Fabrication of TiO2-Reduced Graphene Oxide Nanorod Composition

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Fabrication of TiO2–Reduced Graphene Oxide Nanorod Composition Spreads Using Combinatorial Hydrothermal Synthesis and Their Photocatalytic and Photoelectrochemical Applications Wen-Chung Lu, Li-Chun Tseng, and Kao-Shuo Chang ACS Comb. Sci., Just Accepted Manuscript • DOI: 10.1021/acscombsci.7b00077 • Publication Date (Web): 26 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017

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ACS Combinatorial Science

Fabrication of TiO2–Reduced Graphene Oxide Nanorod Composition Spreads Using Combinatorial Hydrothermal Synthesis and Their Photocatalytic and Photoelectrochemical Applications Wen-Chung Lu1,2, Li-Chun Tseng2, and Kao-Shuo Chang1,2,3,* 1

Institute of Nanotechnology and Microsystems Engineering, National Cheng Kung

University, No.1, University Road, Tainan City 70101, Taiwan. 2

Department of Materials Science & Engineering, National Cheng Kung University. No.1,

University Road, Tainan City 70101, Taiwan. 3

Promotion Center for Global Materials Research, National Cheng Kung University. No.1,

University Road, Tainan City 70101, Taiwan. *

E-mail: [email protected]; Tel.: +886-6-2757575 ext. 62922.

ABSTRACT This study is the first to employ combinatorial hydrothermal synthesis and facile spin-coating technology to fabricate TiO2–reduced graphene oxide (rGO) nanorod composition spreads. The features of this study are 1) the development of a self-designed spin coating wedge, 2) the systemic investigation of the structure–property relationship of the system, 3) the high-throughput screening of the optimal ratio from a wide range of compositions for photocatalytic and photoelectrochemical (PEC) applications, and 4) the effective coupling between the density gradient TiO2 nanorod array and the thickness gradient rGO. The formation of rGO in the fabricated TiO2–rGO sample was monitored through Fourier transform infrared spectrometry. Transmission electron microscopy images also suggested that the TiO2 nanorod surfaces were covered with a thin layer of amorphous rGO. The rutile TiO2 1

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plane evolution along the composition variation was verified through X-ray diffraction. 7% TiO2–93% rGO on the nanorod composition spread exhibited the most promising photocatalytic ability; the corresponding photodegradation kinetics, denoted by the photodegradation rate constant (k), was determined to be approximately 12.7 × 10−3 min−1. The excellent performance was attributed to the effective coupling between the TiO2 and rGO, which improved the charge carrier transport, thus inhibiting electron-hole pair recombination. A cycling test implied that 7% TiO2–93% rGO is a reliable photocatalyst. A photoluminescence spectroscopy study also supported the superior photocatalytic ability of the sample, which was attributed to its markedly poorer recombination behavior. In addition, without further treatment, the sample exhibited excellent PEC stability; the photocurrent density was more than three times higher than that exhibited by the density gradient TiO2 nanorods.

Keywords: TiO2–rGO nanorod composition spread, spin coating, combinatorial hydrothermal synthesis, photocatalysis, photoelectrochemical reaction

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Introduction Green energy sources are imperative, alternative energy sources that can replace nonrenewable fossil fuels in the pursuit of environmental sustainability [1]. The trend of replacing fossil fuels with green energy has been driving the rapid development of various related technologies in recent decades [1-3], and this includes technologies used in geothermal energy, solar energy, tidal energy, and wind power generation. Among the various sources of green energy, solar energy is considered to hold great promise as an alternative energy source [4]. Using semiconductor photocatalysts is an efficient approach for converting solar energy into oxidative potentials and electron–hole (e−–h+) pairs to degrade various types of organic pollutant and to activate photoelectrochemical (PEC) water splitting, respectively [5]. Metal oxides and sulfides [5, 6], complex compounds [7], metal complexes [8], and nonmetallic semiconductors [9] are widely applied as degradation photocatalysts. Among them, TiO2 is promising because it possesses various desirable features, such as a strong adsorption affinity for organic pollutants, low recombination rates, nontoxicity, low cost, favorable optical properties, and oxidizing abilities [10, 11]. However, its wide band gap (Eg, approximately 3.0 eV) limits the scope of its application. To overcome this limitation, various methods have been developed to modify the band structure of TiO2 [10-13]. To achieve a practical PEC reaction, the concerns of corrosion resistance, suitable band edges, and ease of maintenance must be addressed [14]. Various suitable photocathode and photoanode materials have been developed [15, 16]. To achieve either photodegradation or PEC reactions, reducing the e−–h+ pair recombination rate is crucial, because the recombination reaction is usually substantially faster (approximately 10−9 s) than the chemical reaction between TiO2 and pollutants (approximately 10−8–10−3 s) [17]. Thus, considerable efforts have been made to minimize e−– h+ pair recombination to enhance photocatalytic efficiency [18]. 3



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Using TiO2-based nanocomposites is considered an excellent strategy, because e−–h+ pair recombination can be maneuvered by tailoring the energy band alignment between constituent materials [19]. Li et al. [19] reviewed types I, II, and p–n heterojunctions, homojunctions, and the Z-scheme system of various TiO2 semiconductor photocatalysts for water splitting, photodegradation, and CO2 conversion. Recently, highly conductive carbon-based materials [20] or graphene-based materials [21] hybridized with TiO2 have been extensively studied. Morales-Torres et al. [21] studied the important properties of TiO2–graphene, TiO2–graphene oxide (GO), and TiO2–reduced GO (rGO) and reviewed the synthetic methods of these photocatalysts and their various applications to photodegradation, water splitting, dye-sensitized solar cells, and Li-ion batteries. Although high-quality graphene can be fabricated using epitaxial chemical vapor deposition, which is a complex procedure, graphene synthesis is still considered difficult and challenging [22]. The widely applied Scotch tape method (exfoliation from bulk graphite), which is a fabrication approach developed by Novoselov and Geim in 2004 [23, 24], is relatively simple; however, this approach is not desirable for large-scale processes. Thus, GO with oxygen-containing functional groups [22, 25], which is typically synthesized using the Hummers method [26], has been found to be an effective yet inexpensive material; it can thus be used as a precursor for the cost-effective synthesis of graphene-like sheets, in which GO is reduced to graphene by restoring the sp2 hybridized network [22, 27] by using either a chemical or thermal reduction process [25] or reducing agents such as hydrazine (N2H4) [25], NaBH4 [25], and alcohol [25, 28], or UV light irradiation [25]. However, the synthesis of TiO2–graphene-based nanocomposites still requires substantial investigation, because the optimal ratios of components are yet to be systemically determined from a wide composition range to yield nanocomposites with desirable properties. In the present study, high-throughput combinatorial synthesis was applied to resolve the concern. This technique considerably accelerates the exploration of multicomponent or 4


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composite materials [29], because it can synthesize hundreds of samples with various compositions on one substrate simultaneously [30]. The integration of this technique with a rapid screening approach can enable the efficient identification of the potential compositions (ratios) of composites. The combinatorial technique is prevalent in catalyst development and numerous combinatorial techniques have been reported [31]. In addition, Woodhouse et al. [32] reviewed various combinatorial approaches to study functional metal oxide photocatalysts, including ink jet printing, electrochemical scanning light beam analysis, electrochemical synthesis and screening, sol gel routes, and thin film deposition techniques. However, the combinatorial spin coating route for fabricating nanocomposites has not been reported in the literature. Thus, this study is the first to fabricate TiO2–rGO nanorod composition spreads by using combinatorial spin coating and hydrothermal reactions. Their photocatalytic applications were also investigated. In addition to determining the optimal ratios of the nanocomposite constituents, other crucial parameters for effective photocatalysis are the number of surface active sites on TiO2 nanorods and their coupling with rGO. Thus, TiO2 nanorod densities were systemically tuned in this study. Previous studies [12, 33, 34] have validated that determining the appropriate precursor concentration is essential to tailoring the various densities of TiO2 nanorods. Thus, the current study used two concentrations of Ti precursors and a facile spin coater, the application of which has not been reported for fabricating density gradient TiO2 nanorods on a single substrate. This was achieved using a thickness gradient TiO2 seed layer as a template. The sample was then coupled with another thickness gradient rGO to form a TiO2–rGO nanorod composition spread, in which the wide range of composition ratios enabled studying the coupling between TiO2 and rGO. Numerous analytical methods were employed to determine the characteristics of the samples, including the phase, morphology, microstructure, and recombination behavior of photogenerated e−–h+ pairs. Location 5 (7% TiO2–93% rGO) 5



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on the TiO2–rGO nanorod composition spread was rapidly determined to be a promising photocatalyst; its photodegradation capability was almost five times higher than that of Location 1 on the same sample. In addition, all densities of TiO2 nanorods coupled with rGO exhibited much more superior photocatalytic ability to that of the samples without rGO incorporation. These findings indicated that rGO is effective for inhibiting the recombination of e−–h+ pairs in the system. In addition, the PEC study also revealed that the photocurrent at Location 5 was more than double that at the same location in a sample without rGO. This paper describes the validity of applying combinatorial hydrothermal methodology to rapidly optimize the TiO2:rGO ratios to achieve synergistic photocatalysis.

Experimental A Si wafer was used as the substrate; it was cut into pieces sized 5 × 1.2 cm2. Prior to deposition, the Si substrate was cleaned under ultrasonication with acetone, ethanol, isopropyl alcohol, and deionized (DI) water (in that order) for 5 min each. Before deposition, the cleaned substrate was dried under nitrogen gas. A spin coater was used to prepare either a uniform or a thickness gradient TiO2 seed layer as a template for growing homogeneous or density gradient TiO2 nanorods through the hydrothermal route. Two concentrations (0.8 and 1.5 M) of titanium butoxide (TBOT) precursor solution were prepared to grow the aforementioned samples. The 0.8 M TBOT solution was synthesized by mixing 5.26 mL of TBOT, 3 mL of ethanol, and 1.25 mL of diethanolamine, and then stirring the mixture vigorously for 1 h. The prepared solution was then mixed with another solution comprising 0.425 mL of DI water, 8.5 mL of ethanol, and 0.325 mL of HCl; the mixture was stirred for another 15 min and then left to stand for 24 h at room temperature to obtain a homogeneous and stable coating solution. The 1.5 M TBOT solution was prepared using the same strategy, except that 15 mL of TBOT was used. To achieve a substantial density variation of TiO2 6


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nanorods across the substrate, a self-designed spin coater wedge was employed (angle = 21°). Coupling the 0.8 and 1.5 M precursor solutions enabled fabrication of a thickness gradient TiO2 seed layer for use as a template. As shown in the upper part of Fig. 1(a), the cut Si substrate was placed such that its bottom edge (Position 1) was near the center of the self-designed wedge (red). Two 0.8 M TBOT droplets were dispensed at the location presented in the figure following a one-by-one drop spin coating process; each droplet was spun at 3000 rpm for 40 s. The spin-coated substrate was then annealed at 400°C for 1 h in a tube furnace; a schematic (side view) is shown in the lower part of Fig. 1(a). Thereafter, the substrate was indexed at 180° and placed such that its bottom edge (Position 2) was near the center of the self-designed wedge (top portion of Fig. 1(b)). One 1.5 M TBOT droplet was then dispensed and spin coated at 3000 rpm for 40 s. The obtained thickness gradient TiO2 seed layer sample was finally annealed at 700°C for 1 h to enhance its adhesion with Si; the corresponding schematic (side view) is shown in the lower part of Fig. 1(b). The seed layer template was then placed in an autoclave containing a precursor solution of 1.795 g of NaCl, 0.5 mL of TBOT, 19.5 mL of HCl, and 26 mL of DI water for a hydrothermal reaction to grow density gradient TiO2 nanorod arrays. The reaction was allowed to proceed at 160°C for 3 h; the resultant sample was then rinsed with DI water and dried at 40°C for 20 min. As presented in Fig. 1(c), high-density TiO2 nanorods grew at Position 1 (0.8 M-rich), and the density steadily decreased toward Position 2 (1.5 M-rich). The details are displayed in Fig. 3. To fabricate TiO2–rGO nanorod composition spreads, the density gradient TiO2 nanorod sample was placed on a spin coater such that Position 1 was over the vacuum hole (Fig. 2(a)). Numerous GO droplets (prepared using Hummers method [26]) were dispensed at various locations, as follows. Each droplet was spin coated at 3000 rpm for 60 s. One GO droplet was first dispensed near Position 1. Subsequently, the substrate was moved to the left, where approximately one-third of the substrate was positioned over the vacuum hole [Fig. 2(b)]. Two 7



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GO droplets were dispensed using a one-by-one drop spin-coating process. Finally, the substrate was further moved to the left, where approximately two-thirds of the substrate was over the vacuum hole [Fig. 2(c)]; the dispensing process described in the previous step was applied. Thus, the preparation of TiO2–GO nanorod composition spreads was completed; the nanorod composition spreads were then reduced to TiO2–rGO by immersing TiO2–GO in alcohol and illuminating with UV irradiation (10 mW/cm2) for 12 h. Fig. 2(d) shows the schematic (top and side views) of the sample, in which the TiO2 nanorods and rGO are marked in yellow and black, respectively; the color depth represents the thickness of the rGO (deeper colors represent thicker layers). Numerous TiO2–rGO sample sets were reliably fabricated and each set of them was prepared for each specific characterization. Because of the nature of gradient, average properties were obtained at each location using various characterizations. Scanning electron microscopy (SEM) was performed to study the morphology evolution of the samples. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were employed to determine the phases and microstructures. Fourier transform infrared spectrometry (FTIR) was used to study the reduction of GO by monitoring its functional group evolution. Raman spectrometry was also conducted to compliment the FTIR results by recognizing the structural disorder of the samples. The TiO2:rGO ratio was determined through X-ray photoelectron spectroscopy (XPS). The TiO2–rGO nanorod composition spread (5 × 1.2 cm2) was cut into six pieces along the composition variation; each piece was approximately 0.8 × 1.2 cm2 in size. Their photodegradation ability was studied individually by measuring the degradation rate of a methylene blue (MB) solution (2 ppm) under UV irradiation (1.5 mW/cm2). Prior to measurement, the sample solutions were maintained in the dark for the first 30 min to achieve absorption–desorption equilibrium. The data were then collected every 30 min during the reaction. To study the photostability of the samples, a three-run cycling test was conducted. 8


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Between each run, each studied sample was cleaned thoroughly by soaking it in acetone overnight; it was then immersed in the same volume of a fresh MB solution for another run. Photoluminescence spectroscopy (PL) was used to measure the recombination process of photogenerated charges by monitoring the emitted radiation signals. In addition, the photocurrent density (JD) was characterized at a bias of 0.5 V [vs saturated calomel electrode (SCE), E0SCE = 0.241 V vs. NHE (pH = 0) at 25°C)] under noncollimated illumination (1.5 mW/cm2 UV) by using a potentiostat in a three-electrode electrochemical cell, which consisted of a working photoelectrode (the studied sample), a counter electrode (Pt), and an SCE immersed in a 1.0 M NaOH electrolyte solution. Alligator clips were used for making contacts.

Results and Discussion SEM images (top and side views) of the density gradient TiO2 nanorod arrays on a single Si substrate are shown in Fig. 3. The images in Fig. 3(a)–(h) showed eight positions along the density variation direction across the sample area, as depicted in Fig. 3(i). The TiO2 nanorod densities gradually increased from Fig. 3(a) (1.5 M-rich) to Fig. 3(h) (0.8 M-rich). In addition, the nanorods collapsed near the 1.5 M-rich end [(a)–(b)] because of their low-density arrays, leading to a substantial amount of space between the nanorods. Thus, a sufficient number of neighboring nanorods had not grown to support each other along the normal direction of the substrate. By contrast, the nanorods grew vertically near the 0.8 M-rich end. To further illustrate the density variation, the corresponding numbers of vertical nanorods at each location were approximated, the results of which are summarized in Table 1. The number of vertical TiO2 nanorods in Fig. 3(h) was 25 times higher than that in Fig. 3(a). These observations suggested that a wide density variation of TiO2 nanorods was achieved using the facile spin-coating and combinatorial hydrothermal synthesis strategy. 9



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FTIR was employed to study the GO reduction evolution. Fig. 4 presents the FTIR spectra obtained from three representative locations [1.5 M and rGO-rich (purple) end, middle region (black), and 0.8 M-rich end (red)] on the TiO2–rGO nanorod composition spread irradiated with UV for various times (0, 4, 8, and 12 h). In general, the functional groups of C=O (approximately at 1710 cm−1), C=C (approximately at 1520 cm−1), and C–O (approximately at 1000 cm−1) [35] were observed at the three locations before irradiation (0 h); their intensities also gradually intensified from the 0.8 M-rich to 1.5 M and rGO-rich ends. When the sample was illuminated for 4 and 8 h, the intensities of the three functional groups weakened; however, the reduction of the signals near the 1.5 M and rGO-rich end was not as substantial as that observed near the 0.8 M-rich end. These observations indicated that the GO thickness gradient had been fabricated, as outlined in Fig. 2(a)–(c), and implied insufficient GO reduction under these circumstances. When the exposure time was extended to 12 h, the signals of the three functional groups disappeared. This finding suggested that no residual GO was detected across the composition spread area. Thus, an exposure time of 12 h was applied for this study. The GO reduction mechanism was proposed by Williams et al. that the formation of •C2H4OH led to accumulation of electrons in TiO2, which facilitated the reduction of GO [36]. To systemically investigate the phase evolution of TiO2–rGO along the composition variation direction, six positions indexed by Locations 1 (0.8 M-rich) to 6 (1.5 M and rGO-rich), as indicated in the inset of Fig. 5, were characterized using XRD. In general, the planes at (110), (101), (111), and (211) of the rutile TiO2 phase (JCPDS no. 21–1276) were observed for all positions. This finding indicated sufficient numbers of TiO2 nanorods, although the density gradient was artificially fabricated. Among these planes, the intensity of the high-energy facet of (101) (at approximately 36°) was approximately constant at all locations. However, the intensity of another high-energy facet of (111) (at approximately 41°) 10


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decreased slightly from Locations 1 to 6, and that of the low-energy facets of (110) (at approximately 27°) and (211) (at approximately 54°) also intensified. These evolutions can be rationalized as follows: 1) the low density (near Location 6) promoted the growth of individual TiO2 nanorods, because the high concentration of TBOT (1.5 M) enhanced the formation of a substantial amount of complex species in the hydrothermal solutions [12], and 2) high-energy facets grew more predominately than did low-energy facets [37, 38], leading to predominantly low-energy facets [37, 38] on the basis of crystal growth principles. This plane evolution also indicated that the orientation of TiO2 nanorods was textured near Location 1, because predominant planes at (101) and (111) contributed to the excellent alignment of TiO2 nanorod growth. However, poor textures were observed at locations near the other end, as demonstrated by various observable planes attributed to the random growth of TiO2 nanorods (Fig. 3). Furthermore, no notable characteristic peaks attributed to rGO (at approximately 25°) were observed. This finding indicated that amorphous rGO had been incorporated into the sample. TEM was conducted for further analysis. The photodegradation ability at the six locations was also studied under UV irradiation for 90 min (Fig. 6). Self-degradation of MB was measured as a reference (grey, without any TiO2–rGO). The results after the dark-reaction correction are presented in Fig. 6(a). In general, the photodecomposition at Locations 1–4 was not substantial; however, it was noticeably enhanced at Locations 5 and 6. More than 70% of MB was photodegraded after 90 min, particularly at Location 5 (blue). The corresponding photodegradation rate constant (k) for each location was then calculated on the basis of ln(C0/C) (where C0 and C denote the original and residual concentrations of MB, respectively) versus the irradiation time [Fig. 6(b) (red columns)]. Among all locations, Location 5 exhibited the most promising photocatalytic ability (k ≅ 12.7 × 10−3 min−1), which was superior to that reported in a similar study that used TiO2 nanoparticles coupled with rGO nanosheets (k ≅ 9.5 × 10−3 min−1) [39]. Furthermore, 11



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compared with the photocatalytic abilities at the six locations on the density gradient TiO2 nanorod arrays (no rGO incorporation, blue columns), rGO-involved samples exhibited markedly superior photocatalytic ability. This finding implied the effectiveness of rGO incorporation for improving photodecomposition efficiency. The substantial enhancement was not attributed to the high-energy facets of (111) and (101) of TiO2 nanorods, because their contributions did not vary considerably, as evidenced by the XRD results (Fig. 5). The primary reason for the enhancement was the strong coupling between the TiO2 and rGO. This effect was particularly predominant at Locations 5 and 6 because of the low TiO2 nanorod densities, which were accompanied by a large active surface area capable of accommodating a considerable amount of rGO. Given the high conductivity of rGO, the intimate coupling [also indicated by the TEM results, Fig. 10(a)] between the TiO2 and rGO improved the charge carrier transport, thus inhibiting the recombination of e−–h+ pairs in the system. The recombination behavior was further demonstrated by the PL results (Fig. 9). Another notable observation was that the high-density TiO2 nanorods did not necessarily exhibit promising photodegradation capability; thus, a substantial amount of rGO is crucial for improving photodecomposition. Through the composition spread technique, the optimal TiO2:rGO ratios were readily determined. Location 5 was also subjected to a three-run cycling test to evaluate its stability and reusability (Fig. 7). Between each cycle (150 min), the sample was removed from the MB solution and soaked in acetone overnight to remove absorbed MB in TiO2 nanorods. The cleaned sample was then immersed in the same volume of a fresh MB solution for another run. As observed in Fig. 7, the photocatalytic results were consistent throughout the three-run cycling test without observable deterioration. Thus, Location 5 is a reliable photocatalyst for promising environmental sustainability-related applications. XPS was performed to examine the TiO2:rGO ratio at Location 5. The C 1s peak was used 12


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to calibrate the binding energies of the sample. Fig. 8(a) shows that in the C 1s spectrum, four subpeaks were deconvoluted and indexed at approximately 284.4, 285.6, 286.8, and 288.0 eV, and these four subpeaks were attributed to the binding of C–C, C–OH, C–O, and C=O, respectively [40]. To elucidate the reduction of the TiO2–GO to TiO2–rGO, the C 1s spectra of the TiO2–GO was compared [Fig. 8(b)]. The peak intensities of C–OH (285.6 eV), C–O (286.8 eV), and C=O (288.0 eV) [25, 40] exhibited by the TiO2−rGO sample were substantially weaker, indicating the reduction of GO to rGO. The residual C–O related signals observed in the TiO2–rGO were attributed to the sample surface oxidation because no residual GO was detected, as discussed in the FTIR spectra (Fig. 4). To study the bonding between Ti and C in the TiO2–rGO system, the Ti 2p spectra of TiO2−rGO and pure TiO2 were compared [(Fig. 8(c)]. The Ti 2p3/2 peak shifted from approximately 461 eV for pure TiO2 (inset) to approximately 459 eV for the TiO2−rGO, indicating the bonding of Ti−O−C on the TiO2 surface [40]. By comparing the peak area of C−C [Fig. 8(a)] with that of Ti 2p [Fig. 8(c)] and considering the sensitivity factors of C and Ti, the TiO2:rGO ratio was calculated to be approximately 7%:93%. In addition, the deconvolution of the Ti 2p peak indicated the formation of some Ti3+ (approximately 6 %), which might cause the dark current in the PEC reactions [discussed in Fig. 11(b)]. PL emission signals are indicative of the recombination of photoinduced e−–h+ pairs; poor recombination leads to weak PL signals. Thus, PL was used to understand the recombination behavior of photogenerated e−–h+ pairs at Location 5. Location 5 on the other two samples, namely density gradient TiO2 nanorod arrays and TiO2–GO nanorod composition spreads, were measured for comparison. As shown in Fig. 9, the peak located at approximately 410 nm (3.0 eV) was attributed to the band-to-band transition of TiO2 [41, 42]; the broad emission signal that peaked at approximately 530–550 nm was attributed to defect transitions, particularly oxygen vacancies [41, 42]. Location 5 on the TiO2–GO sample exhibited much 13



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stronger peak intensities (blue curve) than did the other two samples. This observation indicated substantial e−–h+ pair recombination, which was induced by the considerable number of defects involved in GO and its poor conductivity. However, both peak intensities decreased markedly when GO was reduced to rGO (red curve) and were weaker than those exhibited by the density gradient TiO2 nanorods (black curve). These observations indicated that much poorer e−–h+ pair recombination occurred in the TiO2–rGO sample. Thus, charge transport was facilitated, which further supported the superior photocatalytic ability at Location 5 and verified the effectiveness of rGO incorporation. TEM was performed to further study the rGO microstructures at Location 5 [Fig. 10(a)]. Location 5 on the density gradient TiO2 nanorods [Fig. 10(b)] was compared. The nanorod diameter was approximately 200 nm. Moreover, a pyramid-like tip structure was clearly observed [Fig. 10(b)], which is consistent with the result described in the literature [37, 38]. Compared with Fig. 10(b), Fig. 10(a) shows that the TiO2 nanorod surfaces were covered by a thin layer of rGO. The high-resolution TEM image (lower right inset), taken at a position near the nanorod surface (blue circle), shows the distinctive crystalline fringes associated with the TiO2 planes at (110), which were attributed to the 0.327 nm d-spacing. The light contrast region depicted by the double-head arrow denotes amorphous rGO. Location 5 on the TiO2–rGO nanorod composition spread can be applied as a photoelectrode material in a PEC cell for water splitting under UV illumination for H2 production. PEC current measurements are an indication of cell performance. To obtain an appropriate applied bias for operating the cell, the photocurrent under UV irradiation (1.5 mW/cm2) versus applied voltage was first characterized. Fig. 11(a) presents the net measured photocurrent values after the dark-current correction. The original I-V curves with and without illumination are presented in the inset. No photocurrent was observed until the bias reached approximately 0.2 V, implying a nonspontaneous water splitting reaction. Thus, to 14


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achieve reasonable photocurrent densities, a bias of 0.5 V was rationally chosen to study cell performance. Three runs (5 min each) and seven cycles for each run (lights on for 20 s and lights off for 20 s for each cycle) were conducted to ensure the photostability of the sample [Fig. 11(b)]. The PEC current rapidly increased at the beginning of each cycle because of the rapid generation of e−–h+ pairs under UV illumination. The current gradually decreased to a constant state at which time equilibrium was reached. The measured current was approximately 9 µA/cm2. The possible source of the dark current may originate from the formation of Ti3+ [43], which caused the minor corrosion of TiO2 in basic electrolyte solutions (1.0 M NaOH). As analyzed in Fig. 8(c), approximately 6 % of Ti3+ was formed. This observation was consistent with the study of facile synthesis of the Ti3+ self-doped TiO2-graphene nanosheet composites with enhanced photocatalysis [40]. By contrast, Fig. 11(c) presents the result obtained from Location 5 on the density gradient TiO2 nanorods, in which a constant photocurrent of approximately 2.7 µA/cm2 was observed throughout the seven-cycle run. The results showed that Location 5 on the TiO2–rGO nanorod composition exhibited more than three times higher photocurrent than that exhibited by Location 5 on the density gradient TiO2 nanorods. This observation was also attributed the intimate coupling between the TiO2 and rGO and inhibition of the recombination of e−–h+ pairs in the system. Although the TiO2–rGO sample was not further treated to reduce charge carrier recombination, it still exhibited superior PEC stability.

Conclusions This study is the first to report the photocatalytic and PEC properties of TiO2–rGO composition spreads fabricated by coupling a density gradient TiO2 nanorod array with a thickness gradient rGO by using a self-designed spin coating wedge and combinatorial hydrothermal synthesis. FTIR was employed to verify the formation of rGO. TEM images 15



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suggested that the TiO2 nanorod surfaces were covered by a thin layer of amorphous rGO. XRD confirmed the plane evolution of the sample along the composition variation. Through the facile composition spread strategy, the optimal TiO2:rGO ratios were readily determined. 7% TiO2–93% rGO on the composition spread exhibited the most promising photocatalytic ability (k ≅ 12.7 × 10−3 min−1), which was superior to that of a similar system described in the literature. The TiO2–rGO sample also exhibited substantially more superior photocatalytic ability to that of the TiO2 sample without rGO. The primary reason was the effective coupling between low-density TiO2 nanorods and substantial amounts of rGO, in which e−–h+ pair recombination was inhibited, thus leading to improved charge carrier transport in the system. The PL study also suggested that much poorer recombination occurred at Location 5 on the TiO2–rGO sample, supporting superior photocatalytic performance. This sample also exhibited reliable photocatalysis, as demonstrated by the cycling test. In addition, without further treatment, the sample exhibited superior PEC stability and a photocurrent density of approximately 9 µA/cm2, which was more than three times higher than that exhibited by the density gradient TiO2 nanorods.

Acknowledgments This study was partially supported by the Ministry of Science and Technology, Taiwan (MOST 104-2221-E-006-025).

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Figure Captions Fig. 1. Schematic of the density gradient TiO2 nanorod fabricated using the self-designed spin coating wedge. (a) Si substrate placement and 0.8 M TBOT dispensing locations (top); resultant thickness gradient TiO2 seed layer (bottom). (b) Si substrate placement and 1.5 M TBOT dispensing locations (top); resultant thickness gradient TiO2 seed layer (bottom). (c) Resultant density gradient TiO2 nanorod arrays after the hydrothermal reaction. Fig. 2. Schematic of the thickness gradient GO fabricated on the density gradient TiO2 nanorod array. (a) One GO droplet was dispensed near Position 1. (b) Two GO droplets were dispensed on approximately one-third of the substrate. (c) Two GO droplets were dispensed on approximately two-thirds of the substrate. (d) Schematic of top and side views of the fabricated TiO2–rGO composition spread. Fig. 3. Top and side view SEM images of density gradient TiO2 nanorod arrays taken at eight locations [(a)–(h)], as depicted in (i). Fig. 4. FTIR spectra obtained at three locations [1.5 M and rGO-rich (purple) end, middle region (black), and 0.8 M-rich end (red)] on the TiO2–rGO nanorod composition spread. Fig. 5. XRD results at six locations, as indicated in the inset, on the TiO2–rGO nanorod composition spread along the composition variation direction. Fig. 6. (a) Photodegradation abilities at six locations on the TiO2–rGO nanorod composition spread. (b) Photodegradation rate constant (k) calculated from each location (red columns). Photodegradation capabilities at the six locations on the TiO2–GO nanorod composition spread were compared (blue columns). Fig. 7. Three-run photodegradation cycling test at Location 5 on the TiO2–rGO nanorod composition spread. Fig. 8. XPS spectra at Location 5 on the TiO2–rGO [(a) and (c)] and TiO2–GO (b) nanorod composition spreads, and the density gradient TiO2 nanorod array [inset of (c)]. (a) C 1s. (b) C 17



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1s. (c) Ti 2p. Fig. 9. PL study at Locations 5 on the TiO2–rGO (red) and TiO2–GO (blue) nanorod composition spreads and the density gradient TiO2 nanorod array (black). Fig. 10. TEM results at Locations 5 on (a) the TiO2–rGO nanorod composition spread and (b) the density gradient TiO2 nanorod array. Fig. 11. PEC performance study. (a) Net photocurrent values at Location 5 on the TiO2–rGO nanorod composition spread after the dark current correction. The inset shows the current with (red) and without (blue) illumination. (b) Photocurrent density obtained at Location 5 on the TiO2–rGO nanorod composition spread. Three runs and seven cycles for each run were conducted. (c) Photocurrent density obtained at Location 5 on the density gradient TiO2 nanorod array.

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Table Captions Table 1. Number of vertical nanorods at each location [(a)–(h)] on the density gradient TiO2 nanorod array.

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Fig. 1 W.-C. Lu et al.

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C1s TiO2-rGO 284.4 eV, C-C

283 285 287 289 Binding Energy (eV)

291

C1s TiO2-GO 287.2 eV, C-O

Ti2p 458.6 eV, Ti4+ TiO2-rGO

456

Intensity (a.u.)

286.8 eV, C-O 285.6 eV, C-OH 288 eV, C=O

281

(b)

(c) Intensity (a.u.)

Intensity (a.u.)

(a)

Intensity (a.u.)

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457.4 eV, Ti3+

281

288 eV, C=O

283 285 287 289 Binding Energy (eV)

458

461.2 eV, Ti4+

461 464 Binding energy (eV)

458 460 Binding Energy (eV)

285.2 eV, C-OH 284.6 eV, C-C

TiO2

291

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Fig. 11 W.-C. Lu et al. 0.001

Net current (A)

0.0015

(b)

illumination dark

0 0

1

0 0

(c)

0.5

1

Potential vs SCE (V)

12 8 4

Current density (µ µA/cm2)

(a)

Current density (µ µ A/cm2)

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#5 of TiO2-rGO

30 20 10

--- 1st

--- 2nd

0 0

100

0 0

100

200

--- 3rd

200

Time (second)

300

Time (second)

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Table 1 W.-C. Lu et al.

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Table of Contents graphic

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Fig. 1. Schematic of the density gradient TiO2 nanorod fabricated using the self-designed spin coating wedge. (a) Si substrate placement and 0.8 M TBOT dispensing locations (top); resultant thickness gradient TiO2 seed layer (bottom). (b) Si substrate placement and 1.5 M TBOT dispensing locations (top); resultant thickness gradient TiO2 seed layer (bottom). (c) Resultant density gradient TiO2 nanorod arrays after the hydrothermal reaction. 254x190mm (300 x 300 DPI)


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Fig. 2. Schematic of the thickness gradient GO fabricated on the density gradient TiO2 nanorod array. (a) One GO droplet was dispensed near Position 1. (b) Two GO droplets were dispensed on approximately onethird of the substrate. (c) Two GO droplets were dispensed on approximately two-thirds of the substrate. (d) Schematic of top and side views of the fabricated TiO2–rGO composition spread. 254x190mm (300 x 300 DPI)



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Fig. 3. Top and side view SEM images of density gradient TiO2 nanorod arrays taken at eight locations [(a)– (h)], as depicted in (i). 254x190mm (300 x 300 DPI)


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Fig. 4. FTIR spectra obtained at three locations [1.5 M and rGO-rich (purple) end, middle region (black), and 0.8 M-rich end (red)] on the TiO2–rGO nanorod composition spread. 254x190mm (300 x 300 DPI)



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Fig. 5. XRD results at six locations, as indicated in the inset, on the TiO2–rGO nanorod composition spread along the composition variation direction. 254x190mm (300 x 300 DPI)


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Fig. 6. (a) Photodegradation abilities at six locations on the TiO2–rGO nanorod composition spread. (b) Photodegradation rate constant (k) calculated from each location (red columns). Photodegradation capabilities at the six locations on the TiO2–GO nanorod composition spread were compared (blue columns). 254x190mm (300 x 300 DPI)



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Fig. 7. Three-run photodegradation cycling test at Location 5 on the TiO2–rGO nanorod composition spread. 254x190mm (300 x 300 DPI)


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Fig. 8. XPS spectra at Location 5 on the TiO2–rGO [(a) and (c)] and TiO2–GO (b) nanorod composition spreads, and the density gradient TiO2 nanorod array [inset of (c)]. (a) C 1s. (b) C 1s. (c) Ti 2p. 254x190mm (300 x 300 DPI)



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Fig. 9. PL study at Locations 5 on the TiO2–rGO (red) and TiO2–GO (blue) nanorod composition spreads and the density gradient TiO2 nanorod array (black). 254x190mm (300 x 300 DPI)


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Fig. 10. TEM results at Locations 5 on (a) the TiO2–rGO nanorod composition spread and (b) the density gradient TiO2 nanorod array. 254x190mm (300 x 300 DPI)



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Fig. 11. PEC performance study. (a) Net photocurrent values at Location 5 on the TiO2–rGO nanorod composition spread after the dark current correction. The inset shows the current with (red) and without (blue) illumination. (b) Photocurrent density obtained at Location 5 on the TiO2–rGO nanorod composition spread. Three runs and seven cycles for each run were conducted. (c) Photocurrent density obtained at Location 5 on the density gradient TiO2 nanorod array. 254x190mm (300 x 300 DPI)


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Table 1. Number of vertical nanorods at each location [(a)–(h)] on the density gradient TiO2 nanorod array. 254x190mm (300 x 300 DPI)


Fabrication of TiO2-Reduced Graphene Oxide Nanorod Composition

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