ARTICLE pubs.acs.org/JPCC
Poly(3-hexylthiophene) Adhesion on Zinc Oxide Nanoneedles Claudia Caddeo,*,†,‡ Roberta Dessì,† Claudio Melis,†,‡ Luciano Colombo,†,‡ and Alessandro Mattoni*,‡ †
Dipartimento di Fisica, Universita di Cagliari, and ‡Istituto Officina dei Materiali del CNR, Unita SLACS, Cittadella Universitaria, I-09042 Monserrato (Ca), Italy
bS Supporting Information ABSTRACT: We study the interface between poly(3-hexylthiophene) and wurtzite ZnO nanoneedles by molecular dynamics simulation. We provide evidence that the polymer is easily adsorbed on the nanostructured surface with the largest binding energy found when the chain is aligned to the needle axis. Helically wrapped polymer configurations are nevertheless possible, and they are found to be metastable with long lifetimes and small polymer mobility on the nanoneedle surface. The wrapped configuration lifetime has been compared with the case of carbon nanotube-based systems at finite temperatures and calculated to be much longer. The dependence of the adhesion energy on the polymer orientation is discussed and explained by a model, including strain, adhesion anisotropy, and nanoneedle shape effects (i.e., the presence of edges between facets).
’ INTRODUCTION Zinc oxide (ZnO) has a potentially large technological impact as an electron acceptor in hybrid photovoltaics13 due to its large electron affinity, good transport properties, ease of fabrication, and its environmentally friendly nature.4,5 ZnO can be synthesized into nanostructures, thus obtaining large surface-to-volume ratios and tuning its electronic properties can be achieved by controlling the size and the shape of the nanoparticles.6,7 So far, several ZnO nanostructures have been fabricated, (e.g., nanoneedles, nanobelts, nanosprings, nanorings, and many others) by means of various methods, including vapor-phase transport and electrochemical deposition.810 In particular, ZnO nanoneedles grown perpendicularly to the substrate have attracted interest because they can be easily synthesized from a solution at low temperature.11,12 Conjugated polymers are an important class of electron donor and hole transporters that can be used in association with nanostructured ZnO. For example, bulk heterojunction solar cells can be obtained from solution by mixing ZnO one-dimensional (1D) nanostructures (e.g., nanorods or nanoneedles) with polymers, such as poly[2-methoxy-5-(30 ,70 -dimethyloctyloxy)-1,4phenylenevinylene] (MDMO-PPV)13 or poly(3-hexylthiophene) (P3HT).7,14,15 In particular, P3HT has been widely investigated because of its good hole mobility and favorable band alignment with ZnO.16,17 The state-of-the-art efficiency of ZnO:P3HT hybrid solar cells is still lower14,15,18 than in all-organic systems with a power conversion efficiency approaching 5%.19 As P3HT and ZnO exhibit good transport properties and should have, in principle, favorable band alignment, the reason of such a low efficiency must be found in the complex morphology18 of the hybrid and in r 2011 American Chemical Society
atomic scale features of the ZnO/P3HT interface. For example, by studying bilayer architectures, it has been experimentally found that the P3HT tends to be amorphous rather than crystalline at the interface with ZnO.20 The crystallinity of the polymer is relevant for the transport and collection of holes. It has been also shown that the optical and transport properties of P3HT are better when the polymer is in its crystalline phase.2123 From the theoretical point of view, first-principles calculations have been used to study the adhesion of a single P3HT molecule on the planar ZnO (1010) surface,16 identifying the preferred molecular orientations. More recently, by using model potential molecular dynamics, planar ZnO/P3HT interfaces have been investigated,24 which confirmed the experimental evidence of polymer disorder at the ZnO interface. On the other hand, other issues, such as the effect of the curvature of the interface (corresponding to important characteristics emerging at the nanoscale), have not been investigated so far. This is important when dealing with ZnO nanostructures where their high surface curvature is expected to affect the polymer organization and adhesion. For instance, it is known that, in the case of carbon nanotubes, complex helical configurations of the polymer can be found,25 depending on temperature, polymer length, presence of solvent, and nanotube chirality.2628 It is, therefore, interesting to investigate whether helical configurations are possible for P3HT on ZnO nanostructures. Whereas in the nanotube case, the hybrid interactions are dominated by dispersive contributions, in the case of the metaloxide, the electrostatic forces are important, and Received: May 6, 2011 Revised: July 6, 2011 Published: July 07, 2011 16833
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Figure 2. (a) Definition of coiling angle around the nanoneedle and (b) rotation angle on the planar surface. Dashed lines in (a) are a guide to the eye to identify the nanoneedle facets. Note that only the backbone atoms are shown for clarity.
Figure 1. Examples of starting (top) and final (bottom) configurations for two different systems. On the left, the coiling angle is 40°, on the right, 70°. The insets show the top view of the systems and the chemical structure of P3HT. In all figures, red is oxygen, gray is zinc, cyan is carbon, yellow is sulfur, and white is hydrogen.
hence, different results can be expected. As a direct experimental observation of single polymer chains on ZnO nanoneedles is very difficult, a theoretical investigation is relevant. In this work, by using model potential molecular dynamics (MD), we study the adhesion of a single P3HT chain on a ZnO nanoneedle (N) and we compare the results with the case of a planar ZnO surface (P). An analytical model for the polymer adhesion on the nanoneedle is given. The stability of helically wrapped polymer configurations is studied at finite temperature and then compared to the case of a carbon nanotube with a similar size.
’ COMPUTATIONAL FRAMEWORK We focus on ZnO wurtzite-structured nanoneedles, with a hexagonal cross section, and grown along the [0001] direction with (1010) facets. They are ideally infinite in length (due to periodic boundary conditions) and have a diameter as small as ∼2.5 nm. The simulation cells have dimensions of 7 nm 7 nm 21.3 nm, along the x, y, and z directions, respectively. The P3HT molecules are up to 64 monomers (mon’s) long (∼24 nm). The simulations cells contain ∼12 000 atoms. To describe P3HT, we used the AMBER force field,29 which includes either bonding (stretching, bending, and torsional) or nonbonding (van der Waals and Coulomb) contributions. The atomic partial charges for P3HT were calculated according to the standard AM1-CM2 method30 and are available in the Supporting Information. The ZnOP3HT interactions are described as a sum of electrostatic and dispersive contributions (of Lennard-Jones type), with parameters taken from the Amber database.29 All the electrostatic contributions were computed by the Ewald sum method with real space cutoff, Fc = 8 Å. Partial charges for ZnO are taken from ref 6 and are available in the
Supporting Information. The interatomic potential for ZnO forces is the sum of an electrostatic term plus a Buckingham-type contribution6 qi qj rij C EZnO ðrij Þ ¼ þ A 3 exp ð1Þ 6 rij F rij where rij is the distance between i and j ions, qi and qj are the charges on ions i and j, respectively, and A, F, and C are the potential parameters, which are taken from ref 6 and available in the Supporting Information. The cutoff distance has been set equal to 8 Å. The simulations are performed using the DL_POLY 3 parallel code.31 The atomic trajectories were calculated by using the velocity Verlet algorithm with a time step as small as 0.5 fs. The temperature was controlled by the Berendsen thermostat with relaxation costant f = 0.5 ps.
’ RESULTS AND DISCUSSION The typical initial configuration for the following MD simulations is represented in Figure 1 (top panels). It consists of a ZnO nanoneedle and a helical-shaped polymer chain wrapped around ZnO. To explore the most favorable configurations, we varied the arrangement of the polymer on the nanoneedle by studying regular helices. Each helix is characterized by the coiling angle θ (defined as in panel (a) of Figure 2), with 10° e θ e 70°. Each system only differs from the others for the coiling angle. The corresponding structures were first relaxed and then annealed at room temperature for ∼3 ns. Two examples are reported in Figure 1 (bottom panels). Upon annealing, the polymer bends around the nanoneedle following its hexagonal section (see insets in bottom panels of Figure 1). The flexible hexyl tails of the polymer are largely distorted with respect to the ideal regioregular geometry (compare top and bottom panels of Figure 1), as a result of the local electrostatic interactions between the partial charges of the chain and the surface ions. The binding energy of the hybrid system, UNB(θ), where N stands for “nanoneedle” and B stands for “binding”, is defined as the difference between the total energy of the bound configuration UNZnO+P3HT(θ) and the total energy of two separate components (UNZnO for the ZnO nanoneedle and U0P3HT for 16834
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Figure 3. Chain strain energy (open triangles, 1) and polymer nanoneedle binding energy (filled squares, 2) as a function of the coiling angle (top scale). Polymersurface binding energy (open squares, 3) as a function of the rotational angle (bottom scale). Lines are a numerical fit (see text).
Table 1. Values of the Fitting Parameters fitting parameter
value
RC βC
1.84 105 [eV/mon] 2.32 103 [eV/mon]
RP
3.28 105 [eV/mon]
βP
2.5 103 [eV/mon]
γP
0.52 [eV/mon]
AP
10249.39 [eV]
dP0
0.31 [Å ]
CP
879.52 [eV 3 Å6]
E
A dE0
4308.98 [eV] 0.36 [Å]
CE
1105.52 [eV 3 Å6]
the straight (0) isolated polymer, respectively. All the energies were taken after relaxing the structures for 50 ps at T = 0 K N N 0 ðθÞ UZnO UP3HT UBN ðθÞ ¼ UZnOþP3HT
ð2Þ
where the θ dependence is introduced by the sole U ZnO+P3HT(θ) contribution. The calculated UNB(θ) values are reported in Figure 3 (filled symbols) as a function of the coiling angle (top scale). The dotted line will be described later. The behavior is nonmonotonic with the highest binding energy occurring at small coiling angles. This means that the straight polymers are more strongly bound to the nanoneedle. A similar result was found for P3HT on single-walled carbon nanotubes (SWCNTs).26 There, it was explained by taking into account the strain energy of the polymer, defined as the energetic cost paid to bend the chain into an helical shape. Here, to separately account for all the possible contributions to the configurational energy of an isolated wrapped polymer, we considered the same conformation of the polymer obtained for the relaxed hybrids, and then we calculate the resulting total energy UC(θ) of the molecule in its present helical conformation. The configurational energy difference UC(θ) U0P3HT is shown in Figure 3 (open triangles), where it is also fitted by a secondorder function of the coiling angle θ (dotted line) N
UC ðθÞ ¼ RC θ2 þ βC θ
ð3Þ
where RC and βC are fitting parameters (values are reported in Table 1).
Figure 4. (top) Snapshots of the investigated structures: (a) planar surface, (b) nanoneedle facet, (c) nanoneedle edge. (bottom) Interaction basin of a thiophene ring on a ZnO planar surface (open squares), on a nanoneedle facet (open triangles), and on a nanoneedle edge (filled squares). The values are the minima among different rotational angles of the ring (on the ring plane) around its center of mass.
This is consistent with the quadratic dependence of the polymer strain energy on the local curvature, as reported elsewhere.32 UC(θ) U0P3HT increases by a fairly small amount in the range of 0 e θ e 70° (actually from 0 to 0.07 eV/mon). At variance, in the same range, the binding energy UNB given in eq 2 gives a larger variation of 0.25 eV per monomer, suggesting that other contributions must be considered to be added to simple coiling. A new feature not considered so far comes from the relative orientation of the polymer backbone with respect to the crystalline substrate. We, therefore, calculate the adhesion of a straight P3HT segment on a ZnO planar surface as a function of the molecular orientation. This is defined by the rotation angle ϕ formed by the polymer backbone and the [0001] direction (see Figure 2, right panel). The corresponding energy, UPB(ϕ) = UPZnO+P3HT(ϕ) UPZnO U0P3HT (where UPZnO is the total energy of the planar ZnO surface), is reported in Figure 3 by open squares. When the P3HT backbone is aligned to the [0110] direction (corresponding to ϕ = 90°), the binding is maximum, corresponding to UPB(ϕ = 90°) = 0.56 eV/mon. The calculated energies can be fitted by a simple quadratic function UBP ðϕÞ ¼ RP ϕ2 þ βP ϕ þ γP
ð4Þ
where RP, βP, and γP are fitting parameters (values are reported in Table 1). This analysis provides evidence that the ϕ dependence must be included in the binding energy of the P3HT on the nanoneedle. When considering a wrapped polymer chain, the average rotation angle on the facets depends on the coiling angle; that is, ϕ = ϕ(θ). For the case of a regular helix considered here, we have ϕ = θ and we can, therefore, replace ϕ with θ everywhere. If applied alone, the anisotropy argument would predict the largest binding at θ ∼ 90°, that is, when the polymer chain is orthogonal to the growth direction of the nanoneedle. This is not the case for UNB, which includes other contributions. For example, as discussed above, the polymer strain favors small coiling angles. Although a better physical insight is provided by the above argument, a model for UNB(θ) only including contributions from coiling and relative orientation features (UPC(θ) and UPB(θ), 16835
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thiophene molecule close to the nanoneedle edge. Their values are reported in Table 1. The above results for the single thiophene molecule can be extended to the case of the P3HT. Whenever we have a polymer wrapped around a nanoneedle, some portions of the chain fall necessarily close to the edges of the nanostructure. According to the above analysis, these portions of the polymer are less bound to the nanoneedle; the bigger the portion of chain close to the edges, the weaker is the binding. The number of times Nedges the edges are met by the polymer depends on the coiling angle θ, and it is larger for larger coiling angles. A simple counting of Nedges can be obtained by dividing the polymer length L by the edge-to-edge distance (Lf)/(sin θ) (where Lf is the facet width) along the coiling direction. We get Nedges ðθÞ ∼
L sin θ Lf
ð6Þ
Eventually, we get a thorough expression for the binding energy UBN(θ), including all the contributions UBN ðθÞ ¼ UC ðθÞ þ UBP ðθÞ þ PeN Nedges ðθÞ
ð7Þ
PNe
Figure 5. Left: starting (top) and final (bottom) configurations for a polymernanoneedle system annealed at T = 500 K for 7 ns. Right: starting (top) and final (bottom) configurations for a polymer SWCNT system annealed at T = 500 K for 50 ps. The insets show the top view of the systems.
respectively) is still unable to fully account for the calculated binding energy; as a matter of fact, we estimate [UPC(θ) + UPB(θ)] UNB(θ) ∼ 0.1 eV/mon, proving that a further correction is actually needed. A meaningful improvement consists in explicitly including the effects of the edges between the facets of the nanoneedle on the energetics of our problem. Edges are extended defects that affect the polymer adhesion. This is clearly shown by a simpler model system formed by just one thiophene molecule for which we calculate the binding energy on both a planar surface and a nanoneedle. In Figure 4, such an energy is plotted versus the distance d between the center of mass of the molecule and the plane of the solid surface for three different configurations (shown in the top panels) corresponding to different symbols. It is found that the binding is favored when the thiophene interacts with the planar surface (empty squares) or with the center of a nanoneedle facet (empty triangles). Actually, the values are almost identical and can be fitted with the same curve (full line). Conversely, the binding is unfavored when the molecule is close to an edge (filled squares) and the adhesion is reduced by PTe ∼ 0.13 eV (dashed line). Both series of data can be fitted by Buckingham-type curves d C EðdÞ ¼ A 3 exp 6 d0 d
ð5Þ
where AP, dP0, and CP refer to the adhesion on the planar surface or on the nanonedle face, while AE, dE0, and CE correspond to the
takes into account the reduction of the polymer where adhesion energy due to the edges. PNe is here used as an adjustable parameter. The best fit is reported as a dashed line in Figure 3 for PNe ∼ 0.011 eV, a value that is smaller than PTe found for the single thiophene. This result suggests that the reduction of the adhesion of P3HT on the ZnO edges is smaller than in the case of a thiophene molecule. This is due to the fact that the polymer is flexible enough to optimize its morphology on the ZnO edge. The analysis carried out at zero temperature shows that the P3HT chain is preferably aligned to the nanoneedle axis. Nevertheless, in real samples, several factors, such as solvent, thermal fluctuations, or synthesis procedures, can induce wrapped configurations. To understand the stability of these configurations, we study the nanoneedleP3HT hybrids during annealings at T = 300 K and T = 500 K. We find that the polymer morphology does not sizably evolve even after a 5 ns long annealing. We never observe a fully unwrapping phenomena both at room temperature or at T = 500 K. This is in contrast with the case of P3HT on SWCNTs,26 which is another important example of an electron acceptor in hybrid solar cells. In Figure 5, we compare the unwrapping process of the same polymer on a SWCNT (top right) or on a ZnO nanoneedle (top left) of similar diameter: the two systems only differ for the chemistry of the nanostructures. After 50 ps, the polymer completely unwraps from the SWCNT (bottom right), while the chains remain wrapped around the ZnO nanoneedle even after 7 ns (bottom left). It is important to remark here that the polymer adhesion on a nanoneedle (0.5 eV/ monomer) is not much larger than in the case of a SWCNT (0.4 eV/monomer). It is just the energy barrier for the polymer diffusion on the surface and for the unwrapping that is higher for the nanoneedle case. This is due to the different nature of the polymer interaction with the ZnO surface, including electrostatic forces not present in the SWCNT case.
’ CONCLUSIONS In conclusion, by molecular dynamics simulations, we analyzed the P3HT adhesion on zinc oxde nanoneedles for different polymer configurations around the nanoneedle. We showed that the binding energy is a function of the polymer coiling angle and it depends on the crystallography of the substrate onto which the 16836
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’ ASSOCIATED CONTENT
bS
Supporting Information. Atomic partial charges and Buckingham potential parameters can be found in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (C.C.), alessandro.mattoni@ dsf.unica.it (A.M.).
’ ACKNOWLEDGMENT This work has been funded by the Italian Institute of Technology (IIT) under Project SEED “POLYPHEMO” and Regione Autonoma della Sardegna under Project “Design di nanomateriali ibridi organici/inorganici per l0 energia fotovoltaica” L.R.7/ 2007. We acknowledge computational support by CYBERSAR (Cagliari, Italy), CINECA (Casalecchio di Reno, Italy), and CASPUR (Rome, Italy).
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