GrapheneâPorphyrin Nanorod Composites for Solar Light Harvesting

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Graphene Oxide - Porphyrin Nanorod Composites for Solar Light Harvesting Amitava Patra, Sadananda Mandal, Bodhiswatta Mondal, Bikash Jana, Sandip K. Nayak, and rajesh bera ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01504 • Publication Date (Web): 18 Jan 2016 Downloaded from http://pubs.acs.org on January 18, 2016

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ACS Sustainable Chemistry & Engineering

Graphene Oxide - Porphyrin Nanorod Composites for Solar Light Harvesting

Rajesh Bera1, Sadananda Mandal1#, Bodhisatwa Mondal1, Bikash Jana1, Sandip K. Nayak2 and Amitava Patra1* 1

Department of Materials Science, Indian Association for the Cultivation of Science, 2a & b

Raja S. C. Mullick Road, Jadavpur, Kolkata 700 032, India 2

Bio-Organic Division, Bhabha Atomic Research Centre, Trombay, Mumbai - 400 085, India

#

Present Address: Department of Chemistry, Vivekananda Mahavidyalaya, Burdwan

Sripally, Burdwan-713103, West Bengal, India *To whom correspondence should be addressed. E-mail: [email protected] Phone: (91)-33-2473-4971, Fax: (91)-33-2473-2805

1

ACS Paragon Plus Environment


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Abstract Well defined organic nanostructures of porphyrin are promising candidate towards photocatalysis, photovoltaics and electronics applications where photoinduced electron transfer process occurs. On the other hand, reduced graphene oxides (RGO) have been attracted much attention in light energy conversion owing to their efficient charge separation property. In this respect, we have demonstrated a composite of one dimensional (1D) nanostructure of 5, 10, 15, 20-tetrakis (4-carboxyphenyl) porphyrin (TCPP) and RGO for enhancing photoinduced charge separation. The composite was characterised by scanning electron microscopy (SEM), UV-visible spectroscopy, fluorescence spectroscopy, time correlated single photon counting (TCSPC), and femtosecond fluorescence upconversion spectroscopy. It is noted that a very fast decay of TCPP NR was observed in TCPP NR/RGO composite due to electron transfer process and the electron transfer rate is found to be 10.0 x 10-4 ps-1 for TCPP NR/RGO system. An increment (1.9 fold) of photocurrent of this composite system under visible light illumination is obtained due to electron transfer from TCPP NR to RGO. This new class of porphyrin based composite structures opens up new possibilities in photocatalytic, solar energy conversion, photovoltaic, and other new emerging applications.

Key Words: Photocurrent.

Porphyrin

Nanorod,

Graphene,

Composite,

Ultrafast

Spectroscopy,

Introduction The investigation of organic based nanostructures is recognized for potential applications because of their unprecedented optical, electronic, chemical, and mechanical 2


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properties.1-6 Self assembled porphyrin aggregates are found to be attractive for designing promising artificial light harvesting systems. The porphyrin nanoaggregates are formed due to various non-covalent interactions7-13 such as hydrogen bonding, π-π stacking, hydrophobic- hydrophobic interaction, electrostatic interactions and van der Waals forces etc. Generally, intermolecular porphyrin-porphyrin interaction occurs in free-base porphyrins which has a nice planarity. It is evident that free base porphyrin is more suitable than metallo porphyrin towards photo induced applications as the metal of the metallo-porphyrin moiety inhibits the transfer of photoexcited electrons to the acceptor to some extent but free base porphyrins do not.14-15 Thus, the nanostructure of free base porphyrin nanoassemblies has received special attention over metallo-porphyrin based nanostructures. J-type (edge-to-edge arrangement) or H-type (face-to-face arrangement) aggregation occurs during the selfassemble porphyrins nanostructure formation which results red or blue shift of their corresponding UV-vis and fluorescence spectrum. Thus, the electronic properties of the excitons can be engineered by tuning the packing of the molecules. It is evident that Jaggregates are mimicking the chlorophyll light-harvesting antenna complexes with a circle configuration of chromophores in the photosynthesis.16 In case of J-aggregation of πconjugated organic molecules, a strong intermolecular π electronic coupling occurs between the coherently aligned chromophores which enhance the coherent electronic delocalization due to the strong intermolecular π-π interactions.17-18 Recently, Hasobe gave an overview on recent advances in supramolecular structures of porphyrins for solar energy conversion and the photophysical interactions between porphyrin and carbon nanotubes/graphene in details.19 The current research is dealing with anchoring porphyrin nanostructure rather than monomer of porphyrin on reduced graphene oxide surface is attractive though it is a challenging issue. The use of nanocrystalline porphyrins for dye sensitized solar cells is highlighted by Nazeeruddin et al.20 Drain et al. have demonstrated the enhanced catalytic properties of porphyrin nanoparticle.21 Ghosh et al. have investigated the exciton and charge transfer in porphyrin aggregates and it is clear that J aggregate improves the photoinduced charge separation in porphyrin/TiO2 interface.22 It is found that the tuning of electronic energy level by changing morphology of porphyrin assemblies and sphere, rods, flakes and flower shaped porphyrin nano-structures are promising for extending light harvesting.12 Guo et al. reported that nanofibers have more photocatalytic efficiency than spherical nano aggregates of ZnTPyP.23 Recently, our group have investigated the photo-catalytic efficiencies of various nanostructures

(nanoparticles,

nanorods,

flakes

and

flowers)

3


of

meso-tetra

(4-


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carboxyphenyl)porphyrin where rod like structure shows higher activity under visible light illumination.24 Thus, emphasis has been given on 1D porphyrin nano structure towards solar energy applications. On the other hand, fullerene, carbon nanotube, graphene and graphene quantum dots based composite with porphyrin are found to be promising for light energy conversion because carbon network promotes the charge transfer of prophyrin molecules.25-29 Special attention has been given on designing reduced GO-porphyrin composite for future electronic and photonic applications. Graphene is a two dimensional atomically thin sheet of sp2 hybridized carbon which has unique physical and electronic properties.30-34 Therefore, the basic understanding on the electronic interaction of organic dyes, semiconducting polymers, porphyrin molecules or inorganic semiconductor nanoparticles with reduced graphene oxide (RGO) is very important to design promising devices.35-37 Fukuzumi and his co-workers synthesised composite nanoclusters of porphyrins and fullerenes with gold nanoparticles for photovoltaics cell.26 Chen et al. functionalised porphyrin molecules covalently on graphene in organic solvent and studied optical properties for possible electron or energy transfer.38 Kamat et al. studied the excited state interaction of cationic 5,10,15,20-tetrakis(1-methyl-4pyridinio)porphyrin tetra(ptoluenesulfonate)(TMPyP) with reduced graphene oxide systems and measured photocurrent

under visible light irradiation.39 Zhu and his co-worker

synthesised surfactant assisted porphyrin-graphene nanocomposite for photocatalytic hydrogen generation.40 Furthermore Tagmatarchis et al. designed porphyrin-based multi chromophoric supramolecules, non-covalently attached on the RGO surfaces for efficient light harvesting.41 All of the preceding examples clearly demonstrate that the porphyrin reduced graphene oxide (RGO) composites are one of the best suited materials for the charge separation of porphyrin where porphyrins act as light harvesting antenna materials. Liu et al. synthesised one dimensional porphyrin nanoassemblies of Zinc 5,10,15,20-tetra(4-pyridyl)21H,23H-porphine on the graphene surface where co-ordinating properties of zinc plays an important role for the formation of this nanostructure and these nanoassemblies are very much efficient for photocatalysis.42 Kang et al. have integrated the porphyrin nanoparticles into a 2D graphene matrix for free-standing nanohybrid films with enhanced visible-light photocatalytic activity.43 Thus, special interest has been paid on designing the reduced GO/porphyrin nanostructure hybrids rather than reduced GO/porphyrin monomer for their unique electronic properties and also advantage of unique 1D nanostructure of a free base

4


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porphyrin where the coherent electronic delocalization could facilitate the electron transfer process. Here, we have synthesised one dimensional porphyrin nanorod from a free base porphyrin, 5, 10, 15, 20-tetrakis (4-carboxyphenyl) porphyrin (TCPP) and designed a composite of porphyrin nanorod/RGO for light harvesting. The formation of TCPP NR/RGO composite has been well characterised by scanning electron microscopy (SEM), UV-visible spectroscopy. We have also investigated the photophysical properties of the composite system using steady state and time resolved spectroscopy. Femtosecond fluorescence upconversion spectroscopic study is being used to understand the electron transfer process between TCPP NR and RGO in the TCPP NR/RGO composite. Furthermore, the transport properties of the porphyrin nanorod/RGO device under visible light illumination have also been investigated. Photocurrent measurements of this device open up a new avenue in solar energy conversion, photovoltaic and optoelectronic applications.

Experimental Section Materials 4-Carboxybenzaldehyde (Sigma-Aldrich), propionic acid (Sigma-Aldrich), pyrrole (Sigma-Aldrich), cetyltrimethylammonium bromide (CTAB) [Alfa Aesar], sodium hydroxide (NaOH) [MECRK], hydrochloric acid (HCl) [MECRK], tetrahydofuran (THF) [MECRK], and de-ionized water (MECRK) were used as received. The molecular structure of 5, 10, 15, 20-tetrakis (4-carboxyphenyl) porphyrin (TCPP) is presented in scheme 1.

Scheme1. Chemical structure of 5, 10, 15, 20-tetrakis (4-carboxyphenyl) porphyrin (TCPP)

Synthesis of 5, 10, 15, 20-tetrakis (4-carboxyphenyl) porphyrin (TCPP): TCPP was synthesized following the previously reported method24. Briefly, to a solution of 4-carboxybenzaldehyde (6.0 g, 0.04 mol) in hot propionic acid (400 ml), freshly 5



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distilled pyrrole (2.68 g, 2.8 ml, 0.04

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mol) was added drop wise. The mixture was then

refluxed for 45 mins, cooled to ambient temperature and kept in the refrigerator overnight. The dark solution was filtered and the brown solid was washed with dichloromethane. The crude solid was purified by column chromatography (silica, eluant CHCl3-MeOH = 8:2) to get pure TCPP as a purple powder (0.180 g, 2.3 %). The sample was well characterized by NMR, IR and UV-vis studies. 1H NMR (CD3OD): δ 8.85 (bs, 8H, pyrrole β-H), 8.47 (d, J = 8.0 Hz, 8H), 8.28 (d, J = 8.1 Hz, 8H); IR: 3314, 2918, 1728, 1694, 1605, 794 cm-1; UV-vis (λmax): 415, 513, 547, 588, 645 nm. Synthesis of TCPP nanorod: TCPP nanorods were synthesised by a modified acid–base neutralization-based surfactant assisted self-assembly method (SAS). In brief, 0.004 g of TCPP was dissolved in 0.5 ml alkaline (0.2 M) solution. In another beaker 0.036 g of CTAB was dissolved in 9.5 ml of water followed by the addition of 0.085 ml conc. HCl. Then, TCPP solution was rapidly added to the acidic solution of CTAB with stirring for 30 minutes. The as prepared nanorods of TCPP were centrifuged at 5000 rpm for 5 minutes to remove the excess surfactant. Synthesis of reduced graphene oxide (RGO) and TCPP nanorod-RGO composites: For TCPP NR/RGO composite, we have synthesised water soluble reduced graphene oxide. Briefly, 10 mg of as prepared graphene oxide (GO) was dissolved in 20 ml distilled water under mild ultrasonication until yellow brown solution appeared. This GO solution was placed in a 50 ml round bottom flask and maintained pH 9. Then 50µL of hydrazine hydrate was added to this GO solution and temperature was raised to 90

0

C for 2 h. The black

precipitated was collected by several times centrifuging and washing with distilled water. This synthesised RGO was dissolved in double distilled water to prepare 0.1 mg/ml RGO solution for further experiment. The prepared reduced graphene oxide was characterised by UV-visible spectroscopy and Raman spectroscopy44-45 (Figure S1 and S2). TCPP nanorod /RGO composite were prepared by adding desired amount of RGO solution to a fixed TCPP NR solution and it was centrifuged at 2500 rpm for 5 minutes to make a compact composite system. The device was fabricated as follows45: First the ITO coated glass was washed by soap solution and then ethanol and 2-butanol respectively step by step and dried at 100˚C under vacuum. A very thin layer of PEDOT: PSS was given on this dried ITO glass by spin coating at 2000 rpm and dried at 120˚ C under vacuum for 2 hours. Here PEDOT: PSS acts as a hole acceptor and it provides good interface between active layer and electrode. Then, a 6


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layer of pure or composite systems that is photoactive layer was spin coated on this modified ITO surface at 2500 rpm for 30 seconds. Then, this ITO carefully was baked at 60˚ C under vacuum for overnight. The completion of device fabrication was done by depositing Al on active layer of ITO which acts as cathode. This fabricated device was used for photocurrent measurement. Characterization: The morphological characters and sizes of TCPP nanorods and TCPP nanorod-RGO composite were done by Field Emission Scanning Electron Microscopy (FE-SEM, JEOL, JSM-6700F). Room temperature optical absorption spectra were taken by a UV-vis spectrophotometer (SHIMADZU). Room temperature photoluminescence spectra were recorded

by

a

Fluoromax-P

(HORIBA

JOBIN

YVON)

photoluminescence

spectrophotometer. For the time correlated single photon counting (TCSPC) measurements, the samples were excited at 405 nm using a pico-second diode laser (IBH Nanoled-07) in an IBH Fluorocube apparatus. The repetition rate was 1 MHz. The fluorescence decays were analyzed using IBH DAS6 software. The following equation was used to analyze the experimental time resolved fluorescence decays, P(t ) : n

P (t ) = b + ∑ α i exp( − i

t

τi

)

(1)

Here, n is the number of discrete emissive species, b is a baseline correction (“dc” offset), and αi and τi are the pre-exponential factors and excited-state fluorescence lifetimes associated with the ith component, respectively. For multi-exponential decays the average lifetime,〈τ〉, was calculated from the following equation: n

< τ >= ∑ a iτ i

(2)

i =1

Where a i = α i / ∑ α i and ai is contribution of the decay component. The investigation of ultrafast spectroscopy was done using a Femtosecond fluorescence upconversion spectrophotometer with a Halcyone ultrafast setup (coherent). The sample was excited with 400 nm wavelength of excitation, pumped using a 800 nm femtosecond (fs) (140fs pulse width, 80 MHz repetition rate) laser pulse (4.4 W) from a Ti:sapphire oscillator (Chameleon,Coherent) coupled to a second harmonic generator (by BBO type I crystal). The emission wavelength (666 nm) and the gate pulse of the fundamental beam (800 nm) are upconverted using a nonlinear crystal (BBO type II). The FWHM of the instrument response 7



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function is about 300 fs. The femtosecond time resolved decay data were fitted using Surface Xplorer 2.3 fitting software. Photocurrent measurement was performed with a 150 W Newport Solar Simulator model 76500 using white light under 1 sun (AM 1.5 G) illumination at 100 mW cm−2. Results and Discussions: Structural Study Figure 1 shows the FE-SEM images of TCPP nanorods (A) and its composite with RGO (B, C). These images unambiguously confirm the formation of porphyrin nanorods by the acid-base neutralisation based method in aqueous medium. The average length of the nanorod is about 150-170 nm with the diameter of ~50 nm. In general, TCPP molecules are insoluble in water. The carboxylic groups of TCPP are de-protonated in alkaline solution to form tetra-carboxylate anion, [TCPP]4- which is soluble in water and form a homogeneous solution. The encapsulation process into the micelle occurs by adding this [TCPP]4- basic aqueous solution to acidic aqueous solution of CTAB under vigorous stirring which gives TCPP nanorods.

(A)

(B)

(C)

RGO

NR

Figure 1: FE-SEM images of TCPP nanorods(A) and TCPP nanorod-RGO composites(B,C)

Here, the cationic surfactant might act as a stabilizer via the electrostatic interaction between the carboxylate anion in the porphyrin molecules and the cationic surfactant. Subsequently, the surfactant might affect the formation mechanism as well as the micelle formation for dispersion of the porphyrin aggregation. During acid-base neutralization, the [TCPP]4- anion becomes hydrophobic TCPPs after protonation. These hydrophobic TCPP molecules are encapsulated into the hydrophobic micelle because of hydrophobic-hydrophobic interaction between porphyrin and CTAB molecules. The formation of assembled structures is driven by 8


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the non-covalent interactions such as hydrophobic-hydrophobic and aromatic π-π stacking between the molecules. The size and shape of the TCPP nanorods remain almost unchanged after the formation of composite with RGO. The zeta potential value of TCPP NR and RGO was 34 mV and -20 mV respectively which were given in supporting information (Figure S3). The composite formation between RGO and TCPP NR is due to electrostatic interaction between the positively charged nanorod and negatively charged surface of the RGO. Photophysical Properties of TCPP NR and TCPP NR-RGO Composites It is evident that the porphyrin nano-assembled structures especially ID nanostructure have unique optical properties and potential application in any kind of artificial photo induced applications. Metalloporphyrins and free base porphyrins feature two well defined absorption regions, so-called high-energy B band (known as Soret band) and low-energy Q bands. Soret absorption band corresponds to S0-S2 electronic transitions, while Q bands correspond to S0-S1 electronic transitions. Both the B and Q bands arise from π-π* electronic transitions.46-47 The photophysical properties of the as prepared TCPP nanorods and TCPP nanorod-RGO composites have been studied by steady state and time resolved spectroscopy. Figure 2A shows the UV-vis spectra of TCPP molecules in THF and TCPP nanorods. In THF solution, TCPP exists as monomer, showing high intensity Soret band at 415 nm and four relatively weak Q bands at 513 nm, 547 nm, 588 nm and 645 nm, respectively.35 In contrast, the UV-vis spectrum of the TCPP nanorod is quite different from the TCPP monomer. The Soret absorption band position slightly red shifted from 415 nm to 425 nm with the spectral broadening. The Q absorption bands are also red shifted along with high absorbance with respect to TCPP monomer. The red shifting and spectral broadening of the absorption bands of TCPP nanorods indicates the J-aggregation of TCPP molecules in the formation of the TCPP nanorods.48-49 Figure 2B shows the photoluminescence spectra of the corresponding TCPP monomer and nanostructures. In THF solution, TCPP monomer shows two emission bands, Q*X00 and Q*X01, with maxima at 650 nm and 715 nm, respectively.24 This spectral pattern is the well known characteristic of the monomeric porphyrin molecules. Both of the maximum and minimum emission bands are red shifted when the TCPP molecules form nanorods. The red shifting of the emission spectra further suggests the Jaggregation of porphyrin molecules in the nanorods.

9



a

(A)

300

a

b

Normalised Intensity (a.u)

Normalised Absorbance

400 500 600 Wavelength (nm)

b

(B)

700 550

600

650

700

750

800

Wavelength (nm)

Figure 2: UV-vis (A) and photoluminescence spectra (B) of TCPP (a) in THF and (b) TCPP

nanorods Now, the optical properties of the TCPP nanorod-RGO composite have been studied by the steady state and time resolved spectroscopy. Figure 3 shows the UV-vis spectra of TCPP nanorods in the presence of different concentrations of RGO. It has been observed that the peak positions of both Soret and Q absorption bands of TCPP nanorods remain unchanged (425 nm) after the formation of TCPP NR-RGO composites. With gradual increase of RGO in the composite systems, all absorption peak positions remain still unaltered but the optical density increases. This observation suggests that the TCPP nanorods morphology did not altered after the composite formation with RGO and it could be seen that the optical density increases due to the increase of concentration of RGO (in fig. 3).

240 µg RGO

Absorbance

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0 µg RGO

(RGO)

300

400 500 Wavelength (nm)

600

700

Figure 3. Absorption spectra of TCPP nanorods in composite with different concentrations of

RGO (0 µg, 60 µg, 100 µg and 240µg) 10


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Steady state photoluminescence study of TCPP nanorods and composite systems have been carried out to investigate the photophysical interaction between TCPP nanorods and RGO in the composite system. Figure 4 shows the fluorescence spectra of TCPP nanorods and TCPP nanorod-RGO composites with varying the concentration of RGO at the excitation wavelength of 405 nm. It has been observed that the TCPP NR exhibits two emission bands at 666 nm and 732 nm. In composite systems, the emission peak positions remain unchanged (666 nm), but the photoluminescence intensity of TCPP nanorods decrease with increasing the concentration of RGO. The photoluminescence quenching efficiency is found to be 89 % for the addition of 240 µg RGO. This photoluminescence quenching signifies the possibility of energy transfer or electron transfer from TCPP nanorods to the RGO. It is important to note that there is no spectral overlap between the emission spectrum of TCPP nanorod and absorption spectrum of RGO (fig. S4). Thus, the energy transfer from TCPP nanorod to RGO is ruled out. Therefore, photoluminescence quenching of TCPP nanorod in the composite system is occurred may be due to the electron transfer from TCPP nanorod to RGO in the TCPP nanorod-RGO composite systems.

a

0 µg RGO solution

b

Intensity (a.u)

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c 240 µg RGO solution d

e

600

650

700 Wavelength (nm)

750

800

Figure 4: Photoluminescence spectra of TCPP NR-RGO composites at different

concentrations of RGO [0 µg (a), 60 µg (b), 100 µg (c), 150 µg (d) and 240µg (e)]

In order to investigate the excited state interactions between TCPP nanorod and RGO in the composite system, we have performed photoluminescence decay time of TCPP monomer TCPP nanorods and TCPP nanorod-RGO composite systems at the excitation wavelength of 405 nm (Figure 5). The decay time measurements are more sensitive than PL 11



quenching efficiencies where errors come from the fluctuations in the lamp intensity. The decay curve of TCPP monomer is fitted by mono-exponential decay with the decay time of 9.83 ns. The decay time of TCPP nanorod was fitted using bi-exponential parameter, where the average decay time is reduced to 6.79 ns.

(a)

1000

(b)

Counts (log)

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(c) 100 (d)

10 0

20

40

60

80

Time (ns)

Figure 5: Photoluminescence decay curves of TCPP monomer (a), TCPP nanorods (b) and

TCPP nanorod-RGO composites [(c) 100µg and (d) 240µg of RGO].

This shortening of decay time confirms the formation of J-aggregated nanostructure of TCPP molecules.24, 50 On the other hand, the decay time of TCPP nanorod-RGO composite is fitted by tri-exponential decay. The corresponding components are 3.8 ns (19 %), 9.47 ns (17 %) and 0.158 (64%) with the average decay time of 2.4 ns. Among three components, the faster component is responsible for the energy/electron transfer from TCPP nanorod to RGO. We have ensured decay time of TCPP NR taking higher amount of RGO (240µg) in composite system where the decay time of faster component decreases significantly. This signifies the electron transfer from TCPP NR to RGO in composite system. We have also checked the decay time of TCPP monomer/RGO composite system in aqueous medium (Figure S5). It can be shown that there is no major change in decay time upon increasing the amount of RGO probably due to feeble interaction as both are in negatively charged materials. All the decay parameters are given in Table 1.

12


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Table 1: Decay parameters of TCPP monomer, TCPP nanorod, TCPP NR-RGO Sample

τ 1(ns)

τ 2(ns)

τ 3(ns)

(a1)

(a2)

(a3)

9.83

-

-

9.83

3.36

9.55

-

6.79

(0.44)

(0.56)

TCPP NR-

3.80

9.47

0.15

2.4

RGO(100µg)

(0.19)

(0.17)

(0.64)

TCPP NR-

2.36

6.37

0.10

RGO(240µg )

(0.08)

(0.06)

(0.86)

TCPP

(ns)

(1) TCPP NR

0.65

Since the photo-excited electron transfer is very fast process, so faster component of the TCPP nanorod-RGO system may be responsible for the electron transfer from TCPP nanorod to RGO. In order to calculate the electron transfer rate quantitatively, the decay times of the TCPP nanorods and TCPP nanorod-RGO composites at different concentrations of RGO have been measured by femtosecond fluorescence upconversion spectrophotometer. Figure 6 shows the ultrafast decay curves of TCPP nanorods and TCPP nanorod-RGO composites at the excitation wavelength of 400 nm and the emission wavelength of 666 nm. The decay curve of TCPP nanorods is fitted by mono-exponential decay with the decay time of 610 ps.

(a)

Normalised Counts

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(b)

(c)

0

50

100 150 Delay Time (ps)

200

250

Figure 6. Ultra fast decay times of TCPP nanorods (a) and TCPP nanorod-RGO composites

[100 µg (b) and 240 µg (c) of RGO] 13



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Page 14 of 22

The decay curves of TCPP nanorod-RGO composites at different concentrations of RGO are also fitted by mono-exponential decay, but the decay time of TCPP NR decreases with the increase of concentration of the RGO. The observed decay times of TCPP NR/RGO composite are 478 ps and 378 ps for the concentration of 100 µg and 240 µg of RGO, respectively. The electron transfer rate is calculated by using the following equation;

K NRD =

1

τ

−

(DA)

1

τ

(3)

(D)

Where, kNRD is the nonradiative excited state decay rate due to electron transfer and τDA and τD represents the fast decay component of TCPP NR/RGO and TCPP NR respectively. The calculated kNRD values of TCPP NR/RGO composite are found to be 4.5 x 10-4 ps-1 and 10.0 x 10-4 ps-1 for the concentration of 100 µg and 240 µg of RGO, respectively. This electron transfer process may have potential application for designing solar cell and other photodriven nanodevices. Photo Current Measurement

This excited state electronic interaction between TCPP NR and RGO can be utilized to fabricate optoelectronic devices. We measured the photocurrent of the devices of TCPP and TCPP/RGO systems (figure 7B) under illumination of visible light. The cross section of the device was shown in supporting information (Figure S6). Kamat et al observed photocurrent increment in RGO-TMPyP electrode using photo electrochemical process.39 Here, it is remarkable that the photo current of the device prepared by RGO/TCPP NR composite system shows 1.9 fold enhancement of photocurrent compared to pure TCPP NR which is shown in figure 7A. Intrinsic graphene does not show any photocurrent under visible light. Here, RGO plays an important role: it helps to separate the photogenerated excitons as it has superior electron accepting and transporting properties. It is noteworthy that there is significant increase of the photocurrent in the presence of light for the composite device. Thus, the change in photocurrent in TCPP NR/RGO composites under visible light illumination is due to excited electron transport from TCPP NR to RGO which is consistent with upconverted fluorescence decay. The probable charge separation process of this system is given in figure 7(C).

14


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(A)

0.3

(B) d

0.2

Al

Active layer

c

0.1

PEDOT:PSS ITO

b 0.0

a

Dark

e

(C)

-

e LUMO

-0.2

Energy

-0.1 Li gh t

Current (µA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RGO

Al

ITO PEDOT:PSS

+ HOMO

-0.3

TCPP NR

-1.0

-0.5

0.0

0.5

1.0

Voltage (Volt)

Figure7. [A] (a) Dark and (c) photo current of TCPP NR. (b) Dark and (d) photo current of

TCPP NR/RGO composite system. [B] Structure of device. [C] Energy level and charge separation process in the system. Conclusion

In summary, we have successfully prepared a composite of a free base porphyrin nanorod and reduced graphene oxide for solar light harvesting. The detailed photophysical properties of these composites are being investigated by using UV-vis spectroscopy, fluorescence spectroscopy, time correlated single photon counting and femtosecond fluorescence upconversion spectroscopy. This nano hybrid system reveals the enhancement of photocurrent of the devices due to sufficient photo induced charge separation. It is still a subject of dominant importance to develop new form architecture of porphyrin to initiate new platforms for solar light harvesting. Supporting Information

UV-Vis spectra of RGO in water, Raman spectra of GO and RGO, Zeta potential of RGO and TCPP NR, Absorption spectra of RGO (a) and emission spectra of TCPP NR (b), PL Decay of TCPP monomer in THF, TCPP NR in water, TCPP monomer/RGO in aqueous medium and SEM Cross section of as prepared device of TCPP NR/RGO composite are given in supporting information. This material is available free of cost from http//:pubs.acs.org.

Acknowledgement

DST is gratefully acknowledged for the financial support. RB, BM and BJ thanks CSIR and SM thanks IACS for awarding the fellowship. 15



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Figure Captions Scheme1. Chemical structure of 5, 10, 15, 20-tetrakis (4-carboxyphenyl) porphyrin (TCPP) Figure 1: FE-SEM images of TCPP nanorods (A) and TCPP nanorod-RGO composites (B, C) Figure 2: UV-vis (A) and Photoluminescence spectra (B) of TCPP (a) in THF and (b) TCPP nanorod. Figure 3. Absorption spectra of TCPP nanorods in composite with different concentrations of RGO (0, 60,100 and 240µg). Figure 4: Photoluminescence spectra of TCPP NR-RGO composite at different concentrations of RGO [0, 60, 100, 150 and 240µg (a-e)]. Figure 5: Photoluminescence decay curves of TCPP monomer (a), TCPP nanorod (b) and TCPP nanorod-RGO composite [(c) 100µg and (d) 240µg of RGO]. Table 1: Decay parameters of TCPP monomer, TCPP nanorod, TCPP NR-RGO Figure 6. Ultra fast decay time of TCPP nanorod (a) and TCPP nanorod-RGO composite [(b) 100 µg and (c) 240 µg of RGO] Figure7. [A] (a) Dark and (c) photo current of TCPP NR. (b) Dark and (d) photo current of

TCPP NR/RGO composite system. [B] Structure of device. [C] Energy level and charge separation process in the system.

21



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TOC

Graphene Oxide - Porphyrin Nanorod Composites for Solar Light Harvesting Rajesh Bera1, Sadananda Mandal1#, Bodhisatwa Mondal1, Bikash Jana1, Sandip K. Nayak2 and Amitava Patra1* The design of porphyrin nanorod-reduced graphene oxide (RGO) composite opens up a new avenue for solar energy conversion.

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GrapheneâPorphyrin Nanorod Composites for Solar Light Harvesting

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