Design Strategy for the Molecular Functionalization of Semiconductor

Subscriber access provided by UNIV OF DURHAM

Article

Design Strategy for the Molecular Functionalization of Semiconductor Photoelectrodes: A Case Study of p-Si(111) Photocathodes for H2 Generation Ashwathi Iyer, Kara Kearney, Shohei Wakayama, Hirotoshi Odoi, and Elif Ertekin Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03948 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 15, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31 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

Langmuir

Design Strategy for the Molecular Functionalization of Semiconductor Photoelectrodes: A Case Study of p-Si(111) Photocathodes for H2 Generation Ashwathi Iyer,†,‡ Kara Kearney,¶,‡ Shohei Wakayama,§ Hirotoshi Odoi,§ and Elif Ertekin∗,¶,‡ †Department of Physics, University of Illinois, 1110 W Green Street, Urbana, IL 61801, USA ‡International Institute for Carbon Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan ¶Department of Mechanical Science and Engineering, University of Illinois, 1204 West Green Street, Urbana, IL 61801, USA §Department of Mechanical and Aerospace Engineering, Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan E-mail: [email protected] Phone: 217-333-8175 Abstract Functionalization of semiconductor photoelectrodes is actively pursued as an approach to improve the efficiency of photoelectrochemical reactions by modulating the semiconductor’s barrier height, but the selection of molecules for functionalization re-

1

ACS Paragon Plus Environment

Langmuir 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

mains largely empirical. We propose a simple but effective design strategy for the organic functionalization of photocathodes for high-efficiency hydrogen generation based on first-principles density functional theory (DFT) calculations. The surface dipole of the functionalized photocathode determines its barrier height, which can be optimized to enhance charge separation at the semiconductor-electrolyte interface. Focusing on p-Si(111) photocathodes functionalized with different mixed aryl/methyl monolayers, we use DFT to systematically investigate the effect of - the presence and distribution of pi bonds, the binding atom (the atom in the functional group that bonds with the Si surface), functional group length, and electrophilic substituent groups - on the surface dipole and charge rearrangement at the functionalized surface. We find that the most important factors affecting the surface dipole are the intrinsic molecular dipole moment of the organic moiety, the presence of electrophilic substituent groups, and the binding atom. Using these findings, a three-step design strategy is proposed for the experimental realization of high-performing functionalized p-Si(111) photocathodes by maximizing the surface dipole.

1

Introduction Achieving high efficiency photoelectrochemical hydrogen evolution (e.g., 2H+ + 2e− →

H2 ) critically depends on precise control of the electronic properties of the semiconductor photocathode. 1 In particular, the barrier height at the semiconductor-electrolyte interface is an important determinant of charge separation at the interface and hence, the efficiency of hydrogen evolution. Functionalization of photoelectrode surfaces with organic molecules is an established technique to increase the barrier height 2,3 while maintaining the stability of the photoelectrode surface. 4 Functionalization produces a surface dipole that generates a built-in electric field, which in turn modulates the barrier height. 5 Focusing on functionalized p-Si(111) photocathodes for the hydrogen evolution reaction (HER), this paper identifies design rules for choosing organic functional groups that max-

2


Page 2 of 31

Page 3 of 31 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

Langmuir

imize the barrier height at the photocathode surface. Previous studies have shown that in the case of functionalized Si photoelectrodes, the barrier height is determined primarily by the surface dipole. 6,7 Therefore, the problem of finding appropriate functional groups can be reformulated in terms of the surface dipole that they produce. In the case of p-Si(111) photocathodes, a positive surface dipole shifts the valence and conduction bands in the semiconductor bulk upwards, increasing the downward band bending at the surface and hence, the barrier for holes to flow from the photocathode into the electrolyte (see Supporting Information (SI)). 2,3,6 Several organic functional groups have been experimentally fabricated on Si(111) and the resulting barrier height has been shown to be sensitive to the functionalization. 2,8–10 Functional groups forming a silicon-carbon bond (Si-C) such as mixed methyl/aldehyde, 8 alkyl chain, 9 and aryl moieties 2,10 are the most commonly studied. In particular, incorporating aryl moieties into the stable methylated p-Si(111) surface (Si-CH3 ) has been shown to lead to a large, positive surface dipole. 2,10 Functional groups with binding atoms other than carbon have also been shown to result in a stable silicon surface, 11–14 but experimental measurements of their surface dipoles are not extensive. Computational studies involving DFT calculations have been used to study the surface bonding chemistry, adsorption energies, and surface dipoles of a variety of surface functionalizations. 15,16 Galli and co-workers have used DFT and many-body perturbation theory (MBPT) to calculate the surface dipole of -H, -CH3 , and halide terminations on Si(111), showing that while DFT does not accurately predict the absolute values of dipoles, it robustly predicts trends in relative surface dipoles, for example, with respect to the well-characterized Si-H surface. 17–19 In particular, relative surface dipoles calculated using DFT-LDA and the more accurate G0 W0 approach (MBPT) were shown to agree. Device-scale modeling has also been used to study the properties of functionalized silicon photoelectrodes. 6 Recently, Ertekin and co-workers used a multi-scale approach by combining DFT and device-modeling to predict the surface dipoles and open-circuit voltages due to a variety of mixed methyl/aryl

3


Langmuir 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

monolayers on Si(111) and identified a linear correlation between the surface dipole and the dipole moment of the corresponding isolated molecule. 3 They also showed that the opencircuit voltage, which is a metric of device efficiency, is a function of the surface dipole. 3 Despite the rich computational and experimental literature on functionalized Si(111) surfaces, there are no comprehensive studies on design rules for maximizing the surface dipole. The focus of this paper therefore, is to identify design rules for stable functional groups on p-Si(111) photocathodes that result in a large, positive surface dipole. The configurational phase space for choosing organic functional groups that satisfy this design criterion is large. Here, we systematically study the dependence of the surface dipole on the following functional group properties: 1. Positions and types of pi bonds in the functional group structure 2. Binding atom (the atom in the functional group that bonds with the Si surface) 3. Functional group length and correspondingly the distance of the main electrophilic or nucleophilic group from the Si(111) surface 4. Electrophilic substituent groups Even though these represent but a fraction of the variations in functional groups that one can consider, our results point to general design rules, which are then used to reverse-engineer a functionalization with the highest surface dipole that we have calculated to date. Scheme 1 shows our proposed approach for designing high-performing surface functionalizations for p-Si(111) HER photocathodes. The simple and clear design strategy is a step towards the experimental realization of optimal device performance by maximizing the surface dipole. This is important because of the difficulty, time, and cost involved in fabricating new functional groups. While the focus of this paper is on functionalized photocathodes, the design strategy proposed here is more generally applicable to applications where surface dipoles play an important role, such as chemical sensing, 20,21 water remediation, 22,23 and CO2 capture. 24 4


Page 4 of 31

Page 5 of 31 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

Langmuir

2

Computational Methods We performed first-principles density functional theory 25,26 (DFT) calculations of func-

tionalized Si(111) photocathodes using VASP. 27–31 The generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) 32 was used to approximate the exchangecorrelation functional and PAW pseudopotentials 33 were used to eliminate core electrons from the simulation. A plane-wave cutoff of 480 eV, an 8 × 2 × 1 mesh for sampling k-space, and a Gaussian smearing of 0.02 eV were used. All structures were relaxed until the forces on each atom were less than 0.01 eV/Å and the energy difference between consecutive steps was less than 10−5 eV. Since DFT-PBE accurately reproduces trends in relative surface dipoles, 19 all surface dipoles are reported here with respect to the Si-H surface. In several cases, we compared our PBE results to those obtained using the non-local Van der Waals functional of Dion et al , 34 but found that while the absolute value of surface dipole shifts significantly, the surface dipole relative to Si-H only slightly increases (by 0.005-0.008 eV), with trends between different functional groups remaining the same. Bader charge analysis 35 is used to investigate the charge rearrangement at the functionalized surface. The net charge on an atom is calculated as the difference between the valence charge of the atom and the computed Bader charge. Thus a positive (negative) net charge indicates electron loss (gain). Functionalized silicon slabs were modeled as 1 × 4 periodic supercells consisting of 16 layers of Si atoms separated in the (111)-direction by a 12 Å-thick vacuum layer (see SI). The supercells are symmetrically terminated by functional groups in order to avoid spurious dipoles in the vacuum region. Unless otherwise specified, all slabs are functionalized at 25% coverage of the functional group. Following Ref., 3 the attachment geometry was chosen such that the plane of the benzene ring lies along the (111) direction. Methyl groups were used to terminate the remaining atop sites (at 75% coverage) to mimic experimental conditions, where unterminated Si surface atoms are methylated to prevent oxide formation. 10 A surface coverage of 25% was chosen in part because experimental coverages of mixed methyl/aryl monolayers have been observed at 1-50% coverage. 10 Moreover, trends in the surface dipole 5


Langmuir 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

Page 6 of 31

across functional groups are not affected by the exact value of the surface coverage chosen as long as it is in the range of 1-50%. 3 Figure 1 shows a schematic of a Si(111) supercell symmetrically terminated by 25% (1-fluoromethyl)phenyl and 75% methyl groups, where the dashed line indicates a single supercell. The computed lattice constant of Si was 3.867 Å, which is in good agreement with the experimental lattice constant (3.840 Å). The surface dipole of the functionalized surface is a combined effect of the charge rearrangement at the surface and the intrinsic molecular dipole moment of the functional group, 19 and is defined as a vector pointing from a positive to a negative charge. The surface dipole is related to a surface potential shift (an experimentally measurable quantity 2,3 ) via the Helmholtz equation, ∆Vsurf =

4πµsurf A

(1)

where µsurf is the surface dipole in the direction of the surface normal ((111)) and A is the surface area of the supercell (see SI). The surface potential shift thus calculated can be approximated as a modulation in the barrier height of the functionalized photocathode 3 (SI). The surface dipoles reported in this work are calculated using a rigorous method called "nanosmoothing" 36 (see SI). Due to the relationship in Equation 1, all surface dipoles are reported in eV. It is important to note that in this work, we have neglected the effects of the water environmnet on the calculated surface dipoles, which is a limitation of the results presented. This is expected to have a significant effect on the absolute value of the surface dipole calculated. 37–40 For instance, Galli and co-workers previously used ab initio molecular dynamics simulations to compare the surface dipoles of functionalized Si in contact with vacuum and with water. 37 They found that the presence of water consistently shifts the band edges by 0.5 eV for Si-H, Si-CH3 , and Si-CF3 . Also in previous work, 3 we observed that for aryl groups, irrespective of the surface termination, there is a systematic offset of 0.5-0.6 eV between theoretical (DFT, vacuum contact) and experimental (Mott-Schottky, water contact) surface dipoles. We therefore suggested that to some extent, the systematic discrepancy between 6


Page 7 of 31 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

Langmuir

theoretical and experimental surface dipoles may be attributed to the absence of a water environment in our simulations. 3 While it is essential to simulate the water environment to accurately predict absolute surface dipoles, relative values of the dipoles can be reasonably established without the electrolyte present. Because the aim of this work is to propose design principles for obtaining the largest possible surface dipole, with particular focus on aryl groups, the overall conclusions of the study are not expected to change in the presence of a water environment.

3

Results and Discussion To begin, in earlier work, 3 we observed an empirical trend indicating a nearly linear

correlation between the surface dipole of the functionalized surface (µsurf ) and the molecular dipole moment of the corresponding isolated molecule (µmol ). Figure 2 shows this trend again, now expanded to include a more comprehensive set of 30 functionalized surfaces. Also shown is a linear fit between µsurf and µmol adapted from Ref. 3 Compared to our earlier work, the trend remains reasonable, but the expanded dataset shows more deviations. Understanding the key functional group features, in addition to µmol , that determine the surface dipole is critical to being able to engineer high-performing functionalizations. To this end, we consider a subset of these functional groups and investigate the impact of pi bonds, binding atom, functional group length, and electrophilic substituent groups. All the functionalized surfaces we considered are stable, with formation energies in the range of -0.28 eV/site and -0.85 eV/site (SI).

3.1

Pi Bonds

The presence of pi bonds in a functional group could influence the charge rearrangement with the silicon substrate. Aryl groups are a common choice for surface functionalization, 2,10,41 but whether the pi bonds in the ring structure lead to an enhancement of the

7


Langmuir 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

surface dipole is not well-understood. In the case of mixed monolayers, pi bonds could also affect the charge rearrangement between different surface moieties. For instance, neighboring methyl groups were found to interact with a recently fabricated high-performing functional group, trifluorophenylacetynyl (TFPA), 10 via pi-hydrogen bonding 42 such that the dipoles induced by this interaction increased the net surface dipole. The effect on the surface dipole of pi bonds in aryl groups, the pi bond type (double or triple), and position relative to the Si surface are analyzed.

3.1.1

Pi Bonds in the Benzene Ring

To understand the effect of the benzene ring, the surface dipole and charge rearrangement due to trifluoromethyl (-CF3 ) and (trifluoromethyl)phenyl (-CF3 Ph) functional groups (Figure 3) are compared. The surface dipoles of Si-CF3 and Si-CF3 Ph are -0.053 eV and 0.504 eV respectively, suggesting that the presence of the aryl group could lead to an enhanced surface dipole. To understand the cause for this difference, net charges on the functionalized systems considered (Figure 3). Interestingly, the bond dipole between Si and the binding C atom flips its sign in Si-CF3 compared to Si-CF3 Ph (red arrows in Figure 3). This is due to the highly electronegative F atoms. In Si-CF3 , each F takes around 1 electron from the C, leaving it with a net charge of 1.65. To allow for favorable bonding with the electron-poor C, the surface Si becomes negatively charged by accepting electrons from neighboring subsurface Si atoms. In Si-CF3 Ph, as in Si-CF3 , each F has a net charge of -0.90, giving rise to an electron-poor C atom, but with a higher net charge of 2.67. Whereas the positive charge on the carbon is compensated by the Si substrate in Si-CF3 , it is not compensated by the ring in Si-CF3 Ph, which shows negligible charge rearrangement. For example, the C at the para-position of the ring, which corresponds to the surface Si atom in Si-CF3 , is nearly charge-neutral. The relative resistance of the benzene ring to a significant charge redistribution is due to its energetically favorable conjugated structure, and the larger bond energies of C=C and

8


Page 8 of 31

Page 9 of 31 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

Langmuir

C-C bonds, 602 kJ/mol and 346 kJ/mol, compared to the Si-Si covalent bond energy, 222 kJ/mol. The interaction dipoles calculated following Ref., 42 of Si-CF3 and Si-CF3 Ph surfaces are 0.09 eV and 0.11 eV respectively, suggesting that the interaction between the aryl group and the neighboring methyl groups does not lead to an appreciable increase in the surface dipole compared to the Si-CF3 surface. Overall, our analysis suggests that the desirable performance of aryl groups, which has been empirically established, arises from the stability of the benzene ring and its relative resistance to significant charge redistributions when bonding with silicon or highly electrophilic substituent groups (-F in this case), rather than from chemical interactions involving the pi bonds. In addition, aryl groups tend to bond to the Si surface with the plane of the benzene ring nearly perpendicular to the surface, 3 thus maximizing the component surface dipole along the (111)-direction. Having theoretically established the favorable effect aryl groups on the surface dipole, the discussion in the rest of the paper focuses on aryl functional groups.

3.1.2

Pi Bond Type and Position

The trifluorophenylacetynyl (TFPA) molecule is used as a starting point for our analysis. Figure 4(a) shows schematics of four molecules labeled for simplicity as TFPA, TFPE (trifluorophenylethylene), A (3-trifluorophenyl-1-propyne), and B (3-trifluorophenyl-2-propyne). These molecules attach to the Si surface through the carbon atom at the lower end. Molecules A, B, and TFPE are realized by modifying TFPA to obtain structures having a different pi bond type or position. TFPA consists of a triple bond in the C-C chain attached to the benzene ring. TFPE has a double bond in place of the triple bond. Molecules A and B have one extra carbon atom in the chain with a triple bond at different positions: closer to the Si surface in A and farther away in B. The carbon atoms forming the pi bond in each molecule are shaded in blue in Figure 4(a). Bader charge analysis is used to understand the effect of changing the type or position of the pi bonds on µmol , the dipole moment of the isolated molecule (x -axis of Figure 4(b)).

9


Langmuir 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

Figure 4(a) also shows the Bader charges on the carbon atoms involved in the pi bond, and the most significant contributions to µmol , denoted by green and red arrows, which represent positive and negative contributions respectively. In all four systems, the net charge in the C-F bonds is nearly equal, suggesting that changing the nature of the pi bond does not affect the bonding of the fluorine atoms to the ring. The net charges shown in Figure 4(a) show that the dipole due to the triple bond in TFPA is opposite to that of the C-F bond dipoles, leading to a net reduction in µmol , whereas in TFPE, the dipole due to the double bond is negligible, leading to an enhanced µmol . The difference in the µmol values of A and B similarly arise from different charge distributions on the chain. The dipole contribution of the pi bond opposes (points along) the C-F bond dipole in A (B). From a design standpoint, modulating the position and type of the pi bonds in the functional group such that the internal dipoles do not oppose each other can lead to an enhancement of µmol and correspondingly, the surface dipole. Figure 4(b) shows the µsurf due to each functionalization as a function of µmol . The surface dipoles largely follow the molecular dipole moments, except for a small discrepancy for TFPA and A, caused by the molecular plane of A being tilted away from the (111)-direction. As a general rule, even upon attachment to the Si surface, singly- and doubly-bonded carbon atoms, to some extent, maintain a tetrahedral and trigonal planar geometry respectively. Therefore, a binding carbon atom in a benzene ring or a triple bond (eg. TFPA) is less likely to induce tilting away from the (111)-direction. Modulating the pi bond type and position as shown tunes µsurf over a range of 0.33 eV. The interaction dipole with neighboring methyl groups due to each functional group is in the range of 0.1-0.2 eV, in good agreement with previous results. 42

3.2

Binding Atom at the Si Surface

The effect of changing the atom directly attached to the silicon surface (referred to as the ’binding atom’) to species other than carbon is studied by comparing the surface dipoles due 10


Page 10 of 31

Page 11 of 31 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

Langmuir

to Si-Toluene, Si-Phenol, Si-Thiophenol, and Si-Aniline, which respectively form an Si-C, SiO, Si-S, and Si-N bond at the silicon surface. Each molecule has the same ring structure and differs only in the binding atom type. A schematic of the functionalized surfaces is shown in Figure 5(a) and the µsurf values of the surfaces are plotted in Figure 5(b). Modifying the binding atom while keeping the rest of the functional group intact offers tunability of the surface dipole over a range 0.46 eV, the most significant enhancement considered yet. The surface dipoles show a weak dependence on the binding atom’s electronegativity, but the correlation, unsurprisingly, is not perfect. For instance Si-Aniline has a more positive surface dipole than Si-Phenol even though nitrogen has a lower electronegativity than oxygen. This discrepancy has been observed in other systems as well 15 and can be understood from the Bader charge analysis of the functionalized systems. Regardless of the binding atom, the surface Si always loses about one electron. 42 This could be due to the stable nature of the Si-Si covalent bonds, which puts a limit on the amount of charge that Si can lose. However, the net charges on the binding atoms (Figure 5(b), inset) show a dependence on the binding atom’s valency. For highly electronegative binding atoms (S, O, and N), the net charge is roughly equal to the number of electrons needed to satisfy the octet rule. Nitrogen, which needs 3 electrons has the largest net charge, leading to the largest surface dipole for Si-Aniline. This suggests that a binding atom with a high electronegativity and the ability to accumulate a large net charge should be chosen. We note that the µsurf trends shown in Figure 5(b) cannot be obtained by considering only the dipole moments of the isolated molecules. For example, the molecular dipole moments of phenol and toluene are 0.06 D and 0.43 D respectively, but Si-Phenol has a more positive surface dipole than Si-Toluene. Our hypothesis that the enhanced surface dipole is due to an increase in net charge in the vicinity of the Si surface has been indirectly validated by experimental contact angle measurements on Si-H and Si-OCH3 surfaces, which show that Si-OCH3 surfaces are more hydrophilic, implying that they have a larger surface charge density. 43

11


Langmuir 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

3.3

Functional Group Length

Earlier studies have suggested that functional group length is an important lever for tuning the electron transfer properties at the Si/organic moiety interface, 41 and possibly also the surface dipole. We consider the effect of functional group length on the surface dipole of Si(111) surfaces functionalized with (n-fluoroalkyl)phenyl groups, where n is the number of carbon atoms in the alkyl chain. The alkyl chain is at the para-position on the ring. Figure 6(a) shows a side view of the molecular structures of (3-fluoropropyl)benzene (left) and (4-fluorobutyl)benzene (right) with the benzene ring enclosed in brackets. Figure 6(b) shows the computed µmol of these molecules (Debye (D)) as a function of the alkyl chain length in angstrom. The number of carbon atoms in the alkyl chain is shown next to each data point. The µmol values show an odd-even oscillatory pattern, with the alkyl chains with an even number of carbon atoms exhibiting a higher µmol . The oscillatory behavior arises from the alternating direction of the carbon-fluorine (C-F) bond (Figure 6(a)). For even-numbered chains, the C-F bond is almost parallel to the plane of the benzene ring, whereas for odd-numbered chains, the bond has a large component perpendicular to the plane of the ring. Figure 6(c) shows the µsurf of the functionalized systems, as a function of alkyl chain length. From Figures 6(b,c), the alkyl chains don’t demonstrate a strong correlation between µmol and µsurf . Although µsurf follows µmol for shorter chain lengths (1-3), the odd-even oscillation in µsurf is significantly suppressed in alkyl chains having 4 or more carbon atoms. This behavior is best explained in a heuristic manner by considering the main contributions to the surface dipole. For each functional group, it is possible to divide the net surface dipole moment into contributions arising from three primary regions: the charge distributions on the benzene ring and the Si substrate, the C-F bond, and the chain itself. Figure 6(d) shows the relaxed geometry of Si-(5-fluoropentyl)phenyl with these three regions indicated. Their contributions to the total surface dipole can be obtained in a qualitative manner by considering the net charges and positions of each atom. By analyzing these charges (see SI), 12


Page 12 of 31

Page 13 of 31 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

Langmuir

we are able to make the following observations: (a) The local dipoles due to the Si-C bond at the Si surface and the C-F bond are independent of the alkyl chain length, and together, always contribute 2.7-2.9 eÅ to the surface dipole. (b) The directionality of the C-F bond does not dramatically affect µsurf as it does µmol because the contribution of the C-F bond to the surface dipole (0.75-0.78 eÅ) is much smaller than that of the C-Si bond (1.7-1.8 eÅ), leading to a suppression of the odd-even oscillation. (c) The charge distribution on the alkyl chain is length-dependent, but for chains with n > 3, the contribution to the surface dipole is small (-0.27 to -0.3 eÅ). However, the value is larger and more length-sensitive for n = 1, 2, 3, which is the main cause for the larger variations in µsurf for these functional groups. The odd-even variation in the surface dipole has been observed on SAM/metal systems 44 and indicates that this effect can be used to tune the barrier height. However, our results suggest that the local dipoles between Si and C at the silicon surface, and the substituent group (-F in this case) and the rest of the functional group should have a more significant effect on µsurf . The effect of modulating the substituent group will be briefly analyzed in the next section.

3.4

Substituent Group

An electrophilic or nucleophilic substituent group on the benzene ring can modulate the molecular dipole moment of the isolated molecule. Correspondingly, its effect on the surface dipole is also significant. Whereas we show that the dipole between the binding atom and the silicon surface accounts for the largest contribution to the net surface dipole, the dipole between the substituent group and the benzene ring offers the greatest tunability of the surface dipole. To demonstrate this, we modify the substituent groups on two functionalizations considered previously in this paper, TFPA and (2-fluoroethyl)phenyl, and compare the resulting µsurf values. In both cases, all the -F groups are replaced by a single -NO2 group at the para position of the ring, as demonstrated in Figure 7, which also shows the µsurf of each functionalization. The more electronegative -NO2 group leads to an increase in µsurf of up 13


Langmuir 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

to 1 eV, the largest tunability range achieved from modulating functional group properties. Using a nucleophilic substituent group on the other hand, would lead to a decrease in µsurf .

4

Design Rules in Practice The configurational phase space of surface functionalizations is large, but our results sug-

gest that the complexity can be reduced by focusing on the most significant contributions to the surface dipole. Table 1 summarizes the range of tunability offered by modulating the different functional group properties investigated in this work. The tunability due to functional group length is considered only for chains with 3 or more carbon atoms since the µmol is a good descriptor of the surface dipole for shorter chains. While the exact range of tunability depends on the specific functional groups considered, the results in Table 1 clearly show that the most important functional group properties for achieving a large surface dipole are electrophilic substituent groups, the presence of an aryl group, and the binding atom. Based on this, we propose a design strategy, shown in Scheme 1, primarily applicable to aryl moieties. Across all the systems considered here, the molecular dipole moment of the isolated molecule is still the most significant determinant of the surface dipole, but the linear correlation breaks down for complicated molecules (Figure 2). Hence, when designing surface functionalizations for optimal performance, the first step is to screen for molecules with a large, positive molecular dipole moment. Molecular dipole moment data is widely available. 45 In the example shown in Scheme 1, chlorobenzene, which has a molecular dipole moment of 1.607 D is chosen. The next step is to choose an electrophilic, but stable substituent group at the para and/or meta positions of the benzene ring to further tune the surface dipole. The fluorine atom at the para position is replaced by the more electrophilic NO2 group. Finally, carbon is replaced with a binding atom that has a higher electronegativity and ability to accumulate a higher net charge. In Schematic 1, an oxygen binding atom is used, to obtain

14


Page 14 of 31

Page 15 of 31 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

Langmuir

nitrophenol. The surface dipole of Si-nitrophenol relative to Si-H is 1.849 eV, the highest that we have calculated to date.

5

Conclusions In conclusion, the effects of - aryl groups, pi bond position and type, binding atom,

functional group length, and substituent group - on the surface dipole of functionalized Si(111) have been analyzed. We find that: 1. Despite the large configurational phase space of feasible functionalizations, the surface dipole of functionalized Si(111) is primarily determined by three properties - intrinsic molecular dipole moment, electrophilic substituent groups, and the binding atom. 2. Other functional group properties such as length or the distribution of pi bonds can be tweaked to fine-tune the surface dipole, but our analysis suggests that these effects will be relatively small. The proposed design strategy is not limited to p-Si(111) HER photoelectrodes. Specific design criteria (eg. sign of desired surface dipole) will be different depending on the dopant type and/or reaction, but the framework and analysis presented here are applicable.

15


Langmuir 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

6

Page 16 of 31

Floats Identify molecules with a high dipole moment

Replace substituent group with a more electrophilic group O

Cl

N

O

-

X=C Chlorobenzene

Replace the binding atom, A, so that the EN difference between Si and A is increased O

X=O

Nitrobenzene

N

O

OH

Nitrophenol

Scheme 1: Design strategy for surface functional groups with a large, positive surface dipole.

16


(-) (+)

(111)

Vacuum

Figure 1: Schematic of a 1 × 4 Si(111) supercell functionalized with 25% (1fluoromethyl)phenyl and 75% methyl groups on both ends. The black dashed line indicates a single supercell. Dark blue, brown, light blue, and magenta atoms represent silicon, carbon, fluorine, and hydrogen respectively. The red arrow shows the direction of a positive surface dipole.

17

Langmuir

NitroPhenol-O

1.5

μsurfΔV wrtsurf Si-H (eV) (eV)

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

Page 18 of 31

□

NitroPA □

● NitroEtPh

TFPE ◆ ○B TFPA ▲ TFPEt FPePh ■▼A ◆CF3Ph FHepPh FEtPh ◇ ■ Pyridine CF3 ▽ ▼ ○ □◆FPrPh ▽ ◇▲ FBuPh △ FNonPh FMePh FHexPh ● -0.5 ○ FOctPh Phenol-O ▽ Thphenol-S ▼ ■ FDecPh ◇ △Ph EtPh-R Aniline-N ▲ EtPh-Br● Phenol-C DiHPyridine-N DiHPyridine-C

0.5

-1.5 △

-1

0

1

2

(D) μμmol mol (D)

3

4

5

Figure 2: Dependence of µsurf (eV) on µmol (Debye) for a set of 30 functional groups. The black dotted line is the linear fit between µsurf and µmol , adapted from Ref. 3 (R2 = 0.91). See SI for IUPAC names corresponding to the labels in this figure.

18


Page 19 of 31 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

Langmuir

-0.9 2.67

-0.04 -0.9 1.65

-1.08

-0.15

1.00

Figure 3: Relaxed structures of Si-CF3 (left) and Si-CF3 Ph (right). Dark blue, brown, light pink, and light blue atoms represent Si, C, H, and F respectively. Net charges on the surface Si atom, the C atom, and a single F atom are shown on the left. On the right, net charges on the surface Si, the C at the 1- and 4-positions on the ring, the C bonded to the F atoms, and a single F atoms are shown. The red arrows denote the orientations of the Si-C bond dipole at the silicon surface.

19


Langmuir

a)

TFPA

TFPE

-0.04

2.28 -2.68

0.02 B

A

2.29 -2.62

2.35 -2.68 b)

Surface potential μsurf wrt Si-H (eV) shift wrt SiH (eV)

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

Page 20 of 31

B

0.70

TFPE 0.60 0.50

TFPA

0.40 2.4

A 2.6

2.8

3.0

3.2

3.4

3.6

3.8

Molecular dipole moment (D) μmol (D)

Figure 4: a) Relaxed molecular structures of TFPA, TFPE (trifluorophenylethylene), A (3-trifluorophenyl-1-propyne), and B (1-trifluorophenyl-1-propyne). Brown, magenta, and light blue atoms represent carbon, hydrogen, and fluorine respectively. The net charges on carbon atoms forming the pi bond (shaded in blue) are shown next to the respective atoms. The green and red arrows schematically represent the most significant contributions to the molecular dipole moment. b) Plot of µsurf (eV) vs. µmol (Debye) for TFPA, TFPE, A, and B.

20


Page 21 of 31

a)

X

CH2, S, O, or NH

Si

Si-Toluene Si-Thiophenol Si-Phenol Si-Aniline

SH b)

Binding Atom (X) C

S

O

μ wrt Si-H (eV) surf Si-H wrt (eV)

0.0 0.0

Surface potential shift wrtpotential Si-H (eV)shift Surface

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

Langmuir

N

-0.2 -0.2 -0.4 -0.4 -0.6 -0.6 C: -0.98 S: -1.78 O: -1.89 N: -3.02

-0.8 -0.8 -1.0 -1.0

Binding Atom Binding Atom Figure 5: a) Schematic of functionalized surfaces with different binding atoms. The atom "X" denotes CH2 , S, O, or NH binding groups corresponding to Si-Toluene, Si-Thiophenol, Si-Phenol, and Si-Aniline surfaces respectively. b) The y-axis shows the µsurf (eV) of Si surfaces functionalized with toluene (red), thiphenol (blue), phenol (magenta), and aniline (orange). Inset: Net charges on the binding atoms.

21


Langmuir

a)

z

Odd

Even b) Molecular dipole μmol (D) moment (D)

2.0

(2)

(4)

(6)

(3)

(5)

(7)

8

10

12

(8) (10)

1.5 1.0

(1)

0.5 0.0

Surface Surface potential potential shift shift

c)

μsurf wrt Si-H (eV) wrt wrt SiH SiH (eV) (eV)

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

Page 22 of 31

1.0 1.0

6

(9) 14

Length of molecule (Å)

0.5 0.5

(2) (4)

0.0 0.0

-0.5 -0.5

(1) 66

(3)

(5)

88

10 10

(6)

(8) (10)

(7) 12 12

(9) 14 14

Lengthofofalkyl alkylchain chain(Å) (Å) Length Alkyl chain length (Å)

d) ~ -0.285 eÅ

Ring + C-F + C-Si ~ 2.8 eÅ

Figure 6: a) Side view of the relaxed structures of (3-fluoropropyl)benzene (left) and (4fluorobutyl)benzene (right). The position of the benzene ring is indicated by brackets. Brown, magenta, and light blue atoms represent carbon, hydrogen, and fluorine respectively. b) Plot of µmol (Debye) of isolated molecules as a function of the alkyl chain’s length (Å). The numbers next to each data point indicate the number of carbon atoms in the alkyl chain. c) Plot of the µsurf (eV) of the functionalized surface as a function of the alkyl chain length (Å). d) Relaxed structure of the Si-(5-fluoropentyl)phenyl surface. The regions shaded in blue indicate contributions to the surface dipole that are independent of the alkyl chain length. The contribution of the alkyl chain’s charge distribution to the surface dipole is also shown. Dark blue atoms represent silicon.

22


Page 23 of 31

O

F

O

N

-

F

F

+0.99 eV TFPA 0.46 eV

NitroPhenylAcetynyl 1.45 eV

H

H

O

H

H

F

F

N -

O

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

Langmuir

H

H

+0.82 eV

(2-fluoroethyl)phenyl 0.13 eV

(2-nitroethyl)phenyl 0.95 eV

Figure 7: Schematics of TFPA, (2-fluoroethyl)phenyl, NitroPhenylAcetynyl, and (2nitroethyl)phenyl functional groups. The latter two are obtained by replacing the -F atoms in the former two with a single -NO2 substituent group at the para-position on the ring. The µsurf relative to Si-H (eV) due to each functionalization is shown next to the corresponding schematic.

23


Langmuir 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

Page 24 of 31

Functional Group Property

Range of Tunability

Functional group length

0.1 eV

Interaction dipole

0.1-0.2 eV

Position/type of pi bond

0.33 eV

Binding atom

0.46 eV

Presence of aryl group

0.56 eV

Substituent group

1 eV

Table 1: The range of tunability of µsurf relative to Si-H offered by the modulation of the functional groups properties considered in this work.

Acknowledgement This work was supported by the International Institute for Carbon Neutral Energy Research (WPI-I2CNER), sponsored by the Japanese Ministry of Education, Culture, Sports, Science and Technology and by the National Science Foundation under Grant No. 1545907.

Supporting Information Available The Supporting Information is available free of charge on the ACS website. Theoretical background, computational details, IUPAC names of functional groups, Bader charge analysis of Si-(5fluoropentyl)Ph, and formation energies.

24


Page 25 of 31 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

Langmuir

References (1) Li, Z.; Luo, W.; Zhang, M.; Feng, J.; Zou, Z. Photoelectrochemical cells for solar hydrogen production: current state of promising photoelectrodes, methods to improve their properties, and outlook. Energy Environ. Sci. 2013, 6, 347–370. (2) Seo, J.; Kim, H. J.; Pekarek, R. T.; Rose, M. J. Hybrid Organic/Inorganic Band-Edge Modulation of p-Si(111) Photoelectrodes: Effects of R, Metal Oxide, and Pt on H2 Generation. Journal of the American Chemical Society 2015, 137, 3173–3176, PMID: 25716423. (3) Kearney, K.; Iyer, A.; Rockett, A.; Staykov, A.; Ertekin, E. Multiscale Computational Design of Functionalized Photocathodes for H2 Generation. Journal of the American Chemical Society 2018, 140, 50–53, PMID: 29271201. (4) Bansal, A.; Li, X.; Lauermann, I.; Lewis, N. S.; Yi, S. I.; Weinberg, W. H. Alkylation of Si Surfaces Using a Two-Step Halogenation/Grignard Route. Journal of the American Chemical Society 1996, 118, 7225–7226. (5) Gleason-Rohrer, D. C.; Brunschwig, B. S.; Lewis, N. S. Measurement of the Band Bending and Surface Dipole at Chemically Functionalized Si(111)/Vacuum Interfaces. The Journal of Physical Chemistry C 2013, 117, 18031–18042. (6) Kearney, K. L.; Rockett, A. A. Simulation of Charge Transport and Recombination across Functionalized Si(111) Photoelectrodes. Journal of The Electrochemical Society 2016, 163, H598–H604. (7) Grimm, R. L.; Bierman, M. J.; O?Leary, L. E.; Strandwitz, N. C.; Brunschwig, B. S.; Lewis, N. S. Comparison of the Photoelectrochemical Behavior of H-Terminated and Methyl-Terminated Si(111) Surfaces in Contact with a Series of One-Electron, OuterSphere Redox Couples in CH3CN. The Journal of Physical Chemistry C 2012, 116, 23569–23576. 25


Langmuir 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

(8) O?Leary, L. E.; Strandwitz, N. C.; Roske, C. W.; Pyo, S.; Brunschwig, B. S.; Lewis, N. S. Use of Mixed CH3?/HC(O)CH2CH2?Si(111) Functionality to Control Interfacial Chemical and Electronic Properties During the Atomic-Layer Deposition of Ultrathin Oxides on Si(111). The Journal of Physical Chemistry Letters 2015, 6, 722–726, PMID: 26262493. (9) Lavi, A.; Cohen, H.; Bendikov, T.; Vilan, A.; Cahen, D. Si-C-bound alkyl chains on oxide-free Si: towards versatile solution preparation of electronic transport quality monolayers. Phys. Chem. Chem. Phys. 2011, 13, 1293–1296. (10) Plymale, N. T.; Ramachandran, A. A.; Lim, A.; Brunschwig, B. S.; Lewis, N. S. Control of the Band-Edge Positions of Crystalline Si(111) by Surface Functionalization with 3,4,5-Trifluorophenylacetylenyl Moieties. The Journal of Physical Chemistry C 2016, 120, 14157–14169. (11) Gao, F.; Teplyakov, A. V. Dehydrohalogenation Condensation Reaction of Phenylhydrazine with Cl-Terminated Si(111) Surfaces. The Journal of Physical Chemistry C 2016, 120, 5539–5548. (12) Zhu, X.-Y.; Boiadjiev, V.; Mulder, J. A.; Hsung, R. P.; Major, R. C. Molecular Assemblies on Silicon Surfaces via Si?O Linkages. Langmuir 2000, 16, 6766–6772. (13) Hacker, C. A.; Anderson, K. A.; Richter, L. J.; Richter, C. A. Comparison of Si?O?C Interfacial Bonding of Alcohols and Aldehydes on Si(111) Formed from Dilute Solution with Ultraviolet Irradiation. Langmuir 2005, 21, 882–889, PMID: 15667163. (14) Hu, M.; Liu, F.; Buriak, J. M. Expanding the Repertoire of Molecular Linkages to Silicon: Si?S, Si?Se, and Si?Te Bonds. ACS Applied Materials & Interfaces 2016, 8, 11091–11099, PMID: 27055056. (15) Arefi, H. H.; Nolan, M.; Fagas, G. Binary functionalization of H:Si(111) surfaces by

26


Page 26 of 31

Page 27 of 31 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

Langmuir

alkyl monolayers with different linker atoms enhances monolayer stability and packing. Phys. Chem. Chem. Phys. 2016, 18, 12952–12963. (16) Arefi, H. H.; Nolan, M.; Fagas, G. Density Functional Theory with van der Waals Corrections Study of the Adsorption of Alkyl, Alkylthiol, Alkoxyl, and Amino-Alkyl Chains on the H:Si(111) Surface. Langmuir 2014, 30, 13255–13265, PMID: 25260071. (17) Li, Y.; Galli, G. Electronic and spectroscopic properties of the hydrogen-terminated Si(111) surface from ab initio calculations. Phys. Rev. B 2010, 82, 045321. (18) Aliano, A.; Li, Y.; Cicero, G.; Galli, G. Structural and Electronic Properties of the Methyl-Terminated Si(111) Surface. The Journal of Physical Chemistry C 2010, 114, 11898–11902. (19) Li, Y.; O?Leary, L. E.; Lewis, N. S.; Galli, G. Combined Theoretical and Experimental Study of Band-Edge Control of Si through Surface Functionalization. The Journal of Physical Chemistry C 2013, 117, 5188–5194. (20) Qi, P.; Vermesh, O.; Grecu, M.; Javey, A.; Wang, Q.; Dai, H.; Peng, S.; Cho, K. J. Toward Large Arrays of Multiplex Functionalized Carbon Nanotube Sensors for Highly Sensitive and Selective Molecular Detection. Nano Letters 2003, 3, 347–351. (21) Kong, J.; Chapline, M. G.; Dai, H. Functionalized Carbon Nanotubes for Molecular Hydrogen Sensors. Advanced Materials 2001, 13, 1384–1386. (22) Chandra, V.; Park, J.; Chun, Y.; Lee, J. W.; Hwang, I.-C.; Kim, K. S. WaterDispersible Magnetite-Reduced Graphene Oxide Composites for Arsenic Removal. ACS Nano 2010, 4, 3979–3986, PMID: 20552997. (23) Xu, J.; Wang, L.; Zhu, Y. Decontamination of Bisphenol A from Aqueous Solution by Graphene Adsorption. Langmuir 2012, 28, 8418–8425, PMID: 22571829.

27


Langmuir 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

(24) Mishra, A. K.; Ramaprabhu, S. Nanostructured polyaniline decorated graphene sheets for reversible CO2 capture. J. Mater. Chem. 2012, 22, 3708–3712. (25) Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys. Rev. 1964, 136, B864– B871. (26) Kohn, W.; Sham, L. J. Self-Consistent Equations Including Exchange and Correlation Effects. Phys. Rev. 1965, 140, A1133–A1138. (27) https://www.vasp.at. (28) Kresse, G.; Hafner, J. Ab initio. Phys. Rev. B 1993, 47, 558–561. (29) Kresse, G.; Hafner, J. Ab initio. Phys. Rev. B 1994, 49, 14251–14269. (30) Kresse, G.; Furthmüller, J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set. Phys. Rev. B 1996, 54, 11169–11186. (31) Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Computational Materials Science 1996, 6, 15 – 50. (32) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–3868. (33) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B 1994, 50, 17953–17979. (34) Dion, M.; Rydberg, H.; Schröder, E.; Langreth, D. C.; Lundqvist, B. I. Van der Waals Density Functional for General Geometries. Phys. Rev. Lett. 2004, 92, 246401. (35) Tang, W.; Sanville, E.; Henkelman, G. A grid-based Bader analysis algorithm without lattice bias. Journal of Physics: Condensed Matter 2009, 21, 084204.

28


Page 28 of 31

Page 29 of 31 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

Langmuir

(36) Junquera, J.; Cohen, M. H.; Rabe, K. M. Nanoscale smoothing and the analysis of interfacial charge and dipolar densities. Journal of Physics: Condensed Matter 2007, 19, 213203. (37) Pham, T. A.; Lee, D.; Schwegler, E.; Galli, G. Interfacial Effects on the Band Edges of Functionalized Si Surfaces in Liquid Water. Journal of the American Chemical Society 2014, 136, 17071–17077, PMID: 25402590. (38) Kearney, K.; Rockett, A.; Ertekin, E. Computational insights into charge transfer across functionalized semiconductor surfaces. Science and Technology of Advanced Materials 2017, 18, 681–692. (39) Cheng, J.; Sprik, M. Aligning electronic energy levels at the TiO2 /H2 O interface. Phys. Rev. B 2010, 82, 081406. (40) Le, J.; Iannuzzi, M.; Cuesta, A.; Cheng, J. Determining Potentials of Zero Charge of Metal Electrodes versus the Standard Hydrogen Electrode from Density-FunctionalTheory-Based Molecular Dynamics. Phys. Rev. Lett. 2017, 119, 016801. (41) Li, F.; Basile, V. M.; Rose, M. J. Electron Transfer through Surface-Grown, FerroceneCapped Oligophenylene Molecular Wires (5?50 Å) on n-Si(111) Photoelectrodes. Langmuir 2015, 31, 7712–7716, PMID: 26153919. (42) Kearney, K.; Iyer, A.; Rockett, A.; Staykov, A.; Ertekin, E. Effect of Surface Coverage and Composition on the Stability and Interfacial Dipole of Functionalized Silicon. The Journal of Physical Chemistry C 2017, 121, 11312–11318. (43) Michalak, D. J.; Amy, S. R.; Aureau, D.; Dai, M.; Estève, A.; Chabal, Y. J. Nanopatterning Si(111) surfaces as a selective surface-chemistry route. Nature Materials 2010, 9, 266 EP –.

29


Langmuir 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

(44) Tao, F.; Bernasek, S. L. Understanding Odd?Even Effects in Organic Self-Assembled Monolayers. Chemical Reviews 2007, 107, 1408–1453, PMID: 17439290. (45) http://cccbdb.nist.gov/DipReCalc.asp.

30


Page 30 of 31

Page 31 of 31 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

Langmuir

Graphical TOC Entry Identify molecules with a high dipole moment Cl

Replace substituent group with a more electrophilic group O

N

O

-

X=C Chlorobenzene

Nitrobenzene

31

Replace the binding atom, A, so that the EN difference between Si and A is increased O

X=O

N

O

OH

Nitrophenol


Design Strategy for the Molecular Functionalization of Semiconductor

Recommend Documents