical absorption for the NPs formed from pristine versus aged powders of TiO2:Ni2+ (15 mol %) was identified from XRD and UV−vis spectroscopy. HAADF−STEM images coupled with EELS confirmed the segregation of cubic NiO NPs from the aged and annealed TiO2:Ni2+ (15 mol %) NPs. This difference due to aging of the dried TiO2:Ni2+ (15 mol %) powders was investigated using FTIR and time-resolved UV−vis absorption spectroscopy. Consistent with the FTIR spectra that show an increase in the surface hydroxyl concentration with aging, the crystal-field splitting energy of the dried powders also increased with exposure time, suggesting the replacement of ethoxy with hydroxyl groups on surface cations. Furthermore, the NiO segregation mechanism in the aged and annealed TiO2:Ni2+ (15 mol %) NPs was elucidated from the TGA−DSC studies. Aging of the dried powders in air results in the formation of Ni(OH)2, which segregate into large NiO clusters upon annealing. The lower calcination temperature of NiO and the higher thermodynamic stability of Ti(OH)4/TiO2 clusters favored the coexistence of NiO and TiO2 phases in the aged and annealed TiO2:Ni2+ (15 mol %) NPs. The effect of annealing time on the doped system was demonstrated by rapidly annealing the dried TiO2:Ni2+ (15 mol %) powders for 30 min. The EELS analysis of these NPs shows a uniform distribution of dopants (Ni2+) with peaks corresponding to Ti, Ni, and O elements, indicating that the doped system was quenched in the metastable state during dopant incorporation. Finally, the photocatalytic degradation of MB was observed under simulated AM 1.5G using these different treatments for the TiO2:Ni2+ (15 mol %) NPs. The difference in the pseudofirst-order rate constants suggests that the photocatalytic responses can be modified by controlling the dopant position via secondary processing parameters of the dried TiO2:Ni2+ (15 mol %) powders. This ability to spatially control the dopant position in the solid host to engineer the optoelectronic properties of the material will be the first step toward the development of next-generation metal oxide-based photoelectric devices with tunable properties.

Figure 6. Photocatalytic degradation of MB under solar light using no catalyst, TiO2, TiO2:Ni2+ (15 mol %) NPs, and NiO-segregated TiO2 NPs. The slope of the ln(Co/Ct) vs t plot gives the pseudo-first-order rate constant.

rate was analyzed according to first-order kinetics, r = kCt (k is the pseudo-first-order rate constant).31,67 Interestingly, the highest degradation rate is obtained for pure TiO2 with the pristine sample having the least activity. This can be attributed to the injection of photoexcited electrons from the MB dye to the titania NPs, increasing the activity of pure TiO2.68,69 Although these results are counter to most literature reports20,30,70 that show higher catalytic activity for the doped TiO2, they can be explained by the formation of oxygen vacancies (Ti3+) or black NiO NPs, that can be occasionally observed in their optical absorption spectra.20,30 Additionally, many of these reports utilize narrow wavelengths for the excitation of the samples,66 instead of using a simulated solar spectrum, driving the degradation only upon Ni−O absorption. In the present case, for the pristine and annealed TiO2:Ni2+ (15 mol %) NPs, the charge carriers trapped by one dopant are combined with charge carriers trapped by another dopant, resulting in poor charge separation in these highly doped TiO2 NPs.21,71 On the other hand, the aged samples, that is, NiO-segregated TiO2 NPs, resulted in degradation rates closer to that of pure TiO2. Although it is documented in the literature that the interband gap Ni 3d states act as recombination centers for highly doped TiO2:Ni2+ NPs,21,31 it is believed that the NiO segregation eliminates these states which results in higher degradation rates. Therefore, the observed variation in the photocatalytic responses of these TiO2:Ni2+ (15 mol %) NPs under AM 1.5G excitation can be attributed to the dopant position, where the positioning of the dopant electronic states within the band gap of the host material dictates the recombination kinetics. Traditionally, the photocatalytic reaction rates are observed to be a function of the doping concentration, where the highest degradation efficiency is achieved for an optimum doping concentration.31 In the present case, lower degradation rates observed for the highly doped TiO2:Ni2+ (15 mol %) NPs compared to the NiO-segregated samples indicate that their

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.9b01067. Low-res TEM images, XRD patterns, Lattice parameter calculations, XPS spectra, HAADF images, EEL spectra, FTIR spectra, and TGA−DSC data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Kerry M. Dooley: 0000-0002-9476-3855 James A. Dorman: 0000-0002-3248-7752 F


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The Journal of Physical Chemistry C Author Contributions

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P.D. and D.H. have performed the synthesis of TiO2 and TiO2:Ni2+ NPs. P.D. has completed the XRD, UV−vis, HRTEM, and TGA−DSC analysis. P.D. and K.M.D. have analyzed the FTIR data. D.C. performed the HAADF imaging along with the acquisition of EEL spectra. P.D. and N.M. did the TEM measurements. P.D. and J.A.D. have planned and contributed to the writing of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.D. acknowledges the Louisiana Board of Regents (LEQSF(2016-19)-RD-A-03) for financial support. N.M. acknowledges the funding from U.S. Department of Energy (DOE) under EPSCOR grant no. DE-SC0012432. STEM analysis was conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. We thank Dr. Kevin McPeak for access to his laboratory, specifically the solar simulator for the photocatalytic measurements.

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