1D TiO2

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Engineered Solution-Liquid-Solid Growth of a “Tree-like” 1D/1D TiO2 Nanotube-CdSe Nanowire Heterostructure: Photoelectrochemical Conversion of Broad Spectrum of Solar Energy Bratindranath Mukherjee, Swagotom Sarker, Eric Crone, Pawan Pathak, and Vaidyanathan Ravi Subramanian ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10200 • Publication Date (Web): 20 Oct 2016 Downloaded from http://pubs.acs.org on October 24, 2016

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ACS Applied Materials & Interfaces

Engineered Solution-Liquid-Solid Growth of a “Tree-like” 1D/1D TiO2 Nanotube-CdSe Nanowire Heterostructure: Photoelectrochemical Conversion of Broad Spectrum of Solar Energy Bratindranath Mukherjee,a,b,§ Swagotom Sarker,a,b,† Eric Crone,a Pawan Pathak,a and Vaidyanathan R. Subramaniana,* AUTHOR ADDRESS a

Department of Chemical and Materials Engineering, University of Nevada, Reno, NV 89557.

Phone: 775-784-4686. Fax: 775-327-5059. E-mail: [email protected]. KEYWORDS: TiO2 Nanotubes, Bismuth Nanoparticles, CdSe Nanowires, Solution-LiquidSolid, Photoelectrochemistry.

ABSTRACT. This work presents a hitherto unreported approach to assemble a 1D oxide-1D chalcogenide heterostructured photoactive film. As a representative system, bismuth (Bi) catalyzed 1D CdSe nanowires are directly grown on anodized 1D TiO2 nanotube (T_NT). A combination of the reductive successive-ionic-layer-adsorption-reaction (R-SILAR) and the solution-liquid-solid (S-L-S) approach is implemented to fabricate this heterostructured assembly, reported in this 1D/1D form for the first time. XRD, SEM, HRTEM, and elemental mapping are performed to systematically characterize the deposition of bismuth on T_NT and

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the growth of CdSe nanowires leading to the evolution of the 1D/1D heterostructure. The resulting “tree-like” photoactive architecture demonstrates UV-visible light-driven electron-hole pair generation. The photoelectrochemical results highlight: i) the formation of a stable n-nheterojunction between TiO2 nanotube and CdSe nanowire, ii) an excellent correlation between the absorbance vis-à-vis light conversion efficiency (IPCE), and iii) a photocurrent density of 3.84 mA/cm2. This proof-of-concept features the viability of the approach for designing such complex 1D/1D oxide - chalcogenide heterostructures that can be of interest to photovoltaics, photocatalysis, environmental remediation, and sensing.

1. INTRODUCTION

Various metal chalcogenides and their nanocomposites1 with metal oxides have been of interest to flexible devices / optoelectronics,2-4 photovoltaics,5 clean fuel generation (such as hydrogen generation from water splitting), and environmental remediation6 (pollutant remediation).7-10 Literature reports presented so far have primarily focused on the nanostructured oxidechalcogenide films that are respectively in either 0D/0D11 or 1D/0D12 configurations. In these configurations, the oxide particulate films prepared on various substrates using sol-gel13 or chamber-based processes14 may be used to anchor quantum dots (QDs). These 0D QDs could be either obtained15-17 or prepared by nucleation and growth using electrochemical18 or successive ionic layer adsorption and reaction (SILAR)19 technique. However, such nanostructured film prepared using 0D materials can possess inter-particle grain boundaries. As a result, charge transport and photocatalytic activity across 0D particulate films will differ from those across 1D oxide films.20,21 Since charge transfer occurs by the process of hopping mechanism across such boundaries, called as nodes here, they function as recombination centres and reduce


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photoelectrochemical activity in 0D films. Alternately, switching to a 1D nanostructure of the same oxide offers reduced inter-particle grain boundaries that can in-principle serve as a better way for a more rapid electron transport process shown in Scheme 1. The photogenerated charge carrier separation is facilitated by the alignment of the conduction bands of an oxide and an overlying visible light harvester such as a quantum dot nanocrystal (QDNC)22 across the established heterojunction. QDNC can participate in redox catalysis and consist of multiple components.23 On the other hand, nanowires, which can be synthesized using solution−liquid−solid method,

10,24-26

can demonstrate interesting optical properties; some of

which differ from QDNCs.27,28 Immobilization of visible light agents on 1D TiO2 nanotubes and ZnO arrays for directional transport of photogenerated carriers from dye or quantum dot based light harvesters of various sizes, is well documented.18,29 It is important to note that a series of reports30-32 on stand-alone (i.e. oxide free) NWs such as CdSe,33 CdTe,34 as well as heterostructured NWs35 consisting of CdS/PbS25 or CdSe/PbSe36 that demonstrated excellent light absorbance and photoelectrochemical properties, are reported. Consequently, the hypotheses of this work are: I) demonstrate if the fabrication of a “proof-of-concept” heterostructured assembly of only 1D components i.e. a 1D oxide and a 1D chalcogenide is possible and II) understand the photogenerated carrier separation by the built-in potential and its transport through the single crystalline backbone of the heterojunction. Towards this end, the first step undertaken was an assembly such a “tree-like” heterostructure. The difficulty associated with the formation 1D/1D nanostructured hybrid is that, unlike QDs, the attachment of the visible light absorbing NWs on 1D oxide is challenging using a bifunctional linker. Further, they cannot be directly grown using the SILAR technique as is the case with many 1D


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oxide/0D chalcogenide systems. However, one can adapt the solution-liquid-solid approach and modify it to assemble NWs on the oxide surface.37 Herein, we report such an approach to fabricate a low melting point metal catalyzed 1D/1D oxide – chalcogenide tree-like architecture by the combination of an R-SILAR and an S-L-S approach. In the first part of the assembly, the R-SILAR technique is illustrated to uniformly deposit seed nanoparticles (NPs) such as Bi. In the second part, the S-L-S approach is leveraged to directly fabricate nanotube-nanowire hybrid, a first-of-its-kind. The photoelectrochemical (PEC) results achieved are promising given that only single component NWs are assembled and fine tuning of the PEC activity is yet to be performed. The work sets the stage for further optimization of the PEC results with multicomponent heterojunction NW using the ion-exchange approach presented in one of our an earlier works.25 2. EXPERIMENTAL SECTION 2.1 Synthesis 2.1.1 Chemicals. All chemicals were obtained commercially and used without additional treatment or purification. These include: ammonium ﬂuoride (ACS reagent, ≥98%), ethylene glycol (99.9%), DI water, acetone, bismuth citrate (high purity grade), tin(II) chloride dehydrate (ACS reagent, 98%), citric acid (Alfa Aesar, 99.5+%), ammonia (Fisher, 26oBaumé), sodium borohydride (powder, 98%), selenium (Acros, 99.5+%), trioctylphosphine (Acros, 90%), oleic acid (TCI Chemicals, 85%), cadmium acetate dihydrate (ACS, 98%), oleylamine (Acros, 90%), pyridine, mercaptoacetic acid (Acros, 98%), iso-propanol, sodium sulphide nonahydrate (ACS reagent, 98%). 2.1.2 Synthesis of anodized TiO2 nanotubes. Synthesis of anodized TiO2 nanotube has been reported in our earlier work.12 Titanium foil (99.7% purity, 0.13 mm thickness) was purchased


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from the Strem Chemicals Inc. 4 cm × 0.8 cm titanium strips were used for anodization. Each strip was polished well and, later, ultrasonicated in DI water, iso-propanol and acetone, separately. Ultrasonication was performed to remove dirt materials and organic residues. Electrolyte used for anodization consisted of a fluorinated solution: ammonium fluoride at 0.5% w/w of ethylene glycol & DI water (10% w/w) of the electrolyte. The cleaned strip was anodized at 40 V (DC) for 1 h in a two-electrode cell set-up; platinum was used as the cathode. The distance between the cathode and the anode was ~1.5 cm. Each anodized sample was ultrasonicated in DI water for 3-5 s. Later, annealing of the anodized TiO2 was performed at 450 o

C in presence of air flow for 2 h to achieve the desired crystalline structure.

2.1.3 Deposition of bismuth nanoparticles on T_NT. Bismuth precursor consists of a mixture of citric acid (20 mM) and bismuth citrate (20 mM) in DI water. In the mixture, ammonia solution was added drop-wise to form a clear solution. NaBH4 (0.1 M) was used as the reducing agent. Bismuth deposition scheme is shown in Scheme S1. A standard size T_NT strip was immersed in the bismuth precursor for a minute. Then, it was immersed in DI water. During the 3rd step of immersion, reduction of bismuth precursor was performed with NaBH4 containing solution. Finally, it was gently placed again in DI water to complete one cycle of bismuth deposition. Deposition was repeated for 10 cycles. Bismuth decorated T_NT (T_NT/Bi) was dried in air prior to its exposure to the S-L-S approach to grow CdSe nanowires. 2.1.4 Deposition of tin nanoparticles on T_NT. A solution of 20 mM tin(II) chloride in DI water was obtained by ultrasonicating the tin salt. Similar to bismuth nanoparticle deposition (Scheme S1), tin nanoparticles were deposited on T_NT using sodium borohydride as the reducing agent. 10 cycles of deposition were performed as part of the comparison with that of


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bismuth using the R-SILAR approach. Tin deposited T_NT i.e. T_NT/Sn was dried in air prior to the growth of CdSe nanowire using the S-L-S approach. 2.1.5 Formation of 1D/1D heterostructure. Synthesis of CdSe nanowire was performed at 280 o

C in a glass centrifuge tube (placed in a digitally controlled customized heating mantle) using

30 mL oleylamine, which works as a solvent, reducing agent, and surfactant. 1 mmol Se was dissolved in 1 mmol tri-octylphosphine designated as TOP using ultrasonication to prepare the Se precursor (TOP/Se). After the addition of 1 mmol cadmium oleate29 to oleylamine at 280 oC, 1 mmol of TOP/Se was also introduced. Immediately, a T_NT/Bi strip was immersed in the reaction medium and Bi catalyzed CdSe NWs were allowed to grow on T_NT. To address the time dependent Bi catalyzed growth of CdSe NWs on T_NT, three distinct synthesis periods (1, 4, and 10 min) were studied. Since Bi catalyzed CdSe nanowires were grown on T_NT in an organic medium, surface treatment of the T_NT/Bi/CdSe samples was carried out by ligand exchange with pyridine (18 h) and a mixture of mercaptoacetic acid & iso-propanol29 (6 h), separately. Surface-treated samples were annealed at 350 oC under N2 gas flow for 2 h to remove the residual organics from the composite surface. Sn catalyzed CdSe nanowires were grown using the T_NT/Sn sample under the same S-L-S approach as well. Since tin has a lower melting point than that of bismuth, Sn catalyzed CdSe nanowires were grown at a reduced synthesis temperature (230 oC) for 10 min. After the synthesis of nanotube-nanowire heterostructured composite, surface treatment was performed to conduct the characterization. Overall process scheme for synthesizing T_NT/Bi/CdSe and T_NT/Sn/CdSe nano-hybrid is illustrated in Scheme 2. 2.2 Optical, surface, and photoelectrochemical characterization. A Hitachi S-4700 scanning electron microscope under high resolution mode was used to obtain the SEM images. HRTEM


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analysis was performed using a JEOL 2100F instrument. Crystallographic feature of the synthesized samples were determined by the X-Ray diffraction pattern; the feature was obtained with the help of a Philips 2045 B/3 diffractometer. Scanning was conducted at a rate of 1 o/min under CuKα radiation. UV-vis. analysis was performed using a Shimadzu UV-2501PC spectrophotometer equipped with diffuse reflectance accessories. Photoelectrochemical measurements were performed with an Autolab PGSTAT 30 electrochemical analyzer. The synthesized samples were used as the anode; Pt and Ag/AgCl/3.4 M KCl were used as the counter electrode and the reference electrode, respectively, under a three-electrode cell set-up. A 0.1 M Na2S aqueous solution was chosen as the electrolyte for photoelectrochemical measurements. A 500 W Newport Xenon lamp was used as the light source with appropriate filter (0.5 M CuSO4 solution) to obtain an intensity of ~100 mW/cm2. Mott-Schottky measurements were also performed under the same electrode set-up (mentioned above) and using 0.1 M Na2S aqueous solution as the electrolyte. 3. RESULTS 3.1 Morphology analysis of T_NT and Bi nanoparticle deposited on T_NT. Figure 1(A) shows the scanning electron microscopy (SEM) image of the nanotubes prepared by anodization. The nanotubes are vertical to the titanium substrate and well-aligned; diameter of the nanotubes is close to 100±5 nm. Distinct inter-tubular spacing is evident. Nanotubes are hollow and juxtaposed next to one another. A cross-section of a representative image of TiO2 nanotubes is presented in Figure S1. Detailed characterization identifying these nanotubes as crystalline T_NT with an anatase phase has been performed previously and reported elsewhere.38-40 The deposits formed by the reductive SILAR approach on T_NT are evaluated using the microscopic techniques as well. Figure 1(B) shows the SEM image of Bi nanoparticle decorated


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T_NT. In contrast to the SEM image of plain T_NT, deposited nanoparticles are distinct, small, and present as discontinuous islands along the surface of T_NT. The size distribution of Bi NPs is between ~20 - 60 nm for 10 cycles of deposition. Although the extent of Bi NP distribution along the T_NT wall is not completely comprehensible from the SEM image, clogging of T_NT pore openings with Bi NPs is entirely absent. Figure 1(C) shows the TEM image of Bi NP decorated T_NT. The contrast in the image indicates the presence of the relatively transparent nanotubes and the dark discontinuous spots as the deposited nanoparticles. It complements the observations made in the SEM analysis. Figure 1(D) presents the HRTEM image of Bi NP decorated T_NT. The presence of fringes indicates that the material is crystalline. The correlation between the lattice spacing of the (012) plane in the HRTEM image and an XRD analysis (discussed later) confirms the presence of Bi. Therefore, the results in Figure 1 highlight that the R-SILAR method can be seen as a successful approach for obtaining Bi NP decorated T_NT. Consequently, this particular method may also be suitable for preparing such nano-islands on other non-planar oxides and intricate surfaces as well. 3.2 Comparative analysis of Bi and Sn catalyzed CdSe nanowire growth on T_NT. Although this section presents tin nanoparticle catalyzed growth of CdSe nanowire using the S-L-S method, which is first of a kind to be reported in the best of our knowledge, an analysis on the difficulties associated with the synthesis/growth of Sn catalyzed nanowires needs to be addressed in comparison with that of Bi. Figure 2(A) features the SEM image of the Bi catalyzed CdSe nanowire growth on T_NT with an inset of corresponding T_NT/Bi sample; while, Sn catalyzed CdSe nanowire on T_NT with an inset of corresponding T_NT/Sn sample is presented in Figure 2(B). From the insets (T_NT/Bi and T_NT/Sn), it is noticed that deposition of Sn leads to a very


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wide range of nanoparticle formation in contrast to Bi nanoparticle size distribution on T_NT under the aforementioned experimental conditions. Moreover, unlike Bi nanoparticles, Sn results in clogging the pore openings of T_NT. As it is mentioned in the experimental section that Sn has a lower melting point (~232 oC) than that of Bi, the goal of introducing Sn as the catalyst instead of using Bi is to achieve the desired heterostructure at a reduced synthesis temperature. When Bi deposited on T_NT is used as the catalyst, distinct CdSe nanowires are observed all over the nanotube surface as shown in the SEM image of Figure 2(A). The nanowires are identifiable, relatively straight, and well-distributed. Specifically, they from a dense “canopy” over T_NT and their length, which varies over several hundred nanometers, generally increases with reaction time (see discussion section). As shown in Figure S2, cross sectional images of T_NT/Bi/CdSe indicate an apparent reduction of the density of CdSe nanowire formation along the length of TiO2 nanotube in contrast to its top. Figure S3 corresponds to the TEM images of Bi catalyzed CdSe nanowire grown inside titanium dioxide nanotubes; elemental mapping of a section of T_NT/Bi/CdSe nano-hybrid is shown in Figure S3(B)-(F). However, the growth of Sn catalyzed CdSe nanowires T_NT indicates non-uniformity and appears to grow as patches as shown in Figure 2(B). Moreover, although Sn catalyzed CdSe nanowires can be grown on T_NT, size control of Sn nanoparticles and growth control of CdSe nanowires remain challenging under the aforementioned experimental condition. However, this analysis indicates that both Sn and Bi can be used to grow nanowires but Bi-catalyzed nanowires are more uniform and broadly distributed. Therefore, the subsequent work reported herein focuses on a systematic analysis of surface, optical, and photoelectrochemical properties of CdSe nanowires grown on T_NT using bismuth as the catalyst. Moreover, SEM and TEM images collectively provide the evidence that


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a combination of the R-SILAR and the S-L-S approach can be successively employed to fabricate a “tree-like” 1D/1D TiO2/Bi/CdSe heterostructure. 3.3 Identification of the crystallinity of the nano-hydrid. X-ray diffraction (XRD) patterns of T_NT, T_NT/Bi, and T_NT/Bi/CdSe are shown in Figure 3. Anatase phase of TiO2 and elemental titanium are indicated in the Figure 2(I) as A and T, respectively. While TiO2 is indexed to the JCPDS card (PDF # 21-1272), titanium foil is indexed to the JCPDS card (PDF # 44-1294). It is noticed that the intense peaks of elemental Ti and TiO2 suppress / overlap the peaks of Bi and CdSe in the XRD patterns. The XRD pattern in Figure 3(III) shows the presence of (100), (101), (110), and (112) planes in the heterostructured sample indicating that the nanowire is crystalline. The nanowires are subsequently identified as the hexagonal CdSe (JCPDS No # 08-0459). Therefore, the outcomes of the XRD patterns compliment the results found from SEM and (HR)TEM analyses. 3.4 Optical properties of T_NT/Bi/CdSe nano-hybrid. The absorption properties of T_NT and T_NT/Bi are shown in Figure S4(A) and Figure S4(B), respectively. Bare T_NT shows its characteristic absorption onset at ~386 nm. However, incorporation of Bi NPs with T_NT hardly changes the absorption feature in comparison with the plain T_NT. It is worthy to mention that the appearance of the broad absorbance in the visible region of the spectrum profile may be attributed to the presence of the Ti3+ state that results during the heat treatment.41 Figure S4(C) presents the absorption spectra of CdSe NW incorporated T_NT, where Bi catalyzed CdSe NWs were grown on T_NT for 1, 4, and 10 min under the S-L-S approach. Further, the representative photographs of T_NT, T_NT/Bi, and the T_NT/Bi/CdSe are also shown in Figure S4(D) I-III. It is observed that the presence of CdSe NWs results in a red shifted absorption onset and an enhancement of absorption in the visible light. Moreover, as the time of growth of CdSe NWs is


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increased, a consequential increase in absorption is noticed in Figure S4(C). S-L-S process assisted CdSe NW grown on T_NT for 10 min shows an absorption onset at ~764 nm. In a related work, the presence of 0D CdSe nanoparticles on TiO2 nanotubes has also demonstrated a red shift in absorbance.12 Thus, the observations noted in this work are consistent with the reports in the literature and are attributed to the growth of CdSe NWs on T_NT. The translation of this absorbed energy will indicate the viability of this1D/1D heterostructure as a photoelectrode. 4. DISCUSSIONS 4.1 Photoelectrochemical responses of the 1D/1D architecture. The earlier section presents the details regarding the assembly of 1D CdSe nanowire decorated 1D TiO2 nanotube, the systematic examination of its surface features, and the optical characterization. As part of the determination of its impact in any application, the film is evaluated as a photoelectrode in an electrochemical cell and its responses are examined. Photoelectrochemical analyses of the 1D/1D T_NT/Bi/CdSe heterostructured film offer insights into the charge generation & transport, the nature of contact between CdSe NW and T_NT as well as the stability of the film under UV-vis. photoillumination.42 The experimental details of PEC measurement is found in section 2.2. j-V measurements offer insights into the response of the film under an external electric bias.43 The j-V plots are obtained using a 3-electrode cell set-up containing 0.1 M Na2S as the electrolyte. A comparison of the j-V characteristics of T_NT and T_NT/Bi/CdSe nano-hybrid is presented in Figure 4(A). In response to photo-illumination, the plain T_NT photoelectrode yields ~0.12 mA/cm2 at 0 V. However, the formation and the growth of CdSe NWs on T_NT for 10 min under the S-L-S approach result in not only a robust increase in current density (3.84 mA/cm2 at 0 V) but also a significant negative shift in the onset potential (-0.84 V vs. Ag/AgCl)


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compared to that of the plain T_NT (-0.77 V vs. Ag/AgCl). 0D oxide-0D chalcogenide films also demonstrated a similar negative shift in the onset potential as evident from previous studies.15,44 This outcome can be attributed to the strategic location of the conduction band edges of CdSe and TiO2 with respect to one another favoring photogenerated charge transport.45 To understand the rapidity of the photogenerated charge transport and the reproducibility of the photoelectrode performance under chopped illumination, j-t plots were generated. The results are presented in Figure 4(B). Upon illumination, T_NT shows an instantaneous photocurrent of 0.1 mA/cm2, while the T_NT/Bi/CdSe photoanode demonstrates a photocurrent of 3.4 mA/cm2. The multiple on-off cycles reveal the photoactivity and reproducibility of the electrodes. This trend in the photocurrent density is consistent with the observations in the j-V results. Incident photon to current conversion efficiency (IPCE) offers insights into the realizing the effectiveness of the electrode that promotes charge separation. The results are presented in Figure 5(A) and obtained using the equation stated below:46,47 ‫(ܧܥܲܫ‬%) =

ଵଶସ଴×௝ ௉(ఒ)×ఒ

(1)

where, j = photocurrent density (A/cm2) at 0 V vs. Ag/AgCl at a certain wavelength (ߣ), P(ߣ) = incident power density at wavelength λ (W/cm2), and λ = wavelength (nm). The IPCE (%) of the 1D-1D T_NT/Bi/CdSe photoanode is estimated to be 12%. In addition, it is to be noted that the IPCE response coincides with the absorption onset, which indicates electron-hole pair generation as a result of the incident light absorption by the photoanode. Mott-Schottky (MS) analysis is beneficial to understand the n-/p-characteristics of the electrode. It is estimated according to the following expression: 25,48 ଵ ஼మ

=

ଶ(௏ି௏೑್ ି

ೖ೅ ) ೐

ఌఢబ ௘ேವ

(2)


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Where, C = capacitance, V = applied bias, Vfb = flat band potential, k = Boltzmann constant, T = absolute temperature, e = charge of an electron, ε = dielectric constant, εo = permittivity in vacuum, area ≈ 1 cm2, and ND = donor density. The MS plots for T_NT and T_NT/Bi/CdSe are shown in Figure 5(B); they indicate the formation of an n-n heterojunction, established by the positive slope of the linear portion of the Mott-Schottky plots.41 Further, order of the donor density, measured from the slope of the plots, follows a trend: ND_T_NT/Bi/CdSe_ photoillumination>ND_T_NT/Bi/CdSe_ dark condition>

ND_T_NT_

photoillumination>ND_T_NT_ dark condition.

The results

imply: i) an incorporation CdSe NW with T_NT results in greater charge generation than that of plain T_NT and ii) UV-vis. photo illumination boosts the charge generation process further due to the contribution from the photo-excited CdSe nanowires. For clarity, MS plots of T_NT and T_NT/Bi/CdSe electrodes are shown separately in Figure S5. The stability of the T_NT/Bi/CdSe photoelectrode was determined by exposing it to continuous photoillumination at a constant bias of 0 V (vs. Ag/AgCl) over an extended period of time as shown in Figure S6. The photocurrent remains stable at ~3.54 mA/cm2 over 400 s. This stable PEC response by the electrode is contributed to i) broad-band absorption of incident light by the directly attached single crystalline CdSe NWs with T_NT resulting in an efficient charge separation process and 1D/1D photogenerated charge transport,49 ii) the negative shift in the open circuit potential,50 and iii) the integrity of the 1D/1D heterostructure hybrid. The mechanism of charge generation and separation in response to photoillumination by the T_NT/Bi/CdSe electrode can be summarized by the following equations.51,52

CdSe

Vis Light  →

CdSe ( e + h )

TiO 2 + CdSe ( e + h ) → TiO 2 ( e ) + CdSe ( h )

(3) (4)


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Therefore, the PEC measurements along with the IPCE(%) and the MS analyses provide a comprehensive picture of the photoelectrochemical performance of the novel 1D/1D T_NT/Bi/CdSe heterostructure. It indicates that the S-L-S growth of CdSe NW on a wide bandgap semiconductor has resulted in i) as high as ~30-fold increase in the photocurrent density compared to that of the plain T_NT photoelectrode, ii) the formation of an effective n-n junction between the 1D T_NT and 1D CdSe, and iii) a stable photocurrent over an extended period of operation. As a result, the full potential of such 1D/1D photoelectrode can be realized by an extensive and systematic study. 4.2 Time dependent Bi catalyzed growth of 1D CdSe on T_NT. The mechanism Bi catalyzed growth of CdSe nanowire is examined by changing the reaction time. The collage of SEM images shown in Figure 6 indicates the evolution and the Bi catalyzed growth of CdSe NWs. The images clearly show the transformation of CdSe from a nanorod to a nanowire. The S-L-S growth of CdSe NWs over 1 min, 4 min, and 10 min indicates that as the duration of reaction time is increased, a significant NW coverage over T_NT may be achieved. Therefore, growth period of NWs is as an important variable to control its length. A generalized summary of the combination of the R-SILAR and the S-L-S approach for the fabrication of similar 1D/1D heterostructure is presented in the scheme of Figure 6(D). NWs synthesized from Cd & Se precursors are generally leaner.53 In response to the time dependent Bi catalyzed growth of CdSe on T_NT as shown in Figure 6, growth mechanism of nanowires10,54-571-5 under the S-L-S process is illustrated in Scheme 3. Bi nanoparticle acts as the seed with its low melting point (271.4 oC) and its surface works as a heterogeneous nucleation site for the semiconductor.10,57 Cd and Se precursors form the CdSe in-situ normally in an organic solution, which is dissolved in molten Bi seed. However, the limited solubility of CdSe


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results in supersaturation.10 When the solubility of CdSe in Bi crosses the thermodynamic limiting value, its solid crystalline phase separates from Bi seed at the liquid droplet-solid-crystal interface. Bi catalyzed process continues as long as the dissolution of CdSe sourced from the decomposition of metallo-organic precursors is maintained; the phase separation facilitates the growth of length of CdSe nanowire, of which diameter is mostly controlled by the size of Bi seed. However, instability of temperature in the reaction medium may result in the kinking of nanowires.54 The growth regime maintains the coexistence of solution (precursors), liquid (Bi seed) and solid (intermittent Bi/CdSe alloyed composition). Therefore, dissolution and phase separation in and from Bi seed are the driving force for the growth of nanowire. In the Bi/CdSe alloy, it is reported that the growth of NW can occur from regions of higher localized CdSe concentration via the interaction with either CdSe or Cd/Se precursors. The presence of Bi at one end indicates that the S-L-S mechanism is the basis of the NW growth.58 Radial and longitudinal growth of CdSe NW is expected to simultaneously occur with time via the S-L-S mechanism as shown in Scheme 3. However, it is noticed that Bi nanoparticles deposited on T_NT have gradually moved to the end of the NW (NW sandwiched between Bi and T_NT); it indicates a modification of the S-L-S approach. 4.3 Broader impact of the 1D/1D architecture and future directions. The NW is the light absorber and T_NT is not only a chemically robust matrix but also remains unaffected by most chemical treatments. Therefore, once the T_NT/Bi/CdSe nanostructured hybrid is assembled, several structural and chemical modifications can be made specific to the NWs to synthesize interesting heterostructures having better light harvesting abilities and carrier generation with possible use in solar energy conversion as illustrated in Scheme S2. For example, cation exchange technique25 can be used to create p-n junction NW (PbSe QD on CdSe NW) for broad


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absorption and efficient charge transport. Alternately, decoration of noble metal nanoparticles (e.g. Au nanoparticles on CdSe NW) on nanowire can contribute to dual benefit of carrier lifetime enhancement and energy transfer to the nanowire owing to unique properties of Au NPs namely electron storage sink and plasmonic absorption. Wu’s group demonstrated that Au nanoparticles sandwiched between TiO2 nanorods and CdS quantum dots increase the chargetransfer lifetime, reduce the trap-state Auger rate, suppress the long-time scale back transfer, and partially compensate the negative effects of the surface trap states.47 Similar benefits can be achieved by coating Au nanoparticles on CdSe nanowires, which, in turn, can enhance photoelectrochemical properties with added functionalities. Finally, cation such as Mn2+ doping can be used for broadband light absorption as well as enhancing lifetime of photogenerated carriers by introducing swallow trap levels below CdSe conduction band.59,60 These approaches may lead to the realization of the “rainbow solar cell”.61 5. CONCLUSION In conclusion, this work highlights: a proof-of-concept i) to form discontinuous bismuth islands on an intricate non-planar surface of TiO2 nanotubes by a reductive SILAR approach with no use of surface adhesives (such as PVA, PVP, etc.), ii) to leverage the solution-liquid-solid approach to assist with the growth of a representative 1D visible light absorber (CdSe nanowire) on the T_NT surface, which establishes a direct contact between the semiconductors, and iii) to fabricate a 1D/1D heterostructure similar to TiO2/Bi/CdSe “tree-like” nano-hybrid. The physical and optical characterization of the heterostructured architecture has been systematically perfromed. Bi NP (~20-60nm) catalyzed growth of CdSe nanowire on T_NT has conclusively been proven using SEM, TEM, elemental mapping, and XRD. The TiO2/Bi/CdSe heterostructure has been used as a photoanode in a photoelectrochemical cell and investigated. The formation of


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the n-n junction in the nano-hybrid yields a stable and promising photocurrent density of 3.84 mA/cm2 under ~AM 1.5 photoillumination. Finally, leveraging the variety of techniques reported in literature for modification and tuning of nanowires, this novel synthesis protocol should set the stage for developing unique 1D/1D oxide-chalcogenide heterostructures that can be used for various applications in photovoltaics, light sensing, and photocatalysis.


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FIGURES

Figure 1. A collage containing electron microscopy images and evidence determining substrate components. It includes (A) SEM image of T_NT, (B) SEM image of T_NT with Bi nanoparticles, (C) TEM image of T_NT/Bi, and (D) determination of the deposits as Bi using HRTEM analysis.


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Figure 2. SEM images of (A) T_NT/Bi/CdSe with an inset of T_NT/Bi [growth of CdSe NW for 10 min] and (B) T_NT/Sn/CdSe with an inset of T_NT/Sn [growth of CdSe NW for 10 min].

Figure 3. XRD patterns of (I) T_NT (II) T_NT/Bi, and (III) T_NT/Bi/CdSe [CdSe NW growth for 10 min]


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Figure 4. (A) j-V profiles of (I) T_NT & (II) T_NT/Bi/CdSe and (B) chronoamperometric profiles (j-t) of (I) T_NT & (II) T_NT/Bi/CdSe are shown, where growth of CdSe NWs is maintained for 10 min under the S-L-S approach.

Figure 5. (A) IPCE profiles of (I) T_NT & (II) T_NT/Bi/CdSe and (B) Mott-Schottky (MS) plots of (I) plain T_NT under dark condition, (II) plain T_NT under UV-vis. photoillumination, (III) T_NT/Bi/CdSe under dark condition, and (IV) T_NT/Bi/CdSe under UV-vis. photoillumination. Growth of CdSe NWs is maintained for 10 min under the S-L-S approach. Note: 1G = 109.


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Figure 6. Time dependent Bi nanoparticle catalyzed growth of CdSe NWs on T_NT under the SL-S approach: SEM image of T_NT/Bi/CdSe heterostructure (A) after 1 min of CdSe NW growth with an inset, (B) after 4 min of CdSe NW growth with an inset, and (C) after 10 min of CdSe NW growth. General concept to fabricate the 1D/1D architecture for efficient solar energy harvesting using the combination of R-SILAR and S-L-S approach is illustrated in (D).


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SCHEMES

Scheme 1. The reduced particle boundaries in oxide-chalcogenide heterostructures of 1D/1D configuration in comparison with 0D/0D nanocomposite, can lead to a more effective charge transport.


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Scheme 2. Step-by-step approach to synthesize the assembly of Bi or Sn catalyzed CdSe nanowire on T_NTs.


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Scheme 3. Bi catalyzed growth mechanism of CdSe nanowire on T_NT.


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ASSOCIATED CONTENT Supporting Information. Scheme of Bi deposition on TiO2 nanotube, cross-sectional images of T_NT and T_NT/Bi/CdSe, TEM image of TiO2/Bi/CdSe & its elemental mapping, UV-vis. spectra of T_NT, T_NT/Bi, and T_NT/Bi/CdSe, where CdSe was grown for 1, 4, and 10 min, a photograph of T_NT, T_NT/Bi, T_NT/Bi/CdSe sample, MS plots, stability plot, and a scheme on broader outlook. This material is available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *

Department of Chemical and Materials Engineering, University of Nevada, Reno, NV 89557.

Phone: 775-784-4686. Fax: 775-327-5059. E-mail: [email protected]. Present Addresses §

Department of Metallurgical Engineering, Indian Institute of Technology (Banaras Hindu

University), Varanasi 221005, India. †

Dept. of Chemical & Materials Engineering, New Mexico State University, 3055 Williams

Ave., Las Cruces, NM 88003, USA. Author Contributions b

Equal contributions to this work.

Funding Sources This work was supported by NSF. Grant Number: CBET 1337050. Notes


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The authors declare no competing financial interest. ACKNOWLEDGMENT RSV would like to thank Dr. M. Ahmedian for assisting with the (HR)TEM images. Dr. Satyajit Gupta is appreciated for the helpful discussions. References. 1. Xiao, F. X.; Miao, J.; Wang, H. Y.; Yang, H.; Chen, J.; Liu, B. Electrochemical Construction of Hierarchically Ordered CdSe-Sensitized TiO2 Nanotube Arrays: Towards Versatile Photoelectrochemical Water Splitting and Photoredox Application. Nanoscale 2014, 6, 6727-6737. 2. Oh, S. J.; Uswachoke, C.; Zhao, T.; Choi, J. H.; Diroll, B. T.; Murray, C. B.; Kagan, C. R. Selective p- and n-Doping of Colloidal PbSe Nanowires To Construct Electronic and Optoelectronic Devices. ACS Nano 2015, 9, 7536–7544. 3. Jin, W.; Zhang, K.; Gao, Z.; Li, Y.; Yao, L.; Wang, Y.; Dai, L. CdSe Nanowire-Based Flexible Devices: Schottky Diodes, Metal–Semiconductor Field-Effect Transistors, and Inverters. ACS Appl. Mater. Interfaces 2015, 7, 13131−13136. 4. Littig, A.; Lehmann, H.; Klinke, C.; Kipp, T.; Mews, A. Solution-Grown Nanowire Devices for Sensitive and Fast Photodetection. ACS Appl. Mater. Interfaces 2015, 7, 12184−12192. 5. Schoen, D. T.; Peng, H.; Cui, Y. CuInSe2 Nanowires from Facile Chemical Transformation of In2Se3 and Their Integration in Single-Nanowire Devices. ACS Nano 2013, 7, 3205–3211.


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TABLE OF CONTENTS.


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Engineered Solution-Liquid-Solid Growth of a “Tree-like” 1D/1D TiO2 Nanotube-CdSe Nanowire Heterostructure: Photoelectrochemical Conversion of Broad Spectrum of Solar Energy Bratindranath Mukherjee,a,b,§ Swagotom Sarker,a,b,† Eric Crone,a Pawan Pathak,a and Vaidyanathan R. Subramaniana,*


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FIGURES

Figure 1. A collage containing electron microscopy images and evidence determining substrate components. It includes (A) SEM image of T_NT, (B) SEM image of T_NT with Bi nanoparticles, (C) TEM image of T_NT/Bi, and (D) determination of the deposits as Bi using HRTEM analysis.


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Figure 2. SEM images of (A) T_NT/Bi/CdSe with an inset of T_NT/Bi [growth of CdSe NW for 10 min] and (B) T_NT/Sn/CdSe with an inset of T_NT/Sn [growth of CdSe NW for 10 min].

Figure 3. XRD patterns of (I) T_NT (II) T_NT/Bi, and (III) T_NT/Bi/CdSe [CdSe NW growth for 10 min]


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Figure 4. (A) j-V profiles of (I) T_NT & (II) T_NT/Bi/CdSe and (B) chronoamperometric profiles (j-t) of (I) T_NT & (II) T_NT/Bi/CdSe are shown, where growth of CdSe NWs is maintained for 10 min under the S-L-S approach.

Figure 5. (A) IPCE profiles of (I) T_NT & (II) T_NT/Bi/CdSe and (B) Mott-Schottky (MS) plots of (I) plain T_NT under dark condition, (II) plain T_NT under UV-vis. photoillumination, (III) T_NT/Bi/CdSe under dark condition, and (IV) T_NT/Bi/CdSe under UV-vis. photoillumination. Growth of CdSe NWs is maintained for 10 min under the S-L-S approach. Note: 1G = 109.


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Figure 6. Time dependent Bi nanoparticle catalyzed growth of CdSe NWs on T_NT under the SL-S approach: SEM image of T_NT/Bi/CdSe heterostructure (A) after 1 min of CdSe NW growth with an inset, (B) after 4 min of CdSe NW growth with an inset, and (C) after 10 min of CdSe NW growth. General concept to fabricate the 1D/1D architecture for efficient solar energy harvesting using the combination of R-SILAR and S-L-S approach is illustrated in (D).


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SCHEMES

Scheme 1. The reduced particle boundaries in oxide-chalcogenide heterostructures of 1D/1D configuration in comparison with 0D/0D nanocomposite, can lead to a more effective charge transport.


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Scheme 2. Step-by-step approach to synthesize the assembly of Bi or Sn catalyzed CdSe nanowire on T_NTs.


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Scheme 3. Bi catalyzed growth mechanism of CdSe nanowire on T_NT.


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1D TiO2

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