Facet‐Dependent Interfacial and Photoelectrochemical Properties of TiO2 Nanoparticles

The photoelectrochemical properties of titanium dioxide (TiO2) nanoparticles are directly related to the presence of certain facets. The synthesis of such particles is challenged using volatile and sensitive precursors utilizing shape capping agents. In this study, an easier two‐step synthesis technique for the preparation of well‐specified morphology TiO2 nanocrystals from TiO2 powder is demonstrated. The facet‐dependent Fermi level and flat band potential of the prepared particles are calculated and their photoelectrochemical properties are investigated by Cu2+ and Pb2+ photo/electrodeposition. From Cu2+ electrodeposition, it is shown that the presence of {100} and {001} facets in cubic morphology facilitates the electrodeposition with progressive nucleation mechanisms, while the presence of {101} facet in octahedral geometry follows instantaneous nucleation mechanisms. The activity of electroreduction is also related to the flat‐band potential which shows the highest activity in Pb2+ electrodeposition for octahedron nanostructure due to the alignment of the reduction potential to the edge of the flat band. Photodeposition of Cu2+ and Pb2+ ions shows identical trends to electrodeposition indicating the facets influence in ion adsorption and structure of the bandgap of morphological TiO2. The findings emphasize the importance of facet‐dependent surface adsorption and bandgap structure designing faceted TiO2 nanoparticles for tailored applications.


Introduction
Titanium dioxide (TiO 2 ) possesses superior photoelectrochemical properties combined with high stability, low cost, and nontoxicity, which made it one of the most studied materials for DOI: 10.1002/admi.202300555photo related applications. [1,2]The anatase phase attracts more interest than the rutile and brookite phases due to its better charge separation and transport properties. [3,4]11][12][13][14] The enhanced activity of certain facets can be related to the variation in coordination and orientation of the atoms which results in different surface energies. [4,8,10]There are three main facets in anatase TiO 2 .Those facets are {101}, {100}, and {001} with calculated average surface energies of 0.44, 0.53, and 0.90 J m −2 respectively. [15][18] It is also shown that different faceted nanostructures possess distinct electronic levels. [19,20]The position of the Fermi level in the vacuum and flat-band potential relative to reference electrodes in electrolyte have shown variation by changing the morphology of TiO 2 . [21,22]Moreover, studies have shown a preferential flow of photogenerated charge carriers toward specific exposed facets. [10,23]btaining high-quality exposed facet nanocrystals with relatively simple preparation methods is required for their widespread application.The preparation techniques for TiO 2 nanoparticles with tailored exposed facets vary from hydrothermal, sol-gel, coprecipitation, and deposition.Several reviews summarize these preparation methods in detail. [22,24,25]In all methods, the aim is to form high-quality, well-specified morphologies with fewer steps and a minimum amount of material.The hydrothermal method is favored for its ease and controllable preparation of the faceted crystals.The morphology of the nanoparticles is controlled by shape-capping agents which are adsorbed to specific facets during nucleation and the growth of the particles.The temperature and pressure of the reaction vessel are adjusted to give the desired size and shape of the nanoparticles.The hydrothermal preparation of morphological TiO 2 nanoparticles from sodium titanate nanowires was demonstrated to be facile to perform with less toxic chemicals.In two different studies, Yang et al. and Erdural et al. prepared octahedral TiO 2 nanoparticles from sodium titanate nanowires using alkali metal ions as the shape-capping agents. [26,27]The method takes advantage of the slow release of the Ti ions during the decomposition of the nanowires which consequences in the controlled growth of the desired facet by the adsorbed shaping agents.It has the advantage of using TiO 2 powder as the precursor which is easier and safer to handle than the commonly used precursors such as Ti(IV) isopropoxide (TTIP), Ti(IV)-n-butoxide (Ti (OBu) 4 ), Ti(IV) acetylacetonate, Ti tetrachloride (TiCl 4 ), and Ti tetrafluoride (TiF 4 ). [28,29]Here, we use sodium titanate nanotubes as a precursor to prepare three different TiO 2 nanoparticle morphologies which are prepared from TiO 2 powder.We use different proportions of F − and Cl − as shape-capping agents to obtain the desired facets.
The shape caping-agents act to stabilize the surface of certain facets by chemical adsorptions.The surface energy level is reduced by adsorption which allows for the growth of the facet. [15]If the caping agent is not used the facet with high energy tend to diminish quickly to minimize the surface energy during the crystal growth process. [30]It has been shown that different halogens and ions tend to adsorb with different strength at different facets which allow for the shape control of the TiO 2 nanoparticles. [31]he reactivity of certain facets of anatase TiO 2 nanocrystal toward several photoinduced reactions such as water oxidation is related to the types and density of the undercoordinated atoms, and charges (holes or electrons). [17,21,32]The change in coordination results in variation in the position of the Fermi level or flat band potential relative to the reaction.The reactivity will not only depend on the surface energy of the facet but also on the electronic band structure of the particle. [10,33]In the literature, different reactions are utilized as probes to determine the reactivity of the facets. [8,10,23,32,33]Nevertheless, there are contradictions among the results which are related to the presence of adsorbed species on the surface of the facet, the use of different-size nanoparticles, and the variation in the electronic band structures among the studies. [8,32]Though the preparation method will have a great influence on the photocatalytic activity of the faceted TiO 2 along with the electronic band structure and the surface properties.
Considering the above-mentioned points, we introduce a twostep preparation method of well-defined anatase TiO 2 nanocrystals starting from commercial TiO 2 powder with three different morphologies as shown in Figure 1.The prepared morphologies are cubic which is dominated by {001} and {100} facets, truncated octahedron with {101} and {100} facets, and octahedron with only {101} facets.X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), and Transmission Electron Microscopy (TEM) analysis were performed to determine the presence of the single crystal anatase TiO 2 with the intended facets.The composition, surface orientation and Fermi levels were characterized by X-ray Photoelectron Spectroscopy (XPS) and Mott-Schottky analysis.After that, we studied the reactivity of the formed facets by the electrodeposition/stripping of Cu and Pb ions.This kind of investigation is used by us for the first time in the literature to investigate the facet-dependent properties of TiO 2 nanoparticles.We demonstrate how the presence of certain facets causes a shift in the reduction potential and change in the nucleation and growth mechanism due to the change in the TiO 2 /electrolyte interface.We also show how the position of the flat band is important for determining the reactivity of the facet toward the photo/electrodeposition of the ions.

Nanoparticles Characterization
The formation of the single crystal TiO 2 nanoparticles in the anatase phase was confirmed by X-ray diffraction patterns shown in Figure 2a.Most of the patterns match the anatase (Joint Committeeon Powder Diffraction Standards (JCPDS) card no.21-1272) pattern indicating the formation of anatase crystals nanoparticles.However, small peaks exist in the pattern which can be clearly seen in Figure S1 (Supporting Information).The peaks are visible around 9.5 0 , 27.5 0 , 36 0 , 42 0 , 62 0 2 in all morphologies and extra for octahedron morphology at 29 0 , 43 0 2 indicating the presence of other crystal structures or impurities.We relate the present of those peaks to the titanate crystal structure which is the precursor used to form the anatase nanoparticles and of rutile crystal structure which is present in the TiO 2 powder that we used as starting material in our synthesis.The XRD pattern for the commercial TiO 2 powder is shown in Figure S1 (Supporting Information).The rutile (JCPDS card no.21-1276) and titanate (JCPDS card no.47-0192) pattern is also shown in Figure 1a which matches all the small peaks presents in the pattern except for the extra peaks in the octahedron morphology.The peaks are related to the presence of impurities from the caping agents used lithium perchlorate (LiClO 4 ).The presence of the titanate patterns originates from the incomplete conversion to the anatase phase during the hydrothermal treatment.
The peak intensity varies in each morphology because of differences in the crystal planes.The ratios of the peak intensities of I (101)/(200) were calculated and shown as bar graphs in Figure 2b.It was found that I (101)/(200) to be similar for octahedral (3.95) and truncated (3.97) particles and lower for cubic (3.64) morphology.This is expected due to the domination of {100} plane on cubic morphology and the domination of {101} plane on the octahedral morphology.Truncated particles showed a similar ratio to octahedral particles due to the existence of both planes in the structure.
The morphology of the formed particles was studied by SEM and TEM.The formation of the intended shapes was confirmed from the obtained images in Figure 2c-e.The presence of different facets in each morphology is demonstrated in Figure 2f-h for cubic, for truncated, and for octahedron morphology.The formation of the exposed facets can be deduced from SEM and TEM images.Cubic particles are expected to have {100} and {001} facets, truncated particles to have {100} and {101} facets, and octahedral particles to form only {101} facets.The formed facets in each morphology were analyzed by X-ray diffraction patterns along the TEM analysis.Figure S2 (Supporting Information) shows the diffraction patterns for each morphology taken from the highresolution TEM of the specified facets.As can be seen, the formation of {100} and {001} facets are formed in cubic particles.{001} and {101} facets on truncated particles and only {101} facets in the octahedron particles.
The particle size distribution histograms obtained from SEM and TEM images are shown in Figure 1i.The octahedron morphology shows a narrow distribution between 35 and 110 nm with an average particle size of 65 nm which was followed by the cubic morphology between 55 and 220 nm with an average particle size of 105 nm.On the other hand, truncated morphology was found to have wide particle size distribution between 90 and 400 nm with average particle size of 218 nm.The average size was measured along the edge of the cubic and truncated particles and the horizontal corners of the octahedron particles.The size variation can be related to the use of different capping agents for each morphology and the different hydrothermal treatment duration.
The composition and surface structure of the synthesized nanoparticles with the exposed facets were characterized using Raman, Fourier Transform Infrared Spectroscopy (FTIR) and XPS. Figure 3a,b shows the FTIR and the Raman spectra of the three morphologies, respectively.The FTIR spectra shows a broad band at 3115 cm −1 which corresponds to the stretching mode of hydroxyl group (O-H) of the water molecules and TiO 2 with also a weak peak at 1635 cm −1 which is attributed to the bending vibration of O-H. [34,35]The peak at 1535 cm −1 can be attributed to the stretching mode of carboxyl (C═O) group from the adsorbed CO 2 molecules. [36]The broad absorption band between 400 and 900 cm −1 are ascribed to the vibration absorption of the titanium oxygen bonds (Ti-O-Ti) in TiO 2 nanoparticles originating from the anatase and titanate phases. [37,38]The extra two bands in the octahedron morphology at 3400 and 1420 cm −1 can be attributed to the presence of perchlorate (ClO4 -) functional group from the caping agent used LiClO 4 .It is reported that the presence of ClO4 -results in a weak hydrogen-bonding , which results in a positive shift in the O-H band. [39]The band at 1420 cm −1 is attributed to the formation of carbonate functional groups with the remaining of Li-ion. [40]aman spectra shown in Figure 2b demonstrate the five characteristic Raman active modes of anatase TiO 2 .The modes observed are E g , E g , B 1g , A 1g , and E g at 148, 197, 397, 51, and 630 cm −1 , respectively.These characteristic vibrational frequencies and their intensity ratios confirmed the domination of anatase TiO 2 .[41,42] However, following the results observed in the XRD, small peaks were observed which can be related to the titanate phase.The most obvious mode of vibration is observed at 276 and 443 cm −1 which are reported for the titanate phase.[43,44] The composition of the TiO 2 nanoparticles and the oxidation state of Ti were investigated by XPS analysis.The XPS survey obtained for each morphology is shown in Figure S4 (Supporting Information) which demonstrates the presence of Ti 2s, 2p, 3s, 3p, and O 1s as main peaks with the presence of C 1s for all the morphologies and F 1s for the cubic morphology.The C 1s peak is related to the adventitious carbon contamination on the surface of the particles which were deconvoluted as shown in Figure S5 (Supporting Information).The main peak was found to be at 284.5 eV for the three particles indicating the absence of surface charging.The presence of the F 1s peak is related to the use of HF as the capping agent which is adsorbed on the surface of {001} and {100} facets.[15]   3 eV respectively can be seen. These vlues as reported by other studies correspond to Ti(IV) in anatase TiO 2 form.[45,46] XPS peak of the O 1s region showed the existence of two forms of oxygen.The higher intensity peak is assigned for the oxide oxygen environment (O-Ti) at 529.6 eV and the lower is for the surface hydroxyl groups (O-H) at 531.5 eV.Octahedral TiO 2 particles showed another peak at 530.8 eV which is assumed to be from the perchlorate-oxygen (O-ClO 4 ) from the precursor (LiClO 4 ) used for the synthesis of octahedron nanoparticles.The ratio of the area under (O-Ti) peak to (O-H) peak was found to be 3.34 for cubic, 1.96 for truncated, and 5.68 for octahedra.
The optical properties of the prepared TiO 2 nanoparticles were determined using reflectance Ultraviolet-Visible Spectroscopy (UV-vis) measurement shown in Figure S6 (Supporting Infor-mation).The bandgap of the particles was calculated from the obtained spectra using Tauc plots which are shown in Figure 4a.The values obtained were very close to 3.22 eV which is the reported bandgap value for the anatase phase [47] with cubic 3.10 eV, truncated 3.14 eV and octahedron 3.22 eV.The small changes in the bandgap values for the cubic and the truncated morphologies can be related to the presence of the F − atoms which are shown to cause small negative shifts in the bandgap values. [48,49]PS valence band spectra provide information about the Fermi level of the particles.The XPS valence band spectra for the three morphologies are shown in Figure 4b.Fermi level energy is calculated by fitting the curve to indicate the zero point which corresponds to the Fermi energy of the particle.As can be seen, the three morphologies show different values for the Fermi energy level with the highest being for the truncated (2.3 eV) followed by the cubic (2.1 eV) and lowest for the octahedral (1.8 eV) particles.These values are smaller than the Fermi level values for reported natural anatase crystals with both (101) and (001) surfaces. [50] To determine the surface charge of the prepared nanoparticles, zeta potential measurements were obtained for the dispersions of the particles in water which are shown in Figure S7 (Supporting Information).The dispersions of the particles showed pH values of 5 and the zeta potential values of −36.9 mV for cubic particles, −25.7 mV for truncated particles and −21.3 mV for octahedral particles.These values agree with the reported surface energies for the existing facets with {001} to have the highest and {101} the lowest. [15,25]

Facet-Dependent Photo/Electrochemical Performance
To investigate the morphology effect on the formed electrode/electrolyte interface and surface adsorption properties, we performed electrochemical measurements both in dark and under UV illumination.First, the flat band potential was determined by Mott-Schottky analysis using Electrochemical Impedance Spectroscopy (EIS) in an acidic environment.Second, facet-dependent photo/electrochemical behavior was investigated by TiO 2 -modified glassy carbon (TiO 2 /GC) electrodes in acidic solutions containing Cu 2+ and Pb 2+ ions.The electrodeposition and stripping behaviors were examined for each morphology with the same conditions and the amount of TiO 2 on the GC electrode.
The flat band level was determined using Mott-Schottky measurements in 0.1 m HClO 4 solution.The Mott-Schottky plots are shown in Figure 4c which were calculated from the EIS measurements of the TiO 2 /FTO electrodes at a potential range of −0.1 to −0.7 V versus Ag/AgCl reference electrode.The EIS Nyquist plots and the equivalent circuit fits are shown in Figure S8 (Supporting Information).The impedance data were fitted with an equivalent circuit model containing a constant phase element representing the capacitance of the TiO 2 space-charge layer connected in parallel to a resistor.The constant phase element was converted to capacitance.The calculated flat band potential values show the lowest value for octahedral particles (−0.54 V) and highest for truncated (−0.67 V) particles and (−0.62 V) for cubic particles.These values match the trend obtained in the Fermi level in the energy band structure and are similar to the reported values in the literature. [55,56]Figure 4e shows the schematic for the position of the flat band potentials relative to the formal potentials of Cu 2+ /Cu and Pb 2+ /Pb reactions versus Ag/AgCl reference electrode.From these positions, we can expect no contribution in the charge transfer from TiO 2 for all morphologies except in the octahedral morphology for Pb 2+ /Pb reaction due to the match in the flat band potential.

Cyclic Voltammetry
Cyclic voltammetry (CV) was performed between 0.3 and −0.3 V versus Ag/AgCl potential range to identify the position of electrodeposition/stripping of Cu 2+ and to verify the morphologydependent electrochemical behavior of the TiO 2 /GC electrodes.Figure 5a shows the CV curves of electrodeposition and stripping of Cu 2+ on the TiO 2 /GC electrode for the three morphologies obtained at a scan rate of 20 mV s −1 .A wide reduction peak is observed between 0 and −0.3 V for all the morphologies with different onset and peak potentials.The onset potential for Cu 2+ on the GC electrode is reported to be at −0.034 V versus Ag/AgCl electrode. [56,57]The observed onset potential is −0.01 V for cubic, −0.04 V for truncated, and −0.10 V for octahedral morphologies.On the positive sweep, sharp stripping peaks are observed between 0.05 and 0.15 V with different intensities and peak values.The stripping curves show high current values for the cubic peak at 0.12 V followed by the truncated octahedron peak at 0.11 V and the octahedron peak at 0.13 V.The charges calculated from the area under the stripping peaks show the highest value for cubic morphology with 1.9 mC followed by truncated morphology with 1.5 mC and octahedral morphology with 1.2 mC.
The same CV measurements were performed under UV irradiation to observe the effect of photoinduced electrons in the elecof Cu 2+ which are shown in Figure 5b.UV irradiation causes a decrease in the onset electrodeposition potential for all morphologies.This decrease in the reduction potential is caused by the photoexcited electrons from the TiO 2 nanoparticles indicating the participation of TiO 2 in the charge transfer reaction.There is an increase in the reduction current values as well as the stripping peaks indicating more reduction of Cu 2+ ions.This increase is explained by the reduction of Cu 2+ by both electrochemical potential from the GC and photoexcited electrons from TiO 2 .
The reduction potential of Cu 2+ lies at the bandgap of the three morphologies of the TiO 2 nanoparticles which forbid their contribution to the charge transfer reaction.The change in the activity of Cu 2+ electrodeposition/stripping for certain morphologies can be related to the change in the electrode/electrolyte interface by the presence of certain facets.The high activity of cubic particles in the electroreduction of Cu 2+ is the result of its high adsorption capacity.The presence of {100} and {001} facets which are known for their high surface energy results in the presence of dense TiO 2 /electrolyte interface condensed with Cu 2+ causing more reduction reaction.The contribution of TiO 2 nanoparticles in charge transfer can be achieved by UV irradiation which shows lower overpotentials and results in higher reduction rates.
The same investigation was also performed for Pb 2+ deposition which appears at more negative potentials. [58]Figure 6a,b shows the CVs of the Pb 2+ electrodeposition and stripping under dark and UV illumination, respectively.The highest activity was observed for cubic nanoparticles which is like the Cu 2+ case.
The onset potential was observed to be −0.504V.However, the same trend was not observed for the other morphologies.Octahedral morphology showed higher activity than truncated morphology with an onset potential of −0.505 V. Truncated morphology showed an onset potential of −0.518 V.The high activity of the octahedral nanoparticles is concluded to result from the edge position of the flat band which was determined by Mott-Schottky as −0.45 V versus Ag/AgCl.This value lies at the reduction potential of Pb 2+ allowing the electrons to be transferred to the conducting band and to participate in the reduction reaction.The effect of the position of the flat band potential can also be seen with the CV results under UV illumination.As can be seen from Figure 6b, octahedral particles showed high activity with lower reduction potential and the highest current in the stripping peak.These results demonstrate the importance of both the surface interaction of the faceted TiO 2 and the electronic band structure in the reduction reaction.
The CV measurements of Cu 2+ and Pb 2+ deposition/stripping on bare GC electrode were performed and overlaid with the TiO 2 modified GC electrodes which are shown in Figure S9 (Supporting Information).The lower onset potential and the higher current values demonstrate the variation in the electrode/electrolyte interface by the semiconducting TiO 2 film on the GC.Moreover, the stability of the morphological TiO 2 nanoparticles were investigated after the CV measurements and found to sustain its exposed facets.The SEM images of the particles after the CV measurements are shown in Figure S11 (Supporting Information).

Chronoamperometry
Chronoamperometric scans were performed by stepping the voltage from the open circuit potential to the reduction potential of the ions.In the TiO 2 -modified GC electrode, the transition from equilibrium to steady state reduction reaction is controlled by the reduction sites and the rate of mass transfer of ions.The current shows a transient response which provides information about the nucleation mechanism and enables the estimation of the diffusion constants using the Cottrell equation. [59] 2+ versus Ag/AgCl performed on the TiO 2 /GC electrodes for the three morphologies.In all morphologies and for both ions, the response curves show a sharp current increase instantaneously after applying the reduction potential which is due to the double-layer charging.Then it is followed by a lower increase in the current response resulting from nucleation and growth at the active reduction sites of the TiO 2 /GC electrode.The higher the current values for the second current increase the more active reduction sites on the surface of the electrode and the stronger the metal ion adsorption on the surface of TiO 2 nanoparticles.The reduction proceeds by the growth of each nucleus forming a hemispherical diffusion zone around it which expands and overlaps with the neighboring nuclei.The maximum current value corresponds to this overlap and the transition from the hemispherical to the linear diffusion behavior. [56,60]After the peak, the current response decreases until it reaches the steady state condition which is controlled by the rate of mass transfer described by the Cottrell equation.
For the reduction of Cu 2+ , chronoamperometric scans show the highest current values for cubic morphology followed by truncated and octahedron particles.This indicates the high adsorption capacity for the cubic morphology with {001} and {100} facets.The presence of the {001} along with {101} facet on the truncated morphology can explain the higher activity from the octahedral morphology which possesses only {101}.These ob-servations are consistent with the reported surface energies of each morphology which are expected to increase the adsorption capacity of the metal ions on the surface of the TiO 2 /GC electrode resulting in a rate increase in the reduction reaction.
The nucleation mechanism can be further investigated by the model developed by Scharifker and Hills which describes the mode of nucleation in an electrochemical reduction under mass transfer limited condition. [60,61]The model was developed to describe the electrochemical nucleation process which was successfully applied in the analysis of the transient current response of the chronoamperometric scans. [57,62]The observed hemispherical diffusion-limited response can be modeled by two limiting cases during nucleation.The first assumes the very rapid nucleation on a limited number of reduction sites called instantaneous nucleation.The second assumes slower nucleation on more reduction sites called progressive nucleation.The density of active sites and the nucleation rate constant is what determine the nucleation model.Since the nucleation rate constant is the same for all the morphologies and the reaction conditions were similar, the difference in the response can be related directly to the density of the reduction sites caused by the change in the morphology of TiO 2 .
From the Scharifker-Hills model, non-dimensional plots of (i/i max ) 2 versus (t/t max ) are used to distinguish between instantaneous and progressive nucleation processes.The comparison of the theoretical instantaneous and progressive values to the experimental ones allows for the deduction of the type of nucleation.Figure 7c shows the nondimensional plot for the three morphologies with the theoretical values.From the figure, we can see that each morphology demonstrates different behavior during nucleation with cubic following progressive nucleation and octahedra following instantaneous nucleation and truncated morphology between the two models.These results indicate the higher active reduction sites and the more adsorption capacity for the cubic morphology resulting in progressive nucleation and the reverse for the octahedron morphology resulting in instantaneous nucleation.The presence of {001} and {100} facets increase the adsorption of the metal ions and the reduction sites while the presence of {101} results in low adsorption and fewer reduction sites.This is demonstrated in the schematic shown in Figure 8.
In the case of Pb 2+ deposition, a similar current response is observed by the chronoamperometric scans at the reduction potential (−0.6 V vs Ag/AgCl) as can be seen in Figure 7b demonstrating double layer charging, nucleation, and growth of the particles followed by diffusion-limited reduction reaction toward steady-state.Though, the same morphology-dependent activity trend was not observed.As can be seen in Figure 7b, octahedron morphology shows the highest current values followed by cubic and truncated morphologies.The high activity of the octahedron TiO 2 /GC electrode is related to the participation of the TiO 2 nanoparticles in the reduction reaction rather than acting as adsorption sites for the Pb 2+ ions as was observed in the CV mea-surements.The flat band potential edge (−0.56 V vs Ag/AgCl) lays at the Pb 2+ reduction potential.Applying a reduction potential of −6.0 V to the electrode causes the injection of electrons into the conducting band of the octahedral TiO 2 nanoparticles which makes them available for the reduction of Pb 2+ .This is not possible for cubic and truncated morphologies which have flat-band edges greater than 0.6 V. Instead, they only act as adsorption sites for Pb 2+ ions with cubic particles to have higher adsorption capacity than the truncated particles like in the case of Cu 2+ reduction.
The nucleation mechanism for Pb 2+ for the three morphologies was found to follow a progressive nucleation mechanism rather than an instantaneous mechanism.The nondimensional (i/i max ) 2 versus (t/t max ) curves for the Pb 2+ reduction is shown in Figure 7d.The progressive mechanism can be explained by the slow nucleation rate of the Pb 2+ ion.
The same chronoamperometric scans were performed under UV illumination.The current response is shown in Figure 7e,f for Cu and Pb respectively.The UV illumination resulted in the disappearance of the increase in the current caused by the nucleation and growth of the Cu 2+ ions indicating the prior formation of the Cu atoms and the progressive reaction toward a steady state by mass transfer limitation.However, in the case of Pb 2+ reduction, all morphologies demonstrated a huge increase in the nucleation and growth currents as can be seen in Figure 7f.This large increase is explained by the participation of the photoexcited electron in the nucleation and growth of the Pb 2+ ions.

Electrochemical Impedance
EIS measurements were performed at the reduction potentials of the ions to study the morphology-dependent change in charge transfer resistance and the formed capacitance on the TiO 2 /GC electrodes.The impedance data were fitted with an equivalent circuit model containing charge transfer resistance connected parallel to the phase element CPE|R which is shown Figure S8 (Supporting Information).Nyquist plots for Cu 2+ deposition with the three morphologies are shown in Figure 9. From the equivalent circuit fits the lowest charge transfer resistance was found to be for cubic morphology with 1.75 kΩ followed by truncated with 2.29 kΩ and octahedral morphology with 2.54 kΩ.By UV illumination the charge transfer resistance was decreased to 0.98 kΩ for cubic, 1.25 kΩ for truncated, and 1.49 kΩ for octahedral morphology.
In the case of Pb 2+ electrodeposition, the lowest charge transfer resistance was for cubic with 1.15 kΩ followed by octahedron with 3.18 kΩ and truncated with 4.85 kΩ.UV illumination decreased the value of the charge transfer resistance, but the trend is the same with 0.62, 1.06, and 1.68 kΩ for cubic, octahedral, and truncated morphologies, respectively.

Differential Pulse Voltammetry
To investigate the effect of the TiO 2 nanoparticles morphologies on the amount of electrodeposition of Cu 2+ and Pb 2+ , we performed differential pulse voltammetry (DPV) analysis for stripping peak around 0.1 V for Cu and −0.4 V for Pb.We also performed DPV measurements after illumination of UV light to investigate the activity of the morphology for photoinduced reduction.
Similar to what we observed in the previous analyses, cubic morphology showed higher activity for Cu 2+ electrodeposition after holding the potential for 30 s at −0.3 V which can be observed from the higher DPV peak shown in Figure 10a.Truncated and octahedral morphologies showed similar responses.This is also the case for the photo deposition of Cu 2+ by UV irradiation which was applied to the electrodes for 5 min before the DPV scan, shown in Figure 10c.The higher peak for cubic morphology indicates the more reduction of Cu 2+ by the photoexcitation of the electrons from the valence band of the TiO 2 particles.
In the Pb 2+ electrodeposition, the octahedron shows higher activity followed by the cubic and truncated morphologies after holding the voltage at −0.55 V for 30 s as shown in Figure 10b.For the same reason mentioned above of the participation of the conducting band electrons, the octahedral morphology shows higher activity.Octahedron also outperforms the other two morphologies in the photo deposition of Pb 2+ after UV irradiation for 5 min, as shown in Figure 10d.This is related to the position of the conducting band edge at the reduction potential of the Pb 2+ reduction potential which allows more electrons to be transferred in Pb 2+ reduction.

Conclusion
In this work, we introduced a two-step preparation method for the well-specified morphologies of TiO 2 nanocrystals from TiO 2 powder.The method is based on first the conversion of the TiO 2 powder to sodium titanate nanowires and second the formation of faceted nanoparticles under a hydrothermal condition in the presence of shape capping agents.The TiO 2 nanocrystals were characterized for the formation of cubic morphology with {100} and {001}, truncated morphology with {001}and{101}, and octahedron morphology with {101} facet.The nanoparticles are composed of mostly anatase crystal structure with some remaining titanate crystal structure as demonstrated by XRD and Raman analysis.XPS studies revealed the formation of surface clean nanoparticles with variation in the position of the Fermi level.Mott-Schottky measurements showed consistent flat band potential values to the Fermi level positions.We showed for the first time in the literature, facet-dependent properties investigation using photoelectrochemical deposition/stripping of transition metal ions in acidic environment.The electrochemical reduction of Cu 2+ showed low overpotential for cubic morphology with progressive nucleation mechanism while high overpotential for the octahedron particles with instantaneous nucleation with truncated particles between the two.The position of the flatband potential was shown to be critical for the Pb 2+ reduction which showed the highest activity for the octahedron particles.The highest activity is assigned to the alignment of the reduction potential to the flat band potential edge.The photo reduction showed the same activity due to the same reason.From the results, it is concluded that the activity of the TiO 2 is greatly affected by the formed facets which are related to the change in the surface energy of the particles and the variation of the electronic bandgap structure.The facet with higher surface energy is responsible for more reduction of the ions because of the densely formed TiO 2 /electrolyte interface.The position of the flat band is important for the reactions with near reaction potential.These findings are important in the design of effective photoelectrochemical catalysts which should consider the surface energy of the particle and the change in the flat band potential.

Experimental Section
The preparation method was performed by a two-step synthesis technique.First, the formation of sodium titanate nanowires was obtained.Second, the conversion of the nanowires to the intended shape of nanoparticles was accomplished.In the first step, commercial anatase TiO 2 powder was converted to sodium titanate nanowires by hydrothermal treatment in a basic solution.100 g of commercial TiO 2 powder (Sigma-Aldrich 325 mesh ≥99% trace metals basis) was dispersed into 10 m of NaOH solution by mixing for 30 min.Then the dispersion was transferred into a Teflon-lined stainless-steel autoclave for heating.The stainless-steel autoclave was heated at 130 °C for 48 h in an oil bath under mixing at 400 rpm.It was then cooled at room temperature and opened.Then the obtained sodium titanate nanowires were washed with 0.1 m nitric acid solution until the pH is 5.The obtained sponge-like sodium titanate was then dried in the oven at 100 °C for 12 h under atmospheric pressure.
The sodium titanate nanowires were then used to prepare the single-crystal nanoparticles.For each morphology, the preparation method was based on dispersing 200 mg of the nanowires in 100 mL of a specific solution containing the shape-capping agents.For the cubic nanoparticles, the solution was made of 2.0 m HCl and 0.03 m NaF.For the truncated octahedral nanoparticles, it was made of 2.0 m HCl and 0.045 m NaF.For the octahedral nanoparticles, the solution was made of 0.1 m LiClO 4 .The dispersions were transferred into a Teflon-lined stainless-steel autoclave for another hydrothermal treatment.The stainless-steel autoclaves were heated at 120 °C for cubic, 160 °C for truncated octahedra, and 200 °C for octahedral nanoparticles for 12 h.The particles are then centrifuged and washed with water until the pH is neutral.After centrifugation, the particles were dried at 80 °C for 2 h.To ensure the cleanliness of the surface of the particles, they were calcinated at 500 °C for 2 h as was determined by thermogravimetric analysis measurement and advised in the literature. [63]

Figure 1 .
Figure 1.Preparation method for the single crystal anatase TiO 2 with two-step preparation technique.First, sodium titanate nanowires form.Second, single crystal nanoparticles with exposed facets are hydrothermally formed using different capping agents.

Figure 2 .
Figure 2. a) XRD patterns of the prepared nanoparticles, anatase, rutile and titanate, b) Peak intensity of the XRD peaks, SEM, and TEM images of e,f) Cubic, d,g) Truncated, e,h) Octahedron, and i) Particle size distribution histograms with average particle size.

Figure 3 .
Figure 3. a) FTIR spectra, b) Raman spectra of the three morphologies and XPS of Ti 2p and O 1s peak fits for c,f) Cubic, d,g) Truncated, e,h) Octahedron morphologies.

Figure 4 .
Figure 4. a) Tauc plots obtained from the reflectance UV-vis measurement, b) XPS valence region fits, c) Mott-Schottky plot obtained from the EIS measurements in 0.1 m HClO 4 solution, d) Energy bandgap diagram and e) Energy flat band potential diagram in electrolyte versus Ag/AgCl electrode for the three morphologies showing Fermi level variation for each morphology.

Figure
Figure3c-h shows the peak convolution of the Ti 2p and O 1s regions.The formation of only one oxidation state Ti(IV) with a doublet peak representing the characteristic spin-orbit splitting of 2p level in the 2p 3/2 and 2p 1/2 components at 458.6 and 464.3 eV respectively can be seen.These values as reported by other studies correspond to Ti(IV) in anatase TiO 2 form.[45,46]XPS peak of the O 1s region showed the existence of two forms of oxygen.The higher intensity peak is assigned for the oxide oxygen environment (O-Ti) at 529.6 eV and the lower is for the surface hydroxyl groups (O-H) at 531.5 eV.Octahedral TiO 2 particles showed another peak at 530.8 eV which is assumed to be from the perchlorate-oxygen (O-ClO 4 ) from the precursor (LiClO 4 ) used for the synthesis of octahedron nanoparticles.The ratio of the area under (O-Ti) peak to (O-H) peak was found to be 3.34 for cubic, 1.96 for truncated, and 5.68 for octahedra.The optical properties of the prepared TiO 2 nanoparticles were determined using reflectance Ultraviolet-Visible Spectroscopy (UV-vis) measurement shown in FigureS6(Supporting Infor-

Figure 5 .
Figure 5. Cyclic voltammetry results for TiO 2 modified glassy carbon electrodes in Cu 2+ solution in a) dark and b) UV irradiation.

Figure 6 .
Figure 6.Cyclic voltammetry results for TiO 2 modifies glassy carbon electrodes in Pb 2+ solution in a) dark and b) UV irradiation.

Figure 7 .
Figure 7. Chronoamperometric scan of TiO 2 nanoparticles on glassy carbon electrode in a) Cu 2+ solution at −0.3 V, b) Pb 2+ solution at −0.6 V. Nondimensional plots of (i/i m ) 2 versus (t/t m ) for c) Cu deposition and d) Pb deposition.Chronoamperometric scan under UV illumination of TiO 2 nanoparticles on glassy carbon electrode in e) Cu 2+ solution at −0.3 V, f) Pb 2+ solution at −0.6 V.

Figure 7a ,
Figure 7a,b shows the chronoamperometric scans from open circuit potentials to −0.3 V for Cu 2+ and −0.6 V for Pb2+  versus Ag/AgCl performed on the TiO 2 /GC electrodes for the three morphologies.In all morphologies and for both ions, the response curves show a sharp current increase instantaneously after applying the reduction potential which is due to the double-layer charging.Then it is followed by a lower increase in the current response resulting from nucleation and growth at the active reduction sites of the TiO 2 /GC electrode.The higher the current values for the second current increase the more active reduction sites on the surface of the electrode and the stronger the metal ion adsorption on the surface of TiO 2 nanoparticles.The reduction proceeds by the growth of each nucleus forming a hemispherical diffusion zone around it which expands and overlaps with the neighboring nuclei.The maximum current value corresponds to this overlap and the transition from the hemispherical to the linear diffusion behavior.[56,60]After the peak, the current response decreases until it reaches the steady state condition which is controlled by the rate of mass transfer described by the Cottrell equation.For the reduction of Cu 2+ , chronoamperometric scans show the highest current values for cubic morphology followed by truncated and octahedron particles.This indicates the high adsorption capacity for the cubic morphology with {001} and {100} facets.The presence of the {001} along with {101} facet on the truncated morphology can explain the higher activity from the octahedral morphology which possesses only {101}.These ob-

Figure 8 .
Figure 8. Schematics demonstrating the nucleation and growth mechanism change by changing the morphology of the TiO 2 particles for a) Cubic and b) Octahedral nanoparticles.

Figure 9 .
Figure 9. Nyquist plots of TiO 2 nanoparticles on glassy carbon electrode at −0.1 V versus Ag/AgCl reference in Cu 2+ solution a) in the dark and c) under UV illumination.Nyquist plots at −0.55 V versus Ag/AgCl reference in Pb 2+ solution b) in the dark and d) under UV illumination.

Figure 10 .
Figure 10.Differential pulse voltammetry of TiO 2 nanoparticles on glassy carbon electrode for a) Cu 2+ solution after holding the voltage at −0.3 V for 30 s, b) Pb 2+ solution after holding the voltage at −0.55 V for 30 s, c) Cu 2+ solution after UV illumination for 5 min, d) Pb 2+ solution after UV illumination for 5 min.