Effects of Carbon Allotrope Interface on the Photoactivity of Rutile One

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Effects of Carbon Allotrope Interface on the Photoactivity of Rutile One-Dimensional (1D) TiO2 Coated with Anatase TiO2 and Sensitized with CdS Nanocrystals Pawan Pathak,† Luis Henrique Israel,† Ellen Jessica Monterio Pereira,† and Vaidyanathan Ravi Subramanian*,† †

Chemical and Materials Engineering, University of Nevada, Reno, Nevada 89557, United States S Supporting Information *

ABSTRACT: The assembly of a large-bandgap one-dimensional (1D) oxide− conductive carbon−chalcogenide nanocomposite and its surface, optical, and photoelectrochemical properties are presented. Microscopy, surface analysis, and optical spectroscopy results are reported to provide insights into the assembly of the nanostructure. We have investigated (i) how the various carbon allotropes (C60), reduced graphene oxide (RGO), carbon nanotubes (CNTs), and graphene quantum dots (GQDs) can be integrated at the interface of the 1D TiO2 and zero-dimensional (0D) CdS nanocrystals; (ii) the carbon allotrope and CdS loading effects; (iii) the impact of the carbon allotrope presence on 0D CdS nanocrystals; and (iv) how they promote light absorbance. Subsequently, the functioning of the integrated nanostructured assembly in a photoelectrochemical cell has been systematically investigated. These studies include (i) chronoamperometry, (ii) impedance measurements or EIS, and (iii) linear sweep voltammetry. The results indicate that the presence of a GQD interface shows the most enhancement in the photoelectrochemical properties. The optimized photocurrent values were respectively noted to be 2.8, 2.2, 1.9, and 1.6 mA/cm2, indicating JGQD > JRGO > JCNT > Jfullerene. Furthermore, the annealing conditions have indicated that ammonia treatment leads to an increase in the photoelectrochemical responses when using any form of the carbon allotropes. KEYWORDS: TiO2 nanorod, carbon allotropes, chalcogenide, photoelectrochemistry, solar energy conversion tions (PV and solar fuel production).24 The chalcogenides can facilitate light absorbance across the visible and infrared ranges, thus covering a significant range of the low energy bandwidth in the solar spectrum, improving the overall efficiency of the nanocomposite.25 The study of chalcogenides with oxides has been well-documented, establishing this area as one of continued interest.22,26−29 Of relevance to the aforementioned materials, carbon is a universal, inexpensive, and unique additive that can aid in charge separation and transport. Its properties have been leveraged in light-driven processes such as solar energy conversion, as well as other energy-related applications such as in fuel cells. Allotropic forms of carbon that have generally been examined are fullerene (C60 and its analogues),30,31 carbon nanotubes (single-walled/ multiwalled),32,33 and graphenes (including reduced graphene oxides (RGOs)).34−38 From a solar energy conversion standpoint, all of these carbon allotropes have demonstrated improvement in photoelectrochemical responses.39−41 In the context of this discussion, they have been tested with oxides and

1. INTRODUCTION Light-driven applications such as photocatalytic clean fuel production,1 electricity generation,2 and environmental remediation3,4 involve utilizing photoactive oxides as a major component. Oxides as the material of choice arise from the fact that they demonstrate multifunctionality, can be assembled as particulate films on substrates using a variety of cost-effective wet chemical and chamber-based processes, and demonstrate wide ranging pH stability.5−7 The assembly of one-dimensional (1D) oxide as films, as opposed to zero-dimensional (0D) oxide films, allows for a reduction in grain boundaries promoting epitaxial charge transport.3,8 This benefit has been one basis for the study of 1D oxides with forms such as nanotubes, nanorods, and nanowires.9−12 The application of these oxides, in combination with other photoactive materials, such colored dyes (in DSSC), QDs (QDSCs), and metals (photocatalysis), has also been successfully demonstrated.13−16 Since efficient photodriven processes require leveraging visible light, the use of chalcogenides as visible-light-harvesting agents are of immense interest.17−20 They display several unique properties, such as (i) size-dependent optical absorbance (driven mainly by quantization effects),21,22 (ii) improved light-toelectrical energy conversion (boosted by multiple exciton generation),23 and (iii) compatibility to wide ranging applica© 2016 American Chemical Society

Received: February 13, 2016 Accepted: April 28, 2016 Published: April 28, 2016 13400

DOI: 10.1021/acsami.6b01854 ACS Appl. Mater. Interfaces 2016, 8, 13400−13409

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ACS Applied Materials & Interfaces

high-resolution transmission electron microscopy (HRTEM) system, and a JEOL 100 system was used to examine the size distribution of the particles. X-ray diffraction (XRD) measurements were performed using a Philips Model XRG 3100 X-ray diffractometer, operated at 35 kV to identify the phase of the material after thermal treatment of the samples. 2.2. Surface Passivation of Rutile TiO2 Nanorods with an Anatase Layer. The approach used for the synthesis of the rutile TiO2 nanorods (r_TiO2) is described elsewhere.44,45 An anatase TiO2 layer was deposited over the TNR using the following procedure. HCl (2 M) was mixed with titanium(IV) butoxide to prepare a precursor solution. A quantity of 0.5 mL of the precursor solution was added to 100 mL of a 0.025 M HCl solution. Two fluoride-doped tin oxide (FTO) slides with TiO2 nanorods were placed at an angle against the wall of the beaker, supporting each other at the base with the conductive side facing up. The beaker was kept inside an oven at a temperature of 90 °C for 1 h. After cooling to room temperature, the sample was washed in DI water and dried in air. The samples were subsequently annealed at 350 °C for 1 h in air. 2.3. Coating of Allotropic Forms of Different Carbon. Single-walled carbon nanotubes (SWCNTs) were dispersed in the dimethylformamide solution at a concentration of 1 mg/mL. The fullerene was dispersed in the toluene solution at a concentration of 1 mg/mL, and RGO was dispersed in water at a concentration of 1 mg/mL. Meanwhile, graphene quantum dots (GQDs) were obtained at a concentration of 0.6 mg/mL after synthesis (synthesis details are given in the Supporting Information). The carbon materials dispersed in solution were ultrasonicated for 1 h for thorough mixing of the sample and then coated on the 1D oxide with a brush. 2.4. Deposition of Chalcogenide. The SILAR approach was used for the deposition of the CdS on the 1D oxide−carbon. The FTO substrates deposited with 1D oxide and coated with carbon were immersed in a 0.5 M Cd(NO3)2 solution in a CH3OH/water mixture (1:1) for 2 min, where the Cd2+ ions adhere on the substrate surface. This was followed by washing with DI water for 2 min, to prevent homogeneous precipitation. The samples were then immersed in 0.5 M Na2S solution for 2 min, where the precursor ions react with S2−. This was followed by cleaning the sample in DI water for 2 min. The steps were repeated to grow subsequent layers via multiple cycles. After the SILAR deposition, the samples were annealed at 350 °C for 3 h either in a nitrogen or ammonia atmosphere. 2.5. Photoelectrochemical (PEC) Characterization. All the PEC measurements were performed using a potentiostat (Autolab, Model PGSTAT 302).46 A Newport 500 W xenon lamp was used as the light source with a 0.5 M CuSO4 solution as a far UV-cutoff filter to reduce the light intensity to ∼90 mW/ cm2 (equivalent to AM 1.5 illumination).47 The three electrode experiments were performed in a quartz cuvette, which included the prepared 1D oxide/carbon/chalcogenide photoanode, a Pt mesh cathode, and a leak-free Ag/AgCl reference electrode. A 0.1 M solution of Na2S was used as the electrolyte.

chalcogenides as well; although the focus has primarily been toward its integration with 0D oxides. Recently, we have shown that 1D oxide−chalcogenide materials can be coupled using RGOs to assemble a unique heterostructure that demonstrates multifunctionality.42,43 As indicated in Scheme 1, the RGO, in the form of a screen, offers a Scheme 1. Addition of a Carbon Allotrope Interface Such As Reduced Graphene Oxide (RGO) between a 1D Oxide and a Chalcogenide Can Provide an Array of Benefits [Adapted from ref 43. Copyright 2014, Wiley, Weinheim, Germany.]

host of benefits in the domain of charge separation and transport. We have shown a proof-of-concept of such a heterostructure assembly, using the popular ZnO in the 1D form. The architecture is photostable and demonstrates photovoltaic, photoelectrochemical, and photocatalytic properties. However, the role of the oxide phases (anatase and rutile) on the effects of charge transport in 1D is yet to be fully understood. In addition, some questions do arise, such as (1) Can all of the carbon allotropes be integrated to form a stable heterostructure using 1D oxide and chalcogenide? (2) And, if so, which one of these allotropic forms is the most suitable for promoting charge transport for this architecture? This work focuses on answering these questions using CdS as the representative chalcogenide prepared using the successive ionic layer adsorption and reaction (SILAR) approach.

2. MATERIALS AND METHODS 2.1. Chemicals. Titanium(IV) butoxide and cadmium nitrate [Cd(NO3)2] were obtained from Sigma−Aldrich. Sodium sulfide nanohydrate (Na2S·9H2O), fullerene (C60), and carbon nanotube (CNT) were obtained from Alfa Aesar. Methanol (CH3OH) and concentrated hydrochloric acid (HCl) were obtained from local suppliers and were used without further purification. Graphite flakes (SP-1) were purchased from Bay Carbon, Inc. Deionized (DI) water was obtained from an inhouse laboratory water purification system (Millipore, Bedford, MA). Optical characterization of the materials was performed using a Shimadzu Model UV-2501PC spectrophotometer in the wavelength range of 300−800 nm. The morphologies and microstructures of the samples were probed using a Hitachi fieldemission scanning electron microscopy (FESEM) system that was equipped with an energy-dispersive spectroscopy (EDS) analyzer (Oxford Instruments). The scanning electron microscopy (SEM) samples were coated with platinum prior to the examination. Transmission electron microscopy (TEM) images of the samples were taken at 100 kV, using a JEOL Model 2100F

3. RESULTS 3.1. Surface and Optical Characterization. 3.1.1. Microscopy, Surface, and Optical Analyses of Oxide Layer. The formation of the oxide layer following the seed mediated approach and the effects of the subsequent surface treatment were examined using microscopy and XRD analysis. The exposure of the seeded substrate to an acidified solution of titanium(IV) precursor under hydrothermal treatment leads to 13401


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Figure 1. (A) SEM image of the nanorods grown using the seed mediated approach, (B) SEM image of the nanorods after surface treatment using the titanium butoxide precursor, (C) XRD over a 2θ range of 20°−80° of (A) indicating rutile TiO2, and (D) absorbance spectra before and after surface treatment. (See Figure S1 in the Supporting Information for anatase TiO2.)

3.1.2. Characterization of the Carbon Allotrope Deposits on the TiO2 Nanorods. The various carbon allotropes were deposited over the TiO2 nanorods using the approach discussed in section 2.3 and imaged using microscopy. The HRTEM image and the corresponding EDS analysis of the 1D oxide−various conductive carbon−chalcogenide nanocomposite is shown in Figure S2 in the Supporting Information. The HRTEM images indicate the general assembly of the a_r_TiO2/C/CdS nanocomposite while the EDS analysis shows that the participating elements are detected once the assembly of the nanocomposite is completed. The SEM analysis was also performed on all of the samples. An important aspect to note early on is the unique physical forms these carbon materials have and are likely to evolve into when deposited on the nanorods. Their distribution and density can be influenced by the stearic hindrances posed by their natural physical dimensions. Since circumventing or controlling them is not an option when we are studying their relative effects, it is expected that these hindrances will play a definitive role in influencing the photoelectrochemical performance of the heterostructure and the mass transport of the electrolyte. Considering this aspect, one approach to perform a comparative evaluation of their contributions that was followed, is to individually optimize the loading of each of the carbon forms on the 1D TiO2 nanorods prior to the deposition of the chalcogenide. Since SWNT and fullerenes were commercially obtained and used as is, their individual properties were not independently characterized. The characterization of the in-house synthesized RGO has been systematically performed using FTIR and Raman spectroscopy and reported elsewhere.43 The procedure followed for GQD synthesis was obtained from published literature, and the Supporting Information discusses the confirmation of the synthesized product as GQDs. Briefly, the ultraviolet−visible light (UV-vis) absorbance of the in-house synthesized product is shown in Figure S3(A) in the Supporting Information is consistent with the results reported elsewhere,48−50 indicating the formation of GQDs. Furthermore, GQDs show luminescent properties, as indicated in the photograph shown in Figure S3(B)

the evolution and growth of the 1D structure, as indicated in Figure 1A. The directional growth and epitaxial arrangement of several physically similar nanorods with distinctly smooth side walls, inter-rod spacing, and square cross section, are evident from the SEM image. The physical dimensions of the rods are noted to be 50 ± 5 nm laterally and ∼4 μm in length. Once these nanorods are subjected to the titanium precursor treatment, the formation of surface deposits with evidence of an increase in surface roughness, especially along the side walls, is noted as indicated in Figure 1B. However, there is no distinguishable change in the overall physical dimensions of the nanorod, which suggests that the treatment leads to a very thin layer of coverage. It is also evident that the surface passivation step does not significantly affect the spacing between the rods. XRD analysis of the 1D nanorods was performed to determine the crystallinity and phase before and after surface treatment. The presence of distinct peaks suggests that both seed-grown and surface-passivated nanorods become polycrystalline after thermal treatment. The sample peaks were indexed to the ⟨101⟩, ⟨002⟩, and ⟨112⟩ planes, using the standard JCPDS database, as shown in Figure 1C, and that indicates that the 1D nanorods belong to the rutile TiO2 phase (Joint Committee on Powder Difrraction Standards (JCPDS) File No. 88-1175). The samples formed after the surface passivation step did not show any significant change in the XRD patterns. This could be attributed to the deposition of a very thin layer of precursor (consistent with the SEM observations). Therefore, to identify this phase a thicker layer was formed from the same precursor solution using multiple deposition steps. The XRD analysis of these deposits indicated the formation of a polycrystalline film identified as TiO2 of the anatase phase, as shown in Figure S1 in the Supporting Information (JCPDS File No. 84-1826). The corresponding absorbance spectra of the rutile rod versus the anatase-coated rutile rod (labeled henceforth as r_TiO2 and a_r_TiO2, respectively) are shown in Figure 1D. The absorbance of the two oxide nanorods are almost the same, with an overlapping onset at ∼413 nm. This observation indicates that the surface passivation layer does not distinguishably alter the optical properties of the TiO2 nanorod. 13402


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consistent with CdS deposition. After the completion of 15 SILAR cycles, the absorbance appears to remain constant, indicating that a maximum in CdS loading has been reached. Any further increase in the number of the SILAR cycles actually led to a decrease in the CdS loading on the carbon-free a_r_TiO2 (as indicated in Figure 2 with 20 SILAR cycles). This observation can be attributed to the lack of adsorption of the ions to form the deposits. A similar trend in the general chalcogenide absorbance profile on both 0D and 1D oxides has been documented elsewhere.44,51 3.1.4. Optimization of the Carbon Allotrope Deposits and CdS on the 1D a_r_TiO2. To optimize the carbon allotrope content, various coatings of the carbon were deposited first on the a_r_TiO2. The CdS then was prepared on the a_r_TiO2/C composite. At this point, the number of SILAR cycles was kept at a constant of 15 cycles, based on the results from section 3.1.3. The absorbance spectra of the a_r_TiO2/C/CdS samples thus prepared with only a variation in the carbon coating is presented in Figure 3. The baseline C-free samples [a_TiO2/CdS] shows a smaller absorbance compared to samples with the carbon attributable to greater CdS deposition in the presence of carbon. For RGO, fullerene, and CNT, two coatings with concentration of 1 mg/mL resulted in an optimal absorbance (note RGO optimization has been reported elsewhere43). Whereas, for GQDs, the optimal absorbance was obtained at 10 coatings with a GQD solution having a concentration of 0.6 mg/mL. Any further increase in the number of carbon coating decreased the absorbance. This is attributed to a decrease in the CdS deposition. There are a few noteworthy takeaways from the general profile of the absorbance spectra of the nanocomposite. First, the onset absorbance indicates that the CdS nanocrystals are successfully deposited on the 1D TiO2/C. The presence of any form of carbon allotrope does not impede the deposition of the CdS on the a_r_TiO2. In fact, in some cases, it may be providing additional sites for the CdS deposition and distribution. Second, upon close examination of Figures 2 and 3, it may be noted that, in the presence of any forms of carbon, there is an increase in visible absorbance. For example, the absorbance at 450 nm is 0.62 and 0.75 with a_r_TiO2/CdS and a_r_TiO2/C/CdS, respectively. This aspect is likely to be beneficial since an enhancement in absorbance is the first step toward a better solar-to-electric conversion, as well as boosting photocatalytic activity. 3.1.5. Microscopy and Surface Analyses of the Overlying Carbon Allotropes. The SEM images after the deposition of the different carbon allotropes on the a_r_TiO2 nanorods are shown in Figure 4. The image in Figure 4A shows that the RGO is

in the Supporting Information. To confirm the luminescent properties, a photoluminescence analysis was performed (λex = 340 nm) and shown in Figure S3(C) in the Supporting Information. This observation is also consistent with the published literature48 on GQD synthesis, confirming that the material produced in this work is GQD. 3.1.3. Optical Analyses of Chalcogenide Formation on Carbon-Free a_r_TiO2 Nanorods. The SILAR approach was used to prepare the chalcogenide deposits. The reasons for the choice of this process are (i) it is simple and scalable, (ii) it leads to stoichiometric chalcogenide deposits, and (iii) it forms nanocrystals of similar sizes in the presence of some of the carbon forms, as evident from our earlier studies.51−53 The SILAR approach leads to the formation of varying density chalcogenide deposits by the process of ionic interaction with every subsequent cycle.53 Therefore, optimizing the number of chalcogenide deposition cycles is required for maximizing the chalcogenide coating on the 1D TiO2. Using UV-vis analysis, the absorbance spectra of the carbon-free a_r_TiO2 with the SILAR deposited samples was tracked and the results reported in Figure 2. With an increase in the number of SILAR cycles, the optical

Figure 2. Absorbance spectra of the anatase-coated rutile TiO2 and CdS nanocrystals formed by varying the number of SILAR deposition cycles. The number of cycles performed were (a) 5, (b) 10, (c) 15, and (d) 20.

absorbance increases in the visible region and stabilizes at an outer diameter (OD) of 0.62. The XRD analysis of the deposits formed from the SILAR process is shown in Figure S4 in the Supporting Information and indicates the presence of ⟨002⟩, ⟨110⟩, and ⟨201⟩ peaks, characteristic to CdS nanocrystal formation (JCPDS File No. 77-2303). The absorbance pattern of the SILAR deposits in Figure 2 demonstrates an onset at a wavelength of 564 nm and is also

Figure 3. Figure shows the absorbance spectra of the a_r_TiO2 with (A) C60, (B) GQD, and (C) CNT deposits followed by CdS deposition using the SILAR approach. The SILAR cycles were kept constant at 15 for the different carbon allotropes. The ordinate scale is the same for panels (A), (B), and (C). 13403


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deposit formed appears to be markedly influenced by the underlying TiO2/C. For example, the CdS seems to assemble as a continuum of a cluster on the GQD and RGO, compared to the fullerene and the CNT. Despite the complex nature of the final heterostructure, this work illustrates an approach that can be used when scaling up is considered, since several steps involve bench easy benchtop wet chemistry. 3.2. Photoelectrochemical Characterization. 3.2.1. Activity of the TiO2 Nanorods. The performances of the r_TiO2 and the a_r_TiO2 were evaluated by using them as an electrode (photoanode) in a photoelectrochemical (PEC) cell with Pt as a counter electrode and Ag/AgCl as the reference electrode. The chronoamperometry results of the 1D nanorods are shown in Figure 6. The rods display an immediate step change in the Figure 4. SEM images of the a_r_TiO2 nanorods containing the various carbon allotropes: (A) RGO, (B) CNT, (C) GQD, and (D) C60.

mostly present over the nanorod surface as a screen. The image in the inset is shown at a lower RGO loading to indicate that the nanorods are underlying, with respect to the RGO position; as if the RGO is suspended by these rods (a detailed characterization of this assembly is reported elsewhere).44 Figure 4B indicates the presence of long and slender dispersed tubes, characteristic of the CNT, interspersed between the nanorods. Figure 4C shows the small fragments of the nanosheet deposited on the surface of 1D nanorods suggesting the formation of the GQD. Figure 4D shows the image after fullerene deposits; the images do not indicate any distinguishable differences from a fullerene-free sample. This could be attributed to the very small dimensions of fullerene in comparison to the other carbon forms. 3.1.6. Microscopy Analysis of the Chalcogenide Deposits on a_r_TiO2/C. The CdS deposits formed using the optimized SILAR cycles on the carbon-deposited a_r_TiO2 was again examined using microscopy. These images are shown for each of the carbon allotropes in Figure 5. The aspects common to all of the samples are (i) a general decrease in the TiO2 inter-rod spacing and (ii) an absence of any distinguishable clustering of the CdS nanocrystals. The latter conclusion is based on the fact that the onset of absorbance with C-free and C-based films are the same, i.e., 550 nm, as is evident from Figures 2 and 3. Upon closer examination of the SEM images, the nature of the CdS

Figure 6. Figure shows the photoelectrochemical (PEC) responses of (a) the rutile TiO2 nanorod and (b) anatase TiO2-coated rutile TiO2 nanorod under UV-vis illumination with intermittent light “on−off” cycles. [PEC conditions: CE, Pt; RE, Ag/AgCl; electrolyte, 0.1 N Na2S.]

photocurrent magnitude, indicating that they are instantaneously photoactive. The direction of the current is consistent with ntype TiO2.54,55 The photocurrent after the anatase layer formation shows an increase in current from 0.22 mA/cm2 to 0.45 mA/cm2. An ∼100% increase indicates that the presence of an overlaying anatase layer is beneficial to photoactivity of the 1D r_TiO2. The anatase phase of TiO2 has an indirect bandgap, whereas rutile phase has a direct bandgap.56 As a result, anatase phase has a higher photoactivity than the rutile phase of TiO2. Furthermore, upon observing the optical spectra one can rule out the increase in surface area of nanorods. Finally, the multiple on− off illumination cycles demonstrate a reproducible photocurrent indicating that the seed-mediated growth followed by the deposition approach leads to a stable composite 1D oxide film. 3.2.2. Linear Sweep Voltammetry (LSV) of Carbon-Free TiO2 with CdS Deposits. The CdS-deposited TiO2 nanorods (rutile and anatase + rutile) were tested using LSV. As indicated in the linear sweep voltammetry (LSV) curve of Figure 7, a doubling in the current at zero potential and a slight negative shift in the apparent flat band potential is noted simultaneously with the addition of the anatase coating on the r_TiO2, consistent with the inferences drawn from Figure 6. However, a much larger negative shift in the apparent flat band potential is noted when CdS is deposited on the a_r_TiO2. This shift is attributable to the more negative conduction band edge of CdS.2,52,57 Furthermore, an increase in the number of SILAR cycles correspondingly increases the photocurrent. Beyond an optimal loading of 15 cycles, the absorbance is reduced, as indicated in the XYY plot of Figure S5 in the Supporting Information, and so does the photocurrent, indicating that CdS is not deposited beyond the 15

Figure 5. Figure shows the SEM images of SILAR-treated CdS formed over a_r_TiO2 containing the various carbon allotropes: (A) RGO, (B) CNT, (C) GQD, and (D) C60. 13404


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CdS, a_r_TiO2/CNT/CdS and a_r_TiO2/fullerene/CdS annealed in nitrogen is increased by 240%, 180%, 125%, and 83%, respectively, compared to the 1.2 mA/cm2 noted with a_r_TiO2/CdS in Figure 7. The fact that the presence of the carbon allotropes increases the photocurrent and simultaneously leads to a shifts of the apparent flatband, indicates that the carbon allotropes at the interface of the TiO2 and the CdS facilitates improved charge separation as well as charge transportation. Thus, the increase in the current noted in sections 3.2.3 and 3.2.4 is attributed to (i) the greater distribution of the CdS (concluded from absorbance studies of Figures 2 and 3) and (ii) the ability of the carbon to promote charge transport. The carbon allotropes used here serves the purpose of electron shuttling, as indicated in earlier works using metal nanoparticles.58 Besides, metal nanoparticles can assist with other functions besides charge transport such as light absorbance59 and electron flow regulation60 and should be considered if such multifunctionality is desired. Alternatively, the carbon allotropes could be a costeffective option to metal nanoparticles should electron transport be the only criteria.

Figure 7. Current density−voltage (J−V) plot of (a) rutile TiO2 nanorod, (b) anatase TiO2-coated rutile TiO2 nanorod, and (c) anatase TiO2-coated rutile TiO2 nanorod with CdS deposits. [PEC conditions: CE, Pt; RE, Ag/AgCl; electrolyte, 0.1 N Na2S.]

cycles limit. It is also to be noted, that the anatase coating showed higher current at all SILAR cycles compared to the anatase-free rutile TiO2. Nevertheless, the photocurrent using the a_r_TiO2 with the CdS shows a maximum of ∼1.2 mA/cm2 at 0 V with respect to Ag/AgCl. 3.2.3. Photocurrent Response with Various Carbon Allotropes. The photocurrent was then examined with samples containing the various forms of carbon allotropes and the overlying CdS. The comparative results of the chronoamperometry studies are shown in Figure 8A. All samples show

4. DISCUSSION 4.1. Annealing Effects: Effects of N 2 and NH 3 Annealing. All the samples tested so far had been annealed under reductive conditions using N2. However, it is known that all the forms of carbon used here respond differently under different reductive conditions. For example, we had recently reported that the surface functionalities (such as −OH and −COOH) that are present in RGO can be removed or reduced when annealed under reductive conditions that are reactive.43 To evaluate if changing the reductive medium can have an effect on photoelectrochemical responses, we have studied the effects of samples annealed in the presence of ammonia (NH3). The results of this analysis are shown in Figures 9A and 9B. The

Figure 8. (A) PEC responses showing the multiple “on−off” cycles and (B) J−V characteristics of the various carbon loaded samples between a_r_TiO2 nanorod and CdS nanoparticles annealed under nitrogen. [PEC conditions: CE, Pt; RE, Ag/AgCl; electrolyte, 0.1 N Na2S.]

instantaneous and reproducible photocurrent upon illumination and in the same direction as the underlying TiO2 (confirming that the assembly also demonstrates n-type characteristics). The optimized photocurrent values are respectively noted to be 2.8, 2.2, 1.9, and 1.6 mA/cm2, indicating the following: JGQD > JRGO > JCNT > Jfullerene

Figure 9. (A) PEC responses showing the multiple “on−off” cycles and (B) J−V characteristics of the various carbon-loaded a_r_TiO2 samples containing the CdS nanocrystals annealed under ammonia [PEC conditions: CE, Pt; RE, Ag/AgCl; electrolyte, 0.1 N Na2S.]

addition of RGO leads to a current of 2.2 mA/cm2 upon N2 annealing. Whereas, annealing in an NH3 atmosphere under similar conditions, results in a photocurrent of 3.4 mA/cm2, as shown in Figure 9A. This is a 180% higher photocurrent increase. The increase in photocurrent due to annealing in the presence of NH3 may be attributed to the incorporation of nitrogen among the carbon atoms.61−63 Such an effect can lead to the localized electronic states leading to more-effective charge transport. Figure 9B shows the corresponding LSV of the a_r_TiO2/C/ CdS. The trends in the LSV results correspond to the trends noted in Figure 9A: JGQD > JRGO > JCNT > Jfullerene

Thus, compared to the carbon-free TiO2/CdS, the presence of any of the four carbon allotropes uniformly indicate an increase in the photocurrent beyond the 1.2 mA/cm2 noted for the C-free samples. 3.2.4. Linear Sweep Voltammetry with Various Carbon Allotropes. The corresponding LSV response of the electrodes with each of the carbon additives is shown in Figure 8B. The J−V responses follow the same trend with various carbon allotropes as the photocurrent responses. At 2.7 mA/cm2, the GQD shows the highest current with the TiO2/CdS. A corresponding shift in the apparent flat band potential by a magnitude of 0.9 V is also noted. The photocurrent of a_r_TiO2/GQD/CdS, a_r_TiO2/RGO/ 13405


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multiple vibrational spectra; this decreases the Franck−Condon factor in the coupling of the two states and as a result relaxation rate decreases.48,49 This effect could also contribute to improved charge transport properties in nanocomposites that contain GQD. 4.3. Surface Analysis of the 1D-TiO2/GQD/CdS and Mechanism of Charge Transport. HRTEM and fast Fourier transform (FFT) analysis of the TiO2/GQD/CdS assembly was performed and shown in Figure 11. The microscopy image

The photocurrent of the a_r_TiO2/GQD/CdS sample annealed under NH3 shows an ∼8-fold increase in the photocurrent, compared to a_r_TiO2. These results indicate that a_r_TiO2/ GQD/CdS sample annealed under NH3 significantly enhances the photoelectric response compared to samples annealed in N2. The results of the corresponding open-circuit voltage (Voc) for each of the samples are shown in Table S1 in the Supporting Information. The increase in the Voc of a_r_TiO2/GQD/CdS alludes to a more-efficient charge separation of photoelectrons in a_r_TiO2/GQD/CdS with NH3 treatment. 4.2. Electrochemical Impedance Analysis. To further evaluate the effects of NH3 treatment, one must fully understand the electronic properties of the composite assembly. The standard (P)EC characterization techniques are generally focused on providing information only on the overall characteristics. Alternately, the electrochemical impedance spectroscopy measurements provide complementary information on the extent of charge transport in the composite photoelectrodes. To this end, the Nyquist plot and Bode plot of the sample with and without GQDs are presented in Figure 10. The charge-

Figure 11. Figure shows the TEM images of the (A) TiO2/GQD/CdS (inset shows the EDX analysis at various spots), (B) GQD indicating an average particle size of 3 ± 1 nm (inset shows the fast Fourier transform (FFT) diffraction pattern), and (C) CdS indicating an average particle size of 7 ± 1 nm (inset shows FFT diffraction pattern). Figure 10. Figure shows the results of the electrochemical impedance measurements on a_r_TiO2 nanorod with CdS nanocrystals containing GQD annealed under nitrogen and ammonia are shown. These include the (A) Nyquist plot, and (B) Bode plot representing the variation in the phase with respect to frequency for the samples annealed in the presence of nitrogen and ammonia.

shown in Figure 11A indicates a representative section of the sample with the participating species: CdS, GQD, and TiO2. Figures 11B and 11C show the location of the GQD [ϕ = 3 ± 1 nm, 0.214 nm: ⟨100⟩ plane of carbon] and the CdS [ϕ = 7 ± 1 nm, 0.33 nm: ⟨002⟩ plane of CdS], respectively, and the crystallinity of the latter (see inset in Figure 11C). The images demonstrate the existence of a synergistic coupling between the TiO2, GQD, and CdS mainly because of the good mixing afforded by the similarity in the dimensions of the GQD and CdS. Consequently, the enhancement in the photoelectrochemical response is realized due to the boosting of photogenerated charge transport from the CdS to the TiO2 by the GQD as indicated in Scheme 2. 4.4. Decoupling the Contributors to Photocurrent Enhancement and Mechanistics of the Process. This work alludes to the intimate mixing of the GQD between the oxide and the chalcogenide and the excellent charge mobility of graphene as the dual basis for the observed efficient performance of the oxide−C-chalcogenide composite. Furthermore, it is noteworthy to mention that a property called “photon downconversion”, where GQD can absorb the shorter wavelength and then emit longer wavelength photons, has been recently reported.67 The role of this phenomena in promoting enhanced photoelectrochemical response must be studied separately in a systematic manner. From the results drawn in the preceding sections, the following key contributors to the systematic photocurrent enhancement can be inferred: (i) the addition of an anatase TiO2 layer on the underlying rutile TiO2, (ii) broadening of the light absorbance bandwidth with the addition of visible light harvester (CdS),

transfer resistance is proportional to the diameter of the arc of the Nyquist plot at constant bias voltage.64,65 The complex impedance plane or Nyquist plot suggests that the charge transfer resistance is lowest for the NH3-annealed sample having GQDs, in comparison to the sample without GQDs and sample with GQDs annealed in the presence of N2. Furthermore, from the Bode phase plot, quantitative information such as the average lifetime of the electron can be calculated using the following equation:12,66 1 τ = πf p 2 where f p is the characteristic frequency of the samples, relative to electrochemical reaction at the sample and the electrolyte interface. The electron lifetimes of the samples with GQDs annealed in NH3, GQDs annealed in N2, and without GQDs are estimated to be 260, 50, and 32 ms, respectively, which indicates that the electron transfer process is more effective in the presence of GQD annealed in NH3. This result provides the basis for the observed increase in the photocurrent discussed in section 3. Furthermore, if the size of a semiconductor crystal is less than the Bohr radius of the bulk material, the electron distribution will be changed. As a result, the size-dependent electronic properties such as bandgap and the energy relaxation dynamics can be observed. In GQDs, energy spacing in electronic states is much larger than the vibrational frequency required for emission of 13406


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5. CONCLUSIONS The assembly of a heterostructure involving one-dimensional (1D) TiO2 prepared using a seed-mediated approach and CdS, using the successive ionic layer adsorption and reaction (SILAR) approach, is presented. The addition of the carbon allotropes (carbon nanotubes (CNTs), fullerenes, graphene quantum dots (GQDs), and reduced graphene oxide (RGO)) as an interfacial layer between the TiO2 and CdS is examined. Surface and optical studies indicate that carbon can be successfully integrated as a part of the heterostructure and it allows for a greater absorbance of the light upon deposition of the CdS using 15 SILAR cycles. The PEC properties of the TiO2/CdS can be improved by (i) introducing a thin anatase layer on the surface of 1D TiO2 of rutile form (which increase the photocurrent ∼2-fold) and the carbon interfacial layer. While all carbon allotropes shows an increase in the PEC response, the presence of the GQDs at the TiO2/CdS interface results in the highest improvement in the photoelectrochemical responses of the nanocomposite. Furthermore, thermal annealing in the presence of ammonia is shown to enhance the performance of all the carbon forms: A 19fold increase in the photocurrent response is reported with the a_r_TiO2/GQD/CdS nanocomposite, compared to the r_TiO2 sample. Other combinations worthy of consideration in this configuration could be 1D ZnO, with alternative visible-light harvesters of the general formula XY (where X = Cd, Pb, or Zn and Y = S, Se, or Te). Note that such combinations do require the use of precursors amenable to assembly using the technique presented here.

Scheme 2. Integration of GQD (a Conducting Nanocarbon) with CdS (Visible Light Harvester) over an Anatase-Coated Titania Nanorod Leads to the Transport of the Photogenerated Charges Most Effectively Compared to Other Carbon Allotropesa

a

The energy levels of CdS nanoparticles, titania, and GQD are qualitative and not drawn up to the exact scale.

(iii) inclusion of a carbon layer in the form of GQD as the charge transport interface, and (iv) a reductive thermal treatment to enhance charge transport.

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Specifically, a 2-fold, 5-fold, 12-fold, and 19-fold increase has been noted with the addition of anatase, overlayer, CdS deposition, GQD addition, and varying annealing conditions, as summarized in Figure 12. It remains to be seen how the presented approach can be leveraged for light-driven applications such as photocatalysis and photovoltaics, using other oxides or chalcogenides.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b01854. Details on the synthesis, XRD of the samples, HRTEM images, EDS data, GQD characterization details, and a

Figure 12. Bar graph identifying and summarizing each of the steps that lead to the boosting of the light-to-energy conversion using the 1D a_r_TiO2 as the base with the various carbon allotropes and visible-light harvesting agent (CdS nanocrystals). 13407


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graph correlating the absorbance with photocurrent (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.S.V. gratefully acknowledges National Science Foundation funding (NSF-CBET-1337050). L.I. and E.P. participated in the laboratory work during a visit sponsored by the Brazilian Scientific Mobility Program (BSMP). The undergraduate funding from the BSMP program is appreciated. The authors would like to thank Dr. Mojtaba Ahmedian with assistance in obtaining HRTEM images.

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