Facile Synthesis of [101]-Oriented Rutile TiO2 Nanorod Array on FTO

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Facile synthesis of [101]-oriented rutile TiO2 nanorod array on FTO substrate with a tunable anatase-rutile heterojunction for efficient solar water-splitting Hogiartha Sutiono, Alok M. Tripathi, Hung-Ming Chen, Ching-Hsiang Chen, Wei-Nien Su, Liang-Yih Chen, Hongjie Dai, and Bing-Joe Hwang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01066 • Publication Date (Web): 18 Oct 2016 Downloaded from http://pubs.acs.org on October 23, 2016

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ACS Sustainable Chemistry & Engineering

Facile synthesis of [101]-oriented rutile TiO2 nanorod array on FTO substrate with a tunable anatase-rutile heterojunction for efficient solar water-splitting Hogiartha Sutiono1, Alok M. Tripathi1, Hung-Ming Chen1, Ching-Hsiang Chen2, Wei-Nien Su2, LiangYih Chen1,*, Hongjie Dai4, Bing-Joe Hwang1,3,* 1

Department of Chemical Engineering, National Taiwan University of Science and Technology, 43,

Section 4, Keelung Road, Taipei 106, Taiwan 2

Graduate Institute of Applied Science and Technology, National Taiwan University of Science and

Technology, 43 Keelung Road, Sec. 4, Taipei 10617, Taiwan 3

National Center for Synchrotron Radiation Research Centre, Hsinchu, Taiwan

4

Department of Chemistry and Laboratory for Advanced Materials, Stanford University, Stanford,

California, USA *Corresponding authors email address: [email protected] (Liang-Yih Chen) and [email protected] (Bing-Joe Hwang) KEYWORDS: Anatase, Heterojunction, Photocatalyst, Rutile, Tip Enhanced Raman Spectroscopy, Titanium Dioxide, Water-Splitting

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ABSTRACT: Generating sustainable energy source through photoelectrochemical (PEC) watersplitting requires a suitable photocatalyst. [101]-oriented rutile TiO2 nanorod (NR) array in heterojunction with anatase on fluorine doped tin oxide (FTO) substrate is successfully prepared using a facile single step hydrothermal process. The presence of anatase phase over predominant rutile NRs’ surface is confirmed by transmission electron microscopy and tip enhanced Raman spectroscopy. Solar water-splitting performances of anatase-rutile heterojunction with low energy (101) and high energy (001) rutile facets are compared. Low energy (101) facet rutile-anatase heterojunction shows higher photoconversion efficiency of 1.39% at 0.49 VRHE than high energy (001) facet rutile-anatase heterojunction (0.37% at 0.73 VRHE). The mechanism for enhanced photocatalytic activity of low energy (101) facet rutile-anatase heterojunction has been proposed. The role of NaCl in tuning the anatase portion, morphology and PEC water-splitting performance has also been studied.

INTRODUCTION The groundbreaking research into PEC water-splitting by Fujishima and Honda in 19721, has led to TiO2 being used as a green and sustainable photocatalyst material. Subsequent to their work, the use of TiO2 has attracted tremendous global interest as a means of improving the solar energy conversion efficiency of water to generate hydrogen. During illumination, TiO2 photoelectrodes generate electron and hole pairs. The photogenerated electrons migrate to the Pt counter-electrode to reduce water, thereby producing H2, whereas the holes oxidize water to produce O2 on the TiO2 photoelectrode (typically with an external bias being applied). The use of TiO2 offers several advantages, e.g. it is inexpensive, chemically stable, and non-toxic; additionally, it has usefully placed band-edge positions, and a high photo-corrosion resistance. A method developed by Liu and Aydil2, has become a standard procedure to prepare rutile TiO2 nanorod (NR) arrays on fluorine doped tin oxide (FTO) substrates. This procedure not only offers intimate contact between TiO2 NRs and the FTO substrate but also results in the formation of a direct pathway for the photogenerated electron to reach the electron collector. Some studies have adapted and ACS Paragon Plus Environment

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enhanced this procedure using techniques such as surface decoration3-4, oxygen vacancy creation5, and elemental dopant incorporation6-7. Design of nanostructured catalyst with preferential orientation has resulted in enhanced PEC properties due to variation in surface energy and band edge positions8-9. The procedure proposed by Liu and Aydil produces [001]-oriented rutile TiO2 NR array2. A similar orientation was observed by other group4-5, 10, but Wang et al., had observed [110]-oriented rutile TiO2 NR with 1D and 3D structures11. Additionally, coupling with two different semiconductors also can enhance the photocatalyst’s performance. Anatase and rutile are the two most commonly used TiO2 phases for PEC water-splitting applications. Their differing semiconducting properties, e.g. band-edge positions and band-gaps, if successfully combined may offer functional synergisms as suggested in the literature3-4,

12-13

. By

coupling the anatase and rutile phases, charge recombination can be suppressed, which in turn improves the photocatalytic ability. This is understandable since the conduction band of the anatase phase is slightly higher which permits photogenerated electrons to be transferred to the rutile phase. Yang et al., synthesized rutile TiO2 NR arrays on FTO substrates and then decorated them with TiO2 nanoparticles (NPs) in rutile and anatase phases (sequentially located on the NR’s surface) through a series of preparation steps3. As a result, the PEC water-splitting performance was improved by the enlargement of the surface area, resulting in better light absorption and scattering, and enhanced charge separation. Combining the anatase and rutile phases within the crystal structure is not normally regarded as a facile synthesis procedure. A series of preparation steps has to be performed to create a rutile-anatase heterojunction in the photoanode. Several synthesis routes have been developed to achieve this – as summarized in Table S1. Kho et al.12 successfully prepared TiO2 NPs with the presence of anatase and rutile phases using a flame spray pyrolysis method. Although they could bring these phases together, the preparation steps were relatively complex and required an extra step to deposit the photocatalyst on the substrate. As mentioned before, the procedure developed by Liu and Aydil, which has been widely used and modified by many research groups, involves creating a rutile-anatase heterojunction by decorating ACS Paragon Plus Environment

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the rutile TiO2 NRs with anatase phase3-4, 10, 12-13. These approaches involve multiple annealing steps, pre-treatment of FTO and chemical etching thereby making it difficult to maintain a consistent quality. An ideal solution would be the development of a simple, facile, and effective method to produce an anatase-rutile heterojunction for solar water-splitter and other photocatalytic applications. In this work, we report a facile synthesis to prepare a rutile-anatase heterojunction with low energy rutile facet. We successfully tuned the NR’s diameter, NR number density and anatase portion in heterojunction using a simple approach. Additionally, PEC water splitting performance of all possible combinations of rutile-anatase heterojunction has been studied.

EXPERIMENTAL SECTION Preparation of TiO2 Nanorods Crystal Arrays Basically, there were two methods used in this study to prepare rutile TiO2 NR arrays. The difference between them was the orientation of FTO’s conducting side during synthesis. In this report, the method where FTO substrate was placed at an angle against Teflon-liner’s wall with conducting side facing down will be called as original procedure (method by Liu and Aydil2). The second method used here FTO substrate was placed on the bottom of a Teflon-liner with the conducting side facing up; this will be called as modified procedure (modification of original procedure). The more detailed steps will be explained as follow. Titanium (IV) isopropoxide (0.7 ml) (TTiP, Acros Organics, 98+%) was added into 40 ml aqueous hydrochloric acid (HCl) solution (20 ml of deionized (DI) water + 20 ml of concentrated HCl (Scharlau, reagent grade, 37%)) and magnetically stirred for 5 minutes. Subsequently, 0-1 ml (0; 0.25; 0.5; 0.75; and 1 ml) saturated aqueous NaCl solution was added into the mixture and stirred for another 5 minutes. In this study, it must be noted that there was no NaCl added into the growth solution of original procedure. Prior to the hydrothermal process, the FTO (1.5 × 2 cm2) was ultrasonically cleaned in a mixture of DI water, acetone, and ethanol with a volume ratio of (1:1:1). After cleaning, the FTO substrate was placed into a Teflon-lined stainless steel autoclave in two different ways. As mentioned before, for the original procedure the FTO was placed at an angle against Teflon-liner’s wall ACS Paragon Plus Environment

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with conducting side facing down, whereas for the modified procedure the FTO was placed on the bottom of a Teflon-liner with the conducting side facing up. After which the prepared growth solution was gently poured into the Teflon-liner. The hydrothermal synthesis was carried out at 150 °C in (held for 18 hours) in an electric oven without stirring and then allowed to cool to room temperature. The TiO2 on the FTO substrate was rinsed with DI water and allowed to dry. Lastly, it was annealed at 500 °C for 30 minutes (heating rate of 5 °C min-1) in air using an electric furnace. Material Characterizations The morphology investigation was carried out using a field emission scanning electron microscope (FE-SEM) on JEOL JSM 6500F. Lattice structural information was extracted using a transmission electron microscope (TEM) Philips Tecnai G2 F20 FEG-TEM with a lacy carbon coated TEM grid. Xray diffraction (XRD) was used to determine the crystal structure of the film. The XRD spectra were recorded in a Bruker D2 Phaser with Cu Kα radiation (λ = 1.5406 Å) from 20° to 70° at a scanning speed of 4.8° min-1. Optical Measurements Wavelength-dependent optical absorption properties were obtained using an integrated sphere JASCO (ISV-469) V 560 UV-vis spectrophotometer. The absorption plus scattering was calculated from these measurements with the formula A + S = 100 – T – R, with A the absorption, S the scattered light that was not accounted for in the measurement, T the total transmittance, and R the total reflectance. To eliminate the contribution of the substrate, a bare FTO was similarly examined to normalize the results. Raman Spectroscopy Raman spectroscopy was performed on the UniRAM system (UniNanoTech) utilizing excitation wavelength of 532 nm with 100 mW laser output and CCD with 1024×256 pixels for signal collection. To prevent degradation, the exposure time was 5 seconds with 10 accumulations. An objective lens 100X/0.95 (Olympus America Inc.) was used and the measurement was carried out at room temperature in a dark room. For point wise Raman measurements, the sample was scratched from the thin film grown on the FTO substrate and dispersed on lacy carbon coated TEM grids with the sample being ACS Paragon Plus Environment

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moved by an auto-controlled stage in the UniRAM system. Before taking measurements from our samples the Raman band of a silicon wafer at 520 cm-1 was used as the standard reference to calibrate the spectrometer. Tip Enhanced Raman Spectroscopy (TERS) In order to obtain the near field Raman signal on the surface of the TiO2 nanorods crystal, tip enhanced Raman spectroscopy (TERS) was used. Atomic force microscope (AFM) was combined with the microscope of the Raman spectrophotometer. The AFM tip was coated with gold to enhance the Raman signal. This characterization was performed on UniRAM microscope Raman system combined with a Nanonics MV4000 AFM scanning stage. Green laser light (532nm) was used as the excitation light source. To prevent degradation, an exposure time of 1 second with 5 accumulations was used. An objective lens 50X/0.55 N.A. was used with the measurements being carried out at room temperature in dark conditions. Before measurements were made the Raman band of a silicon wafer at 520 cm-1 was used as the standard reference to calibrate the spectrometer. Photoelectrochemical Measurements PEC measurements for the TiO2 nanorods crystal arrays were made using a potentiostat (Autolab PGSTAT302N) in a three-electrode configuration. We used TiO2 as the working electrode, a Pt wire as the counter electrode, Ag/AgCl in saturated KCl aqueous solution as the reference electrode, and 1 M KOH aqueous solution as the electrolyte. Simulated solar light at 100 mW cm-2 from a Hg (Xe)-500 W lamp (200-2500 nm; Model No. 66142; Newport, USA) was generated using a solar simulator (Model No. 66901; Newport, USA) coupled with a AM 1.5 G filter. The illuminated and wetted areas of the TiO2 nanostructured arrays were equal and well-defined as 1.5×1.5 cm2. The measured sample was mounted to the PEC cell in front of the quartz window and illuminated by the AM 1.5 G simulated solar light irradiation at 100 mW cm-2. In order to hasten the transport of photogenerated holes to the electrode/electrolyte interface to be consumed by the adsorbed reactants14, the measured sample was irradiated from the front of the sample instead of from the rear side. Throughout the PEC measurements, pure Argon gas was introduced to purge the produced gases. During the linear sweep voltammetry ACS Paragon Plus Environment

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measurements, the applied potential was swept linearly from –1 to 0.8 V vs Ag/AgCl at a scan rate of 10 mV s-1. The stability test for the measured sample was carried out by holding a constant applied potential at 0.204 V vs. Ag/AgCl or 1.23 VRHE for 1500 seconds, where light and dark was alternated every 100 seconds. RESULTS AND DISCUSSION Material Characterization First of all, the samples prepared using the original and the modified procedures (in the absence of NaCl) will be compared thoroughly. The morphology of each sample was investigated using FE-SEM (Figure 1a and S1). The cross-sectional FE-SEM image of TiO2 NR array prepared using the modified procedure (Figure 1a) indicates that morphology of NR array remain unchanged after modifying FTO substrate orientation during synthesis. In the original procedure, the conducting side of FTO substrate kept facing down during synthesis. It could be advantageous to prevent star-like structures formed on the top TiO2 NRs as it could happen, if the conducting side kept facing up. However, when the modified procedure used in this study, the star-like structures that formed on the top TiO2 NRs were very less (Figure S1d), not as much as reported by other research group11. Other physical characterizations such as XRD, TEM, and SAED for as-prepared TiO2 NR arrays (after heat treatment) are summarized in Error! Reference source not found.b-d. The XRD patterns (Figure 1b) of both TiO2 NR arrays prepared using original and modified procedures indicate that all peaks agree well with the tetragonal rutile phase (SG, P42/mmm; JCPDS No.88-1175, a = b = 0.4517 and c = 0.2940). The only difference observed in both procedure is the difference in relative peak intensities of (002) and (101) planes. This difference indicates that both NRs have different growth orientation after changing the FTO substrate orientation during synthesis. The modified procedure has adopted a preferred growth direction of lower energy facet (101) while in the original procedure (proposed by Liu and Aydil2), has preferred growth in [001] direction. This might be due to the different exposure of the nucleation center on the FTO’s conducting side to the accessible precursor volume. As consequence, the face-up position will get more


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access to precursor volume than face-down position. Thus, the high energy facet can be easily passivated by the precursor molecules resulting in low energy facet NR structure. On the other hand, the face-down position provides only high energy facet NR due to lack of accessible precursor volume.

Figure 1. (a) FE-SEM image of TiO2 NR array prepared using the modified procedure. (b) XRD patterns of TiO2 NR arrays prepared using the original procedure (red) and the modified procedure (black). Corresponding FEG-TEM images of samples prepared using (c) the modified procedure and (d) the original procedure. Insets are corresponding SAED patterns and low magnification FEG-TEM images. Both samples compared here prepared in the absence of NaCl. TEM and SAED patterns (Figures 1c and d) for both samples also show the presence of mainly rutile phase. The high resolution TEM images of the modified (Figure 1c) and the original (Figure 1d) procedures have shown d-spacing of 0.259 nm and 0.284 nm for (101) and (001) planes, respectively. This result indicates that the modified procedure could bring a TiO2 NRs array with unique crystallographic orientation that differs from the original procedure. The high resolution TEM images confirm the preferred growth direction of NR array is in [101] direction when prepared using the modified procedure. In contrast, the original procedure gives [001] preferred growth direction, as observed in XRD patterns and high resolution TEM image shown in Figure 1b and d, respectively. The SAED patterns shown in insets of Figure 1c and d also show the diffraction points for respective planes of preferred growth direction for each sample. This is very interesting, because the modified procedure could easily modulate the orientation of the TiO2 NRs crystal and could bring different crystal facets which have different properties. It is understandable because each facet has a unique atomic


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arrangement which results in particular electronic properties, surface energy, and photocatalytic properties as well. In addition, optical characterization was performed on these two samples and the result is depicted in Figure S2. It can be seen that the sample prepared using the modified procedure has an improved absorption plus scattering (A + S = 100 – R – T). The TiO2 NR array with [001] orientation has an absorption band edge at 418 nm while the [101]-oriented TiO2 NR array has an absorption band edge at 423 nm. This result dignifies the enhancement in optical properties of [101]-oriented rutile TiO2 NR array over [001]-oriented rutile TiO2. An advanced NR array photocatalyst should be engineered in a particular way to maintain a proper separation between neighboring NRs. By doing so, the number of surface-to-surface contact between NRs in the photoelectrode system can be lowered, which might provide more accessible photocatalytic active area. NaCl is known to improve thin-film’s quality by controlling the diameter of growing NR which prevent NRs surfaces coalescing2, 15. Here, we added various amounts of saturated aqueous NaCl solution (0-1 ml) into the crystal growth solution (modified procedure). As a result, the average of NR’s diameter was reduced and the number of NR per unit area was increased with respect to the amount of saturated aqueous NaCl solution added – see Figure S3 and Table S2. An additional conspicuous result observed that rutile TiO2 NR array prepared using the modified procedure has a smaller average of NR’s diameter and more NRs per unit area compared to the sample prepared using the original procedure. Furthermore, XRD analysis was also performed on all samples prepared using the modified procedure with various amounts of NaCl. XRD patterns shown in Figure S4 indicate that the crystallographic properties of the obtained samples remain unchanged as NaCl added into the crystal growth solution. Optical characterization was also performed to evaluate the light absorption properties of the samples, which were prepared with five different amounts of NaCl in the crystal growth solution. As a result, the optical properties of these samples are slightly improved as can been in Figure S5. Detailed surface observation of the sample prepared using the modified procedure in the presence of 0.5 ml saturated aqueous NaCl solution through FEG-TEM shows the existence of irregular features over TiO2 NR’s surface – see Figure 2a-c. Irregular features in the form of dark spots, distributed in a ACS Paragon Plus Environment

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discrete manner all over the NR’s surface. Dark field TEM image of single TiO2 NR also shows the presence of these irregular features as dark spots over lighter background of rutile phase, as shown in Figure 2b. These irregular features are expected to be amorphous TiO2 anatase phase. A similar result was also observed in the previous in situ TEM studies performed on TiO2 growth which concluded that the amorphous anatase interphase acts as a nucleation site for the formation of a crystalline rutile phase16-19. We expect the growth of NR array in the present case has followed the similar mechanism. Raman spectroscopy measurement is commonly used to investigate various phases of TiO2 nanostructures. This technique is very useful to reveal and distinguish individual phases (rutile or anatase) even in very small quantity. To identify the irregular feature observed in the high resolution TEM image, Raman spectra were taken from the bottom to the top of a single NR surface (at consecutive eight points) – see Figure 2d. The spectra generated from the eight points, displayed in Figure 2e, show strong peaks at 610 cm-1 and 443 cm-1 for the A1g and Eg modes of rutile TiO2, respectively, thereby confirming that the predominant TiO2 crystal structure is rutile. Nevertheless, the peak located between 150 cm-1 and 300 cm-1 is quite asymmetric – this range in the TiO2 Raman spectra corresponds to the phonon scattering mode of rutile at 239 cm-1 and the Eg mode of anatase at 197 cm-1. Another hump is also visible below 150 cm-1 in all the Raman spectra taken over the entire TiO2 NR which corresponds to the characteristic Eg mode of anatase. The presence of the asymmetric peak from 150 to 300 cm-1 and the hump below 150 cm-1 strongly indicates the presence of anatase phase on the TiO2 NR’s surfaces. Additionally, the asymmetric peak is also observed (Figure S6a) when rutile TiO2 NR arrays prepared using the original and the modified procedures (without NaCl) were conducted to Raman spectroscopy measurement. This finding is an addition to the process proposed by Liu and Aydil2.


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Figure 2. FEG-TEM (a) bright field and (b) dark field images of single TiO2 NR, (c) high resolution TEM image from the middle edge, (d) dark field optical image of single NR on lacy carbon coated TEM grid (200X magnification), (e) Raman spectra from eight linear points over NR shown in (d). Point 1 from bottom and point 8 from top of the NR. All samples prepared using the modified procedure in the presence of 0.5 ml saturated aqueous NaCl solution. Furthermore, in order to provider a stronger evidence of anatase phase over rutile TiO2 NRs tipenhanced Raman spectroscopy (TERS) has been performed. TERS has very high spatial resolution and single molecular sensitivity due to precise positioning of scanning probe at analyzed surface and confinement of electromagnetic field at scanning probe tip20. The black and red lines, shown in Figure 3a and b, describe the characteristic of outer and inner regions of the TiO2 NRs samples, respectively. Figure 3a, showing the characteristics of sample prepared using the modified procedure in the absence of NaCl, reveals that the outer and inner region of the TiO2 NR array were similar (indicated by the overlapping black and red lines of TERS). The asymmetric peak between 150 and 300 cm-1 and the hump below 150 cm-1 also appear in the spectra indicating the presence of anatase phase. It should be ACS Paragon Plus Environment

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noted that the addition of NaCl into the crystal growth solution is very useful to modulate anatase to rutile phase conversion in the outer regions, as indicated in Figure 3b – as can be seen from the enhanced Raman peak at 144 cm-1 which can be attributed to the Eg mode of anatase phase. Blue lines in Figure 3b and c (inset) show the difference between tip-in and tip-out data. Thus, Raman results confidently confirm the presence of anatase phase, distributed in a discrete manner, on the surface of rutile TiO2 NRs as observed in FEG-TEM (illustrated in Figure 3c). Such discrete distribution of anatase over the surface of rutile may help to enhance the photocatalytic activity.

Figure 3. TERS spectra of TiO2 NR for samples prepared using a modified procedure in the presence of (a) 0 ml, and (c) 0.5 ml saturated aqueous NaCl solution, (c) schematic diagram: discrete distribution of anatase over rutile NRs’ surface. To estimate the presence of anatase on rutile TiO2 NRs in different samples prepared using the modified procedure with different concentrations of NaCl, Raman spectra were performed on thin film TiO2 NR arrays, as shown in Figure S6b. The peak asymmetry (150-300 cm-1) continues as in case of a single TiO2 nanorod in Figure 2e. Nevertheless, there is a hidden peak as indicated by the shoulder near the 235 cm-1 Raman shift (inset of Figure S6b). Inset of Figure S6a also shows the shouldering peak for samples prepared using the original and the modified procedures (without NaCl added). Fitting was performed based on two-peak deconvolution using PeakFit software for all Raman spectra. The fitting results for all samples, displayed in Figure S7 and S8, show that the hidden peak can be attributed to the Eg mode of TiO2 anatase phase located around 197 cm-1 (all obtained parameters are summarized in ACS Paragon Plus Environment

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Table S3). The percentage estimation of each phase in the TiO2 NR arrays was calculated by taking the ratio of the peak areas at 197 and 235 cm-1 which correspond to the anatase and rutile phases, respectively and tabulated in Table 1 and S3. A fascinating fact is observed, after changing the FTO substrate orientation during the synthesis, the anatase portion changes (9.9% and 4.7% for the original and the modified procedures, respectively). Further addition of NaCl in the modified procedure shows change in the anatase portion. An optimum anatase portion of 9.3% is obtained when 0.5 ml saturated aqueous NaCl solution added. This analysis shows that the amount of the anatase phase can be easily tuned by controlling the amount of NaCl added. It is noteworthy that the preparation procedure proposed in this study can provide a shortcut (by eliminating the need for extra treatment) to create a tunable anatase-rutile heterojunction. All TiO2 NR arrays with anatase-rutile heterojunction discussed above were conducted to PEC water-splitting performance tests.

PEC Water-Splitting Performance Photoelectrochemical (PEC) testing was used to evaluate water-splitting performances of all prepared samples. Significantly enhanced PEC water-splitting performance shown in J-V curves from TiO2 NR array photoanode, made using the modified procedure, is shown in Figure 4a – included for reference is material made using the original procedure of Liu and Aydil2 (both samples prepared in the absence of NaCl). Unexpectedly, the sample which was prepared using the modified procedure (black line) shows a much higher photocurrent density compared to the sample prepared using the original procedure (red line) (2.32 mA cm-2 and 1.18 mA cm-2 at 1.23 VRHE, respectively). The photoconversion efficiency for these two samples are also calculated using Equation (1)5 – see Figure S9. The sample prepared using the modified procedure had a better efficiency of 1.39% at low potential of 0.49 VRHE (vs. 0.37% at 0.73 VRHE for sample prepared using the original procedure). Remarkably, a TiO2 NR array prepared using our modified procedure in the absence of NaCl addition, had a conversion efficiency of 1.39%, surpassing the reported efficiency for pristine TiO25, i.e. ~ approximately 6-fold higher. This shows that PEC water-splitting performance was significantly improved by a very simple modification – namely, ACS Paragon Plus Environment

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the orientation of the FTO substrate during the hydrothermal synthesis. When compared to other published reports focused on pristine TiO2 NRs systems3-7, this result is remarkably significant, as it shows steeper and higher photocurrent density increases together with a higher saturation photocurrent density. A steeper increase of photocurrent density indicates better charge transfer in the semiconductor and electrolyte interface. The saturation photocurrent density of 2.32 mA cm-2 at 1.23 VRHE is quite significant, as to our best knowledge no comparable result (in pristine TiO2 material) has been previously published. At this point the unique crystallographic orientation obtained through the modified procedure clearly offers a significant improvement in water-splitting performance. It is generally known that anatase phase has superior photocatalytic properties than its rutile counterpart21-22. Photocatalyst materials that consist of anatase-rutile heterojunction have been reportedly shown synergistic effect on enhanced photocatalytic performance3-4,

12-13, 22

. This synergistic effect can be achieved due to their

semiconducting properties such as Fermi level shift during irradiation, band-gap and the band-edge position, that can suppress the recombination between photogenerated electrons and holes3-4, 12-13, 22. According to electrochemical impedance study, the conduction band position of anatase lies 0.2 eV above that of rutile23. In addition to charge separation, photocatalyst materials with well-faceted crystal could drive electrons transfer between facets due to their surface energy difference8-9, 24. It can be understood by the fact that crystal facets with unique atomic arrangement will have a specific surface energy. This also could lead to a dramatic band-gap difference for a particular rutile TiO2 system25. Combining heterojunction components which have distinctly different energy level could promote the charge separation and enhance the photocatalytic activity. In case of TiO2 photocatalyst material, higher surface energy facets used to have higher conduction band position that result in wider band-gap26-27. The surface energy of rutile (001) and (101) facets are 1.32 and 1.08 J m-2, respectively28. Therefore, the conduction band position of rutile TiO2 (001) facet will lie above (101) facet, as shown in Figure 4b. During the illumination, photogenerated electrons will be excited from the valence band to the conduction band. Then, the photogenerated electron in the anatase conduction band will ACS Paragon Plus Environment

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spontaneously move to lower energy rutile conduction band suppressing the charge recombination. Energy diagrams of rutile (001) and (101) facets which are in heterojunction with anatase phase are displayed in Figure 4b. As shown in Figure 4b, the energy level difference between conduction bands of anatase and rutile (001) facet (∆E1) is lower than the energy level difference between conduction bands of anatase and rutile (101) facet (∆E2). The energy levels difference (∆E2) is considered large enough to drive charge separation between anatase phase and rutile (101) facet (prepared using the modified procedure). As a result, a more efficient charge separation and improved PEC water-splitting performance can be obtained (Figure 4a and Table 1). Table 1. Photoelectrochemical properties of TiO2 anatase-rutile heterojunction. Sample

Original Procedure

Preferred Orientation

Saturated Aqueous NaCl Solution Added (ml)

Anatase Portion (%)

Photocurrent Density at 1.23 VRHE (J, mA/cm2)

Photoconversion Efficiency (η, %); (at Potential)

[001]

0.00

9.9

1.18

0.37; (0.73 VRHE)

0.00

4.7

2.32

1.39; (0.49 VRHE)

0.25

7.1

2.37

1.49; (0.48 VRHE)

0.50

9.3

2.45

1.67; (0.44 VRHE)

0.75

7.3

2.51

1.60; (0.47 VRHE)

1.00

5.9

2.43

1.47; (0.49 VRHE)

Modified Procedure

[101]

NaCl was selected as an additive to modulate surface-to-surface contact between TiO2 NRs on the FTO substrate. The saturated aqueous NaCl solution was added into the growth solution in amounts ranging from 0-1 ml. As mentioned, the addition of NaCl reduced the diameter of NRs meaning that surface-to-surface contact between NRs can be minimized (as indicated by reduced average of NRs’ diameter, Table S2), which has been shown to improve the TiO2 PEC water-splitting performance. As shown in Figure 4c, the PEC water-splitting performance was enhanced. The photocurrent increase becomes steeper and the saturation photocurrent also becomes higher with respect to the amount of ACS Paragon Plus Environment

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NaCl. Additionally, the onset potential (inset of Figure 4b) also shifted in a negative direction in response to the NaCl additions, which is reasonable as the gap between NRs becomes wider. This allows the electrolyte solution to more easily access the NRs’ surfaces, which in turn promotes the water-splitting performance by improving charge transfer at the semiconductor/electrolyte interface. As the amount of accessible NRs surface increases, the charge transfer at the semiconductor/electrolyte also improves. Moreover, the amount of NaCl added into the crystal growth solution also can be used to tune the amount of the anatase portion in the heterojunction. The steeper photocurrent increase and the negative-shifted-onset potential indicate that improved charge separation can be achieved by tuning the amount anatase portion in the heterojunction. In this study, the steepest photocurrent increase and the highest saturation photocurrent were obtained with a sample prepared with the addition of 0.5 ml and 0.75 ml saturated aqueous NaCl solution, respectively (summarized in Table 1). It is noteworthy that photocurrent density has increased with respect to the anatase portion in the heterojunction. During irradiation both heterojunction components (anatase and rutile) will generate electrons and holes. The photogenerated electron in anatase conduction band will be transferred to the rutile conduction band enhancing the charge separation. The increase of anatase portion means that more heterojunctions in the NR structure have been created which results in the increase of charge separation efficacy. Thus, the energy difference between the conduction bands of two heterojunction components at the surface and the amount of heterojunction can be responsible for enhanced photocurrent.


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Figure 4. (a) J-V curves comparison under dark conditions and under illumination. Samples prepared using the modified and the original procedures (in the absence of NaCl). (b) Energy diagram of anataserutile heterojunction on different rutile TiO2 facets (left panel for the original procedure – (001) facet, right panel for the modified procedure – (101) facet). PEC water-splitting performances of samples prepared using modified procedure with varying amount of NaCl addition (c) J-V curve comparison under dark and illuminated conditions (inset: onset potential), (d) Photoconversion efficiency. Equation 1 is used to calculate the photoconversion efficiency, (e) Correlation between photoconversion efficiency and anatase portion vs. amount of saturated NaCl aqueous solution added in the crystal growth solution, (f) Current density-time response (stability test) measured at a constant applied potential of 1.23 VRHE.

η=

J (1.23 − V ) × 100% P

(1)

The photoconversion efficiency is calculated using Equation (1)5 – see Figure 4d and Table 1. It is noteworthy that the photoconversion efficiency can be easily tuned and optimized using NaCl, as shown in Figure 4e. As the amount of NaCl added increases, the amount of anatase portion in the heterojunction increases with an optimum portion of 9.3% (0.5 ml saturated aqueous NaCl solution ACS Paragon Plus Environment

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added). To be an efficient water-splitter, a PEC water-splitting system requires both a low onset potential and a low saturation photocurrent potential. With these two features the required applied bias needed to maximize the generated photocurrent can be reduced. As mentioned above, the addition of NaCl into the crystal growth solution enhances the performance of the TiO2 NR array. The sample prepared in the presence of 0.5 ml saturated NaCl solution had highest efficiency 1.67% at a low potential (0.44 VRHE). It is understandable that the sample having the steepest increase in photocurrent has better charge separation and collection;29-30 and this result reflects a material performance superior to the most efficient TiO2 photoanode material previously reported5. In addition, this result also implies that TiO2 NR array (prepared using the modified procedure) with more anatase portion in its heterojunction can offer an enhanced charge separation which in turn could improve the photocatalytic properties. In contrast, the result exhibited by the sample prepared using the original procedure, which has 9.9% anatase portion, does not show a significant improvement. Although these two samples (sample prepared using the modified procedure with 0.5 ml saturated aqueous NaCl solution and sample prepared using original procedure (in the absence of NaCl)) had a comparable amount of the anatase portion (9.9% and 9.3%, respectively), it is apparent that the presence of anatase phase on the NRs’ surfaces is not the only factor enhancing the PEC water-splitting performance. The sample prepared using modified procedure had a much better performance due to its [101] orientation that offers a more efficient charge separation. It can be understood as it has a lower surface energy and importantly, it works perfectly with anatase phase that has higher conduction band position – see Figure 4b. Besides having good efficiency, the photo-water-splitter needs to have good performance stability during utilization. This can be seen from the stability of the photo-water-splitter in generating photocurrent with respect to time. The generated photocurrent commonly decays after a time because charge transfer and charge separation within the system are not well-maintained, due to surface charge accumulation, or charge recombination. Additionally, the nature of the semiconductor is also important because some of the semiconductor is not chemically stable or has poor resistance towards photocorrosion. In order to evaluate stability, the photocurrent density-time (J-t) response was determined; ACS Paragon Plus Environment

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see Figure 4f, which shows that the photocurrent is quite stable over time. Figure S10 shows that the samples do not have the transient photocurrent typically present for bare TiO2 photoelectrodes as reported previously3. The transient photocurrents occur due to surface accumulation of photoexcited holes that cannot be consumed directly by the reactants on the surface, prior to their recombination with photogenerated electrons from the conduction band. To improve the transient behavior, anatase TiO2 nanoparticles, having a higher conduction band energy level, were decorated on TiO2 NRs to enhance the charge separation and stability3, 13. In contrast, our work adopts a single-step-synthesis procedure to prepare [101]-oriented rutile TiO2 NR array with a tunable anatase-rutile heterojunction and overcome this charge separation problem. The incident photon-to-current conversion efficiency (IPCE) (shown in Figure S11) of a PEC watersplitting photoelectrode is associated with the efficiencies of three fundamental processes, namely charge

generation

efficiency,

charge

transfer

(collection

or

injection)

efficiency

at

the

electrode/electrolyte interface, and charge transport efficiency within the material31. Figure S5b shows that our strategy to add NaCl into the growth solution has little impact on absorption or charge generation efficiency. However, the charge transfer efficiency at the electrode/electrolyte interface can be enhanced by using this approach. Figure 4c shows the increasing photocurrent gradient with charge transfer at the electrode/electrolyte interface (on the photocatalyst’s surface) where the reaction occurs. The last factor affecting the IPCE value is the charge transport within the material, for which both electrons and holes transport are important and affected by bulk recombination processes. The photocurrent generated was quantified by measuring the amount of charge transport from the material to the FTO substrate, external wire, and the current detector. In this ‘journey’, the photogenerated charge, in this case is electron, can recombine or become trapped by crystal defects in the bulk material. From Figure 4c, one can see that as the amount of saturated aqueous NaCl solution increases the saturation photocurrent also increases. This implies that charge transport within material is relatively good. The spectra in Figure S11 reveal that the IPCE value over the entire UV region is enhanced relative to the amount of NaCl. At a wavelength of 365 nm, each photoanode attained its highest IPCE value, ACS Paragon Plus Environment

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whereas at wavelengths higher than 400 nm negligible IPCE values matching the band-gap energy (3.0 eV) of rutile TiO2 were observed. The presence of the anatase phase on the rutile TiO2 NRs is advantageous for charge separation which in turn can improve the PEC water-splitting performance as illustrated in Figure S12. By using our procedure the optimized anatase portion of 9.3% in heterojunction with [101]-oriented rutile can be obtained. But, for best rational composition to properties, the process needs to be improved.

CONCLUSION A novel approach has been established to synthesize a [101]-oriented rutile TiO2 NR array in heterojunction with anatase phase on FTO substrate. The addition of NaCl provides a capability to tune the anatase phase portion in the nano-engineered heterojunction. Additionally, with increasing amount of NaCl, the density of NR per unit area increased and the average diameter of NRs decreased. This feature also brings advantages in harvesting solar irradiation and benefits the overall conversion efficiency. The introduction of low energy (101) rutile facet in heterojunction with high energy anatase phase exhibited a 2-fold increase in photocurrent density. This enhanced PEC water-splitting performance also reflected in improved photoconversion efficiency.

ASSOCIATED CONTENT Supporting Information available: SEM images, XRD, optical characterization, TEM images, and Raman spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Author Contributions The manuscript was written through contributions of all authors.

Notes


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The authors declare no competing financial interest.

ACKNOWLEDGMENT The financial supports from the Ministry of Science and Technology(MOST) (103-2221-E-011-156MY3, 103-3113-E-011-001, 101-3113-E-011-002, 101-2923-E-011-001-MY3), the Ministry of Economic

Affairs

(MOEA)

(101-EC-17-A-08-S1-183),

and

the

“Top

University

Projects”((100H451401) and “Global Networking Talent 3.0 Plan” of Ministry of Education (MOE), as well as the facilities supports from the National Taiwan University of Science and Technology (NTUST) are acknowledged.

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For Table of Contents Use Only

Facile synthesis of [101]-oriented rutile TiO2 nanorod array on FTO substrate with a tunable anatase-rutile heterojunction for efficient solar water-splitting Hogiartha Sutiono, Alok M. Tripathi, Hung-Ming Chen, Ching-Hsiang Chen, Wei-Nien Su, Liang-Yih Chen, Hongjie Dai, Bing-Joe Hwang

Facile synthesis of [101]-oriented rutile TiO2 NR array with a tunable anatase-rutile heterojunction for sustainable hydrogen production using an eco-friendly photocatalyst, water and sunlight


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Facile Synthesis of [101]-Oriented Rutile TiO2 Nanorod Array on FTO

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