Nanophotoactivity of Porphyrin Functionalized Polycrystalline ZnO

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Nanophotoactivity of Porphyrin Functionalized Polycrystalline ZnO Films Celia Rogero,*,† David F. Pickup,† Jaime Colchero,‡ Eneko Azaceta,§ Ramón Tena-Zaera,§ and Elisa Palacios-Lidón*,‡ †

Centro de Física de Materiales (CSIC-UPV/EHU), Material Physics Center (MPC) and Donostia International Physics Center, 20018 San Sebastian, Spain ‡ Departamento Física, Facultad de Química (Campus Espinardo), Universidad de Murcia, E-30100 Murcia, Spain § Materials Department, IK4-CIDETEC, 20009 San Sebastian, Spain S Supporting Information *

ABSTRACT: Kelvin probe force microscopy in darkness and under illumination is reported to provide nanoscale-resolved surface photovoltage maps of hybrid materials. In particular, nanoscale charge injection and charge recombination mechanisms occurring in ZnO polycrystalline surfaces functionalized with Protoporphyrin IX (H2PPIX) are analyzed. Local surface potential and surface photovoltage maps not only reveal that upon molecular adsorption the bare ZnO work function increases, but also they allow study of its local dependence. Nanometer-sized regions not correlated with apparent topographic features were identified, presenting values significantly different from the average work function. Depending on the region, the response to the light excitation is different, distinguishing two relaxation processes, one faster than the other. This behavior can be explained in terms of electrons trapped closed to the molecule−semiconductor interface or electrons pushed into the ZnO bulk, respectively. Moreover, the origin of these differences is correlated with the H2PPIX−ZnO bonding and molecules configuration and aggregation. The chenodeoxycholic acid (CDCA) coadsorption leads to a more homogeneous surface potential distribution, confirming the antiaggregate effect of this additive, while the surface photovoltage is mostly dominated by the slow relaxation component. This work reveals the complexity of real device architectures with ill-defined surfaces even in a relatively simple system with only one type of dye molecule and hightlights the importance of nanoscale characterization with appropriate tools. KEYWORDS: Kelvin probe force microscopy, surface photovoltage, ZnO, porphirin, dye−semiconductor interface

1. INTRODUCTION The possibility of tuning material surface properties is crucial for the design and fabrication of tailored devices in which interfaces play a fundamental role. For this reason, the functionalization of surfaces has received great interest in the scientific community.1 Particular attention has been paid to the chemisorption of small molecules on semiconducting oxides. In this case, the molecule adsorption involves strong covalent bonds that modify the electronic properties of the surfaces. Macroscopically, this leads to a change in the work function of the semiconductor and variations in surface properties such as conductance.2,3 Microscopically, a covalently bonded molecule on a surface modifies the electronic properties of both components. Theoretical and experimental evidence agree that due to the electronic coupling between the molecular and surface orbitals, the molecule/surface system should be treated as a whole “new system” with possibly quite distinct properties compared to the building blocks. The properties of this coupled semiconductor/molecule heterojunction strongly depends on © 2016 American Chemical Society

the type of surface bonding and on the structure and orientation of the molecular layer.4 In this context, establishing a correlation between the specific molecule/substrate architecture, the electronic behavior, and the surface properties is essential for understanding the basic working principle of devices and to improve the overall response. However, in real devices, including ill-defined surfaces, it may be difficult to obtain real nanoscopic information and to extrapolate data that complement the characterization performed with macroscopic techniques. This problem can be overcome using scanning Kelvin probe force microscopy (KPFM) to map the topography and the surface potential (SP) simultaneously with lateral resolution of a few tens of nanometers.5 Moreover, when KPFM is combined with controlled sample illumination, it becomes an ideal tool to Received: March 23, 2016 Accepted: June 15, 2016 Published: June 15, 2016 16783

DOI: 10.1021/acsami.6b03544 ACS Appl. Mater. Interfaces 2016, 8, 16783−16790

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ACS Applied Materials & Interfaces measure photoinduced effects at the nanoscale,6−11 As in the classical surface photovoltage (SPV) technique, the lightinduced changes of SP are measured and analyzed with the surface photovoltage (SPV) theory. An extensive review on the working principle and applications of this technique can be found in ref 12. Information on the band bending near the semiconductor surface and/or the built in voltage at the buried interface inside a heterojunction, the surface dipole, carrier density, and carrier diffusion length can be obtained. Zinc Oxide (ZnO) is a cheap, biocompatible alternative to be used as transparent electrode in optoelectronic devices, biosensors, and photocatalysis. Currently, it is one of the most prominent semiconductors due to its high mobility, large band gap, and exciton binding energy. ZnO exhibits a rich family of nanostructures with wide potential applications. In addition, it is possible to prepare atomically smooth surfaces, enabling the basic investigation of charge transfer processes at the ZnO/organic semiconductor heterojunction with welldefined interface geometry and energy.13 The carboxylic group is one of the frequently used anchor groups in biosensors and material surface functionalization that allows the enhancement of electron injection into the semiconductor.14,15 Among the different carboxylic functionalized molecules, the porphyrins may be noted because ZnO/porphyrin hybrid materials have been proposed for different applications such as solar cells,16,17 light emitting diodes,18 gas detectors,19 photocatalysis,20 and nanomedicine.21 Nevertheless, the performance of the ZnO/ porphyrin system in some applications, such as solar cells, is still far below that compared with the analogue TiO2-based system.16,22 Therefore, further knowledge into the governing mechanisms occurring in the ZnO/porphyrin systems is needed. In this context, the ZnO/protoporphyrin IX (PP) nanocomposites, which have been proposed in nanocomposite form for nanomedicine21 and photocatalytic20 applications seem to be an appropriate model for nanoscale resolution studies. In this work, KPFM combined with external illumination is used to investigate the nanophotoactivity of ZnO polycrystalline thin films functionalized with Protoporphyrin IX (H2PPIX). By mapping the local SP and SPV, information on the dye−ZnO charge injection and charge recombination mechanisms is extracted. Different photoactive nanometric regions have been detected. These regions are not ZnO-facet dependent and are randomly distributed within the sample surface. Two different charge relaxation processes in the H2PPIX−ZnO interface have been determined and associated with the molecular arrangement. The contribution of each of them to the SPV determines the different photoactivity observed in the two regions.

Figure 1. (a and b) Topography images of bare ZnO and H2PPIX/ ZnO samples, respectively (z scale 215 nm). (c and d) SP images of both surfaces acquired “in darkness”. SP images are presented overimposed with the topography to visualize any correlation between them. (e) Profiles of the SP images along the white dotted lines, together with their mean SP values. To highlight the shifts of the SP around the mean values, the color scales of the original images, topright corner in c and d, have been readjusted around the mean SP for each sample, to illustrate the rescaled color codes. mixed solution of H2PPIX:CDCA in a 2:1, ratio, respectively. Immobilization of the dye on the ZnO surface was carried out by immersing the ZnO samples in the solutions. After 20 min the ZnO samples were removed and thoroughly rinsed with ethanol to remove physisorbed molecules and blow-dried with nitrogen. The resulting samples do not present apparent topological changes compared to bare ZnO (Figure 1b), discharging detectable chemical etching and large molecular agglomeration.24 Morphology and SP were studied at ambient conditions using a homemade Scanning Force Microscopy (SFM) mounted on an inverted optical microscope (Nikon Eclipse TE2000-E) that allows controlled sample illumination using platinum coated silicon tips (Olympus, k = 3 N/m and f = 75 kHz).25,26 Freely available WSxM software has been used for image acquisition and processing.27 The SFM was operated in frequency modulation mode (FM-SFM), that is, the frequency shift channel is used for the topography feedback with oscillation amplitudes of 1 nm and tip−sample distance of about 7 nm. As described in ref 28, with these operation parameters, measurements are performed in the conservative regime, needed to acquire quantitative highly resolved SP measurements. KPFM images were acquired in frequency modulation KPFM mode (FM-KPFM) with an AC voltage of 500 mV at 7 kHz. More details of the KPFM set up and operation are described elsewhere.28 In our setup, the DC and AC bias are applied to the sample. By definition, the SP is the difference between the sample and the tip work function, SP = Wsample − ϕtip, where Wsample and ϕtip are the sample and the tip work functions,

2. MATERIALS AND METHODS The ZnO films were deposited on commercial glass/FTO substrates (TEC15, Hartford Glass) by spray pyrolysis technique.23 A zinc acetate solution (0.1 M Zn[C2H3O2]·2H2O, 0.2 M C2H4O6 in 25:75 (v/v) water/ethanol mixture) was sprayed on the substrates, which were held at ∼350 °C. The spraying process consisted of 50 cycles (1 cycle means 5 s with spray and 5 s without spray). The samples were annealed in air at 450 °C during 1 h. As can be seen in Figure 1a, the resulting ZnO thin films consist of polydisperse aggregates of closedpacked nanograins of few tens of nanometers. Protoporphyrin IX (H2PPIX), chenodeoxycholic acid (CDCA), and ethanol (anhydrous) were purchased from Sigma-Aldrich and used without further purification. Two immobilization solutions were formulated in ethanol; a 0.6 mM pure H2PPIX solution and a 0.6 mM 16784


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ACS Applied Materials & Interfaces respectively. Assuming that ϕtip is not varying during the experiments, changes in the SP can be directly correlated with Wsample changes. All the measurements presented in this work have been obtained with the same tip. Keeping the tip integrity during the experiment is fundamental to achieve quantitative and reproducible results. To ensure the reliability of the measurements, a control step was introduced: ZnO bare sample was used as reference and topography and SP was measured in between any sample characterization. Reproducibility was checked on different sets of samples with different tips. Although the relative amount of the different regions slightly varies between similar samples, the photoactivity of each specific region is the same independently of the sample. Moreover, similar measurements on samples stored in darkness in air atmosphere for several months leads to similar results without presenting sighs of degradation. To study the photoinduced SP changes, the sample were illuminated using a collimated monochromatic LED (λ = 405 nm, E = 3 eV) (Thorlab M405L2) through the ZnO electrode. This photon energy is smaller than the ZnO bandgap at room temperature (i.e., 3.35 eV) but at the maximum of the H2PPIX narrow absorption band. This means that, while little absorption takes place in the transparent electrode, an efficient H2PPIX molecule excitation is expected. By comparing the light-induced SP response of the bare ZnO and the H2PPIX functionalized samples, the photoexcitation of the H2PPIX molecules and injection of the photogenerated charges into the ZnO can be analyzed. The irradiation intensity was set to 6 × 1017 photons· s−1·cm−2 during periods of about 2 s, and the illuminated area was about 1 mm2. This intensity was chosen to be in the photosaturation regime (higher intensity does not produce larger SPV), but not too high, to avoid sample degradation. Due to the small light absorption on the ZnO and its relatively high thermal conductivity, the heat dissipation is efficient for short illumination periods. Hence, at this moderate light intensity, no significant change in sample temperature due to illumination is expected. In addition to the sample degradation, the local heating may result in the activation of some additional pathways for the photogenerated charges. In our setup, the SPV is defined as SPV = (SPon − SPoff), where SPon and SPoff are the SP under illumination and in darkness, respectively. Starting from a sample that has been kept in darkness for a long time, and therefore can be considered completely relaxed, the examined area was illuminated to determine the SPV by using a “two pass method”.26 In this “two pass” method each SP image x-horizontal line is acquired twice, first in darkness (OFF pass) and then under illumination (ON pass). That is, the illumination protocol consists of cyclic periods of illumination and darkness (2 s each). In this way, two SP(t) images, one in darkness (SPoff(t) image) and one under illumination (SPon(t) image), are simultaneously recorded. Hence, the SPV(t) image, defined as SPV(t) = (SPon(t) − SPoff(t)) can be directly obtained by subtracting the corresponding images. Notice that, in the present experiments, the tip is spatially scanned in a sample region (x,y), and the SP images crosstalk time dependence and spatial information. As widely explained in ref 26, to obtain the spatially averaged mesoscale SP(t) curve from the SP images, the average SP of each horizontal line is plotted vs the mean line acquisition time. This scanning option has been used to gain statistical information from large sample regions in one single experiment. However, to check the reliability of our results, similar experiments have been performed in one spatial line. In this way, the image y-axis is directly related with the time. This type of experiments leads to exactly similar results (see Supporting Information). In the present work, this “two pass” method has been used to distinguish between fast and slow relaxation processes. Notice that generally SPon(t) and SPoff(t)) depends on the previous illumination (relaxation) time that determines the degree of excitation of the system at a given time t. Therefore, from the time evolution of these signals, it is possible to distinguish between “fast” and “slow” processes. A process faster than the acquisition time (∼30 ms) is seen as a jump in the SP signal while a slow process presents a monotonic time evolution.12

3. RESULTS AND DISCUSSION 3.1. Local Surface Potential in Darkness. In order to determine the SPV, it is necessary to first characterize the SP of the samples without any external illumination. This “in darkness” equilibrium state will be used as reference to quantify the light induced changes in the following sections. Figure 1c and d shows the SP images for the ZnO surface before and after H2PPIX functionalization. From the SP image of the bare ZnO (Figure 1c), a mean SP value of about −580 mV can be extracted (Figure 1e). The SP is homogeneously distributed (RMS 30 mV) and only small brighter spots appear within the sample surface (pink dots in the image) (Figure 1c). These spots, smaller than the grain size, are ascribed to filled localized surface electron trap states.29 The mean SP of the ZnO/H2PPIX sample is shifted to around SPZnO_H2PPIX= −100 mV, larger work function compared to the bare surface (Figure 1e). Moreover, its distribution around the mean SP value is more inhomogeneous. SP regions with higher and lower SP values are now regions of several grains size (pink and blue regions in Figure 1d) with SP ZnO_H2PPIX_pink= −50 mV and SPZnO_H2PPIX_blue = −210 mV, respectively (see profile in Figure 1e). Since the functionalized H2PPIX surface basically has the same surface morphology as that of the bare ZnO (Figure 1b), the differences in the SP should be associated with a different molecule−substrate coupling. In more detail, the semiconductor work function Ws is defined as follows:12 Ws = (Ec − E F)b + χ − Δϕs − eVs

(1)

where Ec is the energy of the conduction band bottom, EF is the Fermi level, χ is the electron affinity, Δϕs is the surface dipole, and Vs is the semiconductor surface band bending. Since the first two terms are inherent bulk properties, the SP changes associated with the molecular adsorption (ΔSPads) are correlated with variations of the semiconductor work function due to changes on the band bending (ΔVS) and/or on surface dipole (Δ(Δϕs)), ΔSPads = SPZnO−H2PPIX − SPZnO−Bare = Δ(Δϕs) + eΔVs (2)

In the present case, where samples consist of polycrystalline films covered with chemically anchored H2PPIX, both terms may contribute to ΔSPads. On the one hand, a change in the band bending may be due to the carboxylic molecule−ZnO bond that induces “extrinsic” surface states and/or pasivates “intrinsic” ones. It has been reported for different ZnO faces that the amount and position of these surface states depend on the bonding configuration as well as on the surface coverage.14 On the other hand, the covalent molecule−substrate bond induces a surface dipole whose magnitude depends on the ionicity of the bond. Particularly, it is known that the contribution to the surface dipole of the carboxylic bond strongly depends on the molecular tail but not much on the bonding configuration.30 Moreover, surface coverage and molecular organization also contribute to the Δ(Δϕs) term by the intrinsic dipole moment of the molecules. Hence, in the ZnO/H2PPIX sample where only one type of molecule is present, differences between pink and blue SP regions should be related to regions where the molecule−semiconductor bond is different (different band bending) and/or where the molecule arrangement and coverage varies (different surface dipole). 16785


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Figure 2. Image of the SP for the bare ZnO (images size = 1 × 1 μm2, z scale = 300 mV). SPoff (a) and under illumination SPon (b), images acquired during the “two pass” method. (c) SPoff(t) and SPon(t) curves obtained from (a) and (b), respectively. The t = 0 has been set as the moment when the light is initially switched on (dashed line in (a) and (b)). The relaxation of the SPoff(t) once the light is completely switched off has been included (with the SP image inserted). (d) SPV image (z scale 300 mV).

and black profile on the left panel of Figure 3c), while in other extended regions (blue circle in Figure 3a and black profile on the right panel of Figure 3c) the SPoff value is in between the “in darkness” SP and the SPon value. In addition, it should be highlighted that although the initial “in darkness” SP value is different for pink and blue regions, under illumination, their SPon mostly coincide (purple dots in Figure 3c). Notice that these two regions correspond to the regions discussed before which presented higher (pink) and lower (blue) SP with respect to the mean SPZnO_H2PPIX value before any illumination. Hence, it can be concluded that these two types of regions have not only a different initial “in darkness” SP but also different photoinduced response. In order to disentangle the origin of this different light induced behavior and the relation with the interaction and organization of the molecules within the ZnO surface, we analyze more in detail the factors that play a role in the SPoff(t), SPon(t), and SPV(t) signals (Figure 3c and e). To analyze the pink and blue SP regions separately, the spatially line averaged analysis used before is not valid, since different regions coexist within the sample. Therefore, a mask-based analysis has been applied. In more detail, instead of calculating directly the averaged SP for each image horizontal line, based on the different photoactivity, the SPV image (Figure 3d) has been used to generate two complementary masks selecting the “pink” and “blue” regions, respectively. Then, these masks are applied to the SPoff, SPon, and SPV images, and the regions are analyzed separately using the same line average processing explained before.8 The first point that should be addressed is the fact that, under illumination, the entire surface presents a similar SPon, i.e., similar work function. In other words, which factors determine that the illuminated SP value is the same for the pink and blue areas? Remembering eq 1, the SP is determined by the sum of two factors: surface dipole and band bending (Δϕon s or eVson). Under direct photoexcitation, electrons are transferred from the excited molecule to the semiconductor conduction

To further study the ZnO/H2PPIX system and the charge transfer processes, the SP and the SPV of the samples has been studied under illumination (λ = 405 nm) using the “two pass” method explained in the Materials and Methods. 3.2. Photoinduced Surface Potential Changes: Charge Injection and Charge Relaxation. When the bare ZnO sample is initially irradiated, the mean SPon(t = 0), is shifted up by about +45 mV, as seen in the color change in Figure 2b and the abrupt step in the SP(t) signal of Figure 2c. The explanation for this small SPon shift can be found in the fact that this photon energy is close to the ZnO bandgap absorption edge, and therefore sub-bandgap electron transitions can take place between the valence band and the empty localized states near the conduction band, shifting the SP to higher values.31 Moreover, the fact that the SPon(t) and SPoff(t) signals of the “two pass” method are essentially identical (Figure 2c), and consequently the SPV image (Figure 2d) has no significant features, indicating that the excited electrons do not have enough time to relax during the “in darkness” part of the two pass cycle. Once the light is completely switched off, several tens of minutes are needed to recover the initial SP, confirming that electrons are trapped in long-lived states (Figure 2c). The functionalization of the ZnO surface with H2PPIX molecules radically changes the semiconductor light-induced response (Figure 3). The first difference is that, under illumination, the mean SPOn ZnO_H2PPIX is shifted to lower values (change to darker color in Figure 3b) opposite from the behavior of the bare surface. This confirms the excitation of the H2PPIX molecules and the electron−hole pair splitting by injecting the electron into the ZnO. Since an SPV signal implies spatial charge separation, excitation-recombination processes within the molecule would not lead to any measurable SPV.12 The second difference is that, with the molecules, the SPoff and SPon images acquired with the “two pass” method (Figure 3a and b) are not equivalent. The SPoff image (Figure 3a) presents regions, where the SPoff value almost recovers the initial “in darkness” SP value instantly (pink circle in Figure 3a 16786


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Figure 3. Image of the SP for the ZnO_H2PPIX sample acquired during the “two pass” method (images size = 1 × 1 μm2, z scale = 300 mV), SPoff (a) and under illumination SPon(b). (c) SPoff(t) and SPon(t) curves for the pink (left panel) and blue (right panel) SP regions, (d) SPV image (z scale = 300 mV) (e) SPV(t) signal for the high (top panel) and low (bottom panel) SP regions.

band or to surface localized states.32 The SPon steady-state is achieved when thermodynamic equilibrium between electron injection and electron recombination is established (Figure 4), keeping in mind that the SPon differs from the relaxed “in darkness” SPoff only if the injection time of the excited electrons is faster than the recombination one, leading to a net charge density at the surface generated by the positively charged oxidized molecules. This charged layer, together with the electrons at the semiconductor surface, generates a positive surface dipole that decreases the work function, and therefore magnitude is shifts the SPon to lower values. The Δϕon s proportional to the surface charge density and in our case, to the surface density of excited molecules in the illuminated state. Under illumination, the band-bending change is negligible and, as will be shown below, the pink and blue SP regions present a different “in darkness” band bending. This means that the surface charge density is large enough, Δϕon s to become the dominant contribution to the SPon signal. Hence, we can conclude that in the H2PPIX_ZnO system, the SPon of the

illuminated excited steady-state is mainly determined by the density of excited H2PPIX molecules which is basically independent of the specific molecular bonding and/or configuration over the surface. The second open question is why the SPV(t) signal is different in pink and blue SP regions, in other words, how the surface dipole and the surface band bending affect the photoinduced response of the two different regions. When the light is initially switched on, two instant SPVs are found in the pink and the blue SP regions, SPVpink(t = 0) ≈ −300 mV and SPVblue (t = 0) ≈ −100 mV (green arrow in Figure 3c and e). Since SPon(t), which is dominated by the photodipole contribution, is similar for both regions, it is clear that the SPV(t) differences have to be related with the corresponding SPoff(t) signal (black dots of Figure 3c). In addition, the other photoresponse difference between the pink and the blue SP regions arises on the SP relaxation during the OFF part of the “two pass” illumination protocol. In both regions, the SP relaxation has a fast and a slow component, as 16787


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≈ − 200 mV) and restores most of the initial “in darkness” SP. In blue SP regions, this fast contribution is only about 50% (SPfast blue ≈ − 50 mV) of the initial SPVblue (blue arrows in Figure 3c and e). The remaining SP corresponds to a slow relaxation slow component with SPslow pink ≈ − 50 mV and SPdark ≈ − 50 mV in pink and blue regions, respectively (red arrows in Figure 3c. and 3e). The origin of these fast and slow SP relaxation components can be understood as follows. Under illumination (Figure 4b), the injected electron (1) can be transferred away to the semiconductor bulk, (2) can be trapped in a surface state before recombination, or (3) can quickly recombine by tunneling back to the dye molecule.32 When the illumination is switched off, the system relaxes to equilibrium by recombining the excited dye molecules (typically in the fs−ps time scale) and by restoring the ZnO surface “in darkness” relaxed state. The photoinjected electrons that stayed closer to the surface (processes (2) and (3)) instantaneously (in our time scales) recombine with the oxidized molecules. On the contrary, during relaxation the photoinjected electrons that were transferred into the semiconductor bulk (process (1)) should return to the surface. Since the ZnO is the only electron supplier to restore the final charge equilibrium, if the surface barrier is high, then bulk electrons would take a long time to overcome the surface potential barrier and reach the surface to restore the equilibrium. This difference in the time scale processes leads to the slow and fast charge relaxation component, respectively. The fact that the SPfast contribution is smaller in blue SP regions, while the SPslow contribution is basically the same for both (SPslow ≈ −50 mV), points toward a similar nature of the slow recombination process. The larger SPfast component together with the larger “in darkness” SP value found in the pink SP regions indicate that the density of surface states and, therefore, the band-bending, is larger than in the blue SP regions. As explained above, this difference in the band-bending should be due to different carboxylic bonding configuration. Hence, we can conclude that in pink and blue SP regions, the molecule bonding type is different and that the bonding configuration determines the region’s photoresponse.

Figure 4. Schematic illustration of the band structure of the semiconductor−dye system. (a) In darkness relaxed state and (b) steady-state under illumination. Red and blue arrows correspond to excitation and recombination processes, respectively. Only the relevant contributions to the SPV signal have been included.

shown in the corresponding SPoff(t) and SPV(t) signals (Figure 3c and e). There is a fast contribution because SPon and SPoff signals are different, and there is a slow contribution because none of the regions recover the initial “in darkness” SP values, indicating that system has not completely relaxed during the OFF part of the two pass illumination protocol. However, their magnitude is different depending on the region. In pink SP regions, about 80% of the total SPVpink is quickly relaxed (SPfast pink

Figure 5. (a) SP image of the ZnO/H2PPIX:CDCA overimposed with the corresponding topography. The bottom panel shows the z color scale of the SP image together with a profile around the mean SP value. (b) SPoff and SPon images acquired during the “two pass” method and SPV image obtained from them (z scale = 300 mV). (c) SPoff(t) and SPon(t) signals (d) SPV(t) signal. All the images are 1 × 1 μm2 size. 16788


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ACS Applied Materials & Interfaces 3.3. ZnO/H2PPIX:CDCA Sample. To gain further insight into the charge transfer processes on H2PPIX functionalized ZnO surfaces, chenodeoxycholic acid (CDCA) was coimmobilized with the H2PPIX. The coadsorption of CDCA and dye molecules on semiconductor oxides avoids molecular aggregation and favors the upright molecule configuration.33 Figure 5a shows the topography AFM image of this surface, together with the SP. The coadsoption of CDCA does not modify the ZnO surface morphology compared to the bare ZnO and the ZnO/ H2PPIX samples, but again it has profound effects on the sample’s photoresponse. The means the “in darkness” SPH2PPIX:CDCA sample is shifted about +340 mV with respect to the bare one but shifted downward, about −150 mV and −50 mV compared to the SPH2PPIX_pink and SPH2PPIX_blue, respectively. The SPH2PPIX:CDCA distribution is more homogeneous than that of the SPH2PPIX one. Apart from the homogeneous background, a few high and low SP regions are found (Figure 5). The high SP regions present the same “in darkness” SP and SPV as the high SP regions of the ZnO/H2PPIX sample and can be ascribed to pure H2PPIX regions. The low SP regions do not show any photoactivity, and they are probably due to pure CDCA regions. To quantify the effect of the CDCA coadsoption on the photoinduced sample response, we will focus only on the homogeneous areas where H2PPIX and CDCA molecules coexist. The corresponding SPoff(t) and SPon(t) and the SPV(t) are shown in Figure 5. Initially, a SPVZnO_H2PPIX (t = 0) ≈ −134 mV is found. Moreover, the SP relaxation is mainly due to a slow contribution as seen from the SPon(t) and SPoff(t) signals. Although the overall SPV(t) behavior is similar to that of “blue” SP regions of the ZnO_H2PPIX (SPVblue(t = 0) ≈ −100 mV vs SPVZnO_H2PPIX (t = 0) ≈ −134 mV), the addition of CDCA molecules reduces the fast component (SPfast ZnOH 2PPIX:CDCA ≈ − 30 fast mV vs SPblue ≈ − 50 mV) while the slow one increases slow (SPslow ZnOH 2PPIX:CDCA ≈ − 100 mV vs SPblue ≈ − 50 mV). It is wellknown that the use of CDCA as an additive results in a significant improvement of the cell efficiency in DSSC.34 Recalling that SPfast and SPslow relaxation components are related to photoinjected electrons filling surface states and electrons pushed into the bulk, respectively, we can conclude that CDCA reduces the density of surface states. Until now, CDCA has been generally used as coadsorbent in DSSCs. However, this finding recommends the extension of its use to other applications such as nanomedicine and photocatalysis where the charge transfer efficiency may be crucial too. Taking into account the antiaggregate effect of the CDCA, it has been found that pink SP regions in ZnO/H2PPIX and ZnO/H2PPIX:CDCA samples are related to H2PPIX aggregates and that the addition of CDCA effectively inhibits their formation. Moreover, these aggregates show a completely different photoresponse behavior compared to the H2PPIX:CDCA mixture that is related not only to a different molecule organization but also to a different molecule bonding configuration.

adsorbed molecules, the SP in darkness and the SPV signal are highly inhomogeneous and can be correlated with regions where the H2PPIX−ZnO bond type and molecular aggregation differ. These SP regions are randomly distributed within the sample surface and do not show any ZnO−facet dependence. Under illumination, the SP distribution becomes homogeneous, indicating that the main contribution to the excited SP comes from the surface positive charge density, due to the charged oxidized molecules, that is independent of the specific region. The origin of the photoactivity differences is found in the charge excitation and relaxation processes that contribute to the SP signal: a fast and a slow component that are related to photoinjected electrons filling surface states and electrons pushed into the bulk, respectively. The coadsoption of CDCA mostly inhibits the formation of high SP regions, confirming that these regions are due to H2PPIX aggregates. In addition, the SPV signal is dominated by the slow contribution while the fast one is reduced. To completely disentangle the origin of the different SP contributions and their implication in the device performance, further studies such as the spectral dependence of the SP and field-induced SPV measurements are needed. However, this work shows that even in “a priori” simple systems with only one type of molecule and a narrow absorption band, the sample presents a rich nanostructure that strongly determines the sample photoactivity. KPFM makes possible to study a semiconductor/dye interface with high lateral resolution which allows detection of spatial variations in the surface photovoltage and surface potential, studying the photoactivity of the different regions individually. These results highlight the importance of the nanoscale characterization of real device architectures to complement the macroscopic techniques that could explain ambiguous results and the lack of reproducibility.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b03544.

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Description of the “two pass” method (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (C.R.). *E-mail: [email protected] (E.P.-L.). Funding

This work has been partially funded by the Ministerio de Economia y Competitividad and (MINECO, Spain (Grants MAT2013-47192-C3-2-R, MAT2013-46593-C6-4-P), FEDER (EU) through the projects ENE2013-48816-C5-1-R -C02-01, Force for Future CSD2010-00024, Fundacion Seneca 15324/ PI/10, and Basque Department of Education (Grant IT-62113). Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS The nanoscale characterization of polycrystalline ZnO thin films functionalized with H2PPIX and H2PPIX:CDCA shows that, although the surface morphology is not substantially altered by molecule immobilization, the work function and photoactivity drastically change. In samples with only H2PPIX

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Nanophotoactivity of Porphyrin Functionalized Polycrystalline ZnO

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