Thin Solid Films 565 (2014) 155–164

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Investigation of phosphonic acid surface modifications on zinc oxidenanoparticles under ambient conditions

Rosalynn Quiñones a,⁎, Kate Rodriguez b,1, Robbie J. Iuliucci b,1

a Marshall University, Chemistry Department, 1 John Marshall Drive. Huntington, WV 25755, USAb Washington and Jefferson College, Chemistry Department, 60 South Lincoln St, Washington, PA 15301, USA

⁎ Corresponding author. Tel.: +1 304 696 6731; fax: +E-mail address: quinonesr@marshall.edu (R. Quiñones

1 Tel.: +1 724 503 1001x6132; fax: +1 724 223 6155.

http://dx.doi.org/10.1016/j.tsf.2014.06.0570040-6090/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 17 May 2013Received in revised form 11 June 2014Accepted 14 June 2014Available online 6 July 2014

Keywords:Self-assemblyThin filmsZinc oxideNanoparticlesPhosphonic acidHydrogen bondingSolid-state nuclear magnetic resonanceSurface coverage

Zinc oxide (ZnO) nanoparticles have emerged as a fascinating metal oxide semiconductor nanomaterial duelargely to their wide array of properties that can be altered by surface modification. For example electrical andphotonic properties include a range of conductivity frommetallic to insulating (n-type and p- type conductivity),wide-band gap semiconductivity, room-temperature ferromagnetism, and chemical-sensing. Recently there hasbeen much interest in the electronic and photonic properties of ZnO nanostructures as foreseeable applicationsinclude solar cells and laser diodes. For such purposes, controlling the surface functionalization is importantand can be tailored by the chemical attachment of organic acids to the surface. The oxide surface readily reactswith organics forming self-assembled alkylphosphonate films. In this study, ZnO nanoparticles were modifiedusing self-assembly thin films with phosphonic functional head groups. The amount of organic acid used inpreparation of the thin film was shown to be important to the nanoparticle surface coverage. The modifiedZnO nanoparticles were then characterized using infrared spectroscopy, powder X-ray diffraction, solid-state nu-clear magnetic resonance, and scanning electron microscopy-energy dispersive X-ray spectroscopy. The interfa-cial bonding was identified by spectroscopy analysis to be the bidentate and tridentate motifs between thephosphonic head group and the oxide surface. Work function modification was measured using Ultravioletphotoelectron spectroscopy. The influences of temperature, humidity, and solvent rinse on the stability of thesurface modifications were performed.

1 304 696 3243.).

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, increasing attention has been paid to zinc oxide(ZnO) nanoparticles as a result of their wide breadth of applications,from improving the corrosion resistance of chemical coatings to servingas sources of UV light emission [1,2]. Several unique properties of ZnO,such as its wide band gap (3.37 eV), high exciton binding energy(~60 meV), and stable wurtzite structure have made it an ideal candi-date for these and other optical and electronic applications [1,3,4]. Thegrowing interest in nanotechnology has increased the focus on ZnOnanostructures. ZnO nanorods, for example, have been shown to havesignificant usefulness in the creation and maintenance of solar cells[5–8]. Recent studies have also centered on the modification of ZnOnanoparticles via surface-level attachment of various molecules andthe subsequent characterization of the altered nanoparticles. In particu-lar, it has been shown that organic acids may be attached to ZnOnanoparticles and that these attachments have a significant impact

on properties such as the electronic configuration of ZnO nanoparticles[9].

Organic acid attachments are chemically bonded onto the surface ofnanoparticles via the formation of self-assembly organic thin films. Thinfilms (monolayer and multilayers) have emerged as an alternativeand useful strategy for arrangingmolecular components on various sur-faces [6,10–12]. They are formed spontaneously by chemisorption,yielding robust, well-defined structures on substrates [11,13,14]. Self-assembled films are particularly useful when they contain modifiabletail groups; these bifunctional thin films provide an effective and inex-pensive method of tailoring the surface properties of the nanoparticles[15–18].

The formation of organic self-assembled films on metal oxide sur-faces has been widely investigated for a variety of applications. For ex-ample, the attachment of organic acid to and the synthesis of titaniumdioxide (TiO2) nanoparticles have attracted immense interest, as hasthe attachment of alkanethiols onto gold surfaces [19–24]. Phosphonicacids have been used to anchor dyes to TiO2 anatase thin films in thepreparation of solar cells, modify TiO2 membranes, enhance the adhe-sion of thin polymer films to titanium, and improve bone binding totitanium dental or orthopedic implants [25–27]. The study of these or-ganic attachments onto ZnO nanoparticles, however, has been more

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limited. Alkanephosphonic acids have additionally been shown tomod-ify planar ZnO surfaces, while providing increased corrosion resistanceas compared to alkanethiols [28,29]. Interface thin films with self-assembling properties have been used to improve the charge transferbetween organic layers and metal oxides through covalently bondingthe thin film onto the surface of the ZnO [6]. The ZnO thin film canhave several effects, including passivation of the surface charge trapsto improve forward charge transfer, tuning of the energy level offsetbetween semiconductors and organic layers, and an effect on theupper organic layer morphology [6,11,28].

Due to the potential of applications of self-assembly organic thin filmson ZnO, an in-depth study is necessary. In this study, phosphonic acid-functionalized self-assembled thin films were used to modify the ZnOnanoparticle surface. Various phosphonic acids with slight variationin chain lengths and polarities were explored: octadecylphosphonicacid (ODPA), 16-phosphonohexadecanoic acid (COOH-PA), and (12-phosphonododecyl) phosphonic acid (Di-PA). The molecular structuresof these acids are displayed in Fig. 1. The nature of the interface phenom-ena is not well understood for these bifunctional acids [17,30–32]. Bothfunctional groups, the carboxylic and phosphonic acids can form stronghydrogen bonds and both can chemical bond to the oxide surface [10,33–35]. The hydrogen bonding may enable layering or aggregation [32].In COOH-PA, a competitive situation arises and in the Di-PA bridgingcan occur. Therefore, in this study methyl, carboxylic and phosphonicacids terminated thin films were analyzed to further understand theseinteractions. Furthermore, a study with different amount of ODPAand an understanding about surface coverage was attained.

Factors such as the means by which the phosphonic acid headgroups attach to ZnO and the alkyl chain ordering of the organic thinfilms contribute to the overall surfacemodification of the nanoparticles;for this reason, confirmation of attachment and characterization of bothbonding and ordering are immensely important [28]. The surfacemodified nanoparticles were characterized using Attenuated TotalReflectance Infrared Spectroscopy (ATR-IR), powder X-ray diffraction(PXRD), scanning electron microscopy–energy dispersive X-rayspectroscopy (SEM/EDX), and solid state nuclear magnetic reso-nance (SS-NMR). Surface modified samples were stored for twelvemonths, exposed to acidic/basic solutions, and solvents to prove sta-ble and durable films were created. This paper is a comprehensivestudy of self-assembly alkylphosphonic acid film chemically bondedon the ZnO nanoparticle surface. Ultraviolet Photoelectron Spectros-copy (UPS) was used to determine electronic structure changes in-duced by the presence of these thin films to report work function.Work function, which is relevant to the integration of electronicdevices, was shown to be highly depended on coverage, and phos-phonic acid type [36–39].

2. Experimental section

2.1. Methods and materials

ZnOnanopowder (particles b 100 nm in size), ODPA, (97.0%), COOH-PA, (97.0%), (Di-PA (97%), and isopropenyl chloroformate (IPCF, 97%)

Fig. 1.Molecular structures of organic a

were purchased from Sigma Aldrich. Tetrahydrofuran (THF, HPLCgrade), hexanes (ACS reagent grade), methanol (MeOH, 200 proof),nitric acid (ACS reagent grade), and sodium hydroxide pellets werepurchased from PharmCo-Aaper. All reagents were used as receivedwithout further purification.

2.2. Preparation of the samples

For preparation of the adsorbed molecules, 0.35 g of ZnO nanoparti-cles was dispersed in 30 mL of THF by sonication. Then, differentamounts ranging from 0.01 to 0.09mmol of each organic acid were dis-solved by sonication in 6 mL THF. The 30 mL ZnO solutions were com-bined with the 6 mL acid solutions. After sonication, the mixtureswere left stirring for 48 h and were allowed to evaporate at room tem-perature. The dry samples were dispersed again in 15 mL of THF andfurther sonicated for 15 min and then filtered using gravimetric filtra-tion (filter paper #42) to eliminate the solvent [17,40–42]. This rinsetechnique is known to eliminate acid molecules weak physicallybound to the nanoparticles. The samples were allowed to dry at roomtemperature.

2.3. Stability tests

In order to assess the stability of the self-assembled films on the sur-face of the nanoparticles, three stability tests were performed: solvent,time, and humidity. For the solvent tests, 250 mg of each of the surfacemodified nanoparticles (0.045 and 0.09 mmol samples) was dissolvedin 2–3 mL of hexane. The same solvent test was done with methanol.Then, the samples were left in their respective solutions for 4 days,dried, and stored in sealed glass vials at ambient temperatures. Toexamine durability, the samples were analyzed every month by ATR-IR for a year following their preparation date. To test resistance to hu-midity, separate samples were analyzed after storage under 10% ± 6%,35% ± 6%, 94% ± 3% and 100% ± 1% relative humidity, respectively.The temperatures of these various humidity samples were held con-stant (23 ºC ± 0.5 ºC) for 7 days before ATR-IR analysis.

2.4. Characterization of the films

2.4.1. ATR-IRATR-IR was performed using a Thermo Scientific Nicolet IR200 and

was used to analyze the alkyl chain ordering and bonding mode of themolecules to the surface. The unmodified ZnO nanoparticles wereused as the background spectra for analysis purposes. Typically, 1024scans were collected.

2.4.2. PXRDPXRDwas used to characterize the bulk andmodified nanoparticles.

Diffraction patterns obtained for the modified samples were comparedwith patterns from the database to ensure ZnO was the principle com-ponent of the nanoparticles. The PXRD patterns of sampleswere obtain-ed using a PANalytical X'pert PRO MPD powder X-ray diffractometerwith the X'celerator detector operating at 45 kV and 40 mA in the

cids bonded to ZnO nanoparticles.

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Bragg–Brentano geometry and using Cu KR radiation. Scans wereperformed from 5° to 80° 2 θ with a step with of 0.017°.

2.4.3. SEMSEMwas performed using a Hitachi S-3400 N scanning electron mi-

croscope. The samples were mounted onto double-sided carbon tape,which was adhered to an aluminum specimen holder and examinedunder high vacuum conditions. A 2.0 kV accelerating voltage and a9.0 mmworking distance were employed.

2.4.4. SEM/EDXSEM/EDX was performed using an FEI NanoSEM 630 FESEM. The

EDX was collected with an Oxford Silicon Drift X-Ray Detector anddata were analyzed using the Oxford Aztec Energy Analyzer software.The chamber of the SEM was held under high vacuum conditions.Accelerating voltages ranged from 3 to 10 kV. SEM/EDXwas used to an-alyze the surface composition of the nanoparticles, obtain informationabout particle size, and elemental composition analysis.

2.4.5. SS-NMRSS-NMR spectra were acquired with a Varian Inova spectrometer

and 9.39 T Oxford magnet. Samples were packed into 4 mm zirconiarotors. A Varian T3 double resonance MAS probe was tuned to a 1H fre-quency of 399.814 MHz and a 13C frequency of 100.543 MHz. Magic-angle-spinning (MAS) of 10 kHz was employed. The 1H/13C cross-polarization (CP) pulse sequence used a 2.1 μs 90° 1H pulse. Protonmag-netization transferred in 5 ms under Hartman–Hahn conditions. A50 ms acquisition length was used to digitize a 50 kHz wide spectrum.The signal was average until desired signal-to-noise ratio was achieved.The FIDwas Fourier transformedwith 16Hz of line broadening and zerofilled with 32 k points. The 13C spectra were referenced externally toTMS by measuring the resonance frequency of the methyl carbon in 3-methylglutaric acid at 18.84 ppm.

For 31P SS-NMR analyses, MAS probewas tuned to a 1H frequency of399.814 MHz and a 31P frequency of 161.865 MHz. The 1H/31P cross-polarization pulse sequence used a 2.1 μs 90° 1H pulse. Proton magneti-zation transferred in 3 ms under Hartman–Hahn conditions. A 100 msacquisition length was used to digitize a 100 kHz wide spectrum. Thesignal was average until desired signal-to-noise ratio was achieved.The FID was Fourier transformed with 16 Hz of line broadening andzero filled with 32 k points. The 31P spectra were referenced externallyby measuring the resonance frequency of aqueous 85% phosphoricacid.

2.4.6. UPSThe work functions were measured by UPS. Surfaces were modified

as described above and placed into a PHI 5000 VersaProbe ESCAMicroprobe system (ULVAC-PHI) with a windowless He dischargelight source (HIS 13 VUV source, Focus GmbH) that providedHe (I) excitation at 21.2 eV. The UPS had a base pressure of 5.0× 10−2 mbar and a current of 80 mA. The power for the VUV sourcewas 39 W with a 1.7 mm beam diameter and incident angle of 45°.The take-off angle of the photoelectronwas set at 90° and a pass energyof 1.175 V was used. Three to five areas of each sample were character-ized using the UPS.

3. Results and discussion

3.1. Attenuated total reflectance infrared spectroscopy (ATR-IR)

ATR-IR was used to distinguish chemisorbed from physisorbedorganic thin film on the surface of the nanoparticles. In addition, chang-es on phosphonic head group IR frequencies indicate the manner bywhich the self-assembly thin films were bonded to the surface and thealkyl chain stretches reflect their trans/gauche conformation. The ZnOnanoparticles were modified via surface chemical adsorption of three

organic acids with phosphonic functional groups (ODPA, COOH-PA,and Di-PA). This led to the formation of self-assembled organic thinfilms on the surface of the nanoparticles. Since ZnO by itself is freefrom organic IR stretches, spectra readily reveal the thin film formation.Rinsing nanoparticles in THF followed by sonication is an effectivemeans to remove weakly bound physisorbed films [40,43,44]. Thepresence of organic stretches (2800 to 3000 cm−1) that exist aftersonication as shown in Fig. 2A, C, and F verifies that the attachmentswere both stable and chemically bonded.

In the spectra of the modified ZnO, the C-H stretches of the methy-lene group are used as the reference peaks for the phosphonate thinfilm organization. The C-H stretch has two common vibrations: asymmetric stretch corresponding to a peak at ~2850 cm−1 and anasymmetric stretch corresponding to a peak at ~2918 cm−1 [14,45].These two wavenumbers can be shifted depending on the alkyl chainconformation. Peaks shifted to higher values (νCH2 asym N 2918 cm−1

and νCH2 sym N 2848 cm−1) indicate a disordered thin film with gaucheconfiguration in the alkyl chain. When wavenumber values shift tolower values (νCH2 asym ≤ 2918 cm−1), the thin film is considered or-dered, with all trans alkyl chain conformation [13,14,45]. In this study,the values of νCH2 after rinsing and sonication were νCH2 asym /νCH2 sym =2918 cm−1/2848 cm−1, 2917 cm−1/2848 cm−1, and 2918 cm−1/2848 cm−1 for nanoparticles modified with ODPA, COOH-PA, and Di-PA, respectively (Fig. 2A, C, and F). This indicates that the attachmentsof phosphonic acids were strongly-bonded and with alkyl chains inboth the trans/gauche conformations.

The nature of the interaction between the phosphonic head groupand the surface can be determined from the shifts and broadening ofthe phosphonic acid (νP¼O, νP−O, νPOH) stretches. The existence of P-Ostretching either corresponds to bi- and/or tridentate bonding ordenotes a multidentate phosphonate-metallic ion interaction [17,32].Specifically, the PO region of bulk ODPA control shows νP¼O =1225 cm−1 and 1215 cm−1, νP−O = 1077 cm−1 and 1003 cm−1, andνPOH =957 cm−1, 948 cm−1, and 932 cm−1(Fig. 2B, black line). In con-trast, the PO region of the modified ZnO-ODPA 0.045 mmol displayedpeaks that were shifted from those of the bulk control ODPA(Fig. 2B,red line). While the PO region proves that the thin film is chemicallybonded to the surface, the bonding type (bi-or tridentate) is uncertain.For ZnO-ODPA, P = O stretching occurred at νP¼O = 1170 cm−1,indicating a shift towards formation of phosphonate bonding on the sur-face. Other PO stretchingwithin this region for ZnO-ODPA0.02mmolwasobserved at values of 1116 cm−1, 1050 cm−1, 980 cm−1, and 951 cm−1

(Fig. 2B, gray line). These stretches, although within the P-O andP-OH regions, cannot be absolutely verified and the exact nature of thebonding of ODPA to the ZnO surface cannot be deduced [46,47].

COOH-PA was bonded onto ZnO surface in which the infraredstretches were observed at νP−O = 1147 cm−1 and 1091 cm−1, andνPOH = 1064 cm−1 and 923 cm−1 compared to bulk control COOH-PAthat has stretches of νP¼O = 1213 cm−1, νP−O = 1076 cm−1 and1007 cm−1, andνPOH =951 cm−1 and 933 cm−1(Fig. 2D). The carbonylregion in the IR was analyzed and a peak at 1702 cm−1, which corre-sponds to the νC¼O of free carboxylic acid was observed for ZnO-COOH-PA (Fig. 2E). Bridging, which is a phenomenon that occurswhen both the head (i.e., phosphonate group) and tail ends (i.e., carbox-ylate group) of a molecule attach to the surface of interest, does not ap-pear to be occurring here. Some bands attributable to the formation ofsurface carboxylate bonds were observed as previously reported,although the presence of hydrogen-bonded carboxylic acid groups isexpected [48,49].

The IR spectra of bulk Di-PA control show peaks at νP¼O =1217 cm−1, νP−O = 1080 cm−1 and 1018 cm−1, and νPOH =995 cm−1 and 949 cm−1; while spectra of Di-PA bond to ZnO surfaceshow peaks at νP¼O =1230 cm−1, νP−O =1153 cm−1 and 1064 cm−1,and νPOH = 926 cm−1 (Fig. 2G). These types of stretches indicate thatthe Di-PA is chemically bonding to the surface of the nanoparticles byat least one phosphonic acid group [13,46]. The possibility of bridging

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Table 1Summary of results from infrared spectroscopy are listed.

Organic acids P –O region (cm-1)

ZnO – ODPA (multilayer) 1170, 1116, 1050, 980, 951ZnO – ODPA (monolayer) 1163, 1110, 1060, 1034,ZnO – COOH-PA 1147,1091, 1064, 923ZnO – Di-PA 1153, 1064. 926

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where both phosphonic acid groups chelate to the surface is plausible.These data are summarized in Table 1.

The modifications of ZnO created by different amounts (0.01, 0.02,0.045 and 0.09 mmol) of acid resulted inmodest changes to the IR spec-tra. The relative reflectance was observed to be proportional to the ap-plied amount, which suggests a change in surface coverage. However,no significant changes in wavenumbers were observed indicating thatthe bonding type remained consistent, the one exception being themodified ZnO that was created by the lowest applied amount of acid(0.01 mmol). The changes noted in the C-H region for this sample is ex-plained by a more disordered monolayer. This could be expected as alower coverage would promote disordered films. This conclusion isfurther supported by the SEM/NMR data (see below).

Fig. 3. Scanning electron microscope images of 0.01 mmol ZnO-ODPA (A), 0.045 mmol ZnO-O(C and D).

Fig. 2. Infrared spectra of ZnO nanoparticles modifiedwith ODPA, COOH-PA, and Di-PA are displfrom the PO region (B, D, and G). The C-O region (E) is shown only for the COOH-PA treated sarespectively. Black lines are the acid control samples.

3.2. Scanning electron microscope

The shape and physical structure of particles modified and unmodi-fied with organic phosphonic acids were observed using a SEM /EDX.SEM image of the ZnO modified shown in Fig. 3 indicated that therewas little or no change to the nanoparticles morphology during themodification of the ZnO surface by chemically adsorbed phosphonicacid films compared to surface of ZnO unmodified. It could be observedby the EDX phosphorus Kα1 analysis (Fig. 3C, and D) that the films wereuniformly distributed across the surface of each nanoparticle withmodest aggregation. A distribution of cluster sizes is apparent in theSEM images. Measurement of the particle diameters demonstrates anincrease with the average as well as an increase in the distribution ofdiameters upon attachment of phosphonic acid molecules on ZnOnanoparticles. These measurements are summarized in Table 2.The modifications of ZnO with phosphonic acid at different amountare revealed in Fig. 3A and B images of ZnO-ODPA (0.01 mmol,0.045 mmol). It was worth pointing out that the 0.045 mmol was ad-vantageous for uniform coverage of phosphonic acid film on ZnO nano-particles as observed by EDX phosphorus Kα1 analysis (Fig. 3D). For thisreason, 0.045 mmol samples were chosen to examine when analyzingthe remaining two organic acids (COOH-PA and Di-PA). The fact thatsurface modified nanoparticles are more agglomerated was previously

DPA nanoparticles (B), and phosphorus Kα1 of 0.045 mmol ODPA on ZnO analysis by EDX

ayed. The spectral regions corresponding to C-H stretches (A, C, and F) are shown separatemple. Red and gray lines refer to nanoparticles treated with 0.045 and 0.02 mmol of acid,

Table 2SEM data summary of particle sizes of ZnO unmodified and modified (0.045mmol acidconcentrations) with phosphonic acid thin film.

Modifications Average Particle Size (nm) Particle Distribution (±nm)

ZnO 106 27ZnO – ODPA 145 53ZnO – COOH-PA 161 61ZnO – Di-PA 194 83

Fig. 4. Overlay of PXRD patterns shown for unmodified ZnO (black) and surface modifiedwith the various phosphonic acids ZnO- COOH-PA (green), ZnO-ODPA (red), and ZnO-Di-PA (blue). The expanded region displays the pattern for angles 5 to 30 Theta.

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explained by the so-called “zipper effect” of the surface-bound alkylchains.[49,50] This effect is caused by dense, well-packed moleculesand thus, the agglomerates are difficult to break apart, seemingly “zip-pered” together. Thus, an increase in particle diameter is expected.

Besides the use of EDX to visualize the surface coverage, the electronbeam can be held fixed to quantitatively measure elemental composi-tion. Themorphologies and spotted elemental composition of phospho-rus Kα1 of ODPA on ZnO are shown in Fig. 3D. As expected the EDXelemental analysis revealed the presence of zinc, oxygen, and phospho-rus ions in all examined sections of ZnO-ODPA, ZnO-COOH-PA, andZnO-Di-PA samples, respectively, whereas the unmodified sample re-vealed only zinc and oxygen ions. (Table 3) The percent phosphorouscomposition was found to be proportional to the applied molar ratioof organic acid. Surface coverage could be estimated based on averageparticle size and the relative elemental phosphorous composition. TheEDX spectrum further confirmed that the ZnO surface was modifiedusing various phosphonic acid films. The results following EDX analysisof these sections are quantified in Table 3. The analysis of ODPA surfacecoverage from the EDX data resulted in values of 29%, 167%, and 231%.Thin films created by amounts of the organic acid ranging from 0.045mmol or higher were calculated to bemultilayer films (182%–248% sur-face coverage). Incomplete coverage (surface coverage below 80%) wasdetermined by films prepared by 0.01 mmol acid. A 60%–85% surfacecoverage is expected for samples prepared using 0.02 mmol acid.

3.3. Powder X-ray diffraction

The X-ray powder diffraction patterns of unmodified and modifiednanocrystalline ZnO are shown in Fig. 4. Observed (unmodified-blackline) and calculated (*) X-ray powder diffraction patterns are alsoshown in Fig. 4. All XRD results display the same pattern from 30° to80° two θ range, which corresponds to hexagonal wurtzite ZnO phase[49]. The wurtzite ZnO phase is free of any peaks below 30°. The low in-tensity peaks in the 5°–25° region are indicative of the specific function-al groups of the organic acid attachment. An expansion of the smallangle region of the diffraction pattern of the unmodified ZnO comparedto modified samples is shown in Fig. 4 (top right). Moreover, diffractionpatterns of each bulk phosphonic acid control displayed similar diffrac-tion patterns (not shown). Therefore, chemical binding of the phos-phonic organic layers at the surface was accomplished [31,32,41,51,52]. No traces of secondary phases (cubic zincblende) are observed onthe ZnO nanoparticles. Indeed, all the XRD patterns show that the

Table 3SEM/EDX data summary of ZnO unmodified and modified with phosphonic acid film andelemental analysis.

Modifications EDX elementalcomposition (%)

Surface Coverage (%)a

Zn O P

ZnO 58.22 41.78 0.00 __0.010 mmol ZnO – ODPA 56.58 43.28 0.14 290.045 mmol ZnO – ODPA 54.99 44.21 0.79 1670.090 mmol ZnO – ODPA 57.87 40.98 1.15 2310.045 mmol ZnO – COOH-PA 60.71 38.61 0.67 1480.045 mmol ZnO – Di-PA 60.48 38.19 1.33 161

a Estimated coverage from elemental analysis.

phosphonic acid modified the structure and remained at the surface ofthe ZnO nanoparticles even after sonication.

3.4. SS-NMRSS-NMR can be a powerful tool for assessing chemical reactions at

surfaces as well to analyze different functional materials of interest. Inorder to better characterize the grafting of the alkylphosphonic acidsat the surface of ZnO nanoparticles, 13C and 31P SS-NMR experimentswere carried out.

3.4.1. 13C SS-NMRThe methylene 13C chemical shift of phosphonate thin film reveals

the conformation of alkyl chains [41]. The downfield methylene shiftof an ordered film (all trans conformation)moves upfield upon disorder(increasing number of gauche conformations) due to theγ-gauche effect[31]. The domain size of the ordered and disordered chains is reflectedin the 13C spectra because the relative populations of trans and gaucheconformations influence the 13C chemical shift of the interiormethylenecarbon [17,18,31,53]. Both trans and gauche domains for the phosphonicacid thin films adsorbed on ZnO surface studied here are revealed by the13C CP/MASNMR spectra. In Fig. 5, as previously observed for other self-assembly systems, the methylene resonances broaden and shift uponadsorption [31,41].

Fig. 5. Solid-state 13C CP-MAS NMR spectra of ODPA modified with different concentra-tions and bulk control.

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For the bulk ODPA control, the methylene carbons resonated be-tween 22.8 and 34 ppm (Fig. 5). Themodifications of ZnOwith differentamounts (0.01, 0.02, 0.045 and 0.09 mmol) of ODPA to form films wereanalyzed by SS-NMR. The series of spectra with varying amounts of theacid displays an interesting feature. Disordered gauche domains shiftupfield. The upfield shift of the methyl group at the chain terminals ascompared to the methyl in bulk ODPA control has also been explainedby gauche conformation [17,32]. The distribution of trans and gauchedomains results primarily in two broad peaks. Two methylene compo-nents are clearly evident in the ZnO spectrumwith the surfacemodifiedwith the least concentrated (0.01 mmol)ODPA solution. In addition, thegauche domain is shifted upfield the furthest (31.3 ppm) than the othersamples. According to the estimates based on the SEM data, only a frac-tion of the surface is covered. The NMR spectrum (Fig. 5) displays thatthe film is deposited in two ways with trans/gauche distributions. Asmore acid is deposited, ordering increases as evident in both the loss

Fig. 6. Solid-state 13C CP-MAS NMR spectra of COOH-PA methylene region (A), COOH-PAcarbonyl region (B), and Di-PA (C) modifications and bulk controls.

of the gauche peak (30.7 ppm) and the narrowing of the trans peak(33.2 ppm), which are seen in the 0.02 mmol spectrum. As the amountwas increased to 0.045 mmol, ODPA adsorbed on ZnO displays a singleintense component at 33.7 ppm for methylene groups suggesting highconformational order with few gauche defects. The 33.7 ppm peak ap-pears to split as the applied amount further increases to 0.09 mmol.While the development of the peak splitting may suggest more gaucheconformations, the surface coverage at this point is above 180% andthe possibility of multilayer film being present may explain the addi-tional peak at 33.0 ppm [10].

Similar changes to themethylene resonances occur for the COOH-PAsystem. For bulk COOH-PA control, the methylene resonates between34 and 38ppm for the all trans conformation (Fig. 6A) [17,32].When ap-plied to the ZnO surface, the methylene α to the COOH-PA (peaks at38 ppm) disappears upon adsorption, while the interior methylene po-sitions shift to an intense peak at 33.0 ppm (Fig. 6A). The upfield shift ofthe methylene group suggests some gauche conformation at the chainterminals as compared to the methylene in bulk COOH-PA control. An-other major spectral change was observed in the carbonyl region. Thebulk COOH-PA control has a peak at 184 ppm while the COOH-PAafter modification shifts upfield to 182 ppm (Fig. 6B). The carbonylpeak would shift downfield by 3 ppm if bonded to the ZnO surface.The carbonyl peak in stearic acid, 181.9 ppm for the bulk, moves to185.5 ppm when chemically bonded to ZnO surface (spectra notshown). It is plausible that this upfield shift may be caused by hydrogenbondingwithin the thinfilm orwith additional layers,whichwould giverise to a more extensive but also a more disrupted hydrogen-bondednetwork due to the carboxylic acid terminated layer or layers [17,51].

The ZnO surface modified with Di-PA exhibited themethylene reso-nance between 26 and 33 ppm with an intense peak at 33.2 ppm(Fig. 6C). This corresponds to trans/gauche conformations, comparedto bulk Di-PA control that showed a dominate methylene resonance at35 ppm [10,28,32]. Multilayer growth must be considered, similar toZnO-COOH-PA, since in this spectrum bifunctional phosphonic acid at0.045 mmol was used to modify the ZnO surface [10,12,51,53].

3.4.2. 31P SS-NMRThe 31P chemical shift of the phosphonic head group can reflect the

diversity of potential surface bonding (mono-, bi-, or tridentate) motifsand hydrogen bonding environments [53]. Due to the fact that the 31Pchemical shift is influenced by hydrogen bonding and other associativeinteractions, the variation of the isotropic shifts was further examined.All of the control spectra corresponding to the alkylphosphonic acidsdisplay narrow 31P peaks indicating homogenous and ordered mate-rials. Upon adsorption to ZnO, multiple broad peaks between 38 and

Fig. 7. Solid-state 31P CP-MAS NMR spectra of ODPA modified with different concentra-tions, and bulk control.

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22 ppm appear that is typical of self-assembly phosphonate thin filmson metal oxide nanoparticles. The upfield shift is associated with thechemically bonded phosphonic acid film with the ZnO surfaces[17–19,34,54,55].

The ZnO samples modified with ODPA shows many 31P resonancesbetween 22.0 and 35.9 ppm reflecting the diversity of potential bond-ing. (Fig. 7) An X-ray photoelectron spectroscopy (XPS) study ofsingle-crystal ZnO substrates modified with N-hexylphosphonic acidrevealed that the primary bonding form is tridentate phosphonatechemisorbed on ZnO surface [28]. ZnO films deposited onto silicon orgold substrates and modified with ODPA were also proposed to havetridentate bonding to the surface [28]. In this study, at lower concentra-tion (0.01 and 0.02 mmol), only a single resonance at 35.7–35.9 ppm isobserved, suggesting one preferred type of surface bonding that corre-sponds to thepreviously proposed tridentate. Additional resonances de-velop between 33.1 and 35.9 ppm when the coverage goes above 100%(0.045 mmol). This could be the development of an extra bidentatebonding due to multilayer formation as surface coverage increases orcaused by the increased presence of intramolecular interactions [12,17,18,55–57]. Finally, more upfield peaks emerge at higher coverage.The film created by 0.09 mmol ODPA showed multiple resonances at22.0 ppm in addition to new resonances between 33.1 ppm and 36.5ppm. These upfield peaks may be caused by the presence of excesssurfactant and the nature of the interaction between the phosphonicacid headgroup and the metal oxide surface [29].

31P CP/MAS-NMR spectra of the ZnO modified nanoparticles withCOOH-PA (Fig. 8A) are compared to the spectra of the SS 31P spectraof the bulk COOH-PA control. The COOH-PA 31P chemical shift changesfrom a single peak at 30.8 ppm to multiple peaks ranging from 31.4 to28.7 ppmupon adsorption on the ZnOnanoparticles with the resonancebroadened as compared to the unbound acid. The broadening can beexplained as a distribution of binding sites andmodes of the phosphonicacid headgroup onheterogeneous surfaces [53,58]. Similar to theODPA-ZnO system, the increased number of peaks present when COOH-PAadsorbs may be caused by different types of surface bonds and bondingsites on ZnO surface [33]. The use of carboxyalkylphosphonic acid tomodify ZnO nanowires and bare ZnO wafers has been previouslydescribed as resulting from either a tridentate chelating binding, atridentate bridging binding, or a bidentate binding moiety based on X-ray photoelectron spectroscopy (XPS) and FT-IR [17,31,59].

The spectra of the ZnO modified by Di-PA show peaks at 31.4and 29.4 ppm; one peak shifted upfield compared to Di-PA control(31.4 ppm). (Fig. 8B) An upfield shift has been associated with theacid bonded to the ZnO surface [10,12]. Therefore, it is reasonable to as-sociate the 29.4 ppm peak observed here to the phosphate involved inthe surface bonding. The peak positioned at 31.4 ppm has a similar

Fig. 8. Solid-state 31P CP-MAS NMR spectra of COOH-PA

shift to that of the bulk Di-PA control may correspond to the tail phos-phate group. The presence of a resonance with a shift similar to thebulk suggests that the terminal phosphonic acid is available to be func-tionalized for further chemical reactions. Previous FT-IR study demon-strated the “free acid” capable of carrying out chemical reactions atthe interface [10,34]. A multilayer stack may be composed of Di-PAmolecules due to strong hydrogen bonds, resulting in better molecularstability and stronger interactions with the ZnO surface than methylterminated layers [28]. However, covalent binding of Di-PA to the ZnOsurface combined with strong affinity hydrogen bonding among theterminal groups or hydrogen bonding onto the ZnO surface maycomplicate/allow for precise conclusion regarding the type of surfacebonding [17,53].

3.5. UPSThe work function of ZnO nanostructures has been reported to be

4.2–4.4 eV [59–63]. It has been demonstrated that surface potential isdirectly proportional to the effectivework function [62,64]. Here, the ef-fective work function of ZnO and ZnO modified surface was measuredby using ultraviolet photoelectron spectroscopy (UPS). The unmodifiedZnO nanoparticles measured 4.4 eV in agreement with the literaturevalue. The work functions of the methyl-terminated modified samplesranges from 3.4 to 5.4 eV depending of surface coverage and film thick-ness (Table 4). Work functions of metal oxide surfaces can also be opti-mized over a continual range by changing the acid concentration usedto prepare the film as previously observed [39,65]. The work functiondecreases as the surface coverage of ODPA also decreases. Furthermore,the work function tends to increase as multilayer formation occurs. Theincrease is due to surface defects being minimized by the growth ofphosphonic multilayers. Carboxylic and phosphonic acid-terminatedphosphonic acid layerswere also analyzed; theirwork function is signif-icantly higher, 5.4 to 5.9 eV. This effect of the electronegative terminatedfilms has been previously demonstrated and explained by surfacedipoles generated by the organic film [62,63,66].

3.6. Stability test analysisThe stability of thin films on metals is a concern especially for long-

term applications [14,29,40,43,45,67–69]. The stability of self-assemblyorganic thin films systems on and their affinity toward metal oxidemainly depend on different factors such as the type of head groupsand their affinity toward metal oxide, hydrogen bonds functionalizedtail groups of the molecules, and van der Waals interactions betweenalkyl chains.[43,44] The quality of the organic thin films can be moni-tored under several different conditions. The samples exposed to airunder atmospheric conditions were analyzed each month for twelveconsecutive months. The self-assembly thin films remained trans/

(A) and Di-PA (B) modifications and bulk controls.

Table 4Work functions (Φ) of ZnO unmodified andmodifiedwith phosphonic acid films by usingUPS analysis.

Modifications Work function Φ ± 0.1 eV)

ZnO 4.40.010 mmol ZnO – ODPA 3.30.020 mmol ZnO – ODPA 3.90.045 mmol ZnO – ODPA 4.00.090 mmol ZnO – ODPA 4.40.020 mmol ZnO – COOH-PA 5.40.045 mmol ZnO – COOH-PA 5.60.020 mmol ZnO – Di-PA 5.60.045 mmol ZnO – Di-PA 5.9

163R. Quiñones et al. / Thin Solid Films 565 (2014) 155–164

gauche conformation (2918/2848 cm−1) and bonded to the surface asindicated by the lack of change in the IR spectra over the course oftwelvemonths. Samples exposed to polar or nonpolar solvents (metha-nol and hexane) also had no effect on the stability of the thin films. SS-NMR analysis, performed on samples aged for twelve months, alsoremained unchanged. Based on these ATR-IR and SS-NMR data, theself-assembled films appear to be stable. The modified samples werethermally annealed to 100 °C and the films remained intact after amonth of analysis.

Relative humidity has been shown to affect the performance of solarcells as well as influence corrosion rates [70–72]. Here, IR spectroscopywas used to show the critical level of humidity for OPDA modified ZnOsurface. The samples, stored at 50% relative humidity showed no changein the IR spectra (2918/2848 cm−1). However, ODPA films (0.01, 0.02,0.045, and 0.09 mmol) stored at 95% and 100% relative humidity dem-onstrated an increase in disorder of the film based on the IR frequencies(2920/2851 cm−1). Therefore, water adsorbed on the surface could cor-rode the stable organic layer from the ZnO surface.

4. Conclusion

In this study, ZnO nanoparticles were modified by the attachmentof the phosphonic head groups with three different organic tails(methyl, phosphonic, and carboxylic terminated functional groups)by self-assembly. Through characterization by ATR-IR and PXRD,the alkylphosphonic acid molecules were found to attach via thephosphonic acid and form strong uniform covalent bonds on the sur-face of the ZnO nanoparticles without causing secondary phases. Thethin film on the particle proved durable as the film remained intact tothe surface after ambient storage (1 year) and even exposure tosolvent rinses and acid/alkaline bath. This study demonstrates astraight forward method of surface modification that will enablethe tailoring of the Zn oxide physical and chemical properties by re-actions with the terminal functionality of the thin film. Tail groupfunctionality can affect thin film formation and stability; thus, thesurface's hydrophobicity can be altered by the selection of tailgroup (hydrophobic when terminated by an organic chain and hy-drophilic when terminated with an acid). Moreover, hydrogen bond-ing between acid terminal groups in self-assembled thin film isthought to enhance the film's stabilization.

SS-NMR was helpful in gaining a better understanding in thebonding of the phosphonic acid head group on the ZnO surface. The13C SS-NMR spectra revealed alkyl chain order based on the methyleneshifts that vary depending on existing in gauche or trans conformations.The order was observed to increase with increasing surface coverage. Inaddition, the 13C spectrum of COOH-PA suggests that the carboxylic acidis not involved with bonding at the surface. A diversity of phosphonicbonding is evident by the numerous 31P chemical shifts. The singlepeak in the low coverage spectra of ODPA suggests that the downfieldpeak corresponds to the surface bonding, which has previously beenattributed to a tridentate bonding.[28] The appearance of upfield

resonances suggests the formation of additional bonding (i.e.,bidentate) or intermolecular surface interactions.

The effects of themodifications on the particle size of the ZnO nano-particles were investigated via SEM. This analysis showed, as expected,that the attachment of the organic thin films slightly increased theaverage particle size. Carboxylic and phosphonic acid-terminated thinfilms are capable of hydrogen bonding among each other (intermolecu-lar aggregation) as well as to polar surfaces or adsorbate layers. Nearlymonodisperse ZnO nanoparticles were properly functionalized sub-strates with good control of the surface coverage. Using diphosphonicacid, multilayer films could be formed with precisely controllablethickness and layer sequence, which can be utilized for different futureapplications. Work functions can be tuned by adjusting the surfacecoverage, film thickness, and type of deposited acids.

Acknowledgments

The National Science Foundation (NSF) Grant award CHE-0956755and internal funding from Washington & Jefferson College providedfunding for this research. We want to thank the Penn State Nano-fabrication Facility for the SEM/EDX that was purchased using fundsfrom the NSF, Grant No. CHE-0923183. We gratefully acknowledgeDr. Jennifer A. Aitken and Kimberly Rosmus (Duquesne University) forthe assistance with the PXRD and the Hitachi SEM. Instrumentationswere purchased using the following NSF grants; DUE-0511444 for thePXRD and MRI-CHE-0923183 for the Hitachi SEM. Finally, the authorswould also like to thank Dr. Weiqiang Ding from West Virginia Univer-sity (WVU) Shared Research Facilities for the use of the UPS.

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