Adsorption and Polymerization of Formaldehyde on Cu(100) Todd R. Bryden and Simon J. Garrett* Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824-1322 ReceiVed: July 14, 1999; In Final Form: October 7, 1999 The adsorption of formaldehyde (H 2 CO) on clean Cu(100) at 85 K has been studied using electron energy loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD). For coverages up to (1.06 ( 0.22) × 10 15 H 2 CO molecules/cm 2 , formaldehyde spontaneously polymerized to form a monolayer of disordered poly(oxymethylene) (POM), arranged with the chain directions parallel to the surface plane. Thermal decomposition/desorption of the polymer monolayer occurred by three routes, producing peaks in temperature-programmed desorption (TPD) at approximately 177, 200, and 215 K. The lowest temperature peak was exclusively associated with production of H 2 and CO in approximately equal proportions. The two higher temperature peaks were produced by molecular H 2 CO generated via depolym- erization of the polymer. The 200 and 215 K features displayed zero- and first-order desorption kinetics, corresponding to estimated activation energies for depolymerization of 75 ( 10 and 53.9 ( 0.5 kJ/mol, respectively. The presence of two polymer desorption peaks is attributed to chain conformational differences present within the monolayer and has not been previously observed in studies of formaldehyde adsorption on metal surfaces. Large exposures of H 2 CO on this surface formed multilayers of molecular formaldehyde on top of the first polymer layer. The second layer desorbed at 105 K and subsequent layers at ∼100 K. 1. Introduction The desire to create unique surfaces with specific chemical or physical properties has focused attention on monolayers and multilayers of organic molecules or polymers. Through innova- tive synthetic design, polymeric materials can combine high chemical, mechanical, and thermal stability with tailored adhe- sion, wettability, tribological, electronic, or optical properties. Hence, macromolecular thin films are promising candidates for a wide range of technological applications including corrosion protection 1,2 and chemical sensing media. 3 Additionally, the low surface free energy and negligible aqueous solubility of most polymers means they may be employed as materials for in vivo applications. 4 Ordered polymer films with crystalline character are espe- cially important for technological applications since they generally possess superior optical and electronic properties compared to their amorphous counterparts. In particular, interest in photonic materials 5,6 has focused a great deal of effort on the development of techniques for producing crystalline, mac- romolecular thin films on solid surfaces. The polymer thin film may be deposited onto a solid surface either preformed or as a monomer adlayer followed by subsequent in situ polymerization. Methods investigated to date include the deposition and po- lymerization of unsaturated Langmuir-Blodgett (LB) films 7,8 and, more recently, self-assembled monolayers (SAMs). 9-14 In these cases, the physical structure of the polymer film is known to be influenced by the atomic arrangement of the underlying solid substrate. For example, it has been known for some time that deposition of poly(oxymethylene), -(CH 2 -O) n -, from solution onto cleaved alkali halide single crystals generates extended polymer chains aligned with specific substrate crystal directions. 15 A similar effect has been observed in studies of the polymerization of ether-containing molecules upon adsorp- tion onto a graphite surface. 16 The preferred directions appear to strongly correlate with minimal lattice mismatch between the macromolecular chain and surface unit cells. There are relatively few fundamental studies of small molecule polymerization reactions on surfaces. There is some precedent for thermal polymerization of H 2 CO to poly(oxy- methylene) (POM) on several metal surfaces: O/Ag(110), 17 Ni- (110), 18 Pt(111), 19 Pd(111), 20 O/Rh(111), 21 O/Pd(111), 22 NiO- (100), 23 and Cu(110). 24 Thermal polymerization of acetaldehyde on O/Ag(111) has also been observed. 25 Ultraviolet photons or low-energy electrons have been shown to initiate polymerization of TCNQ, 26 dinitrobenzene, 27 thiophene, 28,29 formaldehyde, 30,31 and styrene 32 on metal surfaces. For most of the systems that have been investigated to date, little is known about the adsorbed monolayer structure prior to polymerization, the polymer morphology, or the details of the reaction mechanism. Moreover, the correlation between the atomic arrangement of the substrate and polymer has not been systematically studied. We are interested in elucidating the polymerization initiation, propagation, and termination mech- anisms operative for ordered monolayers of unsaturated small molecules and the influence of surface electronic and crystal- lographic structure on the polymer film order, chain length, conformation and direction(s) in relation to specific crystal directions. Ultimately, control of some or all of these processes may enable the design of high-quality crystalline polymer thin films. To our knowledge, the possibility of controlling polym- erization through the use of a surface acting as a reaction “template” (lattice-controlled or topotactic reaction) has not been investigated. We present here results from an experimental investigation of the adsorption and polymerization of formaldehyde (H 2 CO) adsorbed on Cu(100). The gas, solution, and solid-state chem- istry of formaldehyde is well-known. 33,34 Formaldehyde has a low activation barrier to polymerization (11.7 kJ/mol), 33 and * Corresponding author. E-mail: garrett@cem.msu.edu. 10481 J. Phys. Chem. B 1999, 103, 10481-10488 10.1021/jp992353u CCC: $18.00 © 1999 American Chemical Society Published on Web 11/10/1999