Face-Selective Etching of ZnO during Attachment of Dyes - The

Article pubs.acs.org/JPCC

Face-Selective Etching of ZnO during Attachment of Dyes E. Palacios-Lidón,*,† D. F. Pickup,‡,§ P. S. Johnson,∥ R. E. Ruther,⊥ R. Tena-Zaera,# R. J. Hamers,⊥ J. Colchero,† F. J. Himpsel,∥ J. E. Ortega,‡,§,∇ and C. Rogero*,‡,∇ †

Centro de Investigación en Optica y Nanofísica (CIOyN), Universidad de Murcia, E-30100 Murcia, Spain Centro de Física de Materiales (CSIC-UPV/EHU), Material Physics Center (MPC), E-20018 San Sebastian, Spain § Departamento de Física Aplicada I, Universidad del País Vasco UPV/EHU, E-20018 San Sebastian, Spain ∥ Department of Physics and ⊥Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706-1390, United States # Energy Division, IK4-CIDETEC, E-20009 San Sebastian, Spain ∇ Donostia International Physics Center, E-20018 San Sebastian, Spain ‡

ABSTRACT: Scanning force microscopy and near-edge X-ray absorption spectroscopy are used to study the attachment of Protoporphyrin IX dye molecules to the low-index single-crystal faces of ZnO, namely, the polar (0001̅) face and the nonpolar (101̅0) face. We found that surface etching depends on the crystal face for various dye immobilization procedures: whereas the polar (0001̅) surface remains nearly unaltered, the nonpolar (101̅0) surface is strongly etched by acidic solutions; these results demonstrate the selective attachment of dye molecules to ZnO surfaces, showing that the dye attachment is extremely sensitive to both the exposed facet and the immobilization protocol. This could be a factor in the surprisingly poor efficiency observed for ZnO-based dyesensitized solar cells, especially those using ZnO nanorod arrays.

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ZnO (7.5%)13 are still modest compared to those of TiO2based devices (12.3%).14 A chemical attack on the ZnO surface during the functionalization process has been suggested as a possible cause for this unexpected low efficiency.15,16 For DSC applications, a semiconductor surface is functionalized with a molecular dye, typically consisting of organometallic molecules. The molecules are usually attached to a semiconducting electron-acceptor material, such as n-type TiO2 and ZnO, after being dissolved in an organic solvent containing the dye molecule along with a coadsorbent species, usually chenodeoxycholic acid (CDCA).17−21 The presence of CDCA has been shown to improve device efficiency in TiO2-based devices, and this has been traced to a reduction of electron− hole recombination losses;22 an upward shift of the Fermi level of TiO2;23 and a reduction of dye aggregation, which facilitates charge separation.24,25 This functionalization procedure has been extended to ZnO devices. Because ZnO is known to be unstable under acidic conditions,11,14,26−29 the surface is prone to being damaged in the presence of CDCA. A similar instability has been observed for carboxylic acid functional groups, which are often used to link molecular dyes to the semiconductor surface. The aim of this work was to analyze the influence of the functionalization process on various ZnO surfaces. Proto-

INTRODUCTION ZnO is an attractive semiconductor material because of its many favorable properties, including high transparency; a wide band gap; piezoelectricity; strong room-temperature luminescence; and, most importantly, low cost. Furthermore, ZnO can be grown anisotropically to obtain a great variety of nanostructures.1−4 This unique combination of properties makes ZnO an appealing material for applications in transistors, light-emitting diodes, sensors, and solar cells.1,4 In particular, ZnO has generated substantial interest as a possible alternative to the wide-band-gap semiconductor TiO2 in dye-sensitized solar cells (DSCs).5−9 DSCs are seen as promising candidates for the next generation of inexpensive but efficient solar-energy conversion devices. To date, the most successful DSCs have incorporated TiO2 as the electron-acceptor material. However, such DSCs are susceptible to electron−hole recombination prior to charge separation, which reduces the cell efficiency.9,10 ZnO has a band gap and physical properties similar to those of TiO2 but with an electron mobility that is 2−3 orders of magnitudes higher, which is expected to reduce recombination effects in DSCs. Moreover, the anisotropic crystallographic structure of ZnO allows for the formation of one-dimensional nanostructures such as single-crystal nanowires and nanorods growing in the direction of the c axis.11 These one-dimensional structures have been shown to exhibit favorable electron-transport properties,12 leading to extensive research on incorporating ZnO into DSC architectures. However, the highest conversion efficiencies with © 2013 American Chemical Society

Received: May 7, 2013 Revised: August 2, 2013 Published: August 6, 2013 18414

dx.doi.org/10.1021/jp4045447 | J. Phys. Chem. C 2013, 117, 18414−18422

The Journal of Physical Chemistry C

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angle of incidence was varied from normal incidence (s polarization) to 60° from normal (predominantly p polarization). The photon energies at the N 1s and O 1s edges were calibrated using the sharp 2p-to-3d transition in TiO2 (rutile) at 458.0 eV as a secondary standard.34,35 This value was established by measuring TiO2 powder side by side with gasphase N2 trapped inside an irradiated imide. The first vibrational line of N2 at 400.9 eV36,37 served as the primary standard. All spectra were normalized to the incident photon flux. To remove the effects of beam fluctuations and decay, the sample current was divided by the current from a mesh coated in situ with Au. After this division, a linear background was subtracted using an extrapolation of the pre-edge signal. This normalization produced a signal proportional to the density of N or O atoms. Scanning Force Microscopy (SFM). Topography images were acquired at room temperature and ambient conditions using a Nanotec Electronica SFM system with a phase lock loop (PLL)/dynamic measurement board. Topography images were obtained working in amplitude modulation noncontact dynamic mode with silicon tetrahedral tips [Olympus Optical Co. LDT, OMCL-AC series, long cantilevers (length 240 μm), nominal force constant of 2 N/m, resonance frequency of 70 kHz]. Relatively small oscillation amplitudes (10-nm peak− peak) and a small reduction of oscillation amplitude were used (about 90% of the free oscillation amplitude). The phaselocked loop of our dynamic measurement board was enabled to keep the tip sample system always at resonance. In this way, measurements were performed in the attractive regime of tip− sample interaction without touching the surface.38 The SFM images were processed with the freely available WSXM program.39

porphyrin IX (H2PPIX) dye molecules were immobilized at two single-crystal ZnO surfaces, the polar, oxygen-terminated (0001̅) surface and the nonpolar (101̅0) surface. Although most DSC studies have been performed using Ru-based dyes, the highest efficiency has been achieved using a zinc porphyrinbased dye.14 In addition to avoiding the rare metal Ru, such planar molecules allow the degree of molecular orientation to be studied by polarization-dependent X-ray absorption spectroscopy.30 Our selection of single-crystal surfaces was motivated by common ZnO-based DSC architectures, such as ZnO nanowire arrays.2,3,11 The lateral facets of such ZnO nanowires consist of the nonpolar (101̅0) surface . The polar (0001)̅ ZnO face exhibits the highest dye uptake reported so far.11,30 We investigated various immobilization solutions by mixing H2PPIX with the coadsorbent CDCA in ethanol using different CDCA/H2PPIX molar ratios. Near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and scanning force microscopy (SFM) were used to characterize both the initial polar (0001̅) and nonpolar (101̅0) ZnO surfaces exposed to these dye solutions. The polar (0001̅) surface was found to exhibit only small changes after exposure, regardless of which solution was used. However, the nonpolar (101̅0) face was found to undergo surface etching and loss of crystallinity with increasing CDCA content. These results suggest an attack of acidic solutions on the nonpolar (101̅0) side faces of ZnO nanowires as a possible reason for the low efficiencies of ZnO DSCs.

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EXPERIMENTAL DETAILS Sample Preparation. Most characterization studies were performed on ZnO single crystals. These were prepared from polished ZnO wafers with (1010̅ ) and (0001)̅ orientations (CrysTec GmbH), which were annealed at 1100 °C in a ceramic ZnO kiln made from a ZnO sputter target (99.999%, Kurt J. Lesker Co.). The crystals were annealed in ambient air for 3 h to produce surfaces with well-defined terraces and steps.31 Some experiments were also carried out on ZnO nanorod arrays that were obtained by electrodeposition from the reduction of dissolved molecular oxygen in zinc chloride solutions.32,33 Further details on ZnO nanorod electrodeposition can be found elsewhere.30 Protoporphyrin IX (H2PPIX), chenodeoxycholic acid (CDCA), and ethanol were purchased from Sigma-Aldrich and used without further purification. Stock formulations (6 mM ) of H2PPIX and CDCA in ethanol were prepared and subsequently mixed to produce two 0.6 mM solutions with CDCA/H2PPIX ratios of 1:2 and 2:1. A 0.6 mM H2PPIX solution, with no CDCA, was also prepared, which, for convenience, we denote as 0:3. Finally, a CDCA/H2PPIX solution with a 2:1 ratio was prepared at twice the concentration (1.2 mM) and is denoted as 4:2. Immobilization of the dye on the ZnO surface was carried out by immersing the ZnO sample in one of the prepared solutions for 20 min, with occasional agitation. After 20 min, the ZnO sample was removed and thoroughly rinsed with ethanol to remove any physisorbed molecules. Near-Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy. The measurements were conducted at the VLS-PGM beamline of the Synchrotron Radiation Center (SRC) in Madison, WI. Radiation damage was minimized by using filters.34 NEXAFS spectra were measured by collecting the total electron yield. To determine the quality of the crystalline surface and the orientation of the molecules, the

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RESULTS Clean Surface. ZnO exhibits the wurtzite structure, which can be schematically described as alternating planes of O and Zn ions stacked along the c axis. Because of the lack of inversion symmetry of this structure, spontaneous polarization appears in the direction parallel to the −c ⃗ axis. Thus, depending on the crystal face, the surfaces can be polar or nonpolar. In ZnO, each Zn2+ ion is surrounded by four O2− ions and vice versa. The polar faces correspond to cutting the crystal structure along the Zn or O planes. The nonpolar surfaces contain an equal number of O and Zn ions, because they are formed by breaking the same number of oxygen and zinc bonds. Most studies related to the characterization of ZnO surfaces have been performed under ultra-high-vacuum (UHV) conditions. It has been proposed that the surface morphology, such as hydrogen adsorption or removal of Zn atoms, obtained under UHV, depends on the growth conditions and surface preparation treatment (such as the temperature and atmosphere during sample annealing).40−46 However, to offer a true low-cost alternative for small-molecule devices, fabrication should be carried out in ambient conditions where the presence of water and airborne contamination due to atmospheric exposure will modify the surface morphology, chemical activity, and electronic properties of the ZnO surface.47 Therefore, prior to molecular immobilization, it is important to characterize the pristine crystal surfaces. As mentioned, two different ZnO single-crystal surfaces were investigated in the present work: the bare polar surface (0001̅), which is nominally O-terminated, and the nonpolar (101̅0) face. If the wet chemical etching of ZnO depends not only on 18415



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to the mixing of O 2p and Zn 4d states.52,53 The most noticeable changes occur in the contributions labeled B−D. Peaks B and D are more intense for c ⃗ ⊥ E⃗ , whereas peak C appears stronger for c ⃗ ∥ E⃗ . The O 1s edge NEXAFS spectra for the nanorod ZnO samples confirmed that the nanorods were single-crystal columns with the c ⃗ axis perpendicular to the surface. The spectra shown in Figure 1 measured for c ⃗ ⊥ E⃗ and c ⃗ ∥ E⃗ present the same dichroism as observed for the single-crystal surfaces, with the polarization dependence the same as that measured for the polar sample for the c ⃗ axis perpendicular to the surface. To obtain information about the surface morphology, SFM measurements were performed. Figure 2 shows the SFM

the etchant properties but also on the exposed facets, then distinct behavior should be expected for the two faces.48−51 Using NEXAFS spectroscopy, it is possible to investigate the crystalline structure of surfaces because the spectra depend on the crystallographic orientation relative to the polarization of the soft X-rays (which is parallel to the electric field vector). In noncubic systems, the orientation dependence of NEXAFS data provides valuable information on the anisotropic properties of crystals, such as wurtzite ZnO, which has a preferred c axis and alternating hexagonal planes of Zn and O. Figure 1 shows the O

Figure 1. O 1s NEXAFS edges for the ZnO (0001)̅ and (1010̅ ) crystal faces and ZnO nanorods measured for the −c⃗ axis parallel and perpendicular to the electric field, E⃗ (c⃗ ∥ E⃗ and c⃗ ⊥ E⃗ , respectively). The strong polarization dependence demonstrates the anisotropy of the ZnO crystal structure and a well-ordered surface structure.

1s NEXAFS edges for the two crystal faces (0001)̅ and (101̅0) measured at two different electric field orientations with respect to the −c ⃗ axis of the ZnO. The direction of the linearly polarized incident light was changed from perpendicular to close to grazing, forming an angle of 60°, which modified the angle of the electric field E⃗ with respect to the −c ⃗ axis. For the polar face, the situation changed from c ⃗ ⊥ E⃗ at normal incidence to almost parallel (for simplicity, we denote this configuration as c ⃗ ∥ E⃗ ) for grazing incidence. Because the polar and nonpolar faces are perpendicular to each other, the configurations were reversed for (101̅0): c ⃗ ⊥ E⃗ at grazing incidence and c ⃗ ∥ E⃗ at normal incidence. For both faces, the c ⃗ ∥ E⃗ and c ⃗ ⊥ E⃗ spectra differ from each other, in agreement with calculated spectra for this material.52,53 The spectra observed for the polar face are inverted with respect to those measured for the nonpolar face, as expected, taking into account that the faces are oriented perpendicularly to one another. (The polar face is perpendicular to the c ⃗ axis, whereas the nonpolar face is parallel.) In Figure 1, the main peak positions are labeled A−D. The range from 530 to 539 eV, comprising peaks A−C, can mainly be assigned to O 2p hybridized with Zn 4s states, whereas the range from 539 to 550 eV, incorporating peak D, is associated with O 2p mixed with Zn 4p states. Finally, the range above 550 eV is attributed

Figure 2. (a,b) Low- and (c,d) high-magnification SFM images of the polar (0001)̅ and nonpolar (1010̅ ) surfaces, respectively, illustrating the topography differences between the two ZnO crystal surfaces. The lower panels correspond to the profiles in panels c and d.

images of the ZnO polar (0001̅) and nonpolar (101̅0) faces. The polar surface (Figure 2a) exhibits large terraces extending for several hundreds of nanometers (500−1000 nm), running along the (1010̅ ) direction and separated by double or quadruple steps (step heights of 0.5 ± 0.15 and 1.0 ± 0.2 nm, respectively, as shown in the left profile of Figure 2). The steps are mainly triangular, with the vertices forming an angle of 120° with the (101̅0) direction.40 High-magnification images (Figure 2c) show that the terraces are full of pits with diameters of 10−30 nm. These pits appear at higher concentration at the quadruple step edges, giving the edge an irregular shape, whereas at the double-step edge, only a few appear. Further examination of the surface reveals that the terraces are not perfectly flat; instead, they appear to be rough and covered with blurry protruding features. Under UHV conditions, the clean 18416



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(0001̅) surface displays hexagonal holes within the flat terraces.40 It is well-known that, when this surface is exposed to water, it becomes hydroxylated through dissociative adsorption of water molecules on the oxygen vacancies or through the interaction of atomic hydrogen with surface oxygen atoms.54,55 Although a systematic study on how water adsorption modifies the morphology of the (0001̅) surface has not yet been performed, a corresponding study of the polar Zn-terminated (0001) surface showed that water absorption destroys the triangular structures characteristic of this facet, resulting in a rough surface.55,56 Therefore, the pitting, protrusion, and irregular shape of the quadruple steps are probably due to water adsorption. The SFM image of the nonpolar ZnO(101̅0) surface in Figure 2b shows long terraces parallel to the (1210) direction with a lateral size of a few hundred nanometers along the (0001̅) direction. Rounded multi-atom steps with a great variety of heights ranging from 1.8 to 5.5 nm (or even greater) separate these large terraces. As mentioned previously, the final surface morphology depends strongly on the preparation conditions and, particularly, on the annealing temperature: Increasing this temperature causes small square-shaped terraces to evolve into long terraces, separated by step bunches. Rectangularlike islands of monatomic height (monatomic step = 0.26 nm) can be distinguished within the terraces. These islands are commonly found on clean (101̅0) surfaces under UHV conditions.40 Neither the step edges nor the border of the islands are sharp, but instead, they appear buried. Also, as observed for the (0001̅) surface, the terraces are rough with small protuberances, which is again attributed to water chemisorption and/or oxygen hydroxylation.55 Molecular Immobilization. Figure 3 shows SFM topography images of ZnO(0001̅) surfaces after immersion in three 0.6 mM solutions containing different CDCA/H2PPIX relative concentrations: 0:3, 1:2, and 2:1. In the lowmagnification images (Figure 3a−c), it can be seen that, after molecular immobilization, the overall appearance of the surfaces is very similar, independent of the relative CDCA/ H2PPIX concentration. It is noticeable that the terraces are shorter and not as straight and parallel to the (101̅0) direction as in the pristine ZnO surface. The triangular step edges become rounded with wavy features. Moreover, the height of the steps increases, from two atomic units in the pristine samples to four atomic units or even higher on the immobilized surfaces. This tendency is independent of the relative concentration of CDCA to H2PPIX. Even an increase in the overall molar concentration of the 1:2 CDCA/H2PPIX solution to 2:4 CDCA/H2PPIX (1.2 mM solution) did not lead to any appreciable change in the sample appearance (images not shown). Therefore, the SFM results indicate that, under the mild acidic conditions used here, any molecular etching that takes place is mainly perpendicular to the triangular monatomic steps, leaving the (0001̅) upper surface essentially unaltered. NEXAFS measurements confirmed these results, demonstrating that surface crystallinity was preserved. Figure 4a shows the O 1s edges of three ZnO(0001̅) surfaces after immersion in three solutions with varying CDCA/H2PPIX ratio and overall molar concentration: 0:3, green; 1:2, brown; and 2:4, blue spectra. The top and bottom panels summarize the measurements for c ⃗ ⊥ E⃗ and c ⃗ ∥ E⃗ , respectively. The spectra have been manually offset for clarity, although the superposition of the spectra is also included to demonstrate that the results

Figure 3. (a−c) SFM topography images and (d−f) their corresponding magnifications of the polar ZnO(0001̅) surface after immersion in three 0.6 mM solutions with different CDCA/H2PPIX relative concentrations: (a,d) 0:3, (b,e) 1:2, and (c,f) 2:1. Inset: Profile measured for the image in panel f. The surface remained almost unaltered after various acidic treatments.

measured for c ⃗ ∥ E⃗ and c ⃗ ⊥ E⃗ perfectly match the corresponding spectra for the bare surfaces (black dotted lines). This indicates that the surfaces maintained the (0001̅) orientation and that they did not suffer any strong etching or that, if they did undergo etching, it proceeded layer by layer. The most notable change in the surface morphology with respect to the pristine sample is the absence of pitting both within the terraces and at the step edges, as can be seen in the high-magnification SFM images (Figure 3d−f). This result indicates that, during molecular immobilization, the adsorbed water is removed from the surface and the presence of the newly adsorbed dye molecules prevents subsequent surface− water interactions when the sample is re-exposed to ambient conditions. On the 2:1 CDCA/H2PPIX sample (Figure 3f), some large holes of a few hundred nanometers can be seen. Their depth was measured to be approximately 1.5 nm (profile shown in the inset), and taking into account the molecular dimensions, 1.5 × 1.3 nm2, these areas can be attributed to uncovered areas. Moreover, this result suggests that the dye molecules adopt an upright monolayer configuration to cover the ZnO surface. 18417



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and oriented with respect to the surface. Because the dipole matrix element associated with π* excitations has its maximum for the electric field vector perpendicular to the plane of the central tetrapyrrole ring of H2PPIX, this observation shows that the molecules are preferentially oriented perpendicular to the surface, as illustrated at the bottom of Figure 4b. This is in good agreement with the SFM images and with previous observations for this polar face.30 Therefore, the final aspect of the Zn(0001̅) polar surface after the immobilization of the dye molecules appears to be independent of the immobilization solution used. Changing the CDCA/H2PPIX ratio or the overall concentration of the dye solution was not found to affect the final surface result. In all cases, immobilization had little effect on the underlying ZnO structure and resulted in uniform coverage of dye molecules, which adopted an upright orientation. The homogeneous nature of the surface images together with the step height of the uncovered areas (1.5 nm) observed in Figure 3f indicates the adsorption of a monolayer of dye molecules. The situation was found to be radically different for the (101̅0) surface. Comparison of SFM topography images of the pristine ZnO surfaces (Figure 2d) with those for samples prepared using different dye immobilization conditions, shown in Figure 5, demonstrates that the morphology of the (101̅0) face is modified by the interaction with the acidic CDCA/ H2PPIX solutions. Increasing the relative CDCA concentration appears to have a more significant effect. Figure 5a−c shows different details of the nonpolar surface after immersion in 0:3 CDCA/H2PPIX molecular solution. The images show that, in the absence of CDCA, the sample appearance is very similar to that of the pristine (101̅0) ZnO surface. Again multi-atom steps (higher than four atomic units) appear with a great variety of heights from one step to another. The squarelike islands of monatomic height and buried edges can also be distinguished on the terraces as observed in the clean sample. The only apparent difference between the pristine and 0:3 CDCA/ H2PPIX samples is that the terraces become straighter and narrower, whereas the steps are no longer rounded. The O 1s edge for this sample presents no extra features compared to the bare surface, as shown in Figure 6a for the spectra measured for c ⃗ ⊥ E⃗ (comparing the dotted black and green spectra). There is only an intensity decrease, probably because of the reduction of the terrace size. These results indicate that, during immobilization using the 0:3 CDCA/H2PPIX solution, slight etching takes place at the steps, whereas the upper (101̅0) surface is unaltered. Upon increasing the relative concentration of CDCA more significant changes appear. After immersion in the 1:2 CDCA/ H2PPIX molecular solution (Figure 5d−f), the large range of step heights disappears, with the sample exhibiting steps of mainly single or double atomic units. The new surface is characterized by long stripes along the (1210) direction, whereas the terraces have narrowed, changing from several hundred nanometers to about 100 nm along the (0001̅) direction. The density of the rectangular monatomic islands substantially increases compared with that on the pristine ZnO surface (detailed in Figure 5f), and the island edges become sharper. In fact, in some cases, it is visible that the islands are no longer rectangular but have a slightly truncated triangular shape. These topographic changes were also detected in the NEXAFS results. As shown in Figure 6a, the similarity between reference spectra before immersion (dotted black line) and

Figure 4. (a) O 1s and (b) N 1s NEXAFS spectra for the ZnO(0001)̅ surface after immersion in three solutions with different CDCA/ H2PPIX relative concentrations: 0:3 and 1:2 for 0.6 mM solution and 2:4 for 1.2 mM solution. Spectra were measured for c⃗ ⊥ E⃗ and c⃗ ∥ E⃗ . Spectra are offset for clarity, and insets show the spectra without the shifts to illustrate the good matches among them. The similarity of the spectra of the bare ZnO surface and the surface after various treatments demonstrates that the surface remained essentially unchanged.

The immobilization of porphyrin molecules on ZnO was confirmed by XPS (data not shown) and NEXAFS spectroscopy. The latter also confirmed the upright orientation of the dye molecules relative to the surface. The detection of the N 1s NEXAFS edge (N 1s core level in XPS) is the unambiguous indication of the presence of the molecules. N atoms are considered the fingerprint element of the dye because only Protoporphyrin IX molecules contain N (neither CDCA nor the solvent contain N). Figure 4b shows the N 1s edges for three ZnO(0001̅) surfaces after immobilization with 0:3, 1:2, and 2:4 CDCA/H2PPIX solutions (green, brown, and blue spectra, respectively) measured for c ⃗ ⊥ E⃗ (top) and c ⃗ ∥ E⃗ (bottom). The spectra present two well-defined π* peaks below 400 eV and a broad σ* peak at around 407 eV. In all cases, the spectral shapes resemble the expected result for Protoporphyrin IX molecules.30 Moreover, the polarization dependence, that is, the dependence of the relative intensities of the π* and σ* peaks on the angle of incident light, indicates how the molecules are ordered 18418



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Figure 5. (a−i) SFM topographic images at different magnifications illustrating the morphological modifications observed for the nonpolar ZnO(1010̅ ) surface after immersion in three 0.6 mM solutions with different CDCA/H2PPIX relative concentrations: (a−c) 0:3, (d−f) 1:2, and (g− i) 2:1. (j) SFM image of the nonpolar ZnO(101̅0) surface after immersion in 1.2 mM solution (2:4 CDCA/H2PPIX).

rectangular because only one side of the rectangle has a welldefined straight edge whereas the other has an irregular shape, even more so than in the case for the 1:2 sample. Because these edges are perpendicular to the polar (0001) direction, this result points toward polar-dependent etching. Nevertheless, the most significant feature is the presence of pits that appear mainly at the island borders and at the step edges. The depth of these large holes is about 2−3 nm and cannot be associated with gaps in the molecular monolayer. It is more probable that they arise as a consequence of strong surface etching because of the higher CDCA concentration. The O 1s edge measured with c ⃗ ∥ E⃗ for the ZnO(101̅0) surface after immobilization using the 2:1 solution (red spectrum in Figure 6a) supports this assignment, as a significant change in spectral shape is observed, with splitting of peak B and a change in its relative intensity with respect to peak C. This demonstrates that the surface morphology of the ZnO(101̅0) nonpolar surface is very sensitive to the concentration of CDCA in the immobilization conditions, which is contrary to what was observed for the (0001̅) polar surface. Peaks A and B seem to be split. Based on previous theoretical calculations, 52 peak A splits as a consequence of the disordered structures and, peak B probably splits because of the presence on the surface of other orientations. Therefore, the NEXAFS spectra for c ⃗ ⊥ E⃗ show that peaks A and B decrease and peak C increases (as summarized in the inset of the bottom panel of Figure 6a), and those for c ⃗ ∥ E⃗ show a splitting of peaks A and B (as summarized the inset in the top panel of Figure 6a). All of these changes demonstrate that nonpolar ZnO(101̅0) is etched by the acidic nature of the

those measured after immersion in the 1:2 CDCA/H2PPIX solution (brown spectra) is not as good as for the ZnO(0001)̅ surface. For both measured configurations (c ⃗ ∥ E⃗ and c ⃗ ⊥ E⃗ ), there are significant changes. For c ⃗ ⊥ E⃗ , in addition to a general intensity loss in the spectra, the relative intensity of peak B decreases, whereas that of peak C slightly increases, and peak A seems to split. Similar splitting is also detected for c ⃗ ∥ E⃗ . Taking into account the facts that the intensity of peak B increases when c ⃗ is parallel to E⃗ , peak C is more intense with c ⃗ perpendicular to E⃗ , and the splitting of peak A can be related to disordered structures,52,53 these results suggest that there is a loss of sample crystallinity due to surface etching. Increasing the overall solution concentration from 1:2 CDCA/H2PPIX in 0.6 mM solution to 2:4 CDCA/H2PPIX in 1.2 mM solution did not result in any significant changes from those already discussed. The surface aspect is very similar in both samples, as the SFM images in Figure 5e,j illustrate. The increase in the molecular concentration mainly affects the rectangular islands, which appear to be better resolved because of an increase in height (∼0.5 nm, around two atomic steps) and display sharper edges. In the O 1s edge measured for c ⃗ ⊥ E⃗ , the C peak is better resolved (blue spectrum in Figure 6a). Apparently, increasing the relative CDCA concentration further, in the 2:1 CDCA/H2PPIX solution, yielded SFM images generally similar to those of the 1:2 sample, as illustrated in Figure 5g−i. In the medium magnified image, Figure 5h, one can observe long stripes separated by monatomic steps with rectangular islands. However, at higher magnification, in Figure 5i, a detailed inspection reveals some interesting differences. The islands are not completely 18419



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This is an effect of etching, as defects in the surface induce defects in the adsorbed molecular layer. It is interesting to note that the N 1s spectral features resemble those of Protoporphyrin IX molecules for both ZnO surfaces with all treatments. There is no evidence of the characteristic spectral shifts due to metalation by surface Zn atoms, as observed previously under different preparation conditions.30 In that case, solutions without CDCA were used for the immobilization of H2PPIX molecules on ZnO nanorods. However, sonication was not used during the immobilization process, and the immobilization took much longer (90 min compared to 20 min in the present work).

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CONCLUSIONS In summary, we found that the dye immobilization protocols commonly used in TiO2-based DSCs cannot be transferred directly to ZnO-based devices. Because of the instability of the nonpolar ZnO(101̅0) face under acidic solutions, the process of attaching dye molecules can lead to surface etching of ZnO that destroys its crystallinity. The morphology of the oxygenterminated (0001̅) facet, on the other hand, is preserved. The adsorption of dye molecules is shown to actually stabilize the polar (0001̅) surface, preventing adsorption of water and keeping the surface stable over a period of months. For both the (0001̅) and (101̅0) surfaces, the orientation of the molecules remains independent of the CDCA concentration. The upright configuration increases the molecular coverage and favors charge injection from the dye molecules into the substrate, as required in a DSC. The observed face-selective etching has consequences for ZnO DSCs based on nanorod arrays. These are oriented parallel to the c axis, and their largest facets are the (101̅0) side faces. They are strongly etched during the immobilization of the dye, which explains the relatively low efficiency. To confirm this hypothesis, it would be interesting to perform nanoscale studies of the charge-transfer processes and to investigate other dye immobilization protocols employing less acidic solutions designed specifically for ZnO nanorods.

Figure 6. (a) O 1s and (b) N 1s NEXAFS edges for the ZnO(101̅0) surface after immersion in 0:3, 1:2, and 2:1 CDCA/H2PPIX 0.6 mM solutions and 2:4 1.2 mM solution. Spectra, measured for c⃗ ∥ E⃗ and c⃗ ⊥ E⃗ , are offset, and insets show the spectra without the shifts to illustrate the matches among them. The polarization dependence of the spectra was reduced after immersion, and new components appeared in peaks A and B, demonstrating loss of crystallinity at the surface.

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AUTHOR INFORMATION

Corresponding Authors

*E.P.-L.: Tel.: +34868888551. E-mail: [email protected]. *C.R.: Tel.: +34943015804. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge funding support from the Spanish Goverment (MAT2010-21156-C03-01, C03-03, MAT2010-21267-C02-01, PIB2010US-00652), the Basque Government (IT-621-13), the Seneca Foundation (15324/PI/10), and the CONSOLIDER Program ‘Force for Future’ (CSD2010-00024). This work was supported in the United States by the NSF under Awards CHE-1026245 and DMR-0537588 (Synchrotron Radiation Center). R.T.-Z. and E.P.-L. acknowledge support from the Program “Ramon y Cajal” of the MINECO.

immobilization solution, promoting disorder and the formation of a new crystallographic orientation on the surface. This effect is multiplied by the presence of CDCA on the surface. Finally, looking at further details of the high-magnification SFM images (Figure 5c,f,i), the sample surface is covered with blurry protruding features, independently of the relative concentration of CDCA. At this scale, there are no appreciable differences compared with the bare ZnO(101̅0) sample, indicating that molecules are homogenously covering the surface. The N 1s NEXAFS edge data, shown in Figure 6b, verify that, independently of the immobilization solution, the spectral dependence on the direction of incident light (and hence the electric field) again indicates that the molecules adopt an upright orientation. It is noticeable that the matching between the spectra is not as good for the nonpolar (101̅0) face as for the polar (0001̅) surface (insets in Figure 6b, top and bottom panels, where spectra are plotted without any offset).

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Face-Selective Etching of ZnO during Attachment of Dyes - The

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